Constructive quantum interference in a bis-copper six-porphyrin nanoring

Sabine Richert,Ilya Kuprov,Jonathan Cremers,Christiane R Timmel,Martin D Peeks,Harry L Anderson

doi:10.1038/ncomms14842

Sabine Richert, Ilya Kuprov + Show 4 more

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https://doi.org/10.1038/ncomms14842

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Abstract

The exchange interaction, J, between two spin centres is a convenient measure of through bond electronic communication. Here, we investigate quantum interference phenomena in a bis-copper six-porphyrin nanoring by electron paramagnetic resonance spectroscopy via measurement of the exchange coupling between the copper centres. Using an analytical expression accounting for both dipolar and exchange coupling to simulate the time traces obtained in a double electron electron resonance experiment, we demonstrate that J can be quantified to high precision even in the presence of significant through-space coupling. We show that the exchange coupling between two spin centres is increased by a factor of 4.5 in the ring structure with two parallel coupling paths as compared to an otherwise identical system with just one coupling path, which is a clear signature of constructive quantum interference.

Highlights

The exchange interaction, J, between two spin centres is a convenient measure of through bond electronic communication
We present solutions to both these problems: We implement the scheme shown in Fig. 1 by testing the through-bond exchange coupling, J, between two paramagnetic centres, which can be measured accurately by electron paramagnetic resonance (EPR), and we lock the molecular wire into a well-defined conformation by using supramolecular assembly on a radial template
The ring represents a molecular analogue of the two-path model in Fig. 1 in which transmission between the two copper spin centres is possible through two identical, parallel pathways

Summary

Introduction

The exchange interaction, J, between two spin centres is a convenient measure of through bond electronic communication. If the system behaves coherently, constructive quantum interference is expected to give a total conductance of GAB1⁄44 G1 (refs 4,8) This scenario has been tested experimentally for charge transport through single molecules. Similar experiments compared the conductance of a carbobenzene macrocycle (dAB 1⁄4 0.8 nm) with a single-path reference molecule to give GAB % 40 G1, in this case the higher conductance of the two-path system is partly a consequence of its greater conformational rigidity[10]. These results highlight the challenges involved in using single-molecule charge transport measurements to probe quantum coherence. It is difficult to synthesize pairs of molecules that have identical

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