Towards the design of molecular cells for quantum cellular automata: critical reconsideration of the parameter regime...

In this article, we present a critical discussion of the parametric regimes required for reaching the functionality of the two-electron square-planar tetrameric mixed-valence (MV) complexes as molecular cells in quantum cellular automata (QCA). Previous studies on molecular QCA were restricted by the limit case of strong Coulomb interaction that was supposed to be the only way to ensure such two key requirements for functioning QCA cells as bistability and switchability. It was thus assumed that the site-to-site electron transfer energy t should be much smaller than the energy U of the Coulomb repulsion between the two excess electrons (strong-U limit defined by the inequality U ≫ t). Unlike those studies, here, we develop a generalized theoretical approach within which no restricting assumptions are implied on the relative strength of the intracell Coulomb interaction, electron transfer, the vibronic coupling with "breathing" modes of redox sites and the external electrostatic field of the driver cell acting on the working cell. We demonstrate that dominating Coulomb repulsion is not the only source of bistability and switchability, but such key features of QCA cell can be reached even in systems in which the strong-U limit is violated, provided that the vibronic coupling is strong enough. Such a reconsideration of the parameter regime for achieving proper functionality is expected to essentially enlarge the family of MV molecules, which can be used as molecular QCA cells.