Molecular Luminescence Spectrometry

Introduction to Luminescence Spectrometry

  • Excited molecules emit light as they return to their ground state.
  • Photoluminescence
    • Excited state obtained by a molecule absorbing a photon.
    • Two main types:
    • Fluorescence
    • Phosphorescence
  • Chemiluminescence
    • Excited state obtained by a chemical reaction.

Fluorescence and Phosphorescence Theory

  • General Properties:
    • Can occur in simple or complex structures and in solid, liquid, or gas states.
    • If emitted light has the same wavelength as the absorbed light, it is called resonance radiation.
  • Resonance Fluorescence
    • More often, emitted light is at a longer wavelength (BB) than absorbed light.
    • Shift in wavelength is termed the Stokes shift.

Energy States in Fluorescence and Phosphorescence

  • Ground Singlet State (S0)
  • Excited Singlet State (S1)
  • Excited Triplet State (T1)
  • Processes involved:
    • Internal Conversion
    • Vibrational Relaxation
    • Intersystem Crossing
    • Absorption
    • Fluorescence
    • Phosphorescence

Variables Affecting Fluorescence and Phosphorescence

  • Quantum Yield or Quantum Efficiency (Φ):

    • Defined as the ratio of the number of molecules that luminesce to the total number of excited molecules.
    • Mathematical expression:
    • \Phi = \frac{kf}{kf + ki + k{ec} + k{ic} + k{pa} + k_a}
    • Where:
    • $k_f$: fluorescence rate constant
    • $k_i$: internal conversion rate constant
    • $k_{ec}$: rate constant for energy transfer
    • $k_{ic}$: intersystem crossing rate constant
    • $k_{pa}$: photon absorption rate constant
    • $k_a$: other rate constants

  • Transition Types:

    • Rare transitions: σ^*
      ightarrow σ
    • More common transitions: л^*
      ightarrow л and л^*
      ightarrow n

  • Quantum Efficiency and Transition Type:

    • Fluorescence primarily from л^*
      ightarrow \pi
    • Phosphorescence primarily from л^*
      ightarrow n

Fluorescence and Structure

  • Most intense fluorescence occurs in:
    • Aromatic compounds with low p^*
      ightarrow p transitions.
    • High conjugation yields increased fluorescence.
    • Most unsubstituted aromatics fluoresce, with increase in ring complexity enhancing fluorescence.
    • Simple heterocycles do not fluoresce; however, fused-ring structures do.
  • Effect of Substitution on Benzene Ring
    • Shifts in absorption and fluorescence peaks, influencing efficiency.

Structural Rigidity Effects

  • An example layout:
    • $C{13}H{10}$ (fluorene)
    • $C{12}H{10}$ (biphenyl)
    • Other derivatives

Temperature, Solvent Effects, and pH Impact

  • Effects of pH on fluorescence:
    • Observed through resonance forms of aniline and its anilinium ion.

Concentration and Quenching Effects

  • As concentration increases, fluorescence intensity changes:
    • Quenching: reduction of fluorescence intensity due to energy transfer or absorption by other molecules.

Spectra in Luminescence Analysis

  • Excitation Spectra
    • Scan excitation at a certain emission wavelength (BB_em).
  • Emission Spectra
    • Scan emission at a certain excitation wavelength (BB_ex).
  • Synchronous Spectra
    • Simultaneous scanning of both excitation and emission wavelengths.
  • Total Luminescence Spectra
    • Incorporates all luminescence data.

Instrumentation

  • Key components:
    • Sample
    • Excitation Source
    • Emission Selector
    • Transducer: converts light into electrical signals.
    • Electronics/Computer Data System for analysis.

Applications of Luminescence Spectrometry

  • Offers lower limits of detection compared to absorption spectrophotometry.
  • Recognized as one of the most sensitive analytical techniques available.
  • However, precision and accuracy are poorer than those of absorption spectrophotometry.

Inorganic and Organic Species Analysis

  • Direct Methods: Form fluorescent chelators to measure emission.
  • Indirect Methods: Measure the quenching impact of substances being determined.
  • Organic compounds with aromatic rings are typically fluorescent, whereas aromatic heterocycles are often phosphorescent.

Chemiluminescence

  • Highly sensitive analytical method applicable to gases (e.g., atmospheric pollutants like ozone and nitrogen oxides) and liquids (e.g., reactions with strong oxidizing agents).