ESR STUDY OF LOPHYL FREE-RADICALS IN DRY FILMS

Photolysis of '-chlorophenyl)-4,4'-5,5'-tetraphenylbisimidazoles (D) i nto corresponding lophyl free radicals (R(.)) was studied by steady-st ate and time-resolved (TR) CW ESR in dry films of poly(methyl methacry late) (PMMA) at 6-400 K and in a frozen Liquid solution at 77 K. Stead y-state ESR experiments demonstrate the formation of spatially proxima te electronically interacting radical pairs (RPs) R(.)...R(.) as well as the formation of spatially isolated free radicals R(.). The ratio o f amounts of photogenerated R(.) to R(.)...R(.) depends upon the tempe rature and the properties of the film. This ratio increases with incre asing 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 (ZF S) values of 2D approximate to 250 G (corresponding to intraradical di stance 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 roo m, and even at elevated, temperatures (up to 400 K) as well as ESR spe ctra of RPs of two types (r approximate to 6 and 8 Angstrom) at reduce d temperatures. Thus, the measurement of ZFS parameters of RPs produce d by the photolysis of D in films can be used to characterize photopol ymer film. TR ESR spectra of the triplet state of D were observed in f ilms at 53 K Simulation of the spectra leads to estimates of 2D approx imate to 2200 G and E approximate to 165 G. Decay kinetics of the loph yl radicals in films was satisfactorily described by polychromatic sec ond-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).