Photochemistry of Aza-1,2-Diarylethylenes

Abstract

The photophysical properties and photochemistry of various 1,2-diarylethylenes which have aza-aromatics such as pyridine, pyrazine, quinoline and quinoxaline have been investigated. The photochemistry of aza-stilbenes reflects mostly the photophysical properties of aza-aromatic ring itself in the molecule. Styrylpyridine (StP) and dipyridylethylene (DPE) which have pyridine rings (PHI(f)=0.00; PHI(ic)=1.0) show internal conversion to be the major deactivation process (PHI(ic)=0.5-0.99) in the excited singlet state, and a singlet mechanism has been proposed for the direct photoisomerization. On the contrary, the major deactivation process for bispyrazinylethylene (BPyE) which has two pyrazine rings (PHI(f)=0.00; PHI(isc)=1.0) is S1-->T intersystem crossing followed by photoisomerization through the triplet state. The solvent effects on the fluorescence of azastilbenes indicate that StPs and DPEs have the lowest 1(pi, pi*) state which is extensively mixed with the 1(n, pi*) state lying just above it in polar solvents, while BPyE has the lowest 1(n, pi*) state in all the solvents. Similar results were observed in the aza analogues of 2-styrylnaphthalene such as pyrazinylquinoxalinylethylene and 2-styrylquinoline.