Bound Excitons in Sr$_{2}$CuO$_{3}$
  We investigated temperature dependent optical spectra of the one-dimensional chain compound Sr$_{2}$CuO$_{3}$. The charge transfer transition polarized along the chain direction shows a strongly asymmetric line shape as expected in one-dimensional extended Hubbard model. At low temperature the charge transfer peak shows a large blue shift and reveals additional sharp peaks at the gap. Even though many spectroscopic studies suggest that this material can not have a bound exciton based on the one-dimensional extended Hubbard model, we attribute the additional sharp peaks to excitons, which come to exist due to the long range Coulomb interaction. 
K. W. Kim, G. D. Gu, C. C. Homes, T. W. Noh