Electron Spin Relaxation Mechanisms Of Atomic Hydrogen Trapped In Silsesquioxane Cages: The Role Of Isotope Substitution

Encapsulated atomic hydrogen in silsesquioxane cages is a promising candidate for applications in emerging technologies like spin-based quantum computing, magnetic field sensing, and atomic clock devices. Previous studies on different polyhedral octasilsesquioxanes (POSS) of the type Si8O12R8 have shown that key parameters for quantum computing like electron spin relaxation times T-1 and T-M depend strongly on the type of peripheral organic substituents. Herein we examine for the first time the effect of deuterium isotopic substitution on the spin relaxation properties of H@h(72)Q(8)M(8), the derivative with R = OSi(CH3)(3), by applying pulsed electron paramagnetic resonance (EPR) methods on its deuterated analogues H@d(72)Q(8)M(8) and D@d(72)Q(3)M(8). For the latter species we measure a phase memory time of 60 mu s at 190 K, the largest obtained so far for this family of molecular spins. We show that substitution of peripheral hydrogen atoms with deuterium reveals high-temperature relaxation mechanisms that were previously hidden by proton nuclear spin diffusion. Unusually short T-M values observed for all deuterated species even at liquid helium temperatures are discussed in terms of tunneling reorientation of methyl groups.