Discerning the internal spatial heavy-atom effect on the organic phosphorescence of 9-phenylcarbazole by transient absorption spectroscopy

Enhancing spin-orbital coupling by heteroatom and heavy-atom effects plays a critical role in organic room-temperature phosphorescence. Herein, a nitrogen-hybridized 9-phenylcarbazole (1), and its bromine-substituted derivatives on the 9-phenyl moiety (ortho-4, meta-3, para-2) and carbazole moiety (5, 6, 7), are studied by transient absorption (TA) spectroscopy. For 9-phenylcarbazole, the presence of excited-state-absorption (610 nm) and triplet-triplet-absorption (400 nm) signals in TA spectroscopy indicates the occurrence of intersystem crossing (ISC) with a lifetime of about 10 ns. After Br substitution in the 9-phenyl moiety, the ISC lifetime follows the order of compound 2 (4.9 ns) > compound 3 (1.9 ns) > compound 4 (0.74 ns), implying that the heavy-atom effect enhances with decreasing distance between the Br atom and core carbazole moiety. For carbazole-substituted derivatives, the ISC lifetimes of compounds 5 and 6 significantly decrease to 0.05171 and 0.01185 ns, respectively, demonstrating that the bromine substituent in the carbazole core results in a more efficient heavy-atom effect. However, the lifetime of the triplet exciton decreases with the enhancement of ISC efficiency. Compared to compound 5, the ISC lifetime of biphenyl-structured compound 7 (53.35 ps) shows a slight change, whereas the triplet-exciton lifetime of compound 7 (183.3 ns) increases by approximately 40 times. The decrease in the Hung-Rhys factor and reorganization energies confirms that the biphenyl structure hinders the bond motions of the carbazole moiety and restrains the nonradiative decay, leading to a significant increase in the triplet-exciton lifetime. The prolonged triplet lifetime validates that enhanced structural rigidity can significantly improve the phosphorescence efficiency without affecting the heavy-atom effect, which provides a strategy for balancing phosphorescence lifetime and quantum yields.