Selective interaction between Crystal Violet Lactone and Fe(3+)/Fe(2+) in methanol leads to a reversible ionochromic colour change. This interaction can be controlled electrochemically, in order to achieve reversible colour changes with high contrast between colourless and dark blue solutions. The presented system is proposed as an alternative electrochromic solution. (C) 2009 Elsevier Ltd. All rights reserved.

The rate and equilibrium constants of the network of chemical species involving the dye 2'-methoxyflavylium tetrafluoroborate were characterized using stopped flow and flash photolysis in both acidic and alkaline aqueous solution. The trans-chalcone is the thermodynamic stable species in acidic solutions 2 < pH < 7; irradiation of trans-chalcone at low pH leads to the corresponding coloured flavylium cation. The system reverts to its initial state in a few hours, in the dark. The kinetics of the various steps in the system were determined; flash photolysis revealed that the rate determining process of flavylium formation is the ring closure to give the corresponding hemiketal. In alkaline medium the ionized transchalcone is the stable form and clear evidence for the existence of a hemiketal species was obtained. A write-read-erase cycle can be performed with this compound. (c) 2008 Elsevier Ltd. All rights reserved.

Flavylium salts contain the basic structure and show a pH-dependent sequence of reactions identical to natural anthocyanins, which are responsible for most of the red and blue colors of flowers and fruits. In this work we investigated the effect of the water-soluble molecular clips C1 and C2 substituted by hydrogen phosphate or sulfate groups on the stability and reactions of the flavylium salts 1-4 by the use of UV-vis absorption, fluorescence, and NMR spectroscopy as well as of the time-resolved pH jump and flash photolysis methods. Clip C1 forms highly stable host-guest complexes with the flavylium salts 1 and 2 and the quinoidal base 3A in methanol. The binding constants were determined by fluorometric titration to be log K = 4.1, 4.7, and 5.6, respectively. Large complexation-induced (1)H NMR shifts of guest signals, Delta delta(max), indicate that in the case of the flavylium salts 1 and 2 the pyrylium ring and in the case of the quinoidal base 3A the o-hydroxyquinone ring are preferentially bound inside the clip cavity. Due to the poor solubility of these host-guest complexes in water, the association constants could be only determined in highly diluted aqueous solution by UV-vis titration experiments for the complex formation of clip C1 with the flavylium salt 3AH(+) at pH = 2 and the quinoidal base 3A at pH = 5.3 to be log K = 4.9 for both complexes. Similar results were obtained for the formation of the complexes of the sulfate-substituted clip C2 with flavylium salt 4AH(+) and its quinoidal base 4A which are slightly better soluble in water (log K = 4.3 and 4.0, respectively). According to the kinetic analysis (performed by using the methods mentioned above) the thermally induced trans-cis chalcone isomerization (4Ct -> 4Cc) and the H(2)O addition to flavylium cation 4AH(+) followed by H(+) elimination leading to hemiketal 4B are both retarded in the presence of clip C2, whereas the photochemically induced trans-cis isomerization (4Ct -> 4Cc) is not affected by clip C2. The results presented here are explained with dominating hydrophobic interactions between the molecular clips and the flavylium guest molecules. The other potential interactions (ion-ion, cation-pi, pi-pi, and CH-pi), which certainly determine the structures of these host-guest complexes to a large extent, seem to be of minor importance for their stability.

In the framework of supramolecular chemistry, a three component system constituted by Fe(3+)/Fe(2+), crystal violet lactone (CVL) and a spiropyran (SPI) leads to three coloured states, cyan, magenta and yellow, as well as a transparent state, each independently addressable by light and electrical input. The system profits from fine speciation Fe(3+)/Fe(2+) complexes with CVL and SPI. Redox stimulus operates the metal, leading to different coloured complexes (ionochromism), while light stimulus operates the SPI component (photochromism).

2-Styryl-1-benzopyrylium derivatives exhibit deeper hues and absorption spectra that are substantially red-shifted when compared with their 2-phenyl-1-benzopyrylium analogues. They follow the same pH and light-dependent network of chemical reactions previously described for 2-phenyl-1-benzopyrylium compounds. In this work, the photochromic properties of 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium chloride are reported. This compound was fully characterized by UV-vis absorption, fluorescence emission, pH jumps, and flash photolysis, and its properties were compared with the analogue 7,4'-dihydroxyflavylium (7-hydroxy-2-(4-hydroxyphenyl)-1- benzopyrylium). The trans-chalcones of both compounds lacking the hydroxyl in position 2 were synthesized and used as model compounds since they exhibit cis-trans isomerization but cannot be involved in the other processes resulting from the ring closure. The transient absorption of two triplets attributed to the chalcones Ct/Ct(-), and a tautomer was detected by nanosecond flash photolysis, independent of the existence of the 2-hydroxyl substituent. The experimental results are compatible with the main formation of cis-chalcone from the singlet state. In the case of the styryl derivatives, the fraction of triplet formed from excitation of Ct is much higher, and the fraction of isomerization is much smaller. For this reason, the photochemistry of 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium in water is much less efficient than that of its parent 7,4'-dihydroxyflavylium; however, in the presence of CTAB micelles, intense red colors can be obtained upon irradiation, confirming the usefulness of this family of compounds as photochromic systems.

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.