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By the means of density functional theory calculations, we find that CO 2 activation via reverse water−gas shift (r-WGS) follows different elementary steps on different metals (Pt, Rh, Ni, Cu, Ag, and Pd). We relate these differences to the interactions between the adsorbed oxygen and the metals, which strongly affect the dissociation activation energy. In particular, CO 2 dissociation is favored on metals that present high affinity toward oxygen. As the O interaction with the metals weakens, CO 2 hydrogenation becomes more favored at the expenses of the dissociation. We found that the binding energy of oxygen scales almost linearly with the difference between the activation energy of the two competing paths, and therefore this quantity can be used as a simple descriptor to discriminate which of the two mechanisms is dominant on different metals. Such findings allow rationalization of the different catalytic cycles reported in the literature for the r-WGS reaction on metal surfaces.

We describe the photochemical behavior of aqueous solutions containing the Co(CN) 6 3complex ion and the trans-chalcone form Ct of the 4′-methoxyflavylium ion (AH + ). It is shown that, under the experimental conditions used, the photochemical reaction leading from Ct to AH + shows an off-on-off behavior upon excitation with a continuous light source or with successive light flashes. The described systems perform as threshold devices and as XOR (eXclusive OR) logic gates. They behave as rudimental artificial neuron-like systems in the sense that their outputs (formation of the AH + species, with a consequent change in the spectroscopic properties) are the result of an elaboration of two distinct (light) inputs by chemical reactions in solution.

A series of novel redox-active and photoactive ruthenium(II) and osmium(II) bipyridyl-, ferrocene-, and cobaltocenium-containing macrocyclic receptors with the dual capability of selectively sensing anionic guest species via electrochemical and optical methodologies have been prepared. Single-crystal X-ray structures of 7·Cl-, 7·2Br-, and 13·2OAc- highlight the importance of hydrogen bonding and respective macrocyclic cavity size to the anion recognition process in the solid state. Proton NMR titration studies in deuterated DMSO solutions reveal these receptors form strong and remarkably selective complexes with Cl-, H2PO4 -, and OAc- anions dependent upon the flexibility, topology, and size of the receptor cavity. Cyclic and square-wave voltammetric investigations have demonstrated these receptors to electrochemically recognize Cl-, H2PO4 -, and OAc- anions. Photophysical studies reveal emission spectral recognition of Cl- in acetonitrile solutions is displayed by 7−12. With the hetero-dinuclear receptors 8, 9, and 12, the rate constants of the energy transfer process responsible for the quenching of the luminescent ruthenium excited state significantly decreased in the presence of chloride anion.

In aqueous solution (2 < pH < 8) the thermodynamically stable form of the 4‘-methoxyflavylium ion (AH + ) is its hydrated derivative trans-4‘-methoxychalcone, C t . The C t compound shows a broad absorption band with λmax = 350 nm. In acid medium, irradiation of C t with near-UV light causes strong spectral changes with five isosbestic points and appearance of a very intense band in the visible region with maximum at 435 nm, corresponding to the AH + form. It has been shown that irradiation of C t causes a trans → cis photoisomerization reaction (Φ = 0.04 at λexc = 365 nm), which is followed by 100% conversion of the cis-chalcone form (C c ) to the AH + ion. The AH + ion is photochemically inactive and thermally inert in acid medium (half-life of the back conversion at 25 °C in the dark is 815 days at pH 1.0 and 20 h at pH 4.3, respectively). At high temperature (>50 °C) and/or pH ≥3, however, AH + can be quantitatively converted back to C t (half-life of 15 min at pH 4.0 and 60 °C). Owing to this unique behavior, this represents a novel molecular system in which the color can be controlled by light and changes in temperature and/or pH. The ability to photochemically convert the stable and colorless C t form to the kinetically inert and colored AH + form, and the possibility to reconvert AH + to C t at high temperature or by a pH jump make the system well-suited as the basis for an optical memory device with multiple storage and nondestructive readout capacity through a write−lock−read−unlock−erase cycle.

Luminescent dendrimers are currently attracting much attention since coupling luminescence and dendrimer research topics can lead to valuable new functions. Indeed, luminescence is a valuable tool to monitor both basic properties and possible applications (sensors, displays, lasers), and dendrimers are macromolecular compounds exhibiting a well-defined chemical structure with the possibility of containing selected chemical units in predetermined sites and of encapsulating ions or neutral molecules in their internal dynamic cavities. In this paper we will review recent advances in this field focusing our attention on their properties in fluid solution related to light harvesting, changing the "color" of light, sensing with signal amplification, quenching and sensitization processes, shielding effects, elucidation of dendritic structures and superstructures, and investigation of dendrimer rotation in solution.