EXCITED-STATE PROPERTIES OF TRANS-1-(9-ANTHRYL)-2-(4-R-PHENYL)ETHYLENES WITH ELECTRON-DONATING AND ELECTRON-ACCEPTING SUBSTITUENTS (R=N(CH3)2, OCH3, CH3, BR, CN, AND NO2)

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 st udied 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 deter mined by steady-state and transient techniques. The solvent polarity h as a large effect on the Stokes shift for (H3C)2N- and O2N-StA, the tw o derivatives with the strongest electron-donating and -accepting subs tituents. The smallest changes in PHI(f) and PHI(T), in comparison wit h the case of parent 9-StA, are caused by the Br or CH3 groups. For O2 N- 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 pola rity. 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 a ctivated and competes with fluorescence and intersystem crossing. For NC-, H3CO-, and (H3C)2N-StA at room temperature trans --> cis photoiso merization occurs with substantial quantum yield (PHI(t --> c)) in tho se solvents in which the presence of the postulated A-state efficienc y reduces fluorescence and intersystem crossing. A contribution of a t riplet mechanism to trans --> cis photoisomerization can be excluded t hroughout. A special case is O2N-StA, which exhibits virtually no phot ochemistry in both nonpolar and polar solvents, but PHI(t --> c) is up to 0.2 in slightly polar solvents, e.g., toluene.