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

The Merrifield model of electronic excitations in a linear molecular crystal is applied to investigate the relation between the bound electron–hole pairs (charge transfer excitons) and the corresponding continuum of unbound states. It is demonstrated that, contrary to the conventional explanation, the difference of 0.2–0.3 eV between the electric and optical band gap observed for polyacene crystals is a consequence of charge delocalization rather than lattice relaxation. The results also suggest that in some molecular crystals and polymers, moderate structural disorder may strengthen the photoconductive response instead of suppressing it, which suggests a new strategy in the search for efficient photoconductors.