On-Surface Stereochemical Characterization of a Highly Curved Chiral Nanographene by Noncontact Atomic Force Microscopy and Scanning Tunneling Microscopy

A highly distorted chiral nanographene structure composed of triple corannulene-fused [5]helicenes is prepared with the help of the Heck reaction and oxidative photo-cyclization with an overall isolated yield of 28%. The com-plex three-dimensional (3D) structure of the bowl-helix hybrid nanostructure is studied by a combination of non-contact atomic force microscopy (AFM) and scanning tunneling microscopy (STM) on the Cu(111) surface, density functional theory calculations, AFM/STM simulations, and high-performance liquid chromatography-electronic circular dichroism analysis. This examination reveals a molecular structure in which the three bowl-shaped corannulene bladesd position themselves in a C3-symmetric fashion around a highly twisted triphenylene core. The molecule appears to be shaped like a propeller in which the concave side of the bowls face away from the connected [5]helicene motif. The chirality of the nanostructure is confirmed by the direct visualization of both MMM and PPP enantiomers at the single-molecule level by scanning probe microscopies.These results underline that submolecular resolution imaging by AFM/STM is a powerful real-space tool for the stereochemical characterization of 3D curved chiral nano-graphene structures.

Ministry of Education Singapore under the AcRF Tier 1 (MOE T1 RG11/21) and AcRF Tier 2 (MOE-T2EP10221-0002); Deutsche Forschungsgemeinschaft via grants (nos. SCHI 619/13 and EB535/1-1); the GRK (Research Training Group) 2204 “Substitute Materials for Sustainable Energy Technologies”; the LOEWE Program of Excellence of the Federal State of Hesse (LOEWE Focus Group PriOSS “Principles of On-Surface Synthesis”); National Natural Science Foundation of China (grant nos. 21790053, 51821002, and 22072103); National Major State Basic Research Development Program of China (grant nos. 2017YFA0205000 and 2017YFA0205002); Collaborative Innovation Center of Suzhou Nano Science & Technology, 111 Project; National Supercomputer Centre, Sweden; Wroclaw Centre for Networking and Supercomputing, Poland