A theoretical study on the low-lying excited states of 2,2′:5′,2"-terthiophene and 2,2′:5′,2":5",2′"-quaterthiophene

The nature and properties of the low-lying singlet and triplet valence excited states of 2,2'.5',2"-terthiophene (terthiophene) and 2,2'.5',2":5",2"'-quaterthiophene (tetra thiophene) are discussed on the basis of high-level ob initio computations. The spectroscopic features determined experimentally for short alpha-oligothiophenes are rationalised on theoretical grounds. Special attention is devoted to the nonradiative decay process through intersystem crossing (ISC) from the singlet to the triplet manifold, which is known to be relatively less efficient in tetrathiophene. Along the geometry relaxation of the S-1 state of terthiophene, the S-1 and T-2 states become degenerate, which leads to a favourable situation for the occurrence of ISC The parallel process is expected to be less favoured in tetrathiophene because of the less efficient spin-orbit coupling and the increase of the S-1-T-2 energy gap.