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Abstract:
Searching for new pi-conjugated charge-transfer systems, the electronic
structure of a new acceptor-donor pair derived from coronene
(C(24)H(12)) was investigated by ultraviolet photoelectron spectroscopy
(UPS). The acceptor coronene-hexaone (C(24)H(6)O(6), in the following
abbreviated as COHON) and the donor hexamethoxycoronene (C(30)H(24)O(6),
abbreviated as HMC) were adsorbed as pure and mixed phases on gold
substrates. At low coverage, COHON adsorption leads to the appearance of
a charge-transfer induced interface state 1.75 eV below the Fermi
energy. At multilayer coverage the photoemission intensity of the
interface state drops and the valence spectrum of neutral COHON appears.
The sample work function decreases from 5.3 eV (clean Au) to 4.8 eV
(monolayer) followed by an increase to 5.6 eV (multilayer). The
formation of a significant interface dipole due to charge-transfer at
the metal-organic interface is possibly accompanied by a change in
molecular orientation. HMC on Au exhibits no interface state and the
sample work function decreases monotonically to ca. 4.8 eV (multilayer).
The UPS spectra of individual donor and acceptor multilayers show good
agreement with density functional theory modeling. In donor/acceptor
mixed. films the photoemission signal of the donor (acceptor) shifts to
higher (lower) binding energy. This trend is predicted by the
calculation and is anticipated when charge is transferred from donor to
acceptor. We propose that mixed. films of COHON and HMC constitute a
weak charge-transfer system.
