Towards Luminescent Vanadium(II) Complexes with Slow Magnetic Relaxation and Quantum Coherence

Molecular entities with doublet or triplet ground states find increasing interest as potential molecular quantum bits (qubits). Complexes with higher multiplicity might even function as qudits and serve to encode further quantum bits. Vanadium(II) ions in octahedral ligand fields with quartet ground states and small zero-field splittings qualify as qubits with optical read out thanks to potentially luminescent spin-flip states. We identified two V2+ complexes [V(ddpd)(2)](2+) with the strong field ligand N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine (ddpd) in two isomeric forms (cis-fac and mer) as suitable candidates. The energy gaps between the two lowest Kramers doublets amount to 0.2 and 0.5 cm(-1) allowing pulsed EPR experiments at conventional Q-band frequencies (35 GHz). Both isomers possess spin-lattice relaxation times T-1 of around 300 mu s and a phase memory time T-M of around 1 mu s at 5 K. Furthermore, the mer isomer displays slow magnetic relaxation in an applied field of 400 mT. While the vanadium(III) complexes [V(ddpd)(2)](3+) are emissive in the near-IR-II region, the [V(ddpd)(2)](2+) complexes are non-luminescent due to metal-to-ligand charge transfer admixture to the spin-flip states.

Automated summary

Molecular entities with doublet or triplet ground states are being increasingly studied as potential qubits. Vanadium(II) ions in octahedral ligand fields with quartet ground states are considered as qubits with potential luminescent spin-flip states. Two V2+ complexes, [V(ddpd)(2)](2+), with the strong field ligand N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine (ddpd) in two isomeric forms (cis-fac and mer), were identified as suitable candidates. The energy gaps between the two lowest Kramers doublets allow pulsed EPR experiments at conventional Q-band frequencies. Both isomers have relatively long spin-lattice relaxation times and a phase memory time at low temperatures. Furthermore, the mer isomer shows slow magnetic relaxation in an applied field. The [V(ddpd)(2)](2+) complexes are non-luminescent due to charge transfer admixture to the spin-flip states.