(Invited) Corannulene-Based Molecular Nanographenes: Synthesis, Reduction, Supramolecular Complexation and Photophysical Properties

The outstanding electronic properties of graphene-based materials make them one of the most promising assets for application in the electronics industry of the XXI century. Encouraged by these properties, a great number of materials scientists are currently engaged in the development of the emergent field of 2D materials. In the last decade, the reduction of the dimensionality to 1D (nanoribbons) and 0D (nanographenes NGs, or graphene quantum dots, GQDs) have become an important research topic by their singular properties. In this regard, the quantum confinement of the electrons and the edge effects of these graphene derived structures makes possible the bandgap opening and, therefore, the appearance of a semiconductor behavior. The bottom-up preparation of these new structures, specifically employing stepwise organic synthesis, lead to monodisperse molecular nanographenes with atomically controlled morphology and, therefore, to the control of photophysical properties at will. An example of this synthetic management is the insertion of non-hexagonal rings in the honeycomb pattern of NGs, endowing Gaussian curvature to these structures and making molecules with bowl or saddle shapes. Our research group has described the wet synthesis of curved corannulene-based nanographenes by a stepwise Sonogashira–[4+2]–Scholl strategy. [1] Using bromocorannulene 1 as starting material, and depending on the temperature and the oxidant agent used in the Scholl reaction, a [6]helicene corannulene-based nanographene 2 or a positively-negatively curved nanographene 3 were prepared. These molecular curved NGs present interesting photophysical properties such as thermal activated delayed fluorescence (TADF) or dual emission. [2] Additionally, the multi-electron acceptor nature of molecular NGs allows the reduction with alkali metals that normally remains hosted in the pockets of curved nanographenes leading to potential energy storage systems. Within a collaborative work, the reduction of nanographene 2 with Na metal has recently been performed. [3] The formation of a “naked” dianion C 76 H 64 2– ( 2 2– ) solvent separated from the two cationic moieties was observed affording unprecedented selectively reduced species. DFT calculations revealed significant changes in the electron density and the inversion of the ring current aromaticity. Finally, concave–convex interactions between curved nanographenes and fullerenes are known to lead to supramolecular complexes with special interest as photoinduced electron transfer systems, mimicking the photosynthetic process. In a collaborative work, [4] the supramolecular complexation of nanographene 3 with C 60 was experimentally monitored by H NMR spectroscopy and studied by DFT calculations.The formation of complexes with stoichiometries 2:1, 1:1 and 1:2 is possible. Furthermore, when the complex was irradiated at 460nm, the formation of the C 60 radical anion is observed in an amazing photoinduced electron transfer process. References 1 J. M Fernández-García, P. Evans, S. Medina Ribero, I. Fernández, D. García-Fresnadillo, J. Perles, J. Casado, N. Martín , J. Am. Chem. Soc. 2018 , 140, 17188. 2 Manuscript in preparation. 3 Z. Zhou, Y, Zhu, J. M. Fernández-García, Z. Wei, I. Fernández, M. Petrukhina, N. Martín et al ., Chem. Commun. 2022 , 58 , 5574. 4 S. Zank, J. M. Fernández-García, A. J. Stasyuk, A. A. Voityuk, M. Krug, M. Solà, D. Guldi and N. Martín , Angew. Chem. Int. Ed. 2022 , 61 , e202112834 Figure 1