Pulsed EPR spectroscopy of the photoexcited triplet states of thiophene oligomers in frozen solution

We present a triplet EPR study on photoexcitations in two different series of well defined thiophene oligomers. The first series consists of the so-called 'end-capped' oligothiophenes with chain length n = 2-6. In the second series dodecyl chains are linked to alpha-oligothiophenes with n = 3, 6, and 8. Measurements of the triplet EPR spectra were performed using pulsed EPR spectroscopy. The characteristic lineshape of the EPR spectra provides evidence that photoexcitation leads to molecular triplet states in all compounds. We determined the fine structure parameters D and E and the population of the triplet eigenfunctions P-x, P-y, P-z performing spectra simulations. The chain length dependence and the conformation dependence are discussed.