title: Impact of water on the cis-trans photoisomerization of hydroxychalcones.
abstract: The photochromism of a 2-hydroxychalcone has been studied in CH3CN and H2O/CH3OH (1/1, v/v), as well as in analogous deuterated solvents using steady-state (UV-vis absorption, (1)H and (13)C NMR) and time-resolved (ultrafast transient absorption and nanosecond flow flash photolysis) spectroscopies. Whereas the irradiation of trans-chalcone (Ct) under neutral pH conditions leads to the formation of the same final chromene derivative (B) in both media, two distinct photochemical mechanisms are proposed in agreement with thermodynamic and kinetic properties of the chemical reaction network at the ground state. Following light excitation, the first steps are identical in acetonitrile and aqueous solution: the Franck-Condon excited state rapidly populates the trans-chalcone singlet excited state (1)Ct* (LE), which evolves into a twisted state (1)P*. This excited state is directly responsible for the photochemistry in acetonitrile in the nanosecond time scale (16 ns) leading to the formation of cis-chalcone (Cc) through a simple isomerization process. The resulting cis-chalcone evolves into the chromene B through a tautomerization process in the ground state (τ = 10 ms). Unlike in acetonitrile, in H2O/CH3OH (1/1, v/v), the P* state becomes unstable and evolves into a new state attributed to the tautomer (1)Q*. This state directly evolves into B in one photochemical step through a consecutive ultrafast tautomerization process followed by electrocyclization. This last case represents a new hypothesis in the photochromism of 2-hydroxychalcone derivatives.
authors.affiliation: REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal. yoann.leydet@gmail.com
authors.forename: Yoann
authors.initials: Y
authors.lastname: Leydet
authors.forename: Pinar
authors.initials: P
authors.lastname: Batat
authors.forename: Gediminas
authors.initials: G
authors.lastname: Jonusauskas
authors.forename: Sergey
authors.initials: S
authors.lastname: Denisov
authors.forename: João Carlos
authors.initials: JC
authors.lastname: Lima
authors.forename: A Jorge
authors.initials: AJ
authors.lastname: Parola
authors.forename: Nathan D
authors.initials: ND
authors.lastname: McClenaghan
authors.forename: Fernando
authors.initials: F
authors.lastname: Pina
mesh_terms: D000097:Acetonitriles / Q000737:chemistry; D047188:Chalcones / Q000737:chemistry*; D000432:Methanol / Q000737:chemistry; D015394:Molecular Structure; D055668:Photochemical Processes; D013237:Stereoisomerism; D014867:Water / Q000737:chemistry*
chemical_list: D000097:Acetonitriles; D047188:Chalcones; D014867:Water; C013031:2'-hydroxychalcone; D000432:Methanol; C032159:acetonitrile