Effects of electrostatic field on the phase properties of mixed-stack organic charge-transfer compounds.

Abstract

We have studied the effects of an external electric field on the phase properties of mixed-stack organic charge-transfer complexes through illustrative calculations based on the ionic crystal model, paying special attention to the roles of electrostatic energy. The ground-state charge 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-dimensional bulk crystal, and thin film. It has been found for TTF-CA bulk crystals at room temperature and ambient pressure that the applied electric field beyond approximately ${10}^{6}$ V/cm would induce a transition from the neutral phase to the charge-separated phase in which positively and negatively charged donor-acceptor pairs, ${\mathit{D}}^{+}$${\mathit{A}}^{0}$ and ${\mathit{D}}^{0}$${\mathit{A}}^{\mathrm{\ensuremath{-}}}$, separately 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 mixed-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-complex systems in terms of employing compounds other than TTF-CA or of varying temperature, pressure, and film thickness to approach the systems to the neutral-ionic phase boundary.