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Applied Surface Science 313 (2014) 841–849

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om ep age: www.elsev ier .com/ locate /apsusc

olvent- and guest-responsive supramolecular self-assembly of,3,5-tris(10-carboxydecyloxy) benzene by scanning tunnelingicroscopy

ihua Cui, Xinrui Miao ∗, Li Xu, Wenli Deng ∗

ollege of Materials Science and Engineering, South China University of Technology, Wushan Road, Tianhe District, Guangzhou 510640, PR China

r t i c l e i n f o

rticle history:eceived 9 May 2014eceived in revised form 9 June 2014ccepted 15 June 2014vailable online 20 June 2014

eywords:elf-assemblyoadsorption monolayerydrogen bondingcanning tunneling microscopyolid/liquid interface

a b s t r a c t

Two-dimensional hydrogen-bonded networks formed in the self-assembly of 1,3,5-tris(10-carboxydecyloxy) benzene (TCDB) show regular solvent- and guest-induced supramolecular structuralproperties, which have been presented by scanning tunneling microscopy at the liquid–solid interfaceat ambient conditions. TCDB acting as a host template can entrap solvent molecules or �-electron-conjugated guest molecules to fabricate the flexible co-adsorption architectures, which are subject tothe balance between the hydrogen bonding of the host lattice and the van der Waals forces betweenthe host and the guest molecules. Hydrogen bonding among TCDB molecules is crucial to stabilize thehost networks to settle the system into a global minimum of Gibbs free energy. We also find a strongcorrelation between the structural parameters and the physical properties of the solvent. Statisticalanalysis shows that the unit cell volume of TCDB dissolved in nonpolar 1-phenylotane and n-tetradecaneshrank significantly compared with that of host–guest system, which fully reflects the coadsorption

effect of nonpolar solvent molecules. Our results identify that the kinetic effect of adsorption/desorptionas well as the solvent viscosity comes into play in tuning the two-dimensional self-assembled structures.Furthermore, mechanical calculations demonstrate that TCDB incline to adsorb with a larger dipole con-figuration in nonpolar solvents due to its dissolvability. It is believed that the results are of significanceto supramolecular host–guest chemistry and surface science.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

During the last two decades, supramolecular self-assembly asn important so-called “bottom-up” method in nanotechnology,as been widely studied and considered to be one of the mostromising ways to hit the ultimate target of producing new func-ional nano-materials with precision [1–3]. Thanks to the powerfultomic scale resolution of scanning tunneling microscopy (STM) inetecting single molecules and self-assembled monolayers (SAMs)4], tremendous progress on the structural control of various two-imensional (2D) highly ordered supramolecular architectures haseen made in lab research recently, with its potential applications

n cocrystallization [5,6], guest molecule adsorption [7–9], chiralrdering induction [9–12], and molecular electronics [13]. How-ver, the mechanism of controlling factors in the assembled process

∗ Corresponding authors. Tel.: +86 020 22236708.E-mail addresses: msxrmiao@scut.edu.cn (X. Miao), wldeng@scut.edu.cn

W. Deng).

ttp://dx.doi.org/10.1016/j.apsusc.2014.06.087169-4332/© 2014 Elsevier B.V. All rights reserved.

remains to be fully understood. Especially, the delicate interactionin governing the characteristics of supramolecular architecturessuch as coadsorption and host–guest also keenly demand fur-ther dedicated investigation [14–16], which is beneficial for theconstruction mechanism of novel heterogeneous molecular archi-tectures. Gutzler [17], Lackinger [18], de Feyter [19,20] and Samori[21] thoroughly modeled the thermodynamics at the liquid/solidinterface. Fundamental understanding of the driving forces andthe physical chemistry nature of host–guest systems offer us greatopportunities to clarify the noncovalent interactions such as hydro-gen bonding [7,8,22,23], dipole–dipole interaction [3,24–27] andvan der Waals forces [9,28], which are well known to play compet-itive and synergistic roles in the dynamic process.

Up to now, not only the well-documented modeling toexplain the kinetic and thermodynamic mechanism of the sol-vent effect, but also the interesting molecular nanoporous surface

structures function as a template to host the guest molecules,further to achieve a desirable molecular pattern with control-lable symmetry and structure have been demonstrated [29,30].Many groups reported significant supramolecular assemblies,

842 L. Cui et al. / Applied Surface Science 313 (2014) 841–849

Table 1Chemical structures of TCDB and solvents.

HOOC(H2C)10O

O(CH2)10COO H

O(CH2)10COO H

1,3,5- tris(10 -ca rbox ydec ylox y)-benze ne(TCDB )

1-phenylotane toluene n-tetradecane 1,2-dichloroethane heptanoic acid

sol vent CH3 CH 2Cl 2 O

OH

Table 2Physical properties of the used solvents and the different types of coadsorption effect.

Chemicals Boiling point (◦C) Viscosity(mN s m2)

Dielectric constant (ε/ε0) Dipole moment (Debye) aCoadsorption effect

1-Phenylotane 261–263 2.61 2.26 (20) 0/0.660 Yn-Tetradecane 253.7 2.18 2.01 (20) 0.0002 YToluene 110.6 0.623

0.5232.36 (25) 0.375 H-G

Dichloromethane 240 0.43 6.49 (20) 3.4 H-GHeptanoic acid 222–224 2.72 2.59 (22)

3.04 (30)1.570 H-G

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a Y” and “H-G” represent solvent and guest molecules respectively.

hich were formed by multi-carboxylic group compounds. Zengt al. [7,8,17,31–33] reported that the host matrix, 1,3,5-tris(10-arboxydecyloxy)-benzene (TCDB) can recognize and capture flatolecules containing large �-conjugated moieties such as CuPc,

60, and coronene [34–36]. Their results demonstrated that TCDBerved as an excellent host template to adjust itself to accom-odate guest molecules. Previous studies have shown that the

ydrogen bonding among TCDB molecules is crucial to stabilizehe host networks, and the van der Waals force is the dominantntermolecular interactions to organize into homogeneous stableost–guest networks [34–37]. However, to the best of our knowl-dge, in spite of the well-documented thermal-annealing [35] andhoto-induced [36] effects, there are no previous reports about theolvent effect on the 2D host–guest molecular networks of TCDB.olubility of the solute in particular solvent and kinetic parame-ers such as viscosity modulating surface mobility and adsorptiontability play important roles in the self-assembled process. Onequilibrium state can be broken down by thermodynamic param-ters, such as solvation enthalpy [20], hydrophobic forces [38],ffinity of solvent molecules to the substrate, or kinetic parame-ers such as viscosity and thus mobility of the molecules in solution39,40]. Up to now, although a series of in-depth investigations haveeen achieved [20,38,39], no conclusive theory is applied to explainhe solvent effect.

Herein, to further elucidate solvent effect on the assembledharacteristics of the nanoporous molecular networks, we are com-itted to systematically revealing the solvent and the template

ffect of TCDB through the formation of well-defined 2D networks

ith the addition of corrole and fluorenone in polar solvents.

he chemical structures of solvents and their physical proper-ies used in this work are listed in Tables 1 and 2, respectively.igh-resolution scanning tunneling microscopy (STM) images

fully reveal the distinct structural features of the host–guestsystems in five representative solvents: 1-phenylotane, toluene,n-tetradecane, dichloromethane and heptanoic acid. The selec-tion principle is motivated by their chemical characteristics.1-Phenylotane is a nonpolar solvent with both aromatic andaliphatic moieties, which has a long alkyl chain. Toluene anddichloromethane are strong polar and volatile solvents. n-Tetradecane is a nonpolar aliphatic solvent. Heptanoic acid servesas nonvolatile polar alkylated acid to explore the functional group

COOH. In this contribution, not only the characteristics of hydro-gen bonding between TCDB host lattices can be revealed, but alsothe non-equilibrium process involving intermolecular and interfa-cial interactions is considered, which are governed by a competitionbetween kinetic and thermodynamic factors.

2. Experimental

Detailed procedures of synthesizing 1,3,5-tris(10-carboxydecyloxy) benzene (TCDB) used in this study were similarto the method described in previous reports [34]. 1-Phenylotane,n-tetradecane, toluene, dichloromethane and heptanoic acid wereused as received from Aldrich. The preparation of the sampleswas as follows. First, TCDB was dissolved in different solvents ata concentration between 10−4 and 10−5 M, and then a drop ofthe mixture was deposited on a freshly cleaved surface of highlyoriented pyrolytic graphite (HOPG, quality ZYB, Bruker, USA). Afterobtaining the STM images of TCDB assembled adlayer clearly, adrop of corrole solution was applied onto the same HOPG. The

physical monolayer was spontaneously formed, and then theSTM experiment was carried out. All of the images were recordedwithin 5 h at the liquid–solid interface at ambient conditions usinga Multimode Nanoscope IIIa SPM (Bruker, USA). STM tips were

L. Cui et al. / Applied Surface Scie

Fig. 1. The proposed molecular model of TCDB network including two kinds ofhydrogen bonds. Two black squares (a) and a black ellipse (b) point out the differenth

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extending longitudinally with another chain from TCDB. The

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ydrogen bonds of the host networks.

repared by mechanical cutting from Pt/Ir wire (80:20, diameter.2 nm). All of the STM images were obtained using the constanturrent mode. Each specific tunneling condition was shown in thegure captions. The experiments were repeated in several sessionssing different tips and samples to check for the reproducibilitynd to avoid experimental artifacts. The images were shownithout further processing except flatting to move the tilting

ffect of the HOPG substrate plane.Theoretical calculations were performed with Materials Stu-

io 5.5 using density functional theory (DFT) as implemented inhe DMol3 package. Calculations of binding energy and dipole

oment are conducted by Perdew–Burk–Ernzerh (PBE) function,hich is used to describe exchange and correction. All of the com-

utations are all-electron spin restricted ones and are performedith minimal basis set and medium integration mesh. The conver-

ence thresholds for energy and electron density in self-consistent

ig. 2. (a) Constant current STM image (20 nm × 20 nm) of 2D self-assembly of TCDB at 1TM image (15 nm × 15 nm) of the monolayer of TCDB physisorbed at the 1-phenyloctauggested molecular model for the ordered adlayer based on the STM image.

nce 313 (2014) 841–849 843

iterations are 1.0 × 10−5 a.u. and 1.0 × 10−3 a.u. for gradient anddisplacement in geometry optimizations.

The possible hydrogen bonding interactions in the supramolec-ular adlayers were also calculated. Physisorption of the adlayers onthe graphite surface was modeled by Molecular Mechanics (MM)approach. The DREIDING force field, as implemented in the FORCITEtool pack of Materials Studio was used, because it is particularlyadapted to account for the hydrogen bonds that promote the self-assembled molecules.

3. Results

As previously reported [33–37], TCDB could assemble into large-area and homogeneous tetragonal cavities upon physisorption.The details of the cavity are worthy of discussion, because thehydrogen bonding plays a dominating role in stabilizing the hostnetworks. The proposed molecular models (Fig. 1) illustrate twotypes of hydrogen bonds among TCDB host template. Every twoTCDB molecules form a dimer connected via dimeric hydrogenbonds between carboxyl groups (Fig. 1a). DFT calculations revealthat two TCDB molecules interact through two pairs of O H· · ·Ohydrogen bonds as indicated by the black squares in Fig. 1a, furtherto form a cavity with each other. It is suggested that the length ofthe hydrogen bond is ∼2.53 A, and the hydrogen bond interactionof Fig. 1a is about 19.61 kcal mol−1 [35].

It is noticed that the third alkyl chain attached to the core

chains approach closely, thus the cavities in neighboring rows canbe connected through another pair of O H· · ·O hydrogen bonds(as indicated by a black ellipse in Fig. 1b) to construct the network.

-phenyloctane/HOPG interface. Iset = 497 pA and Vbias = 800 mV. (b) High-resolutionne/HOPG interface. The imaging conditions are Iset = 497 pA and Vbias = 785 mV. (c)

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44 L. Cui et al. / Applied Surfa

heoretical analysis shows that the measured length of the hydro-en bond is ∼2.55 A and the hydrogen bonding interaction is about0.32 kcal mol−1 for the host lattice.

.1. Assembled structure of TCDB in 1-phenyloctane

Fig. 2 presents representative STM images of the resultingssembly of TCDB in 1-phenyloctane after we applied a drop ofCDB saturated solution on the HOPG surface. Repeated scans withifferent tips reach the same supramolecular patterns. Bright spotsnd dark troughs can be distinguished easily from the STM image.wing to their higher electronic density, aromatic moieties areften observed to show higher tunneling efficiency, which resultsn a bright–dark contrast in the images [28]. Therefore, the largerright spots correspond to the benzene cores of TCDB, while thetriped troughs are contributed to the alkyl chains. Not all phenylroups appear with the same brightness, indicating that they doot have the same orientation with respect to the graphite lat-ice. The high-resolution STM image (Fig. 2b) shows the detailedelf-assembled structure. The alkyl chains can be divided intowo subsets depending on their orientation and contrast. On oneand, along the packed orientation of phenyl core in TCDB, twoO(CH2)10COOH side chains of vertically adjacent TCDB molecules

orm a dimer which is connected via double hydrogen bondsetween carboxyl groups. The hydrogen bonds are shown with dark

ntermittent junction resolved from the STM image. On the otherand, along the lamellar axis, the four linear alkyl chains indicatedy pink bars in Fig. 2c occupy the upper right and lower left sidesf the quadrilateral cavity. They are attached to the two diagonalCDB molecules showing dark troughs. The presence of parallelacked alkyl chains can stabilize the van der Waals interaction withhe graphite substrate, which clearly has an effect on the orderingf the alkyl chains: the alkyl chains along the molecular stackingirection of a run parallel with one of the major symmetry axesf graphite; those perpendicular to the direction of a are orientedpproximately perpendicular to the major graphite axis. To maxi-ize the substrate coverage favored for the reduced enthalpy, the

nterdigitated alkyl chains will pack in the approximately parallelrrangement giving rise to a closely packed structure.

On the basis of the adlayer symmetry and intermolecular dis-ances, a unit cell mesh is superimposed on the image in Fig. 2b.

e can clearly see that all the cavities in TCDB adlayer are filledith 1-phenyloctane molecules, which are shown as bright spots

n STM images due to the highly delocalized electrons of benzeneores. In the tetragonal cavity, paired benzene head groups occupyiagonally close to the phenyl cores of TCDB because of steric effect.he adjacent alkyl chains of 1-phenyloctane in the same cavity packarallel to each other. The length of alkane backbone is measuredo be 1.1 ± 0.1 nm, which is consistent with that of 1-phenyloctane

olecule. The unit cell parameters of the network are determinedo be a = 1.9 ± 0.1 nm, b = 5.5 ± 0.1 nm, and = 79 ± 1◦. A structural

odel for the adlayer is sketched in Fig. 2c which is in perfectgreement with the STM results.

.2. Assembled structure of TCDB in n-tetradecane

A highly ordered TCDB adlayer is also observed using n-etradecane as the solvent (Fig. 3a), which is different from thessembled structure of TCDB dissolved in 1-phenloctane ascribed tohe coadsorbed solvent molecules. The 2D self-assembled adlayerf TCDB has been examined in n-tetradecane with an appropriateolution concentration, as the solution concentration has no effect

n the structural formation. A high-resolution STM image (Fig. 3b)hows that two parallel arranged solvent molecules are entrappedn the cavities of TCDB. The length of the long rods extracted fromhe image is 1.2 ± 0.1 nm, in accordance with the theoretical length

nce 313 (2014) 841–849

of n-tetradecane (about 1.4 nm). Considering the consistency ofthe length, solution composition and steric constraint, the longrods are attributed to the n-tetradecane molecules coadsorbed inthe assembled adlayer as a counterpart. On the basis of the STMimages, a structural model for the tetragonal network is proposedin Fig. 3c. The unit cell is superimposed on the image in Fig. 2b witha = 1.7 ± 0.1 nm, b = 2.9 ± 0.1 nm and = 66 ± 1◦.

3.3. STM observation of TCDB host–guest structures at theliquid–solid interface

In the assembly of TCDB, previous reports have presented thatmost of planar guest molecules adopted a “face on” style [33–37]. Itis interesting to further investigate the formation of 2D host–guestassembly in nonpolar volatile solvents such as toluene and polarvolatile solvents dichloromethane. Here we report the cavitiesformed by TCDB host template to achieve the inclusion of corroleand fluorenone. We have tried to introduce corrole and fluorenoneinto the networks because of their steric size and significance insurface science.

3.3.1. Assembled structure of toluene/TCDB/corroleFig. 4a shows a stable molecular adlayer of TCDB/corrole

host–guest assembly using toluene as the solvent. When cor-role molecules are entrapped in the cavities of TCDB networks,highly uniform arrays of host–guest architecture are formed. Moredetailed structures can be observed from the high-resolution STMimage (Fig. 4b). A unit cell for the cavity is outlined with the param-eters of a = 2.0 ± 0.1 nm, b = 3.4 ± 0.1 nm and = 77 ± 1◦. There is noapparent difference from above observed TCDB coadsorption struc-tures using 1-phenyloctane and n-tetradecane as solvents. We canclearly identify that most of the cavities of TCDB are filled withcorrole dimers, which are stacked face to face along columns par-allel to the substrate. The guest molecules absorb nearly upright topack in the parallel rows. Though the molecule–substrate interac-tion is diminished in comparison with the planar adsorption, theintermolecular van der Waals force and �–� stacking interactionsbetween the aromatic surfaces can stabilize the structure. Twelveschematic corrole molecular are denoted as red and yellow bars inthe high-resolution image. Furthermore, we quantify the size of therods in detail. The inner width of the cavity is 1.7 ± 0.1 nm, whichis similar to the lattice constant about 1.6 nm (Fig. 4d) of closelypacked corrole columnar array [41]. This phenomenon suggeststhat spontaneous aggregation of corrole molecules take place inorder to construct host–guest architecture in the coadsorption pro-cess. The coincidence of the size and shape of corrole arrayed withthe TCDB cavities confirms the entrapment of corrole molecules. Toshow the adsorption structure more vividly, a model for the arraystructure of the two-dimensional host–guest unit is presented inFig. 4c.

3.3.2. Assembled structures of dichloromethane/(hydroxyl)fluorenone/TCDB and heptanoic acid/hydroxyl fluorenone//TCDB

We also investigated the host–guest assembly usingdichloromethane and heptanoic acid as the solvents. Large-scaleSTM images show the uniform arrangements almost free of defects(Fig. 5a and c). The TCDB molecules self-organize into a similarnetwork adlayer to the assembly adlayer of toluene/corrole/TCDB.To explore the structural detail, a moderate concentration ofhydroxyl fluorenone/dichloromethane is chosen to investigatethe self-assembly process. Distinctive arrays of ordered structuresinside the TCDB cavity adopting “edge-on” stacking styles are

observed in host–guest monolayers, which are ascribed to theintermolecular �–� stacking interactions of fluorenone dimers(Fig. 5b). The result demonstrates that the conjugated and highlydelocalized �-systems play a key role in stabilizing the monolayer.

L. Cui et al. / Applied Surface Science 313 (2014) 841–849 845

Fig. 3. (a) Large-scale STM image (35 nm × 35 nm) of TCDB monolayer formed at n-tetradecane/HOPG interface. Iset = 497 pA and Vbias = 990 mV. (b) High-resolution STMimage (15 nm × 15 nm) of the assembled structure. Iset = 532 pA and Vbias = 700 mV. (c) Suggested molecular model for the ordered adlayer based on the STM image.

Fig. 4. (a) Constant current STM image (30 nm × 30 nm) of TCDB template in toluene after addition of corrole. Iset = 497.4 pA and Vbias = 800 mV. (b) High-resolution STM image(22 nm × 22 nm) of TCDB/corrole assembled adlayer on the HOPG surface. Red and yellow bars represent the �–� stacked corrole dimers. Iset = 497 pA and Vbias = 785 mV. (c)Structural model for the host–guest adlayer. (d) Molecular size of corrole guest molecules.

846 L. Cui et al. / Applied Surface Science 313 (2014) 841–849

Fig. 5. Large-scale STM images (a) (30 nm × 30 nm) and (c) (50 nm × 50 nm) of dichloromethane/hydroxyl fluorenone/TCDB and heptanoic acid/hydroxyl fluorenone/TCDBadlayers, respectively. Iset = 475 pA and Vbias = 610 mV. High-resolution STM images of (b) (18 nm × 18 nm) and (d) (15 nm × 15 nm) show the host–guest assembled adlayersolved in dichloromethane and heptanoic acid, respectively. The imaging conditions are Iset = 497 pA and Vbias = 602 mV. (e) Tentative molecular model to enhance visuali ular s

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dentification of hydroxyl fluorenone/TCDB dissolved in dichloromethane. (f) Molec

n the basis of symmetry and intermolecular distance, a unitell is superimposed on the image in Fig. 5b with a = 2.1 ± 0.1 nm,

= 4.1 ± 0.1 nm, and = 79 ± 2◦, which closely resemble that ofhe corrole/TCDB network in Fig. 4. The length of the brighttripe located inside the cavity is 0.9 ± 0.1 nm, which confirms the

ntrapment of fluorenone molecules into the TCDB networks. Thealculations convince us that the structural model for the adlayerroposed (Fig. 5e) is in good accordance with the experimentalesults.

ize of hydroxyl fluorenone guest molecules.

Fig. 5d shows the high-resolution self-assembly of heptanoicacid/hydroxyl fluorenone/TCDB, in which tetragonal cavities arealigned in well-ordered columns. The cavities of TCDB are filledwith hydroxyl fluorenone dimers, which are ordered in “edge-on”stacking styles ascribed to a typical �–� stacking interaction. It

is interesting to find that heptanoic acid only exerts as a disper-sant without coadsorption or apparent structural transformationof the TCDB adlayer compared with other coadsorption assembly,although the introduction of -COOH is taken into consideration. The

L. Cui et al. / Applied Surface Scie

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ig. 6. Schematic model of molecular self-assembly at the liquid/solid interface.

entative model is detailed in Fig. 5e, which demonstrates that thessembled adlayer of heptanoic acid/hydroxyl fluorenone/TCDB ishe same as the host–guest assembly of dichloromethane/hydroxyluorenone/TCDB with only tiny discrepancy in the size of the cav-

ty. The unit cell parameters as outlined at the bottom-left of Fig. 6ds a = 2.1 ± 0.1 nm, b = 4.2 ± 0.1 nm, and = 98 ± 1◦.

Overall, although TCDB can accommodate the solvent and guestolecules as the result of steric commensurability, the nature of the

oadsorbed molecules makes the assembled morphology different.ompared with the planner adsorption conformations of solventolecules, �-stacked dimer-entrapped architectures were apt to

e formed after the introduction of corrole or hydroxyl fluorenonecting as the guest molecules.

. Discussion

The vital role of solvents in 2D coassembly process at theiquid–solid interface has been extensively studied in supramolec-lar chemistry. It is commonly known that the microenvironmentf the adsorbates can be tuned by the polarity of the solvents. Aypical host–guest structure could be introduced by finely tun-ng the interaction of the adsorbate molecules via solvent polarity,

hich is similar to the solvent molecules entrapped coadsorptionffect. The formation of 2D SAMs involves the thermodynamic andinetic equilibrium of the host/guest complex. The equilibriums determined by the desolvation of host and guest molecules as

ell as the solvation of the host–guest complex. The host–guestupramolecular inclusion process involves the breaking downf the solvent cages of the host and guest molecules, and re-stablishing the solvent cage for the coadsorption complex. Theolvent affects the mobility of the molecules, e.g., by affectinghe adsorption–desorption dynamics. In this dynamic process, the

obility of the molecules is affected by the solvation energy [16]molecule–solvent interaction) and possibly also by solvent viscos-ty [42].

Although the TCDB adlayer with the entrapment of solvent anduest molecules has the similar 2D tetragonal cavity structure, theize of the unit cell and the detailed molecular configuration areifferent. The experimental results can be summarized as follows:1) the coadsorption effect of nonpolar solvent molecules and theuest molecules in volatile solvents, (2) the structural evolution ofhe different coadsorption configurations, (3) the polarity of TCDB

olecules in different coassembled structures.

.1. Effect of solvent coadsorption

We would like to discuss the solvent-induced coadsorptionffect inferred from the observation of TCDB dissolved in 1-henyloctane and n-tetradecane. The crucial and dominant forceor the formation of solvent coadsorption structures is the steric

nce 313 (2014) 841–849 847

effect. For the arrangement of tetragonal cavity, which is sur-rounded by alkyl arms attached to the phenyl core of TCDB dimers,it is suitable to entrap solvent molecules critically for the size andshape. In addition, the solvophobic effect of these nonpolar solventsfavors the formation of porous structures, which can help to sustainthe void structure by filling the porous. Solvent viscosity comesinto play in determining the assembled structure of 1-phenylotaneand n-tetradecane. The diffusion constant of TCDB decreases asthe viscosity of the liquid rises up, and the solubility of TCDB willdecrease because of the solvophobic effect. Higher deposition rateslead to a preference of the more densely packed structure. Fig. 6shows an idealized scheme of self-assembly from the solutionphase. In view of the total energy, densely packed assemblyis most frequently favored especially when the intermolecularinteraction lacks directionality, which allows all alkyl chains of themonodendrons to be fully adsorbed on the graphite surface.

4.2. Structural properties of solvent- and guest-coadsorption ofTCDB

The structural difference of 2D coadsorbed adlayer is listedin Table 3. We can find that the structure parameters of 1-phenylotane/TCDB, n-tetradecane/TCDB, toluene/corrole/TCDB,dichloromethane/hydroxyl fluorenone/TCDB and heptanoicacid/hydroxyl fluorenone/TCDB networks are comparable. Accord-ing to literature and calculations, the polarity of the solventsused in this work has a trend of dichloromethane > heptanoicacid > 1-phenylotane ≈ toluene > n-tetradecane. For convenience,1-phenylotane and n-tetradecane are sorted as class A, whiletoluene, dichloromethane and heptanoic acid are assorted as classB.

It can be inferred from Table 3 that the unit cell parametersdiffer significantly in b direction, as is the same case of the angle˛, and both of them show a decrease from class A to class B sol-vents. In contrast, there is little change in a direction. The hostcavity dimensions of TCDB are greatly reduced when the pairedsolvent molecules are entrapped in them, which fully reflect thecoadsorption effect in class A solvent. However, as explored fromstructural analysis, the host cavities of TCDB expand significantlywhen the guest molecules (corrole and hydroxyl fluorenone) areenclosed within them. It is commonly known that an appreciableexpansion or contraction of the host cavities occurs accompaniedby the inclusion of guest molecule, which depends on the appro-priate sized cores linked through directional flexible alkyl-chaininterdigitation. Here, we suppose that the distinctly different struc-tural formation of 2D host–guest self-assembly should be largelyrelated to the �–� stacking of guest molecules. In order to accom-modate the large-sized corrole molecules with �-conjugated cores,the host molecules have to expand through the connection of theneighboring alkyl chains linking the cavities in response, whichreveal the strength of the hydrogen bonding formed between thehost lattices. As a whole, these comparisons of structural parame-ters show that the TCDB networks must adjust the alkyl chains toaccommodate the guest molecular dimers. Besides, the calculatedhydrogen bonding connection is stable in this system.

4.3. Conformational stability of isolated molecules andmorphology character of polarity: the nature of the solvent

Based on the experiments, a semi-empirical quantum mechan-ical calculation of geometry energy of a single TCDB molecule withdifferent angles of the ether oxygen bond was carried out in the

gas phase as illustrated in Fig. 7. We calculated the binding energy(ETCDB) of TCDB in different self-assembled structures as a functionof the ether oxygen bond angles. Regular changes occur in the anglebetween two horizontally and laterally oriented alkane chains of

848 L. Cui et al. / Applied Surface Science 313 (2014) 841–849

Table 3Unit cell parameters, unit cell areas and molecular area densities of TCDB adlayers in different solvents.

Solvent a (nm) b (nm) (◦) S (nm2 per molecule) Density Ether oxygen angle of TCDB (◦)

1-Phenylotane 1.7 ± 0.1 2.9 ± 0.1 62 ± 1 1.70 2.34 64 ± 1n-Tetradecane 1.8 ± 0.2 3.0 ± 0.2 65 ± 1 1.87 2.13 67 ± 1Toluene 2.0 ± 0.1 3.4 ± 0.1 77 ± 1

1,2-Dichloroethane 2.1 ± 0.1 4.1 ± 0.1 79 ± 2

Heptanoic acid 2.1 ± 0.1 4.2 ± 0.1 98 ± 1

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ig. 7. Theoretical calculation of ETCDB in different conformations and the corre-ponding dipole moment for the rotation of the ether oxygen bond of TCDB moleculen the gas phase.

CDB (as shown in Fig. 1). We can learn that ETCDB is sensitive to thengle, thus it is worthy of discussion about the different assembledtructures in this system. With the changes of the angle (denoteds above) from 60◦ to 75◦, ETCDB first decreases until reach a min-mum at about 67◦, and it begins to increase after that. The Y-likeonfiguration at the angle of 67◦ has the lowest binding energy thusaking it the most stable state. On the basis of the calculated TCDBono-molecular polarity in different configurations, the solvent

olarity greatly affects the assembled structures of TCDB SAMs.ur STM observation indicates that the TCDB molecule is likely todopt a higher polarity of Y-like configuration in class A than thatn class B. It is suggested that the solvent effect involves the polarityf the solvents, the solvophobic and solvophilic effects. Moreover,he competition between kinetics and thermodynamics plays a keyole.

On one hand, TCDB molecules are inclined to assemble in nonpo-ar solvents such as 1-phenyloctane and n-tetradecane at a low bentngle (63.61◦ for 1-phenyloctane and 66.57◦ for n-tetradecane). Weuppose that in these nonpolar solvents, TCDB molecules preferttaching to themselves via the van der Waals forces dominated byhe dipole–dipole interaction and hence they would deposit on theOPG surface. Meanwhile, the Y-like configuration at a low bentngle has a larger dipole moment and forms a larger van der Waalsorce. This can explain why low bent angles of the ether oxygenond of TCDB exist in nonpolar solvents. The theoretical calcula-ion of ETCDB demonstrates that the angle of the ether oxygen bondt about 67◦ is most energetically favored. The densely packed poly-orphs become thermodynamically preferred in view of Gibbs free

nergy of adsorption in these solvent-coadsorbed monolayers.On the other hand, the kinetic effect might also play an

mportant role in the self-assembled monolayers. ConsideringCDB contains polar carboxyl groups, the polar solvents suchs dichloromethane and heptanoic acid have good solubility.

CDB molecules with high dipole moment (3.75–4.15 Debye) areore likely to stay in polar solvents because of solvophilic effect

ather than in nonpolar solvents. It is speculated that when theolvent–solute interactions are strong, the desolvation energy cost

2.96 1.34 74 ± 13.76 1.06 76 ± 24.48 0.89 73 ± 1

for the TCDB molecules is high so that they are very mobile at theinterface and could not interact well with the substrate to formstable assembled adlayer. It is well known that there is an equi-librium between the adsorption and desorption in the solution.The solvophilic effect will increase the desorption rate at the solu-tion/solid interface, thus lowering the deposition rate (as shownin Fig. 6). Accordingly those remaining TCDB molecules with smalldipole moment at a high bent angle tend to be more kineticallycontrolled, thus it is much easier to interact with the guest andsolvent molecules to form SAMs. This solvophilic effect has beenwidely used to reveal the solvent effect in 2D self-assembly sys-tem [42]. For example, Z-like configuration has more polarity thanthe V-like one and prefers staying in polar solvent to assemblingon the HOPG surface. This can explain why the molecule adopted alow polarity V-like configuration in polar solvents and a high polar-ity Z-like configuration in nonpolar solvents. Then, as the dipolealignments vary with the host–guest co-assembled configurations,we can understand the guest-induced polarity evolution based onthe structural distortion. From the calculated dipole moments as afunction of ether oxygen angles, it is known that the dipole momentvaries with the host–guest coassembled configurations. For theguest-entrapped system, TCDB host molecule should adjust itselfto accommodate these �–� stacking corrole/fluorenone dimersas guest molecules, and the cavity surrounded by the alkyl armsof the TCDB dimer goes with the enlargement of the unit cell.The experimental and calculated results indicate that the arrange-ment of TCDB host–guest assembly adopts the low polarity of TCDBmolecules.

5. Conclusions

In summary, we have investigated the 2D coassembly effects ofTCDB dissolved in nonpolar 1-phenylotane and n-tetradecane withhigh viscosity. Corrole and hydroxyl fluorenone were also intro-duced as guest molecules when volatile toluene, dichloromethane,and heptanoic acid were used as the solvents. STM studies ofthe resulting monolayers revealed that the nature of solvent hada dramatic effect on the ordering of SAMs, which could func-tion via either its polarity or other properties such as viscosityand the solubility of the assembled molecules. It is illustratedthat TCDB could form extremely versatile 2D tetragonal cavi-ties at room temperature. On one hand, the nonpolar solventsserved as a counter-part could participate in the assembly forma-tion through co-adsorption effect because of the solvent–substrateinteraction and steric constraints. On the other hand, when intro-ducing corrole or hydroxyl fluorenone into the system, �-stackeddimer-entrapped architectures were formed. Meanwhile, the unitcell volume of those dissolved in such nonpolar solvents as 1-phenylotane and n-tetradecane shrank significantly, comparedwith those of host–guest systems. DFT calculations of the hydro-gen bonding in the dimers-entrapped architectures indicate thatthe bonding is strong enough to keep the regular tetragonal cav-

ities. The hydrogen bonding among TCDB molecules is crucial tostabilize the assembled networks. In addition, mechanical calcu-lations point out that TCDB is inclined to adopt the larger dipoleconfiguration in nonpolar solvents than in polar solvents, and the

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hermodynamics and kinetic parameters play some roles in tun-ng the 2D self-assembled structures. Furthermore, this typicaltudy would deepen our understanding of the solvent effect onupramolecular structures and may help us to better control andesign new SAMs for future applications of molecular devices.

cknowledgements

Financial supports from the National Program on Key Basicesearch Project (2012CB932900), the National Natural Scienceoundation of China (21103053 and 51373055) and the Fundamen-al Research Funds for the Central Universities (SCUT) are gratefullycknowledged.

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