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 S 2 , a stilbenic excited state. The differences in behavior are traced to the diminished S 1 /S 2 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 S 1 state in t ‐ NPE B and the stilbenic S 2 state in t ‐ BPE .