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 ca tions or arrays of quantum dots. If the coupling is strong enough to overco me the gaps, charge can migrate. If the coupling is too weak, the charge re mains localized. In the intermediate case, the charge is shown to be locali zed over a finite, connected, subset of sites. Degenerate perturbation theo ry provides a suitable zero-order basis for this intermediate regime. In a time dependent language, in the domain-localized regime, the charge migrate s over a limited range of states. Also discussed is an effect of electron c orrelation, the so-called Coulomb blockade, on charge localization with com putational examples. The experimental probing of the domain-localized regim e is considered. Probes of the energy dependence of the local density of st ates such as scanning tunneling microscopy (STM) of arrays of quantum dots and photoelectron spectroscopy (PES) of chromophore bearing molecules are s uggested.