Photochemical Reactions — Light-Driven Chemistry
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Photochemical reactions are initiated by the absorption of light (photons) rather than heat. When a molecule absorbs a photon with sufficient energy, an electron is promoted to an excited state, making the molecule more reactive than in its ground state. Photochemistry governs processes as diverse as photosynthesis, vision, photography, atmospheric ozone chemistry, and the degradation of plastics in sunlight.
Reaction Mechanism
The Grotthuss-Draper law states that only absorbed light can cause chemical change. The Stark-Einstein law states that each molecule undergoing photochemical change absorbs exactly one photon (primary process). After absorption, the excited molecule can undergo several pathways: fluorescence (re-emit light), internal conversion (convert to heat), intersystem crossing (change spin state to a longer-lived triplet state), or chemical reaction. Quantum yield measures the efficiency — moles of product per moles of photons absorbed.
Everyday Examples
Sunburn is a photochemical reaction — UV-B light damages DNA in skin cells. Sunscreen absorbs UV photons before they reach skin. Photosynthesis uses chlorophyll to absorb red and blue light, driving the reaction 6CO2 + 6H2O -> C6H12O6 + 6O2. Photography (both film and digital) relies on photochemical conversion of light into chemical or electrical signals.
Importância industrial
Semiconductor lithography uses photochemical reactions in photoresists to pattern circuits at nanometer scales — enabling modern microchips. UV curing of coatings and adhesives is faster and more energy-efficient than thermal curing. Photocatalysis using titanium dioxide can break down pollutants in water and air. Solar cells use photovoltaic effects (related to photochemistry) to generate over 1,300 TWh of electricity globally.
Safety Note
UV light sources used in photochemistry can cause severe eye and skin damage. Always use UV-blocking goggles and shields. Some photochemical reactions generate reactive oxygen species (singlet oxygen, hydroxyl radicals) that are highly toxic. Store photosensitive reagents in amber bottles away from light.
Dye-Sensitized Solar Cell (Grätzel Cell)
Dye + hν → Dye* → Dye⁺ + e⁻ (into TiO₂)
In a Grätzel cell, a ruthenium dye absorbs sunlight and injects an electron into the conduction band of nanocrystalline TiO₂. …
Photochemical Smog Formation (NO₂ Photolysis)
NO₂ → NO + O·
Sunlight (< 420 nm) photolyzes nitrogen dioxide to produce nitric oxide and an oxygen atom. The oxygen atom then reacts …
Photochromism of Spiropyran
Spiropyran (colorless) → Merocyanine (colored)
UV light causes the C–O bond in spiropyran to break, opening the ring to form the planar, conjugated merocyanine isomer …
Chlorophyll Fluorescence
Chlorophyll + hν (blue/red) → Chlorophyll* → Chlorophyll + hν (red)
Chlorophyll absorbs blue (430 nm) and red (660 nm) light, reaching an excited singlet state. Most energy is transferred to …
Photodegradation of Methylene Blue by TiO₂
C₁₆H₁₈ClN₃S + TiO₂ + hν → CO₂ + H₂O + degradation products
TiO₂ photocatalysis generates hydroxyl radicals and superoxide ions that mineralize organic pollutants like methylene blue into CO₂, water, and simple …
Photodissociation of Chlorine Gas
Cl₂ → 2Cl·
UV or visible light (< 500 nm) cleaves the Cl–Cl bond homolytically to produce two chlorine free radicals. This is …
Silver Bromide Photodecomposition
2AgBr → 2Ag + Br₂
Silver bromide is more photosensitive than silver chloride and was the primary photosensitive material in photographic film. Blue and UV …
Cis-Trans Isomerization of Retinal
11-cis-Retinal → all-trans-Retinal
When a photon strikes rhodopsin in rod cells of the retina, it causes the 11-cis-retinal chromophore to isomerize to the …
Photosynthesis
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
The most important photochemical reaction on Earth. Plants, algae, and cyanobacteria use chlorophyll to capture sunlight and convert carbon dioxide …
Ozone Formation by UV Light
3O₂ → 2O₃
Ultraviolet radiation (wavelength < 240 nm) splits O₂ molecules in the stratosphere. The resulting oxygen atoms combine with intact O₂ …
Silver Chloride Photodecomposition
2AgCl → 2Ag + Cl₂
Silver chloride darkens upon exposure to light as photons provide enough energy to reduce Ag⁺ to metallic silver. This photosensitivity …
Vitamin D₃ Photosynthesis in Skin
7-Dehydrocholesterol → Previtamin D₃ → Vitamin D₃
UV-B radiation (280–315 nm) converts 7-dehydrocholesterol in the skin to previtamin D₃ through a photochemical ring-opening reaction. Previtamin D₃ then …
Photocatalytic Water Splitting (TiO₂)
2H₂O → 2H₂ + O₂
Titanium dioxide absorbs UV light and generates electron-hole pairs that can split water into hydrogen and oxygen. Discovered by Fujishima …
Luminol Chemiluminescence
C₈H₇N₃O₂ + H₂O₂ → 3-aminophthalate + N₂ + hν (blue light)
Luminol reacts with hydrogen peroxide in the presence of a catalyst (typically iron from hemoglobin in blood) to produce 3-aminophthalate …
Photolysis of Hydrogen Peroxide
H₂O₂ → 2OH·
UV light (< 300 nm) cleaves the O–O bond in hydrogen peroxide to generate two hydroxyl radicals, among the most …
Firefly Bioluminescence (Luciferin Oxidation)
Luciferin + ATP + O₂ → Oxyluciferin + AMP + PPᵢ + CO₂ + hν
Firefly luciferase catalyzes the ATP-dependent oxidation of luciferin, producing oxyluciferin in an electronically excited state that emits yellow-green light (560 …
Photoisomerization of Azobenzene
trans-Azobenzene → cis-Azobenzene
UV light (340 nm) converts the thermodynamically stable trans isomer of azobenzene to the cis form through rotation around the …
Photolysis of Water (Vacuum UV)
H₂O → H· + OH·
Vacuum UV radiation (< 185 nm) directly dissociates water molecules into hydrogen and hydroxyl radicals. This reaction is significant in …
Photoreduction of CO₂ to Methanol
CO₂ + 3H₂O → CH₃OH + 3/2 O₂
Photocatalysts can reduce CO₂ to methanol using sunlight and water, mimicking photosynthesis to produce liquid fuel. Copper-modified TiO₂ and other …
Photoelectric Effect (Cesium)
Cs + hν → Cs⁺ + e⁻
Photons with energy exceeding the work function of cesium (2.1 eV, wavelength < 590 nm) eject electrons from the metal …
Photocatalytic NOx Removal
NOₓ + O₂ + H₂O → HNO₃
TiO₂ photocatalyst on building surfaces oxidizes nitrogen oxides (NOx) from vehicle exhaust to nitric acid (HNO₃), which is then washed …
Ozone Decomposition by UV
2O₃ → 3O₂
UV-C radiation (< 240 nm) and UV-B (240–320 nm) break ozone back into molecular oxygen and atomic oxygen. This absorption …
Photopolymerization of Acrylates
n CH₂=CHCOOR → [–CH₂–CH(COOR)–]ₙ
UV or visible light activates a photoinitiator that generates free radicals, which then initiate chain polymerization of acrylate monomers. The …
UV Curing of Epoxy Resins
Epoxy monomer + Photoinitiator → Cross-linked polymer
Cationic photoinitiators generate strong acids upon UV exposure, which catalyze the ring-opening polymerization of epoxy groups. Unlike free-radical photopolymerization, cationic …
Photochemical Cycloaddition ([2+2])
2 CH₂=CH₂ → Cyclobutane
UV light promotes one alkene to its excited state, allowing a symmetry-forbidden [2+2] cycloaddition that cannot occur thermally according to …