Effective promotion of steric hindrance effect on singlet-triplet transition of para-linked carbazole-biphenyl by transient absorption spectroscopy.

The para-linked carbazole-biphenyl (CBP) is commonly utilized in phosphorescent organic light-emitting diodes. This study investigates the steric hindrance and heavy-atom effects in CBP derivatives through transient absorption spectroscopy. In contrast to CBP, CBP derivatives shows new triplet-triplet absorption signals and isosbestic points, accompanied by the decay of excited state absorption signal, which indicates the occurrence of intersystem crossing (ISC). The experimental ISC lifetimes for mCBP, CDBP, and CPB-2Br are 8 ns, 7.4 ns, and 0.103 ns respectively, aligning with the increased theoretical spin-orbit coupling constants (ξ) of S1 → T1 (0.032 cm-1 < 0.034 cm-1 < 1.26 cm-1). Notably, compared to CDBP (0.75 cm-1, 0.3 μs), the lower ξ(T1, S0) of mCBP (0.014 cm-1) extending the experimental triplet-exciton lifetimes (τT) to 1.97 μs. The τT (1.92 μs) of CBP-2Br is prolonged due to the significantly reduced recombination energy (2073.52 cm-1). This study provides insights into prolonging the lifetime of halogen-free phosphorescent molecules.