The syntheses, thermodynamic and kinetic properties of a series of 2-styryl-1-benzopyrylium compounds are reported. This family of compounds was found to follow the same pH- and light-dependent network of chemical reactions previously described for flavylium (2-phenyl-1-benzopyrylim) compounds. However, 2-styryl-1-benzopyrylium compounds exhibit absorption spectra substantially red shifted when compared with flavylium analogues (up to 90 nm). In particular, a photochromic system switching from yellow to light blue based on derivatives of natural anthocyanins is for the first time documented. (C) 2008 Elsevier Ltd. All rights reserved.

The analysis of different historic mauve samples-mauve salts and dyed textiles-was undertaken to establish the exact nature of the iconic dye produced by W.H. Perkin in the nineteenth century. Fourteen samples from important museum collections were analyzed, and it was determined that. in contrast to the general wisdom that mauveine consists of C, and C, structures. Perkin's mauveine is a complex mixture of at least thirteen methyl derivatives (C, to C(28)) with a 7-amino5-phenyl-3-(phenylamino)phenazin-5- ium core. A fingerprint was established in which mauveines A or B were dominant, and in which mauveines B2 and C(25) were found to be important tracers to probe the original synthesis. Counterion analysis showed that all the mauve salts should be dated after 1862. Perkin's original recipe could be identified in three textile samples, and in these cases, mauveines A and C25 were found to he the major chromophores. These are now shown to be the samples containing the "original mauve".

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.

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.

A new fluorescent macrocyclic structure (0) bearing two naphthalene units at both ends of a cyclic polyaminic chain containing two phenanthroline units was investigated with potentiometric and fluorescence (steady-state and time-resolved) techniques. The fluorescence emission spectra show the simultaneous presence of three bands: a short wavelength emission band (naphthalene monomer), a middle emission band (phenanthroline emission), and a long-wavelength band. All three bands were found to be dependent on the protonation state of the macrocyclic unit (including the polyaminic and phenanthroline structures). The existence of the long-wavelength emission band is discussed and is shown to imply that a bending movement involving the two phenanthroline units leads to excimer formation. This is determined by comparison with the excimer emission formed by intermolecular association of 1,10-phenanthroline. With ligand L1, excimer formation occurs only at pH values above 4. At very acidic pH values, the protonation of the polyamine bridges is extensive leading to a rigidity of the system that precludes the bending movement. The interaction with metal cations Zn(II) and Cu(II) was also investigated. Excimer formation is, in these situations, increased with Zn(II) and decreased with Cu(II). The long-emission band is shown to present a different wavelength maximum, depending on the metal, which can be considered as a characteristic to validate the use of ligand L1 as a sensor for a given metal.