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)­(Me3tctpy)]2+, and [Ru­(Cl-L1)­(Me3tctpy)]2+ is oxidized prior to the RuIII/II process, and the one-electron-oxidized forms of these complexes
show intense RuII → 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)­(Me3tctpy)]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.