Stability of Guest-Incorporated 2D Molecular Networks - The Journal of

Oct 18, 2016 - The same protocol seems reasonable to fabricate hybrid monolayers yet typically results in segregated domains. Demonstrated herein is a...
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Stability of Guest-Incorporated 2D Molecular Networks Shern-Long Lee, Chih-Hsun Lin, Kum-Yi Cheng, Yen-Chen Chen, and Chun-hsien Chen* Department of Chemistry and Center for Emerging Material and Advanced Devices, National Taiwan University, Taipei, Taiwan, 10617 S Supporting Information *

ABSTRACT: Molecular self-assembly, taking advantage of reversible intermolecular interactions, represents an efficient method to prepare ultrathin films exhibiting minimal packing defects. The same protocol seems reasonable to fabricate hybrid monolayers yet typically results in segregated domains. Demonstrated herein is a host−guest concept in which guest molecules are hosted in homogeneously patterned voids at the liquid−solid interface. However, 2D open lattices with low packing densities often suffer poor stability. In this study, the concept is realized by a 2D porous network assembled via 1,3,5-tris(4-carboxyphenyl)benzene (BTB) whose stability is significantly enhanced by hosting spatially matched pentacene or its analogues. The conformal contact between the nearest neighbors optimizes intermolecular interactions. Simulation results of molecular mechanics for a simplified model suggest that the hybrid lattice is about 250 kcal/mol per BTB pore more stable than guests such as coronene and Cu-phthalocyanine.

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mM were on the same order of magnitude.28 The different packing, attributed to the stabilization gained by coadsorbed solvent molecules with 9A being the strongest and 7A the weakest, suggests that the unoccupied porous network is less stable than the row assembly.28 Phase transitions from porous to close-packed structures can be activated by raising the temperature via adding a 10 μL droplet of ∼70 °C solvent29 or a thermostated sample stage.28 The transition temperature was unveiled at 43 °C in 8A and 55 °C in 9A.28 Importantly, Lingenfelder et al.30 noted that the porous-to-close-packed transformation eventually took place for all BTB concentrations examined (down to 5 μM) and concluded that the porous network was favored kinetically rather than thermodynamically. The temperature-induced transition was also found in ultrahigh vacuum (UHV) for metallic substrates such as Cu(111)35 and Ag(111),27 attributed to the deprotonation of the carboxylic acids.35 The transition was, however, irreversible in UHV. In addition, the unsteadiness of the hexagonal pores was attested by the structural self-adjustment to adapt guest molecules of cobalt phthalocyanine on graphene on Ir(111) in UHV.36 Alternatively, at the solid−liquid interface, phase transitions of BTB can be controlled reversibly by switching the electric polarity of the HOPG substrate, with a positive substrate bias generating the close-packed phase (Ebias > +0.4 V) and a negative bias yielding the honeycomb porous structure (< −0.35 V).29,30 Even with the presence of guest molecules such as relatively large nanographene (∼2 nm in diameter), phase transition due to electric polarity is reversible and furnishes mostly in 5 s,29 suggesting insignificant improvement of the

INTRODUCTION Spontaneous assembly is a well-received approach for molecular nanopatterning.1−5 Most examples take advantage of the weak, yet multiple, noncovalent interactions to reversibly optimize the synergic interplays which drive molecules arranged orderly.6−9 Subtle changes of the intermolecular interactions may act collectively and lead to polymorphic formation,10−14 especially for open adlattices, suggesting insufficient structural stability.15 Typical two-dimensional (2D) assemblies are composed of single components; relatively limited are the instances of multicomponents which presumably involve perplexing mechanisms and configure sophisticated motifs.16−25 Among those exhibiting 2D polymorphism, 1,3,5tris(4-carboxyphenyl)benzene (BTB, Figure 1) lately garners considerable attention mainly due to the presence of porous networks which attract perpetual interests from a broad range of research fields, including host−guest chemistry.26−30 In this study of formulating robust bicomponent assemblies, we demonstrate an interlock concept31 of having shape-matched guest molecules incorporated into the porous host of BTB. 2D lattices of BTB that have been reported were recently summarized by Lingenfelder et al. and were categorized into porous and close-packed phases.30 The latter includes oblique,32,33 compact,29,30,32 and row28 (viz., adapting a nearly upright orientation) with the order of increasing packing densities.34 The dominant structure depends on experimental parameters such as BTB concentration, solvent, substrate, temperature, and electric polarity of the substrate. Specifically, Gutzler and co-workers found that saturated BTB on graphite formed exclusively row in 1-heptanoic acid (7A), honeycomb pores in 1-nonanoic acid (9A), and a row-pore mixture in 1octanoic acid (8A) at room temperature, although the respective saturated concentrations of 0.77, 0.50, and 0.75 © XXXX American Chemical Society

Received: September 20, 2016 Revised: October 18, 2016 Published: October 18, 2016 A

DOI: 10.1021/acs.jpcc.6b09538 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Structures of the building block (BTB), the assembled porous network, and guest molecules.

Figure 2. Dependence of BTB assemblies on the electric polarity of HOPG substrate. (a) An STM scheme illustrating the sample-biased configuration in which the STM tip was the virtual ground. Images of typical BTB assemblies for substrate being (b) negatively (Ebias, −0.85 V) and (c) positively biased (Ebias, +0.85 V). (d) Real-time imaging of phase transition of BTB upon the change of substrate polarity. The electric polarity of substrate is denoted on the images. Lower panels display magnified images for the porous network at the left corner, close-packed structure at the , , and α: (b) porous motif, 3.2 (±0.3) nm, 3.1 (±0.2) nm, right corner, and models to manifest the detailed structure. Unit-cell parameters of 59° (±3°); (c) compact motif: 2.4 (±0.2) nm, 1.6 (±0.3) nm, 77° (±3°). Imaging conditions: Ebias, ± 0.85 V; itunneling, 150 pA. Other conditions: solvent, 8A; concentration of BTB, 1.0 μM in 8A.

−1.20 to 1.20 V and from 50 to 800 pA, respectively. The reported lattice parameters were calibrated by the unit-cell vectors of the underlying HOPG using an SPIP software (scanning probe image processor, Image Metrology ApS). The chemicals were commercially available and were used as received, including octanoic acid (8A, ≥ 99%), 1-phenyloctane (≥98%), 1,3,5-tris(4-carboxyphenyl)benzene (BTB, ≥ 99%), coronene (≥97%), Cu-phthalocyanine (CuPc, ≥ 99%), tetracene (≥99%), pentacene (≥99%), 6,13-pentacenedione (≥99%), 5,7,12,14-pentacenetetrone (≥98%), quinacridone (≥98%), and irregularly shaped polyaromatics such as picene (≥96%) and 3,4-benzopyrene (≥96%). BTB and guest molecules were weighed and dissolved in 8A and 1-phenyloctane, respectively. For host−guest systems, the solutions were premixed with the host and guest solutions (1:1 (v/v)) and a 10 μL droplet was placed on the HOPG surface for STM imaging. HyperChem 7.5 was employed to perform calculations of Molecular Mechanics (MM) Force Field for the stabilization energy of host−guest assemblies. A two-layer sheet of graphene (2500 carbons) was constructed to represent the graphite

structural stability by the inclusion of large polyaromatic compounds. The approach of creating robust and coassembled monolayers via porous templates is unattempted. Given that the behaviors of guest molecules in BTB pores are relatively unexplored, herein, the stability of guest-incorporated 2D networks is interrogated by the above-mentioned procedures of switching electric polarity and thermal treatment. Among the guest molecules examined, pentacene and its analogues do not allow the otherwise observed phase transition. This finding is supported by simulations of molecular mechanics.

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EXPERIMENTAL SECTION STM experiments were performed at the liquid−solid interface at room temperature (ca. 25 °C) using a MultiMode NanoScopeIIIa (Bruker) operating in the sample-biased (Figure 2a) and constant-current modes. STM tips were mechanically cut Pt/Ir wires (80%/20%, diameter 0.25 mm). HOPG (highly orientated pyrolytic graphite) was purchased from Advanced Ceramics (ZYB grade, Advanced Ceramics Inc.). Imaging conditions of Ebias and itunneling were ranged from B

DOI: 10.1021/acs.jpcc.6b09538 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 3. Images of (a) BTB−coronene and (b) BTB−CuPc under a negative sample bias and phase transition of BTB−CuPc (c) upon reversing electric polarity and (d) 2 min later. In panel c, the blue dashed line and the yellow arrow indicate the change of electric polarity and the imaging , , and α: (a) 3.1 (±0.2) nm, 3.2 (±0.2) nm, 60° (±2°); (b) 3.2 (±0.2) nm, 3.2 (±0.3) nm, 62° direction, respectively. Unit-cell parameters of (±3°). The green stars in (a, b) manifest the BTB pores. Inset of the lower panel of (b) illustrates the side view of two CuPc stacked in a BTB pore. Other conditions: Ebias, (a, b) −0.85 V, (d) +0.85 V; itunneling, 150 pA; solvent, 8A; concentration, 1.0 μM BTB, 1.0 mM coronene, 1.0 mM CuPc.

Coassembly of BTB with Unspecific Guest Molecules. There are only three literature studies associated with BTB− guest complexation, including BTB hosting coronene at the HOPG−fatty acid interface29,37 and phthalocyanine under UHV environments.36 Accordingly, this section starts from the two molecules. Panels a and b of Figure 3, respectively, display STM images for guest molecules of coronene and CuPc hosted in the BTB template. The bright features encompassed within hexameric BTB are distinct from the dark voids that appeared in Figure 2b. The green stars depicted in Figure 3a,b indicate that the BTB network is unaffected by the insertion of coronene or CuPc molecules, different than the UHV example in which the honeycomb pores are flexibly distorted to accommodate two CoPc per site.36 For the case of coronene, Lackinger and coworkers37 suggested that the bright features represent three rotating guest molecules based on the results of MD simulations (molecular dynamics). Hence, the movements of coronene make it impossible to be resolved at the submolecular level.29,37 It is worthwhile to note that the dimension of BTB voids is substantially larger than those of the guests (coronene, 1.0 nm;37 CuPc, 1.3 nm36). The relative dimensions are illustrated in the lower panels of Figure 3a,b. Similarly to the case of coronene, the somewhat spacious pores render additional room for CuPc to rotate or translate and lead to the shape of the bright feature being circular rather than square in STM images. The arrows in Figure 3b indicate that some guest sites are brighter than others. Those are tentatively attributed to cofacially stacked CuPc molecules (inset of Figure 3b).

substrate. The BTB porous network and guest molecules were placed 3.5 Å above the first layer of graphene. The adlattice structures of host alone and host−guest complexes were optimized by MM+, and the corresponding energy gained by the coassembled guest molecules was derived.

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RESULTS AND DISCUSSION BTB Assemblies Prior to Hosting Guest Molecules. Before the exploration of host−guest hybridized 2D assemblies, we first examined the structures of BTB alone which were subsequently subjected to the aforementioned stability test, namely, the stimulus of electric polarity. Figure 2b shows the formation of honeycomb pores, taking place in a very dilute solution (1.0 μM), even though it is not the thermodynamically favored form.27,28 At the lower panels of Figure 2b, an enlarged view and an illustration manifest the interconnections weaved through the terminal carboxylic groups. The repeated dimension of ca. 3.1 nm is consistent with literature reports.33 Note that Figure 2b was obtained at a negative potential of −0.85 V (substrate-biased). Panels c and d of Figure 2 demonstrate the instability of the porous structure via applying a positive polarity at the substrate.29,30 For a substrate bias of +0.85 V, STM unveils the close-packed structure shown in panel c. Panel d exemplifies a rapid switching which started immediately after the electric polarity was changed. The prompt reorganization, identical to literature results,29,30 is fully reversible. It was inferred and supported by simulations that the open structure might be stabilized by having the void space incorporated with coadsorbed solvent molecules28 which, however, are unresolved by literature studies28−30,32,33 nor this present inspection. C

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The Journal of Physical Chemistry C The circular features are ascribed to the mobile guest molecules in the space-confined nanowells. Such results are also reminiscent of weak host−guest interactions which are further supported by the following phase transitions switched via electric-field polarity. Figure 3c shows that the characteristics of BTB−guest hybrids become less distinct upon flipping Ebias from a negative potential to a positive one. Figure S1 demonstrates that the switching behavior is reversible and that CuPc molecules take the cavities after the formation of BTB pores. Close-packed structures develop mostly in 5−10 s or occasionally in 2 min (Figure 3d), slightly slower than that without guest molecules. Other scrutinized polyaromatic guests include rectangular tetracene and irregularly shaped picene and 3,4-benzopyrene (Figure 1). None of them exhibit pronounced interactions with the hexameric BTB voids. Coassembly of BTB with Pentacene and Its Analogues. In sharp contrast to the aforementioned systems, a distinct 2D motif is developed by placing on HOPG a 10 μL aliquot containing 1.0 μM BTB and 1.0 mM pentacene (Figure 4a). A high-resolution image is presented in Figure 4c where the green star can be regarded as six triangles and one hexagon. Each triangle contains one BTB molecule. By joining together, they delineate the hexameric BTB cavity, namely, the hexagon which now hosts three pentacene and one BTB molecule. Unlike the rotating circles of mobile guests such as coronene and CuPc, pentacene is immobilized in the BTB matrix, evident by the better defined molecular shape of a rectangle which exhibits nodal characteristics resembling the distribution of electron clouds of the HOMO level. It is intriguing that BTB molecules exhibit two types of tunneling efficiency, viz., apparent heights in STM images. Taking the green star in Figure 4c as an example, although the triangles all represent BTB molecules, the brightness goes strong-and-weak alternately for the neighboring ones. The model of Figure 4b illustrates another layout that a brighter BTB (in yellow) can be found at the center of six radially spread pentacene molecules and a dim one (in blue) is triangularly bounded by three pentacene. The tunneling efficiency of BTB is likely affected by the degree of lateral interactions with conjugated pentacene. Nonetheless, the possibility of the difference arising from BTB adsorption sites on HOPG lattice is not ruled out.38 With careful inspection of BTB with the stronger tunneling efficiency (i.e., yellow ones in Figure 4b), it can be found 10−15% BTB (e.g., yellow circles in Figure 4c) adapting an angle of 60° off from those at equivalent lattice sites. Figure 4d illustrates that, although the symmetry of the BTB template appears defective, the number of H-bonding remains the same as that of a perfect lattice and thus strengthens the stability of the BTB−pentacene coassembly. The robustness of the hybrid films is interrogated by the tests of positive sample bias (Figure 4e,f) and the above-mentioned thermal treatment by adding a 10 μL droplet of warm solution29 (∼70 °C, Figure S3). Opposite to all literature examples that the close-packed structure develops promptly, the BTB−pentacene motif is unaffected by these stimuli and thus is demonstrated to be a highly stable 2D lattice. MM (molecular mechanics) simulations are employed to evaluate the stability of the BTB−pentacene coassembly. The computations were modeled by having guest molecules incorporated in a single unit of hexameric BTB on two layers of graphene (Figure S4). The results show that the BTB− pentacene coassembly (−623 kcal/mol) is about 250 kcal/mol

Figure 4. (a−d) BTB−pentacene coassembly and (e, f) the stability evaluated by applying a positive bias. The model of panel b utilizes yellow and blue color to represent BTB molecules exhibiting strong and weak tunneling efficiency. To elaborate the defective sites (yellow circles) in panel (c), panel (d) utilizes yellow molecular models to highlight those BTB that rotate by 60° from the comparable position of vertices. In panel (e), the blue dashed line marks the location where the electric polarity was changed. The yellow arrow indicates the , 3.3 (±0.2) nm; imaging direction. Unit-cell parameters (panel b): , 3.4 (±0.2) nm; α, 60° (±2°). Other conditions: Ebias, (a, c) −0.85 V, (f) +0.85 V; itunneling, 150 pA; solvent, 8A; concentrations, 1.0 μM BTB, 1.0 mM pentacene.

per BTB pore more stable than those of BTB hosting three coronene (−366 kcal/mol) and one CuPc molecule (−378 kcal/mol). It seems plausible to ascribe the discrepancy to whether the guest molecules are immobilized or mobile because the former gains additional hydrogen bonding (Figure S5), van der Waals attractions, and adsorbate−substrate interactions. However, the magnitude to which the first two types of interactions can contribute is too small to account for the energy of ∼250 kcal/ mol. Specifically, for (aryl)CH···O(carbonyl) hydrogen bonds, Lackinger and co-workers37 derived ∼2.2 kJ/mol (equivalent to 0.53 kcal/mol) by MM calculations from a similar host−guest pair of one coronene enclosed by six 1,3,5-benzenetricarboxylic acid. For van der Waals interactions, the typical range is only 0.4−4 kJ/mol (equivalent to 0.1−1 kcal/mol). Although a larger adsorbate confers stronger adsorbate−substrate interactions, given the fact that a nanographene composed of 19 aromatic rings also rotates within the BTB pores,29 the static BTB−pentacene assembly is unlikely to be driven by D

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biased potentials. For the presence of irregularly shaped guests, the BTB pores remain vacant, indicative of weak host−guest interactions. Guest molecules of coronene and CuPc are found incorporated in the BTB pores. However, STM images cannot resolve submolecular features and suggest that the molecules are mobile within the nanowells of hexameric BTB. With coronene and CuPc hosted by BTB pores, the honeycomb takes slightly longer periods (5−10 s) to transform toward the close-packed structure after the electric polarity of the substrate is changed. For pentacene and its analogues, the components of the coassemblies are well resolved and the 2D structures are unaffected by the stability test. MM simulations show that the BTB−pentacene lattice is significantly more stable than its BTB−coronene and BTB−CuPc counterparts. Overall, our study demonstrates that stable hybrid assemblies can be built from spatially complementary host and guest molecules.

pentacene−HOPG interactions alone. Hence, the stability may result from the structural complementarity between the shape and size of pentacene and BTB. Such an interlocked conformation immobilizes the guest molecules and develops adsorbate−substrate interactions which are reinforced by intermolecular attractions such as van der Waals force and hydrogen bonding between (aryl)CH and carboxylic acids (Figure S5). Pentacene analogues are examined to elaborate the importance of a congruently matched molecular conformation to the stability of the BTB coassembly. The structures of 6,13pentacenedione and 5,7,12,14-pentacenetetrone are sketched in Figure 1. Figure 5 presents typical images of the hosted 2D

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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/acs.jpcc.6b09538. Additional STM experiments of stability test and images of quinacridone and MM simulations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +886-2-33664191. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank NTU, MOE, and MOST (Taiwan) for financial support. NTU (105R89641), MOST (103-2113-M002-006-MY3, 104-2113-M-002-017-MY2).

Figure 5. STM images and models for coassemblies of BTB with (a) 6,13-pentacenedione and (b) 5,7,12,14-pentacenetetrone. Other conditions are the same as those in Figure 4a.

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framework in which the guest molecules adapt lattice positions identical to those of pentacene. The images also show that the orientation of some BTB molecules is different from the others, the same as those circled or colored in yellow in Figure 4c,d. The coassembled lattices remain unaffected after being subjected to the stability tests of switching electric polarity and thermal treatment (not shown). For nonplanar quinacridone, an analogue of pentacene whose two sp2-hybridized carbon are replaced by sp3-hybridized nitrogen atoms (Figure 1), STM results reveal segregated domains of stacked quinacridone39 and the unoccupied porous BTB honeycomb (Figure S6). The fact that quinacridone and BTB interact differently than analogues of pentacene manifests how the conformation of guest molecules can optimize host−guest interactions and result in stable hybrid assemblies.

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CONCLUSIONS The 2D porous honeycomb composed of BTB building blocks is revisited. A novel aspect of stable host−guest coassemblies is discovered by polyaromatic guest molecules with shapes of circle, square, rectangle, and irregular ones. For plain BTB, the open structure responds immediately to the stability test of thermal treatments and electric polarity switching to positively E

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DOI: 10.1021/acs.jpcc.6b09538 J. Phys. Chem. C XXXX, XXX, XXX−XXX