Resistive memories based on Rose Bengal and related xanthene derivatives: insights from modeling charge transport properties

We present a computational study on the electrical bistability behavior of four xanthene derivatives based on sodium fluorescein. Intra- and intermolecular charge transfer parameters are computed and employed to rationalize the efficiencies experimentally determined for resistive memory devices based on these organic materials. Charge injection at the electrode/organic interface in the presence of pristine and reduced molecular species is estimated by comparing the electron affinities of xanthene derivatives with the work function of commonly employed electrodes, and bulk charge transport is modeled assuming a charge hopping regime. It is shown that the OFF state is injection limited and that the efficiency of the bistability phenomenon is governed by sizeable intramolecular reorganization energies associated with electron transfer. The computed results reveal that the combined role of electron attractor groups and conformational degrees of freedom contributes to a more favorable level alignment at the interface and to the desired increase of the intramolecular reorganization energies. These optimal conditions are fulfilled for Rose Bengal. It is expected that the interrelated role of molecular parameters, conformational degrees of freedom and electron attractor character of substituents disclosed by this study might be used to formulate general structure-property relationships for the design of new, more efficient restive memory devices based on molecular semiconductors.