Highly Ordered Thin Films of Octasubstituted Phthalocyanines Paul Smolenyak, ² Rebecca Peterson, ² Ken Nebesny, ² Michael To 1 rker, ‡ David F. O’Brien,* ,² and Neal R. Armstrong* ,² Contribution from the Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721, and Institute fu ¨ r Angewandte Photophysik, Technical UniVersita ¨ t Dresden, Dresden, Germany ReceiVed May 6, 1999 Abstract: Phthalocyanines (Pc) modified at eight positions with benzyloxyethoxy groups (CuPc(OC 2 OBz) 8 and H 2 Pc(OC 2 OBz) 8 ) form discotic mesophases in the bulk and self-assemble into cofacial rodlike aggregates in monolayers. These Pc aggregates form close packed arrays which are simple to process into highly coherent multilayer thin films. When these self-organizing molecular systems are compressed on the surface of an LB trough, the Pc columns align parallel to the compression barriers and form a rigid bilayer film (ca. 5.6 nm thick). Sections of these bilayer films can be mechanically removed from the trough surface with complete retention of integrity of the remaining film. This unprecedented behavior appears to arise from the multiple π-π interactions between adjacent Pcs, resulting in unusually strong noncovalent interactions. AFM and STM studies of monolayer and bilayer films show that they consist of Pc columns which are coherent over distances of 50-100 nm. Thin films consisting of up to 15 bilayers (ca. 84 nm) can be quickly built by a “mechanical stamping” process, and small-angle X-ray scattering characterization of these film materials confirms their unusual coherence. Large electrical and optical anisotropies are observed in multilayer films of CuPc(OC 2 - OBz) 8 and H 2 Pc(OC 2 OBz) 8 which are comparable to those previously reported for thin film assemblies based on side-chain-modified silicon phthalocyanine polymers. Introduction Molecules which form discotic mesophases with significant long-range order and anisotropic electrical and optical properties continue to be sought for applications in electrical or optical logic circuits, electroluminescence devices, and electrochromic displays, among others. 1-16 Side-chain-modified phthalocyanines form a major class of discotic mesophase materials, and there are multiple examples of these molecules which aggregate or polymerize into rodlike assemblies with extremely interesting optical and electrical properties. Tailoring conductivity, optical band gap, luminescence energy and efficiency in thin films and “fibers” of these materials, through changes in their composition and molecular architecture, has been widely demonstrated. 1,6,8-10,14 Ordered organic thin films of these materials can often show large anisotropies in these properties, 1,2,5,8-10 which might be their most important advantage, if simple processing conditions can sustain these anisotropies in the macroscopic material. Highly ordered thin films have been previously created using prepolymerized silicon phthalocyanines (PcPS), modified at either the 4 or 8 positions on each Pc with solubilizing side chains, forming rodlike units, each unit containing up to 100 Pcs. 1-5 These polymeric rods pack with excellent uniformity in orientation, parallel to the vertical dipping direction used for Langmuir-Blodgett thin film fabrication, and their large optical and electrical anisotropies form the basis for comparison of these properties in the materials reported here. The disk-shaped monomeric 2,3,9,10,16,17,23,24-octakis(2- benzyloxyethoxy)phthalocyaninato copper, and its metal free analogue (CuPc(OC 2 OBz) 8 and H 2 Pc(OC 2 OBz) 8 (Figure 1)), have recently been shown to exhibit a discotic mesophase in the bulk material, and a unique degree of self-assembly into coherent rodlike structures on the Langmuir trough. 12,13 Al- though many different side-chain-modified Pcs and other discotic molecules have been synthesized, few of these side chains have been terminated with aromatic functionality, and for those that have, the thin film properties either were not reported or did not show the properties of the Pcs reported here. 10,15,16 We are aware of only one other monomeric side- chain-modified Pc (possessing chiral centers in the side chain), 8 * To whom correspondence should be addressed. E-mail: nra@ u.arizona.edu; dfobrien@u.arizona.edu. ² University of Arizona. ‡ Technical Universita ¨t Dresden. (1) Wegner, G. Mol. Cryst. Liq. 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