Molecular Luminescence Spectroscopy Notes

Molecular Luminescence Spectroscopy

  • Luminescence: Emission of light after nonthermal excitation.
  • Types of Luminescence:
  • Photoluminescence: Emission after excitation by light/photons.
  • Fluorescence: Immediate light emission upon excitation.
  • Phosphorescence: Delayed light emission due to triplet state transitions.
  • Chemiluminescence: Light emission from a chemical reaction.
  • Electrochemiluminescence: Light emission generated by electrochemical reactions.
  • Bioluminescence: Light produced by living organisms.
  • Others include Triboluminescence, sonoluminescence, radioluminescence.

Molecular Photoluminescence

  • Electronic States:

  • Singlet States: Denoted as S₁, where all electrons are paired.

  • Triplet State: Denoted as T₁, where two electrons are unpaired.

  • Process:

  • Absorption of photons results in transitions from the ground state to excited singlet or triplet states.

  • After absorption, fluorescence (singlet to singlet transition) or phosphorescence (singlet to triplet transition) can occur.

Nonradiative Processes Competing with Fluorescence

  1. Vibrational Relaxation:
  • Transition to the lowest vibrational state before fluorescence.
  • Generally, the wavelength absorbed is shorter than that emitted (Stokes shift).
  1. Internal Conversion:
  • Nonradiative transitions between electronic states of the same multiplicity, often singlet to singlet.
  • Requires overlap in vibrational states between them.
  1. Predissociation & Dissociation:
  • High vibrational energy can cause bond breaking.
  1. External Conversion (Collisional Deactivation):
  • Energy transfer to another molecule, leading to quenching of fluorescence.
  1. Intersystem Crossing:
  • Transition between states of different multiplicities (e.g., singlet to triplet).
  • Can be slow due to the spin flip but may be expedited by interactions.

Fluorescence Quantum Yield (φf)

  • Formula: φf = (Quant. Emitted)/(Quant. Absorbed)
    φf = kf / (kf + knr)
    Where:
  • kf = fluorescence rate constant.
  • knr = sum of all nonradiative rate constants.
  • Larger kf values indicate more efficient fluorescence processes due to shorter lifetimes.

Transitions Causing Fluorescence

  • Common Transitions:
  • π → π* transitions are most common and effective for fluorescence.
  • n → π* and σ* transitions are generally less efficient, often resulting in predissociation.

Effects of Conjugation on Fluorescence

  • Increased conjugation can enhance fluorescence and produce longer emission wavelengths.
  • Examples:
  • Benzene: φf = 0.3, λEm ≈ 290 nm
  • Naphthalene: φf = 0.2, λEm ≈ 320 nm
  • Anthracene: φf = 0.005, λEm ≈ 400 nm

Effects of pH on Fluorescence

  • Protonation changes orbitals and affects fluorescence:
  • Quinine is fluorescent only in its protonated state.
  • Aniline is fluorescent; however, the anilinium ion is not.

Effect of Structural Rigidity on Fluorescence

  • More rigid structures tend to exhibit higher fluorescence efficiency.
  • Structural rigidity limits internal conversion to the ground state, enhancing emission.

Effects of Temperature & Viscosity

  • Lower Temperature: Increases fluorescence by limiting nonradiative processes.
  • Higher Viscosity: Also increases fluorescence by reducing collisional deactivation.

Deviations from Linearity in Fluorescence

  • Deviations from linearity occur due to:
  • Inner filter effects (loss of intensity due to sample absorbance).
  • Self-quenching at higher concentrations.

Instrumentation for Fluorescence Measurement

  • Fluorescence Instrumentation Components:

  • Light source, usually low-pressure Hg lamps or lasers.

  • λ selectors and detectors to measure emission intensity.

  • Arrangement often has excitation and emission angles at 90° to each other.

  • Excitation & Emission Spectra:

  • Excitation spectrum holds emission λ constant while varying excitation λ.

  • Emission spectrum keeps excitation λ constant and varies emission λ.

Self-Absorption and Self-Quenching

  • Self-absorption: Occurs when the emitted wavelength can still be absorbed by the same analyte, reducing observed fluorescence intensity.
  • Self-quenching: At high concentrations, certain analyte molecules can suppress the fluorescence of others nearby.

Phosphorescence and Chemiluminescence

  • Phosphorescence: Characterized by a lower rate constant (kp); typically difficult to measure accurately due to rapid nonradiative decay.

  • Chemiluminescence: Similar to fluorescence but is driven by chemical reactions rather than photon absorption, like in luminol lighting reactions in the presence of certain catalysts.

  • Overall, understanding these mechanisms and their effects on fluorescence is critical for applications in analytical chemistry and materials science.