Radiative decay processes are crucial in photochemistry, involving light emission from excited molecules. Fluorescence and phosphorescence are key phenomena, differing in their excited state origins and emission timescales.

Understanding these processes is vital for applications like microscopy, medical treatments, and lighting technology. Factors influencing decay rates, such as molecular structure and environment, play a significant role in determining their behavior and usefulness.

Radiative Decay Processes

Process of fluorescence

• Fluorescence emits light from excited singlet state within nanoseconds
• Jablonski diagram shows excitation, vibrational relaxation, and emission
• Stokes shift measures energy loss between excitation and emission wavelengths
• Quantum yield calculates fluorescence efficiency (emitted/absorbed photons)
• Fluorescence lifetime averages excited state duration before emission
• Spectra exhibit mirror image rule between absorption and emission (longer wavelength)

Mechanism of phosphorescence vs fluorescence

• Phosphorescence emits light from excited triplet state over longer periods (microseconds to seconds)
• Intersystem crossing facilitates singlet to triplet transition via spin-orbit coupling
• Triplet states have lower energy and longer lifetime than singlet states
• Phosphorescence differs from fluorescence:
  1. Longer emission lifetime
  2. Lower quantum yield
  3. Larger Stokes shift
  4. Often requires low temperatures or rigid environments

Factors influencing radiative decay rates

• Einstein coefficients quantify spontaneous ($A_{21}$) and stimulated ($B_{12}$, $B_{21}$) emission/absorption rates
• Oscillator strength measures transition probability between states
• Franck-Condon principle relates vibrational wavefunction overlap to transition probability
• Environmental factors affect decay rates (solvent polarity, viscosity, temperature)
• Structural factors impact decay (molecular size, conjugation, heavy atoms)
• Competing non-radiative processes influence decay (internal conversion, vibrational relaxation, energy transfer)

Applications of radiative decay principles

• Fluorescence microscopy enables high-resolution biological imaging (cellular components)
• Fluorescent probes measure environmental conditions (pH, ion concentrations)
• Photodynamic therapy treats diseases using photosensitizers (cancer)
• LED technology provides energy-efficient lighting through electroluminescence
• Glow-in-the-dark materials utilize phosphorescent pigments (safety signs, toys)
• Analytical techniques employ fluorescence for chemical analysis and molecular dynamics studies
• Laser technology harnesses population inversion and stimulated emission (medicine, industry, research)