Excited states properties of some phenoxathiin derivatives

Several properties of the excited singlet state (natural lifetimes, fluores cence quantum yields, solvatochromicity, etc.) of some 3-substituted phenox athiin derivatives (methyl, formyl, acetyl) were investigated by means of a bsorption 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 p olarity on the fluorescence maxima of 3-formyl- and 3-acetylphenoxathiin, r ationalized in terms of the Lippert-Mataga equation, points at an increase in the dipole moment in the excited state. A specific behavior in protic so lvents was also noticed. AMI calculations for the ground and excited states support the experimental data and explain the higher efficiency of the car bonyl containing derivatives in terms of a different nature of the first ex cited singlet. It was established that the enhanced radiative decay for the se compounds is due to the presence of new low-lying vacant pi-molecular or bital situated between the frontier orbitals of phenoxathiin. The theoretic al results reflect also the charge transfer character of the excited states in agreement with the solvatochromicity already discussed. (C) 1999 Elsevi er Science S.A. All rights reserved.