Exciton Binding Energy in small organic conjugated molecule
  For small organic conjugated molecules the exciton binding energy can be calculated treating molecules as conductor, and is given by a simple relation BE \approx e2/(4{\pi}{\epsilon}0{\epsilon}R), where {\epsilon} is the dielectric constant and R is the equivalent radius of the molecule. However, if the molecule deviates from spherical shape, a minor correction factor should be added. 
Pabitra K. Nayak
cond-mat.mtrl-sci