Molecular Electronics

A molecular wire is best understood as a molecular bridge that can move charge rapidly and efficiently over many chemical bond lengths. In assembling electron donor-bridge-acceptor (D-B-A) systems for molecular electronics, one must design the system to adopt the most efficient charge transport mechanism possible, one which maintains this efficiency as the bridge is lengthened. Long-distance charge transfer (CT) is intrinsically a nonadiabatic process in which the CT rate is dictated by some combination of strongly distance-dependent coherent transport and weakly distance-dependent incoherent charge hopping, Figure 1. Optimizing the molecular structure to accentuate the latter process is key to producing wire-like behavior in molecules. To achieve this goal one must 1) isolate the contributions of each mechanism to CT, 2) find the link between these contributions and the energy levels of the system, and 3) choose redox components (donors, bridges, and acceptors) that drive the system toward incoherent behavior at long distances. A recent example is shown Figure 2.

Photoinduced Charge Transport in a Molecular Wire

This D-B-A system uses a series of p-phenylene (Phn) oligomers, where n = 1-5, to link a phenothiazine (PTZ) electron donor to a perylene-3,4:9,10-bis(dicarboximide) (PDI) electron acceptor, Figure 2. Selective photoexcitation of PDI within PTZ-Phn-PDI results in charge separation to produce a spin-coherent singlet radical ion pair (RP), 1(PTZ+.-Phn-PDI-.), which subsequently undergoes radical pair intersystem crossing (RP-ISC) to yield 3(PTZ+.-Phn-PDI-.). The triplet RP then recombines to give almost exclusively the lowest excited triplet state of PDI (3*PDI). The RP-ISC mechanism is well-known to account for triplet state formation within photosynthetic reaction centers and in selected biomimetic systems as well.

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