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Photochemical reaction
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- photochemical reaction, a chemical reaction initiated by the absorption of energy in the form of light. The consequence of molecules ’ absorbing light is the creation of transient excited states whose chemical and physical properties differ greatly from the original molecules.
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What happens if a molecule absorbs light?
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Jan 30, 2023 · This page explains what happens when organic compounds absorb UV or visible light, and why the wavelength of light absorbed varies from compound to compound.
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- PC1. Absorbance
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Light is composed of photons. As photons shine through the solution, some of the molecules catch the photons. They absorb the light. Generally, something in the molecule changes as a result. The molecule absorbs energy from the photon and is left in an excited state.
The electrons will absorb the energy of the light wave and change their energy state. There are several options that could happen next, either the electron returns to the ground state emitting the photon of light or the energy is retained by the matter and the light is absorbed.
In the light-dependent reactions, which take place at the thylakoid membrane, chlorophyll absorbs energy from sunlight and then converts it into chemical energy with the use of water. The light-dependent reactions release oxygen as a byproduct as water is broken apart.
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photochemical reaction, a chemical reaction initiated by the absorption of energy in the form of light. The consequence of molecules’ absorbing light is the creation of transient excited states 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 electrons, hydrogen atoms, protons, or their electronic excitation energy to other molecules. Excited states are stronger acids and stronger reductants than the original ground states.
It is this last property that is crucial in the most important of all photochemical processes, photosynthesis, upon which almost all life on Earth depends. Through photosynthesis, plants convert the energy of sunlight into stored chemical energy by forming carbohydrates from atmospheric carbon dioxide and water and releasing molecular oxygen as a byproduct. Both carbohydrates and oxygen are needed to sustain animal life. Many other processes in nature are photochemical. The ability to see the world starts with a photochemical reaction in the eye, in which retinal, a molecule in the photoreceptor cell rhodopsin, isomerizes (or changes shape) about a double bond after absorbing light. Vitamin D, essential for normal bone and teeth development and kidney function, is formed in the skin of animals after exposure of the chemical 7-dehydrocholesterol to sunlight. Ozone protects Earth’s surface from intense, deep ultraviolet (UV) irradiation, which is damaging to DNA and is formed in the stratosphere by a photochemical dissociation (separation) of molecular oxygen (O2) into individual oxygen atoms, followed by subsequent reaction of those oxygen atoms with molecular oxygen to produce ozone (O3). UV radiation that does get through the ozone layer photochemically damages DNA, which in turn introduces mutations on its replication that can lead to skin cancer.
The use of photochemistry by humans began in the late Bronze Age by 1500 bce when Canaanite peoples settled the eastern coastline of the Mediterranean. They prepared a purple fast dye (now called 6,6’-dibromoindigotin) from a local mollusk, using a photochemical reaction, and its use was later mentioned in Iron Age documents that described earlier times, such as the epics of Homer and the Pentateuch. In fact, the word Canaan may mean “reddish purple.” This dye, known as Tyrian purple, was later used to colour the cloaks of the Roman Caesars.
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Types of Chemical Reactions
In the simplest photochemical process, excited states can emit light in the form of fluorescence or phosphorescence. 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 George Stokes noticed that a quinine solution exposed to a lightning flash gave off a brief blue glow, which he called fluorescence. Stokes realized that lightning gave off energy in the form of UV light. The quinine molecules 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.)
In the 16th century Florentine sculptor Benvenuto Cellini recognized that a diamond 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 fluorescence by the length of time it persists. Synthetic inorganic phosphors were prepared in 1603 by cobbler-alchemist Vincenzo Cascariolo of Bologna by reducing the natural mineral barium sulfate with charcoal to synthesize barium 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 Bologna stones) and make their own phosphor. Subsequent work by Italian astronomer Niccolò Zucchi in 1652 demonstrated that the phosphorescence 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 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.
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Jan 23, 2023 · The molecule absorbs energy from the photon and is left in an excited state. The more of these molecules there are in the solution, the more photons will be absorbed. If there are twice as many molecules in the path of the light, twice as many photons will be absorbed.
A pigment molecule in the photosystem absorbs one photon, a quantity or “packet” of light energy, at a time. A photon of light energy travels until it reaches a molecule of chlorophyll. The photon causes an electron in the chlorophyll to become “excited.”
