Molecular Trapping Phenomenon of the 2-D Assemblies of Octa

J. Phys. Chem. B 2002, 106, 12569-12574

12569

Molecular Trapping Phenomenon of the 2-D Assemblies of Octa-Alkoxyl-Substituted Phthalocyanine Studied by Scanning Tunneling Microscopy Yuhong Liu, Shengbin Lei, Shuxia Yin, Sailong Xu, Qiyu Zheng, Qingdao Zeng, Chen Wang,* Lijun Wan, and Chunli Bai* Center for Molecular Science, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080 China ReceiVed: January 17, 2002; In Final Form: June 17, 2002

In this paper, we illustrate the trapping effect of 2-D assembly of octa-alkoxyl-substituted phthalocyanine for individual molecules of phthalocyanine, prophyrins, and calix[8]arene. It is observed that single molecules are trapped in quadratic lattices rather than hexagonal lattices, and domain boundaries are preferential trapping sites as compared with the internal domain sites. The observed trapping behaviors could be considered in analogue to the phenomena of point defects, dislocations, and grain-boundary segregations in solid-state materials.

1. Introduction The adsorption and assembly of organic molecules on the surfaces of semiconductors and metals is a subject of considerable interest. Many researchers have dedicated themselves to investigating a large number of organic species with all kinds of surface analytical methods. These investigations have advanced the knowledge of the microscopic behavior of adsorption processes and adsorbate structures, such as the binding site of individual molecules with respect to the substrate lattices, conformational states of individual molecules, and periodicity of ordered surface molecular structures, as well as defects and domains that are present in ordered structures.1-8 Previously, we have shown the stabilizing effect of linear alkanes in the adsorption of copper (II) octa-alkoxyl-substituted phthalocyanine (CuPcOC8) on HOPG.9 Distinctly different packing symmetries (quadratic and hexagonal symmetries) were observed and attributed to the intermolecular and adsorbatesubstrate interactions. In particular, it was noticed that, in quadratic symmetry domains, the interdigitated alkyl parts of four molecules in the same unit cell can enclose to form a square cavity, with the side width determined by the length of octaalkoxyl-substituted groups. It is conceived that monolayers of octa-alkoxyl-substituted phthalocyanine could have the potential to function as templates for trapping individual molecules on an inert substrate at room temperature. The current study is motivated to study the interaction between different molecular species, in particular, using the selfassembled structure of octa-alkoxyl-substituted phthalocyanine (PcOC8) on HOPG as the template. The additionally introduced species have nearly planar structures and have been known to have rich chemical properties. The experimental observation confirmed that the assembly structure of PcOC8 could indeed help to immobilize different types of molecules and display differed immobilization efficiencies at various structural sites. Phthalocyanines are of particular interest as organic semiconductors, electronically active organic molecules, and ana* To whom correspondence should be addressed. Fax: (86)-10-62557908. E-mail: [email protected] (C.W.); [email protected] (C.L. Bai).

logues of biological molecules such as chlorophyll. Because of their relevance to molecular electronics, the mechanism leading to a well-ordered monolayer of phthalocyanine on interfaces is of great interest. Copper phthalocyanine has been investigated on highly oriented pyrolytic graphite (HOPG) and MoS2,8 Cu (100),10 GaAs (110),11 Si (100) (2 × 1),12 Si (100), and Si (111),7 and the close-packed and the rowlike phase have been observed. The nearly quadratic arrangement is also reported for phthalocyanine molecules formed on graphite, MoS2, and ionic crystal surfaces with the MBE method.8,13 Another type of molecule studies in this work is calixarenes. Calixarenes are ideal starting materials for the synthesis of various types of host molecules and can also act as building blocks for the construction of larger molecular systems with defined structures and functions. Their potential applications range from use as highly specific legends for analytical chemistry, sensor techniques, and medical diagnostics to their use in the decontamination of wastewater, the construction of artificial enzymes, the synthesis of new materials for nonlinear optics of for ultrathin layers, and sieve membranes with molecular pores.14 Therefore, a great deal of experiment technologies and approaches are used in studying the preparation and characterization of self-assembled calixarenes. However, it is noted that there is a lack of direct experimental evidence that has been obtained on the molecular orientation and packing arrangement.14-17 Recently, Reinhoudt and co-workers reported atomic force microscopy (AFM) studies on self-assembled monolayers of calix[4]resorcinarene and calix[4]arene sulfide adsorbates on Au(111) substrate.18,19 To the best of our knowledge, there is still no report describing the STM images of calixarenes on HOPG under ambient condition. Herein, we present the direct observation of the trapping phenomenon of two-dimensional (2-D) assemblies of octaalkoxyl-substituted phthalocyanine using planar phthalocyanine, porphyrins, and calix[8]arene as model species. The trapped molecules both inside the assembly and domain boundaries could endure repeated scanning because of the immobilization effect of 2-D assemblies of octa-alkoxyl-substituted phthalocyanine.

10.1021/jp0201682 CCC: $22.00 © 2002 American Chemical Society Published on Web 11/12/2002

12570 J. Phys. Chem. B, Vol. 106, No. 48, 2002 SCHEME 1: Molecular Structure of PcOC8

Liu et al. SCHEME 4: Molecular Structure of PyPP

SCHEME 5: Molecular Structure of TCPP

SCHEME 2: Molecular Structure of Pc

SCHEME 6: Molecular Structure of Calix[8]

SCHEME 3: Molecular Structure of PP

2. Experiment The molecules in this study are illustrated in Schemes 1-6. Octa-alkoxyl-substituted phthalocyanine (purity>95%) denoted as PcOC8 (Scheme 1), phthalocyanine (Pc, Scheme 2), and porphrin (PP, Scheme 3) were obtained from Aldrich and used without further purification. 5,10,15,20-Tetra(4-pyridyl)por-

phyrin (PyPP) is purchased from Fluka and used without further purification. Its chemical structure is shown in Scheme 4. 5,10,15,20-Tetrakis(4-carboxylphenyl)-21H,23H-porphyrin (TCPP, Scheme 5) is purchased from Acros Co. and used without any purification. The p-tert-butylcalix[8]arene (Calix[8]) were synthesized following the method previously reported,20 which is first synthesized through the so-called one step procedure and then its lower rim hydroxyl groups are further functionalized to

Molecular Trapping Phenomenon of Phthalocyanine

Figure 1. STM image of PcOC8 molecular domains on the HOPG (scan size, 57.6 nm × 57.6 nm; tunneling condition, -534 mV, 559 pA).

partially immobilize the conformation. Scheme 6 shows its chemical structure. The solutions (molar ratio about 1:1 for all samples) of mixed PcOC8/Pc molecules and PcOC8/PP molecules were made in a solvent of toluene (HPLC-grade, Aldrich) with a concentration of less than 1mM. A droplet of the solution was deposited onto a freshly cleaved surface of highly oriented pyrolytic graphite (quality ZYB, Digital Instruments). The experiment was performed on Nanoscope IIIa SPM (Digital Instruments, Santa Barbara, CA) at room temperature in ambient conditions. STM tips were mechanically formed Pt/ Ir wire (90/10). 3. Results and Discussion In this work, octa-alkoxyl-substituted phthalocyanine and phthalocyanine were coadsorbed from solution onto HOPG. Figure 1 presents the assembly structure of solely PcOC8 molecules. The distance between adjacent rows is measured to be 2.7 nm ( 0.1 nm (noted as L). The cavities enclosed by each unit cell are homogeneously distributed in the assembly. Figure 2a presents a typical image of a single Pc molecule trapped in the PcOC8 molecular assembly prepared by coadsorption of above-mentioned mixture solution. The structure of Pc molecules is identical to the phthalocyanine core of PcOC8. It can be seen that the height of Pc molecules is lower than the height for the phthalocyanine core of PcOC8, because the phthalocyanine core of PcOC8 is supported by eight octaalkoxyl-substituted groups. The difference is measured to be about 0.1 nm, which is slightly smaller than the length of the C-H bond of CH2. The height of octa-alkoxyl-substituents is measured to be about 0.2 nm, which is in accordance with twice the length of the C-H bond of CH2. The experimental results also revealed little correlation of the number of inserted single Pc molecules with the initial molar ratio of PcOC8 to Pc (and

J. Phys. Chem. B, Vol. 106, No. 48, 2002 12571 that for other species in the following studies). This is possibly due to the preferential adsorption and assembling of PcOC8 molecules on the substrate as discussed later. The single Pc molecule is surrounded by four groups of alkyl substituent of four PcOC8 molecules in the same unit cell. It can be seen that the inclusion of the Pc molecule is associated with little distortion of the original unit cell. Figure 2b illustrates the proposed molecular arrangement according to molecular mechanics simulation as a schematic illustration of quasi-4-fold domain structures, confirming the full inclusion of the Pc molecule without lattice deformation. Figure 2c shows the left side-view of Figure 2b. For the legible visual field, it only displays one PcOC8 molecule in the top left corner of Figure 2b and one Pc molecule. It can be seen that the center of the Pc molecule is slightly lower than the center of the PcOC8 molecule. In the current study, the PcOC8 and Pc molecules are coadsorbed onto HOPG. Because of the enhanced interaction of PcOC8 with the substrate (as the result of octa-alkoxylsubstitution groups9,21), the PcOC8 molecules are considered to adsorb and assemble preferentially on the substrate. We inclined to consider that the Pc molecules were trapped during the formation of the assembly of PcOC8 on HOPG surface. Figure 3a displays the insertion of one row of Pc molecules with the end point clearly visible. The intercalation structure of Pc molecules changes the separation between neighboring molecule rows, resembling the characteristic dislocations found in solid state materials. This insertion only affects adjacent rows of PcOC8, namely, it is a short-range effect. The distance from the center of PcOC8 molecule to the center of the adjacent Pc molecule is about 2.1 nm ( 0.1 nm, which is smaller than the distance between two neighboring PcOC8 molecules because Pc molecules have no octa-alkoxyl-substituted group. Figure 3b shows a large-scale monolayer assembly of PcOC8 with Pc. The high-contrast molecules correspond to PcOC8 molecular array. The side to side width of the central high-contrast moieties is about 1.2 nm ( 0.1 nm. In this figure, one could discern two rows of Pc molecules (noted as I and II) inserted into the PcOC8 array and widen the distance of PcOC8 molecules from 2.7 nm ( 0.1 nm (noted as L2) to 3.5 nm ( 0.1 nm (noted as L1). The molecularly intercalated structure of lines I and II only result in displacement of adjacent PcOC8 rows. Next to the lines of Pc molecules, PcOC8 molecules in the domains have a quasi4-fold arrangement. Figure 3c shows that Pc molecules can intercalate into both straight and curved boundaries of PcOC8 domains, reminiscent of the extensively studied grain-boundary segregation phenomena in metal and semiconductor materials. The same situation can be observed in the monolayer of PcOC8 and PP (Figure 4a), PyPP (Figure 4b), and TCPP (Figure 4c). With the intercalation of PP, PyPP, and TCPP molecules in the boundary of PcOC8 domains, the distances between neighboring PcOC8 molecular rows are widened, similar to that observed in Figure 3. The differences in the structures with and without the intercalation of PP, PyPP, and TCPP molecules are summarized in Table 1. The distances between neighboring PcOC8 molecular rows with the intercalation structure are noted as L1, whereas the distances between neighboring PcOC8 molecular rows without intercalating by any molecule are noted as L2. From the data in Table 1, one could observe the correlation between the sizes of the trapped molecule with the variation of L1. In this experiment, we cannot find that PP and its derivative molecules trapped in the PcOC8 molecular assembly and do

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Liu et al.

Figure 2. (a) Higher resolution STM image of an individually trapped Pc molecule (scan area, 12.3 nm × 12.3 nm; tunneling condition, -700 mV, 253 pA). (b) Schematic representation of the quasi-4-fold arrangement of PcOC8 intercalated by a single Pc molecule as deduced from STM image. (c) The left-side view of the molecular monolayer.

not affect the lattice of PcOC8 molecules (analogous situation displayed in Figure 2a). However, these situations maybe exist. The individual molecules of functionlizd p-tert-butylcalix[8]arene are also observed along the boundary of PcOC8 domains at HOPG surface by using STM. Figure 5a shows a large scale image of the calix[8]arene molecules trapped along the domain boundary of PcOC8. With the intercalation of Calix[8] molecules in the boundary of PcOC8 domains, the distance of neighbor PcOC8 molecules in the different domains changes to 3.7 nm (noted as L1), whereas the distance of two lines of PcOC8 molecules without Calix[8] molecules keeps 2.7 nm (noted as L2). The higher resolution STM image of the intercalating structure is displayed in Figure 5b. It is obvious that each Calix[8] molecule appears in a “calix”. The diameter of the “calix” is measured to be about 1.6 nm ( 0.1 nm. The surrounding circular protrusions can be attributed to the eight tert-butyl groups, and the depression in the STM images can be assigned as molecular cavity of calix[8]arene molecule,

suggesting that in situ STM may be used to probe the molecular recognition process. On the basis of above analysis, it can be concluded that the calix[8]arene molecules are in cone conformation and upright on HOPG surface. The intercalation associated spacing increase between molecular rows is summarized in Table 1 and plotted in Figure 6. It is evident that, with the increase of the intercalating molecular size, the distances of neighboring PcOC8 rows also increase correspondingly. It is also noticed that the increase in spacing of PcOC8 arrays is smaller than the molecular size listed in Table 1, except of TCPP. This could be an indication of the flexibility of the PcOC8 lattice. The insertion of molecules in the initially packed lattice would lead to enhanced repulsive interaction between molecules. On the other hand, the overlapped of alkyl parts could readjust to accommodate additional molecules and compensate the associated the increment of repulsion interaction. Therefore, L1 does not need to increase with full size of the molecular size.

Molecular Trapping Phenomenon of Phthalocyanine

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Figure 4. STM images of (a) PP (scan area, 18.4 nm × 18.4 nm; tunneling condition, -579 mV, 864 pA), (b) PyPP (scan area, 19 nm × 19 nm; tunneling condition, -738 mV, 571 pA), (c) TCPP (scan area, 12.4 nm × 12.4 nm; tunneling condition, -534 mV, 559 pA) molecules trapped along the domain boundaries of PcOC8.

TABLE 1: Distances between Neighboring PcOC8 Molecular Rows with and without the Intercalation Structurea Figure 3. Large scale STM images of the intercalated structures of PcOC8 and Pc. (a) One row of Pc molecules entered into the PcOC8 array (scan area, 45 nm × 45 nm; tunneling condition, -539 mV, 365 pA). (b) Two rows of Pc inserted into the PcOC8 array (scan area, 94.7 nm × 94.7 nm; tunneling condition, -483 mV, 953 pA). (c) STM image of Pc molecules trapped along the straight and curve domain boundaries of PcOC8 (scan area, 80.9 nm × 80.9 nm; tunneling condition, -700 mV, 331 pA).

The above observations could draw close relevance to the prevalent experimental and theoretical studies of point defects, dislocations, and grain-boundary segregations in solid-state materials 22-24. It has been well established that these structural sites could substantially affect the electrical and mechanical properties of the polycrystalline materials. Though the species reported in this work have not been subjected to rigorous theoretical analysis, we believe that analogous consideration (especially structural models and diffusion processes) could be adapted to the analysis of molecular defects and grain-boundary

Pc PP PyPP TCPP Calix[8]

molecular size (side to side)

L1

L2

1.2 nm ( 0.1 nm 0.7 nm ( 0.1 nm 1.4 nm ( 0.1 nm 1.7 nm ( 0.1 nm 1.6 nm ( 0.1 nm

3.5 nm ( 0.1 nm 3.3 nm ( 0.1 nm 3.6 nm ( 0.1 nm 3.9 nm ( 0.1 nm 3.7 nm ( 0.1 nm

2.7 nm ( 0.1 nm 2.7 nm ( 0.1 nm 2.7 nm ( 0.1 nm 2.7 nm ( 0.1 nm 2.7 nm ( 0.1 nm

a The distance between neighboring PcOC8 molecular rows with the intercalation structure is noted as L1, whereas the distance between neighboring PcOC8 molecular rows without intercalating by any molecule is noted as L2.

segregations in self-assembled molecular structures. Such study could be potentially important in assessing the fabrication of SAM-based electrical and opticelectrical devices. The presented observations are deemed as preliminary in elucidating the molecular behavior at domain/grain boundaries. Further explorations, including theoretical simulations, could also enrich the general interests in grain-boundary segregation at the molecular level.

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Liu et al. calix[8]arene were trapped under ambient conditions on HOPG. This result showed that the cavities in the monolayer of octaalkoxyl-substituted phthalocyanine can be used for immobilizing organic species on substrates. The trapping efficiency is significantly higher at the domain boundaries than within domains. Acknowledgment. The authors thank the financial support from National Natural Science Foundation (Grant Nos. 20073053 and 29825106) and the Foundation of the Chinese Academy of Sciences. The support from National Key Project on Basic Research (Grant G2000077501) is also gratefully acknowledged. The authors are also indebted to the constructive comments from the reviewers of the manuscript. References and Notes

Figure 5. (a) Large scale STM image of the Calix[8] molecules trapped along the domain boundary of PcOC8 (scan area, 30 nm × 30 nm; tunneling condition, -440 mV, 570 pA). (b) A higher resolution STM image of a row of Calix[8] trapped along the domain boundary of PcOC8 (scan area, 14.2 nm × 14.2 nm; tunneling condition, -555 mV, 325 pA)

Figure 6. Correlation between molecular size and increment of spacing between PcOC8 molecular rows.

4. Conclusions Using a monolayer of octa-alkoxyl-substituted phthalocyanine as a template matrix, individual phthalocyanine, porphyrins, and

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