Decomposition of Molecular Charge and Spin Transfer Global Indexes into Atomic Group Contributions

We recently introduced a model for decomposing the global charge transfer (CT) excitation indexes proposed by Le Bahers, Adamo, and Ciofini (Le Bahers, T. J. Chem. Theory Comput. 2011, 7, 2498-2506) into contributions from molecular subdomains (Gatti, C. J. Phys. Chem. A 2022, 126, 6314-6328), together with a software tool, DOCTRINE (atomic group Decomposition Of the Charge TRansfer INdExes), which implements this approach. DOCTRINE has been successfully applied to several excited states (ESs) of a push-pull compound in different solvent environments. In this work, we extend our previous model to spin-polarized systems by introducing, in addition to the global CT excitation indexes, their analogous electron spin transfer (ST) indexes. These can also be decomposed into chemically significant contributions from molecular subdomains. This extension provides a set of related CT and ST descriptors, enabling a visual and quantitative differentiation of the behavior of electronic charge and spin transfers. The updated DOCTRINE_SPIN version of the software now includes computation of ST indexes and their associated descriptors, broadening the applicability of the method to spin-resolved electronic excitations. Our CT and ST decomposition model is applicable to any partitioning of real space, whether fuzzy or disjoint and exhaustive. However, we apply it in terms of chemically relevant molecular subdomains based on the Atoms in Molecules (AIM) Bader's basins, taking advantage of associating intra- and inter-subdomain contributions with rigorously defined quantum objects that retain clear chemical meaning. The model allows for a quantitative evaluation of subdomain contributions to the CT, the ST, and their excitation lengths, and to the charge- and spin-transfer dipole moments. Although these global indexes can be derived either from electron and spin density increments or from their depletions upon excitation, the subdomain contributions obtained from the two distributions generally differ. This distinction helps to determine whether a given property's contribution from a subdomain is dominated by one of the distributions or whether both play a significant role. As an initial application of our spin-polarized model extension, we selected a π-conjugated (acceptor-donor-acceptor) compound (TMTQ), composed of a central 1,6-methano[10]annulene (M10A) and 5-dicyanomethyl-thiophene (DT) peripheries in an exo geometry. TMTQ exhibits a singlet-triplet energy gap of only 4.9 kcal/mol, with the singlet state being more stable than the triplet. This small energy gap arises from the different weights of nearly degenerate mesomeric structures with distinct electron delocalization patterns. The electronic charge (and spin) transfers occurring upon excitation of the singlet and triplet ground states (GS) (S₀ and T₁) to their first five excited states (S₁-S₅ and T₂-T₆) are characterized and compared, highlighting their distinct features, the role of ST on CT when both transfers are possible, and the resulting effects on electron and spin delocalizations.