Bis-tridentate ruthenium complexes with a redox-active amine substituent: Electrochemical, spectroscopic, and DFT/TDDFT studies

Seven bis-tridentate ruthenium complexes with a general formula of [Ru(N-L1)(L2)]²⁺ (the anions are PF₆^-) have been prepared, where N-L1 is a 2,2':6',2′-terpyridine (tpy) derivative with a redox-active di-p-anisylamino (MeO-L1), di-p-tolylamino (Me-L1), or bis(p-chlorophenyl)amino (Cl-L1) substituent and L2 is tpy, bis(N- methylbenzimidazolyl)pyridine (Mebip), or trimethyl 2,2':6',2′- terpyridine-4,4',4′-tricarboxylate (Me₃tctpy). The electrochemical results suggest that the amine segment of [Ru(MeO-L1)(tpy)] ²⁺, [Ru(MeO-L1)(Mebip)]²⁺, [Ru(MeO-L1)(Me ₃tctpy)]²⁺, and [Ru(Cl-L1)(Me₃tctpy)] ²⁺ is oxidized prior to the Ru^III/II process, and the one-electron-oxidized forms of these complexes show intense Ru^II → N^•+ metal-to-ligand charge transfer (CT) transitions around 1200-1500 nm. In contrast, the ruthenium ion is more easily oxidized than the amine segment in [Ru(Cl-L1)(Mebip)]²⁺ and only weak ligand-to-metal CT transitions are observed after one-electron oxidation. The one-electron-oxidation processes of [Ru(Me-L1)(tpy)]²⁺ and [Ru(Cl-L1)(tpy)]²⁺ are possibly associated with both the amine segment and ruthenium component. The substitution of the amine group expands the visible absorptions of these complexes, as a result of the presence of additional intraligand CT transitions. The complex [Ru(Cl-L1)(Me ₃tctpy)]²⁺ with multiple electron-withdrawing substituents emits at 732 nm with a quantum yield of 1.3%. DFT calculations have been performed to provide information on the frontier orbital energy alignment and spin distributions of one-electron-oxidized forms. The absorption spectra have been rationalized with the aid of TDDFT calculations.