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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 π-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.

A series of novel linear conjugates of helical peptides and oligo(phenyleneethynylene) (OPE) were synthesized and studied on the dipole effect of the helical peptide moieties on the electronic structure of the OPE. The helical peptides have a generalized formulation of multiple repeats of l-alanine and α-aminoisobutyric acid, (Ala-Aib)nor m, which are connected to OPE in series of (Ala-Aib)n-OPE-(Ala-Aib)mabbreviated by 2nOPE2m, (n, m) = (4, 0), (4, 4), (8, 0), (8, 4), (8, 8). The conjugates having one or two hexadecapeptides formed well-packed and vertically oriented self-assembled monolayers on gold as revealed by infrared-reflection absorption spectroscopy, cyclic voltammetry, and ellipsometry. Absorption spectra of the OPE moiety in the SAMs showed a bathochromic shift of ca. 25 nm from a reference conjugate of 11-mercaptoundecanoic acid and OPE (C11OPE). The shift is consistent with the density functional theory calculations, showing that an external electric field directed along the molecular axis diminishes the highest occupied molecular orbital−lowest unoccupied molecular orbital gap of OPE. We thus conclude that the electric field generated by the peptide dipoles effectively modulate the electronic structure of the OPE moiety in the SAMs.