Excited‐State Proton Transfer in Push‐Pull N‐Methyl Pyridium Reversible Super‐Photoacids Ruled by Intramolecular Hydrogen‐Bond‐Like Interactions

Three push-pull N-methyl pyridium phenols have been thoroughly investigated for their photoacid behavior through stationary and time-resolved spectroscopic techniques. Fluorimetric titrations revealed a clear excited-state proton transfer (ESPT) with negative pKa* values for the two molecules with the phenol attached in ortho relative to the double bond, so as to be regarded as super-photoacids. Conversely, no proton transfer was observed when the hydroxyl group was in the para position. Quantum-mechanical calculations confirmed that intramolecular hydrogen-bond-like interactions, only attainable in the ortho isomers, are strengthened by the charge transfer process occurring upon excitation, resulting in enhanced acidity. Ultrafast transient absorption and fluorescence up-conversion (UC) spectroscopies allowed direct observation of the ESPT process on the picosecond time scale, in agreement with the negative pKa*. In addition, nanosecond laser flash photolysis unraveled the recombination of the conjugated base with the proton once returned to the ground state. The time for this recombination was slowed down by one order of magnitude in the presence of a proton scavenger. These findings demonstrated both the reversible nature of these super-photoacids, thus regulating the pH of the nearby environment through light irradiation, and their ability to give a proton to a second molecule, for potential photocatalytic applications.