The 𝑝𝐾 𝑎 of an acid is determined by measuring the different concentrations of the acid and the conjugate base species in solutions at different pH values. UV-Vis spectroscopy is a useful analytical method for such experiments. For the measurement of 𝑝𝐾 𝑎 ℎ𝑣 , time-resolved spectroscopy, fluorescence titration, and the Förster thermodynamic cycle method are options. The Förster cycle (Scheme 1A) utilizes the difference in the vertical electronic transition energies between the photoacid and the conjugate base (𝛥𝐸 𝑎𝑐𝑖𝑑 and 𝛥𝐸 𝑏𝑎𝑠𝑒 in Equation ), which returns the Δ𝑝𝐾 𝑎 between the S0 and S1 states. 𝛥𝑝𝐾 The energies required in Equation 1 can be obtained from either absorption or fluorescence spectroscopy data. However, using absorption data in the Förster cycle does not consider the geometry relaxation in the excited state. An alternative is the Born-Haber thermodynamic cycle (Scheme 1B), which can be used for calculating both 𝑝𝐾 𝑎 and 𝑝𝐾 𝑎 ℎ𝑣 , provided the Gibbs free energies of all species involved are calculated in the gas phase and in the solvent phase (water). In that regard, the Gibbs free energies of protons can be obtained from experimental values in the literature. In Equation , 𝛥𝐺 ° is the reaction free energy in the deprotonation of a (photo)acid.