Structural changes of photochromic pyrazolones were first verified by an X-ray crystallographic method directly. A tautomeric transformation between enol and keto form isomers occurs, accompanied by proton transfer during the photochromic processes.

The global freshwater scarcity crisis demands sustainable solutions aligned with circular economy principles. Solar-driven steam generation (SSG) has emerged as a promising approach to obtain freshwater from seawater or wastewater using solar energy. However, its widespread application relies on the development of energy-efficient, eco-friendly, and high-performance photothermal conversion materials. Herein, we present a sustainable strategy for converting autumn-fallen plane tree leaves into a photothermal material (AC-800) via KOH activation at 800 °C. AC-800 exhibits 91% broadband absorption (250–2500 nm). A light-absorbing layer fabricated by vacuum filtration was used for SSG tests. Under 1 sun irradiation, AC-800 achieves an evaporation rate of 1.5441 kg·m⁻²·h⁻¹ with 87.1% solar-to-vapor efficiency and a surface temperature of 48.3 °C. Ten repetitive cycles of experiments using AC-800 has demonstrated the cycling stability of SSG. Desalinated water meets World Health Organization (WHO) drinking water standards, and organic dye removal from wastewater in distilled water reaches ~100%. This low-cost, eco-friendly strategy advances sustainable SSG, with potential in seawater desalination and wastewater treatment to support circular economy objectives.