Seeking for Optimal Excited States in Photoinduced Electron-Transfer Processes─The Case Study of Brooker's Merocyanine.

Material design enters an era in which control of electrons in atoms, molecules, and materials is an essential property to be predicted and thoroughly understood in view of discovering new compounds with properties optimized toward specific optical/optoelectronic applications. π-electronic delocalization and charge separation/recombination enter notably into the set of features that are highly desirable to tailor. Diverse domains are particularly relying on photoinduced electron-transfer (PET), including fields of paramount importance such as energy production through light-harvesting, efficient chemoreceptive sensors, or organic field-effect transistors. In view of completing the arsenal of strategies in this area, we selected Brooker's merocyanine─a typical [D-π-A] compound─as the case study and examined from time-dependent density functional theory the opportunity offered by selected excited states to reach a suited manipulation of the charge transfer (CT) extent. In addition to the consideration of diagnostic tools able to spot the charge amount (i.e., magnitude of electron fraction) transferred upon excitation (), the spatial extent associated with such an electronic transition or CT length (), as well as the corresponding variation in dipole moment between the ground and the excited states (), further analysis of the excitation process was undertaken. The advantage of going beyond the above-mentioned molecular indicators─which can be considered as PET global indices─was explored on the basis of a partitioning of the electron density. Relevant insight was gained on the relation these global indices have with the evolution of (local) features characterizing either chemical bond or electron delocalization upon vertical excitations.