Low-energy optical switching of SO2 linkage isomerisation in single crystals of a ruthenium-based coordination complex.

Single crystals that behave as optical switches are desirable for a wide range of applications, from optical sensors to read-write memory media. A series of ruthenium-based complexes that exhibit optical switching in their single-crystal form via SO2 linkage photoisomerisation are of prospective interest for these technologies. This study explores the optical switching behaviour in one such complex, trans-[Ru(SO2)(NH3)4(H2O)]tosylate2 (1), in terms of its dark and photoinduced crystal structure, as well as its light and thermal decay characteristics, which are deduced by photocrystallography, single-crystal optical absorption spectroscopy and microscopy. Photocrystallography results reveal that a photoisomerisation level of 21.5(5)% is achievable in 1. Biphasic photochromic crystals of 1 were generated by applying green and then red light to switch on and off the η2-(OS)O photoisomer in different regions of a crystal. Heat is a known alternative to its thermal decay, whereby a method is demonstrated that employs optical absorption spectra to determine its activation energy of 30 kJ mol-1. This low-energy barrier to optical switching agrees well with computational studies on 1, as well as being comparable to activation energies in ruthenium-based nitrosyl linkage photoisomers that also display solid-state optical switching.

Solar-powered nanomechanical transduction from crystalline molecular rotors.

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.

Photoconversion bonding mechanism in ruthenium sulfur dioxide linkage photoisomers revealed by in situ diffraction.

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.

Nanoscale water condensation on click-functionalized self-assembled monolayers.

We have examined the nanoscale adsorption of molecular water under ambient conditions onto a series of well-characterized functionalized surfaces produced by Cu(I)-catalyzed alkyne-azide cycloaddition (CuAAC or "click") reactions on alkyne-terminated self-assembled monolayers on silicon. Water contact angle (CA) measurements reveal a range of macroscopic hydrophilicity that does not correlate with the tendency of these surfaces to adsorb water at the molecular level. X-ray reflectometry has been used to follow the kinetics of water adsorption on these "click"-functionalized surfaces, and also shows that dense continuous molecular water layers are formed over 30 h. For example, a highly hydrophilic surface, functionalized by an oligo(ethylene glycol) moiety (with a CA = 34°) showed 2.9 Å of adsorbed water after 30 h, while the almost hydrophobic underlying alkyne-terminated monolayer (CA = 84°) showed 5.6 Å of adsorbed water over the same period. While this study highlights the capacity of X-ray reflectometry to study the structure of adsorbed water on these surfaces, it should also serve as a warning for those intending to characterize self-assembled monolayers and functionalized surfaces to avoid contamination by even trace amounts of water vapor. Moreover, contact angle measurements alone cannot be relied upon to predict the likely degree of moisture uptake on such surfaces.

Bis{µ-2-[(pyridin-2-yl)imino-meth-yl]phenolato}bis-[(2-formyl-phenolato)copper(II)].

The asymmetric unit of the title compound, [Cu(2)(C(12)H(9)N(2)O)(2)(C(7)H(5)O(2))(2)], contains two independent (2-formyl-phen-olato){2-[(pyridin-2-yl)imino-meth-yl]phenolato}copper(II) mol-ecules that form pseudocentrosymmetric dimers via inter-actions between the Cu and pyridyl N atoms of independent monomers. The square-planar geometry of the Cu atoms in the monomer thus becomes square-based pyramidal in the dimer. The crystal studied was an inversion twin, with unequal populations of 0.353 (17) and 0.647 (17).