Neutral d8 metal complexes with intervalence charge-transfer transition trigger an effective NIR-II photothermal conversion for solar-driven desalination

Abstract

Solar steam generation (SSG) has been a promising solution to water scarcity by evaporating seawater at a low cost and in an eco-friendly way. However, small molecule-based photothermal materials utilized for SSG interfacial layers have been limited owing to their narrow absorption and low solar-thermal conversion. In this work, three neutral d⁸ transition-metal (nickel, palladium, and platinum) bis(dithiolene) complexes with an intervalence charge-transfer that induces the second near-infrared (NIR-II) absorption and square-planar geometries favoring intermolecular stacking were synthesized and identified. The metal complex-adsorbed SSG interfacial layer exhibits broad absorption from visible to NIR-II regions for effective solar-light harvesting as well as a high photothermal conversion temperature ranging 155–186 °C under 1064 nm of laser irradiation for 16 s via advancing nonradiative heat release. Among them, the best solar energy-to-vapor conversion efficiency of 95.64% and a water evaporation rate of 1.406 kg m⁻² h⁻¹ under one sun irradiation were achieved for nickel complex-based SSG devices. This study provides a strategy for designing solar-thermal conversion materials based on NIR-II organometallic complexes and further successfully advancing them into high-performing solar-driven evaporation applications.