Organic donor–acceptor cocrystals can improve the light-harvesting ability in visible or near-infrared regions, having a good photothermal conversion efficiency in some cases. While the photothermal conversion mechanism of organic cocrystals is still ambiguous. Here, the fluorescent molecule pyrene with a wide energy gap is selected as a donor, and the conjugated tetracyanide molecules are chosen as the acceptors. Pyrene and tetracyanide molecules are readily self-assembled into cocrystals (pyrene-tetracyanobenzene (PBC), pyrene-tetracyanoethylene (PEC), and pyrene-tetracyanoquinodimethane (PQC)) through intermolecular charge transfer. By changing the framework of acceptors, the optical properties of these cocrystals are tuned from photoluminescence (PBC) to photothermal conversion (PEC and PQC). PEC and PQC have an excellent photothermal conversion efficiency under near-infrared laser (808 nm) irradiation, its values can reach 80.9 and 83.3%, respectively. Based on the intermolecular interactions of cocrystals, femtosecond transient absorption, and excited-state theoretical calculation studies, the excellent photothermal conversion is attributed to the free rotation of the −C­(CN)2 group, which opens a tailormade channel for the effective nonradiative decay of the excited charge-transfer state. This study paves a way to design organic donor–acceptor cocrystal materials with high photothermal conversion efficiency.

Organic cocrystal exhibits excellent photothermal conversion (PTC), but how the intermolecular interactions of cocrystals regulate the PTC is obscure. Here, two isomeric donor molecules (phenanthrene and anthracene) and two electron-withdrawing molecules (7,7,8,8,8-tetracyanodimethylquinone and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinone dimethane) are self-assembled into the four cocrystals (PTQ, PFQ, ATQ, and AFQ). By changing the molecular configuration of the donor and the electron-withdrawing ability of the acceptor, the intrinsic influencing factors of the intermolecular interaction on the PTC were explored. Under near-infrared laser (808 nm) irradiation, the PTC efficiencies of PTQ, PFQ, AFQ, and ATQ are 35.85, 44.74, 57.00, and 60.53%, respectively. Based on the single-crystal X-ray diffraction, ultrafast time-resolved transient absorption, and excited-state theoretical calculations, we found that the π–π stacking in ATQ and AFQ is conducive to promoting the near-infrared light-harvesting ability and the p−π interaction of cocrystals can regulate the nonradiative rotation of −C­(CN)2 groups, resulting in a tunable near-infrared PTC via the isomeric cocrystals. Accordingly, the evaporation rate of the porous polyurethane-AFQ foam can reach 1.33 kg·m–2·h–1 in the simulated solar-driven water evaporation system. This work provides a strategy to boost the PTC by the intermolecular interactions of cocrystal materials.

Organic photothermal cocrystals, integrating the advantages of intrinsic organic cocrystals and the fascinating photothermal conversion ability, hold attracted considerable interest in both basic science and practical applications, involving photoacoustic imaging, seawater desalination, and photothermal therapy, and so on. However, these organic photothermal cocrystals currently suffer individual cases discovered step by step, as well as the deep and systemic investigation in the corresponding photothermal conversion mechanisms is rarely carried out, suggesting a huge challenge for their further developments. Therefore, it is urgently necessary to investigate and explore the rational design and synthesis of high‐performance organic photothermal cocrystals for future applications. This review first and systematically summarizes the organic photothermal cocrystal in terms of molecular classification, the photothermal conversion mechanism, and their corresponding applications. The timely interpretation of the cocrystal photothermal effect will provide broad prospects for the purposeful fabrication of excellent organic photothermal cocrystals toward great efficiency, low cost, and multifunctionality.

Invited for this month's cover is the group of Shun-Li Chen and Ming-De Li at the Shantou University. The image shows that one electron can be transferred easily from donor to acceptor unit to obtain integer-charge-transfer cocrystals for realizing high-efficient solar-harvesting and photothermal conversion. The Research Article itself is available at 10.1002/cssc.202300644.

Inspired by the concept of ionic charge-transfer complexes for the Mott insulator, integer-charge-transfer (integer-CT) cocrystals are designed for NIR photo-thermal conversion (PTC). With amino-styryl-pyridinium dyes and F4TCNQ (7,7',8,3,5, serving as donor/acceptor (D/A) units, integer-CT cocrystals, including amorphous stacking "salt" and segregated stacking "ionic crystal", are synthesized by mechanochemistry and solution method, respectively. Surprisingly, the integer-CT cocrystals are self-assembled only through multiple DÀ A hydrogen bonds (CÀ H•••X (X=N, F)). Strong chargetransfer interactions in cocrystals contribute to the strong light-harvesting ability at 200-1500 nm. Under 808 nm laser illumination, both the "salt" and "ionic crystal" display excellent PTC efficiency beneficial from ultrafast (~2 ps) nonradiative decay of excited states. Thus integer-CT cocrystals are potential candidates for rapid, efficient, and scalable PTC platforms. Especially amorphous "salt" with good photo/thermal stability is highly desirable in practical large-scale solar-harvesting/conversion applications in water environment. This work verifies the validity of the integer-CT cocrystal strategy, and charts a promising path to synthesize amorphous PTC materials by mechanochemical method in one-step.