STM Observation of Alkyl-Chain-Assisted Self-Assembled Monolayers

May 25, 2004 - From Chaos to Order: Chain-Length Dependence of the Free Energy of Formation of Meso-tetraalkylporphyrin Self-Assembled Monolayer Polym...
0 downloads 6 Views 170KB Size
Download PDF
5454

Langmuir 2004, 20, 5454-5459

STM Observation of Alkyl-Chain-Assisted Self-Assembled Monolayers of Pyridine-Coordinated Porphyrin Rhodium Chlorides Taichi Ikeda,†,| Masumi Asakawa,† Midori Goto,† Koji Miyake,‡ Takao Ishida,§ 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, Nanotechnology Research Institute (NRI), AIST, Tsukuba Central 4, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8562, Japan, and New Energy and Industrial Technology Development Organization (NEDO), Sunshine 60 Building, 3-1-1 Higashi Ikebukuro, Toshima-Ku, Tokyo, 170-6028, Japan Received February 18, 2004. In Final Form: April 14, 2004 Alkyl-chain-assisted self-assembled monolayers of pyridine-coordinated porphyrin rhodium chlorides were observed at the solid-liquid interface by scanning tunneling microscopy (STM). The resolved images at a molecular level were obtainable in the pure solution of pyridine-coordinated porphyrin rhodium chloride with four triacontyl groups [Rh(C30OPP)(Cl)(Py)]. In the case of pyridine-coordinated porphyrin rhodium chloride with four octadecyl groups [Rh(C18OPP)(Cl)(Py)], the STM images were not obtainable in the pure solution of Rh(C18OPP)(Cl)(Py) but obtainable in the mixture containing Rh(C18OPP)(Cl)(Py) and free porphyrin C18OPP. On the basis of the mixed self-assembled monolayer analysis, the apparent difference in the adsorption free energy between Rh(CnOPP)(Cl)(Py) and CnOPP (∆Gapp) was calculated. The calculated ∆Gapp values for C18OPP and C30OPP mixed systems were quite different. The disadvantage of the adsorption free energy of Rh(C18OPP)(Cl)(Py) makes it difficult to obtain molecularly resolved images of Rh(C18OPP)(Cl)(Py), and the large adsorption energy due to the long alkyl chains enabled us to obtain molecularly resolved images of Rh(C30OPP)(Cl)(Py).

Introduction The molecular alignment of single molecular devices on a substrate is an essential issue in the fields of nanoscience and nanotechnology.1 Self-assembly is a promising technique to create the molecular-scale patterns on the substrate rapidly and conveniently.2 Self-assembled monolayers on the substrate can be categorized into two approaches, namely, chemisorption and physisorption. These self-assembled monolayers are represented by organothiols on a noble metal surface (organothiol selfassembled monolayers)3 and by alkyl-chain-substituted compounds on a graphite surface (alkyl-chain-assisted selfassembled monolayers),4 respectively. Although physisorbed monolayers are comparatively less stable than chemisorbed monolayers, physisorbed monolayers are more suitable for single molecular patterning on the substrate. In organothiol self-assembled monolayers, it is * Corresponding author. Fax: +81-29-861-4545. E-mail: [email protected]. † NARC, AIST. ‡ AMRI, AIST. § NRI, AIST. | NEDO. (1) (a) Tour, J. M. Acc. Chem. Res. 2000, 33, 791. (b) Pease, A. R.; Jeppesen, J. O.; Stoddart, J. F.; Luo, Y.; Collier, C. P.; Heath, J. R. Acc. Chem. Res. 2001, 34, 433. (c) Yasuda, S.; Nakamura, T.; Matsumoto, M.; Shigekawa, H. J. Am. Chem. Soc. 2003, 125, 16430. (2) Lehn, J. M. Supramolecular Chemistry: Concept and Perspectives; VCH: Weinheim, 1995. (3) Reviews: (a) Ulman, A. Chem. Rev. 1996, 96, 1533. (b) Chiang, S. Chem. Rev. 1997, 97, 1083. (c) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (d) Yang, G.; Liu, G. J. Phys. Chem. B 2003, 107, 8746. (4) Reviews: (a) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600. (b) Giancarlo, L. C.; Flynn, G. W. Annu. Rev. Phys. Chem. 1998, 49, 297. (c) De Feyter, S.; De Schryver, F. C. Chem. Soc. Rev. 2003, 32, 139.

difficult to control the symmetry and periodicity of molecular arrangement on the substrate.5 On the other hand, molecular patterning by alkyl-chain-assisted selfassembled monolayers has been studied for more than a decade by scanning tunneling microscopy (STM).4,6-13 To date, molecular design to control two-dimensional selfassembly on the substrate (molecular architecture in the (5) (a) Ishida, T.; Mizutani, W.; Akiba, U.; Umemura, K.; Inoue, A.; Choi, N.; Fujihira, M.; Tokumoto, H. J. Phys. Chem. B 1999, 103, 1686. (b) Smith, R. K.; Reed, S. M.; Lewis, P. A.; Monnell, J. D.; Clegg, R. S.; Kelly, K. F.; Bumm, L. A.; Hutchison, J. E.; Weiss, P. S. J. Phys. Chem. B 2001, 105, 1119. (c) Zareie, M. H.; Ma, H.; Reed, B. W.; Jen, A. K.-Y.; Sarikaya, M. Nano Lett. 2003, 3, 139. (6) (a) McGonigal, G. C.; Bernhardt, R. H.; Thomson, D. J. Appl. Phys. Lett. 1990, 57, 28. (b) McGonigal, G. C.; Bernhardt, R. H.; Yeo, Y. H.; Thomson, D. J. J. Vac. Sci. Technol. 1991, B9 (2), 1107. (7) (a) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (b) Rabe, J. P.; Buchholz, S. Phys. Rev. Lett. 1991, 66, 2096. (c) Rabe, J. P.; Buchholz, S.; Askadskaya, L. Synth. Met. 1993, 54, 339. (d) Stabel, A.; Rabe, J. P. Synth. Met. 1994, 67, 47. (e) Stabel, A.; Herwig, P.; Mu¨llen, K.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1995, 34, 1609. (f) Stabel, A.; Heinz, R.; De Schryver, F. C.; Rabe, J. P. J. Phys. Chem. 1995, 99, 505. (8) (a) De Feyter, S.; Grim, P. C. M.; van Esch, J.; Kellogg, R. M.; Feringa, B. L.; De Schryver, F. C. J. Phys. Chem. B 1998, 102, 8981. (b) De Feyter, S.; Gesquie`re, A.; Abdel-Mottaleb, M. M.; Grim, P. C. M.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mu¨llen, K. Acc. Chem. Res. 2000, 33, 520. (c) Abdel-Mottaleb, M. M. S.; GomarNadal, E.; De Feyter, S.; Zdanowska, M.; Veciana, J.; Rovira, C.; Amabilino, D. B.; De Schryver, F. C. Nano Lett. 2003, 3, 1375. (d) De Feyter, S.; Gesquie`re, A.; Klapper, M.; Mu¨llen, K.; De Schryver, F. C. Nano Lett. 2003, 3, 1485. (9) (a)Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99, 6608. (b) Venkataraman, B.; Flynn, G. W.; Wilbur, J. L.; Folkers, J. P.; Whitesides, G. M. J. Phys. Chem. 1995, 99, 8684. (c) Cyr, D. M.; Venkataraman, B.; Flynn, G. W.; Black, A.; Whitesides, G. M. J. Phys. Chem. 1996, 100, 13747. (d) Giancarlo, L. C.; Fang, H.; Rubin, S. M.; Bront, A. A.; Flynn, G. W. J. Phys. Chem. B 1998, 102, 10255. (e) Giancarlo, L.; Cyr, D.; Muyskens, K.; Flynn, G. W. Langmuir 1998, 14, 1465. (f) Wintgens, D.; Yablon, D. G.; Flynn, G. W. J. Phys. Chem. B 2003, 107, 173.

10.1021/la049577a CCC: $27.50 © 2004 American Chemical Society Published on Web 05/25/2004

STM Observation of Pyridine-Coordinated Porphyrins

lateral direction against the underlying substrate) has been intensively studied;4c,8d,11d,13 however, less attention has been paid to expand the molecular architecture in the vertical direction against the underlying substrate.11e Therefore, we focused on alkyl-chain-assisted adsorption and self-assembly of porphyrins13 because metalloporphyrins can bind axial ligand molecules.14 Two-dimensional arrays of metalloporphyrins on the substrate can be utilized as a molecular template to immobilize desired organic molecules through their axial coordination to the central metal of metalloporphyrins. In this study, we confirmed self-assembled monolayers of pyridine-coordinated metalloporphyrins by using STM. Although STM observations of metalloporphyrins have been conducted in several research groups,15 no report has been published on ligand-coordinated metalloporphyrins. Therefore, we have discussed the requirements to obtain the STM images of pyridine-coordinated metalloporphyrins in self-assembled monolayers. Experimental Section General. Tetracarbonyl-di-µ-chlororhodium(I) ([Rh(CO)2Cl]2) (Sigma-Aldrich Co.), 1,2-dichlorobenzene, 4-hydroxybenzaldehyde, phenyloctane, pyridine, pyrrole (Wako Pure Chemical Industries), 1-bromooctadecane, and 1-chlorotriacontane (Tokyo Chemical Industry Co.) were purchased from commercial sources. All chemical reagents were used without further purification. 5,10,15,20-Tetra(4-octadecyloxyphenyl)porphyrins (C18OPP) and (10) (a) Claypool, C. L.; Faglioni, F.; Goddard, W. A., III; Gray, H. B.; Lewis, N. S.; Marcus, R. A. J. Phys. Chem. B 1997, 101, 5978. (b) Claypool, C. L.; Faglioni, F.; Goddard, W. A., III; Lewis, N. S. J. Phys. Chem. B 1999, 103, 7077. (c) Claypool, C. L.; Faglioni, F.; Matzger, A. J.; Goddard, W. A., III; Lewis, N. S. J. Phys. Chem. B 1999, 103, 9690. (11) (a) Yin, S.; Wang, C.; Xu, Q.; Lei, S.; Wan, L.; Bai, C. Chem. Phys. Lett. 2001, 348, 321. (b) Wang, Z.; Zeng, Q.; Luan, Y.; Wu, X.; Wan, L.; Wang, C.; Lee, G. U.; Yin, S.; Yang, J.; Bai, C. J. Phys. Chem. B 2003, 107, 13384. (c) Lu, J.; Zeng, Q.; Wang, C.; Wan, L.; Bai, C. Chem. Lett. 2003, 32, 856. (d) Xu, S.; Zeng, Q.; Lu, J.; Wang, C.; Wan, L.; Bai, C. Surf. Sci. 2003, 538, L451. (d) Gong, J.; Lei, S.; Wan, L.; Deng, G.; Fan, Q.; Bai, C. Chem. Mater. 2003, 15, 3098. (e) Lei, S.; Wang, C.; Fan, X.; Wan, L.; Bai, C. Langmuir, 2003, 19, 9759. (12) (a) Wawkuschewski, A.; Cantow, H.-J.; Magonov, S. N.; Mo¨ller, M.; Liang, W.; Whangbo, M.-H. Adv. Mater. 1993, 5, 821. (b) Watel, G.; Thibaudau, F.; Cousty, J. Surf. Sci. 1993, 281, L297. (c) Liu, T.-L.; Parakka, J. P.; Cava, M. P.; Kim, Y.-T. Synth. Met. 1995, 71, 1989. (d) Mu¨ller, H.; Petersen, J.; Strohmaier, R.; Gompf, B.; Eisenmenger, W.; Vollmer, M. S.; Effenberger, F. Adv. Mater. 1996, 8, 733. (e) Lee, H. S.; Iyengar, S.; Musselman, I. H. Langmuir 1998, 14, 7475. (f) Azumi, R.; Go¨tz, G.; Ba¨uerle, P. Synth. Met. 1999, 101, 569. (g) Le Poulennec, C.; Cousty, J.; Xie, Z. X.; Mioskowski, C. Surf. Sci. 2000, 448, 93. (h) Ho¨ger, S.; Bonrad, K.; Mourran, A.; Beginn, U.; Mo¨ller, M. J. Am. Chem. Soc. 2001, 123, 5651. (i) Kaneda, Y.; Stawasz, M. E.; Sampson, D. L.; Parkinson, B. A. Langmuir 2001, 17, 6185. (j) Hoepper, S.; Chi, L.; Fuchs, H. Nano Lett. 2002, 2, 459. (k) Jin, J.; Yang, W.; Li, Y.; Li, L.; Zhao, Y.; Jiang, L.; Li, T. New J. Chem. 2003, 27, 1463. (l) Su, C.; Lee, L.-X.; Chen, C.-F. Synth. Met. 2003, 137, 865. (m) Qui, D.; Ye, K.; Wang, Y.; Zou, B.; Zhang, X. Langmuir 2003, 19, 678. (13) (a) Qiu, X.; Wang, C.; Zeng, Q.; Xu, B.; Yin, S.; Wang, H.; Xu, S.; Bai, C. J. Am. Chem. Soc. 2000, 122, 5550. (b) Wang, H.; Wang, C.; Zeng, Q.; Xu, S.; Yin, S.; Xu, B.; Bai, C. Surf. Interface Anal. 2001, 32, 266. (14) Sanders, J. K. M.; Bampos, N.; Watson, Z. C.; Darling, S. L.; Hawley, J. C.; Kim, H.-J.; Mak, C. C.; Webb, S. J. The Porphyrin Handbook; Academic Press: San Diego, 2000; Vol. 3, p 1. (15) (a) Nagahara, L. A.; Manivannan, A.; Yanagi, H.; Toriida, M.; Ashida, M.; Maruyama, Y.; Hashimoto, K.; Fujishima, A. J. Vac. Sci. Technol., A 1993, 11, 781. (b) Manivannan, A.; Nagahara, L. A.; Yanagi, H.; Fujishima, A. J. Vac. Sci. Technol., B 1994, 12, 2000. (c) Tao, N. J.; Cardenas, G.; Cunha, F.; Shi, Z. Langmuir 1995, 11, 4445. (d) Jung, T. A.; Schlitter, R. R.; Gimzewski, J. K.; Tang, H.; Joachim, C. Science 1996, 271, 181. (e) Gimzewski, J. K.; Jung, T. A.; Cuberes, M. T.; Schlittler, R. R. Surf. Sci. 1997, 386, 101. (f) Jung, T. A.; Schlitter, R. R.; Gimzewski, J. K. Nature 1997, 386, 696. (g) Gimzewski, J. K.; Joahim, C. Science 1999, 283, 1683. (h) Fujita, D.; Ohgi, T.; Deng, W. L.; Neji, H.; Okamoto, T.; Yokoyama, S.; Kamikado, K.; Mashiko, S. Surf. Sci. 2000, 454, 1021. (i) Scudiero, L.; Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 11899. (j) Scudiero, L.; Barlow, D. E.; Mazur, U.; Hipps, K. W. J. Am. Chem. Soc. 2001, 123, 4073. (k) Yoshimoto, S.; Tada, A.; Suto, K.; Narita, R.; Itaya, K. Langmuir 2003, 19, 672. (l) Deng, W. L.; Hipps, K. W. J. Phys. Chem. B 2003, 107, 10736.

Langmuir, Vol. 20, No. 13, 2004 5455

Figure 1. Chemical structure of (a) CnOPP and (b) Rh(CnOPP)(Cl)(Py) (n ) 18 and 30). 5,10,15,20-tetra(4-triacontyloxyphenyl)porphyrins (C30OPP) [Figure 1a] were prepared according to ref 16. Column chromatography was carried out on a Wakogel C-400HG (Wako Pure Chemical Industries). 1H and 13C NMR spectra were recorded on a Bruker AVANCE 400 spectrometer (400 and 100 MHz for 1H and 13C NMR, respectively) with use of residual solvent as the internal standard. CnOPP. 1H NMR for CnOPP (400 MHz, CDCl3): δ -2.72 (s, NH), 0.91 (t, CH3), 1.25-1.50 (m, CH2), 1.63 (m, CH2), 2.00 (m, CH2), 4.27 (t, OCH2), 7.29 (d, phenyl), 8.12 (d, phenyl), 8.89 (s, pyrrole). 13C NMR (100 MHz, CDCl3): δ ) 14.6, 23.2, 26.8, 29.9, 30.0-30.3, 32.4, 68.9, 113.2, 120.3, 135.0, 136.1, 159.5. C18OPP: Calcd for C116H174N4O4‚H2O: C, 81.64; H, 10.39; N, 3.28. Found: C, 81.55; H, 10.14; N, 3.25. C30OPP: Calcd for C164H270N4O4: C, 83.40; H, 11.52; N, 2.37. Found: C, 83.18; H, 11.61; N, 2.24. Rh(CnOPP)(Cl)(Py). C18OPP (70 mg, 4.2 × 10-5 mol) and [Rh(CO)2Cl]2 (33 mg, 8.5 × 10-5 mol) were dissolved in dry toluene (100 mL). The mixture was stirred at 80 °C for 5 h. The solvent was evaporated, and the residue was purified by silica gel chromatography (CH2Cl2/hexane, 7/3). The recovered product was dissolved in a small amount of CH2Cl2, and then methanol was added. The precipitated product was recovered by filtration and dried under reduced pressure [Rh(C18OPP)(Cl) yield: 38 mg, 50%]. Rh(C18OPP)(Cl) (37 mg, 2.0 × 10-5 mol) was dissolved in a small amount of chloroform, and then 2 µL of pyridine was added. The product was purified by silica gel chromatography (CH2Cl2/hexane, 7/3). The recovered product was dissolved in a small amount of CH2Cl2, and then methanol was added. The precipitated product was recovered by filtration and dried under reduced pressure [Rh(C18OPP)(Cl)(Py) yield: 36 mg, 92%]. Rh(C30OPP)(Cl)(Py) was synthesized by the same procedure. 1H NMR for Rh(C OPP)(Cl)(Py) (400 MHz, CDCl ): δ 0.91 (t, n 3 CH3), 0.98 (d, pyridine), 1.25-1.55 (m, CH2), 1.64 (m, CH2), 2.00 (m, CH2), 4.27 (t, OCH2), 5.05 (t, pyridine), 6.02 (t, pyridine), 7.25 (d, phenyl), 7.29 (d, phenyl), 8.01 (d, phenyl), 8.23 (d, phenyl), 8.93 (s, pyrrole). 13C NMR for Rh(CnOPP)(Cl)(Py) (100 MHz, CDCl3): δ ) 23.2, 26.8, 29.9, 30.1-30.4, 32.4, 68.9, 112.9, 113.2, 121.3, 122.4, 132.7, 134.9, 135.7, 136.3, 143.1, 146.4, 159.4. Rh(C18OPP)(Cl)(Py): Calcd for C121H177ClN5O4Rh‚H2O: C, 75.61; H, 9.39; N, 3.64. Found: C, 75.85; H, 9.21; N, 3.62. Rh(C30OPP)(Cl)(Py): Calcd for C169H273ClN5O4Rh: C, 78.76; H, 10.68; N, 2.72. Found: C, 78.47; H, 10.68; N, 2.65. STM Observation. STM measurements were conducted by a Nanoscope IIIa multimode SPM (Digital Instruments) with commercially available Pt/Ir tips (80/20). STM observation was conducted at the solid-liquid interface, directly in the droplet of the solution. Phenyloctane and 1,2-dichlorobenzene were used as solvents for C18OPP and C30OPP systems, respectively, because of the poor solubility of C30OPP in phenyloctane. The experimental (16) (a) Kugimiya, S.; Takemura, M. Tetrahedron Lett. 1990, 31, 3160. (b) Bruce, D. W.; Dunmur, D. A.; Santa, L. S.; Wali, M. A. J. Mater. Chem. 1992, 2, 363.

5456

Langmuir, Vol. 20, No. 13, 2004

Ikeda et al.

procedure was as follows: preparation of the stock sample solution, STM observation of a freshly cleaved graphite surface, placement of a droplet of the sample solution on the graphite surface, incubation for 5 min, and then STM observation of selfassembled monolayers. The concentrations of the solutions were set at 2.0 × 10-4 M for C18OPP, 2.0 × 10-6 M for C30OPP, and 4.0 × 10-6 M for Rh(C30OPP)(Cl)(Py). Mixed self-assembled monolayers were prepared by using the mixture containing CnOPP and Rh(CnOPP)(Cl)(Py). The total concentrations of C18OPP and C30OPP mixed systems were fixed at 2.0 × 10-4 and 4.0 × 10-6 M, respectively. The mixing ratios were set to 75:25, 50:50, 25:75, and 10:90 [CnOPP/Rh(CnOPP)(Cl)(Py)]. The number ratios of CnOPP and Rh(CnOPP)(Cl)(Py) in mixed self-assembled monolayers were obtained by counting each molecule in the STM image. A total of 20 images (50 nm × 50 nm) were used for the mixed monolayer analysis at each condition. X-ray Crystallography. Pyridine-coordinated 5,10,15,20tetra(phenyl)porphyrin rhodium chloride [Rh(TPP)(Cl)(Py)] was synthesized and analyzed by X-ray crystallography as a model compound for Rh(CnOPP)(Cl)(Py). X-ray crystallography was conducted by a Rigaku AFC7R diffractometer with graphite monochromated Mo KR radiation. The structure was solved by direct methods (SIR92) and expanded by Fourier techniques. Crystal Data for Rh(TPP)(Cl)(Py). [C49H33ClN5Rh]‚[CHCl3]: Mr ) 949.57, orthorhombic, space group P212121 (no. 19), a ) 17.547(2), b ) 25.181(2), c ) 9.817(1) Å, V ) 4337.7(8) Å3, Z ) 4, Fcalcd ) 1.454 g cm-3, µ(Mo KR) ) 6.81 cm-1, F(000) ) 1928.00, T ) 193 K; red prismatic crystal, 0.20 × 0.10 × 0.15 mm, R1 ) 0.056 [I > 2.0σ(I)], ωR2 (F2) ) 0.155, goodness of fit ) 1.20.

Results and Discussion Design and Synthesis of Pyridine-Coordinated Metalloporphyrin. To obtain the STM images of pyridine-coordinated metalloporphyrins at room temperature, the selection of a central metal is important. Umezawa et al. reported the discrimination of Ni(II) and Zn(II) porphyrins on a graphite surface by using STM gold tips chemically modified with 4-mercaptopyridine.17 The coordination bonds of Ni(II) and Zn(II) to pyridine are so weak that STM observation was successful without attaching metalloporphyrins to pyridine-coated STM tips. Furthermore, Bai et al. reported site-selective adsorption of benzoic acid using self-assembled monolayers of trialkylamine as the molecular template.11e Although benzoic acid binds to the amino group through a weak hydrogen bond, the molecular image of benzoic acids was missing within several minutes after starting STM observation. These results suggested that strong binding between the ligand molecule and the molecular template is essential for molecularly resolved STM images. Since the coordination bonds of popular metals such as Cu(II) and Zn(II) are weak at room temperature, we selected porphyrin Rh(III) chloride (Figure 1). The strong coordination bond of Rh(III) with pyridine allows us to obtain pure pyridinecoordinated complex by column chromatography at room temperature.18 The preparations of CnOPP and Rh(CnOPP)(Cl)(Py) were confirmed by 1H, 13C NMR, and elemental analysis. STM Observation of Rh(CnOPP)(Cl)(Py) SelfAssembled Monolayers. Panels a and b of Figure 2 are the STM images of C18OPP and C30OPP self-assembled monolayers, respectively. We were able to confirm wellordered arrays with a characteristic lamellar structure of 2-fold symmetry. The image shows that the porphyrins are aligned side by side in the bright stripes and they are separated by the alkyl chains from the neighboring (17) Ohshiro, T.; Ito, T.; Bu¨hlmann, P.; Umezawa, Y. Anal. Chem. 2001, 73, 878. (18) (a) Asakawa, M.; Ikeda, T.; Yui, N.; Shimizu, T. Chem. Lett. 2002, 174. (b) Ikeda, T.; Asakawa, M.; Goto, M.; Nagawa, Y.; Shimizu, T. Eur. J. Org. Chem. 2003, 3744.

Figure 2. STM images of self-assembled monolayers at the solid-liquid interface (35 nm × 35 nm): (a) C18OPP solution; (b) C30OPP solution; (c) Rh(C18OPP)(Cl)(Py) solution; (d) Rh(C30OPP)(Cl)(Py) solution; (e) a mixture containing C18OPP and Rh(C18OPP)(Cl)(Py) (molar ratio 1:9); (f) a mixture containing C30OPP and Rh(C30OPP)(Cl)(Py) (molar ratio 1:9). Tunneling conditions: I ) 30 pA, V ) -1000 mV. Temperature: 293 K.

porphyrin array. In the dark areas, the alkyl chains are interdigitated.11 The lattice parameters of the unit cell a × b and β were 4.03 ( 0.2 × 1.97 ( 0.1 nm and 103 ( 6° for C18OPP and 5.40 ( 0.1 × 2.02 ( 0.1 nm and 95 ( 3° for C30OPP. Details of the molecular arrangement are discussed in refs 13 and 19. STM observation was conducted for the pyridinecoordinated products [Rh(CnOPP)(Cl)(Py)] on the graphite surface. When we attempted to obtain clear STM images in the pure Rh(C18OPP)(Cl)(Py) solution, an occasional glimpse of the self-assembled structure was possible. In most cases, however, we obtained only noisy images [Figure 2c]. On the other hand, molecularly resolved STM images were obtainable in the pure Rh(C30OPP)(Cl)(Py) solution [Figure 2d]. It is considered that the larger adsorption energy due to the longer alkyl chains should contribute to the formation of Rh(C30OPP)(Cl)(Py) selfassembled monolayers.19 The lattice parameters of the unit cell a × b and β were 5.45 ( 0.2 × 2.03 ( 0.1 nm and 95 ( 4° for Rh(C30OPP)(Cl)(Py), indicating that the molecular packing feature of Rh(C30OPP)(Cl)(Py) is almost the same as that of C30OPP. Panels a and b of Figure 3 show apparent height distributions of the bright spots in C30OPP and Rh(C30OPP)(Cl)(Py) self-assembled monolayers, respectively. The average height of Rh(C30OPP)(Cl)(Py) was clearly more than that of C30OPP. The average heights of the bright spots in C30OPP and Rh(C30OPP)(Cl)(Py) self-assembled monolayers were 0.20 ( 0.03 and 0.43 ( 0.04 nm, respectively. This finding strongly suggests that the spots aligned in Figure 2d represent pyridine-coordinated (19) Miyake, K.; Ikeda, T.; Asakawa, M.; Shimizu, T.; Ishida, T.; Sasaki, S. AIP Conf. Proc. 2003, 696, 537.

STM Observation of Pyridine-Coordinated Porphyrins

Langmuir, Vol. 20, No. 13, 2004 5457

Figure 4. A section profile of the STM image for mixed selfassembled monolayers consisting of C18OPP and Rh(C18OPP)(Cl)(Py). The mixing molar ratio in the solution is 1:9 [C18OPP/ Rh(C18OPP)(Cl)(Py)].

Figure 5. Molecular structure for the model compound Rh(TPP)(Cl)(Py) based on X-ray crystallographic analysis: (R) 1.11 nm, (β) 0.59 nm, (γ) 0.39 nm, (δ) 0.69 nm, and () 0.41 nm. The dihedral angles of the pyridine plane and four phenyl planes with respect to the least-squares plane of the porphyrin core are 84.7° and 71.5° (average), respectively. Figure 3. Apparent height distributions of adsorbed molecules observed by STM: (a) C30OPP self-assembled monolayers, (b) Rh(C30OPP)(Cl)(Py) self-assembled monolayers, and (c) mixed self-assembled monolayers consisting of C18OPP and Rh(C18OPP)(Cl)(Py). All of the data were extracted from height profiles of STM images at the tunneling conditions of I ) 30 pA and V ) -1000 mV. Apparent height was defined as the height difference between the alkyl chain and porphyrin parts. The average heights were (a) 0.20 ( 0.03 nm (N ) 300), (b) 0.43 ( 0.04 nm (N ) 400), and (c) 0.20 ( 0.03 and 0.45 ( 0.03 nm (N ) 330).

porphyrins. As can be seen at the upper-right corner of Figure 2b, the broad bright spots were often detectable in C30OPP STM images. The size of these spots was larger than that of porphyrins. These broad bright spots were considered to correspond to the stacked dimer of the C30OPP molecules. The porphyrin molecules stack easily through π-π interactions.20 The weak interaction between the C30OPP molecules in dichlorobenzene is responsible for the broadening of these spots. The apparent height of the broad bright spots was ca. 0.61 nm [Figure 3a]. In Figure 2d, all of the adsorbed molecules were Rh(C30OPP)(Cl)(Py). However, the molecules with the height of ca. 0.21 nm were detectable in STM images of Rh(C30OPP)(Cl)(Py) [Figure 3b]. These molecules are attributable to C30OPP or pyridine dissociated product [Rh(C30OPP)(Cl)]. Although we confirmed the purity of the sample by NMR and elemental analysis, it is difficult to exclude the possibility that these molecules are impurities in the sample. Furthermore, we have no idea how much STM measurement affects the equilibrium of the coordination bond between rhodium and pyridine. The size of the bright spots in Figure 2d was similar to that of the porphyrin part. No broad bright spot was detectable in the STM images of Rh(C30OPP)(Cl)(Py), because the axially coordinated ligands (chlorine and pyridine) hinder the π-π stacking between porphyrins. In the solution state, the coordinated pyridine molecule has fast rotational motion on the rotational axis of the (20) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525.

Npyridine-Rh bond.21 The round shape of the bright spot for Rh(C30OPP)(Cl)(Py) may arise from a time-averaged image of the rotational pyridine molecule.22 STM Observation for Mixed Self-Assembled Monolayers. We analyzed mixed self-assembled monolayers consisting of CnOPP and Rh(CnOPP)(Cl)(Py) in order to obtain additional evidence for our consideration that the bright spots in Figure 2d correspond to pyridinecoordinated product. Interestingly, molecularly resolved STM images could be obtained in the mixture containing C18OPP and Rh(C18OPP)(Cl)(Py) [Figure 2e], in contrast with the noisy images in the pure Rh(C18OPP)(Cl)(Py) solution [Figure 2c]. Two species coexisting in selfassembled monolayers (relatively dark and bright spots) are easily differentiated. The number ratio of the relatively bright spots on the graphite surface increased with increasing the fractional concentration of Rh(C18OPP)(Cl)(Py) in the solution, suggesting that the dark and bright spots correspond to C18OPP and Rh(C18OPP)(Cl)(Py), respectively. Mixed self-assembled monolayers were also obtainable in the mixture containing C30OPP and Rh(C30OPP)(Cl)(Py) [Figure 2f]. Figure 4 shows the section profile of mixed selfassembled monolayers. It is evidently clear that the relatively dark and bright spots in mixed self-assembled monolayers have different constant heights. Figure 3c shows apparent height distributions of the spots in mixed self-assembled monolayers consisting of C18OPP and Rh(C18OPP)(Cl)(Py). The average heights for the relatively dark and bright spots were 0.20 ( 0.03 and 0.45 ( 0.03 nm, respectively. These values are well consistent with the results obtained for the solutions of C30OPP and Rh(C30OPP)(Cl)(Py) (0.20 ( 0.03 and 0.43 ( 0.04 nm). Figure 5 shows the molecular structure of the model compound in the solid state [Rh(TPP)(Cl)(Py)]. The difference in height between the top of the coordinated pyridine and the edges of four phenyl groups is 0.39 nm, which is larger than the STM height difference between (21) (a) Nakamura, M.; Ikezaki, A. Chem. Lett. 1995, 733. (b) Momot, K. I.; Walker, F. A. J. Phys. Chem. A 1997, 101, 2787. (22) Gimzewski, J. K.; Joachim, C.; Schlittler, R. R.; Langlais, V.; Tang, H.; Johannsen, I. Science 1998, 281, 531.

5458

Langmuir, Vol. 20, No. 13, 2004

Ikeda et al.

Figure 6. Relationship between the fractional surface coverage of CnOPP (θfree) and the fractional concentration of CnOPP in the solution [ffree ) Cfree/(Cfree + CPy)] (average ( standard deviation): (O) mixed monolayers consisting of C18OPP and Rh(C18OPP)(Cl)(Py); (b) mixed monolayers consisting of C30OPP and Rh(C30OPP)(Cl)(Py). A total of 20 images (50 nm × 50 nm) were used for this analysis at each concentration. The dotted and dashed curves were calculated assuming ∆Gapp ) 5.1 and 1.5 kJ mol-1, respectively.

the dark and the bright spots (0.23-0.25 nm). STM contrast does not reflect the real height of adsorbed molecules.23 The apparent height distribution in Figure 3c indicated that the stacked dimer of C18OPP also existed in mixed self-assembled monolayers (ca. 0.62 nm). Apparent Difference in the Adsorption Free Energy between CnOPP and Rh(CnOPP)(Cl)(Py). We assumed that the fractional surface coverage of CnOPP and Rh(CnOPP)(Cl)(Py) depends on the Boltzmann distribution. The apparent difference in the adsorption free energy between CnOPP and Rh(CnOPP)(Cl)(Py) was applied (∆Gapp), because tunneling current and bias may change the real equilibrium state of coadsorption. The relationship between the fractional surface coverage (θ, θfree + θPy ) 1) and the concentration of the solution (C) can be expressed as eq 1.

(

)

θfree Cfree ∆Gapp ) exp θPy CPy RT

(1)

where R and T are the gas constant and temperature (K), respectively. The subscripts free and Py represent CnOPP and Rh(CnOPP)(Cl)(Py), respectively. Through least-squares fitting, we confirmed that the results of mixed self-assembled monolayer analysis could be expressed by eq 1 (Figure 6). The calculated ∆Gapp values for C18OPP and C30OPP mixed systems were 5.1 and 1.5 kJ mol-1, respectively. In both cases, the adsorption of Rh(CnOPP)(Cl)(Py) was comparatively less stable than that of CnOPP. Interestingly, the calculated ∆Gapp values for C18OPP and C30OPP mixed systems were quite different. Some factors can rationalize this result. On the basis of a molecular symmetry argument,24 the adsorption of CnOPP on the substrate is more favorable than that of Rh(CnOPP)(Cl)(Py) because CnOPP has two equivalent faces to adsorb to the surface while Rh(CnOPP)(Cl)(Py) has only one. We believe that the “chlorine side”, not the “pyridine side”, of Rh(CnOPP)(Cl)(Py) will attach to the surface because the adsorption and the crystallization of the alkyl groups on the graphite surface are seriously affected in the latter case. In the case of the C30OPP mixed system, the calculated ∆Gapp value (1.5 kJ mol-1) was (23) Wiesendanger, R. Scanning Probe Microscopy and Spectroscopy: Method and Application; Cambridge University Press: New York, 1994. (24) Bailey W. F.; Monahan, A. S. J. Chem. Educ. 1978, 55, 489.

almost the same as RT ln 2 [1.7 kJ mol-1 (T ) 293 K)]. Therefore, the ∆Gapp value for the C30OPP mixed system is attributable to the entropy factor. On the other hand, the entropy factor alone does not explain the calculated ∆Gapp value for the C18OPP mixed system (5.1 kJ mol-1). The additional unfavorable factor for the molecular adsorption was divided into two factors: the enthalpy factor arising from the molecular structure and the STM-induced factor. The molecular structure of Rh(CnOPP)(Cl)(Py) may affect the enthalpy gain in the adsorption process. The crystal structure of the model compound, Rh(TPP)(Cl)(Py), is helpful to understand the molecular structure of Rh(CnOPP)(Cl)(Py) on the graphite surface. The chlorine atom protrudes from the lower edges of four phenyl groups by ca. 0.11 nm (Figure 5). This structural environment does not allow four phenyl groups to attach stably to the surface. Furthermore, the electric field induced by STM affects stable attachment of Rh(CnOPP)(Cl)(Py) through the dipole moment (Rhδ+‚‚‚Clδ-).25 The calculated ∆Gapp value for the C30OPP mixed system indicated that the enthalpy factor arising from the molecular structure and the STM-induced factor were apparently negligible in the C30OPP mixed system. We considered that the large adsorption energy of Rh(C30OPP)(Cl)(Py) should hinder the molecular exchange between self-assembled monolayers and supernatant. The fractional surface coverage of C30OPP and Rh(C30OPP)(Cl)(Py) might be decided by the first adsorption of these species on the substrate, and the system could not reach the real equilibrium state within the short time even if STM measurement changed the equilibrium state in the system. In the cases of C30OPP systems, STM observation is limited within the short time (about 30 min) because of the fast evaporation of the solvent (dichlorobenzene). The disadvantage of the adsorption free energy of Rh(C18OPP)(Cl)(Py) makes it difficult to obtain molecularly resolved images of Rh(C18OPP)(Cl)(Py). In contrast, how was it possible to obtain molecularly resolved images in mixed self-assembled monolayers? In our experiment, Rh(C18OPP)(Cl)(Py) is likely to keep moving on the graphite surface without growing into a large monolayer. Figure 2c suggested the adsorbed molecules to be in the two-dimensional gas or liquid state.26 We confirmed that no molecularly resolved image of Rh(C18OPP)(Cl)(Py) was obtainable using a solution with higher concentration (up to 1 mM). Presumably, the desorption rate of Rh(C18OPP)(Cl)(Py) is competitive with the rate of monolayer growth in the pure Rh(C18OPP)(Cl)(Py) solution. In the mixture containing C18OPP and Rh(C18OPP)(Cl)(Py), selfassembled monolayers of C18OPP could grow up; then the molecular exchange between C18OPP in self-assembled monolayers and Rh(C18OPP)(Cl)(Py) in the supernatant might be possible. The residence time of Rh(C18OPP)(Cl)(Py) in mixed self-assembled monolayers might be longer than that of Rh(C18OPP)(Cl)(Py) in the twodimensional gas or liquid state, because the adsorbate in the monolayer becomes stable due to the two-dimensional crystallization energy between the close-packed alkyl chains.27 The longer residence time allows us to obtain the molecularly resolved image of mixed self-assembled monolayers. (25) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W., Jr.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303. (26) Berner, S.; Brunner, M.; Ramoino, L.; Suzuki, H.; Gu¨ntherodt, H.-J.; Jung, T. A. Chem. Phys. Lett. 2001, 348, 175. (27) Yin, S.; Wang, C.; Qiu, X.; Xu, B.; Bai, C. Surf. Interface Anal. 2001, 32, 248.

STM Observation of Pyridine-Coordinated Porphyrins

Conclusion We obtained molecularly resolved images of alkyl-chainassisted self-assembled monolayers of Rh(C30OPP)(Cl)(Py). In the case of Rh(C18OPP)(Cl)(Py), STM images were not obtainable in the pure Rh(C18OPP)(Cl)(Py) solution but obtainable in the mixture containing C18OPP and Rh(C18OPP)(Cl)(Py). We confirmed that the apparent STM height of Rh(CnOPP)(Cl)(Py) was more than that of CnOPP. The apparent difference in the adsorption free energy was calculated on the basis of mixed selfassembled monolayer analysis. In the C18OPP mixed system, the fractional surface coverage of C18OPP and Rh(C18OPP)(Cl)(Py) was determined by the enthalpy factor arising from the molecular structure and the STM-induced factor in addition to the entropy factor arising from the molecular symmetry (∆Gapp ) 5.1 kJ mol-1). On the other hand, the fractional surface coverage of C30OPP and Rh(C30OPP)(Cl)(Py) was apparently determined by only the entropy factor (∆Gapp ) 1.5 kJ mol-1 = RT ln 2). This result was considered to be due to the large adsorption energy of Rh(C30OPP)(Cl)(Py) on the substrate. This study is a significant step toward a new molecular alignment

Langmuir, Vol. 20, No. 13, 2004 5459

method of functional molecules on the substrate. The dihedral angle between the pyridine plane and the leastsquares plane of the porphyrin core is 84.7° (Figure 5), indicating that the pyridine molecule stands perpendicularly relative to the underlying substrate. Since the coordinated molecules are separated from one another by at least 2 nm, our method is useful to analyze the properties of individual molecules as well as the effect of a nanoscale periodic array on molecular interactions and functions. The STM observation of porphyrin rhodium chlorides with the other ligand molecules is now in progress. 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: X-ray crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. LA049577A