Spectroscopic and optical imaging studies of fullerene complexes and single-walled carbon nanotubes
Photophysical and optical imaging studies were performed on fullerene molecular complexes and individual single-walled carbon nanotubes (SWNTs). First, we investigated the reversible dimerization reaction of the newly discovered isomer of C60 oxide, [5,6]-open C60O. This oxide was found to undergo spontaneous dimerization in solution to form a new isomer of C120O2, which was structurally and photophysically characterized. This C120O2 compound can be easily converted back to its [5,6]-C60O precursor under optical irradiation. These compounds represent a unique fullerene system in which composition can be easily controlled through adjustment of concentration, temperature, and light exposure. Further, we describe a new aspect of fullerene-porphyrin interactions. The effect manifests itself in a vast increase of the fullerene triple-singlet radiative rate. Strong emission, that is C70 phosphorescence, appears in the near-infrared (NIR) wavelength region. We carefully characterized C 70-palladium octaethylporphyrin (PdOEP) supramolecular interactions and also found a similar effect with other fullerenes. The complex formation mechanism and its photophysical characterization are described. Third, we present NIR-fluorescence microscopy as a versatile method to visualize and study individual SWNTs. We demonstrate observation of individual nanotubes in a variety of environments including solid polymeric films and liquid media. SWNT identities are confirmed with spectroscopic and optical anisotropy measurements. Also, we demonstrate optical length measurements of individual nanotubes that were at least several micrometers long. Emission spectra of different parts of a single nanotube have been examined for the first time. Finally, we discuss future uses of SWNTs as novel nanoscale fluorescence markers. They supersede conventional fluorophores, which are fluorescent dyes or quantum dots (QD), in terms of both optical anisotropy and photostability. The relative ease of their detection allows one to perform a number of studies at the single nanoparticle level. Unrestricted translational and rotational motions of SWNTs are recorded and analyzed. The observed variation of translational diffusion coefficients reflects the length distribution of SWNTs in the sample. Rotational diffusion constants were found to correlate well with SWNTs translational coefficients. Promising directions for future research are outlined.
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