Dead-Time Free Measurement of Dipole–Dipole Interactions between Electron Spins

TLDR

A four‑pulse DEER experiment is introduced to determine interradical distances at nanoscopic scales. The method uses a four‑pulse DEER sequence that removes dead‑time artifacts and, with a pulse train in the detection period, improves signal‑to‑noise, enabling characterization of broad electron‑electron distance distributions. Tests on two nitroxide biradicals confirm a measurable distance range of 1.5–8 nm, and spectra of an ionic spin probe reveal intra‑ and inter‑cluster interactions, providing cluster‑size information not obtainable with earlier techniques.

Abstract

A four-pulse version of the pulse double electron-electron resonance (DEER) experiment is presented, which is designed for the determination of interradical distances on a nanoscopic length-scale. With the new pulse sequence electron-electron couplings can be studied without dead-time artifacts, so that even broad distributions of electron-electron distances can be characterized. A version of the experiment that uses a pulse train in the detection period exhibits improved signal-to-noise ratio. Tests on two nitroxide biradicals with known length indicate that the accessible range of distances extends from about 1.5 to 8 nm. The four-pulse DEER spectra of an ionic spin probe in an ionomer exhibit features due to probe molecules situated both on the same and on different ion clusters. The former feature provides information on the cluster size and is inaccessible with previous methods.