Steering the Achiral into Chiral with a Self-Assembly Strategy

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Steering the Achiral into Chiral with a Self-Assembly Strategy Huanjun Song, Hao Zhu, Zhichao Huang, Yajie Zhang, Wenhui Zhao, Jing Liu, Qiwei Chen, Cen Yin, Lingbo Xing, Zhantao Peng, Peilin Liao, Yongfeng Wang, Yuan Wang, and Kai Wu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b02683 • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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Steering the Achiral into Chiral with a SelfAssembly Strategy Huanjun Song1, 4, Hao Zhu1, Zhichao Huang1, Yajie Zhang1 ,2, Wenhui Zhao1, Jing Liu1, Qiwei Chen1, Cen Yin1, Lingbo Xing1, Zhantao Peng1, Peilin Liao3,*,Yongfeng Wang2,*, Yuan Wang1,*, and Kai Wu1,* 1 BNLMS,

College of Chemistry and Molecular Engineering, Peking University, Beijing 100871,

China. 2 Key

Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics,

Peking University, Beijing 100871, China. 3

School of Materials Engineering, Purdue University, West Lafayette, IN 47907, USA.

4 Research

Institute of Aerospace Special Materials and Processing Technology, Beijing, 100074,

China. *E-mail: [email protected], [email protected], [email protected], [email protected]. ABSTRACT Chirality transfers from self-assembly of achiral titanyl phthalocyanine (TiOPc) to its topsitting TiOPc molecule has been successfully achieved. The TiOPc molecules first assemble into a porous network on Au(111) that contains periodic chiral voids, each being fenced by four axially rotating TiOPc molecules in upward adsorption geometry where their ending O atoms exclusively point away from the substrate. Additional top-sitting TiOPc molecule turns out to be chiral upon adsorption on a chiral void with its ending O atom towards the substrate. The chirality of the topsitting TiOPc is associated with a charge transfer between its indole rings and the ending O atoms of the underlying TiOPc molecules that form the chiral void, resulting in asymmetric electronic

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density of the indole rings in the top-sitting molecule and accordingly the chirality of the molecular orbitals. Such a scenario also validates for other planar achiral metallo-phthalocyanines such as copper phthalocyanine that become chiral upon adsorption on the chiral voids in the underlying TiOPc assembly, indicating that the chirality transfer mechanism from assembly to top-sitting molecule is not uncommon. KEYWORDS: chirality transfer, self-assembly, TiOPc, molecular orbital, scanning tunneling microscopy, density functional theory

Chirality transfer is of fundamental importance in heterogeneous catalysis for asymmetric synthesis, nonlinear optical and electronic devices, and bio-active molecules in life sciences.1-8 Studies in the past decades showed that molecular chirality could be directionally and hierarchically transferrable among various dimensional structures, from single molecules (zerodimensional)9-12 to linear chains (one-dimensional),13-15 assembling networks (two-dimensional, 2D)8,16-19 and stereoscopic stacking structures (three-dimensional, 3D).20-23 2D chirality of molecular self-assemblies fabricated on various surfaces by deposition of chiral molecules8,16-18 has been of great interest in surface chemistry. Achiral molecules have also been employed as building blocks to construct 2D chiral assemblies via dictation of single crystal surfaces,24,

25

addition of chiral co-adsorbates26 or specific bond formations among molecules,27 Chirality transfer from molecule to assemblies or aggregates was extensively reported in previous studies1315,20-23,25,26,28-34

where chirality was unidirectionally transferred from low to high dimensions.

While 2D chiral assemblies could be routinely achieved on various surfaces via such a chirality transfer,26, 28-31 its reverse transfer route, i.e., from assembly to molecule sitting atop, has hardly been achieved. The chirality transfer from the assembly to its top-sitting molecule could in return affect chirality of the molecular orbitals of the involved molecules that plays a key role in mediation of their physical and chemical properties. Molecular chirality generally originates from either geometric conformation or electronic state of the involved molecule, which accordingly influences its optical properties.10 Titanyl phthalocyanine (TiOPc) has been of tremendous interest because of its highest efficiency as a photoconductor in xerographic devices.35 For instance, 3D Y-form TiOPc crystal possesses the


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highest photosensitivity towards near-infrared light with a quantum yield of photogeneration above 90%.36,37 It’s well known that the intermolecular charge transfer in the TiOPc crystal plays a key role in developing high-performance photoconductors and has long been proposed to be accessible by scanning tunneling microscopy (STM).35 Although the geometric structure of the TiOPc molecules can be explored at the single molecule level,11,38-44 the insight of intermolecular charge transfer and change of the electronic states are still elusive. In this study, the chirality transfer from assembly to its top-sitting molecule was successfully achieved by experiments. The achiral TiOPc molecules self-assembled into a molecular network that consisted of chiral voids formed by four TiOPc molecules and hence possessed organizational chirality, though individual TiOPc molecules in the assembly were still achiral. When evaporated onto the assembly, additional TiOPc molecules exclusively became chiral. Both STM measurements and density functional theory (DFT) calculations showed that the chirality of each top-sitting TiOPc molecule was induced by its underlying chiral void in the assembly. The achieved chirality, attributed to chirality of the molecular orbital, was electronic rather than conformational, as confirmed by DFT calculations. The calculation results showed that a slight charge transfer took place from the indole rings of the top-sitting TiOPc molecule to the upwardpointing O atoms of the underlying TiOPc molecules, leading to the chirality transfer from the underlying TiOPc assembly to its top-sitting molecules. The same chirality transfer mechanism was further validated by the experimental facts that planar and achiral copper phthalocyanine (CuPc) sitting at the chiral voids of the underlying TiOPc assembly also became electronically chiral. RESULTS AND DISCUSSION Figure 1a schematically shows two TiOPc molecules in different adsorption geometries on Au(111). The left molecule with its ending O atom pointing downward, i.e., towards the substrate, is denoted as O-down molecule, while the right one with its O atom pointing upward, i.e., away from the surface, is named O-up molecule. Figure 1b depicts a representative STM image of fourlobed TiOPc molecules in the first monolayer. Two types of molecules with different apparent heights in the center can be feasibly identified. The bright-centered molecule marked by the dashed square in Figure 1b is assigned as O-up TiOPc, while the dim-centered one highlighted by the dashed circle, as O-down TiOPc. The O-up TiOPc molecules by far outnumber the O-down TiOPc 3 Environment ACS Paragon Plus

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ones, which was judged based on a statistical analysis of about 5000 molecules from 20 images acquired from different areas of the sample. A square unit cell highlighted by the white square possesses a measured side length of 1.36 ± 0.04 nm. Its molecular model is given in Figure 1c.

Figure 1. Molecular structure and STM image of TiOPc. (a) Side-view molecular structures showing the Odown (left) and O-up (right) orientations, respectively. (b) Constant-current image of the TiOPc network on Au(111). Scanning conditions: bias voltage = 1.0 V, feedback current = 30 pA, scanning area = 5.4 × 5.4 nm2. (c) Corresponding structural model for the TiOPc network in (b). Green-centered molecular structures highlight the O-down geometry while red-centered ones, the O-up geometry.

A large-scale image of a highly ordered TiOPc assembly is shown in Figure 2a. The appearance of the herringbone structure indicates that the reconstruction of the underlying Au(111) surface survives the TiOPc assembly. The image can be divided into various domains, marked by R and L, respectively.10 These domains contain voids of opposite chirality (Figures 2c and 2d). Each pinwheel-like void is formed by four O-up TiOPc molecules, as pointed out by the white dashed arrows highlighting that the voids in the R and L unit cells appear counter-clockwise and clockwise, respectively. The chiral voids are similar to those raveled in previous studies.10, 24 Each chiral domain can be as large as 100 × 100 nm2 in area. Pure homo-chiral network extending across the monoatomic steps that separate adjacent terraces can be clearly identified in the STM images


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(Figure 3), indicating that the steps do not interrupt the uniform homo-chiral networks in different domains.

Figure 2. Constant-current STM images showing the chirality of the porous networks formed by monolayered TiOPc assemblies. (a) Large-scale STM image of the TiOPc monolayer consisting of R and L domains (0.8 V, 60 pA, 114.0 × 114.0 nm2). (b) STM image showing the boundary between R and L domains, as highlighted by the white dashed line extending along the [11-2] direction of the substrate (1.2 V, 20 pA, 8.7 × 8.7 nm2). (c) and (d) Enlarged STM images of the R and L domains with their unit cells marked by dashed squares, dashed white arrows marked the lobes pointing to oxygen atoms. (1.0 V, 20 pA, 3.5 × 3.5 nm2).

Figure 3. STM images of the assembled monolayer across the monoatomic steps on Au(111). (a) Large-scale STM image (1.0 V, 50 pA, 48.7 × 48.7 nm2), and (b) enlarged STM image (0.8 V, 30 pA, 20.0 × 20.0 nm2).

At the boundary of the R and L domains, there appears a type of line defects, as marked by the dashed lines in Figures 2a and 2b. The lattice orientations of the Au(111) surface are marked by three white arrows in the image, enabling to recognize that the boundary line defects extend along


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the [11-2] and [2-1-1] directions. The experimental facts that the molecules near the boundaries remain ordered and intact in their morphology allow feasible symmetry identification of the first assembled monolayer. At the assembly level, the assembled structures separated by the line defects display two kinds of enantiomorphism for the voids which are mirror-symmetric with respect to the [-1-12] substrate lattice direction, as shown by the white dashed squares in Figure 2b. At the molecular level, individual TiOPc molecules in the R section line up along [0-1-1] and [-211] directions while their counterparts in the L section, along [-101] and [-12-1] directions, showing the mirror symmetry and dislocation with respect to the axis parallel to [-1-12]. A symmetry analysis according to previous work24 demonstrates that the TiOPc assembly consists of enantiomeric voids in R and L domains.

Figure 4. STM images of the induced chiral TiOPc molecules in the second layer. (a) STM image showing both r and l chiral TiOPc molecules in the top layer (0.1 V, 20 pA, 9.7×9.7 nm2). White dashed line pointing along the [11-2] direction indicates the boundary between R and L domains. White dashed squares highlight the R and L chiral voids in the first assembly layer. (b) and (c) STM images of l chiral TiOPc molecules sitting on L domain (1.2 V, 30 pA, 12.0 × 12.0 nm2) and r chiral TiOPc molecules sitting on R domain (1.2 V, 30 pA, 12.0 × 12.0 nm2). (d) Bias-dependent constant-height STM images of individual TiOPc molecules of l and r chirality (3.7 × 3.7 nm2). The feedback loop is off. The blue arrows in the panels acquired at 1.4 V mark the [11-2] direction that symmetrically bisects the chiral molecule.


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Further evaporation of TiOPc led to the appearance of sparsely distributed TiOPc molecules in the top layer, as shown in Figure 4a. The Pc planes of the top TiOPc molecules sitting at voids formed by four underlying TiOPc molecules are parallel to the substrate surface. The depicted two top-sitting TiOPc molecules which are rotated by ± 55° against [11-2] of the substrate with their Ti=O bonds downward and inserted into the voids in the underlying TiOPc assembly. More strikingly, the two top-sitting TiOPc molecules adopt opposite chirality, marked as r and l, respectively. Figure 4d shows constant height STM images of both enantiomeric molecules at various scanning sample bias voltages. Over the bias range from 1.2 to 1.6 V, the top-sitting TiOPc molecules appear chiral. The enantiomers are mirror-symmetric with respect to the axis parallel to [11-2], as marked by the blue arrows in the STM images acquired at 1.4 V. The chirality contrast in the STM images becomes increasingly intensified at bias voltages from 1.2 to 1.6 V and eventually fades out at the bias of 1.8 V. Such a bias-dependent chirality behavior strongly implies that the appeared chirality in this system originates from the molecular electronic state rather than conformational geometry. Meanwhile, the change in the chiral morphology from 0.8 to 1.6 V also confirms that the electronic state plays an important role in generating the chirality of the topsitting TiOPc molecules. Similar behavior has been previously observed with CuPc molecules adsorbed on Ag(100).10 CuPc is intrinsically achiral in geometric conformation, but displays chirality at negative biases and fades out at positive biases. Both high-resolution STM images and ab initio calculations confirm that the chiral CuPc pattern is due to their asymmetric electronic structure rather than geometric conformation. Since similar behavior occurs for the present TiOPc/Au(111) system, namely, the chirality fades in at some biases and out at others, we therefore attribute the chirality origin of the top-sitting TiOPc molecules to their electronic structures. Figures 4b and 4c show the enantiomeric TiOPc domains. Nearly all TiOPc molecules in the top layer adopt the O-down adsorption geometry which is more energetically favorable. Single handedness exists for individual top-sitting TiOPc molecules and the underlying assembly as well. The Rr and Ll domains, where Rr stands for the domain with R assembly chirality and r molecule chirality and Ll represents the opposite situation, establish an unambiguous relationship for the chirality of the top-sitting molecule and underlying assembly. In the assembly, chirality does not exist for an individual TiOPc molecule, which is consistent with the intrinsic achiral TiOPc geometric structure. The chirality of the assembly is in coincidence with that of the top-sitting molecule, suggesting that the top-sitting molecule chirality be governed by the assembly chirality. 7 Environment ACS Paragon Plus

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Figure 5. Chirality transfer from assembly to single molecules. (a) and (b) STM images showing that a chiral TiOPc molecule in the top layer is moved away by the tip with a 3.5 V bias pulse (a: 1.0 V, 100 pA, 8.7 × 8.7 nm2; b: 1.2 V, 100 pA, 8.7 × 8.7 nm2). (c) and (d) STM images showing that an additional TiOPc molecule is dropped onto the chiral void in the first assembly layer (c: 0.9 V, 30 pA, 13.0 × 13.0 nm2; d: 0.8 V, 30 pA, 12.5 × 12.5 nm2).

The strong and univocal connection between chirality of the top-sitting molecule and the underlying assembly actually indicates the occurrence of chirality transfer from the assembly to its top-sitting molecules. To further prove such a chirality transfer mechanism, STM manipulation experiments are performed, as shown in Figure 5. On the one hand, a top-sitting molecule with r chirality (Figure 5a) was moved away by the STM tip, and the left void in the assembly as indeed in R chirality. The four molecules surrounding the void exclusively adopt the O-up geometry (Figure 5b). On the other hand, by dropping a TiOPc molecule on to the void of the underlying assembly with R chirality (Figure 5c), an individual molecule with r chirality is created (Figure 5d). Such STM manipulation experiments clearly demonstrate that the top-sitting TiOPc molecule chirality is dictated by the void chirality in the underlying assembly. The chirality of the top TiOPc molecule transferred from the underlying assembly is attributed to induced chirality of the molecular orbital, which is supported by the dI/dV mapping and DFT calculations. Figures 6a and 6b are the DFT-optimized molecular models of the Ll and Rr domains. Figures 6c and 6d show the dI/dV mapping of the top TiOPc molecule with l and r chirality,


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respectively. The density of states (DOS) corresponds to the LUMO of the top TiOPc molecule which is about 1.6 eV above the Femi level. The DOS looks obvious chirality from the perspective of four lobes. In addition, the chiral feature can also be identified from the simulated contour plot of charge density over the range from the Fermi level to 1.4 eV for the top TiOPc molecule, which is consistent with the dI/dV mapping (Figures 6e and 6f). Thus, the chirality of the molecular orbital is invoked from intrinsic achiral molecule affected by the underlying chiral assembly.

Figure 6. DFT-optimized molecular models (a and b), dI/dV mapping (c and d: constant height mode,1.6V, 2.0 × 2.0 nm2. The feedback loop is off.) and contour plot of charge density over the range from Fermi level to 1.4 eV (e and f). The top TiOPc molecule in (a, c, e) exhibits l chirality, while the top TiOPc molecule in (b,d,f) possesses r chirality. Red arrows mark the clockwise or counterclockwise swirling of the asymmetric electron density of each lobe.

The calculation results in Figure 7 show that the electron density of the O atom in the assembling TiOPc molecule slightly increases upon adsorption of the top-sitting TiOPc and the electron density of the nearest lobe of the top-sitting TiOPc decreases, indicating that a charge 9 Environment ACS Paragon Plus

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transfer occurs between the assembled and top-sitting TiOPc molecules. Since each O atom of the underlying assembled TiOPc molecules locates at the left/right side of the corresponding lobe of the top-sitting TiOPc molecule, the  orbital electron state of the corresponding lobe is polarized to be asymmetric due to charge transfer, resulting in the l/r electron chirality of the top-sitting TiOPc molecule. Therefore, the chirality transfer from the TiOPc assembly to its top-sitting TiOPc molecule in this study stems from the charge transfer.

Figure 7. Density difference plot obtained by subtracting electron density of isolated top layer molecule and bottom layer assembly from the electron density of the full model. Red and green colors refer to increase and decrease in the electron density, respectively.

In previous reports, the chirality transfer undergoes via either chiral substrates or chiral precursors. For example, the chirality of CuPc on Ag(100)10 and ZnPc on Cu(100)12 originates from the substrate-molecule interaction that induces mismatch between the component atoms in the adsorbed molecules and the underlying metal substrate lattices. Another example is that heptahelicene molecules become chiral on Cu(111), which is attributed to the instinct geometric structures.7,8,30 However, our TiOPc/Au(111) system presents a different mechanism that the chirality of the voids in TiOPc assembly governs that of the top-sitting TiOPc molecules. Moreover, the chiral assembly can induce the chirality of the molecular orbital of an intrinsic achiral molecule, which provides insights into the electronic states of achiral molecules in a chiral assembly environment and therefore affects the optical and chemical properties. Moreover, similar experimental observation occurred when CuPc molecules adsorbed on the chiral voids in the underlying TiOPc assembly, as shown in Figure 8. On the L/R chiral voids in the TiOPc assembly, the top-sitting CuPc molecules appear in l or r chirality at a bias of -0.8 V


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and disappear at -1.2 V (Figures 8a, 8b and 8c). High-resolution images of two top-sitting CuPc molecules at different biases (Figure 8c) indicate that their chirality originates from their electronic states as well. STS in Figure 8d present the electronic densities of states for the CuPc molecules in Figure 8a and the TiOPc molecules (DOS) in Figure 4a, which clearly shows the DOS difference between the CuPc and TiOPc molecules.

Figure 8. STM images of the induced chiral CuPc molecules in the second layer. (a) STM images showing CuPc molecules with Ll chirality at bias of -0.8 V (15.0 × 15.0 nm2). (b) STM images showing CuPc molecules with Rr chirality at bias of -0.8 V (15.0 × 15.0 nm2). (c) High resolution images of individual CuPc molecules at bias of -0.8 V and -1.2 V (2.6 × 2.6 nm2 for each image). (d) STS pattern of CuPc and TiOPc molecules in the second layer.

CONCLUSION In summary, chiral voids were formed in the assembly of achiral TiOPc molecules on Au(111). The chiral voids in the assembly transferred their chirality to top-sitting TiOPc molecules, realizing assembly-to-top-molecule chirality transfer. The induced chirality of the top-sitting TiOPc molecule was electronic rather than conformational according to both STM experiments and DFT calculations. In concurrence with the chirality transfer, intermolecular charge transfer occurred


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between the top-sitting and underlying TiOPc molecules. Such a chirality transfer mechanism also worked for CuPc which became electronically chiral upon adsorption at the voids in the TiOPc assembly on Au(111). The intermolecular charge transfer may help understand the high performance of TiOPc-based photoconductors. The chiral electron states of the molecules induced by the assembly chirality provide insights into the change of the molecular orbital and may further help understand surface asymmetric chemistry. EXPERIMENTAL METHODS All experiments were performed on an ultrahigh vacuum (UHV) low-temperature STM system (Unisoku, USM1300S3HE) with a base pressure below 3×10-10 Torr. The employed substrate was Au(111) single crystal which was cleaned by cycles of 2 keV Ar+ ion sputtering and subsequent annealing at 770 K. TiOPc molecules were prepared and purified according to previous treatment procedure,45 and then evaporated at 500 K onto the Au(111) substrate kept at room temperature (RT). The deposition rate was about 1~2 monolayers (ML) per minute, as monitored by a quartz balance (Inficon, SQM-160). After TiOPc deposition, the sample was swiftly transferred to the low-temperature STM chamber. CuPc (Sigma-Aldrich, 97% purity) was evaporated onto substrate covered by 1 ML TiOPc at 570 and 580 K, respectively. To prepare the STM tip, a polycrystalline tungsten wire was electrochemically etched and thermally cleaned by an e-beam heater. All STM images were acquired at 4.2 K which was achieved by liquid helium cooling, and finally processed with WSxM software.46 COMPUTATIONAL DETAILS We performed density functional theory calculations using the Vienna Ab initio Simulation Package (VASP, version 5.4.4).47, 48 Spin-polarized calculations were performed with the PerdewBurke-Ernzerhof (PBE)49 exchange-correlation functional and the all-electron projector augmented-wave (PAW) method.50 The Tkatchenko-Scheffler method51 was employed to account for the van der Waals interactions. The structure model contains five TiOPc molecules in total. Four of the TiOPc molecules form a square planar structure, with Ti-O bonds aligned to the same direction. The last TiOPc is placed on top, with the Ti-O bond pointing towards the center of the four bottom TiOPc molecules. A unit cell of 45 Å  45 Å  17 Å was used, which ensures at least 12 Å of separation between boundary atoms of adjacent unit cells. A kinetic energy cutoff of 400


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eV was used for the plane wave basis set. The gamma point was used for the k-point sampling. Gaussian smearing with a finite width of 0.1 eV was used to improve convergence near the Fermi level. Energy convergence for electronic wave function was set to 110-4 eV, and geometrical optimization stopped when forces on atoms were below 0.025 eV/Å. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected], [email protected], [email protected]. Author Contributions KW initiated the project. HS conducted the experiments. ZH, YZ, WZ, JL, QC, CY, LX and ZP helped in the STM experiments. HS and WK analysed the data and wrote the manuscript. PL carried out the calculations and participated in data analyses. YFW helped in STS measurements and joined the result discussions. YW supplied the molecules and participated in the manuscript writing. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENTS This work was jointly supported by NSFC (21821004, 91527303 and 21333001) and MOST (2017YFA0204702), China. REFERENCES 1. Mallat, T.; Orglmeister, E.; Baiker, A., Asymmetric Catalysis at Chiral Metal Surfaces. Chem. Rev. 2007, 107, 4863-4890. 2. Hutchings, G. J., Heterogeneous Asymmetric Catalysts: Strategies for Achieving High Enantioselection. Ann. Rev. Mater. Res. 2005, 35, 143-166. 3. Verbiest, T.; Elshocht, S. V.; Kauranen, M.; Hellemans, L.; Snauwaert, J.; Nuckolls, C.; Katz, T. J.; Persoons, A., Strong Enhancement of Nonlinear Optical Properties Through Supramolecular Chirality. Science 1998, 282, 913-915.


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TABLE OF CONTENTS/ABSTRACT GRAPHIC


Steering the Achiral into Chiral with a Self-Assembly Strategy

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