The stable radical cation of thiophene annelated with bicyclo[2.2.2]octene and its reaction with triplet oxygen to give a protonated cation of 2-butene-1,4-dione derivative.

The first isolable salt of the thiophene radical cation was prepared from the derivative annelated with two bicyclo[2.2.2]octene units, and its reaction with triplet oxygen was found to give a novel cation of a proton-chelating 2-butene-1,4-dione derivative with remarkable stability.