Superlattice of Self-Assembled Silver-Thiolates: The Formation of Unique Sublayers Induced by Mixed <i>N</i> -Alkyl Chains

A new type of superlattice has been found in blended metal-thiolate crystals, AgSC_<i>n</i>H₂<i>_n</i> _{+ 1} and AgSC_<i>m</i>H_{2<i>m</i> +} ₁ (<i>n</i>, <i>m</i> = 6-18, <i>n</i> < <i>m</i>, <i>n</i> and <i>m</i> are even), with various chain length mismatches (η = (<i>m</i> - <i>n</i>)/<i>n</i>), precipitated from a mixture of hot solutions of individual thiolates. The superlattice consists of alternating sublayers of crystalline thiolate lamellae and chain-end regions with distinct properties. Diffraction patterns suggest that the superlattice stacks are highly periodic and well registered. Carbons from these two alternating sublayers are exposed to different chemical environments, as shown by nuclear magnetic resonance measurements. The chain-end region exhibits a new material state that is ordered but behaves like an amorphous state in calorimetric measurements, without a chain-melting transition. Its thickness, estimated from X-ray diffraction and calorimetric analysis, coincides with the chain length difference between -C_<i>n</i>H_{2<i>n</i> +} ₁ and -C_<i>m</i>H_{2<i>m</i> +} ₁ ligands, presumably indicating that the chain-end regions consist of surplus parts of the longer alkyl chains protruding from adjacent lamellar layers. Mixed thiolates exhibit a melting point depression of the crystalline lamellae in comparison with the individual components. Triggered by the existence of the unique nonmelting layer, this effect is described by the Gibbs-Thomson model. The synthetic procedure yields good-quality crystals with η up to 1 over a wide range of mixing ratios, which is employed as a control parameter to continuously tune the structure of the superlattice.