The essential role of H-F substitution in the electron-phonon interactions and electron transfer in the negatively charged acenes.

Abstract

The single charge transfer through acenes, partially H-F substituted acenes, and fluoroacenes is discussed. The reorganization energies between the neutral molecules and the corresponding monoanions for partially H-F substituted acenes lie between those for acenes and fluoroacenes. The delocalization of the lowest unoccupied molecular orbitals (LUMO) by substituting hydrogen atoms by fluorine atoms with the highest electronegativity in every element is the main reason why the reorganization energy between the neutral molecule and the monoanion for partially H-F substituted acenes lies between those for acenes and fluoroacenes. This result implies that the negatively charged partially H-F substituted acenes would be better conductors with rapid electron transfer than the negatively charged fluoroacenes if we assume that the overlap of the LUMO between partially H-F substituted acenes is not significantly different from that between two neighboring fluoroacenes. The structures of the monoanions of acenes, fluoroacenes, and partially H-F substituted acenes are optimized under D2h geometry, and the Jahn-Teller effects in the monoanions of benzene and fluorobenzene are discussed. The vibration effect onto the charge transfer problem is also discussed. The C-C stretching modes around 1500 cm(-1) are the main modes converting the neutral molecules to the monoanions in acenes, fluoroacenes, and partially H-F substituted acenes. It can be confirmed from the calculational results that the C-C stretching modes around 1500 cm(-1) the most strongly couple to the LUMO in these molecules. The main reason why the total electron-phonon coupling constants (lLUMO) for the monoanions of acenes in which four outer hydrogen atoms are substituted by fluorine atoms are larger than those for the monoanions of acenes in which several inner hydrogen atoms are substituted by fluorine atoms is suggested. The relationships between the electron transfer and the electron-phonon interactions are discussed. The plot of the reorganization energies against the lLUMO values is found to be nearly linear. In view of these results, the relationships between the normal and superconducting states are briefly discussed.