Tesis para optar al grado de Magíster en Ciencias, Mención Física / The electron localization function (ELF) is a scalar field that accounts of the excess of electronic kinetic energy due to Pauli repulsion between electrons with the same spin. With this function, it is possible to divide real space in regions (basins) where electronic localization is high and Pauli repulsion is low (up-down electron pairs). From a phenomenological point of view, bifurcation points of the localization domains (points that belong to certain basins of a molecule) can be used to describe the rupture and formation of chemical bonds. Moreover, topological analysis of ELF allows us performing a statistical analysis of the electronic population of basins in a molecule. In this work, by using density functional theory with an hybrid exchange-correlation functional, we describe the electron localization along the intramolecular proton transfer in the Salicidene Methilamine molecule (SMA). First we do it in the ground state, in order to acquire physical insight of the process. Later, by means of time-dependent density functional theory (TD-DFT) in the linear response regime, we perform an equivalent analysis in the first excited state, for which we propose a way to compute ELF in excited states using TD-DFT. We show how the electronic population and other properties of interest of the basins associated with the atoms and bonds involved in the proton transfer change during the reaction. Finally, we choose this system because, after photoexcitation and proton transfer process, SMA suffers a large Stokes shift followed by a "closed" photocycle that ends with the molecule in its original ground state. This makes molecules like SMA good prospects for molecular photoswitches. The main contribution of this thesis is that this is the first time that the ELF developed and successfully used to explain chemical bonding in excited states.