Issue 24, 2020

Anomalous chemically induced electron spin polarization in proton-coupled electron transfer reactions: insight into radical pair dynamics

Abstract

Time-resolved electron paramagnetic resonance (TREPR) spectroscopy has been used to study the proton coupled electron transfer (PCET) reaction between a ruthenium complex (Ru(bpz)(bpy)2) and several substituted hydroquinones (HQ). After excitation at 355 nm, the HQ moiety forms a strong hydrogen bond to the exposed N atoms in the bpz heterocycle. At some point afterwards, a PCET reaction takes place in which an electron from the O atom of the hydrogen bond transfers to the metal center, and the proton forming the hydrogen bond remains on the bpz ligand N atom. The result is a semiquinone radical (HQ˙), whose TREPR spectrum is strongly polarized by the triplet mechanism (TM) of chemically induced dynamic electron spin polarization (CIDEP). Closer examination of the CIDEP pattern reveals, in some cases, a small amount of radical pair mechanism (RPM) polarization. We hypothesize that when the HQ moiety has electron donating groups (EDGs) substituted on the ring, S–T RPM polarization is observed in HQ˙. These anomalous intensities are accounted for by spectral simulation using polarization from S–T mixing. The generation of S–T RPM is attributed to slow radical separation after PCET due to stabilization of the positive charge on the ring by EDGs. Results from a temperature dependence support the hypothesis.

Graphical abstract: Anomalous chemically induced electron spin polarization in proton-coupled electron transfer reactions: insight into radical pair dynamics

Article information

Article type
Edge Article
Submitted
11 Maijs 2020
Accepted
27 Maijs 2020
First published
28 Maijs 2020
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY license

Chem. Sci., 2020,11, 6268-6274

Anomalous chemically induced electron spin polarization in proton-coupled electron transfer reactions: insight into radical pair dynamics

A. M. Brugh and M. D. E. Forbes, Chem. Sci., 2020, 11, 6268 DOI: 10.1039/D0SC02691C

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

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