Self-organising amphiphilic oligothiophene block co-polymers
A new route to oligothiophene-PEO-block-co-polymers has been developed, in which well-defined a-oligothiophene blocks (from bithiophene to sexithiophene) alternate with poly(ethylene oxide) blocks. These materials show high solubility in common organic solvents. Sexithiophene derivatives with short PEO chains attached at the a-positions have been synthesised as model systems for the above sexithiophene polymer. PEO side-chains as short as pentaethylene glycol had a good solubilising effect on the otherwise insoluble sexithiophene. All compounds prepared exhibited surface activity, as shown by monolayer experiments on a Langmuir trough. Sexithiophene model compounds with short PEO side-chains could be compressed to form a two-dimensional condensed phase. X-ray diffraction of melt cast films of the oligothiophene polymers showed that a minimum oligothiophene block-length of five thiophenes (pentathiophene) was necessary to fully disturb PEO crystallisation. The sexithiophene polymer formed regions of crystalline/aggregated sexithiophene in the bulk. UV/Vis and fluorescence studies in solution indicated that the oligothiophene segments were molecularly dissolved in good solvents like chloroform. Aggregation of the oligothiophenes occurred in THF/water mixtures, which was consistent with observed shifts of the UV absorption maxima towards the blue and quenching of the fluorescence. An oligothiophene length of four thiophenes (quaterthiophene) was necessary to observe this aggregation phenomenon. A sexithiophene derivative with chiral pentaethylene glycol side chains showed a large circular dichroism effect in aqueous solution and no effect in chloroform solution, indicating the formation of a chiral superstructure upon aggregation.