Photochemistry and DNA-affinity of some stilbene and distyrylbenzene analogues containing pyridinium and imidazolium iodides

The relaxation properties of the excited states of some trans-1,2-diarylethene analogues (where one aryl group is a methylpyridinium or dimethylimidazolinium group and the other one is a π-excessive furyl or pyrrolyl group) and two all-trans-distyrylbenzene analogues (where the central ring is a methylpyridinium group and the side rings are furyl or methylpyrrolyl groups) have been investigated in buffered (pH 7) aqueous solutions. The compounds of the diarylethene series generally undergo efficient trans → cis photoisomerization while the yield of the radiative deactivation is very small at room temperature. The corresponding distyrylbenzenes display small yields of radiative/reactive pathways and mainly deactivate by internal conversion. The solvent effect on the spectral behaviour indicates the occurring of intramolecular charge transfer (“push–pull” compounds) which can induce interesting non-linear optical properties.Some experiments on the interactions with DNA, which might affect the cell metabolism, showed a modest binding affinity for the compounds with one ethenic bond and two aromatic rings. The complexation constant increases substantially in compounds with two ethenic bonds (three aromatic rings) and in the halogen-substituted compounds. The formation of ligand–DNA complexes affects only slightly the competition of the radiative/reactive relaxation from the lowest excited singlet state.