title: Hydroxypyridinechromene and pyridinechalcone: two coupled photochromic systems.
abstract: Substitution of the phenyl group in 2-hydroxychalcones by a 4-pyridine unit dramatically changes the network of chemical reactions of this compound: trans-chalcone-type (Ct), cis-chalcone-type (Cc), and a hemiketal (hydroxy-4-pyridinechromene) (B) and their protonated forms are formed, but the presence of a flavylium-type cation could not be detected even at very acidic pH values. Moreover, whereas in 2-phenyl-2-benzopyrylium compounds B and Cc are generally elusive species whose kinetic processes in aqueous solutions occur on the sub-second timescale, in the present compound these species equilibrate on a timescale four orders of magnitude lower. Complete characterization of the equilibrium and kinetics of the reaction network could thus be achieved by (1)H NMR spectroscopy and UV/Vis spectrophotometry. The network of chemical reactions exhibits cis-trans photoisomerization, as well as photochromism between the hemiketal and the chalcone-type species. The irradiation of Ct in MeOH/H(2)O (1:1) at 365 nm produces B almost quantitatively through two consecutive photochemical reactions: Ct-->Cc photoisomerization followed by Cc-->B photo ring closure with a global quantum yield of 0.02. On the other hand, irradiation of B at 254 nm leads to a photostationary state composed by 80 % Ct and 20 % B, with a quantum yield of 0.21.
authors.affiliation: Departamento de Química, REQUIMTE, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Portugal.
authors.forename: Yoann
authors.initials: Y
authors.lastname: Leydet
authors.forename: A Jorge
authors.initials: AJ
authors.lastname: Parola
authors.forename: Fernando
authors.initials: F
authors.lastname: Pina
mesh_terms: D001578:Benzopyrans / Q000138:chemical synthesis* / Q000737:chemistry; D047188:Chalcones / Q000138:chemical synthesis* / Q000737:chemistry; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D055668:Photochemical Processes; D011725:Pyridines / Q000138:chemical synthesis* / Q000737:chemistry; D013237:Stereoisomerism; D013816:Thermodynamics
chemical_list: D001578:Benzopyrans; D047188:Chalcones; D011725:Pyridines