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J. Phys. Chem. B 2001, 105, 10838-10841
Surface Stabilized Porphyrin and Phthalocyanine Two-Dimensional Network Connected by Hydrogen Bonds S. B. Lei, C. Wang,* S. X. Yin, H. N. Wang, F. Xi, H. W. Liu, B. Xu, L. J. Wan, and C. L. Bai* Center of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, China ReceiVed: February 14, 2001; In Final Form: July 19, 2001
Using carboxyl functionalized porphyrin and phthalocyanine as building blocks and alkane derivatives as coadsorbates, two-dimensional hydrogen-bonded networks were formed on graphite surface and observed by scanning tunneling microscopy. The configuration of the hydrogen bonding in the 2-D structure of 5,10,15,20-tetrakis (4-carboxylphenyl)-21H,23H-porphyrin (TCPP) is found to be different from that in the 3-D structure in bulk crystal. The difference of these structures from that expected from the bulk crystal is attributed to the effect of minimization of surface free energy in the 2-D system, which leads to close packing symmetry of molecules on the substrate.
Introduction Construction of molecular nano-structures has progressed remarkably in the past decade. The art of constructing molecular nanostructures is the intermolecular interactions in all range. 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 Much attention and effort have been dedicated to hydrogen-bond connected systems in both 3-D and 2-D nanostructures.2-15 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 directionally constructing low dimensional molecular structures. As a closely related topic, utilizing metallomacrocycles as building blocks, expansible clathrates with guest components of versatile size were demonstrated.2-8 Most of these crystals are sustained by stacking interactions and hydrogen bonding. In the channels among the porphyrin blocks, in most cases, solvent molecules or other small molecules exist, which serve as a template. The vast majority of these structures consist of offset-stacked porphyrin layers with an average interlayer distance of 4.5 Å. The clathrate structure formed by zinc tetra(4-carboxylphenyl) porphyrin (ZnTCPP) is a typical instance of this type of crystals. In this crystal, porphyrin blocks are connected by hydrogen bonding between carboxyl groups dimers in the same layer and the interlayer is connected by π-π interaction. Four porphyrin units and eight hydrogen bonds encircle any given pore within the network. The scale of the channel is up to 16 Å × 21 Å, and the space occupancy is only 39%.4 It has also been demonstrated that the hydrogen-bonded system can be assembled on a solid support with high stability,
for a single component such as adenine,12 and a 2-component system of 5-alkoxyisophthalic acid and diazine.15 In the 2-D system the surface support and adsorbate-substrate interaction could be substantial in determining the assembly characteristics. It is interesting to see if the adsorption structure of these porphyrin blocks mentioned above will resemble the intralayer alignment in the bulk 3-D crystal. In this work, using alkane derivatives as coadsorbate, 5,10,15,20-tetrakis (4-carboxylphenyl)-21H,23H-porphyrin (TCPP) and copper(II) 2,3,9,10,16,17,23,24-octakis (carboxyl)-29H,31H-phthalocyanine (CuPc8C) were immobilized on a graphite surface and the characteristic hydrogen bonded 2-D structure of TCPP was compared with the 3-D structures of ZnTCPP previously reported.4 This is of special interest for surface-oriented functionalizations or devices. The observed difference in assembling behavior is attributed to the effect of minimization of surface free energy. Experimental Section The materials used in the experiments, TCPP, 1-iodooctadecane, and stearic acid are purchased from Acros Co. and used without further purification. The solvent used is toluene (HPLC grade, Aldrich Inc.). CuPc8C were synthesized following the method previously reported.16 The ratio of aromatic species to alkane derivatives is about 1:3 and the concentration is less than 1 mM. Samples are prepared by depositing a droplet of the solution on a freshly cleaved HOPG surface and allow the solvent to evaporate. STM experiments were carried out on a Nano IIIa SPM system (Digital Instruments, Santa Barbara, CA) operating with constant current mode under ambient conditions. Tips are mechanically formed Pt/Ir wires (90/10). HyperChem version 5.02 for Win95/NT was used to optimize the conformation of adsorbate molecules and to simulate molecular models of the 2-D adsorption structures. If noted the STM images presented here were only processed by “Flatten” command and no further filtering were carried out. Results and Discussion
* Authors to whom correspondence should be addressed. Fax: 86 10 6255 7908. (Bai) E-mail:
[email protected]. (Wang) E-mail: wangch@ infoc3.icas.ac.cn.
Figure 1 presents the molecular structures of the organic species under study. In this work we allowed a binary mixture
10.1021/jp0105701 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/16/2001
Porphyrin and Phthalocyanine 2-D Network
J. Phys. Chem. B, Vol. 105, No. 44, 2001 10839
Figure 1. Molecular structure of TCPP(1) and CuPc8C(2).
of TCPP and stearic acid to co-deposit on the surface of HOPG. Two-dimensional islands of TCPP can readily be observed on the surface of HOPG, which were surrounded by lamellae of stearic acid (Figure 2a), these data suggest that the TCPP molecules form a submonolayer on the surface. In the TCPP domains, molecules are arranged in 4-fold symmetry and molecularly sharp boundaries can be observed. It was noticed that adsorption of TCPP alone on the surface of HOPG does not yield observable molecular images. It appears TCPP molecules are highly mobile and no stable ordered structures could be detected. This is not surprising for investigations have shown that CuPc, which has a size similar to that of TCPP, cannot adsorb on graphite with enough stability to allow STM imaging under ambient condition.17 But when coadsorbed with stearic acid, as shown in Figure 2, high-resolution images of TCPP molecules can be obtained within the 2-D domains, the resolution is comparable to that obtained for the alkanesubstituted porphyrin.18 It is now known that the STM contrast is not given by the contribution of the LUMO or the HOMO alone. Numerous theoretical works together with detailed comparisons with experimental STM images had shown that a large amount of molecular orbitals are necessary to recover the experimental contrasts. Since the image in this work was obtained under positive sample bias, the image contrast is considered to be determined by orbitals close to the LUMO. In the image, the disk-shaped structure of TCPP molecules in the center is attributed to the porphyrin ring of TCPP, and the four extending wings are attributed to the four phenyl groups. The distance between two adjacent TCPP molecules is measured to be 1.90 ( 0.05 nm. Figure 3b shows a model of the arrangement of TCPP based on hydrogen bonding, which was deduced from the 4-fold symmetry and intermolecular distance measured from the STM image using HyperChem 5.02 on a PC computer. In this arrangement, TCPP molecules are connected by cyclic hydrogen bonds composed by four carboxyl groups from four adjacent TCPPs, the H‚‚‚O distance is 1.80 Å, which is in the range of a typical hydrogen bond length. The distance between adjacent TCPP molecules is 1.83 nm in this model, in good
Figure 2. STM images of the 2-D structure formed by TCPP and stearic acid. (a) A large-scale image shows the boundary of the TCPP domain and stearic acid lamellae. (b) High-resolution STM image of the TCPP hydrogen-bonded network, tunneling condition: 852.3 mV, 1.089 nA.
accordance with the distance measured from the STM image. The same cyclic hydrogen bonds has also been observed in the work of Gerner et al.15 Figure 3 shows three possible arrangements of TCPP. In the arrangement shown in Figure 3a, there is no hydrogen bond and the arrangement is based upon the criteria of close packing, which is similar to the reported structure of 5,10,15,20-tetrakis (N-methylpyridinium-4-yl)-21H,23H-porphine tetrakis (p-toluensulfonate) (TMPyP) on I-Au(111) surface.19 On I-Au(111), each TMPyP molecule tilts for 45° and fits one of its phenyl groups into the void between two phenyl groups of adjacent molecules (Figure 3a). Apparently, the arrangement of TMPyP follows the criteria of close packing in order to minimize the surface free energy. In the arrangement shown in Figure 3a, the intermolecular distance is 1.63/1.75 nm, respectively, and the symmetry is nearly 6-fold. In the arrangement of TCPP as shown in Figure 3b, TCPP molecules are connected by cyclic hydrogen bond composed of four carboxyl groups instead of closest packing. The intermolecular distance in this arrangement is very consistent with that measured from the STM image (1.83
10840 J. Phys. Chem. B, Vol. 105, No. 44, 2001
Lei et al.
Figure 4. (a). STM image of an ordered network of CuPc8C, imaging condition: 700 mV, 590 pA, and the image has been “lowpassed” in order to reduce noise. This image is slightly distorted. Insert: Fourier transform of the image shows the 4-fold symmetry. (b) and (c) Possible alignment of CuPc8C in the 2-D domain and at the domain boundary.
TABLE 1: Parameters of the Crystal Cell of the 2-D Structures Mentioned in the Text a/nm TCPP
Figure 3. (a) A schematic representation of an alignment of TCPP follows the need of closest packing as TMPyP without any hydrogen bond between molecules. (b) Possible hydrogen-bonded network deduced from the STM images. (c) Configuration of TCPP representing the arrangement of ZnTCPP in a projection plane in the 3-D crystal co-crystallized with nitrobenzene.
vs 1.90 nm). Though the hydrogen bond cannot be directly observed by STM, it could be deduced from the fact that TCPP arranged in 4-fold symmetry, which is very different from the arrangement without hydrogen bonding.19 Carboxylic acids in general, and TCPP in particular, are known to form in the bulk diverse patterns of intermolecular hydrogen bonding. But when adsorbed on the substrate, the conformation of TCPP is restricted as affected by the molecule-substrate interaction and the nearly perpendicular orientation of the phenyl rings with respect to the porphyrin core will be changed to nearly parallel. This orientation variation has also been recognized in earlier studies on n-alkylcyanobiphenyl (nCB).20 It should be noted that the arrangement shown in Figure 3b may not be the only possibility for assembled surface structure, the presented assembly structure is the most likely arrangement that resembles the structure shown in Figure 2b. It is also noted that the suggested tetrameric configuration of TCPP is only possible when phenyl rings are nearly coplanar with porphyrin cores upon adsorption on graphite as discussed above. Considering the presented STM results in this work do not have high enough resolution to directly observe the orientation of the phenyl groups, the arrangement of TCPP in Figure 2b should be considered qualitatively. This arrangement is also different from the reported hydrogen-bonded structure in the 3-D crystal lattice of ZnTCPP (Figure 3c).4 The distance between the adjacent TCPP molecules in that arrangement is 2.26 nm, apparently larger than that measured from STM images, and the orientation of TCPP in the unit cell of these two arrangements is also different (the parameters of the related alignment of TCPP are listed in Table 1). Obviously, the head-to-head hydrogen
b/nm
θ/°
1.75
1.63
64
(without hydrogen bond, Figure 3a) 1.83 1.83 90 (with hydrogen bond, Figure 3b) 2.26 2.26 90 (arrangement similar to a projected plane of the 3-D crystal of ZnTCPP, Figure 3c) 1.90 ( 0.05 1.90 ( 0.05 90 ( 5 (measured from STM image)
CuPc8C 1.71
1.71
90
(in the alignment of Figure 4b) 1.82 ( 0.05 1.82 ( 0.05 90 ( 6 (measured from STM image)
bonding of carboxyl groups in the 3-D arrangement has much larger voids. But when adsorbed on the surface, such large voids will lead to the decrease of surface coverage, thus increasing surface free energy and is energetically unfavorable. Therefore Figure 3b is the favored configuration for the observed molecular assembly shown in Figure 2. The arrangement of TCPP in the 2-D structure is apparently a compromise of hydrogen binding and the minimization of the surface free energy. Another molecule investigated in this work is CuPc8C whose structure is also presented in Figure 1. In this work we used a binary mixture of CuPc8C and 1-iodooctadecane in order to achieve the stable adsorption structure of CuPc8C. Twodimensional islands of CuPc8C can readily be observed on the surface of HOPG, which were surrounded by lamellae of 1-iodooctadecane. In the CuPc8C domains, molecules are arranged in nearly 4-fold symmetry. As shown in Figure 4, STM images of CuPc8C molecules can be obtained within the 2-D domains. In the image, the CuPc8C molecules appear with 4-fold symmetry, consistent with the 4-fold symmetric molecular structure as illustrated in Figure 1. The protruded structure on the corner of the CuPc8C molecule is attributed to the phenyl groups attached to the conjugated porphyrin ring. The distance between two adjacent CuPc8C molecules is measured to be 1.82 ( 0.05 nm. Figure 4b shows
Porphyrin and Phthalocyanine 2-D Network a model of the arrangement of CuPc8C, which was deduced from the intermolecular distance measured from the STM image. In this arrangement, CuPc8C molecules are connected by hydrogen bonds between carboxyl groups from adjacent CuPc8Cs, the distance between adjacent CuPc8C molecules is 1.71 nm, in accordance with the distance measured from the STM image. As shown in Figure 4, a domain boundary can be observed in the middle of the 2-D domain of CuPc8C. At this domain boundary, the CuPc8C molecules in the upper left shifted for half a molecular width with respect to the lower half domain. A molecular model of this domain boundary is shown in Figure 4c. Depending on the molecular structure of CuPc8C, intermolecular hydrogen bonds can also be formed in this arrangement. So this kind of domain boundary presents another type of hydrogen-bonded assembly structure on surface support. The 4-fold symmetry in this arrangement is quite different from the reported 4-fold symmetric adsorption structure of unsubstituted copper phthalocyanine on the graphite surface.17 On HOPG, each CuPc molecule tilts for a certain angle and fits one of its phenyl groups into the void between two phenyl groups of adjacent molecules. In that 2-D structure the intermolecular distance measured was 1.45 ( 0.05 nm. Apparently, the arrangement of CuPc follows the criteria of close packing in order to minimize the surface free energy. While in the arrangement of CuPc8C, as shown in Figures 4b and 4c, CuPc8C molecules are connected by hydrogen bonds composed by two carboxyl groups instead of the closest packing and CuPc8C molecules do not tilt with respect to the cell vector. This arrangement in the 2-D structure is apparently dominated by hydrogen bonding rather than spatial hinder. Discussion It has been illustrated by a number of groups that the substrate could exert appreciable influence on the adsorption structure of organic molecules and the 2-D adsorption structure may be very different from that of the bulk 3-D crystal due to the adsorbate-substrate interaction. Gerner et al.15 has revealed that in the adsorption of pure 5-alkoxyisophthalic acid, the 2-D structure is appreciably different from that expected from a 3-D crystal, while in the case of supra-molecular assembly of this acid with diazine, the 2-D structure is identical to a projection plane of the corresponding 3-D crystal. It is well established that the adsorption process of molecules on substrate surface is controlled by many factors, among which the interactions of adsorbate-adsorbate and adsorbate-substrate are most important. Whereas the determining factor is the minimization of free energy of the total system, the request for the minimization of surface free energy is always dominated in the surface adsorption system and thus leads to the close packing configuration of molecules on the surface, which could be drastically different from bulk crystals. Examples can be seen in the case of the adsorption of alkane and alkane derivatives on surface of HOPG.21,22 The difference between the 2-D and 3-D structure of 5-alkoxyisophthalic acid is also raised because the arrangement in the 3-D crystal does not meet the requirement of 2-D close packing on the surface. In the present work, the request for minimization of surface free energy demands the closest packing, thus the arrangement of TCPP in the 3-D structure with large voids is not favorable. But entirely following the spatial dominated arrangement of TMPyP (Figure 3a) will completely destroy the hydrogen bonds and also lead to increase
J. Phys. Chem. B, Vol. 105, No. 44, 2001 10841 of system energy, therefore the final adsorption structure of hydrogen-bonded networks is a compromise of these factors. For CuPc8C, the situation is similar. Though the crystal data of CuPc8C is not available, the structure different from that expected for molecules without hydrogen bonding also leads to the same conclusion that the final structure is a compromise of hydrogen bonding and close packing requirements. Conclusion Assemblies of TCPP and CuPc8C were characterized by STM operating under ambient conditions on HOPG. Submolecular resolution was achieved on these molecules. In the 2-D adsorption structure, TCPP arrange in an appreciably different way from that in the 3-D crystal. We consider this difference is caused by the minimization of surface free energy in the adsorption process. The results obtained on CuPc8C also support this point of view. These results demonstrate hydrogen bonding and 2-D closest packing principles both play important roles in the adsorption process and the final adsorption structure is determined by the equilibrium of these two factors. Acknowledgment. The authors are thankful for the financial support from National Natural Science Foundation (Grant no. 29825106, 20073053), and the National Key Project on Basic Research (Grant G2000077501). References and Notes (1) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (2) Kumer, R. K.; Balasubramanian, S.; Goldberg, I. Inorg. Chem. 1998, 37, 541. (3) Abrahams, R. F.; Hoskins, B. F.; Michall, D. M.; Robson, R. Nature 1994, 369, 727. (4) Diskin-Posner, Y.; Goldberg, I. Chem. Commun. 1999, 1961. (5) Bhyrappa, P.; Wilson, S. R.; Suslick, K. S. Supermol. Chem. 1998, 9, 169. (6) Byrn, M. P.; Curtis, C. J.; Hsion, Y.; Khan, S. I.; Sawin, P. A.; Tendick, S. K.; Terzis, A.; Strouse, C. E. J. Am. Chem. Soc. 1993, 115, 9480. (7) Diskin-Posner, Y.; Krishna, R.; Goldberg, I. New J. Chem. 1999, 23, 885. (8) Byrn, M. P.; Curtis, C. J.; Goldberg, I.; Hsion, Y.; Khan, S. I.; Sawin, P. A.; Tendick, S. K.; Strouse, C. E. J. Am. Chem. Soc. 1991, 113, 6549. (9) Barth, J. V.; Wechesser, J.; Cai, C.; Gunter, P.; Burgi, L.; Jeandupeux, O.; Kern, K. Angew. Chem., Int. Ed. 2000, 39, 1230. (10) Gottarelli, G.; Masiero, S.; Mezzina, E.; Pieraccini, S.; Rabe, J. P.; Samori, P.; Spada, G. P. Chem. Eur. J. 2000, 6, 3242. (11) Etter, M. C. Acc. Chem. Res. 1990, 23, 120. (12) Sowerby, S. J.; Edelwirth, M.; Reiter, M.; Heckl, W. M. Langmuir 1998, 14, 5195. (13) Feyter, S.; Gesquiere, A.; Abdel-Mottaleb, M. M.; Grim, P. C. M.; Schryver, F. C.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mullen, K. Acc. Chem. Res. 2000, 33, 520. (14) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M. Chem. Commun. 1999, 1197. (15) Eichhorst-Gerner, K.; Stabel, A.; Moessner, G.; Declerq, D.; Valiyaveettil, S.; Enkelmann, V.; Mullen, K.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1996, 35, 1492. (16) Boston, Q. R.; Bailar, J. C. Inorg. Chem. 1972, 11, 1578. (17) Xu, B.; Yin, S.; Wang, C.; Qui, X.; Zeng, Q.; Bai, C. J. Phys. Chem. B 2000, 104, 10502. (18) Qiu, X. H.; Wang, C.; Zeng, Q. D.; Xu, B.; Yin, S. X.; Wang, H. N.; Xu, S. D.; Bai, C. L. J. Am. Chem. Soc. 2000, 122, 5550. (19) Kunitake, M.; Batina, N.; Itaya, K. Langmuir 1995, 11, 2337. (20) Smith, D. P. E.; Horber, J. K. H.; Binning, G.; Nejoh, H. Nature 1990, 344, 641. (21) Cyr, D. M.; Venbataraman, B.; Flynn, G. W.; Black, A.; Whitesides, G. W. J. Phys. Chem. 1996, 100, 13747. (22) Giancarlo, L.; Cyr, D.; Muyskens, K.; Flynn, G. W. Langmuir 1998, 14, 1465.
