Basic types of photochemical reaction

The chemical behavior of molecules usually depends on the weakest of the outer electrons, and the molecules in the electron excited state differ from the molecules in the ground state in terms of energy and the wave function of the electrons, and therefore their chemical reaction capacity is also different. The chemical reactions that occur in electron-excited molecules excited by light are called photochemical reactions to distinguish them from the chemical reactions that occur in ordinary ground-state molecules. Photochemical reaction is more selective than thermal reaction and has two characteristics: first, the reaction rate mainly depends on the intensity of light and is less affected by temperature; Second, light enables certain non-spontaneous processes.
Photochemical reactions include photolysis reaction, photooxidation or reduction reaction, photopolymerization reaction, photoisomerization reaction, photosubstitution reaction and photoaddition reaction. Photochemical reactions can be monomolecular reactions, such as photolysis and photoisomerization reactions, or bimolecular reactions, such as dimerization between excited molecules and ground state molecules, REDOX reactions, substitution reactions, and addition reactions between excited molecules and other molecules. It is not difficult to understand that photochemical reactions can only occur during the lifetime of the excited electron state. The above photochemical reactions are often encountered in fluorescence analysis and may cause more serious problems
The influence is mainly photolytic reaction and photooxidation or reduction reaction.
4.1.2.1 Unimolecular dissociation
It is of great theoretical and practical significance to cut organic monomolecules by photochemical reaction and make the molecules undergo homolytic cleavage to obtain free radicals. Free radicals have active chemical properties, ideal reaction rate and yield, and play an important role in photochemical reactions [1]. This kind of chemical reaction is spontaneous, in the reaction process each state is determined by the previous state, the starting point of the whole reaction, that is, the reactant, its structure and its environment (reaction conditions), determine the reaction of all the Liao.
4.1.2.2 Photooxidation reaction
Photooxidation reaction is the addition reaction of organic molecules and oxygen under the action of light irradiation and photosensitizer. The photooxidation reaction is mild and does not require chemical pure reagents and heavy metal catalysts. It has been frequently reported in the synthesis of fine chemicals such as drugs, flavors, detergents and dyes, such as the oxidation of alcohols to aldehydes, ketones, cocodiene to cyclic peroxides. It has also been reported in the preparation of aryl acids. For different reaction systems and reactants · Just changing the light wavelength and solvent can have obvious effects. Compared with the traditional oxidation reverse point, photooxidation reaction has obvious advantages and the environmental pollution caused by it is small. Therefore, it is predicted that the application range of photooxidation will be further expanded.
The influence is mainly photolytic reaction and photooxidation or reduction reaction.
4.1.2.3 Photoreduction reaction
Photoreduction reaction is a photocatalytic process in which organic molecules extract hydrogen atoms from hydrogen donors. At present, more research is on? For example, the photoreduction reaction of biphenyl ketone in diphenylcarbinol can produce tetraphenyl glycol, and the photoreduction reaction saves a large number of metal catalysts required in the traditional reduction reaction, such as Pt, Pb, Mn and Co, and the conditions are milder, which saves energy consumption and avoids the harm of heavy metal catalysts to the human body. From the environmental point of view, photoreduction reaction has incomparable advantages over traditional methods.
4.1.2.4 Photosubstitution reaction
The most common photosubstitution reaction is the fatty photochlorination reaction, which refers to the chlorination process, such as chloromethane, dichloromethane, chloroform, and carbon tetrachloride. In addition, photochlorosulfonation, photooxo sulfonation, photonitrosation and phobophosphorylation are all substitution reactions that take place by photocatalysis. A variety of cleaners, insecticides, antioxidants and intermediates can be prepared by photogovernment reaction. These reactions have the characteristics of milder, higher recovery and better selectivity than other ways, and some products can not even be produced by non-photooxidation reaction, so it has great potential in industrial production.
4.1.2.5 Photopolymerization reaction
Photopolymerization is a method of photochemical polymerization of monomers. The light source used is mainly high pressure and medium pressure water N eight constant-z plus plus (continuous light). Photopolymerization has been applied in photography, printing and printed circuits, because it does not require traditional initiators to reduce the generation of pollution
Photopolymerization can be divided into pure photopolymerization and photopolymerization.
(1) Pure photopolymerization 'It has the characteristics of condensation polymerization reaction, meaning cup especially phase * * heart large * the energy of the most t and when divided, tie up the chemical pair containing inverse energy groups. In the polymerization process, the monomer molecules directly absorb the corresponding wavelength of light, or are excited by the energy transfer of the photosensitizer. Chemical coupling reaction occurs
Should form a new chain link, also known as photosensitive polymerization; The degree of polymerization of the polymer obtained by the extrapolate method is relatively low.
Photosensitizer is a substance that can be photoexcited and open to polymerization of Zr-like and H monomers. The actual application of the photoinitiator king must have a network of substances that are activated by the book of Jude and generate active centers (free radicals or ions) and trigger monomer polymerization. actual
The system composed of benzophenone or anthracene titi and hydrogen donor can produce free radicals through photochemical hydrogen extraction reaction, in which the free radicals produced by hydrogen donor mainly play an initiating role; The second is benzoin and acetophenone derivatives. At present, this kind of reaction is mainly used for polymer modification or synthesis of prepolymers with such reactive functional groups for optical imaging systems.
The common mechanism of photosensitive polymerization is that as long as dyes or initiators or photosensitivities capture a photon, active groups such as free radicals or cations will be generated, leading to monomer polymerization. Or the photosensitizer itself can not directly form free radicals, but the light absorption after the energy transfer to the monomer or initiator to trigger polymerization, two-photon polymerization mechanism and single photon is basically the same, only the photon absorption mechanism is different. The general requirements of double photoprephotopolymerization photosensitive initiators are as follows: having a common cabinet large r bond, electron withdrawing group, electron donating group, the longer the conjugate large r bond, the larger the absorption cross section; The dipole moment and polarizability of molecules have great influence on the absorption cross section of molecules.
(2) photoinduced polymerization, including free radical polymerization and ionic polymerization, is the polymerization caused by photoexcitation of photoinitiator, this method has the characteristics of low polymerization temperature, high reaction selectivity and easy control, can occur the reaction that can not be carried out by general molecules, and expand the means of obtaining polymers. Compared with the commonly said free radical polymerization, the main difference is the difference in initiation mode, that is, the active center is generated by the photochemical method, the polymerization activation energy is low (generally below 5kcal/molO), and the polymerization degree increases with the reaction temperature. Its chain growth is a chain reaction, and the polymerization quantum yield of this method can be as high as 102 ~103.
4.1.2.6 Photocatalytic reaction
There is still some debate about the term photocatalysis, with some arguing that the term photocatalytic reaction is unscientific because it implies that
Light is a catalyst in a reaction, whereas in fact it is only a reactant in a chemical reaction. Now people still use the term photocatalysis, which should be understood as the organic combination of photochemistry and catalyst, and light and catalyst are necessary conditions for triggering and promoting photocatalytic oxidation reactions. Photocatalytic reaction is a way of interaction between light and matter, a fusion of light reaction and catalytic reaction, and a chemical reaction carried out under the simultaneous action of light and catalyst.
4.1.2.7 Photosynthesis
In 1905, the British botanist F.F. Blackman proposed that photosynthesis includes the "light reaction" that requires light and the "dark reaction" that does not require light, and the two are interdependent, the energy absorbed during the light reaction is supplied to the dark reaction to synthesize polysaccharides containing high energy. The 20th century
In the second generation, O. Warburg further proposed that in light reactions it is not the temperature that is north of the degree F expansion + the probability you use for photosynthesis. Photophosphorylation is photosynthesis Neil's work changed the long-held view that photosynthesis must release oxygen and expanded the concept of photosynthesis. Photophosphorylation is photosynthesis
Plays an important role in energy transfer processes. In 1954, DI Arnon, while studying carbon dioxide assimilation with spinach chloroplasts, found that chlorophyll was excited by light to produce electrons, phosphorylated and coupled to produce ATP during the transfer process, and the electrons returned to the chlorophyll molecule to continue the above process, which is called cyclic photophosphorylation.
The discovery of two photoreaction systems in photosynthesis has promoted the further study of photophosphorylation. This work was done primarily by American plant physiologists
R. Emerson and his collaborators from the 1940s to his H. 1T Ermen9 showed that the photosynthetic efficiency of both simultaneous irradiation is higher than that of a single irradiation (about 650nm) than that of the long-wave region (about 700nm). In 1957 they found that the combination of irradiation produced more photosynthesis than single irradiation
High efficiency. Based on their work, as well as that of others, R. Hill et al., UK, suggested that there may be
(about 700nm) excitation, System II relies on higher energy red light (about 650nm). Non-cyclic photophosphorylation is a strong supporting example. A "Z diagram" was also designed to express the synergistic effect of the two photoreaction systems, which set off a boom in the study of the structure and function of the two photoreaction systems, and promoted the study of the primary reaction and the photolytic problem of water, the core problems of photosynthesis.