Page 44 - Instrumental Analysis - Pharm D Clinical- 07-PA403
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Spectrofluorimetry
• Effect of structural rigidity: fluorescence is particularly favored in rigid
molecules. Rigidity lowers the rate of non-radiative relaxation to the point
where relaxation by fluorescence has time to occur.
• For example, the quantum efficiency of fluorene is nearly 1.0 while that of
biphenyl is about 0.2. This is due to the increased rigidity provided by the
bridging methylene group in fluorene.
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The oxygen bridge in fluorescein provides rigidity that is not present
in phenolphthalein. Therefore, fluorescein is highly fluorescent, while
phenolphthalein is almost non-fluorescent.
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