Fundamentals in the Design of Molecular Electronic Devices: Long-Range Charge Carrier Transport and Electronic Coupling

Virtually all molecular electronic devices rely on the interplay between spatial organization and electronic motion to attain the resolution and responsiveness necessary for practical applications.1 An efficient optical computer,2 for example, will need a set of precisely positioned chromophores that can be individually addressed and that can reliably produce detectable electrical signals. The device itself must be relatively large compared with a simple molecule, but tiny on a macroscopic scale, to preserve the possibility of chemical accessibility, structural complexity, and general utility. Such arrays are often referred to as “nanoscopic”, and their potential utility in devices is one important reason for worldwide interest in “nanotechnology”.3 Long-range nanoscopic ordering can be attained with either specifically designed block copolymers or low molecular weight species that can arrange themselves into supramolecular arrays, e.g., liquid crystals, micelles, vesicles, tubules, etc.4 An efficient optoelectronic device will also incorporate an energy gradient that permits vectorial electron motion and signal amplification,5 ideally with sufficient time resolution to permit each electronic signal to be differentiated from an alternately placed neighbor. These requirements are not easily met, particularly not simultaneously. In principle, the required chromophore positioning can be attained by either of two methods: by sequential bondby-bond synthesis or by taking advantage of spontaneous self-assembly or aggregation into stable twoand threedimensional structures. The structures of the resulting assemblies can then be inferred from spectroscopic or kinetic measurements. From the known distance dependencies of energy transfer6 and electron-transfer quenching7 or from the observed shifts of spectral energies and intensities, the local order and the character of interchromophore interactions are then established.8 Both assembly techniques have been used in our group, and some examples are presented in this Account to illustrate the comparative advantages and difficulties associated with each. This Account is thus an attempt to describe to readers new to this area the key design features for model compounds relevant to this class of materials. We also illustrate the utility of photoinduced electron transfer in the characterization of these compounds. We do not yet understand in detail the complexities of either the intramolecular electronic interactions operative in structurally complex molecules or the influence of noncovalent intermolecular interactions on the properties of these same species in three-dimensonally ordered arrays. If we are to ever exploit the practical applications of the optoelectronic materials, however, we must first understand how to control these key intermolecular interactions. In the following sections, therefore, we survey the efforts of our research group in the last several years to define the several factors that must be incorporated into a practical molecular electronic device. Specifically, we consider (1) how to construct complex nanostructures with precise positioning of the photoresponsive chromophores, (2) how a nanoscale energy cascade can dictate the direction of electron flow, and (3) how electrical fields felt along the migratory pathway of a charge carrier influence its mobility.