Chiral Boron–Formazanates as a Multifunctional NIR Chiroptical Platform: Modular Synthesis, Photothermal Conversion, and Reversible Acid–Base Chiroptical Switching

Chiral organic materials with near-infrared (NIR) optical activity are highly sought after for advanced photonic and optoelectronic applications but remain rare. Here, we report two complementary types of chiral boron-formazanates: 1) mono-formazanates bearing 1,1'-bi-2-naphthol (BINOL)-derived chiral ligands at boron, and 2) bis-formazanates with embedded chiral backbones, both prepared in enantiopure form (ee > 95%) via modular synthetic routes without chiral resolution. To our knowledge, these represent the first reported examples of optically pure formazanates. This enables direct evaluation of their intrinsic optical and chiroptical properties, which include broadband NIR circular dichroism (CD) activity and, in selected cases, NIR-II fluorescence, aggregation-induced emission (AIE), NIR circularly polarized luminescence (CPL) activity beyond 800 nm, and fully reversible acid-base chiroptical switching. Mono-formazanates show high light-to-heat conversion and serve as efficient photothermal catalysts for red-light-mediated polymerization, while bis-formazanates exhibit amplified chiroptical responses via exciton-like coupling. TD-DFT calculations unveiled the mechanistic aspects of broadband Cotton effects in mono-formazanates, exciton-like coupling in bis-formazanates, and protonation-induced inversion of chiroptical signals. This work establishes chiral formazanates as a versatile molecular platform for next-generation NIR-active chiroptical materials with potential utility in bioimaging, sensing, photothermal therapy, and responsive chiroptical devices.