Radical-Modulated Triplet Emission in Optically Homogeneous Blue Graphene Quantum Dots

Graphene oxide and graphene quantum dots (GOQDs) have attracted considerable interest due to their tunable bandgap and aqueous solubility, which are governed by their oxygen content, surface functional groups, and dimensional attributes. In this study, we report the synthesis of blue-emitting graphene quantum dots (b-GOQDs) derived from highly oxidized graphene oxide (hoGO) possessing an oxidized carbon content of 58%, achieved via mild peroxide oxidation in an aqueous medium. The elevated oxygen content facilitates controlled cleavage within the graphene lattice under mild conditions, yielding b-GOQDs that exhibit excitation-independent emission spectra indicative of a high degree of emitter homogeneity. Their photoluminescence, which resembles molecular-like emission, originates primarily from uniform surface defects (associated with the loss of sp2 carbon centers in the graphitic core), with a quantum yield of 0.17. We further investigate the long-lived photophysical behavior of these b-GOQDs in the microsecond to millisecond regime, as such defect states are known to support triplet excitons. At cryogenic temperatures, the ratio of long-lived to short-lived excited states approaches 6:1, highlighting the potential of these materials for applications requiring extended emission lifetimes. Additionally, we demonstrate 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)-mediated multichannel triplet harvesting via P-type delayed fluorescence and phosphorescence. Notably, an afterglow lasting up to 25 s was observed, exhibiting bimolecular characteristics and significant quenching in the presence of TEMPO, which is known to enhance phosphorescence through intersystem crossing. This work emphasizes the critical influence of synthetic methodology on the photophysical properties of b-GOQDs and introduces a novel approach for tuning their emission characteristics, thereby advancing their potential for incorporation into nontoxic, long-lifetime optoelectronic applications.