Photoisomerization dynamics of N-1-methyl-2-(tolylazo) imidazole and the effect of complexation with Cu(ii)

Abstract

Azo-compounds containing an imidazole moiety have the potential to photoregulate biofunctions, such as gene-expression and enzymatic action. Photoinduced isomerization of the azo-backbone is the vital process for such applications, but the photoisomerization dynamics of azo-imidazole compounds has not been well explored. We investigated the photoisomerization dynamics of trans-N-1-methyl-2-(tolylazo) imidazole (trans-MTAI) using femtosecond transient absorption spectroscopy following photoexcitation to the S₂ state. Time evolution of the transient electronic spectra and the global analysis of the temporal profiles reveal a three state relaxation (S₂ → S₁ → S₀) process in different kinds of solvents. The lifetime of the S₂ state is independent of the viscosity of solvent, whereas that of the S₁ state becomes longer with increasing solvent viscosity. This observation clearly indicates that the large amplitude motion that leads to the trans → cis isomerization occurs only in the S₁ state and relaxation of the S₂ state is not associated with the isomerization process. We have also investigated the excited state dynamics of [Cu(trans-MTAI)₂]Cl₂ to examine the effect of complexation with the metal ion on the isomerization dynamics of trans-MTAI. It is observed that the photoinduced isomerization of the azo-backbone in trans-MTAI is completely inhibited upon complexation with Cu(II).