Photophysics of Ru(II) Dyads Derived from Pyrenyl-Substitued Imidazo[4,5-f][1,10]phenanthroline Ligands

The photophysicS of a Series of Ru(II) dyads based on the 2-(1-pyrerry1)-IH-imidazo[42,5-f][1,10]-phenanthroline ligand was investigated. The ability of these metal coinplexes to intercalate DNA and induce cell death upon photoactivation makes them attractive photosensitizers for a range of photobiological applications, Including photodynamic therapy. In the present study, time-resolved transient absorption and emission spectroscopy were used to Interrogate the photoinduced processes that follow metal-to-ligand charge transfer excitation of the complexes in solution. It Was found that energy transfer to pyrene-localized intraligand triplet states, facilitated by torsional motion of the pyrene moiety relative to the imidazo[4,5-f] [1,10]phenanthroline ligand was an important relaxation pathway governing the photophysical dynamics RI this class of compounds. Biphasic decay kinetics were assigned to spontaneous (pre-equilibrium) and delayed emission, arising from an equilibrium established between (MLCT)-M-3 and (IL)-I-3 states. TDDFT calculations supported these interpretations.