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]
作者:Licheng Sun、Helmut Goerner
DOI:10.1021/j100145a012
日期:1993.10
The der-ay pathways of the lowest excited singlet state (1t*) of trans-1-(9-anthryl)-2-(4-R-phenyl)ethylenes (trans-R-StAs, where R = N(CH3)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 (tau(f)) and quantum yield (PHI(f)) as well as the yield (PHI(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 PHI(f) and PHI(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, PHI(f) and tau(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 (PHI(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 PHI(t --> c) is up to 0.2 in slightly polar solvents, e.g., toluene.