Excited-State Properties of trans-1-(9-Anthryl)-2-(4-R-phenyl)ethylenes with Electron-Donating and -Accepting Substituents (R=N(CH3)2, OCH3, CH3, Br, CN, and NO2)

Article abstract of DOI:10.1021/j100145a012

The decay pathways of the lowest excited singlet state (1t*) of trans-1-(9-anthryl)-2-(4-R-phenyl)ethylenes (trans-R-StAs, where R=N(CN3)2, OCH3, CH3, Br, CN, and NO2 on the 4-position of the styryl) were studied in solution at room and low temperatures.Fluorescence lifetime (τ_f) and quantum yield (Φ_f) as well as the yield (Φ_T) and spectral and kinetic properties of the lowest triplet state were determined by steady-state and transient techniques.The solvent polarity has a large effect on the Stokes shift for (H3C)2N- and O2N-StA, the two derivatives with the strongest electron-donating and -accepting substituents.The smallest changes in Φ_f and Φ_T, in comparison with the case of parent 9-StA, are caused by the Br or CH3 groups.For O2N- and (H3C)2N-StA, and to a lesser degree for NC- and H3CO-StA, Φ_f and τ_f become significantly smaller with increasing solvent polarity.Efficient deactivation by charge transfer via an excited singlet state with essentially trans configuration (A*) is proposed for these derivatives.The 1t*->A* relaxation pathway in polar solvents is activated and competes with fluorescence and intersystem crossing.For NC-, H3CO-, and (H3C)2N-StA at room temperature trans->cis photoisomerization occurs with substantial quantum yield (Φ_t->c) in those solvents in which the presence of the postulated A*-state efficiency reduces fluorescence and intersystem crossing.A contribution of a triplet mechanism to trans->cis photoisomerization can be excluded throughout.A special case is O2N-StA, which exhibits virtually no photochemistry in both nonpolar and polar solvents, but Φ_t->c is up to 0.2 in slightly polar solvents, e.g., toluene.