Symmetric,Unsymmetrical, and Asymmetric [7]‐, [10]‐, and [13]Helicenes

Fully aromatic helicenes with more than one pitch‐length are illustrious synthetic targets with potential applications in advanced optical devices and nano‐electronics. The task of extending the length of fully conjugated helicenes past one pitch length is challenging. Now, the synthesis of a series of azaoxa[7]‐, [10]‐, and [13]helicenes is described. The synthesis is based on iterative oxidative furan formation between 3,6‐dihydroxycarbazoles and/or 2‐naphthols. The flexibility of the presented method allows the convenient and scalable synthesis of symmetric, unsymmetrical, and asymmetric homo‐chiral structures. The [13]helicenes can be synthetically functionalized both at the termini and the periphery. The full range of helicenes were characterized using NMR and optical spectroscopy (UV/Vis, fluorescence, and CD) along with single‐crystal X‐ray crystallography. The enantiomers of the [13]helicenes are the longest optically pure helicenes isolated to date.     

Discrimination between Conglomerates and Pseudoracemates Using Surface Coverage Plots in 2D Self‐Assemblies at the Liquid–Graphite Interface

The two‐dimensional (2D) molecular ordering of two photochromic diarylethenes was investigated using scanning tunneling microscopy at the liquid–graphite interface. The racemic mixture of the closed‐ring isomer of one of the diarylethenes showed spontaneous separation of its enantiomers upon 2D crystallization, whereas that of the other diarylethene formed a pseudoracemic crystal in which two enantiomers coexist in a 2D ordering domain. The mixing of enantiomers in 2D assemblies can be analyzed by the dependence of the surface coverage on the concentration of enantiomers at different enantiomeric ratios.     

Steering On‐Surface Reactions by a Self‐Assembly Approach

4,4′‐Bis(2,6‐difluoropyridin‐4‐yl)‐1,1′:4′,1′′‐terphenyl (BDFPTP) molecules underwent dehydrocyclization and covalent coupling reactions on Au(111) according to scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations. Self‐assembly of the reactants in well‐defined molecular domains prior to reaction could greatly enhance the regioselectivity of the dehydrocyclization reaction and suppress defluorinated coupling, demonstrating that self‐assembly can efficiently steer on‐surface reactions. Such a strategy could be of great importance in surface chemistry and widely applied to control on‐surface reactions.     

On‐Surface Evolution of meso‐Isomerism in Two‐Dimensional Supramolecular Assemblies

Chiral structures created through the adsorption of molecules onto achiral surfaces play pivotal roles in many fields of science and engineering. Here, we present a systematic study of a novel chiral phenomenon on a surface in terms of organizational chirality, that is, meso‐isomerism, through coverage‐driven hierarchical polymorphic transitions of supramolecular assemblies of highly symmetric π‐conjugated molecules. Four coverage‐dependent phases of dehydrobenzo[12]annulene were uniformly fabricated on Ag(111), exhibiting unique chiral characteristics from the single‐molecule level to two‐dimensional supramolecular assemblies. All coverage‐driven phase transitions stem from adsorption‐induced pseudo‐diastereomerism, and our observation of a lemniscate‐type (∞) supramolecular configuration clearly reveals a drastic chiral phase transition from an enantiomeric chiral domain to a meso‐isomeric achiral domain. These findings provide new insights into controlling two‐dimensional chiral architectures on surfaces.