Computational methods: The Gaussian16 quantum chemistry package was used for density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations reported in this work. The counter ion tetrafluoroborate was omitted from the calculations, as we verified that its presence did not change the conformers obtained in the geometry optimizations (Fig. ). Several functionals were used (CAM-B3LYP, ωB97X-D, B3LYP and M06-2X), with the basis set 6-311+G(d,p). Dispersion corrections were considered by including the GD3 version of Grimme's dispersion model. Stationary points were characterized as minima through frequency calculations. Implicit solvent effects were modeled by the Solvent Model Based on Density (SMD). For the investigation of the conformers (computed NMR data) the solvent modelled was acetonitrile, given the best solubility of HDMAPAN-(BF4)2 in that solvent. For the structures used in the 𝑝𝐾 𝑎 determination, the solvent modelled was water. The SMD method was chosen based on a comparative computational study reporting the superior (in general) performance of SMD against other implicit solvent models. The Gibbs free energies of proton utilized in the Born-Haber cycle equations were taken from the literature: gas phase, -6.28 kcal mol -1 , and solvation -265.9 kcal mol -1 . The multicenter index (MCI) and the aromatic fluctuation index (FLU) were used to assess the (anti)aromatic character of the photoacid in the S0 and S1 states. Only these two indices were chosen (among several aromaticity indices) because they are the most robust to assess singlet excited state (anti)aromaticity. FLU and MCI indices were obtained with the ESI program. Calculations with (fractional) point charges were performed in vacuum at the CAM-B3LYP/6-31+G(d,p) level and using the Q-Chem package. 64