Physics

Quantum yield

Organic chemistry

Excited state

Atomic physics

Chemistry

Helicene

Photodynamic therapy

BODIPY

Photosensitizer

Molecule

Singlet state

Quantum mechanics

Intersystem crossing

Photochemistry

Triplet state

Fluorescence

Angewandte Chemie International Edition

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 =1.76×10 5 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.

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