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Questions and Answers
What is the Stokes shift a measure of?
What is the Stokes shift a measure of?
Which of the following best describes the order of energy differences during the processes of absorption, fluorescence, and phosphorescence?
Which of the following best describes the order of energy differences during the processes of absorption, fluorescence, and phosphorescence?
In terms of wavelengths, which of the following statements is true?
In terms of wavelengths, which of the following statements is true?
What happens to energy upon the transition from absorption to fluorescence?
What happens to energy upon the transition from absorption to fluorescence?
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How does non-radiative decay influence fluorescence and phosphorescence?
How does non-radiative decay influence fluorescence and phosphorescence?
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What term describes the emission of light from an electronically excited state when stimulated by light?
What term describes the emission of light from an electronically excited state when stimulated by light?
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Which of the following is NOT a type of photoluminescence?
Which of the following is NOT a type of photoluminescence?
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In which relaxation process is energy lost as thermal energy to the solvent?
In which relaxation process is energy lost as thermal energy to the solvent?
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What reflects the transition between states with the same spin quantum numbers?
What reflects the transition between states with the same spin quantum numbers?
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Which process involves emission of energy in the form of electromagnetic radiation?
Which process involves emission of energy in the form of electromagnetic radiation?
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What is the role of external conversion in energy relaxation?
What is the role of external conversion in energy relaxation?
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Which of the following statements about chemiluminescence is true?
Which of the following statements about chemiluminescence is true?
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What phenomenon does the Jablonski diagram primarily illustrate?
What phenomenon does the Jablonski diagram primarily illustrate?
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What characterizes the lifetime of fluorescence compared to phosphorescence?
What characterizes the lifetime of fluorescence compared to phosphorescence?
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What is the process of internal conversion (IC) primarily known for?
What is the process of internal conversion (IC) primarily known for?
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Which transition must occur for phosphorescence to happen?
Which transition must occur for phosphorescence to happen?
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In which condition does fluorescence typically occur?
In which condition does fluorescence typically occur?
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What determines the intensity of emitted fluorescence?
What determines the intensity of emitted fluorescence?
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What is a key difference in the emissions from excitation and emission spectra?
What is a key difference in the emissions from excitation and emission spectra?
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Which of the following best describes molecular rigidity's effect on photoluminescence?
Which of the following best describes molecular rigidity's effect on photoluminescence?
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What is the relationship between the shortest wavelength in the fluorescence spectrum and the absorption spectrum?
What is the relationship between the shortest wavelength in the fluorescence spectrum and 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
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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.
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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 (τ) 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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Description
Explore the fascinating concepts of photoluminescence, including fluorescence and phosphorescence. Understand the differences between these processes and how spectrofluorimetry measures emitted light. This quiz covers the mechanics of electronic states and energy relaxation processes.
