Molecular state and distribution of fullerenes entrapped in sol-gel samples

A novel synthetic method that can encapsulate fullerene molecules (pure C60, pure C70, or their mixture) over a wide range of concentrations ranging from micromolar to millimolar in hybrid glass by a sol-gel method without any time-consuming, complicated, and unwanted extra steps (e.g., addition of a surfactant or derivatization of the fullerenes) has been successfully developed. The molecular state and distribution of encapsulated fullerene molecules in these sol-gel samples were unequivocally characterized using newly developed multispectral imaging techniques. The high sensitivity (single-pixel resolution) and ability of these instruments to record multispectral images at different spatial resolutions (approximately 10 microm with the macroscopic instrument and approximately 0.8 microm with the microscopic instrument) make them uniquely suited for this task. Specifically, the imaging instruments can be used to simultaneously measure multispectral images of sol-gel-encapsulated C60 and C70 molecules at many different positions within a sol-gel sample in an area either as large as 3 mm x 4 mm (with the macroscopic imaging instrument) or as small as 0.8 microm x 0.8 microm (with the microscopic instrument). The absorption spectrum of the fullerene molecule at each position can then be calculated either by averaging the intensity of a 15 x 15 square of pixels (which corresponds to an area of 3 mm x 4 mm) or from the intensity of a single pixel (i.e., an area of about 0.8 microm x 0.8 microm), respectively. The molecular state and distribution of fullerene molecules within sol-gel samples can then be determined from the calculated spectra. It was found that spectra of encapsulated C60 and C70 measured at five different positions within a sol-gel sample were similar not only to one another but also to spectra measured at six different times during the sol-gel reaction process (from t = 0 to 10 days). Furthermore, these spectra are similar to the corresponding spectra of monomeric C60 or C70 molecules in solution. Similarly, spectra of sol-gel samples containing a mixture of C60 and C70 were found to be the same at five different positions, as well as similar to spectra calculated from an average of the spectra of C60 and C70 either encapsulated in a sol-gel or in solution. It is evident from these results that C60 and C70 molecules do not undergo aggregation upon encapsulation into a sol-gel but rather remain in their monomeric state. Furthermore, entrapped C60 and C70 molecules in their monomeric state were distributed homogeneously throughout the entire sol-gel samples. Such a conclusion can be readily, quickly, and easily obtained, not with traditional spectroscopic techniques based on the use of a single-channel detector (absorption, fluorescence, infrared, Raman) but rather with the newly developed multispectral imaging technique. More importantly, the novel synthetic method reported here makes it possible, for the first time, to homogenously entrap monomeric fullerene molecules (C60, C70, or their mixture) in a sol-gel at various concentrations ranging from as low as 2.2 mM C60 (or 190 microM C70) to as high as 4.2 mM C60 (or 360 microM C70).