Impact of Vibrations and Electronic Coherence on Electron Transfer in Flat Molecular Wires
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Impact of Vibrations and Electronic Coherence on Electron Transfer in Flat Molecular Wires Oscar Grånäs1,2, Grigory Kolesov1 and Efthimios Kaxiras1 1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA 2 Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Box 516, SE-75120 Uppsala, Sweden ABSTRACT Electron transfer in molecular wires are of fundamental importance for a range of optoelectronic applications. The impact of electronic coherence and ionic vibrations on transmittance are of great importance to determine the mechanisms, and subsequently the type of wires that are most promising for applications. In this work, we use the real-time formulation of time-dependent density functional theory to study electron transfer through oligo-pphenylenevinylene (OPV) and the recently synthesized carbon bridged counterpart (COPV). A system prototypical of organic photovoltaics is setup by bridging a porphyrin-fullerene dyad, allowing a photo-excited electron to flow between the Zn-porphyrin (ZnP) chromophore and the C60 electron acceptor through the molecular wire. The excited state is described using the fully self-consistent ∆-SCF method. The state is then propagated in time using the real-time TD-DFT scheme, while describing ionic vibrations with classical nuclei. The charge transferred between porphyrin and C60 is calculated and correlated with the velocity autocorrelation functions of the ions. This provides a microscopic insight to vibrational and tunneling contributions to electron transport in linked porphyrin-fullerene dyads. We elaborate on important details in describing the excited state and trajectory sampling. INTRODUCTION From a computational perspective, the processes involved in excitation and charge transfer in organic materials is extremely difficult to model ab initio. Recently several efforts have been made to address this problem.[1,2] Often vibrational and electronic properties are tightly coupled, rendering adiabatic methods less reliable. The description of the excited electrons also requires a method able to capture for example excitonic effects. Together with the fact that statistics over a significant number of trajectories often has to be accounted for, implies that first-principles modelling in combination with statistical modeling is often necessary. Of particular interest in molecular electronics and energy harvesting, are photoactive molecules like porphyrins, a key component in photosynthesis. To harvest the energy of the absorbed light the electron-hole pair has to be separated, the transport of charge relevant for separation and recombination of electrons and holes is what we aim to address. A particularly suitable testbed for evaluating critical components of a computational method is the donor-bridge-acceptor system ZnP-OPV/COPV-C60,[3,4] where minor changes in the bridge leads to order of magnitude changes in charge transfer times.[5] We report an ongoing study aiming to address questions regarding i) how
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