  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.
