Photochemical reaction | Light-Induced Chemical Changes

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class="md-content"> <div class="md-article-container template-desktop infinite-pagination"> <div class="infinite-scroll-container article last"> <article class="article-content container-lg qa-content px-0 pt-0 pb-40 py-lg-20 content md-expanded" data-topic-id="457736"> <div class="grid gx-0"> <div class="col-auto"> <div class="topic-left-rail md-article-drawer position-relative d-flex border-right-sm border-left-sm open"> <div class="drawer d-flex flex-column open"> <div class="left-rail-section-content"> <div class="topic-left-rail-header text-truncate bg-gray-50 position-relative text-right d-flex align-items-center"> <div class="tlr-title px-20 py-15 text-left"> <em class="material-icons text-gray-400 d-lg-none" data-icon="toc"></em> <a class="font-serif font-weight-bold text-black link-blue" href="https://www.britannica.com/science/photochemical-reaction">photochemical reaction</a> </div> <button aria-label="Close" class="js-sections-close-button btn-link btn-sm btn d-lg-none position-absolute top-0 p-10 right-0" > <em class="material-icons font-26" data-icon="close"></em> </button> </div> <div class="section-content pl-10 pr-20 pl-sm-50 pr-sm-60 pl-lg-5 pr-lg-10 pt-10 pt-lg-0 bg-gray-50 clear-catfish-ad"> <div class="toc mb-20"> <div class="font-serif font-14 font-weight-bold mx-15 mb-15 mt-20"> Table of Contents </div> <ul class="list-unstyled my-0" data-level="h1"><li data-target="#ref1"><div class="pl-25"><a class="link-gray-900 w-100" href="/science/photochemical-reaction">Introduction</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref277508"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/photochemical-reaction#ref277508">History</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref277509"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/photochemical-reaction/Consequences-of-photoexcitation">Consequences of photoexcitation</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref277510"><a class="w-100 link-gray-900" href="/science/photochemical-reaction/Consequences-of-photoexcitation#ref277510">Luminescence</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277511"><a class="w-100 link-gray-900" href="/science/photochemical-reaction/Photosensitization">Photosensitization</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277512"><a class="w-100 link-gray-900" href="/science/photochemical-reaction/Photosensitization#ref277512">Chemiluminescence</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277513"><a class="w-100 link-gray-900" href="/science/photochemical-reaction/Photosensitization#ref277513">Photoprotection</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277514"><a class="w-100 link-gray-900" href="/science/photochemical-reaction/Photosensitization#ref277514">Photodissociation</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277515"><a class="w-100 link-gray-900" href="/science/photochemical-reaction/Photoisomerization">Photoisomerization</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277516"><a class="w-100 link-gray-900" href="/science/photochemical-reaction/Photoisomerization#ref277516">Photorearrangement</a></li></ul></div></li><li data-target="#ref277517"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/photochemical-reaction/Photoisomerization#ref277517">Photochemical steps in photosynthesis</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li></ul> <a class="toc-extra-link link-gray-900" href="https://www.britannica.com/science/photochemical-reaction/additional-info">References & Edit History</a> <a class="toc-extra-link link-gray-900" href="/facts/photochemical-reaction">Related Topics</a> </div> <div class="tlr-media-slider pb-10 mb-30"> <a class="section-header link-gray-900 font-serif font-14 font-weight-bold mb-10 mx-10" href="https://www.britannica.com/science/photochemical-reaction/images-videos">Images & Videos</a> <div class="slider js-slider position-relative d-inline-flex align-items-center mw-100 "> <div class="slider-container js-slider-container overflow-hidden d-flex overflow-hidden text-nowrap ml-15"> <a href="https://cdn.britannica.com/60/95660-050-154C44E8/Chain-fluorescent-tunicates.jpg" data-href="/media/1/457736/110383" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/60/95660-004-0BEB79D7/Chain-fluorescent-tunicates.jpg" 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transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="186952">How to prevent beer from skunking</span><span>Understand why beer skunks, or spoils, the role light plays in this, and how to prevent it.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/photochemical-reaction/images-videos">See all videos for this article</a></figcaption></figure></div></div></div><button disabled="true" class="prev-button js-prev-button position-absolute btn btn-circle shadow btn-lg btn-blue-dark m-20"><span class="material-icons" data-icon="keyboard_arrow_left"></span></button><button disabled="true" class="next-button js-next-button position-absolute btn btn-circle shadow btn-lg btn-blue-dark m-20"><span class="material-icons" data-icon="keyboard_arrow_right"></span></button></div><p class="topic-paragraph"><strong><span id="ref499176"></span>photochemical reaction</strong>, a <a href="https://www.britannica.com/science/chemical-reaction" class="md-crosslink " data-show-preview="true">chemical reaction</a> initiated by the absorption of <span id="ref499177"></span><a href="https://www.britannica.com/science/energy" class="md-crosslink " data-show-preview="true">energy</a> in the form of <span id="ref499180"></span><a href="https://www.britannica.com/science/light" class="md-crosslink " data-show-preview="true">light</a>. The consequence of <span id="ref499183"></span><a href="https://www.britannica.com/science/molecule" class="md-crosslink " data-show-preview="true">molecules</a>’ absorbing light is the creation of <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="transient" href="https://www.merriam-webster.com/dictionary/transient" data-type="MW">transient</a> <span id="ref1006457"></span><a href="https://www.britannica.com/science/excited-state" class="md-crosslink ">excited states</a> whose chemical and physical properties differ greatly from the original molecules. These new chemical species can fall apart, change to new structures, combine with each other or other molecules, or transfer <a href="https://www.britannica.com/science/electron" class="md-crosslink " data-show-preview="true">electrons</a>, <a href="https://www.britannica.com/science/hydrogen" class="md-crosslink " data-show-preview="true">hydrogen</a> <a href="https://www.britannica.com/science/atom" class="md-crosslink " data-show-preview="true">atoms</a>, <a href="https://www.britannica.com/science/proton-subatomic-particle" class="md-crosslink " data-show-preview="true">protons</a>, or their electronic <span id="ref499202"></span><a href="https://www.britannica.com/science/excitation" class="md-crosslink " data-show-preview="true">excitation</a> energy to other molecules. Excited states are stronger <a href="https://www.britannica.com/science/acid" class="md-crosslink " data-show-preview="true">acids</a> and stronger reductants than the original <span id="ref1006475"></span><a href="https://www.britannica.com/science/ground-state" class="md-crosslink ">ground states</a>.</p><span class="marker MOD1 mod-inline"></span><span class="marker PREMOD2 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="96765" data-asm-type="image"><div class="md-assembly-wrapper card-media " data-type="image"><a href="https://cdn.britannica.com/09/76809-004-40BD51F3/Antarctic-ozone-hole-September-17-2001.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/457736/96765"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/09/76809-004-40BD51F3/Antarctic-ozone-hole-September-17-2001.jpg?w=300"><img src="https://cdn.britannica.com/09/76809-004-40BD51F3/Antarctic-ozone-hole-September-17-2001.jpg?w=300" alt="ozone depletion" data-width="550" data-height="412" loading="eager"></picture><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/09/76809-004-40BD51F3/Antarctic-ozone-hole-September-17-2001.jpg" data-href="/media/1/457736/96765">ozone depletion</a><span>Antarctic ozone hole, September 17, 2001.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div></figcaption></figure></div></div><p class="topic-paragraph">It is this last property that is crucial in the most important of all photochemical processes, <span id="ref499181"></span><a href="https://www.britannica.com/science/photosynthesis" class="md-crosslink " data-show-preview="true">photosynthesis</a>, upon which almost all <a href="https://www.britannica.com/science/life" class="md-crosslink " data-show-preview="true">life</a> on <a href="https://www.britannica.com/place/Earth" class="md-crosslink " data-show-preview="true">Earth</a> depends. Through photosynthesis, <a href="https://www.britannica.com/plant/plant" class="md-crosslink " data-show-preview="true">plants</a> convert the energy of sunlight into stored <a href="https://www.britannica.com/science/chemical-energy" class="md-crosslink autoxref " data-show-preview="true">chemical energy</a> by forming <a href="https://www.britannica.com/science/carbohydrate" class="md-crosslink " data-show-preview="true">carbohydrates</a> from atmospheric <a href="https://www.britannica.com/science/carbon-dioxide" class="md-crosslink " data-show-preview="true">carbon dioxide</a> and <a href="https://www.britannica.com/science/water" class="md-crosslink " data-show-preview="true">water</a> and releasing molecular <a href="https://www.britannica.com/science/oxygen" class="md-crosslink " data-show-preview="true">oxygen</a> as a byproduct. Both carbohydrates and oxygen are needed to <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="sustain" href="https://www.britannica.com/dictionary/sustain" data-type="EB">sustain</a> animal life. Many other processes in nature are photochemical. The ability to <span id="ref1006501"></span><a href="https://www.britannica.com/science/vision-physiology" class="md-crosslink " data-show-preview="true">see</a> the world starts with a photochemical reaction in the <a href="https://www.britannica.com/science/human-eye" class="md-crosslink " data-show-preview="true">eye</a>, in which retinal, a molecule in the photoreceptor cell <span id="ref1006506"></span><a href="https://www.britannica.com/science/rhodopsin" class="md-crosslink " data-show-preview="true">rhodopsin</a>, isomerizes (or changes shape) about a double bond after absorbing light. <span id="ref1006549"></span><a href="https://www.britannica.com/science/vitamin-D" class="md-crosslink " data-show-preview="true">Vitamin D</a>, essential for normal <a href="https://www.britannica.com/science/bone-anatomy" class="md-crosslink " data-show-preview="true">bone</a> and <a href="https://www.britannica.com/science/tooth-anatomy" class="md-crosslink " data-show-preview="true">teeth</a> development and <a href="https://www.britannica.com/science/kidney" class="md-crosslink " data-show-preview="true">kidney</a> function, is formed in the <a href="https://www.britannica.com/science/human-skin" class="md-crosslink " data-show-preview="true">skin</a> of animals after exposure of the chemical 7-dehydrocholesterol to sunlight. <span id="ref499182"></span><a href="https://www.britannica.com/science/ozone" class="md-crosslink " data-show-preview="true">Ozone</a> protects Earth’s surface from intense, deep <span id="ref499194"></span><a href="https://www.britannica.com/science/ultraviolet-radiation" class="md-crosslink " data-show-preview="true">ultraviolet (UV) irradiation</a>, which is damaging to <span id="ref1006482"></span><a href="https://www.britannica.com/science/DNA" class="md-crosslink " data-show-preview="true">DNA</a> and is formed in the <span id="ref499243"></span><a href="https://www.britannica.com/science/stratosphere" class="md-crosslink " data-show-preview="true">stratosphere</a> by a photochemical <a href="https://www.britannica.com/science/dissociation" class="md-crosslink autoxref " data-show-preview="true">dissociation</a> (separation) of molecular oxygen (O<sub>2</sub>) into individual oxygen atoms, followed by <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="subsequent" href="https://www.britannica.com/dictionary/subsequent" data-type="EB">subsequent</a> reaction of those oxygen atoms with molecular oxygen to produce ozone (O<sub>3</sub>). UV radiation that does get through the <a href="https://www.britannica.com/science/ozone-layer" class="md-crosslink autoxref " data-show-preview="true">ozone layer</a> photochemically damages DNA, which in turn introduces <a href="https://www.britannica.com/science/mutation-genetics" class="md-crosslink " data-show-preview="true">mutations</a> on its replication that can lead to <a href="https://www.britannica.com/science/skin-cancer" class="md-crosslink " data-show-preview="true">skin cancer</a>.</p><span class="marker MOD2 mod-inline"></span><span class="marker PREMOD3 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="109640" data-asm-type="image"><div class="md-assembly-wrapper card-media " data-type="image"><a href="https://cdn.britannica.com/34/63034-050-448D8D81/type-metal-oxide-semiconductor-transistor-sequence-operations-circuit.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/457736/109640"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/34/63034-050-448D8D81/type-metal-oxide-semiconductor-transistor-sequence-operations-circuit.jpg"><img src="https://cdn.britannica.com/34/63034-050-448D8D81/type-metal-oxide-semiconductor-transistor-sequence-operations-circuit.jpg?w=300" alt="making a microchip" data-width="1526" data-height="1600" loading="eager"></picture><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/34/63034-050-448D8D81/type-metal-oxide-semiconductor-transistor-sequence-operations-circuit.jpg" data-href="/media/1/457736/109640">making a microchip</a><span>The sequence of operations in making one type of integrated circuit, or microchip, called an n-channel (containing free electrons) metal-oxide semiconductor transistor. First, a clean p-type (containing positively charged “holes”) silicon wafer is oxidized to produce a thin layer of silicon dioxide and is coated with a radiation-sensitive film called a resist (a). The wafer is masked by lithography to expose it selectively to ultraviolet light, which causes the resist to become soluble (b). Light-exposed areas are dissolved, exposing parts of the silicon dioxide layer, which are removed by an etching process (c). The remaining resist material is removed in a liquid bath. The areas of silicon exposed by the etching process are changed from p-type (pink) to n-type (yellow) by exposure to either arsenic or phosphorus vapour at high temperatures (d). Areas covered by silicon dioxide remain p-type. The silicon dioxide is removed (e), and the wafer is oxidized again (f). An opening is etched down to the p-type silicon, using a reverse mask with the lithography-etching process (g). Another oxidation cycle forms a thin layer of silicon dioxide on the p-type region of the wafer (h). Windows are etched in the n-type silicon areas in preparation for metal deposits (i).</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div></figcaption></figure></div></div><p class="topic-paragraph">Photochemical reactions and the properties of excited states are also critical in many commercial processes and devices. <a href="https://www.britannica.com/technology/technology-of-photography" class="md-crosslink " data-show-preview="true">Photography</a> and <a href="https://www.britannica.com/technology/electrophotography" class="md-crosslink " data-show-preview="true">xerography</a> are both based upon photochemical processes, while the manufacture of <a href="https://www.britannica.com/science/semiconductor" class="md-crosslink " data-show-preview="true">semiconductor</a> chips or the preparation of masks for printing <a href="https://www.britannica.com/topic/newspaper" class="md-crosslink " data-show-preview="true">newspapers</a> relies on UV light to destroy molecules in selected regions of <a href="https://www.britannica.com/science/polymer" class="md-crosslink " data-show-preview="true">polymer</a> masks.</p><span class="marker MOD3 mod-inline"></span></section> <span class="marker h2"></span><section data-level="1" id="ref277508"><h2 class="h1">History</h2> <span class="marker PREMOD4 mod-inline"></span><p class="topic-paragraph">The use of photochemistry by humans began in the late <a href="https://www.britannica.com/event/Bronze-Age" class="md-crosslink " data-show-preview="true">Bronze Age</a> by 1500 <span class="text-smallcaps">bce</span> when Canaanite peoples settled the eastern coastline of the Mediterranean. They prepared a purple fast <span id="ref1006729"></span><a href="https://www.britannica.com/technology/dye" class="md-crosslink " data-show-preview="true">dye</a> (now called 6,6’-dibromoindigotin) from a local <a href="https://www.britannica.com/animal/mollusk" class="md-crosslink " data-show-preview="true">mollusk</a>, using a photochemical reaction, and its use was later mentioned in <a href="https://www.britannica.com/event/Iron-Age" class="md-crosslink " data-show-preview="true">Iron Age</a> documents that described earlier times, such as the epics of <a href="https://www.britannica.com/biography/Homer-Greek-poet" class="md-crosslink " data-show-preview="true">Homer</a> and the Pentateuch. In fact, the word <a href="https://www.britannica.com/place/Canaan-historical-region-Middle-East" class="md-crosslink " data-show-preview="true"><em>Canaan</em></a> may mean “reddish purple.” This dye, known as <span id="ref1006749"></span><a href="https://www.britannica.com/science/Tyrian-purple" class="md-crosslink " data-show-preview="true">Tyrian purple</a>, was later used to colour the <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="cloaks" href="https://www.britannica.com/dictionary/cloaks" data-type="EB">cloaks</a> of the Roman Caesars.</p><span class="marker MOD4 mod-inline"></span> <span class="marker PREMOD5 mod-inline"></span><p class="topic-paragraph">In the simplest photochemical process, <span id="ref1006458"></span><a href="https://www.britannica.com/science/excited-state" class="md-crosslink ">excited state</a>s can emit light in the form of fluorescence or <span id="ref499212"></span><a href="https://www.britannica.com/science/phosphorescence" class="md-crosslink " data-show-preview="true">phosphorescence</a>. In 1565, while investigating a Mexican wood that relieved the excruciating pain of urinary stones, Spanish physician Nicolás Monardes made an aqueous (water-based) extract of the wood, which glowed blue when exposed to sunlight. In 1853 English physicist <span id="ref1006769"></span><a href="https://www.britannica.com/biography/Sir-George-Gabriel-Stokes-1st-Baronet" class="md-crosslink " data-show-preview="true">George Stokes</a> noticed that a <a href="https://www.britannica.com/science/quinine" class="md-crosslink " data-show-preview="true">quinine</a> solution exposed to a <a href="https://www.britannica.com/science/lightning-meteorology" class="md-crosslink " data-show-preview="true">lightning</a> flash gave off a brief blue glow, which he called <span id="ref499210"></span><a href="https://www.britannica.com/science/fluorescence" class="md-crosslink " data-show-preview="true">fluorescence</a>. Stokes realized that lightning gave off energy in the form of UV light. The quinine <a href="https://www.britannica.com/science/molecule" class="md-crosslink " data-show-preview="true">molecules</a> absorbed this energy and then reemitted it as less-energetic blue radiation. (Tonic water also glows blue because of quinine, which is added to provide a bitter taste.)</p><a class="link-module shadow-sm d-block qa-quiz-module" href="/quiz/ins-and-outs-of-chemistry" data-link-module-iframe-link=""> <img loading="lazy" src="https://cdn.britannica.com/81/151881-131-9E9DD08E/hand-person-fluid-beaker-flask-experiments-Chemistry.jpg" alt="A person's hand pouring blue fluid from a flask into a beaker. Chemistry, scientific experiments, science experiments, science demonstrations, scientific demonstrations." class="rounded-sm mr-15" width="70" /> <div class="line-clamp clamp-5"> <div class="module-title bg-green">Britannica Quiz</div> <div class="font-weight-semi-bold mt-5">Ins and Outs of Chemistry</div> </div> </a><span class="marker MOD5 mod-inline"></span> <span class="marker PREMOD6 mod-inline"></span><p class="topic-paragraph">In the 16th century Florentine sculptor <span id="ref1006829"></span><a href="https://www.britannica.com/biography/Benvenuto-Cellini-Italian-artist" class="md-crosslink " data-show-preview="true">Benvenuto Cellini</a> recognized that a <span id="ref1006830"></span><a href="https://www.britannica.com/topic/diamond-gemstone" class="md-crosslink " data-show-preview="true">diamond</a> exposed to sunlight and then placed into the shade gave off a blue glow that lasted for many seconds. This process is called phosphorescence and is distinguished from <span id="ref1006310"></span><a href="https://www.britannica.com/science/fluorescence" class="md-crosslink " data-show-preview="true">fluorescence</a> by the length of time it persists. <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="Synthetic" href="https://www.merriam-webster.com/dictionary/Synthetic" data-type="MW">Synthetic</a> inorganic <a href="https://www.britannica.com/science/phosphor" class="md-crosslink " data-show-preview="true">phosphors</a> were prepared in 1603 by cobbler-alchemist <span id="ref1006849"></span><a href="https://www.britannica.com/biography/Vicenzo-Cascariolo" class="md-crosslink ">Vincenzo Cascariolo</a> of Bologna by reducing the natural mineral <a href="https://www.britannica.com/science/barium" class="md-crosslink " data-show-preview="true">barium</a> sulfate with <a href="https://www.britannica.com/science/charcoal" class="md-crosslink " data-show-preview="true">charcoal</a> to synthesize <span id="ref1006850"></span><a href="https://www.britannica.com/science/barium-sulfide" class="md-crosslink ">barium</a> sulfide. Exposure to sunlight caused the phosphor to emit a long-lived yellow glow, and it was sufficiently regarded that many traveled to Bologna to collect the mineral (called <span id="ref1006869"></span><a href="https://www.britannica.com/science/Bologna-stone" class="md-crosslink " data-show-preview="true">Bologna stones</a>) and make their own phosphor. Subsequent work by Italian astronomer <span id="ref1006870"></span><a href="https://www.britannica.com/biography/Niccolo-Zucchi" class="md-crosslink " data-show-preview="true">Niccolò Zucchi</a> in 1652 demonstrated that the <span id="ref1006318"></span><a href="https://www.britannica.com/science/phosphorescence" class="md-crosslink " data-show-preview="true">phosphorescence</a> is emitted at longer wavelengths than needed to excite the phosphor; for instance, blue phosphorescence follows UV excitation in diamonds. In addition, in 1728 Italian physicist <span id="ref1006909"></span>Francesco Zanotti showed that phosphorescence keeps the same colour even when the colour of the excitation radiation is altered to increasing energy. These same properties are also true of fluorescence.</p><span class="marker MOD6 mod-inline"></span> <span class="marker PREMOD7 mod-inline"></span><p class="topic-paragraph">The modern era of organic photochemistry began in 1866, when Russian chemist <span id="ref1006929"></span>Carl Julius von Fritzche discovered that a concentrated <a href="https://www.britannica.com/science/anthracene" class="md-crosslink " data-show-preview="true">anthracene</a> solution exposed to <span id="ref499186"></span><a href="https://www.britannica.com/science/ultraviolet-radiation" class="md-crosslink " data-show-preview="true">UV</a> <span id="ref499178"></span><a href="https://www.britannica.com/science/radiation" class="md-crosslink " data-show-preview="true">radiation</a> would fall from the solution as a <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="precipitate" href="https://www.britannica.com/dictionary/precipitate" data-type="EB">precipitate</a>. This precipitation happens because the anthracene molecules join together in pairs, or dimers, which are no longer soluble.</p><div class="module-spacing"> <DIV class="marketing-INLINE_SUBSCRIPTION marketing-content" data-marketing-id="INLINE_SUBSCRIPTION"><style> .student-promo-banner-wrapper { container-type: inline-size; margin-bottom: 15px; } @container (min-width: 475px) { .student-promo-banner { flex-direction: row; } .student-promo-banner-img-wrapper { margin-bottom: 0; margin-right: 10px; justify-content: flex-start; } .student-promo-banner-text-wrapper { text-align: left; margin-bottom: 0px; margin-left: 10px; } .student-promo-banner-button-wrapper { margin-right: 0; } }</style> <div class="student-promo-banner-wrapper"> <div class="student-promo-banner d-flex flex-column align-items-center bg-blue rounded p-20"> <div class="student-promo-banner-img-wrapper mb-20 mr-0 d-flex justify-content-center"> <img class="rounded" style="max-width: 100px; min-width: 80px" src="https://cdn.britannica.com/marketing/BlueThistle.webp" /> </div> <div class="student-promo-banner-text-wrapper ml-0 mb-10 text-center text-white"> <div class="h2 mb-10">Get Unlimited Access</div> <div class="h4 font-weight-semi-bold">Try Britannica Premium for free and discover more.</div> </div> <div class="student-promo-banner-button-wrapper d-flex justify-content-center align-items-center ml-auto mr-auto"> <a class="btn btn-m btn-orange" href="https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=inline-cta&utm_campaign=black-friday-2024">Subscribe</a> </div> </div> </div> </DIV></div><span class="marker MOD7 mod-inline"></span> <span class="marker PREMOD8 mod-inline"></span><p class="topic-paragraph">In the 19th and early 20th centuries, scientists developed a fundamental understanding of the basis for <span id="ref1006311"></span><a href="https://www.britannica.com/science/fluorescence" class="md-crosslink " data-show-preview="true">fluorescence</a> and phosphorescence. The foundation was the realization that the materials (dyes and phosphors) must have the capability of absorbing optical radiation (the <span id="ref1006889"></span><a href="https://www.britannica.com/science/Grotthuss-Draper-law" class="md-crosslink ">Grotthus-Draper law</a>). German chemist <span id="ref1006930"></span><a href="https://www.britannica.com/biography/Robert-Bunsen" class="md-crosslink " data-show-preview="true">Robert Bunsen</a> and English chemist <span id="ref1006949"></span><a href="https://www.britannica.com/biography/Henry-Enfield-Roscoe" class="md-crosslink ">Henry Roscoe</a> demonstrated in 1859 that the amount of fluorescence or phosphorescence was determined by the total amount of optical radiation absorbed and not the energy content (i.e., the wavelength, colour, or frequency) of the radiation. In 1908 German physicist <span id="ref1006950"></span><a href="https://www.britannica.com/biography/Johannes-Stark" class="md-crosslink " data-show-preview="true">Johannes Stark</a> realized that absorption of radiation was a consequence of a <span id="ref499191"></span><a href="https://www.britannica.com/science/quantum-chromodynamics" class="md-crosslink " data-show-preview="true">quantum</a> <span id="ref499213"></span><a href="https://www.britannica.com/science/transition-atomic-physics" class="md-crosslink " data-show-preview="true">transition</a>, and this was further extended by German physicist <span id="ref499188"></span><a href="https://www.britannica.com/biography/Albert-Einstein" class="md-crosslink " data-show-preview="true">Albert Einstein</a> in 1912 to include the <a href="https://www.britannica.com/science/conservation-of-energy" class="md-crosslink " data-show-preview="true">conservation of energy</a>—the <a href="https://www.britannica.com/science/internal-energy" class="md-crosslink autoxref " data-show-preview="true">internal energy</a> introduced to the molecule by absorption must be equal to the total of the energies of each individual process of energy <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="dissipation" href="https://www.merriam-webster.com/dictionary/dissipation" data-type="MW">dissipation</a>. <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="Implicit" href="https://www.merriam-webster.com/dictionary/Implicit" data-type="MW">Implicit</a> in the previous sentence is the <a href="https://www.britannica.com/science/photochemical-equivalence-law" class="md-crosslink autoxref " data-show-preview="true">photochemical equivalence law</a>, also called the Stark-Einstein law, which states that a single molecule may absorb exactly one <span id="ref499190"></span><a href="https://www.britannica.com/science/photon" class="md-crosslink " data-show-preview="true">photon</a> of light. The amount of energy absorbed by a substance is the product of the number of photons absorbed and the energy of each photon, but it is the radiation intensity and the number of absorbed photons per second, and not their energy, that determine the extent of photochemical processes.</p><div class="one-good-fact-module"> </div><span class="marker MOD8 mod-inline"></span> <span class="marker PREMOD9 mod-inline"></span><p class="topic-paragraph">The contemporary <a href="https://www.britannica.com/science/quantum-mechanics-physics" class="md-crosslink " data-show-preview="true">quantum mechanical</a> description of the absorption of optical radiation involves promotion of an electron from a low-energy <a href="https://www.britannica.com/science/orbital" class="md-crosslink " data-show-preview="true">orbital</a> to a more energetic orbital. This is <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="synonymous" href="https://www.britannica.com/dictionary/synonymous" data-type="EB">synonymous</a> with saying that the molecule (or atom) is promoted from its <span id="ref1006476"></span><a href="https://www.britannica.com/science/ground-state" class="md-crosslink ">ground state</a> (or lowest energy state) to an <span id="ref1006459"></span><a href="https://www.britannica.com/science/excited-state" class="md-crosslink ">excited state</a> (or higher energy state). This excited-state molecule often has drastically different properties from the ground-state molecule. In addition, a molecule’s excited state is short-lived because a sequence of events will either return it to its original ground state or form a new chemical species that will eventually reach its own ground state.</p><span class="marker MOD9 mod-inline"></span></section> <span class="marker end-of-content"></span><span class="marker after-article"></span></div> <div id="chatbot-root"></div> </div> </div> </div> <div class="ai-dialog-placeholder"></div> </div> </div> <aside class="col-md-da-320"></aside> </div> </div> </div> </div> </article></div> </div></div> </div> </main> <div id="md-footer"></div> <noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-5W6NC8" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript> <script type="text/javascript" id="_informizely_script_tag"> var IzWidget = IzWidget || {}; (function (d) { var scriptElement = d.createElement('script'); scriptElement.type = 'text/javascript'; scriptElement.async = true; scriptElement.src = "https://insitez.blob.core.windows.net/site/f780f33e-a610-4ac2-af81-3eb184037547.js"; var node = d.getElementById('_informizely_script_tag'); node.parentNode.insertBefore(scriptElement, node); } )(document); </script>  <script> window.ap3c = window.ap3c || {}; var ap3c = window.ap3c; ap3c.cmd = ap3c.cmd || []; ap3c.cmd.push(function() { ap3c.init('ZO4siT4cLwnykPnzZWJtd3Byb2Q', 'https://engage.email.britannica.com/'); ap3c.track({v: 0}); }); ap3c.activity = function(act) { ap3c.act = (ap3c.act || []); ap3c.act.push(act); }; var s, t; s = document.createElement('script'); s.type = 'text/javascript'; s.src = "https://engage.email.britannica.com/app.js"; t = document.getElementsByTagName('script')[0]; t.parentNode.insertBefore(s, t); </script> <script class="marketing-page-info" type="application/json"> {"pageType":"Topic","templateName":"DESKTOP","pageNumber":1,"pagesTotal":4,"pageId":457736,"pageLength":1140,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-130/js/libs/jquery-3.5.0.min.js?v=3.130.14"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-130/dist/topic-page.js?v=3.130.14' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"A","adLeg":"A","userType":"ANONYMOUS","pageType":"Topic","pageSubtype":null,"articleTemplateType":"PAGINATED","gisted":false,"pageNumber":1,"hasSummarizeButton":false,"hasAskButton":false} </script> <script type="text/javascript"> EBStat={accountId:-1,hostnameOverride:'webstats.eb.com',domain:'www.britannica.com', json:''}; </script> <script type="text/javascript"> ( function() { $.ajax( { dataType: 'script', cache: true, url: '//www.britannica.com/webstats/mendelstats.js?v=1' } ) .done( function() { try {writeStat(null,EBStat);} catch(err){} } ); })(); </script> <div id="bc-fixed-dialogue"></div> </body> </html>

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Photochemical reaction | Light-Induced Chemical Changes | Britannica