Abstract
<jats:p> Developing near-infrared (NIR) photothermal materials for solar energy harvesting and biomedical technologies is highly desirable, yet achieving both outstanding conversion efficiency and long-term stability remains a critical challenge for small-molecule systems. Here, we report a homochiral binuclear iridium(III) complex featuring an electron-deficient 1,2,4,5-tetrazine, which narrows the HOMO-LUMO gap, inducing strong and exceptionally broad NIR absorption extending up to 1200 nm in solution and 1500 nm in the solid state. The intrinsic flexibility of the tetrazine bridge enables pronounced internal butterfly-like conformational dynamics of the complex in solution. These molecular vibrations couple with low-energy charge-transfer transitions, driving highly efficient non-radiative decay of the photoexcited state. In the solid phase, the competitive interplay between anion···π and lone pair···π interactions targeting the tetrazine core serves as a structural switch, locking or unlocking the "butterfly" vibrations across different crystalline solvatomorphs. This local arrangement directly dictates the macroscopic photothermal performance and operational photostability of the material. The complex achieves a record-high photothermal conversion efficiency of up to 94.9% in powder, 93.3% on cellulose paper (808 nm laser), and 68.3% (980 nm laser) in solution, alongside superior operational photostability demonstrated by FTIR, UV-Vis-NIR spectroscopy and X-ray diffraction. When applied to solar-driven water evaporation, the system delivers an impressive efficiency of 86.6% (water evaporation rate of 1.275 kg m <jats:sup>−2</jats:sup> h <jats:sup>−1</jats:sup> ). Overall, this work establishes bridging 1,2,4,5-tetrazines and crystal engineering as powerful tools for designing high-performance organometallic photothermal materials. </jats:p>