Thermodynamics and Kinetics of Protonated Merocyanine Photoacids in Water

01 June 2020, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


Metastable-state photoacids (mPAHs) are chemical species whose photo-activated state is long-lived enough to allow for proton diffusion. Liao’s photoacid (1) 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 1 in water, however, still remains poorly understood. Herein, we provide in-dept insights on the thermodynamics and kinetics of 1 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 (Ka) of the open protonated form (MCH) is followed by isomerization (Kc) of the open deprotonated form (MC) to the closed spiropyran form (SP) – i.e., in the absence of light, the ground state acidity can be expressed as KaGS = Ka(1+Kc). On the other hand, under powerful and continuous light irradiation we were able to assess, for the first time experimentally, the dissociation constant (KaMS) of the protonated metastable state (cis-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 other two compounds bearing a shorter (ethyl, 2) and a longer (butyl, 3) alkyl-1-sulfonate bridge. We found that the pKa remains constant, whereas both pKc and pKaMS 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 1‒3 in water. The superior hydrolytic stability and water solubility of compound 3, together with its relatively high pKaGS (low Kc), allowed us to achieve fully reversible jumps of 2.5 pH units over 18 consecutive cycles (6 hours).


molecular photoswitches


Comments are not moderated before they are posted, but they can be removed by the site moderators if they are found to be in contravention of our Commenting Policy [opens in a new tab] - please read this policy before you post. Comments should be used for scholarly discussion of the content in question. You can find more information about how to use the commenting feature here [opens in a new tab] .
This site is protected by reCAPTCHA and the Google Privacy Policy [opens in a new tab] and Terms of Service [opens in a new tab] apply.