Near‐Quantitative Photothermal Conversion in Non‐Fluorescent Diradicaloid Organic Molecules for Efficient Solar Energy Harvesting

Abstract Organic small molecules have emerged as promising photothermal materials for solar energy harvesting due to their structural tunability and diverse optoelectronic properties. However, achieving photothermal conversion efficiencies (PCEs) exceeding 90% in such systems remains a significant challenge, largely limited by residual fluorescence and suboptimal non‐radiative decay pathways. Here, a molecular design strategy is reported that combines inherently non‐fluorescent diradicaloid cores with electron‐donating substituents to facilitate non‐radiative decay and enhance PCE. It is demonstrated that the PCE can be effectively tuned from 64.9% (nitro‐substituted) to a near‐quantitative 94.3% (dimethylamine‐substituted). Moreover, the equilibrium temperature of dimethylamine functionalized diradicaloid can be elevated to record breaking 350 °C in organic materials under 1 W cm −2 808 nm laser, and lifted to 103 °C under one sun irradiation when loaded into polyurethane. This exceptional performance is attributed to a small energy gap, strong donor–acceptor interaction, and active molecular motion that together promote efficient vibronic relaxation and internal conversion. Furthermore, these molecules exhibit broadband absorption across 300–2000 nm, enabling a high water evaporation efficiency of 98.52% under one sun and facilitating high‐voltage output in solar thermoelectric generators. This work presents a robust design strategy for high‐efficiency organic photothermal materials, offering new opportunities for solar‐driven thermal energy harvesting and conversion technologies.