Efficient electron transfer across hydrogen bond interfaces by proton-coupled and -uncoupled pathways

Tao Cheng,Nathan J Patmore,Dong Xue Shen,Hong Li Zhang,Huo Wen Chen,Miao Meng,Yi Yang Wu,Shan Feng Zou,Yi Qin,Lijiu Cao,Chun Y Liu,Suman Mallick

doi:10.1038/s41467-019-09392-7

Tao Cheng, Nathan J Patmore + Show 10 more

Open Access

https://doi.org/10.1038/s41467-019-09392-7

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Journal: Nature Communications	Publication Date: Apr 4, 2019
Citations: 44	License type: open-access

Affiliation: Jinan University, University of Huddersfield

Abstract

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.

Highlights

H bonds able to transport electrons and (ii) whether the ET process occurs with the help of the proton
It is found that the rate constants and ET distances (Rab) of 1c+ and 2c+ fit well the linear relationship between ln(kET) and Rab for MV {[Mo2]–bridge–[Mo2]}+complexes with the same [Mo2] donor and acceptor but varying π conjugated bridges, in accordance with the decay law (Eq (6)) in the frame of the McConnel superexchange mechanism 46, kET 1⁄4 k0 expðÀβRabÞ; ð6Þ
For the three D–B–A series with different bridges, DFT calculations on the simplified models, generated by replacing the Ar groups on the formamidinate ligand with a hydrogen atoms, show that the HOMO and HOMO-1 arise from the phase-out and phase-in combinations of the δ orbitals of the two Mo2 units, respectively (Supplementary Figure 10)

Summary

Introduction

H bonds able to transport electrons and (ii) whether the ET process occurs with the help of the proton. Mixed-valence (MV) D–B–A molecules, which have identical D and A sites differing only in formal oxidation states, are valuable experimental models for study of electron selfexchange reaction with Marcus–Hush theory[11,12], which has been successfully used to evaluate intramolecular ET through covalently bonded bridges[13,14]. Recent efforts have produced various examples of hydrogen bonded MV complexes, in which self-complementary HB interactions are used to bridge the electron donor and acceptor[15,16,17], with the aim to evaluate the D–A EC optically upon analysis of the intervalence charge transfer (IVCT) absorbance This optical feature has only been observed in few examples of hydrogen bonded MV compounds, and lack of a test-bed series of compounds has hindered the kinetic study of thermal ET in electron self-exchange reactions with zero driving force (−ΔG° = 0)[5,16,17]

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