Intermolecular proton shuttling in excited state proton transfer reactions: insights from theory

The mechanism of base to base intermolecular proton shuttling occurring in the excited state proton transfer reaction between 7-hydroxy-4-(trifluoromethyl) coumarin (CouOH) and concentrated 1-methylimidazole base (1-Meld) in toluene solution is disclosed here by means of a computational approach based on Density Functional Theory (DFT) and Time Dependent DFT (TD-DFT). These methods allow us to characterize both the ground and excited state potential energy surfaces along the proton shuttling coordinate, and to assess the nature of the emitting species in the presence of an excess of 1-Meld. As a result, the tautomerism of CouOH is found to be photo-activated and, from a mechanistic point of view, the calculations clearly show that the overall driving force of the entire shuttling is the coumarin photoacidity, which is responsible for both the first proton transfer event and the strengthening of the following chain mechanism of base to base proton hopping.