Impact of Water on the Cis鈥揟rans Photoisomerization of Hydroxychalcones

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• 作者：Yoann Leydet ; Pinar Batat ; Gediminas Jonusauskas ; Sergey Denisov ; Jo茫o Carlos Lima ; A. Jorge Parola ; Nathan D. McClenaghan ; Fernando Pina
• 刊名：The Journal of Physical Chemistry A
• 出版年：2013
• 出版时间：May 23, 2013
• 年：2013
• 卷：117
• 期：20
• 页码：4167-4173
• 全文大小：426K
• 年卷期：v.117,no.20(May 23, 2013)
• ISSN：1520-5215

文摘

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鈥搗is absorption, ¹H and ¹³C 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鈥揅ondon excited state rapidly populates the trans-chalcone singlet excited state ¹Ct* (LE), which evolves into a twisted state ¹P*. 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 (蟿 = 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 ¹Q*. 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.