Chirally twisted oligo(phenyleneethynylene) by cyclization with alpha-helical peptide.

A novel cyclic conjugate of a helical decapeptide and oligo(phenyleneethynylene) (OPE), C-OPE10, was synthesized. The conformation and the optical properties of the cyclic conjugate were studied by circular dichroism (CD), absorption, and emission spectroscopies. In the cyclic conjugate, the rotational motion around the molecular axis of the OPE moiety was hindered to take a chirally twisted conformation, which is a distorted form from the coplanar conjugated structure, as revealed by observation of an induced negative Cotton effect of the OPE moiety. Molecular simulation using time dependant-density functional theory indicated a right-handed twist conformation of the OPE moiety for the negative Cotton effect. This conjugate therefore provides a new way to obtain a pi-conjugated compound having main-chain chirality. The optical properties of the OPE moiety taking the twist conformation in the cyclic conjugate are also discussed in depth.