Table Relative energies of twisted and folded conformers of HDMAPAN and DMAPAN in the S0 and S1 states. (TD-)CAM-B3LYP-D3/6-311+G(d,p) level, (SMD, water). Considering that the HDMAPAN has the twisted conformer lowest in energy in S0, and that the conjugate base DMAPAN has the twisted conformer as the lowest in energy in S1, we computed the 𝑝𝐾 𝑎 ℎ𝑣 of HDMAPAN considering only the twisted conformers. Following the same Born-Haber thermodynamic-cycle procedure as in the determination of the 𝑝𝐾 𝑎 values, and using water as the solvent, we found the 𝑝𝐾 𝑎 ℎ𝑣 to be -12.3. If only the twisted conformers are considered in the determination of the S0 state 𝑝𝐾 𝑎 , we obtain 𝑝𝐾 𝑎 = 1.5. Therefore, there is a jump of 13.8 units of 𝑝𝐾 𝑎 between the S0 and S1 states, indicating strong photoacidity. We could also estimate the 𝑝𝐾 𝑎 ℎ𝑣 through the Förster cycle, using Equation . The calculated energies of the vertical excitations of tw-HDMAPAN and tw-DMAPAN for the transition S0 → S1 are 2.75 and 2.10 eV, respectively. Using these energies in Equation , we found the Δ𝑝𝐾 𝑎 = -11.0, and therefore the 𝑝𝐾 𝑎 ℎ𝑣 = -9.5, which also is indicative of a strong photoacidic character.