Photochemistry is the branch of chemistry concerned with chemical transformations initiated by the absorption of electromagnetic radiation. It examines what happens when molecules absorb photons and enter electronically excited states that can possess very different energies, geometries and chemical reactivities from their ground states. Absorbed energy may lead to fluorescence, phosphorescence, internal conversion, intersystem crossing, energy transfer, electron transfer, bond cleavage or chemical reaction.
Photochemical processes can involve ultraviolet, visible or other regions of the electromagnetic spectrum depending on the molecular energy levels and experimental system. Photochemistry is distinct from ordinary thermal chemistry because light provides the initial excitation energy, although thermal processes can subsequently influence the fate of excited molecules and reaction intermediates. The field connects molecular spectroscopy with organic synthesis, atmospheric science, biology, materials science, medicine and energy research.
The first stage of many photochemical processes is photon absorption. The energy of a photon is determined by its frequency and wavelength, so shorter wavelength radiation corresponds to greater photon energy. A molecule absorbs radiation when the photon energy matches an allowed transition between electronic states. The initially excited molecule can undergo rapid vibrational relaxation before other processes occur. Fluorescence generally involves emission from an excited singlet state, while phosphorescence commonly involves emission from a triplet state after intersystem crossing.
Internal conversion transfers energy between electronic states of the same spin multiplicity, whereas intersystem crossing changes spin multiplicity. These processes compete with chemical reactions and determine the lifetime and fate of the excited state. Jablonski diagrams are frequently used to represent these pathways conceptually. Time resolved spectroscopy can measure excited state lifetimes and reveal processes that occur too rapidly to observe through conventional steady state measurements.
Photochemical reactions can follow mechanisms that are inaccessible or inefficient under ordinary thermal conditions. Excitation changes electron distribution and can alter bond strengths, acidity, redox properties and molecular geometry. An excited molecule may therefore undergo electron transfer, hydrogen abstraction, rearrangement, fragmentation or addition reactions that are difficult to achieve from the ground state. Quantum yield is an important measure of photochemical efficiency and relates the number of specified chemical or physical events to the number of photons absorbed.
Quantum yields can exceed one when chain reactions allow a single initiating photon to produce multiple chemical events. Low quantum yields can result from rapid nonreactive relaxation, quenching, competing reactions or poor absorption. Photochemical mechanisms are therefore determined not simply by whether a molecule absorbs light but by what happens after absorption and how efficiently the excited state reaches a chemically productive pathway.
Natural systems depend heavily on photochemistry. Photosynthesis uses light absorption to initiate electron transfer and energy conversion processes that ultimately support the synthesis of energy rich biological compounds. Vision involves a photochemical change in light sensitive molecules, particularly the photoisomerisation of retinal within visual proteins. Atmospheric photochemistry is driven by sunlight and includes reactions involving oxygen, ozone, nitrogen oxides and numerous organic compounds. Ultraviolet radiation can initiate molecular transformations that influence atmospheric composition and pollutant lifetimes.
Sunlight also causes photochemical degradation of dyes, pharmaceuticals, polymers and other substances exposed to the environment. The rate of degradation depends on wavelength, molecular absorption, oxygen concentration, temperature, humidity, catalysts and surrounding materials. Understanding environmental photochemistry is therefore important for predicting pollutant persistence and the long term stability of chemical products exposed to natural radiation.
Photochemistry has many technological applications. Photopolymerisation uses light to initiate polymer formation or cross linking and is used in coatings, inks, adhesives, dental materials, imaging systems and additive manufacturing. Photolithography uses controlled light exposure to create microscopic patterns and is fundamental to semiconductor fabrication. Photodynamic therapy uses light activated chemical systems to generate reactive species that can damage selected biological targets. Photocatalysis uses light to activate a catalyst and promote chemical transformations, including oxidation and reduction processes.
Titanium dioxide is widely investigated as a photocatalytic material, while many molecular and semiconductor systems have been developed for visible light driven reactions. Solar fuel research uses photochemical and photoelectrochemical processes to convert solar energy into chemical energy, including attempts to produce hydrogen or transform carbon dioxide into energy rich products.
Photochemical behaviour is strongly affected by the molecular and environmental surroundings. Solvent polarity, hydrogen bonding, viscosity and temperature can influence excited state energies and lifetimes. Oxygen can quench excited states or participate in subsequent oxidation reactions, while other molecules can act as quenchers by removing excitation energy or accepting electrons. Sensitisation occurs when one species absorbs radiation and transfers energy or an electron to another species that does not efficiently absorb the available wavelength.
Photosensitisers are therefore important in many biological and technological systems. Light penetration also becomes an important practical issue because strongly absorbing reaction mixtures can create concentration gradients in photon availability. Photochemical reactor design consequently differs from ordinary thermal reactor design because the spatial distribution of light can be as important as temperature, concentration and mixing.
Photochemistry must be distinguished from photophysics. Photophysics describes physical processes involving electronically excited molecules without permanent chemical transformation, whereas Photochemistry specifically concerns chemical change caused or initiated by light. The two areas are nevertheless closely connected because photophysical processes determine whether an excited molecule remains available long enough to react. A reaction performed under illumination is not automatically photochemical because the light may simply heat the system or may not be absorbed by the reacting species.
Evidence for a genuine photochemical pathway can include wavelength dependence, action spectra, quantum yield measurements and appropriate control experiments. Photochemistry also differs from thermal activation because photon absorption produces discrete electronic excitation rather than merely increasing the average translational energy of molecules. This distinction is particularly important when interpreting mechanisms and designing selective light driven reactions.
Modern Photochemistry combines spectroscopy, physical chemistry, organic chemistry, materials science, biology and computational modelling. Ultrafast spectroscopy can monitor energy redistribution, charge separation and bond changes over extremely short timescales, providing detailed information about the earliest events following photon absorption. Visible light photoredox chemistry has expanded synthetic possibilities by using excited catalysts to mediate electron transfer and generate reactive intermediates under comparatively mild conditions.
Other research areas include artificial photosynthesis, photocatalytic environmental remediation, optical materials, solar energy conversion and light responsive molecular systems. Limitations include inefficient photon penetration, unwanted side reactions, photodegradation of catalysts, oxygen sensitivity and difficulties in scaling laboratory photochemical reactions to industrial volumes. Nevertheless, Photochemistry provides a distinctive method for controlling chemical reactivity because the energy, timing and wavelength of light can be manipulated externally, allowing molecular transformations that can differ substantially from those accessible through purely thermal chemistry.
