Photostable Anisometric Lanthanide Complexes as Promising Materials for Optical Applications

Andrey Knyazev; Maxim Karyakin; Yuriy Galyametdinov

doi:10.3390/photonics6040110

Photostable Anisometric Lanthanide Complexes as Promising Materials for Optical Applications

Knyazev, Andrey;Karyakin, Maxim;Galyametdinov, Yuriy 2019-10-25 00:00:00 hv photonics Article Photostable Anisometric Lanthanide Complexes as Promising Materials for Optical Applicationsy 1 , 1 1 , 2 Andrey Knyazev * , Maxim Karyakin and Yuriy Galyametdinov Kazan National Research Technological University, 68 Karl Marx, Kazan 420015, Russia; belgesto@list.ru (M.K.); yugal2002@mail.ru (Y.G.) Zavoisky Physical-Technical Institute, FRC Kazan Scientiﬁc Centre of RAS, 10/7 Sibirsky tract, Kazan 420029, Russia * Correspondence: knjazev2001@mail.ru y This paper is an extended version of our paper published in: Knyazev, A.A.; Galyametdinov, Y.G. Luminescent materials based on anisometric lanthanide complexes. In Proceedings IV International Conference on Applications in Optics and Photonics (AOP 2019), Lisbon, Portugal, 31 May–4 June 2019. Received: 17 September 2019; Accepted: 22 October 2019; Published: 25 October 2019 Abstract: Uniform luminescent ﬁlms with high optical quality are promising materials for modern molecular photonics. Such ﬁlm materials based on -diketonate complexes of lanthanides have the following application problem: rapid luminescence degradation under UV radiation, low thermostability, poor mechanical properties, and aggregation propensity. An alternative approach to solving these problems is the use of anisometric analogues of -diketonate compounds of lanthanides (III). The main advantage of such compounds is that they do not crystallize because of long hydrocarbon substituents in the structure of complexes, so they can be used to fabricate thin nano-, micro-, and macroscale uniform ﬁlm materials by a melt-processing technique at relatively low temperatures, as well as by spin-coating. The method of fabrication of microscale luminescent ﬁlm materials with controlled optical properties from anisometric analogues of Ln(DBM) Phen and Ln(bzac) Phen 3 3 complexes (Ln = Eu, Tb) is proposed in this paper. Within the framework of this research, we developed original ﬁlms which are highly uniform and transparent. An important advantage of these ﬁlms is their high photostability and potential for applications as reusable luminescent sensors and light converters. Keywords: lanthanide complexes; luminescence; thin ﬁlms; luminescent sensors 1. Introduction Molecular photonics is a modern branch of science and its active areas of study are development and fabrication of highly functional luminescent materials and devices [1–7]. Among many dierent types of luminescent compounds (e.g., organic compounds, quantum dots, metal-ligand complex), lanthanide (III) coordination compounds are very promising candidates for development of luminescent molecular photonics materials and devices due to their unique photophysical properties, such as a narrow-band luminescence with a high quantum yield and a long lifetime [8]. Lanthanide (III) complexes with aromatic -diketonate ligands are among the most interesting coordination compounds of lanthanides because they combine intensive monochromatic luminescence with attractive chemical characteristics (such as a relatively easy synthesis, good solubility in many basic solvents and a capability of incorporation into various host matrices, such as polymers and liquids crystals). Due to these properties, such compounds have a strong potential for a broad variety of applications. Uniform luminescent ﬁlms with high optical quality are promising materials for modern molecular photonics. Such ﬁlm materials based on -diketonate complexes of lanthanides have the following Photonics 2019, 6, 110; doi:10.3390/photonics6040110 www.mdpi.com/journal/photonics Photonics 2019, 6, 110 2 of 10 application problem: rapid luminescence degradation under UV radiation, low thermostability, poor mechanical properties and propensity to aggregation [9,10]. A possible solution is incorporating these compounds into host matrices (e.g., organic polymers, ionic liquids, zeolites, and etc.) [11–13]. There are also other methods of fabricating ﬁlm materials based on these compounds, such as electro deposition and sol-gel technology [1–3]. Another problem of many ﬁlm materials based on -diketonate complexes of lanthanides is that their adsorption band is limited by the UV range. Therefore, UV sources are required to excite luminescence in such ﬁlms, such sources are unsuitable in many applications for certain reasons (such as low brightness and high cost of such devices as compared to visible light sources). Thus, an urgent problem of molecular photonics is to broaden this adsorption band so it can include the visible light range. The solutions for this problem oered today are known to be based on the following approaches: synthetic modiﬁcation of -diketonate ligands and coordination of complex chromophores to tris( -diketonates) of lanthanides(III) with capabilities of ecient visible light adsorption. For example, it is challenging to select a proper solvent for both a matrix and a complex, where, in addition, this complex must not dissociate. An alternative approach to solution of these problems is the use of anisometric analogues of -diketonate compounds of lanthanides (III) [14,15]. The main advantage of such compounds is that they do not crystallize because of long hydrocarbon substituents in the structure of complexes and possess thermostable properties [16], so they can be used to fabricate thin nano-, micro- and macroscale uniform ﬁlm materials by a melt-processing technique at relatively low temperatures, as well as by spin-coating [17]. The ﬁlms produced by a melt-processing technique require a much simpler technology for fabrication of luminescent materials based on lanthanide compounds as compared to available analogues, because no solvents or photostabilizing matrices are needed in this case. Such ﬁlms have the following technological advantages: intense luminescence, high optical transmittance (up to 95%) in nearly entire visible and near IR ranges, high resistance to UV light, and controllability of their photophysical properties (e.g., the absorption band width and the luminescence intensity) [18,19]. In this paper, we discuss the opportunities of application of microscale ﬁlms based on anisometric europium(III) and terbium(III) -diketonate complexes as materials for photonics and optics. 2. Materials and Methods The synthesis of anisometric complexes was performed according to the technique developed earlier by our group. CHN elemental microanalysis was performed on a EA-1110 apparatus by CE Instruments. Europium elemental microanalysis was performed on a Bruker S8 TIGER X-ray ﬂuorescence spectrometer (Billerica, MA, USA). The thermal behavior of complexes was studied by polarized optical microscopy (POM) on an Olympus BX51 microscope equipped with a LINKAM high-precision heating system and by dierential scanning calorimetry (DSC) (Olympus America, Melville, NY, USA). DSC thermograms were recorded on a Mettler-Toledo DSC 1 Star module in the heating-cooling mode at the 10 C/min scan rate. Thin nanoscale ﬁlms of complexes were prepared from toluene solution (1 10 mol/L) by spin coating on a WS-650 MZ-23NPP Laurell Spin Coater (Laurell Technologies Corporation, North Wales, PA, USA). The transmittance spectrum at room temperature was recorded on a Perkin-Elmer Lambda 35 spectrophotometer. The luminescence spectra at room temperature were measured by a Varian Cary Eclipse spectroﬂuorometer. The photoluminescence excitation spectra of the vitriﬁed ﬁlms were recorded at room temperature on an experimental setup consisting of a MDR-2 excitation monochromator and a MDR-12 recording monochromator. A DKSL-1000 xenon lamp was used as an excitation source. The light signal passing through the MDR-12 monochromator was detected using a FEU-79 photomultiplier tube. The input and the output slit widths of the MDR-2 monochromator were set to 10 nm. The excitation spectra were recorded in the 230–430 nm range by varying the excitation light wavelength and monitoring the Photonics 2019, 6, x FOR PEER REV IEW 3 o f 10 Photonics 2019, 6, 110 3 of 10 spectra w er e r ecor ded in the 230 –430 nm r ange by varying the excitation light w avelength and monitoring the photoluminescence intensity at 545 nm and 612 nm, r espectively. To study the p photoluminescence hotostability of the intensi obtainty edat fil545 m m nm ater and ials,612 thenm, UVG respectively L-58 Manu.aT l o UV study -lamthe p (3photostabi 65 nm) w itlity h thof e 6the W capacity w as used. obtained ﬁlm materials, the UVGL-58 Manual UV-lamp (365 nm) with the 6 W capacity was used. Th The e p powders ow der s of of anisometric anisometriccomplexes complexeswer w ee r esynthesized synthesized accor acco ding r ding to tthe o thpr e p ocedur r ocedu e rdescribed e descr ibe in d ithe n th pr eevious pr eviopapers us pap[ e20 r s, 21 [20 ].,2 A 1]warm . A w a solution r m soluof tioLnCl n of Ln6H Cl3 · O 6H (0.1 2 Ommol) (0.1 min mo ethanol l) in eth was ano slowly l w as sadded lowly 3 2 a dr dopwise ded dr oto pw aihot se talcoholic o a hot al solution coholic s containing olution con 0.3 taimmol ning 0of .3 m-diketone mol of β -[d 22 ik], et0.1 onemmol [22], of 0.1Phen, mmoand l of P 0.3 hemmol n, and of 0KOH. .3 mmThe ol oformed f KOH.light-yellow The for med pr lecipitate ight -yellwas ow p ﬁlter r ecip ed, itapuriﬁed te w as fin ilte hot r edalcohol , pur ifie and d in dried hot alcohol and dr ied under vacuum. Then, the pr oduct w as dissolved in toluene and the obtained under vacuum. Then, the product was dissolved in toluene and the obtained solution was ﬁltered s dried olutio under n w asvacuum. filter ed dr ied under vacuum. Tr is[1-(4-(4-pr opylcyclohexyl)phenyl)decane-1,3-dione](1,10-phenanthr oline) terbium(III) Tris[1-(4-(4-propylcyclohexyl)phenyl)decane-1,3-dione](1,10-phenanthroline) terbium(III) ( (Tb(CPDK Tb (CP DK3–7)3)pphen). h en). YY i e ield: ld: 771% 1% (0 (0,107 ,107 g g). ). M Melting elting p point: oint: 1130 30 °C C. . E Elemental lemental a analysis nalysis (%): (%): c calculated alculated 3–7 3 for C87H119N2O6Tb : C, 72.17; H, 8.28; N, 1.93. Found: C, 71.68; H, 8.55; N, 1.95. for C H N O Tb: C, 72.17; H, 8.28; N, 1.93. Found: C, 71.68; H, 8.55; N, 1.95. 87 119 2 6 Tr Tris[1-phenyl-3-(4-(4-pr is[1-phenyl-3-(4-(4-pr o opylcyclohexyl)phenyl)pr pylcyclohexyl)phenyl)pr o opane-1,3-dionato](1,10-phenanthr pane-1,3-dionato](1,10-phenanthr ooline) line) eur opium(III) (Eu (CP Dk3-Ph)3Phen). Yield : 59% (0.087 g). Melting point: 142 °C. El emental analysis europium(III) (Eu(CPDk ) Phen). Yield: 59% (0.087 g). Melting point: 142 C. Elemental analysis 3-Ph 3 ( (%): %): c calculated alculated for for C C84 H H 89N N 2O O 6 EEu u (% (%): ): CC, , 73 73.40; .40; H H, , 6.6.53; 53; N N, , 22.04; .04; EEu,11.06. u ,11.06. FFound ound (% (%):C, ):C, 773.01; 3.01; H H, , 66.88; .88; 84 89 2 6 N, 2.12; Eu, 11.00. N, 2.12; Eu, 11.00. Tr Tris[1-(4-(4-pr is[1-(4-(4-propylcyclohexyl)phenyl)octane-1,3-dionato](1,10-phenanthr opylcyclohexyl)phenyl)octane-1,3-dionato](1,10-phenanthr ooline) line) e eur ur o opium(III) pium(III) (Eu(CP DK3-5)3Phen). Yield : 70% (0,105 g). Melting point: 110 °C. Elem ental analysis (%): Calcd (Eu(CPDK ) Phen). Yield: 70% (0,105 g). Melting point: 110 C. Elemental analysis (%): Calcd 3-5 3 f forC or C81H H 107Eu EuN N2O6O : C : ,C, 7171.61; .61; H,H, 7.9 7.95; 5; NN, , 2.0 2.06; 6; Eu Eu,11.20. ,11.20. FoFound: und: C, C, 7171.16;H, .16;H, 8.3 8.31; 1; N,N, 2.0 2.02; 2; EuEu, , 1111.50. .50. 81 107 2 6 The sensitive luminescent material w as pr epar ed fr om a pow der of the Eu (CP Dk 3–5)3Phen , The sensitive luminescent material was prepared from a powder of the Eu(CPDk ) Phen, 3–5 3 E Eu(CPDk u (CP Dk3-Ph)3)Ph Phen en oor r Tb Tb(CPDk (CP Dk3–7 )3p ) h phen en co complexes mplex es bby y aa m melt-pr elt -pr o ocessing cessing t te ec chnique. hnique. P Powders ow der s o of f 3-Ph 3 3–7 3 complexes w er e melted b etw een tw o quartz plates w ith a size of 7 × 15 × 0.5 mm placed on the stage complexes were melted between two quartz plates with a size of 7 15 0.5 mm placed on the o stage f an of Olan ym Olympus pus BX-5BX-51 1 polapolarizing r izing micmicr r osco oscope. pe. Afte After r hea heating ting upup toto th the e is isotr otr oopic pic lliquid iquid t transition ransition temper atur e, the sample w as quickly cooled dow n to r oom temperatur e to obtain vitr ifi ed films w ith temperature, the sample was quickly cooled down to room temperature to obtain vitriﬁed ﬁlms with 3 3 μ m m, , 6 6.1 .1 μm m or or 20 20 μm m thickness, thickness, which w hich wer w ere e sandwiched sandw iched between betw een two tw o quartz quar tz plates. plates. To To c contr ontr o ol l thickness, w e used 3 μm, 6.1 μm and 20 μm spacer s, r espectively. thickness, we used 3 m, 6.1 m and 20 m spacers, respectively. 3. Results and Discussion 3. Results and Discussion The structure of anisometric -diketonate complexes of lanthanides(III) is demonstrated in The str uctur e of anisometr ic β-diketonate complexes of lan thanides(III) is demonstr ated in Figure 1. Figur e 1. (a) (b) F Figure igure 1. 1. T The he s scheme cheme o of f the the anisometric a nis ometric analogues a na logues of of ((aa )) Ln(DBM) Ln(DBM)3Phen Phe n and a nd ((b b)) Ln(bzac) Ln(bza c)3Phen Phe n 3 3 c complexes omple xe s (Ln (Ln = = E Eu, u, T Tb). b). The thermal behavior of powders of anisometric complexes was studied by polarized optical The ther mal behavior of pow der s of anisometr ic complexes w as studied by polar ized optical microscopy (POM) and dierential scanning calorimetry (DSC). The presence of alkyl and cyclohexane micr oscopy (POM) and di ffer ential scanning calor imetry (DS C). The pr esence o f alkyl and substituents in the chemical structure of -diketonate ligands of anisometric -diketonate lanthanide(III) cyclohexane substituents in the chemical str uctur e of β-diketonate ligands of anisometr ic complexes prevents them from crystallization and allows for considerable reduction of the melting β-diketonate lanthanide(III) complexes pr events them fr om cr ystallization and allows for temperature to avoid decomposition at melting that is typical for their non-anisometric analogues consider able r eduction of the melting temper atur e to avoid decomp osition at melting that is typical Photonics 2019, 6, x FOR PEER REV IEW 4 o f 10 Photonics 2019, 6, x FOR PEER REV IEW 4 o f 10 for their non-anisometr ic analogues (Table 1). The phase tr ansitions deter mined by the POM method have been confir med by the DSC m ethod, as illustr ated in Figur e 2 for the Eu (CP Dk 3–Ph)3phen Photonics 2019, 6, 110 4 of 10 for their non-anisometr ic analogues (Table 1). The phase tr ansitions deter mined by the POM method complex. Acc or ding to the DS C heating scan, the isotr opic tr ansition temper atur e w as 142°C. The have been confir med by the DSC m ethod, as illustr ated in Figur e 2 for the Eu (CP Dk 3–Ph)3phen sample is vitr ified upon cooling, as can be concluded fr om the absence of the exother mic complex. Acc or ding to the DS C heating scan, the isotr opic tr ansition temper atur e w as 142°C. The (Table 1). The phase transitions determined by the POM method have been conﬁrmed by the DSC cr ystallization peaks in the DS C cooling scan. The combination of such pr operties allow s for sample is vitr ified upon cooling, as can be concluded fr om the absence of the exother mic method, as illustrated in Figure 2 for the Eu(CPDk ) phen complex. According to the DSC heating 3–Ph 3 fabr icating vitr ified luminescent films w ith high optical quality fr om melts of p ow der s of these cr ystallization peaks in the DS C cooling scan. The combination of such pr operties allow s for scan, the isotropic transition temperature was 142 C. The sample is vitriﬁed upon cooling, as can be substances (see Figur e 3 inser t). These films ha ve almost no cr ystalline effects, accor ding to the fabr icating vitr ified luminescent films w ith high optical quality fr om melts of p ow der s of these concluded from the absence of the exothermic crystallization peaks in the DSC cooling scan. The polar ized optical micr oscopy data [23]. The obtained vitr ified films w er e stable and r emained substances (see Figur e 3 inser t). These films ha ve almost no cr ystalline effects, accor ding to the combination of such properties allows for fabricating vitriﬁed luminescent ﬁlms with high optical transpar ent at r oom temper atur e over sever al months. It is impor tant to note that such films cannot polar ized optical micr oscopy data [23]. The obtained vitr ified films w er e stable and r emained quality from melts of powders of these substances (see Figure 3 insert). These ﬁlms have almost no be pr oduced fr om non-anisometric analogues of β-diketonate lanthanide(III) complex es because of transpar ent at r oom temper atur e over sever al months. It is impor tant to note that such films cannot crystalline eects, according to the polarized optical microscopy data [23]. The obtained vitriﬁed ﬁlms their str ong aggr egation pr opensity and high melting temper atur es. be pr oduced fr om non-anisometric analogues of β-diketonate lanthanide(III) complex es because of were stable and remained transparent at room temperature over several months. It is important to note their str ong aggr egation pr opensity and high melting temper atur es. that such ﬁlms cannot be produced from non-anisometric analogues of -diketonate lanthanide(III) complexes because of their strong aggregation propensity and high melting temperatures. DSC POM DSC POM Cooling Cooling -1 Heating -2 I -1 Heating -3 -2 40 60 80 100 120 140 160 180 200 Temperature (° C) -3 40 60 80 100 120 140 160 180 200 Temperature (° C) Figure 2. Diffe re ntia l sca nning ca lorime try (DSC) the rmogra m of Eu(CPDk3 -Ph)3Phe n. POM = Figure pola rize 2.dD oip ff te ic re an l t m iailcs rc oasn cn oip ny g.c alorimetry (DSC) thermogram of Eu(CPDk ) Phen. POM = polarized 3-Ph 3 Figure 2. Diffe re ntia l sca nning ca lorime try (DSC) the rmogra m of Eu(CPDk3 -Ph)3Phe n. POM = optical microscopy. pola rize d optica l micros copy. (a) (b) Figure 3. A microscale ﬁlm of Tb(CPDk ) Phen complexes between two quartz plates (a); Figure 3. A micros ca le film of Tb(CPDk3 – 7)3Phe n comple xes be twee n two quartz pla tes (а); (a) 3–7 3 (b) luminescence lumine sce nce excitation e xcita tion spectra s pectra o off T Tb(CPDk b(CPDk3 – 7) )3P Phen he n ccomplex omple x fi ﬁlms lms w with ith 3, 6.1 3, 6.1, , a and nd 20 20 µm m t thickness hickne ss 3–7 3 Figure 3. A micros ca le film of Tb(CPDk3 – 7)3Phe n comple xes be twee n two quartz pla tes (а); between be twe e n quartz qua rtz plates pla te sr recor ecor ded dedat a t545 545 nm nm wavelength wa ve le ngth and a ndr r oom oomtemperatur te mperatue re( b ( b ).). lumine sce nce e xcita tion s pectra of Tb(CPDk3 – 7)3Phe n comple x films with 3, 6.1, a nd 20 µm thickne ss Table 1. Melting temperatures of anisometric Ln(III) complexes and their commercial analogues. be twe e n qua rtz pla te s recorded a t 545 nm wa ve le ngth a nd room te mperature ( b). T able 1.Me lting te mpe ratures of a nis ome tric Ln(I I I ) comple xe s a nd the ir comme rcia l a na logue s . Complex MeltingPoint C Quantum Yield % Complex Melting Point °С Quantum Yield % T able 1.Me lting te mpe ratures of a nis ome tric Ln(I I I ) comple xe s a nd the ir comme rcia l a na logue s . Eu(DBM) Phen 185–187 decomposition [24] 4.9 [25] Eu(DBM)3Phen 185–187 decomposition [24] 4.9 [25] Complex Melting Point °С Quantum Yield % Eu(bzac) Phen 192–194 decomposition [24] 18 [26] Eu(bzac)3Phen 192–194 decomposition [24] 18 [26] Eu(CPDk ) Phen 110 [15] 30–32 [27] Eu(DBM)3Phen 185–187 decomposition [24] 4.9 [25] 3-5 3 Eu(CPDk3-5)3Phen 110 [15] 30−32 [27] Eu(CPDk )Phen 142 [19] 25–27 [27] 3-Ph Eu(bzac)3Phen 192–194 decomposition [24] 18 [26] Eu(CPDk3-Ph)Phen 142 [19] 25−27 [27] Tb(CPDk ) Phen 130 [22] - 3-7 3 Eu(CPDk3-5)3Phen 110 [15] 30−32 [27] Eu(CPDk3-Ph)Phen 142 [19] 25−27 [27] Heat Flow (mW) Heat Flow (mW) Photonics 2019, 6, x FOR PEER REV IEW 5 o f 10 Tb (CPDk3-7)3Phen 130 [22] - Photonics 2019, 6, 110 5 of 10 The technology of fabr ication of micr oscale vitr ified films fr om pow der s of anisometr ic β-diketonate lanthanide(III) complex es is ver y simple to implement because no solvents or solid The technology of fabrication of microscale vitriﬁed ﬁlms from powders of anisometric matr ices for photostabilization of complexes ar e used, as opposed to tr aditional appr oaches to -diketonate lanthanide(III) complexes is very simple to implement because no solvents or solid fabr ication of film mater ials based on lanthanide complexes. The follow ing pr ocedur e w as used to matrices for photostabilization of complexes are used, as opposed to traditional approaches to fabr icate our films: a small amount of a pow der complex w as placed betw een tw o quar tz plates and fabrication of ﬁlm materials based on lanthanide complexes. The following procedure was used to an isotr opic melt was cr eated by heating. Subsequent r apid cooling d ow n to r oom te mper atur e fabricate our ﬁlms: a small amount of a powder complex was placed between two quartz plates and an r esults in the glass tr ansition of the melt and for mation of a solid and unifor m film. This technology isotropic melt was created by heating. Subsequent rapid cooling down to room temperature results in allows for contr olling the thickness of films by spacer s, such as polystyr ene micr ospher es w ith the varglass ious d transition iameter . of the melt and formation of a solid and uniform ﬁlm. This technology allows for controlling the thickness of ﬁlms by spacers, such as polystyrene microspheres with various diameter. A peculiar featur e of vitr ified films is that their adsor ption (excitation) band width can be manA ipu peculiar lated byfeatur varyieng of th vitriﬁed e melt tﬁlms hickne is ssthat . Du e their to stadsorption r ong adsor p (excitation) tion capaciband ty of fwidth ilms wcan ith t be he manipulated by varying the melt thickness. Due to strong adsorption capacity of ﬁlms with the thickness exceeding 3 μm in the 200−380 nm r ange, it w as possible to detect only the long wave edge thickness of the adexceeding sor ption s3 p ec m tr u in m the . Lu 200 mi n380 escenm ncer e ange, xcitait tio was n sp possible ectr a arto e m detect or e in only for m the ativ long e bewave causeedge they of the adsorption spectrum. Luminescence excitation spectra are more informative because they allow allow for detecting changes (fr om var ious thicknesses of the films) in the entir e spectral range for (20detecting 0–450 nm changes ). Figur e (fr 3om b ilvarious lustr ates thicknesses the influen of ce the of ﬁlms) thickn in esthe ses entir of th eespectral Tb (CP D range k3–7 )3P (200–450 hen filmnm). s on Figure 3b illustrates the inﬂuence of thicknesses of the Tb(CPDk ) Phen ﬁlms on their excitation their excitation spectr a. The excitation spectra of a 3 μm film (monitor ed at 545 nm) consist of a 3–7 3 spectra. br oad ba The nd w excitation ith the ma spectra ximumof at a 35 36 nm m ﬁlm cor r e (monitor spondined g toat si545 nglenm) t -sinconsist glet tran of sita iobr ns oad in liband gands with . The the maximum at 356 nm corresponding to singlet-singlet transitions in ligands. The increase of the incr ease of the film thickness to 6 μm br oadens the excitation spectr um band to the violet visible ﬁlm r ang thickness e and shto ifts 6 t h m e br moadens aximum the of excitation the spectr spectr um. F um ur tband her in to crthe ease violet of thvisible e film range thicknand ess t shifts o 20 the μm maximum of the spectrum. Further increase of the ﬁlm thickness to 20 m creates a distinguished cr eates a distinguished shoulder of the spectr um in the 393 nm ar ea. β -dicar bonyl compounds ar e shoulder know n of to the forspectr m asum sociin ate the s i 393 n c nm oncar en ea. tr a te -dicarbonyl d solutions compounds . For exam ar p ele known , moleto cuform lar aassociates ssociates in of −2 concentrated solutions. For example, molecular associates of benzoylacetone in cyclohexane solution benzoylacetone in cyclohexane solution (С = 1 × 10 М) ar e r esponsible for the long-wavelength (C ba= nd 1 wi 10 th th M) e m ar ae xirm esponsible um at 400 for nm the in long-wavelength the absor ption a band nd ex with citatithe on maximum spectr a. Diat me 400 r ic nm spec in ies the in absorption and excitation spectra. Dimeric species in similar molecular systems were detected by similar molecular systems w er e detected by NMR spectr oscopy. W e consider that the featur es NMR menti spectr oned oscopy above .in W de icconsider ate clearthat ly ththe at th featur e thies ckn mentioned ess of the Tb above (CP indicate Dk3–7 )3Ph clearly en film that s inthe fluethickness nces their of the Tb(CPDk ) Phen ﬁlms inﬂuences their local structure (the ratio between individual molecules local str uctur e (the ratio betw een individual molecules and aggr egates) [2 8]. The pr esence of 3–7 3 and aggr aggr egategates) ed com [p 28 le ].xe The s in pr tesence he film of s aggr w ith egated var iocomplexes us thicknein ssethe s isﬁlms the with factovarious r r espon thicknesses sible for th ise the factor responsible for the long-wave “wing” of the excitation spectra of the ﬁlms. Thus, variation long-w ave “w ing” of the excitation spectr a of the films. Thus, var iation of the r atio betw een of inthe divi ratio dualbetween molecule individual s and aggrmolecules egates in tand he lo aggr cal egates str uctuin r e the of flocal ilms str (atuctur the se ta of geﬁlms of th(at eir the fabr stage icatio of n) their fabrication) allows for manipulating the width of absorption and excitation bands and creating allows for manipulating the width of absor ption and excitation bands and cr eating optically optically transpar e transpar nt lumin ent escluminescent ent mater ialsmaterials, , w hich ar e which capabar lee ocapable f absor bof ing absorbing light in a light br oad in r a anbr ge oad inclrange uding including the visible violet light. The transmittance of the vitriﬁed ﬁlms reaches 95% in the wavelength the visible violet light. The tr ansmittance of the vitrified films r eaches 95% in the w avelength r ange range of 450of –1450–1000 000 nm. A nm. ggr eAggr gatedegated films d ﬁlms emon demonstrate str ate complcomplete ete light a light bsor p absorption tion in thein enthe tir e entir UV e raUV nge range (Figure 4a). Thanks to all these properties, the ﬁlms are not sensitive to sunlight or artiﬁcial (Figur e 4a). Thanks to all these pr oper ties, the films ar e not sensitive to sunlight or artificial illumination (such as ﬁlament lamps, halogen, and luminescent lamps) in the visible spectral range, illumination (such as filament lamps, halogen , and luminescent lamps) in the visible spectr al r ange, which are intensive sources of optical disturbances. It is important to note that such ﬁlms cannot be w hich ar e intensive sour ces of optical disturbances. It is impor tant to note that such films cannot be fabricated from non-anisometric analogues of -diketonate lanthanide (III) complexes because of their fabr icated fr om non-anisometr ic analogues of β-diketonate lanthanide (III) complexes because o f strong crystallization propensity and high melting temperatures. their str ong cr ystallization pr opensity and high melting temper atur es. (a) (b) Figure 4. Cont. Photonics 2019, 6, x FOR PEER REV IEW 6 o f 10 Photonics 2019, 6, 110 6 of 10 (c) Figure 4. Transmittance(a) and luminescence spectra excited at 337 nm (b) of the 6.1 m thick Figure 4. Tra nsmitta nce (a) a nd luminesce nce s pectra e xcite d a t 337 nm (b) of the 6.1 µm thick vitriﬁed ﬁlms at room temperature; (c) photostability of Eu(CPDk ) Phen complex ﬁlms fabricated by vitrifie d films a t room tempe ra ture ; (c) photosta bility of Eu(CP 3-5 Dk3 3 -5)3Phe n comple x films fa brica te d melt-processing (6.1 m thick) and spin-coating techniques. by me lt-proce ssing (6.1 µm thick) a nd s pin-coa ting te chniques. Aggregated ﬁlms are the most promising candidates for various applications. For example, Aggr egated films ar e the most pr omising candidates for var ious applications. For example, aggregated ﬁlms of Tb(CPDk ) Phen and Eu(CPDk ) Phen complexes are characterized by 3–7 3 3–5 3 aggr egated films of Tb (CP Dk3–7 )3Phen and Eu (CP Dk3–5)3Ph en complexes ar e char acterized by br oad broad luminescence excitation bands in the 250–420 nm range, which correspond to -diketonate luminescence excitation bands in the 250–420 nm r ange, w hich corr espond to β -diketonate ligand-centered singlet-singlet transitions [18]. The 365 nm UV excitation of Tb(CPDk ) Phen and 3–7 3 ligand-center ed singlet -singlet tr ansitions [18]. The 365 nm UV excitation of Tb (CP Dk 3–7)3Phen and 3+ 3+ Eu(CPDk ) Phen ﬁlms initiates monochromatic luminescence of Tb and Eu ions with 545 nm 3+ 3+ 3–5 3 Eu (CP Dk3–5)3Ph en films initiates monochr omatic luminescence of Tb and Eu i ons w ith 545 nm and 612 nm maximums, respectively (Figure 4b). Absence of emission from the ligands (which is and 612 nm maximums, r espectively (Figur e 4b). Absence o f emission fr om the li gands (w hich is expected to occur in the wavelength range of 450–670 nm) [19] suggests that the ligand-to-metal energy expected to occur in the wavelength r ange of 450 –670 nm) [19] suggests that the ligand-to-metal transfer process is very ecient in the ﬁlm. ener gy tr ansfer pr ocess is ver y efficient in the film. It should be noted that intensive absorption of the ﬁlms in the 390–405 nm wavelength range It should be noted that intensive absor ption of the films in the 390–405 nm wavelength r ange allows for using them not only as UV light sources but also as purple LEDs. allow s for using them not only as UV light sour ces but also as pur ple LEDs. Photostability of luminescent materials is a very important criterion for their application. Photostability of luminescent mater ials is a very important criter ion for their application. A A signiﬁcant problem, which hinders broad application of ﬁlm materials based on -diketonate significant pr oblem, w hich hinder s br oad application of film mater ials based on β-diketonate lanthanide complexes, is irreversible reduction of their luminescence intensity under UV radiation. lanthanide complexes, is ir r ever sible r eduction of their luminescence intensity un der UV r adiation. This process is particularly intensive in the presence of atmospheric oxygen. The inﬂuence of long-time This pr ocess is particular ly intensive in the pr esence o f atmospher ic oxygen. The influenc e o f UV irradiation on the luminescence intensity of vitriﬁed ﬁlms based on anisometric -diketonate long-time UV ir radiation on the luminescence intensity of vitrified films based on anisometr ic lanthanide (III) complexes was studied. For reference, the photostability of ﬁlms fabricated by β-diketonate lanthanide (III) c omplexes w as stud ied. For r efer ence, the photostability of films spin-coating was also characterized. The results for the ﬁlms of Eu(CPDk ) Phen complexes are 3–5 3 fabr icated by spin-coating w as also char acter ized. The r esults for the films o f Eu(CP Dk 3–5 )3Phen represented in Figure 4c. The UV source was the UVGL-58 Handheld UV Lamp with 365 nm wavelength complexes ar e r epr esented in Figur e 4c. The UV sour ce w as the UVGL -58 Handheld UV Lamp w ith and 6 W power. 365 nm w avelength and 6 W pow er . UV radiation does not change the luminescence intensity of the ﬁlms fabricated from the complexes UV r adiation does not change the luminescence intensity of the films fabr icated fr om the by the melt-processing technique even after the 10-h exposure, whereas the intensity of ﬁlms fabricated complexes by the melt-pr ocessing technique even after the 10 -h exposur e, w her eas the intensity of by the spin-coating technique is reduced almost 5 times in the same exposure conditions. It may be films fabricated by the spin-coating technique is r educed almost 5 times in the same exposur e the eect of photooxidation of the ﬁlms under UV radiation. Thus, the technology of fabrication of conditions. It may be the effect of photooxidation of the films und er UV r adiation. Thus, the ﬁlm materials by melting between quartz glasses isolates a luminophore from atmospheric oxygen technology of fabr ication of film mater ials by melting betw een quartz glasses isolates a luminophor e and, therefore, increases photostability of ﬁlms and prevents them from photooxidation. From the fr om atmospher ic oxygen and, ther efor e, incr eases photostability of films and pr events them fr om application point of view, a photostable luminescent vitriﬁed Eu(CPDk ) Phen ﬁlm is primarily 3–5 3 photooxidation. Fr om the application point of view , a photostable luminescent vitr ified interesting for its capability to perform multiple functions at the same time. Firstly, it completely Eu (CP Dk3–5)3Ph en film is pr imarily inter esting for its capability to per for m multiple functi ons at the absorbs the light energy in the entire UV range (200–385 nm) and is not destructed by the UV radiation, same time. Fir stly, it completely absor bs the light ener gy in the entir e UV r ange (200 –385 nm) and is so it can be used as a molecular UV ﬁlter. Secondly, this material can eciently transform the light not destr ucted by the UV r adiation, so it can be used as a molecular UV filter . Secondly, this mater ial 3+ energy in the 280–415 nm range into intensive monochromatic orange-red luminescence of Eu ions can efficiently tr ansfor m the light ener gy in the 280–415 nm range into intensive monochr omatic 3+ (the absolute quantum yield of luminescence of Eu ions is ~30% [29]), so it can be an ecient 3+ 3+ or ange-r ed luminescence of Eu ions (the absolute quantum yield of luminescence of Eu ions is light-transforming material and the source of monochromatic orange-red light. Finally, this material ~30% [29]), so it can be an efficient light -tr ansfor ming mater ial and the sour ce of monochr omatic can reversibly change luminescence intensity and quenching time in the 298–348 K temperature range or ange-r ed light. Finally, this mater ial can r ever sibly change luminescence intensity and quenching time in the 298–348 K temper atur e r ange (Figur e 5a), so it can become an efficient w or king el ement Photonics 2019, 6, 110 7 of 10 Photonics 2019, 6, x FOR PEER REV IEW 7 o f 10 of multiple use luminescent ther mometer s. This mater ial posses ses the follow ing technological (Figure 5a), so it can become an ecient working element of multiple use luminescent thermometers. advantages over its close analogues: total r esistance to the UV r adiation, high optical quality, total This material possesses the following technological advantages over its close analogues: total resistance pr otection fr om the contact with atmospher ic oxygen, and the highest aver age absolute temper atur e to the UV radiation, high optical quality, total protection from the contact with atmospheric oxygen, and −1 sensitivity of the luminescence decay time (6.5 μs∙K ) among temper atur e-sensitive film mater ials the highest average absolute temperature sensitivity of the luminescence decay time (6.5 sK ) among based on nonmesogenic eur opium(III) β -diketonate complexes, w hich efficiently absorb light in the temperature-sensitive ﬁlm materials based on nonmesogenic europium(III) -diketonate complexes, violet r ange of the visible spectr um. The value of the r elative temper atur e sens itivity of the which eciently absorb light in the violet range of the visible spectrum. The value of the relative (r) −1 −1 (r) 1 luminescence decay time S τ var ies fr om −0.28%·K at 298 K to −1.6%·K at 348 K w ith τRef = 537µ s. temperature sensitivity of the luminescence decay time S varies from 0.28%K at 298 K to −1 (r) 1 1 Its aver age value is −1.2%·K . It is impor tant to note that the aver age S τ value is on e of the lar gest 1.6%K at 348 K with = 537s. Its average value is 1.2%K . It is important to note that Ref (r) r epor ted for the temper atur e-sensitive films based on the eur opium(III) b -diketonate complexes, the average S value is one of the largest reported for the temperature-sensitive ﬁlms based on the w hich ar e efficient absor ber s of light in the violet r egion [30]. europium(III) b-diketonate complexes, which are ecient absorbers of light in the violet region [30]. A luminescent mater ial made of a vitrified Tb (CP DK3–7)3Phen 6.1 µ m thick film also offer s A luminescent material made of a vitriﬁed Tb(CPDK ) Phen 6.1 m thick ﬁlm also oers 3–7 3 inter esting pr operties for br oad application in var ious ar eas. This film is an efficient absor ber of the interesting properties for broad application in various areas. This ﬁlm is an ecient absorber of the light ener gy in the br oad 280 −405 nm r ange, w hich is then conver ted into the monochr omatic gr een light energy in the broad 280405 nm range, which is then converted into the monochromatic green 3+ 3+ luminescence of Tb ions. In the 143−253 K temper atur e range, this mater ial can r eversibly change luminescence of Tb ions. In the 143253 K temperature range, this material can reversibly change the the luminescence qu enching time (Fi gur e 5b), so it can also become an effici ent w or king element o f luminescence quenching time (Figure 5b), so it can also become an ecient working element of multiple multiple use luminescent ther mometer s [29]. A temper atur e sensitive mater ial made of th e use luminescent thermometers [29]. A temperature sensitive material made of the Tb(CPDK ) Phen 3–7 3 Tb (CP DK3–7)3Phen film is easy to pr oduce, insensitive to oxygen, char acter ized by high optical ﬁlm is easy to produce, insensitive to oxygen, characterized by high optical quality, and resistant quality, and r esistant to destr uctive effects of UV r adiation. It is an efficient absor ber of violet light to destructive eects of UV radiation. It is an ecient absorber of violet light and possesses high −1 and possesses high temper atur e sensitivity to a lumi nescence quenching time (3.3 μs∙К ). The temperature sensitivity to a luminescence quenching time (3.3 sK ). The relative temperature (r) −1 (r) 1 1 r elative temperatur e sensitivity of the average luminescence decay time S τ is −0.4%·K at 253 K and sensitivity of the average luminescence decay time S is0.4%K at 253 K and1.4%K at 143 K −1 −1 −1.4%·K at 143 K w ith τRef= 373 µ s. Its mean value is −0.9%·K . T o the best of our know ledge, ther e with = 373 s. Its mean value is 0.9%K . To the best of our knowledge, there are no reports Ref ar e no r epor ts descr ibing the temper atur e-sensitive luminescent films (oper ating below r oom describing the temperature-sensitive luminescent ﬁlms (operating below room temperature) based on temper atur e) based on ter bium(III) β -diketonate complexes w ith the similar char acter istics. terbium(III) -diketonate complexes with the similar characteristics. (a) (b) Figure 5. Re vers ible cha nges in the lumine sce nce deca y time (m onitore d a t 612 nm) for the Figure 5. Reversible changes in the luminescence decay time (monitored at 612 nm) for the Eu(CPDK3 – 5)3Phe n film (a) a nd in the luminesce nce deca y time (monitore d a t 545 nm) for the Eu(CPDK ) Phen ﬁlm (a) and in the luminescence decay time (monitored at 545 nm) for the 3–5 3 Tb(CPDK3 – 7)3Phe n (b) unde r the e xcita tion by the 337 nm pulse d nitrogen lase r with the 0.17 mW Tb(CPDK ) Phen (b) under the excitation by the 337 nm pulsed nitrogen laser with the 0.17 mW 3–7 3 a vera ge output powe r in consecutive hea ting -cooling cycles . Sta ndard de via tions a re s hown a s error average output power in consecutive heating-cooling cycles. Standard deviations are shown as ba rs . error bars. The temperature sensitivity of luminescence properties of the fabricated ﬁlm materials was studied The temper atur e sensitivity of luminescence pr oper ties of the fabr icated film mater ials w as by measuring their lifetimes. As opposed to the luminescence intensity, the luminescence lifetime studied by measuring their lifetimes. As opp osed to the lu minescence intensity, the luminescence parameter does not depend on measurement conditions and the degradation coecient value, so lifetime par ameter does not dep end on measur ement conditions and the degr adation coefficient it can be used for more reliable and accurate determination of temperature. Thermally sensitive value, so it can be used for mor e r eliable and accur ate deter mination of temper atur e. Ther mally luminescent properties of the Tb(CPDk ) Phen ﬁlms were studied in the 143–253 K temperature 3–7 3 sensitive luminescent pr operties of the Tb (CP Dk3–7)3Phen films w er e studied in the 143 –253 К range (Figure 6a) because no signiﬁcant changes of the luminescence lifetime were observed in the temper atur e r ange (Figur e 6a) because no significant changes of the luminescence lifetime w er e upper range of 253–284 K, while cooling below 143 K led to irreversible changes in the local structure of obser ved in the upper range of 253–284 К, w hile cooling below 143 К led to irr eversible changes in the local str uctur e of films. Th er mally sensitive luminescent pr oper ties of the Eu(CP Dk 3–5 )3Phen Photonics 2019, 6, 110 8 of 10 Photonics 2019, 6, x FOR PEER REV IEW 8 o f 10 Photonics 2019, 6, x FOR PEER REV IEW 8 o f 10 ﬁlms. Thermally sensitive luminescent properties of the Eu(CPDk ) Phen ﬁlms were studied in the 3–5 3 films w er e studied in the 253–348 K temper atur e r ange (Figur e 6b) because the luminescence lifetime films w er e studied in the 253–348 K temper atur e r ange (Figur e 6b) because the luminescence lifetime 253–348 K temperature range (Figure 6b) because the luminescence lifetime was thermally irresponsive w as thermally irr esponsive at temper atur es below 253 К, w hile the 353 К temper atur e tur ned out to w as thermally irr esponsive at temper atur es below 253 К, w hile the 353 К temper atur e tur ned out to at temperatures below 253 K, while the 353 K temperature turned out to be the softening threshold of be the softening thr eshold of the Eu(CPDk3–5)3Phen films. be the softening thr eshold of the Eu(CPDk3–5)3Phen films. the Eu(CPDk ) Phen ﬁlms. 3–5 3 (a) (b) (a) (b) Figure 6. Te mpe ra ture de pe nde nce of the luminesce nce life time of the Tb (CPDk3 – 7)3Phe n films (a) Figure 6. Temperature dependence of the luminescence lifetime of the Tb (CPDk ) Phen ﬁlms (a) Figure 6. Te mpe ra ture de pe nde nce of the luminesce nce life time of the Tb (CPDk3 – 7)3Phe n films (a) 3–7 3 a nd the Eu(CPDk3 – 5)3Phe n (b) comple xe s. and the Eu(CPDk ) Phen (b) complexes. a nd the Eu(CPDk3 – 5)3Phe n (b) comple xe s. 3–5 3 3+ 3+ We previously demonstrated [29] that the major quenching route of luminescence of Tb ions in W e pr eviously demonstr ated [29] that the maj or quenching r oute of luminescence of Tb 3+ ions W e pr eviously demonstr ated [29] that the maj or quenching r oute of luminescence of Tb ions 5 3+ 5 3+ the Tb(CPDK ) Phen ﬁlm is the energy back transfer from the D level 5 of the Tb ion 3+to the T1 state in the Tb (CPDK3–7)3Phen film is the ener gy back tr ansfer fr om the D4 level of the Tb ion to the T1 3–7 3 4 in the Tb (CPDK3–7)3Phen film is the ener gy back tr ansfer fr om the D4 level of the Tb ion to the T1 of the ligands and the subsequent non-radiative relaxation. The paper [30] shows that the low-lying state of the ligands and the subsequ ent non -r adiative r elaxation. The paper [30] show s that the state of the ligands and the subsequ ent non -r adiative r elaxation. The paper [30] show s that the ligand-to-metal charge transfer (LMCT) states are responsible for the strong luminescence quenching low -lying ligand-to-metal char ge transfer (LMCT) states ar e r esponsible for the str ong luminescence low -lying ligand-to-metal char ge transfer (LMCT) states ar e r esponsible for the str ong luminescence from the D level in the Eu(CPDK ) Phen ﬁlm at high temperatures. quenching fr om the 5 D0 level in the Eu(CPDK3–5)3Phen film at high temper atur es. 0 3–5 3 quenching fr om the D0 level in the Eu(CPDK3–5)3Phen film at high temper atur es. At room temperature, the vitriﬁed Tb(CPDK ) Phen ﬁlm demonstrates substantial increase At r oom temper atur e, the vitr ified Tb (CP DK3–7)3Ph en film demonstr ates substantial incr ease of 3–7 3 At r oom temper atur e, the vitr ified Tb (CP DK3–7)3Ph en film demonstr ates substantial incr ease of 3+ 3+ of the luminescence intensity of the Tb ions under UV laser radiation (Figure 7) that is untypical the luminescence intensity of the Tb3+ ions under UV laser r adiation (Figur e 7) that is untypical for the luminescence intensity of the Tb ions under UV laser r adiation (Figur e 7) that is untypical for for classical -diketonate complexes [18]. In the context of potential applications, it is interesting classical β-diketonate complexes [18]. In the context of potential applications, it is inter esting that the classical β-diketonate complexes [18]. In the context of potential applications, it is inter esting that the that the modiﬁed luminescence brightness of the ﬁlms is sustainable for several months, while the modified luminescence br ightness of the films is sustainable for sever al months, w hile the films modified luminescence br ightness of the films is sustainable for sever al months, w hile the films ﬁlms return to their initial state after thermal processing. Experiments conﬁrmed that it is possible r etur n to their initial state after thermal pr ocessing. Exp er iments confi r med that it is possible to r etur n to their initial state after thermal pr ocessing. Exp er iments confi r med that it is possible to to perform multiple switching of the luminescence brightness. The ﬁlms with such characteristics per for m multiple sw itching of the luminescence br ightness. The films w ith such char acteristics per for m multiple sw itching of the luminescence br ightness. The films w ith such char acteristics provide potential solutions for creating brand-new molecular photonics devices, such as multiple use pr ovide potential solutions for cr eating br and-new molecular photonics devices , such as multiple pr ovide potential solutions for cr eating br and-new molecular photonics devices , such as multiple luminescent UV sensors, which can “remember” their modiﬁed state for several months. use luminescent UV sensor s, w hich ca n “r emember ” their modified state for sever al months. use luminescent UV sensor s, w hich ca n “r emember ” their modified state for sever al months. Figure 7. Lase r control a nd tempe ra ture s witching of lumine sce nce inte ns ity in photos ta ble Figure 7. Laser control and temperature switching of luminescence intensity in photostable transparent Figure 7. Lase r control a nd tempe ra ture s witching of lumine sce nce inte ns ity in photos ta ble tra ns pa rent film ba s ed on the te rbium(III) β -dike tonate comple x. tﬁlm ra nsbased pa rent on filthe m bterbium(III) a s ed on the t e-diketonate rbium(III) βcomplex. -dike tonate comple x. 4. Conclusions 4. Conclusions In this paper , w e pr oposed a simple method for cr eating micr oscale luminescent film mater ials In this paper , w e pr oposed a simple method for cr eating micr oscale luminescent film mater ials w ith contr olled optical pr operties based on anisometr ic compounds of Tb (III) and Eu (III) for w ith contr olled optical pr operties based on anisometr ic compounds of Tb (III) and Eu (III) for applications in optics and photonics. The pr esence of long alkyl substituents in the str uctur e of applications in optics and photonics. The pr esence of long alkyl substituents in the str uctur e of Photonics 2019, 6, 110 9 of 10 4. Conclusions In this paper, we proposed a simple method for creating microscale luminescent ﬁlm materials with controlled optical properties based on anisometric compounds of Tb(III) and Eu(III) for applications in optics and photonics. The presence of long alkyl substituents in the structure of ligands prevents them from destruction by melting, reduces melting and glass transition temperatures, and minimizes conditions that favor formation of crystalline defects. The anisometric complexes synthesized within the framework of this research are capable of transforming UV radiation into visible light. Thus, this research resulted in fabrication of highly photostable and uniform transparent ﬁlms with possible application as reusable sensors and light converters. Author Contributions: The work described in this article is the collaborative development of all authors. Conceptualization, A.K. and Y.G.; Methodology, A.K. and Y.G.; Software, M.K.; Validation, A.K., M.K. and Y.G.; Formal Analysis, M.K.; Investigation, A.K., M.K. and Y.G.; Resources, Y.G.; Data Curation, A.K. and Y.G.; Writing-Original Draft Preparation, A.K. and M.K.; Writing-Review & Editing, Y.G.; Supervision, A.K. and Y.G. Funding: This research was funded by Russian Science Foundation (grant No. 18-13-00112). Acknowledgments: The authors would like to thanks the Russian Science Foundation (grant No. 18-13-00112) for funding this research work. Conﬂicts of Interest: The authors declare no conﬂict of interest. References 1. Chu, T.; Zhang, F.; Wang, Y.; Yang, Y.; Ng, S.W. A novel electrophoretic deposited coordination supramolecular network ﬁlm for detecting phosphate and biophosphate. Chem. A Eur. J. 2017, 23, 7748–7754. [CrossRef] [PubMed] 2. Wang, Z.; Liu, H.; Wang, S.; Rao, Z.; Yang, Y. 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Lapaev, D.V.; Nikiforov, V.G.; Saﬁullin, G.M.; Lobkov, V.S.; Knyazev, A.A.; Krupin, A.S.; Galyametdinov, Y.G. Changes in luminescent properties of vitriﬁed ﬁlms of terbium (III) -diketonate complex upon UV laser irradiation. J. Lumin. 2016, 175, 106–112. [CrossRef] 20. Knyazev, A.A.; Karyakin, M.E.; Krupin, A.S.; Romanova, K.A.; Galyametdinov, Y.G. Inﬂuence of Eu(III) complexes structural anisotropy on luminescence of doped conjugated polymer blends. Inorg. Chem. 2017, 56, 6067–6075. [CrossRef] 21. Galyametdinov, Y.G.; Knyazev, A.A.; Dzhabarov, V.I.; Cardinaels, T.; Driesen, K.; Görller-Walrand, C.; Binnemans, K. Polarized luminescence from aligned samples of nematogenic lanthanide complexes. Adv. Mater. 2008, 20, 252–257. [CrossRef] 22. Knyazev, A.A.; Dzhabarov, V.I.; Lapaev, D.V.; Lobkov, V.S.; Haase, W.; Galyametdinov, Y.G. New nematogenic -diketones for synthesis of lanthanidomesogens. Russ. J. Gen. Chem. 2010, 80, 756–760. [CrossRef] 23. Lapaev, D.V.; Nikiforov, V.G.; Saﬁullin, G.M.; Lobkov, V.S.; Knyazev, A.A.; Krupin, A.S.; Galyametdinov, Y.G. UV laser-induced enhancement of photoluminescence intensity in vitriﬁed terbium(III) -diketonate complex ﬁlm in air. J. Lumin. 2018, 194, 407–413. [CrossRef] 24. Binnemans, K. Rare-earth beta-diketonates. In Handbook on the Physics and chemistry of Rare Earths; Gschneidner, K.A., Bünzli, J.-C., Pecharsky, V.K., Eds.; Elsevier: Amsterdam, The Netherlands, 2005; Volume 35, pp. 107–272. [CrossRef] 25. Nockemann, P.; Beurer, E.; Driesen, K.; Van Deun, R.; Van Hecke, K.; Van Meervelt, L.; Binnemans, K. Photostability of a highly luminescent europium -diketonate complex in imidazolium ionic liquids. Chem. Commun. 2005, 134, 4354–4356. [CrossRef] [PubMed] 26. Faustino, W.M.; Junior, S.A.; Thompson, L.C.; De Sá, G.F.; Malta, O.L.; Simas, A.M. Theoretical and experimental luminescence quantum yields of coordination compounds of trivalent europium. Internat. J. Quant. Chem. 2005, 103, 572–579. [CrossRef] 27. Romanova, K.A.; Datskevich, N.P.; Taidakov, I.V.; Vitukhnovskii, A.G.; Galyametdinov, Y.G. Luminescent characteristics of some mesogenic tris( -diketonate) europium(III) complexes with Lewis bases. Russ. J. Phys. Chem. A 2013, 87, 2108–2111. [CrossRef] 28. Nikolov, P.; Fratev, F.; Petkov, I.; Markov, P. Dimer ﬂuorescence of some -dicarbonyl compounds. Chem. Phys. Lett. 1981, 83, 170–173. [CrossRef] 29. Lapaev, D.V.; Nikiforov, V.G.; Lobkov, V.S.; Knyazev, A.A.; Galyametdinov, Y.G. A photostable vitriﬁed ﬁlm based on a terbium (III) -diketonate complex as a sensing element for reusable luminescent thermometers. J. Mater. Chem. C 2018, 6, 9475–9481. [CrossRef] 30. Lapaev, D.V.; Nikiforov, V.G.; Lobkov, V.S.; Knyazev, A.A.; Galyametdinov, Y.G. Reusable temperature-sensitive luminescent material based on vitriﬁed ﬁlm of europium(III) b-diketonate complex. Opt. Mater. 2018, 75, 787–795. [CrossRef] © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Photonics Multidisciplinary Digital Publishing Institute http://www.deepdyve.com/lp/multidisciplinary-digital-publishing-institute/photostable-anisometric-lanthanide-complexes-as-promising-materials-KrSLOq0oBF

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Publisher: Multidisciplinary Digital Publishing Institute
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ISSN: 2304-6732
DOI: 10.3390/photonics6040110
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Abstract

hv photonics Article Photostable Anisometric Lanthanide Complexes as Promising Materials for Optical Applicationsy 1 , 1 1 , 2 Andrey Knyazev * , Maxim Karyakin and Yuriy Galyametdinov Kazan National Research Technological University, 68 Karl Marx, Kazan 420015, Russia; belgesto@list.ru (M.K.); yugal2002@mail.ru (Y.G.) Zavoisky Physical-Technical Institute, FRC Kazan Scientiﬁc Centre of RAS, 10/7 Sibirsky tract, Kazan 420029, Russia * Correspondence: knjazev2001@mail.ru y This paper is an extended version of our paper published in: Knyazev, A.A.; Galyametdinov, Y.G. Luminescent materials based on anisometric lanthanide complexes. In Proceedings IV International Conference on Applications in Optics and Photonics (AOP 2019), Lisbon, Portugal, 31 May–4 June 2019. Received: 17 September 2019; Accepted: 22 October 2019; Published: 25 October 2019 Abstract: Uniform luminescent ﬁlms with high optical quality are promising materials for modern molecular photonics. Such ﬁlm materials based on -diketonate complexes of lanthanides have the following application problem: rapid luminescence degradation under UV radiation, low thermostability, poor mechanical properties, and aggregation propensity. An alternative approach to solving these problems is the use of anisometric analogues of -diketonate compounds of lanthanides (III). The main advantage of such compounds is that they do not crystallize because of long hydrocarbon substituents in the structure of complexes, so they can be used to fabricate thin nano-, micro-, and macroscale uniform ﬁlm materials by a melt-processing technique at relatively low temperatures, as well as by spin-coating. The method of fabrication of microscale luminescent ﬁlm materials with controlled optical properties from anisometric analogues of Ln(DBM) Phen and Ln(bzac) Phen 3 3 complexes (Ln = Eu, Tb) is proposed in this paper. Within the framework of this research, we developed original ﬁlms which are highly uniform and transparent. An important advantage of these ﬁlms is their high photostability and potential for applications as reusable luminescent sensors and light converters. Keywords: lanthanide complexes; luminescence; thin ﬁlms; luminescent sensors 1. Introduction Molecular photonics is a modern branch of science and its active areas of study are development and fabrication of highly functional luminescent materials and devices [1–7]. Among many dierent types of luminescent compounds (e.g., organic compounds, quantum dots, metal-ligand complex), lanthanide (III) coordination compounds are very promising candidates for development of luminescent molecular photonics materials and devices due to their unique photophysical properties, such as a narrow-band luminescence with a high quantum yield and a long lifetime [8]. Lanthanide (III) complexes with aromatic -diketonate ligands are among the most interesting coordination compounds of lanthanides because they combine intensive monochromatic luminescence with attractive chemical characteristics (such as a relatively easy synthesis, good solubility in many basic solvents and a capability of incorporation into various host matrices, such as polymers and liquids crystals). Due to these properties, such compounds have a strong potential for a broad variety of applications. Uniform luminescent ﬁlms with high optical quality are promising materials for modern molecular photonics. Such ﬁlm materials based on -diketonate complexes of lanthanides have the following Photonics 2019, 6, 110; doi:10.3390/photonics6040110 www.mdpi.com/journal/photonics Photonics 2019, 6, 110 2 of 10 application problem: rapid luminescence degradation under UV radiation, low thermostability, poor mechanical properties and propensity to aggregation [9,10]. A possible solution is incorporating these compounds into host matrices (e.g., organic polymers, ionic liquids, zeolites, and etc.) [11–13]. There are also other methods of fabricating ﬁlm materials based on these compounds, such as electro deposition and sol-gel technology [1–3]. Another problem of many ﬁlm materials based on -diketonate complexes of lanthanides is that their adsorption band is limited by the UV range. Therefore, UV sources are required to excite luminescence in such ﬁlms, such sources are unsuitable in many applications for certain reasons (such as low brightness and high cost of such devices as compared to visible light sources). Thus, an urgent problem of molecular photonics is to broaden this adsorption band so it can include the visible light range. The solutions for this problem oered today are known to be based on the following approaches: synthetic modiﬁcation of -diketonate ligands and coordination of complex chromophores to tris( -diketonates) of lanthanides(III) with capabilities of ecient visible light adsorption. For example, it is challenging to select a proper solvent for both a matrix and a complex, where, in addition, this complex must not dissociate. An alternative approach to solution of these problems is the use of anisometric analogues of -diketonate compounds of lanthanides (III) [14,15]. The main advantage of such compounds is that they do not crystallize because of long hydrocarbon substituents in the structure of complexes and possess thermostable properties [16], so they can be used to fabricate thin nano-, micro- and macroscale uniform ﬁlm materials by a melt-processing technique at relatively low temperatures, as well as by spin-coating [17]. The ﬁlms produced by a melt-processing technique require a much simpler technology for fabrication of luminescent materials based on lanthanide compounds as compared to available analogues, because no solvents or photostabilizing matrices are needed in this case. Such ﬁlms have the following technological advantages: intense luminescence, high optical transmittance (up to 95%) in nearly entire visible and near IR ranges, high resistance to UV light, and controllability of their photophysical properties (e.g., the absorption band width and the luminescence intensity) [18,19]. In this paper, we discuss the opportunities of application of microscale ﬁlms based on anisometric europium(III) and terbium(III) -diketonate complexes as materials for photonics and optics. 2. Materials and Methods The synthesis of anisometric complexes was performed according to the technique developed earlier by our group. CHN elemental microanalysis was performed on a EA-1110 apparatus by CE Instruments. Europium elemental microanalysis was performed on a Bruker S8 TIGER X-ray ﬂuorescence spectrometer (Billerica, MA, USA). The thermal behavior of complexes was studied by polarized optical microscopy (POM) on an Olympus BX51 microscope equipped with a LINKAM high-precision heating system and by dierential scanning calorimetry (DSC) (Olympus America, Melville, NY, USA). DSC thermograms were recorded on a Mettler-Toledo DSC 1 Star module in the heating-cooling mode at the 10 C/min scan rate. Thin nanoscale ﬁlms of complexes were prepared from toluene solution (1 10 mol/L) by spin coating on a WS-650 MZ-23NPP Laurell Spin Coater (Laurell Technologies Corporation, North Wales, PA, USA). The transmittance spectrum at room temperature was recorded on a Perkin-Elmer Lambda 35 spectrophotometer. The luminescence spectra at room temperature were measured by a Varian Cary Eclipse spectroﬂuorometer. The photoluminescence excitation spectra of the vitriﬁed ﬁlms were recorded at room temperature on an experimental setup consisting of a MDR-2 excitation monochromator and a MDR-12 recording monochromator. A DKSL-1000 xenon lamp was used as an excitation source. The light signal passing through the MDR-12 monochromator was detected using a FEU-79 photomultiplier tube. The input and the output slit widths of the MDR-2 monochromator were set to 10 nm. The excitation spectra were recorded in the 230–430 nm range by varying the excitation light wavelength and monitoring the Photonics 2019, 6, x FOR PEER REV IEW 3 o f 10 Photonics 2019, 6, 110 3 of 10 spectra w er e r ecor ded in the 230 –430 nm r ange by varying the excitation light w avelength and monitoring the photoluminescence intensity at 545 nm and 612 nm, r espectively. To study the p photoluminescence hotostability of the intensi obtainty edat fil545 m m nm ater and ials,612 thenm, UVG respectively L-58 Manu.aT l o UV study -lamthe p (3photostabi 65 nm) w itlity h thof e 6the W capacity w as used. obtained ﬁlm materials, the UVGL-58 Manual UV-lamp (365 nm) with the 6 W capacity was used. Th The e p powders ow der s of of anisometric anisometriccomplexes complexeswer w ee r esynthesized synthesized accor acco ding r ding to tthe o thpr e p ocedur r ocedu e rdescribed e descr ibe in d ithe n th pr eevious pr eviopapers us pap[ e20 r s, 21 [20 ].,2 A 1]warm . A w a solution r m soluof tioLnCl n of Ln6H Cl3 · O 6H (0.1 2 Ommol) (0.1 min mo ethanol l) in eth was ano slowly l w as sadded lowly 3 2 a dr dopwise ded dr oto pw aihot se talcoholic o a hot al solution coholic s containing olution con 0.3 taimmol ning 0of .3 m-diketone mol of β -[d 22 ik], et0.1 onemmol [22], of 0.1Phen, mmoand l of P 0.3 hemmol n, and of 0KOH. .3 mmThe ol oformed f KOH.light-yellow The for med pr lecipitate ight -yellwas ow p ﬁlter r ecip ed, itapuriﬁed te w as fin ilte hot r edalcohol , pur ifie and d in dried hot alcohol and dr ied under vacuum. Then, the pr oduct w as dissolved in toluene and the obtained under vacuum. Then, the product was dissolved in toluene and the obtained solution was ﬁltered s dried olutio under n w asvacuum. filter ed dr ied under vacuum. Tr is[1-(4-(4-pr opylcyclohexyl)phenyl)decane-1,3-dione](1,10-phenanthr oline) terbium(III) Tris[1-(4-(4-propylcyclohexyl)phenyl)decane-1,3-dione](1,10-phenanthroline) terbium(III) ( (Tb(CPDK Tb (CP DK3–7)3)pphen). h en). YY i e ield: ld: 771% 1% (0 (0,107 ,107 g g). ). M Melting elting p point: oint: 1130 30 °C C. . E Elemental lemental a analysis nalysis (%): (%): c calculated alculated 3–7 3 for C87H119N2O6Tb : C, 72.17; H, 8.28; N, 1.93. Found: C, 71.68; H, 8.55; N, 1.95. for C H N O Tb: C, 72.17; H, 8.28; N, 1.93. Found: C, 71.68; H, 8.55; N, 1.95. 87 119 2 6 Tr Tris[1-phenyl-3-(4-(4-pr is[1-phenyl-3-(4-(4-pr o opylcyclohexyl)phenyl)pr pylcyclohexyl)phenyl)pr o opane-1,3-dionato](1,10-phenanthr pane-1,3-dionato](1,10-phenanthr ooline) line) eur opium(III) (Eu (CP Dk3-Ph)3Phen). Yield : 59% (0.087 g). Melting point: 142 °C. El emental analysis europium(III) (Eu(CPDk ) Phen). Yield: 59% (0.087 g). Melting point: 142 C. Elemental analysis 3-Ph 3 ( (%): %): c calculated alculated for for C C84 H H 89N N 2O O 6 EEu u (% (%): ): CC, , 73 73.40; .40; H H, , 6.6.53; 53; N N, , 22.04; .04; EEu,11.06. u ,11.06. FFound ound (% (%):C, ):C, 773.01; 3.01; H H, , 66.88; .88; 84 89 2 6 N, 2.12; Eu, 11.00. N, 2.12; Eu, 11.00. Tr Tris[1-(4-(4-pr is[1-(4-(4-propylcyclohexyl)phenyl)octane-1,3-dionato](1,10-phenanthr opylcyclohexyl)phenyl)octane-1,3-dionato](1,10-phenanthr ooline) line) e eur ur o opium(III) pium(III) (Eu(CP DK3-5)3Phen). Yield : 70% (0,105 g). Melting point: 110 °C. Elem ental analysis (%): Calcd (Eu(CPDK ) Phen). Yield: 70% (0,105 g). Melting point: 110 C. Elemental analysis (%): Calcd 3-5 3 f forC or C81H H 107Eu EuN N2O6O : C : ,C, 7171.61; .61; H,H, 7.9 7.95; 5; NN, , 2.0 2.06; 6; Eu Eu,11.20. ,11.20. FoFound: und: C, C, 7171.16;H, .16;H, 8.3 8.31; 1; N,N, 2.0 2.02; 2; EuEu, , 1111.50. .50. 81 107 2 6 The sensitive luminescent material w as pr epar ed fr om a pow der of the Eu (CP Dk 3–5)3Phen , The sensitive luminescent material was prepared from a powder of the Eu(CPDk ) Phen, 3–5 3 E Eu(CPDk u (CP Dk3-Ph)3)Ph Phen en oor r Tb Tb(CPDk (CP Dk3–7 )3p ) h phen en co complexes mplex es bby y aa m melt-pr elt -pr o ocessing cessing t te ec chnique. hnique. P Powders ow der s o of f 3-Ph 3 3–7 3 complexes w er e melted b etw een tw o quartz plates w ith a size of 7 × 15 × 0.5 mm placed on the stage complexes were melted between two quartz plates with a size of 7 15 0.5 mm placed on the o stage f an of Olan ym Olympus pus BX-5BX-51 1 polapolarizing r izing micmicr r osco oscope. pe. Afte After r hea heating ting upup toto th the e is isotr otr oopic pic lliquid iquid t transition ransition temper atur e, the sample w as quickly cooled dow n to r oom temperatur e to obtain vitr ifi ed films w ith temperature, the sample was quickly cooled down to room temperature to obtain vitriﬁed ﬁlms with 3 3 μ m m, , 6 6.1 .1 μm m or or 20 20 μm m thickness, thickness, which w hich wer w ere e sandwiched sandw iched between betw een two tw o quartz quar tz plates. plates. To To c contr ontr o ol l thickness, w e used 3 μm, 6.1 μm and 20 μm spacer s, r espectively. thickness, we used 3 m, 6.1 m and 20 m spacers, respectively. 3. Results and Discussion 3. Results and Discussion The structure of anisometric -diketonate complexes of lanthanides(III) is demonstrated in The str uctur e of anisometr ic β-diketonate complexes of lan thanides(III) is demonstr ated in Figure 1. Figur e 1. (a) (b) F Figure igure 1. 1. T The he s scheme cheme o of f the the anisometric a nis ometric analogues a na logues of of ((aa )) Ln(DBM) Ln(DBM)3Phen Phe n and a nd ((b b)) Ln(bzac) Ln(bza c)3Phen Phe n 3 3 c complexes omple xe s (Ln (Ln = = E Eu, u, T Tb). b). The thermal behavior of powders of anisometric complexes was studied by polarized optical The ther mal behavior of pow der s of anisometr ic complexes w as studied by polar ized optical microscopy (POM) and dierential scanning calorimetry (DSC). The presence of alkyl and cyclohexane micr oscopy (POM) and di ffer ential scanning calor imetry (DS C). The pr esence o f alkyl and substituents in the chemical structure of -diketonate ligands of anisometric -diketonate lanthanide(III) cyclohexane substituents in the chemical str uctur e of β-diketonate ligands of anisometr ic complexes prevents them from crystallization and allows for considerable reduction of the melting β-diketonate lanthanide(III) complexes pr events them fr om cr ystallization and allows for temperature to avoid decomposition at melting that is typical for their non-anisometric analogues consider able r eduction of the melting temper atur e to avoid decomp osition at melting that is typical Photonics 2019, 6, x FOR PEER REV IEW 4 o f 10 Photonics 2019, 6, x FOR PEER REV IEW 4 o f 10 for their non-anisometr ic analogues (Table 1). The phase tr ansitions deter mined by the POM method have been confir med by the DSC m ethod, as illustr ated in Figur e 2 for the Eu (CP Dk 3–Ph)3phen Photonics 2019, 6, 110 4 of 10 for their non-anisometr ic analogues (Table 1). The phase tr ansitions deter mined by the POM method complex. Acc or ding to the DS C heating scan, the isotr opic tr ansition temper atur e w as 142°C. The have been confir med by the DSC m ethod, as illustr ated in Figur e 2 for the Eu (CP Dk 3–Ph)3phen sample is vitr ified upon cooling, as can be concluded fr om the absence of the exother mic complex. Acc or ding to the DS C heating scan, the isotr opic tr ansition temper atur e w as 142°C. The (Table 1). The phase transitions determined by the POM method have been conﬁrmed by the DSC cr ystallization peaks in the DS C cooling scan. The combination of such pr operties allow s for sample is vitr ified upon cooling, as can be concluded fr om the absence of the exother mic method, as illustrated in Figure 2 for the Eu(CPDk ) phen complex. According to the DSC heating 3–Ph 3 fabr icating vitr ified luminescent films w ith high optical quality fr om melts of p ow der s of these cr ystallization peaks in the DS C cooling scan. The combination of such pr operties allow s for scan, the isotropic transition temperature was 142 C. The sample is vitriﬁed upon cooling, as can be substances (see Figur e 3 inser t). These films ha ve almost no cr ystalline effects, accor ding to the fabr icating vitr ified luminescent films w ith high optical quality fr om melts of p ow der s of these concluded from the absence of the exothermic crystallization peaks in the DSC cooling scan. The polar ized optical micr oscopy data [23]. The obtained vitr ified films w er e stable and r emained substances (see Figur e 3 inser t). These films ha ve almost no cr ystalline effects, accor ding to the combination of such properties allows for fabricating vitriﬁed luminescent ﬁlms with high optical transpar ent at r oom temper atur e over sever al months. It is impor tant to note that such films cannot polar ized optical micr oscopy data [23]. The obtained vitr ified films w er e stable and r emained quality from melts of powders of these substances (see Figure 3 insert). These ﬁlms have almost no be pr oduced fr om non-anisometric analogues of β-diketonate lanthanide(III) complex es because of transpar ent at r oom temper atur e over sever al months. It is impor tant to note that such films cannot crystalline eects, according to the polarized optical microscopy data [23]. The obtained vitriﬁed ﬁlms their str ong aggr egation pr opensity and high melting temper atur es. be pr oduced fr om non-anisometric analogues of β-diketonate lanthanide(III) complex es because of were stable and remained transparent at room temperature over several months. It is important to note their str ong aggr egation pr opensity and high melting temper atur es. that such ﬁlms cannot be produced from non-anisometric analogues of -diketonate lanthanide(III) complexes because of their strong aggregation propensity and high melting temperatures. DSC POM DSC POM Cooling Cooling -1 Heating -2 I -1 Heating -3 -2 40 60 80 100 120 140 160 180 200 Temperature (° C) -3 40 60 80 100 120 140 160 180 200 Temperature (° C) Figure 2. Diffe re ntia l sca nning ca lorime try (DSC) the rmogra m of Eu(CPDk3 -Ph)3Phe n. POM = Figure pola rize 2.dD oip ff te ic re an l t m iailcs rc oasn cn oip ny g.c alorimetry (DSC) thermogram of Eu(CPDk ) Phen. POM = polarized 3-Ph 3 Figure 2. Diffe re ntia l sca nning ca lorime try (DSC) the rmogra m of Eu(CPDk3 -Ph)3Phe n. POM = optical microscopy. pola rize d optica l micros copy. (a) (b) Figure 3. A microscale ﬁlm of Tb(CPDk ) Phen complexes between two quartz plates (a); Figure 3. A micros ca le film of Tb(CPDk3 – 7)3Phe n comple xes be twee n two quartz pla tes (а); (a) 3–7 3 (b) luminescence lumine sce nce excitation e xcita tion spectra s pectra o off T Tb(CPDk b(CPDk3 – 7) )3P Phen he n ccomplex omple x fi ﬁlms lms w with ith 3, 6.1 3, 6.1, , a and nd 20 20 µm m t thickness hickne ss 3–7 3 Figure 3. A micros ca le film of Tb(CPDk3 – 7)3Phe n comple xes be twee n two quartz pla tes (а); between be twe e n quartz qua rtz plates pla te sr recor ecor ded dedat a t545 545 nm nm wavelength wa ve le ngth and a ndr r oom oomtemperatur te mperatue re( b ( b ).). lumine sce nce e xcita tion s pectra of Tb(CPDk3 – 7)3Phe n comple x films with 3, 6.1, a nd 20 µm thickne ss Table 1. Melting temperatures of anisometric Ln(III) complexes and their commercial analogues. be twe e n qua rtz pla te s recorded a t 545 nm wa ve le ngth a nd room te mperature ( b). T able 1.Me lting te mpe ratures of a nis ome tric Ln(I I I ) comple xe s a nd the ir comme rcia l a na logue s . Complex MeltingPoint C Quantum Yield % Complex Melting Point °С Quantum Yield % T able 1.Me lting te mpe ratures of a nis ome tric Ln(I I I ) comple xe s a nd the ir comme rcia l a na logue s . Eu(DBM) Phen 185–187 decomposition [24] 4.9 [25] Eu(DBM)3Phen 185–187 decomposition [24] 4.9 [25] Complex Melting Point °С Quantum Yield % Eu(bzac) Phen 192–194 decomposition [24] 18 [26] Eu(bzac)3Phen 192–194 decomposition [24] 18 [26] Eu(CPDk ) Phen 110 [15] 30–32 [27] Eu(DBM)3Phen 185–187 decomposition [24] 4.9 [25] 3-5 3 Eu(CPDk3-5)3Phen 110 [15] 30−32 [27] Eu(CPDk )Phen 142 [19] 25–27 [27] 3-Ph Eu(bzac)3Phen 192–194 decomposition [24] 18 [26] Eu(CPDk3-Ph)Phen 142 [19] 25−27 [27] Tb(CPDk ) Phen 130 [22] - 3-7 3 Eu(CPDk3-5)3Phen 110 [15] 30−32 [27] Eu(CPDk3-Ph)Phen 142 [19] 25−27 [27] Heat Flow (mW) Heat Flow (mW) Photonics 2019, 6, x FOR PEER REV IEW 5 o f 10 Tb (CPDk3-7)3Phen 130 [22] - Photonics 2019, 6, 110 5 of 10 The technology of fabr ication of micr oscale vitr ified films fr om pow der s of anisometr ic β-diketonate lanthanide(III) complex es is ver y simple to implement because no solvents or solid The technology of fabrication of microscale vitriﬁed ﬁlms from powders of anisometric matr ices for photostabilization of complexes ar e used, as opposed to tr aditional appr oaches to -diketonate lanthanide(III) complexes is very simple to implement because no solvents or solid fabr ication of film mater ials based on lanthanide complexes. The follow ing pr ocedur e w as used to matrices for photostabilization of complexes are used, as opposed to traditional approaches to fabr icate our films: a small amount of a pow der complex w as placed betw een tw o quar tz plates and fabrication of ﬁlm materials based on lanthanide complexes. The following procedure was used to an isotr opic melt was cr eated by heating. Subsequent r apid cooling d ow n to r oom te mper atur e fabricate our ﬁlms: a small amount of a powder complex was placed between two quartz plates and an r esults in the glass tr ansition of the melt and for mation of a solid and unifor m film. This technology isotropic melt was created by heating. Subsequent rapid cooling down to room temperature results in allows for contr olling the thickness of films by spacer s, such as polystyr ene micr ospher es w ith the varglass ious d transition iameter . of the melt and formation of a solid and uniform ﬁlm. This technology allows for controlling the thickness of ﬁlms by spacers, such as polystyrene microspheres with various diameter. A peculiar featur e of vitr ified films is that their adsor ption (excitation) band width can be manA ipu peculiar lated byfeatur varyieng of th vitriﬁed e melt tﬁlms hickne is ssthat . Du e their to stadsorption r ong adsor p (excitation) tion capaciband ty of fwidth ilms wcan ith t be he manipulated by varying the melt thickness. Due to strong adsorption capacity of ﬁlms with the thickness exceeding 3 μm in the 200−380 nm r ange, it w as possible to detect only the long wave edge thickness of the adexceeding sor ption s3 p ec m tr u in m the . Lu 200 mi n380 escenm ncer e ange, xcitait tio was n sp possible ectr a arto e m detect or e in only for m the ativ long e bewave causeedge they of the adsorption spectrum. Luminescence excitation spectra are more informative because they allow allow for detecting changes (fr om var ious thicknesses of the films) in the entir e spectral range for (20detecting 0–450 nm changes ). Figur e (fr 3om b ilvarious lustr ates thicknesses the influen of ce the of ﬁlms) thickn in esthe ses entir of th eespectral Tb (CP D range k3–7 )3P (200–450 hen filmnm). s on Figure 3b illustrates the inﬂuence of thicknesses of the Tb(CPDk ) Phen ﬁlms on their excitation their excitation spectr a. The excitation spectra of a 3 μm film (monitor ed at 545 nm) consist of a 3–7 3 spectra. br oad ba The nd w excitation ith the ma spectra ximumof at a 35 36 nm m ﬁlm cor r e (monitor spondined g toat si545 nglenm) t -sinconsist glet tran of sita iobr ns oad in liband gands with . The the maximum at 356 nm corresponding to singlet-singlet transitions in ligands. The increase of the incr ease of the film thickness to 6 μm br oadens the excitation spectr um band to the violet visible ﬁlm r ang thickness e and shto ifts 6 t h m e br moadens aximum the of excitation the spectr spectr um. F um ur tband her in to crthe ease violet of thvisible e film range thicknand ess t shifts o 20 the μm maximum of the spectrum. Further increase of the ﬁlm thickness to 20 m creates a distinguished cr eates a distinguished shoulder of the spectr um in the 393 nm ar ea. β -dicar bonyl compounds ar e shoulder know n of to the forspectr m asum sociin ate the s i 393 n c nm oncar en ea. tr a te -dicarbonyl d solutions compounds . For exam ar p ele known , moleto cuform lar aassociates ssociates in of −2 concentrated solutions. For example, molecular associates of benzoylacetone in cyclohexane solution benzoylacetone in cyclohexane solution (С = 1 × 10 М) ar e r esponsible for the long-wavelength (C ba= nd 1 wi 10 th th M) e m ar ae xirm esponsible um at 400 for nm the in long-wavelength the absor ption a band nd ex with citatithe on maximum spectr a. Diat me 400 r ic nm spec in ies the in absorption and excitation spectra. Dimeric species in similar molecular systems were detected by similar molecular systems w er e detected by NMR spectr oscopy. W e consider that the featur es NMR menti spectr oned oscopy above .in W de icconsider ate clearthat ly ththe at th featur e thies ckn mentioned ess of the Tb above (CP indicate Dk3–7 )3Ph clearly en film that s inthe fluethickness nces their of the Tb(CPDk ) Phen ﬁlms inﬂuences their local structure (the ratio between individual molecules local str uctur e (the ratio betw een individual molecules and aggr egates) [2 8]. The pr esence of 3–7 3 and aggr aggr egategates) ed com [p 28 le ].xe The s in pr tesence he film of s aggr w ith egated var iocomplexes us thicknein ssethe s isﬁlms the with factovarious r r espon thicknesses sible for th ise the factor responsible for the long-wave “wing” of the excitation spectra of the ﬁlms. Thus, variation long-w ave “w ing” of the excitation spectr a of the films. Thus, var iation of the r atio betw een of inthe divi ratio dualbetween molecule individual s and aggrmolecules egates in tand he lo aggr cal egates str uctuin r e the of flocal ilms str (atuctur the se ta of geﬁlms of th(at eir the fabr stage icatio of n) their fabrication) allows for manipulating the width of absorption and excitation bands and creating allows for manipulating the width of absor ption and excitation bands and cr eating optically optically transpar e transpar nt lumin ent escluminescent ent mater ialsmaterials, , w hich ar e which capabar lee ocapable f absor bof ing absorbing light in a light br oad in r a anbr ge oad inclrange uding including the visible violet light. The transmittance of the vitriﬁed ﬁlms reaches 95% in the wavelength the visible violet light. The tr ansmittance of the vitrified films r eaches 95% in the w avelength r ange range of 450of –1450–1000 000 nm. A nm. ggr eAggr gatedegated films d ﬁlms emon demonstrate str ate complcomplete ete light a light bsor p absorption tion in thein enthe tir e entir UV e raUV nge range (Figure 4a). Thanks to all these properties, the ﬁlms are not sensitive to sunlight or artiﬁcial (Figur e 4a). Thanks to all these pr oper ties, the films ar e not sensitive to sunlight or artificial illumination (such as ﬁlament lamps, halogen, and luminescent lamps) in the visible spectral range, illumination (such as filament lamps, halogen , and luminescent lamps) in the visible spectr al r ange, which are intensive sources of optical disturbances. It is important to note that such ﬁlms cannot be w hich ar e intensive sour ces of optical disturbances. It is impor tant to note that such films cannot be fabricated from non-anisometric analogues of -diketonate lanthanide (III) complexes because of their fabr icated fr om non-anisometr ic analogues of β-diketonate lanthanide (III) complexes because o f strong crystallization propensity and high melting temperatures. their str ong cr ystallization pr opensity and high melting temper atur es. (a) (b) Figure 4. Cont. Photonics 2019, 6, x FOR PEER REV IEW 6 o f 10 Photonics 2019, 6, 110 6 of 10 (c) Figure 4. Transmittance(a) and luminescence spectra excited at 337 nm (b) of the 6.1 m thick Figure 4. Tra nsmitta nce (a) a nd luminesce nce s pectra e xcite d a t 337 nm (b) of the 6.1 µm thick vitriﬁed ﬁlms at room temperature; (c) photostability of Eu(CPDk ) Phen complex ﬁlms fabricated by vitrifie d films a t room tempe ra ture ; (c) photosta bility of Eu(CP 3-5 Dk3 3 -5)3Phe n comple x films fa brica te d melt-processing (6.1 m thick) and spin-coating techniques. by me lt-proce ssing (6.1 µm thick) a nd s pin-coa ting te chniques. Aggregated ﬁlms are the most promising candidates for various applications. For example, Aggr egated films ar e the most pr omising candidates for var ious applications. For example, aggregated ﬁlms of Tb(CPDk ) Phen and Eu(CPDk ) Phen complexes are characterized by 3–7 3 3–5 3 aggr egated films of Tb (CP Dk3–7 )3Phen and Eu (CP Dk3–5)3Ph en complexes ar e char acterized by br oad broad luminescence excitation bands in the 250–420 nm range, which correspond to -diketonate luminescence excitation bands in the 250–420 nm r ange, w hich corr espond to β -diketonate ligand-centered singlet-singlet transitions [18]. The 365 nm UV excitation of Tb(CPDk ) Phen and 3–7 3 ligand-center ed singlet -singlet tr ansitions [18]. The 365 nm UV excitation of Tb (CP Dk 3–7)3Phen and 3+ 3+ Eu(CPDk ) Phen ﬁlms initiates monochromatic luminescence of Tb and Eu ions with 545 nm 3+ 3+ 3–5 3 Eu (CP Dk3–5)3Ph en films initiates monochr omatic luminescence of Tb and Eu i ons w ith 545 nm and 612 nm maximums, respectively (Figure 4b). Absence of emission from the ligands (which is and 612 nm maximums, r espectively (Figur e 4b). Absence o f emission fr om the li gands (w hich is expected to occur in the wavelength range of 450–670 nm) [19] suggests that the ligand-to-metal energy expected to occur in the wavelength r ange of 450 –670 nm) [19] suggests that the ligand-to-metal transfer process is very ecient in the ﬁlm. ener gy tr ansfer pr ocess is ver y efficient in the film. It should be noted that intensive absorption of the ﬁlms in the 390–405 nm wavelength range It should be noted that intensive absor ption of the films in the 390–405 nm wavelength r ange allows for using them not only as UV light sources but also as purple LEDs. allow s for using them not only as UV light sour ces but also as pur ple LEDs. Photostability of luminescent materials is a very important criterion for their application. Photostability of luminescent mater ials is a very important criter ion for their application. A A signiﬁcant problem, which hinders broad application of ﬁlm materials based on -diketonate significant pr oblem, w hich hinder s br oad application of film mater ials based on β-diketonate lanthanide complexes, is irreversible reduction of their luminescence intensity under UV radiation. lanthanide complexes, is ir r ever sible r eduction of their luminescence intensity un der UV r adiation. This process is particularly intensive in the presence of atmospheric oxygen. The inﬂuence of long-time This pr ocess is particular ly intensive in the pr esence o f atmospher ic oxygen. The influenc e o f UV irradiation on the luminescence intensity of vitriﬁed ﬁlms based on anisometric -diketonate long-time UV ir radiation on the luminescence intensity of vitrified films based on anisometr ic lanthanide (III) complexes was studied. For reference, the photostability of ﬁlms fabricated by β-diketonate lanthanide (III) c omplexes w as stud ied. For r efer ence, the photostability of films spin-coating was also characterized. The results for the ﬁlms of Eu(CPDk ) Phen complexes are 3–5 3 fabr icated by spin-coating w as also char acter ized. The r esults for the films o f Eu(CP Dk 3–5 )3Phen represented in Figure 4c. The UV source was the UVGL-58 Handheld UV Lamp with 365 nm wavelength complexes ar e r epr esented in Figur e 4c. The UV sour ce w as the UVGL -58 Handheld UV Lamp w ith and 6 W power. 365 nm w avelength and 6 W pow er . UV radiation does not change the luminescence intensity of the ﬁlms fabricated from the complexes UV r adiation does not change the luminescence intensity of the films fabr icated fr om the by the melt-processing technique even after the 10-h exposure, whereas the intensity of ﬁlms fabricated complexes by the melt-pr ocessing technique even after the 10 -h exposur e, w her eas the intensity of by the spin-coating technique is reduced almost 5 times in the same exposure conditions. It may be films fabricated by the spin-coating technique is r educed almost 5 times in the same exposur e the eect of photooxidation of the ﬁlms under UV radiation. Thus, the technology of fabrication of conditions. It may be the effect of photooxidation of the films und er UV r adiation. Thus, the ﬁlm materials by melting between quartz glasses isolates a luminophore from atmospheric oxygen technology of fabr ication of film mater ials by melting betw een quartz glasses isolates a luminophor e and, therefore, increases photostability of ﬁlms and prevents them from photooxidation. From the fr om atmospher ic oxygen and, ther efor e, incr eases photostability of films and pr events them fr om application point of view, a photostable luminescent vitriﬁed Eu(CPDk ) Phen ﬁlm is primarily 3–5 3 photooxidation. Fr om the application point of view , a photostable luminescent vitr ified interesting for its capability to perform multiple functions at the same time. Firstly, it completely Eu (CP Dk3–5)3Ph en film is pr imarily inter esting for its capability to per for m multiple functi ons at the absorbs the light energy in the entire UV range (200–385 nm) and is not destructed by the UV radiation, same time. Fir stly, it completely absor bs the light ener gy in the entir e UV r ange (200 –385 nm) and is so it can be used as a molecular UV ﬁlter. Secondly, this material can eciently transform the light not destr ucted by the UV r adiation, so it can be used as a molecular UV filter . Secondly, this mater ial 3+ energy in the 280–415 nm range into intensive monochromatic orange-red luminescence of Eu ions can efficiently tr ansfor m the light ener gy in the 280–415 nm range into intensive monochr omatic 3+ (the absolute quantum yield of luminescence of Eu ions is ~30% [29]), so it can be an ecient 3+ 3+ or ange-r ed luminescence of Eu ions (the absolute quantum yield of luminescence of Eu ions is light-transforming material and the source of monochromatic orange-red light. Finally, this material ~30% [29]), so it can be an efficient light -tr ansfor ming mater ial and the sour ce of monochr omatic can reversibly change luminescence intensity and quenching time in the 298–348 K temperature range or ange-r ed light. Finally, this mater ial can r ever sibly change luminescence intensity and quenching time in the 298–348 K temper atur e r ange (Figur e 5a), so it can become an efficient w or king el ement Photonics 2019, 6, 110 7 of 10 Photonics 2019, 6, x FOR PEER REV IEW 7 o f 10 of multiple use luminescent ther mometer s. This mater ial posses ses the follow ing technological (Figure 5a), so it can become an ecient working element of multiple use luminescent thermometers. advantages over its close analogues: total r esistance to the UV r adiation, high optical quality, total This material possesses the following technological advantages over its close analogues: total resistance pr otection fr om the contact with atmospher ic oxygen, and the highest aver age absolute temper atur e to the UV radiation, high optical quality, total protection from the contact with atmospheric oxygen, and −1 sensitivity of the luminescence decay time (6.5 μs∙K ) among temper atur e-sensitive film mater ials the highest average absolute temperature sensitivity of the luminescence decay time (6.5 sK ) among based on nonmesogenic eur opium(III) β -diketonate complexes, w hich efficiently absorb light in the temperature-sensitive ﬁlm materials based on nonmesogenic europium(III) -diketonate complexes, violet r ange of the visible spectr um. The value of the r elative temper atur e sens itivity of the which eciently absorb light in the violet range of the visible spectrum. The value of the relative (r) −1 −1 (r) 1 luminescence decay time S τ var ies fr om −0.28%·K at 298 K to −1.6%·K at 348 K w ith τRef = 537µ s. temperature sensitivity of the luminescence decay time S varies from 0.28%K at 298 K to −1 (r) 1 1 Its aver age value is −1.2%·K . It is impor tant to note that the aver age S τ value is on e of the lar gest 1.6%K at 348 K with = 537s. Its average value is 1.2%K . It is important to note that Ref (r) r epor ted for the temper atur e-sensitive films based on the eur opium(III) b -diketonate complexes, the average S value is one of the largest reported for the temperature-sensitive ﬁlms based on the w hich ar e efficient absor ber s of light in the violet r egion [30]. europium(III) b-diketonate complexes, which are ecient absorbers of light in the violet region [30]. A luminescent mater ial made of a vitrified Tb (CP DK3–7)3Phen 6.1 µ m thick film also offer s A luminescent material made of a vitriﬁed Tb(CPDK ) Phen 6.1 m thick ﬁlm also oers 3–7 3 inter esting pr operties for br oad application in var ious ar eas. This film is an efficient absor ber of the interesting properties for broad application in various areas. This ﬁlm is an ecient absorber of the light ener gy in the br oad 280 −405 nm r ange, w hich is then conver ted into the monochr omatic gr een light energy in the broad 280405 nm range, which is then converted into the monochromatic green 3+ 3+ luminescence of Tb ions. In the 143−253 K temper atur e range, this mater ial can r eversibly change luminescence of Tb ions. In the 143253 K temperature range, this material can reversibly change the the luminescence qu enching time (Fi gur e 5b), so it can also become an effici ent w or king element o f luminescence quenching time (Figure 5b), so it can also become an ecient working element of multiple multiple use luminescent ther mometer s [29]. A temper atur e sensitive mater ial made of th e use luminescent thermometers [29]. A temperature sensitive material made of the Tb(CPDK ) Phen 3–7 3 Tb (CP DK3–7)3Phen film is easy to pr oduce, insensitive to oxygen, char acter ized by high optical ﬁlm is easy to produce, insensitive to oxygen, characterized by high optical quality, and resistant quality, and r esistant to destr uctive effects of UV r adiation. It is an efficient absor ber of violet light to destructive eects of UV radiation. It is an ecient absorber of violet light and possesses high −1 and possesses high temper atur e sensitivity to a lumi nescence quenching time (3.3 μs∙К ). The temperature sensitivity to a luminescence quenching time (3.3 sK ). The relative temperature (r) −1 (r) 1 1 r elative temperatur e sensitivity of the average luminescence decay time S τ is −0.4%·K at 253 K and sensitivity of the average luminescence decay time S is0.4%K at 253 K and1.4%K at 143 K −1 −1 −1.4%·K at 143 K w ith τRef= 373 µ s. Its mean value is −0.9%·K . T o the best of our know ledge, ther e with = 373 s. Its mean value is 0.9%K . To the best of our knowledge, there are no reports Ref ar e no r epor ts descr ibing the temper atur e-sensitive luminescent films (oper ating below r oom describing the temperature-sensitive luminescent ﬁlms (operating below room temperature) based on temper atur e) based on ter bium(III) β -diketonate complexes w ith the similar char acter istics. terbium(III) -diketonate complexes with the similar characteristics. (a) (b) Figure 5. Re vers ible cha nges in the lumine sce nce deca y time (m onitore d a t 612 nm) for the Figure 5. Reversible changes in the luminescence decay time (monitored at 612 nm) for the Eu(CPDK3 – 5)3Phe n film (a) a nd in the luminesce nce deca y time (monitore d a t 545 nm) for the Eu(CPDK ) Phen ﬁlm (a) and in the luminescence decay time (monitored at 545 nm) for the 3–5 3 Tb(CPDK3 – 7)3Phe n (b) unde r the e xcita tion by the 337 nm pulse d nitrogen lase r with the 0.17 mW Tb(CPDK ) Phen (b) under the excitation by the 337 nm pulsed nitrogen laser with the 0.17 mW 3–7 3 a vera ge output powe r in consecutive hea ting -cooling cycles . Sta ndard de via tions a re s hown a s error average output power in consecutive heating-cooling cycles. Standard deviations are shown as ba rs . error bars. The temperature sensitivity of luminescence properties of the fabricated ﬁlm materials was studied The temper atur e sensitivity of luminescence pr oper ties of the fabr icated film mater ials w as by measuring their lifetimes. As opposed to the luminescence intensity, the luminescence lifetime studied by measuring their lifetimes. As opp osed to the lu minescence intensity, the luminescence parameter does not depend on measurement conditions and the degradation coecient value, so lifetime par ameter does not dep end on measur ement conditions and the degr adation coefficient it can be used for more reliable and accurate determination of temperature. Thermally sensitive value, so it can be used for mor e r eliable and accur ate deter mination of temper atur e. Ther mally luminescent properties of the Tb(CPDk ) Phen ﬁlms were studied in the 143–253 K temperature 3–7 3 sensitive luminescent pr operties of the Tb (CP Dk3–7)3Phen films w er e studied in the 143 –253 К range (Figure 6a) because no signiﬁcant changes of the luminescence lifetime were observed in the temper atur e r ange (Figur e 6a) because no significant changes of the luminescence lifetime w er e upper range of 253–284 K, while cooling below 143 K led to irreversible changes in the local structure of obser ved in the upper range of 253–284 К, w hile cooling below 143 К led to irr eversible changes in the local str uctur e of films. Th er mally sensitive luminescent pr oper ties of the Eu(CP Dk 3–5 )3Phen Photonics 2019, 6, 110 8 of 10 Photonics 2019, 6, x FOR PEER REV IEW 8 o f 10 Photonics 2019, 6, x FOR PEER REV IEW 8 o f 10 ﬁlms. Thermally sensitive luminescent properties of the Eu(CPDk ) Phen ﬁlms were studied in the 3–5 3 films w er e studied in the 253–348 K temper atur e r ange (Figur e 6b) because the luminescence lifetime films w er e studied in the 253–348 K temper atur e r ange (Figur e 6b) because the luminescence lifetime 253–348 K temperature range (Figure 6b) because the luminescence lifetime was thermally irresponsive w as thermally irr esponsive at temper atur es below 253 К, w hile the 353 К temper atur e tur ned out to w as thermally irr esponsive at temper atur es below 253 К, w hile the 353 К temper atur e tur ned out to at temperatures below 253 K, while the 353 K temperature turned out to be the softening threshold of be the softening thr eshold of the Eu(CPDk3–5)3Phen films. be the softening thr eshold of the Eu(CPDk3–5)3Phen films. the Eu(CPDk ) Phen ﬁlms. 3–5 3 (a) (b) (a) (b) Figure 6. Te mpe ra ture de pe nde nce of the luminesce nce life time of the Tb (CPDk3 – 7)3Phe n films (a) Figure 6. Temperature dependence of the luminescence lifetime of the Tb (CPDk ) Phen ﬁlms (a) Figure 6. Te mpe ra ture de pe nde nce of the luminesce nce life time of the Tb (CPDk3 – 7)3Phe n films (a) 3–7 3 a nd the Eu(CPDk3 – 5)3Phe n (b) comple xe s. and the Eu(CPDk ) Phen (b) complexes. a nd the Eu(CPDk3 – 5)3Phe n (b) comple xe s. 3–5 3 3+ 3+ We previously demonstrated [29] that the major quenching route of luminescence of Tb ions in W e pr eviously demonstr ated [29] that the maj or quenching r oute of luminescence of Tb 3+ ions W e pr eviously demonstr ated [29] that the maj or quenching r oute of luminescence of Tb ions 5 3+ 5 3+ the Tb(CPDK ) Phen ﬁlm is the energy back transfer from the D level 5 of the Tb ion 3+to the T1 state in the Tb (CPDK3–7)3Phen film is the ener gy back tr ansfer fr om the D4 level of the Tb ion to the T1 3–7 3 4 in the Tb (CPDK3–7)3Phen film is the ener gy back tr ansfer fr om the D4 level of the Tb ion to the T1 of the ligands and the subsequent non-radiative relaxation. The paper [30] shows that the low-lying state of the ligands and the subsequ ent non -r adiative r elaxation. The paper [30] show s that the state of the ligands and the subsequ ent non -r adiative r elaxation. The paper [30] show s that the ligand-to-metal charge transfer (LMCT) states are responsible for the strong luminescence quenching low -lying ligand-to-metal char ge transfer (LMCT) states ar e r esponsible for the str ong luminescence low -lying ligand-to-metal char ge transfer (LMCT) states ar e r esponsible for the str ong luminescence from the D level in the Eu(CPDK ) Phen ﬁlm at high temperatures. quenching fr om the 5 D0 level in the Eu(CPDK3–5)3Phen film at high temper atur es. 0 3–5 3 quenching fr om the D0 level in the Eu(CPDK3–5)3Phen film at high temper atur es. At room temperature, the vitriﬁed Tb(CPDK ) Phen ﬁlm demonstrates substantial increase At r oom temper atur e, the vitr ified Tb (CP DK3–7)3Ph en film demonstr ates substantial incr ease of 3–7 3 At r oom temper atur e, the vitr ified Tb (CP DK3–7)3Ph en film demonstr ates substantial incr ease of 3+ 3+ of the luminescence intensity of the Tb ions under UV laser radiation (Figure 7) that is untypical the luminescence intensity of the Tb3+ ions under UV laser r adiation (Figur e 7) that is untypical for the luminescence intensity of the Tb ions under UV laser r adiation (Figur e 7) that is untypical for for classical -diketonate complexes [18]. In the context of potential applications, it is interesting classical β-diketonate complexes [18]. In the context of potential applications, it is inter esting that the classical β-diketonate complexes [18]. In the context of potential applications, it is inter esting that the that the modiﬁed luminescence brightness of the ﬁlms is sustainable for several months, while the modified luminescence br ightness of the films is sustainable for sever al months, w hile the films modified luminescence br ightness of the films is sustainable for sever al months, w hile the films ﬁlms return to their initial state after thermal processing. Experiments conﬁrmed that it is possible r etur n to their initial state after thermal pr ocessing. Exp er iments confi r med that it is possible to r etur n to their initial state after thermal pr ocessing. Exp er iments confi r med that it is possible to to perform multiple switching of the luminescence brightness. The ﬁlms with such characteristics per for m multiple sw itching of the luminescence br ightness. The films w ith such char acteristics per for m multiple sw itching of the luminescence br ightness. The films w ith such char acteristics provide potential solutions for creating brand-new molecular photonics devices, such as multiple use pr ovide potential solutions for cr eating br and-new molecular photonics devices , such as multiple pr ovide potential solutions for cr eating br and-new molecular photonics devices , such as multiple luminescent UV sensors, which can “remember” their modiﬁed state for several months. use luminescent UV sensor s, w hich ca n “r emember ” their modified state for sever al months. use luminescent UV sensor s, w hich ca n “r emember ” their modified state for sever al months. Figure 7. Lase r control a nd tempe ra ture s witching of lumine sce nce inte ns ity in photos ta ble Figure 7. Laser control and temperature switching of luminescence intensity in photostable transparent Figure 7. Lase r control a nd tempe ra ture s witching of lumine sce nce inte ns ity in photos ta ble tra ns pa rent film ba s ed on the te rbium(III) β -dike tonate comple x. tﬁlm ra nsbased pa rent on filthe m bterbium(III) a s ed on the t e-diketonate rbium(III) βcomplex. -dike tonate comple x. 4. Conclusions 4. Conclusions In this paper , w e pr oposed a simple method for cr eating micr oscale luminescent film mater ials In this paper , w e pr oposed a simple method for cr eating micr oscale luminescent film mater ials w ith contr olled optical pr operties based on anisometr ic compounds of Tb (III) and Eu (III) for w ith contr olled optical pr operties based on anisometr ic compounds of Tb (III) and Eu (III) for applications in optics and photonics. The pr esence of long alkyl substituents in the str uctur e of applications in optics and photonics. The pr esence of long alkyl substituents in the str uctur e of Photonics 2019, 6, 110 9 of 10 4. Conclusions In this paper, we proposed a simple method for creating microscale luminescent ﬁlm materials with controlled optical properties based on anisometric compounds of Tb(III) and Eu(III) for applications in optics and photonics. The presence of long alkyl substituents in the structure of ligands prevents them from destruction by melting, reduces melting and glass transition temperatures, and minimizes conditions that favor formation of crystalline defects. The anisometric complexes synthesized within the framework of this research are capable of transforming UV radiation into visible light. Thus, this research resulted in fabrication of highly photostable and uniform transparent ﬁlms with possible application as reusable sensors and light converters. Author Contributions: The work described in this article is the collaborative development of all authors. Conceptualization, A.K. and Y.G.; Methodology, A.K. and Y.G.; Software, M.K.; Validation, A.K., M.K. and Y.G.; Formal Analysis, M.K.; Investigation, A.K., M.K. and Y.G.; Resources, Y.G.; Data Curation, A.K. and Y.G.; Writing-Original Draft Preparation, A.K. and M.K.; Writing-Review & Editing, Y.G.; Supervision, A.K. and Y.G. Funding: This research was funded by Russian Science Foundation (grant No. 18-13-00112). Acknowledgments: The authors would like to thanks the Russian Science Foundation (grant No. 18-13-00112) for funding this research work. Conﬂicts of Interest: The authors declare no conﬂict of interest. References 1. Chu, T.; Zhang, F.; Wang, Y.; Yang, Y.; Ng, S.W. A novel electrophoretic deposited coordination supramolecular network ﬁlm for detecting phosphate and biophosphate. Chem. A Eur. J. 2017, 23, 7748–7754. [CrossRef] [PubMed] 2. Wang, Z.; Liu, H.; Wang, S.; Rao, Z.; Yang, Y. 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Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Photostable Anisometric Lanthanide Complexes as Promising Materials for Optical Applications

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Photostable Anisometric Lanthanide Complexes as Promising Materials for Optical Applications

Photostable Anisometric Lanthanide Complexes as Promising Materials for Optical Applications

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