Cluster anions of 2,2'- bithiophene, (2T)(n)(-), were produced up to n similar to 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 similar to 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.