Thienothiophene

In organic chemistry, thienothiophene is any of several compounds consisting of two fused thiophene rings. They have the molecular formula C₆H₄S₂. Three constitutional isomers have been synthesized: thieno[3,2-b]thiophene, thieno[2,3-b]thiophene, and thieno[3,4-b]thiophene. The other isomer features S(IV) and is less stable.^[1][2] 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.^[3] More efficient syntheses of this and the other two stable thienothiophenes involve cyclization reactions of substituted thiophenes.^[1]

Three thienothiophenes, being aromatic and bicyclic, are often compared to naphthalene. They and their benzo derivatives are the topic of research in molecular electronics.^[4] They nor their derivatives are found naturally.

• Isomers of thienothiophene
• 
  Thieno[3,2-b]thiophene
  CAS RN 251-41-2
  m.p. 56.0-56.5 °C
  white solid.
• 
  Thieno[2,3-b]thiophene
  CAS RN 250-84-0
  b.p. 102 °C at 16 mmHg
  colorless oil.
• 
  Thieno[3,4-b]thiophene
  CAS RN 250-65-7
  m.p. 7.0-7.5 °C
  colorless oil.
• 
  2λ⁴δ²-Thieno[3,4-c]thiophene
  CAS RN 24976-21-4
  not isolated.^[5]