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Questions and Answers
What is the difference between fluorescence and phosphorescence in terms of lifetime of the excited state?
What is the difference between fluorescence and phosphorescence in terms of lifetime of the excited state?
What is the process by which an excited molecule returns to its ground state?
What is the process by which an excited molecule returns to its ground state?
What is the term for the emission of EMR by atoms?
What is the term for the emission of EMR by atoms?
What is the process by which an electron in an excited singlet state relaxes to the ground state?
What is the process by which an electron in an excited singlet state relaxes to the ground state?
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What type of luminescence involves the emission of EMR by a chemical reaction?
What type of luminescence involves the emission of EMR by a chemical reaction?
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What is the diagram that shows the phenomena of fluorescence and phosphorescence?
What is the diagram that shows the phenomena of fluorescence and phosphorescence?
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What is the difference between singlet and triplet states?
What is the difference between singlet and triplet states?
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What is the term for the emission of EMR by molecules?
What is the term for the emission of EMR by molecules?
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What is the total spin of a molecule in its ground state?
What is the total spin of a molecule in its ground state?
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What is the term for the energy state in which the electron spins are parallel?
What is the term for the energy state in which the electron spins are parallel?
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Which type of deactivation process involves the conversion of an excited electron to a lower energy sublevel?
Which type of deactivation process involves the conversion of an excited electron to a lower energy sublevel?
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What is the process by which a molecule passes from a higher excited energy level to a lower excited energy level?
What is the process by which a molecule passes from a higher excited energy level to a lower excited energy level?
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What is the process that involves the interaction and energy transfer between the excited molecule and the solvent or other solutes?
What is the process that involves the interaction and energy transfer between the excited molecule and the solvent or other solutes?
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What is the process by which the spin of an excited electron is reversed?
What is the process by which the spin of an excited electron is reversed?
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What type of spectrum is obtained when the intensity of emitted radiation is plotted versus wavelength?
What type of spectrum is obtained when the intensity of emitted radiation is plotted versus wavelength?
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What is the condition that enhances the probability of intersystem crossing?
What is the condition that enhances the probability of intersystem crossing?
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What is observed when the excitation and emission spectra of a compound are plotted on the same chart?
What is observed when the excitation and emission spectra of a compound are plotted on the same chart?
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What is the quantum yield or quantum efficiency (f) of a highly fluorescent molecule?
What is the quantum yield or quantum efficiency (f) of a highly fluorescent molecule?
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What is the main reason for the loss of linearity in the plot of fluorescence intensity versus concentration at high concentrations?
What is the main reason for the loss of linearity in the plot of fluorescence intensity versus concentration at high concentrations?
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What type of functional groups are commonly associated with intense fluorescent behavior?
What type of functional groups are commonly associated with intense fluorescent behavior?
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What is the term for the displacement of the emission band to longer wavelengths?
What is the term for the displacement of the emission band to longer wavelengths?
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What is the ratio of the number of photons emitted to that absorbed in a fluorescent process?
What is the ratio of the number of photons emitted to that absorbed in a fluorescent process?
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What is the formula for calculating fluorescence intensity?
What is the formula for calculating fluorescence intensity?
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What is the term for the decrease in fluorescence due to the collision of excited molecules?
What is the term for the decrease in fluorescence due to the collision of excited molecules?
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What is the main limitation of the Mercury-arc lamp in spectrofluorometry?
What is the main limitation of the Mercury-arc lamp in spectrofluorometry?
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What is the purpose of the wavelength selector in a spectrofluorometer?
What is the purpose of the wavelength selector in a spectrofluorometer?
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What is the absorption maximum of fluorescein?
What is the absorption maximum of fluorescein?
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What is the effect of halogen substitution, especially with bromine and iodine, on fluorescence?
What is the effect of halogen substitution, especially with bromine and iodine, on fluorescence?
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What type of lamp is commonly used when spectrum scanning is required?
What type of lamp is commonly used when spectrum scanning is required?
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Why is it important to measure the emitted radiation at 90° from the path of the exciting beam?
Why is it important to measure the emitted radiation at 90° from the path of the exciting beam?
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Why do certain heterocyclic compounds, such as pyridine, thiophene, pyrrole, and furan, not fluoresce?
Why do certain heterocyclic compounds, such as pyridine, thiophene, pyrrole, and furan, not fluoresce?
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What is the effect of molecular structure on fluorescence?
What is the effect of molecular structure on fluorescence?
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What is the purpose of the cell in a spectrofluorometer?
What is the purpose of the cell in a spectrofluorometer?
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What is the reason for the increased fluorescence of 8-hydroxyquinoline when it forms a zinc complex?
What is the reason for the increased fluorescence of 8-hydroxyquinoline when it forms a zinc complex?
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What type of detector is commonly used in spectrofluorometry?
What type of detector is commonly used in spectrofluorometry?
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Why do nonrigid molecules tend to lose their absorbed energy through nonradiative means?
Why do nonrigid molecules tend to lose their absorbed energy through nonradiative means?
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What is the advantage of using fluorimetry for analyzing compounds that are intrinsically fluorescent?
What is the advantage of using fluorimetry for analyzing compounds that are intrinsically fluorescent?
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What is the purpose of the second wavelength selector in a spectrofluorometer?
What is the purpose of the second wavelength selector in a spectrofluorometer?
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What is the effect of the presence of a methylene group in fluorene on its fluorescence?
What is the effect of the presence of a methylene group in fluorene on its fluorescence?
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What is the general trend observed in the fluorescence of compounds with fused ring systems?
What is the general trend observed in the fluorescence of compounds with fused ring systems?
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Study Notes
Molecular Fluorescence Spectroscopy
- Molecular absorption spectrophotometry and molecular emission spectrophotometry are types of spectroscopy
- Photoluminescence is a process in which a substance absorbs light and emits it as a different wavelength
- Fluorescence and phosphorescence are types of photoluminescence
- Fluorescence has a shorter lifetime (10^-7 s) compared to phosphorescence (100 s)
Theory of Molecular Fluorescence
- An excited molecule can return to its ground state through radiative and non-radiative deactivation processes
- Radiative deactivation involves the emission of photons (fluorescence)
- Non-radiative deactivation involves vibrational relaxation, internal conversion, external conversion, and intersystem crossing
Jablonski Diagram
- The Jablonski diagram shows the energy levels of a molecule and the possible transitions between them
- Singlet and triplet states are types of energy states
- Singlet state: the total spin of the molecule is zero (S = 0)
- Triplet state: the total spin of the molecule is one (S = 1)
Deactivation Processes
- Radiative deactivation: involves the emission of photons
- Fluorescence: occurs when an electron in an excited singlet state relaxes to the ground state
- Phosphorescence: occurs when an electron in an excited triplet state relaxes to the ground state
- Non-radiative deactivation: involves the conversion of energy into heat
- Vibrational relaxation
- Internal conversion
- External conversion
- Intersystem crossing
Excitation and Emission Spectra
- Excitation spectra: a plot of the intensity of emitted light vs the wavelength of radiation used to excite the molecule
- Emission spectra: a plot of the intensity of emitted light vs the wavelength of emitted light
- The excitation and emission spectra have a mirror image relationship
Quantum Yield
- The quantum yield (f) is the ratio of the number of molecules that fluoresce to the total number of excited molecules
- f approaches unity (f = 1) for highly fluorescent molecules and zero (f = 0) for non-fluorescent molecules
Quantitative Fluorimetry
- F = 2.3 K ebc I0 is the equation for quantitative fluorimetry
- A plot of fluorescence intensity vs concentration is linear at low concentrations
- Deviation from linearity occurs at high concentrations due to self-absorption and self-quenching
Factors Affecting Fluorescence
- Molecular structure: compounds with aromatic functional groups, fused rings, and rigid planar structures exhibit intense fluorescence
- Halogen substitution can decrease fluorescence due to intersystem crossing
- Structural rigidity favors fluorescence
Instrumentation
- A spectrofluorometer consists of a source of energy, a wavelength selector, a cell, and a detector
- The source of energy can be a mercury-arc lamp or a high-pressure xenon lamp
- The wavelength selector can be a filter or a monochromator
- The cell is typically a tetragonal or cylindrical tube made of transparent material
- The detector is usually a photomultiplier type
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Description
This quiz covers the concepts of molecular fluorescence, spectroscopy, and absorption and emission of electromagnetic radiation. It includes topics such as UV-Vis spectrophotometry, photoluminescence, and atomic absorption and emission spectroscopy.
