Spin dynamics and long phase-memory times in structurally distinct non-oxido vanadium(IV) complexes with rigid ligand frameworks relevant to molecular qubits

Abstract

We report two chemically very similar non-oxido V(IV) complexes that feature distorted octahedral and distorted trigonal prismatic coordination geometries, as elucidated by single-crystal X-ray diffraction. Both compounds are thermally stable and show no degradation in the presence of air or water. An in-depth echo-detected field-sweep (EDFS) ESR spectroscopic study in frozen solution at temperatures below 10 K revealed resolved hyperfine interactions with the spin-bearing ⁵¹V nucleus, along with additional couplings to neighboring ¹⁴N and ¹H nuclei detected in ESEEM experiments. Investigation of the spin dynamics, and of the potential influence of the distinct coordination geometries, yielded long spin-lattice relaxation times (T₁) exceeding 100 ms at 5 K, which decreased to about 40 µs at 50 K. These long T₁ times therefore do not limit the spin coherence, and consequently the phase-memory times (T_m), at very low temperatures. The latter ranged from 5 to 8 µs and exhibited negligible temperature dependence within the investigated range. However, the use of a deuterated solvent increased T_m by approximately one order of magnitude to 52 µs at 5 K, though it also resulted in a more pronounced temperature dependence. Coherent spin manipulation was demonstrated by echo-detected transient nutation experiments, revealing well-defined Rabi oscillations with frequencies in the range of 13 MHz at an attenuation level of 9 dB and figures of merit exceeding 10³ in deuterated solvent. Cluster correlation expansion (CCE) simulations reproduce the experimentally observed trends and identify the surrounding nuclear spin bath as the dominant decoherence source under dilution-limit conditions. No significant differences attributable to the distinct coordination geometries or structural variations in the ligand backbone were observed. These findings establish the present vanadium(IV) complexes as competitive candidates for molecular spin qubits and promising targets for future surface-deposition studies