Rotation in an Enantiospecific Self‐Assembled Array of Molecular Raffle Wheels

Abstract Tailored nano‐spaces can control enantioselective adsorption and molecular motion. We report on the spontaneous assembly of a dynamic system—a rigid kagome network with each pore occupied by a guest molecule—employing solely 2,6‐bis(1H‐pyrazol‐1‐yl)pyridine‐4‐carboxylic acid on Ag(111). The network cavity snugly hosts the chemically modified guest, bestows enantiomorphic adsorption and allows selective rotational motions. Temperature‐dependent scanning tunnelling microscopy studies revealed distinct anchoring orientations of the guest unit switching with a 0.95 eV thermal barrier. H‐bonding between the guest and the host transiently stabilises the rotating guest, as the flapper on a raffle wheel. Density functional theory investigations unravel the detailed molecular pirouette of the guest and how the energy landscape is determined by H‐bond formation and breakage. The origin of the guest's enantiodirected, dynamic anchoring lies in the specific interplay of the kagome network and the silver surface.


Introduction
Inspired by biomolecular rotors that are omnipresent in nature,artificial molecular rotors have drawn attention in the field of nanoscience due to their potential application as functional molecular nanomachines.M ounting such devices on as urface in analogy to natural motors which operate at interfaces can expand their applicability.T his has triggered intense scientific interest in the last decades. [1] Thepioneering work of Gimzewski and co-workers demonstrated and established am ethodological approach for the characterisation of such rotors by combining real space visualisation by scanning tunnelling microscopy (STM) with theoretical studies. [2] It has been shown that the molecular motion on surfaces can be tuned by molecule-substrate interactions, [3] as well as by supramolecular interactions. [4] While isolated rotors have been the subject of intensive investigations,t he programmed assembly and motion of molecular rotors is less explored, despite having advantages in applications such as novel sensors and signal processing. [5] Different approaches have been shown to achieve this goal, including the use of bimetallic dislocation networks [6] and molecular networks, [7] to control the positioning of molecular rotors directly on the surface,aswell as the two-dimensional (2D) self-assembly of molecular platforms,which were utilised for the arrangement of axially attached rotating units. [8] Here we will demonstrate the regular assembly and motion of caged molecular rotors with enantiospecific stationary positions.Anin-depth analysis will reveal that the energy landscape of the rotation is influenced by ac ombination of hydrogen bonds to the host and site-specific molecule-substrate interactions.

Results and Discussion
Our investigation was initiated by the discovery of an intriguing self-assembly upon depositing bpp-COOH (2,6bis(1H-pyrazol-1-yl)pyridine-4-carboxylic acid) on Ag(111). Thebpp-COOH molecules were too mobile to image by STM investigations performed at 200 Ka nd above,f ollowing deposition at room temperature (RT). However,a nnealing at 373 Ks tabilised ac omplex arrangement which could be imaged at RT ( Figure 1A). To identify the long-range periodic features,weinspected the corresponding fast Fourier transform (FFT) showing distinct spots ( Figure 1B). These correspond to au nit cell which can be approximated by the epitaxial matrix of 82 À26 on the Ag(111) lattice,c orresponding to ah exagonal overlayer twisted anticlockwise by 148 8 with respect to the substrate lattice. [9] Correlating the reciprocal space periodicities to the real-space data, we identify am otif containing four molecules.T hree of these molecules are arranged regularly in ak agome pattern (Figure 1A,highlighted in yellow). Thefourth one appears in the pore of the kagome pattern. Theorientation of the molecules in the pores is non-periodic,c ausing local variations in the long-range order. [10] Consecutive images revealed alterations of the same guest molecule between different orientations ( Figure 1D and Supporting Information Movies S1). In contrast to aB rownian ratchet, [11] no preferred directionality of the switching is expected for the thermally activated motion of the guest molecule in thermal equilibrium. An analysis of the rotational events showed no clear preferential direction (see Supporting Information Figure S1). Thus the supramolecular kagome is considered as a2Dmatrix hosting single guest molecules in various orientations,r eminiscent of the assemblies of atriangular discotic liquid crystal on graphite [7a] and trimeric supramolecules of bisphenol Ao nA g(111). [7e] To identify the chemical state of the molecules within this self-assembly,w ec arried out X-ray photoelectron spectroscopy (XPS) measurements.T he most informative core-level region is the O1sr egion (Figure 2, left). Thes ignal is composed of three components in ratios of % 6:1:1, in order of increasing binding energy.T he higher binding energy components in ar atio of 1:1a re consistent with signals from the hydroxy and the carbonyl Oatoms of the carboxylic acid moiety (bpp-COOH), while the lower binding energy component can be assigned to Oatoms in carboxylate moieties (bpp-COO À ). [12] TheC 1s signal (Figure 2, right) further supports the presence of carboxylate (orange) and carboxylic acid (green) moieties in aratio of approximately 3:1(detailed fitting parameters and respective assignment are summarised in Supporting Information Table S1). As the self-assembled structure contains "stators" (arranged in ak agome pattern) and "rotors" in the same 3:1ratio,weinfer that the "stators" are chemically modified bpp-COO À molecules (bottom inset in Figure 2), and the "rotors" are pristine bpp-COOH molecules (top inset in Figure 2). We therefore associate the stabilisation of the structure after annealing to 373 Kwith the formation of the carboxylate species,w hich is not present after the RT deposition (Supporting Information Figure S2).
After identifying the chemical state of the molecular components,w ea nalysed their relative arrangement. From  the STM contrast, we can identify the orientation of the pyridine-carboxy moiety (hereafter head group). However, the pyrazole orientation is not directly evident and similar contrast can arise from different planar rotamers ( Figure 1C and Supporting Information Figure S3). Thehighly symmetric nature of the kagome network suggests syn,syn-or anti,anticonfigurations of the Natoms in adjacent pyridine-pyrazole units.A st he anti-anti conformation is the most stable in the solid state, [13] we do not anticipate the expression of adifferent rotamer after room temperature deposition. To ensure that this is also the preferred conformation on the surface following the deprotonation, we performed DFT modelling of the supramolecular network on Ag(111) with both configurations.T he anti,anti-structure is favoured by 1.86 eV per unit cell. Ac omparison of the hydrogen bonding schemes of the kagome syn,syn-and the anti,anti-rotamers can rationalise al arge energy difference between the two structures (see Supporting Information Figure S4). In addition, the preferred orientation of the guest molecule matches the STM images only for the anti,anti-configuration (see larger pore of kagome pattern in Figure 3and Supporting Information Figure S5 for DFT geometry optimisation of the syn,syn-structure). Thefcc hollow site was determined as energetically favoured for the pyridine of both anti,anti-bpp-COOH and bpp-COO À (Supporting Information Table S2) and was used to position the molecular units forming the kagome structure on Ag(111). Importantly,t heoretically simulated STM images,o btained from DFT calculations,a lso reproduce the relative contrast difference of the head group compared to the pyrazole side groups.T he pyridine-carboxylic acid appears brighter than the pyridine-carboxylate,r eaffirming the assignment of chemically modified bpp-COO À tectons constituting the 2D kagome pattern on Ag(111) ( Figure 3B,C).
Subsequently,w ea nalysed the orientation of the guest molecules in our "frozen" system ( Figure 4B)b yi nspection of STM images acquired at 4K(see Supporting Information Figure S6 for an example). In general, this is found to be  within 158 8 of the head-to-head orientation between the guest molecule and the host species.W eexpressed this as the angle of rotation of the guest molecule within the pore,w ith 08 8 being ag uest carboxylic acid to host carboxylate head-tohead arrangement (indicated in the blue frame of Figure 4B). Remarkably,wefound aclear signature of an enantiospecific interaction for the guest molecules:7 1% were rotated clockwise,2 6% were not rotated and 3% were rotated anticlockwise.T he clockwise angle ((CW), orange in Figure 4B)i mplies that the carboxylic Hatom is expressed predominantly as as ingle surface enantiomer.I ts hould be noted that bpp-COOH is an achiral molecule,b ut confinement of the carbonyl carbon of bpp-COOH to the surface plane gives rise to two surface enantiomers depending on the position of the carboxylic Hatom. Fort he remaining molecules,t he position of the carboxylic Hatom of the bpp-COOH guest cannot be deduced from the orientation (26 %, blue in Figure 4B), whereas only 3% exhibit clearly the anticlockwise bpp-COOH enantiomer ((ACW), green in Figure 4B). Therefore,t he host network induces chiral organisation of the guest molecules.T he origin of this effect can be found by looking at the registry of the kagome network on the substrate.
Enantiospecific molecular interactions on solid surfaces may stem from chiral steps or kinks [14] or by molecular chiral modifiers. [15] Unlike other reported 2D kagome networks, [16] the one reported here is (if considering the unsupported layer) achiral, as is the bare Ag(111) surface.The origin of the enantiospecific interaction can be found in the twist when superposing the supramolecular structure to the substrate symmetry. [17] Indeed, the superposition of two layers with atwist angle has been shown to give rise to chiral films [18] with topologically protected chiral 1D edge states. [19] Here,t he rotated registry of the kagome network with respect to the substrate leads to CW-a nd ACW-domains of the kagome network, which correspond to clockwise and anticlockwise rotations,respectively,ofthe kagome network with respect to the substrate lattice vectors ( Figure 4A). Note that the CW/ ACW network is not to be confused with (CW)/(ACW) guest. Figure 4B shows an analysis of solely ACW-domains at 4K. The CW-a nd ACW-domains have opposite chiral induction effects on the guest molecule,r esulting in different enantioselectivity (Supporting Information Figure S7). Importantly, the chiral induction is present even at RT,although the exact rotation angle of the guest has ab roader distribution (Supporting Information Figure S8) between 08 8 and AE 158 8.
To verify the origin of this chiral induction, we investigated the effect of the Ag(111) substrate.D FT energy optimisation of the supramolecular structure in the absence of the Ag substrate shows that the (CW)-and (ACW)-bpp-COOH guest orientations are energetically degenerate,both have aminimum at 08 8;and in addition, reflection planes at 08 8 and 608 8 transform the energy landscape of the (CW) into that of the (ACW). Similarly,onthe bare achiral Ag(111) surface (with no kagome network present), the rotational energy landscapes of the (CW) and (ACW) have energetically degenerate minima;t hese however occur at different orientation angles for the two surface enantiomers and away from 08 8.A ccordingly,t he reflection planes now occur at different angles.T his means that the degeneracyo ft he minima of the (CW) and (ACW) enantiomers is broken when one adds the effect of both the substrate and the kagome network (Supporting Information Figure S9). Consequently,i tc an be inferred that it is the non-zero twist angle between the kagome network and the Ag(111) which breaks the symmetry between (CW) and (ACW) and is thus responsible for the guest chiral induction.
Themost frequently observed orientation of guest to host molecule matches well with the DFT-optimised guest geometry.H aving observed the switching of the position of the molecule in consecutive STM images at RT ( Figure 1A and Supporting Information Movies S1), we considered this am odel system for ac onfined rotation controlled by Hbonds.T ob etter understand the dynamics of this system, we performed adual experimental and theoretical investigation.
We acquired systematically time-resolved STM data of the switching events as af unction of the temperature, tunnelling bias and tunnelling current. We find that the range of tunnelling conditions employed here does not influence the switching events,s ow ec an use STM to follow the natural thermally activated rotation of the guest molecules within the nanopore (Supporting Information Figure S10). However,the possibility of tip-induced activated rotation under different tunnelling conditions exists.N oc orrelation of the rotations and positions of the neighbouring caged guests was noticed, we therefore considered them as independent rotors.W e recorded the frequencyo fr otation events by % 1208 8 as afunction of temperature (see plot in Figure 5Aand Methods in Supporting Information). With an Arrhenius equation fitting,the barrier is determined to be 0.95 eV AE 0.07 eV with acorresponding pre-exponential factor of % 2.3 10 14AE1 s À1 .
Our theoretical investigations provided am ore detailed view of this process.I ne xcellent agreement with the experimentally deduced value,arotational barrier of 0.94 eV is computed ( Figure 5). Them ain contribution to this barrier is the lateral bonding of the rotating guest to the kagome network pore (0.77 eV for rotation in the network alone,Supporting Information Figure S9). We identify different intermolecular H-bonds in the charge redistribution maps corresponding to the charge density difference of the separate molecular constituents and the Ag substrate from the adsorbed host-guest system ( Figure 5C). Thes ignature of an attractive interaction is al ine of alternating yellow and green lobes,which represent isosurfaces of the same value of electron accumulation and depletion, respectively. [20] The simulated 158 8 steps of a3 608 8 rotation are animated in Supporting Information Movie S2. Five strong hydrogen bonds can be identified in the second most frequently observed geometry ( Figure 5C,d irect head-to-head, 08 8), where the kagome structure maximises its interaction with the guest molecule.Atthe global energy minimum, where the interaction of the guest molecule and the substrate is also optimised (0.07 eV lower in energy), three stronger and two weaker hydrogen bonds form, as illustrated by the charge redistribution visualised in Figure 5C,1 5 8 8.A sm entioned above,t he twist angle between Ag(111) and the kagome network is responsible for this energy minimum. Thee nergy maximum is found at aposition where the guest does not have significant bonding with the host ( Figure 5C,6 0 8 8).
We also show that the charge transfer between the guest molecule and the host (comprising the kagome network and the Ag surface) correlates well with the height of the rotational barrier:t he charge transfer is greatest at the minimum of the rotational energy landscape,and least for the most unfavourable guest orientation at 608 8 (Supporting Information Figure S11). Comparison with the confinement of an (ACW)-bpp-COOH guest in an ACW-domain of the kagome network and its rotation (Supporting Information Figure S12) further supports the observation of chiral induction.
Interestingly,b ys crutinising the optimised DFT geometries,the adaptability of the host with respect to the guest dynamic position in the optimised geometries becomes evident ( Figure 5B and Supporting Information Movie S2). Displacements of 0.78 can be detected for the centre of the pyridine ring, in line with dynamic porous features of responsive coordination polymers. [21] As af ootnote,w ea lso noticed as tatistically insignificant number (less than 1%)o f guests rotating too fast to be monitored with our scanning speed in our experimental data. We tentatively attribute these to potential bpp-COO À guests.C onsideration of bpp-COO À guests by DFT found indeed asignificantly reduced barrier of 0.11 eV for rotation of such guest molecules.H owever,t he bpp-COOH guest is significantly favoured by 0.81 eV/unit cell (Supporting Information Figure S13), further reinforcing our interpretation.

Conclusion
We report on the spontaneous self-assembly of bpp-COO À molecules on aw ell-defined surface into as table kagome network filled with bpp-COOH guests in its pores. Theregistry of the kagome network on the Ag(111) substrate induces enantioselective organisation of the guest molecule inside the pores of the kagome network. Ther otation of the guest molecule inside the pore is controlled by the kagome network:t he chemical structure of the pore wall provides flappers that pin the guest molecule and prevent its unhindered rotation. Thespontaneous formation of the host-guest assembly reported in this study is am odel system for the Figure 5. Dynamics of caged bpp-COOH guest in the kagome network. A) Arrheniusp lot of rotation frequency determined by STM data in order to deduce the energy barrier.B)DFT-derived energy relative to the optimised geometry (guest at 158 8)i nthe path leading to aguest rotation of 1208 8 for (CW)-guestinanACW-domain.T he rotation angle represents the clockwise offset from the head-to-head orientation. C) The panels display the optimised geometries and superposedd ifferential charge distribution at different rotation angles of the guest. (C, N, O, H, and Ag atoms are shown in black, blue, red, white and grey,respectively. Yellow and green lobes indicate electron accumulation and electron depletion, respectively,with isosurface values of 1.2 10 À3 ebohr À3 .) investigation of delicate,d ynamic,a nd enantiospecific interactions in confined spaces,w hich could help design enantioselective heterogeneous catalysts.W ef urther anticipate that such ordered large-scale networks with embedded individual switching units could be promising components for developing molecular nanodevices.