☀️Photochemistry Unit 1 – Photochemistry: Light-Matter Interactions

Key Concepts and Definitions

• Photochemistry studies chemical reactions and physical changes that occur when molecules absorb light
• Light behaves as both a wave and a particle, with wavelength ($\lambda$), frequency ($\nu$), and energy ($E$) related by $E = h\nu = hc/\lambda$
• Photons are discrete packets of light energy that can be absorbed or emitted by molecules
• Electronic transitions involve the excitation of electrons from a lower energy state to a higher energy state
• Vibrational transitions involve changes in the vibrational energy levels of a molecule
• Rotational transitions involve changes in the rotational energy levels of a molecule
• Selection rules determine which transitions are allowed based on the symmetry and spin of the molecular orbitals involved
• Franck-Condon principle states that electronic transitions occur vertically on a potential energy diagram, without changes in nuclear coordinates

Light Properties and Behavior

• Light exhibits wave-particle duality, behaving as both a wave and a particle depending on the context
• Wavelength ($\lambda$) is the distance between two consecutive peaks or troughs of a light wave
• Frequency ($\nu$) is the number of wave cycles that pass a fixed point per unit time
• Energy ($E$) of a photon is directly proportional to its frequency and inversely proportional to its wavelength
  • $E = h\nu = hc/\lambda$, where $h$ is Planck's constant and $c$ is the speed of light
• Electromagnetic spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays
• Visible light has wavelengths between 400 nm (violet) and 700 nm (red)
• Ultraviolet light has shorter wavelengths and higher energy than visible light
• Infrared light has longer wavelengths and lower energy than visible light

Molecular Energy Levels

• Molecules have discrete energy levels corresponding to electronic, vibrational, and rotational states
• Electronic energy levels are associated with the arrangement of electrons in molecular orbitals
  • Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are important in photochemistry
• Vibrational energy levels are associated with the stretching and bending motions of chemical bonds
• Rotational energy levels are associated with the rotation of the molecule about its axes
• Born-Oppenheimer approximation assumes that electronic, vibrational, and rotational motions can be treated separately
• Jablonski diagram is a graphical representation of molecular energy levels and transitions between them
• Singlet and triplet states differ in the spin orientation of the excited electron relative to the ground state electron

Absorption and Emission Processes

• Absorption occurs when a molecule absorbs a photon and is excited to a higher energy state
• Absorption spectrum shows the wavelengths or frequencies of light that a molecule can absorb
• Beer-Lambert law relates the absorption of light to the concentration of the absorbing species and the path length of the sample
  • $A = \epsilon bc$, where $A$ is absorbance, $\epsilon$ is molar absorptivity, $b$ is path length, and $c$ is concentration
• Emission occurs when a molecule relaxes from a higher energy state to a lower energy state, releasing a photon
• Fluorescence is the emission of light from a singlet excited state to a singlet ground state
  • Occurs on a nanosecond timescale and is spin-allowed
• Phosphorescence is the emission of light from a triplet excited state to a singlet ground state
  • Occurs on a microsecond to second timescale and is spin-forbidden
• Stokes shift is the difference between the maximum absorption and emission wavelengths of a molecule

Photochemical Reactions

• Photochemical reactions are chemical reactions initiated by the absorption of light
• Primary photochemical process is the initial excitation of a molecule by light absorption
• Secondary photochemical processes are the subsequent reactions of the excited molecule or its products
• Photodissociation is the breaking of a chemical bond by light absorption (e.g., photolysis of water)
• Photoisomerization is the light-induced conversion of a molecule from one isomeric form to another (e.g., cis-trans isomerization of retinal in vision)
• Photocycloaddition is the formation of a cyclic product from two or more molecules by light absorption (e.g., photodimerization of thymine in DNA damage)
• Photoreduction and photooxidation involve electron transfer between an excited molecule and another species
• Photosensitization occurs when an excited molecule transfers its energy to another molecule, which then undergoes a photochemical reaction

Quantum Yield and Efficiency

• Quantum yield ($\Phi$) is the number of molecules reacted or products formed per photon absorbed
  • $\Phi = \frac{\text{molecules reacted or products formed}}{\text{photons absorbed}}$
• Quantum efficiency is the ratio of the quantum yield to the maximum possible quantum yield (usually 1)
• Factors affecting quantum yield include competing processes (e.g., fluorescence, intersystem crossing), concentration, and temperature
• Actinometry is the measurement of the number of photons absorbed by a chemical system using a reference compound with a known quantum yield
• Photochemical equivalence law states that for every photon absorbed, one molecule reacts or one product molecule is formed
• Photon flux is the number of photons passing through a given area per unit time
• Photon fluence is the total number of photons delivered to a given area

Applications in Research and Industry

• Photodynamic therapy uses light-activated drugs (photosensitizers) to selectively destroy cancer cells or other targeted tissues
• Photopolymerization is the light-induced formation of polymers from monomers (e.g., dental composites, 3D printing)
• Photolithography uses light to pattern photoresists for the fabrication of microelectronic devices and integrated circuits
• Photocatalysis employs light-activated catalysts to drive chemical reactions (e.g., water splitting for hydrogen production, CO2 reduction)
• Photochromism is the reversible color change of a material upon light exposure (e.g., photochromic lenses, data storage)
• Photosynthesis is the biological process of converting light energy into chemical energy in plants and other organisms
• Photodegradation is the breakdown of materials (e.g., plastics, dyes) by light, which can be harnessed for environmental remediation

Experimental Techniques and Instrumentation

• UV-Vis spectroscopy measures the absorption of light by a sample in the ultraviolet and visible regions
  • Used to determine the concentration of absorbing species, identify compounds, and study reaction kinetics
• Fluorescence spectroscopy measures the emission of light from a sample following excitation
  • Provides information on the electronic structure, environment, and dynamics of molecules
• Time-resolved spectroscopy techniques (e.g., flash photolysis, pump-probe) allow the study of short-lived excited states and reaction intermediates
• Laser spectroscopy uses tunable lasers to selectively excite specific molecular transitions and probe their properties
• Photoacoustic spectroscopy detects the sound waves generated by the absorption of light in a sample
• Circular dichroism spectroscopy measures the differential absorption of left and right circularly polarized light, providing information on the chirality and conformation of molecules
• Quantum chemical calculations (e.g., density functional theory) can predict the electronic structure, spectra, and reactivity of molecules
• Photochemical reactors are designed to optimize light delivery and control reaction conditions for efficient photochemical synthesis or processing