Ionic vs Free Radical Pathways in the Direct and Sensitized Photochemistry of 2-(4‘-Methoxynaphthyl)-4,6-bis(trichloromethyl)-1,3,5-triazine:  Relevance for Photoacid Generation

• Published in: Journal of the American Chemical Society Vol. 121; no. 26; pp. 6167 - 6175
• Main Authors: Pohlers, G, Scaiano, J. C, Step, E, Sinta, R
• Format: Journal Article
• Language: English
• Published: American Chemical Society 07.07.1999
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja983371c

The photochemistry and photophysics of the title compound (2), which finds application as a photoacid generator in photoresist formulations, has been investigated using a combination of laser flash photolysis work and emission spectroscopy, as well as evaluation of acid formation. The studies were carried out in polar (acetonitrile) and nonpolar (cyclohexane, benzene) solvents, employing both direct excitation and sensitized conditions using isopropylthioxanthone (ITX) and acetone as sensitizers. In nonpolar medium, photolysis of 2 follows a conventional mechanism involving C−Cl cleavage. In contrast, direct excitation of 2 in acetonitrile leads to C−Cl heterolysis with formation of the cation, which can be readily identified by its rapid quenching by nucleophiles such as halide anions, azide, and methanol. Interestingly, 2 + must rearrange for its spectroscopic and kinetic parameters to be consistent with carbocation structures. Several possible structures are proposed for 2 + and its rearranged isomers. Photolysis of 2 in the presence of ITX or acetone also leads to the carbocation, but the yields increase by more than 1 order of magnitude, indicating a higher efficiency of the triplet reaction compared to the singlet reaction. No evidence for electron transfer between 2 and ITX could be found; i.e., the sensitization is entirely due to T−T energy transfer. The quantum yield of HCl generation upon direct excitation (monitored in the microsecond time scale) is very low (0.007); sensitization increases it to 0.111 (acetone) and 0.074 (ITX). In nonpolar solvents, the primary photochemical step in the direct photolysis of these compounds is the homolysis of one of the carbon−chlorine bonds; the chlorine atoms formed in this reaction can be detected by complexation with benzene. Contrary to the ionic pathway, the quantum yield of the homolysis is rather insensitive to sensitization.