Metal-induced supramolecualr chirality in an optically active polythiophene aggregate.
Chiral polythiophenes (PTs), in sharp contrast to other optically active polymers, exhibit optical activity in the pi-pi* transition region which is derived from the chirality of the main chain when they self-assemble to form a supramolecular pi-stacked aggregate with intermolecular interactions in a poor solvent or in a film. We now report that the regioregular, optically active PT poly[(R)-3-[4-(4-ethyl-2-oxazolin-2-yl)phenyl]thiophene] (poly-1) exhibits unique split-type induced circular dichroism (ICD) in the pi-pi* transition region of the main chain upon complexation with various metal salts such as trifluoromethanesulfonates of copper(I), copper(II), silver(I), and zinc(II), and iron(II) perchlorate in chloroform, which is a good solvent for poly-1. The appearance of ICD and slight changes in the UV/Vis spectra (no color change), except for the zinc salts, indicated that the chirality may not be induced by chiral pi-stacked aggregates of poly-1, but by the chirality of the main chain, for example, a predominantly one-handed helical structure induced by intermolecular coordination of the oxazoline groups to metal ions. The sign of the Cotton effect depends on the metal salt; most metal salts induced ICDs with similar Cotton-effect patterns, while zinc salts caused an inversion of the signs of the Cotton effect of poly-1 accompanied by a gradual red shift in the absorption of up to 125 nm. The changes in the conformation and the size of the poly-1 aggregates induced by different metal salts were also investigated by (1)H NMR titrations, static light scattering (SLS), atomic force microscopy (AFM), and membrane filtration. On the basis of these results, we propose a possible model for the chiral supramolecular aggregates of poly-1 with metal salts.