Real-time revealing of triplet state evolution in dibenzothiophene derivatives using ultrafast spectroscopy

Aromatic heterocyclic dibenzothiophene (DBT) and carbazole (Cz) are commonly utilized as the core molecule for modulating luminescence properties. This research focuses on the investigation of the monomer DBT and its derivative 2,8-Bis(9H-carbazol-9-yl) dibenzothiophene (DBT-2Cz) through transient absorption (TA) spectroscopy. The excited state absorption (ESA) signal of DBT peaks at 628 nm within 3 ps, followed by decay with the emergence of triplet-triplet absorption (TTA) at 391 nm. The altered spectra and the presence of an isosbestic point at 480 nm suggest the intersystem crossing (ISC) from singlet to triplet states in 958.1 ps. In comparison, the TA spectroscopy and ISC lifetime (1 ns) of DBT-2Cz exhibit minor alterations. The TTA signal lifetime (6.4 µs) is longer than that of DBT (2.7 µs) due to an additional decay lifetime (827.5 ns), corresponding to the transition from high triplet to low triplet states. Unlike the direct S₁→T₁ transition in DBT, DBT-2Cz transitions to the T₂ state due to a small energy difference (0.16 eV), similar orbital transition component (HOMO→LUMO), and a high spin-orbit coupling value (0.271 cm^-1). Moreover, the internal conversion process from T₂ to T₁ consistent with the transition in the fitted TTA signal, leads to a longer triplet state lifetime (6.4 µs) for DBT-2Cz. This work offers new insights for the design of phosphorescent materials with prolonged lifetimes.