Synthesis, Spectroscopy and Photophysical Behavior of Graphene Quantum Dots

Chromophore aggregates are very important in both natural and artificial light harvesting, where interacting chromophores are optimally aligned for efficient energy funneling to the reaction center. While our understanding of the exciton size and migration in these aggregates has been significantly improved through steady-state and time resolved 1D/2D laser spectroscopic studies, numerous questions regarding coherent and incoherent energy transfer in these systems remain unanswered. Here, we take the advantages of recent developments in bottom-up synthesis of well-defined graphene quantum dots (GQDs) and their aggregates to investigate the exciton size and migration in model systems. Two model GQDs namely HBC and CQD were synthesized using methodology developed previously by Mϋllen and co-workers. The aggregation behavior of GQDs into long fibers was studied using steady-state UV/Vis spectroscopy, X-ray scattering, as well as electron microscopy. Exciton size and dynamics in the GQD assemblies was obtained using time-resolved pump-probe spectroscopy. Our experimental results indicate that the exciton is delocalized over up to two molecules in both HBC and CQD assemblies which agrees with the results obtained by theoretical calculations. Exciton dynamics results indicate that the exciton can migrate up to 16 nm in HBC and up to 3 nm in CQD assemblies. Subsequently, a pyridine moiety was added to HBC core to enable its coordination to cobaloxime catalyst. Steady-state and time-resolved spectroscopies enabled us to determine the energy and electron transfer pathways in the chromophore-catalyst assembly. The excited state lifetime of HBC was found to be significantly quenched in the presence of cobaloxime which was an outcome of Dexter energy transfer and Marcus electron transfer in CoQD-A. Further research involves synthesizing covalently attached GQD and catalyst assemblies to create robust system for fuel-forming reactions.