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

Abstract

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[4,5i]dipyrido[3,2-a:2′,3′-c]phenazine) on the photophysics of the respective ruthenium(II) complex [(tbbpy) 2 Ru(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 3 MLCT and 3 LC states. The introduction of the methyl group increases the lifetime of the 3 LC 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.

Exploring the Excited States of a Hexa-peri-hexabenzocoronene-Substituted Dipyridophenazine Ligand and Its Metal Complexes

“…11,12 Recently, the use of an imidazolefunctionalized dppz was shown to produce long-lived emission from an imidazole to phenazine ligand-centered state with lifetimes of ≤18.3 μs. 13,14 Hexabenzocoronene (HBC) is a highly symmetric polyaromatic hydrocarbon (PAH) consisting of 42 carbons, which has the potential to act as a donating group owing to the high density of π electrons and a large bandgap. HBCs have a larger bandgap compared with those of other PAHs because of a stable resonance structure with the π electrons located on separated rings known as Clar's aromatic sextet.…”

“…Polythiophene and thiazole donors have also been shown to generate 3 ILCT states. Fused ring systems have also demonstrated charge transfer transitions with tetrathiafulvalene substituents resulting in long-lived dark ligand to ligand charge transfer (LLCT) states. , Recently, the use of an imidazole-functionalized dppz was shown to produce long-lived emission from an imidazole to phenazine ligand-centered state with lifetimes of ≤18.3 μs. , …”