Functionalization of Single-Walled Carbon Nanotubes with Coumarin-Labeled Polymers

Abstract

Single-walled carbon nanotubes (SWNTs) are a new class of materials that have recently attracted a great deal of interest because of their unique structural, mechanical, and electronic properties. Also, SWNTs have a high potential for a number of technological applications, including molecular electronics, emissive devices, and photovoltaic devices. To fully utilize their unique properties, control of the solubility, processibility, and functionality of SWNTs is required. Therefore chemical functionalization of SWNTs using a variety of methods, in either covalent or noncovalent manner, has been developed to produce soluble nanotube composites coupled with various chemical moieties. To explore the possibility of making potential soluble nanotube-based materials for solar cells, SWNTs were functionalized with organic chromophore-labeled polymers via a radical coupling process. The organic chromophore was used to absorb light to produce photo-induced electrons, while the polymer chains were used for improving the solubility of SWNTs. These novel chromophore-labeled polymers were made by stable free radical polymerization (SFRP), either using a synthetic chromophore-functionalized styrenic monomer or by derivatizing well-defined polystyrenes. Specifically, the chromophores employed in this investigation were commercially available 7-hydroxycoumarin and coumarin-343. In order to carry out fluorescence studies of SWNT-coumarin composites systematically, various factors were probed by (1) altering polystyrene lengths between the SWNT and the coumarin; (2) changing the distribution of coumarins along the polymer chain, in the form of either a block or random copolymer; (3) placing single coumarins on the surface of SWNTs. All of these resulting polymer functionalized SWNTs were found to be soluble in certain organic solvents such as CHCl3. Different absorption behaviors have been observed for SWNTs functionalized with 7-hydroxycoumarin containing copolymers. Fluorescence was still observable for all of these composites, and the pi-pi interactions between coumarins and nanotubes were believed to be responsible for the broadening of emission bands of the resulting composites.