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.

Hybrid photochromic mesoporous materials based on MCM-41 and SBA-15 were synthesized by covalent attachment of 3'-butoxy-7-hydroxyflavylium (Fl-OH) and 3'-butoxy-7-metoxyflavylium (Fl-OCH3) hydrogensulfates. The pristine materials were initially grafted with 3-chloropropyl groups, reacted with 3'-hydroxyacetophenone and finally condensed with appropriate salicylaldehydes to yield the new hybrids MCM-41-Fl-OH and SBA-15-Fl-OCH3. The materials were characterized by powder X-ray diffraction, N-2 adsorption, solid-state C-13 CPMAS NMR spectroscopy, and thermogravimetric and elemental analyses, which confirm the successful covalent bonding of the flavylium moieties with loadings of 16.90 +/- 0.05% and 11.78 +/- 0.04% (w/w) for MCM-41-Fl-OH and SBA-15-OCH3, respectively. Flavylium compounds originate in solution a multiequilibria reaction network than can be actuated by pH and light, defining pH-coupled photochromic systems. The new hybrids show pH-dependent reflectance spectra resembling those observed in solution, but shifted to higher pH ranges, indicating a higher stability of the grafted flavylium cations. Irradiation of these materials equilibrated at adequate pH values where the photoisomerizable trans-chalcones predominate shows formation of the respective flavylium cations that recover back to the initial compositions upon standing in the dark, leading these new organic-inorganic hybrids as pH-dependent photochromic materials. (C) 2013 Elsevier Inc. All rights reserved.

Substitution of the phenyl group in 2-hydroxychalcones by a 4-pyridine unit dramatically changes the network of chemical reactions of this compound: trans-chalcone-type (Ct), cis-chalcone-type (Cc), and a hemiketal (hydroxy-4-pyridinechromene) (B) and their protonated forms are formed, but the presence of a flavylium-type cation could not be detected even at very acidic pH Values. Moreover, whereas in 2-phenyl-2-benzopyrylium compounds B and Cc are generally elusive species whose kinetic processes in aqueous solutions occur oil the sub-second time-scale, in the present compound these species equilibrate on a timescale four orders of magnitude lower. Complete characterization of the equilibrium and kinetics of the reaction network could thus be achieved by (1)H NMR spectroscopy and UV/Vis spectrophotometry. ne network of chemical reactions exhibits cis-trans photoisomerization, as well as photochromism between the hemiketal and the chalcone-type species. The irradiation of Ct in MeOH/ H(2)O (1:1) at 365 nm produces B almost quantitatively through two Consecutive photochemical reactions: Ct -> Cc photoisomerization followed by Cc -> B photo ring Closure with a global quantum yield of 0.02. On the other hand, irradiation of B at 254 nm leads to it photostationary state composed by 80% Ct and 20%, B. with a quantum yield of 0.21.

The compound 7,4'-dihydroxy-5-methoxyflavylium (dracoflavylium) was identified as the major red colorant in samples of the resin "dragon's blood", extracted from the tree Dracaena draco. The complex network of reversible chemical reactions that dracoflavylium undergoes in aqueous solution is fully described; for the first time, all the equilibrium constants that enable a complete characterisation of the system have been obtained (K'(a)=1.6 x 10(-4), K-a1 = 1.0 x 10(-4), K-a2 = 3.2 x 10(-8), K-Ct1 = 1.0 x 10(-7), K-Ct2 = 1.3 x 10(-10)). It is concluded that the red colour is due to a stable quinoid base, A, which is the major species at pH 4-7. It is further shown that this compound does not fit the commonly accepted definitions of anthocyanidin nor 3-deoxyanthocyanidin. Similarly to synthetic flavylium salts, the natural compound 7,4'-dihydroxy-5-methoxyflavylium gives rise to several species (multistate system) reversibly interconverted by external stimuli, such as pH.