(Invited) Chiral Graphene Quantum Dots

Abstract

Graphite is an inexpensive starting material for the production of graphene or graphene quantum dots (GQDs) considering a broad range of chemical and physical methods. Besides a simple fabrication method and a low production cost, GQDs have received considerable attention because of their electronic and optical properties, fine biocompatibility and low toxicity, which hold great promise for applications such as bioimaging, medical diagnosis, photovoltaic devices and catalysis. Nevertheless, when considering the potential applications of GQDs, chirality is an important aspect that can severely influence the performance and that has not been addressed so far. Natural processes have many examples where chiral compounds have a major role in molecular recognition, chemistry, biology and medicine, and an understanding of the fundamental concepts relevant to chirality in nanostructures is important for the further advancement of nanoscience and nanotechnology. Chiral quantum dots (CdS, CdSe, CdTe, ZnS) have been prepared by a fast microwave-induced heating of the corresponding precursors in the presence of enantiomerically pure stabilizing ligands or via the conventional hot injection technique followed by a phase transfer in the presence of an appropriate chiral stabilizer. In the examples reported to date, the chiral stabilizing ligands and the induced chirality effects have a crucial role in the optical properties, in particular in luminescence sensing and chiral recognition of enantiomers. In contrast, in the case of carbon nanostructures, chirality has been barely explored. Our research group has reported the highly efficient synthesis of enantiomerically pure derivatives of fullerene and endohedral fullerenes with total control of the stereochemical outcome using metallic catalysis and/or organocatalysts under very mild conditions.1 In this communication, to the best of our knowledge, we proof for the first time the principle that chiral graphene quantum dots (CGQDs) can be obtained by reaction of oxidized GQDs with enantiomerically pure (R) or (S)-2-phenyl-1-propanol and that their chirality can be efficiently transferred to the supramolecular assemblies formed with small molecules such as pyrene. The structural properties of the obtained chiral namomaterials have been investigated considering thermogravimetric analysis (TGA), X-ray diffraction (XRD) and different spectroscopic (NMR, FTIR, Raman, UV-Vis, fluorescence) and microscopic (TEM, AFM) techniques. As a result of the covalent functionalization, we proof the concept that GQDs could become chiral and that this property can be transferred to a supramolecular structure built with pyrene molecules, where the CGQDs/pyrene ensembles show a characteristic chiroptical response depending on the configuration of the organic ligands introduced.2