Electric Field-Induced Chirality Switching Of The Self-Assembled Domains

Chirality is ubiquitous in nature and plays an important role in physics,chemistry,and biology due to its potential applications in enantioselective heterogeneous catalysis and enantiospecific sensors.Using scanning tunneling microscopy(STM),some recent investigations have provided deeper insights into the chiral recognition,chiral separation,and chiral amplication.In principle,chirality can be expressed at two different levels: point chirality(local chiral motif)and organizational chirality(chiral domains).In particular,chiral phase transition about point chirality and organizational chirality has attracted enormous interest in recent years.For example,the chiral switching of individual adsorbates,and chiral phase transitions of two-dimensional(2D)molecular domains,are the most fascinating topics in surface chirality studies.Chiral switching provides an efficient pathway towards the formation of homochiral domains and 2D chiral phase transition.Weigelt et al.reported the chiral switching of a prochiral molecule on Au(111)between the two enantiomeric forms when it under goes thermally induced conformational change.Choi et al.studied the reversible switching of isolated azobenzene molecules on Au(111)between the transand cis-conformations via inelastic electron tunneling(IET).In propene/Cu(211)system,Parschau et al.demonstrated that the two distinct enantiomeric states of the prochiral propene can be interconverted by IET.The absolute configuration of the adsorbate complex can be switched under the STM tip by IET into the opposite configuration.About the 2D chiral phase transition,Bohringer et al.found a conglomerate-racemate transition driven by the molecular coverage in nitronaphthalene/Au(111)system.A similar situation was also observed for PVBA on Cu(100),where the homochiral domains with clockwise(CW)and counterclockwise(CCW)motifs were found when the coverages remain below a critical value.When the critical coverage was exceeded,the abrupt transition to the racemic structure was observed.Yang et al.reported a chiral phase transitionof quinacridone derivative on Au(111)from a conglomerate of homochiral domains into a racemic close-packed structure with increasing coverage.The absolute configuration of single enantiomer was recognized and the enantio selective adsorption was identified.Recently,Seibel et al.identified the main factor favoring the conglomerate crystallization over racemate crystallization.It was demonstrated that the racemic heptahelicene on Ag(100)surface shows a transition from homochiral nuclei to larger racemic motifs,although the extended homochiral phase exhibits higher density.It is noticed that nearly all the reported 2D chiral phase transitions are limited to the conversion from a homochiral conglomerate to a hetero chiral racemate and vice versa.The driving force is arisen from the molecular coverage.Chiral switching has been found only at the level of point chirality of individual adsorbates.To the best of our knowledge,there has been so far no report on the switching of organizational chirality occurred in the 2D molecular domains.So it remains unclear whether the 2D molecular domains can switch their chirality from R-to S-enantiomer or vice versa.If yes,what kind of external stimulation should be provided to drive such kind of switching in the 2D adsorb domains?STM has been proven to be a powerful tool not only to obtain the spatial information at atomic scale,but also to manipulate single atoms,molecules,and nanostructures on surfaces.Aside from driving the chiral switching of individual molecules,STM tip can be used to initiate nonlocal chemical reactions by injecting hot electrons,to displace the monatomic steps over large distances via the attractive interaction,to provoke trans-cis isomerization by electric field in the tunneling junction,and to enable the field-induced electro migration of In and Cs atom son surfaces.It was recognized that the mass transport is driven by the electric field along the surface rather than a direct current through the sample.Thus one can expect a noticeable effect of the STM tip induced electric field on the adsorbates with a net charge or polarizability.Herein,we present the electric field-induced chirality switching of the self-assembled domains of copper phthalocyanines(CuPc)molecules on Cd(0001)surface.CuPc is a planar symmetric molecule with a single Cu ion in the center of a macrocycle and four outer benzene rings attached to the macrocycle.On Cd(0001),the CuPc molecules show a series of self-assembled structures such as the gas-like phase,network phase,square phase,and compact phase.In particular,chirality switching of CuPc network from S-to R-enantiomer has been achieved by applying a voltage pulse.This behavior indicates that a considerable low barrier for interconversion between the R-and S-enantiomeric domains,and an enhanced tip-molecule interaction due to the charge transfer.Our results demonstrate that not only the point chirality of individual adsorbates but also the organizational chirality of self-assembled domains can be switched by the electric filed from STM tip.