Intermolecular association and supramolecular organization in dilute solution .1. Regioregular poly(3-dodecylthiophene)

Static and dynamic polarized and depolarized light scattering characterization of dilute solutions of regioregular poly(3-dodecylthiophene) is described to study intermolecular association as a function of the thermal history of the solutions. It is shown that metastable aggregation obtains under all of the conditions studied, including temperatures as high as 65 degrees C. Under some conditions, the aggregated moiety appears to have a disklike shape at room temperature, with appreciable depolarized scattering, attributed to an extended chain structure, consistent with the formation of the lamellar suprastructure characteristic of the bulk, with the polythiophene chains adopting an extended conformation and forming a nematic phase in a polythiophene lamella faced by lamellae rich in alkyl chains. Under other conditions, especially at low temperature, elongated supramolecular structures are formed, with the polythiophene chains in an extended conformation. The reversible thermochromic effect is associated with enhanced order of the alkyl side chains with decreasing temperature, facilitating coplanar conformers in the polythiophene backbones, with the attendant enhancement in the pi-pi* transitions of the thiophene ring electronic absorption spectra. This behavior is analogous to the events in the thermochromic event in the bulk. Such supramolecular structures could intervene in normal film casting solution processing, with effects on the electronic or optical properties of the cast film. This interpretation of the light scattering data suggests strategies to enhance or suppress lamella formation in solvent-cast films.