Inverting the Rhodamine Paradigm: Closed-Form Fluorescence with 280 nm Stokes Shift Drives Plastic Circularity.

Rhodamine derivatives exhibiting inverted open-closed form fluorescence behavior redefines conventional photochemical paradigms while illuminating new structure-property relationships and fascinating application potentials. Herein, we report a donor-acceptor engineering strategy that activates closed form emission in rhodamines, achieving unprecedented Stokes shifts (>280 nm) while overcoming aggregation-caused quenching. The new class of rhodamines with inverted open-close form emission behavior are created through simultaneous substitution of N,N-diethyl groups with indole (donor) and conversion of spiro-lactam to benzene sulfonamide (acceptor). The closed structure was verified by single crystal X-ray diffraction and the intramolecular spatial steric hinderance increased after indole substitution, which avoids exciton coupling and leads to bright fluorescence in solid state (absolute quantum yield>25%). Density functional theory calculations confirmed inverted intramolecular charge-transfer (ICT) characteristics of the synthesized dyes versus rhodamine B, where moderate ICT gives higher fluorescence and strong ICT quenches fluorescence. Leveraging the unique photophysical properties of the novel rhodamines, a closed-loop plastic recycling scheme is proposed, where the dye in closed state is used for fluorescence tracing and the labelled unrecyclable plastics are utilized as encryption ink after acid treatment (open state). This finding highlighted a new insight for engineering fluorescence of rhodamine dyes and contributes to plastics sustainability.