Long‐Range Ordered Fluorescein Nanosheets Defying the Fluorescence‐Electron Transfer Trade‐Off Through Weak J‐Coupling and Vibrational Suppression

Achieving bright fluorescence in dye aggregates while maintaining efficient electron transfer remains a fundamental challenge due to aggregation‐caused quenching (ACQ). DCFPy/Ag nanosheets (DCFPy/Ag NS) are presented as self‐assembled from the fluorescein‐based dye (DCFPy) coordinated with Ag⁺ that adopt a long‐range ordered structure featuring weak J‐coupling. These nanosheets exhibit a high fluorescence quantum yield (Φ F = 66.4%) in the aggregated state, comparable to monomeric DCFPy (Φ F = 66.9%), and uniquely generate superoxide anions (O 2 •− ) under light irradiation, a property absent in the dye alone. This synergistic dual functionality arises from ordered structural features that suppress non‐radiative decay via minimized defects, while weak J‐coupling enables incoherent exciton migration to sustain radiative transitions and avoid rapid quenching typical of strongly coupled aggregates. Concurrently, Ag + coordination enhances electron transfer and reduces vibrational dissipation as indicated by lattice contraction. By integrating intense emissions with efficient photochemical activity, DCFPy/Ag NS overcome the conventional trade‐off between fluorescence and electron transfer in molecular aggregates. This work highlights the critical role of supramolecular order and metal coordination in designing advanced photochemical materials for applications including fluorescence imaging, phototherapy, optoelectronics, and photocatalysis.