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

We demonstrate the use of yellow and green fluorescence (Y-FL and G-FL) obtained from polymorphic crystals as signals in an add-drop filter (P-ADF). This innovation leverages two distinct crystalline polymorphs, green (G) and yellow (Y), of 5'-(9-ethyl-9H-carbazol-3-yl)-[2,2'-bithiophene]-5-carbaldehyde (CZ-CHO), which exhibit RTP and TADF, respectively. The differing emissive behaviors are attributed to unique intermolecular interactions and crystal packing in each polymorph, which also provide mechanical flexibility. A functional optical circuit, comprising two Y-FL and one G-FL crystal waveguides, is constructed utilizing an AFM-tip-based micromanipulation technique. The circuit configuration enables the G-FL signal emitted from the waveguide to be modulated and transduced into a Y-FL output. Remarkably, the circular shape of the single crystal acts as a ring resonator, and allows for clockwise or anticlockwise signal routing. Our findings establish a new paradigm for polymorph-driven photonic circuit design and open avenues for tunable signal processing using TADF/RTP emissive materials.