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2007, 111, 27-31 Published on Web 12/10/2006
Self-Assembled Fibers of a Discotic Phthalocyanine Derivative: Internal Structure, Tailoring of Geometry, and Alignment by a Direct Current Electric Field Volodimyr Duzhko* and Kenneth D. Singer Department of Physics, Case Western ReserVe UniVersity, CleVeland, Ohio 44106 ReceiVed: October 23, 2006; In Final Form: NoVember 21, 2006
The self-assembly of discotic molecules of a metal-free alkoxy-substituted phthalocyanine derivative, 2,3,9,10,16,17,23,24-octakis(octyloxy)-29H,31H-phthalocyanine, in organic solvents was studied by optical microscopy, UV-vis spectroscopy, and scanning electron microscopy. These molecules self-assemble into fibers with well-defined internal structure and controlled external geometry on submicron length scales in dodecane solution. Spectroscopic evidence suggests a tilted molecular arrangement at low temperatures and a cofacial arrangement upon heating, which is consistent with the orthorhombic and ordered columnar hexagonal lattices for the solid or liquid crystalline mesophases of the neat material, respectively. At temperatures above the phase transition, molten side chains of the alkoxy-substituted phthalocyanine in a solvent with similar chemical structure facilitates the splitting of the fibers along their long axis into molecular fibers with characteristic diameters at the nanoscale. Strong interaction of the self-assembled fibers with a direct current electric field allows for their homeotropic or planar alignment on the conductive substrates for controlling the orientation needed for various optoelectronic device architectures. Our studies are consistent with a model describing the self-assembly of single molecular stacks due to π-π interaction of the phthalocyanine cores (tilted or cofacial), as well as additional stack aggregation due to side chains interdigitations as a result of van der Waals interaction.
The self-assembly of organic molecules in solution is nature’s mechanism for the construction of living cells. Imitation of the structures and functionality of highly efficient biological systems enables new concepts in the engineering of devices with specific photophysical properties, including nonlinear optical elements,1 electro-optic cells,2 electronic and optoelectronic devices,3,4 and sensors.5 Both structure and property arise from tailoring on a molecular scale. The synthesis of artificial building blocks and the availability of noncovalent bonding mechanisms in solution yield the opportunity to engineer complex functional architectures with specific properties and cost-effective implementation into efficient devices. Control of noncovalent assembly in solution and on surfaces has been the subject of a number of recent studies.6-10 The first examples of optoelectronic devices making use of controlled morphology on the molecular scale demonstrated record efficiencies of photovoltaic (PV) cells3,4 and the highest achieved mobilities in field-effect transistors (FETs). Their quasi-onedimensional molecular arrangement leads to efficient charge carrier transport. Controlled homeotropic or planar alignment of the molecular wires with respect to the substrates required for PV cells or FETs, respectively, is usually achieved using either amphiphilic molecules in Langmuir-Blodgett films11 or hydrophilic/hydrophobic surface treatments.12 External electric13 and magnetic fields14 are revealing practical approaches to controlling the alignment of discotic molecules. Small-molecular-weight organic materials of phthalocyanine (Pc) derivatives * Corresponding author. E-mail:
[email protected].
10.1021/jp066936x CCC: $37.00
as well as others, such as triphenylenes,15 porphyrines,14 perylene diimides,16,17 and perfluorinated dendrons,18 demonstrate satisfactory environmental and photostability, strong absorption of light in the visible spectral range, and efficient charge carrier transfer properties at intermolecular length scales.19,20 The columnar stacks of discotic Pc molecules with efficient overlap of π-orbitals along the stacking direction and low reorganization energy21 provide efficient anisotropic electronic transport channels along the molecular columns in the liquid crystalline mesophases with hole mobilities on the order of 10-1 cm2/Vs.22 Solution-based self-assembly of mesogens is a new fabrication tool and design concept for building complex functional architectures with tailored properties. In this letter, we demonstrate that (i) discotic molecules of a metal-free alkoxy-substituted Pc self-assemble into molecular fibers of well-defined internal structure in dodecane solution, and (ii) the external geometry of the fibers and their alignment on the substrates can be precisely controlled by processing conditions. Our work continues the line of fundamental studies aimed at building artificial heterogeneous molecular systems for specific photophysical and optoelectronic properties. The compound 2,3,9,10,16,17,23,24-octakis(octyloxy)29H,31H-phthalocyanine (H2Pc-OC8), consisting of a discshaped metal-free Pc core peripherally substituted with eight n-alkoxy side chains, was purchased from Aldrich (batch #462209). The synthesis of this compound has been previously described,23 and its chemical structure is shown in Figure 1. We purified the as-bought powders by columnar chromatography with chloroform. The solutions were prepared by weighing © 2007 American Chemical Society
28 J. Phys. Chem. C, Vol. 111, No. 1, 2007
Letters
Figure 1. Chemical structure of an H2Pc-OC8 molecule.
the powder H2Pc-OC8 and dissolving it in a given volume of a solvent with subsequent stirring for a few hours. Less concentrated solutions were prepared by dilution of the stock solution. Toluene or dodecane (all from Fisher, laboratory grade) was selected based on their chemical structure consisting of a π-conjugated system or linear hydrocarbon chain, respectively. In toluene, the solute is expected to interact with the π-conjugated system, while, in dodecane, the solute is expected to interact with the aliphatic tails. These differing interactions can produce different types of aggregates depending on the details of the relative interactions: solute-solute versus solute-solvent versus solvent-solvent. Dodecane and toluene were selected since they produced the structures desired for this study. This choice of solvents arose from our previous studies of the selfassembly of triphenylene-based molecular nanostructures and the role of intermolecular interactions between the solute and solvent molecules.24 Optical microscopy images were taken with an Olympus BX60 optical microscope. Spectroscopic studies were conducted with a Varian CARY500 UV-vis-NIR spectrophotometer in the absorption mode, normalized to an identical cuvette filled with a solvent. A set of quartz cuvettes with thicknesses of 10 and 1 mm as well as self-made glass cuvettes (Corning, Micro Slides) with a thickness of 120 µm were used. Direct current (dc) electric field alignment of self-assembled nanofibers in solution was studied using self-made cells of indium-tin oxide (ITO)-coated glass plates separated by an insulating cover glass (Corning) of 150 µm thickness assembled with epoxy. An SRC PS350 high-voltage power supply was used as the electric field source. All the measurements involving varying temperatures were done with a combination of the aforementioned equipment and an Instec STC200 hotstage, which provides temperature control with
