Characterization of chain molecular assemblies in long-chain, layered silver thiolates: A joint infrared spectroscopy and x-ray diffraction study

The first direct characterization of structures of bi-molecular chain assemblies in a self-consistent series of pillared, layered organic-inorganic long-chain silver (n-alkane)thiolates, (AgS(CH2)(n)CH3; n = 5, 6, 9, 11, 15, and 17), is reported using the combined application of infrared transmission spectroscopy and powder X-ray diffraction. The structural attributes elucidated:include quantitative estimates of average chain orientation, chain conformation, chain-chain translational order, interpenetration of the contiguous layers, as well as void characteristics in the chain matrix. The evidence presented here establishes that the layered chain assemblies sandwiched between the inorganic Ag-S backbones in a double-layer arrangement are comprised of an ordered packing of all-trans-extended chains. The average chain in each assembly is oriented vertically away from the quasi-hexagonal Ag-S lattice, in a two-dimensional pseudo-monoclinic arrangement of domains of 60-70 translationally correlated chains. Small interpenetration between the contiguous layers leads to the formation of regularly spaced 1D channels or corridors. The three-dimensional network of 1D channels alternates between the chain layers. All the chain structural characteristics deduced here are, in good conformity with those implied in the model proposed earlier by Dance and co-workers. The present results, together with the previous X-ray analysis for comparable short-chain AgSRs, are used to propose a two-step, hierarchical self-assembly mechanism for the:formation of silver (n-alkyl) thiolates. It is proposed that the primary self-assembly process involves the organization of Ag+ and RS- species into puckered sheets of quasi-hexagonally symmetric 2D lattices, with the chain substituents extending on each side. The subsequent self-assembly of these 2D building blocks in the third dimension via complementary stacking appears to complete the formation of sandwiched bimolecular chain assemblies.