AbstractThe photochemical and photophysical properties of the extended conformers of trans‐ and cis‐1‐(2‐naphthyl)‐2‐phenylethenes (t‐ and c‐NPEB) 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‐NPEB 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‐NPEB suggests that charge transfer involves primarily the naphthalenic S1 state in t‐NPEB and the stilbenic S2 state in t‐BPE.