A symmetric bis(flavylium) constituted by two 7-hydroxyflavylium moieties linked by a methylviologen bridge was synthesized. The thermodynamic and kinetics of the network of chemical reactions involving bis(flavylium) and the model compound 7-hydroxy-4'-methylflavylium was completely characterized by means of direct and reverse pH jumps (stopped flow) and flash photolysis. Both compounds follow the usual pH-dependent network of chemical reactions of flavylium derivatives. The equilibrium species of the model compound are the flavylium cation (acidic species) and the trans-chalcone (basic species) with an apparent pK(a)' = 2.85. In the case of the bis(flavylium) it was possible to characterize by (1)H NMR spectroscopy three species with different degrees of isomerization: all flavylium, flavylium-trans-chalcone, and all trans-chalcone. Representation of the time-dependent mole fraction distribution of these three forms after a pH jump from equilibrated solutions of all-flavylium cation (lower pH values) to higher pH values, shows that formation of trans-chalcone is not completely stochastic (two independent isomerizations), the isomerization of one flavylium showing a small influence on the isomerization of the other. The radical of the methyl viologen bridge is formed upon reduction of the bis(trans-chalcone) with dithionite. The system is reversible after addition of an oxidant in spite of the occurrence of some decomposition.
ISI Document Delivery No.: 783PGTimes Cited: 2
Cited Reference Count: 28
Diniz, Ana M. Pinheiro, Carlos Petrov, Vesselin Jorge Parola, A. Pina, Fernando
Pina, Fernando/C-8161-2011; Parola, A. Jorge/F-4048-2010; Diniz, Ana /C-6705-2013; Caparica, cqfb_staff/H-2611-2013; REQUIMTE, AL/H-9106-2013; Chaves, Pedro/K-1288-2013
Parola, A. Jorge/0000-0002-1333-9076;
FCT-MCTES, Portugal; FCT-MCTES [PTDC/QUI/67786/2006, PTDC/QUI-QUI/104129/2008, SFRH/BD/48226/2008, SFRH/BPD/18214/2004]
We thank J. C. Lima and C. A. T. Laia for fruitful discussions and LabRMN at FCT-UNL and Rede Nacional de RMN (supported with funds from FCT-MCTES, Portugal) for access to the facilities. FCT-MCTES is also acknowledged for financial support through projects PTDC/QUI/67786/2006 and PTDC/QUI-QUI/104129/2008 and grants SFRH/BD/48226/2008 (A.D.) and SFRH/BPD/18214/2004 (V.P.).
Wiley-blackwell
Malden