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.

Persistent spectral holes burned in the system octaethylporphin in poly(styrene) exhibit a symmetrical broadening varying in a linear fashion upon application of a static electric field. This effect is due to permanent electric-dipole moments induced in the dye molecules by the electric "matrix field. " The average value of the dipole-moment difference p between the excited and the ground state of the guest molecules, which can be deduced from the broadening, shows a distinct increase from the blue to the red edge of the inhomogeneous absorption band, thus reflecting the varying dye-matrix interaction for centers with different solvent shift. A detailed analysis of this variation in the framework of a microscopic theory, based on a recent publication by Laird and Skinner [J. Chem. Phys. 90, 3274 (1989)], leads to the conclusion that the solvent shift of the absorption lines and also the p variation across the inhomogeneous band is largely dominated by the dispersion interaction. The electrostatic contribution to the line shift is smaller by about 2 orders of magnitude.

Long electron spin coherence lifetimes are essential for applications in quantum information science and electron paramagnetic resonance, for instance, for nanoscale distance measurements in biomolecular systems using double electron−electron resonance. We experimentally investigate the decoherence dynamics under the Hahn echo sequence of the organic radical d 18 -TEMPO in a variably deuterated frozen water:glycerol matrix. The coherence time (phase memory time) T M scales with proton concentration as [ 1 H] −0.65 . For selectively deuterated matrices, decoherence is accelerated in the presence of proton clustering, that is, substantial short-range density in the proton−proton radial distribution functions (<3 Å). Simulations using molecular dynamics and many-body spin quantum dynamics show excellent agreement with experiment and show that geminal proton pairs such as CH 2 and OH 2 groups are major decoherence drivers. This provides a predictive tool for designing molecular systems with long electron spin coherence times.

Abstract. Double electron–electron resonance (DEER) is a pulse electron paramagnetic resonance (EPR) technique that measures distances between paramagnetic centres. It utilizes a four-pulse sequence based on the refocused Hahn spin echo. The echo decays with increasing pulse sequence length 2(τ1+τ2), where τ1 and τ2 are the two time delays. In DEER, the value of τ2 is determined by the longest inter-spin distance that needs to be resolved, and τ1 is adjusted to maximize the echo amplitude and, thus, sensitivity. We show experimentally that, for typical spin centres (nitroxyl, trityl, and Gd(III)) diluted in frozen protonated solvents, the largest refocused echo amplitude for a given τ2 is obtained neither at very short τ1 (which minimizes the pulse sequence length) nor at τ1=τ2 (which maximizes dynamic decoupling for a given total sequence length) but rather at τ1 values smaller than τ2. Large-scale spin dynamics simulations based on the coupled cluster expansion (CCE), including the electron spin and several hundred neighbouring protons, reproduce the experimentally observed behaviour almost quantitatively. They show that electron spin dephasing is driven by solvent protons via the flip-flop coupling among themselves and their hyperfine couplings to the electron spin.

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