Exploring excited state dynamics in benzocarbazole derivatives through transient absorption spectroscopy

Abstract

Increasing the spin-orbit coupling (SOC) constant by introducing heteroatoms is crucial approach for achieving efficient pure organic room-temperature phosphorescent (RTP). This research focused on the molecules 3,3″-Di(9H-carbazol-9-yl)-1,1':3′,1″-terphenyl (DCzTp) and 2,6-Bis[3-(9H-carbazol-9-yl)phenyl]pyridine (DCzPPy) using transient absorption spectroscopy experiments. For DCzTp, the femtosecond spectroscopy revealed an excited state absorption (ESA) signal at 630 nm, which reached a maximum within 1.3 ps, followed by decay of the ESA signal and appearance of triplet-triplet absorption (TTA) signal at 405 nm. An isosbestic point at 465 nm indicated the presence of intersystem crossing (ISC). In nanosecond spectroscopy, the TTA signal reached its maximum within 23 ns, and then the triplet state lifetime (τ_TTA) decayed within 1.9 μs. DCzPPy exhibited faster ISC lifetime (τ_ISC = 5.5 ns) and longer τ_TTA (4.9 μs) compared to DCzTp. Theoretical simulations demonstrated that DCzTp transitions from the lowest singlet excited state (S₁) to the lowest triplet excited state, while DCzPPy transitions from S₁ to the higher triplet excited state (T₂). Notably, due to the heteroatom effect, the SOC constant of DCzPPy (0.27 cm⁻¹) was greater than that of DCzTp (0.23 cm⁻¹), leading to a faster τ_ISC (5.5 ns vs. 11.4 ns). Additionally, DCzPPy exhibited an additional triplet state internal conversion process (1.1 μs), leading to a longer τ_TTA (4.9 μs vs. 1.9 μs). This research provides valuable insights into how heteroatoms enhance RTP efficiency in pure organic molecules.