J. Phys. Chem. B 2008, 112, 4865-4869
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ARTICLES Electrical Conductivity Studies on Discotic Liquid Crystal-Ferrocenium Donor-Acceptor Systems P. Suresh Kumar, Sandeep Kumar, and V. Lakshminarayanan* Raman Research Institute, C. V. Raman AVenue, SadashiVanagar, Bangalore 560 080, India ReceiVed: October 4, 2007; In Final Form: February 6, 2008
The dispersion of electron-deficient ferrocenium ions was studied in the electron-rich media of two different triphenylene-based columnar hexagonal liquid-crystalline phases. These composites were characterized using polarizing optical micrography (POM), differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), visible absorption spectroscopy, and dc and ac conductivity measurements. It was found that these composites form donor-acceptor systems that enhance the quasi-one-dimensional conductivity of the discotic system without altering the hexagonal columnar mesophase. The absorbance spectra confirm the formation of a charge-transfer complex between the electron-rich discotic molecules and the electron-deficient ferrocenium ions.
Introduction Discotic liquid crystals (DLCs) are unique nanoarchitectures that consist of disks stacked on top of each other to form columns. These different columns constitute a two-dimensional lattice to form various phases such as hexagonal and rectangular. The separation of the cores within a column is of the order of 0.35 nm, and there is a considerable overlap of π-orbitals. As long flexible aliphatic chains surround the core, the separation of the columns is usually 2-4 nm, depending on the lateral chain length. Therefore, interactions between neighboring molecules within the same column are much stronger than interactions between neighboring columns. Consequently, it is expected that the electrical conductivity along the column, σ|, should be higher than that in the perpendicular direction, so that these materials can be considered quasi-one-dimensional conductors.1 However, because of the large band gap of about 4 eV, they are insulators like any other organic material. Moreover, they can be made conducting by being doped with a small amount of electron donor or acceptor, depending on the electronic nature of the core. Some early studies on the conducting properties of DLCs were performed in the 1980s, and the electrical properties of many phthalocyanine and triphenylene discotic systems have been studied.1-18 It has been shown that, when electron-rich hexaalkoxytriphenylene discotics are doped with NOBF4, the conductivity increases significantly.4 Similarly, the doping of electron-deficient tricycloquinazoline discotic with potassium enhances the dc electrical conductivity to 10-4 S/m.12 Hexahexylthiotriphenylene (HHTT), which has a very high photoinduced charge mobility,16 has been studied extensively for electrical conductivity by doping with iodine10,17 and trinitrofluorenone (TNF).11 Doping of HHTT with a small amount of TNF was found to enhance the σ|| conductivity by more than a factor of 107.11 1-D photoconductivity induced by * Corresponding author. E-mail:
[email protected]. Tel.: +91-8023610122 ext. 366. Fax: +91-80-23610492.
the formation of donor-acceptor pairs has also been studied.18-23 Hexabenzocoronenes also exhibit quasi-one-dimensional conductivity in their nanotubular form through either oxidation with nitrosonium tetrafluoroborate or functionalization with electron acceptors such as TNF.24,25 Ferrocenium cation, the oxidized state of the ferrocene molecule, contains an iron(III) ion sandwiched between two cyclopentadiene rings. It is known that ferrocene can act as an electron donor.26-28 The behavior of the electrochemically formed ferrocenium in different nonaqueous solvents has also been studied.29 Ferrocenium can interact with β-cyclodextrin and DNA systems and has been shown to exhibit antitumor properties.30-33 The liquid-crystalline behavior of several ferrocenium derivatives has also been reported.34-38 Even though ferrocenium salts have been extensively used in many systems, the incorporation of ferrocenium in discotic liquid-crystalline matrix has not yet been reported. Therefore, in this work, we studied the dc conductivity properties of two triphenylene-based discotic liquid crystals doped with ferrocenium species. Experimental Section The two DLCs hexahexyloxytriphenylene (HAT6) and hexahexylthiotriphenylene (HHTT) were synthesized by following the literature method.39 The structures of these two molecules are shown in Figure 1. Ferrocenium tetrafluoroborate (FcTFB) was purchased from Aldrich and was used without further purification. The DLCs were dissolved in dichloromethane and stirred with FcTFB to form a mixture, and slow evaporation of the solvent resulted in the formation of a dispersion. We prepared samples with three different compositions for the purposes of comparison, using 1%, 10%, and 50% by weight of FcTFB with each of the DLCs. The composites were characterized using polarizing optical microscopy, small-angle X-ray scattering, differential scanning calorimetry, visible spectroscopy, and electrical conductivity experiments.
10.1021/jp709704x CCC: $40.75 © 2008 American Chemical Society Published on Web 03/29/2008
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Figure 1. Chemical structures of the discotic molecules used in the study, HAT6 and HHTT.
The polarizing optical microscopy (POM) images were obtained using an Olympus POM instrument, coupled with a Mattler heater. The samples were sandwiched between a glass slide and a cover slip. The sample was heated to the isotropic phase, and the textures were imaged during cooling. The transition temperatures and associated enthalpy values were determined by differential scanning calorimetry (DSC; PerkinElmer, model Pyris 1D) at a scanning rate of 5 °C min-1 for both heating and cooling. The apparatus was calibrated using indium (156.6 °C) as a standard. Small-angle X-ray studies have been carried out using an X-ray diffractometer (Rigaku, UltraX 18) operating at 50 kV and 80 mA using Cu KR radiation having a wavelength of 1.54 Å. Samples were prepared by filling a capillary with the pure discotic systems or the composites and then sealing it. All scattering studies were carried out at 80 °C, and the diffraction patterns were collected on a two-dimensional Marresearch image plate. A model SD 2000 spectrophotometer (Ocean Optics, Dunedin, FL) fitted with a tungsten lamp source and a cell having a path length of 1 cm was employed to measure the visible absorbance spectra. The dc conductivity studies of the composites were carried out in ITO-coated glass sandwich cells (10 mm × 5 mm) with a thickness of either 60 or 4 µm. Current measurements were carried out using a Keithley picoammeter (model 480) along with a constant dc voltage source and a temperature controller. Ionic conductivity measurements were carried out using a lock-in amplifier (Stanford Research Systems model SR830) at 1 kHz frequency on cooling from the isotropic phase.
Kumar et al.
Figure 2. Polarizing optical texture of 10% FcTFB/HAT6 showing the columnar hexagonal phase of the system, imaged during cooling from the isotropic phase.
Results and Discussion
Figure 3. DSC traces of (A) (a) pure HAT6, (b) 1% Fc+/HAT6, and (c) 10% Fc+/HAT6 and (B) (a) pure HHTT, (b) 1% Fc+/HHTT, and (c) 10% Fc+/HHTT.
Polarizing Optical Microscopy. Figure 2 shows one of the typical polarizing optical micrographic textures obtained for the composite containing 10% FcTFB in HAT6. A similar texture was also obtained for the FcTFB composite with HHTT. As is obvious from the figure, the hexagonal columnar (Colh) phase of both HAT6 and HHTT was maintained even in the presence of ferrocenium as a dopant. At very high concentrations of dopant (for example, for the 50% FcTFB composite), a phase separation was observed. The phase transition temperatures of the composites were found to be a function of the ferrocenium concentration. Differential Scanning Calorimetry. The composites were characterized by DSC analysis for the measurement of the phase transition temperatures and the enthalpies of the transitions. Figure 3A,B shows the DSC data for the composites and the
pure discotic systems for HAT6 and HHTT, respectively. It is clear from the plots that the phase transition profiles are not significantly changed even after the addition of ferrocenium ions at low concentrations. We also observed that the Colh phase becomes destabilized at very high concentrations of Fc+ ion, which is in accordance with the POM results. The phase transition temperatures were found to shift to lower temperature with increasing dopant concentration. The enthalpy of the phase transition decreases with increasing dopant concentration. At 50% Fc+ concentration, the Colh phase was not observed. The highly electron-deficient ferrocenium ions form a charge-transfer complex with the electron-rich triphenylene core, as is evident from the UV-visible spectroscopic studies. The formation of the complex reduces the electron density of the core molecules. In addition, the steric hindrance of the ferrocenium ions is the
Electrical Conductivity of DLC Donor-Acceptor Systems
Figure 4. Scattering vector, q, versus intensity profiles obtained from small-angle X-ray scattering studies of the composites (a) 10% Fc+/ HAT6 and (b) 10% Fc+/HHTT. The intensity is on an arbitrary scale. The insets show enlargements of the 1/x3 and 1/x4 peaks from the main plot.
TABLE 1: d Spacing (Å) for the Colh Phases of HAT6 and HHTT Measured from the SAXS Studies before and after Doping sample
pure
10% FcTFB
HAT6 HHTT
20.15 18.02
18.16 18.47
major destabilizing factor in the composite systems, whereas charge-transfer complex formation of ferrocenium with the electron-rich triphenylene core enhances the stability. This is further supported by the fact that the enthalpy of the phase transition decreases with increasing ferrocenium concentration. Small-Angle X-ray Scattering. Small-angle X-ray scattering studies were carried out in 1-mm capillary tubes (Hamton Research, Aliso, Viejo, CA) at 80 °C, and all of the scattering studies were performed during cooling from the isotropic phase. The SAXS patterns support our conclusions from the POM studies that the pure discotic samples as well as the 1% and 10% FcTFB-doped samples retain the Colh phase. The typical patterns obtained for the samples are shown in Figure 4. The d spacing and the lattice parameters calculated from the SAXS patterns are summarized in Table 1. The d values of 18.02 Å for HHTT and 20.15 Å for HAT6 as calculated from the SAXS patterns are in good agreement with the literature values.40 The main scattering peak in the pattern shown here corresponds to the 2D plane having Miller indices of (1,0) of the Colh phase. The other two peaks of lower intensity are shown in the inset. The ratio of the d values corresponding to the three peaks is 1:(1/x3):(1/x4) showing hexagonal order. As is clear from the table, the doping of ferrocenium in the discotic matrix did not disturb the lattice parameters as there was a negligible change in the d spacing. We previously proposed that gold nanoparticles occupy the interdomain spacing without altering the d spacing of the columns.41 In a similar manner, we argue that the ferrocenium molecules occupy the interdomain spacing in the discotic matrix. We rule out intercolumn spacing because of the relatively larger size of ferrocenium ions as compared to iodine, for example.10 This observation is also consistent with the fact that no significant alteration of the d spacing was found even after doping. Visible Absorption Spectra. Visible absorbance spectral studies of the composites were carried out in dichloromethane medium with a composition of 3 mg of the composite in 2 mL of dichloromethane. The solution was sonicated for 5 min, and the absorbance of the solution was recorded immediately with pure dichloromethane as a reference. Figure 5 shows the
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Figure 5. Visible absorption spectra of the composites (a) 1% Fc+/ HAT6, (b) 10% Fc+/HAT6, (c) 50% Fc+/HAT6, (d) pure ferrocene, and (e) pure FcTFB.
absorbance spectra for different composites, in addition to those of pure ferrocenium and ferrocene. It is clear from the figure that, in the composites, there is a blue shift in the absorbance of the ferrocenium group, which is due to the formation of a charge-transfer complex. Also, there is a shoulder peak in the 400-500 nm region. Because ferrocenium is a very strong electron acceptor, charge transfer from the electron-rich discotic molecules occurs, resulting in the formation of donor-acceptor pairs. Pure ferrocenium ions absorb at around 600 nm, whereas pure ferrocene absorbs at around 460 nm. In the present case, a blue shift was observed in the absorption that can be attributed to the occurrence of charge transfer from triphenylene to ferrocenium ion in the composite. It is known that ferrocenium ions are unstable in an electron-rich environment such as highly polar solvents.28 The extent of the blue shift was shown to depend on the concentration of Fc+ ion and the polarity of the solvent. In the present study, the absorbance had a higher blue shift at lower concentrations for both HAT6 and HHTT composites, as shown in the Figure 5. For the Fc+/HAT6 composites, the 1%, 10%, and 50% samples show absorbances at wavelengths of 358, 362, and 383 nm, respectively, whereas pure HAT6 absorbs at 277 nm.42 This might be due to the fact that, at lower concentrations of Fc+, complete charge transfer between the donor and acceptor molecules occurs. Similar behavior was observed for the Fc+/HHTT composite as well. The charge-transfer band in ferrocenium results from transfer from the e1u bonding ligand level to the empty e2g metal level.43 Conductivity. The dc conductivity studies of the composites were performed in ITO-coated glass sandwich cells of 60-µm thickness. Conductivity measurements were carried out using a Keithley picoammeter (model 480) along with a constant dc voltage source and a temperature controller. We also carried out the measurements in a 4-µm separation cell. A slow cooling in the small cell from the isotropic phase results in the homeotropic alignment of the sample. This method of cooling from the isotropic phase in sandwiched cells has proven to be a common method for the homeotropic alignment of discotic liquid crystals.44-62 In the present study, the alignment was confirmed using polarizing optical microscopy, where the aligned sample does not show textures. The conductivity values of both aligned and unaligned samples are in agreement. The aligned sample, of course, shows a larger electrical conductivity because of its facile electron transport route as compared to that of the unaligned sample. The dc conductivity values of the samples were measured as a function of temperature, and the reported values were obtained
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Figure 7. (a) dc and (b) ac conductivity plots for the 10% FcTFB/ HAT6 system upon cooling from the isotropic phase.
Figure 6. dc conductivity plots of different composites as a function of the temperature for (A) (a) 1% Fc+/HAT6, (b) 10% Fc+/HAT6 unaligned samples, (c) 1% Fc+/HAT6, and (d) 10% Fc+/HAT6 aligned samples and (B) (a) 1% Fc+/HHTT and (b) 10% Fc+/HHTT unaligned samples.
during cooling from the isotropic phase. Figure 6A,B shows plots of conductivity as a function of temperature for the HAT6 and HHTT systems, respectively, after doping. There is a large increment in the conductivity values of the system after doping. This can be explained by the charge transport that takes place after the addition of Fc+ ions to the discotic phase. Both HAT6 and HHTT have very low dc conductivities in their pure state that could not be measured with reasonable accuracy in the present setup. It can be seen that the conductivity increases by several orders of magnitude after doping. This increase is also dependent on the concentration of the dopant. A similar trend was observed for the aligned sample also in the direction of column axis for the HAT6 system, as is obvious from Figure 6A. The HHTT system also was found to exhibit similar behavior. To understand the nature of the conductivity of these systems, we also measured the ionic conductivity by ac measurements for all of these samples. Figure 7 shows a comparison of ionic conductivity for a typical sample containing 10% Fc+ in HAT6. From the figure, the ionic conductivity was found to be about 2 orders of magnitude less than the dc conductivity. This shows that the mechanism operating in these systems is mainly electron transport rather than ion transport by the ferrocenium ions or the BF4- counterions. Ferrocene has an iron(II) ion sandwiched between the two cyclopentadiene units, and this organometallic compound obeys the 18-electron rule.63 On the other hand, the ferrocenium cation has 17 electrons and therefore acts as a strong electron acceptor. The a1g molecular orbital of the ferrocenium ion has an unpaired electron, which explains its high electron-accepting nature. After the charge transfer from triphenylene, the composite forms an electron-hole pair, which enhances the dc conductivity of the
composite. Whereas the electron-accepting properties of ferrocenium have been well studied in different solvent systems, its behavior in discotic liquid-crystalline systems is of greater interest as such systems form quasi-one-dimensional molecular wires. We calculated the mole fractions of the composites by taking 0.35 nm as the intercolumn spacing and 18.3 Å as the d spacing for the discotic molecules. For the 10% FcTFB/HAT6 composition, the molar ratio of ferrocenium ions to discotic molecules was estimated to be about 1:3, which is much higher than the corresponding weight ratio. On the other hand, the volume fraction of ferrocenium ions to discotic molecules was calculated to be about 1:200 from the known molecular volumes of HAT6 and ferrocenium.36,64 This also explains the fact that no significant change in the d spacing and no disruption of the hexagonal order occur even after doping of ferrocenium to the extent of 10%. We also argue that, for the 50% composite, the molar ratio is too large, so that there is a disruption of the columnar phase because of steric effects. From the reported values in literature, we calculated that the reduction potential of the ferrocenium redox couple is +0.6 V with respect to HAT6.65 The more positive reduction potential explains the electron transfer from HAT6 to ferrocenium in the mixture and therefore their role as donor-acceptor pair. We propose that the electronic conductivity occurs by the hopping mechanism where the electron-rich triphenylene core donates an electron to the electron-deficient ferrocenium ion, thereby forming an electron-hole pair. The hopping conductivity of the electron-hole pair under the action of an applied dc electric field brings about the electronic conductivity. A similar mechanism was postulated to occur in discotic molecule-TNF pairs, discotic molecule-iodine pairs, and discotic molecule-gold nanoparticle systems.10,25,41 The conductivity increment in the systems after the addition of ferrocenium species also brings several interesting aspects. Recently, Power et al. reported a large increment in the conductivity by about 4 orders of magnitude in the case of the dispersion of salts in alcohol media. In our system, however, we found a large increment in the electronic conductivity upon the addition of ferrocenium species. The system described here provides a model for studying the interactions of donoracceptor systems, interactions of other organometallic compounds in discotic systems, π-π interactions between donoracceptor moieties, and similar interactions. It is also worth mentioning that, in the present systems, the dopants occupy the interdomain spacing as confirmed by the SAXS studies, and in this way they differ from many of the other reported dopants such as iodine or potassium, which occupy the intercolumnar space.10,25
Electrical Conductivity of DLC Donor-Acceptor Systems Acknowledgment. We thank Mrs. K. N. Vasudha for technical support in the characterization of the samples. References and Notes (1) Kumar, S. Chem. Soc. ReV. 2006, 35, 83. (2) Andre, J.-J.; Holczer, K.; Petit, P.; Riou, M.-T.; Clarisse, C.; Even, R.; Fourmigue, M.; Simon, J. Chem. Phys. Lett. 1985, 115, 463. (3) Belarbi, Z.; Sirlin, C.; Simon, J.; Andre, J.-J. J. Phys. Chem. 1989, 93, 8105. (4) Arikainen, E. O.; Boden, N.; Bushby, R. J.; Clements, J.; Movaghar, B.; Wood, A. J. Mater. Chem. 1995, 5, 2161. (5) Dyre, J. C. J. Appl. Phys. 1988, 64, 2456. (6) Markovitsi, D. Mol. Cryst. Liq. Cryst. 2003, 397, 89. (7) Boden, N.; Bushby, R. J.; Cooke, G.; Lozman, O. R.; Lu, Z. J. Am. Chem. Soc. 2001, 123, 7915. (8) Yoshio, M.; Mukai, T.; Ohno, H.; Kato, T. J. Am. Chem. Soc. 2004, 126, 994. (9) Imrie, C. T.; Inkster, R. T.; Lu, Z.; Ingram, M. D. Mol. Cryst. Liq. Cryst. 2004, 408, 33. (10) Vaughan, G. B. M.; Heiney, P. A.; McCauley, J. P., Jr.; Smith, A. B., III. Phys. ReV. B 1992, 46, 2787. (11) Balagurusamy, V. S. K.; Krishna Prasad, S.; Chandrasekhar, S.; Kumar, S.; Manickam, M.; Yelamaggad, C. V. Pramana 1999, 53, 3. (12) Boden, N.; Borner, R. C.; Bushby, R. J.; Clements, J. J. Am. Chem. Soc. 1994, 116, 10807. (13) Piecocki, C.; Simon, J.; Andre, J.-J.; Guillon, D.; Petit, P.; Skoulios, A.; Weber, P. Chem. Phys. Lett. 1985, 122, 124. (14) Bushby, R. J.; Donovan, K. J.; Kreouzis, T.; Lozman, O. R. OptoElectron. ReV. 2005, 13, 269. (15) Boden, N.; Bushby, R. J.; Clements, J.; Movaghar, B.; Donovan, K. J.; Kreouzis, T. Phys. ReV. B 1995, 52, 13274. (16) Adam, D.; Schuhmacher, P.; Simmerer, J.; Haussling, L.; Siemensmeyer, K.; Etzbachi, K. H.; Ringsdorf, H.; Haarer, D. Nature 1994, 371, 141. (17) Boden, N.; Bushby, R. J.; Clements, J. J. Mater. Sci. Mater. Electron. 1994, 5, 83. (18) Markovitsi, D.; Bengs, H.; Ringsdorf, H. J. Chem. Soc., Faraday Trans. 1992, 88, 1275. (19) Pisula W.; Kastler, M.; Wasserfallen, D.; Robertson, J. W. F.; Nolde, F.; Kohl, C.; Mullen, K. Angew. Chem., Int. Ed. 2006, 45, 819. (20) Fron, E.; Bell, T. D. M.; Vooren, A. V.; Schweitzer, G.; Cornil, J.; Beljonne, D.; Toele, P.; Jacob, J.; Mullen, K.; Hofkens, J.; Van, der Auweraer, M.; De Schryver, F. C. J. Am. Chem. Soc. 2007, 129, 610. (21) Sulzberg, T.; Cotter, R. J. J. Org. Chem. 1970, 35, 2762. (22) Percec, V.; Glodde, M.; Bera, T. K.; Milura, Y.; Shiyanovskaya, I.; Singer, K. D.; Balagurusamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp, A.; Spiess, H.-W.; Hudson, S. D.; Duan, H. Nature 2002, 417, 384. (23) Pacsial, E. J.; Alexander, D.; Alvarado, R. J.; Tomasulo, M.; Raymo, F. M. J. Phys. Chem. B 2004, 108, 19307. (24) Hill, J. P.; Jin, W.; Kosaka, A.; Fukushima, T.; Ichihara, H.; Shimomura, T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T. Science 2004, 304, 1481. (25) Yamamoto, Y.; Fukushima, T.; Suna, Y.; Ishii, N.; Saeki, A.; Seki, S.; Tagawa, S.; Taniguchi, M.; Kawai, T.; Aida, T. Science 2006, 314, 1761. (26) Brand, J. C. D.; Snedden, W. Trans. Faraday Soc. 1957, 53, 894. (27) Togni, A.; Hobi, M.; Rihs, G.; Rist, G.; Albinati, A.; Zanello, P.; Zech, D.; Keller, H. Organometallics 1994, 13, 1224. (28) Salman, H. M. A.; Mahmoud, M. R.; Abou-El-Wafa, M. H. M.; Rabie, U. M.; Crabtree, R. H. Inorg. Chem. Commun. 2004, 7, 1209. (29) Hurvois, J. P.; Moinet, C. J. Organomet. Chem. 2005, 690, 1829. (30) Lu, C.-S.; Ren, X.-M.; Hu, C.-J.; Zhu, H.-Z.; Meng, Q.-J. Chem. Pharm. Bull. 2001, 49, 818. (31) Moozyckine, A. U.; Bookham, J. L.; Deary, M. E.; Davies, D. M. J. Chem. Soc., Perkin Trans. 2 2001, 2, 1858.
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