3D Electron Diffraction Uncovers the Structural Origin of Delayed Dual Emission in a Heavy-Atom-Free Carbazole Hexamer

Abstract

The coexistence of thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) offers a powerful yet rarely realized strategy for harvesting triplet excitons in purely organic materials. However, understanding the structural factors governing such delayed dual emission remains limited. Here, we unveil the structure-property correlation in a spacer- and heavy-atom-free carbazole hexamer (Cz-H) by utilizing three-dimensional electron diffraction (3D ED) and ultrafast time-resolved spectroscopy. Remarkably, solid-state Cz-H exhibits phosphorescence-dominated delayed dual emission, in contrast to its reference trimer (Cz-T), which displays an equitable TADF-RTP. Despite its micro-crystalline nature (∼5.7 × 105 times smaller crystal volume than Cz-T) arising from a nonplanar aromatic framework, the molecular packing of Cz-H was successfully resolved using 3D ED. The analysis reveals a rigid supramolecular network reinforced by C─H···π interactions, which likely suppresses nonradiative decay, facilitating triplet generation. Femtosecond and nanosecond transient absorption spectroscopy confirm enhanced intersystem crossing (ISC) in Cz-H (𝑘Cz−H𝐼⁢𝑆⁢𝐶 > 𝑘Cz−T𝐼⁢𝑆⁢𝐶). Quantum-chemical calculations further indicate a 7.8-fold increase in spin–orbit coupling and a significant reduction in ΔEST from 22.6 to 8.5 meV in Cz-H relative to Cz-T. These findings demonstrate that 3D ED provides direct structural insights, enabling the correlation between molecular packing-induced supramolecular rigidity and triplet generation in organic emitters.