Remote Electrostatic Repulsion Trigged by Excited State Antiaromaticity Relief as Origin of Photoacidity in an Organic Dye

Photoacids are molecules which become (more) acidic under photoirradiation, and they find valuable applications in organic synthesis and biological chemistry. We herein report a new photoacid, HDMAPAN-(BF4)2, which functions by a different photoacidity mechanism than existing ones. HDMAPAN-(BF4)2, consisting of one dibenzotropylium and one anilinium moiety connected together by a biaryl C-C bond, shows an exceptionally low computed Delta_pKa(hv) (-12.3), which is within the range of superphotoacids. Surprisingly, despite the electronic S0 to S1 transition being located in the dibenzotropylium unit, the photoexcitation affects the acidity of the proton on the anilinium unit. Our computational investigations using TD-DFT reveal that the photoacid is functioning by a new mechanism, in which the excited state antiaromatic character of the dibenzotropylium unit in the S1 state unleashes a local reorganization of the charge distribution, alleviating the destabilizing antiaromatic character. Upon excitation, the positive charge within the dibenzotropylium system is localized on the carbons closest to the anilinium ring, creating an increased electrostatic repulsion against the acidic proton. Placing spacers between the dibenzotropylium and the anilinium moieties gradually reduced the computed Delta_pKa, supporting the electrostatic repulsion between the two ring systems as the main source to the strong photoacidity in HDMAPAN-(BF4)2. Finally, using DFT calculations with point charges, we show that a fractional positive charge indeed destabilizes the anilinium system when it is in close proximity.