Excited state proton transfer reacitons and DFT: old and new challenges

The fascinating world of Excited State Proton Transfer (ESPT) reactions is experiencing a renewed enthusiasm due to the advance of techniques such as the femtosecond stimulated Raman spectroscopy.[1] The theoretical-computational chemistry is challenged by this progress, and it is called for a new effort to interpret and, when possible, to forecast the experimental results. The simulations of excited and ground state PT share difficulties and limits related to the classical or quantum treatment of the proton dynamics, especially when methods rooted in Density Functional Theory (DFT) are considered. Moreover, other problems emerging from the photo-induced nature of the ESPT can be envisaged. These reactions are in fact triggered by the electronic and/or nuclear rearrangements upon excitation, and can be competitive with radiative and/or non radiative decay processes. This complexity can be critical for many ingredients of the modeling, starting from a proper characterization of different potential energy surfaces. This contribution is a flavor of our investigations of different ESPT reactions treated by time-dependent DFT. Our work is focused on two aspects of the ESPT simulation: the use of excited state ab-initio dynamics, and the set up of new kind of analysis capable to individuate promoting forces activated by the electronic excitation.[2,3] These tools exploit the definition of electronic density based indices, ad hoc designed to follow ESPT in a simple and effective manner.[3] Examples of application will be photo acid-base reactions in solvent and the excited state proton shuttle in fluorescent proteins.