Morphology controlled nanostructures self-assembled from phthalocyanine derivatives bearing alkylthio moieties: effect of sulfur-sulfur and metal-ligand coordination on intermolecular stacking

To investigate the effect of sulfur-sulfur and metal-ligand coordination on the molecular structure and morphology of self-assembled nanostructures, metal-free 2,3,9,10,16,17,23,24-octakis(isopropylthio)phthalocyanine H(2)Pc(β-SC(3)H(7))(8) (1) and its copper and lead congeners CuPc(β-SC(3)H(7))(8) (2) and PbPc(β-SC(3)H(7))(8) (3) are synthesized and fabricated into organic nanostructures by a phase-transfer method. The self-assembly properties are investigated by electronic absorption and Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Experimental results reveal different molecular packing modes in these aggregates, which in turn result in self-assembled nanostructures with different morphologies ranging from nanobelts for 1 through nanoribbons for 2 to cluster nanoflowers for 3. Intermolecular π-π and sulfur-sulfur interactions between metal-free phthalocyanine 1 lead to the formation of nanobelts. The additional Cu-S coordination bond between the central copper ion of 2 and the sulfur atom of the adjacent molecule of 2 in cooperation with the intermolecular π-π stacking interaction increases the intermolecular interaction, and results in the formation of long nanoribbons for 2. In contrast to compounds 1 and 2, the special molecular structure of complex 3, together with the intermolecular π-π stacking interaction and additional Pb-S coordination bond, induces the formation of Pb-connected pseudo-double-deckers during the self-assembly process, which in turn further self-assemble into cluster nanoflowers. In addition, good semiconducting properties of the nanostructures fabricated from phthalocyanine derivatives 1-3 were also revealed by I-V measurements.     

A molecular approach to self-assembly of trimethylsilylacetylene derivatives on gold

We recently discovered that a linear multifunctional trimethylsilylacetylene (TMSA) compound forms long-range and highly stable self-assembled monolayers (SAMs) on reconstructed Au(111). To better understand the interactions governing self-assembly in this new system, we synthesized a series of homologue organosilanes and performed scanning tunneling microscopy (STM) measurements at the Au(111)/n-tetradecane interface. The four TMSA-terminated linear silanes that we synthesized self-assemble in similar ways on gold, with the molecules standing upright on the surface. In contrast, compounds with a slightly modified terminal group but the same polyunsaturated linear chain above the TMSA head do not self-assemble. In particular, substituting a methyl group of TMSA with a more bulky one prevents self-assembly. Removing the C triple bond C triple bond of TMSA or substituting the Si atom by a C atom also hinders self-assembly. Finally, substituting one methyl group of TMSA by a hydrogen atom induces self-assembly but in a different geometry, with the molecules lying flat on the gold surface in a quasi-epitaxy mode. Our molecular approach demonstrates the key role played by the TMSA head in self-assembly, its origin being twofold: 1) the TMSA layers are commensurate to the Au(111) adlattice along the <112> direction, and 2) the C triple bond C triple bond of TMSA activates the Si atom and induces the creation of a surface Si-Au chemical bond. The highly stable TMSA-based SAMs appear then as promising materials for applications in surface modification.     

Supramolecular self-assembly initiated by solid-solid wetting

We present a preparation method for self-assembled supra-molecular monolayers of unsubstituted organic semiconductors and pigments on a solid substrate, applicable under ambient conditions. The deposition is based on a solid-solid wetting phenomenon, whereas the subsequent layer growth proceeds according to standard models. Molecular adsorption results from direct contact of the compound in a nanocrystalline state with the solid surface. Based on complementary force field calculations, we propose that molecules disintegrate from the crystalline state and adsorb on the surface because of a gain in binding energy. The preparation method is exemplified by means of a linear hydrogen-bonded system, namely quinacridone (QAC) on graphite. In addition, the chosen system allows us to actively guide the self-assembly after deliberate removal of molecules from a predefined area.