Terpyridine ruthenium–triarylamine asymmetrical mixed-valence systems: Syntheses, (Spectro) electrochemistry and theoretical calculations

A series of asymmetrical terpyridine ruthenium-TAA redox-active organometallic complexes was prepared and characterized. The bonding and electronic properties of these complexes were investigated by spectro-electrochemistry and theoretical calculations. Electrochemical results revealed that four complexes exhibited two successive single-electron redox processes, in which the first-step electrochemical behavior of tpyRu-ethynyl-TAA complexes occurred at the triarylamine moiety, the first-step based-metal oxidation of Cl-tpyRu-TAA complex was decided, and their half-wave potential difference ∆E decreased gradually. The near-infrared (NIR) spectroscopic results of mixed-valence states obtained by spectro-electrochemistry showed that they all have NIR multiple absorption characteristics. The maximum absorption wavelength of singly-oxidized tpyRu-ethynyl-TAA molecules redshifted with bridge conjugate extension, whereas Cl-tpyRu-TAA mixed-valence state only displayed weak absorption at high-energy region, as later confirmed by TDDFT calculation. The electron coupling parameters Hab obtained from the inter-valence charge transfer analysis of mixed-valence states by the Marcus–Hush theory decreased gradually with the extension of the bridge chain, consistent with the electrochemical results. In addition, DFT calculation reflected that the HOMO contribution in bridge chain of neutral molecules increased with the conjugate bridge extension, and the spin density distribution of mixed-valence states was mainly localized on the aryl bridge and NAr2 and featured the less metal character and hence less charge delocalization. The above results concluded that the degree of electronic coupling and the order of redox processes are greatly affected by conjugated bridge and the coordination model between terpyridine and ruthenium.