Position-dependent phenylenediamine precursor enables selective triplet state formation in carbon dots.

Room-temperature phosphorescent (RTP) carbon dots (CDs) have attracted increasing attention for their long-lived excited states and applications in information encryption and anti-counterfeiting. However, the relationship between precursor molecular configuration and triplet-state formation remains poorly understood. Herein, we demonstrate that positional isomer engineering of phenylenediamine precursors enables selective triplet-state generation in CDs. Three structurally similar but spatially distinct precursors, o-phenylenediamine (oPD), m-phenylenediamine (mPD), and p-phenylenediamine (pPD), were employed to construct CDs with boric acid, succinic acid, and γ-aminopropyltriethoxysilane as cooperative structural regulators. Notably, only mPD-derived CDs (m-CDs) exhibit stable green RTP, with an emission maximum of 513 nm, a lifetime of 104.79 ms, and a phosphorescence quantum yield of 32%, whereas pPD- and oPD-derived CDs display violet and yellow fluorescence, respectively. Experimental analyses and theoretical calculations reveal that the weaker intermolecular interactions of mPD facilitate precursor rearrangement and suppress excessive self-condensation, promoting the incorporation of B/O- and Si/O-containing structural units during carbonization. These interactions generate a favorable local chemical environment with enhanced n-π⁎ transitions and continuous π-π⁎/n-π⁎ electronic-state coupling, facilitating intersystem crossing. Meanwhile, the rigid B/Si-O-based network effectively restricts molecular motion and suppresses triplet-state nonradiative decay. This work provides molecular-level insights into precursor configuration-dependent triplet-state regulation and offers a strategy for designing high-performance phosphorescent CDs.