Phase Transition of Two-Dimensional Chiral Supramolecular Nanostructure Tuned by Electrochemical Potential

Abstract

Molecular chiralty and phase transition of p-phenylenedi(rcyanoacrylicacid) di-n-ethyl ester (p-CPAEt) assembled on Au(111) have been studied in the electric double layer region in 0.1 M HClO 4 by electrochemical scanning tunneling microscopy (ECSTM) technique. Three types of chiral supramolecular nanostructures were resolved at differently charged interfaces. Within a potential range (0.65 V < E < 0.8 V, region I), a close-packed physisorbed adlayer of chiral stripe pattern, with the (3 × 6) structure, has been observed. At more negative potential (0.2 V < E e 0.65 V, region II), the stripe patterns gradually dissolved, and two types of new chiral network structures (3 7 × 4 7) and (3 7 × 3 7) evolved on reconstructed and unreconstructed surfaces, respectively. On the basis of the high-resolution STM images, it was tentatively proposed that three types of chiral supramolecular nanostructures were formed by two-dimensional adorption-induced chiral p-CPAEt species together with lateral hydrogen-bonding interaction (CsH · · · N≡C). Intriguingly, ECSTM images allow in situ monitoring of the phase transition process of these chiral adlayers driven by the electrochemical potential. The detailed dynamic results showed that the chiral two-dimensional adlayers could be reversibly tuned purely by the applied electrode potential.Phenomena associated with chirality have held a deep fascination for scientists since Pasteur manually separated left-handed and right-handed sodium ammonium tartrate crystals. 1 It is one of the most attractive areas in chemistry because of the important applications in heterogeneous and asymmetric catalysis, 2-5 enantiomeric selectivity, 6 and chemical and pharmaceutical industries. 7 In recent years, the study of the expression of the chirality on solid surfaces 8-14 is receiving more and more attention, especially with the development of scanning tunnelling microscopy (STM), which is able to directly visualize the chirality at atomic/ submolecular resolution at various interfaces. On solid surfaces, chirality can be encountered by adsorption of chiral molecules or be induced through chiral ordering. For example, the racemic mixtures were found to spontaneously separate into chiral domains on Cu(111), 15 Au(111), 8 and highly oriented pyrolytic graphite (HOPG). 10 The supramolecular chiral channels were formed by the chiral tartaric acids on a Cu(110) surface. 13 Furthermore, for simple geometrical reasons, three-dimensional (3D) achiral molecules may become two-dimensional (2D) chiral species upon adsorption due to the reduced symmetry at the surface. This case is called adsorption-induced chirality. [16][17][18][19] Bernasek et al. observed the adsorption-induced chirality for 1-octadecanol on HOPG due to the mismatch between molecules and substrate. 16 Mol. Catal. A: Chem. 1997, 115, 473-493. (6) Maki, J. J.; Kauranen, M.; Persoons, A. Phys. Rev. B 1995, 51, 1425-1434. (7) Cai, C.; Brune, H.; Grünter, P.; Klaus, K.

In-situ STM observation of the phase transition of two-dimensional 2,5-distyrylpyrazine nanostructure adsorbed on Au(111) in an electrochemical environment

“…The rod length was measured to be approximately 1.8 nm, in agreement with the theoretical length of the DSP molecule. The series of bright protrusions in a linear configuration are a contrast feature typical of STM images for a planar-oriented conjugated aromatic molecule physisorbed on a substrate surface [20,21]. A similar STM contrast and stripe pattern was also reported for a DSP derivative adsorbed in a flat configuration on highly orientated pyrolytic graphite [22].…”

Molecular Dynamics in Two-Dimensional Supramolecular Systems Observed by STM

“…This behavior was also observed by UHV-STM [38]. In addition to the order–order transition of achiral rod-like molecules, other 2D chiral structures have also been constructed [39]. …”

Direction of the polymorphic form of entacapone using an electrochemical tuneable surface template

“…Moreover, it is known that CPAEt molecules that contain two cyano groups gives three different nanopatterns on Au(111) electrodes with respect to the applied potential region selected. 37 These authors have also shown that these patterns can be reversibly switched through the polarisation. These results are particularly interesting in that the meta-stable -form of entacapone has only previously been prepared under non-equilibrium conditions by anti-solvent drown out or by interphase conversion from another polymorph form .…”

“…For example, the charge density at an electrode surface can be fine-tuned by an external electric field which can change the configuration and strength of surface adsorption and even result in the formation of different surface structures. [28][29][30][31][32][33][34][35][36][37] To the authors' knowledge there has been no published research in which low magnitude dc polarisation has been applied to a SAM template with the aim to effect the direction and control of the polymorphic form. Previous studies have, though, demonstrated that strong electric fields can be used to enhanced crystal nucleation, 38,39 promote the preferred orientation of crystals 40 and to direct polymorphs.…”