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  1. Effects of aromaticity in cations and their functional groups on the temperature dependence of low-frequency spectrum

    We investigate the temperature dependence of low-frequency spectra in the frequency range of 0.3–200 cm -1 for ionic liquids (ILs) whose cations possess two systematically different cyclic groups, using femtosecond Raman-induced Kerr effect spectroscopy. The target ILs are bis(trifluoromethylsulfonyl)amide [NTf 2]– salts of 1-cyclohexylmethyl-1-methylpyrrolidinium [CHxmMPyrr] +, 1-cyclohexylmethyl-3-methylimidazolium [CHxmMIm] +, N-cyclohexylmethylpyridinium [CHxmPy]+, 1-benzyl-1-methylpyrrolidinium [BzMPyrr] +, 1-benzyl-3-methylimidazolium [BzMIm] +, and N-benzylpyridinium [BzPy] + cations. The aim of this study is to better understand the effects of aromaticity in the cations’ constituent groups on the temperature-dependent low-frequency spectral features of the ILs. The low-frequency spectra of these ILs are temperature dependent, but themore » temperature-dependent spectrum of [CHxmMPyrr][NTf 2] is different from that of other ILs. While [CHxmMPyrr][NTf 2] shows spectral changes with temperature in the low-frequency region below 50 cm -1, the other ILs also show spectral changes in the high-frequency region above 80 cm -1 (above 50 cm -1 in the case of [BzMPyrr][NTf 2]). We conclude that the spectral change in the low-frequency region is due to both the cation and anion, while the change in the high-frequency region is attributed to the red shift of the aromatic ring librations. On the basis of the plots of the first moment of the spectra vs. temperature, we found that the first moment of the low-frequency spectrum of the IL whose cation does not have an aromatic ring is less temperature dependent than that of the other ILs. However, the intrinsic first moment, the first moment at 0 K, of the low-frequency spectrum is governed by the absence or presence of a charged aromatic group, while a neutral aromatic group does not have much influence on determining the intrinsic first moment.« less
  2. Pulse Radiolysis and Computational Studies on a Pyrrolidinium Dicyanamide Ionic Liquid: Detection of the Dimer Radical Anion

    A pulse radiolysis study on pyrrolidinium cation based ionic liquids is presented here in this paper. Time-resolved absorption spectra for 1-methyl-1-propylpyrrolidinium dicyanamide (DCA) at 500 ns after the electron pulse show broad absorption bands at wavelengths below 440 nm and at 640 nm. In pyrrolidinium bis(trifluoromethylsulfonyl)imide (NTf 2) and tris(perfluoroethyl)trifluorophosphate (FAP) ILs, the transient absorption below 440 nm is much weaker. The absorption at 500 ns, which increases with wavelength from 500 nm to beyond 800 nm, was assigned to the tail of the solvated electron NIR absorption spectrum, since it disappears in the presence of N 2O. In themore » DCA IL, the presence of a reducing species was confirmed by the formation of pyrene radical anion. The difference in the transient species in the case of the DCA IL compared to other two ILs should be due to the anion, with cations being similar. In pseudohalide ILs such as DCA, radicals are formed by direct hole trapping by the anion (X + h + → X•), followed by addition to the parent anion. Prediction of the UV/vis absorption spectra of the dimer radical anion by computational calculation supports the experimental results. The oxidizing efficiency of (DCA) 2•– and its reduction potential (E(DCA)2•–/(2DCA–)) have been determined.« less
  3. Photoinduced Bimolecular Electron Transfer in Ionic Liquids: Cationic Electron Donors

    Recently, we have reported a systematic study of photoinduced electron-transfer reactions in ionic liquid solvents using neutral and anionic electron donors and a series of cyano-substituted anthracene acceptors [Wu, B.; Maroncelli, M.; Castner, E. W., Jr.Photoinduced Bimolecular Electron Transfer in Ionic Liquids. J. Am. Chem. Soc.139, 2017, 14568]. In this paper, we report complementary results for a cationic class of 1-alkyl-4-dimethylaminopyridinium electron donors. Reductive quenching of cyano-substituted anthracene fluorophores by these cationic quenchers is studied in solutions of acetonitrile and the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Varying the length of the alkyl chain permits tuning of the quencher diffusivities in solution.more » The observed quenching kinetics are interpreted using a diffusion-reaction analysis. Finally, together with results from the prior study, these results show that the intrinsic electron-transfer rate constant does not depend on the quencher charge in this family of reactions.« less
  4. Improving the radiation hardness of graphene field effect transistors

    Ionizing radiation poses a significant challenge to the operation and reliability of conventional silicon-based devices. In this paper, we report the effects of gamma radiation on graphene field-effect transistors (GFETs), along with a method to mitigate those effects by developing a radiation-hardened version of our back-gated GFETs. We demonstrate that activated atmospheric oxygen from the gamma ray interaction with air damages the semiconductor device, and damage to the substrate contributes additional threshold voltage instability. Our radiation-hardened devices, which have protection against these two effects, exhibit minimal performance degradation, improved stability, and significantly reduced hysteresis after prolonged gamma radiation exposure. Finally,more » we believe this work provides an insight into graphene's interactions with ionizing radiation that could enable future graphene-based electronic devices to be used for space, military, and other radiation-sensitive applications.« less
  5. Development of nanosecond time-resolved infrared detection at the LEAF pulse radiolysis facility

    When coupled with transient absorption spectroscopy, pulse radiolysis, which utilizes high-energy electron pulses from an accelerator, is a powerful tool for investigating the kinetics and thermodynamics of a wide range of radiation-induced redox and electron transfer processes. The majority of these investigations detect transient species in the UV, visible, or near-IR spectral regions. Unfortunately, the often-broad and featureless absorption bands in these regions can make the definitive identification of intermediates difficult. Time-resolved vibrational spectroscopy would offer much improved structural characterization, but has received only limited application in pulse radiolysis. In this paper, we describe in detail the development of amore » unique nanosecond time-resolved infrared (TRIR) detection capability for condensed-phase pulse radiolysis on a new beam line at the LEAF facility of Brookhaven National Laboratory. The system makes use of a suite of high-power, continuous wave external-cavity quantum cascade lasers as the IR probe source, with coverage from 2330-1051 cm⁻¹. The response time of the TRIR detection setup is ~40 ns, with a typical sensitivity of ~100 µOD after 4-8 signal averages using a dual-beam probe/reference normalization detection scheme. As a result, this new detection method has enabled mechanistic investigations of a range of radiation-induced chemical processes, some of which are highlighted here.« less

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