Electric-Field-Mediated Reversible Transformation between

Jul 8, 2019 - By using an oriented electric field in a scanning tunneling microscope, one can locally control the condensation of boronic acids at the...
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Electric-Field-Mediated Reversible Transformation between Supramolecular Networks and Covalent Organic Frameworks Zhen-Feng Cai,†,∥ Gaolei Zhan,*,†,∥ Lakshya Daukiya,† Samuel Eyley,§ Wim Thielemans,§ Kay Severin,‡ and Steven De Feyter*,†

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†

Department of Chemistry, Division of Molecular Imaging and Photonics, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium ‡ Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland § Department of Chemical Engineering, Renewable Materials and Nanotechnology Group, Campus Kortrijk, KU Leuven, Etienne Sabbelaan 53, 8500 Kortrijk, Belgium S Supporting Information *

disassociated state, thereby becoming sticky and soft.8 The reversible state transformations in these boroxine-containing systems are achieved by the addition/removal of water. As for single-layer covalent organic frameworks (sCOFs), the reversibility of the polymerization was demonstrated by combined effects of temperature and water.18 In addition, by introducing a photosensitive group to the backbone of the precursor units, a photoresponsive sCOF was obtained.28 The boroxine bonds in this sCOF could be cleaved by irradiation and then be repaired by annealing. Besides, the controllable boroxine/boronic acid equilibrium can also be guided by the addition/removal of Lewis bases.25,31 We reasoned that electric field effects could affect boronic acid-based DCC too, potentially reversibly, and this may even occur locally. For this end, the use of scanning tunneling microscopy (STM) was considered a good choice, as it combines localized control of a switchable electric field, and high-resolution imaging. The idea of applying an electric field together with STM to obtain switchable surfaces has been successfully demonstrated by taking advantage of reversible dynamic interactions, such as hydrogen bonds,32,33 electrostatic interactions,34 π−π stacking,35 and metal−ligand interactions.36 In addition to these monocomponent systems, switching has also been demonstrated for bicomponent supramolecular systems37 and host−guest systems.38 To date, most of the studies about electric-field-induced phase transitions are based on the dynamics of noncovalent interactions. To the best of our knowledge, electric-fieldinduced switchable surfaces based on reversible covalent bonds are not explored yet. Herein, we have designed a surface model system to demonstrate the bidirectional guidance of a DCC system by an EEF, illustrated in Figure 1. By reversing the direction of the electric field that exists between the STM tip and a conductive solid substrate, one can locally control the on-surface polymerization/depolymerization at a liquid/solid interface. Consequently, the reversible transformation between selfassembled molecular networks (SAMNs) and sCOFs can be monitored at the molecular level.

ABSTRACT: By using an oriented electric field in a scanning tunneling microscope, one can locally control the condensation of boronic acids at the liquid/solid interface. The phase transition between self-assembled molecular networks and covalent organic frameworks is controlled by changing the polarity of the applied bias. The electric-field-induced phase transformation is reversible under ambient conditions.

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lectrostatic forces are long-range interactions that play a key role in most chemical systems in nature. Reactions involving redox chemistry are considered to be electric field responsive because of charge-separated processes.1,2 Theoretical studies suggest that an oriented electric field can also affect other types of reactions.3−7 For instance, recent research shows that a bimolecular carbon−carbon bond-forming reaction such as a Diels−Alder reaction involving reagents of ostensibly negligible polarity can be induced and accelerated by an oriented external-electric field (EEF).1 The EEF can affect chemical processes by stabilizing/destabilizing covalent species which have minor charge-separated resonance contributors.3,4 Thus, electric field effects make it possible to manipulate the kinetics and/or thermodynamics of chemical reaction processes. Dynamic covalent chemistry (DCC) of polymeric materials has attracted great interest in recent years because of its potential to obtain responsive and self-healing surfaces/ materials.8−14 Unlike conventional covalent bond formation, the DCC concept takes advantage of the reversible nature of bond formation, for example of disulfide,15 acetal,16 ester,9,10 imine,17 and boroxine bonds,8,18−24 to facilitate the generation of new covalent structures under thermodynamic control. Boronic acids are appealing for DCC, as the formation of the boroxine ring usually takes place under mild conditions.25 Boroxine ring formation has been applied to the synthesis of two-dimensional (2D) and three-dimensional molecular architectures, covalent organic frameworks (COFs), selfhealing systems, and responsive surfaces.8,26−30 For instance, a strong and stiff polymeric material cross-linked by dynamiccovalent boroxine bonds can be shifted toward the © XXXX American Chemical Society

Received: May 21, 2019 Published: July 8, 2019 A

DOI: 10.1021/jacs.9b05265 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society

phase A is mainly stabilized by van der Waals interactions between phenyl groups and weak B−O···H interactions. Under the same conditions, another self-assembled phase A′ also exists on the surface. The detailed description of phase A′ can be found in Figure S2. Covalently Bonded Networks of TBPBA. It is noteworthy that the noncovalent phases (A, A′) were observed when monitoring the surface with STM at positive sample bias. Interestingly, by flipping the polarity of the sample bias from positive to negative and continuous scanning the same area, a regular porous phase B emerged on the surface (Figures 3, S3).

Figure 1. Schematic illustration of the electric-field-induced reversible transformation between self-assembled molecular networks (SAMNs) and covalent organic frameworks (COFs).

Figure 3. (a) High-resolution STM image of porous phase B at 1octanoic acid/HOPG interface. Imaging conditions: Iset = 200.0 pA, Vbias = −0.60 V. (b) Structural model proposed for phase B.

A triangular shaped molecular building block (left top panel, Figure 1) used in this study, 1,3,5-tris(4-biphenylboronic acid) benzene (TBPBA),39 consists of three arms rotated by 120° and ended by boronic acid groups. The distance between two adjacent boron atoms of TBPBA is 2.1 nm based on density functional theory (CASTEP simulation). Self-Assembled Molecular Networks of TBPBA. The room-temperature deposition of TBPBA solution (5 × 10−5 M) on a highly oriented pyrolytic graphite (HOPG) surface gave rise to two types of self-assembled phases A and A′. The dominant phase A (Figures 2, S1) is close-packed and consists of triangular shaped features. Each triangle feature can

Structural analysis of the calibrated STM images (Figures 3a, S4) shows a hexagonal network with a unit cell vector of 2.3 ± 0.1 nm, which is in excellent agreement with the 2.35 nm size (predicted by density functional theory calculations) of the covalently bonded three-membered rings formed by TBPBA. Thus, the covalent formation of boroxine-linked sCOFs-1 was confirmed. Linear or bended features inside the cavities of sCOFs-1 indicate the coadsorption of 1-octanoic acid. The proposed structural model of sCOFs-1 is shown in Figure 3b. In addition, another polymeric phase B′ coexists on the surface and detailed analysis is shown in Figures S5−S6. In previous studies, annealing is required to create boroxinebased COFs. Here, the reaction takes place at room temperature with the assistance of an STM tip, which provides a local reaction environment so that the molecules positioned underneath can react with each other without thermal activation. The advantage of using triboronic acid is that only three molecules are required to form the macrocyclic sCOF substructure rather than six molecules for the case of diboronic acids derivatives.40 Thus, less boroxine bond formation/breaking is involved for the construction/decomposition of sCOFs, which makes the error correction easier to take place. However, it is still quite challenging to obtain highly ordered sCOFs from C3 symmetric building blocks compared to the linear analogues, which probably relates to the reduced on-surface mobility of the larger building blocks. Transition from SAMNs to COFs. The phase transformation from A to B is time-dependent, as illustrated in Figure 4. The initial image (Figure 4a) was obtained at positive bias (+0.3 V), and the entire scanning area was covered by phase A. Upon continuous scanning the same region at opposite bias (−0.3 V) and the analysis of sequential STM images (Figure 4b−e), phase A gradually shrinks starting from its domain boundaries/edges, which are labeled by white dashed curves. Meanwhile, some polymeric phases emerge and gradually organize into an ordered hexagonal network. A closer

Figure 2. (a) High-resolution STM image of the self-assembled phase A at the 1-octanoic acid/HOPG interface. Imaging conditions: Iset = 50.0 pA, Vbias = 0.50 V. (b) Proposed structural model of phase A.

be assigned to a single TBPBA molecule, as the end-to-end distance between two bright arms (L, outlined in Figure 2a) of such feature is measured to be 2.1 ± 0.2 nm, which is in agreement with the calculated value (2.1 nm) of the distance between two adjacent boron atoms. The unit cell is a rhombus with cell parameters: a = 2.9 ± 0.2 nm, b = 2.7 ± 0.2 nm, γ = 96 ± 5°. Each unit cell includes two TBPBA molecules, and the packing density is 0.25 molecule/nm2. The assembly of B

DOI: 10.1021/jacs.9b05265 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society

Figure 4. (a−f) Sequential STM images showing the phase transition from SAMNs to sCOFs-1. (g) Histogram of the time evolution of coverage for sCOFs-1 and SAMNs. Imaging conditions: (a) Scan size = 70 × 70 nm2, Iset = 300 pA, Vbias = 0.30 V; (b−f) Scan size = 70 × 70 nm2, Iset = 200 pA, Vbias = −0.30 V.

Figure 5. (a−f) Sequential STM images of the sCOFs-1 in response to the polarity of EEF. (g) Histogram of the time evolution of coverage for sCOFs-1 and SAMNs. Imaging conditions: (a) Scan size = 60 × 60 nm2, Iset = 300 pA, Vbias = −0.30 V; (b−f) Scan size = 60 × 60 nm2, Iset = 300 pA, Vbias = 0.30 V.

view of this transformation process can be found in Figure S7. Cross section analysis reveals that no bilayer structures are involved in this process (Figure S8). After 130 min, phase A disappears completely and the surface is dominated by ordered domains of phase B (Figure 4f). Figure 4g shows the histograms of the coverage changes of these two phases over time. In order to know whether the formation of phase B occurred locally, a zoom-out STM image with a larger scanning area (160 × 160 nm2, Figure S9) was recorded after a sequence of STM images (80 × 80 nm2), revealing that the transformed area was mainly surrounded by phase A. Further consecutive scans at negative sample bias in this larger region decreased the coverage of phase A (Figure S9d). The whole process is slow under experimental conditions, which is in sharp contrast to STM tip-induced phase transformation for molecular assembly systems.37,38 The difference could be explained by the slow error-correction process during the crystallization of COFs: cycles of covalent bond formation/cleavage are required. Transition from COFs to SAMNs. The reversed process can also be induced. Covalently bonded sCOFs-1 can be destroyed and gradually transformed into SAMNs at positive sample bias. Figure 5a shows the initial state of the sCOFs-1. The polarity of the EEF was then changed from negative to positive. The in situ transformation process from sCOFs-1 to compact SAMNs was recorded in subsequent images of the same region (Figure 5). Some fuzzy features observed are attributed to the dynamic exchange. At several sites, domains of phase A appear and then grow. After 14 min, almost all sCOFs-1 domains are transformed into phase A (Figure 5f and 5g). The coverage of sCOF-1 decreases in time, while the

coverage of SAMNs increases. Compared to the initial state, no sCOFs-1 domains can be observed. The induced SAMNs are surrounded by disordered adlayers of TBPBA, demonstrating the local control over the DCC system by EEF (Figure S10). A similar phase transformation between SAMNs and sCOFs was also observed with the smaller building block 1,3,5-tris(4phenylboronic acid) benzene (Figures S11−S14), indicating that this phenomenon is not just limited to one compound. The transition from SAMNs to sCOFs-1 implies the polymerization of boronic acid derivatives, while the reversed process implies a hydrolysis of the boroxine links. Such a reversible switch can be controlled efficiently for a relatively wide sample bias range (from −0.6 to −0.1 V for negative bias, and from 0.1 to 0.6 V for positive bias). In contrast to experiments carried out at the 1-octanoic acid/graphite interface, at the air/solid interface, at positive bias voltage, ex situ synthesized sCOFs (Figures S15−S16) cannot transform into SAMNs, and vice versa (Figures S17−S19). This result indicates that the EEF itself is not enough to induce the conversion between sCOFs and SAMNs. To gain further insight into the mechanistic aspects, we then investigate the role of solvent. Upon adding a droplet of 1-octanoic acid on ex situ synthesized sCOFs-1, and continued scanning at positive bias conditions, the sCOFs-1 gradually transformed into SAMNs (Figure S20). However, when the added solvent is replaced from 1-octanoic acid to 1-phenyloctane, no phase transition was observed (Figure S21). This result reveals the crucial role of 1-octanoic acid in electric-field-mediated phase conversion. Indeed, adsorption of hydrophobic 1-octanoic acid molecules was clearly observed inside the cavity of the B and C

DOI: 10.1021/jacs.9b05265 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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B′ phase (Figures 3, S5), though only at negative bias. Such coadsorption behavior will improve the stability of the porous 2D polymer and mediate the local concentration of water. EEF could indirectly affect the water content at the 1-octanoic acid/ HOPG interface, due to the electric-field-induced adsorption/ desorption of the solvent molecules at negative/positive sample bias, respectively. This is in line with the fact that the local concentration of water affects the chemical equilibrium of dehydration reactions.18,19 In conclusion, two new sCOFs with different pore sizes were constructed on the HOPG surface based on dynamic boroxine formation. The on-surface polymerization process can be initiated locally, at room temperature, by an electric field, under the tip of a scanning tunneling microscope. Under the same conditions, except for the direction of the electric field, the sCOFs can be converted, locally again, into SAMNs of its monomer. Reversible structural transitions between sCOFs and SAMNs were triggered by changing the polarity of the electric field. The mechanism of the electric-field-induced reversibility of boroxine formation needs further exploration. Attempts to investigate multicomponent systems and other linkages (such as boronate ester and imine bond) for sCOFs formation are underway. This ability to control chemical reactions with electric fields could be used in switchable molecular architectures, and lays a foundation for the development of smart surfaces and smart/self-healing polymeric materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b05265. Experimental methods and supporting figures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Samuel Eyley: 0000-0002-1929-8455 Wim Thielemans: 0000-0003-4451-1964 Kay Severin: 0000-0003-2224-7234 Steven De Feyter: 0000-0002-0909-9292 Author Contributions ∥

Z.-F.C. and G.Z. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Fund of Scientific Research − Flanders (FWO), in part by FWO under EOS 30489208, and Internal Funds KU Leuven. We thank Dr. E. Sheepwash for synthesizing the triboronic acids. W.T. and S.E. acknowledge financial support through the Accelerate3 project from the Interreg Vlaanderen-Nederland program and Flanders Innovation & Entrepreneurship and from KU Leuven Internal Grant 3E180424. D

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DOI: 10.1021/jacs.9b05265 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX