Tunable Singlet-Triplet Energy Splitting of Graphene Quantum Dots

Abstract

Afterglow emissions from long-lived excited states, such as room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF), are highly promising in the fields of display, bioimaging, and data security. However, it is challenging to achieve a single material that simultaneously exhibits both RTP and TADF properties with their relative strengths changed in a tunable manner. Inorganic phosphors or organic compounds have been widely explored for afterglow materials, but they often require expensive rare-earth metals, high reaction temperatures, or complicated synthetic processes. Herein, we propose a new design strategy, where the singlet-triplet energy splitting (∆EST) is tuned by varying the degree of oxidation, i.e., the ratio of oxygenated carbon to sp2 carbon (= OC) in graphene quantum dots (GQDs)/graphene oxide quantum dots (GOQDs). A series of GQDs is prepared to have various OC values ranging from 4.6% to 59.6%, by which ∆EST was changed from 0.37 eV to 0.13 eV, leading to a dramatic afterglow transition from RTP to TADF. Being incorporated into boron oxynitride matrix material, the low oxidized (OC = 4.6%) GQD exhibits RTP lifetime as long as 783 ms, and the highly oxidized (OC = 59.6%) GOQD exhibits TADF lifetime as long as 125 ms. Collective photoluminescence measurements and photophysical kinetics regarding each system reveal that OC is responsible for the transition of afterglow characteristics. In addition, the time-dependent density functional theory elucidates the variation in the afterglow properties are attributed in part to the distorted molecular geometry and the change of spin-orbit coupling matrix element value by an increment of oxygen functional groups. Finally, we demonstrate anti-counterfeiting and multilevel information security, through the long-lived RTP and TADF properties from the oxidation-controlled GQDs, illustrating the immense potential in various fields.