Controllable and Exceptionally Efficient Spin-Orbit Charge-Transfer Intersystem Crossing in Twisted π-Conjugated Perylene Bisimides for High-Performance Photochemical Applications.

Herein, we present a rationally designed heavy-atom-free photosensitizer by incorporating spin-orbit charge-transfer intersystem crossing (SOCT-ISC) characteristics into a twisted π-conjugated perylene bisimide (PBI) framework (FQAOPBI). The compound allows direct excitation of the charge transfer (CT) state, whose energy can be modulated by solvent polarity, providing controlled access to distinct photophysical pathways and promoting triplet formation. This discovery not only addresses the unpredictability of ISC efficiency in twisted π-conjugated systems but also overcomes the fundamental design constraints of SOCT-ISC, specifically regarding molecular orthogonality and energy dissipation during the CT process. The resulting photosensitizer, FQAOPBI, exhibits broad absorption (300-600 nm), higher triplet excited state energy (1.5 eV), a long-lived triplet excited state (101 µs), and an unprecedented ISC efficiency (ΦΔ = 95%). Notably, this PBI derivative was utilized for the first time in energy-transfer-based upconversion, achieving a remarkable efficiency of 13.1%. Furthermore, as a benchmark twisted π-conjugated material, FQAOPBI was shown to be a highly efficient photocatalyst for green light-driven photooxidation and atom-transfer radical polymerization. These findings are expected to inspire innovative approaches to photosensitizer design, as well as to play a significant role in promoting diverse photochemical applications.