Sustainable Triplet‐State Engineering in Cotton‐Derived Carbon Dots: Mg‐Based Matrices Enable Multicolor Room‐Temperature Phosphorescence

Sustainable production of color-tunable, room-temperature phosphorescent (RTP) carbon dots (CDs) from abundant biomass sources presents significant scientific and technological challenges. Here, a facile strategy is presented for fabricating multicolor RTP CDs through controlled thermal treatment of natural cotton in Mg(NO₃)₂·6H₂O. By precisely controlling the calcination temperature at 300-500 °C, three distinct RTP materials are obtained: yellow-emitting CDs@Mg(NO₃)₂/Mg₃(OH)₄(NO₃)₂-300, cyan-emitting CDs@MgO-400, and blue-emitting CDs@MgO-500. Systematic investigations reveal that the emission color is governed by the relative contributions of carbon core states and surface functional groups, which can be modulated by the calcination temperature. The rigid Mg-based matrices provide spatial confinement and form covalent/hydrogen bonds with CDs, enabling efficient intersystem crossing and suppressing non-radiative decay pathways. The resulting materials exhibit exceptional RTP performance, including long lifetimes of up to 483 ms, high phosphorescence quantum yields reaching 12.4%, and remarkable stability under various conditions. Leveraging their unique excitation-dependent emission and time-resolved decay characteristics, sophisticated applications of these materials are demonstrated in multilevel data encryption, advanced anti-counterfeiting, and dynamic password systems. This work not only provides fundamental insights into triplet-state engineering of CDs but also establishes a sustainable platform for designing next-generation optical materials with potential applications in security, displays, and beyond.