Robust organic radical cations with near-unity absorption across solar spectrum

Developing low-energy-gap materials for efficient photothermal conversion provides promising candidates for solar energy utilization. Herein, we explore the feasibility of employing robust organic radical cations as near-unity solar absorbers for practical seawater evaporation. Gram-scale organic radical cations are straightforwardly synthesized through single-electron oxidation. The open-shell structure and intervalence charge-transfer characteristics of radicals enable near-unity absorption of full solar spectral irradiance. Femtosecond transient absorption spectroscopy reveals that the intervalence charge-transfer electron relaxes non-radiatively in femtoseconds, with a rapid rate of 5.26 × 10¹²s^-1. Notably, the radical cations exhibit exceptional stability, attributed to para-position protection, spin delocalization, and frontier orbital inversion. By simply soaking cellulose paper, a highly efficient interfacial evaporation system is established. Under one sunlight irradiation, the system achieves a remarkable solar-to-vapor conversion efficiency of 97.2%. This work offers new perspectives on designing robust radical systems and developing efficient photothermal conversion materials.