# Configuration–packing synergy enabling integrated crystalline-state RTP and amorphous-state TADF

## TL;DR

A new molecule shows different light-emitting behaviors in crystalline and amorphous states, enabling control of phosphorescence and fluorescence.

## Contribution

The study demonstrates a single molecule that exhibits room-temperature phosphorescence in crystalline form and thermally activated delayed fluorescence in amorphous form.

## Key findings

- Crystalline-state RTP is observed with a lifetime of ≈0.39 s and thermal quenching.
- Amorphous-state TADF is characterized by delayed fluorescence with temperature-dependent lifetimes.
- The molecule's structure enables phase-dependent emission behavior through ΔEST and intermolecular interactions.

## Abstract

A twisted D–π–A molecule, PI-Cz 1, was designed and synthesized using phthalimide as the acceptor, carbazole as the donor, and a phenylene bridge. Single-crystal X-ray diffraction revealed a markedly non-coplanar skeleton. Calculations based on the crystallographic geometry and frontier-orbital analysis indicate that the HOMO and LUMO are localized on the carbazole and phthalimide fragments, respectively, affording a small singlet–triplet energy gap. In the solid state, compound 1 exhibits pronounced phase dependence: powder samples display room-temperature delayed emission with principal bands at 550/600 nm and a lifetime of ≈0.39 s that undergoes strong thermal quenching, diagnostic of room-temperature phosphorescence. In contrast, amorphous films show no RTP; their delayed component grows with temperature and shares the same peak position as the prompt emission, consistent with thermally activated delayed fluorescence (TADF). Correlating temperature-dependent lifetimes with phase characterization indicate that, in amorphous environments lacking ordered π–π stacking and rigid confinement, the small ΔEST promotes reverse intersystem crossing, yielding delayed fluorescence; whereas in powder states, intermolecular interactions enhance spin–orbit coupling and crystallinity suppresses nonradiative decay, thereby activating RTP. This work achieves an integrated “crystalline-state RTP–amorphous-state TADF” regulation within a single molecule.