Photochemistry of quinoxalinylpyridylethylenes

Abstract

Photophysical properties and conformational equilibrium of trans-1-(2-Quinoxalinyl)-2-(n``-pyridinyl) ethylene (2,n-QxPE or n,2-PQxE, n=2 or 3), aza-derivatives of 2-styrylnaphthalene, have been investigated under various conditions in order to explain the role of nitrogen heteroatom in the photochemistry of 2-styrylnaphthalenes. The UV absorption spectra of 2,n``-QxPE in nonpolar solvents are similar to those of 2-styrylquinoxaline (2-StQx) and 2-pyrazinylquinoxalinylethylene (2-PyQxE). However, they show somewhat different shape from the corresponding hydrocarbon (2-StN) and other aza-derivatives probably due to increase of planarity and larger induction effect of nitrogen atom in naphthalene ring than benzene ring. Fluorescence quantum yields of 2,n``-QxPE in nonpolar solvents are very low (0.001) compared to that of 2-StN suggesting that vibronic mixing between (n,$\pi^\star$) and ($\pi,\pi^\star$) states make the radiationless decay processes such as internal conversion and intersystem crossing very efficient. However, the fluorescence maxima were blue shifted in contrast to 2-StQx and other aza-analogues indicating the lowest $^1(n, \pi^\star$) state. On changing the excitation and emission wavelengths, the fluorescence emission and excitation maxima shifted probably due to the conformational equilibrium between conformers originating from the rotation of aromatic ring(s) around the quasi-single bond between the aryl and ethylenic carbon atoms. From the solvent effects and comparative study with conformationally restricted 3-Me-2,2``-QxPE and 3-Me-2,3-QxPE, the shorter-wavelength absorbing conformer is minor contributor in nonpolar solvents while the major one in polar solvents. The blue shift of fluorescence maxima as solvent polarity increases can be explained by the conformational equilibrium in the ground state. To prove the existence of distinct conformers of aza-2-styrylnaphthalene in solution, direct detecting technique can be very useful either to...