Ultrafast Interfacial Electron Transfer from Graphene Quantum Dot to 2,4-Dinitrotoluene

Ultrafast photoinduced electron transfer (PET) from a photoexcited graphene quantum dot (GQD*) to an electron-deficient molecule 2,4-dinitrotoluene (DNT) is studied in a water-methanol mixture (1:1 by volume) at different temperatures (5 degrees C-60 degrees C). The temperature-dependent study reveals that quenching of GQD emission by DNT is a complex process, where use of collisional or static quenching alone in the fitting model cannot fit the entire time regime of the kinetics. Irrespective of temperature, the collisional quenching rate obtained from the TCSPC lifetime quenching study appears at the upper limit of the bimolecular diffusion-controlled rate of the medium. The weak solubility of DNT in the polar solvents leads to GQD-DNT complex formation through hydrophobic interactions, allowing one to obtain a diffusion-free ultrafast PET timescale of the complex using a femtosecond upconversion setup. GQD-DNT complex formation is manifested by the heat change in the isothermal titration calorimetry (ITC) study upon mixing of DNT with GQD. Findings of our work would reinforce the understanding of the interfacial charge transfer process of GQD and thereby expand the promises to its real applications, especially in sensing and photovoltaics where materials with ultrafast PET are highly desirable.