Structural and solvent effects on the fluorescence properties of benzodihydropyranones

Abstract

Fluorescence properties of benzodihydropyranones (BDP) varied markedly with substituents and solvents. 6-Methoxy-BDP (3) showed φ_f= 0.2–0.4 while 7-methoxy-BDP was non-fluorescent in methanol. Both unsubstituted BDP (1) and 6-methyl-BDP (2) were weakly fluorescent (φ_f= 10^–3–10^–2) in methanol, but (2) had high φ_f(0.2–0.4) in aqueous methanol. The fluorescence wavenumber of (3) was decreased linearly, while φ_f increased with solvent polarity E_T(30). 2-Aryl-BDP showed a lower φ_f than 2-alkyl-BDP. Compared with BDP, substituted acetophenones had a lower φ_f by one order of magnitude. The results are discussed in terms of relative ordering of low-lying excited states, intramolecular charge transfer in S₁(π,π*), heterolytic C–O bond cleavage, meta-transmission effects of the methoxy group, etc.