Reversible photoacids based on hydroxyarenes have been previously used in light-responsive acid-catalyzed systems. Recently, the Ardo Group used a hydroxyarene photoacid covalently bound to a cation-exchange membrane to demonstrate a visible-light-driven proton pump that can potentially be used for direct sunlight-driven desalination of salt water (JACS 2017 139 11726). Since then, using kinetic models I showed that the most effective sunlight-driven desalination devices require extreme ground-state and the excited-state acidities, with pKa ≈ 14 and pKa* ≈ 0, respectively. In my presentation, I will discuss the synthesis and characterization of a novel donor-π-acceptor photoacid in which heterocyclic amines are used as proton donors. Remarkably, pH titrations coupled with Henderson–Hasselbalch analysis of absorption data revealed that this model photoacid dye has a near-ideal pKa ≈ 13.8. Various electron acceptor and electron donor groups were strategically incorporated into the dye motif to tune the pKa from ~10 to ~16. Interestingly, pKa* values derived from pH titrations coupled with photoluminescence measurements revealed that the observed pKa* was rather insensitive to the nature of the dye. Through a straightforward analysis of fundamental rate equations, for the first time we show clearly that the observed pKa* from photoluminescence titrations does not report on a thermodynamic property but instead is due to kinetic branching between excited-state proton transfer to hydroxides and excited-state relaxation to the ground-state. This was evident from the fact that the Henderson–Hasselbalch fits of photoluminescence data were often highly non-ideal, requiring a sharper-than-theoretical rise in the sigmoidal function. These observations validate our unique hypothesis where for kinetic reasons the extent of photoacidity resulting in proton transfer directly to hydroxide changes as a function of pH. This further clarifies the processes by which excited-state proton transfer can occur and identifies limiting conditions necessary for an effective photoacid in a light-driven proton pump.

Unique photochemistry of new strongly alkaline heterocyclic amine-based donor-π-acceptor photoacids

2019 Fall National Meeting

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