Accelerated Charge Recombination in MOF Cocrystals Enables Efficient NIR-Driven Photothermal Polymerization

Understanding charge transfer across donor-acceptor interfaces is essential for designing high-performance optoelectronic materials. Herein, we construct host-guest metal-organic framework (MOF) cocrystals employing a calcium-based naphthalenediimide (NDI) host and electron-rich aromatic guests to investigate photoinduced charge transfer at well-defined crystalline donor-acceptor interfaces. Challenging the conventional view that a high concentration of radical anions is typically associated with high photothermal conversion efficiency, our study uses electron paramagnetic resonance, surface photovoltage spectroscopy, and femtosecond transient absorption microscopy to show that a more extended conjugated plane in the guest accelerates charge recombination (charge-transfer state lifetime: 201 vs. 348 ps). This leads to a MOF cocrystal with a photothermal conversion efficiency twice that of its counterpart, despite the latter exhibiting significantly higher NDI radical concentration. Moreover, this MOF cocrystal displays good aqueous stability and retains a photothermal conversion efficiency of 65.6% in aqueous media, enabling efficient near-infrared light-driven photothermal polymerization in both organic and aqueous phases. Notably, this photopolymerization proceeds even under natural sunlight irradiation and across biological tissue barriers. This work provides a novel design principle for donor-acceptor materials, in which promoting charge recombination can be strategically exploited to develop high-performance photothermal materials for solar energy utilization.