Gomes, R, Parola AJ, Laia CAT, Pina F.
2007.
Efficient photochromism from the network of chemical reactions of 7,4 '-dihydroxyflavylium in CTAB micelles, 2007. Photochemical & Photobiological Sciences. 6:1003-1009.
AbstractDifferently from water,effcient photochromism with a strong colour contrast has been observed for the multistate compound 7,4'- dihydroxyflavylium in the presence of cetyl trimethylammonium bromide ( CTAB) micelles. Two states are responsible for the photochromism: trans-chalcone ( inside the micelle) in the dark, and flavylium cation, AH(+), ( in bulk water) upon irradiation. The kinetics of the system was characterized by. ash photolysis and pH jumps. Evidence that the photochemical process leading to AH(+) occurs in the micelle was achieved. The best colour contrast is obtained at pH = 1.5, from a solution practically colourless in the dark, to an intense yellow upon irradiation ( quantum yield 0.4). The system is completely reversible with a lifetime of 38 min at room temperature, and exhibits a reasonable stability. A kinetic model capable of fitting the data from thermal entrance of the compound into the micelle, its ejection to bulk water upon irradiation and quantum yields of the photochemical process is proposed.
Laia, CAT, Parola AJ, Folgosa F, Pina F.
2007.
Multistate reaction kinetics of 6-hydroxy-4 '-(dimethylamino)flavylium driven by pH. A stopped-flow study, 2007. Organic & Biomolecular Chemistry. 5:69-77.
AbstractThe synthetic flavylium salt 6-hydroxy-4'-(dimethylamino)flavylium hexa. uorophosphate displays a set of pH-driven chemical reactions in aqueous solutions, involving the formation of hemiketal species and chalcones with cis and trans configurations. Such reactions were studied by steady-state and transient UV-Vis spectroscopy and by stopped-flow techniques. A novel and more generalized kinetic scheme is developed, in order to take account of possible acid/base pairs that occur in the network of chemical reactions as the pH is changed. It is found that the formation of the hemiketal species by hydration of the. avylium is slow, and it is not possible to isolate each process that leads to the formation of the cis-chalcone ( hydration and tautomerization reactions). The cis/trans isomerization reaction of cis-chalcone is slow, and the system takes several hours to reach equilibrium after a pH jump at room temperature. In basic conditions, negatively charged trans-chalcones are dominant. Comparison with other. avylium compounds shows that the hydration process is affected mainly by the amino group, while the hydroxyl group influences the tautomerization and isomerization reactions.
Pina, F, Parola AJ, Melo MJ, Laia CAT, Afonso CAM.
2007.
Photochromism of 7-(N, N-diethylamino) 4 '-hydroxyflavylium in a water-ionic liquid biphasic system, 2007. Chemical Communications. :1608-1610.
AbstractPhotochromism of trans-4-(N,N-diethylamino)-2,4'-dihydroxychalcone, with formation of the photoproduct 7-(N,N-diethylamino)-4'-hydroxyflavylium, is promoted in the ionic liquid phase of a water/[bmim][PF6] biphasic system.
Gomes, R, Parola AJ, Laia CAT, Pina F.
2007.
Promoting photochromism on flavylium derived 2-hydroxychalcones in aqueous solutions by addition of CTAB micelles, 2007. Journal of Physical Chemistry B. 111:12059-12065.
AbstractA strategy to obtain photochromism from the network of chemical reactions originated by flavyliurn compounds in solution is described. This strategy is particularly useful for flavyliurn salts bearing amino groups which give rise to a variety of beautiful colors but lack photochemistry in water. The trans-chalcone of 7-(N,Ndiethylamino)-4'-hydroxyflavylium interacts strongly with CTAB micelles defining a yellow dark state. Upon irradiation, the system switches to a pink-red state emerging from the flavylium cation that is formed inside the micelle and ejected to the bulk aqueous phase. The photochemical product reverts back to the transchalcone adduct with the micelle in the dark. The thermodynamics as well as the kinetics of the photochromic system were studied in detail. The best color contrast is obtained at pH = 4.25 with (D = 0.001 and a recovery lifetime of approximately 3 h. This photochromic system works with no need of changing the pH, which constitutes an important improvement over previously described systems dependent on pH jumps.
Roque, A, Lima JC, Parola AJ, Pina F.
2007.
Substitution and solvent effects in the chalcones isomerization barrier of flavylium photochromic systems, 2007. Photochemical & Photobiological Sciences. 6:381-385.
AbstractUseful 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).
Parola, AJ, Lima JC, Pina F, Pina J, de Melo JS, Soriano C, Garcia-Espana E, Aucejo R, Alarcon J.
2007.
Synthesis and photophysical properties of dansyl-based polyamine ligands and their Zn(II) complexes, 2007. Inorganica Chimica Acta. 360:1200-1208.
AbstractThe 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.