Chemical and photophysical properties of amine functionalized bis‐NHC‐pyridine‐RuII complexes

Amine substitution in the backbone of a room temperature (r. t.) luminescent C^N^C ruthenium(II) complex alters the excited energy landscape such that at low temperature (l.t.) the typical luminescent relaxation of the ³MLCT (metal-to-ligand charge transfer) to the ¹GS (ground state) can occur, but at r. t. a dark relaxation pathway across the ³MC (metal centered) state is activated and a higher photostability is observed.

Abstract

The effects of backbone amine functionalization in three new homoleptic C^N^C type ruthenium(II) complexes bearing a tridentate bis-imidazole-2-ylidene pyridine ligand framework are characterized and studied by single crystal diffraction, electrochemistry, optical spectroscopy and transient absorption spectroscopy in combination with ab initio DFT calculations. Functionalization by dimethylamine groups in 4-position of the pyridine backbone significantly influences the properties of the complexes as revealed by comparison with the unfunctionalized references. As a result of the amine functionalization, a higher molar absorption coefficient of the MLCT bands, a decreased photoluminescence quantum yield at room temperature together with a shortened excited state lifetime but an improved photostability is observed. Introduction of electron donating and withdrawing groups at the NHC unit modifies the electronic and optical properties, such as the oxidation potential, absorption and emission properties, and the lifetimes of the excited states.

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