Molecular chains and carpets of sexithiophenes onAu(111)

Abstract

The two organic molecular materials ␣-sexithiophene ͑6T͒ and ␣ , -dihexylsexithiophene ͑DH6T͒ adsorbed on Au͑111͒ in the ͑sub͒monolayer range were investigated by scanning tunneling microscopy ͑STM͒ in order to explore the effect of alkyl substitution on the self-assembly at surfaces. Metal substrate step edges are identified as preferred nucleation sites for 6T, while stable nucleus formation for DH6T occurs at kinks of the Au͑111͒ herringbone reconstruction. At low coverage, 6T forms continuous chains of single-molecular width along Au step edges, involving molecular conformation changes by rotations around C-C bonds of neighboring thiophene units. In contrast, DH6T exhibits no ordered structures in the submonolayer range. At monolayer coverage, substantially different structures were observed for the two molecules. 6T forms rows of molecules with parallel long molecular axes, whereas DH6T forms lines along these axes, where the conjugated cores are embedded in a matrix of hexyl chains. Because of different preferred nucleation sites, 6T forms a continuous molecular carpet on extended Au͑111͒ terraces, whereas DH6T resembles a patchworklike carpet as domain boundaries are induced by the Au͑111͒ herringbone surface structure, leading to reduced domain sizes. Alkylation of 6T thus drastically changes the adsorption behavior and the resulting layer structure on the Au surface. These results should be valuable for developing new directed self-assembly schemes on prepatterned surfaces.

Balancing Intermolecular and Molecule–Substrate Interactions in Supramolecular Assemblies

“…The persistence of the surface-state in the case of DIP deposition reflects the weakness of the molecule-substrate interaction, as observed with most other organic molecules such as, e.g., PTCDA, CuPc, FePc, HBC, NTCDI, BDG, 6T, DH6T, pentacene, etc. [9,23,31,[34][35][36][37][38][39] A much stronger (and rarer) interaction becomes evident from the data for the F 16 CuPc layers. It is plausible that this interaction arises from a partial charge transfer from Au to F 16 CuPc.…”

Step-by-step growth of epitaxially aligned polythiophene by surface-confined reaction

“…The mostly upright orientation is somewhat surprising for thiophene-derived units. Flat or slightly tilted orientations, enabling aromatic-metal interactions, have been consistently observed for oligothiophenes on Ag (23), Au (24,25), and Cu(110) (26,27). The upright orientation for dehalogenated EDOT is stabilized by two C-Cu bonds, as well as by strong Cu-S interactions and weak van der Waals contacts with the adjacent molecules in the stack.…”

Commensurate surface structures and concertedcis-trans-isomerization within ordered monolayers ofα-sexithiophene on Ag(001)

“…In contrast to these expectations, up to now ordered struc- tures of 6T have been observed exclusively in all-trans configuration on metal substrates. 1,6,10,13,17,21,22,25 Consequently, if the molecules do not remain in the all-trans state after sublimation, they must change back to the all-trans configuration during or after adsorption.…”

“…Their monolayer growth has been investigated on various noble-metal surfaces and highly oriented graphite. 1,3,[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] While on many oxide surfaces, with a weak molecule-substrate interaction, -conjugated molecules are found to adsorb in an upright standing configuration, 3,23,24 on metal substrates, such as Cu, Ag or Au, molecules in the first layer have been found to be flat lying. 1,6,10,13,17,25 On Ag͑110͒ and Ag͑111͒ 6T seems to adsorb into commensurate structures with the molecular axis parallel to the high-symmetry ͓001͔ and ͓110͔ direction, respectively, based on near-edge absorption fine structure ͑NEXAFS͒ and reflection high-energy electron diffration ͑RHEED͒ results.…”