☀️Photochemistry Unit 14 – Photochemistry: Materials Applications

Key Concepts and Definitions

• Photochemistry studies chemical reactions and physical changes induced by light (ultraviolet, visible, or infrared)
• Light-matter interactions involve the absorption, emission, or scattering of photons by atoms, molecules, or materials
  • Absorption occurs when a photon's energy is taken up by an atom or molecule, exciting electrons to higher energy states
  • Emission happens when an excited electron returns to a lower energy state, releasing a photon
  • Scattering involves the redirection of photons by atoms, molecules, or particles without absorption
• Photochemical reactions are chemical transformations initiated by the absorption of light, leading to the formation of new chemical species or changes in the properties of materials
• Quantum yield is a measure of the efficiency of a photochemical reaction, defined as the number of molecules reacted per photon absorbed
• Photosensitizers are molecules that absorb light and transfer the energy to other molecules, initiating photochemical reactions
• Photochromic materials reversibly change color or opacity when exposed to light of specific wavelengths
• Photopolymerization is a process where light initiates the formation of polymers from monomers, enabling rapid and spatially controlled fabrication of 3D structures

Light-Matter Interactions

• Light-matter interactions form the basis of photochemistry and involve the exchange of energy between photons and atoms, molecules, or materials
• The absorption of light by molecules can lead to electronic transitions, where electrons are excited from their ground state to higher energy states
  • The energy of the absorbed photon must match the energy difference between the ground and excited states
  • The absorption of light follows the Beer-Lambert law, which relates the attenuation of light to the properties of the material through which it travels
• Excited molecules can return to their ground state through various processes, such as fluorescence, phosphorescence, or non-radiative relaxation
  • Fluorescence involves the emission of a photon as the electron returns to the ground state from a singlet excited state
  • Phosphorescence occurs when the electron undergoes intersystem crossing to a triplet excited state before emitting a photon and returning to the ground state
• Energy transfer can occur between molecules, where an excited molecule (donor) transfers its energy to a nearby molecule (acceptor) through dipole-dipole interactions (Förster resonance energy transfer) or electron exchange (Dexter energy transfer)
• Light scattering by molecules or particles can be elastic (Rayleigh scattering) or inelastic (Raman scattering), providing information about the structure and properties of the material

Photochemical Reactions in Materials

• Photochemical reactions in materials involve the use of light to initiate chemical transformations or modulate the properties of the material
• Photoisomerization is a common photochemical reaction where molecules undergo a change in their geometric structure upon light absorption (azobenzenes, stilbenes)
  • Cis-trans isomerization of azobenzenes can be used to control the shape and properties of polymers or liquid crystals
• Photocleavage reactions involve the breaking of chemical bonds upon light absorption, which can be used for the controlled release of molecules or the degradation of materials
  • Photolabile protecting groups (o-nitrobenzyl, coumarins) are used in organic synthesis and drug delivery to release active compounds upon light exposure
• Photocycloaddition reactions, such as [2+2] cycloadditions, can be used to create cyclic structures from unsaturated molecules (alkenes, alkynes) upon light irradiation
• Photoinduced electron transfer reactions involve the transfer of an electron from a donor to an acceptor molecule upon light absorption, forming charge-separated states
  • These reactions are crucial in photovoltaic devices and photocatalysis for solar energy conversion and chemical synthesis
• Photodynamic therapy utilizes photosensitizers that generate reactive oxygen species upon light exposure to selectively destroy cancer cells or pathogenic microorganisms

Photosensitive Materials and Their Properties

• Photosensitive materials undergo changes in their physical, chemical, or optical properties when exposed to light
• Photoresists are light-sensitive polymers used in photolithography for the fabrication of microelectronic devices and micro/nanostructures
  • Positive photoresists become more soluble in developer solutions upon light exposure due to the photochemical degradation of the polymer
  • Negative photoresists become less soluble in developer solutions upon light exposure due to photocrosslinking or photopolymerization
• Photochromic materials exhibit reversible color changes when exposed to light of specific wavelengths
  • Organic photochromic compounds (spiropyrans, diarylethenes) undergo structural transformations upon light absorption, leading to a change in their absorption spectra
  • Inorganic photochromic materials (silver halides, tungsten oxides) exhibit color changes due to the formation of color centers or changes in oxidation states upon light exposure
• Photorefractive materials experience a change in their refractive index when exposed to non-uniform illumination, enabling applications in holography and optical data storage
  • The photorefractive effect arises from the generation, transport, and trapping of charge carriers in the material, creating a space-charge field that modulates the refractive index
• Photoluminescent materials absorb light and emit photons at a longer wavelength, finding applications in displays, lighting, and sensing
  • Quantum dots are semiconductor nanocrystals with size-dependent photoluminescence properties, offering narrow emission spectra and high quantum yields

Applications in Solar Energy Conversion

• Solar energy conversion involves the use of photochemical processes to harness energy from sunlight and convert it into electrical or chemical energy
• Photovoltaic devices, such as solar cells, convert light into electricity through the photovoltaic effect
  • In organic photovoltaics, conjugated polymers or small molecules absorb light, generating excitons that dissociate into free charge carriers at donor-acceptor interfaces
  • Dye-sensitized solar cells use a photosensitizer (ruthenium complexes) adsorbed on a mesoporous semiconductor (TiO2) to inject electrons upon light absorption, while a redox electrolyte regenerates the dye
• Artificial photosynthesis aims to mimic the natural process of photosynthesis to produce fuels or valuable chemicals from sunlight, water, and CO2
  • Photocatalytic water splitting uses semiconductor photocatalysts (TiO2, SrTiO3) to absorb light and generate electron-hole pairs that drive the reduction and oxidation of water to produce hydrogen and oxygen
  • Photocatalytic CO2 reduction employs photocatalysts (Cu2O, CdS) to convert CO2 into carbon-based fuels (methanol, methane) or value-added chemicals (formic acid, ethylene) using sunlight as the energy source
• Solar thermal energy storage utilizes photochemical reactions to store solar energy in the form of chemical bonds, which can be released as heat on demand
  • Molecular solar thermal systems use photoswitchable molecules (norbornadienes, azobenzenes) that undergo isomerization upon light absorption, storing energy in their metastable states

Photopolymerization and 3D Printing

• Photopolymerization is a process where light initiates the formation of polymers from monomers, enabling rapid and spatially controlled fabrication of 3D structures
• In stereolithography, a focused laser beam selectively polymerizes a photosensitive resin layer-by-layer to create 3D objects based on a digital model
  • The photopolymerization is typically initiated by a photoinitiator (benzophenones, phosphine oxides) that generates free radicals upon light absorption
• Digital light processing (DLP) 3D printing uses a digital micromirror device to project a dynamic mask of light onto the photopolymer resin, polymerizing an entire layer at once
• Two-photon polymerization (2PP) enables the fabrication of sub-micron features by using a focused femtosecond laser to induce two-photon absorption in the photoresist
  • The quadratic dependence of two-photon absorption on light intensity confines the polymerization to a small focal volume, allowing for high-resolution 3D printing
• Photopolymerization is also used in dental restorations, where a photosensitive composite resin is cured using a blue light source to create fillings or crowns
• Photopolymer-based 3D printing has applications in bioprinting, where biocompatible photopolymers are used to create scaffolds for tissue engineering or drug delivery systems

Photochromic Materials and Smart Windows

• Photochromic materials change color reversibly when exposed to light of specific wavelengths, enabling their use in smart windows and adaptive optical devices
• Organic photochromic compounds, such as spiropyrans and diarylethenes, undergo structural transformations upon light absorption, leading to a change in their absorption spectra
  • Spiropyrans switch between a colorless closed form and a colored open form (merocyanine) upon UV and visible light exposure
  • Diarylethenes exhibit photochromism due to the reversible photocyclization reaction between their open and closed forms
• Inorganic photochromic materials, such as silver halides and tungsten oxides, exhibit color changes due to the formation of color centers or changes in oxidation states upon light exposure
  • Silver halide-based photochromic glasses darken when exposed to UV light due to the formation of silver nanoparticles, and return to their original state when exposed to visible light or heat
• Smart windows incorporate photochromic materials to regulate the amount of light and heat entering a building, improving energy efficiency and occupant comfort
  • Photochromic smart windows automatically darken when exposed to intense sunlight, reducing glare and heat gain, and become transparent again under low light conditions
• Photochromic lenses, used in eyeglasses and camera lenses, darken when exposed to UV light, providing protection against harmful radiation and glare
• Coupling photochromic materials with other stimuli-responsive materials (thermochromic, electrochromic) can create multi-functional smart windows that adapt to various environmental conditions

Emerging Trends and Future Directions

• The integration of photochromic materials with other stimuli-responsive materials (thermochromic, electrochromic) is an emerging trend in the development of multi-functional smart windows and adaptive optical devices
• Photochromic organic-inorganic hybrid materials, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), offer unique opportunities for tailoring photochromic properties and enhancing switching speeds and cycling stability
• The development of near-infrared (NIR) photochromic materials is gaining attention for applications in bioimaging, night vision devices, and secure optical communications
  • NIR photochromic compounds, such as dihydroazulenes and dihydropyrenes, undergo reversible photoisomerization in the NIR region, enabling deeper tissue penetration and reduced background interference
• Photoactuators and photomechanical materials, which convert light energy into mechanical motion, are being explored for applications in soft robotics, artificial muscles, and microfluidic devices
  • Azobenzene-containing polymers and liquid crystal elastomers exhibit reversible shape changes upon light exposure due to the photoisomerization of azobenzene moieties
• The combination of photochemistry with other advanced technologies, such as nanotechnology, 3D printing, and artificial intelligence, is expected to lead to the development of novel smart materials and devices with enhanced functionalities
• Photochemical processes are being investigated for environmental applications, such as photocatalytic air and water purification, and the degradation of pollutants and microplastics
• The development of more efficient and stable photocatalysts for solar fuel production and CO2 reduction is a key focus in the field of artificial photosynthesis, aiming to address energy and environmental challenges
• Photochemical methods are being explored for the synthesis of complex molecules and materials, offering new routes for drug discovery, materials science, and nanotechnology