Spectroscopy and Molecular Fluorescence
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Questions and Answers

What is the difference between fluorescence and phosphorescence in terms of lifetime of the excited state?

  • Fluorescence is immediate, phosphorescence is delayed
  • Fluorescence is delayed, phosphorescence is immediate
  • Fluorescence has a longer lifetime, phosphorescence has a shorter lifetime
  • Fluorescence has a shorter lifetime, phosphorescence has a longer lifetime (correct)

    • What is the process by which an excited molecule returns to its ground state?

  • Deactivation or relaxation (correct)
  • Photoluminescence
  • Absorption of EMR
  • Emission of EMR

    • What is the term for the emission of EMR by atoms?

  • Molecular absorption spectrophotometry
  • Luminescence
  • Molecular emission spectrophotometry
  • Atomic emission spectroscopy (correct)

    • What is the process by which an electron in an excited singlet state relaxes to the ground state?

    <p>Fluorescence</p> Signup and view all the answers

    What type of luminescence involves the emission of EMR by a chemical reaction?

    <p>Chemiluminescence</p> Signup and view all the answers

    What is the diagram that shows the phenomena of fluorescence and phosphorescence?

    <p>Jablonski Diagram</p> Signup and view all the answers

    What is the difference between singlet and triplet states?

    <p>Singlet states have paired electrons, triplet states have unpaired electrons</p> Signup and view all the answers

    What is the term for the emission of EMR by molecules?

    <p>Molecular emission spectrophotometry</p> Signup and view all the answers

    What is the total spin of a molecule in its ground state?

    <p>0</p> Signup and view all the answers

    What is the term for the energy state in which the electron spins are parallel?

    <p>Triplet state</p> Signup and view all the answers

    Which type of deactivation process involves the conversion of an excited electron to a lower energy sublevel?

    <p>Vibrational relaxation</p> Signup and view all the answers

    What is the process by which a molecule passes from a higher excited energy level to a lower excited energy level?

    <p>Internal conversion</p> Signup and view all the answers

    What is the process that involves the interaction and energy transfer between the excited molecule and the solvent or other solutes?

    <p>External conversion</p> Signup and view all the answers

    What is the process by which the spin of an excited electron is reversed?

    <p>Intersystem crossing</p> Signup and view all the answers

    What type of spectrum is obtained when the intensity of emitted radiation is plotted versus wavelength?

    <p>Emission spectrum</p> Signup and view all the answers

    What is the condition that enhances the probability of intersystem crossing?

    <p>The lowest vibrational energy level of the lowest excited singlet state is almost identical to that of the triplet excited state</p> Signup and view all the answers

    What is observed when the excitation and emission spectra of a compound are plotted on the same chart?

    <p>A mirror image relationship between the excitation and emission spectra</p> Signup and view all the answers

    What is the quantum yield or quantum efficiency (f) of a highly fluorescent molecule?

    <p>f = 1</p> Signup and view all the answers

    What is the main reason for the loss of linearity in the plot of fluorescence intensity versus concentration at high concentrations?

    <p>Both self-absorption and self-quenching</p> Signup and view all the answers

    What type of functional groups are commonly associated with intense fluorescent behavior?

    <p>Aromatic functional groups</p> Signup and view all the answers

    What is the term for the displacement of the emission band to longer wavelengths?

    <p>Stock's shift</p> Signup and view all the answers

    What is the ratio of the number of photons emitted to that absorbed in a fluorescent process?

    <p>Quantum yield</p> Signup and view all the answers

    What is the formula for calculating fluorescence intensity?

    <p>F = 2.3 K eb c I0</p> Signup and view all the answers

    What is the term for the decrease in fluorescence due to the collision of excited molecules?

    <p>Self-quenching</p> Signup and view all the answers

    What is the main limitation of the Mercury-arc lamp in spectrofluorometry?

    <p>It cannot be used for spectrum scanning</p> Signup and view all the answers

    What is the purpose of the wavelength selector in a spectrofluorometer?

    <p>To select the most intense emitted wavelength</p> Signup and view all the answers

    What is the absorption maximum of fluorescein?

    <p>498 nm</p> Signup and view all the answers

    What is the effect of halogen substitution, especially with bromine and iodine, on fluorescence?

    <p>It decreases the fluorescence</p> Signup and view all the answers

    What type of lamp is commonly used when spectrum scanning is required?

    <p>High pressure xenon lamp</p> Signup and view all the answers

    Why is it important to measure the emitted radiation at 90° from the path of the exciting beam?

    <p>To minimize the error due to scattering of light from the walls of the cell</p> Signup and view all the answers

    Why do certain heterocyclic compounds, such as pyridine, thiophene, pyrrole, and furan, not fluoresce?

    <p>Their lowest transition is an n-p* system, which is rapidly converted to a triplet, preventing fluorescence</p> Signup and view all the answers

    What is the effect of molecular structure on fluorescence?

    <p>The quantum yield increases with the increase of fused rings</p> Signup and view all the answers

    What is the purpose of the cell in a spectrofluorometer?

    <p>To contain the sample</p> Signup and view all the answers

    What is the reason for the increased fluorescence of 8-hydroxyquinoline when it forms a zinc complex?

    <p>Due to the increase in molecular rigidity</p> Signup and view all the answers

    What type of detector is commonly used in spectrofluorometry?

    <p>Photomultiplier</p> Signup and view all the answers

    Why do nonrigid molecules tend to lose their absorbed energy through nonradiative means?

    <p>Due to their ability to undergo vibrational relaxation or degradation</p> Signup and view all the answers

    What is the advantage of using fluorimetry for analyzing compounds that are intrinsically fluorescent?

    <p>It is more sensitive than absorption spectroscopy</p> Signup and view all the answers

    What is the purpose of the second wavelength selector in a spectrofluorometer?

    <p>To select the most intense emitted wavelength</p> Signup and view all the answers

    What is the effect of the presence of a methylene group in fluorene on its fluorescence?

    <p>It increases the fluorescence</p> Signup and view all the answers

    What is the general trend observed in the fluorescence of compounds with fused ring systems?

    <p>The fluorescence increases with the increase of fused rings</p> Signup and view all the answers

    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.

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