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Synthesis and Characterization of a Symmetric Bis(7‐hydroxyflavylium) Containing a Methyl Viologen Bridge
Author(s) -
Diniz Ana M.,
Pinheiro Carlos,
Petrov Vesselin,
Parola A. Jorge,
Pina Fernando
Publication year - 2011
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201003726
Subject(s) - isomerization , chemistry , chalcone , mole fraction , kinetics , photochemistry , medicinal chemistry , organic chemistry , catalysis , physics , quantum mechanics
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 p K ′ 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.