Fully Conjugated Thiophene-Fused Oligo-BODIPYs: A Class of Intensely Near-Infrared Absorbing, Arc-Shaped Materials with up to 31 Linearly-Fused Rings

Structurally well-defined large π-conjugated systems attract significant interest in molecular materials both for their unique electronic/photophysical properties and unexplored structure-property relationships arising from synthetic challenges. Herein, we address this challenge by leveraging a series of polycondensed π-system doping with B, N and S heteroatoms. In our approach, a series of fully conjugated thiophene-fused oligo-BODIPYs with atomic precision have been efficiently synthesized through the combination of intermolecular S_NAr reactions followed by intramolecular aromatic oxidative couplings from halogenated BODIPY precursors. The largest architecture is a fully fused BODIPY octamer, featuring a coplanar backbone of 31 linearly fused rings. The extended π-conjugation causes a dramatic shift of the absorption event from about 500 nm (monomer) to 822 nm (octamer) with extremely high molar absorptivities reaching 800,000 M^-1 cm^-1, as well as maintaining intense fluorescence intensity (Φ_FL up to 0.32), long triplet lifetime (τ_T = 0.61-15.4 μs), efficient triplet quantum yields (Φ_T = 0.24-0.81) and good singlet oxygen generation abilities. More interestingly, due to the weak aromaticity of thiophene, oligo-BODIPYs exhibit triplet state localization as their conjugation length increases, where the triplet energy remains constant while the singlet energy decreases significantly. Notably, intense near-infrared thermally activated delayed fluorescence (TADF) is observed even in tetramers, hexamers, and octamers. Our findings not only present a new series of heteroatom-doped condensed π-systems but also establish a precise regulation mechanism for singlet-triplet energy levels in molecules with large rigid π-conjugated structures. Furthermore, this work provides a novel strategy for designing next-generation TADF molecules with narrowband emission.