Thermodynamics and kinetics of protonated merocyanine photoacids in water

Abstract

Metastable-state photoacids (mPAHs) are chemical species whose photo-activated state is long-lived enough to allow for proton diffusion. Liao's photoacid (<b>1</b>) represents the archetype of mPAHs, and is being widely used on account of its unique capability to change the acidity of aqueous solutions reversibly. The behavior of <b>1</b> in water, however, still remains poorly understood. Herein, we provide in-depth insights on the thermodynamics and kinetics of <b>1</b> in water through a series of comparative ¹H NMR and UV-Vis studies and relative modelling. Under dark conditions, we quantified a three-component equilibrium system where the dissociation (<i>K</i> _a) of the open protonated form (MCH) is followed by isomerization (<i>K</i> _c) of the open deprotonated form (MC) to the closed spiropyran form (SP) - <i>i.e.</i>, in the absence of light, the ground state acidity can be expressed as <i>K</i> ^GS _a = <i>K</i> _a(1 + <i>K</i> _c). On the other hand, under powerful and continuous light irradiation we were able to assess, for the first time experimentally, the dissociation constant (<i>K</i> ^MS _a) of the protonated metastable state (<i>cis</i>-MCH). In addition, we found that thermal ring-opening of SP is always rate-determining regardless of pH, whereas hydrolysis is reminiscent of what is found for Schiff bases. The proposed methodology is general, and it was applied to two other compounds bearing a shorter (ethyl, <b>2</b>) and a longer (butyl, <b>3</b>) alkyl-1-sulfonate bridge. We found that the p<i>K</i> _a remains constant, whereas both p<i>K</i> _c and p<i>K</i> ^MS _a linearly increase with the length of the alkyl bridge. Importantly, all results are consistent with a four-component model cycle, which describes perfectly the full dynamics of proton release/uptake of <b>1-3</b> in water. The superior hydrolytic stability and water solubility of compound <b>3</b>, together with its relatively high p<i>K</i> ^GS _a (low <i>K</i> _c), allowed us to achieve fully reversible jumps of 2.5 pH units over 18 consecutive cycles (6 hours).