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A new charge-transfer complex in UHV co-deposited tetramethoxypyrene and tetracyanoquinodimethane
UHV-deposited films of the mixed phase of tetramethoxypyrene and
tetracyanoquinodimethane (TMP1-TCNQ1) on gold have been studied using
ultraviolet photoelectron spectroscopy (UPS), X-ray-diffraction (XRD), infrared
(IR) spectroscopy and scanning tunnelling spectroscopy (STS). The formation of
an intermolecular charge-transfer (CT) compound is evident from the appearance
of new reflexes in XRD (d1= 0.894 nm, d2= 0.677 nm). A softening of the CN
stretching vibration (red-shift by 7 cm-1) of TCNQ is visible in the IR
spectra, being indicative of a CT of the order of 0.3e from TMP to TCNQ in the
complex. Characteristic shifts of the electronic level positions occur in UPS
and STS that are in reasonable agreement with the prediction of from DFT
calculations (Gaussian03 with hybrid functional B3LYP). STS reveals a HOMO-LUMO
gap of the CT complex of about 1.25 eV being much smaller than the gaps (>3.0
eV) of the pure moieties. The electron-injection and hole-injection barriers
are 0.3 eV and 0.5 eV, respectively. Systematic differences in the positions of
the HOMOs determined by UPS and STS are discussed in terms of the different
information content of the two methods.Comment: 20 pages, 6 figure