Ab initio study of the photochemical isomerization of thiophene derivatives

Abstract

The photochemical isomerization reactions of thiophene, thiophene-2-carbonitrile, and 2-phenylthiophene were studied using ab initio methods. The results are in agreement with the previously reported data obtained through semiempirical methods. Triplet excited thiophene is a π,σ ∗ triplet with LSOMO at −9.94 eV and HSOMO at −9.51 eV and the biradical intermediate is a π,π ∗ species with LSOMO at −10.12 eV and HSOMO at −4.82 eV. In this case, the singlet excited state can evolve giving the Dewar thiophene, while the corresponding excited triplet state cannot be obtained. Furthermore, the triplet state cannot be converted into the biradical intermediate because this intermediate shows a higher energy than the triplet state, thus preventing the formation of the cyclopropenyl derivatives. Triplet excited thiophene-2-carbonitrile is a π,π ∗ species. It shows the LSOMO at −11.38 eV and the HSOMO at −7.36 eV. In this case, the direct irradiation involves the population of the excited singlet state, and then the formation of the Dewar isomer is possible. The intersystem crossing to the triplet state can occur; however, its interconversion into the corresponding biradicals cannot be efficient considering that the biradicals show a higher energy, even if for a little amount, than that of the triplet state. Triplet excited 2-phenylthiophene is a π,π ∗ species. It shows the LSOMO at −9.32 eV and the HSOMO at −6.24 eV. In this case, the direct irradiation involves the population of the excited singlet state, and then the formation of the Dewar isomer is possible. The energy of the excited singlet state was obtained from the UV absorption of the substrate. The intersystem crossing to the triplet state cannot occur; because it shows higher energy than the corresponding singlet state. Furthermore, its interconversion into the corresponding biradicals cannot be efficient considering that the biradicals show the same energy of the triplet state. The high efficiency of this reaction can be explained on the basis of low energy of the Dewar isomer.