Delocalized Triplet State for Efficient Generation of Superoxide Radical.

Directing photodynamic action toward the Type-I pathway requires suppressing singlet oxygen formation. A key strategy employs strongly coupled multichromophoric systems, but this is counteracted by accelerated nonradiative decay that hinders intersystem crossing (ISC). Herein, we address this issue through the rational design of asymmetric boron dipyrromethene (BODIPY) dimers. Through a combination of time-resolved spectroscopy and quantum calculations, we elucidate the structure-dependent dynamics and identify two key design principles: strong interchromophoric coupling and molecular asymmetry. The former establishes an exciton-delocalized framework essential for a low-energy triplet state. The latter, arising from chemical inequivalence, enables efficient mixing between the delocalized exciton (DE) and an intramolecular charge-transfer (ICT) state, significantly enhancing spin-orbit coupling (SOC). Although the SOC-enhanced ISC rate remains slower than that of the nonradiative decay, it becomes sufficiently competitive to facilitate triplet formation. The resulting delocalized triplet exciton lies below the singlet oxygen energy gap, ensuring Type-I pathway preference. Consequently, our work not only deciphers key relaxation pathways, but also delivers a framework for developing high-performance Type-I photosensitizers.