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

Abstract

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 spinflip states. We identified two V 2 + complexes [V(ddpd) 2 ] 2 + with the strong field ligand N,N'-dimethyl-N,N'-dipyridine-2-ylpyridine-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 À 1 allowing pulsed EPR experiments at conventional Q-band frequencies (35 GHz). Both isomers possess spin-lattice relaxation times T 1 of around 300 μs and a phase memory time T M of around 1 μs 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.

Electronic Structure and Excited-State Dynamics of the NIR-II Emissive Molybdenum(III) Analogue to the Molecular Ruby

“…While the meridional configuration of I and II was preserved in the synthesis of 1 and 2 , [3] 3+ was exclusively isolated with a cis-facial coordination of the ligands regardless of whether the synthesis was performed at room temperature (route A) or at 82 °C (routes B and C; Scheme ). This contrasts with a previous report on [V­(ddpd) 2 ] 2+ , where the cisfac isomer was obtained as the kinetic and the mer isomer as the thermodynamic product at 22 and 82 °C, respectively . Similarly, [Co­(ddpd) 2 ] 2+ could be isolated as cisfac and mer isomers by rapid and slow crystallization, respectively. , DFT calculations on [3] 3+ suggest only a slight stabilization of the mer isomer (ca.…”

“…The calculations yielded D = −6.6 cm −1 and E = 1.65 cm −1 , which reasonably agrees with the magnitude of the experimental ZFS but gives the wrong sign for D. The same behavior has been observed for the vanadium(II) analogue of cisfac- [3] 3+ , where the calculated ZFS parameters showed good agreement with the experimental data for selected active spaces, while for others, the sign of D was inverted. 81 In summary, SQUID magnetometry and EPR spectroscopy yielded a consistent picture of the electronic ground state of cisfac- [3] 3+ with a total electron spin S of 3 / 2 , ZFS parameters of D = 6.7(1) cm −1 and E = 0.8(5) cm −1 , g xx,yy,zz = [1.61(3), 2.12(5), 1.90(3)], and an exchange coupling constant of J = −0.7 cm −1 (Table 2). Literature reports on the spin-Hamiltonian parameters of monomeric molybdenum(III) complexes are scarce, with many studies focusing on polynuclear compounds or clusters.…”

“…There is, however, a notable shift of the two states of the 2 E parentage to higher energy in the two facial isomers (Figure S50). In previous reports, e.g., on the ddpd complexes of chromium(III) 40 and vanadium(II), 81 the positions of the metal-centered 4 A 2 → 4 T 2 absorption bands that correspond to the ligand-field splitting were identified using magnetic circular dichroism (MCD), supplemented by CASSCF calculations. So, in an attempt to better resolve the energetically close-lying electronic transitions in cisfac- [3] 3+ , we recorded its MCD response at 1.8 K and 5 T (Figure S22).…”

“…This contrasts with a previous report on [V(ddpd) 2 ] 2+ , where the cisfac isomer was obtained as the kinetic and the mer isomer as the thermodynamic product at 22 and 82 °C, respectively. 81 Similarly, [Co(ddpd) 2 ] 2+ could be isolated as cisfac and mer isomers by rapid and slow crystallization, respectively. 82,83 DFT calculations on [3] 3+ suggest only a slight stabilization of the mer isomer (ca.…”

Photodynamics of the Molecular Ruby [Cr(ddpd)2]3+

“…Furthermore, the emission wavelengths could be shifted far into the near-infrared II region to 1067 nm by using anionic dpc ligands (dpc = 3,6-di-tert-butyl-1,8-di(pyridine-2-yl)carbazolato), although [Cr(dpc) 2 ] + exhibited phosphorescence only at 77 K, while emission was quenched at room temperature [ 22 ]. Using the isoelectronic vanadium(II) in place of chromium(III) does not produce a strong enough ligand field, as [V(ddpd) 2 ] 2+ is not luminescent [ 23 ]. Vanadium(III) complexes of ddpd, in contrast, are emissive [ 24 , 25 ]; the different electron configuration of the vanadium(III) center, however, gives rise to a completely different phosphorescence mechanism [ 26 ].…”