Excited-state properties of trans-(9-anthryl)ethylenes. Effects of geometric distortion about single bond

The singlet- and triplet-related photophysical properties of four trans-1,2-diarylethylenes, containing methyl, phenyl, 1-naphthyl, and 9-anthryl group as one substituent and 9-anthryl group as the other, have been studied by steady-state and time-correlated fluorescence measurements, laser flash photolysis, and pulse radiolysis. The results show pronounced effects of the photoexcitation-induced geometric distortion (about the anthryl-to-ethylene "single" bond) in rendering fluorescence and triplet-triplet absorption spectra broad and structureless, and in enhancing radiationless processes from singlets, triplets, and exciplexes (with N,N-dimethylaniline). The shortening of triplet lifetimes, and the triplet quenching behaviors toward ferrocene, di-tert-butylnitroxy radical, and oxygen, are also explainable in terms of the torsional relaxation; these effects are similar to, but less pronounced than, those arising from double bond twisting in smaller arylethylene triplets. No photophysical anomaly attributable to ground-state rotamerism is observed in fluid solutions