Photoprocesses in diphenylpolyenes. Oxygen and heavy-atom enhancement of triplet yields

Abstract

Results, obtained by nanosecond laser flash photolysis (337.1 nm) and fluorescence measurements, are presented concerning triplet-triplet spectra and excited-state dynamics of three all-trans ..cap alpha..,..omega..-diphenyl-substituted polyenes, Ph(CH=CH)/sub n/Ph (n = 2-4), in various solvents and in the presence of oxygen and heavy-atom-containing molecules. The spectra of triplets generated by both direct excitation and energy transfer, display the characteristics of an intense Franck-Condon allowed transition with vibrational spacings in the range 1200-1600 cm/sup -1/ and show solvent-effect and chain-length dependence very similar to those observed for the strongly allowed band system in ground-state absorption. The quantum yields of occupation of lowest triplet are small (0.5-3%) in deaerated solvents such as cyclohexane, methanol, and benzene, but become significantly enhanced in heavy-atom-containing solvents (e.g., ethyl bromide and bromobenzene) and in the presence of oxygen and heavy-atom species. The intersystem crossing rate constants for T/sub 1/reverse arrowS/sub 1/ and S/sub 0/reverse arrowT/sub 1/ are in the ranges 7 x 10/sup 5/ - 9 x 10/sup 7/ and 2 x 10/sup 4/ - 7 x 10/sup 5/ s/sup -1/, respectively, and exhibit a decreasing trend as the polyene chain length is increased. Based on kinetic data concerning singlet quenching and triplet yield enhancement by O/submore » 2/ and ethyl iodide in methanol and cyclohexane, the fraction of the singlet quenching events that result in triplet generation is found to range from 0.1 to 0.8. The correlation (or lack of it) between intersystem crossing rate constants and those for bimolecular quenching of singlet and triplet is discussed in the context of the energy gap law observed in similar studies for aromatic hydrocarbons.« less