T₁ Anisotropy Elucidates Spin Relaxation Mechanisms in an S = 1 Cr(IV) Optically Addressable Molecular Qubit

Paramagnetic molecules offer unique advantages for quantum information science owing to their spatial compactness, synthetic tunability, room-temperature quantum coherence, and potential for optical state initialization and readout. However, current optically addressable molecular qubits are hampered by rapid spin-lattice relaxation (T₁) even at sub-liquid nitrogen temperatures. Here, we use temperature- and orientation-dependent pulsed electron paramagnetic resonance (EPR) to elucidate the negative sign of the ground state zero-field splitting (ZFS) and assign T₁ anisotropy to specific types of motion in an optically addressable S = 1 Cr(o-tolyl)₄ molecular qubit. The anisotropy displays a distinct sin²(2θ) functional form that is not observed in S = 1/2 Cu(acac)₂ or other Cu(II)/V(IV) microwave addressable molecular qubits. The Cr(o-tolyl)₄ T₁ anisotropy is ascribed to couplings between electron spins and rotational motion in low-energy acoustic or pseudoacoustic phonons. Our findings suggest that rotational degrees of freedom should be suppressed to maximize the coherence temperature of optically addressable qubits.