Cobalt-Dioxolene Redox Isomers: Potential Spintronic Devices

Abstract

Cobalt-dioxolene complexes may show redox isomerism (valence tautomerism). This phenomenon is due to intramolecular electron transfer within the cobalt(III)-catecholato moiety yielding a cobalt(II)-semiquinonato species. The relative stability of the two redox isomers is determined by temperature and it is possible to stimulate the interconversion from the stable isomer to the metastable one upon irradiation at appropriate wavelengths. Since the energies of the two isomers are different, irradiation is followed by a decay process to the ground state. Theory predicts that dynamics of relaxation should be basically controlled by the energy gap between the two interconverting isomers. A series of cobalt complexes of general formula [Co(Me n tpa)(diox)]PF·solv(diox = 3,5-di-tert-butyl-1,2-dioxolene, solv = ethanol, toluene) has been prepared using as an ancillary ligand the tripod-like Me n tpa (n = 0, 1, 2, 3), derived by tris(2-pyridylmethyl)amine (tpa) by successive introduction of methyl groups into 6-position of pyridine moieties. The steric hindrance induced by this substitution modulates the redox properties of the metal acceptor, thus determining the charge distribution of the metal-dioxolene moiety at room temperature. Optically induced redox isomerism was observed by irradiating at cryogenic temperatures the Co(Me n tpa)(diox)PF complexes (n = 0, 1, 2). The different kinetics of relaxation of the photo-induced metastable phases are related to the respective free energy changes of the interconversion, as estimated by cyclic voltammetry experiments at room temperature, and to the different lattice interactions, as supported by structural data. These results show the importance of molecular techniques in the perspective of controlling the relaxation properties of photo-induced metastable species.