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

ABSTRACT 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.