3: Molecular Luminescence
- Page ID
- 111543
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After completing this unit the student will be able to:
- Describe the difference between a singlet and triplet state.
- Draw an energy level diagram and identify the transitions that correspond to absorption, fluorescence, internal conversion, radiationless decay, intersystem crossing and phosphorescence.
- Explain why phosphorescence emission is weak in most substances.
- Draw a diagram that shows the layout of the components of a fluorescence spectrophotometer.
- Describe the difference between a fluorescence excitation and emission spectrum.
- Draw representative examples of fluorescence excitation and emission spectra.
- Describe a procedure for measuring phosphorescence free of any interference from fluorescence.
- Justify why fluorescence measurements are often more sensitive than absorption measurements.
- Describe the meaning and consequences of self-absorption.
- Identify variables including the effect of pH that can influence the intensity of fluorescence.
- Identify the features that occur in organic molecules that are likely to have high fluorescent quantum yields.
- Compare two molecules and determine which one will undergo more collisional deactivation.
Luminescent methods refer to a family of techniques in which excited state species emit electromagnetic radiation. Among luminescent methods are various sub-categories that include the processes of fluorescence, phosphorescence, chemiluminescence, bioluminescence and triboluminescence. Among these different sub-categories, fluorescence spectroscopy is by far the most common technique used for analysis purposes. You are no doubt familiar with fluorescent lights. This unit will allow you to understand how such a light works.
- 3.5: Quantum Yield of Fluorescence
- The quantum yield is a ratio that expresses the number of species that fluoresce relative to the total number of species that were excited. Earlier we said that anything that reduces the number of excited state species that undergo fluorescence is said to quench the fluorescence. The expression for the quantum yield will depend on the rate constants for the different processes that can occur for excited state species.