13.2 Fluorescence and phosphorescence

Luminescence phenomena are fascinating light emission processes that occur when materials absorb energy. Fluorescence and phosphorescence are two key types, differing in their emission timescales and underlying mechanisms. These processes have wide-ranging applications in science and technology.

From bioimaging to display technologies, luminescence plays a crucial role in modern life. Understanding the factors affecting luminescence efficiency, like quantum yield and quenching, is essential for optimizing its applications. This knowledge opens doors to innovative uses in various fields.

Luminescence Phenomena

Mechanisms of fluorescence and phosphorescence

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• Fluorescence and phosphorescence are types of photoluminescence
  • Photoluminescence involves emission of light from a substance after absorbing photons (light)
• Fluorescence
  • Occurs when a substance emits light after absorbing light or other electromagnetic radiation
  • Involves emission from excited singlet states
  • Typically has a short lifetime ranging from nanoseconds ($10^{-9}$ to $10^{-7}$ seconds)
• Phosphorescence
  • Occurs when a substance emits light after absorbing light or other electromagnetic radiation
  • Involves emission from excited triplet states
  • Has a longer lifetime than fluorescence, ranging from milliseconds to hours
• Underlying mechanisms involve
  • Absorption of photons excites electrons to higher energy states
  • Relaxation of electrons back to the ground state results in the emission of photons
  • Intersystem crossing enables transition between singlet and triplet states

Fluorescence vs phosphorescence characteristics

• Fluorescence characteristics
  • Short lifetime (nanoseconds)
  • Emission from singlet states
  • Higher energy emission compared to phosphorescence
• Phosphorescence characteristics
  • Longer lifetime (milliseconds to hours)
  • Emission from triplet states
  • Lower energy emission compared to fluorescence
• Fluorescence applications
  • Fluorescence microscopy enables imaging of biological samples (cells, tissues)
  • Flow cytometry allows analyzing cell populations based on fluorescent markers
  • Fluorescence spectroscopy helps studying molecular interactions and conformations
  • Fluorescent labeling tags molecules for detection and tracking (proteins, nucleic acids)
• Phosphorescence applications
  • Glow-in-the-dark materials used in emergency signage, toys, and decorations
  • Oxygen sensing based on phosphorescence quenching by oxygen
  • Organic light-emitting diodes (OLEDs) utilize phosphorescent materials for display technologies and solid-state lighting

Factors in luminescence efficiency

• Quantum yield represents the ratio of emitted photons to absorbed photons
  • Influenced by competing non-radiative processes (internal conversion, intersystem crossing)
• Fluorescence and phosphorescence lifetime depend on
  • Rate of radiative and non-radiative transitions
  • Factors such as temperature, solvent, and the presence of quenchers
• Quenching reduces fluorescence or phosphorescence intensity through
  • Collisional quenching deactivates excited states by collisions with quencher molecules
  • Static quenching forms non-fluorescent complexes between fluorophores and quenchers
  • Energy transfer enables non-radiative transfer of excitation energy from a donor to an acceptor molecule

Applications of luminescence phenomena

• Bioimaging applications
  • Fluorescent proteins (GFP) enable labeling and tracking proteins in living cells
  • Fluorescent dyes allow staining specific cellular structures or molecules (nucleus, mitochondria)
  • Fluorescence resonance energy transfer (FRET) helps studying protein-protein interactions and conformational changes
• Display technologies
  • Organic light-emitting diodes (OLEDs) use phosphorescent materials for efficient and flexible displays (smartphones, TVs)
  • Quantum dots are nanoscale semiconductor crystals with size-dependent fluorescence properties
• Other applications include
  • Environmental monitoring for detecting pollutants and contaminants (oil spills, heavy metals)
  • Art conservation for analyzing pigments and binders in paintings
  • Forensic science for detecting trace evidence and bodily fluids (fingerprints, blood stains)