Theoretical Study of Staging Structure in the Neutral-Ionic Transition of Charge-Transfer Complexes

Abstract

Accompanying the neutral-ionic transition, the co-existence of the neutral and ionic states has been observed in mixed-stack charge-transfer (CT) complexes, (3,3',5,5')-tetramethylbenzidine-(7,7,8,8)tetracyanoquinodimethane (TMB-TCNQ), dimethyltetrathiafulvalene- p -chloranil (DMTTF-CA), and tetrathiafulvalene- p -chloranil (TTF-CA). We calculated electrostatic energies in these crystals in order to investigate the stability of the staging structure proposed for the co-existing state, that is, a superstructure consisting of an alternative sequence of the ionic and neutral domains. We took account of intramolecular charge distributions and the extent of molecules. It was found that the staging state is slightly stable than the uniform ionic and neutral states in TMB-TCNQ. The stability is determined by the difference of the intramolecular charge distribution between the ionic and neutral states. This allows the staging state even under the attractive interaction between the stacking chains, in contrast to the original model of the state. It was found that the interaction is attractive in the three materials.