EFFECTS OF ELECTROSTATIC-FIELD ON THE PHASE PROPERTIES OF MIXED-STACKORGANIC CHARGE-TRANSFER COMPOUNDS

We have studied the effects of an external electric field on the phase properties of mixed-stack organic charge-transfer complexes through i llustrative calculations based on the ionic crystal model, paying spec ial attention to the roles of electrostatic energy. The ground-state c harge configurations of tetrathiafulvalene-p-chloranil (TTF-CA) under electric fields have been explored mainly by the Monte Carlo simulated annealing method in the cases of one-dimensional chain, three-dimensi onal bulk crystal, and thin film. It has been found for TTF-CA bulk cr ystals at room temperature and ambient pressure that the applied elect ric field beyond approximately 10(6) V/cm would induce a transition fr om the neutral phase to the charge-separated phase in which positively and negatively charged donor-acceptor pairs, D(+)A(0) and D(0)A(-), s eparately appear near both ends of DA stack chains. Investigating the phase structure of charge-transfer complexes through the variations in ionization energy, Madelung energy, and external field, we have also found that the applied field can cause a neutral-ionic transition of m ixed-stack organic charge-transfer compounds in the close vicinity of neutral-ionic phase boundary. We then discuss the feasibility of this electric-field-induced neutral-ionic transition in actual organic-comp lex systems in terms of employing compounds other than TIE-CA or of va rying temperature, pressure, and film thickness to approach the system s to the neutral-ionic phase boundary.