Excited states properties of some phenoxathiin derivatives

Abstract

Several properties of the excited singlet state (natural lifetimes, fluorescence quantum yields, solvatochromicity, etc.) of some 3-substituted phenoxathiin derivatives (methyl, formyl, acetyl) were investigated by means of absorption and steady-state fluorescence spectroscopy. The results point out that the emission properties are dependent on the type of the substituent, the most efficient being 3-acetylphenoxathiin. The effect of the solvent polarity on the fluorescence maxima of 3-formyl- and 3-acetylphenoxathiin, rationalized in terms of the Lippert–Mataga equation, points at an increase in the dipole moment in the excited state. A specific behavior in protic solvents was also noticed. AMI calculations for the ground and excited states support the experimental data and explain the higher efficiency of the carbonyl containing derivatives in terms of a different nature of the first excited singlet. It was established that the enhanced radiative decay for these compounds is due to the presence of new low-lying vacant π-molecular orbital situated between the frontier orbitals of phenoxathiin. The theoretical results reflect also the charge transfer character of the excited states in agreement with the solvatochromicity already discussed.