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Columnar Discotic Liquid-Crystalline Oxadiazoles as Electron-Transport Materials† Ya-Dong Zhang,‡ Kim G. Jespersen,§ Michael Kempe,| Julia A. Kornfield,| Stephen Barlow,‡ Bernard Kippelen,*,§ and Seth R. Marder*,‡,§ Department of Chemistry and Optical Sciences Center, University of Arizona, Tucson, Arizona 85721, and Department of Chemical Engineering, 210-41 Caltech, Pasadena, California 91125 Received January 28, 2003. In Final Form: April 7, 2003 A range of discoid species with benzene or triazine cores and three (trialkoxyaryl)oxadiazole arms have been synthesized. 1,3,5-tris{5-[3,4,5-tris(octyloxy)phenyl]-1,3,4-oxadiazol-2-yl}benzene has been studied in detail; it exhibits a columnar discotic liquid-crystalline mesophase between 38 and about 210 °C. The time-of-flight electron mobility at room-temperature varies from about 10-3 to 10-4 cm2 V-1 s-1, indicating these materials may find applications in organic electronics.
Introduction Organic semiconducting materials with high carrier mobilities are desirable for various electronic applications including light-emitting diodes, solar cells, and field-effect transistors. Compounds that form columnar discotic liquid-crystalline mesophases offer the possibility of increased carrier (hole or electron) mobility relative to amorphous materials, as a result of good intermolecular orbital overlap within the stacks, while being readily processed by wet methods.1-4 However, there is a paucity of readily processible high-mobility electron-transport liquid-crystalline materials, and there are only a few reports of electron mobility measurements in columnar discotic liquid crystals.5 1,3,4-Oxadiazoles are one of the more widely studied classes of organic electron-transport materials;6 mobilities as high as about 10-3 cm2 V-1 s-1 have been reported (at 50+ °C),7 and they have proven to be effective electron-transport agents in organic lightemitting diodes.8 The oxadiazole functionality has been incorporated into various calamitic liquid-crystalline systems, both as additives9 and as mesogens.7,10-12 More recently, several discotic oxadiazole mesogens have been * Authors to whom correspondence should be addressed. E-mail:
[email protected] (S.R.M.);
[email protected]. edu (B.K.). † This paper is in memory of our colleague and friend, the late David O’Brien, and is a part of the Langmuir special issue dedicated to him. ‡ Department of Chemistry, University of Arizona. § Optical Sciences Center, University of Arizona. | Caltech. (1) Adam, D.; Closs, F.; Frey, T.; Funhoff, D.; Haarer, D.; Ringsdorf, H.; Schumacher, P.; Siemensmeyer, K. Phys. Rev. Lett. 1993, 70, 457. (2) Boden, N.; Bushby, R. J.; Clements, J.; Movghar, B.; Donovan, K. J.; Kreouzis, T. Phys. Rev. B 1995, 52, 13274. (3) Simmerer, J.; Gluesen, B.; Paulus, W.; Kettner, A.; Schuhmacher, P.; Adam, D.; Etzbach, K. H.; Siemensmeyer, K.; Wendorff, J. H. Adv. Mater. 1996, 8, 815. (4) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (5) For examples of the application of electron-transporting columnar discotic liquid crystals in organic electronic devices, see: Seguy, I.; Destruel, P.; Bock, H. Synth. Met. 2000, 111-112, 15. Percec, V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanovskaya, L.; Singer, K. D.; Balagurusamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp, A.; Spiess, H.-W.; Hudson, S. D.; Duan, H. Nature 2002, 419, 384. (6) Tokuhisa, H.; Era, M.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1995, 66, 3433. (7) Tokuhisa, H.; Era, M.; Tsutsui, T. Adv. Mater. 1998, 10, 404. (8) Adachi, C.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1989, 55, 1489.
reported. The oxadiazole moiety has been used as the discotic core of species forming columnar mesophases.13 Several studies have focused on threefold symmetric discoid (“starburst”) tris(oxadiazole) derivatives with benzene cores; these materials have been reported to form amorphous films,14,15 or nematic liquid crystals,16,17 rather than columnar mesophases. This article reports on the synthesis of a series of new tris(oxadiazole) species that form columnar discotic liquid-crystalline mesophases, focusing on the liquid-crystalline properties and electron mobility of 1,3,5-tris{5-[3,4,5-tris(octyloxy)phenyl]-1,3,4oxadiazol-2-yl}benzene (4a; Scheme 1). Results and Discussion We have synthesized a range of discoid oxadiazole molecules with benzene and triazine cores, as shown in Scheme 1; 4a was produced using standard synthetic procedures for oxadiazoles.18 The experimental details and characterizing data for 1a-4a are given in the Supporting Information (1a has been recently reported by other workers).13 In this article, we concentrate on the mesomorphic and electronic properties of 4a. Differential scanning calorimetry revealed phase transitions at 38 and 211 °C upon heating, with the transitions being observed at 205 and 21 °C upon cooling. The material was examined using polarizing optical microscopy on a hot stage to determine the nature of the phase transitions. Upon heating to 38 °C, the material melts, as was evidenced by a rapid change in birefringence (Figure 1). At temperatures above 211 °C, it becomes a low-viscosity (free(9) Mochizuki, H.; Hasui, T.; Tsutsumi, O.; Kanazawa, A.; Shiono, T.; Ikeda, T.; Adachi, C.; Taniguchi, Y.; Shirota, Y. Mol. Cryst. Liq. Cryst. 2001, 365, 129. (10) Tokuhisa, H.; Era, M.; Tsutsui, T. Chem. Lett. 1997, 303. (11) Tokuhisa, H.; Era, M.; Tsutsui, T. Appl. Phys. Lett. 1998, 72, 2639. (12) Mochizuki, H.; Hasui, T.; Kawamoto, M.; Shiono, T.; Ikeda, T.; Adachi, C.; Taniguchi, Y.; Shirota, Y. Chem. Commun. 2000, 1923. (13) Lai, C. K.; Ke, Y.-C.; Su, J.-C.; Shen, C.; Li, W.-R. Liq. Cryst. 2002, 29, 915. (14) Bettenhausen, J.; Strohriegl, P. Adv. Mater. 1996, 8, 507. (15) Bettenhausen, J.; Strohriegl, P.; Brutting, W.; Tokuhisa, H.; Tsutsui, T. J. Appl. Phys. 1997, 82, 4957. (16) Kim, B. G.; Kim, S.; Park, S. Y. Tetrahedron Lett. 2001, 42, 2697. (17) Kim, B. G.; Kim, S.; Park, S. Y. Mol. Cryst. Liq. Cryst. 2001, 370, 391. (18) Hetzheim, A.; Mo¨ckel, K. Adv. Heterocycl. Chem. 1966, 7, 183.
10.1021/la0341456 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/10/2003
Oxadiazoles
Langmuir, Vol. 19, No. 16, 2003 6535 Scheme 1. Synthesis of Discoid Oxadiazole-Containing Molecules
Figure 1. Photographs showing optical textures of a sample of 4a taken at (a) 35 and (b) 44 °C during the warming of the sample.
flowing) isotropic fluid. The optical texture observed between 38 and 211 °C (Figure 1b) is consistent with a discotic columnar mesophase19 and is quite different from the patterns seen for discotic nematic liquid crystals.20 The high viscosity of the phase is also consistent with a columnar mesophase. The similarities of the optical textures observed at temperatures above and below the melting point (Figure 1) indicate that the columnar structure of the mesophase is retained in the low-temperature phase (presumably a glassy or crystalline solid). A sample of 4a was prepared by cooling the material between two ITO-on-glass electrodes from above the isotropic melting point to room temperature (20 °C), with no attempt being made to align the stacks of the columnar material. The microscopy studies discussed above indicate that the solid material obtained in this way re(19) Chandrasekhar, S. In Advances in Liquid Crystals; Brown, G. H., Ed.; Academic Press: New York, 1982; Vol. 5, p 47. (20) Kumar, S.; Varshney, S. K. Angew. Chem., Int. Ed. Engl. 2000, 39, 3140.
tains the stacked structure of the columnar discotic mesophase. The electron mobility was measured in air by the conventional time-of-flight method.21 Figure 2a shows a typical normalized transient photocurrent measured as a function of time at room temperature with an applied field of 40 V µm-1. Because the transient photocurrents were dispersive, the transit times were determined from the double logarithmic plot of the transient photocurrent.22 The transit time in this case was defined as the point of time when the slopes of the photocurrent at short and long times undergo a significant change (see inset of Figure 2a). The electron drift mobility µ was calculated from the transit time tt according to the equation µ ) d2/ttV, where d is the sample thickness and V the applied voltage. As is shown in Figure 2b, the electron mobility was found to decrease with increasing electric field. This (21) Borsenberger, P. M.; Weiss, D. S. Organic Photoreceptors for Xerography; Marcel Dekker: New York, 1998. (22) Scher, H.; Montroll, E. W. Phys. Rev. B 1975, 12, 2455.
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Figure 2. Time-of-flight electron mobility data for a 20-µmthick sample of 4a at room temperature: (a) transient photocurrent measured at an applied field of 40 V µm-1 (inset, log-log plot of the photocurrent with the transit time marked by an arrow) and (b) field dependence of the electron mobility.
behavior is contrary to the usual behavior of amorphous organic photoconductors, in which mobility is often welldescribed by the disorder formalism of Borsenberger, Ba¨ssler, and co-workers,23-25 which predicts the functional dependence of the mobility, µ, on the applied field, E, to be given by
µ ) µ′ exp(βE1/2)
(1)
where µ′ is the zero-field mobility; β ) C[(σ/kBT)2 - Σ2]; σ and Σ characterize the energetic and positional disorders, respectively; kB is the Boltzmann constant; and C is a constant. β is typically positive for amorphous organic photoconductors, including several oxadiazoles (for example, see ref 15); negative values have been found in a few cases, including an oxadiazole-functionalized polymer.26 The implication of the observed negative β is that the positional disorder is unusually large or the energetic (23) Ba¨ssler, H. Phys. Status Solidi B 1993, 175, 15. (24) Borsenberger, P. M.; Magin, E. H.; van der Auweraer, M.; de Schryver, F. C. Phys. Status Solidi A 1993, 140, 9. (25) Ba¨ssler, H. Mol. Cryst. Liq. Cryst. 1994, 252, 11.
disorder is unusually low; in contrast, the stacking found in the columnar mesophases would suggest low positional disorder, and the dispersive transients seen for 4a suggest a relatively high degree of energetic disorder. The Borsenberger-Ba¨ssler model was, however, developed for three-dimensional transport, and it is not clear whether it is appropriate to apply this model to more anisotropic systems. Recently reported Monte Carlo simulations for a one-dimensional system, in which positional disorder is neglected,27 may be more appropriate for the current system. These suggest a low-field regime in which mobility decreases with increasing field and a higher-field regime in which the same functional dependence is found as that in eq 1 but in which β is given by β ) C[(σ/kBT)2 - Σ0(σ)], where Σ0(σ) is a phenomenological constant depending upon σ and can be positive or negative. In addition, electrically induced effects on the morphology or alignment could potentially complicate the field dependence of the data. Further study will be necessary to establish the most appropriate model for transport in these particular systems. In summary, we have synthesized a class of oxadiazoles that form columnar mesophases over large temperature ranges, with solid-liquid-crystal transitions close to room temperature; one example, 4a, has been found to exhibit high electron mobility in the solid state. It should be noted that the mobility measurements were conducted without any attempt being made to align the directors of the columnar material. We anticipate that improved processing, perhaps involving the use of surface-modifying alignment agents, may allow us to attain even higher electron mobilities. Furthermore, it is interesting to note that the molecules 4 have rather small cores compared to those of most other high-carrier-mobility columnar materials. Work is currently in progress to improve the alignment in these mesophases and to explore their utility in a variety of organic electronic applications. Acknowledgment. This material is based upon work supported in part by the STC Program of the National Science Foundation under Agreement No. DMR-0120967. We also gratefully acknowledge the NSF for a CAREER grant to B.K. and the Air Force Office of Scientific Research (through the LC MURI), the Office of Naval Research, the Defense Advanced Research Program Agency, and the National Renewable Energy Laboratory for financial support. We also thank the reviewers for helpful comments. Supporting Information Available: Experimental details for the synthesis of compounds 1a-4a and time-of-flight mobility measurements. This material is available free of charge via the Internet at http://pubs.acs.org. LA0341456 (26) Hou, S. J.; Gong, X.; Chan, W. K. Macromol. Chem. Phys. 1999, 200, 100. (27) Bleyl, I.; Erdelen, C.; Schmidt, H.-W.; Haarer, D. Philosoph. Magn. B 1999, 79, 463.
