Unveiling photoinduced electron transfer in cobalt(iii)-R-pyridine complexes anchored to anatase nanocrystals: photoluminescence and magnetic studies

Abstract

In this study, we synthesized mixed ligand complexes of the cis-[Co(tn)₂(Rpy)Br]Br₂ type using a novel mechanochemical approach. Characterization involved spectral measurements and single crystal X-ray diffraction analysis, confirming the structure of the cis-[Co(tn)₂(4-Mepy)Br]Br₂ complex. The single crystal refinement data revealed a monoclinic crystal system with a distorted octahedral geometry. The choice of the sixth ligand influenced the emission and magnetic properties, showing a ferromagnetic character in the Co(III)-complex environment. We investigated efficient electron transfer to the cobalt(III) center using TiO₂ nanoparticles under UV-light irradiation. The adsorption characteristics of cis-[Co(tn)₂(Rpy)Br]Br₂ in aqueous 2-propanol varied, leading to surface compound formation. Under UV irradiation, the anatase surface exhibited remarkable adsorption capabilities, facilitating efficient electron transfer to the Co(III) center and resulting in a high photoefficiency for Co(II) formation. Our study has put forward a model for interfacial electron transfer (IET), taking into account the overlap between the TiO₂ conduction band and the acceptor level of the Co center, as well as the electronic coupling between the donor level of the Ti center and the acceptor level of the Co center. This model sheds light on the accumulation of electrons for reducing the adhered complex ion. The IET process was corroborated by the conversion of 2-propanol into acetone, as verified by ¹H NMR technique. Overall, our findings provide novel insights into the role of the Rpy moiety in modifying the structure of the TiO₂-cobalt(III)-Rpy compound and propose a mechanism for IET reactions, thus advancing the field.