Click-Functionalizable Tetrathiafulvalene Tetrathiolate Diradicaloids as Modular NIR-II Fluorophores

Near-infrared (NIR) emissive fluorophores are appealing for biological imaging and sensing applications, particularly in the lower-energy NIR-II region (1000-1700 nm). Compared to light in the visible and NIR-I regions, NIR-II light undergoes less absorption and scattering by biological tissues, enabling deeper tissue-penetration and higher imaging resolution. Tetrathiafulvalene-tetrathiolate (TTFtt)-bridged bimetallic diradicaloid complexes exhibit bright NIR-II emission with high photoluminescence quantum yields (PLQYs) relative to other NIR-II fluorophores. These complexes are also particularly modular, as the capping ligands on the metal centers can be substituted to impart desired properties. Here, azide-functionalized bis(imino)acenaphthene (BIAN) capping ligands are introduced to leverage and expand this modularity. BIAN-capped TTFtt complexes exhibit chemical and photophysical properties comparable with previously reported TTFtt complexes. BIAN backbones functionalized with an azide group further enable Huisgen cycloaddition "click" chemistry for the attachment of diverse functional moieties while preserving the intrinsic NIR-II photophysical properties of the TTFtt core. As one example, a sulfonate clicked TTFtt complex (1-C3) displays enhanced solubility and increased photoluminescence (PL) intensity in aqueous solutions compared to other TTFtt complexes. Notably, 1-C3 also exhibits pH-dependent PL intensity, suggesting its potential utility as an NIR-II-emitting pH sensor. Overall, these results demonstrate the dramatically expanded modularity of TTFtt complexes through the use of clickable-BIAN ligands.