Controllable Singlet–Triplet Energy Splitting of Graphene Quantum Dots through Oxidation: From Phosphorescence to TADF

Long-lived afterglow emissions, such as room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF), are beneficial in the fields of displays, bioimaging, and data security. However, it is challenging to realize a single material that simultaneously exhibits both RTP and TADF properties with their relative strengths varied in a controlled manner. Herein, a new design approach is reported to control singlet-triplet energy splitting (∆E_ST ) in graphene quantum dots (GQD)/graphene oxide quantum dots (GOQDs) by varying the ratio of oxygenated carbon to sp² carbon (γ_OC ). It is demonstrated that ∆E_ST decreases from 0.365 to 0.123 eV as γ_OC increases from 4.63% to 59.6%, which in turn induces a dramatic transition from RTP to TADF. Matrix-assisted stabilization of triplet excited states provides ultralong lifetimes to both RTP and TADF. Embedded in boron oxynitride, the low oxidized (4.63%) GQD exhibits an RTP lifetime (τ^T _avg ) of 783 ms, and the highly oxidized (59.6%) GOQD exhibits a TADF lifetime (τ^DF _avg ) of 125 ms. Furthermore, the long-lived RTP and TADF materials enable the first demonstration of anticounterfeiting and multilevel information security using GQD. These results will open up a new approach to the engineering of singlet-triplet splitting in GQD for controlled realization of smart multimodal afterglow materials.