Chiral Recognition Of On-surface Helicene Self-assembly

Chiral recognition among stereo molecules is of essential importance in many chemical and biochemical processes.The complexity necessitates investigating manageable model systems for unveiling the fundamental principles of chiral recognition at molecular level.Owning to the unique ?-conjugated helical structure and potential applications in optoelectronic devices,helicene has been intensively investigated recently.Scanning probe microscopy,which possesses sub-molecular resolution,is employed to explore self-assembly behaviors of helicenes.The self-assembly of P-[6]H and rac-[6]H is investigated on Au(111).A novel aggregates of helicene,asymmetric trimer,has been observed when the coverage of rac-[6]H is below medium.Manual separation is utilized to split the trimer up,followed by combined STM/AFM measurement to confirm its handedness.The result indicates the trimer is composed of two [6]H with the same handedness and one with the opposite helicity.Theory calculations tells the rationality of the aggregates.While the aggregate changes from heterochiral trimer to homochiral trimer and spontaneous resolution is achieved when the coverage comes to monolayer.All of the experimental phenomenon shows great difference with former reports of [5]H and [7]H.Hence the structure of carbohelicene has an great impact on the chiral recognition,which usually arises from ?-? interaction in non-functional helicenes.Then S type azahelicene is explored on Au(111)to construct on-surface network structure for trial.4-aza-S[7]H and test molecule clarify the hydrogen bond exists when they self-assemble respectively.Two kinds of trimers arising from different assembly mechanisms are observed.And the long-range ordered network constructed by new designed 4,4'-aza-S[7]H,which has experienced spontaneous resolution,confirms the former conclusion once again.Nc-AFM is employed to detect the location of hydrogen bond in network.Compared with two kinds of trimer formed by 4-aza-S[7]H sample,only one kind of trimer constructed by 4,4'-aza-S[7]H highlights the importance of steric hindrance on periodic structure.And the changed size of network by changing substrate tells the significance of substrate for self-assembly.It provides a new proposal for regulating network sizes.Two kinds of helicenes self-assembling in different forms are explored.Scanning probe microscopy measurements promote the conclusion of mechanism.Our findings will provide valuable insights for understanding the intermolecular recognition of helical molecules.