P. Petelenz et G. Mazur, Band gap and binding energies of charge-transfer excitons in organic molecular crystals, CHEM P LETT, 301(3-4), 1999, pp. 223-227
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. T
he 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 t
he 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. (C) 1999 Elsevier
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