Photoprocesses in 4-nitro- and 2,4-dinitro-substituted trans-1-styrylnaphthalene, trans-9-styrylanthracene and related systems

The photophysical and photochemical properties of four trans-1-(R)-2-(2,4-d initrophenyl)ethylenes (R: 1-naphthyl: IIa, 9-anthryl: IIb, styryl: IIc and 4-(2',4'-dinitro)-trans-stilbenyl : IId) were studied in solution. The qua ntum yield of fluorescence (Phi(f)) is small for IIa-IId in 2-methyltetrahy drofuran (MTHF) at room temperature, but enhanced on going to -196 degrees C. Phosphorescence of singlet molecular oxygen was observed in several solv ents and its quantum yield is substantial. The properties of the triplet st ate (T-T absorption spectra and triplet lifetimes) of IIa-IId at 25 and -19 6 degrees C are described. The quantum yield of trans --> cia photoisomeriz ation (Phi(t-->c)) is small with the exception of IIc, where Phi(t-->c) app roximate to 0.3. Trans --> cis photoisomerization occurs via the triplet me chanism, but its contribution to the overall deactivation is reduced by int ramolecular electron transfer to the nitro group. The results for IIa and I Ib are compared with those of two mononitro analogues, R: l-naphthyl (Ia) a nd 9-anthryl (Ib). The Stokes shift and Phi(f) of Ia and Ib depend strongly on the solvent polarity. The effects of intramolecular charge transfer in mononitro- and dinitro-arylstyrenes are discussed. (C) 1999 Elsevier Scienc e S.A. All rights reserved.