Photoluminescence and Spectrofluorimetry

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 (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 (λ_ex) and emission wavelengths (λ_em).
• 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..