Other meanings of Photochemistry
CHEMISTRY
Photochemistry is the branch of chemistry concerned with chemical reactions initiated or influenced by light. It explains how absorbed photons create electronically excited molecules whose reactivity can differ sharply from that of their ground states.
Photochemistry begins when a molecule absorbs a photon whose energy matches an allowed electronic transition. The molecule then reaches an excited singlet or, after intersystem crossing, triplet state; these states may undergo fluorescence, phosphorescence, energy transfer, electron transfer, rearrangement, or bond cleavage.1 The Grotthuss–Draper principle states that absorbed light, rather than merely incident light, is the effective photochemical agent. The Stark–Einstein law further relates one absorbed photon to one primary photochemical event, although subsequent chain reactions can produce larger quantum yields. Solvent polarity, oxygen, temperature, concentration, molecular structure, and wavelength all influence the outcome. Photochemical kinetics therefore often differs from ordinary thermal kinetics, and excited-state lifetimes become central experimental quantities.
Photochemical research combines spectroscopy, controlled illumination, and chemical analysis to connect excited states with products. Absorption spectra identify wavelengths that can be used selectively, while time-resolved fluorescence, transient absorption, and electron-paramagnetic-resonance methods follow intermediates from femtoseconds to seconds. A Jablonski diagram summarizes the competition among internal conversion, intersystem crossing, fluorescence, phosphorescence, and reactive pathways. Quantum yield measures the efficiency of a photochemical process relative to the number of photons absorbed, but its value depends on how the event is defined and detected. Oxygen can quench triplet states or generate reactive oxygen species, making degassing and atmosphere control important. NIST spectral and thermochemical databases support the identification and comparison of molecular species used in such studies.3
Photochemistry underpins processes ranging from photosynthesis and vision to atmospheric ozone chemistry, photopolymerization, imaging, and synthetic organic chemistry. In photoredox catalysis, visible light excites a catalyst that transfers an electron or an energy quantum to a substrate, enabling bond constructions under comparatively mild conditions. Ultraviolet radiation drives important atmospheric reactions, including ozone formation and destruction, while sunlight also initiates degradation of pollutants and natural organic matter.4 Industrial photopolymerization cures coatings, dental resins, inks, and three-dimensional-printing materials. Medical photodynamic therapy uses a photosensitizer, light, and molecular oxygen to produce cytotoxic reactive oxygen species. Solar-fuel research applies related principles to water splitting and carbon-dioxide reduction, although efficiency, selectivity, and long-term stability remain practical challenges.
Photochemical behavior can change dramatically when molecules are confined, aggregated, or placed near an interface. Solid-state reactions may produce products that are inaccessible in solution because crystal packing controls which atoms can approach one another. In photochemistry, a reaction can also be initiated indirectly: a sensitizer absorbs light and transfers energy or an electron to a second molecule that does not absorb efficiently at the operating wavelength. Light intensity is not always proportional to product formation, because excited-state saturation, reabsorption, oxygen depletion, and radical-chain propagation can intervene. Wavelength-selective chemistry is used in lithography and optical data recording, while photochromic molecules reversibly change structure and color under illumination. Atmospheric photochemistry likewise connects local molecular reactions with planetary-scale ozone, aerosol, and climate processes.2
Photochemistry is distinct from photobiology, although the two fields overlap wherever light-driven molecular reactions occur in living systems.
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