Spin-orbit charge transfer-induced intersystem crossing (SOCT-ISC) is of particular interest for preparation of heavy atom-free triplet photosensitizers. Up to now, examples for SOCT-ISC dyads are limited and electron donor/acceptor SOCT-ISC dyads showing strong visible-light harvesting are rare. Herein, we studied the photophysics of a series of Bodipy-anthracene (BDP-An) compact dyads, especially the triplet state electron spin dynamics with the time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The electronic coupling matrix elements (V DA *) between the 1CT (charge transfer) state and 1LE (locally excited) state are in the range 773–1545 cm–1. For one dyad, we observed three triplet states simultaneously with TREPR, that is triplet states confined on the anthracene (3An) and the Bodipy (3BDP) moieties as well as a 3CT state. Based on the electron spin polarization of these three triplet states and the optical experiments, the SOCT-ISC mechanism is confirmed and the radical pair-ISC mechanism as the main ISC channel was excluded. Triplet–triplet annihilation-induced delayed fluorescence was observed for the dyads, which is rare for Bodipy fluorophores.

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The effect of thienyl substitution on the intersystem crossing (ISC) of a few arenes was studied using steady state and time-resolved transient absorption and emission spectroscopies, as well as DFT/TDDFT computations. We found that the phenyl and thienyl substituents generally induce red-shifted absorptions for the chromophores, and the DFT/TDDFT computations show that the red-shifted absorption and emission are due to the increased HOMO and the reduced LUMO energy levels. Nanosecond transient absorption spectra indicate the formation of a triplet state, the triplet state lifetime is up to 282 μs, and the singlet oxygen quantum yields (Φ) are up to 60%. DFT/TDDFT computations indicate that introducing the thienyl substituent alters the relative singlet/triplet excited state energy levels, and the energy level-matched S/T states are responsible for the enhanced ISC of the thienyl compounds. This information is useful for the design of heavy atom-free triplet photosensitizers and for the study of the fundamental photochemistry of organic compounds.