Self-Assembly and Disassembly of Regioregular, Water Soluble Polythiophenes: Chemoselective Ionchromatic Sensing in Water

Abstract

The development of polythiophene (PT) derivatives with essentially 100% head-to-tail (HT) couplings1,2 has led to the discovery of materials that self-assemble both in solution and in the solid state. Light-scattering studies on regioregular, HTpoly(3-dodecylthiophene) (HT-PDDT) show that a supramolecular organization occurs via intermolecular aggregation.3 This solution aggregation is precursive to a microcrystalline, self-assembled structure which has been characterized by X-ray studies.1,2,4 Polythiophenes possessing this metastable organization evidence high electrical conductivities1,2,5-8 and very small bandgaps. Previous work in our lab has shown that both conformational order and solid state organization in regioregular PT derivatives are remarkably sensitive to the placement and nature of the substituent chains.1,6,7,9 This sensitivity provides the opportunity to design new conducting polymers in which self-assembly is controlled by environmental factors. One remarkable example is HT-2,5-poly(thiophene-3-propionic acid) (3, PTPA). This polymer was designed such that upon deprotonation, water soluble polythiophene salts are formed.10 Both 3 and its partially deprotonated derivative 5 undergo protein-like hydrophobic assembly, with subsequent H-bond stabilization, to generate a self-assembled conducting polymer aggregate. Varying the size of the counterion in the carboxylate polymer changes the effective steric bulk of the substituent. Small cations favor a self-assembled (purple) state and large cations can completely disrupt the aggregated phase. In fact, a remarkable chemoselective counterion size-dependent chromaticity (Figure 1) is observed for polymer 5. The UV-vis λmax can be varied over a 130 nm range (from purple to yellow) simply by changing the countercation. Initially, we prepared the carboxylate precursor polymer 2 using the Ni(dppp)Cl2 cross-coupling method for the synthesis of HT polythiophenes.5,6,9 This method leads to low molecular weights (Mn ) 1.5-3.0 K), low yields (<5%), and polymers that show variable ionic chromaticity. However, the regioregular oxazoline polythiophene (2) (Mn ) 8 K, PDI ) 1.2) is prepared (Scheme 1) from monomer11 1 in high yields (84%)