Steric hindrance prolongs phosphorescence while maintaining bromination-accelerated intersystem crossing in...

Benzo[c]fluorene (BzSBF) provides a useful scaffold for room-temperature phosphorescence. Herein, DiMe-BzSBF, Br-DiMe-BzSBF, 2Br-DiMe-BzSBF, and 2Br-2Ph-BzSBF were investigated to elucidate the roles of bromination and steric hindrance by transient absorption spectroscopy. For DiMe-BzSBF, the singlet excited-state absorption (SESA) signal at 535 nm peaked within 1.6 ps and subsequently decayed, accompanied by the emergence of triplet excited-state absorption (TESA) at 445 nm and a persistent isosbestic point at 470 nm, implying an intersystem crossing (ISC) within 2.2 ns. Upon bromination, the ISC lifetime decreased from 1.1 ns in Br-DiMe-BzSBF to 0.69 ns in 2Br-DiMe-BzSBF, while the corresponding phosphorescence lifetime decreased from 531.84 μs to 338.21 μs, indicating that increasing bromination accelerates ISC but shortens the phosphorescence lifetime. However, the 2Br-2Ph-BzSBF obtained by introducing 7,7'-diphenyl groups onto the dibrominated scaffold retained a short ISC lifetime of 0.62 ns but exhibited a prolonged phosphorescence lifetime of 608.5 μs. The calculated Huang-Rhys (HR) factors followed the order 2Br-2Ph-BzSBF (5.92) < DiMe-BzSBF (10.53) < Br-DiMe-BzSBF (15.27) < 2Br-DiMe-BzSBF (16.51), indicating that bromination strengthens vibronic coupling, whereas further introduction of 7,7'-diphenyl groups weakens and disperses this coupling, consistent with the prolonged phosphorescence lifetime. For DiMe-BzSBF, the undetectable phosphorescence is attributed to insufficient triplet population, while appreciable structural reorganization and vibronic coupling further favor nonradiative decay before phosphorescence can occur. These results demonstrate that steric hindrance prolongs the phosphorescence lifetime without sacrificing the fast ISC enabled by bromination, thereby overcoming the bromination-induced trade-off between accelerated ISC and shortened phosphorescence lifetime.