Influence of the Protonation State on the Excited-State Dynamics of Ruthenium(II) Complexes with Imidazole π-Extended Dipyridophenazine Ligands

Abstract

Ruthenium(II) complexes, like [(tbbpy)₂Ru(dppz)]²⁺ (<b>Ru-dppz</b>; tbbpy = 4,4'-di-<i>tert</i>-butyl-2,2'-bipyridine, dppz = dipyrido-[3,2-a:2',3'-c]phenazine), have emerged as suitable photosensitizers in photoredox catalysis. Since then, there has been ongoing interest in the design of π-extended <b>Ru-dppz</b> systems with red-shifted visible absorption maxima and sufficiently long-lived excited states independent of the solvent or pH value. Herein, we explore the photophysical properties of protonation isomers of the linearly π-extended [(tbbpy)₂Ru(<b>L</b>)]²⁺-type complexes bearing a dppz ligand with directly fused imidazole (<b>im</b>) and methyl-imidazole units (<b>mim</b>) as <b>L</b>. Steady-state UV-vis absorption, resonance Raman, as well as time-resolved emission and transient absorption spectroscopy reveal that <b>Ru-im</b> and <b>Ru-mim</b> show desirable properties for the application in photocatalytic processes, <i>i.e</i>., strong visible absorbance and two long-lived excited states in the ³ILCT and ³MLCT manifold, at pH values between 3 and 12. However, protonation of the (methyl-)imidazole unit at pH ≤ 2 unit causes decreased excited-state lifetimes and an emission switch-<i>off.</i>