Electron Localization Function in Excited States: The Case of the Ultrafast Proton Transfer of the Salicylidene Methylamine

The physical characterization of the chemical bond in the ground state has been a central theme to theoretical chemistry. Among many techniques, quantum chemical topology (QCT) has emerged as a robust technique to understand the features of the chemical bond and electron organization within molecules. One consolidate tool within QCT is the topological analysis of the electron localization function (ELF). Most research on ELF and chemical bond has focused either on singlet ground states or the first excited triplet. However, most photochemical reactions and photophysical processes occur in excited states with the same spin-symmetry as the ground state. In this work, we develop a proposal on how to compute the ELF in excited states of any symmetry within linear-response time-dependent density functional theory. Then, we study the evolution of the chemical bonds in the ground- and excited-state intramolecular proton transfer (ESIPT) of a prototypal Schiff base (the salicylidene methylamine). We found that th...