Photochemistry of styrylquinoxaline

Abstract

Photophysical and photochemical properties and conformational equilibrium of styrylquinoxaline(StQx), an aza-analogue of 2-styrylanaphthalene, have been investigated under various conditions in order to elucidate the role of nitrogen heteroatom in the photochemistry of 2-styrylnaphthalene. Fluorescence quantum yields of StQx in nonpolar solvents are very low compared to that of corresponding hydrocarbon due to the presence of (n,$\pi^{\ast}$) state which can lead to rapid radiationess decay processes such as internal conversion and/or intersystem crossing. However, the fluorescence quantum yields increases drastically with increasing solvent polarity. This behavior is related to non-radiative processes becoming less important because of a weakening of the vibronic coupling between the lowest($\pi,\pi^{\ast}$) state and (n,$\pi^{\ast}$) state in polar solvents. The fluorescence spectra and quantum yields vary on changing the excitationwavelength. Fluorescence of StQx decays bi-exponentially which is related to the equilibrium between two conformers originating from the rotation of the quinoxaline ring around the quasi-single bond between the quinoxaline ring and ethylenic carbon atom. Two isoenergetic conformers for StQx are expected from semiempirical molecular orbital calculations(AM1 and CNDO/SCI). The photophysical properties of two conformers of StQx are estimated by deconvolution analysis of StQx fluorescence decay curve. The Photophysical properties of conformationally restricted analogues agree with those of two conformers. The direct trans$\rightarrow$cis photoisomerization of StQx proceeds through both the singlet and triplet excited manifolds, mainly via the triplet manifold at room temperature. The quantum yields of photoismerization decrease with increasing solvent polarity in contrast to that of fluorescence. Laser flash photolysis studies suggest that the decrease of intersystem crossing rate in polar solvents leads to inefficient photoisomerizati...