Self-Assembly of the Chiral Donor-Acceptor Molecule DCzDCN on Cu(100)

Donor-acceptor (D-A) structured molecules are essential components of organic electronics. The respective molecular structures of these molecules and their synthesis are primarily determined by the intended area of application. Typically, D-A molecules promote charge separation and transport in organic photovoltaics or organic field-effect transistors. D-A molecules showing a larger twist angle between D and A units are, e.g., essential for the development of high internal quantum efficiency in organic light-emitting diodes. A prototypical molecule of this D-A type is DCzDCN (5-(4,6-diphenyl-1,3,5-triazine-2-yl)benzene-1,3-dinitrile). In most cases, these molecules are only investigated regarding their electronic and structural interaction in bulk aggregates but not in ultrathin films supported by a metallic substrate. Here, we present growth and electronic structure studies of DCzDCN on a Cu(100) surface. We used a complementary approach through the use of scanning tunneling microscopy/spectroscopy (STM and STS), ultraviolet/inverse photoemission spectroscopy (UPS and IPES), and single-molecule density functional theory (DFT) calculations. This method combination enabled us to investigate the adsorption geometry (STM) and the local electronic states near the Fermi energy (E_F) of a single adsorbed molecule (using STS) and to compare these data with the integral overall electronic structure of the DCzDCN/Cu(100) interface (using UPS/IPES). The orientation of the molecules with the donor part toward the substrate results in a chiral resolution at the interface due to the molecular as well as the substrate symmetry and additional strong molecular electrostatic forces induced by the charge distribution of the twisted dicarbonitrile part. Thus, the formation of various bulk-unlike homochiral structures and the appearance of hybrid interface states modify the molecular electronic properties of the DCzDCN/Cu(100) system, e.g., the transport gap by -1.3 eV compared to that of a single DCzDCN molecule. This may be useful not only for optoelectronic applications but also in organic spintronics.