Inorganic chemistry, thienothiophene is any of several compounds consisting of two fused thiophene rings. They have the molecular formulaC6H4S2. Three constitutional isomers have been synthesized: thieno[3,2-b]thiophene, thieno[2,3-b]thiophene, and thieno[3,4-b]thiophene. Other isomers feature S(IV) and are less stable.[1] Thieno[2,3-b]thiophene was the first member of the series to be isolated. It was obtained in very low yield upon heating citric acid, a source of a six-carbon linear chain, with P4S10.[2] More efficient syntheses of this and the other two stable thienothiophenes involve cyclization reactions of substituted thiophenes.[3]
Three thienothiophenes, being aromatic and bicyclic, are often compared to naphthalene. They are the topic of academic research. They have no commercial applications nor are they or their derivatives found naturally.
![Thieno[3,2-b]thiophene CAS RN 251-41-2 m.p. 56.0-56.5 °C white solid.](http://en.m.wikipedia.org/wiki/File:Thienothiophene251-41-2.png)
![Thieno[2,3-b]thiophene CAS RN 250-84-0 b.p. 102 °C at 16 mmHg colorless oil.](http://en.m.wikipedia.org/wiki/File:Thienothiophene250-84-0.png)
![Thieno[3,4-b]thiophene CAS RN 250-65-7 m.p. 7.0-7.5 °C colorless oil.](http://en.m.wikipedia.org/wiki/File:Thienothiophene250-65-7.png)
![2λ4δ2-Thieno[3,4-c]thiophene CAS RN 24976-21-4 not isolated.[4]](http://en.m.wikipedia.org/wiki/File:24976-21-4S%28IV%29.png)