title: Synthesis and characterization of a symmetric bis(7-hydroxyflavylium) containing a methyl viologen bridge.
abstract: 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.
authors.affiliation: REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal.
authors.forename: Ana M
authors.initials: AM
authors.lastname: Diniz
authors.forename: Carlos
authors.initials: C
authors.lastname: Pinheiro
authors.forename: Vesselin
authors.initials: V
authors.lastname: Petrov
authors.forename: A Jorge
authors.initials: AJ
authors.lastname: Parola
authors.forename: Fernando
authors.initials: F
authors.lastname: Pina
mesh_terms: D002599:Chalcone / Q000737:chemistry*; D004227:Dithionite / Q000737:chemistry*; D005419:Flavonoids / Q000138:chemical synthesis*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D016877:Oxidants / Q000737:chemistry*; D010269:Paraquat / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D005419:Flavonoids; D016877:Oxidants; C561452:bis(7-hydroxyflavyliume); D004227:Dithionite; D002599:Chalcone; D010269:Paraquat