Metal-Free Triplet Photosensitizers via Arene–BODIPY Charge Transfer and Iodine-Induced Spin Conversion

BODIPY derivatives are promising scaffolds for triplet photosensitizers because of their strong absorption and tunable absorption range, but their dominant fluorescence decay and weak spin–orbit coupling hinder efficient intersystem crossing (ISC). Here, we report iodine-substituted BODIPY photosensitizers bearing anthracene, pyrene, and perylene units (BI-Ant, BI-Pyr, and BI-Per), designed to combine the heavy-atom effect with the spin–orbit charge-transfer ISC (SOCT-ISC) process. Spectroscopic and computational analyses revealed charge-transfer character and efficient triplet formation in all three compounds. Transient absorption measurements showed that BI-Ant and BI-Pyr undergo an ISC process to BODIPY-centered triplet state, whereas BI-Per exhibits intermediate strong charge-transfer-state evolution and the fastest ISC process. Solvent-dependent results further indicate that charge-transfer-state stabilization accelerates ISC process, supporting an additional SOCT contribution. These findings provide an effective strategy for developing highly efficient metal-free triplet photosensitizers.