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

Abstract

The photophysical and photochemical properties of four trans-1-(R)-2-(2,4-dinitrophenyl)ethylenes (R: 1-naphthyl: IIa, 9-anthryl: IIb, styryl: IIc and 4-(2’,4’-dinitro)- trans-stilbenyl: IId) were studied in solution. The quantum yield of fluorescence ( Φ f) is small for IIa– IId in 2-methyltetrahydrofuran (MTHF) at room temperature, but enhanced on going to −196°C. Phosphorescence of singlet molecular oxygen was observed in several solvents and its quantum yield is substantial. The properties of the triplet state (T–T absorption spectra and triplet lifetimes) of IIa– IId at 25 and −196°C are described. The quantum yield of trans → cis photoisomerization ( Φ t → c ) is small with the exception of IIc, where Φ t → c ≈ 0.3. Trans → cis photoisomerization occurs via the triplet mechanism, but its contribution to the overall deactivation is reduced by intramolecular electron transfer to the nitro group. The results for IIa and IIb are compared with those of two mononitro analogues, R: 1-naphthyl ( Ia) and 9-anthryl ( Ib). The Stokes shift and Φ f of Ia and Ib depend strongly on the solvent polarity. The effects of intramolecular charge transfer in mononitro- and dinitro-arylstyrenes are discussed.