Photophysics of Ruthenium(II) Complexes with Thiazole π-Extended Dipyridophenazine Ligands.

Transition-metal-based donor-acceptor systems can produce long-lived excited charge-transfer states by visible-light irradiation. The novel ruthenium(II) polypyridyl type complexes and based on the dipyridophenazine ligand () directly linked to 4-hydroxythiazoles of different donor strengths were synthesized and photophysically characterized. The excited-state dynamics were investigated by femtosecond-to-nanosecond transient absorption and nanosecond emission spectroscopy complemented by time-dependent density functional theory calculations. These results indicate that photoexcitation in the visible region leads to the population of both metal-to-ligand charge-transfer (MLCT) and thiazole (tz)-induced intraligand charge-transfer (ILCT) states. Thus, the excited-state dynamics is described by two excited-state branches, namely, the population of (i) a comparably short-lived phenazine-centered MLCT state (τ ≈ 150-400 ps) and (ii) a long-lived ILCT state (τ ≈ 40-300 ns) with excess charge density localized on the phenazine and tz moieties. Notably, the ruthenium(II) complexes feature long-lived dual emission with lifetimes in the ranges τ ≈ 40-300 ns and τ ≈ 100-200 ns, which are attributed to emission from the ILCT and MLCT manifolds, respectively.