We demonstrate that water is almost universally present on apparently dry self-assembled monolayers, even on those considered almost hydrophobic by conventional methods such as water contact goniometry. The structure and kinetics of nanoscale water adsorption onto these surfaces were investigated using X-ray and neutron reflectometry, as well as atomic force microscopy. Condensation of water on hydrophilic surfaces under ambient conditions formed a dense sub-nanometre surface layer; the thickness of which increased with exponentially limiting kinetics. Tapping mode AFM measurements show the presence of nanosized droplets that covered a small percentage (∼2%) of the total surface area, and which became fewer in number and larger in size with time. While low vacuum pressures (∼10-8 bar) at room temperature did nothing to remove the adsorbed water from these monolayers, heating to temperatures above 65 °C under atmospheric conditions did lead to evaporation from the surface. We demonstrate that water contact angle measurements are not necessarily sensitive to the presence of nanoscale adsorbed water and do not vary with time. For the most part they are a poor indicator of the kinetics and the amount of water condensation onto these surfaces at the molecular level. In summary, this study reveals the need to exclude air containing even trace amounts of water vapor from such surfaces when characterizing using techniques such as X-ray reflectometry. 2011 The Royal Society of Chemistry. We demonstrate that water is almost universally present on apparently dry self-assembled monolayers, even on those considered almost hydrophobic by conventional methods such as water contact goniometry. The structure and kinetics of nanoscale water adsorption onto these surfaces were investigated using X-ray and neutron reflectometry, as well as atomic force microscopy. Condensation of water on hydrophilic surfaces under ambient conditions formed a dense sub-nanometre surface layer; the thickness of which increased with exponentially limiting kinetics. Tapping mode AFM measurements show the presence of nanosized droplets that covered a small percentage ($2%) of the total surface area, and which became fewer in number and larger in size with time. While low vacuum pressures ($10 À8 bar) at room temperature did nothing to remove the adsorbed water from these monolayers, heating to temperatures above 65 C under atmospheric conditions did lead to evaporation from the surface.We demonstrate that water contact angle measurements are not necessarily sensitive to the presence of nanoscale adsorbed water and do not vary with time. For the most part they are a poor indicator of the kinetics and the amount of water condensation onto these surfaces at the molecular level. In summary, this study reveals the need to exclude air containing even trace amounts of water vapor from such surfaces when characterizing using techniques such as X-ray reflectometry.

Three new ruthenium-sulfur dioxide linkage photoisomeric complexes in the [Ru(NH(3))(4)(SO(2))X]Cl(2)·H(2)O family (X = pyridine (1); 3-chloropyridine (2); 4-chloropyridine (3)) have been developed in order to examine the effects of the trans-ligand on the nature of the photo-induced SO(2) coordination to the ruthenium ion. Solid-state metastable η(1)-O-bound (MS1) and η(2)-side S,O-bound (MS2) photoisomers are crystallographically resolved by probing a light-induced crystal with in situ diffraction. This so-called photocrystallography reveals the highest known photoconversion fraction of 58(3)% (in 1) for any solid-state SO(2) linkage photoisomer. The decay of this MS1 into the MS2 state was modeled via first-order kinetics with a non-zero asymptote. Furthermore, the MS2 decay kinetics of the three compounds were examined according to their systematically varying trans-ligand X; this offers the first experimental evidence that the MS2 state is primarily stabilized by donation from the S-O(bound) electrons into the Ru dσ-orbital rather than π-backbonding as previously envisaged. This has important consequences for the optoelectronic application of these materials since this establishes, for the first time, a design protocol that will enable one to control their photoconversion levels.

A photoinduced solid-state SO₂ isomerism drives a larger mechanical change (benzene-ring rotation) in a neighbouring ion (i.e., the system acts as a solar-powered molecular transducer). The ring rotation and SO₂ photoisomerisation are observed using in situ X-ray crystallography and are controllable, reproducible, and metastable at low temperatures. This discovery presents a new range of materials for solar-energy-based molecular transduction.

Thermally reversible solid-state linkage SO 2 photoisomers of three complexes in the [Ru(NH 3 ) 4 SO 2 X]tosylate 2 family are captured in their metastable states using photocrystallography, where X = pyridine (1), 3-Cl-pyridine (2), and 4-Cl-pyridine (3). This photoisomerism exists only in the single-crystal form; accordingly, the nature of the crystalline environment surrounding the photoactive species controls its properties. In particular, the structural role of the tosylate anion needs to be understood against possible chemical influences due to varying the trans ligand, X. The photoexcited geometries, photoconversion levels, and thermal stabilities of the photoisomers that form in 1−3 are therefore studied. 1 and 2 yield two photoisomers at 100 K: the Obound end-on η 1 -SO 2 (MS1) configuration and the side-bound η 2 -SO 2 (MS2); 3 exhibits only the more thermally stable MS2 geometry. The decay kinetics of the MS2 geometry for 1−3 demonstrate that the greater the free volume of the GS SO 2 ligand for a given counterion, the greater the MS2 thermal stability. Furthermore, a rationalization is sought for the SO 2 phototriggered molecular rotation of the phenyl ring in the tosylate anion; this is selectively observed in 2, manifesting as nanomechanical molecular transduction. This molecular transduction was not observed in 1, despite the presence of the MS1 geometry due to the close intermolecular interactions between the MS1 SO 2 and the neighboring tosylate ion. The decay of this anionic molecular rotor in 2, however, follows a nontraditional decay pathway, as determined by time-resolved crystallographic analysis; this contrasts with the well-behaved first-order kinetic decay of its MS1 SO 2 phototrigger.

The photophysical properties associated with solid-state Ru-based SO 2 linkage photoisomerism are shown to differ with single-molecule recognition; this stands to afford superior optical resolution for data storage applications. Two compounds, [Ru(NH 3 ) 4 SO 2 X]tosylate 2 (X = isonicotinamide (1) and isonicotinic acid (2)), yield crystal structures, each with two chemically identical but crystallographically distinct Ru-based complexes (Ru01 and Ru02) and thus two different photoisomerizable SO 2 environments. It was found that the SO 2 photoconversion fraction in each crystallographically independent complex differed by over 20% (for 1: 51.0(12) % in Ru01 and 28.6(9) % in Ru02). These photophysical differences between neighboring molecules were attributed to the larger free volume around the groundstate SO 2 in Ru01, allowing for higher photoconversion, i.e., a "lock-and-key" environment controls the photochemistry. Furthermore, the η 2 -side-S,O-bound (MS2) metastable photoisomer in Ru01 was 20 K more thermally stable in both 1 and 2; photoinduced intermolecular interactions are shown to dictate this thermal stability.