The photoluminescence of some β-nitrostyrenes. Excited state energies in unsaturated nitro-compounds

Abstract

The luminescences of various β-nitrostyrenes, XC₆H₄CHC(R)NO₂, where X = H,Cl, OCH₃, or N(CH₃)₂and R = H or CH₃, are reported. When X = H, Cl, or OCH₃ only phosphorescence is observed in glassy media at 77K whereas when X = N(CH₃)₂ only a fluorescence, strongly dependent in intensity on solvent polarity and temperature, is detectable. An interpretation is given supported by PPP–SCF–MO–Cl calculations. It is argued that the singlet and triplet n,π* states are located around 2.95 and 2.50 eV respectively in every case and that the energy of the intramolecular charge-transfer π,π* excited singlet state, which is sensitive to substituents and solvent polarity, determines the resulting photophysical behaviour.