Toward a Quantitative Assessment of Electronic Transitions’ Charge-Transfer Character

We hereby report studies devoted to a topological descriptor of photoinduced electronic charge density variation. Our novel index, symbolized as ϕS, consists in the detachment and attachment densities overlap, where the detachment density physically depicts the electron density removed from the ground state of a molecule during the transition while the attachment density consists in the rearranged density in the excited state. Our method provides a simple and efficient way to quantitatively evaluate how easy the charge-separation is made upon the chromophore’s light absorption. Furthermore, this model can be applied for instance to address a comment on new push–pull dyes charge-transfer ability in order to assess their potentiality as candidates for light absorption-based devices. Moreover, the ϕS assessment allows us to perform some methodological diagnostic tests concerning the use of long-range corrected exchange–correlation functional in a time-dependent density functional theory (TDDFT) framework. This paper relates the ϕS descriptor’s mathematical foundations from various perspectives (detachment/attachment densities or natural transition orbitals), together with its application to several types of chromophores. Connections and divergences with a formerly proposed index are finally evidenced.