Photochemistry
The study of chemical reactions, isomerizations and physical behavior that may occur under the influence of ultraviolet or visible light is called as Photochemistry. Photochemistry is study of interactions between atoms, small molecules, and light. The pillars of photochemistry are UV/VIS spectroscopy, photochemical reactions in the organic chemistry and photosynthesis in biochemistry. There are significant processes based in the photochemistry principles. One case is photosynthesis, which some plants use light to produce glucose in the chloroplasts to contribute to cell metabolism. Photochemistry is concerned with the absorption, excitation and emission of photons by atoms, molecules, atomic ions, molecular ions, etc. The first law of photochemistry, known as the Grotthuss-Draper law, states that light should be absorbed by a chemical substance in order for a photochemical reaction to take place. The 2nd law of photochemistry, the Stark-Einstein law, states that for each photon of light absorbed by a chemical system, only 1 molecule is activated for a photochemical reaction. This is called as the photoequivalence law and was derived at the time when the quantum theory of light was being developed. The simplest photochemical process is seen with the absorption and emission of a photon by a gas phase atom such as sodium. The glucose is used by the plant's mitochondria to produce adenosine triphosphate. Medicine bottles are made with darkened glass to prevent the medicine itself from reacting with light chemically. In fireflies, an enzyme in abdomen works to produce bioluminescence. The mercaptans are produced by photochemical addition of hydrogen sulfide (H2S) to alpha olefins. Among their several uses as a chemical reagent these mercaptans are used to provide a distinctive odor to otherwise odorless natural gas.
Many polymerizations are started by photoinitiatiors which decompose upon absorbing light to produce the required free radicals for Radical polymerization some photochemical pathways allow the synthesis of few classes of chemical compounds, like cyclobutanes, stereospecific compounds, which cannot be easily prepared using conventional organic synthesis. Photochemistry can also be introduced to laymen as a reaction which proceeds with the absorption of light. Normally a reaction occurs when a molecule gains the necessary activation energy to undergo change. In the case of photochemical reactions light gives the activation energy. The activation energy can be provided in the form of heat or a spark. When a molecule is irradiated with a frequency which matches the energy difference between excited and ground state, a photochemical reaction may occur. These types of reactions differ considerably from the traditional reactions. Therefore reactions can occur which are highly endothermic in the ground state. This can allow reactions which are not possible for the electronic reasons in the ground state. Photochemical reaction includes singlet and triplet states. Thermal reactions usually show singlet states. In the photochemical reaction intermediates can be formed which are not accessible at thermal conditions. Photochemical reactions include electronic reorganization initiated by electromagnetic radiation.