Altering the Static Dipole on Surfaces through Chemistry: Molecular Films of Zwitterionic Quinonoids

The adsorption of molecular films made of small molecules
with a large intrinsic electrical dipole has been explored. The data
indicate that such dipolar molecules may be used for altering the
interface dipole screening at the metal electrode interface in organic
electronics. More specifically, we have investigated the surface electronic
spectroscopic properties of zwitterionic molecules containing 12π
electrons of the p-benzoquinonemonoimine type, C6H2(···NHR)2(···O)2 (R = H (1), n-C4H9 (2), C3H6–S–CH3 (3), C3H6–O–CH3 (4), CH2–C6H5 (5)), adsorbed on Au. These molecules are stable
zwitterions by virtue of the meta positions occupied by the nitrogen
and oxygen substituents on the central ring, respectively. The structures
of 2–4 have been determined by single
crystal X-ray diffraction and indicate that in these molecules, two
chemically connected but electronically not conjugated 6π electron
subunits are present, which explains their strong dipolar character.
We systematically observed that homogeneous molecular films with thickness
as small as 1 nm were formed on Au, which fully cover the surface,
even for a variety of R substituents. Preferential adsorption toward
the patterned gold areas on SiO2 substrates was found with 4. Optimum self-assembling of 2 and 5 results in ordered close packed films, which exhibit n-type character,
based on the position of the Fermi level close to the conduction band
minimum, suggesting high conductivity properties. This new type of
self-assembled molecular films offers interesting possibilities for
engineering metal–organic interfaces, of critical importance
for organic electronics.