Cross Surface Ambipolar Charge Percolation in Molecular Triads on Mesoscopic Oxide Films

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Abstract
We report on cross surface ambipolar charge percolation within a monolayer of a molecular triad adsorbed on semiconducting or insulating mesoscopic metal oxide films. The triad consists of a triphenlyamine (TPA) donor and a perylenemonoimide (PMI) acceptor connected by a bithiophene (T2) bridge. The self-assembled PMI-T2-TPA monolayer exhibits p-type or n-type conduction depending on the potential that is applied to the conducting glass (FTO) electrode supporting the oxide films. Cross surface electron transfer is turned on at around −1.24 V (vs Fc+/Fc) where the PMI moiety is electroactive. The color of the film changes from red to blue during the reduction of the PMI. By contrast, lateral hole transfer is turned on at around 0.8 V (vs Fc+/Fc) where the TPA moiety becomes electroactive. The stepwise oxidation of the T2-TPA units at 0.79 and 1.28 V (vs Fc+/Fc) is associated with a color change of the film from red to black. Cyclic voltammetric as well as chronocoulometric and spectroelectrochemical measurements were applied to determine the percolation threshold for cross surface charge transfer and the diffusion coefficients for the electron and hole hopping process. The effect of oxide surface states on the lateral charge motion was also investigated.