Charge-pair states in organic molecular crystals: localized vs. delocalized description

The Merrifield model of electronic excitations in a linear molecular crysta l is applied to investigate the relation between the bound electron-hole pa irs (charge transfer excitons) and the corresponding continuum of unbound s tates. It is demonstrated that, contrary to the conventional explanation, t he difference of 0.2-0.3 eV between the electric and optical band gap obser ved for polyacene crystals is a consequence of charge delocalization rather than lattice relaxation. The results also suggest that in some molecular c rystals and polymers, moderate structural disorder may strengthen the photo conductive response instead of suppressing it, which suggests a new strateg y in the search for efficient photoconductors. (C) 2000 Elsevier Science S. A. All rights reserved.