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

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

Chem Sci. 2020 May 28;11(24):6268-6274. doi: 10.1039/d0sc02691c. eCollection 2020 Jun 28.

Authors

Alexander M Brugh¹, Malcolm D E Forbes¹

Affiliation

¹ Department of Chemistry , Center for Photochemical Sciences , Bowling Green State University , Bowling Green , OH 43403 , USA . Email: forbesm@bgsu.edu.

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.