Template-Assisted Proximity for Oligomerization of Fullerenes

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Template-Assisted Proximity for Oligomerization of Fullerenes Kum-Yi Cheng, Shern-Long Lee, Ting-Yang Kuo, Chih-Hsun Lin, YenChen Chen, Ting-Hao Kuo, Cheng-Chih Hsu, and Chun-hsien Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00314 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Langmuir

Template-Assisted Proximity for Oligomerization of Fullerenes Kum-Yi Cheng, Shern-Long Lee,† Ting-Yang Kuo,‡ Chih-Hsun Lin, Yen-Chen Chen, Ting-Hao Kuo, Cheng-Chih Hsu and Chun-hsien Chen* Department of Chemistry and Center for Emerging Material and Advanced Devices, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan ABSTRACT: Demonstrated herein is an unprecedented porous template-assisted reaction at the solid-liquid interface involving bond formation, which is typically collision-driven and occurs in the solution and gas phases. The template is a TMA (trimesic acid) monolayer with two-dimensional pores that host fullerenes, which otherwise exhibit an insignificant affinity to an undecorated graphite substrate. The confinement of C84 units in the TMA pores formulates a proximity that is ideal for bond formation. The oligomerization of C84 is triggered by an electric pulse via an STM (scanning tunneling microscope) tip. The spacing between C84 moieties becomes 1.4 nm, which is larger than the edge-to-edge diameter of 1.1~1.2 nm of C84 due to the formation of intermolecular single bonds. In addition, the characteristic mass-to-charge ratios of dimers and trimers are observed by mass spectrometry. The experimental findings shed light on the active role of spatially tailored templates in facilitating the chemical activity of guest molecules.

Introduction Porous monolayers are two-dimensional (2D) assemblies whose void size, shape, and inherited functional groups can be tuned via the selection of a variety of molecular building blocks.1-4 The porous template offers a versatile platform for improving the fundamental understanding of supramolecular chemistry, such as host-guest recognition and 2D co-crystallization.5-9 The pores confine molecular motion and facilitate, for example, the molecularly resolved imaging of unimolecular photoisomerization of a thiophene-bridged diarylethene.10 The isolated compartments have been suggested to function as reaction vessels1,2,11 which has yet to be realized. In addition, reactions involving pore-assisted bond formation are unexplored. To promote chemical reactions, it is typical to raise the concentrations of reactants and thus to increase the frequency of collisions that bring the reactants into sufficient proximity, a prerequisite for the subsequent formation or cleavage of chemical bonds. Alternatively, the required conditions can be provided by 2D porous templates in which the confined space around the reactants creates intermolecular proximity at the solid-liquid interface. To prove this concept of pore-assisted reactions, our model study examines an electrically driven [2+2] cycloaddition of fullerenes12 assembled by a porous monolayer of TMA13-16 (trimesic acid, Figure 1a). This cycloaddition reaction has been observed by UHV-STM (ultrahigh vacuum scanning tunneling microscope) studies17 on C60 multilayers deposited by thermal evaporation. When the tip was scanned over the C60 multilayers (Ebias = −2.0 V, tip-grounded), the probed sites exhibited dents, indica-

tive of a shortened distance across 2~5 layers due to the oligomerization of C60.18 The mechanism proposed by Nakayama and co-workers12 involves inelastic scattering of tunneling electrons in which one or two electrons are excited from lower states and fill the bonding orbital of the reaction intermediate through the negatively charged substrate (for details, see the Supporting Information of Reference 12).

Results and Discussion Figure 1a illustrates the porous template. TMA possesses three-fold symmetry with carboxylic acid functional groups at the meta positions, which form a honeycomb structure networked via hydrogen bonding.14,19-20 The distance between the centers of neighboring pores is approximately 1.7 nm,20 and subtraction of the van der Waals spacing of the TMA framework leads to an internal diameter of the cavity of ~1.0-1.1 nm.9,21 Among the commercially available fullerenes, the size of ellipsoidal C84 (1.1~1.2 nm)22,23 is seemly suitable for fitting in the voids of the TMA host. However, the coverage of full monolayers is unattainable by spontaneous adsorption. Similar findings were reported in the literature, where it was shown that fullerenes did not reside at the TMA pores but were wedged at the narrower defect sites of the domain boundaries.24 Herein, we will present a simple yet crucial procedure for improving the coverage for less-than-ideal host-guest pairs. Sufficient proximity between C84 units is thus reached to promote subsequent bond formation oligomerization prompted by an electric pulse. Figure 1b shows the typical honeycomb features of TMA monolayers on HOPG (highly oriented pyrolytic graphite). The details of the pores and submolecular lobes are

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resolved (inset), consistent with literature reports.21,25 A 2.0-μL aliquot of C84 was placed on TMA-preadsorbed HOPG. The coverage was low even when the saturation concentration (0.5 mM,26 Figure 1c) was employed, although 20-nm C84 patches were observed occasionally (Figure S1e). A different preparation scheme performed by placing mixtures of TMA and saturated C84 on bare HOPG yielded similar results of low C84 coverages. Given that [2+2] cycloaddition reactions require close proximity between reactants, it is necessary to increase the coverage of C84. A nearly full monolayer of C84 was obtained, as presented in Figure 1d. This improvement was achieved by the removal of solution with a piece of tissue paper (Figure S1).27-30 In Figure 1d, the defects outlining the C84 islands are domain boundaries across which the C84 units are not aligned in parallel. Although there are point defects and line defects, this is the first reported fullerene assembly to achieve a full monolayer by utilizing the traditional effortless drop-cast method.

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coronenes in porous monolayers of BTB (1,3,5-tris(4carboxyphenyl)benzene).5,19,28,31

Figure 2. Electric pulse-triggered oligomerization of C84. STM images of a C84 assembly on the porous TMA template a) prior to and after application of an electric pulse of b) −3.5 V and e) −4.0 V for 10 μs. Histograms of the interfullerene spacing c) before and d) after the pulse. To obtain the distribution, each histogram was measured from ~600 C84 in four 2 experiments. Image size: 30 x 30 nm . Imaging conditions (Ebias, itunneling): −0.90 V, 0.10 nA.

Figure 1. TMA porous layer and improvement in the C84 coverage. a) Model of the TMA template and STM images of b) the TMA template and C84 adsorption c) prior to and d) after the removal of solvent with a piece of Kimwipe. The internal diameter of the cavity and the center-to-center distance are 9, 21 20 ~1.0-1.1 nm and ~1.7 nm , respectively. Solutions: (b) sat26 urated TMA (0.65 mM) in octanoic acid; (c, d) saturated C84 24 (0.5 mM) in a mixed solvent of octanoic acid and phe2 nyloctane (v/v, 1:1). Image size: (b) 30 x 30 nm (inset, 5 x 5 2 2 nm ); (c, d) 80 x 80 nm . Imaging conditions (Ebias, itunneling): (b) −0.90 V, 0.30 nA; (c, d) −0.90 V, 0.10 nA.

It is worth nothing that the images of C84 exhibit facets (Figure 2a) or lobes (Figure S2) corresponding to the molecular orbitals of fullerenes.17,18,23 This observation implies that the host-guest interactions stabilize C84 and restrict its movement. Otherwise, the guest molecules would not be resolved in detail but would appear as round spheres, such as those observed for the spinning of one square Cu-phthalocyanine or the rotation of three

By taking advantage of the proximity facilitated by the prepatterned TMA pores, an electric pulse more negative than −3.5 V (in 10 μs, tip-grounded) enables C84 oligomerization to occur. For example, panels a and b in Figure 2 were acquired at the same location prior to and after applying an electric pulse, which transformed the morphological features dramatically. In panel a, although the C84 units appear to be densely arranged, the circumferences of the units are outlined distinctly. Neighboring C84 units become fused after application of a −3.5 V/10 μs pulse with the tip-substrate spacing defined by the tunneling conditions of 0.9-V Ebias and 0.1-nA itunneling. The outline of individual C84 units in panel b becomes smeared in the direction from the lower left to the upper right, in which the fullerenes turn into strings. Figure S3 is a histogram of the formation frequency of the oligomers, in which dimers and trimers are the most abundant and undecamers are the longest. The spacing between C84 moieties in the oligomers is determined by the peak-to-peak separation of their section profiles and was found to be 1.4 nm, slightly larger than the aforementioned van der Waals contact distance of 1.1~1.2 nm for C84 monomers.32 This change is consistent with the increase of 0.2 nm from monomeric C60 (0.7 nm) to bridged C60 (0.9 nm) and is ascribed to the formation of single bonds, which involve the protrusion of sp3 carbon atoms from the fullerene surface.33-35 In

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Langmuir terms of images a and b of Figure 2, the spacing distribution initially peaks at 1.7 nm (corresponding to the centerto-center spacing of neighboring TMA pores) and becomes 1.4 nm after oligomerization (see Figure 2c and d). This decrease in spatial separation between the nearest C84 units is not due to deterioration of the tip shape by the electric pulse because the tip shape governs the resolution, not the spacing. In addition, undistorted TMA lattices are well resolved (e.g., Figures 2e and S6a-c, copresent with oligomerized C84) after the deliberate application of strong pulses. At a sufficient distance from the pulsed position (e.g., ~1 μm), the images of monomeric C84 always exhibit a distinct circumference similar to that of Figure 2a. Moreover, the images in Figure S7 reveal the presence of a phase boundary that divides the oligomerized and monomeric C84 units (but with broken domains), and thus, the observation of oligomerization is not an artifact associated with pulse-induced reshaping of the STM tip. For experiments without the porous TMA host, even at the saturation concentration of C84, electric pulses initially produce bare HOPG due to desorption of the low density of C84 units (e.g., Figure 1c). After the collection of a few imaging frames, monomeric C84 readsorbs sparsely. No features resembling oligomerization are found, indicating the importance of the proximity imparted by the TMA template in the bond formation reaction. The products of the monolayer reactions at the liquidgraphite interface can be identified by laser desorption/ionization time-of-flight mass spectrometry (LDITOF-MS),36 which was therefore applied to demonstrate the success of the template-assisted reactions. Figure 3 was obtained on HOPG with TMA-hosted C84 monolayers without introducing an additional matrix. The carbon clusters ionized as singly charged C84+ ions (Figure S11). No apparent peaks with mass-to-charge ratios (m/z) larger than 1008 were found prior to treatment with electric pulses (Figure 3a). The inset of Figure 3b reveals the formation of dimers and trimers with peaks approximately at multiple integers of C84. The intensity of the monomer peak is orders of magnitude stronger than that of the dimer peak, ascribed to (1) the relatively larger laser beam spot (with a diameter of 40-50 μm or 2 x 103 μm2) than the oligomerized area (~17 μm2) generated by the STM tip and (2) the distinct differences in the efficiency of desorption/ionization, in which smaller molecules are ionized more easily. In the m/z range below m/z 1008 in Figure 3a, there are a series of peaks that differ by m/z 24, equivalent to two carbon atoms (Figure S12). This is well known laser-induced fragmentation behavior of fullerenes37,38 and is explained by the loss of C2 units at sites with five-membered rings, such as pentalene, pyracyclene, and indene.39,40 Although interesting, this feature is not pertinent to the present study, and thus, the detailed rationale is provided in the Supporting Information (Figure S14). STS (scanning tunneling spectroscopy) was carried out to probe the intrinsic electronic properties of oligomeric C84 at the solid-liquid interface. The spectra were typical-

ly analyzed based on the assumption that the tip bears a uniform electronic structure; however, the electronic structure is tip shape-dependent and accordingly could be altered by the electric pulses applied in this study. Furthermore, room temperature operation is associated with concerns about where exactly the STS spectra are sampled because thermal drift affects the relative lateral position of the tip to the surface. These problems were alleviated by the following tactics. The spectra were collected after pulse-triggered oligomerization for both the monomers and oligomers at a distance far away from and a location underneath the point at which the pulse was applied. Hence, the tip employed to acquire the STS spectra was not subjected to additional pulses. Regarding thermal drift, which was slower than 1 nm/min for our instrument, we followed literature procedures41,42 to limit the acquisition time per spectrum to 20 msec to minimize the problem. Even with these preventative measures, the above concerns still prevented from quantitative analysis of the spectra. Nonetheless, STS offers in situ information that is not obtainable with other methods.

Figure 3. LDI-TOF-MS spectra of C84 hosted by TMA pores on HOPG (a) prior to and (b) after application of electric pulses via the STM tip. For the sample preparation, oligomerization was performed by applying 40 pulses in a rectangle with an 8-column and 5-row formation, where neighboring sites were separated by ca. 0.6 μm. The insets show the m/z range of 1500−4000 derived from the full spectrum

Figure 4. Averaged STS spectra of the C84 monomers (black) and oligomers (red). The spectra were obtained after an electric pulse was applied to preserve the tip geometry. The spectra of the oligomers were measured first, and then the tip was positioned 1 μm away from the pulsed location for the monomers. Each panel was obtained with its own tip (i.e., traces obtained from different tips were not pooled together). Three panels are presented to show the general spectral features. The traces were the average of raw spectra, which are shown in Figure S9. No additional smoothing process was performed.

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Displayed in Figure 4 are STS spectra where one tip was utilized for each panel to obtain the traces of monomers (black) and oligomers (red) in one experiment. The raw data of the traces are shown in the Supporting Information (Figure S9). Three panels are presented herein to show the typical features of the spectra. The HOMOLUMO gap was ca. 0.8−0.9 V for the monomers, and it became wider after oligomerization. Explicitly, the onset potentials for the HOMOs (ca. −1.2 V) of the monomers and oligomers essentially overlap, yet the LUMO of the oligomers is ~0.2 V more positive than that of the monomers. The FWHM (full width at half maximum) of the LUMO appeared slightly broader and less defined after oligomerization, which was attributed to the relaxed molecular symmetry and the inhomogeneity of the oligomeric structrues.43 Spectroscopic studies show that the HOMO-LUMO gaps for dimeric C60 and C70 in the solution phase are 14,230 cm−1 (equivalent to 1.76 eV)44 and 1.80 eV,45 respectively, approximately 0.14 eV narrower than the respective monomeric forms of 15,300 cm−1 (~1.90 eV)44 and 1.94 eV.45 Although no corresponding information is available for dimeric C84, interestingly, the opposite is trend is observed in Figure 4, where the band gap is larger gap for C84 oligomers. In an STS study of C60 on Au(111), Torrente and co-workers46 demonstrated that a stronger adsorbatesubstrate interaction leads to a narrower HOMO-LUMO gap due to stronger energy level alignment. For example, the use of tetraphenyladamantane to lift C60 from Au(111) and to decouple the C60-Au(111) hybridization resulted in a HOMO-LUMO gap that was larger by ~0.2 eV.46 Similarly in the present study, because oligomeric C84 units do not align in parallel with the TMA pores (vide supra, Figure 2e), the oligomers do not sit inside the pores and thus lose the strong C84-HOPG interactions that occur with the monomers (Figures 2a, S2). Oligomerization occurs only when Epulse ≤ −3.5 V (e.g., Figures 2b and S5), consistent with the cycloaddition mechanism proposed by Nakayama et al. in which the bonding orbital of the intermediate is stabilized by electrons excited from lower states due to the inelastic scattering of tunneling electrons (vide supra).12 Other possibilities for the origin of the stabilizing electrons have not been ruled out; for example, another possibility is that the electron is captured transiently during an elastic process such as those of orbital-mediated tunneling.47 It was found that a stronger voltage and thus a larger current create a larger crater-like circumference in a monolayer of C84 oligomers (Figure S5). As the current increases, the density of ionized or polarized molecules increases, and repulsion can result in desorption. The motion might push a C84 unit against its neighbors and enable extensive propagation of the reaction. Alternatively, the motion can be pressure-driven and arise from the shockwaves generated by the electric pulse.48-50 It is interesting to note that although the fullerenes appear to be splattered, the TMA underlayer remains intact on HOPG (Figures 2c and S6).

Conclusions In conclusion, this study demonstrates for the first time the concept of pore-assisted chemical reactions. Control experiments without the TMA host show no oligomeric products, even in a saturated C84 solution. Densely packed fullerenes are created by a novel solvent removal scheme at the solid-liquid interface. STM images of the submolecular lobes of C84 reveal their non-rotating feature, which is attributed to the tight fit of the guest molecules in the space defined by the underlying TMA and the neighboring C84. Such close proximity fulfills one of the prerequisites of bond formation reactions. Oligomerization can then be initiated by an electric pulse and propagates across hundreds of nanometers. The resulting oligomers appear linear, with trimers being the most abundant type of oligomers and undecamers the longest. The occurrence of oligomerization is confirmed by mass spectrometry, where the molecular weights indicate the formation of trimers, and by STS, which reveals a larger HOMO-LUMO gap ascribed to weaker adsorbatesubstrate interactions that hamper the energy level alignment. The reaction occurs only when the applied bias reaches −3.5 V or more negative (tip-grounded). The mechanism has been proposed to include [2+2] cycloaddition between fullerenes, in which the reaction intermediate is stabilized by electrons injected via the substrate. The porous template is thus shown to be a platform that bring reactants together. The concept put forward in this study will allow the pores to be tailored for hosting reactions involving multiple molecules or components.

Experimental Section Solutions of TMA (Sigma-Aldrich, 98%) and fullerenes (C84, SES Research, 98%) were respectively prepared in OA (octanoic acid, TCI, 98%) and a mixed solvent containing OA, PO (1-phenyloctane, TCI, 98%), and TCB (1,2,4-trichlorobenzene, Sigma-Aldrich, 98%). Specifically, for the latter, due to the concerns of solubility, fullerenes were dissolved in TCB and subsequently introduced into a mixture of OA and PO to make up a solvent of TCB/OA/PO (v/v/v, 0.01:1:1). As a simple procedure of purification, C84 was centrifuged, and the supernatant was used to avoid impurities with large molecular weights. To assemble the porous template, a 2.0-μL aliquot of saturated TMA was placed on freshly cleaved HOPG (Bruker, ZYB-grade). After the addition of another 2.0-μL aliquot of the aforementioned fullerene solution, a piece of folded Kimwipes was placed against the edge of the HOPG susbtrate27,29,30 to absorb the solvent and increase the coverage of the fullerenes. STM images were acquired by a MultiMode NanoScopeIIIa instrument(Bruker) with typical imaging parameter values for Ebias and itunneling of −0.9 V and ranging from 10 pA to 0.3 nA, respectively. Homemade Pt/Ir tips were mechanically cut (80%/20%, diameter of 0.25 mm). The images were analyzed by SPIP software (scanning probe image processor, Image Metrology ApS). Mass spectra were obtained with a MALDI-TOF/TOF mass spectrometer (Autoflex Speed MALDI TOF/TOF system,

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Langmuir Bruker Daltonics) equipped with a third harmonic of Nd:YAG SmartBeamTM-II laser (355 nm). All the LDI MS spectra were acquired without matrix application. Operating conditions: positive polarity and linear mode; mass range of 500-6000 m/z; matrix suppression up to 450 Da; laser attenuator offset at 80% of the maximum laser power; laser repetition rate of 1 kHz.

ASSOCIATED CONTENT Supporting Information Figures S1-S14 with experimental details of STS and MS, data analysis, and additional STM images for the effect of the solvent removal method, the effect of the pulse strength, and boundaries of monomeric and oligomeric C84. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

ORCID Chun-hsien Chen: 0000-0001-5507-3248 Cheng-Chih Hsu: 0000-0002-2892-5326

Present Addresses †Institute for Advanced Study, Shenzhen University, Shenzhen, Guangdong, China 518060. ‡Department of Natural Sciences and Sustainable Development, Ministry of Science and Technology, Taipei, Taiwan 10622.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by grants from the National Taiwan University and MOST. The authors thank the group of Professor Sheng-Hsien Chiu for the purified C84, Ms. Fung-Mei Chen for the preparation of the TOC graph, and Mr. ErChien Horng for experimental assistance.

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Langmuir SYNOPSIS TOC Enzyme-like pockets that facilitate chemical reactions are imitated with a 2D porous host, which, together with a novel solvent removal tactic, drives C84 to form a monolayer at the solid-liquid interface. The prearranged close proximity of C84 enables subsequent oligomerization, exemplifying the applications of tailored porous templates with enzymatic activities.

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