ESR STUDY OF LOPHYL FREE-RADICALS IN DRY FILMS

Photolysis of 2-(2'-chlorophenyl)-2'-(2'-chlorophenyl)-4,4'-5,5'-tetraphenylbisimidazoles (D) into corresponding lophyl free radicals (R(.)) was studied by steady-state and time-resolved (TR) CW ESR in dry films of poly(methyl methacrylate) (PMMA) at 6-400 K and in a frozen Liquid solution at 77 K. Steady-state ESR experiments demonstrate the formation of spatially proximate electronically interacting radical pairs (RPs) R(.)...R(.) as well as the formation of spatially isolated free radicals R(.). The ratio of amounts of photogenerated R(.) to R(.)...R(.) depends upon the temperature and the properties of the film. This ratio increases with increasing temperature, and at the same temperature the ratio is larger in films containing a plasticizer (triacetin). At low temperatures (6-200 K) RPs are observed and are characterized by zero-field splitting (ZFS) values of 2D approximate to 250 G (corresponding to intraradical distance of r approximate to 6 Angstrom) and E less than or equal to 10 G. Films which do not contain a plasticizer demonstrate ESR spectra of RPs with 2D approximate to 100 G (r approximate to 8 Angstrom) at room, and even at elevated, temperatures (up to 400 K) as well as ESR spectra of RPs of two types (r approximate to 6 and 8 Angstrom) at reduced temperatures. Thus, the measurement of ZFS parameters of RPs produced by the photolysis of D in films can be used to characterize photopolymer film. TR ESR spectra of the triplet state of D were observed in films at 53 K Simulation of the spectra leads to estimates of 2D approximate to 2200 G and E approximate to 165 G. Decay kinetics of the lophyl radicals in films was satisfactorily described by polychromatic second-order kinetics with a rectangular distribution of reactivity. The estimated maximum difference in activation energies for a film without a plasticizer is ca. 4 kcal/mol, suggesting a range of recombination rate constants of ca. 10(3).