Effect of temperature on the triplet states of methyl- and phenyl-substituted biphenyl molecules

Abstract

The temperature dependence of the quantum yields Φ c→t and Φ t→c of the cis → trans and trans → cis isomerization respectively of 1,3-pentadine sensitized by phenyl- and methyl-substituted biphenyls has been studied in fluid media. The triplet-state quantum yields for the substituted biphenyls increased with decreasing temperature but approached maximum values at the lowest temperatures. The sums of the fluorescence and triplet quantum yields Φ F and Φ T for 4-methylbiphenyl, 3-methylbiphenyl, 4,4′-dimethyl-biphenyl and 3,3′-dimethylbiphenyl were less than unity at all temperatures and the nature of the temperature-dependent non-radiative process(es) is discussed. An attempt has been made to compare triplet-state quantum yields determined by the olefin isomerization technique with those obtained using sensitized biacetyl phosphorescence.