Photochemistry of aza-derivatives pf trans-2-styrylnaphthalene

Abstract

The absorption spectra of trans-2-styrylquinoxaline(StQx) conformer. Fluorescence quantum yield of StQx in various solvents are somewhat different from those of the corresponding hydrocarbon (StN) and other aza-analogues probably because of an increase in planarity of one StQx conformer. Fluorescence quantum yield of StQx in n-hexane is quite small, 0.002 indicating that vibronic mixing between (n,$\pi^{\star}$) and ($\pi$,$\pi^{\star}$) state makes the nonradiative internal conversion and intersystem crossing to be favorable. On changing the excitation and emission wavelengths, the fluorescence emission and excitation maxima in polar solvents are changed significantly possibly to an equilibrium between the two conformers originating from the rotation of the quinoxalyl group about the quasi-single bond between the quinoxalyl group and ethylenic carbon atom. MM2 force field calculation suggested that the postulated conformers are different from each other in energy and planarity and are separated by a barrier of about 4 kcal/mole. From the solvent effects and comparative study with conformationally restricted MStQx, it was verified that the conformer B which has a good planarity and the lowest ($\pi,\pi^{\star}$) state, gives weak fluorescence in n-hexane while non-planar conformer A which has the lowest (n,$\pi^{\star}$) state gives no fluorescence in nonpolar solvents. With increasing solvent polarity, the solvent induced relaxation processes become important and stabilization of ($\pi,\pi^{\star}$) state reduces vibronic coupling leading to the strong fluorescence in both conformers. Shift of conformational equilibrium to particular one conformer was also demonstrated by changing external factors such as crystal force, temperature, and solubility. The emission from crystalline StQx is independent on excitation wavelength indicating only one conformer is present. Decreasing temperature of solvent medium displayed slight equilibrium shift to more stable one (co...