Journal of Photochemistry and Photobiology A: Chemistry 269 (2013) 1–8 Contents lists available at SciVerse ScienceDirect Journal of Photochemistry and Photobiology A: Chemistry journal h om epa ge: www.elsevier.com/locate/jphotochem On the photostationary state of the flavylium network of chemical reactions Fernando Pina ∗ , Raquel Gomes, Nuno Basílio, César A.T. Laia REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal a r t i c l e i n f o Article history: Received 20 March 2013 Received in revised form 11 June 2013 Accepted 15 June 2013 Available online 10 July 2013 Keywords: Photochromism Flavylium Photoisomerization a b s t r a c t Calculation of the quantum yields from the photostationary state, in the cases where the thermal back reaction competes efficiently with the photoproducts formation is described. The system can be mathe- matically treated as a single equilibrium involving two opposite reactions, one driven by light of constant intensity and the other due to the thermal back reaction. By analogy with excited state redox potential calculations, the photostationary state can be defined by a constant, which is function of the number of absorbed photons by the cis and trans forms, the respective reaction quantum yields, the light inten- sity, irradiated volume, as well the rates of the thermal forward and backward thermal reactions (in the absence of light). Application of this concept to the flavylium network of chemical reactions is presented. The model compound 4 ′ ,7-dihydroxyflavylium was used to test the model. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The kinetic elucidation methods of photochromic systems, as for example spiropyrans, have been subjected to some attention [1]. The calculation of the quantum yields in the case of a cis–trans photo-isomerization in competition with a thermal back reaction is not an easy task if the thermal and photochemical processes occur in the same range of rates. However, the situation is even more complex in the case of the flavylium network of chemical reactions in aqueous solutions, because the cis–trans isomerization is connected to other reactions involving the photoproduct, Scheme 1, including acid/base reactions [2]. Scheme 1. Network of chemical reactions of 4 ′ ,7-dihydroxyflavylium. ∗ Corresponding author. Tel.: +351 212948355; fax: +351 212948550. E-mail address: fp@fct.unl.pt (F. Pina). 1010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotochem.2013.06.009