Charge dynamics through pi-stacked arrays of conjugated molecules: effect of dynamic disorder in different transport/transfer regimes

Abstract

We provide further computational evidence that the electronic coupling between pi-stacked molecules is strongly modulated by the thermal motions at room temperature, not only in supramolecular flexible systems (like DNA) but also in molecular crystals. The effect of this modulation on the charge dynamics is different for different transfer/transport mechanisms and depends on the modulation timescale. In the case of charge transfer (CT) between a donor and an acceptor, the effect of electronic coupling fluctuations introduces a corrective term in the expression of the rate constant (different for adiabatic and non-adiabatic CT). For the transport in molecular crystals, this fluctuation can be the limiting factor for the charge mobility. Although the fluctuation of the electronic coupling is similar in magnitude for all systems containing molecular pi-stacking, its importance for the charge dynamics increases with the decrease of the reorganization energy.

Keywords:

• Charge transfer
• Organic semiconductor
• Quantum dynamics
• Electron-phonon coupling

Acknowledgements

I am grateful to Prof. G. Orlandi for his help in the study of the charge transport mechanism in organic materials and to Prof. A. Nitzan and Prof. M. Ratner for illuminating discussions on the theory of electron tunneling through fluctuating bridges. This work is supported by the Research Council UK.

Notes

We have seen that, in DNA, the same qualitative behavior for V _DA is obtained using the approximate McConnell expression or the more rigorous matrix partition method.

To simplify the notation we have assumed V _DA>0, but the same final result is obtained if V _DA < 0.