A theoretical study on the photoisomerization of thiophene

The photoisomerization of thiophene (3–5 and 2–4 transpositions of carbon atoms) has been studied with ab initio SCF and CI calculations. A possible reaction mechanism from the lowest excited singlet state ¹B₂ of thiophene can be proposed from potential surface via Dewar thiophene. The ¹B₂ state of thiophene would easily convert to the biradical intermediate by almost one step. The internal conversion of this species to the S₀ state would cause to the transposition of carbon atoms. The effect of phenyl substituent is also discussed.