AbstractThe aggregation‐caused quenching has always limited the high concentration and solid‐state applications of carbon nanodots. While the aggregation‐induced emission effect, dominated by intramolecular motion, may be an effective means to solve this problem. Here, hydrophobic solid‐state red‐light carbon nanodots (M‐CDs) with 95% yield are synthesized by a one‐step hydrothermal method using 2,2'‐dithiodibenzoic acid as the carbon source and manganese acetate as the dopant source. The disulfide bond of 2,2'‐dithiodibenzoic acid serves as the symmetry center of molecular rotation and Mn catalyzes the synthesis of M‐CDs, which promotes the formation of the central graphitic carbon structure. The M‐CDs/agar hydrogel composites can achieve fluorescence transition behavior because of the special fluorescence transition properties of M‐CDs. When this composite hydrogel is placed in water, water molecules contact with M‐CDs through the network structure of the hydrogels, making the aggregated hydrogels of M‐CDs fluorescence orange‐red under 365 nm excitation. While in dimethyl sulfoxide, water molecules in the hydrogels network are replaced and the M‐CDs fluoresce blue when dispersed, providing a potential application in information encryption. In addition, high‐performance monochromatic light‐emitting diode (LED) devices are prepared by compounding M‐CDs with epoxy resin and coating them on 365 nm LED chips.