Triplet Exciton Convergence toward Charge-Transfer States Enables Long-Wavelength Room-Temperature Phosphorescence from Carbon Dots

Long-wavelength room-temperature phosphorescent carbon dots (CDs), especially the red phosphorescent ones, have garnered significant attention due to their low-energy emission and potential applications. However, the achievement of red phosphorescence in CDs presents a substantial challenge, and the underlying emission mechanism remains largely elusive. This work proposes a construction strategy for long-wavelength phosphorescence of CDs driven by matrix-induced low-energy charge transfer (CT) excited states. By guiding the directional migration and aggregation of intrinsic triplet excitons of CDs toward CT-related low-energy triplets, long-wavelength phosphorescence emission can be achieved without relying solely on traditional regulation of the intrinsic energy levels of phosphorescent centers. Based on this design philosophy, using (R)-methylbenzylamine and pyromellitic acid as precursors, long-wavelength room-temperature phosphorescent CDs were successfully constructed via a one-step hydrothermal method. Combining femtosecond transient absorption spectroscopy, single-crystal structure analysis, transmission electron microscopy, X-ray photoelectron spectroscopy, and theoretical calculations, it was confirmed that the formation of low-energy CT states induced by matrix effects drives the migration and aggregation of intrinsic triplet excitons toward CT-related low-energy triplets, constituting the key mechanism for long-wavelength phosphorescence generation. The broad applicability of this strategy was further validated using two different representative matrix systems. Therefore, the design strategy for long-wavelength CDs phosphorescence proposed in this study provides a new theoretical framework and design approach for the rational development of long-wavelength and even near-infrared CD-based phosphorescent materials.