Developing Effective Descriptors for Photoacidity and Photobasicity

While photoacids and photobases are known to experience an enhanced thermodynamic driving force for proton transfer upon electronic excitation, the underlying physical origin for this phenomenon is not fully understood. Here, we describe our efforts to develop descriptors for photoacidity and photobasicity based on intramolecular charge transfer, excited-state anti-aromaticity, and an energy decomposition analysis (EDA) of their hydrogen bonding complexes with H2O. We demonstrate that intermolecular charge-transfer descriptors are generally less effective than those involving excited-state anti-aromaticity or hydrogen-bonding EDA. Overall, our work provides new insights into the properties of known photoacids and photobases, while also supporting efforts to identify novel photoacids and photobases with an increased thermodynamic driving force for excited-state proton transfer.