ARTICLE pubs.acs.org/JPCC
Two-Dimensional Self-Assembled Molecular Structures Formed by the Competition of van der Waals Forces and DipoleDipole Interactions Li Xu,†,§ Xinrui Miao,†,§ Xiao Ying,‡ and Wenli Deng*,† †
College of Materials Science and Engineering, and ‡College of Sciences, South China University of Technology, Guangzhou 510640, China
bS Supporting Information ABSTRACT: The self-assembly of 2,7-bis(10-ethoxycarbonyl-decyloxy)9-fluorenone (BEF) has been investigated by scanning tunneling microscopy at the liquid/solid interface. The coexistence and reversible transformation of linear structure and cyclic network were observed. Solvents with different polarity had little effect on the self-assembly of BEF on the HOPG surface. The observed structural transformation was found to be driven by voltage pulses applied to the STM tips. The influence of dipoledipole interactions and van der Waals (vdWs) interactions on the different packing patterns was scrutinized by comparing the two-dimensional self-assembly of 2,7-bis(10-ethoxycarbonyldecyloxy)-9-fluorene, 2,7-ditetradecyloxy-9-fluorenone, and 2,7-ditetradecyloxy-9-fluorene with that of BEF molecule. The formation of different morphologies depended critically not only on the molecular conformation, but the different dipoledipole interactions of the fluorenone unit and two ester alkoxy chains. The computer simulation and calculation of different morphologies are useful tools to dissect and explain the formation mechanism. On the basis of the comparative experiments and calculation, we could conclude that for the linear structure, the vdWs forces between the molecules were more important than the dipoledipole interactions; on the contrary, the intermolecular dipoledipole interactions induced the cyclic network.
’ INTRODUCTION Fluorene- or fluorenone-based materials due to their efficient applications as active components for OLEDs,1,2 photovoltaic cells,3,4 or field effect transistors5,6 have been studied widely. It is now full recognized that, in addition to the chemical structure of the molecules, the supramolecular organization and morphology in the solid states has a significant influence on those optoelectronic properties. For example, the main properties of organic field effect transistors arise mainly from the organization of the very first monolayer of conjugated molecules deposited on top of the dielectric surface, as charge transport occurs mostly through a very thin conducting channel at the semiconductor insulator interface.7 Therefore, it is of prime importance to understand and control the supramolecular assembly in the thin adlayers of conjugated molecules.8 Up to date, a minority of work has focused on the fluorenone-based materials and their organized properties.9,10 Supramolecular fabrications with two-dimensional (2D) cyclic networks are of great interest because such patterns provide the possibility to immobilize functional guest molecules. The spontaneous formation and transformation of patterns with hexagonal,1115 honeycomb,14,1619 or Kagome geometries2023 have been obtained at the liquid/solid interface. The morphologies, as well as the drastic structural change often induced by subtle changes r 2011 American Chemical Society
in molecular structure,2426 concentration,27,28 or solvent,2931 are usually explained by the moleculemolecule and molecule substrate interactions. During the self-assembly, hydrogen bonding, dipolar interactions, metalorganic coordination, or van der Waals (vdWs) interactions are always present as competing forces to dominate the supramolecular order. In most cases, the monocomponent cyclic networks are built by the vdWs interactions based on alkyl-chain interdigitation32 and hydrogen bonding.33,34 Surprisingly, the dipole-induced cyclic networks by a single central dipole moment have never been probed to the best of our knowledge. The Zimmt research group reported that the position of the ether oxygen atoms could generate repulsive dipole interactions between one or two pairs of ether groups in adjacent physisorbed molecules.35,36 Moreover, it was revealed that side chains with slight differences in the position of ether group or length produced a variety of morphologies and vastly different extents of polymorphism. Recently, Mu et al. reported that the antiparallel intermolecular dipoledipole interactions were the main driving force in the formation of 2D porous nanostructures.37 However, in these studies, they only focused on the dipoledipole interactions Received: October 18, 2011 Revised: December 11, 2011 Published: December 13, 2011 1061
dx.doi.org/10.1021/jp210000e | J. Phys. Chem. C 2012, 116, 1061–1069
The Journal of Physical Chemistry C between the substituent groups, such as the side chains and CF3. No studies have been reported to investigate the synergetic effect of dipoledipole interactions of both the conjugated moiety and the side chains on the 2D self-assembled nanostructure by scanning tunneling microscopy (STM). Our previous studies have investigated that the formation of the self-assembled motifs of 1,3,5-tris(10-ethoxycarbonyldecyloxy)benzene (TECDB) molecules with three ester alkoxy chains could be controlled by solvents,29 and the honeycomb network was obtained only with coadsorption of a guest template.16 We concluded that the structural transition of TECDB was attributed to the polarity of the ester alkoxy chains. To further understand the synergetic effect of dipoledipole interactions of both the conjugated moiety and the side chains on the 2D self-assembled nanostructure, in this work, we designed and synthesized a molecule [(2,7-bis(10ethoxycarbonyldecyloxy)-9-fluorenone, BEF, Figure 1a], which includes a fluorenone core with strong polarity (dipole moment, μ = 3.05 D)38 and two ester alkoxy chains with weaker polarity. The side chains also act as a functional linking group able to form surface-assisted intermolecular interactions by interdigitation. We found that by adjusting the solution concentration, the coexistence of linear structure and cyclic network could be obtained. In addition, the solvent with different polarity had little effect on the self-assembly of BEF on the HOPG surface. In an effort to understand the origin of the morphological change and identify the dipoledipole interactions that control the morphology and polymorphism, 2,7-ditetradecyloxy-9-fluorenone (DOF, Figure 1b), 2,7-ditetradecyloxy-9-fluorene (DTF, Figure 1c), and 2,7-bis(10ethoxycarbonyldecyloxy)-9-fluorene were synthesized by changing the ending group of the side chains and the molecular cores. In particular, to interpret the self-assembled mechanism of BEF on the HOPG surface, computer simulation and calculation were carried out. This is the first example to find the importance of dipoledipole interactions in both the aromatic cores and the ester alkoxy chains to control the self-assembled monolayers that were unexplored previously. We expect that designing the intermolecular dipoledipole interactions might drive the molecular assembly and enable fine control over the supramolecular architectures at the liquid/solid interface.
’ EXPERIMENTAL SECTION 2,7-Bis(10-ethoxycarbonyldecyloxy)-9-fluorenone (BEF), 2, 7-ditetradecyloxy-9-fluorenone (DOF), 2,7-ditetradecyloxy-9fluorene (DTF), and 2,7-bis(10-ethoxycarbonyldecyloxy)-9fluorene used in this study were synthesized as described in the literature.39 1-Phenyloctane (Aldrich), octanoic acid (TCI), and n-tetradecane (TCI) were used as received. Prior to imaging, the BEF or DTF was dissolved (concentration = 104106 M) in different solvents, and a drop of this solution was applied on a freshly cleaved surface of HOPG (quality ZYB, Digital Instruments, Santa Barbara, CA). Different solution concentrations of BEF, DOF, or DTF in 1-phenyloctane were probed to evaluate the concentration-dependent structural change of the 2D monolayer. The stable physisorbed monolayer was spontaneous formed, and then the STM experiment was carried out. All of the images obtained at the liquidsolid interface were recorded within 5 h after dropping a solution of BEF, DOF, or DTF molecule. All experiments were performed at ambient conditions using a Nanoscope IIIa Multimode SPM (Digital Instruments, Santa Barbara, CA) operating in constant-current mode. STM tips
ARTICLE
Figure 1. Space-filling structures of (a) 2,7-bis(10-ethoxycarbonyldecyloxy)-9-fluorenone (BEF), (b) 2,7-ditetradecyloxy-9-fluorenone (DOF), and (c) 2,7-ditetradecyloxy-9-fluorene (DTF) with outlined molecular cores. Parts a2 and c show the optimized conformations of BEF and DTF calculated by Gaussian W03, respectively. Parts a and c are the self-assembled conformations of BEF and DTF, respectively. Colors corresponding to the elements: H, gray; C, blue; O, red.
were prepared by mechanical cutting from Pt/Ir wire (80:20, diameter 0.2 mm). The experiments were repeated in several sessions using different tips and samples to check for the reproducibility and to avoid experimental artifacts. The images were corrected for drift using the recorded HOPG images for calibration purposes. Molecular models of the observed assembled structure were built by Materials Studio 4.4. The model of monolayer was constructed by placing the molecules according to the intermolecular distances and angles obtained from the analysis of STM images. Molecular Mechanics was applied for the structural optimization calculation. The theoretical calculations were performed by semiempirical calculation of Gaussian 03W after the structural optimization. The tunneling parameters are given in the corresponding figure captions.
’ RESULTS AND DISCUSSION Self-Assembly of BEF in 1-Penyloctane. Figure 2 shows typical large-scale STM images, recorded at the solidliquid interface, of monolayers formed from BEF solutions with differnent concentrations of 1.2 104, 2 105, and 5 106 mol L1 in 1-phenyloctane on the HOPG surface, respectively. Under higher concentration (>1.2 104 mol L1), the BEF molecule tends to form a linear structure on the HOPG surface, as shown in Figure 2a. Under lower concentration (
