Dipole effects on molecular and electronic structures in a novel conjugate of oligo(phenyleneethynylene) and helical peptide

A novel conjugate of a helical nonapeptide and an oligo(phenyleneethynylene) (OPE) having a nitro group at a molecular terminal was synthesized. Both components have a dipole. The peptide has a disulfide group at the N-terminal for immobilization on gold. In order to investigate the electric field effect of the helical peptide dipole on the OPE and molecular structure by the dipole–dipole interaction between the two components, the electronic structure of the OPE was spectroscopically studied in solution, the self-assembled monolayer on gold, and Langmuir–Blodgett (LB) layers on a fused quartz surface. The absorption maximum (λmax) of the OPE component in chloroform is red-shifted by 4 nm from the reference OPE derivative without the helical peptide component. The red shifts of the OPE component are also observed in the LB monolayer and bilayer compared with that of the self-assembled monolayer. The observed dipole effect of the peptide on the OPE electronic structure was quantitatively discussed with ab initio calculations. Antiparallel orientation on the dipole directions of the peptide and the OPE components is considered to explain the red shifts via the dipole effect on the electronic structure of the OPE.