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Single Molecular Arrays of Phthalocyanine Assembled with Nanometer Sized Alkane Templates S. B. Lei, C. Wang,* S. X. Yin, and C. L. Bai* Center of Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China ReceiVed: June 11, 2001; In Final Form: September 22, 2001
It is demonstrated that single molecular arrays of phthalocyanines (Pc) can be assembled with templates of 2-dimensional alkane lamella of nanometer scale. By optimizing stoichiometry of the binary mixture, large areas of uniform single molecular arrays can be formed in submicrometer scale. This work provides a molecular scale illustration of self-assembly of binary molecular systems based on van der Waals interaction, and a method to design and assemble single molecular arrays with submicrometer length scale.
Introduction Molecular nano-structures have been remarkably progressed in the past decade. The art of constructing molecular nanostructures is the intermolecular interactions in all ranges (van der Waals, electrostatic, covalent) and molecular symmetry. The understanding of the assembling principles is helpful to provide venues to bring different molecular species together to form either physical structures such as templates, or provide the basis for local chemical processes such as nano-chemistry. For molecular self-assemblies, the involved interactions are noncovalent types that are relatively weak and reversible with respect to experimental conditions. The interactions in this category include electrostatic, van der Waals, hydrogen bonding, hydrophobic interaction, etc.1 It appears that much attention and effort have been dedicated to hydrogen-bond-connected systems in both 3-D and 2-D nanostructures.2-7 The hydrogen bonds have the advantage of selectivity and directionality, which are important in building biological nanostructures. For other interactions, such as van der Waals and hydrophobic interaction, the lack of directional selectivity makes them generally difficult to be applied in constructing molecular structures. While assemblies are widely studied in 3-D sructures,4-9 using the concept to form 2-D structures is less explored, in which the surface support could be substantial in determining the assembly characteristics. This is of special interest for surface-oriented functionalizations or devices. The assemblies of single-component systems, such as alkanethiols, have been studied rather thoroughly,10-14 the 2-D assembly process of multicomponent systems is not analyzed systematically, as compared to 3-D structures, in which much understanding has been accumulated for multicomponent systems. Phthalocyanines are of interest as an organic semiconductor, electronically active organic molecules, and also because of its similarity to biologic molecules such as chlorophyll. Because of its relevance to the realization of molecular electronics, the mechanism leading to a well-ordered monolayer of phthalocyanine on an interface causes great interest. Copper phthalocyanine has been investigated on HOPG and MoS2(15), * Authors to whom correspondence should be addressed. Fax: 86 10 6255 7908 (C.W.). E-mail:
[email protected] (C.L.B.); wangch@ infoc3.icas.ac.cn (C.W.).
Cu(100),16 GaAs(110),17 Si(100)(2 × 1),18 Si(100), and Si(111),19 and submolecular resolution was obtained on some substrates. In our lab, copper phthalocyanine has been successfully imaged with submolecular resolution on graphite by using long chain alkanes as a buffer layer.20 It is well-known that long chain alkanes adsorb from nonpolar solutions to form close packed monolayers on graphite. Scanning tunneling microscopy (STM), neutron diffraction, and molecular dynamics simulations have been used to study the structures of the alkane monolayer.21-26 Other substituted alkanes such as carboxylic acid, alcohols, dialkyl-substituted benzene, disulfides, thiols, amines, and halides have also been studied by STM.10-14 These alkane derivatives form 2D lamellae with polar groups paired together. This is an indication that the pairing of polar groups could be a favorable arrangement in view of free energy, which governs the molecular self-assembly processes (both entropy and enthalpy). For the multicomponent systems, the molecular assemblies could result in quite different results, and phase separation is rather common which is directly related to the intermolecular forces.27,28 Alkane lamellae can be formed through 2-D crystallization of alkane parts on solid surfaces (HOPG, Au(111)), and hydrophilic or electrostatic interactions are responsible for assembling end groups together29 (also hydrogen bonds in -OH, -COOH). Reported attempts of adsorption of alkanes with other species normally lead to phase separation.27 Little attempt has been seen on achieving molecular nanostructures based upon van der Waals or electrostatic interactions due to reasons discussed above. The reported tests indicate the micro-phase separation is prevalent. It is noticed that these combinary systems could provide a rich range of molecular interaction, and are worthy of systematic studies. In this work, the structures formed by copper phthalocyanine (CuPc) and phthalocyanine (Pc) with n-octadecyl mercaptan (C18SH) and Pc with 1-iodooctadecane, 1-bromooctadecane, 1-chlorooctadecane (later noted as C18X (X ) Cl, Br, I), and octadecyl cyanate (C18CN) were investigated. Besides the domains of segregated Pc and alkane derivatives, a new phase of uniform assembly of Pc with alkane derivatives has been found. The existence of various phases on the surface was found to depend on the molar ratio of Pc to alkane derivatives.
10.1021/jp012200i CCC: $20.00 © 2001 American Chemical Society Published on Web 11/15/2001
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Figure 1. (a) A large-scale image of the uniform assembly of Pc with C18SH when the molar ratio has been adapted to 1:3 (667 mV, 1.019 nA). (b) Coexistence of phase I (domain of pure thiol) and phase II (uniform assembly) when the molar ratio was below 1:3 (-431 mV, 677 pA). (c) Coexistence of phase II and phase III (pure Pc domain) when the molar ratio was above 1:3 (715 mV, 1.136 nA), (inserted) high-resolution image obtained on the Pc domain.
Experiment C18X (X ) Cl, Br, I), C18CN, C18SH, CuPc, and Pc were purchased from Acros, and pyrazine and 2,2′-bipyridine were purchased from Fluka and used without further purification. The solvent used in the experiments was toluene (HPLC grade, Acros) and the concentrations of all the solutions used were less than 1 mM. Samples were prepared by dipping a drop of the above solution on a freshly cleaved HOPG. Binary mixtures of different molar ratio were used in order to control the dominant structure on the samples. STM characterizations were performed on a Nano IIIA nanoscope (Digital Instrument Co.) operated under ambient conditions. The reaction of C18X with Pc (using pyrazine and 2,2′bipyridine as comparison) was tested by dropping a droplet of AgNO3 solved in DMSO to the toluene solution of C18X, C18X
with Pc, C18X with pyrazine, and C18X with 2,2′-bipyridine. The concentration of these solutions was in the same order as that used in STM characterization. Molecular mechanics simulation was carried out on a SGI workstation using software package of Insight II. On the basis of the monolayer simulations, the adsorption of CuPc was modeled on three positions with HOPG fixed. The energy minimization was performed using ESFF force field until RMS was less than 0.01 kcal/A. Model constructions and calculations were accomplished by Builder and Discover package of Biosym. Results and Discussion Several types of 3-D supermolecular crystals using porphyrin as building blocks have been prepared.4-9 In this work we will demonstrate a strategy using HOPG as the supporting surface
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Figure 2. High-resolution image of the uniform assembly of Pc with C18SH (560 mV, 1.019 nA). The imaging condition was optimized for the observation of Pc. Parameters defining this assembly were defined on this image.
to obtain 2-D supermolecular structures formed by phthalocyanine and alkane derivatives. C18SH and C18X (X ) Cl, Br, I, CN) were tested in this work and striking results have been obtained. Phase separation via intermolecular interaction is extensively studied in both small and high-molecular-weight systems. The co-deposition of binary solution on substrate surface often results in phase segregation, like that reported for 8CB/tetracontane and 8CB/PTCDA.27,28 In this work, phthalocyanine and alkane derivatives were coadsorbed from solution onto HOPG, a new phase of homogeneous molecular assembly of Pc with C18SH and C18X, was found in addition to the typical segregated domains. By adjusting the molar ratio of Pc to alkane derivatives to about 1:3, homogeneous intercalation of Pc in the length scale of submicrometers can become the main form of Pc on a graphite surface. Figure 1a shows a homogeneous molecular assembly of Pc with C18SH. In the image, the bright lines correspond to Pc molecular lines, the line width is about 1.5 nm, consistent with the side width of the Pc molecule, and the dark bands in the image correspond to C18SH lamellae, the 2.5 nm width is in accordance with the molecular length of C18SH. On the surface the molecular lines can extend to several hundred nanometers. On the large-scale image, domain boundary can be detected. The angle between the orientations of these domains is measured to be nearly 60° or 120°, this indicates the orientation of the domain is determined by the registry of alkane chains of thiols to the substrate. By altering the molar ratio of Pc to C18SH, multi-row bands of Pc (phase III) or single molecular line of Pc (phase II) surrounded by C18SH lamellae (phase I) can be obtained corresponding to concentration of Pc above or below the optimal ratio, respectively, as shown in Figure 1, parts b and c. High-resolution images obtained on the multi-row band regions of Pc are consistent with the adsorption structure of Pc reported on HOPG,15 Pcs joggle together in the molecular array as in the 2D domain.
Though, the uniform assembly structures can become dominant by adjusting the molar ratio of Pc to alkane derivatives to nearly 1:3, one can still observe structures as shown in Figure 1, parts b and c, in some area of the surface. This is attributed to the fluctuation of the molar ratio of Pc to alkane derivatives during the volatilization of the solvent, and this is considered to be an indication that the deposition from solution is a dynamic equilibrium process, where kinetic factors are attributed to the nonuniform regions. The homogeneous assembly of Pc with C18SH is stable enough to endure repeated scanning and high-resolution images could be obtained on this assembly as shown in Figure 2. The Pc molecule in the line can be observed to tilt for 32 ( 4° from the lamellae direction and the C18SH molecules adsorb on the surface with its carbon chain parallel to the surface and nearly perpendicular to the Pc lines (75 ( 6°). The repeating period (l) is measured to be 4.0 ( 0.2 nm, in accordance with the length of one C18SH molecule plus the width of one Pc molecule. It can also be counted that on each side of one Pc molecule there exist three C18SH molecules (Figure 2), so the Pc vs C18SH ratio can be measured to be 1:3 from the STM image in the intercalated structure. From the section profile the width of the trough between two Pc molecular lines (corresponding to the carbon chains of C18SH) can be measured as 2.5 ( 0.1 nm, in good accordance with the length of C18SH molecule (2.65 nm). This evidence rules out the possibility that Pc adsorbs along the SH headgroups. The co-deposition of CuPc with C18SH has also been investigated. The assembling behavior of CuPc and parameters of the assembly structure is nearly the same as that of Pc. This indicates that the copper ion does not influence the assembly behavior of phthalocyanine. Similar assembling behavior was also observed in other alkane derivatives, with model systems of C18X (X ) Cl, Br, I, CN). When coadsorbed with Pc, octadecyl halides all form molecularly intercalated structures with Pc, the parameters of
Single Molecular Arrays of Phthalocyanine
J. Phys. Chem. B, Vol. 105, No. 49, 2001 12275 TABLE 1: Parameters of the Single Molecular Arrays of H2Pc Formed with Functionalized Alkanes C18Cl/H2Pc C18Br/H2Pc C18I/H2Pc C18CN/H2Pc C18SH/H2Pc
l(nm)
θ(°)
φ(°)
4.0 ( 0.2 4.1 ( 0.2 4.1 ( 0.1 4.0 ( 0.2 4.0 ( 0.2
25 ( 6 28 ( 5 28 ( 6 26 ( 6 32 ( 4
69 ( 3 83 ( 5 82 ( 4 88 ( 3 75 ( 6
The tilt of the Pc molecules can be clearly observed. The carbon chains oriented parallel to the substrate surface can also be seen. It can be measured from the STM image that the Pc vs C18X ratio is 1:3 in the intercalated structure. On some high-resolution STM images, CH2 groups of C18X molecule can be counted. From these evidences we can conclude that Pc and C18X molecules are in the same layer and intercalation has been formed. For the reason that the reaction of halides with nitrogen in aromatic heterocycle compounds is a classical organic reaction, so the possibility that C18X has reacted with the nitrogen atom on Pc should be considered. The reaction of the solution of AgNO3 with C18X, C18X and Pc, C18X and pyrazine, and C18X and 2,2′-bipyridine has been tested, and the results are shown in Table 2. In summary, no sign of turbidity appeared when AgNO3 has been added into solutions of C18Br, C18Cl, C18Br and C18Cl with Pc, but turbidity immediately appears when it is added into solution of C18X with pyrazine and 2,2′bipyridine. In the case of C18I and C18I with Pc, 5 min after the addition of AgNO3, turbidity appears, no evidence of acceleration has been observed in the case of C18I with Pc. This indicates the emergence of turbidity in these two solutions is due to the reaction of C18I with AgNO3, not the reaction of C18I with Pc. From all the evidence it can be concluded that the interaction of C18X with Pc is intermolecular, not covalent. The co-deposited structure of C18CN with Pc has also been tested. In this case, Pc molecules also form single molecular arrays with C18CN (Figure 4c). The ratio of Pc to C18CN in the intercalated structure is also measured to be 1:3. The parameters measured from the STM images have been shown in Table 1. In summary, this structure is very similar to that of Pc with C18X, it is difficult to distinguish these structures from appearance of the STM images, and the difference of their parameters is very tiny. Discussion Figure 3. (a) Large scale STM image of the uniform assembly of Pc with C18I (-547 mV, 1.239 nA), (b) high-resolution image of the assembly (-828 mV, 1.168 nA).
the arrangement are mostly the same. By adjusting the molar ratio of Pc to octadecyl halides to about 1:3, uniform assembly can be the dominate form on the surface. The situation is similar with that of the binary of Pc and C18SH, so we will describe these structures in short. Figure 3a shows a large scale view of the structures formed by the coadsorption of Pc and 1-iodooctadecane. We can see in the 300 nm × 300 nm area only homogeneous assembly can be observed, in the case of C18Br and C18Cl, things are the same. The repeating periods of these assemblies are 4.1 ( 0.1, 4.1 ( 0.2, 4.0 ( 0.2, respectively. These repeating periods are nearly the same, and this is in accordance with the tiny difference between the van der Waals radius of Cl, Br, and I (Cl: 180pm, Br: 195pm, I: 215pm). The parameters that measured from the STM images of these three systems have been shown in Table 1. High-resolution images of these assemblies are shown in Figure 3b and Figure 4, parts a and b.
According to the above STM results, the groove between C18X lamellae appears to be a favorable site for Pc molecules to stay. On the basis of the observations, it is plausible to suggest that when C18X is excessive, the coadsorption of Pcs with C18X results in uniform assembly of C18X and Pc molecules which are surrounded by lamellae formed by C18X. As the stoichiometry of Pc increases, Pc molecule arrays extend in the boundary of C18X lamellae and the size of uniform assemblies also increases until Pc is excessive. At this time, Pc molecules crystallize along the Pc arrays of the uniform assembly and multi-row Pc bands formed. This provides a qualitative illustration of the assembling process and rigorous elucidation of this process needs quantitative simulations. Besides the systems demonstrated above, the co-deposition of Pc with other alkane derivatives such as octadecanol and stearic acid was also studied and homogeneous assembly has not been found,30 thus we consider the interaction between Pc and the end group of the alkane derivative and the strength of interaction between functional groups of alkane derivatives play an important role in the assembling process.
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TABLE 2: Reaction Results of AgNO3 with the Solution of C18X and Binary Mixtures of C18X with H2Pc, Pyrazine, and 2,2′-Bipyridine C18Cl C18Br C18I
H2Pc
pyrazine
2,2′-bipyridine
bare
responseless responseless yellow precipitation (5 min)
white precipitation white precipitation yellow precipitation
yellow precipitation yellow precipitation yellow precipitation
responseless responseless yellow precipitation (5 min)
Figure 4. High-resolution STM image of uniform assembly formed by Pc with (a) C18Br (-668 mV, 1.405 nA), (b) C18Cl (-525 mV, 1.020 nA) and (c) C18CN (-800 mV, 926 pA).
Because self-assembled structures represent thermodynamic minima, they are formed by reversible association of a number of individual molecules, and the interplay of enthalpy and entropy (∆H and ∆S) in their formation is more important than in synthesis process based on formation of covalent bonds.1 Accurate simulation of the assembly behavior of Pc with C18X requires consideration of not only the interaction between the adsorbates and substrate but also interactions between similar as well as dissimilar molecules, i.e., the Pc-Pc, alkane derivative-alkane derivative, and Pc-alkane derivative interaction. The contribution of entropy during the transition from the solution state to assembly structures should also be considered, this brings great complexity in the theoretical simulation of the
system. Since in our study, the model systems are only different in their end groups of alkane part, based on the equation1
T∆S)T(∆Strans + ∆Sconf) ∼ RT ln[c]solution/ [c]pure + RT ln 1/3 the contribution of entropy includes ∆Strans and ∆Sconf, in which ∆Strans is the loss in transitional entropy and is based exclusively on consideration of concentration [c]solution and [c]pure. ∆Sconf is the loss in conformational entropy in freezing a freely rotating bond. Since the starting concentrations [c]solution used in this work are all in the order of millimolar and the molecular structures are nearly identical, the contributions of entropy associated with
Single Molecular Arrays of Phthalocyanine the formation of self-assembled structures to the free energy ∆G may be considered comparable for the model systems in this work. Therefore, one can focus on the enthalpy effect while the entropy effect and shape-dependent effect of the assembly can be neglected for good approximations. The enthalpy is determined by intermolecular interaction, including the interaction between the similar molecules and dissimilar molecules, i.e., the energy change by bringing molecules together into a particular assembly pattern. This effect is more pronounced in the current study since all alkane derivatives have the same alkane chain skeleton, only different in end groups. So the selection of the present system may greatly simplify the simulation process, whereas the quantitative simulation still need full scale investigation. The molecular mechanics simulation of this part is in progress and will be presented in a future report. It is conceivable that further studies are interesting such as varying end groups with different hydrophilic or steric interaction, as well as substrate effect, etc. It is also considered that the study could provide a complimentary approach for constructing single molecular wires, in addition to the nicely demonstrated synthetic route.31 This approach could assemble hundreds of Pc molecules into highly directional arrays. As the molecules in contact with each other with spaces less than the mean free path of conductive electrons, it is believed the assembled molecular arrays could essentially serve as conductive molecular wires. Conclusion In this work, single molecular arrays of Pc formed with template of functionalized alkanes has been explored and characterized by STM. Single molecular arrays of Pc have been obtained with a template of C18X (X ) Cl, Br, I, CN) and C18SH. Three different phases can be formed depending on the molar ratio of Pc to alkane derivatives. By adjusting the molar ratio to about 1:3, uniform assembly can become dominate on the surface. Because of the semiconducting properties of the Pc molecule, the single molecular lines obtained in this work are of interest in the potential application of molecular devices and nano-electronics. It is envisioned that by altering the length of the carbon chains of the functionalized alkanes and the structure of Pc molecules, molecular lines of different properties and width could be prepared. References and Notes (1) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312.
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