Quantitative Structure-Based Prediction of Electron Spin Decoherence in Organic Radicals

Abstract

The decoherence, or dephasing, of electron spins in paramagnetic molecules limits sensitivity and resolution in electron paramagnetic resonance spectroscopy, and it represents a challenge for utilizing paramagnetic molecules as qubit units in quantum information devices. For organic radicals in dilute frozen aqueous solution at cryogenic temperatures, electron spin decoherence is driven by neighboring nuclear spins. Here, we show that this nuclear-spin-driven decoherence can be quantitatively predicted from the molecular structure and solvation geometry of the radicals. We use a fully deterministic quantum model of the electron spin and up to 2000 neighboring protons with a static spin Hamiltonian that includes nucleus−nucleus couplings. We present experiments and simulations of two nitroxide radicals and one trityl radical, which have decoherence time scales of 4−5 μs below 60 K. We show that nuclei within 12 Å of the electron spin contribute to decoherence, with the strongest impact from protons 4−8 Å away.

Exploring the Organometallic Route to Molecular Spin Qubits: The [CpTi(cot)] Case

“…Indeed, the Ti−H distances of ca. 3 Å in 1 fall well inside the spin diffusion barrier involving 1 H nuclei [21] . This barrier was theoretically predicted around 4 Å and experimentally found in the 4–6 Å range for other molecular S= 1/2 systems [22] .…”

“…3 in 1 fall well inside the spin diffusion barrier involving 1 H nuclei. [21] This barrier was theoretically predicted around 4 and experimentally found in the 4-6 range for other molecular S = 1/2 systems. [22] Their contribution to decoherence is therefore limited, although the rotation of the rings is not fully hindered.…”

Probing Vibrational Symmetry Effects and Nuclear Spin Economy Principles in Molecular Spin Qubits

“…We proved experimentally that a coherence time enhancement of porphyrazine-based complexes might be achieved by replacing peripheral hydrogen atoms of [Cu(Pc)] with thiadiazole units of [Cu(TTDPz)], partially highlighting that hydrogen atoms placed around the 6 Å spin diffusion barrier affect the coherence time of the central ion. 38 , 40 On the other hand, the temperature dependence of T 1 was rationalized by ab initio calculations, allowing us to gain microscopic insights into spin–lattice relaxation mechanisms. In the limit of the assumptions mentioned above, the model adopted to fit the temperature dependence of T 1 data 69 revealed that the major difference between the two compounds lies in the efficiency of the direct mechanism dominating the low-temperature regime.…”

“…Following what has been reported in studies conducted on a V IV O series of metal complexes and organic radicals, the critical radius may be reasonably expected to be in the 4–8 Å length scale for molecular spin qubits. 38 , 40 Benzylic hydrogens of the Pc ligand, which account for the shortest Cu–H distance (5.89 Å), should then be considered noninnocent ( Figure 1 ).…”

The decay of the refocused Hahn echo in double electron–electron resonance (DEER) experiments

“…However, this concentration reduction leads to a loss in absolute signal intensity and may significantly prolong the experiment runtime, and therefore, there is an optimal concentration for the best SNR (Jeschke and Polyhach, 2007). Another mechanism that strongly contributes to dephasing is nuclear spin diffusion, which is driven by magnetic nuclei that are coupled to the electron spin and among themselves (Brown, 1979;Canarie et al, 2020;Huber et al, 2001;Lenz et al, 2017;Milov et al, 1972;Mims, 1972;Salikhov and Tsvetkov, 1979;Zecevic et al, 1998). The dephasing by this mechanism is enhanced in particular by nuclei with a large gyromagnetic ratio, such as protons.…”