Characterizations of the redox orbitals of mixed-ligand 2,2′-bipyridine–2-(2′-pyridyl)quinoline ruthenium(II) complexes

Abstract

Electronic absorption spectra from the u.v. to the near-i.r. region, as well as e.s.r. spectra, of the mixed-ligand complexes [Ru(pquin)_x(bipy)_3–x]^2–n(where x= 1–3, n= 0–3, pquin = 2-(2′-pyridyl)quinoline, and bipy = 2,2′-bipyridine) are reported. Besides indicating localization of the electrochemically added electrons into separate ligands, the electronic spectra suggest the existence of π orbitals centred primarily on a quinoline portion of the pquin ligand. The absorption in the 4 000 cm^–1 region has been assigned to an interligand charge transfer which has the net effect of optically transferring a redox electron between the ligands. The e.s.r. spectra are characterized by a lack of spin interaction between the redox electrons, consistent with the localization of these electrons on separate ligands, and by a lack of hyperfine splitting. Furthermore, absence of general temperature-dependent e.s.r. line broadening, associated with a thermal electron hop from a reduced ligand to an unreduced ligand, is rationalized on the basis of donor π*–acceptor π* orbital mismatch.