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

in: Journal of Physical Chemistry A (2021)
Müller, Carolin; Isakov, Dajana; Rau, Sven; Dietzek, Benjamin
Ruthenium(II) complexes, like [(tbbpy)2Ru(dppz)]2+ (Rudppz; tbbpy = 4,4-di-tert-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 ð-extended Ru-dppz 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 ð-extended [(tbbpy)2Ru(L)]2+-type complexes bearing a dppz ligand with directly fused imidazole (im) and methyl-imidazole units (mim) as L. Steady-state UV−vis absorption, resonance Raman, as well as timeresolved emission and transient absorption spectroscopy reveal that Ru-im and Ru-mim show desirable properties for the application in photocatalytic processes, i.e., strong visible absorbance and two longlived 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-off.

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