Supramolecular assembly of conjugated polymers:: From molecular engineering to solid-state properties

pi-Conjugated polymers are central ingredients in the development of organic electronics and it is well-known that the degree of order in thin films of those materials is a major factor governing the performances of the devices (field effect transistors, light-emitting diodes, photovoltaic cells). In this review, we present some approaches for the control of the molecular assembly of conjugated polymers into well-defined nanostructures. We consider self-assembly leading to thin deposits from compounds molecularly dispersed in a solution and we focus on the relationship between the supramolecular interactions between the individual pi-conjugated molecules/polymers and the microscopic morphology appearing upon aggregation. We describe approaches to control the self-assembly of conjugated systems in the solid-state via (i) the interactions with the substrate surface; (ii) the design of a well-defined molecular structure; and (iii) the use of block-copolymers comprising conjugated and non-conjugated blocks. To understand how supramolecular interactions influence the microscopic morphology, we developed a joint experimental-theoretical approach, combining scanning probe microscopy characterization of thin deposits and force-field molecular modeling of supramolecular aggregates and adsorbates. With this methodology, a strong correlation is established between the polymer molecular structure, the degree of supramolecular order, and the solid-state photoluminescence properties (for light-emitting applications), or the charge transport properties (for the design of field-effect transistors). Since the construction of small-size devices requires that the spatial growth of the thin deposits be perfectly controlled, we illustrate in the last section of the paper how soft lithographic methods and nanorubbing can be used to orient the formation of the self-assembled conjugated nanostructures in a confined space. (C) 2007 Elsevier B.V. All rights reserved.