Impact of Water on the Cis–Trans Photoisomerization
of Hydroxychalcones
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Abstract
The
photochromism of a 2-hydroxychalcone has been studied in CH<sub>3</sub>CN and H<sub>2</sub>O/CH<sub>3</sub>OH (1/1, v/v), as well
as in analogous deuterated solvents using steady-state (UV–vis
absorption, <sup>1</sup>H and <sup>13</sup>C 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 <sup>1</sup><b>Ct</b>* (LE), which evolves
into a twisted state <sup>1</sup><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 H<sub>2</sub>O/CH<sub>3</sub>OH (1/1, v/v), the <b>P</b>* state becomes
unstable and evolves into a new state attributed to the tautomer <sup>1</sup><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