The new photoacid HDMAPAN-(BF4)2 exhibits a unique feature in which the S0 → S1 transition is not located in the moiety containing the acidic proton, and yet, the compound shows a large enhancement of the acidity in its S1 excited state (photoacidity). Combining experimental and computational techniques, we were able to clarify the conformational structures and electronic features of HDMAPAN-(BF4)2, and such characterization was essential to further understand its photoacidity. Importantly, visible light triggers a large Δ𝑝𝐾 𝑎 of 13.8 units in HDMAPAN-(BF4)2, a feature that could be utilized to catalyze organic synthesis under visible light activation, especially considering that the photoacid character is also present in the triplet statea state that has longer lifetime compared to singlet states.