A “Backcasting” Strategy for Elucidating the Origin of Excitation‐Wavelength Dependent‐Independent Transition of Carbon Dots through Al ³⁺ ‐Enhanced Fluorescence

Abstract Luminescent materials have only been categorized into excitation‐wavelength dependence (Ex‐De) and excitation‐wavelength independence (Ex‐InDe). Among them, carbon dots (CDs), a state‐of‐the‐art optical nanomaterial, yield fluorescence that is either Ex‐De or Ex‐InDe. However, it remains a challenge to elucidate the origin of Ex‐De/Ex‐InDe by a simple method, let alone realizing the reversible transition of Ex‐De to Ex‐InDe in the same CDs. Herein, a simple “backcasting” strategy is designed and, for the first time, realizes the reversible transition of Ex‐De to Ex‐InDe in the same CDs. In detail, Al 3+ combined the surface sites of CDs and suppressed the Ex‐De luminescence caused by the surface‐states of CDs, resulting in a pronounced (≈29 folds) and fast (<10 s) fluorescence enhancement. Comparative experiments and density functional theory (DFT) calculations are performed to speculate on the possible surface sites of CDs and the mechanism of Al 3+ ‐enhanced fluorescence. The reversible transitions of Ex‐De to Ex‐InDe are realized by the “de‐chelating‐chelating” effect. Interestingly, these phenomena are universally present in fluorescent organic molecules and CDs, and have excellent applications in “and” logic gates and the detection of trace Al 3+ . This work sheds light on new perspectives for understanding the emission origin of CDs and designing luminescent materials with various optical properties.