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

Alexander M Brugh,Malcolm D E Forbes

doi:10.1039/d0sc02691c

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.

Highlights

Proton-coupled electron transfer (PCET) reactions are important to many naturally occurring chemical processes.[1]
We recently reported a detailed study of a photoinduced PCET reaction involving [Ru(II)(bpy)2(bpz)]2+ with hydroquinone (HQ) (Scheme 1).[7,8]
Our results suggest that with electron donating groups (EDGs), the electron transfer portion of the PCET reaction is moving ahead slightly faster, creating an electron de cient oxygen that leads to a weakening of the H-bond, but slowly so that radical separation leads to time in the mixing region

Summary

Introduction

Proton-coupled electron transfer (PCET) reactions are important to many naturally occurring chemical processes.[1]. Mechanistic details and structure-reactivity studies of the reactive intermediate involved in thermal and excited state PCET reactions remain a research topic of high interest

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Results

Conclusion