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.
@article{arxiv.1208.3972,
title = {Exciton Binding Energy in small organic conjugated molecule},
author = {Pabitra K. Nayak},
journal= {arXiv preprint arXiv:1208.3972},
year = {2012}
}