Photoluminescence and Spectrofluorimetry

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Questions and Answers

What is the Stokes shift a measure of?

  • Intensity of fluorescence relative to absorption
  • Change in energy between ground and excited states
  • Rate of thermal energy loss during excitation
  • Difference in wavelength between absorption and fluorescence spectra (correct)

Which of the following best describes the order of energy differences during the processes of absorption, fluorescence, and phosphorescence?

  • ΔE (fluorescence) > ΔE (phosphorescence) > ΔE (absorption)
  • ΔE (absorption) > ΔE (fluorescence) > ΔE (phosphorescence) (correct)
  • ΔE (fluorescence) > ΔE (absorption) > ΔE (phosphorescence)
  • ΔE (phosphorescence) > ΔE (absorption) > ΔE (fluorescence)

In terms of wavelengths, which of the following statements is true?

  • λphosphorescence < λfluorescence < λabsorption
  • λfluorescence < λabsorption < λphosphorescence
  • λfluorescence = λabsorption = λphosphorescence
  • λabsorption < λfluorescence < λphosphorescence (correct)

What happens to energy upon the transition from absorption to fluorescence?

<p>Some energy is lost as thermal energy before fluorescence (B)</p> Signup and view all the answers

How does non-radiative decay influence fluorescence and phosphorescence?

<p>Decreased non-radiative decay increases fluorescence and phosphorescence (C)</p> Signup and view all the answers

What term describes the emission of light from an electronically excited state when stimulated by light?

<p>Photoluminescence (D)</p> Signup and view all the answers

Which of the following is NOT a type of photoluminescence?

<p>Thermoluminescence (D)</p> Signup and view all the answers

In which relaxation process is energy lost as thermal energy to the solvent?

<p>Vibrational Relaxations (C)</p> Signup and view all the answers

What reflects the transition between states with the same spin quantum numbers?

<p>Internal Conversion (A)</p> Signup and view all the answers

Which process involves emission of energy in the form of electromagnetic radiation?

<p>Radiative Process (C)</p> Signup and view all the answers

What is the role of external conversion in energy relaxation?

<p>Conversion to kinetic energy through collisions (A)</p> Signup and view all the answers

Which of the following statements about chemiluminescence is true?

<p>It occurs due to a chemical reaction. (D)</p> Signup and view all the answers

What phenomenon does the Jablonski diagram primarily illustrate?

<p>Relaxation processes of excited states (C)</p> Signup and view all the answers

What characterizes the lifetime of fluorescence compared to phosphorescence?

<p>Fluorescence ceases immediately after excitation. (C)</p> Signup and view all the answers

What is the process of internal conversion (IC) primarily known for?

<p>Relaxing electrons through vibrational states nonradiatively. (D)</p> Signup and view all the answers

Which transition must occur for phosphorescence to happen?

<p>Intersystem crossing. (A)</p> Signup and view all the answers

In which condition does fluorescence typically occur?

<p>In liquid mediums. (C)</p> Signup and view all the answers

What determines the intensity of emitted fluorescence?

<p>The concentration of the analyte. (B)</p> Signup and view all the answers

What is a key difference in the emissions from excitation and emission spectra?

<p>Emission spectra are analyzed while fixing the excitation wavelength. (B)</p> Signup and view all the answers

Which of the following best describes molecular rigidity's effect on photoluminescence?

<p>Increased rigidity can enhance fluorescence properties. (C)</p> Signup and view all the answers

What is the relationship between the shortest wavelength in the fluorescence spectrum and the absorption spectrum?

<p>The shortest wavelength in fluorescence is the longest wavelength in absorption. (A)</p> Signup and view all the answers

Flashcards

Luminescence

The emission of light from a substance when it returns to its ground state from an excited state.

Photoluminescence

A type of luminescence where the excitation source is light.

Fluorescence

A type of photoluminescence where the excited molecule quickly returns to its ground state, emitting light.

Phosphorescence

A type of photoluminescence where the excited molecule takes longer to return to its ground state, emitting light over time.

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Chemiluminescence

A type of luminescence where the excitation source is a chemical reaction.

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Nonradiative Processes

Processes where absorbed energy is dissipated as heat or transferred between electronic states without light emission.

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Radiative Processes

Processes where absorbed energy is released as electromagnetic radiation (light).

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Jablonski Diagram

A diagram that illustrates the energy levels and transitions involved in fluorescence and phosphorescence.

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Stokes Shift

The difference in wavelength between the peak of the absorption and fluorescence spectra for the same electronic transition.

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What happens to the energy absorbed by the molecule?

Some energy is lost to the surroundings as heat, usually through collisions with solvent molecules.

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How are absorption, fluorescence, and phosphorescence wavelengths related?

The wavelength of absorption is shorter than the wavelength of fluorescence, which is shorter than the wavelength of phosphorescence.

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Absorption

Process where a molecule transitions from its ground state to an excited state by absorbing energy, usually light.

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Relaxation (Decay)

Process where an excited molecule returns to its ground state, releasing energy.

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Vibrational Relaxation

A type of relaxation where the excited molecule loses energy through vibrations, typically within 10-12 seconds.

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Internal Conversion (IC)

A non-radiative relaxation process where the excited molecule transitions through vibrational states to reach the ground state, usually within 10-12 seconds.

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Intersystem Crossing

A process where an excited molecule changes its spin state from singlet to triplet, leading to phosphorescence.

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Mirror Image Excitation and Emission Spectra

The relationship where the shortest wavelength in the fluorescence spectrum corresponds to the longest wavelength in the absorption spectrum.

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Study Notes

Photoluminescence and Spectrofluorimetry

  • Photoluminescence is the emission of light from an electronically excited state (LUMO) returning to the ground state (HOMO).
  • If the excitation source is light, it's called photoluminescence, which includes fluorescence and phosphorescence.
  • If the excitation source is a chemical reaction, it's called chemiluminescence.
  • Fluorescence occurs when an excited molecule returns to its ground state emitting light. This process is fast (10-9 to 10-6 seconds).
  • Phosphorescence is the emission of light from an excited state, but the process is slower (10-3 to 1 seconds) because it involves a change in electron spin.
  • Spectrofluorimetry is a technique used to measure fluorescence.
  • The intensity of emitted light is proportional to the analyte concentration.

Relaxation Processes

  • Non-radiative relaxation processes: These are processes where the excited molecule loses energy without emitting light.
    • Vibrational relaxation: Energy lost as heat to the solvent.
    • Internal conversion: The excited electron moves to a lower energy level in the same electronic state.
    • External conversion: Energy is transferred to other molecules by collisions.
    • Intersystem crossing: A transition occurs between states with different spin quantum numbers.
  • Radiative relaxation processes: These processes involve the emission of light.
    • Fluorescence: The excited molecules return to the ground state emitting light promptly.
    • Phosphorescence: The excited molecules emit light after a longer time, often involving a change in electron spin.

Jablonski Diagram

  • A Jablonski diagram is a simplified energy level diagram illustrating electronic and vibrational energy levels of molecules.
  • It shows singlet and triplet states, as well as transitions between them.
  • It graphically depicts vibrational relaxation, internal conversion, fluorescence, intersystem crossing, and phosphorescence.

Stokes Shift

  • The Stokes shift is the difference in wavelength between the absorption and fluorescence spectra.
  • The Stokes shift shows that the emitted light has a longer wavelength than the absorbed light.
  • This is because the molecule loses some energy as heat to the solvent before emitting light.

Fluorescence vs. Phosphorescence

  • Fluorescence has a shorter lifetime, typically less than 10-9 seconds.
  • Phosphorescence has a longer lifetime, typically seconds or longer.
  • Fluorescence occurs at or near room temperature.
  • Phosphorescence tends to occur at low temperatures.
  • Fluorescence is usually more intense than phosphorescence.

Spectrometric Methods of Analysis (UV-Visible Regions)

  • Different types of energy transitions associated with various regions of the electromagnetic spectrum (X-rays, UV/Vis, Infrared, Microwave, Radiofrequencies).
  • Methods include molecular absorption, atomic absorption, molecular luminescence (emission), and atomic emission.

Interpretation of Energy Diagram

  • Absorption: transitioning from ground state to an excited state.
  • Relaxation (Decay): excited state to ground state, through vibrational relaxations and internal conversions, releasing energy as heat.
  • Emission: Release of light from excited state (fluorescence or phosphorescence).
  • Fluorescence: fast process (10-9 to 10-6 seconds).
  • Phosphorescence: slower process (10-3 to 1 seconds), depending on the flipping of electron spin during intersystem crossing.

Spin Orientations

  • Spin orientations are crucial in determining the type and speed of relaxation processes.
  • Singlet and triplet states, involving paired or unpaired electrons, respectively, affect the likelihood and duration of fluorescence and phosphorescence.

Rates of Absorption and Emission

  • A table showing the timescale for various transitions (e.g., light absorption, internal conversion, vibrational relaxation, intersystem crossing, fluorescence, phosphorescence, non-radiative decay) is presented.

Differences between Fluorescence and Phosphorescence

  • Fluorescence lifetimes (Ï„) are significantly shorter.
  • Fluorescence typically occurs at room temperature.
  • Fluorescent emission is often in the visible region.
  • Phosphorescence can persist for longer periods and usually occurs at lower temperatures.

Spectrofluorometry Technique

  • The intensity of emitted fluorescence light is directly proportional to the analyte concentration.
  • One measures excitation wavelengths (λex) and emission wavelengths (λem).
  • An instrument called a spectrofluorimeter is used for such measurements.
  • Spectrofluorimetry charts are used to plot the intensities of the emitted light vs. the emission and excitation wavelength.

Effect of Molecular Structure

  • Molecular rigidity promotes fluorescence.
  • Rigid molecules such as fluorescein exhibit stronger fluorescence compared to similar but more flexible molecules like phenolphthalein..

Assignment

  • The assignment asks for a comparison between spectrophotometers and spectrofluorimeters, spectrophotometry and spectrofluorimetry, as well as comparing fluorescence and phosphorescence..

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