Photoacidity

Photoacidity is the increase in acidity of a molecule when it is electronically excited by light, relative to its ground state. After absorption of a photon, electronic distribution changes in a way that often stabilizes the conjugate base, lowering the excited-state pKa (pKa*) and making proton donation favorable in the excited state.

This excited-state proton transfer can occur to solvent molecules or nearby bases on very fast timescales and is a central feature of many photochemical processes. The extent of photoacidity depends on factors such as solvent polarity and proticity, hydrogen-bonding networks, and substituents that modulate the stability of the deprotonated species in the excited state.

Classic examples are certain phenols that exhibit strong photoacidity and photoacid fluorophores such as pyranine (HPTS), which are used as fluorescent pH indicators because their emission responds to proton transfer in the excited state. Photoacidity finds applications in photoredox catalysis, studies of proton-coupled electron transfer, and fluorescence-based pH sensing. It is also leveraged in microfluidic and photocatalytic systems where light-driven proton transfer can influence reaction outcomes or signaling.

Measurement and interpretation typically rely on spectroscopic methods that monitor the excited-state acid-base equilibrium, such as time-resolved fluorescence or absorption techniques, to estimate pKa* values and ESPT rates. The concept complements traditional ground-state acid–base chemistry and helps explain proton transfer phenomena in light-driven chemical and biological processes.