Photocurrent generation under a large dipole moment formed by self-assembled monolayers of helical peptides having an N-ethylcarbazolyl group

Self-assembled monolayers (SAMs) of alpha-helical peptides carrying an N-ethylcarbazolyl (ECz) group were prepared. The helical peptide SAMs on a gold surface were characterized by quartz crystal microbalance measurements, cyclic voltammetry, and impedance spectroscopy, showing that a tridecapeptide SAM with the N-terminal binding to gold was packed more densely than that with the C-terminal binding. The helix tilt angles from the surface normal in these peptide SAMs were found to be about 40 degrees on the basis of Fourier transform infrared reflection-absorption spectroscopy. Photocurrent generation of these peptide SAMs in an aqueous solution was investigated by photoexcitation of ECz groups either in the presence of an electron donor or acceptor. In the presence of methyl viologen (electron acceptor), electron donation from a gold surface to an ECz group was observed upon photoexcitation. On the other hand, in the presence of triethanolamine or ethylenediaminetetraacetic acid (electron donor), the direction of photocurrent was reversed. In the case of the anodic photocurrent generation using triethanoamine, the electron donation from an ECz group to a gold surface was accelerated by the helix dipole moment directing toward a gold surface. Furthermore, the electronic coupling between an ECz group and a gold surface through the peptide molecule was found to occur more strongly than that through a saturated hydrocarbon. The helical peptides were therefore shown to be an excellent medium for electron transfer.