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‐styrylidenebenze]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 .