CHIROPTICAL PROPERTIES OF REGIOREGULAR CHIRAL POLYTHIOPHENES

The synthesis and (chir)optical properties of a highly regioregular optically active 3-substituted polythiophene are reported. Using the McCullough method a regioselectivity of more than 98% is found for the head-to-tail coupling in 3-[2-((S)-2-methylbutoxy)ethyl]polythiophene 1. Contrary to the small effects observed for the regiorandom analogue 2, made by FeCl3-polymerization, 1 shows a strong induced optical activity in its pi-pi* transition, provided the polymer is in its almost coplanar associated form. This association with a strong conformational rigidity, a prerequisite for a stable chirality from a helical structure of 1, is found in poor-solvents as well as in the solid state. In a good solvent like CHCl3 the disordered non-planar structure is optically inactive in its backbone. The transfer to an associated form is accompanied with solvatochromism and thermochromism in absorption, while significant changes in fluorescence behaviour and in the circular dichroism spectra are observed. All techniques show fine structure in the almost coplanar state while all absorptions are exciton coupled, reflecting the Davydov splitting of the coupled excited states.