| Abstract
| - The electronic absorption and fluorescence spectra, quantum yields for fluorescence (Φf) and trans → cisphotoisomerization (Φtc), and fluorescence lifetimes of trans-4-(N-arylamino)-4‘-cyanostilbenes (2H, 2Me,2OM, 2CN, and 2Xy with aryl = phenyl, 4-methylphenyl, 4-methoxyphenyl, 4-cyanophenyl, and 2,5-dimethylphenyl, respectively), trans-4-(N-methyl-N-phenylamino)-4‘-cyanostilbene (2MP), trans-4-(N,N-diphenylamino)-4‘-cyanostilbene (2PP), trans-4-(N-methyl-N-phenylamino)-4‘-nitrostilbene (3MP), and threering-bridged analogues 2OMB, 2MPB, and 3MPB are reported. Whereas fluorescence and torsion of thecentral double bond account for the excited decay of the majority of these donor−acceptor substituted stilbenesin both nonpolar and polar solvents (i.e., Φf + 2Φtc ∼ 1), exceptions are observed for 2OM, 3MP, and3MPB in solvents more polar than THF and for 2Me and 2MP in acetonitrile as a result of the formation ofa weakly fluorescent and isomerization-free twisted intramolecular charge transfer (TICT) state (i.e., Φf +2Φtc ≪ 1). The TICT state for 2OM, 2Me, and 2MP results from the torsion of the stilbenyl-anilino C−Nsingle bond, but the torsion of the styryl-anilino C−C bond is more likely to be responsible for the TICTstate formation of 3MP and 3MPB. In conjunction with the behavior of aminostilbenes 1, a guideline basedon the values of Φf and Φtc for judging the importance of a TICT state for trans-stilbenes is provided.Accordingly, the TICT state formation is unimportant for the excited decay of trans-4-(N,N-dimethylamino)-4‘-cyanostilbene (DCS). In contrast, our results support the previously proposed TICT state for trans-4-(N,N-dimethylamino)-4‘-nitrostilbene (DNS).
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