Communication pubs.acs.org/cm
Application to Photoreactive Materials of Photochemical Generation of Superbases with High Efficiency Based on Photodecarboxylation Reactions Koji Arimitsu* and Ryosuke Endo Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan S Supporting Information *
KEYWORDS: photobase generator, anionic UV curing, superbase, photodecarboxylation
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Compound 3 can be prepared by simply mixing 1 with a corresponding base molecule, 1,5-diazabicyclo[5.4.0]undec-5ene (DBU), 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), TBD, imino-tris(dimethylamino)phosphazene (P1), or cyclohexylamine (CyA). According to this concept, we can prepare a variety of photobase generators 3a,14 3b,14 3c, 3d, and 3e15 that photochemically generate superbases, as well as aliphatic amines. Although ketoprofen derivatives offer many advantages, they have one limitation, which stems from their poor absorption above 300 nm. We recognized that the structurally related xanthone chromophore has much better absorption above 300 nm. In addition, xanthone acetic acid (2) also undergoes photodecarboxylation reactions with high quantum yield Φ350 = 0.64.16 Therefore, we designed novel photobase generators 4 as well as 3 (Scheme 1). Our primary purpose in this paper is to demonstrate photochemical generation of superbases from salts (3 and 4) and to show that a novel anionic UV-curing system without postexposure baking is realized by the combination of 4c with an epoxy/thiol formulation17,18 comprising epoxy monomer 5 and thiol monomer 6 (Scheme 2).
nly a few articles have mentioned photoreactive materials relying on base-catalyzed transformations, although a large number of investigations concerning analogous systems utilizing acid-catalyzed reactions, such as chemically amplified photoresists1 and cationic UV-curing materials,2 have been reported. This is probably due to relatively low quantum yields for photobase generation and weaker basicity of photogenerated bases, leading to low photosensitivity of photoreactive materials sensitized with photobase generators. Furthermore, many of the photobase generators reported are generally prepared via several synthetic steps.3−11 Recently, Sun et al. have reported bicyclic guanidinium tetraphenylborate as a novel photobase generator to generate a strong base, 1,5,7triazabicyclo[4.4.0]dec-5-ene (TBD).12 Although the tetraphenylborate is an attractive photobase generator, the quantum yield of the tetraphenylborate (Φ254 = 0.18) is not very high. Therefore, we have studied photodecarboxylation reactions to develop novel photobase generators to release strong bases with a high quantum yield. It is well-known that ketoprofen (1) undergoes photodecarboxylation reactions with high quantum yield, Φ313 = 0.75 upon UV irradiation (Scheme 1).13 If a salt consisting of 1 and a superbase could undergo a photodecarboxylation reaction, a free superbase should be produced with high efficiency. This idea led us to carry out the molecular design of a novel photobase generator 3 as given in Scheme 1.
Scheme 2. Application of 4c to Anionic UV-Curing Systems without Heat Treatment
Scheme 1. Photoinduced Generation of Strong Bases from Photobase Generators 3 and 4
Received: July 8, 2013 Revised: October 18, 2013 Published: October 29, 2013 © 2013 American Chemical Society
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Chemistry of Materials
Communication
photodecarboxylation reactions of 3 and 4 proceeded in methanol, leading to the generation of free bases. A thin film of polystyrene containing 4c was spin-coated on a CaF2 plate and irradiated with 365 nm light. The absorption band arising from the carboxylate of 4c at 1372 cm−1 in the FTIR spectrum decreased after UV irradiation, as shown in Figure 3. This means that photodecarboxylation of 4c proceeded
Salts 3 (or 4) were prepared by mixing 1 (or 2) and a strong base (DBU, DBN, TBD, or P1) in organic solvents at room temperature. Photodecomposition of 3 and 4 was investigated in methanol by UV−vis absorption spectral measurements when 3 and 4 were irradiated with 254 and 365 nm light from a Hg−Xe lamp, respectively. The absorption band at the longer wavelength side of λmax at 255 nm decreased slightly, and a new band appeared at 230 nm when 3c in methanol was irradiated with 254 nm light. These results suggest that photodecarboxylation of 3c proceeded in methanol. 3a, 3b, and 3d decomposed quite similarly. Figure 1 shows UV spectral
Figure 3. Power dependence for the consumption of the carboxylate of 4c in a polystyrene film during 365 nm light irradiation.
smoothly in a polystyrene film. Other carboxylates behaved in a similar manner. In addition, the quantum yield for photobase generation of 4c (Φ365 = 0.38) in a polystyrene film was determined.19 The quantum yield is much higher than conventional photobase generators generating organic strong bases. These results show that 3 and 4 underwent photodecarboxylation reactions in solution and a polymer matrix upon UV irradiation, leading to the formation of free superbases. A mixture of 5 and 6 is expected to be cured in the presence of 4c upon UV irradiation, owing to the ring-opening addition of thiolate anions to epoxy groups.17,18 Thiolate anions should be generated by photobase-induced deprotonation reactions of 6. This anionic UV curing was achieved by exposing the coating films consisting of 4c, 5, and 6 to 365 nm light.20 Figure 4 shows the UV-curing behavior of coating films consisting of monomers (5 and 6) and a catalytic amount of 4c. The hardness of the coatings was monitored by the pencil-scratch method based on JIS K5400.21 The pencil hardness was evaluated by scratching UV-cured coatings with pencils, the hardness of which is arranged as follows: 6B (softest), 5B, 4B,
Figure 1. UV spectral changes of 4c (2.0 × 10−5 mol/L) in methanol following 365 nm light irradiation.
changes of 4c in methanol upon irradiation with 365 nm light. As shown in Figure 1, no drastic spectral changes were observed, although the absorption band of λmax at 240 nm decreased slightly. This suggests that drastic structural changes of 4c did not occur except for the photodecarboxylation reaction of 4c. Therefore, it was necessary to detect free bases generated from 4c. The photobase-generating ability of 4c was confirmed by using phenol red indicator.12 Upon addition of irradiated methanol solution of 4c to a methanol solution of phenol red, a new band at 562 nm appeared, assigned to the deprotonated phenol red after reaction with a released base, and its intensity increased with an increase in irradiation energy (Figure 2). These results show that photoinduced decomposition of 4c leads to the generation of free bases. 4a, 4b, and 4d behaved in the same way. These results indicate that
Figure 4. Pencil hardness of coating films comprising 5 and 6 sensitized with 10 wt % of 4c and 4e as a function of irradiation energy from 365 nm light without postexposure baking.
Figure 2. UV−vis spectral changes of phenol red solution upon addition of a solution of 4c being irradiated as a function of irradiation dose. 4462
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Communication
(5) Ito, K.; Nishimura, M.; Sashio, M.; Tsunooka, M. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1793. (6) Cameron, J. F.; Fréchet, J. M. J. J. Am. Chem. Soc. 1996, 118, 12925. (7) Nishikubo, T.; Kameyama, A.; Toya, Y. Polym. J. 1997, 29, 450. (8) Kaneko, Y.; Sarkar, A. M.; Neckers, D. C. Chem. Mater. 1999, 11, 170. (9) Jensen, K. H.; Hansen, J. E. Chem. Mater. 2002, 14, 918. (10) Blanc, A.; Bochet, C. G. J. Am. Chem. Soc. 2004, 126, 7174. (11) Dietliker, K.; Misteli, K.; Studer, K.; Lordelot, C.; Carroy, A.; Jung, T.; Benkhoff, J.; Sitsmann, E. Proc. RADTECH UV&EB Technical Conference 2008 2008, 10. (12) Sun, X.; Gao, J. P.; Wang, Z. Y. J. Am. Chem. Soc. 2008, 130, 8130. (13) Costanzo, L. L.; De Guidi, G.; Condorelli, G.; Cambria, A.; Fama, M. Photochem. Photobiol. 1989, 50, 359. (14) A preliminary report: Arimitsu, K.; Endo, R. J. Photopolym. Sci. Technol. 2010, 23, 135. (15) A preliminary report: Arimitsu, K.; Kushima, A.; Endo, R. J. Photopolym. Sci. Technol. 2009, 22, 663. (16) Blake, J. A.; Gagnon, E.; Lukeman, M.; Scaiano, J. C. Org. Lett. 2006, 8, 1057. (17) Nichols, M. E.; Seubert, C. M. Proc. Am. Chem. Soc. Div. Polym. Mater. Sci. Eng. Symp. 2009, 101, 1141. (18) Seubert, C. M.; Nichols, M. E. J. Coat. Technol. Res. 2010, 7, 615. (19) Φ was determined according to the literature.3 (20) A UV-curable resin solution was prepared by dissolving 0.50 g (1.2 × 10−3 mol) of 5, 0.67 g (1.2 × 10−3 mol) of 6, and 4c (10 mol % relative to 5) in methanol. The solution was coated on glass plates and prebaked at 60 °C on a hot stage for 30 s to give a film. UV curing was achieved by exposing the films to 365 nm light. The pencil hardness of the films was evaluated by scratching UV-cured coatings with pencils according to JIS K5400. (21) JIS K5400 defined by Japanese Industrial Standards is a simple method to test the scratch hardness of coatings. In this test, pencils in a range of 6B to 9H hardness grade are used. The pencil is moved scratching over the surface of the coating at a 45° angle with a constant pressure. (22) An anionic UV-curable resin solution was prepared by dissolving 0.50 g (1.2 × 10−3 mol) of 5, 0.67 g (1.2 × 10−3 mol) of 6, and 4c (10 mol % relative to 5) in methanol. The solution was coated on an OHP sheet and prebaked at 60 °C on a hot stage for 30 s to give 14 μmthick film. UV curing was achieved by exposing the film to 365 nm light (10 J/cm2). A radical UV-curable resin solution was prepared by dissolving 1 g of PETA and 5 wt % of Irgacure 819 in methanol. UV curing was achieved in a similar manner.
3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H, 7H, 8H, and 9H (hardest). Surprisingly, the coating film showed a level of 3H after 365 nm light irradiation with an exposure dose of 1000 mJ/cm2 without postexposure baking, which is quite enough for practical use. This means that the anionic UV curing material sensitized with 4c is highly sensitive, taking into account the fact that conventional anionic UV curing materials were cured by heating at 150−200 °C for ca. 1 h after UV irradiation. By contrast, coating films showed a level lower than 6B when we use 4e, which generates cyclohexylamine (CyA). This is because the basicity of CyA is too weak to induce deprotonation reactions of thiol monomer 6. Radical UV curing materials that are well established in the marketplace have drawbacks because of high volume shrinkage and oxygen inhibition. Our anionically cured film showed high transparency and no volume shrinkage, in contrast to a conventional radical UV curing system, which showed large volume shrinkage, as given in Figure 5.22
Figure 5. Comparison of volume shrinkage between anionic UV curing and radical UV curing systems.
In conclusion, we present novel photobase generators (3 and 4) based on photodecarboxylation reactions. These photobase generators can be easily prepared by mixing ketoprofen (1) or xanthone acetic acid (2) with corresponding superbases. We have also developed a novel anionic UV curing system without postexposure baking, which consists of an epoxy monomer, a thiol monomer, and the photosuperbase generator.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental methods and preparation of photobase generators. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Ito, H. Adv. Polym. Sci. 2005, 172, 37. (2) Crivello, J. V. J. Coat. Technol. 1991, 63, 35. (3) Cameron, J. F.; Fréchet, J. M. J. J. Am. Chem. Soc. 1991, 113, 4303. (4) Fréchet, J. M. J. Pure Appl. Chem. 1992, 64, 1239. 4463
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