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 S(2) state. Time evolution of the transient electronic spectra and the global analysis of the temporal profiles reveal a three state relaxation (S(2) → S(1) → S(0)) process in different kinds of solvents. The lifetime of the S(2) state is independent of the viscosity of solvent, whereas that of the S(1) 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 S(1) state and relaxation of the S(2) state is not associated with the isomerization process. We have also investigated the excited state dynamics of [Cu(trans-MTAI)(2)]Cl(2) 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).
