The key energy scales of Gd-based metallofullerene determined by resonant inelastic x-ray scattering spectroscopy

Yu-Cheng Shao,Yi-Sheng Liu,Shih-Wen Huang,Jinghua Guo,Way-Faung Pong,Shangfeng Yang,Yi-De Chuang,L Andrew Wray,Wang Song

doi:10.1038/s41598-017-08685-5

Abstract

Endohedral metallofullerenes, formed by encaging Gd inside fullerenes like C80, can exhibit enhanced proton relaxitivities compared with other Gd-chelates, making them the promising contrast agents for magnetic resonance imaging (MRI). However, the underlying key energy scales of GdxSc3−xN@C80 (x = 1–3) remain unclear. Here, we carry out resonant inelastic x-ray scattering (RIXS) experiments on GdxSc3−xN@C80 at Gd N4,5-edges to directly study the electronic structure and spin flip excitations of Gd 4f electrons. Compared with reference Gd2O3 and contrast agent Gadodiamide, the features in the RIXS spectra of all metallofullerenes exhibit broader spectral lineshape and noticeable energy shift. Using atomic multiplet calculations, we have estimated the key energy scales such as the inter-site spin exchange field, intra-atomic 4f–4f Coulomb interactions, and spin-orbit coupling. The implications of these parameters to the 4f states of encapsulated Gd atoms are discussed.

Highlights

Using atomic multiplet calculations, we have estimated the key energy scales such as the inter-site spin exchange field, intra-atomic 4f–4f Coulomb interactions, and spin-orbit coupling
Jex in these Gd-based magnetic resonance imaging (MRI) contrast agents are at best calculated theoretically. These energy scales are significant parameters for determining the zero-field splitting (ZFS), an important factor that will influence the electronic relaxation of Gd spin (T1e, 2e) under the magnetic field[13, 14]. Since these key energy scales can affect the energetics of elementary excitations[15], they can be determined by measuring the excitations associated with Gd using inelastically scattered x-rays under Gd resonance conditions
To understand the meaning of distinctive behaviors of peak positions in these RIXS spectra, we examine the dependence of atomic multiplet simulations on different physical parameters

Summary

Introduction

We have estimated the key energy scales such as the inter-site spin exchange field, intra-atomic 4f–4f Coulomb interactions, and spin-orbit coupling. Unlike in Gd-chelates where water molecules (or H+) can come in contact with the central Gd3+ ion, the Gd3+ is well shielded by the carbon cage in metallofullerenes (EMFs) Relaxation mechanisms such as the inner sphere Gd3+-H+ spin interaction and the water exchange between inner and outer spheres are not applicable to EMFs. Instead, much of the discussions about the mechanisms behind their enhanced relaxitivities are on the rotational correlation time τR and the proton-fullerenol interaction (see Fig. 1(a) for the schematic illustration of some relaxation parameters)[11]. Much of the discussions about the mechanisms behind their enhanced relaxitivities are on the rotational correlation time τR and the proton-fullerenol interaction (see Fig. 1(a) for the schematic illustration of some relaxation parameters)[11] Information such as the spin state and valence state of Gd in the fullerene and their interactions with carbon cage have been experimentally obtained[12], important energy scales like spin-orbit coupling (SOC), intra-atomic 4f–4f Coulomb interactions, and inter-atomic spin exchange field www.nature.com/scientificreports/. Since these key energy scales can affect the energetics of elementary excitations[15], they can be determined by measuring the excitations associated with Gd using inelastically scattered x-rays under Gd resonance conditions

Methods

Results

Conclusion