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. 2019 Apr 24;20(8):2018.
doi: 10.3390/ijms20082018.

Chiral Recognition of Hexahelicene on a Surface via the Forming of Asymmetric Heterochiral Trimers

Hong Zhang  1   2   3 , Hong Liu  4 , Chengshuo Shen  5 , Fuwei Gan  6 , Xuelei Su  7 , Huibin Qiu  8 , Bo Yang  9 , Ping Yu  10
Affiliations

Chiral Recognition of Hexahelicene on a Surface via the Forming of Asymmetric Heterochiral Trimers

Hong Zhang et al. Int J Mol Sci. .

Abstract

Chiral recognition among helical 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 the molecular level. Here, we reported chiral recognition in the self-assembly of enantiopure and racemic hexahelicene on a Au(111) surface. Combing scanning tunneling microscopy (STM) and atomic force microscopy (AFM) measurements, the asymmetric heterochiral trimers were observed as a new type of building block in racemic helicene self-assembly on Au(111). The intermolecular recognition of the heterochiral trimer was investigated upon manual separation so that the absolute configuration of each helicene molecule was unambiguously determined one by one, thus confirming that the trimer was "2+1" in handedness. These heterochiral trimers showed strong stability upon different coverages, which was also supported by theoretical calculations. Our results provide valuable insights for understanding the intermolecular recognition of helical molecules.

Keywords: STM; assembly; chiral recognition; helicene; nc-AFM.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
(a) Ball-and-stick models of left-handed hexahelicene (M-[6]H) and right-handed hexahelicene (P-[6]H), (cyan: carbon; grey: hydrogen). (b) Scanning tunneling microscopy (STM) image of P-[6]H adsorbed on Au(111) (V = 300 mV, I = 10 pA). (c–e) Noncontact atomic force microscopy (nc-AFM) images of P-[6]H monomer measured at different tip-sample distances of −3 Å (c), −2.4 Å (d) and −2.1 Å (e). Scale bars: 5 Å.
Figure 2
Figure 2
(a) Overview STM image of P-[6]H. (1000 mV, 50 pA). (b) Zoomed-in STM image of P-[6]H tetramer (300 mV, 10 pA). (c) nc-AFM images of P-[6]H tetramer measured at tip-sample distances of −3.1 Å. (d) Overview STM image of rac-[6]H (1000 mV, 10 pA) and the model of two trimers with opposite handedness, (gold, yellow: top and second/third layer of Au(111), cyan: carbon, grey: hydrogen). (e) Zoomed-in STM images of two types of [6]H trimers A and B (300 mV, 10 pA). (f) nc-AFM images of the trimer A measured at tip-sample distances of −1.6 Å and trimer B measured at tip-sample distances of −1.65 Å. Scale bars: 5 Å (b,c,e,f), 10 nm (a,d), CO tip.
Figure 3
Figure 3
Manual separation of an asymmetric trimer into three isolated helicene monomers. (a–d) STM images of lateral manipulation process for separating the trimer into one monomer and one dimer. (e–h) STM images of lateral manipulation process for separating the dimer into two monomers. (h) STM image of successful manual separation of the trimer to three isolated monomers. Scanning parameters: 300 mV, 10 pA, scale bars 5 nm.
Figure 4
Figure 4
(a,b) Overview STM images before and after manual separation of the trimer (scale bars: 5 nm). The island in the overview image is a NaCl island, which was used to pick up the CO molecule for tip functionalization. (c,d) STM images, nc-AFM images of each helicene molecule manually separated from the trimer. Scale bars: 5 Å, CO tip. (300 mV, 10 pA for all STM images).
Figure 5
Figure 5
The side, top view (ball-stick model) and top view (space-filling model) of binding configurations of (a) homochiral dimer, (b) heterochiral dimer, (c) heterochiral trimer A, (d) trimer B and (e) homochiral tetramer, with corresponding binding energy for each [6]H labeled below.
Figure 6
Figure 6
(a) Overview STM image of rac-[6]H at high coverage. (1800 mV, 10 pA). (b,c) STM and nc-AFM images of a typical single chain of rac-[6]H self-assembly composed of the trimers in A-B-B-B sequence. (d,e) STM and nc-AFM images of a typical double chain of rac-[6]H self-assembly composed of the trimer pairs in AB-BA-AA-AB sequence. One representative trimer pair BA is marked in the rectangular box. Scale bars: 5 nm (a), 5 Å (b–e).
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