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 assem blies in a self-consistent series of pillared, layered organic-inorganic lo ng-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 transmissio n spectroscopy and powder X-ray diffraction. The structural attributes eluc idated:include quantitative estimates of average chain orientation, chain c onformation, chain-chain translational order, interpenetration of the conti guous layers, as well as void characteristics in the chain matrix. The evid ence presented here establishes that the layered chain assemblies sandwiche d between the inorganic Ag-S backbones in a double-layer arrangement are co mprised of an ordered packing of all-trans-extended chains. The average cha in in each assembly is oriented vertically away from the quasi-hexagonal Ag -S lattice, in a two-dimensional pseudo-monoclinic arrangement of domains o f 60-70 translationally correlated chains. Small interpenetration between t he contiguous layers leads to the formation of regularly spaced 1D channels or corridors. The three-dimensional network of 1D channels alternates betw een the chain layers. All the chain structural characteristics deduced here are, in good conformity with those implied in the model proposed earlier b y Dance and co-workers. The present results, together with the previous X-r ay analysis for comparable short-chain AgSRs, are used to propose a two-ste p, hierarchical self-assembly mechanism for the:formation of silver (n-alky l) thiolates. It is proposed that the primary self-assembly process involve s the organization of Ag+ and RS- species into puckered sheets of quasi-hex agonally symmetric 2D lattices, with the chain substituents extending on ea ch side. The subsequent self-assembly of these 2D building blocks in the th ird dimension via complementary stacking appears to complete the formation of sandwiched bimolecular chain assemblies.