The success of CD-RTP with PAHs depends on the formation of a proposed three-component complex of CD, lumiphor, and external heavy atom. The internal cavity of the CD must be large enough to accommodate both the lumiphor, which must fit at least partially inside, and the heavy atom.
Figure 2 shows the luminescence spectra for phenanthrene included in β-CD, with and without the addition of the heavy-atom DBE. Because the heavy-atom moiety enhances spin-orbit coupling and the rate of inter-system crossings, addition of DBE serves to quench much of the fluorescence while intensifying the phosphorescence.
The physical dimensions of the CD cavities, relative to the geometry of the individual PAH, determined the most effective CD form for each analyte. For example, although the phenanthrene molecule will fit inside the cavity of the α-CD, which has an inner diameter of 0.6 nm, there is apparently not enough room remaining to accommodate the DBE molecule. Both phenanthrene and DBE molecules, however, can be included in the larger β-CD cavity, which has an inner diameter of 0.78 nm. Figure 3 compares the luminescence spectra for the same solution of phenanthrene and DBE in α- and β-CD. Anthracene, which is larger than phenanthrene, showed only weak phosphorescence in βCD, but γ-CD, with an inner diameter of 1.0 nm, induced stronger emissions.
The geometrical limitations imposed by CDRTP may be exploited as a selective means of analysis based on molecular size and shape. For example, it is possible to discriminate between naphthalene, which can be induced to phosphoresce in β-CD, and its bulkier derivative 1-phenylnaphthalene, which shows only fluorescence under the same conditions.
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