Shielding of Electron Acceptors Coordinated to Rhenium(I) by Carboxylato Groups. Intraligand and Charge-Transfer Excited-State Properties of fac-(‘Anthraquinone-2-carboxylato')(2,2′-bipyridine)tricarbonylrhenium (fac-[ReI(aq-2-CO2)(2,2′-bipy)(CO)3

Abstract

The photophysical and photochemical properties of (OC-6-33)-(2,2′-bipyridine-κN1,κN1′)tricarbonyl(9,10-dihydro-9,10-dioxoanthracene-2-carboxylato-κO)rhenium (fac-[ReI(aq-2-CO2)(2,2′-bipy)(CO)3]) were investigated and compared to those of the free ligand 9,10-dihydro-9,10-dioxoanthracene-2-carboxylate (=anthraquinone-2-carboxylate) and other carboxylato complexes containing the (2,2′-bipyridine)tricarbonylrhenium ([Re(2,2′-bipy)(CO)3]) moiety. Flash and steady-state irradiations of the anthraquinone-derived ligand (λexc 337 or 351 nm) and of its complex reveal that the photophysics of the latter is dominated by processes initiated in the Re-to-(2,2′-bipyridine) charge-transfer excited state and 2,2′-bipyridine- and (anthraquinone-2-carboxylato)-centered intraligand excited states. In the reductive quenching by N,N-diethylethanamine (TEA) or 2,2′,2″-nitrilotris[ethanol] TEOA, the reactive states are the 2,2′-bipyridine-centered and/or the charge-transfer excited states. The species with a reduced anthraquinone moiety is formed by the following intramolecular electron transfer, after the redox quenching of the excited state: [ReI(aq−2−CO2)(2,2′-bipy.)(CO)3]−⇌[ReI(aq−2−CO2.)(2,2′-bipy)(CO)3]− The photophysics, particularly the absence of a ReI-to-anthraquinone charge-transfer excited state photochemistry, is discussed in terms of the electrochemical and photochemical results.