Spectroscopic\nSignatures of Phonon Character in Molecular\nElectron Spin Relaxation

Spin–lattice\nrelaxation constitutes a key challenge\nfor\nthe development of quantum technologies, as it destroys superpositions\nin molecular quantum bits (qubits) and magnetic memory in single molecule\nmagnets (SMMs). Gaining mechanistic insight into the spin relaxation\nprocess has proven challenging owing to a lack of spectroscopic observables\nand contradictions among theoretical models. Here, we use pulse electron\nparamagnetic resonance (EPR) to profile changes in spin relaxation\nrates (<i>T</i><sub>1</sub>) as a function of both temperature\nand magnetic field orientation, forming a two-dimensional data matrix.\nFor randomly oriented powder samples, spin relaxation anisotropy changes\ndramatically with temperature, delineating multiple regimes of relaxation\nprocesses for each Cu(II) molecule studied. We show that traditional <i>T</i><sub>1</sub> fitting approaches cannot reliably extract\nthis information. Single-crystal <i>T</i><sub>1</sub> anisotropy\nexperiments reveal a surprising change in spin relaxation symmetry\nbetween these two regimes. We interpret this switch through the concept\nof a spin relaxation tensor, enabling discrimination between delocalized\nlattice phonons and localized molecular vibrations in the two relaxation\nregimes. Variable-temperature <i>T</i><sub>1</sub> anisotropy\nthus provides a unique spectroscopic method to interrogate the character\nof nuclear motions causing spin relaxation and the loss of quantum\ninformation.