Understanding the Charge Transfer at the Interface of Electron Donors and Acceptors: TTF-TCNQ as an Example

Charge transfer between an electron donor and an electron acceptor is widely accepted as being independent of their relative configurations if the interaction between them is weak; however, the limit of this concept for an interacting system has not yet been well established. Our study of prototypical electron donor-acceptor molecules, tetrathiafulvalene-tetracyanoquinodimethane, using density functional theory based on an advanced functional, clearly demonstrates that for interacting molecules, their configurational arrangement is as important as their individual electronic properties in the asymptotic limit to determine the charge transfer direction. For the first time, we demonstrate that by changing their relative orientation, one can reverse the charge transfer direction of the pair, causing the molecules to exchange roles as donor and acceptor. Our theory has important implications for understanding the interfacial charge-transfer mechanism of hybrid systems and related phenomena.