Photochemistry and Photophysics of the 3-Styrylidenebenz[e]indanes

The photochemical and photophysical properties of the extended conformers of trans- and cis-1-(2-naphthyl)-2-phenylethenes (t- and c-NPE_B ) are strikingly different than those of their rigid analogues, trans- and cis-3-styrylidenebenz[e]indane (t- and c-BPE). The fluorescence quantum yield and lifetime at 25°C in methylcyclohexane drop from 0.76 and 22.2 ns in t-NPE_B to 0.0051 and 0.25 ns in t-BPE, and there are complementary changes in the photoisomerization quantum yields. In both cases, photoisomerization occurs in S₂ , a stilbenic excited state. The differences in behavior are traced to the diminished S₁ /S₂ energy gap on the alkyl substitution afforded by the five-membered ring in BPE. The effect of viscosity on the torsional relaxation of t-BPE, evaluated in glycerol/methanol mixtures at 25°C and in 95/5 and 99.9/0.1 glycerol/methanol (% v/v) as a function of T, is well accounted for by the medium-enhanced barrier model. Absorption and fluorescence spectra of t-BPE in pure glycerol reveal aggregate formation. Quenching of t-BPE fluorescence in methylcyclohexane by tri-n-butylamine (TBA) leads to exciplex fluorescence. Comparison with analogous results for t-NPE_B suggests that charge transfer involves primarily the naphthalenic S₁ state in t-NPE_B and the stilbenic S₂ state in t-BPE.