Prolonged Luminescence Lifetime of a Dual Emissive Ruthenium Dipyridophenazine-Type Complex in Aprotic and Protic Solvents

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A recently reported ruthenium(II) complex bearing an extended dipyridophenazine ligand exhibits unusual long-lived dual emission at room temperature. In this study, the effect of the introduction of a methyl protecting group to the imidazole moiety of this ligand (L1, 11-methyl-11H-imidazo[4,5-i]dipyrido[3,2-a:2′,3′-c]phenazine) on the photophysics of the respective ruthenium(II) complex [(tbbpy)₂Ru(L1)]²⁺ (C1) is demonstrated by means of electrochemistry, UV/vis absorption and emission spectroscopy, as well as emission lifetime measurements, and transient absorption spectroscopy on the nanosecond time scale. At room temperature, C1 shows dual emission both in aprotic and in protic solvents with time constants of 1.1/34.2 and 1.2/8.4 μs, respectively. These lifetimes are assigned to the emission from ³MLCT and ³LC states. The introduction of the methyl group increases the lifetime of the ³LC state in C1 almost by a factor of 2 in acetonitrile solution compared to the previously reported compound. Accordingly, the newly introduced methyl group is described as a protecting group for the imidazole moiety of the heterocyclic ligand, which enables prolonged lifetimes of the dual emissive complex in protic solvents. The stabilization of the electronic structure is further underlined by the enhanced stability toward electrochemical reduction as evidenced by cyclic voltammetry.