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Self-Organization of Low-Symmetry Adjacent-type Metallophthalocyanines Having Branched Alkyl Chains Mutsumi Kimura,*,† Hiroyuki Ueki,† Kazuchika Ohta,† Hirofusa Shirai,*,‡ and Nagao Kobayashi‡ Department of Functional Polymer Science, Faculty of Textile Science and Technology, Shinshu UniVersity, Ueda 386-8567, Japan, and Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Sendai 980-8587, Japan ReceiVed February 2, 2006. In Final Form: March 10, 2006 Low-symmetry, adjacent-type metallophthalocyanines 1 and 2 with four branched alkyl chains on one side and a chiral bridging segment on the other were synthesized, and their self-organization properties were investigated. The synthesized adjacent-type phthalocyanines were liquid-crystalline and exhibited a phase transition from the crystalline phase to the mesophase below room temperature. X-ray diffraction indicated that the molecules are stacked in onedimensional columnar aggregates with a hexagonal arrangement. The self-organization behavior of zinc complex 1 and cobalt complex 2 was also investigated with a monolayer experiment at the air-water interface. The adjacent-type phthalocyanines formed a stable monolayer at the air-water interface, and the monolayers could be transferred onto quartz substrates by a Y-type deposition. UV-vis, XRD, and CD measurements for the resulting Langmuir-Blodgett films indicated that 1 and 2 had different molecular orientations.
Introduction Self-organization of π-conjugated molecules into well-defined structures is an important process for the construction of molecular-based electronic and optical devices, e.g., electronic wires, switches, electroluminescence devices, field-effect transistors, and photovoltaic devices.1-9 Control of the spatial arrangement and orientation of building units within organized systems enhances the macroscopic properties and functions of these devices.10 π-Conjugated disk-shaped molecules such as triphenylenes, hexa-peri-hexabenzocoronens, porphyrins, and phthalocyanines have been widely studied as molecular building units for the construction of these devices.11-14 Since the discovery of the discotic liquid-crystalline phases of disk-shaped molecules by Chandrasekhar et al. in 1977,15 the development of discotic * To whom correspondence should be addressed. Mailing address: Department of Functional Polymer Science, Faculty of Textile Science and Technology, Shinshu University, Ueda 386-8567, Japan (M.K.). Tel. and fax: +81-268-21-5499 (M.K.). E-mail:
[email protected] (M.K.). † Shinshu University. ‡ Tohoku University. (1) Electronic Materials: The Oligomer Approach; Mullen, K., Wegner, G., Ed.; Wiley-VCH: New York, 1998. (2) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. (3) Whiteside, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (4) Rivera, J. M.; Martin, T.; Rebek, J., Jr. Science 1998, 279, 1021. (5) Brunsveld, L.; Zhang, H.; Glasbeek, M.; Vekemans, J. A. J. M.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 6175. (6) Bourroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; MacKay, R. N.; Friend, R. H.; Burn, P. L.; Homes, A. B. Nature 1990, 347, 539. (7) Sariciftci, S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474. (8) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H. Nature 1995, 376 498. (9) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685. (10) Rep, D. B. A.; Roelfsema, R.; van Esch, J. H.; Schoonbeek, F. S.; Kellog, R. M.; Feringa, B. L.; Palstra, T. T. M.; Klapwijk, T. M. AdV. Mater. 2000, 12, 563. (11) Liu, C.-Y.; Pan, H.-L.; Fax, M. A.; Bard, A. J. Science 1993, 261, 897. (12) Liu, C.-Y.; Pan, H.-L.; Fox, M. A.; Bard, A. J. Chem. Mater. 1997, 9, 1422. (13) Gregg, B. A.; Fox, M. A.; Bard, A. J. J. Phys. Chem. 1989, 93, 4227. (14) Schmidt-Mende, L.; Fechtenkotter, A.; Mullen, K.; Moons, E.; Friend, R. H.; MacKrnzie, J. D. Science 2001, 293, 1119.
liquid-crystalline materials composed of rigid planar central cores and flexible hydrocarbon chain tails has received growing interest.16-20 Some of these discotic liquid-crystalline materials have high charge carrier mobility in the liquid-crystalline phases.21-23 The efficiency of charge carrier mobility strongly depends on the stacking distance and the arrangement among the disk-shaped molecules within the columnar assemblies. For example, long-range ordering along the column of the discotic hexagonal (Dh) and helical phases of hexa-hexylthiotriphenylene displayed a high charge carrier mobility of 0.1 cm2 V-1 s-1.24 Phthalocyanines (Pcs) and their metal complex derivatives (MPcs) have been investigated a great deal in regard to electronic, photonic, and magnetic fields because of their high stability, the presence of intense π-π* transitions in the visible region, and their redox activity.18 Phthalocyanines substituted with linear or branched alkyl, alkoxymethyl, or alkoxy chains form a major class of discotic liquid-crystalline materials.17 Phthalocyanine rings are stacked in one-dimensional columns through strong intermolecular π-π interactions in the solid and liquid-crystalline phases. The type of side chain, its length, the central metal ion, and the symmetry of the Pc molecule influence the self-organized structures. Although self-organization properties of Pcs with D4h symmetry have been extensively examined, few examples of low-symmetry Pcs are known. The preparation and spectroscopic (15) Chandrasekar, S.; Sadashiva, B. K.; Suresh, K. A. Pramana 1977, 9, 471. (16) Chandrasekar, S. Liq. Cryst. 1993, 14, 3. (17) Simon, J.; Bassoul, P. In PhthalocyaninessProperties and Applications; Leznoff, C. C., Lever, A. B. P., Eds.; VCH: New York, 1992; Vol. 2, p 223 and related references therein. (18) McKeown, N. B. Phthalocyanine Materials Synthesis, Structure and Function; Cambridge University Press: Cambridge, U.K., 1998 and related references therein. (19) Gregg, B. A.; Fox, M. A.; Bard, A. J. J. Chem. Soc., Chem. Commun. 1988, 1134. (20) Herwig, P.; Kayser, C. W.; Mullen, K.; Spiess, H. W. AdV. Mater. 1996, 8, 510. (21) Adam, D.; Schuhmacher, P.; Simmerer, J.; Haussling, L.; Siemensmeyer, K.; Etzbachi, K. H.; Ringsdorf, H.; Haarer, D. Nature 1994, 371, 141. (22) van de Craats, A. M.; Warman, J. M. AdV. Mater. 2001, 13, 130. (23) van de Craats, Warman, J. M.; de Haas, M. P.; Adam, D.; Simerer, J.; Haarer, D.; Schuhmacher, P. AdV. Mater. 1996, 8, 823. (24) van de Craats, A. M.; Warman, J. M.; Mullen, K.; Greerts, Y.; Brand, J. AdV. Mater. 1998, 10, 36.
10.1021/la060330i CCC: $33.50 © 2006 American Chemical Society Published on Web 04/29/2006
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properties of “adjacent”-type Pc derivatives have been previously reported. Condensations of bridged units containing two phthalonitriles produced side-linked 1,11,15,25-tetrasubstitited Pcs.25-27 We studied the synthesis and self-organization properties of adjacent-type MPcs 1 and 2 decorated with long branched alkyl chains and an optically active bridging unit. We considered that the low symmetry of the MPc molecules provides new opportunities for the construction of well-defined supramolecular organizations. Experimental Section Materials and Methods. All solvents were dried before use. The precursors 4,5-dibromocatechol,28 1-bromo-3,7,11,15-tetramethylhexadecane,29 and (-)-2,3-O-isopropylidene-D-threitol30 were synthesized by the literature method. Adsorption column chromatography was performed using silica gel or activated alumina. Gel permeation chromatography was carried out on a JAI recycling preparative HPLC instrument using CHCl3 as the eluent. Analytical thin-layer chromatography was performed on commercial Merck plates coated with silica gel F254. NMR spectra were recorded on a Bruker AVANCE 400 FTNMR spectrometer operating at 399.65 and 100.61 MHz for 1H and 13C, respectively, in CDCl . Chemical shifts are reported in ppm 3 downfield from internal (CH3)4Si. FT-IR spectra were recorded on a Shimadzu IRPrestige-21 spectrophotometer. Absorption and fluorescence spectra were measured on a Shimadzu MultiSpec1500 spectrophotometer and a JASCO FP-750 spectrometer, respectively. Circular dichroism (CD) spectra were recorded on a JASCO J-600 spectropolarimeter. X-ray diffraction (XRD) patterns were measured with Cu KR radiation on a Rigaku XRD-DSC diffractometer. Mass spectra were obtained on a PerSeptive Biosystems Voyager DE-Pro spectrometer with dithranol as the matrix. Differential scanning calorimetry (DSC) analyses were carried out on a Seiko DSC 220 apparatus. The measurements were carried out under nitrogen atmosphere with a heating and cooling rate of 10 °C/min. For polarization microscopy, an Olympus BX-51 polarization microscope equipped with a Mettler Toledo FP82HT hot stage was used. Monolayers at the air-water interface were studied by measuring pressure-area (π-A) isotherms using a Kyowa FACE Langmuir trough equipped with a Kyowa HBM AP film balance. Triply distilled water was used as the subphase. A sample was dissolved in chloroform (spectroscopic quality, ca. 1 mg/mL) and spread on the water surface at 25 °C, and isotherms were recorded at a compression rate of 16.7 cm2/min. Deposition of monolayers onto a glass substrate was carried out by vertically dipping the substrate at a speed of 10 mm/min through the monolayer. The monolayer was equilibrated for 2 h at a surface pressure of 25 mN/m and a temperature of 25 °C. Glass substrates were cleaned ultrasonically with chloroform, treated with concentrated sulfochromic acid at 80 °C for 2 h, and washed with distilled water. Syntheses. Compound 3. 4,5-Dibromocatechol (4.6 g, 17.0 mmol) and potassium carbonate (11.7 g, 85.0 mmol) were added to acetone (100 mL), and nitrogen gas was bubbled through this mixture for 5 min at room temperature. 1-Bromo-3,7,11,15-tetramethylhexadecane (13.7 g, 38.0 mmol) was added dropwise to this solution with stirring and refluxed under nitrogen for 48 h. After being cooled, the reaction mixture was poured into water (300 mL), and the aqueous layer was extracted with 3 × 50 mL of ethyl acetate, dried over MgSO4, filtered, and concentrated under vacuum. The residue was purified by column chromatography [silica gel, petroleum ether/ (25) Leznoff, C. C.; Drew, D. M. Can. J. Chem. 1996, 74, 307. (26) Kobayashi, N. J. Chem. Soc., Chem. Commun. 1998, 487. (27) Kobayashi, N.; Miwa, H.; Isgo, H.; Tomura, T. Inorg. Chem. 1999, 38, 479. (28) van Nostrum, C. F.; Picken, S. J.; Schouten, A.-J.; Nolte, R. J. M. J. Am. Chem. Soc. 1995, 117, 9957. (29) Gramatica, P.; Manitto, P.; Speranza, G. Tetrahedron 1987, 43, 4481. (30) Curtis, W. D.; Laidler, D. A.; Stoddart, J. F.; Jones, G. H. J. Chem. Soc., Perkin Trans. 1 1977, 1756.
Kimura et al. CH2Cl2 (9:1 v/v)] to afford 1,2-dibromo-4,5-(3,7,11,15-tetramethylhexadecenoxy)benzene as a colorless liquid (2.7 g, 19%). TLC: Rf ) 0.48 [petroleum ether/CH2Cl2 (9:1 v/v)]. 1H NMR (CDCl3): δ ) 7.06 (s, 2H, Ar), 3.67 (m, 4H, OCH2), 1.82 (m, 4H, CH), 1.12-1.68 (m, 44H, CH2), 0.83-0.93 (m, 30H, CH3). A mixture of 1,2-dibromo-4,5-(3,7,11,15-tetramethylhexadecenoxy)benzene (2.7 g, 3.3 mmol) and CuCN (1.5 g, 16.0 mmol) in dry DMF (25 mL) was refluxed under a nitrogen atmosphere for 7 h. The reaction mixture was cooled to room temperature and poured into 100 mL of 10 wt % aqueous ethylenediamine solution. Air was bubbled through the mixture for 8 h. Subsequently, the reaction mixture was extracted with CH2Cl2, and the extracts were washed with water several times. The organic layer was separated, dried over MgSO4, filtered, and concentrated. The residue was purified by column chromatography [silica gel, petroleum ether/CH2Cl2 (1:1 v/v)]. Recrystallization from ethanol gave compound 3 as a white solid (0.85 g, 36%). TLC: Rf ) 0.40 [petroleum ether/CH2Cl2 (1:1 v/v)]. MALDI-TOF-MS (dithranol): m/z ) 722 ([M + H]+, 100%). Calcd for C48H84N2O2: 720.65. 1H NMR (CDCl3): δ ) 7.11 (s, 2H, Ar), 4.08 (m, 4H, OCH2), 1.90 (d, J ) 7.5 Hz, 2H, CH), 1.64 (m, 4H, CH), 1.19-1.34 (m, 42H, CH2), 0.83-1.07 (m, 30H, CH3). 13C NMR (CDCl3): δ ) 152.7, 115.9, 115.7, 108.4, 68.2, 39.4, 37.4, 37.3, 35.7, 35.6, 32.8, 29.9, 27.9, 24.8, 24.5, 24.4, 22.7, 22.6, 19.7, 19.6. FT-IR (KBr): 2229 cm-1 (-CN). Compound 4. Potassium carbonate (4.3 g, 30.0 mmol), 3-nitrophthalonitrile (2.4 g, 14.0 mmol), and (-)-2,3-O-isopropylideneD-threitol (1.0 g, 6.17 mmol) were stirred in dry DMF (5 mL) at room temperature under a nitrogen atmosphere for 48 h. The reaction mixture was poured into water (100 mL), and the aqueous layer was extracted with ethyl acetate. After being dried over MgSO4, the organic layer was evaporated, and the residue was purified by column chromatography (silica gel, CHCl2). Recrystallization from methanol gave a white solid (1.85 g, 73%). 1H NMR (CDCl3): δ ) 7.69 (dd, J ) 7.2 Hz, 2H, Ar), 7.39 (d, J ) 7.8 Hz, 2H, Ar), 7.35 (d, J ) 8.8 Hz, 2H, Ar), 4.51-4.59 (m, 4H, OCH2), 4.40 (m, 2H, OCH), 1.51 (s, 6H, CH3). 13C NMR (CDCl3): δ ) 160.7, 134.9, 125.8, 117.1, 116.9, 113.1, 110.9, 104.9, 75.7, 69.0, 26.8. FT-IR (KBr): 2228 cm-1 (-CN). Anal. Calcd for C23H18N4O4: C, 66.66; H, 4.38; N, 13.52. Found: C, 66.7; H, 4.5; N, 13.3. Asymmetric ZnPc 1. A mixture of 3 (0.3 g, 0.42 mmol), 4 (69.0 mg, 0.17 mmol), and ZnCl2 (46.0 mg, 0.33 mmol) in 2-(dimethylamino)ethanol (3 mL) was stirred and slowly heated. Then, the mixture was refluxed under a nitrogen atmosphere for 48 h. After the mixture had cooled, methanol was added, and the precipitate was filtered off. The residue was purified by column chromatography (activated alumina, CHCl3) and recycling preparative HPLC (CHCl3) to afford compound 1 as a green solid (21.0 mg, 13%). MALDITOF-MS (dithranol): m/z ) 1922 ([M + H]+, 100%). Calcd for C119H190N8O8Zn: 1920.82. 1H NMR (CDCl3): 8.84 (br, 10H, Ar), 4.1-4.6 (m, 14H, OCH, and OCH2), 2.23 (m, 4H, CH2), 1.98 (m, 8H, CH2), 1.02-1.65 (m, 90H, CH2 and CH3), 0.83-1.05 (m, 60H, CH3). Asymmetric CoPc 2 was synthesized by the same procedure as used for 1 in the presence of CoCl2. Yield: 44.0 mg (28%). MALDITOF-MS (dithranol): m/z ) 1917 ([M + H]+, 100%). Calcd for C119H190N8O8Co: 1916.93. 1H NMR (CDCl3): 8.85 (br, 10H, Ar), 4.0-4.5 (m, 14H, OCH, and OCH2), 2.24 (m, 4H, CH2), 1.97 (m, 8H, CH2), 1.05-1.64 (m, 90H, CH2 and CH3), 0.85-1.03 (m, 60H, CH3). Symmetric ZnPc 5. A mixture of 3 (0.1 g, 0.14 mmol) and ZnCl2 (10.0 mg, 73 µmol) in 2-(dimethylamino)ethanol (3 mL) was stirred and slowly heated. Then, the mixture was refluxed under a nitrogen atmosphere for 48 h. After the mixture had cooled, methanol was added, and the precipitate was filtered off. The residue was purified by column chromatography (activated alumina, CHCl3) to afford compound 5 as a green solid (21.0 mg, 13%). MALDI-TOF-MS (dithranol): m/z ) 2947 ([M + H]+, 100%). Calcd for C192H336N8O8Zn: 2945.4. 1H NMR (CDCl3): 8.84 (br, 8H, Ar), 4.65 (m, 16H, OCH2), 2.24 (m, 8H, CH2), 1.98 (m, 16H, CH2), 1.02-1.65 (m, 168H, CH2 and CH3), 0.83-1.05 (m, 120H, CH3).
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Scheme 1. Synthetic Pathways of Phthalonitriles 2 and 3 and MPcs 1, 2, and 5
Figure 1. Absorption spectra of 1 in CHCl3 and CHCl3/MeOH at ratios of 5:5, 4:6, 3:7, 2:8, and 1:9. The inset shows the absorbance changes of the Q-bands of 1 (b) and 5 (9). [complex] ) 5.0 µM.
Results and Discussion Low-symmetry zinc and cobalt phthalocyanines 1 and 2 were prepared from the two phthlonitriles 3 and 4, as shown in Scheme 1. Phthalonitrile 3 was obtained from 3,4-dibromocatechol and 1-bromo-3,7,11,15-tetramethylhexadecane.28 This branched hexadecane chain was prepared by hydrogenation and bromination of commercially available phytol.29 A phytanyl chain was chosen for the aliphatic side chains of the Pcs instead of linear alkyl chains because of their low transition temperature and influence on the structure and stability of organization.31 Liu et al. reported on the liquid-crystalline behavior of hexa-peri-hexabenzocoronene (HBC) with six phytanyl chains.32 The temperature for the transition from the discotic mesophase to the isotropic liquid was reduced by ca. 200 °C by changing from linear unbranched alkyl chains to the branched phytanyl chains. The other phthalonitrile 4 was prepared from the aromatic nucleophilic substitution reaction between 3-nitrophthalonitrile and (-)-2,3O-isopropylidene-D-threitol.33 Zinc and cobalt phthalocyanines 1 and 2 were prepared using a mixed tetracyclization of 3 and 4 in a 5:2 molecular ratio in the presence of ZnCl2 or CoCl2 in 2-(dimethylamino)ethanol. The required phthalocyanines 3 and 4 were separated by column chromatography and gel permeation chromatography. In addition, symmetrical ZnPc 5 was also prepared from 3 by the same procedure as used for 1. All of the compounds were characterized by MALDI-TOF mass spectrometry and 1H NMR spectroscopy. The resulting adjacenttype MPcs 1 and 2 had four phytanyl chains on one side and an optically active bridging unit on the other and exhibited good solubility in many organic solvents except for alcohols. We compared the aggregation behavior of ZnPcs 1 and 5 in CHCl3 and a mixture of CHCl3 and methanol. The shape and location of the Q-band are known to be an indicator for determining the aggregation behavior of MPcs.34 The absorption (31) Schiller, S. M.; Naumann, R.; Lovejoy, K.; Kunz, H.; Knoll, W. Angew. Chem., Int. Ed. 2003, 42, 208. (32) Liu, C.-Y.; Fechtenkotter, A.; Watson, M. D.; Mullen, K.; Bard, A. J. Chem. Mater. 2003, 15, 124. (33) Snow, A. W.; Jarvis, N. L. J. Am. Chem. Soc. 1984, 106, 4706.
spectra of 1 and 5 in CHCl3 had a strong sharp peak at 680 nm with a shoulder at 630 nm (Figure 1). These bands are attributed to the Q-bands for monomeric ZnPc.34 When methanol was admixed, these bands vanished, and a new absorption band appeared at 620 nm. This spectral change was attributed to the formation of dimeric or oligomeric Pc species.34 The planar phthalocyanine rings became closer with increased solvent polarity. The absorbance at the Q-band of symmetric ZnPc 5 decreased steeply above 60 vol % methanol as shown in the inset of Figure 1. In contrast, the absorbance of low-symmetry ZnPc 1 gradually decreased. The aggregation properties in solution were influenced by the symmetry of the Pc molecule as well as the number of side chains. Since the first report on the discotic liquid-crystalline behavior of Pcs by Piechocki et al.,35 liquid-crystalline Pcs and MPcs have been synthesized by the substitution of eight long alkyl chains at the 2, 3, 9, 10, 17, 23, and 24 positions or the 1, 4, 8, 11, 15, 18, 22, and 25 positions.36-43 The comparative thermal properties of 1, 2, and 5 were investigated using differential scanning calorimetry (DSC) and temperature-controlled X-ray diffraction (XRD) measurements. During the heating run from -150 to 250 °C, the symmetrical ZnPc 5 with eight phytanyl chains exhibited two endothermic peaks at -79 and 121 °C. The subsequent cooling and heating showed both peaks to be reversible. When a sample of 5 was placed between two glass slides at room temperature, the soft substance was found to be birefringent, indicating that 5 exhibits liquid-crystalline behavior below room temperature. When 5 was heated above 120 °C, the birefringent texture disappeared, and a thermodynamically stable (34) Stilman, M. J.; Nyokong, T. In PhthalocyaninessProperties and Applications; Leznoff, C. C., Lever, A. B. P., Eds.; VCH: New York, 1989; Vol. 1, p 135. (35) Piechochi, C.; Simon, J.; Skoulios, A.; Guillon, D.; Weber, P. J. Am. Chem. Soc. 1982, 104, 5245. (36) Piechocki, C.; Simon, J. NouV. J. Chim. 1985, 9, 159. (37) Ohta, K.; Yamaguchi, N.; Yamamoto, I. J. Mater. Chem. 1998, 8, 2637. (38) Engelkamp, H.; Middelbeek, S.; Nolte, R. J. M. Science 1999, 284, 785 (39) van Nostrum, C. F.; Bosman, A. W.; Gelinck, G. H.; Schouten, P. G.; Warman, J. M.; Kentgens, A. P. M.; Devillers, M. A. C.; Meijerink, A.; Picken, S. J.; Sohling, U.; Schouten, A.-J.; Nolte, R. J. M. Chem. Eur. J. 1995, 1, 171. (40) Clarkson, G. J.; Cook, A.; McKeown, N. B.; Treacher, K. E.; Ali-Adib, Z. Macromolecules 1996, 29, 913. (41) Kimura, M.; Kuroda, T.; Ohta, K.; Hanabusa, K.; Shirai, H.; Kobayashi, N. Langmuir 2003, 19, 4825. (42) Cherodian, A. S.; Davies, A. N.; Richardson, R. M.; Cook, M. J.; McKeown, N. B.; Thomson, A. J.; Feijoo, J.; Ungar, G.; Harrison, K. J. Mol. Cryst. Liq. Cryst. 1991, 1, 103. (43) Cook, M. J.; Heeney, M. J. Chem. Eur. J. 2000, 6, 3958.
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Figure 3. Surface pressure vs surface area isotherms for 1 (solid line), 2 (broken line), and 5 (dashed line) at a subphase temperature of 25 °C.
Figure 2. Temperature-controlled XRD patterns of (a) 5 and (b) 1 at 100 °C.
isotropic liquid was formed. Upon slow cooling (2 °C/min) from the isotropic liquid, digitate stars appeared from the undercooled liquid under a temperature-controlled polarizing optical microscope (TPOM). The observed texture was characteristic of a hexagonal columnar mesophase Dh.44 The endothermic peaks at -79 and 121 °C observed in the DSC measurements correspond to the transition from a crystal to a mesophase and the clearing point, respectively. The solid structure of 5 at room temperature was also established by XRD measurements. The XRD pattern was characterized by sharp reflections of 3.24, 1.87, 1.62, and 1.22 nm (Figure 2a). This diffraction pattern could be due to the reflections from a two-dimensional hexagonal lattice with a lattice constant of a ) 3.74 nm. Furthermore, the broad and diffuse halo around 0.43 nm can be ascribed to the liquidlike disorder in the alkyl chains. A broad band can be observed around 0.34 nm due to the average stacking distance between phthalocyanine rings. The introduction of highly branched eight phytanyl chains in a ZnPc core resulted in a significant reduction of the transition points as compared to the reported values of 2,3,9,10,16,17,23,24-octa-n-alkoxy MPcs.17 Low-symmetry Pcs 1 and 2 exhibited only one reversible transition point at -68 °C over the range from -100 to 250 °C detected by DSC. Because the sample did not form an isotropic liquid, identifying the organized structure by TPOM was not possible. However, this phase above the transition point was mobile and distorted under external pressure. An X-ray diagram of 1 at 100 °C shows two reflections at 2.90 and 1.64 nm with a spacing ratio of 1:1/x3 indicating that the organized structure of 1 consists of a hexagonal structure (Figure 2b). The lattice constant (a ) 3.35 nm) almost correlates with the molecular dimensions of 1 derived from CPK models. The Langmuir-Blodgett (LB) technique is a method for fabricating ordered molecular structures from organic functional molecules.45 Well-organized thin films have been created from MPcs with solubilizing side chains through the LB technique.46 Liquid-crystalline MPcs can assemble into coherent rodlike columnar structures on an air-water interface, and the rodlike (44) van der Pol, J. F.; Neelemen, E.; Zwikker, J. W.; Nolte, R. J. M.; Drenth, W.; Aerts, J.; Visser, R.; Picken, S. J. Liq. Cryst. 1989, 6, 577. (45) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991.
assemblies can be transferred as a monolayer uniformly onto a substrate.47 Electrical and optical anisotropies have been observed in LB films made of MPcs. Nolte and co-workers reported the construction of well-organized and stable LB films from liquidcrystalline metal-free Pcs modified at eight positions with optically active alkyl side chains.39 O’Brien, Armstrong, and co-workers investigated the anisotropic electrical conductivity of multilayered LB films consisting of rodlike Pc assemblies.48 Katz and coworkers also reported extremely large optical nonlinearities of octaazaphthalocyanine with optically active [7] helices upon the formation of helical columnar structures within the LB films.49 Furthermore, Cook and co-workers studied the formation of LB films from amphiphilic Pcs substituted in the R-positions with six or seven hydrophobic alkyl chains and one or two hydrophilic chains terminated with carboxylic acid or hydroxyl groups.50,51 The construction of LB films from adjacent-type MPcs has not previously been reported. Because the type of side chain, its length, the central metal ion, and the symmetry of the Pc molecule influence the organized structures within LB films, we performed monolayer studies on 1 and 2. The pressure-surface area (π-A) isotherms for 1 and 2 on pure water at 25 °C are shown in Figure 3. Whereas octasubstituted Pcs and MPcs bearing long alkyl chains formed well-ordered, uniform monolayers on the surface of water, symmetrical ZnPc 5 did not show a steep increase of pressure for the formation of monolayers. This suggests that the conformational disordering and steric crowding of the phytanyl chains in 5 inhibited the formation of rodlike columnar aggregates among the Pc cores through intermolecular π-π interactions at the air-water interface. Both 1 and 2 formed fairly stable and reproducible films at the water-air interface. The adjacent-type MPcs 1 and 2 exhibited different π-A curves as shown in Figure 3. Extrapolation of the steepest part of the π-A curves to zero pressure gave occupied areas per molecule of 1.8 and 2.8 nm2/ molecule for 1 and 2, respectively. The collapsed film pressures were 43 mN/m for 1 and 38 mN/m for 2. The occupied area per (46) Cook, M. J.; Chambrier, I. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 17, p 37 and related references therein. (47) Fujiki, M.; Tabei, H.; Kurihara, T. Langmuir 1988, 4, 1123 (48) Smolenyak, P.; Peterson, R.; Nebesny, K.; Torker, M.; O’Brien, D. F.; Armstrong, N. R. J. Am. Chem. Soc. 1999, 121, 8628. (49) Fox, J. M.; Katz, T. J.; Elshocht, S. V.; Verbiest, T.; Kauranen, M.; Persoons, A.; Thongpanchang, T.; Krauss, T.; Brus, L. J. Am. Chem. Soc. 1999, 121, 3453. (50) Cook, M. J.; McMurdo, J.; Miles, D. A.; Poynter, R. H.; Simmons, J. M.; Haslam, S. D.; Richardson, R. M.; Welford, K. J. Mater. Chem. 1994, 4, 1205. (51) McKeown, N. B.; Cook, M. J.; Thomson, A. J.; Harrison, K. J.; Daniel, M. F.; Richardson, R. M.; Roser, S. J. Thin Solid Films 1988, 159, 395.
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Figure 5. Molecular arrangement models of (a) 1 and (b) 2 within LB films on the substrate.
Figure 4. (a) Absorption and (b) circular dichroism spectra of 40 layers of 1 (solid line) and 2 (broken line) on quartz substrates.
molecule showed the molecular arrangements on the air-water interface. The arrangements of MPc molecules on the surface of water depend on the balance between π-π interactions among MPcs and the interaction of the MPc ring plane with the water surface.52 When the interaction of MPc ring plane with the water surface is stronger than the π-π interactions among MPcs, the Pc ring planes will orient flat on the water surface (face-on). If MPcs assemble into rodlike columnar aggregates through π-π interactions, the Pc planes are vertically oriented with respect to the water-air interface with a monolayer thickness (edge-on). Assuming the molecular dimensions of synthesized low-symmetry MPcs 1 and 2 from CPK molecular models, the occupied area per molecule is 2.9 nm2/molecule for the face-on orientation and 1.7 nm2/molecule for the edge-on orientation. The observed area per molecule for 2 with a cobalt phthalocyanine ring is in agreement with the estimated area for the face-on orientation on the air-water interface. This suggests that the interaction of the cobalt phthalocyanine core in 2 with the water surface inhibits stacking of MPc molecules. In contrast, 1 with a zinc phthalocyanine core exhibited a small value for the area compared to that of 2, suggesting that the planes of the phthalocyanine molecules are oriented perpendicular to the water surface with the side chains extended into the air. Monolayers of 1 and 2 were transferred onto a hydrophobic glass substrate by vertically dipping the substrate through the monolayer at a surface pressure of 25 mN/m. Y-type depositions occurred for 1 and 2 with a transfer ratio of about 1.0 with both upstroke and downstroke dipping. Absorption spectra of 40layer LB films of 1 and 2 are shown in Figure 4a. Absorption spectroscopy provides information on the type of aggregated structures. The LB films made of 1 and 2 exhibited different spectral shapes in the region of the Q-band, indicating different aggregation types for 1 and 2 in the LB films. Compared to the dilute solution spectrum of 1 in CHCl3, shown in Figure 1, the Q-band of the LB film was broad, and new shoulder peaks at 648 and 753 nm appeared. The shape of the Q-band indicates that zinc phthalocyanines in the LB film are stacked into (52) Katayose, M.; Tai, S.; Kamijima, K.; Hagiwara, H.; Hayashi, N. J. Chem. Soc., Perkin Trans. 2 1992, 403.
cylindrical columns driven by the intermolecular π-π interactions.53 Furthermore, the presence of a shoulder peak at 753 nm suggests that the phthalocyanine planes were stacked in an eclipsed conformation within the one-dimensional stacks.54 XRD measurements on a sample containing 40 layers of 1 on a glass substrate showed a single intense Bragg peak with a d spacing of 3.11 nm. This spacing is approximately equal to that of the (100) reflection as observed in the mesophase, indicating the hexagonal close packing of cylindrical columns within the LB film. The chiral substituent in 1 affects the arrangement of Pc rings in the cylindrical columnar aggregates.55 Circular dichroism (CD) spectra of 1 in CHCl3 solution and LB films are shown in Figure 4b. Whereas a solution of 1 in CHCl3 did not give a CD spectrum, the LB film displayed CD activity. The CD spectrum in the Q-band region shows the change of sign from positive to negative for the LB film made of 1 on going from longer to shorter wavelength. To check the linear dichroism (LD) effect, we measured the CD spectra in the same locations for a rotation of 90°. Because no difference was observed between the two CD spectra, we concluded that the LD contribution is essentially negligible for the observed CD activity in the LB film of 1. This CD sign suggests the presence of a right-handed helical arrangement of the transition dipoles of the phthalocyanine rings within the cylindrical columnar aggregates.38,39,56,57 In contrast, the maximum of the Q-band for the LB film of 2 is broadened and red-shifted compared to the Q-band of a chloroform solution of 2 (Figure 4a). This spectral change can be attributed to the formation of slipped cofacial dimers between two Pc units within the LB film. Similar spectral changes were obtained for LB films of dihydroxysilicon phthalocyanines. XRD measurements of the LB film made of 2 yielded no Bragg peaks, and the film did not show CD activity, indicating no formation of helical columnar assemblies within the LB film. From these observations, we propose the molecular orientation models of 1 and 2 within the LB film on the substrate illustrated in Figure 5. Judging from a comparison of UV-vis, XRD, and CD analyses for the LB films of 1 and 2, the central metal of the phthalocyanine rings has a very strong influence on the molecular orientation. Zinc phthalocyanine 1 assembled into one-dimensional columns through the π-π interactions between phthalocyanine rings and rotated along the columnar axis under the effect of the chiral bridging segment. By contrast, CoPc 2 formed an alternating multilayer structure consisting of phthalocyanine dimer layers and a dense layer of phytanyl chains. (53) Hassan, B. M.; Li, H.; McKeown, N. B. J. Mater. Chem. 2000, 10, 39. (54) Farren, C.; FitzGerald, S.; Beeby, A.; Bryce, M. R. Chem. Commun. 2002, 572. (55) Harada, N.; Nakanishi, K. Circular Dichroic SpectroscopysExciton Coupling in Organic Stereochemistry; Oxford University Press: Oxford, U.K., 1983. (56) Kobayashi, N.; Kobayashi, Y.; Osa, T. J. Am. Chem. Soc. 1993, 115, 10994. (57) Kimura, M.; Muto, T.; Takimoto, H.; Wada, K.; Ohta, K.; Hanabusa, K.; Shirai, H.; Kobayashi, N. Langmuir 2000, 16, 2078.
5056 Langmuir, Vol. 22, No. 11, 2006
Conclusion We synthesized symmetrical and low-symmetry MPcs with either eight or four highly branched alkyl chains. Condensations of bridged units containing two phthalonitriles produced adjacenttype MPcs with four alkyl chains on one side and a chiral bridging segment on the other. Symmetrical ZnPc 5 and low-symmetry MPcs 1 and 2 exhibited liquid-crystalline behavior below room temperature, and the mesophase was identified by X-ray diffraction as a hexagonal phase. Langmuir-Blodgett (LB) films of adjacent-type MPcs 1 and 2 were prepared by using the vertical dipping technique, and the molecular orientations of MPcs within the LB films were investigated by UV-vis, XRD, and CD
Kimura et al.
measurements. These results demonstrate that the molecular orientations within the LB films are altered by the changes of the central metal. The control of molecular orientation within LB films makes the adjacent-type MPcs interesting compounds for future applications, e.g., in highly sensitive gas sensors and in electronic and ferroelectronic molecular devices. Acknowledgment. This work was partially supported by the CLUSTER Program and the 21st Century COE Program from the MEXT of Japan. LA060330I
