Size dependence of triplet and singlet states of α-oligothiophenes

Energetic positions of singlet and triplet states of small alpha-oligothiophenes with n = 2, 3 and 4 monomer units were measured by photodetachment photoelectron spectroscopy (PD-PES), phosphorescence and time resolved pump-probe spectroscopy. The triplet energy of T-1 and T-2 were determined experimentally for the first time in the gas phase. The triplet manifold could be extended including the levels observed by triplet-triplet absorption. The transition energies of singlet and triplet states decrease with increasing size n of the oligothiophenes following the extended free electron model(FEMO). The Delta E(n) curves have different slopes for the singlet S-1 and triplet T-2 states with an intersection at n = 2. The mutual positions of S-1 and T-2 states at equal energy for 2T explain the extremely high triplet quantum yield of 2T and its successive decrease for larger nT. The present measurements, combined with literature data, allow for the first time a complete picture to be drawn of all electronic states which are relevant for photodynamic processes in oligothiophenes with n = 1-4.