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Photochromic Properties of 3‐Methyl‐Substituted Flavylium Salts
Author(s) -
Roque Ana,
Lodeiro Carlos,
Pina Fernando,
Maestri Mauro,
Ballardini Roberto,
Balzani Vincenzo
Publication year - 2002
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/1099-0690(200208)2002:16<2699::aid-ejoc2699>3.0.co;2-i
Subject(s) - chemistry , steric effects , chalcone , moiety , photochromism , substituent , protonation , tetrafluoroborate , benzene , ion , photochemistry , medicinal chemistry , stereochemistry , organic chemistry , ionic liquid , catalysis
The photochromic properties of some 3‐methyl‐substituted synthetic flavylium compounds have been investigated. The main structural feature in these compounds is the existence of a steric effect, caused by the methyl substituent, that forces the phenyl ring to move out of coplanarity with the benzopyrylium moiety. The X‐ray structure of 3‐methylflavylium tetrafluoroborate shows a torsion angle of 40.4° between the benzopyrylium and the benzene ring. In 3‐methyl‐substituted synthetic flavylium compounds, the steric effect impedes the formation of trans ‐chalcone. In the case of the 4′‐hydroxy‐3‐methylflavylium ion, for example, the trans ‐chalcone form could only be obtained (up to a maximum yield of 60%) by protonation of the trans ‐chalcone anion, obtained in turn by irradiation of anionic cis ‐chalcone in basic solution. The multistate/multifunctional properties of the 4′‐hydroxy‐3‐methylflavylium compound are also discussed in detail. For the 4′,7‐dihydroxy‐3‐methylflavylium ion, a detailed investigation of the kinetic and thermodynamic properties of the various forms was performed by means of pH jumps and photochemical experiments. (© Wiley‐VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)