Band gap and binding energies of charge-transfer excitons in organic molecular crystals

The observed difference of 0.2–0.3 eV between the optical and electric band gap in organic solids was commonly attributed to vibrational relaxation. The recently calculated values [I.V. Brovchenko, Chem. Phys. Lett. 278 (1997) 355] of the lattice relaxation energy in the vicinity of a charge carrier or a charge-transfer state never exceed 0.02–0.03 eV, demonstrating that the conventional rationalization of the difference is no longer tenable. An alternative explanation is proposed where the difference is interpreted in terms of charge delocalization and the past controversies are attributed to inconsistencies in treating this contribution to energy.