Efficient Photoacids Based upon Triarylamine Dialkylsulfonium Salts Wenhui Zhou, Stephen M. Kuebler, Dave Carrig, Joseph W. Perry,* and Seth R. Marder* Contribution from the Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721 Received May 14, 2001. Revised Manuscript Received October 29, 2001 Abstract: New triarylamine dialkylsulfonium salts that are photosensitive in the near-ultraviolet have been prepared. The quantum yields of photoacid generation were found to be ∼0.5 and are independent of the counterion. On the other hand, the efficiencies of the sulfonium salts toward the photopolymerization of cyclohexene oxide depend on the counterion and the sulfonium substituents. Photopolymerization kinetic studies demonstrate that these triphenylamine sulfonium salts are highly efficient cationic photoinitiators. Introduction Photoacids are used extensively in both positive- and nega- tive-tone photoresist formulations in the microelectronics in- dustry. 1 In particular, iodonium and sulfonium salts have found widespread use as initiators for cationic photopolymerization of epoxides and vinyl ethers, 2 and for the cleavage of tert- butoxycarbonyl, t-BOC, esters in chemically amplified resists. 3 Saeva 4 has shown that for arylsulfonium salts, the photofrag- mentation can be initiated either by direct excitation of a π-σ* (sulfur-carbon bond) transition or by excitation of a π-π* transition followed by intramolecular electron transfer to the σ* orbital of the sulfur-carbon bond. In both cases, subsequent homolytic cleavage of the sulfur-carbon bond leads to forma- tion of a cation-radical on the sulfur-containing fragment, and a neutral radical (Scheme 1, top). These species undergo secondary reactions, such as radical coupling or H-transfer, to produce Brønsted acid, which are important in the decomposi- tion of some sulfonium salts. In the past, this reaction has been facilitated by stabilizing the radical fragment. 4-7 In contrast, phenyldimethyl sulfonium salts having a σ* LUMO are not photoactive toward acid generation. This has been attributed to the strength of the S-CH 3 bond (or, in other words, the instability of a methyl radical), which allows radiative and nonradiative decay pathways to compete effectively with C-S bond cleavage. 6 Provided that the energy of the excited state is sufficient enough to populate σ*, it then follows from the Hammond postulate that both the driving force and rate for bond homolysis should be strongly affected by the stability of both the radical cation and the neutral radical. Herein, we explore the hypothesis that the quantum efficiency of photoacid genera- tion can also be enhanced by incorporating triarylamine func- tionalities that stabilize the radical cation intermediate formed upon homolysis of the carbon-sulfur bond of aryl dialkyl sulfonium salts (Scheme 1, middle and bottom). Triarylamine sulfonium salts were selected as the initial targets to test our hypothesis because (i) triarylamine radical cations are known to be very stable; (ii) protonated triarylamino groups are extremely strong acids (Ph 3 N + H, pK a )-5), 8 thus the presence of this amine functionality should not inhibit reactions such as the ring-opening polymerization of epoxides; and (iii) the conditions used in the preparation of aryl dialkyl sulfonium salts 9 are compatible with those of the triarylamine functional group. Experimental Section Absorption spectra were recorded on a Hewlett-Packard model 8453 spectrophotometer. 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