N-Heterotriangulenes
(N-HTAs) represent a class of functional
molecules
with high potential for optoelectronic materials, for example, as
electron donating compounds in donor/acceptor (D/A) systems. The capability
of two different N-HTAs, N-HTA 550 and N-HTA 557, the latter containing
an additional 7-membered ring, to act as electron donors at interfaces
with strong tetracyanoquinodimethane (TCNQ and F4TCNQ) acceptors is
studied using high-resolution electron energy loss spectroscopy in
combination with state-of-the-art quantum chemical calculations. For
TCNQ/N-HTA bilayer systems adsorbed on Au(111), low-energy (<2.5
eV) electronic transitions which are attributed to charge transfer
(CT) states for all four D/A combinations are identified. Based on
substantial quantum chemical calculations, a generation of ground
state CT complexes is excluded. Instead, CT in the excited state,
in which an electron-stimulated CT from the N-HTAs to TCNQs is the
underlying process, is proposed. The energies of the CT states are
determined by the values of the ionization potential and electron
affinity of the involved donor and acceptor.