Efficient Spin–Orbit Charge‐Transfer Intersystem Crossing and Slow Intramolecular Triplet–Triplet Energy Transfer in Bodipy–Perylenebisimide Compact Dyads and Triads

Abstract

AbstractBodipy (BDP)‐perylenebisimide (PBI) donor–acceptor dyads/triad were prepared to study the spin–orbit charge‐transfer intersystem crossing (SOCT‐ISC). For BDP‐PBI‐3, in which BDP was attached at the imide position of PBI, higher singlet oxygen quantum yield (ΦΔ=85 %) was observed than the bay‐substituted derivative BDP‐PBI‐1 (ΦΔ=30 %). Femtosecond transient absorption spectra indicate slow Förster resonance energy transfer (FRET; 40.4 ps) and charge separation (CS; 1.55 ns) in BDP‐PBI‐3, while for BDP‐PBI‐1, CS takes 2.8 ps. For triad BDP‐PBI‐2, ultrafast FRET (149 fs) and CS (4.7 ps) process were observed, the subsequent charge recombination (CR) takes 5.8 ns and long‐lived 3PBI* (179.8 μs) state is populated. Nanosecond transient absorption spectra of BDP‐PBI‐3 show that the CR gives upper triplet excited state (3BDP*) and subsequently, via a slow intramolecular triplet energy transfer (14.5 μs), the 3PBI* state is finally populated, indicating that upper triplet state is involved in SOCT‐ISC. Time‐resolved electron paramagnetic resonance spectroscopy revealed that both radical pair ISC (RP ISC) and SOCT‐ISC contribute to the ISC. A rare electron spin polarization of (e, e, e, e, e, e) was observed for the triplet state formed via the RP ISC mechanism, due to the S−T+1/T0 states mixing.