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Progress in the synthesis of squaric acid

Dec 09, 2021Leave a message

Squaraic acid (3, 4. Dihydroxy 3-cyclobutene-1,2-dione, 1) is one of the representative substances in oxygen compounds (Oxocarbons), which is derived from its quaternary ring structure and strong acidity The name was first synthesized by Cohen et al in 1959. Because of the unique chemical structure and active chemical properties of squaric acid, chemists from various countries have conducted in-depth and extensive research on the chemistry of squaric acid.

 

Squaric acid is a strong dibasic acid, pK = 0.5, pK 2-3.5. Due to the strong hydrogen bond between molecules, its melting point is very high (>300"C), and its solubility in cold water is only 2. Sprenger proposed that squaric acid molecules have a resonance structure of l-, lb, and lc, which explains the resonance structure The good stability and strong electrophilic reactivity of the squaric acid molecule are improved.

 

Squaric acid 1


The infrared spectrum of the squaric acid dianion (I) shows that there is no carbonyl characteristic peak in the molecule, LC. AO's calculations on the energy and strength of the city prove that the molecule is a symmetrical structure. According to Huckel's rule, the squaric acid divalent anion is aromatic, and West incorporates it into the carbon-oxygen aromatic system with the general chemical formula C.O: one. Other carbon and oxygen compounds also include dihydroxy cycloacetone divalent anion (I), rosinate divalent anion (II) and crotonic acid divalent ion (IV).

 

Squaric acid 2


As a typical molecule in the aromatic system of carbon oxides, squaraine is very active in chemical properties and can react with many compounds and is called a universal intermediate. Its simple cocoon derivatives such as squaraine ester, squaraic acid amide and squaraic acid chloride also show good chemical activity. I is formed by the condensation of squaraine and electric-rich aromatic rings such as aromatic amines, phenols, and ammonia-containing heterocyclic compounds. Squaraine cyanine compounds are a new type of near-infrared dyes with excellent properties. Due to their excellent optical properties and good light and thermal stability, their research has become one of the hotspots in the research of functional dyes in recent years. The optical disc made of the cyanine squaraine dye coating film has been used in practical applications, and it has shown good application performance in the fields of liquid crystal display, organic solar battery research, non-linear optical materials, LB research and so on. In addition, because many squaraine dyes have strong fluorescence emission in the near-infrared region, they can be used as such good near-infrared fluorescent probes for the analysis of vital substances, and thus can be applied to near-infrared fluorescence immunoassay. , DNA sequencing and other frontier fields of life sciences.


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