Polarity dependence of the radiative and nonradiative rates of flavone derivatives comprising structurally similar amino moieties: change in the nature of the emitting state

Abstract

Photophysical behavior of several structurally related electron donor-acceptor flavone derivatives, which have been synthesized and characterized, has been studied as a function of the polarity of the media. Significant variation of the absorption and fluorescence response of the systems has been observed with change in the polarity of the medium. The results show an increase in the radiative rate constant and a decrease in the nonradiative rate constant of the systems with increase in the polarity of the media. This finding has been attributed to the change in the nature of the emitting state from a mixed n−π^∗ and π−π^∗ state to a dominant π−π^∗ state with increase in the polarity of the medium. The results of single-crystal diffraction studies and theoretical calculations based on density functional method support the idea of close proximity of the n−π^∗ and π−π^∗ states and the change in their relative contributions toward the emission process with the polarity of the medium. Laser flash photolysis studies show that the triplet state is not involved in the variation of the fluorescence response of the systems.