Mechanistic Studies of Photoacid Generation from Substituted 4,6-Bis(trichloromethyl)-1,3,5-triazines

The photochemistry and photophysics of 2-methyl- ( 1 ), 2-(2‘-furylethylidene)- ( 2 ), and 2-[(4‘-methoxy)styryl]-4,6-bis(trichloromethyl)-1,3,5-triazine ( 3 ), three compounds that find application as photoacid generators in photoresist formulations, have been investigated under conditions of direct excitation and using various phenothiazine derivatives as photosensitizers. C−Cl bond cleavage is confirmed by laser flash photolysis as the primary photochemical step in the direct photolysis of these compounds; the chlorine atoms formed in this reaction can be detected by complexation with chloride anions or benzene. In the case of 1, this photodissociation occurs with a lifetime of <20 ns. The quantum yield of bond breaking for 1 after direct excitation as determined by laser flash photolysis and steady-state irradiations was found to be 30−40 times greater than for 2 and 3 . The mechanism under sensitized conditions involves electron transfer from the excited phenothiazines to the triazines as the primary photochemical step; the rate constants for excited-state quenching exceed 10 9 M - 1 s - 1 for the T 1 state and 10 10 M - 1 s - 1 for the S 1 state. The rate constants correlate with the free energy for electron transfer estimated from the excited-state energies and the oxidation potential of the sensitizers.