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Characterization of covalency of intermolecular interactions in the van der Waals distance limit remains challenging because the interactions between molecules are weak, dynamic, and not measurable. Herein, we approach this issue in a series of supramolecular mixed-valence (MV) donor(D)-bridge(B)-acceptor(A) systems consisting of two bridged Mo

A series of three Mo2 dimers bridged by a meta-phenylene group has been studied in terms of electronic coupling (EC) and electron transfer (ET) in comparison with the para isomers. Optical analyses on the mixed-valence complexes indicate that by replacing a para-phenylene bridge with a meta one, the EC between the two Mo2 centers is dramatically weakened; consequently, the ET rates (ket ) are lowered by two to three orders of magnitude. In the para series, the EC parameters (Hab ) and ET rates (ket ) are greatly affected by O/S atomic alternation of the bridging ligand. However, for the meta analogues, similar EC and ET parameters are obtained, that is, Hab =300-400 cm-1 and ket ≈109 s-1 . These results suggest that through-σ-bond and/or through-space coupling channels become operative as the π conjugation is disabled. DFT calculations reveal that destructive quantum interference features seen for the meta series arise from the cancellation of two π-conjugated coupling pathways.

Thermal electron transfer through hydrogen bonds remains largely unexplored. Here we report the study of electron transfer through amide-amide hydrogen bonded interfaces in mixed-valence complexes with covalently bonded Mo2 units as the electron donor and acceptor. The rate constants for electron transfer through the dual hydrogen bonds across a distance of 12.5 Å are on the order of ∼ 1010 s−1, as determined by optical analysis based on Marcus–Hush theory and simulation of ν(NH) vibrational band broadening, with the electron transfer efficiencies comparable to that of π conjugated bridges. This work demonstrates that electron transfer across a hydrogen bond may proceed via the known proton-coupled pathway, as well as an overlooked proton-uncoupled pathway that does not involve proton transfer. A mechanistic switch between the two pathways can be achieved by manipulation of the strengths of electronic coupling and hydrogen bonding. The knowledge of the non-proton coupled pathway has shed light on charge and energy transport in biological systems., Thermal electron transfer across hydrogen bond remains largely unexplored. Here the authors demonstrate that electron self-exchange through hydrogen bonds is highly efficient and can proceed either via the known proton-coupled pathway or an overlooked proton-uncoupled pathway

SummaryOur knowledge on the chemistry across molecular and supramolecular levels is insufficient. Here, we show that inclusion of a guest molecule through quadrupolar interactions leads to a chemical transformation of 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. The D-A electron transfer is optically studied under the Marcus-Hush theoretic framework. It is found that in aromatic solvents, the electronic coupling matrix elements (Hab) and electron transfer rate constants (ket) are dramatically reduced. DFT calculations and electrostatic potential map analysis indicate that in the solution, an aromatic molecule is encapsulated in the cleft of the D-B-A array; quadrupole-quadrupole interaction between the guest molecule and the C4H2S group evokes 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, thus, sharing the general principles developed in molecular chemistry. This study illustrates the role of site-specific interactions of the aromatic residues in biological systems in governing the charge transfer processes.

A series of three Mo