Femtosecond transient absorption spectroscopy on the thermally activated delayed fluorescence of bis[4-(9-H-carbazole)phenyl] sulfone

Abstract

Tracking the time-evolution process of singlet-triplet conversion is essential to achieve effective thermally activated delayed fluorescence (TADF). The femtosecond transient absorption (fs-TA) spectroscopy of Bis[4-(9-H-carbazole)phenyl] Sulfone (CZ-DPS) in dichloromethane (DCM) shows that the excited state absorption (ESA) signal (375 nm) and the stimulated emission (SE) signal (430 nm) reach their maximum values within 4.1 ps. Subsequently, along with the decay of the ESA and SE peaks, a noticeable triplet-triplet absorption (TTA) signal appears at 428 nm and the intensity gradually increases. Furthermore, an isosbestic point at 400 nm between ESA and TTA signals indicates the occurrence of the intersystem crossing (ISC) process from singlet to triplet states, and its time constant is 3.3 ns. Moreover, the observed long-lived lifetime of 12.1 μs in DCM solution is assigned to the delayed fluorescence. The large spin-orbit coupling (SOC) constant (1.039 cm−1) facilitates the ISC process of the CZ-DPS from the first excited state (S1) to the second triplet-state (T2). Furthermore, the small energy difference (ΔES1T2 = 0.03 eV) induces the effective reversed ISC process from the T2 to S1 states to achieve delay fluorescence emission. This work presents the time-resolved spectra changes and TADF mechanism of CZ-DPS molecule.