Scanning Tunneling Microscopy Observation of Self-Assembled

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Langmuir 2008, 24, 12877-12882

12877

Scanning Tunneling Microscopy Observation of Self-Assembled Monolayers of Strapped Porphyrins Taichi Ikeda,*,†,§,| Masumi Asakawa,†,⊥ Koji Miyake,‡ Midori Goto,† and Toshimi Shimizu*,†,⊥ Nanoarchitectonics Research Center (NARC), National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, AdVanced Manufacturing Research Institute (AMRI), AIST, 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan, and New Energy and Industrial Technology DeVelopment Organization (NEDO), MUZA Kawasaki Center Tower, 1310 Omiya, Saiwai, Kawasaki, Kanagawa 212-8554, Japan ReceiVed May 16, 2008. ReVised Manuscript ReceiVed August 11, 2008 In this paper, we reveal that the free-base and zinc strapped porphyrins possessing long alkyl chains, C24OPP-HQ and Zn(C24OPP-HQ), respectively, can be arranged on surfaces. We used scanning tunneling microscopy (STM) to observe alkyl-chain-assisted self-assembled monolayers (SAMs) of these strapped porphyrins at the solid-liquid interface. STM images revealed that the strapped benzene moiety was detectable on the porphyrin core: that is, the strapped porphyrins could be differentiated from nonstrapped analogues. We compared the population of the nonstrapped porphyrin (C24OPP) and either of the strapped porphyrins C24OPP-HQ or Zn(C24OPP-HQ) in the mixed SAMs. We then confirmed that Zn(C24OPP-HQ) is more favorably incorporated in the mixed SAMs than C24OPP-HQ. From 1H NMR spectroscopic and X-ray crystallographic analyses, we concluded that the factors increasing the population of Zn(C24OPP-HQ) in the mixed SAMs are the enhanced rigidity of the porphyrin core by the zinc coordination and the flat structure of the porphyrin moiety in the saddle conformation. This study demonstrates that strapped porphyrins possessing long alkyl chains are available to arrange the functional modules on the surface via chemical modification on the strapped moiety.

Introduction Alkyl-chain-assisted self-assembled monolayers (SAMs) have been studied for over two decades.1 In early stages, the pioneering researchers reported self-assembled structures formed from simple alkanes1a,b,2 or amphiphiles.1,3 Recently, alkyl-chain-assisted SAMs have developed into a powerful tool for arranging functional molecules onto surfaces, for example, fused4 and branched5 aromatic derivatives, porphyrins,6 phthalocyanines,7 fullerenes (C60),8 ligand derivatives for transition metal coordination,9 ferrocene,10 tetrathiafulvalene derivatives,11 [2]catenanes,12 semiconductive materials,13 and donor-acceptor dyads * To whom correspondence should be addressed. Telephone: +81 29 861 4544. Fax: +81 29 861 4545. E-mail: [email protected] (T.I.); [email protected] (T.S.). † NARC, AIST. ‡ AMRI, AIST. § NEDO. | Present address: Organic Nanomaterials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. ⊥ Present address: Nanotube Research Center, AIST, Tsukuba central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan. (1) Reviews (a) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600–1615. (b) Giancarlo, L. C.; Flynn, G. W. Annu. ReV. Phys. Chem. 1998, 49, 297–336. (c) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290–4302. (2) (a) McGonigal, G. C.; Bernhardt, R. H.; Thomson, D. J. Appl. Phys. Lett. 1990, 57, 28–30. (b) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424–427. (3) (a) Okawa, Y.; Aono, M. Nature 2001, 409, 683–684. (b) Xu, S. L.; Wang, C.; Zeng, Q. D.; Wu, P.; Wang, Z. G.; Yan, H. K.; Bai, C. L. Langmuir 2002, 18, 657–660. (c) Hoeppener, S.; Chi, L. F.; Fuchs, H. ChemPhysChem 2003, 4, 494–498. (d) Mourran, A.; Beginn, U.; Zipp, G.; Mo¨ller, M. Langmuir 2004, 20, 673–679. (e) Mamdouh, W.; Uji-i, H.; Ladislaw, J. S.; Dulcey, A. E.; Percec, V.; De Schryver, F. C.; De Feyter, S. J. Am. Chem. Soc. 2006, 128, 317–325. (4) (a) Stabel, A.; Herwig, P.; Mu¨llen, K.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1995, 34, 1609–1611. (b) Tahara, K.; Furukawa, S.; Uji-I, H.; Uchino, T.; Ichikawa, T.; Zhang, J.; Mamdouh, W.; Sonoda, M.; De Schryver, F. C.; De Feyter, S.; Tobe, Y. J. Am. Chem. Soc. 2006, 128, 16613–16625. (c) Feng, X. L.; Wu, J. S.; Ai, M.; Pisula, W.; Zhi, L. J.; Rabe, J. P.; Mu¨llen, K. Angew. Chem., Int. Ed. 2007, 46, 3033–3036. (d) Wei, Y. H.; Tong, W. J.; Zimmt, M. B. J. Am. Chem. Soc. 2008, 130, 3399–3405.

and triads.14 Although some groups have developed the programmed patterning of the functional modules using DNA scaffolds,15 which are quite effective at forming complex patterns, alkyl-chain-assisted SAMs are quite complementary because they allow the ready formation of an infinitely sized regular pattern on the surfaces without the need for special equipment or techniques for synthesizing DNA sequences and weaving up DNA textures. These two approaches provide researchers with the choice between programmed or regular patterning. (5) (a) Loi, S.; Butt, H. J.; Hampel, C.; Bauer, R.; Wiesler, U. M.; Mu¨llen, K. Langmuir 2002, 18, 2398–2405. (b) Tomovic´, Z.; Van Dongen, J.; George, S. J.; Xu, H.; Pisula, W.; Lecle`re, P.; Smulders, M. M. J.; De Feyter, S.; Meijer, E. W.; Schenning, A. P. J. J. J. Am. Chem. Soc. 2007, 129, 16190–16196. (c) Lensen, M. C.; Elemans, J. A. A. W.; van Dingenen, S. J. T.; Gerritsen, J. W.; Speller, S.; Rowan, A. E.; Nolte, R. J. M. Chem.sEur. J. 2007, 13, 7948–7956. (6) (a) 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, 5550–5556. (b) Wang, H. N.; Wang, C.; Zeng, Q. D.; Xu, S. D.; Yin, S. X.; Xu, B.; Bai, C. L. Surf. Interface Anal. 2001, 32, 266–270. (c) Katsonis, N.; Vicario, J.; Kudernac, T.; Visser, J.; Pollard, M. M.; Feringa, B. L. J. Am. Chem. Soc. 2006, 128, 15537–15541. (7) (a) Qiu, X. H.; Wang, C.; Yin, S. X.; Zeng, Q. D.; Xu, B.; Bai, C. L. J. Phys. Chem. B 2000, 104, 3570–3574. (b) Klymchenko, A. S.; Sleven, J.; Binnemans, K.; De Feyter, S. Langmuir 2006, 22, 723–728. (c) Ye, T.; Takami, T.; Wang, R. M.; Jiang, J. Z.; Weiss, P. S. J. Am. Chem. Soc. 2006, 128, 10984– 10985. (8) Nakanishi, T.; Miyashita, N.; Michinobu, T.; Wakayama, Y.; Tsuruoka, T.; Ariga, K.; Kurth, D. G. J. Am. Chem. Soc. 2006, 128, 6328–6329. (9) (a) De Feyter, S.; A.-Mottaleb, M. M. S.; Schuurmans, N.; Verkuijl, B. J. V.; van Esch, J. H.; Feringa, B. L.; De Schryver, F. C. Chem.sEur. J. 2004, 10, 1124–1132. (b) Mourran, A.; Ziener, U.; Mo¨ller, M.; Breuning, E.; Ohkita, M.; Lehn, J. M. Eur. J. Inorg. Chem. 2005, 2641–2647. (c) Kikkawa, Y.; Koyama, E.; Tsuzuki, S.; Fujiwara, K.; Miyake, K.; Tokuhisa, H.; Kanesato, M. Langmuir 2006, 22, 6910–6914. (d) Schmittel, M.; Kalsani, V.; Ja¨ckel, F.; Rabe, J. P.; Bats, J. W.; Fenske, D. Eur. J. Org. Chem. 2006, 3079–3086. (10) Wedeking, K.; Mu, Z. C.; Kehr, G.; Fro¨hlich, R.; Erker, G.; Chi, L. F.; Fuchs, H. Langmuir 2006, 22, 3161–3165. (11) A.-Mottaleb, M. M. S.; G.-Nadal, E.; Surin, M.; Uji-i, H.; Mamdouh, W.; Veciana, J.; Lemaur, V.; Rovira, C.; Cornil, J.; Lazzaroni, R.; Amabilino, D. B.; De Feyter, S.; De Schryver, F. C. J. Mater. Chem. 2005, 15, 4601–4615. ¨ nsal, O.; Godt, A.; Rabe, J. P. ChemPhysChem (12) Samorı´, P.; Ja¨ckel, F.; U 2001, 2, 461–464.

10.1021/la801508k CCC: $40.75  2008 American Chemical Society Published on Web 10/16/2008

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The porphyrin derivative is one of the most valuable functional molecules because of its potential application in solar cells,16 catalysts,17 and molecular wires.18 Thus, scanning tunneling microscopy (STM) measurements of the porphyrins on the surface have been extensively conducted.19 In 2000, Bai et al. obtained STM images of alkyl-chain-assisted SAMs consisting of the porphyrin derivative CnOPP at the solid-liquid interface.6a,b Their finding prompted us to study the SAMs of chemically modified CnOPP derivatives.20 Two approaches are available for arranging functional modules on a molecular scaffold of CnOPP: axial coordination and covalent linking approaches. Using the former approach, we reported that the alkyl-chain-assisted SAMs formed from the pyridine-coordinated rhodium porphyrin Rh(CnOPP)(Cl)(Py).20a Because the axial coordination bond between the Rh(III) ion and the pyridine unit is sufficiently strong to allow the complex to be isolated,21 STM images of the pyridinecoordinated porphyrin were obtainable under ambient conditions. The choice of suitable metal ions for the axial coordination approach is, however, limited. For instance, we cannot use popular metal ions such as Fe(II) and Zn(II) ions because their coordination bonds are too labile to allow isolation of their complexes. In fact, no STM images of pyridine-coordinated species have been obtained at room temperature in the case of Zn(II) porphyrins.22 Therefore, to immobilize more complex functional modules onto surfaces, the covalent linking approach is a more powerful alternative to axial coordination. In a previous paper, we reported the synthesis and characterization of C18OPP-HQ, a strapped porphyrin possessing C18 alkyl chains, as the first example of a porphyrin-based SAM scaffold prepared using the covalent linking approach.20b Although we obtained STM images of C18OPP-HQ SAMs on graphite surfaces, the quality of these images was poor because of the instability of the self-assembled structures. It has been found that the stability of alkyl-substituted SAMs depends on the length of the alkyl chains.6b,20a,23 To obtain (13) (a) Ba¨uerle, P.; Fischer, T.; Bidlingmeier, B.; Stabel, A.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1995, 34, 303–307. (b) Gong, J. R.; Zhao, J. L.; Lei, S. B.; Wan, L. J.; Bo, Z. S.; Fan, X. L.; Bai, C. L. Langmuir 2003, 19, 10128–10131. (c) Samorı´, P.; Francke, V.; Enkelmann, V.; Mu¨llen, K.; Rabe, J. P. Chem. Mater. 2003, 15, 1032–1039. (d) Gesquie`re, A.; Jonkheijm, P.; Hoeben, F. J. M.; Schenning, A. P. H. J.; De Feyter, S.; De Schryver, F. C.; Meijer, E. W. Nano Lett. 2004, 4, 1175–1179. (e) Azumi, R.; M.-Osteritz, E.; Boese, R.; B.Buchholz, J.; Ba¨uerle, P. J. Mater. Chem. 2006, 16, 728–735. (f) Lei, S. B.; Deng, K.; Yang, Y. L.; Zeng, Q. D.; Wang, C.; Ma, Z.; Wang, P.; Zhou, Y.; Fan, Q. L.; Huang, W. Macromolecules 2007, 40, 4552–4560. (14) (a) Uji-i, H.; Miura, A.; Schenning, A.; Meijer, E. W.; Chen, Z. J.; Wu¨rthner, F.; De Schryver, F. C.; Van der Auweraer, M.; De Feyter, S. ChemPhysChem 2005, 6, 2389–2395. (b) Samorı´, P.; Fechtenko¨tter, A.; Reuther, E.; Watson, M. D.; Severin, N.; Mu¨llen, K.; Rabe, J. P. AdV. Mater. 2006, 18, 1317–1321. (15) (a) Park, S. H.; Yin, P.; Liu, Y.; Reif, J. H.; LaBean, T. H.; Yan, H. Nano Lett. 2005, 5, 729–733. (b) Zheng, J. W.; Constantinou, P. E.; Micheel, C.; Alivisatos, A. P.; Kiehl, R. A.; Seeman, N. C. Nano Lett. 2006, 6, 1502–1504. (16) (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40–48. (b) Guldi, D. M. Chem. Soc. ReV. 2002, 31, 22–36. (c) Balaban, T. S. Acc. Chem. Res. 2005, 38, 612–623. (d) Nakamura, Y.; Aratani, N.; Osuka, A. Chem. Soc. ReV. 2007, 36, 831–845. (17) Lane, B. S.; Burgess, K. Chem. ReV. 2003, 103, 2457–2473. (18) (a) Robertson, N.; McGowan, C. A. Chem. Soc. ReV. 2003, 32, 96–103. (b) Ikeda, T.; Lintuluoto, J. M.; Aratani, N.; Yoon, Z. S.; Kim, D.; Osuka, A. Eur. J. Org. Chem. 2006, 3193–3204. (19) (a) Grill, L.; Dyer, M.; Lafferentz, L.; Persson, M.; Peters, M. V.; Hecht, S. Nat. Nanotechnol. 2007, 2, 687–691. (b) Scudiero, L.; Hipps, K. W. J. Phys. Chem. C 2007, 111, 17516–17520. (c) Wakayama, Y.; Hill, J. P.; Ariga, K. Surf. Sci. 2007, 601, 3984–3987. (d) Yoshimoto, S.; Honda, Y.; Ito, O.; Itaya, K. J. Am. Chem. Soc. 2008, 130, 1085–1092. (20) (a) Ikeda, T.; Asakawa, M.; Goto, M.; Miyake, K.; Ishida, T.; Shimizu, T. Langmuir 2004, 20, 5454–5459. (b) Ikeda, T.; Asakawa, M.; Miyake, K.; Shimizu, T. Chem. Lett. 2004, 33, 1418–1419. (21) (a) Asakawa, M.; Ikeda, T.; Yui, N.; Shimizu, T. Chem. Lett. 2002, 174– 175. (b) Ikeda, T.; Asakawa, M.; Goto, M.; Nagawa, Y.; Shimizu, T. Eur. J. Org. Chem. 2003, 3744–3751. (22) (a) Williams, F. J.; Vaughan, O. P. H.; Knox, K. J.; Bampos, N.; Lambert, R. M. Chem. Commun. 2004, 1688–1689. (b) Otsuki, J.; Seki, E.; Taguchi, T.; Asakawa, M.; Miyake, K. Chem. Lett. 2007, 36, 740–741. (23) Miyake, K.; Ikeda, T.; Asakawa, M.; Shimizu, T.; Ishida, T.; Sasaki, S. AIP Conf. Proc. 2003, 696, 537–544.

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Figure 1. Chemical structures of (a) C24OPP, (b) C24OPP-HQ, and (c) Zn(C24OPP-HQ), with the labeling scheme for the 1H NMR spectra.

higher quality STM images, we synthesized a strapped porphyrin possessing even longer alkyl chains. In this paper, we report the STM images of a strapped porphyrin possessing C24 alkyl chains (C24OPP-HQ, Figure 1b) and its zinc complex (Zn(C24OPPHQ), Figure 1c). These high-resolution STM images allowed us to distinguish between strapped and nonstrapped porphyrins (e.g., C24OPP, Figure 1a). Indeed, STM observations allowed us to compare the populations of C24OPP-HQ and Zn(C24OPP-HQ) in the mixed SAMs.

Experimental Section General. The alkyl-substituted strapped porphyrin C24OPP-HQ and its zinc complex Zn(C24OPP-HQ) were synthesized according to the procedures described in previous reports.20b The model compound TPP-HQ and its zinc complex were also synthesized using similar procedures. 1H NMR and 13C NMR spectra were recorded using a Bruker AVANCE 400 spectrometer (400 and 100 MHz for 1H NMR and 13C NMR spectra, respectively) with the signal of the residual solvent as the internal standard. STM Observation. STM measurements were conducted using a Nanoscope IIIa multimode SPM (Digital Instruments) with commercially available Pt/Ir tips (80/20). STM images were recorded at the solid-liquid interface, directly in the droplet of the solution. 1,2-Dichlorobenzene was used as the solvent. The experiment was performed in the following sequence: preparation of the stock sample solution, STM observation of a freshly cleaved graphite surface, placement of the sample solution on the graphite surface, incubation for 5 min, and STM observation of SAMs at room temperature (293 K). The concentration of each solution was set at 4.0 × 10-5 M for the strapped porphyrin SAMs. Mixed SAMs were prepared from mixtures containing C24OPP and either C24OPP-HQ or Zn(C24OPPHQ). The total concentration in the solution for the mixed SAMs was 2.0 × 10-5 M. The number ratios of C24OPP and C24OPP-HQ or Zn(C24OPP-HQ) in the mixed SAMs were obtained by counting the number of each molecule in the STM images. A total of 20 images (50 nm × 50 nm) taken at different positions were used for the mixed monolayer analysis under each condition. X-ray Crystallography. The strapped tetraphenylporphyrin (TPPHQ) and its zinc complex [Zn(TPP-HQ)] were synthesized and analyzed through X-ray crystallography as model compounds for C24OPP-HQ and Zn(C24OPP-HQ), respectively. Diffraction data were collected on a Bruker Smart Apex diffractometer using graphitemonochromated Mo KR radiation. Data reduction was performed using SAINTPLUS.24 Empirical absorption correction was applied with SADABS.25 The structure determination was performed through direct methods and refinements with full-matrix least-squares on F2 using the SHELXTL software package.26

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Figure 2. 1H NMR spectra of (a) C24OPP-HQ and (b) Zn(C24OPP-HQ). Conditions: CDCl3, 400 MHz, RT.

Figure 3. X-ray crystallographic structures of the model compounds.

Results and Discussion Synthesis and Characterization of Strapped Porphyrin Derivatives. The strapped porphyrin was synthesized through the condensation of alkyl-substituted dialdehyde and alkylsubstituted dipyrromethane derivatives.20b Figure 2 displays 1H NMR spectra of C24OPP-HQ and Zn(C24OPP-HQ). The disappearance of the signal for the NH protons at δ ) -2.62 ppm, found in the former spectrum, indicates the perfect Zn coordination in the latter compound. For the precursor of the strapped porphyrin, the chemical shifts of the methylene protons R and β and the aromatic proton g are 4.30, 4.41, and 6.91 ppm, respectively. In the strapped porphyrin C24OPP-HQ, the chemical shifts of these protons are 2.81, 3.84 and 3.65 ppm, respectively. The upfield shift of these protons arises from the shielding effect of the porphyrin core, indicating that the strapped moiety is located above the porphyrin core. In comparison with C24OPP-HQ, the greater upfield shifts of the signals for protons R and g of Zn(C24OPP-HQ) indicate that the strapped benzene moiety in Zn(C24OPP-HQ) lies closer to the porphyrin core. X-ray Structures of Strapped Porphyrin Derivatives. Figure 3 presents the solid state structures of the model compounds TPP-HQ and Zn(TPP-HQ). These structures clearly revealed that the strapped moiety resides over the porphyrin core. The distances between the centroid of the strapped benzene unit and the mean plane of the porphyrin core are 4.45 and 4.37 Å for TPP-HQ and Zn(TPP-HQ), respectively. The closer positioning of the strapped benzene moiety to the porphyrin core in Zn(TPPHQ) relative to that in TPP-HQ in the solid state is consistent with the discussions in the previous section. Although the tilt

Figure 4. (a) STM image of a C24OPP SAM at the solid-liquid interface (25 nm × 25 nm). Tunneling conditions: I ) 30 pA; V ) -600 mV. (b) High-magnification STM image of a C24OPP SAM. The blue arrows indicate the alkyl chains. The contrast of the image is enhanced to see the alkyl chains. (c) Proposed structure of C24OPP SAM.

angles of the strapped benzene units relative to the mean plane of the porphyrin core are 70 and 64° for TPP-HQ and Zn(TPPHQ), respectively, in the solid state we suppose that these values fluctuate considerably in solution. X-ray structures indicated that TPP-HQ and Zn(TPP-HQ) possess ruffle and saddle deformations, respectively, of their porphyrin cores.27 Their different deformation geometries result in different orientations of their mesophenyl groups. The average deviations of the hydrogen atoms at the 4-positions of the meso-phenyl groups from the mean plane of the porphyrin core are 1.07 and 0.52 Å for TPP-HQ and Zn(TPP-HQ), respectively. We suppose that the conformational differences between TPP-HQ and Zn(TPP-HQ) would affect the populations of C24OPP-HQ and Zn(C24OPP-HQ) in the mixed SAMs. STM Observation of C24OPP Self-Assembled Monolayers. Figure 4a displays a STM image of a C24OPP SAM formed on a graphite surface. The well-ordered array having a characteristic lamellar structure of 2-fold symmetry was similar to those reported previously.6 The bright and dark stripes correspond to the aromatic units and alkyl chains, respectively. We can clearly detect four alkyl chains along each porphyrin core (Figure 4b), which (24) (a) Smart, v. 5.625; Bruker AXS Inc.: Madison, WI, 1999. (b) SAINTPLUS, v. 6.22; Bruker AXS Inc.: Madison, WI, 2001. (25) Sheldrick, G. M. SADABS; University of Gottingen: Germany, 1996. (26) Sheldrick, G. M. SHELXTL, v. 6.12; Bruker AXS Inc.: Madison, WI, 2000. (27) (a) Scheidt, W. R.; Lee, Y. J. Struct. Bonding (Berlin, Ger.) 1987, 64, 1–70. (b) Haddad, R. E.; Gazeau, S.; Pecaut, J.; Marchon, J. C.; Medforth, C. J.; Shelnutt, J. A. J. Am. Chem. Soc. 2003, 125, 1253–1268.

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Figure 7. (a) STM image of Zn(C24OPP-HQ) SAM (25 nm × 25nm). (b) High-magnification STM image of Zn(C24OPP-HQ) molecule. Tunneling conditions: I ) 30 pA; V ) -600 mV.

Figure 5. STM height distributions for (a) C24OPP, (b) C24OPP-HQ, and (c) Zn(C24OPP-HQ). All data were obtained from the section profiles of STM images observed under the tunneling conditions: I ) 30 pA; V ) -600 mV. STM height difference is defined as the height gap between the alkyl chain region and the highest point of the porphyrin core. The average heights are (a) 0.07 ( 0.01 nm, (b) 0.29 ( 0.09 nm, and (c) 0.29 ( 0.06 nm.

Figure 6. (a) STM image of C24OPP-HQ SAM (25 nm × 25 nm). (b) High-magnification STM image of C24OPP-HQ molecule. Tunneling conditions: I ) 30 pA; V ) -600 mV.

indicates the interdigitation of the C24OPP alkyl chains in the dark stripe regions. Figure 4c provides a schematic representation of the self-assembled structure. The experimental lattice parameters of the unit cell a × b and β are (4.8 ( 0.2) × (2.0 ( 0.1) nm and 100 ( 2°, respectively. These values are consistent with those predicted for the model in Figure 4c (a × b ) 4.8 × 2.0 nm; β ) 102°). The average height difference between the bright (aromatic) and the dark (alkyl) regions in the STM image of C24OPP is 0.07 ( 0.01 nm (Figure 5a). STM Observation of Strapped Porphyrins C24OPP-HQ and Zn(C24OPP-HQ). Figures 6a and 7a display STM images of C24OPP-HQ and Zn(C24OPP-HQ) SAMs, respectively. In these STM images, we were not able to detect the detailed packing structure of the alkyl chains. The experimental lattice parameters

of the unit cell a × b and β for C24OPP-HQ and Zn(C24OPP-HQ) are (4.9 ( 0.3) × (2.0 ( 0.1) nm, 99 ( 6° and (4.8 ( 0.2) × (2.0 ( 0.1) nm, 100 ( 4°, respectively. These values are almost the same as those for C24OPP. This result strongly suggests that the packing structures of C24OPP-HQ and Zn(C24OPP-HQ) alkyl chains are identical to that of C24OPP (Figure 4c). The strapped porphyrin has two nonidentical faces: one faces the strap (strap-face), while the other is free (free-face). In the SAMs of C24OPP-HQ and Zn(C24OPP-HQ), we suppose that the free-face contacts to the graphite surface. As evidence supporting this claim, note that the adsorption of alkyl-substituted tetraphenylporphyrin is much weaker than that of alkyl-substituted phthalocyanines6,7 because the protrusion of the meso-phenyl groups disrupts the stable attachment of the porphyrin core to the surface. In the case of the strapped porphyrin, the adsorption of the strapped face would be seriously disrupted by the strap moiety. The adsorption of the free-face is, however, identical to that of C24OPP. Figures 6b and 7b display the high-magnification STM images of C24OPP-HQ and Zn(C24OPP-HQ) molecules, respectively. Whereas the center of the C24OPP porphyrin core has no feature in its STM image (Figure 4b), a bright spot is detectable at the center of the strapped porphyrin core. In the case of Zn(C24OPPHQ), we clearly observe a central bright spot and four orthogonally located lobes. We assigned the former and the latter to the strapped benzene unit and four meso-phenyl groups, respectively. As for C24OPP-HQ, it should be noted that the shapes of the bright spots are not uniform in the STM image (Figure 6a). After many painstaking trials, we concluded that it is impossible to suppress the fluctuations of these strap units for C24OPP-HQ STM images. We believe that these fluctuations are attributable to the flexibility of the strapped moiety. The fluctuation of the strapped benzene unit in Zn(C24OPP-HQ) is suppressed in comparison to C24OPPHQ. The zinc coordination results in a more rigid porphyrin core that suppresses the rotational movement of the meso-phenyl groups.28 As we mentioned above in the sections of 1H NMR and X-ray crystallographic characterizations, the strapped benzene unit is positioned closer to the porphyrin core in Zn(C24OPPHQ) than it is in C24OPP-HQ. This factor may also contribute to suppress the fluctuation of the strapped benzene unit in Zn(C24OPP-HQ). The average height differences between the bright spots (strapped benzene unit) and the dark stripes (alkyl chains) for C24OPP-HQ and Zn(C24OPP-HQ) are 0.29 ( 0.09 and 0.29 ( 0.06 nm, respectively (Figure 5b and c). The larger standard deviation reflects the larger fluctuation of the strapped benzene unit in C24OPP-HQ. The height difference we obtained through STM analysis is not comparable to that found in the X-ray structure, because the STM contrast depends on local (28) Freitag, R. A.; Whitten, D. G. J. Phys. Chem. 1983, 87, 3918–3925.

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Figure 8. STM image of a mixed SAM comprising C24OPP and Zn(C24OPP-HQ) (25 nm × 25 nm). Tunneling conditions: I ) 30 pA; V ) -600 mV.

conductivity at the observed point and does not reflect the real height of the observed structure.29 STM Observation of Mixed Self-Assembled Monolayers. We prepared the mixed SAMs from mixtures of C24OPP and either C24OPP-HQ or Zn(C24OPP-HQ). Figure 8 displays a STM image of a mixed SAM comprising C24OPP and Zn(C24OPPHQ). We were able to differentiate two kinds of porphyrins, namely, porphyrins having a bright spot at the porphyrin center and those without a bright spot. The former and the latter entities correspond to Zn(C24OPP-HQ) and C24OPP, respectively. We obtained the surface coverage (θ) of each component (θfree + θstrap ) 1) by counting their numbers in the STM image. We confirmed that the surface coverage of each component depends on the fractional concentration of the solution used to prepare the mixed SAMs (Cfree + Cstrap ) 1). We assumed that the relationship of these parameters could be expressed using an index ∆P as follows.

Cfree θfree ) exp(∆P) θstrap Cstrap

(1)

where the subscripts “free” and “strap” represent C24OPP and the strapped porphyrin C24OPP-HQ [or Zn(C24OPP-HQ)], respectively. The ∆P value is an indicator to see which one is the major component in the mixed SAMs. When the populations of the strapped and nonstrapped porphyrins in the mixed SAMs are the same, ∆P equals zero. A positive (negative) ∆P value means that the nonstrapped porphyrin C24OPP is the major (minor) component in the mixed SAMs. Although eq 1 was derived based on Langmuir-type adsorption, the ∆P value does not relate to the physical value of the adsorption free energy. This is because the ∆P value is determined by not only the thermodynamics but also the kinetics of the adsorption process. We were rarely able to detect the positional change of the strapped porphyrins in the mixed SAMs in the successive STM images. This result indicates that the population of the adsorbate is strongly affected by the kinetics of the adsorption process. Figure 9 provides a plot of the relationship between the fractional surface coverage of C24OPP and the molar fraction of C24OPP in the solution. From least-squares fitting of the data to eq 1, we calculated the ∆P values to be +0.49 and +0.16 for the mixed SAMs of C24OPP-HQ and Zn(C24OPP-HQ), respectively. In both cases, the strapped porphyrin is a minor component in the mixed SAMs. On the basis of molecular symmetry, the (29) Wiesendanger, R. Scanning ProVe Microscopy and Spectroscopy: Method and Application; Cambridge University Press: New York, 1994.

Figure 9. Relationship between the fractional coverage of C24OPP on the surface (θfree) and the molar fraction of C24OPP in the solution [Cfree/ (Cfree + Cstrap)] (average ( standard deviation): (O) mixed SAMs comprising C24OPP and C24OPP-HQ; (b) mixed SAMs comprising C24OPP and Zn(C24OPP-HQ).

adsorption of C24OPP onto the substrate is more favorable than that of the strapped porphyrins because C24OPP has two equivalent faces capable of adsorbing to the surface, whereas the strapped porphyrins have only one apiece. As mentioned above, only the free-face of the strapped porphyrin can form stable attachments to the graphite surface. The degree of free rotational motion of the meso-phenyl groups also affects the ∆P value. In C24OPP, four meso-phenyl groups can rotate with respect to the porphyrin core, which would disrupt the stable attachment of C24OPP to the surface. In contrast, the rotational motion of two meso-phenyl groups in the strapped porphyrin is prohibited mechanically by the presence of the strap, which imparts an advantage to the adsorption of C24OPP-HQ over that of C24OPP. Presumably, the balance between these two factors (symmetry factors and the degree of free rotational motion of meso-phenyl groups) decides the value of ∆P. The smaller ∆P value of Zn(C24OPP-HQ) than that of C24OPPHQ indicates that Zn(C24OPP-HQ) is more favorably incorporated in the mixed SAMs. This difference arises from the effect of the zinc coordination. As we mentioned above, zinc coordination increases the rigidity of the porphyrin core and restricts the rotational motion of the meso-phenyl groups.28 In addition, the saddle conformation of the porphyrin core in Zn(C24OPP-HQ) affords a flatter structure of the porphyrin core than that of C24OPP-HQ as evidenced from X-ray crystallographic analysis. The flatter molecule forms more-stable SAMs. These two factors (the restricted rotational freedom of the meso-phenyl groups and the flatter porphyrin core) may contribute to more favorable incorporation of Zn(C24OPP-HQ) in the mixed SAMs in compared to C24OPP-HQ. One other possibility is that a direct stabilizing interaction exists between the Zn(II) ion and the graphite surface. Because the association constants of the fullerene (C60) binding to the host molecules consisting of the free-base and zinc porphyrins are comparable,30 we suppose that the interaction between the Zn(II) ion and the graphite surface is very little.

Conclusion We have recorded STM images of SAMs formed from both strapped porphyrins and nonstrapped porphyrins. Through analysis of their mixed SAMs, we confirmed that zinc-coordinated strapped porphyrin [Zn(C24OPP-HQ)] is more favorably incor(30) Tashiro, K.; Aida, T. Chem. Soc. ReV. 2007, 36, 189–197.

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porated in the mixed SAMs in comparison to the free-base strapped porphyrins (C24OPP-HQ). We supposed that two factors contribute to favorable incorporation of Zn(C24OPP-HQ) in the mixed SAMs: (i) the more rigid porphyrin core by the zinc coordination that restricts rotational movements of the mesophenyl groups and (ii) the flatter structure of the porphyrin core in the saddle conformation. We have demonstrated that the covalent linking approach is an effective means of arranging functional modules on surfaces. We can expect that any kind of functional module could be arranged on the surfaces through chemical modification of the strapped moiety. Our method will

Ikeda et al.

be useful for analyzing the properties of individual functional molecules and fabricating sensor arrays on surfaces. Acknowledgment. This study was supported by the Industrial Technology Research Grant Program of the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Supporting Information Available: Full spectroscopic and X-ray data for the compounds; X-ray crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. LA801508K