The photochemical behavior of trans-4-(N-arylamino)stilbene (1, aryl = 4-substituted phenyl) in solvents more polar than THF is strongly dependent on the substituent in the N-aryl group. This is attributed to the formation of a twisted intramolecular charge transfer (TICT) state for those with a methoxy (1OM), methoxycarbonyl (1CO), or cyano (1CN) substituent but not for those with a methyl (1Me), hydrogen (1H), chloro (1Cl), or trifluoromethyl (1CF) substituent. On the basis of the ring-bridged model compounds 3-6, the TICT states for 1CN and 1CO result from the twisting of the anilino-benzonitrilo C-N bond, but for 1OM it is from the twisting of the stilbenyl-anilino C-N bond, both of which are distinct from the TICT states previously proposed for N,N-dimethylaminostilbenes.

The synthesis, crystal structure, and fluorescence behavior of acetylene-bridged pentiptycene dimer (2), trimer (3), and tetramer (4) are reported. For comparison, a phenylene-pentiptycene-phenylene three-ring system (5) is also investigated. As a result of the unique intrachain pentiptycene-pentiptycene interactions in 3 and 4, their twisted conformers are populated in polar solvents and at low temperatures, and the phenomenon of nonequilibration of excited rotational conformers is observed. Twisting of the pi-conjugated backbones leads to blue-shifted absorption and fluorescence spectra and increased fluorescence quantum yields and lifetimes. The fluorescence spectra of 2-4 undergo small red shifts but large intensity variations in the 0-1 vs 0-0 bands on going from solutions to thin solid films, which can be accounted for by the reabsorption effect. However, the reduction in fluorescence quantum yields for 2-4 in films vs solutions is mainly attributed to efficient interchain exciton migration to nonfluorescent energy traps. In contrast, the behavior of nonequilibration of excited rotamers is not observed for 5 in solutions. Compound 5 forms J-type aggregates through terminal phenylene pi-stackings in the solid state, resulting in a new absorption band at 377 nm and large red shifts of the structured fluorescence spectra.

The electronic absorption and fluorescence spectra, quantum yields for fluorescence (Phi(f)) and trans --> cis photoisomerization (Phi(tc)), 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 three ring-bridged analogues 2OMB, 2MPB, and 3MPB are reported. Whereas fluorescence and torsion of the central double bond account for the excited decay of the majority of these donor-acceptor substituted stilbenes in both nonpolar and polar solvents (i.e., Phi(f) + 2Phi(tc) approximately 1), exceptions are observed for 2OM, 3MP, and 3MPB in solvents more polar than THF and for 2Me and 2MP in acetonitrile as a result of the formation of a weakly fluorescent and isomerization-free twisted intramolecular charge transfer (TICT) state (i.e., Phi(f) + 2Phi(tc) << 1). The TICT state for 2OM, 2Me, and 2MP results from the torsion of the stilbenyl-anilino C-N single bond, but the torsion of the styryl-anilino C-C bond is more likely to be responsible for the TICT state formation of 3MP and 3MPB. In conjunction with the behavior of aminostilbenes 1, a guideline based on the values of Phi(f) and Phi(tc) 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).