Odd–Even Effect in the Hydrophobicity of <i>n</i>‑Alkanethiolate Self-Assembled Monolayers Depends upon the Roughness of the Substrate and the Orientation of the Terminal Moiety

The origin of the odd–even effect in properties of self-assembled monolayers (SAMs) and/or technologies derived from them is poorly understood. We report that hydrophobicity and, hence, surface wetting of SAMs are dominated by the nature of the substrate (surface roughness and identity) and SAM tilt angle, which influences surface dipoles/orientation of the terminal moiety. We measured static contact angles (θ_s) made by water droplets on <i>n</i>-alkanethiolate SAMs with an odd (SAM^O) or even (SAM^E) number of carbons (average θ_s range of 105.8–112.1°). When SAMs were fabricated on smooth “template-stripped” metal (M^TS) surfaces [root-mean-square (rms) roughness = 0.36 ± 0.01 nm for Au^TS and 0.60 ± 0.04 nm for Ag^TS], the odd–even effect, characterized by a zigzag oscillation in values of θ_s, was observed. We, however, did not observe the same effect with rougher “as-deposited” (M^AD) surfaces (rms roughness = 2.27 ± 0.16 nm for Au^AD and 5.13 ± 0.22 nm for Ag^AD). The odd–even effect in hydrophobicity inverts when the substrate changes from Au^TS (higher θ_s for SAM^E than SAM^O, with average Δθ_{s |<i>n</i> – (<i>n</i> + 1)|} ≈ 3°) to Ag^TS (higher θ_s for SAM^O than SAM^E, with average Δθ_{s |<i>n</i> – (<i>n</i> + 1)|} ≈ 2°). A comparison of hydrophobicity across Ag^TS and Au^TS showed a statistically significant difference (Student’s <i>t</i> test) between SAM^E (Δθ_{s |Ag evens – Au evens|} ≈ 5°; <i>p</i> < 0.01) but failed to show statistically significant differences on SAM^O (Δθ_{s |Ag odds – Au odds|} ≈ 1°; <i>p</i> > 0.1). From these results, we deduce that the roughness of the metal substrate (from comparison of M^AD versus M^TS) and orientation of the terminal −CH₂CH₃ (by comparing SAM^E and SAM^O on Au^TS versus Ag^TS) play major roles in the hydrophobicity and, by extension, general wetting properties of <i>n</i>-alkanethiolate SAMs

Putting Cocrystal Stoichiometry to Work: A Reactive Hydrogen-Bonded “Superassembly” Enables Nanoscale Enlargement of a Metal–Organic Rhomboid via a Solid-State Photocycloaddition

Enlargement of a self-assembled metal–organic rhomboid is achieved via the organic solid state. The solid-state synthesis of an elongated organic ligand was achieved by a template directed [2 + 2] photodimerization in a cocrystal. Initial cocrystals obtained of resorcinol template and reactant alkene afforded a 1:2 cocrystal with the alkene in a stacked yet photostable geometry. Cocrystallization performed in the presence of excess template resulted in a 3:2 cocrystal composed of novel discrete 10-component hydrogen-bonded “superassemblies” wherein the alkenes undergo a head-to-head [2 + 2] photodimerization. Isolation and reaction of elongated photoproduct with Cu­(II) ions afforded a metal–organic rhomboid of nanoscale dimensions that hosts small molecules in the solid state as guests