Our computational prediction of the 𝑝𝐾 𝑎 in the S0 state was verified experimentally. Thus, we measured the UV-Vis absorption spectra of aqueous solutions of DMAPAN-BF4 with different pH values, ranging from 1.57 to 8.46 (Figure ). The same was done for aqueous solutions of HDMAPAN-(BF4)2 within the pH range from 0.15 to 8.46 (Fig. , ESI), confirming that these structures are interconvertible and dependent of the pH of the medium. The series of spectra show three isosbestic points at wavelengths of 291 nm, 324 nm and 470 nm. The plot of absorbance at 313 nm against the pH could be fitted to a sigmoidal curve (Boltzmann) in which the point of inflection ( χ 0) corresponds to the 𝑝𝐾 𝑎 value (Fig. ). We calculated the 𝑝𝐾 𝑎 The small Stokes shift indicates that there are merely small changes in conformation in the excited state of HDMAPAN-(BF4)2. Indeed, the two methyl groups in positions 3 and 5 of the anilinium moiety hinder the interconversion between the tw-HDMAPAN and fd-DMAPAN in the excited state. For that reason, we considered the twisted conformer of both acid and conjugate base in the excited state. We optimized the S1 state structures of both acid and conjugate base at TD-CAM-B3LYP-D3/6-311+G(d,p) level, and investigated the relative energies of twisted and folded conformers for the acid and conjugate base in the S0 and S1 states in the solvent phase (water, Table ). The relative energies for the S0 state agree with the earlier conclusions about the conformers fd-DMAPAN and tw-HDMAPAN being the most favorable structures in that state, with a strong preference for the twisted form in the protonated system (25 kcal mol -1 ) and nearly isoenergetic between twisted and folded DMAPAN conformers. On the other hand, computational results reveal that in the S1 state, both the acid and the conjugate base have the twisted conformer as the most stable, by respectively 11.4 and 25.9 kcal mol -1 .