posted on 2024-02-11, 16:04authored byRobert Ranecki, Benedikt Baumann, Stefan Lach, Christiane Ziegler
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 (EF) 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.