The development of dye-sensitized solar cells, metalloenzyme photocatalysis or biological labeling heavily relies on the design of metalbased photosensitizes with directional excitations. Directionality is most often predicted characterizing manually excitations via canonical
frontier orbitals. Although widespread, this traditional approach is, at the very least, cumbersome and subject to personal bias, as well as
limited in many cases. Here, we demonstrate how two orbital-free photophysical descriptors allow an easy and straightforward
quantification of the degree of directionality in electron excitations using chemical fragments. As proof of concept we scrutinize the effect
of 22 chemical modifications on the archetype [Ru(bpy)3]
2+ with a new descriptor coined “substituent-induced exciton localization” (SIEL),
together with the concept of “excited-electron delocalization length” (EEDLn). Applied to quantum ensembles of initially excited singlet
and the relaxed triplet metal-to-ligand charge-transfer states, the SIEL descriptor allows quantifying how much and whereto the exciton
is promoted, as well as anticipating the effect of single modifications, e.g. on C-4 atoms of bpy units of [Ru(bpy)3]
2+. The general
applicability of SIEL and EDDLn is further established by rationalizing experimental trends through quantification of the directionality of
the photoexcitation. We thus demonstrate that SIEL and EEDL descriptors can be synergistically employed to design improved
photosensitizers with highly directional and localized electron-transfer transitions.</p
