Langmuir 2006, 22, 7947-7951
7947
Controlled Self-Assembly of Triphenylene-Based Molecular Nanostructures Volodimyr Duzhko, Hefei Shi, and Kenneth D. Singer* Department of Physics, Case Western ReserVe UniVersity, CleVeland, Ohio 44106
Alexander N. Semyonov and Robert J. Twieg Department of Chemistry, Kent State UniVersity, Kent, Ohio 44242 ReceiVed March 21, 2006. In Final Form: July 18, 2006 Molecular nanostructures of the disc-shaped molecule hexapentyloxytriphenylene have been fabricated on length scales ranging from 30 nm to 1.5 µm following self-assembly arising from π-π interactions in organic solvents. The size and density of the molecular nanostructures deposited onto glass and indium tin oxide-coated glass substrates were characterized by atomic force microscopy. Dynamic light scattering and spectroscopic evidence of predeposition aggregation in solution are presented, suggesting that the nanostructures are organized in solution and then deposited onto the substrate. Correlations between the relative solvent polarity and the size of molecular nanostructures as well as between the solute concentration in dilute solutions and their density on the substrate are discussed.
Introduction Low-molecular-weight organic materials are gaining increasing interest following the demonstration of efficient organic lightemitting diodes1 and for their potential application in cost-effective organic photovoltaic cells2 and field-effect transistors.3 Because of the limited range of exciton diffusion in organic materials, typically up to 10 nm in polymers and 100 nm in small-molecule materials,4 access to a donor-acceptor interface on the length scale of the exciton diffusion length is an important limiting factor for the efficiency of optoelectronic devices.5 Consequently, nanoscale blends of electron-donating and electron-accepting materials exhibiting the proper energy relationship between the lowest unoccupied molecular orbitals and the highest occupied molecular orbitals of the constituent materials might lead to a substantial increase in the efficiency of photovoltaic cells.6 Therefore, control of the morphology of heterogeneous molecular systems on the length scale of the exciton diffusion length and, in particular, creation of columnar arrays, would improve the performance of electronic and optoelectronic devices.7,8 Additionally, such structures would facilitate an understanding of the fundamental photophysical processes of exciton/charge carrier generation, electronic transport, and recombination in heterogeneous molecular systems. Molecular self-assembly is an increasingly important theme in the construction of functional organic materials. Control of noncovalent assembly in solution and on surfaces has been the subject of a number of recent studies.7,9-13 Disc-shaped molecules * Corresponding author. E-mail:
[email protected]. (1) Forrest, S. R. Nature 2004, 428, 911. (2) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183. (3) Dimitrakopoulos C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (4) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 3693. (5) Halls, J. J. M.; Friend, R. H. In Clean Electricity from PhotoVoltaics; Archer, M., Hill, R., Eds.; Imperial College Press: London, 2001. (6) Halls, J. J. M.; Cornil, J.; dos Santos, D. A.; Silbey, R.; Hwang, D.-H.; Holmes, A. B.; Bredas, J. L.; Friend, R. H. Phys. ReV. B 1999, 60, 5721. (7) Schmidt-Mende, L.; Fechtenkoetter, A.; Muellen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (8) Yang, F.; Stein, M.; Forrest, S. R. Nat. Mater. 2005, 37. (9) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (10) Wu¨rthner, F.; Thalacker, C.; Sautter, A.; Scha¨rtl, W.; Ibach, W.; Hollricher, O. Chem.sEur. J. 2000, 6, 3871. (11) Sinks, L. E.; Rybtchinski, B.; Iimura, M.; Jones, B. A.; Goshe, A. J.; Zuo, X. B.; Tiede, D. M.; Li, X. Y.; Wasielewski, M. R. Chem. Mater. 2005, 17, 6295.
of hexapentyloxytriphenylene (HAT5) consisting of the triphenylene core peripherally substituted with flexible alkoxy side chains are known to self-assemble into spatially ordered systems with hexagonal columnar symmetry in a liquid-crystalline mesophase.14,15 The columnar stacks of discotic molecules with efficient overlap of π orbitals provide efficient anisotropic electronic transport channels along the molecular columns.16 These materials are emerging as a new class of organic optoelectronic systems because of their combination of selfassembly and optoelectronic properties. In addition to selfassembly in liquid-crystalline mesophases, solution-based selfassembly will enlarge the number of design and fabrication tools for building complex functional architectures. A number of selfassembly mechanisms (H bonding, metal coordination, and van der Waals interactions) allow programmable engineering of organic heterogeneous self-assemblies on the molecular scale. Our aim in this letter is threefold: (1) We demonstrate the self-assembly of organic discotic molecules of HAT5 into molecular nanostructures due to π-π interactions on various size scales in a number of organic solvents such as chloroform (CF), tetrahydrofuran (THF), and toluene (TL). (2) Dynamic light scattering and spectroscopic studies are used to test the hypothesis that the self-assembly of discotic monomers occurs in solution prior to deposition, with the resulting nanostructures deposited onto the substrate. (3) We studied the size distribution and density of the molecular nanostructures formed on the glass and ITO-coated glass substrates using tapping-mode atomic force microscopy. The examples provided are illustrative of the range of sizes and density of the nanostructures on various substrates and indicate methods for their control. Experimental Section Materials. The HAT5 used in this study was prepared by VOCl3mediated trimerization of 1,2-bis-pentyloxybenzene followed by (12) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139. (13) Mamdouh, W.; Uji-i, H.; Ladislaw, J. S.; Dulcey, A. E.; Percec, V.; De Schryver, F. C.; De Feyter, S. J. Am. Chem. Soc. 2006, 128, 317. (14) Adam, D.; Schuhmacher, P.; Simmerer, J.; Haussling, L.; Siemensmeyer, K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Nature 1994, 371, 141. (15) Bushby, R. J.; Boden, O. R. Curr. Opin. Colloid Interface Sci. 2002, 7, 343. (16) Duzhko, V.; Semyonov, A.; Twieg, R. J.; Singer, K. D. Phys. ReV. B 2006, 73, 064201.
10.1021/la060763l CCC: $33.50 © 2006 American Chemical Society Published on Web 08/15/2006
7948 Langmuir, Vol. 22, No. 19, 2006
Letters
Figure 1. Chemical structure of the HAT5 molecule. realkylation. Purification steps included multiple chromatographies and recrystallizations that have already been described elsewhere.16 The chemical structure of the HAT5 molecule is shown in Figure 1. The molecule consists of a triphenylene core peripherally substituted with six n-pentyloxy groups. For the fabrication of selfassembled nanostructures, HAT5 was dissolved in an organic solvent (CF, THF, or TL, all from Aldrich) at various weight percents (0.1, 0.01, or 0.001 wt %) and stirred for several hours. The alkoxy groups enhance the solubility of these molecules in low-polarity organic solvents. The solutions were then drop cast onto substrates (glass or ITO-coated glass) tilted from the horizontal at an angle of 30° and allowed to flow. We note that this procedure is essential for obtaining a large-area homogeneous material distribution over the substrate as compared with drop casting onto a horizontal substrate. Dropping onto a tilted substrate ensured that the meniscus line moved homogeneously across the substrate surface as it descended. Measurements of receding contact angles using the Wilhelmy method with pure TL, THF, and CF on glass and ITO-coated glass substrates revealed ∼0° contact angles for all of these solvents, indicating that the solvents fully wet the surfaces. The glass and ITO-coated glass substrates were cleaned prior to deposition in an ultrasonic bath with alkonox (Alkonox), deionized water, acetone, methanol, and 2-propanol (Aldrich) with subsequent drying in a nitrogen stream. Following deposition, the samples were heated to 100 °C for 30 min and then cooled to room temperature at a rate of 60 °C/h. All of the measurements were made under ambient conditions within 1 to 2 h following the final fabrication step. Methods. The morphology of HAT5 nanostructures was studied with a multimode scanning probe microscope (Nanoscope IV, Digital Instruments) in the tapping-mode regime. A silicon cantilever (Nanoworld NCH) with a nominal radius of
