Two Emissive Long-Lived Excited States of an Imidazole-Functionalized Ruthenium Dipyridophenazine Complex

Abstract

A ruthenium(II) polypyridine-type complex based on the dipyridophenazine ligand with a directly fused imidazole unit (<b>L1</b>, dipyrido[3,2-<i>a</i>:2',3'-<i>c</i>]phenazine-10,11-imidazole) has been synthesized, and its electrochemical and photophysical properties have been studied. The cyclic voltammogram of [Ru(tbbpy)₂(<b>L1</b>)]²⁺ (<b>C1</b>) (tbbpy is 4,4'-<i>tert</i>-butyl-2,2'-bipyridine) shows a cathodic shift of the phenazine-based reduction process compared to similar molecules, while the first detected reduction wave (-1.34 V vs Fc/Fc⁺) is assigned to the imidazole unit within the molecule. On the basis of the TD-DFT calculations, the strong visible absorption band exhibited by <b>C1</b> is assigned to a metal-to-ligand charge transfer (MLCT) transition with a concurrent ligand-centered (LC) transition. At room-temperature, <b>C1</b> features emission (Φ = 0.04) from its lowest excited states with time constants of 1.2 and 18.3 μs. These lifetimes are assigned to emission processes from the ³MLCT and ³LC state, respectively. This is the first time that a long-lived dual emission has been observed for a ruthenium(II) complex bearing a directly fused extended π-system. Furthermore, the emission of <b>C1</b> is quenched upon water addition. In contrast to related compounds based on a dipyridophenazine ligand, the excited state energy is not shifted, and the lifetime is drastically decreased to 169 ns.