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 Ru1 and Ru2 based on the dipyridophenazine ligand (L0) 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)-M-1) and thiazole (tz)-induced intraligand charge-transfer ((ILCT)-I-1) 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)-M-3 state (tau approximate to 150-400 ps) and (ii) a long-lived 3ILCT state (tau approximate to 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 tau(Em,1) approximate to 40-300 ns and tau(Em,2) approximate to 100-200 ns, which are attributed to emission from the (ILCT)-I-3 and (MLCT)-M-3 manifolds, respectively.

Automated summary

Transition-metal-based donor-acceptor systems can produce long-lived excited charge-transfer states by visible-light irradiation. The excited-state dynamics of the novel ruthenium(II) polypyridyl type complexes Ru1 and Ru2 were studied, revealing the presence of two excited-state branches with short-lived and long-lived excited states. Additionally, the complexes exhibit long-lived dual emission attributed to emission from different energy levels.