Electron Spin Relaxation of Hole and Electron Polarons in π‑Conjugated Porphyrin Arrays: Spintronic Implications

Electron spin resonance (ESR) spectroscopic line shape analysis and continuous-wave (CW) progressive microwave power saturation experiments are used to probe the relaxation behavior and the relaxation times of charged excitations (hole and electron polarons) in <i>meso</i>-to-<i>meso</i> ethyne-bridged (porphinato)­zinc­(II) oligomers (<b>PZn</b>_{<b><i>n</i></b>} compounds), which can serve as models for the relevant states generated upon spin injection. The observed ESR line shapes for the <b>PZn</b>_{<b><i>n</i></b>} hole polaron (<b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^<b>+•</b>) and electron polaron (<b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^{<b>–•</b>}) states evolve from Gaussian to more Lorentzian as the oligomer length increases from 1.9 to 7.5 nm, with solution-phase <b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^<b>+•</b> and <b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^{<b>–•</b>} spin–spin (<i>T</i>₂) and spin–lattice (<i>T</i>₁) relaxation times at 298 K ranging, respectively, from 40 to 230 ns and 0.2 to 2.3 μs. Notably, these very long relaxation times are preserved in thick films of these species. Because the magnitudes of spin–spin and spin–lattice relaxation times are vital metrics for spin dephasing in quantum computing or for spin-polarized transport in magnetoresistive structures, these results, coupled with the established wire-like transport behavior across metal–dithiol-<b>PZn</b>_{<b><i>n</i></b>}–metal junctions, present <i>meso</i>-to-<i>meso</i> ethyne-bridged multiporphyrin systems as leading candidates for ambient-temperature organic spintronic applications

Design, Synthesis, and Characterization of Metal–Organic Frameworks for Enhanced Sorption of Chemical Warfare Agent Simulants

Metal–organic frameworks (MOFs) and specifically the UiO family of MOFs have been extensively studied for the adsorption and degradation of chemical warfare agents (CWAs) and their simulants. We present a combined experimental and computational study of the adsorption of dimethyl methylphosphonate (DMMP), a CWA adsorption simulant, in functionalized UiO-67. We have used density functional theory (DFT) to design functionalized MOFs having a range of binding energies for DMMP. We have selected three different functionalized MOFs for experimental synthesis and characterization from a total of eight studied with DFT. These three MOFs were identified as having the weakest, intermediate, and strongest binding energies for DMMP of the set, as predicted by our DFT calculations. We find that the order of predicted binding energies agrees with data from temperature-programmed desorption experiments. Moreover, the values of the binding energies are also in good agreement. This serves as a proof of concept that ab initio calculations can guide experiments in designing MOFs that exhibit a higher affinity for CWAs and their simulants. One surprising outcome of this work is that reactions between DMMP and the three functionalized UiO-67 MOFs were not observed under ultrahigh-vacuum conditions for the exposure of DMMP of up to 9000 L. This lack of reactivity is attributed to the low levels of defects in the materials used

Anomalous Infrared Intensity Behavior of Acetonitrile Diffused into UiO-67

UiO-67 metal–organic frameworks (MOFs) show promise for use in a variety of areas, especially in industrial chemistry, as stable and customizable catalyst materials often driven by catalytically active defects (coordinatively unsaturated metal sites) present within the MOF crystallite. Thermal activation, or postsynthetic thermal treatment, of MOFs is a seldom used method to induce catalytically active defects. To investigate the effect of thermal activation on defect concentration in UiO-67, we performed Fourier transform infrared (FT-IR) spectroscopy studies of adsorbed CD3CN, a versatile infrared active probe molecule. Our results suggest that, under cryogenic, ultrahigh-vacuum conditions, CD3CN must be thermally diffused into UiO-67 to successfully detect binding sites and defects. Below dehydroxylation temperatures, blueshifted ν­(CN) modes of diffused CD3CN indicate multiple avenues of hydrogen bonding within UiO-67, as well as binding to Lewis acid sites, assigned to be coordinately undersaturated Zr4+, consistent with in situ FT-IR of adsorbed CO. FT-IR of CD3CN diffused into UiO-67 activated to 623 K shows that while thermal activation eliminates hydrogen-bonding moieties, it also induces stronger Lewis acid sites, identified by a blueshifted ν­(CN) doublet. Through density functional theory (DFT) calculations, we demonstrate that the ν­(CN) doublet is a result of CD3CN interacting with two distinct nodal defect sites present in dehydroxylated UiO-67. Additionally, the infrared cross sections of CD3CN's ν­(CN), ν­(CD)s and ν­(CD)as modes change, primarily due to hydrogen bonding, when diffused into UiO-67, as confirmed through DFT calculations. The nonlinear IR cross section behavior suggests that the Beer–Lambert law cannot trivially extrapolate the concentration of an analyte diffused into a MOF. These studies reveal the impact of postsynthetic thermal treatment on the concentration and type of Lewis acid defects in UiO-67 and caution the simple use of integrated infrared absorbance as a metric of analyte concentration within a MOF