Fluorescence of Cyclic Adducts of Fullerene
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ABSTRACT
Five cyclic adducts
C 60 C 4 H 6, C 60 C 5 H 6 , C 60 N1-C0 2,
C60NCO 2Et, and C60CHCO 2Et have been
synthesized and their absorption and fluorescence spectral properties investigated and compared with those of the parent. Breaking the structural symmetry of C60 apparently resulted in enhancing the quantum yield two to three-folds in some adducts and thus made fluorescence much easier to be measured at room temperature. New absorption bands and altered fluorescence spectra were observed in the adducts. The Stokes' shifts of the adducts were small, about 4-5 nrm, compared to 68 nm for the parent compound. All the adducts exhibited a single fluorescence lifetime about the same as C60 (ca. 1.3 ns). Aliphatic solvents had little influence on the absorption or fluorescence spectral profile except on the molar absorptivity whereas aromatic and polar solvents strongly interacted with the adducts causing a peak broadening effect.
INTRODUCTION The electronic absorption and fluorescence properties of C 6 0 and related compounds are valuable to a fundamental understanding of the electro-optical and photochemical properties of fullerene molecules. The absorption spectra of C60 under various conditions (e.g., solid film, in different solvents or temperature) have been characterized.' However, luminescence properties of C6 o have only been studied mostly at low temperature. 2 Because of the very low fluorescence quantum yield (ca. 2 x 10-4), the fluorescence spectra of fullerene in high resolution at room temperature has only been reported recently.3 The lifetime was found to be ca. 1.2 ns for C60. In the present investigation, we studied the absorption properties of C60 and five adducts and observed that all six compounds fluoresced at room temperature. Presumably, because the modification reduced the symmetry of the fullerene structure from C2h to C 2v or C., and converted some vibronic-forbidden states to the allowed states, increasing their populations and transition probabilities. The absorption, excitation, and emission spectra, quantum yields, and lifetimes of these compounds in various solvents were examined. The results presented here may provide a better understanding of the factors, e.g. structure moieties and solvent interaction, that might affect and favor the fluorescence emission of fullerene compounds at room temperature.
EXPERIMENTAL C60 (M 99.5%, MER Corp.) was used without further purification. The adducts (Fig. 1) C60NHCO 2 (IV), C60NCO 2Et (V), and C60CHCO 2Et (VI) were synthesized according to the published procedures and purified on Buckyclutcher I columns (Regis Chenm) by using the HPLC method. 4 All solvents used were of reagent or spectroscopic grade. The purity of all fuillerene adducts used for the spectroscopic work was double-checked C60C4H 6 (I1), C 60C 5H6 (In),
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Mat. Res. Soc. Symp. Proc. Vol. 359 @1995 Materials Research Society
by using 13C-NMR and mass spectrometry. UV-vis absorption spectra were measured in a scanning spectrophotometer (Hitachi U-2000). The fluorescence
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