Probing the Noninnocent π‑Bonding Influence of N‑Carboxyamidoquinolate Ligands on the Light Harvesting and Redox Properties of Ruthenium Polypyridyl Complexes

Electronic and photophysical characterization is presented for a series of bis-heteroleptic [Ru­(bpy)2(R-CAQN)]+ complexes where CAQN is a bidentate N-(carboxyaryl)­amidoquinolate ligand and the aryl substituent R = p-tolyl, p-fluorobenzene, p-trifluoromethylbenzene, 3,5-bis­(trifluoromethyl)­benzene, or 4-methoxy-2,3,5,6-tetrafluorobenzene. Characterized by a strong noninnocent Ru­(dπ)–CAQN­(π) bonding interaction, density functional theory (DFT) analysis is used to estimate the contribution of both atomic Ru­(dπ) and ligand CAQN­(π) manifolds to the frontier molecular orbitals of these complexes. UV–vis absorption and emission studies are presented where the noninnocent Ru­(dπ)–CAQN­(π) bonding scheme plays a major role in defining complex electronic and photophysical properties. Oxidation potentials are tuned over a range of 0.92 V with respect to the [Ru­(bpy)3]2+ reference system, hereafter referred to as 12+, by varying the degree of R-CAQN fluorination while maintaining consistently strong and panchromatic visible absorption properties. Electron paramagnetic resonance (EPR) spectroscopy is employed to experimentally map delocalization of the unpaired electron/electron–hole within the delocalized Ru­(dπ)–CAQN­(π) singly occupied valence molecular orbital of the one-electron oxidized complexes. EPR data is complemented experimentally by UV–vis–NIR spectroelectrochemistry, and computationally by molecular orbital Mulliken contributions and spin-density analysis. It is ultimately demonstrated that the CAQN ligand framework provides a simple yet broad synthetic platform in the design of redox-active transition metal chromophores with a range of electronic and spectroscopic characteristics hinting at the diversity and potential of these complexes toward photochemical and catalytic applications.