Journal of Photochemistry and Photobiology A: Chemistry 220 (2011) 4–10 Contents lists available at ScienceDirect Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem Photochromism of naphthoflavylium. On the role of 4-OH hemiketal in flavylium network Raquel Gavara, Vesselin Petrov, Virginia López, Fernando Pina ∗ REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal article info Article history: Received 28 October 2010 Received in revised form 28 February 2011 Accepted 5 March 2011 Available online 22 March 2011 Keywords: Photochromism Hemiketals Multistate systems Naphthoflavylium Chalcones abstract Flavylium compounds suffer in aqueous solution the nucleophilic addition of water at moderately acidic pH values (hydration reaction). The hydration is possible in two positions, namely at position 2, form- ing hemiketal B2, and at position 4, forming hemiketal B4. B2 can subsequently evolve to give the cis- and trans-chalcone species. At the present work the network of chemical reactions involving the naphthoflavylium compound in aqueous solution was studied by means pH jumps, stopped flow, contin- uous irradiation and flash photolysis. The equilibrium and rate constants of the system were calculated through a mathematical model. The species B4 has a kinetic effect similar to the one observed for the quinoidal base (for flavylium dyes bearing acidic groups), i.e. B4 is a kinetic product retarding the rate of equilibration. Flash photolysis experiments in comparison with reverse pH jumps results show that the appearance of the flavylium ion is faster in the photochemical-induced process than in the thermal one, suggesting an additional photochemical pathway (besides photoisomerization) after the excitation of the trans-chalcone. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The network of chemical reactions involving anthocyanins and related compounds has been shown to exhibit versatile applica- tions [1–3]. Anthocyanins are the ubiquitous colorants used by Nature to obtain most of the red and blue colors in flowers and fruits [4]. Flavylium compounds are synthetic analogues of anthocyanins and present in aqueous solution the same general network of chem- ical reactions (Scheme 1) [5]. Flavylium ion (AH + ) is the most stable species in acidic solutions but when the pH is increased, the sys- tem evolves to a network of interconnected species. An important difference with anthocyanins, is that the trans-chalcone (Ct) is the most stable species at moderately acidic and neutral pH values. This form has the interesting property of photoisomerizing to the cis-chalcone (Cc) that at the appropriate pH value can then evolve to AH + or its quinoidal base (A), the most colorful species in the network. This photoreaction is reversible and the system reverts back to Ct in the dark. Therefore, both pH and light can be used for producing AH + and/or A from Ct in a reversible way. Due to these interesting properties flavylium compounds have been claimed as efficient photochromic systems [3] as well as models for optical memories and switches [6,7] the main factor ruling the final appli- cation being the magnitude of the cis-trans thermal barrier which depends strongly on the substituents. ∗ Corresponding author. Tel.: +351 212948355; fax: +351 212948550. E-mail address: fjp@dq.fct.unl.pt (F. Pina). The kinetics of the global process connecting AH + and Ct can be studied by means of pH jumps and flash photolysis experi- ments. When solutions of AH + (pH < 1) are submitted to a pH jump to higher values, (direct pH jumps) the quinoidal base is the first species to appear, because proton transfer to give A from AH + is by far the fastest process (at least five orders of magnitude) taking place in the network. One peculiarity of the system is the fact that at moderately acidic solutions A is a kinetic product that delays the formation of the final products (essentially Ct). In other words, A is not reactive (unless in basic solutions) towards Ct formation because the hemiketal B2 results exclusively from the hydration of AH + . This means that at higher final pH values of the direct pH jump (pH > pK a ) more A and less AH + are formed and by consequence the rate to reach the final equilibrium becomes slower. On the other hand, direct pH jumps to lower pH’s (pH < pK a ) results in more AH + (at the limit no reaction takes place), lower concentration of Cc to give Ct and the rate also decreases. The final result is a compro- mise between these two contradictory effects, a bell shape curve, Scheme 1 [8]. Regarding to the flash photolysis of Ct, three distinct kinetic pro- cesses are detected: (i) the faster is the formation of Cc from Ct that occurs during the flash, [9] (ii) the second is bi-exponential and corresponds to the formation of AH + /A (depending on pH). In some cases it is not possible to separate these two pro- cesses if one is much slower than the other because the former becomes the rate determining step, (iii) finally the photoproducts AH + /A revert back to Ct with a rate that follows the bell shape curve. 1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochem.2011.03.002