Time-resolved excited-state dynamics of star-shaped carbazole-based room temperature phosphorescent molecule by ultrafast absorption spectroscopy

Star-shaped room-temperature phosphorescent (RTP) molecules have attracted much attention due to high luminescence efficiency by inhibiting nonradiative transition of triplet excitons. In this work, the photophysical processes of 1,3,5-tri(9h-carbazol-9-phenyl)benzene (TCzP) and tri(4-carbazol-9-phenyl)amine (TCzTa), with benzene and triphenylamine (TPA) as central cores respectively and carbazole (Cz) as peripheral groups, are investigated. The excited state absorption (ESA) signal (625 nm) of TCzP reaches maximum intensity within 8.8 ps in the femtosecond (fs) transient absorption (TA) spectroscopy and then decays significantly with the enhancing of triplet-triplet absorption (TTA) signal (425 nm). These evolution processes of spectral signal and the appearance of isosbestic point at 450 nm together confirm the intersystem crossing (ISC) process within 5.1 ns. The TTA signal gradually disappears in the nanosecond (ns) TA spectroscopy with a phosphorescence lifetime (tau ph) of 2 mu s. Compared to TCzP, TCzTa exhibits similar spectral evolution behavior with faster ISC of 2.1 ns and lower tau ph of 0.95 mu s. The theoretical simulation shows that the nitrogen atoms in the TPA core of TCzTa leads to a significantly increased spin-orbit coupling constant of S1 -> Tn (0.86 cm-1) and T1 -> S0 (0.08 cm-1) than that of TCzP (0.25 cm-1 and 0.05 cm-1), resulting in the shorter experimental ISC rate constants (2.1 ns) and tau ph (0.95 mu s). This work provides reasonable insights for understanding the real-time spectral signal evolution of star-shaped carbazole-based RTP molecules.