Organic stoichiometric cocrystals with a subtle balance of charge-transfer degree and molecular stacking towards high-efficiency NIR photothermal conversion

Charge-transfer (CT) stoichiometric cocrystals are promising choice of organic materials for unveiling the structure-property relationship. However, due to the contradiction between large CT degree required for strong NIR absorption and flexible molecular stacking, construction of stoichiomorphism-based cocystals with near-infrared (NIR) photothermal property remains challenging. Herein, the first example of stoichiomorphism-based photothermal cocrystals were accomplished through the adaptive assembly of 3,3′,5,5′-tetramethylbenzidine (TMB) donor and 1,2,4,5-tetracyanobenzene (TCNB) acceptor. The selective cocrystallization could be controlled by varying the donor-acceptor stoichiometries via a surfactant-assisted method, resulting in two cocrystals with 1:1 (T1C1) and 1:2 (T2C1) stoichiometries. The absorbance intensity of T1C1 at 808 nm was nearly twice that of T2C1, while the photothermal conversion efficiency (PCE) of the former was 60.3% ± 0.6%, approximately 80% of that for the latter (75.5% ± 2.6%), which might be caused by the different intermolecular interactions in distinct molecular stacking patterns. Notably, both excellent PCEs of stoichiometric cocrystals were attributed to the nonradiative transition process, including internal conversion and charge dissociation processes, as elucidated by femtosecond transient absorption spectroscopy measurements. Furthermore, T1C1 was used as an NIR heater for preparing agarose-based photothermal hydrogel, showing great potential for light-controlled in-situ gelation. This strategy of balancing the CT degree and molecular packing orientation not only uncovered the relationship between stoichiometric stacking and photothermal property, but also provided an opportunity to develop advanced organic optoelectricmaterials.