Photobehaviour of some 1-heteroaryl-2-(1-methylpyridinium-2-yl)ethene iodides (free and complexed with DNA)

The fluorescence emission and the photoreactivity of three trans-1-heteroaryl-2-(1-methylpyridinium-2-yl)ethene iodides, where the heteroaryl group is thiophen-2-yl, 5-bromothiophen-2-yl or 1-methylpyrrol-2-yl, have been investigated in water and acetonitrile (MeCN). The main relaxation pathway of the lowest excited singlet states of these compounds leads to trans → cis photoisomerization with high quantum yields, particularly for the two thiophen derivatives. The intervention of a thermal isomerization, important only above room temperature, was observed in acetonitrile. The yield of the radiative deactivation is very small and becomes substantial in rigid matrices at low temperature only. Preliminary experiments on the binding of these pyridinium salts to DNA and on its effect on the photoreactivity showed a modest binding affinity to DNA with formation of complexes which did not affect significantly the photoisomerization process.