title: Relaxation phenomena of dithiolene type radical ligands for quantum computation
Creators: name: McGuire, Jake
Publisher: name: University of Glasgow
Year: 2020
Subjects: subject: Qubit, dithiolene, radical, electron, spin, EPR, resonance, relaxation, magnetism, quantum computing.

description: The intrinsic redox activity of the dithiolene ligand is used here as the novel spin host in a prototype molecular electron spin qubit where the traditional roles of the metal and ligand components in coordination complexes are inverted. A series of paramagnetic bis(dithiolene) complexes with group 10 metals– nickel, palladium, platinum – provides a backdrop to investigate the spin dynamics of the organic ligand radical using pulsed EPR spectroscopy. The temperature dependence of the phase memory time (TM) is shown to be dependent on the identity of the diamagnetic metal ion, with the short times recorded for platinum a consequence of a diminishing spin-lattice (T1) relaxation time driven by spin-orbit coupling. The utility of the radical ligand spin center is confirmed when it delivers one of the longest phase memory times ever recorded for a molecular two-qubit prototype. A bis(dithiolene)gold complex is presented as a model for an organic molecular electron spin qubit attached to a metallic surface that acts as a conduit to electrically address the qubit. A two-membered electron transfer series is developed of the formula [AuIII(adt)2]1–/0, where adt is a redox-active dithiolene ligand that is sequentially oxidized as the series is traversed while the central metal ion remains AuIII and steadfastly square planar. One-electron oxidation of diamagnetic [AuIII(adt)2]1– produces an S = 1/2 charge-neutral complex, [AuIII(adt)(adt•)] which is spectroscopically and theoretically characterized with a near negligible Au contribution to the ground state. A phase memory time (TM) of 21 μs is recorded in 4:1 CS2/CCl4 at 10 K, which is the longest ever reported for a coordination complex possessing a third-row transition metal ion. With increasing temperature, TM is dramatically decreased becoming unmeasurable above 80 K as a consequence of the diminishing spin-lattice (T1) relaxation time fuelled by spin-orbit coupling. These relaxation times are 1–2 orders of magnitude shorter for the solid dilution of in isoelectonic [Ni(adt)2] because this material is a molecular semiconductor. Although the conducting properties of this material provide efficient pathways to dissipate the energy through the lattice, it can also be used to electrically address the paramagnetic dopant by tapping into the mild reduction potential to switch magnetism “on” and “off” in the gold complex without compromising the integrity of its structure. These results serve to highlight the need to consider the composition of not just the qubit, but all components of these spintronic assemblies. Addition of Lewis acidic rare earth ions to the bis(dithiooxalato)nickel complex ion generated new charge-neutral heterometallic species where the rare earth M(III) ions (M = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) occupy the O,O′ pocket of both ligands. Together with stabilising hydrotris(pyrazolyl)borate co-ligands on the rare earth ion, chemical reduction of the bridging bis(dithiooxalato)nickel unit led to the first molecular and electronic structure characterisation of the elusive dithiooxalato radical ligand, (dto)3–• for the YIII and GdIII analogues. The central metal was varied down group 10 with lutetium to form a series with which to further investigate the environment of the radical spin.