Patterning of Robust Self-Assembled n-type Hexaazatrinaphthylene-Based Nanorods and Nanowires by Microcontact Printing Hin-Lap Yip, ² Jingyu Zou, ²,§ Hong Ma, ² Yanqing Tian, ² Neil M. Tucker, ‡ and Alex K.-Y. Jen* ,²,‡ Department of Materials Science and Engineering and Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195, and Department of Materials Science, Sichuan UniVersity, Chengdu 610064, People’s Republic of China Received July 11, 2006; E-mail: ajen@u.washington.edu. The fabrication of functional nanostructures using self-assembled one-dimensional (1-D) π-conjugated molecules represents a new method to create devices with single-crystal-like properties. This approach has been proposed as an alternative for carbon nanotubes and inorganic wires-based nanoscale optoelectronic devices. 1 However, there are still several important hurdles that need to be addressed in order to use these materials in active devices. These challenges include a better control of the dimension of the nanostructures and structural robustness of these materials for processing and patterning on surfaces. So far, most of the supra- molecular nanowires reported in the literature are based on p-type π-conjugated materials, and it is essential to develop complementary n-type nanowires for fabricating complete functional devices. 1c Here we report the self-assembling properties of alkylamide- tethered hexaazatrinaphthylenes (HATNA) as the first step toward this direction. The structures of molecules 1 and 2 studied are illus- trated in Figure 1. HATNA is an electron-deficient discotic π-conju- gated molecule that possesses n-type transporting characteristics. It has a C 3h symmetry with well-defined face-to-face columnar packing in its crystal structure. 2a HATNA-based materials have also been theoretically 2b and experimentally 2c-d proven to have high charge mobility. Compound 1 has good solubility in CHCl 3 ; how- ever, it gels easily in mixed solvents such as CHCl 3 /hexane (v/v ) 1:1) owing to enhanced amide-amide hydrogen bonding in a less polar environment, which is one of the main driving forces for the formation of supramolecular nanofibers. 3 The morphological study reveals that 1 forms self-organized nanowires with a diameter of ∼20 nm and a length over 10 μm in the gel (see Supporting Information). To create shape-persistent nanostructures, the fixation of supra- molecular nanowires through chemical or photochemical reaction- induced cross-linking or polymerization at their self-assembled state have been demonstrated. 4 In our study, a similar approach was appli- ed to compound 2 which possesses diacetylene groups that can be converted into polydiacetylene via photopolymerization to create robust nanostructures. The self-assembling properties of 2 are very different from those of 1 in CHCl 3 . The introduction of π-conjugat- ed diacetylene groups further enhances the π-π stacking between the molecules. The shortened flexible alkyl groups also reduce the solvation of the molecules in CHCl 3 . As a result, the binding force between the molecules increases, and these molecules start to aggregate at a lower concentration (∼1 × 10 -5 M in CHCl 3 ). The absorption spectrum of 2 in CHCl 3 shows three characteristic peaks at 407, 386, and 306 nm (Figure 2). The first two peaks correspond to the transitions between the 0-0 and 0-1 states, 5 respectively, and the main peak corresponds to the transition to a higher level. By increasing the concentration from 2 × 10 -6 to 1 × 10 -4 M, all peaks decrease in absorbance. The ratio between the 0-0 and the 0-1 transitions decreases from 1.12 to 0.97, and the main peak absorbance also hypsochromically shifted 4 nm from 306 to 302 nm which indicates the formation of H-aggregates. 5 By fitting the spectroscopic data using the isodesmic (or equal K) model, 6 a plot of fraction of aggregation (R aggr ) as a function of concentration can be established. The calculated average binding constant K for 2 (in CHCl 3 ) from three different wavelengths is 8 × 10 3 M -1 . Controlling the dimension of self-assembled nanostructure in solid-state is essential for further integration of these functional materials into a miniaturized device. Guided by the plot of R aggr versus concentration in Figure 2, we can correlate the solid-state organization of 2 with their self-assembled states in solution through a morphological study of the nanostructures deposited from their corresponding aggregates in solution. The AFM images in Figure 3 show an increase in the columnar stack length with concentration, while the stack diameters in all cases remain unchanged. When molecule 2 was deposited from solutions with low R aggr (Figure 3a-c), nanorod structures with tunable lengths could be obtained. At high R aggr (Figure 3d), nanowires with length over 10 μm were formed. The photo-cross-linking of the diacetylene units in 2 was conducted by exposing the self-assembled nanowires to an 18 W UV lamp in both solid and solution phase. For photo cross-linking in solid phase, the nanowires were drop cast from a 1 × 10 -4 M CHCl 3 solution onto a quartz substrate, and the process was monitored by the time-dependent UV-vis absorption spectrum. The appearance of the absorption peaks at 590 and 536 nm in Figure 4a correspond to the formation of polydiacetylene. 7 Polymerization ² Department of Materials Science and Engineering, University of Washington. ‡ Department of Chemistry, University of Washington. § Department of Materials Science, Sichuan University. Figure 1. Structure of HATNAs. Compound 1 and 2 are tethered with an amide-dodecyl group and an amide-diacetylene-octanyl group, respectively. Figure 2. Concentration-dependent UV-vis spectra from compound 2 in CHCl3 at room temperature. The inset shows the fraction of aggregation as a function of concentration of 2 in CHCl3. Published on Web 09/16/2006 13042 9 J. AM. CHEM. SOC. 2006, 128, 13042-13043 10.1021/ja064934k CCC: $33.50 © 2006 American Chemical Society