Mediation of Electron Transfer by Quadrupolar Interactions: The Constitutional, Electronic, and Energetic Complementarities in Supramolecular Chemistry

Abstract

SummaryStudies of intermolecular interactions enhance our knowledge of chemistry across molecular and supramolecular levels. Here, we show that host-guest quadrupolar interaction has a profound influence on the molecular system. With covalently bonded dimolybdenum complex units as the electron donor (D) and acceptor (A) and a thienylene group (C4H2S) as the bridge (B), the mixed-valence D-B-A complexes are shaped with clefts in the middle of the molecule. Interestingly, in aromatic solvents, the D-A electronic coupling constants (Hab) and electron transfer rates (ket) are dramatically reduced. Theoretical computations indicate that an aromatic molecule is encapsulated in the cleft of the D-B-A array; quadrupole-quadrupole interaction between the guest molecule and the C4H2S bridge evokes a charge redistribution, which increases the HOMO-LUMO energy gap, intervening in the through-bond electron transfer. These results demonstrate that a supramolecular system is unified underlying the characteristics of the assembled molecules through constitutional, electronic, and energetic complementarities.

A single solvating benzene molecule decouples the mixed-valence complex through intermolecular orbital interactions

“…However, for the Class III species near the II–III borderline, which has the lowest intervalence transition energy, both increasing and reducing the extent of EC would cause a blue-shift of the IVCT band ( Tan et al., 2017 ). Therefore, the observed IVCT band shifts toward high energy, together with a great decrease of the IVCT absorption, indicating the strong decoupling effect of benzene ( Mallick et al., 2019 ), in agreement with the electrochemical results. For 1 + , the IVCT band maximum moves from 3,766 to 4,953 cm −1 as the solvent changes from DCM to benzene, showing the largest IVCT blue-shift, i.e., Δ E IT = 1,187 cm −1 ( Table 1 ).…”

“…In benzene, however, the Δ E 1/2 values ( Figure 2 and Table 1 ) tremendously decrease to 82, 190, and 380 mV, respectively, suggesting a significant electronic decoupling effect. These results, resembling the observations for the thienylene bridged analogues in aromatic solvents ( Mallick et al., 2019 ), do not conform to the dielectric continuum model that predicts enhanced EC in less polar solvents. Assuming that the Mo 2 ⋅⋅⋅Mo 2 separations in DCM and benzene remain unchanged, the large decreases of Δ E 1/2 should be attributed to substantial lowering of the resonant contribution to the overall EC ( Crutchley, 1994 ).…”

“…The neutral complexes were converted into the corresponding MV complexes ( 1 + , 2 + , and 3 + ) by chemical oxidation with one equivalent of ferrocenium hexafluorophosphate ( Mallick et al., 2019 ; Tan et al., 2017 ). These cationic complexes were characterized by electron paramagnetic resonance (EPR) spectra in situ .…”

“…These cationic complexes were characterized by electron paramagnetic resonance (EPR) spectra in situ . For the MV complex series, generally termed as [Mo 2 -Mo 2 ] + , the EPR spectra show a pronounced isotropic signal with g ≈ 1.95 ( Figure S11 ), smaller than that of an organic free radical ( g ≈ 2.0023), indicating that the odd electron resides in the δ orbital of the Mo 2 centers ( Mallick et al., 2019 ; Tan et al., 2017 ). Therefore, for the MV series, intramolecular electronic coupling occurs between the two Mo 2 units, which is mediated by the bridging ligand, as evidenced by the varied Δ E 1/2 values ( Xiao et al., 2013 ).…”

“…In Kubiak’s group, it was observed that the non-covalent host–guest interaction between the calix[6]arene and the auxiliary ligands (4-phenyl pyridine) on the Ru 3 -Ru 3 mixed valence (MV) ion caused the electronic coupling to decrease ( Lear and Kubiak, 2007 ). Recently, we demonstrated that an Mo 2 -Mo 2 MV complex is substantially decoupled by quadruple–quadruple interactions between an aromatic solvent molecule and the thienylene bridge of the D-B-A molecule ( Mallick et al., 2019 ). Moreover, ultrafast reorientation of a local solvent molecule coupled to intramolecular ET was observed lately ( Biasin et al., 2021 ).…”