Impact of Electron-Electron Spin Interaction on Electron Spin Relaxation of Nitroxide Diradicals and Tetraradical in Glassy Solvents Between 10 and 300 K

Authors

Hideo Sato, University of Denver
Velavan Kathirvelu, University of Denver
Gaëlle Spagnol, University of Nebraska-Lincoln
Suchada Rajca, University of Nebraska-LincolnFollow
Andrzej Rajca, University of Nebraska - LincolnFollow
Sandra S. Eaton, University of Denver
Gareth R. Eaton, University of Denver

Date of this Version

2008

Citation

J Phys Chem B. 2008 March 13; 112(10): 2818–2828.

Comments

© 2008 American Chemical Society

Abstract

To determine the impact of electron-electron spin-spin interactions on electron spin relaxation rates, 1/T₁ and 1/T_m were measured for nitroxide monoradical, diradical, and tetraradical derivatives of 1,3-alternate calix[4]arenes, for two pegylated high-spin nitroxide diradicals, and for an azine-linked nitroxide diradical. The synthesis and characterization by SQUID (superconducting quantum interference device) magnetometry of one of the high-spin diradicals, in which nitroxides are conformationally constrained to be coplanar with the m-phenylene unit, is reported. The interspin distances ranged from about 5-9 Å, and the magnitude of the exchange interaction ranged from >150 to >0.1 K. 1/T₁ and 1/T_m were measured by long-pulse saturation recovery, three-pulse inversion recovery, and two-pulse echo decay at X-band (9.5 GHz) and Q-band (35 GHz). For a diradical with interspin distance about 9 Å, relaxation rates were only slightly faster than for a monoradical with analogous structure. For interspin distances of about 5-6 Å, relaxation rates in glassy solvents up to 300 K increased in the order monoradical < diradical < tetraradical. Modulation of electron-electron interaction enhanced relaxation via the direct, Raman, and local mode processes. The largest differences in 1/T₁ were observed below 10 K, where the direct process dominates. For the three diradicals with comparable magnitude of dipolar interaction, 1/T_m and 1/T₁ were faster for the molecules with more flexible structures. Relaxation rates were faster in the less rigid low-polarity sucrose octaacetate glass than in the more rigid 4:1 toluene/chloroform or in hydrogen-bonded glycerol glasses, which highlights the impact of motion on relaxation.