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 ϕ<sub><i>S</i></sub>, 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 ϕ<sub><i>S</i></sub> 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 ϕ<sub><i>S</i></sub> 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.