Promoting Nonradiative Decay in α-Cyanopyrrolopyrroles via Bromine-Atom Engineering for Solar Thermoelectric Conversion Applications

Organic photothermal materials have attracted considerable attention in water purification, desalination, and thermoelectric power generation. The radiative decay process critically competes with the dominant nonradiative decay pathways, impairing photothermal performance and limiting their utility in solar–thermal conversion. Herein, by engineering the number and spatial position of Br atoms, a series of α-cyanopyrrolopyrrole-based derivatives (0Br, 1Br(X), 2Br(2X), 2Br(2Y), 3Br(1X-2Y), and 4Br(2X-2Y)) were designed and synthesized. These compounds display extremely weak or no emission characteristics both in solution and in the solid state, which is helpful for converting light energy into heat. The study on structure–property relationships revealed that the presence of Br substituents could efficiently enhance the absorptivity and lower the energy gap of Br-containing molecules, leading to higher photothermal conversion efficiency (PCE) than that of the unbrominated analogue 0Br. In particular, 1Br(X) had the highest PCE of 31.2% under 808 nm irradiation, which was 2.35-fold higher than that of 0Br due to the combined outcome of the heavy-atom effect and enhanced intramolecular charge transfer effect. Additionally, when they were applied in solar-driven water purification, the water evaporation rate of 1.344 kg m–2 h–1, solar-to-vapor efficiency of 92.2%, and voltage output (190 mV) of 1Br(X) were improved to 1.36, 1.35, and 1.28 times over those of 0Br under 1 sun (1.0 kW m–2) exposure. Moreover, 1Br(X) demonstrated excellent capability for water purification from seawater and simulated wastewater. This work provides a simple and effective strategy for the rational design of high-performance organic photothermal materials for diverse applications.