Useful application of photochromic compounds as optical memories implies the existence of a large kinetic barrier between the forms interconverted by light. In the case of flavylium salts, the ground state isomerization barrier between the photoisomerizable chalcone isomers is shown to correlate with the electron donating ability of the substituents, measured by their effects in the H-1 NMR chemical shifts of the aromatic protons. Substitution with electron donating groups in ring A lowers the barrier while substitution at ring B has the opposite effect. However, in water, the observed increase is higher than expected in the case of compound 4', 9-dihydroxychalcone when compared with the analogous 4'-dimethylamino-9-hydroxychalcone, containing a better electron donating group in the same position. Our interpretation is that the water network is providing an efficient pathway to form tautomers. In acetonitrile, unlike water, the expected order is indeed observed: E-a( 4', 9-dihydroxychalcone) = 60 kJ mol(-1) < E-a ( 4'-dimethylamino-9-hydroxychalcone) = 69 kJ mol(-1).

The synthesis, potentiometric studies and photophysical properties of two new polyamine ligands (L1 and L2) possessing the dansyl chromophore were studied in aqueous 0.15 M NaCl. The compounds show the absorption and emissions bands characteristic of the dansylamide fluorophore and both present intramolecular excited state proton transfer at intermediate pH ranges. One of the ligands (L2) strongly coordinates Zn(II) leading to fluorescence quenching. A model compound (L3) of the dansyl moiety was also investigated. (c) 2006 Elsevier B.V. All rights reserved.

A new family of compounds able to promote host-guest interactions with specific molecules (e.g., cyanuric and parabanic acids) and to coordinate metal ions, namely Zn(II) and Cu(II), has been synthesized and fully characterized. The new probes derive from the attachment of two methylaminopyrene units to the carbonyl precursor 2,6-bis(2-formylphenoxymethyl)pyridine. Its signalling properties result from the fluorescence emission properties, which reveal the existence of intramolecular excimer formation. The compounds have showed to be highly sensitive to the solvent and hydrogen ion concentration of the medium. Depending on these, different monomer-to-excimer fluorescence ratio is displayed by the two probes. The compound with a single pyrene unit revealed absence of excimer formation and was used as model compound. The overall results are discussed on the basis of the studied probes as potentially revealing molecular movements, off-on-off fluorescent photoinduced electron transfer (PET), host-guest interactions with specific compounds and of sensing metal ions. (c) 2005 Elsevier B.V. All rights reserved.

The chemistry and photochemistry of the compound 4'-iodo-8-methoxyflavylium tetrafluoroborate was studied in solvent mixtures of water and ethanol. The trans-chalcone form (0) is always the most stable species at equilibrium. In particular, for the first time, Ct and not the flavylium cation is reported as the most stable species in mixtures containing water at pH 1.0. The rate of Ct formation as a function of the water content exhibits a bell-shaped dependence, showing a maximum at approximately 50% water. These trends are explained by a slower cis-trans isomerization in ethanol relative to that in water due to the expected stabilization in polar solvents of the zwitterionic intermediate; on the other hand, the decrease in the concentration of cis-chalcone (Cc) in the pseudoequilibrium with increasing water content, gives rise to a decrease in the rate of Ct formation (k(obs)=k(real)[Cc]). The hydration reaction was found to be much more efficient when water is present in low concentrations. This result has important consequences for the interpretation of the coloring phenomena caused by anthocyanins in plants, as well as for applications of flavylium compounds in the field of optical memories.

Among the Krebs cycle components, just citrate enhances the fluorescence of a new bi(brachial) lariat aza-crown containing appended naphthalene fluorophores.

The outstanding solvatochromic properties of the complex Ru(bpy)(CN)(4)(2-), deriving from the ability of the nitrile goup to participate in H-bond interactions, were explored to develop a sensor for low concentrations of water on common organic aprotic solvents. In dry solvents, the maximum wavenumber of the low energy transition of Ru(bpy)(CN)(4)(2-) presents linear correlations with the Kamlet-Taft solvent acidity parameter (slope = 0.59 +/- 0.02, r > 0.99) and Gutman acceptor number (slope = (1.7 +/- 0.1) x 10(-2), r > 0.99). The slopes and regression coefficients of the correlations measure the sensitivity of the compound to H-bond donation by the solvent and confirm its high specificity towards the sensing of H-bond donating solvents, such as water. In acetonitrile, tetrahydrofuran, N,N-dimethylformamide, dichloromethane and toluene, the sensor presents spectral changes that respond linearly to water concentrations in the range 5-70 ppm and compete favorably with the widely spread Karl-Fisher method with a detection limit of similar to 50 ppm. The high sensitivity and the much simpler spectrophotometric titration, portends the use of this complex as an optical water sensor for determination of low water contents in organic solvents. (c) 2005 Elsevier B.V. All rights reserved.