Negative ion photoelectron spectroscopy of 2,2'-bithiophene cluster anions, (2T)_n^- (n = 1-100)

Cluster anions of 2,2'-bithiophene, (2T)_n ^-, were produced up to n ∼ 500 in the gas-phase. The energetics of the excess electron in the (2T)_n ^- clusters with n =1-100 were explored by negative ion photoelectron spectroscopy. When the vertical detachment energies (VDEs) obtained from the photoelectron spectra were analyzed by a plot against n^-1/3, it has been revealed that the excess electron trapping level thus extrapolated is located at ∼0.8 eV below the conduction band minimum (i.e. LUMO) of the 2T thin film. The large slope of the VDEs vs. n^-1/3 plot suggests that the neutral 2T molecules surrounding the anion core take non-planar twisted conformations with permanent dipole moments, resulting in the exceedingly deep trapping of the excess electron in the 2T cluster anions.