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Synthesis and self-assembly of novel porphyrin molecular wires
Thin Solid Films 499 (2006) 23 – 28 www.elsevier.com/locate/tsf
Synthesis and self-assembly of novel porphyrin molecular wires Masahiro Kawao a,b, Hiroaki Ozawa b,c, Hirofumi Tanaka b,c,d, Takuji Ogawa b,c,d,* a b
Department of Chemistry, Faculty of Science, Ehime University, Matsuyama 790-8577, Japan Research Center for Molecular-Scale Nanoscience, Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan c The Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8787, Japan d CREST, Japan Science and Technology Corporation, Japan Available online 2 September 2005
Abstract Sub-micrometer long butadiyne-linked porphyrin wires were synthesized by oxidative coupling of diethynylporphyrin. The porphyrin wires were analyzed by analytical gel permeation chromatography, absorption spectroscopy and matrix-assisted laser desorption/ionization time of flight mass spectroscopy. Observations of the wire were performed by atomic force microscopy. Self-assembled structures of the wires were observed on highly oriented pyrolytic graphite. Self-assembling features of the porphyrin wires depended on the length of the porphyrin wires and the concentration of the depositing solution. D 2005 Elsevier B.V. All rights reserved. Keywords: Porphyrin wire; k-Conjugated macromolecule; Self-assembled nanostructure
1. Introduction Long k-conjugated molecular wires have been receiving much attention because they can be used as organic conducting materials and nonlinear optical materials [1 – 3]. Measurement of the electric conductivity of individual molecules has been focused on establishing a foundation for molecular electronics and has been performed in various ways, e.g., using a nanofabricated electrode [4,5] and by scanning tunneling microscopy (STM) [6]. However, the measurements are still difficult and only a limited number of successful measurements have been reported so far. Well-ordered structures of organic molecules on devices are required to realize reproducible and reliable functions of the devices. Thus, the fabrication of the molecular nanostructures required for practical molecular electronics entails * Corresponding author. Research Center for Molecular-Scale Nanoscience, Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan. Tel.: +81 564 59 5537; fax: +81 564 59 5535. E-mail addresses: [email protected] (M. Kawao), [email protected] (T. Ogawa). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.07.143
the challenge of assembling molecules into the required well-ordered structure. The self-assembly technique takes advantage of intermolecular interaction or of interaction between molecules and the substrate surface to achieve a spontaneous organization. Thus, assembling k-conjugated macromolecules on substrate surfaces is one of the key technologies for the fabrication of molecular electronic nanodevices [7]. A number of works concerning the synthesis and selfassembly of k-conjugated molecular wires on substrates have been published [8,9]. Among such wires, porphyrin wires have attracted much attention because of the small HOMO – LUMO gap, high stability, and diversity of the substituents and of the center metal. We report the synthesis of novel long k-conjugated porphyrin wires and the first demonstration of the selfassembly properties of the sub-micrometer long porphyrin wires on surfaces of highly oriented pyrolytic graphite (HOPG) and other substrates. The oligo-diethynylporphyrin wires were prepared by copper-catalyzed oxidative coupling reaction of diethynylporphyrin, as shown in Scheme 1 [10 – 13].
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M. Kawao et al. / Thin Solid Films 499 (2006) 23 – 28
2. Experiment 2.1. Materials The porphyrin wires were prepared by a modified version of the reported methods, as shown in Scheme 1. 3,5-Bis(3-methylbutoxy)benzaldehyde (1) was prepared by MnO2 oxidation of the corresponding benzylalcohol [14]. 5,15-Bis(3,5-bis(3-methylbutoxy))phenylporphyrin (2) was prepared by acid-catalyzed condensation of benzaldehyde 1 with dipyrromethane [15]. Diethynylporphyrin (3) was prepared from meso-position-free porphyrin 2 in four steps: bromination [16], metalation with Zn [16], introduction of trimethylsilylacetylene by Sonogashira coupling [17], and deprotection of acetylene units[18]. We prepared oligodiethynylporphyrin (4) by copper-catalyzed oxidative coupling reaction of diethynylporphyrins 3 as follows: Triethylamine (0.5 ml, 5.0 mmol) was added to the THF solution of diethynylporphyrin (3) (0.090 g, 98 mmol) and CuI (0.050 g, 0.26 mmol), and the reaction mixture was stirred at 333 K for 24 h in oxygen atmosphere. The resulting solution was purified by gel permeation chromatography (GPC). The solvent was removed under reduced pressure. A dark green powder of the products was obtained (0.078 g, 87%). The products (reaction mixture hereafter) were purified by preparative scale GPC and analyzed by analytical GPC, absorption spectroscopy, matrix-assisted laser desorption/ ionization time of flight mass (MALDI – TOF– MS) and atomic force microscopy (AFM). 2.2. Measurements
Scheme 1. Synthesis of porphyrin wires. i) TsOH, MeOH, reflux. ii) C5H11Br, K2CO3, acetone, reflux. iii) LiAlH4, THF, reflux. iv) MnO2, toluene, reflux. v) AcOH, MeOH, room temp. vi) TFA, CHCl3. vii) pChloranil, 333 K. viii) NBS, CHCl3, room temp. ix) Zn(AcO)2(2H2O), CHCl3, room temp. x) TMSA, CuI, Pd(PPh3)4, Et3N, THF, reflux. xi) TBAF, THF, room temp. xii) CuI, Et3N, THF, 333 K.
MALDI – TOF –MS spectra were obtained using Applied Biosystems Voyager DE-STR with 7,7,8,8-tetracyanoquinodimethane (TCI) as a matrix. UV – vis absorption spectra were obtained using the Shimadzu UV-3150 spectrometer with pyridine. Analytical GPC experiments were carried out using Shimadzu LC-6A equipped with the MD-2015 detector (JASCO) using two GPC KF-804L columns (critical molecular weight 400,000 Da, Shodex). The tetrahydrofuran (THF) flow rate in all experiments was 1.0 mL/min. The samples for AFM measurements were prepared as follows: the porphyrin wires were dissolved in a solvent mixture (THF/H2O = 10/1, v/v), and the solution was castdeposited onto the substrate surface. We prepared two different concentrations of porphyrin wire solution. One is a solution of porphyrin wires whose absorbance of the soret peak is approximately 0.1 (thin solution). The other is a solution of porphyrin wires whose absorbance of the soret peak is approximately 0.2 (thick solution). These solutions were cast on HOPG, and the solvent evaporated spontaneously. The dried stain on the exterior of the porphyrin wires was observed by AFM. Several substrates such as HOPG, mica, and siliconized SiO2/Si wafers were used for AFM observation. The siliconized
M. Kawao et al. / Thin Solid Films 499 (2006) 23 – 28
3. Results and discussion
1.0
Absorbance / arb. units
SiO2/Si wafer was prepared by treating a thermally oxidized Si wafer with a siliconized reagent (L-25, Fuji Systems Co.). All AFM observations were performed using JSPM-4210 (JEOL) at room temperature in ambient pressure in the tapping mode with a Si cantilever (NSC35/ AIB3, MikroMasch).
25
(i) (h) (g) (f) (e) (d) (c) (b) (a)
0.8
0.6
0.4
0.2
3.1. Analysis of porphyrin wires 0.0
From a solution of reaction mixture, no molecular peaks corresponding to greater than 30 repeat units were detected by MALDI –TOF– MS. On the other hand, analytical GPC clearly indicated the presence of sub-micrometer long molecules. Fig. 1 shows the analytical GPC chart of the porphyrin wires in THF calibrated with standard polystyrene. The charts indicate that the porphyrin wires included molecules heavier than the critical molecular weight (400,000 Da) of the GPC column. Since the molecular weight of one unit of compound 3 is 915, the critical molecular weight corresponds to 437 units of compound 3, which means that the reaction mixture contains molecular wires with length of exceeding 600 nm, because the length of one middle unit of trimer was calculated to be 1.36 nm by MMII calculation. By using preparative-scale GPC, monomers to octamers were obtained in pure forms, and the mixtures, 9¨20-mer, 20¨50-mer, 50¨200-mer, and over200-mer, could be separated. Their molecular weights were determined by analytical GPC. Fig. 2 shows UV – vis absorption spectra of the porphyrin wire molecules separated by preparative-scale GPC. Compared with the monomer (a), over-200-mer (l) showed redshifts of approximately 200 nm in the Q band and 40 nm in the soret band, which indicate a high degree of conjugation.
400
500
600
700
800
900
1000
Wavelength / nm Fig. 2. UV – vis absorption spectra of the separated products in pyridine: (a) monomer, (b) dimer, (c) trimer, (d) tetramer, (e) pentamer, (f) hexamer, (g) heptamer, (h) octamer, (l) over-200-mer.
3.2. Data of porphyrin wires The reaction mixture were purified by preparative scale GPC and analyzed by absorption spectroscopy and
Absorbance /arb. units
100
(a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) (m)
80
60
40
20
0 2 10
3
10
4
10
5
10
6
10
7
10
8
10
Molecular Weight /Da Fig. 1. Analytical GPC charts of the reaction and purified porphyrin wires in THF calibrated by standard polystyrene: (a) monomer, (b) dimer, (c) trimer, (d) tetramer, (e) pentamer, (f) hexamer, (g) heptamer, (h) octamer, (i) 9¨20-mer, (j) 20¨50-mer, (k) 50¨200-mer, (l) over-200-mer, (m) reaction mixture.
Fig. 3. AFM image of the porphyrin oligomers on HOPG: (a) trimer, the white line is an edge of HOPG step; (b) over-200-mer (thin solution).
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M. Kawao et al. / Thin Solid Films 499 (2006) 23 – 28
3.3. Topography of porphyrin wires on various surfaces In order to determine the length of the porphyrin wires, AFM observations of porphyrin wires on HOPG substrates were carried out. Fig. 3 shows AFM images of the porphyrin trimer and over-200-mer (thin solution) on HOPG. In the case of the porphyrin trimer, only dots with height of 0.5¨1 nm and diameter of 20¨40 nm were observed (Fig. 3a). In the case of the long porphyrin wires (over-200-mer), long wire-like images with lengths of 100¨700 nm and heights of 0.5¨2 nm were observed (Fig. 3b). The long wires had linear structures and were aligned along the HOPG lattices. This confirms that the higher oligomer solution contained the sub-micrometer long porphyrin wires. The long porphyrin wires (over-200-mers) were dissolved in THF/H2O (= 10 : 1) solvent to prepare a thick solution. The solution (20 AL) was cast on the HOPG surface and the solvent was allowed to evaporate spontaneously. Two types of self-assembled structures were observed by AFM, as shown in Fig. 4. The distance between the neighboring wires in one structure (Type A)
Frequency
(a)
20 15 10 5 0 0
MALDI – TOF – MS. MALDI – TOF– MS spectral profiles of dimer, trimer and tetramer were obtained by reflector mode. MALDI –TOF– MS spectral profiles of pentamer, hexamer, heptamer and octamer were obtained by linear mode.
5
15
20
25
30
20
25
30
2.0
2.5
3.0
Distance / nm 8 6 4 2 0 0
5
10
15 Distance / nm
(c) 3.0 2.5 Frequency
Dimer: MALDI – TOF – MS for C112H118N8O8Zn2 (calculated: 1830.77), m/ z = 1835.7 [M+]. UV – vis (C5H5N; k max): 436 nm, 718 nm. Trimer: MALDI – TOF – MS for C168H176N12O12Zn3 (calculated: 2745.14), m/z = 2752.0 [M+]. UV – vis (C5H5N; k max): 458 nm, 753 nm. Tetramer: MALDI – TOF – MS for C 224 H 234 N 16 O 16 Zn 4 (calculated: 3659.52), m/z = 3667.7 [M+]. UV – vis (C5H5N; k max): 468 nm, 792 nm. Pentamer: MALDI – TOF – MS for C 278 H 284 N 20 O 20 Zn 5 (calculated: 4541.83), m/z = 4589.5 [M+]. UV – vis (C5H5N; k max): 468 nm, 801 nm. Hexamer: MALDI – TOF – MS for C 334 H 342 N 24 O 24 Zn 6 (calculated: 5456.20), m/z = 5496.3 [M+]. UV – vis (C5H5N; k max): 469 nm, 807 nm. Heptamer: MALDI – TOF – MS for C390 H400 N28 O28 Zn 7 (calculated: 6370.58), m/z = 6411.0 [M+]. UV – vis (C5H5N; k max): 469 nm, 810 nm. Octamer: MALDI – TOF – MS for C 446 H 458 N 32 O 32 Zn 8 (calculated: 7284.95), m/z = 7317.3 [M+]. UV – vis (C5H5N; k max): 471 nm, 817 nm. Over-200-mer: UV – vis (C5H5N; k max): 471.5 nm, 822.5 nm.
10
(b)
Frequency
Fig. 4. AFM images of the porphyrin wires on HOPG. (a) Image of solution of the porphyrin wires (thick solution of over-200-mer). (b) Magnified image of area inside the square in panel (a). Two types of assembling patterns could be observed. Type A is the denser structure.
2.0 1.5 1.0 0.5 0.0 0.0
0.5
1.0
1.5 Height / nm
Fig. 5. Histogram of the height of and distance between the self-assembled wires (over-200-mer) on HOPG: (a) distance in Type A structure, (b) distance in Type B structure, (c) height in the Type B structure.
M. Kawao et al. / Thin Solid Films 499 (2006) 23 – 28
27
of the porphyrin wires can interact more strongly than among themselves. Thus, when the wires touch the HOPG surface directly, the attractive force of the substrate overcomes the sterical repulsive interaction between the neighboring wires and a dense structure (Type A) forms. When the wires are at the top of the array of molecular wires, the attractive interaction with the lower layer molecular wires is too weak to surmount the steric hindrance between the neighboring wires and a sparse structure, as exemplified in Type B, is formed. To better understand the surface effects, substrates comprising mica or siliconized SiO2 on Si wafers were used. Fig. 6 shows AFM images of the porphyrin wires on the mica substrate (thick solution of over-200-mers). In the case of mica substrates, both lumps and wire-like structures were observed. The observed lumps were 100¨600 nm wide and approximately 3 nm high. The length of the observed porphyrin wires was 100¨700 nm. The porphyrin wires aggregated themselves on the mica substrate surface because the mica substrate has a hydrophilic surface and the
Fig. 6. (a) AFM image of the porphyrin wires (thick solution of over-200mer) on mica substrate. (b) Magnified image of panel (a).
was 4.7 T 0.5 nm, and that of the other structure (Type B) was 9.4 T 1.6 nm, as shown in Fig. 5. The molecular height of Type B was 0.4 T 0.1 nm, which is a reasonable diameter for one porphyrin wire. The molecular height of Type A could not be determined because the gaps between each molecular wire were so narrow that the AFM cantilever could not reach the substrate surface. By scrutinizing the AFM images, we found that the Type B structure could be observed only on the upper layer of the molecular stacks, and the Type A structure always existed at the bottom of the layers. Thus, we reasonably hypothesize that the Type A structure was formed directly on the HOPG surface, and the Type B structure was built on the Type A structure. The observed difference in the distances between molecular wires can be explained by this hypothesis. The distances between wires are determined by two interactions; one is the interaction of wires with the lower substrate or molecules, and the other is that among neighboring wires of the same layer. The interaction between the wires and the HOPG surface is a relatively strong attractive force, because HOPG has wide planar k-systems with which the k-system
Fig. 7. (a) AFM image of the porphyrin wires (thick solution of over200-mer) on siliconized SiO2/Si wafer substrate. (b) Magnified image of panel (a).
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M. Kawao et al. / Thin Solid Films 499 (2006) 23 – 28
hydrophobic porphyrin wires are subjected to the repulsive force of the surface. Fig. 7 shows AFM images of the porphyrin wires (thick solution of over-200-mers) on the siliconized SiO2/Si wafer substrate. In this case, both lumps and wire-like structures are again observed. The size of the observed lumps was 100¨500 nm wide and approximately 9 nm high. The length of the observed porphyrin wires was 100¨700 nm. Although the siliconized SiO2/Si wafer has hydrophobic surface properties, the attractive interaction was too weak for the isolated porphyrin wire structure to form. Surfaces that have greater affinity to the porphyrin wires are required to obtain the ordered structure on surfaces. The length of polymer wire can be controlled by the solid-phase reaction [19,20]. For example, it can be achieved by oxidized-coupling of a porphyrin, in which one side of its acetylene is fixed by a solid phase, to diethynylporphyrin, in which one side is free from TMS protection. Therefore, TMS protection was removed by TBAF. Repeating this process will result in a uniform length of polymer wires. Solid-phase synthesis enables accurate control of the length of the porphyrin wires. One ethynyl group of [5,15-bis-(3,5-isopentoxyphenyl)-10,20-diethynylporphinato]zinc(II) capped by a solid phase is connected to [5,15-bis-(3,5-isopentoxyphenyl)-10-ethynyl-20-trimethylsilylethynylporphinato]zinc(II) by copper-catalyzed oxidative coupling. The acetylene unit of the product is deprotected by TBAF. We can control the length of the porphyrin wires by repeating oxidative coupling and deprotection.
4. Summary Butadiyne-linked porphyrin wires with length exceeding 600 nm were synthesized by a simple oxidative coupling reaction. Ordered structures of the porphyrin wires were observed on HOPG substrates. Such ordered structure of the porphyrin wires were not observed on mica or siliconized SiO2/Si wafer substrates. The self-assembled structures composed of the porphyrin wires were formed because of the strong interaction between the porphyrin wires and the HOPG substrate. The detailed investigations of self-
assembled structures and studies on their physical properties are currently in progress.
Acknowledgements One of the authors (HT) thanks Visionarts, Inc., for the financial support.
References [1] A. Tsuda, A. Osuka, Science 293 (2001) 79. [2] T.E.O. Screen, K.B. Lawton, G.S. Wilson, N. Dolney, R. Ispasoin, T. Goodson III, S.J. Martin, D.D.C. Bradley, H.L. Anderson, J. Mater. Chem. 11 (2001) 312. [3] P. Ba¨uerle, Adv. Mater. 5 (1993) 879. [4] J. Park, A.N. Pasupathy, J.I. Goldsmith, C. Chang, Y. Yaish, J.R. Petta, M. Rinkoski, J.P. Sethna, H.D. Abrun˜a, P.L. McEuen, D.C. Ralph, Nature 417 (2002) 722. [5] M.A. Reed, C. Zhou, C.J. Muller, T.P. Burgin, J.M. Tour, Science 278 (1997) 252. [6] M.T. Cygan, T.D. Dunbar, J.J. Arnold, L.A. Bumm, N.F. Shedlock, T.P. Burgin, L. Jones II, D.L. Allara, J.M. Tour, P.S. Weiss, J. Am. Chem. Soc. 120 (1998) 2721. [7] P. Samori, H. Engelkamp, P. de Witte, A.E. Rowan, R.J.M. Nolte, J.P. Rabe, Angew. Chem., Int. Ed. 40 (2001) 2348. [8] N. Katsonis, A. Marchenko, D. Fichou, J. Am. Chem. Soc. 125 (2003) 13682. [9] A.D. Schlu¨ter, J.P. Rabe, Angew. Chem., Int. Ed. 39 (2000) 864. [10] V.S.-Y. Lin, M.J. Therien, Chem. Eur. J. 1 (1995) 645. [11] P.N. Taylor, J. Huuskonen, G. Rumbles, R.T. Aplin, E. Williams, H.L. Anderson, Chem. Commun. (1998) 909. [12] H.L. Anderson, S.J. Martin, D.D.C. Bradley, Angew. Chem., Int. Ed. 33 (1994) 655. [13] M.J. Plater, S. Aiken, G. Bourhill, Tetrahedron 58 (2002) 2415. [14] K. Sugiura, H. Tanaka, T. Matsumoto, T. Kawai, Y. Sakata, Chem. Lett. 24 (1999) 1193. [15] T. Hasobe, H. Imahori, H. Yamada, T. Sato, K. Ohkubo, S. Fukuzumi, Nano Lett. 3 (2003) 409. [16] S.G. DiMagno, V.S.-Y. Lin, M.J. Therien, J. Org. Chem. 58 (1993) 5983. [17] J.J. Gosper, M. Ali, J. Chem. Soc., Chem. Commun. (1994) 1707. [18] T.E.O. Screen, J.R.G. Thorne, R.G. Denning, D.G. Bucknall, H.L. Anderson, J. Mater. Chem. 13 (2003) 2796. [19] S. Huang, J.M. Tour, J. Org. Chem. 64 (1999) 8898. [20] S. Bra¨se, J.H. Kirchhoff, J. Ko¨bberling, Tetrahedron 59 (2003) 885.
Synthesis and self-assembly of novel porphyrin molecular wires Masahiro Kawao a,b, Hiroaki Ozawa b,c, Hirofumi Tanaka b,c,d, Takuji Ogawa b,c,d,* a b
Department of Chemistry, Faculty of Science, Ehime University, Matsuyama 790-8577, Japan Research Center for Molecular-Scale Nanoscience, Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan c The Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8787, Japan d CREST, Japan Science and Technology Corporation, Japan Available online 2 September 2005
Abstract Sub-micrometer long butadiyne-linked porphyrin wires were synthesized by oxidative coupling of diethynylporphyrin. The porphyrin wires were analyzed by analytical gel permeation chromatography, absorption spectroscopy and matrix-assisted laser desorption/ionization time of flight mass spectroscopy. Observations of the wire were performed by atomic force microscopy. Self-assembled structures of the wires were observed on highly oriented pyrolytic graphite. Self-assembling features of the porphyrin wires depended on the length of the porphyrin wires and the concentration of the depositing solution. D 2005 Elsevier B.V. All rights reserved. Keywords: Porphyrin wire; k-Conjugated macromolecule; Self-assembled nanostructure
1. Introduction Long k-conjugated molecular wires have been receiving much attention because they can be used as organic conducting materials and nonlinear optical materials [1 – 3]. Measurement of the electric conductivity of individual molecules has been focused on establishing a foundation for molecular electronics and has been performed in various ways, e.g., using a nanofabricated electrode [4,5] and by scanning tunneling microscopy (STM) [6]. However, the measurements are still difficult and only a limited number of successful measurements have been reported so far. Well-ordered structures of organic molecules on devices are required to realize reproducible and reliable functions of the devices. Thus, the fabrication of the molecular nanostructures required for practical molecular electronics entails * Corresponding author. Research Center for Molecular-Scale Nanoscience, Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan. Tel.: +81 564 59 5537; fax: +81 564 59 5535. E-mail addresses: [email protected] (M. Kawao), [email protected] (T. Ogawa). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.07.143
the challenge of assembling molecules into the required well-ordered structure. The self-assembly technique takes advantage of intermolecular interaction or of interaction between molecules and the substrate surface to achieve a spontaneous organization. Thus, assembling k-conjugated macromolecules on substrate surfaces is one of the key technologies for the fabrication of molecular electronic nanodevices [7]. A number of works concerning the synthesis and selfassembly of k-conjugated molecular wires on substrates have been published [8,9]. Among such wires, porphyrin wires have attracted much attention because of the small HOMO – LUMO gap, high stability, and diversity of the substituents and of the center metal. We report the synthesis of novel long k-conjugated porphyrin wires and the first demonstration of the selfassembly properties of the sub-micrometer long porphyrin wires on surfaces of highly oriented pyrolytic graphite (HOPG) and other substrates. The oligo-diethynylporphyrin wires were prepared by copper-catalyzed oxidative coupling reaction of diethynylporphyrin, as shown in Scheme 1 [10 – 13].
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M. Kawao et al. / Thin Solid Films 499 (2006) 23 – 28
2. Experiment 2.1. Materials The porphyrin wires were prepared by a modified version of the reported methods, as shown in Scheme 1. 3,5-Bis(3-methylbutoxy)benzaldehyde (1) was prepared by MnO2 oxidation of the corresponding benzylalcohol [14]. 5,15-Bis(3,5-bis(3-methylbutoxy))phenylporphyrin (2) was prepared by acid-catalyzed condensation of benzaldehyde 1 with dipyrromethane [15]. Diethynylporphyrin (3) was prepared from meso-position-free porphyrin 2 in four steps: bromination [16], metalation with Zn [16], introduction of trimethylsilylacetylene by Sonogashira coupling [17], and deprotection of acetylene units[18]. We prepared oligodiethynylporphyrin (4) by copper-catalyzed oxidative coupling reaction of diethynylporphyrins 3 as follows: Triethylamine (0.5 ml, 5.0 mmol) was added to the THF solution of diethynylporphyrin (3) (0.090 g, 98 mmol) and CuI (0.050 g, 0.26 mmol), and the reaction mixture was stirred at 333 K for 24 h in oxygen atmosphere. The resulting solution was purified by gel permeation chromatography (GPC). The solvent was removed under reduced pressure. A dark green powder of the products was obtained (0.078 g, 87%). The products (reaction mixture hereafter) were purified by preparative scale GPC and analyzed by analytical GPC, absorption spectroscopy, matrix-assisted laser desorption/ ionization time of flight mass (MALDI – TOF– MS) and atomic force microscopy (AFM). 2.2. Measurements
Scheme 1. Synthesis of porphyrin wires. i) TsOH, MeOH, reflux. ii) C5H11Br, K2CO3, acetone, reflux. iii) LiAlH4, THF, reflux. iv) MnO2, toluene, reflux. v) AcOH, MeOH, room temp. vi) TFA, CHCl3. vii) pChloranil, 333 K. viii) NBS, CHCl3, room temp. ix) Zn(AcO)2(2H2O), CHCl3, room temp. x) TMSA, CuI, Pd(PPh3)4, Et3N, THF, reflux. xi) TBAF, THF, room temp. xii) CuI, Et3N, THF, 333 K.
MALDI – TOF –MS spectra were obtained using Applied Biosystems Voyager DE-STR with 7,7,8,8-tetracyanoquinodimethane (TCI) as a matrix. UV – vis absorption spectra were obtained using the Shimadzu UV-3150 spectrometer with pyridine. Analytical GPC experiments were carried out using Shimadzu LC-6A equipped with the MD-2015 detector (JASCO) using two GPC KF-804L columns (critical molecular weight 400,000 Da, Shodex). The tetrahydrofuran (THF) flow rate in all experiments was 1.0 mL/min. The samples for AFM measurements were prepared as follows: the porphyrin wires were dissolved in a solvent mixture (THF/H2O = 10/1, v/v), and the solution was castdeposited onto the substrate surface. We prepared two different concentrations of porphyrin wire solution. One is a solution of porphyrin wires whose absorbance of the soret peak is approximately 0.1 (thin solution). The other is a solution of porphyrin wires whose absorbance of the soret peak is approximately 0.2 (thick solution). These solutions were cast on HOPG, and the solvent evaporated spontaneously. The dried stain on the exterior of the porphyrin wires was observed by AFM. Several substrates such as HOPG, mica, and siliconized SiO2/Si wafers were used for AFM observation. The siliconized
M. Kawao et al. / Thin Solid Films 499 (2006) 23 – 28
3. Results and discussion
1.0
Absorbance / arb. units
SiO2/Si wafer was prepared by treating a thermally oxidized Si wafer with a siliconized reagent (L-25, Fuji Systems Co.). All AFM observations were performed using JSPM-4210 (JEOL) at room temperature in ambient pressure in the tapping mode with a Si cantilever (NSC35/ AIB3, MikroMasch).
25
(i) (h) (g) (f) (e) (d) (c) (b) (a)
0.8
0.6
0.4
0.2
3.1. Analysis of porphyrin wires 0.0
From a solution of reaction mixture, no molecular peaks corresponding to greater than 30 repeat units were detected by MALDI –TOF– MS. On the other hand, analytical GPC clearly indicated the presence of sub-micrometer long molecules. Fig. 1 shows the analytical GPC chart of the porphyrin wires in THF calibrated with standard polystyrene. The charts indicate that the porphyrin wires included molecules heavier than the critical molecular weight (400,000 Da) of the GPC column. Since the molecular weight of one unit of compound 3 is 915, the critical molecular weight corresponds to 437 units of compound 3, which means that the reaction mixture contains molecular wires with length of exceeding 600 nm, because the length of one middle unit of trimer was calculated to be 1.36 nm by MMII calculation. By using preparative-scale GPC, monomers to octamers were obtained in pure forms, and the mixtures, 9¨20-mer, 20¨50-mer, 50¨200-mer, and over200-mer, could be separated. Their molecular weights were determined by analytical GPC. Fig. 2 shows UV – vis absorption spectra of the porphyrin wire molecules separated by preparative-scale GPC. Compared with the monomer (a), over-200-mer (l) showed redshifts of approximately 200 nm in the Q band and 40 nm in the soret band, which indicate a high degree of conjugation.
400
500
600
700
800
900
1000
Wavelength / nm Fig. 2. UV – vis absorption spectra of the separated products in pyridine: (a) monomer, (b) dimer, (c) trimer, (d) tetramer, (e) pentamer, (f) hexamer, (g) heptamer, (h) octamer, (l) over-200-mer.
3.2. Data of porphyrin wires The reaction mixture were purified by preparative scale GPC and analyzed by absorption spectroscopy and
Absorbance /arb. units
100
(a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) (m)
80
60
40
20
0 2 10
3
10
4
10
5
10
6
10
7
10
8
10
Molecular Weight /Da Fig. 1. Analytical GPC charts of the reaction and purified porphyrin wires in THF calibrated by standard polystyrene: (a) monomer, (b) dimer, (c) trimer, (d) tetramer, (e) pentamer, (f) hexamer, (g) heptamer, (h) octamer, (i) 9¨20-mer, (j) 20¨50-mer, (k) 50¨200-mer, (l) over-200-mer, (m) reaction mixture.
Fig. 3. AFM image of the porphyrin oligomers on HOPG: (a) trimer, the white line is an edge of HOPG step; (b) over-200-mer (thin solution).
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M. Kawao et al. / Thin Solid Films 499 (2006) 23 – 28
3.3. Topography of porphyrin wires on various surfaces In order to determine the length of the porphyrin wires, AFM observations of porphyrin wires on HOPG substrates were carried out. Fig. 3 shows AFM images of the porphyrin trimer and over-200-mer (thin solution) on HOPG. In the case of the porphyrin trimer, only dots with height of 0.5¨1 nm and diameter of 20¨40 nm were observed (Fig. 3a). In the case of the long porphyrin wires (over-200-mer), long wire-like images with lengths of 100¨700 nm and heights of 0.5¨2 nm were observed (Fig. 3b). The long wires had linear structures and were aligned along the HOPG lattices. This confirms that the higher oligomer solution contained the sub-micrometer long porphyrin wires. The long porphyrin wires (over-200-mers) were dissolved in THF/H2O (= 10 : 1) solvent to prepare a thick solution. The solution (20 AL) was cast on the HOPG surface and the solvent was allowed to evaporate spontaneously. Two types of self-assembled structures were observed by AFM, as shown in Fig. 4. The distance between the neighboring wires in one structure (Type A)
Frequency
(a)
20 15 10 5 0 0
MALDI – TOF – MS. MALDI – TOF– MS spectral profiles of dimer, trimer and tetramer were obtained by reflector mode. MALDI –TOF– MS spectral profiles of pentamer, hexamer, heptamer and octamer were obtained by linear mode.
5
15
20
25
30
20
25
30
2.0
2.5
3.0
Distance / nm 8 6 4 2 0 0
5
10
15 Distance / nm
(c) 3.0 2.5 Frequency
Dimer: MALDI – TOF – MS for C112H118N8O8Zn2 (calculated: 1830.77), m/ z = 1835.7 [M+]. UV – vis (C5H5N; k max): 436 nm, 718 nm. Trimer: MALDI – TOF – MS for C168H176N12O12Zn3 (calculated: 2745.14), m/z = 2752.0 [M+]. UV – vis (C5H5N; k max): 458 nm, 753 nm. Tetramer: MALDI – TOF – MS for C 224 H 234 N 16 O 16 Zn 4 (calculated: 3659.52), m/z = 3667.7 [M+]. UV – vis (C5H5N; k max): 468 nm, 792 nm. Pentamer: MALDI – TOF – MS for C 278 H 284 N 20 O 20 Zn 5 (calculated: 4541.83), m/z = 4589.5 [M+]. UV – vis (C5H5N; k max): 468 nm, 801 nm. Hexamer: MALDI – TOF – MS for C 334 H 342 N 24 O 24 Zn 6 (calculated: 5456.20), m/z = 5496.3 [M+]. UV – vis (C5H5N; k max): 469 nm, 807 nm. Heptamer: MALDI – TOF – MS for C390 H400 N28 O28 Zn 7 (calculated: 6370.58), m/z = 6411.0 [M+]. UV – vis (C5H5N; k max): 469 nm, 810 nm. Octamer: MALDI – TOF – MS for C 446 H 458 N 32 O 32 Zn 8 (calculated: 7284.95), m/z = 7317.3 [M+]. UV – vis (C5H5N; k max): 471 nm, 817 nm. Over-200-mer: UV – vis (C5H5N; k max): 471.5 nm, 822.5 nm.
10
(b)
Frequency
Fig. 4. AFM images of the porphyrin wires on HOPG. (a) Image of solution of the porphyrin wires (thick solution of over-200-mer). (b) Magnified image of area inside the square in panel (a). Two types of assembling patterns could be observed. Type A is the denser structure.
2.0 1.5 1.0 0.5 0.0 0.0
0.5
1.0
1.5 Height / nm
Fig. 5. Histogram of the height of and distance between the self-assembled wires (over-200-mer) on HOPG: (a) distance in Type A structure, (b) distance in Type B structure, (c) height in the Type B structure.
M. Kawao et al. / Thin Solid Films 499 (2006) 23 – 28
27
of the porphyrin wires can interact more strongly than among themselves. Thus, when the wires touch the HOPG surface directly, the attractive force of the substrate overcomes the sterical repulsive interaction between the neighboring wires and a dense structure (Type A) forms. When the wires are at the top of the array of molecular wires, the attractive interaction with the lower layer molecular wires is too weak to surmount the steric hindrance between the neighboring wires and a sparse structure, as exemplified in Type B, is formed. To better understand the surface effects, substrates comprising mica or siliconized SiO2 on Si wafers were used. Fig. 6 shows AFM images of the porphyrin wires on the mica substrate (thick solution of over-200-mers). In the case of mica substrates, both lumps and wire-like structures were observed. The observed lumps were 100¨600 nm wide and approximately 3 nm high. The length of the observed porphyrin wires was 100¨700 nm. The porphyrin wires aggregated themselves on the mica substrate surface because the mica substrate has a hydrophilic surface and the
Fig. 6. (a) AFM image of the porphyrin wires (thick solution of over-200mer) on mica substrate. (b) Magnified image of panel (a).
was 4.7 T 0.5 nm, and that of the other structure (Type B) was 9.4 T 1.6 nm, as shown in Fig. 5. The molecular height of Type B was 0.4 T 0.1 nm, which is a reasonable diameter for one porphyrin wire. The molecular height of Type A could not be determined because the gaps between each molecular wire were so narrow that the AFM cantilever could not reach the substrate surface. By scrutinizing the AFM images, we found that the Type B structure could be observed only on the upper layer of the molecular stacks, and the Type A structure always existed at the bottom of the layers. Thus, we reasonably hypothesize that the Type A structure was formed directly on the HOPG surface, and the Type B structure was built on the Type A structure. The observed difference in the distances between molecular wires can be explained by this hypothesis. The distances between wires are determined by two interactions; one is the interaction of wires with the lower substrate or molecules, and the other is that among neighboring wires of the same layer. The interaction between the wires and the HOPG surface is a relatively strong attractive force, because HOPG has wide planar k-systems with which the k-system
Fig. 7. (a) AFM image of the porphyrin wires (thick solution of over200-mer) on siliconized SiO2/Si wafer substrate. (b) Magnified image of panel (a).
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M. Kawao et al. / Thin Solid Films 499 (2006) 23 – 28
hydrophobic porphyrin wires are subjected to the repulsive force of the surface. Fig. 7 shows AFM images of the porphyrin wires (thick solution of over-200-mers) on the siliconized SiO2/Si wafer substrate. In this case, both lumps and wire-like structures are again observed. The size of the observed lumps was 100¨500 nm wide and approximately 9 nm high. The length of the observed porphyrin wires was 100¨700 nm. Although the siliconized SiO2/Si wafer has hydrophobic surface properties, the attractive interaction was too weak for the isolated porphyrin wire structure to form. Surfaces that have greater affinity to the porphyrin wires are required to obtain the ordered structure on surfaces. The length of polymer wire can be controlled by the solid-phase reaction [19,20]. For example, it can be achieved by oxidized-coupling of a porphyrin, in which one side of its acetylene is fixed by a solid phase, to diethynylporphyrin, in which one side is free from TMS protection. Therefore, TMS protection was removed by TBAF. Repeating this process will result in a uniform length of polymer wires. Solid-phase synthesis enables accurate control of the length of the porphyrin wires. One ethynyl group of [5,15-bis-(3,5-isopentoxyphenyl)-10,20-diethynylporphinato]zinc(II) capped by a solid phase is connected to [5,15-bis-(3,5-isopentoxyphenyl)-10-ethynyl-20-trimethylsilylethynylporphinato]zinc(II) by copper-catalyzed oxidative coupling. The acetylene unit of the product is deprotected by TBAF. We can control the length of the porphyrin wires by repeating oxidative coupling and deprotection.
4. Summary Butadiyne-linked porphyrin wires with length exceeding 600 nm were synthesized by a simple oxidative coupling reaction. Ordered structures of the porphyrin wires were observed on HOPG substrates. Such ordered structure of the porphyrin wires were not observed on mica or siliconized SiO2/Si wafer substrates. The self-assembled structures composed of the porphyrin wires were formed because of the strong interaction between the porphyrin wires and the HOPG substrate. The detailed investigations of self-
assembled structures and studies on their physical properties are currently in progress.
Acknowledgements One of the authors (HT) thanks Visionarts, Inc., for the financial support.
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