Blue-green photoluminescence from pyridine-C60 adduct
Introduction
Three dimensional conjugation of the π electron system in C60 makes it a potentially attractive chromophore and fluorophore. However, underivatized pure C60 only fluoresces very weakly in the 620–900 nm region with a quantum yield of 3.2×10−4 and a lifetime of 1.2 ns that are independent of solvents and excitations [1], [2], [3]. Recently, strong photoluminescence and electroluminescence were observed from the derivatized C60[4], [5], leading to a successful fabrication of a fullerene-based white light LED [5].
Optical properties of the dissolved C60 are affected by the nature of solvents [6], [7], [8], [9], [10], some of which form adducts with C60 in the presence of light [11], [12]. Here, we report for the first time that the purple-pinkish color of a freshly prepared C60/pyridine solution changes gradually to brown color and stabilizes thereafter. Unlike the purple-pinkish C60/pyridine solution, which shows no `abnormal' photoluminescence, the brown C60/pyridine solution emits a bright blue-green photoluminescence centered at ca. 470 nm upon UV excitation. We attribute the observed photoluminescence from the brown C60/pyridine solution to new zwitterionic species formed by the adduction of the solvent pyridine molecule(s) to C60, as revealed by -NMR and mass spectra, as well as the solvatochromic effect in the presence of protic solvent (vide infra).
Section snippets
Results and discussion
C60 (99.9%) was dissolved in pyridine (99.9%, Fisher Scientific) sonically. The freshly prepared solution exhibits a purple-pinkish color, consistent with the previous report [6]. However, the color of this solution turns gradually from purple-pinkish into brown and stabilizes thereafter. The presence of light, especially UV radiation, speeds up the color change process. The observed color change of C60/pyridine solution can be accounted for by the change of UV-visible absorption spectra shown
Conclusions
In summary, we observed a color change from purple-pinkish to brown in the C60/pyridine solution. We showed that the brown solution emits a bright blue-green luminescence upon UV excitation. Both the color change and the blue-green luminescence are attributed to the formation of the zwitterionic adducts between pyridine and C60, as supported by its -NMR and mass spectra, as well as the strong solvatochromic effect of the photoluminescence in the presence of protic solvent. We also observed
Acknowledgements
This work is supported by the grants from RGC-HK, HKUST, and NNSFC-RGC. The authors thank Lin Guo, Tingbin Wen and Laura Cao for their helpful discussion and the acquirement of NMR and mass spectra.
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