Electronic Structure of Self-Assembled Peptide Nucleic Acid Thin Films

The electronic structure of self-assembled monolayers (SAMs) of peptide nucleic acid (PNA) formed on Au substrates was investigated. Cys-appended PNA 7-mers of thymine (Cys-T7) were incubated on Au substrates in a nitrogen glovebox attached to a photoemission spectrometer. Ultraviolet and X-ray photoemission spectroscopy (UPS and XPS) measurements on the resulting SAMs revealed the hole injection barrier at the interface and the interface dipole. Electronic structure calculations based on molecular dynamics sampling of the PNA structure yielded the HOMO-LUMO gap and the electronic density of states for PNA. Combined with the UPS data, the theoretical calculation enabled estimation Of the Au charge injection barriers at the interface as well as the assignment of individual UP spectral features to specific molecular orbitals. Interestingly, the dipole moment of the PNA backbone is predicted to polarize PNA MOs, shifting the preferred HOMO localization toward the C-terminus of PNA. Control measurements on Cys-appended, abasic pNA backbone 7-mers (Cys-Bckb7) allowed the identification of the emissions related to the PNA backbone in the UP spectra. The orbital line-up at the interface between the Au substrate and the Cys-PNA indicates a significant interface dipole resulting in the alignment of the Au Fermi level near the center of the PNA HOMO-LUMO gap. This alignment causes large charge injection barriers for both holes and electrons, and thus impedes charge transfer from Au into the Cys-PNA SAM.