(C4H3S)2.CO

from The Collaborative International Dictionary of English v.0.48

Thienone \Thi"e*none\, n. [Thienyl + ketone.] (Chem.)
   A ketone derivative of thiophene obtained as a white
   crystalline substance, {(C4H3S)2.CO}, by the action of
   aluminum chloride and carbonyl chloride on thiophene.
   [1913 Webster]