When dendrimers are not better – rational design of nanolayers for high-performance organic electronic devices

Several generations of carbosilane dendrimers with quaterthiophene end groups were studied by X-ray scattering in small and wide angles, differential scanning calorimetry, polarizing optical and atomic force microscopy and molecular modelling. It was established that the semiconducting properties of such materials are determined by the formation of smectic structures in which aliphatic regions, possessing a low degree of the ordering, alternate with highly ordered herring-bone type crystallites formed by aromatic fragments. The presence of long aliphatic spacers in the dendrimers' structure allows easy formation of such crystallites. Such dendrimers assume flattened conformations, as a smectic mesophase is thermodynamically preferable in a wide temperature range. Only in the dendrimers of the fifth generation, as the density of periphery regions increases substantially, π-π stacking of oligothiophene groups is not enough to hold together, and the molecules take on a spherical shape. As a result, extended conducting conjugated regions do not form, and dendrimers of high generations possess comparatively low semiconducting properties. From the technological point of view, quaterthiophene based carbosilane dendrimers are able to form highly uniform functional films. However, the use of lower generation dendrimers is much more preferable, as additional synthetic steps for the production of higher generation compounds do not lead to the improvement of functional properties.