Deactivating effect of the pyridine n,π* states on the photoreactivity of 5-[2-(pyrid-n-yl)ethenyl]oxazole (n = 2, 3 and 4)

• The chemical and photochemical yields of isomerization and cyclization of 5-[2-(pyrid-n-yl)ethenyl]oxazoles were determined.
• The n,π* states play an important deactivating role dependent of the position (n) of the pyridine moiety.
• QM calculations helped to describe the role of conformers on photoreactivity and the isomerization/cyclization competition.
• A very high regioselectivity was found for the cyclization of one of the conformers with n = 3.
• Solvent photoaddition and formation of a second dihydrophenanthrene-type intermediate were evidenced for compound with n = 4.

This paper describes the results obtained in the study of the photobehaviour of heteroanalogs of stilbene bearing oxazole and pyridine rings at the opposite sides of the ethene moiety. The effect of the positional isomerism of the n-pyridyl group (n = 2, 3 and 4) on the competitive relaxation processes of the excited states (fluorescence, isomerization and cyclization) was investigated and compared with the behaviour previously reported for the 5-styryloxazole analogue.The group in Zagreb prepared the compounds and followed their photochemistry in preparative conditions while the group in Perugia studied the spectral properties of the trans/cis isomers and the cyclization products and measured the quantum yields of the competitive processes in mild conditions with the main aim to clarify the mechanism of the primary stages after excitation. The photobehaviour revealed an important deactivation effect of the n,π* states introduced by the nitrogen atom. This effect reflects the one reported for styrylpyridines, the analogous compounds with a phenyl instead of an oxazolyl ring. Quantum-mechanical Hyperchem calculations proved to be useful to describe the conformational equilibria and the role of conformers on photoreactivity while more refined DFT calculations on the Z isomers allowed the structure dependent competition between their isomerization/cyclization processes and the possible role of intramolecular H-bonds on the deactivation pathways to be explained. For the compound with n = 4, side processes of hydrogen shift in the primary dihydrophenanthrene-like intermediate and of solvent addition accompanying the photocyclization process were evidenced.