Strongly Coupled Cyclometalated Ruthenium-Triarylamine Hybrids: Tuning Electrochemical Properties, Intervalence Charge Transfer, and Spin Distribution by Substituent Effects

Nine cyclometalated ruthenium complexes with a redox-active diphenylamine unit in the para position to the Ru-C bond were prepared. MeO, Me, and Cl substituents on the diphenylamine unit and three types of auxiliary ligands-bis(N-methylbenzimidazolyl) pyridine (Mebip), 2,2':6',2 ''-terpyridine (tpy), and trimethyl-4,4',4 ''-tricarboxylate- 2,2':6',2 ''-terpyridine (Me(3)tctpy)-were used to vary the electronic properties of these complexes. The derivative with an MeO-substituted amine unit and Me3tctpy ligand was studied by single-crystal X-ray analysis. All complexes display two well-separated redox waves in the potential region of +0.1 to +1.0 V versus Ag/AgCl, and the potential splitting ranges from 360 to 510 mV. Spectroelectrochemical measurements show that these complexes display electrochromism at low potentials and intense near-infrared (NIR) absorptions. In the one-electron oxidized form, the complex with the Cl-substituted amine unit and Mebip ligand shows a moderate ligand-to-metal charge transfer at 800 nm. The other eight complexes show asymmetric, narrow, and intense intervalence charge-transfer transitions in the NIR region, which are independent of the polarity of the solvent. The Mebip-containing complexes display rhombic or broad isotropic EPR signals, whereas the other seven complexes show relatively narrow isotropic EPR signals. In addition, DFT and time-dependent DFT studies were performed to gain insights into the spin distributions and NIR absorptions.