Electric Field Effect of Helical Peptide Dipole in Self-Assembled Monolayers on Electronic Structure of Oligo(Phenyleneethynylene)

A series of novel linear conjugates of helical peptides and oligo(phenyleneethynylene) (OPE) were synthesized and studied oil the dipole effect of the helical peptide moieties oil the electronic stricture of (lie OPE. The helical peptides have a generalized formulation of multiple repeats of L-alanine and alpha-aminoisobutyric acid, (Ala-Aib),, which are connected to OPE in series of(Ala-Aib)(n)-OPE-(Ala-Aib)(m) abbreviated by 2nOPE2m, (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-reffection 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-mercaptoudecanoi acid and OPE (CHOPE). The shift is consistent with file density functional theory calculations, showing that all 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.