Resonator-based add-drop filters enabled by flexible polymorphic crystals with TADF-RTP motifs

Area of Science:

• Materials Science
• Optoelectronics
• Crystallography

Background:

• Polymorphism in organic molecules can lead to distinct photophysical properties.
• Organic light-emitting materials are crucial for advanced optical devices.
• Controlling light signals in optical circuits requires precise material design.

Purpose of the Study:

• To utilize distinct crystalline polymorphs of a carbazole derivative for photonic applications.
• To demonstrate a polymorph-dependent add-drop filter (P-ADF) for optical signal processing.
• To explore the potential of Thermally Activated Delayed Fluorescence (TADF) and Room Temperature Phosphorescence (RTP) materials in photonic circuits.

Main Methods:

• Synthesized 5'-(9-ethyl-9H-carbazol-3-yl)-[2,2'-bithiophene]-5-carbaldehyde (CZ-CHO) and identified distinct green (G) and yellow (Y) emitting polymorphs.
• Investigated the photophysical properties (RTP and TADF) and crystal packing of the G- and Y-FL polymorphs.
• Fabricated an optical circuit using AFM-tip-based micromanipulation, integrating G-FL and Y-FL crystal waveguides.

Main Results:

• Demonstrated that different polymorphs of CZ-CHO exhibit distinct fluorescence colors (green and yellow) and emissive mechanisms (RTP and TADF).
• Constructed a functional P-ADF where a green fluorescence signal was modulated and converted to a yellow fluorescence output.
• Utilized the circular shape of single crystals as ring resonators for tunable, bidirectional signal routing (clockwise/anticlockwise).

Conclusions:

• Established a new strategy for photonic circuit design based on controlling material polymorphism.
• Showcased the potential of TADF and RTP emissive materials for advanced, tunable optical signal processing.
• Highlighted the significance of intermolecular interactions and crystal packing in dictating material function for optoelectronic devices.