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 (Me(3)tctpy). The electrochemical results suggest that the amine segment of [Ru(MeO-L1)(tpy)](2+), [Ru(MeO-L1)-(Mebip)](2+), [Ru(MeO-L1) (Me(3)tctpy)](2+), and [Ru(Cl-L1)(Me(3)tctpy)](2+) is oxidized prior to the Ru-III/II in process, and the one-electron-oxidized forms of these complexes show intense R-II -> N center dot+ 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(3)tctpy)](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.