Ionic vs free radical pathways in the direct and sensitized photochemistryof 2-(4 '-methoxynaphthyl)-4,6-bis(trichloromethyl)-1,3,5-triazine: Relevance for photoacid generation

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 emissio n spectroscopy, as well as evaluation of acid formation. The studies were c arried out in polar (acetonitrile) and nonpolar (cyclohexane, benzene) solv ents, employing both direct excitation and sensitized conditions using isop ropylthioxanthone (ITX) and acetone as sensitizers. In nonpolar medium, pho tolysis of 2 follows a conventional mechanism involving C-Cl cleavage. In c ontrast, direct excitation of 2 in acetonitrile leads to C-CI heterolysis w ith formation of the cation, which can be readily identified by its rapid q uenching by nucleophiles such as halide anions, azide, and methanol. Intere stingly, 2(+) must rearrange for its spectroscopic and kinetic parameters t o be consistent with carbocation structures. Several possible structures ar e proposed for 2+ and its rearranged isomers. Photolysis of 2 in the presen ce 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 t riplet reaction compared to the singlet reaction. No evidence for electron transfer between 2 and ITX could be found; i.e., the sensitization is entir ely due to T-T energy transfer. The quantum yield of HCl generation upon di rect excitation (monitored in the microsecond time scale) is very low (0.00 7); sensitization increases it to 0.111 (acetone) and 0.074 (ITX). In nonpo lar solvents, the primary photochemical step in the direct photolysis of th ese compounds is the homolysis of one of the carbon-chlorine bonds; the chl orine atoms formed in this reaction can be detected by complexation with be nzene. Contrary to the ionic pathway, the quantum yield of the homolysis is rather insensitive to sensitization.