Electron spin relaxation of copper(II) complexes in glassy solution between 10 and 120K

Abstract

The temperature dependence, between 10 and 120 K, of electron spin-lattice relaxation at X-band was analyzed for a series of eight pyrrolate-imine complexes and for ten other copper(II) complexes with varying ligands and geometry including copper-containing prion octarepeat domain and S100 type proteins. The geometry of the CuN 4 coordination sphere for pyrrolateimine complexes with R = H, methyl, n-butyl, diphenylmethyl, benzyl, 2-adamantyl, 1-adamantyl, and tert-butyl has been shown to range from planar to pseudo-tetrahedral. The fit to the recovery curves was better for a distribution of values of T 1 than for a single time constant. Distributions of relaxation times may be characteristic of Cu(II) in glassy solution. Long-pulse saturation recovery and inversion recovery measurements were performed. The temperature dependence of spinlattice relaxation rates was analyzed in terms of contributions from the direct process, the Raman process, and local modes. It was necessary to include more than one process to fit the experimental data. There was a small contribution from the direct process at low temperature. The Raman process was the dominant contribution to relaxation between about 20 and 60 K. Debye temperatures were between 80 and 120 K. For samples with similar Debye temperatures the coefficient of the Raman process tended to increase as g z increased, as expected if modulation of spin-orbit coupling is a major factor in relaxation rates. Above about 60 K local modes with energies in the range of 260-360 K (180-250 cm −1 ) dominated the relaxation. For molecules with similar geometry, relaxation rates were faster for more flexible molecules than for more rigid ones. Relaxation rates for the copper protein samples were similar to rates for small molecules with comparable coordination spheres. At each temperature studied the range of relaxation rates was less than an order of magnitude. The spread was smaller between 20 and 60 K where the Raman process dominates, than at higher temperatures where local modes dominate the relaxation.Spin echo dephasing time constants, T m , were calculated from two-pulse spin echo decays. Near 10 K T m was dominated by proton spins in the surroundings. As temperature was increased motion and spin-lattice relaxation made increasing contributions to T m . Near 100 K spin-lattice relaxation dominated T m .

Room temperature quantum coherence in a potential molecular qubit

“…The fact that such a wide range of essential temperature independence of T M is found for 1Cu 0.001% is owing to the fact that T 1 is extremely long at low temperatures. T 1 decreases slowly with increasing temperature and only reaches values comparable to T M (10 ms) above 100 K. We attribute the long T 1 to a relatively rigid lattice, as well as the square planar coordination geometry, which is predicted to give rise to longer T 1 times than a tetrahedral geometry 39 . This then suggests that a design criterion for high-temperature molecular qubits is the engineering of a long T 1 .…”

Electron spin relaxation rates for semiquinones between 25 and 295K in glass-forming solvents

“…Early fundamental work on electron spin lattice relaxation was based on ions in a crystalline lattice, as summarized in references [4–6]. Subsequent studies have found that the characteristic temperature dependence predicted for these solid state mechanisms also is observed in glassy solvents [7–9] for triarylmethyl [10, 11], nitroxyl [12–14], and galvinoxyl [12] radicals, and for Cu(II) [15] and V(IV) complexes [16]. …”

Impact of Chlorine Substitution on Spin–Lattice Relaxation of Triarylmethyl and 1,4-Benzosemiquinone Radicals in Glass-Forming Solvents Between 25 and 295 K

“…Early fundamental work on electron spin lattice relaxation mechanisms focused on ions in a crystalline lattice, as summarized in references [5–7]. Subsequent studies have found that the characteristic temperature dependence predicted for these solid state mechanisms, including the direct, Raman, and local-mode processes, also is observed in glasses [8–10] for triarylmethyl [11, 12], nitroxyl [13–15], galvinoxyl [13] and semiquinone [16] radicals, and for Cu(II) [17] and V(IV) complexes [18]. Above the glass transition temperature, relaxation may be enhanced by tumbling-dependent modulation of g- and A-anisotropy [19–23] and/or by spin rotation [24].…”