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]-substituted polythiophene (I). Contrary to the small effects obsd. for the regiorandom analog of I, made by FeCl3-polymn., 1 shows a strong induced optical activity in its p-p* transition, provided the polymer is in its almost coplanar assocd. form. This assocn. with a strong conformational rigidity, a prerequisite for a stable chirality from a helical structure of I, is found in poor-solvents as well as in the solid state. In a good solvent like CHCl3 the disordered nonplanar structure is optically inactive in its backbone. The transfer to an assocd. form is accompanied with solvatochromism and thermochromism in absorption, while significant changes in fluorescence behavior and in the CD spectra are obsd. 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. [on SciFinder (R)]