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 × 10 ⁵ 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 (> ). Quantum‐chemical calculations further indicate a 7.8‐fold increase in spin–orbit coupling and a significant reduction in ΔE _ST 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.