Substituent Effects on the Electrochemistry and Electronic Coupling of Terphenyl-Bridged Cyclometalated Ruthenium-Amine Conjugated Complexes.

Six terphenyl-bridged cyclometalated ruthenium-amine conjugated complexes (PF)-(PF) were synthesized and studied. Three different substituents, methoxy, methyl, and chloro, were used to vary the electronic nature of the amine unit, and two terminal ligands 2,2':6',2″-terpyridine (tpy) and trimethyl-4,4',4″-tricarboxylate-2,2':6',2″-terpyridine (Metctpy) were used to tune the electronic nature of the ruthenium component. All complexes, except (PF) with the methoxy substituent and Metctpy ligand, display two well-separated redox waves in the potential range of +0.5 to +1.1 V versus Ag/AgCl. The regular electrochemical changes of these complexes help to establish the oxidation order of ruthenium and amine and hence of the direction of the electron transfer in odd-electron state. The degree of electronic coupling was estimated by analyzing the donor-to-acceptor charge transfer band in the near-infrared region obtained by oxidative spectroelectrochemical measurements. Electron paramagnetic resonance analyses and density functional theory calculations were performed on the one-electron oxidized forms to obtain information on the spin distribution of these complexes.