Tuning Donor–Acceptor Stacking in MOFs via Rational Metal Coordination for Enhanced Photothermal Conversion and Solar-Driven Water Evaporation Performance

The photothermal conversion efficiency of metal-organic frameworks (MOFs) is closely correlated with the stacking arrangement of organic ligands. Herein, we systematically investigate two anisostructural MOFs, TTFDPNI-Cd-MOF and TTFDPNI-Co-MOF, constructed from identical redox-active ligands-tetrathiafulvalene-tetrabenzoate (H4TTFTB, donor) and 2,7-di(4-pyridyl)benzo[lmn][3,8]phenanthroline-1,3,6,8-tetraone (DPNI, acceptor)─but distinct metal nodes (Cd2+ versus Co2+). Single-crystal X-ray diffraction analysis reveals that the hexacoordinate Cd2+ center in TTFDPNI-Cd-MOF induces optimal face-to-face π-π stacking between donor (TTF) and acceptor (NDI) moieties, facilitating strong intermolecular charge transfer (CT) interactions. In contrast, the hexacoordinate Co2+ in TTFDPNI-Co-MOF promotes segregated donor-donor (D-D) stacking configurations. Spectroscopic and photothermal characterization demonstrates that the enhanced CT interactions in TTFDPNI-Cd-MOF lead to (i) broader near-infrared absorption (700-1000 nm), (ii) superior photothermal conversion efficiency (47.5% vs 39.4% for Co-MOF), reaching 254 °C under 808 nm laser irradiation (0.7 W cm-2), and (iii) exceptional solar-driven water evaporation performance (96.8% efficiency, 1.78 kg m-2 h-1 under 1 sun illumination). These findings establish a clear structure-property relationship, demonstrating that D-A stacking is an effective strategy for optimizing CT-mediated photothermal processes in MOFs-based materials.