Elucidation of the Intersystem Crossing Mechanism in a Helical BODIPY for Low‐Dose Photodynamic Therapy

Abstract

Intersystem crossing (ISC) of triplet photosensitizers is a vital process for fundamental photochemistry and photodynamic therapy (PDT). Herein, we report the co-existence of efficient ISC and long triplet excited lifetime in a heavy atom-free bodipy helicene molecule. Via theoretical computation and time-resolved EPR spectroscopy, we confirmed that the ISC of the bodipy results from its twisted molecular structure and reduced symmetry. The twisted bodipy shows intense long wavelength absorption (ϵ=1.76×10⁵ m^-1 cm^-1 at 630 nm), satisfactory triplet quantum yield (Φ_T =52 %), and long-lived triplet state (τ_T =492 μs), leading to unprecedented performance as a triplet photosensitizer for PDT. Moreover, nanoparticles constructed with such helical bodipy show efficient PDT-mediated antitumor immunity amplification with an ultra-low dose (0.25 μg kg^-1 ), which is several hundred times lower than that of the existing PDT reagents.