Spectroscopic Signatures of Phonon Character in Molecular Electron Spin Relaxation

Spin-lattice relaxation constitutes a key challenge for the development of quantum technologies, as it destroys superpositions in molecular quantum bits (qubits) and magnetic memory in single molecule magnets (SMMs). Gaining mechanistic insight into the spin relaxation process has proven challenging owing to a lack of spectroscopic observables relative to the many degrees of freedom involved. Here, we use pulse electron paramagnetic resonance (EPR) to profile systematic changes in the spin relaxation anisotropy as a function of temperature for two Cu(II) coordination compounds. For randomly-oriented powder samples, large anisotropy changes arise between 20 and 50 K that delineate multiple regimes of relaxation for each molecule studied. Local mode fitting of the average T1 value fails to consistently extract the anisotropy regime crossover. Single-crystal T1 anisotropy experiments reveal a surprising difference between the symmetry of the spin relaxation tensor in these two regimes. In the high-temperature regime, spin relaxation is fastest and slowest along the principal axes of the molecular g-tensor, which obeys the symmetry of the molecular point group. In the low-temperature regime, spin relaxation is fastest and slowest along arbitrary directions of the g-tensor, instead responding to the crystal packing. We interpret this switch as arising from a change in the localization of the phonons driving spin relaxation at different temperatures, consistent with delocalized lattice phonons in the low-temperature regime and localized molecular vibrations in the high-temperature regime. Variable-temperature T1 anisotropy thus provides a unique method of interrogating the character of nuclear motions causing the spin relaxation process.