Computational Investigation of salicylaldimine-based transition metal (Cu, Co, and Ni) Complexes for Photocatalytic Renewable Energy Conversion and Pollutant Degradation

Abstract The development of efficient and sustainable photocatalysts for solar energy conversion and environmental remediation remains a major scientific challenge. In this work, a comprehensive computational investigation of salicylaldimine-based transition metal complexes (Cu, Co, and Ni) was carried out to evaluate their potential for photocatalytic applications, particularly CO₂ reduction and renewable energy conversion. Density functional theory (DFT) was employed to optimize the geometries and analyze electronic structures, while time-dependent DFT (TD-DFT) was used to elucidate the excited-state properties and UV–visible absorption behavior. The calculated structural parameters reveal strong metal–ligand interactions, with metal substitution significantly influencing coordination geometry and electronic distribution. Frontier molecular orbital analysis shows that the [Co(salen) 2 ] complex possesses the most favorable HOMO–LUMO alignment, facilitating efficient charge separation. TD-DFT results indicate that the Co–salicylaldimine complex exhibits broad and intense visible-light absorption arising from metal-to-ligand charge transfer transitions, whereas the Cu and Ni analogues display predominantly UV-localized absorption. Light harvesting efficiency (LHE) calculations further confirm the superior photon absorption capability of the Co complex in the visible region. Redox thermochemistry and energy conversion (Ec) analysis demonstrate that only the [Co(salen) 2 ] -based complex provides sufficient excited-state reducing power to drive key CO₂ reduction pathways, including the formation of CO, HCOOH, and CH₃OH. The combined photochemical, electronic, and thermodynamic results establish the following photocatalytic performance trend: [Co(salen) 2 ] > [Ni(salen) 2 ] > [Cu(salen) 2 ]. This study highlights the critical role of metal-center selection in tuning the optoelectronic properties of Schiff base complexes and provides molecular-level insights for the rational design of efficient visible-light-driven photocatalysts.