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 T₁ and TM 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@h₇₂Q₈M₈, the derivative with R = OSi(CH₃)₃, by applying pulsed electron paramagnetic resonance (EPR) methods on its deuterated analogues H@d₇₂Q₈M₈ and D@d₇₂Q₈M₈. For the latter species we measure a phase memory time of 60 μs 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 TM values observed for all deuterated species even at liquid helium temperatures are discussed in terms of tunneling reorientation of methyl groups.