Synergistic Linkage Engineering in Donor-Acceptor Covalent Organic Frameworks for NIR-II Absorption and Efficient...

Covalent organic frameworks (COFs) are emerging crystalline porous polymers with precisely defined structures and tunable optoelectronic properties. However, developing COFs that simultaneously achieve NIR-II light absorption and controlled excited-state dynamics remains challenging, limiting their applications in advanced photothermal energy conversion. Herein, we report a strategy that synergistically integrates linkage engineering and donor-acceptor modulation to intrinsically regulate the photophysical behavior of COFs, as demonstrated by the synthesis of a novel donor-acceptor azo-linked COF (Azo-COF-DA). Compared with its imine-linked analogue (Im-COF-DA), Azo-COF-DA exhibits remarkable NIR-II absorption (1500 nm vs 650 nm) and a significantly narrowed bandgap (1.16 eV vs 2.34 eV), highlighting the critical role of azo linkages in maximizing electronic delocalization. Comprehensive spectroscopic and theoretical analyses reveal significantly enhanced vibronic coupling in Azo-COF-DA upon photoexcitation, thereby promoting efficient nonradiative relaxation. Consequently, Azo-COF-DA achieves a temperature increase 10.8-fold higher than that of Im-COF-DA, translating this superior photothermal conversion into a record COF-based thermoelectric output voltage of 262 mV under one-sun irradiation. Furthermore, an integrated solar-driven water evaporation-thermoelectric generator simultaneously achieves a water evaporation rate of 1.44 kg m-2 h-1 and a voltage output of 116 mV. This work reveals the importance of linkage replacement in governing π-electronic delocalization and excited-state relaxation, providing transferable structural design guidelines for tailoring excited-state dynamics in porous organic materials toward high-performance NIR-II photothermal applications.