Intramolecular exciton coupling modulates the convergent singlet-triplet energy gap toward NIR-emissive heavy-atom-free Oligo-BODIPY photosensitizers

Abstract

Heavy-atom-free triplet photosensitizers excited by long-wavelength light are essential for advancing applications in biomedicine and energy. However, the rational design of efficient heavy-atom-free triplet dyes becomes increasingly challenging as molecular engineering shifts absorption to the near-infrared (NIR) region, while still requiring retention of appreciable fluorescence. Herein, we introduce a novel strategy leveraging intramolecular exciton coupling to construct a series of covalently β,β-linked, coplanar BODIPY oligomers (dimer to tetramer), featuring multiple transition dipole moments in a head-to-tail chromophore arrangement. These architectures, lacking pyrrolic substituents, were regioselectively synthesized via a novel palladium(II)-catalyzed dehydrogenative strategy in one-pot. They exhibit intense intramolecular J-type exciton coupling (3584 cm⁻¹), not only enabling tunable absorption/emission (500–800 nm) and good fluorescence quantum yields (0.36–0.44), but also yielding significantly long-lived triplet excited states up to 143 µs, efficient reactive oxygen species (ROS) generation, and highly efficient single-photon absorption-based upconversion (SPA-UC). Theoretical calculations reveal that convergent singlet-triplet energy gaps (ΔE_S–T, from 0.849 to 0.154 eV) are key factors for enhancing the spin–orbit coupling mediated intersystem crossing (SOC-ISC), driven by reduced singlet state energies without significant triplet state perturbation. This work establishes intramolecular exciton coupling in conjugated BODIPY oligomers as a versatile strategy to design heavy-atom-free photosensitizers with simultaneous NIR emission and high triplet yields, unlocking potential in biomedicine, photocatalysis, and beyond.