Characterization
of the Thermal and Photoinduced Reactions
of Photochromic Spiropyrans in Aqueous Solution
- Publication date
- Publisher
Abstract
Six water-soluble spiropyran derivatives
have been characterized
with respect to the thermal and photoinduced reactions over a broad
pH-interval. A comprehensive kinetic model was formulated including
the spiro- and the merocyanine isomers, the respective protonated
forms, and the hydrolysis products. The experimental studies on the
hydrolysis reaction mechanism were supplemented by calculations using
quantum mechanical (QM) models employing density functional theory.
The results show that (1) the substitution pattern dramatically influences
the p<i>K</i><sub>a</sub>-values of the protonated forms
as well as the rates of the thermal isomerization reactions, (2) water
is the nucleophile in the hydrolysis reaction around neutral pH, (3)
the phenolate oxygen of the merocyanine form plays a key role in the
hydrolysis reaction. Hence, the nonprotonated merocyanine isomer is
susceptible to hydrolysis, whereas the corresponding protonated form
is stable toward hydrolytic degradation