Sb(III) corrolates show single/double Soret bands depending on peripheral substituents. To clarify the mechanism, all-electron realistic TDDFT calculations and wave function analyses are performed on a series of Sb(III) corrolates bearing different meso-substituents. The results indicate that the split mainly stems from the large transition energy gap (ΔE) between S0 → S3 (HOMO → LUMO + 1) and S0 → S4 (HOMO-1 → LUMO + 1) excitations, enlarged with the macrocyclic electron density increasing. The large oscillator strength difference (Δf) plays a secondary role in the detectable split at a close ΔE. Sb atom contributes significantly to the inner orbitals (HOMO-2/HOMO-7/HOMO-8). Decomposition of the total transition dipole moment (μT) demonstrates that transitions related to these orbitals induce non-negligible negative contributions to the μT of S0 → S4 excitation, thereby reducing the oscillator strength. The less electropositive Sb atoms trigger the more obvious offsetting effects. In conclusion, electron-donating groups (EDGs) lower the ΔE and Δf of Sb(III) corrolates, leading to splitting Soret bands.
Li et al. (Fri,) studied this question.