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 12 pi electrons of the p-benzoquinonemonoimine type, C6H2((center dot center dot center dot) under bar NHR)(2)((center dot center dot center dot) under barO)(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 6 pi 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 dose packed films, which exhibit n-type character, based on the position of the Fermi level dose to the conduction band minimum, suggesting high conductivity properties. This new type of self-assembled molecular fills offers interesting possibilities for engineering metal organic interfaces, of critical importance for organic electronics.