Targeted Synthesis of NIR Luminescent Rhenium Diimine cis,trans-[Re( NN )(CO)2 (L)2 ]n+ Complexes Containing N-Donor Axial Ligands: Photophysical, Electrochemical, and Theoretical Studies

Abstract

The combined action of ultraviolet irradiation and microwave heating onto acetonitrile solution of [Re( $NN$ )(CO)₃ (NCMe)]OTf ( $NN$ =phenantroline and neocuproine) afforded cis,trans-Re( $NN$ )(CO)₂ (NCMe)₂ ]⁺ acetonitrile derivatives. Substitution of relatively labile NCMe with a series of aromatic N-donor ligands (pyridine, pyrazine, 4,4'-bipyridine, N-methyl-4,4'-bipyridine) gave a novel family of the diimine cis,trans-[Re( $NN$ )(CO)₂ (L)₂ ]⁺ complexes. Photophysical studies of the obtained compounds in solution revealed unusually high absorption across the visible region and NIR phosphorescence with emission band maxima ranging from 711 to 805 nm. The nature of emissive excited states was studied using DFT calculations to show dominant contribution of ³ MLCT (dπ(Re)→π*( $NN$ )) character. Electrochemical (CV and DPV) studies of the monocationic diimine complexes revealed one reduction and one oxidation wave assigned to reduction of the diimine moiety and oxidation of the rhenium center, respectively.

Keywords: Electrochemistry; N-donor ligands; NIR emission; photophysics; rhenium complexes.