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, ¹H and ¹³C NMR) and time-resolved (ultrafast transient absorption and nanosecond flow flash photolysis) spectroscopies. Whereas the irradiation of <i>trans</i>-chalcone (<b>Ct</b>) under neutral pH conditions leads to the formation of the same final chromene derivative (<b>B</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 <i>trans</i>-chalcone singlet excited state ¹<b>Ct</b>* (LE), which evolves into a twisted state ¹<b>P</b>*. This excited state is directly responsible for the photochemistry in acetonitrile in the nanosecond time scale (16 ns) leading to the formation of <i>cis</i>-chalcone (<b>Cc</b>) through a simple isomerization process. The resulting <i>cis</i>-chalcone evolves into the chromene <b>B</b> through a tautomerization process in the ground state (τ = 10 ms). Unlike in acetonitrile, in H₂O/CH₃OH (1/1, v/v), the <b>P</b>* state becomes unstable and evolves into a new state attributed to the tautomer ¹<b>Q</b>*. This state directly evolves into <b>B</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