Toshiya Okazaki

Coaxially Stacked Coronene Columns inside Single-Walled Carbon Nanotubes†

One of the most interesting features of molecular materials is the fact that their physical properties change with the arrangement of the molecules as well as with the properties of the molecules themselves. Self-organization is an efficient pathway through which organic molecules assemble to form well-ordered nanometer-scale objects that are hardly synthesized by conventional chemical reactions. In these systems, two or more molecules are held together and are assembled by means of intermolecular (noncovalent) bonding such as ion–dipole or dipole–dipole interactions, hydrogen bonding, hydrophobic interactions, or p–p stacking. Such molecular self-organization and recognition processes are among the most practical and effective means to facilitate a “bottom-up” approach in nanotechnology. In general, however, fabrication of well-defined organic nanowires or other types of onedimensional (1D) nanostructures with controllable size and morphology is not as far advanced as for their inorganic counterparts. 2] Single-walled carbon nanotubes (SWCNTs) can offer a suitable interior space for the accommodation of molecules. Nanostructures produced by the incorporation of such molecules into SWCNTs are expected to exhibit several superior features. For example, because the diameter of SWCNTs can be adjusted to the size of the molecules, wellordered molecular arrangements more than a micrometer in length can be easily produced. The synthesized molecular arrangements are also expected to be strong and flexible under mechanical strain because the nanotube templates sustain the structure. Furthermore, the already synthesized nanostructures are isolated from reactive species by the tube wall, which leads to the superior durability of the encapsulated molecules. Herein we demonstrate such a 1D SWCNT-templated nanostructure using planar p-conjugated molecules (coronenes). Encapsulated coronenes form nano-scale columns in a way that differs from 3D solid coronenes, thus resulting in electronic and optical properties peculiar to the 1D structure. The production of well-ordered molecular assemblies in SWCNTs can be expected to inspire novel approaches for the synthesis of low-dimensional molecular materials with unique physical properties. The self-organized 1D structure of coronenes inside SWCNTs (coronenes@SWCNTs) was achieved through vapor-phase doping (Figure 1). Coronene confinement in nanometer spaces produces a characteristic columnar structure. Figure 1b,d and Figure S2a (see the Supporting Information) show high-resolution TEM (HRTEM) images of the coronene columns inside SWCNTs; coronene molecules are positioned along the tube axis at almost regular intervals. The formation of the columnar structure is due to two dominant factors: p–p stacking between coronenes and the interaction between the encapsulated coronene and the host SWCNT. In the monoclinic crystal, coronenes have an interplanar distance of 0.34 nm with a tilt angle of about 468 relative to the stacking axis. On the other hand, within SWCNTs, statistical analysis of the HRTEM images shows that the distance between the molecular planes is 0.35 0.03 nm (Figure S2b) and the angle between the molecular plane and the tube axis (q) is approximately 778 (Figure S2c, 1c). Although the interplanar distances are identical between 1D and solid coronenes within the experimental error, a substantial difference is observed for the tilt angle. The observed structure that involves coronene columns was supported by total energy calculations for zig-zag (n, 0) SWCNTs. Figure 1e shows the stabilization energy (DE) of coronenes inside SWCNTs as a function of tube diameter (d), where the molecular plane of coronenes was set to be [*] Prof. T. Okazaki, Y. Iizumi, Dr. S. Okubo, Dr. Z. Liu, Dr. K. Suenaga, Y. Tahara, Dr. M. Yudasaka, Prof. S. Iijima Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST) Tsukuba 305-8565 (Japan) Fax: (+ 81)29-861-6241 E-mail: toshi.okazaki@aist.go.jp

Host-guest interactions in azafullerene (C59N)-single-wall carbon nanotube (SWCNT) peapod hybrid structures

The effect of azafullerene encapsulation on the electronic states of single-wall carbon nanotubes (SWCNTs) is investigated; UV-vis-NIR absorption and photoluminescence spectroscopy shows that the interaction between SWCNTs and the encapsulated azafullerenes is originated from the weak intermolecular forces, which suggests a lack of strong doping effect such as electron transfer between them.