Assembly of Cobalt Phthalocyanine Stacks inside Carbon Nanotubes

Abstract

Organic molecules are rapidly finding their way into electronic architectures and photovoltaic devices. In recent years a pronounced increase in efficiency has been achieved, which is largely due to the virtually unlimited tunability of the building blocks. Indeed, by selecting specific molecular structures and chemically optimizing their interaction with substrates and each other we can dramatically affect the macroscopic properties of novel assemblies. Metallo-organic molecules, like the phthalocyanines, are of particular interest as they can also carry localized magnetic moments. Phthalocyanines are a class of stable, blue-green synthetic pigments which can be synthesized with a variety of metal ions at their center. They are related to the biologically important porphyrin family, which includes molecules like heme and chlorophyll. Within molecular nanotechnology their strong optical absorption is already being exploited in photovoltaic devices containing phthalo-fullerene mixed phases. However, they also display other useful bulk optical properties including dichroism and luminescence, and are used in gas-detection. We have focused our experiments on cobalt phthalocyanine (CoPc) which consists of a planar carbon and nitrogen macrocycle with a single cobalt ion (see Fig. 1, inset). The cobalt ion has a partially filled 3d-orbital and is magnetically active. With this in mind, restricting phthalocyanine assemblies to one or two dimensions raises a variety of exciting issues as to what will happen to the electronic structure and hence to their optical, magnetic and transport properties. A particularly powerful method of forming one-dimensional molecular arrays is the encapsulation in a carbon nanotube. The selection of carbon nanotubes as the container structures is motivated in part by the fact that nanotubes can act as a confining, as well as a shielding, environment. Since we are dealing with planar molecules the precise internal stacking order will be important in assessing the physical properties of this assembly. Previously, planar molecules with a large aspect ratio, such as rectangular perylene-3,4,9,10-tetracarboxylic-3,4,9,10dianhydride (PTCDA) have been shown to align virtually parallel to the nanotube axis, whereas the elliptical fullerene C70 will align their long axis with the nanotube axis in narrow nanotubes (diameter 13.6 A), but stand up in wider nanotubes (≥ 14.9 A). Magnetic effects have been observed recently in the encapsulation of azafullerene (C59N). [11]