Light-generated reactive species

The absorption of a light pulse instantaneously generates reactive species in high concentrations, either tlirough the fomiation of excited species or tlirough photodissociation of suitable precursors. The reaction can... 

There are essentially two types of compounds that are used in the UV curing process to absorb the light and generate reactive species. These are photoinitiators and photosensitizers. 

The polymerization of all these systems can be divided into initiation, propagation, and termination steps. The initiation step involves generation of a reactive species (free radical or acid). During irradiation with UV light, the reactive species are formed by chemical decomposition of a photoinitiator. In electron-beam (EB) curing, reactive species (radicals) are generated by interaction of accelerated electrons with organic compounds. 

Riboflavin or vitamin makes a prosthetic group of flavin enzymes where it can be reversibly reduced by hydrogen atoms (formula [38]). When exposed to light, riboflavin absorbs energy and reacts, via a triplet exited state, with other molecules such as protonated substrates or molecular oxygen, generating reactive species. In the type I photodynamic reaction, energy is transferred from a triplet sensi-tiser to O2 with the formation of O2. 

The idea of this method is to utilize a flash of intense and fast light, usually from a laser, in order to generate reactive species, especially atoms and free radicals. The... 

The word flash has been used in the history of chemistry for many years. For example, flash vacuum pyrolysis [7, 8] is a well-known technique that has been used for chemical synthesis at high temperatures. Flash laser photolysis [9, 10] serves as a powerful method for generating reactive species in a very short time and has been used for mechanistic studies of extremely fast light induced chemical processes which are complete within milliseconds or less. Flash chromatography [11] is one of the most popular techniques for separating and pmifying compounds in organic chemistry laboratories. It should be noted, therefore, that flash chemistry is a new field of chemical synthesis but that the word flash is very common in chemistry. 

The irradiation is usually carried out with light of the near UV region, in order to activate only ihc n n transition of the carbonyl function," thus generating excited carbonyl species. Depending on the substrate, it can be a singlet or triplet excited state. With aromatic carbonyl compounds, the reactive species are usually in a Ti-state, while with aliphatic carbonyl compounds the reactive species are in a Si-state. An excited carbonyl species reacts with a ground state alkene molecule to form an exciplex, from which in turn diradical species can be formed—e.g. 4 and 5 in the following example ... 

Rozanowska, M, Jarvis-Evans, J, Korytowski, W, Boulton, ME, Burke, JM, and Sarna, T, 1995. Blue light-induced reactivity of retinal age pigment. In vitro generation of oxygen-reactive species. J Biol Chem 270, 18825-18830. 

Aminolevulinic acid (ALA HCl, Levulan Kerastick) is indicated for the treatment of nonhyperkeratotic actinic keratosis of the face and scalp. It has two components, an alcohol solution vehicle and ALA HCl as a dry solid. The two are mixed prior to application to the skin. When applied to human skin, ALA is metabolized to protoporphyrin, which accumulates and on exposure to visible light produces a photodynamic reaction that generates reactive oxygen species (ROS).The ROS produce cytotoxic effects that may explain therapeutic efficacy. Local burning and stinging of treated areas of skin due to photosensitization can occur. 

Depending on the type of reactive species generated upon exposure to UV light, photoinitiators are classified as free radical, cationic, and anionic. 

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