Evidence for exciton-exciton binding in a molecular aggregate

We present evidence for a biexciton state in a molecular aggregate. Femtosecond pump-probe spectroscopy of the $H$ aggregate of an epitaxially grown trivalent metal-halogen-phthalocyanine gives distinct photoinduced absorptions to a continuum of two-exciton states and to a bound biexciton state. No signature of the biexciton is found in the $J$ aggregate of a very similar trivalent metal-halogen-phthalocyanine. The strong exciton-exciton interaction in the $H$-aggregate system has its origin in the unique cofacial stacking of the molecular components in this material.