Superexchange, localized, and domain-localized charge states for intramolecular electron transfer in large molecules and in arrays of quantum dots

Superexchange is a longer-range electron-transfer mediated by a nonresonant bridge between the donating and accepting states. We discuss a coupled set of donor/acceptor levels that are not resonant, with special reference to coupling of intermediate strengths. Examples of such systems are peptide cations or arrays of quantum dots. If the coupling is strong enough to overcome the gaps, charge can migrate. If the coupling is too weak, the charge remains localized. In the intermediate case, the charge is shown to be localized over a finite, connected, subset of sites. Degenerate perturbation theory provides a suitable zero-order basis for this intermediate regime. In a time dependent language, in the domain-localized regime, the charge migrates over a limited range of states. Also discussed is an effect of electron correlation, the so-called Coulomb blockade, on charge localization with computational examples. The experimental probing of the domain-localized regime is considered. Probes of the energy dependence of the local density of states such as scanning tunneling microscopy (STM) of arrays of quantum dots and photoelectron spectroscopy (PES) of chromophore bearing molecules are suggested.