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D–π–A–π–D Initiators Based on Benzophenone Conjugate Extension for Two-Photon Polymerization Additive Manufacturing

D–π–A–π–D Initiators Based on Benzophenone Conjugate Extension for... hv photonics Article D––A––D Initiators Based on Benzophenone Conjugate Extension for Two-Photon Polymerization Additive Manufacturing 1 , 2 3 2 , 2 2 2 2 2 Shanggeng Li , Xiaolin Liu , Shuai Zhang *, Yawen Zhou , Xiangyu Wan , Ning Li , Jing Li and Lin Zhang School of Nuclear Science and Technology, University of Science and Technology of China, Hefei 230026, China; lsglsg@mail.ustc.edu.cn Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China; zhouyawen163@163.com (Y.Z.); wanxiangyu_caep@126.com (X.W.); lncaep@163.com (N.L.); jingli0218@163.com (J.L.); zhlmy@sina.com (L.Z.) School of Physics, University of Science and Technology of China, Hefei 230026, China; lxl91003@mail.ustc.edu.cn * Correspondence: zhangshuai_scu@126.com; Tel.: +86-0816-2493148 Abstract: A two-photon polymerization initiator is a kind of nonlinear optical material. With the demand for more efficient initiators in two-photon polymerization additive manufacturing, there are more and more related studies. In this paper, four conjugate-extended two-photon polymerization initiators with different alkane chain lengths were designed and synthesized, and single-photon, two- photon, and photodegradation experiments were carried out. Additive manufacturing experiments illustrated that the designed molecules can be used as two-photon initiators, and the writing speed can achieve 100,000 m/s at a laser power of 25 mW. Through theoretical calculation and experimental comparison of the properties of molecules with different conjugation degrees, it was proven that a certain degree of conjugation extension can improve the initiation ability of molecules; however, this improvement cannot be extended infinitely. Solubility tests of different acrylates showed that molecules with different alkane chain lengths have varying solubility. Changing the molecular alkane chain length may be favorable to adapt to different monomers. Citation: Li, S.; Liu, X.; Zhang, S.; Zhou, Y.; Wan, X.; Li, N.; Li, J.; Zhang, Keywords: optical functional materials; photoinitiator; two-photon polymerization additive L. D––A––D Initiators Based on manufacturing; conjugation extension; solubility Benzophenone Conjugate Extension for Two-Photon Polymerization Additive Manufacturing. Photonics 2022, 9, 183. https://doi.org/ 1. Introduction 10.3390/photonics9030183 Two-photon absorption is a third-order nonlinear optical effect that has a wide range Received: 17 February 2022 of uses. Among them, two-photon polymerization additive manufacturing (TPPAM) is Accepted: 10 March 2022 extensively used in many three-dimensional precision structure manufacturing fields— Published: 13 March 2022 such as those of photonic crystals [1,2], micro-optical elements [3,4], three-dimensional Publisher’s Note: MDPI stays neutral high-density information storage [5–7], biocompatible devices [8–10], etc.—due to its with regard to jurisdictional claims in unique ultra-high-resolution performance. TPPAM has a growing number of applicaitons, published maps and institutional affil- which also highlights the new requirements for its manufacturing speed. Since TPPAM iations. is a manufacturing method in which dots form lines and lines form surfaces, making larger structures takes a lot of time to manufacture. Meanwhile, the speed of writing in manufacturing is extremely significant. A larger writing speed can improve manufacturing efficiency and save time. Copyright: © 2022 by the authors. However, if the writing speed is too high, the resin cannot be irradiated with sufficient Licensee MDPI, Basel, Switzerland. laser energy and polymerized to form a three-dimensional structure. One way to solve this This article is an open access article problem is to increase the power of the laser. Nevertheless, laser power that is too high distributed under the terms and will impose high requirements on the laser and increase production costs. Another way is conditions of the Creative Commons to raise the efficiency of the two-photon polymerization initiator (TPPI). As an important Attribution (CC BY) license (https:// nonlinear material, TPPI absorbs laser energy, undergoes two-photon absorption, generates creativecommons.org/licenses/by/ free radicals, and initiates polymerization [11]. Therefore, it only requires a more efficient 4.0/). Photonics 2022, 9, 183. https://doi.org/10.3390/photonics9030183 https://www.mdpi.com/journal/photonics Photonics 2022, 9, 183 2 of 13 initiator if the speed of writing in manufacturing is to be faster, which can lead to the absorbtion of more photons or the production of more free radicals. As a consequence, the key to the solving this problem is to improve the efficiency of the initiator. In the early days, TPPI used conventional ultraviolet (UV) photoinitiators. UV photoinitiators have active groups and generate free radicals under ultraviolet light irradiation. However, the absorption band of ultraviolet photoinitiators is mainly located in the ultraviolet region, and the two-photon absorption cross section (d ) is also small. The TPA possibility of two-photon absorption is so low that the low initiation efficiency is rendered. Studies have proven that by performing conjugation extension, the d of the molecule TPA can be effectively increased, and the absorption efficiency of light can be raised, thereby improving the efficiency of the initiator [12–14]. Conjugation extension is the introduction of conjugated groups to connect electron acceptors and electron donors, as well as increase the degree of molecular conjugation and intramolecular charge transfer (ICT) [15–17]. Currently, the efficiency of initiators through conjugation expansion has been improved in many studies [18–25]. These studies concluded that molecules make up the structures donor––acceptor (D––A), donor––acceptor––donor (D––A––D), acceptor––donor– –acceptor (A––D––A), etc., would have better ICT and, thus, have larger d and TPA initiator efficients. However, it is clear that there is a limit to the enhancement of molecular initiation properties through conjugation extension. It is interesting to question, then, where the performance limits can be improved using this approach. In addition, the compatibility of molecules and monomers is also essential. The initia- tor molecule must be completely dissolved in the monomer to be effective. Often monomers with different types of structures have different requirements for initiator molecules. For example, the initiators used to print hydrogels need to be water-soluble [26,27]. However, it is very difficult under the condition that one kind of initiator can be widely used in various polar resins. For each monomer, molecular design considering solubility is extremely complicated, and it is not conducive to practical production applications. Therefore, it would be interesting if there was a way to change the structure of the molecule in small amounts, effectively improving the solubility of the molecule with different monomers. The alkane chain is a very easily overlooked part because it hardly affects the chemical properties of the molecule. However, the length of the alkane chain will affect the polarity, the degree of steric hindrance and the geometric configuration of the molecule, and thus, the solubility of the molecule. Moreover, due to the alkane chain hardly affecting the chemical properties of the molecule, changing the length of the alkane chain hardly affects the synthesis method and manufacturing process of the molecule, which greatly reduces the cost of practical application. In view of the above two points, the traditional UV photoinitiator BD-PM with an active structure was conjugated and extended by introducing a benzene ring to obtain the molecules BD-PPM and BM-PPPM, as shown in Figure 1. Among them, BD-PPM molecules have been reported in the literature with similar structures, and certain effects have been achieved [28]. BM-PPPM with two benzene rings has a larger degree of conjugation. In the previous work, the molecules (BM-PTM) with a larger degree of conjugation formed by benzophenone and triphenylamine were studied, and also obtained good results [29]. New groups can be introduced because diphenylamine has one more site on the nitrogen atom than triphenylamine. Hence, the alkane chain will affect the solubility of the molecule, and the length of the alkane chain on the diphenylamine is simply changed for the molecule BM-PPPM, so that initiators suitable for various monomers can be obtained. This paper compares the initiation properties of molecules with different degrees of conjugation and investigated whether BM-PPPM series molecules can be better considered as initiators. Photonics 2022, 9, 183 3 of 14 diphenylamine is simply changed for the molecule BM-PPPM, so that initiators suitable for various monomers can be obtained. This paper compares the initiation properties of molecules with different degrees of conjugation and investigated whether BM-PPPM series molecules can be better considered as initiators. Photonics 2022, 9, 183 3 of 14 diphenylamine is simply changed for the molecule BM-PPPM, so that initiators suitable for various monomers can be obtained. This paper compares the initiation properties of Photonics 2022, 9, 183 3 of 13 molecules with different degrees of conjugation and investigated whether BM-PPPM series molecules can be better considered as initiators. Figure 1. The structural formulas of BD-PM (UV photoinitiator), BD-PPM [28], BM-PPPM and BM- PTM [29]. 2. Theory Calculation The DFT approach can be used to calculate the frontier orbits of conjugated molecules such as D–π–A–π–D and A–π–D–π–A, and to observe the degree of intramolecular charge transfer, so as to pred Figure ict the 1. two The str -p uctural hoton formulas absorp of t BD-PM ion ch (UV aracte photoinitiato risticsr), ofBD-PPM the mo [28 lec ], BM-PPPM ule [30,31 and ]. Figure 1. The structural formulas of BD-PM (UV photoinitiator), BD-PPM [28], BM-PPPM and BM- BM-PTM [29]. PTM [29]. To probe into the electronic properties of molecules with different conjugation extension 2. Theory Calculation degrees, the theoretical calculations were carried out using the DFT approach, and the 2. Theory Calculation The DFT approach can be used to calculate the frontier orbits of conjugated molecules B3LYP functional and 6-31G(d, p) basis sets were used with Gaussian 09 [32]. For The DFT approach can be used to calculate the frontier orbits of conjugated molecules such as D––A––D and A––D––A, and to observe the degree of intramolecular charge molecules with three different degrees of conjugation extension, the degree of transfer such as D , so–asπ–A– to prπ edict –D and the A two-photon –π–D–π–A absorption , and to observe characteristics the degr of ee o the molecule f intramo [l 30 ec ,31 ular ch ]. arge intramolecular charge transfer and the frontier orbital energy were analyzed. To reduce To probe into the electronic properties of molecules with different conjugation extension de- transfer, so as to predict the two-photon absorption characteristics of the molecule [30,31]. grees, the theoretical calculations were carried out using the DFT approach, and the B3LYP the workload of calculations, all alkyl groups were substituted with methyl groups. To probe into the electronic properties of molecules with different conjugation extension functional and 6-31G(d, p) basis sets were used with Gaussian 09 [32]. For molecules with degrees, the theoretical calculations were carried out using the DFT approach, and the It can be seen from Figure 2 that the HOMO orbital was mainly concentrated in the three different degrees of conjugation extension, the degree of intramolecular charge trans- B3LYP functional and 6-31G(d, p) basis sets were used with Gaussian 09 [32]. For amino part of the electron donor and extended to the π bond bridge, while the LUMO fer and the frontier orbital energy were analyzed. To reduce the workload of calculations, molecules with three different degrees of conjugation extension, the degree of all alkyl groups were substituted with methyl groups. orbital mainly extended from the acetylene bridge to the electron acceptor group intramolecular charge transfer and the frontier orbital energy were analyzed. To reduce It can be seen from Figure 2 that the HOMO orbital was mainly concentrated in the benzophenone. With the increases in the conjugation degree, when the BD-PM the workload of calculations, all alkyl groups were substituted with methyl groups. amino part of the electron donor and extended to the  bond bridge, while the LUMO orbital conjugation was extended to BD-PPM, the ICT of the molecule increased significantly. It can be seen from Figure 2 that the HOMO orbital was mainly concentrated in the mainly extended from the acetylene bridge to the electron acceptor group benzophenone. When extending from With BD-PPM the increases toin Bthe M-P conjugation PPM, the degr HO ee, when MO orb the BD-PM ital ch conjugation arge of the was molecule extended to amino part of the electron donor and extended to the π bond bridge, while the LUMO BD-PPM, the ICT of the molecule increased significantly. When extending from BD-PPM to orbital mainly extended from the acetylene bridge to the electron acceptor group was distributed to a further position, along with the introduced conjugated benzene ring, BM-PPPM, the HOMO orbital charge of the molecule was distributed to a further position, benzophenone. With the increases in the conjugation degree, when the BD-PM but the LUMO orbital did not change significantly. This probably indicates that the along with the introduced conjugated benzene ring, but the LUMO orbital did not change conjugation was extended to BD-PPM, the ICT of the molecule increased significantly. expansion of the elec significantly tron donor mainly . This probably affec indicates ts the that the electron distribution of molecules on expansion of the electron donor mainly affects When extending from BD-PPM to BM-PPPM, the HOMO orbital charge of the molecule the electron distribution of molecules on the HOMO orbital. Overall, the designed molecule the HOMO orbital. Overall, the designed molecule BM-PPPM had good ICT performance, was distributed to a further position, along with the introduced conjugated benzene ring, BM-PPPM had good ICT performance, which provides a theoretical basis for its application which provides a theoretical basis for its application as a TPPAM photoinitiator. but the LUMO orbital did not change significantly. This probably indicates that the as a TPPAM photoinitiator. expansion of the electron donor mainly affects the electron distribution of molecules on the HOMO orbital. Overall, the designed molecule BM-PPPM had good ICT performance, which provides a theoretical basis for its application as a TPPAM photoinitiator. Figure 2. Frontier orbital diagrams of molecules with different degrees of conjugation. HOMO: Figure 2. Frontier orbital diagrams of molecules with different degrees of conjugation. HOMO: Highest Occupied Molecular Orbital; LUMO: Lowest Unoccupied Molecular Orbital. (a) BD-PM; Highest Occupied Molecular Orbital; LUMO: Lowest Unoccupied Molecular Orbital. (a) BD-PM; Figure 2. Frontier orbital diagrams of molecules with different degrees of conjugation. HOMO: (b) BD-PPM; (c) BM-PPPM. Highest Occupied Molecular Orbital; LUMO: Lowest Unoccupied Molecular Orbital. (a) BD-PM; (b) BD-PPM; (c) BM-PPPM. The calculated results of the molecular frontier orbital energy of BD-PM were very (b) BD-PPM; (c) BM-PPPM. close to the known reports [33], which proves the rationality of the calculation. While Photonics 2022, 9, 183 4 of 13 the use of the B3LYP functional is suitable for calculations of molecular energies, it also over-planarizes the molecule, and over-delocalizes the wave function. As shown in Table 1, the HOMO energy level of the molecule was significantly increased, the LUMO energy level was significantly decreased, and the band gap was significantly reduced as the BD-PM conjugation was extended to BD-PPM. When BD-PPM was extended to BM-PPPM, the change degree of molecular energy level was reduced significantlyand the band gap was slightly reduced, which is consistent with the result directly observed in Figure 2. This shows that, theoretically, the conjugation of molecules is relatively limited, and it means that the conjugation of the molecule cannot be increased indefinitely by blindly adding conjugated groups. Table 1. Molecular frontier orbital energy. HOMO LUMO Gap Dippole (eV) (eV) (eV) (Debye) BM-PPPM 5.01 1.85 3.16 6.4644 BD-PPM 4.96 1.78 3.18 6.5658 BD-PM 5.71 1.49 4.21 4.2349 3. Experimental Section 3.1. Materials All reagents were obtained from commercial suppliers (Table S1) and used without further purification. 3.2. Physical Measurements Nuclear magnetic resonance (NMR) spectra were obtained on a BRUKER AVANCE 1 13 1 III HD 400 MHz ( H NMR at 400 MHz, C NMR at 100 MHz). The H NMR chemical shifts were measured relative to CDCl (d = 7.26 ppm) as the internal reference. The C NMR chemical shifts were given using CDCl (d = 77.16 ppm) as the internal standard. High-resolution mass spectra (HRMS) were obtained with a Thermo Scientific Q Exactive. Absorption spectra were obtained on a HITACHI U-2910 spectrometer. Fluorescence spectra were collected on a Horiba Fluorolog-3 fluorescence spectrometer. The two-photo fluorescence spectra were obtained with MAITAIHP, spectrum physic maitai, and SD2000. The fluorescence quantum yields were measured using a Hamamatsu C11347-11. The cyclic voltammetry tests were performed on an electrochemical workstation (instrument model: CHI660E). Scanning electron microscopes (SEM) were obtained using a Merlin VP Compact, Carl Zeiss. 3.3. Synthesis Detailed synthetic methods are provided in the Supplementary Materials (Supporting Information) (Figures S1–S3). The synthesis of the final products BM-PPPM was achieved using the classical Sonogashira reaction [34]. BM-PPPM-C6: Purification by column chromatography on silica gel (petroleum ether/ethyl acetate = 10/1, v/v) rendered BM-PPPM-C6 as a yellow solid. H NMR (400 MHz, CDCl ): d = 0.88 (t, J = 6.8 Hz, 6H), 1.25–1.35 (m, 12H), 1.62–1.72 (m, 4H), 3.70 (t, J = 6.8 Hz, 4H), 3.85 (t, J = 7.2 Hz, 4H), 6.75–6.79 (m, 4H), 7.11–7.19 (m, 6H), 7.34–7.40 (m, 8H), 7.56–7.62 (m, 4H), 7.74–7.79 (m, 4H) ppm. C NMR (CDCl , 100 MHz): d = 14.2, 22.8, 26.9, 27.5, 31.8, 52.5, 87.5, 94.1, 111.8, 116.1, 124.5, 125.5, 128.6, 129.8, 130.1, 131.2, 133.0, 136.2, 147.0, 148.8, 195.4 ppm. BM-PPPM-C12: Purification by column chromatography on silica gel (petroleum ether/ethyl acetate = 15/1, v/v) rendered BM-PPPM-C12 as a yellow solid. H NMR (400 MHz, CDCl ): d = 0.88 (t, J = 7.2 Hz, 6H), 1.21–1.33 (m, 36H), 1.62–1.72 (m, 4H), 3.70 (t, J = 8.0 Hz, 4H), 3.85 (t, J = 7.2 Hz, 4H), 6.77 (d, J = 8.8 Hz, 4H), 7.12–7.19 (m, 6H), 7.37 (t, J = 6.8 Hz, 8H), 7.59 (d, J = 8.4 Hz, 4H), 7.77 (d, J = 8.4 Hz, 4H) ppm. C NMR (CDCl , 3 Photonics 2022, 9, 183 5 of 13 100 MHz): d = 14.3, 22.9, 27.2, 27.5, 29.5, 29.6, 29.72, 29.76, 29.77, 29.78, 32.1, 52.5, 87.5, 94.1, 111.7, 116.1, 124.4, 125.5, 128.6, 129.8, 130.1, 131.2, 133.0, 136.1, 147.0, 148.8, 195.4 ppm. BM-PPPM-C16: Purification by column chromatography on silica gel (petroleum ether/ethyl acetate = 15/1, v/v) rendered BM-PPPM-C16 as a yellow oil. H NMR (400 MHz, CDCl ): d = 0.84–0.92 (m, 14H), 1.16–1.28 (m, 46H), 1.77–1.86 (m, 2H), 3.63 (d, J = 7.2 Hz, 4H), 6.78–6.84 (m, 4H), 7.09–7.18 (m, 6H), 7.32–7.40 (m, 6H), 7.56–7.62 (m, 4H), 7.75–7.79 (m, 4H) ppm. C NMR (CDCl , 100 MHz): d = 14.25, 14.28, 22.79, 22.83, 26.46, 26.47, 29.5, 29.7, 29.8, 30.2, 31.66, 31.68, 31.95, 32.0, 36.4, 56.9, 87.6, 94.0, 112.1, 116.9, 124.1, 125.3, 128.6, 129.7, 130.1, 131.2, 132.8, 136.2, 147.6, 149.7, 195.4 ppm. BM-PPPM-C20: Purification by column chromatography on silica gel (petroleum ether/ethyl acetate = 20/1, v/v) rendered BM-PPPM-C20 as a yellow oil. H NMR (400 MHz, CDCl ): d = 0.88 (t, J = 7.2 Hz, 14H), 1.17–1.28 (m, 62H), 1.75–1.85 (m, 2H), 3.70 (d, J = 7.2 Hz, 4H), 6.81 (d, J = 8.8 m, 4H), 7.09–7.18 (m, 6H), 7.31–7.40 (m, 6H), 7.56–7.62 (m, 4H), 7.75–7.79 (m, 4H) ppm. C NMR (CDCl , 100 MHz): d = 14.3, 22.83, 22.85, 26.5, 29.46, 29.51, 29.69, 29.73, 29.78, 29.79, 30.17, 31.67, 32.03, 32.07, 36.34, 56.9, 87.6, 94.0, 112.1, 116.9, 124.1, 125.3, 128.6, 129.7, 130.1, 131.2, 132.9, 136.2, 147.6, 149.7 ppm. 3.4. Resin Preparation To consider the initiation efficiency and solubility of the initiator under different monomers, two monomers—trimethylolpropane triacrylate (TMPTA) and pentaerythritol triacrylate (PETA)—were adopted. They all had three functional groups, which could achieve a high degree of polymerization and avoid the performance differences caused by different functionalities. Photoresist was prepared by mixing TMPTA, PETA, and 4 kinds of initiator molecules with different chain lengths with BM-PPPM (10 mol/g), then stirred overnight at 45 C until clear and free of precipitation. Finally, the mixture was placed in a greenhouse for 24 h to ensure that it did not settle. 3.5. TPPAM Test The TPPAM experiments were carried out by Nanoscribe GT Photonic Professional, in which the repetition rate was 80 MHz and the wavelength was 780  10 nm. The pulse duration was approximately 120 fs. The average power of laser irradiation on the sample plane was about 50 mW. In the experiment, a 63 objective lens (Numerical aperture 1.4, working distance 190 m) was adopted. Meanwhile, a three-dimensional wooden-stick stack structure was designed in the experiment under different laser powers and writing speeds, which aimed to test the molecular initiation efficiency and the manufacturing parameter interval for forming a complete three-dimensional structure. The preparation process adopted an oil-immersion objective lens. After production, the final structure was obtained by being developed twice for 10 min in propylene glycol monomethyl ether acetate (PGMEA), then washed with isopropanol and eventually dried at 40 C. 4. Results and Discussions 4.1. Linear Absorption and Fluorescence Spectra As shown in Figure 3, the ultraviolet absorption and fluorescence spectra of molecules BM-PPPM-C6 and BM-PPPM-C12 are, of course, very close. Owing to its chain length, alkane has little effect on the chemical properties of the molecule, only the properties of BM- PPPM-C6 need to be tested. It can be seen from the absorption spectrum that the molecule has a large ultraviolet absorption peak and molar extinction coefficient near 390 nm. This means that molecules have a great utilization of light near this wavelength. It can be speculated that when two-photon absorption occurs, there will be a greater absorption rate for the 780 nm laser. As can be seen from the fluorescence spectrum of the molecule, the emission wave- length of the fluorescence ehxibits a significant red shift with the increase in the polarity of Photonics 2022, 9, 183 6 of 13 Photonics 2022, 9, 183 6 of 14 the solvent, which indicates that the excitation of molecules has an obvious ICT process. wavelength. It can be speculated that when two-photon absorption occurs, there will be a This is conducive to the nonlinear properties of the molecule. greater absorption rate for the 780 nm laser. Figure 3. Linear absorption and fluorescence spectra of molecules (a) BM-PPPM-C6 and (b) BM- Figure 3. Linear absorption and fluorescence spectra of molecules (a) BM-PPPM-C6 and (b) BM- −5 PPPM-C12 at a concentration of 1 × 10 mol/L and cuvette a cuvette width of 1 cm. THF—tetrahy- PPPM-C12 at a concentration of 1  10 mol/L and cuvette a cuvette width of 1 cm. THF— drofuran; EA—ethyl acetate; DCM—dichloromethane; DMF—N,N-dimethylformamide. tetrahydrofuran; EA—ethyl acetate; DCM—dichloromethane; DMF—N,N-dimethylformamide. As can be seen from the fluorescence spectrum of the molecule, the emission 4.2. Two-Photon Fluorescence Spectra wavelength of the fluorescence ehxibits a significant red shift with the increase in the Two-photon fluorescence spectra can reflect the ability of the molecule to undergo polarity of the solvent, which indicates that the excitation of molecules has an obvious ICT two-photon absorption. As the probability of two-photon absorption is proportional to process. This is conducive to the nonlinear properties of the molecule. the square of the laser power, according to the relationship between the integral area of the two-photon fluorescence and the laser power at different powers, whether or not the 4.2. Two-Photon Fluorescence Spectra molecule has two-photon absorption can be verified. The fluorescence spectrum is shown Two-photon fluorescence spectra can reflect the ability of the molecule to undergo in Figure 4a, and the relationship between its integral area and laser power is shown in two-photon absorption. As the probability of two-photon absorption is proportional to Figure 4b. As can be seen from the fitted line of the logarithmic relationship, its slope the square of the laser power, according to the relationship between the integral area of is around 2 and slightly smaller than 2, indicating that two-photon absorption occupies the two-photon fluorescence and the laser power at different powers, whether or not the Photonics 2022, 9, 183 the molecule has main part two-pho of thetmolecular on absorption excitation can be vemode; rified. Tthis he fluorescen also confirms ce specthat trum the is shown 7 of molecule 14 can, in Figure 4a, and the relationship between its integral area and laser power is shown in indeed, undergo two-photon absorption. Figure 4b. As can be seen from the fitted line of the logarithmic relationship, its slope is around 2 and slightly smaller than 2, indicating that two-photon absorption occupies the main part of the molecular excitation mode; this also confirms that the molecule can, indeed, undergo two-photon absorption. The intensity of two-photon fluorescence at different wavelengths is shown in Figure 4c. The fluorescence quantum yield of BM-PPPM-C6 was 52.1% in DCM. (Figure S4). Combined with the fluorescence quantum yield data, δTPA can be calculated using the up- conversion fluorescence method [35], as shown in Figure 4d. The fluorescence intensity is distinct at different wavelengths, and the calculated two-photon absorption cross section also varies. It can be seen that the molecule obtains a larger two-photon absorption cross section near 800 nm. It makes certain that the molecule has a greater probability of two- photon absorption, which is a prerequisite to being a two-photon initiator. Figure 4. (a) Two-photon fluorescence of molecule BM-PPPM-C6 under diverse laser powers at 800 Figure 4. (a) Two-photon fluorescence of molecule BM-PPPM-C6 under diverse laser powers at nm in DCM; (b) relationship between two-photon fluorescence integration area (Fn) and laser power 800 nm in DCM; (b) relationship between two-photon fluorescence integration area (F ) and laser (mW) of BM-PPPM-C6; (c) two-photon fluorescence of molecule BM-PPPM-C6 at diverse excitation power (mW) of BM-PPPM-C6; (c) two-photon fluorescence of molecule BM-PPPM-C6 at diverse exci- −50 4 wavelengths in DCM; (d) TPACS: δTPA (1 GM = 10 cm ·s/photon) at diverse wavelengths of mole- 50 4 tation wavelengths in DCM; (d) TPACS: d (1 GM = 10 cm s/photon) at diverse wavelengths cule BM-PPPM-C6 in DCM. TPA of molecule BM-PPPM-C6 in DCM. At a wavelength of 800 nm, compared with the molecular BD-PM, the δTPA of BM- PPPM-C6 was 116 GM, while the δTPA of BD-PM was 14 GM [36]. Therefore, the δTPA of the molecule was greatly increased. However, the δTPA of BD-PPM was 336 GM in THF [28], and the δTPA of BD-PTM was about 380 GM [29] in DCM. They were both higher than BM- PPPM-C6. This may be because the fluorescence quantum yield of BD-PPM (52.1%) was much higher than that of BD-PPM (16.0% in DCM) and BD-PTM (40.3% in DCM), result- ing in a smaller calculated value of δTPA. 4.3. Cyclic Voltammograms Cyclic voltammetry can be put to proper use by approximating the orbital energy levels of molecules. The experiment was carried out in a DCM solution of 0.1 mol/L of n- Bu4NPF6. In the experiment, a platinum sheet was employed as the working electrode, platinum wire was used as the counter electrode, an Ag/Ag electrode was used as the reference electrode, and ferrocene was used as the internal standard. The scan rate was set to 0.1 V/s. It can be calculated from Figure 5 that EHOMO = −5.26 eV. Taking into account the approximate conditions used in the theoretical calculations, it can be considered that the experimental results are basically in line with the theoretical ones. From the ultravio- let–visible absorption band edge of the molecule, the optical band gap of the molecule is around 446 nm, Eg = 2.78 eV. If the optical band gap of the molecule is approximately regarded as the molecular energy gap, the calculation can be ELUMO = −2.48 eV. Photonics 2022, 9, 183 7 of 13 The intensity of two-photon fluorescence at different wavelengths is shown in Figure 4c. The fluorescence quantum yield of BM-PPPM-C6 was 52.1% in DCM. (Figure S4). Combined with the fluorescence quantum yield data, d can be calculated using the upconversion TPA fluorescence method [35], as shown in Figure 4d. The fluorescence intensity is distinct at different wavelengths, and the calculated two-photon absorption cross section also varies. It can be seen that the molecule obtains a larger two-photon absorption cross section near 800 nm. It makes certain that the molecule has a greater probability of two-photon absorption, which is a prerequisite to being a two-photon initiator. At a wavelength of 800 nm, compared with the molecular BD-PM, the d of BM- TPA PPPM-C6 was 116 GM, while the d of BD-PM was 14 GM [36]. Therefore, the d of the TPA TPA molecule was greatly increased. However, the d of BD-PPM was 336 GM in THF [28], TPA and the d of BD-PTM was about 380 GM [29] in DCM. They were both higher than TPA BM-PPPM-C6. This may be because the fluorescence quantum yield of BD-PPM (52.1%) was much higher than that of BD-PPM (16.0% in DCM) and BD-PTM (40.3% in DCM), resulting in a smaller calculated value of d . TPA 4.3. Cyclic Voltammograms Cyclic voltammetry can be put to proper use by approximating the orbital energy levels of molecules. The experiment was carried out in a DCM solution of 0.1 mol/L of n-Bu NPF . In the experiment, a platinum sheet was employed as the working electrode, 4 6 platinum wire was used as the counter electrode, an Ag/Ag electrode was used as the reference electrode, and ferrocene was used as the internal standard. The scan rate was set to 0.1 V/s. It can be calculated from Figure 5 that E = 5.26 eV. Taking into account HOMO the approximate conditions used in the theoretical calculations, it can be considered that the experimental results are basically in line with the theoretical ones. From the ultraviolet– visible absorption band edge of the molecule, the optical band gap of the molecule is around 446 nm, E = 2.78 eV. If the optical band gap of the molecule is approximately regarded as Photonics 2022, 9, 183 8 of 14 the molecular energy gap, the calculation can be E = 2.48 eV. LUMO Figure 5. Cyclic voltammetry experiments of molecule BM-PPPM-C6 and ferrocene at a concentra- Figure 5. Cyclic voltammetry experiments of molecule BM-PPPM-C6 and ferrocene at a concentration −3 tion of 1 × 10 mol/L in DCM. of 1  10 mol/L in DCM. 4.4. UV Light Stability 4.4. UV Light Stability Under the ultraviolet light, the photolysis properties of molecules can indicate the Under the ultraviolet light, the photolysis properties of molecules can −5 indicate the sen- sensitivity of molecules to ultraviolet light. Molecules with a concentration of 1 × 10 mol/L are irradiated by ultraviolet light with a wavelength of 365 nm and a light intensity sitivity of molecules to ultraviolet light. Molecules with a concentration of 1  10 mol/L of 50 mW/cm in a DCM solution. The data of the photolysis experiment are shown in are irradiated by ultraviolet light with a wavelength of 365 nm and a light intensity of Figure 6a. It can be seen that the molecules react easily under ultraviolet irradiation, 50 mW/cm in a DCM solution. The data of the photolysis experiment are shown in indicating that they have high photosensitivity, which also means that the molecule Figure 6a. It can be seen that the molecules react easily under ultraviolet irradiation, indi- needed to be kept away from light during use. cating As shown in Fig that they have ure 6b, high the early photosensitivi photolysis pty roce , which ss of the m also olecu means le conform that s the to the molecule needed to first-order kinetic model, and its concentration decays exponentially with time by fitting be kept away from light during use. the relationship between the absorbance of the molecule at 394 nm and the irradiation As shown in Figure 6b, the early photolysis process of the molecule conforms to the time. This has a certain value for studying the principle and kinetic process of molecular first-order kinetic model, and its concentration decays exponentially with time by fitting initiation polymerization. the relationship between the absorbance of the molecule at 394 nm and the irradiation −5 Figure 6. (a) UV degradation characteristics of molecule BM-PPPM-C6 at a concentration of 1 × 10 mol/L. (b) Photodegradation versus time fitting curve. 4.5. TPPAM Experiments After analyzing the optical properties of the molecule, it is confirmed that the molecule has potential as an initiator. However, the actual initiation performance needs to be tested in a two-photon polymerization experiment. The experiment of TPPAM was carried out with the molecule BM-PPPM-C6 as the initiator and TMPTA as the monomer. In Figure 7a, it is shown that the molecule can actually initiate the polymerization of the monomer to form a three-dimensional structure. At a lower writing speed of 100 μm/s, polymerization is still able to initiate when the power is as low as 5 mW. Under an extremely high writing speed of 100,000 μm/s, the power of 30–50 mW is able to initiate Photonics 2022, 9, 183 8 of 14 Figure 5. Cyclic voltammetry experiments of molecule BM-PPPM-C6 and ferrocene at a concentra- −3 tion of 1 × 10 mol/L in DCM. 4.4. UV Light Stability Under the ultraviolet light, the photolysis properties of molecules can indicate the −5 sensitivity of molecules to ultraviolet light. Molecules with a concentration of 1 × 10 mol/L are irradiated by ultraviolet light with a wavelength of 365 nm and a light intensity of 50 mW/cm in a DCM solution. The data of the photolysis experiment are shown in Figure 6a. It can be seen that the molecules react easily under ultraviolet irradiation, indicating that they have high photosensitivity, which also means that the molecule needed to be kept away from light during use. Photonics 2022, 9, 183 8 of 13 As shown in Figure 6b, the early photolysis process of the molecule conforms to the first-order kinetic model, and its concentration decays exponentially with time by fitting the relationship between the absorbance of the molecule at 394 nm and the irradiation time. This has a certain value for studying the principle and kinetic process of molecular time. This has a certain value for studying the principle and kinetic process of molecular initiation polymerization. initiation polymerization. −5 Figure 6. (a) UV degradation characteristics of molecule BM-PPPM-C6 at a concentration of Figure 6. (a) UV degradation characteristics of molecule BM-PPPM-C6 at a concentration of 1 × 10 mol/L. (b) Photodegradation versus time fitting curve. 1  10 mol/L. (b) Photodegradation versus time fitting curve. 4.5. TPPAM Experiments 4.5. TPPAM Experiments After analyzing the optical properties of the molecule, it is confirmed that the molecule After analyzing the optical properties of the molecule, it is confirmed that the has potential as an initiator. However, the actual initiation performance needs to be tested in molecule has potential as an initiator. However, the actual initiation performance needs to b a two-photon e tested in a polymerization two-photon poex lym periment. erization experiment. The The experiment of exTPP periment o AM was f T carried PPAM w out as with carr the ied o molecule ut with BM-PPPM-C6 the molecule as BM the -PP initiator PM-C6 a and s the i TMPT nitiaA tor and as the Tmonomer MPTA as the . In m Figur onom e e 7r. a, it is In shown Figure that 7a, it is the molecule shown th can at the actu molec ally initiate ule can the actua polymerization lly initiate the pol of the ymeriza monomer tion of to th form e a monomer to form a three-dimensional structure. At a lower writing speed of 100 μm/s, three-dimensional structure. At a lower writing speed of 100 m/s, polymerization is still Photonics 2022, 9, 183 9 of 14 polymerization is still able to initiate when the power is as low as 5 mW. Under an able to initiate when the power is as low as 5 mW. Under an extremely high writing speed of extremely high writing speed of 100,000 μm/s, the power of 30–50 mW is able to initiate 100,000 m/s, the power of 30–50 mW is able to initiate polymerization to form a complete three-dimensional structure. Figure 7b,c is the enlarged structure of the area shown. As can polymerization to form a complete three-dimensional structure. Figure 7b,c is the enlarged structure of the area shown. As can be seen, the molecule can be used as an be seen, the molecule can be used as an initiator to obtain a three-dimensional structure initiator to obtain a three-dimensional structure with clear outline and complete structure with clear outline and complete structure under the applicable speed and power conditions, under the applicable speed and power conditions, which proves that the molecule BM- which proves that the molecule BM-PPPM-C6 is successful as an initiator. PPPM-C6 is successful as an initiator. Figure 7. (a) SEM images of structures with initiator BM-PPPM-C6 (10 μmol/g) in TMPTA. (b) Figure 7. (a) SEM images of structures with initiator BM-PPPM-C6 (10 mol/g) in TMPTA. (b) En- Enlarged view of low writing speed area. (c) Enlarged view of high writing speed area. larged view of low writing speed area. (c) Enlarged view of high writing speed area. The designed BM-PPPM series of molecules are equipped with different alkane chain lengths. Although it is generally believed that the chain length of alkane hardly affects the chemical properties of the molecule, the solubility of the molecule and the interaction between the molecule and the monomer will have a certain difference in reality. It is not known whether these differences may have a certain impact on the initiation efficiency of the molecule. Consequently, the polymerization effect of BM-PPPM with different alkane chain lengths in TMPTA was compared, as shown in Figures 8 and S5–S7. It can be seen that under the same initiator concentration, there is no obvious difference in the initiation effect of the designed molecule in TMPTA. Initiator molecules of different chain lengths can be completely dissolved in TMPTA to achieve polymerization. Photonics 2022, 9, 183 9 of 13 The designed BM-PPPM series of molecules are equipped with different alkane chain lengths. Although it is generally believed that the chain length of alkane hardly affects the chemical properties of the molecule, the solubility of the molecule and the interaction between the molecule and the monomer will have a certain difference in reality. It is not known whether these differences may have a certain impact on the initiation efficiency of the molecule. Consequently, the polymerization effect of BM-PPPM with different alkane chain lengths in TMPTA was compared, as shown in Figures 8 and S5–S7. It can be seen that under the same initiator concentration, there is no obvious difference in the initiation effect of the designed molecule in TMPTA. Initiator molecules of different chain lengths Photonics 2022, 9, 183 10 of 14 can be completely dissolved in TMPTA to achieve polymerization. Figure 8. SEM images of structures with initiators (a) BM-PPPM-C12; (b) BM-PPPM-C16; and (c) Figure 8. SEM images of structures with initiators (a) BM-PPPM-C12; (b) BM-PPPM-C16; and BM-PPPM-C20 in TMPTA, 10 μmol/g. (c) BM-PPPM-C20 in TMPTA, 10 mol/g. BM-PPPM is obtained by conjugation expansion, so the changes in the initiating abil- BM-PPPM is obtained by conjugation expansion, so the changes in the initiating ability ity of molecules with diverse degrees of conjugation are interesting. Comparing the initi- ation efficiency of BM-PPPM with BD-PPM and BD-PM in previous work [28,29], it can of molecules with diverse degrees of conjugation are interesting. Comparing the initiation be found that the initiation efficiency of BM-PPPM has been greatly improved compared efficiency of BM-PPPM with BD-PPM and BD-PM in previous work [28,29], it can be found to BD-PPM and BD-PM, which is in line with the expected improvement in initiator effi- that the initiation efficiency of BM-PPPM has been greatly improved compared to BD-PPM ciency due to conjugation extension. Photonics 2022, 9, 183 10 of 13 and BD-PM, which is in line with the expected improvement in initiator efficiency due to Photonics 2022, 9, 183 11 of 14 conjugation extension. However, when BM-PPPM was compared to a more highly conjugated triphenylamine molecule (BM-PTM) [29], it was found that its initiation effect was not inferior; this indi- However, when BM-PPPM was compared to a more highly conjugated triphenyla- cates that simply adding a conjugation group gradually lessened the increase in initiation mine molecule (BM-PTM) [29], it was found that its initiation effect was not inferior; this efficiency as the degree of conjugation increased, just like the diminishing marginal utility indicates that simply adding a conjugation group gradually lessened the increase in initi- ation efficiency as the degree of conjugation increased, just like the diminishing marginal effect. Hence, it should not be achieved simply by conjugation expansion, but requires utility effect. Hence, it should not be achieved simply by conjugation expansion, but re- more in-depth consideration so as to design a larger and more efficient initiator. quires more in-depth consideration so as to design a larger and more efficient initiator. Although molecules with diverse alkane chain lengths have complete solubility in Although molecules with diverse alkane chain lengths have complete solubility in TMPTA, it does not mean that the molecules with different chain lengths are meaningless. TMPTA, it does not mean that the molecules with different chain lengths are meaningless. When using the monomer PETA, it was found that hardly any of the BM-PPPM-C6 can When using the monomer PETA, it was found that hardly any of the BM-PPPM-C6 can be completely dissolved. Additionally, the molecules with longer carbon chains cannot be completely dissolved. Additionally, the molecules with longer carbon chains cannot form a clear and transparent dissolution in PETA. After two days of heating and stirring, form a clear and transparent dissolution in PETA. After two days of heating and stirring, precipitation still exists. This shows that for PETA, the molecule BM-PPPM-C6 alone precipitation still exists. This shows that for PETA, the molecule BM-PPPM-C6 alone can can be used as a suitable initiator. As shown in Figure 9, it is more effective in initiating be used as a suitable initiator. As shown in Figure 9, it is more effective in initiating polymerization in PETA, BM-PPPM-C6. With a power of 15 mW and a speed of up to polymerization in PETA, BM-PPPM-C6. With a power of 15 mW and a speed of up to 100,000 m/s, it can still form a complete three-dimensional structure with only slight 100,000 μm/s, it can still form a complete three-dimensional structure with only slight deformation deformation at at the the edge edges. s. This r This result esult isis be better tter tha thann the the tes test rtesults resultus nder unde the r th same e sam conditions e conditions reported in the recent literature (when the writing speed was 100,000 μm/s, the reported in the recent literature (when the writing speed was 100,000 m/s, the minimum minimum laser power was 44 mW) [15]. Under low speed, low power and a power laser power was 44 mW) [15]. Under low speed, low power and a power condition of 5 mW, condition of 5 mW, polymerization can be realized in the speed range of 100–2000 μm/s, polymerization can be realized in the speed range of 100–2000 m/s, with a larger range with a larger range of polymerization parameters. The above results are excellent, which of polymerization parameters. The above results are excellent, which suggests that this suggests that this molecule can be used in additive manufacturing at high speeds. Com- molecule can be used in additive manufacturing at high speeds. Compared to the molecules pared to the molecules formed by benzophenone and triphenylamine, the test results in formed by benzophenone and triphenylamine, the test results in PETA are not inferior. PETA are not inferior. Figure 9. (a) SEM images of structures with initiators BM-PPPM-C6 in PETA, 10 mol/g. (b–d) Enlarged images of the area shown. Photonics 2022, 9, 183 11 of 13 Meanwhile, in order to illustrate that initiators with different alkane chain lengths have different compatibility in some monomers, the solubility of BM-PPPM series molecules was studied using n-heptane as a solvent. It was found that BM-PPPM-C6 and BM-PPPM-C16 had poor solubility; conversely, BM-PPPM-C20 had good solubility, as shown in Figure S8. BM-PPPM-C12 will separate out after standing for a period of time. At this solvent polarity, longer-chain alkanes are more favorable for dissolution. This entirely illustrates that controlling the alkane chain can effectively regulate the solubility of molecules with different solvents, ensuring that only the change in the alkane chain can meet the needs of different monomers for initiators. In consequence, it can be seen that due to the critical factor of solubility, the length of the alkane chain is not meaningless in the process of being an initiator. The solubility between organics is extremely complicated, and it is tough to accurately determine the degree of solubility. The simple act of introducing carbon chains of different lengths can effectively change the solubility of molecules. However, in the monomer PETA selected in this article, the shortest chain is more conducive to dissolution. Nevertheless, longer alkane chains may have better solubility under certain conditions. Furthermore, only an alkane chain of moderate length can dissolve more effectively. 5. Conclusions In this paper, a BM-PPPM series of D––A––D molecules based on benzophenone groups were designed by conjugated extension. Through theoretical calculations and photophysical experiments, the potential of molecules as nonlinear materials was confirmed. Through TPPAM experiments, it is proven that PETA resin with BM-PPPM molecules as initiators can form complete three-dimensional structures at a writing speed as high as 100,000 m/s and a laser power as low as 15 mW. From a molecular-structure point of view, it is confirmed that the molecular conjugate extension can indeed improve molecular initiation efficiency. However, as the degree of conjugation reaches a certain limit, the improvement that can be brought by adding conjugated groups will be extremely limited. In addition, the chain length of the initiator will obviously affect the solubility with the monomer. By simply changing the alkane chain length, the compatibility relationship between the initiator and the monomer can be enhanced to a certain extent. Supplementary Materials: The supporting information can be downloaded at: https://www.mdpi. com/article/10.3390/photonics9030183/s1. Figure S1: Intermediate product 3 synthesis route. Figure S2: Intermediate product 6 synthesis route. Figure S3: Final product 7 synthesis route. Figure S4: The fluorescence quantum yield of BM-PPPM-C6. Figure S5: SEM images of structures with initiator BM-PPPM-C12 in TMPTA, 10 mol/g. Figure S6: SEM images of structures with initiator BM-PPPM-C16 in TMPTA, 10 mol/g. Figure S7: SEM images of structures with initiator BM-PPPM-C20 in TMPTA, 10 mol/g. Figure S8: Photos showing differences in solubility of different molecules. Table S1: Reagents. Author Contributions: S.L.: Investigation, methodology, testing and analysis, software, writing— original draft; X.L.: writing—review and editing, test and analysis; S.Z.: conceptualization, investi- gation, testing and analysis, data curation, writing—review and editing; Y.Z.: writing—review and editing, test and analysis; X.W.: writing—review and editing, conceptualization, methodology; N.L.: resources; J.L.: resources, conceptualization; L.Z.: resources, supervision. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Applied Basic Research Program of Sichuan Province (no. 2019YJ003), the State Key Laboratory of Environment-Friendly Energy Materials (no. 19kfhg01), the National Natural Science Foundation of China (no. 51803198), and the Presidential Foundation of China Academy of Engineering Physics (no. YZJJLX2020008). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. 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D–π–A–π–D Initiators Based on Benzophenone Conjugate Extension for Two-Photon Polymerization Additive Manufacturing

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hv photonics Article D––A––D Initiators Based on Benzophenone Conjugate Extension for Two-Photon Polymerization Additive Manufacturing 1 , 2 3 2 , 2 2 2 2 2 Shanggeng Li , Xiaolin Liu , Shuai Zhang *, Yawen Zhou , Xiangyu Wan , Ning Li , Jing Li and Lin Zhang School of Nuclear Science and Technology, University of Science and Technology of China, Hefei 230026, China; lsglsg@mail.ustc.edu.cn Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China; zhouyawen163@163.com (Y.Z.); wanxiangyu_caep@126.com (X.W.); lncaep@163.com (N.L.); jingli0218@163.com (J.L.); zhlmy@sina.com (L.Z.) School of Physics, University of Science and Technology of China, Hefei 230026, China; lxl91003@mail.ustc.edu.cn * Correspondence: zhangshuai_scu@126.com; Tel.: +86-0816-2493148 Abstract: A two-photon polymerization initiator is a kind of nonlinear optical material. With the demand for more efficient initiators in two-photon polymerization additive manufacturing, there are more and more related studies. In this paper, four conjugate-extended two-photon polymerization initiators with different alkane chain lengths were designed and synthesized, and single-photon, two- photon, and photodegradation experiments were carried out. Additive manufacturing experiments illustrated that the designed molecules can be used as two-photon initiators, and the writing speed can achieve 100,000 m/s at a laser power of 25 mW. Through theoretical calculation and experimental comparison of the properties of molecules with different conjugation degrees, it was proven that a certain degree of conjugation extension can improve the initiation ability of molecules; however, this improvement cannot be extended infinitely. Solubility tests of different acrylates showed that molecules with different alkane chain lengths have varying solubility. Changing the molecular alkane chain length may be favorable to adapt to different monomers. Citation: Li, S.; Liu, X.; Zhang, S.; Zhou, Y.; Wan, X.; Li, N.; Li, J.; Zhang, Keywords: optical functional materials; photoinitiator; two-photon polymerization additive L. D––A––D Initiators Based on manufacturing; conjugation extension; solubility Benzophenone Conjugate Extension for Two-Photon Polymerization Additive Manufacturing. Photonics 2022, 9, 183. https://doi.org/ 1. Introduction 10.3390/photonics9030183 Two-photon absorption is a third-order nonlinear optical effect that has a wide range Received: 17 February 2022 of uses. Among them, two-photon polymerization additive manufacturing (TPPAM) is Accepted: 10 March 2022 extensively used in many three-dimensional precision structure manufacturing fields— Published: 13 March 2022 such as those of photonic crystals [1,2], micro-optical elements [3,4], three-dimensional Publisher’s Note: MDPI stays neutral high-density information storage [5–7], biocompatible devices [8–10], etc.—due to its with regard to jurisdictional claims in unique ultra-high-resolution performance. TPPAM has a growing number of applicaitons, published maps and institutional affil- which also highlights the new requirements for its manufacturing speed. Since TPPAM iations. is a manufacturing method in which dots form lines and lines form surfaces, making larger structures takes a lot of time to manufacture. Meanwhile, the speed of writing in manufacturing is extremely significant. A larger writing speed can improve manufacturing efficiency and save time. Copyright: © 2022 by the authors. However, if the writing speed is too high, the resin cannot be irradiated with sufficient Licensee MDPI, Basel, Switzerland. laser energy and polymerized to form a three-dimensional structure. One way to solve this This article is an open access article problem is to increase the power of the laser. Nevertheless, laser power that is too high distributed under the terms and will impose high requirements on the laser and increase production costs. Another way is conditions of the Creative Commons to raise the efficiency of the two-photon polymerization initiator (TPPI). As an important Attribution (CC BY) license (https:// nonlinear material, TPPI absorbs laser energy, undergoes two-photon absorption, generates creativecommons.org/licenses/by/ free radicals, and initiates polymerization [11]. Therefore, it only requires a more efficient 4.0/). Photonics 2022, 9, 183. https://doi.org/10.3390/photonics9030183 https://www.mdpi.com/journal/photonics Photonics 2022, 9, 183 2 of 13 initiator if the speed of writing in manufacturing is to be faster, which can lead to the absorbtion of more photons or the production of more free radicals. As a consequence, the key to the solving this problem is to improve the efficiency of the initiator. In the early days, TPPI used conventional ultraviolet (UV) photoinitiators. UV photoinitiators have active groups and generate free radicals under ultraviolet light irradiation. However, the absorption band of ultraviolet photoinitiators is mainly located in the ultraviolet region, and the two-photon absorption cross section (d ) is also small. The TPA possibility of two-photon absorption is so low that the low initiation efficiency is rendered. Studies have proven that by performing conjugation extension, the d of the molecule TPA can be effectively increased, and the absorption efficiency of light can be raised, thereby improving the efficiency of the initiator [12–14]. Conjugation extension is the introduction of conjugated groups to connect electron acceptors and electron donors, as well as increase the degree of molecular conjugation and intramolecular charge transfer (ICT) [15–17]. Currently, the efficiency of initiators through conjugation expansion has been improved in many studies [18–25]. These studies concluded that molecules make up the structures donor––acceptor (D––A), donor––acceptor––donor (D––A––D), acceptor––donor– –acceptor (A––D––A), etc., would have better ICT and, thus, have larger d and TPA initiator efficients. However, it is clear that there is a limit to the enhancement of molecular initiation properties through conjugation extension. It is interesting to question, then, where the performance limits can be improved using this approach. In addition, the compatibility of molecules and monomers is also essential. The initia- tor molecule must be completely dissolved in the monomer to be effective. Often monomers with different types of structures have different requirements for initiator molecules. For example, the initiators used to print hydrogels need to be water-soluble [26,27]. However, it is very difficult under the condition that one kind of initiator can be widely used in various polar resins. For each monomer, molecular design considering solubility is extremely complicated, and it is not conducive to practical production applications. Therefore, it would be interesting if there was a way to change the structure of the molecule in small amounts, effectively improving the solubility of the molecule with different monomers. The alkane chain is a very easily overlooked part because it hardly affects the chemical properties of the molecule. However, the length of the alkane chain will affect the polarity, the degree of steric hindrance and the geometric configuration of the molecule, and thus, the solubility of the molecule. Moreover, due to the alkane chain hardly affecting the chemical properties of the molecule, changing the length of the alkane chain hardly affects the synthesis method and manufacturing process of the molecule, which greatly reduces the cost of practical application. In view of the above two points, the traditional UV photoinitiator BD-PM with an active structure was conjugated and extended by introducing a benzene ring to obtain the molecules BD-PPM and BM-PPPM, as shown in Figure 1. Among them, BD-PPM molecules have been reported in the literature with similar structures, and certain effects have been achieved [28]. BM-PPPM with two benzene rings has a larger degree of conjugation. In the previous work, the molecules (BM-PTM) with a larger degree of conjugation formed by benzophenone and triphenylamine were studied, and also obtained good results [29]. New groups can be introduced because diphenylamine has one more site on the nitrogen atom than triphenylamine. Hence, the alkane chain will affect the solubility of the molecule, and the length of the alkane chain on the diphenylamine is simply changed for the molecule BM-PPPM, so that initiators suitable for various monomers can be obtained. This paper compares the initiation properties of molecules with different degrees of conjugation and investigated whether BM-PPPM series molecules can be better considered as initiators. Photonics 2022, 9, 183 3 of 14 diphenylamine is simply changed for the molecule BM-PPPM, so that initiators suitable for various monomers can be obtained. This paper compares the initiation properties of molecules with different degrees of conjugation and investigated whether BM-PPPM series molecules can be better considered as initiators. Photonics 2022, 9, 183 3 of 14 diphenylamine is simply changed for the molecule BM-PPPM, so that initiators suitable for various monomers can be obtained. This paper compares the initiation properties of Photonics 2022, 9, 183 3 of 13 molecules with different degrees of conjugation and investigated whether BM-PPPM series molecules can be better considered as initiators. Figure 1. The structural formulas of BD-PM (UV photoinitiator), BD-PPM [28], BM-PPPM and BM- PTM [29]. 2. Theory Calculation The DFT approach can be used to calculate the frontier orbits of conjugated molecules such as D–π–A–π–D and A–π–D–π–A, and to observe the degree of intramolecular charge transfer, so as to pred Figure ict the 1. two The str -p uctural hoton formulas absorp of t BD-PM ion ch (UV aracte photoinitiato risticsr), ofBD-PPM the mo [28 lec ], BM-PPPM ule [30,31 and ]. Figure 1. The structural formulas of BD-PM (UV photoinitiator), BD-PPM [28], BM-PPPM and BM- BM-PTM [29]. PTM [29]. To probe into the electronic properties of molecules with different conjugation extension 2. Theory Calculation degrees, the theoretical calculations were carried out using the DFT approach, and the 2. Theory Calculation The DFT approach can be used to calculate the frontier orbits of conjugated molecules B3LYP functional and 6-31G(d, p) basis sets were used with Gaussian 09 [32]. For The DFT approach can be used to calculate the frontier orbits of conjugated molecules such as D––A––D and A––D––A, and to observe the degree of intramolecular charge molecules with three different degrees of conjugation extension, the degree of transfer such as D , so–asπ–A– to prπ edict –D and the A two-photon –π–D–π–A absorption , and to observe characteristics the degr of ee o the molecule f intramo [l 30 ec ,31 ular ch ]. arge intramolecular charge transfer and the frontier orbital energy were analyzed. To reduce To probe into the electronic properties of molecules with different conjugation extension de- transfer, so as to predict the two-photon absorption characteristics of the molecule [30,31]. grees, the theoretical calculations were carried out using the DFT approach, and the B3LYP the workload of calculations, all alkyl groups were substituted with methyl groups. To probe into the electronic properties of molecules with different conjugation extension functional and 6-31G(d, p) basis sets were used with Gaussian 09 [32]. For molecules with degrees, the theoretical calculations were carried out using the DFT approach, and the It can be seen from Figure 2 that the HOMO orbital was mainly concentrated in the three different degrees of conjugation extension, the degree of intramolecular charge trans- B3LYP functional and 6-31G(d, p) basis sets were used with Gaussian 09 [32]. For amino part of the electron donor and extended to the π bond bridge, while the LUMO fer and the frontier orbital energy were analyzed. To reduce the workload of calculations, molecules with three different degrees of conjugation extension, the degree of all alkyl groups were substituted with methyl groups. orbital mainly extended from the acetylene bridge to the electron acceptor group intramolecular charge transfer and the frontier orbital energy were analyzed. To reduce It can be seen from Figure 2 that the HOMO orbital was mainly concentrated in the benzophenone. With the increases in the conjugation degree, when the BD-PM the workload of calculations, all alkyl groups were substituted with methyl groups. amino part of the electron donor and extended to the  bond bridge, while the LUMO orbital conjugation was extended to BD-PPM, the ICT of the molecule increased significantly. It can be seen from Figure 2 that the HOMO orbital was mainly concentrated in the mainly extended from the acetylene bridge to the electron acceptor group benzophenone. When extending from With BD-PPM the increases toin Bthe M-P conjugation PPM, the degr HO ee, when MO orb the BD-PM ital ch conjugation arge of the was molecule extended to amino part of the electron donor and extended to the π bond bridge, while the LUMO BD-PPM, the ICT of the molecule increased significantly. When extending from BD-PPM to orbital mainly extended from the acetylene bridge to the electron acceptor group was distributed to a further position, along with the introduced conjugated benzene ring, BM-PPPM, the HOMO orbital charge of the molecule was distributed to a further position, benzophenone. With the increases in the conjugation degree, when the BD-PM but the LUMO orbital did not change significantly. This probably indicates that the along with the introduced conjugated benzene ring, but the LUMO orbital did not change conjugation was extended to BD-PPM, the ICT of the molecule increased significantly. expansion of the elec significantly tron donor mainly . This probably affec indicates ts the that the electron distribution of molecules on expansion of the electron donor mainly affects When extending from BD-PPM to BM-PPPM, the HOMO orbital charge of the molecule the electron distribution of molecules on the HOMO orbital. Overall, the designed molecule the HOMO orbital. Overall, the designed molecule BM-PPPM had good ICT performance, was distributed to a further position, along with the introduced conjugated benzene ring, BM-PPPM had good ICT performance, which provides a theoretical basis for its application which provides a theoretical basis for its application as a TPPAM photoinitiator. but the LUMO orbital did not change significantly. This probably indicates that the as a TPPAM photoinitiator. expansion of the electron donor mainly affects the electron distribution of molecules on the HOMO orbital. Overall, the designed molecule BM-PPPM had good ICT performance, which provides a theoretical basis for its application as a TPPAM photoinitiator. Figure 2. Frontier orbital diagrams of molecules with different degrees of conjugation. HOMO: Figure 2. Frontier orbital diagrams of molecules with different degrees of conjugation. HOMO: Highest Occupied Molecular Orbital; LUMO: Lowest Unoccupied Molecular Orbital. (a) BD-PM; Highest Occupied Molecular Orbital; LUMO: Lowest Unoccupied Molecular Orbital. (a) BD-PM; Figure 2. Frontier orbital diagrams of molecules with different degrees of conjugation. HOMO: (b) BD-PPM; (c) BM-PPPM. Highest Occupied Molecular Orbital; LUMO: Lowest Unoccupied Molecular Orbital. (a) BD-PM; (b) BD-PPM; (c) BM-PPPM. The calculated results of the molecular frontier orbital energy of BD-PM were very (b) BD-PPM; (c) BM-PPPM. close to the known reports [33], which proves the rationality of the calculation. While Photonics 2022, 9, 183 4 of 13 the use of the B3LYP functional is suitable for calculations of molecular energies, it also over-planarizes the molecule, and over-delocalizes the wave function. As shown in Table 1, the HOMO energy level of the molecule was significantly increased, the LUMO energy level was significantly decreased, and the band gap was significantly reduced as the BD-PM conjugation was extended to BD-PPM. When BD-PPM was extended to BM-PPPM, the change degree of molecular energy level was reduced significantlyand the band gap was slightly reduced, which is consistent with the result directly observed in Figure 2. This shows that, theoretically, the conjugation of molecules is relatively limited, and it means that the conjugation of the molecule cannot be increased indefinitely by blindly adding conjugated groups. Table 1. Molecular frontier orbital energy. HOMO LUMO Gap Dippole (eV) (eV) (eV) (Debye) BM-PPPM 5.01 1.85 3.16 6.4644 BD-PPM 4.96 1.78 3.18 6.5658 BD-PM 5.71 1.49 4.21 4.2349 3. Experimental Section 3.1. Materials All reagents were obtained from commercial suppliers (Table S1) and used without further purification. 3.2. Physical Measurements Nuclear magnetic resonance (NMR) spectra were obtained on a BRUKER AVANCE 1 13 1 III HD 400 MHz ( H NMR at 400 MHz, C NMR at 100 MHz). The H NMR chemical shifts were measured relative to CDCl (d = 7.26 ppm) as the internal reference. The C NMR chemical shifts were given using CDCl (d = 77.16 ppm) as the internal standard. High-resolution mass spectra (HRMS) were obtained with a Thermo Scientific Q Exactive. Absorption spectra were obtained on a HITACHI U-2910 spectrometer. Fluorescence spectra were collected on a Horiba Fluorolog-3 fluorescence spectrometer. The two-photo fluorescence spectra were obtained with MAITAIHP, spectrum physic maitai, and SD2000. The fluorescence quantum yields were measured using a Hamamatsu C11347-11. The cyclic voltammetry tests were performed on an electrochemical workstation (instrument model: CHI660E). Scanning electron microscopes (SEM) were obtained using a Merlin VP Compact, Carl Zeiss. 3.3. Synthesis Detailed synthetic methods are provided in the Supplementary Materials (Supporting Information) (Figures S1–S3). The synthesis of the final products BM-PPPM was achieved using the classical Sonogashira reaction [34]. BM-PPPM-C6: Purification by column chromatography on silica gel (petroleum ether/ethyl acetate = 10/1, v/v) rendered BM-PPPM-C6 as a yellow solid. H NMR (400 MHz, CDCl ): d = 0.88 (t, J = 6.8 Hz, 6H), 1.25–1.35 (m, 12H), 1.62–1.72 (m, 4H), 3.70 (t, J = 6.8 Hz, 4H), 3.85 (t, J = 7.2 Hz, 4H), 6.75–6.79 (m, 4H), 7.11–7.19 (m, 6H), 7.34–7.40 (m, 8H), 7.56–7.62 (m, 4H), 7.74–7.79 (m, 4H) ppm. C NMR (CDCl , 100 MHz): d = 14.2, 22.8, 26.9, 27.5, 31.8, 52.5, 87.5, 94.1, 111.8, 116.1, 124.5, 125.5, 128.6, 129.8, 130.1, 131.2, 133.0, 136.2, 147.0, 148.8, 195.4 ppm. BM-PPPM-C12: Purification by column chromatography on silica gel (petroleum ether/ethyl acetate = 15/1, v/v) rendered BM-PPPM-C12 as a yellow solid. H NMR (400 MHz, CDCl ): d = 0.88 (t, J = 7.2 Hz, 6H), 1.21–1.33 (m, 36H), 1.62–1.72 (m, 4H), 3.70 (t, J = 8.0 Hz, 4H), 3.85 (t, J = 7.2 Hz, 4H), 6.77 (d, J = 8.8 Hz, 4H), 7.12–7.19 (m, 6H), 7.37 (t, J = 6.8 Hz, 8H), 7.59 (d, J = 8.4 Hz, 4H), 7.77 (d, J = 8.4 Hz, 4H) ppm. C NMR (CDCl , 3 Photonics 2022, 9, 183 5 of 13 100 MHz): d = 14.3, 22.9, 27.2, 27.5, 29.5, 29.6, 29.72, 29.76, 29.77, 29.78, 32.1, 52.5, 87.5, 94.1, 111.7, 116.1, 124.4, 125.5, 128.6, 129.8, 130.1, 131.2, 133.0, 136.1, 147.0, 148.8, 195.4 ppm. BM-PPPM-C16: Purification by column chromatography on silica gel (petroleum ether/ethyl acetate = 15/1, v/v) rendered BM-PPPM-C16 as a yellow oil. H NMR (400 MHz, CDCl ): d = 0.84–0.92 (m, 14H), 1.16–1.28 (m, 46H), 1.77–1.86 (m, 2H), 3.63 (d, J = 7.2 Hz, 4H), 6.78–6.84 (m, 4H), 7.09–7.18 (m, 6H), 7.32–7.40 (m, 6H), 7.56–7.62 (m, 4H), 7.75–7.79 (m, 4H) ppm. C NMR (CDCl , 100 MHz): d = 14.25, 14.28, 22.79, 22.83, 26.46, 26.47, 29.5, 29.7, 29.8, 30.2, 31.66, 31.68, 31.95, 32.0, 36.4, 56.9, 87.6, 94.0, 112.1, 116.9, 124.1, 125.3, 128.6, 129.7, 130.1, 131.2, 132.8, 136.2, 147.6, 149.7, 195.4 ppm. BM-PPPM-C20: Purification by column chromatography on silica gel (petroleum ether/ethyl acetate = 20/1, v/v) rendered BM-PPPM-C20 as a yellow oil. H NMR (400 MHz, CDCl ): d = 0.88 (t, J = 7.2 Hz, 14H), 1.17–1.28 (m, 62H), 1.75–1.85 (m, 2H), 3.70 (d, J = 7.2 Hz, 4H), 6.81 (d, J = 8.8 m, 4H), 7.09–7.18 (m, 6H), 7.31–7.40 (m, 6H), 7.56–7.62 (m, 4H), 7.75–7.79 (m, 4H) ppm. C NMR (CDCl , 100 MHz): d = 14.3, 22.83, 22.85, 26.5, 29.46, 29.51, 29.69, 29.73, 29.78, 29.79, 30.17, 31.67, 32.03, 32.07, 36.34, 56.9, 87.6, 94.0, 112.1, 116.9, 124.1, 125.3, 128.6, 129.7, 130.1, 131.2, 132.9, 136.2, 147.6, 149.7 ppm. 3.4. Resin Preparation To consider the initiation efficiency and solubility of the initiator under different monomers, two monomers—trimethylolpropane triacrylate (TMPTA) and pentaerythritol triacrylate (PETA)—were adopted. They all had three functional groups, which could achieve a high degree of polymerization and avoid the performance differences caused by different functionalities. Photoresist was prepared by mixing TMPTA, PETA, and 4 kinds of initiator molecules with different chain lengths with BM-PPPM (10 mol/g), then stirred overnight at 45 C until clear and free of precipitation. Finally, the mixture was placed in a greenhouse for 24 h to ensure that it did not settle. 3.5. TPPAM Test The TPPAM experiments were carried out by Nanoscribe GT Photonic Professional, in which the repetition rate was 80 MHz and the wavelength was 780  10 nm. The pulse duration was approximately 120 fs. The average power of laser irradiation on the sample plane was about 50 mW. In the experiment, a 63 objective lens (Numerical aperture 1.4, working distance 190 m) was adopted. Meanwhile, a three-dimensional wooden-stick stack structure was designed in the experiment under different laser powers and writing speeds, which aimed to test the molecular initiation efficiency and the manufacturing parameter interval for forming a complete three-dimensional structure. The preparation process adopted an oil-immersion objective lens. After production, the final structure was obtained by being developed twice for 10 min in propylene glycol monomethyl ether acetate (PGMEA), then washed with isopropanol and eventually dried at 40 C. 4. Results and Discussions 4.1. Linear Absorption and Fluorescence Spectra As shown in Figure 3, the ultraviolet absorption and fluorescence spectra of molecules BM-PPPM-C6 and BM-PPPM-C12 are, of course, very close. Owing to its chain length, alkane has little effect on the chemical properties of the molecule, only the properties of BM- PPPM-C6 need to be tested. It can be seen from the absorption spectrum that the molecule has a large ultraviolet absorption peak and molar extinction coefficient near 390 nm. This means that molecules have a great utilization of light near this wavelength. It can be speculated that when two-photon absorption occurs, there will be a greater absorption rate for the 780 nm laser. As can be seen from the fluorescence spectrum of the molecule, the emission wave- length of the fluorescence ehxibits a significant red shift with the increase in the polarity of Photonics 2022, 9, 183 6 of 13 Photonics 2022, 9, 183 6 of 14 the solvent, which indicates that the excitation of molecules has an obvious ICT process. wavelength. It can be speculated that when two-photon absorption occurs, there will be a This is conducive to the nonlinear properties of the molecule. greater absorption rate for the 780 nm laser. Figure 3. Linear absorption and fluorescence spectra of molecules (a) BM-PPPM-C6 and (b) BM- Figure 3. Linear absorption and fluorescence spectra of molecules (a) BM-PPPM-C6 and (b) BM- −5 PPPM-C12 at a concentration of 1 × 10 mol/L and cuvette a cuvette width of 1 cm. THF—tetrahy- PPPM-C12 at a concentration of 1  10 mol/L and cuvette a cuvette width of 1 cm. THF— drofuran; EA—ethyl acetate; DCM—dichloromethane; DMF—N,N-dimethylformamide. tetrahydrofuran; EA—ethyl acetate; DCM—dichloromethane; DMF—N,N-dimethylformamide. As can be seen from the fluorescence spectrum of the molecule, the emission 4.2. Two-Photon Fluorescence Spectra wavelength of the fluorescence ehxibits a significant red shift with the increase in the Two-photon fluorescence spectra can reflect the ability of the molecule to undergo polarity of the solvent, which indicates that the excitation of molecules has an obvious ICT two-photon absorption. As the probability of two-photon absorption is proportional to process. This is conducive to the nonlinear properties of the molecule. the square of the laser power, according to the relationship between the integral area of the two-photon fluorescence and the laser power at different powers, whether or not the 4.2. Two-Photon Fluorescence Spectra molecule has two-photon absorption can be verified. The fluorescence spectrum is shown Two-photon fluorescence spectra can reflect the ability of the molecule to undergo in Figure 4a, and the relationship between its integral area and laser power is shown in two-photon absorption. As the probability of two-photon absorption is proportional to Figure 4b. As can be seen from the fitted line of the logarithmic relationship, its slope the square of the laser power, according to the relationship between the integral area of is around 2 and slightly smaller than 2, indicating that two-photon absorption occupies the two-photon fluorescence and the laser power at different powers, whether or not the Photonics 2022, 9, 183 the molecule has main part two-pho of thetmolecular on absorption excitation can be vemode; rified. Tthis he fluorescen also confirms ce specthat trum the is shown 7 of molecule 14 can, in Figure 4a, and the relationship between its integral area and laser power is shown in indeed, undergo two-photon absorption. Figure 4b. As can be seen from the fitted line of the logarithmic relationship, its slope is around 2 and slightly smaller than 2, indicating that two-photon absorption occupies the main part of the molecular excitation mode; this also confirms that the molecule can, indeed, undergo two-photon absorption. The intensity of two-photon fluorescence at different wavelengths is shown in Figure 4c. The fluorescence quantum yield of BM-PPPM-C6 was 52.1% in DCM. (Figure S4). Combined with the fluorescence quantum yield data, δTPA can be calculated using the up- conversion fluorescence method [35], as shown in Figure 4d. The fluorescence intensity is distinct at different wavelengths, and the calculated two-photon absorption cross section also varies. It can be seen that the molecule obtains a larger two-photon absorption cross section near 800 nm. It makes certain that the molecule has a greater probability of two- photon absorption, which is a prerequisite to being a two-photon initiator. Figure 4. (a) Two-photon fluorescence of molecule BM-PPPM-C6 under diverse laser powers at 800 Figure 4. (a) Two-photon fluorescence of molecule BM-PPPM-C6 under diverse laser powers at nm in DCM; (b) relationship between two-photon fluorescence integration area (Fn) and laser power 800 nm in DCM; (b) relationship between two-photon fluorescence integration area (F ) and laser (mW) of BM-PPPM-C6; (c) two-photon fluorescence of molecule BM-PPPM-C6 at diverse excitation power (mW) of BM-PPPM-C6; (c) two-photon fluorescence of molecule BM-PPPM-C6 at diverse exci- −50 4 wavelengths in DCM; (d) TPACS: δTPA (1 GM = 10 cm ·s/photon) at diverse wavelengths of mole- 50 4 tation wavelengths in DCM; (d) TPACS: d (1 GM = 10 cm s/photon) at diverse wavelengths cule BM-PPPM-C6 in DCM. TPA of molecule BM-PPPM-C6 in DCM. At a wavelength of 800 nm, compared with the molecular BD-PM, the δTPA of BM- PPPM-C6 was 116 GM, while the δTPA of BD-PM was 14 GM [36]. Therefore, the δTPA of the molecule was greatly increased. However, the δTPA of BD-PPM was 336 GM in THF [28], and the δTPA of BD-PTM was about 380 GM [29] in DCM. They were both higher than BM- PPPM-C6. This may be because the fluorescence quantum yield of BD-PPM (52.1%) was much higher than that of BD-PPM (16.0% in DCM) and BD-PTM (40.3% in DCM), result- ing in a smaller calculated value of δTPA. 4.3. Cyclic Voltammograms Cyclic voltammetry can be put to proper use by approximating the orbital energy levels of molecules. The experiment was carried out in a DCM solution of 0.1 mol/L of n- Bu4NPF6. In the experiment, a platinum sheet was employed as the working electrode, platinum wire was used as the counter electrode, an Ag/Ag electrode was used as the reference electrode, and ferrocene was used as the internal standard. The scan rate was set to 0.1 V/s. It can be calculated from Figure 5 that EHOMO = −5.26 eV. Taking into account the approximate conditions used in the theoretical calculations, it can be considered that the experimental results are basically in line with the theoretical ones. From the ultravio- let–visible absorption band edge of the molecule, the optical band gap of the molecule is around 446 nm, Eg = 2.78 eV. If the optical band gap of the molecule is approximately regarded as the molecular energy gap, the calculation can be ELUMO = −2.48 eV. Photonics 2022, 9, 183 7 of 13 The intensity of two-photon fluorescence at different wavelengths is shown in Figure 4c. The fluorescence quantum yield of BM-PPPM-C6 was 52.1% in DCM. (Figure S4). Combined with the fluorescence quantum yield data, d can be calculated using the upconversion TPA fluorescence method [35], as shown in Figure 4d. The fluorescence intensity is distinct at different wavelengths, and the calculated two-photon absorption cross section also varies. It can be seen that the molecule obtains a larger two-photon absorption cross section near 800 nm. It makes certain that the molecule has a greater probability of two-photon absorption, which is a prerequisite to being a two-photon initiator. At a wavelength of 800 nm, compared with the molecular BD-PM, the d of BM- TPA PPPM-C6 was 116 GM, while the d of BD-PM was 14 GM [36]. Therefore, the d of the TPA TPA molecule was greatly increased. However, the d of BD-PPM was 336 GM in THF [28], TPA and the d of BD-PTM was about 380 GM [29] in DCM. They were both higher than TPA BM-PPPM-C6. This may be because the fluorescence quantum yield of BD-PPM (52.1%) was much higher than that of BD-PPM (16.0% in DCM) and BD-PTM (40.3% in DCM), resulting in a smaller calculated value of d . TPA 4.3. Cyclic Voltammograms Cyclic voltammetry can be put to proper use by approximating the orbital energy levels of molecules. The experiment was carried out in a DCM solution of 0.1 mol/L of n-Bu NPF . In the experiment, a platinum sheet was employed as the working electrode, 4 6 platinum wire was used as the counter electrode, an Ag/Ag electrode was used as the reference electrode, and ferrocene was used as the internal standard. The scan rate was set to 0.1 V/s. It can be calculated from Figure 5 that E = 5.26 eV. Taking into account HOMO the approximate conditions used in the theoretical calculations, it can be considered that the experimental results are basically in line with the theoretical ones. From the ultraviolet– visible absorption band edge of the molecule, the optical band gap of the molecule is around 446 nm, E = 2.78 eV. If the optical band gap of the molecule is approximately regarded as Photonics 2022, 9, 183 8 of 14 the molecular energy gap, the calculation can be E = 2.48 eV. LUMO Figure 5. Cyclic voltammetry experiments of molecule BM-PPPM-C6 and ferrocene at a concentra- Figure 5. Cyclic voltammetry experiments of molecule BM-PPPM-C6 and ferrocene at a concentration −3 tion of 1 × 10 mol/L in DCM. of 1  10 mol/L in DCM. 4.4. UV Light Stability 4.4. UV Light Stability Under the ultraviolet light, the photolysis properties of molecules can indicate the Under the ultraviolet light, the photolysis properties of molecules can −5 indicate the sen- sensitivity of molecules to ultraviolet light. Molecules with a concentration of 1 × 10 mol/L are irradiated by ultraviolet light with a wavelength of 365 nm and a light intensity sitivity of molecules to ultraviolet light. Molecules with a concentration of 1  10 mol/L of 50 mW/cm in a DCM solution. The data of the photolysis experiment are shown in are irradiated by ultraviolet light with a wavelength of 365 nm and a light intensity of Figure 6a. It can be seen that the molecules react easily under ultraviolet irradiation, 50 mW/cm in a DCM solution. The data of the photolysis experiment are shown in indicating that they have high photosensitivity, which also means that the molecule Figure 6a. It can be seen that the molecules react easily under ultraviolet irradiation, indi- needed to be kept away from light during use. cating As shown in Fig that they have ure 6b, high the early photosensitivi photolysis pty roce , which ss of the m also olecu means le conform that s the to the molecule needed to first-order kinetic model, and its concentration decays exponentially with time by fitting be kept away from light during use. the relationship between the absorbance of the molecule at 394 nm and the irradiation As shown in Figure 6b, the early photolysis process of the molecule conforms to the time. This has a certain value for studying the principle and kinetic process of molecular first-order kinetic model, and its concentration decays exponentially with time by fitting initiation polymerization. the relationship between the absorbance of the molecule at 394 nm and the irradiation −5 Figure 6. (a) UV degradation characteristics of molecule BM-PPPM-C6 at a concentration of 1 × 10 mol/L. (b) Photodegradation versus time fitting curve. 4.5. TPPAM Experiments After analyzing the optical properties of the molecule, it is confirmed that the molecule has potential as an initiator. However, the actual initiation performance needs to be tested in a two-photon polymerization experiment. The experiment of TPPAM was carried out with the molecule BM-PPPM-C6 as the initiator and TMPTA as the monomer. In Figure 7a, it is shown that the molecule can actually initiate the polymerization of the monomer to form a three-dimensional structure. At a lower writing speed of 100 μm/s, polymerization is still able to initiate when the power is as low as 5 mW. Under an extremely high writing speed of 100,000 μm/s, the power of 30–50 mW is able to initiate Photonics 2022, 9, 183 8 of 14 Figure 5. Cyclic voltammetry experiments of molecule BM-PPPM-C6 and ferrocene at a concentra- −3 tion of 1 × 10 mol/L in DCM. 4.4. UV Light Stability Under the ultraviolet light, the photolysis properties of molecules can indicate the −5 sensitivity of molecules to ultraviolet light. Molecules with a concentration of 1 × 10 mol/L are irradiated by ultraviolet light with a wavelength of 365 nm and a light intensity of 50 mW/cm in a DCM solution. The data of the photolysis experiment are shown in Figure 6a. It can be seen that the molecules react easily under ultraviolet irradiation, indicating that they have high photosensitivity, which also means that the molecule needed to be kept away from light during use. Photonics 2022, 9, 183 8 of 13 As shown in Figure 6b, the early photolysis process of the molecule conforms to the first-order kinetic model, and its concentration decays exponentially with time by fitting the relationship between the absorbance of the molecule at 394 nm and the irradiation time. This has a certain value for studying the principle and kinetic process of molecular time. This has a certain value for studying the principle and kinetic process of molecular initiation polymerization. initiation polymerization. −5 Figure 6. (a) UV degradation characteristics of molecule BM-PPPM-C6 at a concentration of Figure 6. (a) UV degradation characteristics of molecule BM-PPPM-C6 at a concentration of 1 × 10 mol/L. (b) Photodegradation versus time fitting curve. 1  10 mol/L. (b) Photodegradation versus time fitting curve. 4.5. TPPAM Experiments 4.5. TPPAM Experiments After analyzing the optical properties of the molecule, it is confirmed that the molecule After analyzing the optical properties of the molecule, it is confirmed that the has potential as an initiator. However, the actual initiation performance needs to be tested in molecule has potential as an initiator. However, the actual initiation performance needs to b a two-photon e tested in a polymerization two-photon poex lym periment. erization experiment. The The experiment of exTPP periment o AM was f T carried PPAM w out as with carr the ied o molecule ut with BM-PPPM-C6 the molecule as BM the -PP initiator PM-C6 a and s the i TMPT nitiaA tor and as the Tmonomer MPTA as the . In m Figur onom e e 7r. a, it is In shown Figure that 7a, it is the molecule shown th can at the actu molec ally initiate ule can the actua polymerization lly initiate the pol of the ymeriza monomer tion of to th form e a monomer to form a three-dimensional structure. At a lower writing speed of 100 μm/s, three-dimensional structure. At a lower writing speed of 100 m/s, polymerization is still Photonics 2022, 9, 183 9 of 14 polymerization is still able to initiate when the power is as low as 5 mW. Under an able to initiate when the power is as low as 5 mW. Under an extremely high writing speed of extremely high writing speed of 100,000 μm/s, the power of 30–50 mW is able to initiate 100,000 m/s, the power of 30–50 mW is able to initiate polymerization to form a complete three-dimensional structure. Figure 7b,c is the enlarged structure of the area shown. As can polymerization to form a complete three-dimensional structure. Figure 7b,c is the enlarged structure of the area shown. As can be seen, the molecule can be used as an be seen, the molecule can be used as an initiator to obtain a three-dimensional structure initiator to obtain a three-dimensional structure with clear outline and complete structure with clear outline and complete structure under the applicable speed and power conditions, under the applicable speed and power conditions, which proves that the molecule BM- which proves that the molecule BM-PPPM-C6 is successful as an initiator. PPPM-C6 is successful as an initiator. Figure 7. (a) SEM images of structures with initiator BM-PPPM-C6 (10 μmol/g) in TMPTA. (b) Figure 7. (a) SEM images of structures with initiator BM-PPPM-C6 (10 mol/g) in TMPTA. (b) En- Enlarged view of low writing speed area. (c) Enlarged view of high writing speed area. larged view of low writing speed area. (c) Enlarged view of high writing speed area. The designed BM-PPPM series of molecules are equipped with different alkane chain lengths. Although it is generally believed that the chain length of alkane hardly affects the chemical properties of the molecule, the solubility of the molecule and the interaction between the molecule and the monomer will have a certain difference in reality. It is not known whether these differences may have a certain impact on the initiation efficiency of the molecule. Consequently, the polymerization effect of BM-PPPM with different alkane chain lengths in TMPTA was compared, as shown in Figures 8 and S5–S7. It can be seen that under the same initiator concentration, there is no obvious difference in the initiation effect of the designed molecule in TMPTA. Initiator molecules of different chain lengths can be completely dissolved in TMPTA to achieve polymerization. Photonics 2022, 9, 183 9 of 13 The designed BM-PPPM series of molecules are equipped with different alkane chain lengths. Although it is generally believed that the chain length of alkane hardly affects the chemical properties of the molecule, the solubility of the molecule and the interaction between the molecule and the monomer will have a certain difference in reality. It is not known whether these differences may have a certain impact on the initiation efficiency of the molecule. Consequently, the polymerization effect of BM-PPPM with different alkane chain lengths in TMPTA was compared, as shown in Figures 8 and S5–S7. It can be seen that under the same initiator concentration, there is no obvious difference in the initiation effect of the designed molecule in TMPTA. Initiator molecules of different chain lengths Photonics 2022, 9, 183 10 of 14 can be completely dissolved in TMPTA to achieve polymerization. Figure 8. SEM images of structures with initiators (a) BM-PPPM-C12; (b) BM-PPPM-C16; and (c) Figure 8. SEM images of structures with initiators (a) BM-PPPM-C12; (b) BM-PPPM-C16; and BM-PPPM-C20 in TMPTA, 10 μmol/g. (c) BM-PPPM-C20 in TMPTA, 10 mol/g. BM-PPPM is obtained by conjugation expansion, so the changes in the initiating abil- BM-PPPM is obtained by conjugation expansion, so the changes in the initiating ability ity of molecules with diverse degrees of conjugation are interesting. Comparing the initi- ation efficiency of BM-PPPM with BD-PPM and BD-PM in previous work [28,29], it can of molecules with diverse degrees of conjugation are interesting. Comparing the initiation be found that the initiation efficiency of BM-PPPM has been greatly improved compared efficiency of BM-PPPM with BD-PPM and BD-PM in previous work [28,29], it can be found to BD-PPM and BD-PM, which is in line with the expected improvement in initiator effi- that the initiation efficiency of BM-PPPM has been greatly improved compared to BD-PPM ciency due to conjugation extension. Photonics 2022, 9, 183 10 of 13 and BD-PM, which is in line with the expected improvement in initiator efficiency due to Photonics 2022, 9, 183 11 of 14 conjugation extension. However, when BM-PPPM was compared to a more highly conjugated triphenylamine molecule (BM-PTM) [29], it was found that its initiation effect was not inferior; this indi- However, when BM-PPPM was compared to a more highly conjugated triphenyla- cates that simply adding a conjugation group gradually lessened the increase in initiation mine molecule (BM-PTM) [29], it was found that its initiation effect was not inferior; this efficiency as the degree of conjugation increased, just like the diminishing marginal utility indicates that simply adding a conjugation group gradually lessened the increase in initi- ation efficiency as the degree of conjugation increased, just like the diminishing marginal effect. Hence, it should not be achieved simply by conjugation expansion, but requires utility effect. Hence, it should not be achieved simply by conjugation expansion, but re- more in-depth consideration so as to design a larger and more efficient initiator. quires more in-depth consideration so as to design a larger and more efficient initiator. Although molecules with diverse alkane chain lengths have complete solubility in Although molecules with diverse alkane chain lengths have complete solubility in TMPTA, it does not mean that the molecules with different chain lengths are meaningless. TMPTA, it does not mean that the molecules with different chain lengths are meaningless. When using the monomer PETA, it was found that hardly any of the BM-PPPM-C6 can When using the monomer PETA, it was found that hardly any of the BM-PPPM-C6 can be completely dissolved. Additionally, the molecules with longer carbon chains cannot be completely dissolved. Additionally, the molecules with longer carbon chains cannot form a clear and transparent dissolution in PETA. After two days of heating and stirring, form a clear and transparent dissolution in PETA. After two days of heating and stirring, precipitation still exists. This shows that for PETA, the molecule BM-PPPM-C6 alone precipitation still exists. This shows that for PETA, the molecule BM-PPPM-C6 alone can can be used as a suitable initiator. As shown in Figure 9, it is more effective in initiating be used as a suitable initiator. As shown in Figure 9, it is more effective in initiating polymerization in PETA, BM-PPPM-C6. With a power of 15 mW and a speed of up to polymerization in PETA, BM-PPPM-C6. With a power of 15 mW and a speed of up to 100,000 m/s, it can still form a complete three-dimensional structure with only slight 100,000 μm/s, it can still form a complete three-dimensional structure with only slight deformation deformation at at the the edge edges. s. This r This result esult isis be better tter tha thann the the tes test rtesults resultus nder unde the r th same e sam conditions e conditions reported in the recent literature (when the writing speed was 100,000 μm/s, the reported in the recent literature (when the writing speed was 100,000 m/s, the minimum minimum laser power was 44 mW) [15]. Under low speed, low power and a power laser power was 44 mW) [15]. Under low speed, low power and a power condition of 5 mW, condition of 5 mW, polymerization can be realized in the speed range of 100–2000 μm/s, polymerization can be realized in the speed range of 100–2000 m/s, with a larger range with a larger range of polymerization parameters. The above results are excellent, which of polymerization parameters. The above results are excellent, which suggests that this suggests that this molecule can be used in additive manufacturing at high speeds. Com- molecule can be used in additive manufacturing at high speeds. Compared to the molecules pared to the molecules formed by benzophenone and triphenylamine, the test results in formed by benzophenone and triphenylamine, the test results in PETA are not inferior. PETA are not inferior. Figure 9. (a) SEM images of structures with initiators BM-PPPM-C6 in PETA, 10 mol/g. (b–d) Enlarged images of the area shown. Photonics 2022, 9, 183 11 of 13 Meanwhile, in order to illustrate that initiators with different alkane chain lengths have different compatibility in some monomers, the solubility of BM-PPPM series molecules was studied using n-heptane as a solvent. It was found that BM-PPPM-C6 and BM-PPPM-C16 had poor solubility; conversely, BM-PPPM-C20 had good solubility, as shown in Figure S8. BM-PPPM-C12 will separate out after standing for a period of time. At this solvent polarity, longer-chain alkanes are more favorable for dissolution. This entirely illustrates that controlling the alkane chain can effectively regulate the solubility of molecules with different solvents, ensuring that only the change in the alkane chain can meet the needs of different monomers for initiators. In consequence, it can be seen that due to the critical factor of solubility, the length of the alkane chain is not meaningless in the process of being an initiator. The solubility between organics is extremely complicated, and it is tough to accurately determine the degree of solubility. The simple act of introducing carbon chains of different lengths can effectively change the solubility of molecules. However, in the monomer PETA selected in this article, the shortest chain is more conducive to dissolution. Nevertheless, longer alkane chains may have better solubility under certain conditions. Furthermore, only an alkane chain of moderate length can dissolve more effectively. 5. Conclusions In this paper, a BM-PPPM series of D––A––D molecules based on benzophenone groups were designed by conjugated extension. Through theoretical calculations and photophysical experiments, the potential of molecules as nonlinear materials was confirmed. Through TPPAM experiments, it is proven that PETA resin with BM-PPPM molecules as initiators can form complete three-dimensional structures at a writing speed as high as 100,000 m/s and a laser power as low as 15 mW. From a molecular-structure point of view, it is confirmed that the molecular conjugate extension can indeed improve molecular initiation efficiency. However, as the degree of conjugation reaches a certain limit, the improvement that can be brought by adding conjugated groups will be extremely limited. In addition, the chain length of the initiator will obviously affect the solubility with the monomer. By simply changing the alkane chain length, the compatibility relationship between the initiator and the monomer can be enhanced to a certain extent. Supplementary Materials: The supporting information can be downloaded at: https://www.mdpi. com/article/10.3390/photonics9030183/s1. Figure S1: Intermediate product 3 synthesis route. Figure S2: Intermediate product 6 synthesis route. Figure S3: Final product 7 synthesis route. Figure S4: The fluorescence quantum yield of BM-PPPM-C6. Figure S5: SEM images of structures with initiator BM-PPPM-C12 in TMPTA, 10 mol/g. Figure S6: SEM images of structures with initiator BM-PPPM-C16 in TMPTA, 10 mol/g. Figure S7: SEM images of structures with initiator BM-PPPM-C20 in TMPTA, 10 mol/g. Figure S8: Photos showing differences in solubility of different molecules. Table S1: Reagents. Author Contributions: S.L.: Investigation, methodology, testing and analysis, software, writing— original draft; X.L.: writing—review and editing, test and analysis; S.Z.: conceptualization, investi- gation, testing and analysis, data curation, writing—review and editing; Y.Z.: writing—review and editing, test and analysis; X.W.: writing—review and editing, conceptualization, methodology; N.L.: resources; J.L.: resources, conceptualization; L.Z.: resources, supervision. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Applied Basic Research Program of Sichuan Province (no. 2019YJ003), the State Key Laboratory of Environment-Friendly Energy Materials (no. 19kfhg01), the National Natural Science Foundation of China (no. 51803198), and the Presidential Foundation of China Academy of Engineering Physics (no. YZJJLX2020008). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. 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Journal

Photonics – Multidisciplinary Digital Publishing Institute

Published: Mar 13, 2022

Keywords: optical functional materials; photoinitiator; two-photon polymerization additive manufacturing; conjugation extension; solubility

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