Tailormade Nonradiative Rotation Tuning of the Near-Infrared Photothermal Conversion in Donor–Acceptor Cocrystals

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.