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

Area of Science:

• Organic Chemistry
• Materials Science
• Photophysics

Background:

• Designing molecules with tunable photophysical properties is crucial for advanced optoelectronic applications.
• Controlling emission mechanisms like phosphorescence and thermally activated delayed fluorescence (TADF) in a single material remains a challenge.

Purpose of the Study:

• To design and synthesize a novel twisted donor-acceptor molecule (PI-Cz 1) capable of exhibiting different emission behaviors.
• To investigate the phase-dependent photophysical properties, specifically room-temperature phosphorescence (RTP) and TADF, of the synthesized molecule.

Main Methods:

• Synthesis of the twisted D-π-A molecule PI-Cz 1.
• Single-crystal X-ray diffraction for structural analysis.
• Photophysical measurements including steady-state and time-resolved emission spectroscopy.
• Computational analysis (frontier-orbital analysis) to understand electronic structure.

Main Results:

• The synthesized molecule PI-Cz 1 possesses a non-coplanar structure with localized HOMO and LUMO on donor (carbazole) and acceptor (phthalimide) units, respectively, leading to a small singlet-triplet energy gap.
• Powder samples exhibited RTP with a lifetime of ~0.39 s, characterized by strong thermal quenching.
• Amorphous films displayed temperature-dependent delayed emission consistent with TADF, without RTP.

Conclusions:

• The molecule PI-Cz 1 demonstrates a unique ability to switch between RTP and TADF based on its solid-state phase (crystalline vs. amorphous).
• Intermolecular interactions and crystalline confinement in powder states favor RTP, while amorphous environments lacking these features promote TADF.
• This work provides a strategy for integrating crystalline-state RTP and amorphous-state TADF within a single molecular design.