Features of Spin Exchange in Short-Chain Nitroxide Biradicals in Ionic Liquids

The intramolecular electron spin exchange has been studied by electron paramagnetic resonance (EPR) spectroscopy in the room temperature ionic liquids (RTIL) 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF₆) and 1-octyl-3-methylimidazolium tetrafluoroborate (omimBF₄) for three nitroxide biradicals with analogous structures of the connecting bridge between two >N–O· centers as a function of temperature. Temperature variations of the isotropic nitrogen hyperfine splitting constant a, and exchange integral values |J/a| were determined from EPR spectra and analyzed. Thermodynamic parameters of the conformational rearrangements were obtained. The spin exchange in rather rigid short-chain biradicals dissolved in omimBF₄ and bmimPF₆ was compared with that in toluene solutions. Interesting features of the spin exchange in biradicals in RTIL were observed and explained as a result of the specific intramolecular conformational transitions. Examples when rather rigid biradical molecules become flexible under an influence of RTIL are reported.     

Unexpected Features of the Intramolecular Spin Exchange in Imidazoline Nitroxide Biradicals Dissolved in Ionic Liquids

Three imidazoline-type nitroxide biradicals of the similar composition R ₅ ^NO –CH=N–N=CH–R ₅ ^N , B1, R ₅ ^NO –CH=N–N=C(CH₃)–R ₅ ^N , B2, and R ₅ ^N –C(CH₃)=N–N=C(CH₃)–R ₅ ^N , B3, with R ₅ ^N and R ₅ ^NO denoting, respectively, the nitroxide rings 1-oxyl-2,2,5,5-tetramethyl-3-imidazoline and 1-oxyl-2,2,5,5-tetramethyl-3-N–oxide imidazoline, have been studied by X-band electron paramagnetic resonance (EPR) spectroscopy. Variations of the intramolecular electron spin exchange in these biradicals dissolved in ethanol and the room temperature ionic liquid bmimBF₄ were characterized as a function of temperature by means of the analysis of the EPR lines shape. Thermodynamic parameters of the conformational rearrangements in ethanol were calculated. Analyzing the EPR spectra of these biradicals in bmimBF₄, it was revealed that the two-conformational model does not describe their conformational transitions. Moreover, the observed EPR spectra are not central symmetric especially at low temperatures that cannot be described and explained in the framework of the current theory of the intramolecular spin exchange. Probable reasons of this “strange” behavior are discussed.     

Behavior of Short Nitroxide Biradical in Room Temperature Ionic Liquids

The intramolecular electron spin exchange in short nitroxide biradical O=S(OR₆)₂ (I), where OR₆ is 1-oxyl-2,2,6,6-tetramethyl-4-oxypiperidine, dissolved in the room temperature ionic liquids (RTILs) 1-octyl-3-methylimidazolium hexafluorophosphate (omimPF₆), 1-octyl-3-methylimidazolium tetrafluoroborate (omimBF₄), 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF₄), and 1-ethyl-3-methylimidazolium tetrafluoroborate (emimBF₄) has been studied by electron paramagnetic resonance (EPR) spectroscopy as a function of temperature. Temperature variations of the isotropic nitrogen hyperfine splitting constant a were measured from EPR spectra. Thermodynamic parameters of the conformational rearrangements were calculated and compared with literature data. These intramolecular movements in rather rigid short-chain biradical I dissolved in four different RTILs are described well by the Debye–Stokes–Einstein law. Unrestricted density-functional-theory calculations of the geometry and electronic structure of the biradical were carried out using the ORCA program package, and showed that the O=S< group is available for the interaction with anions and cations of RTIL. The possible mechanism of such conformational transitions in biradical I in RTIL is discussed.     

Rotational and Translational Mobility of Nitroxide Spin Probes in Ionic Liquids and Molecular Solvents

Rotational and translational movements of 1-oxyl-2,2,6,6-tetramethyl-4-oxypiperidine (TEMPOL) spin probe in the room temperature ionic liquid (RTIL) 1-octyl-3-methylimidazolium tetrafluoroborate (omimBF₄) and in two molecular solvents, 1-propanol and isopropyl benzene (cumene), have been studied by X-band electron paramagnetic resonance (EPR) spectroscopy. Rotational correlation times τ _c of spin probes and the intermolecular spin exchange rate constants k _e were measured from EPR spectra at different temperatures and TEMPOL concentrations, and compared with the published data. The τ _c values were calculated both by known equations and from the EPR spectra simulation. Rotation movements of TEMPOL in omimBF₄ cannot be described by the model of the isotropic Brownian diffusion, which is valid for conventional solvents. The correct modeling of EPR spectra in RTIL can be achieved with the assumption of different rotational mobility of the spin probe around different molecular axes. The rotational, D _rot, and translational, D _tr, diffusion coefficients were calculated from τ _c and k _e values. The Debye–Stokes–Einstein law is valid in all three solvents while the dependence of D _tr on T/η is not linear in Stokes–Einstein coordinates. The effective activation energy E _rot^a of the rotational movements in omimBF₄ is noticeably higher than the corresponding values for conventional solvents, while the effective activation energies E _tr^a of the translational movements are comparable in all solvents studied.     

The Structure and EPR Behavior of Short Nitroxide Biradicals Containing Sulfur Atom in the Bridge

Two short nitroxide biradicals of similar composition: S(OR₆)₂ (1) and O=S(OR₆)₂ (2), where OR₆ is 1-oxyl-2,2,6,6-tetramethyl-4-oxypiperidine, have been studied by electron paramagnetic resonance spectroscopy, and X-ray structural analysis. Variations of the intramolecular electron spin exchange in the biradicals, dissolved in toluene and ethanol, as a function of temperature were characterized by changes in the isotropic ¹⁴N hyperfine splitting constant a, values of the exchange integral $ \left| J \right|, $ and compared with the X-ray structural data. Thermodynamic parameters of the conformational rearrangements were calculated. Geometry optimization and spin density distribution calculations of biradicals 1 and 2 were carried out on the DFT/UB3LYP/cc-pVdz and DFT/ROPBE/N07D levels of theory. Structural rigidity and probable differences in biradicals behavior are discussed.     

The Solvent Effect on Spin Exchange in Long-Chain Nitroxide Biradicals

Intramolecular electron spin exchange, as a function of temperature and the solvent viscosity, polarity and relaxation properties of the solvent molecules, has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in two long-chain flexible nitroxide biradicals existing in fluid solutions in three spectroscopically different spatial conformations. Certain thermodynamic parameters of the conformational rearrangements were calculated from the EPR spectra. Spin exchange in two biradicals dissolved in five different alcohols was compared with that in a nonpolar solvent (toluene), polar protic (water) and aprotic (acetonitrile), and with thermodynamic characteristics of the solvents. Distinct correlations were found between macroscopic (solvent viscosity, polarity) and microscopic (solvent longitudinal relaxation time) characteristics of solvents, and thermodynamic parameters of the intramolecular conformational transitions. Authors' address: Van Anh Tran, Institute of Physical and Theoretical Chemistry, Graz University of Technology, Technikerstrasse 4, 8010 Graz, Austria     

Solvation and Rotational Diffusion of Solutes in Room Temperature Ionic Liquids as Studied by EPR Spectroscopy with Nitroxide Spin Probing Method

In this article, we briefly introduced our studies on solvation and rotational diffusion of solutes in room temperature ionic liquids (RTILs) by electron paramagnetic resonance with nitroxide spin probing method. Most of the rotational correlation times for the nitroxide radicals are within the range calculated on the basis of Stokes–Einstein–Debye hydrodynamic theory with stick and slip boundary conditions or Gierer–Wirtz theory except for smaller solutes in some RTILs with smaller BF₄ and PF₆ anions. In RTILs with 1-butyl-3-methylimidazolium as cation and BF₄ or PF₆ as anion, nitroxide radicals undergo rotational diffusion like supercooled liquids and nitroxide radical with smaller volume rotationally slips.     

1,3-二烷基咪唑离子液体-水二元混合物的溶致显色参数和优先溶剂化行为

Binary mixtures of 1,3-dialkylimidazolium based ionic liquids (ILs) and water were selected as solvent systems to investigate the solute-solvent and solvent-solvent interactions on the preferential solvation of solvatochromic indicators at 25℃. Empirical solvatochromic parameters, dipolarity/polarizability (π^*), hydrogen-bond donor acidity (α), hydrogen-bond acceptor basicity (β), and Reichardt''s polarity parameters (E_T^N) were measured from the ultraviolet-visible spectral shifts of 4-nitroaniline, 4-nitroanisole, and Reichardt''s dye. The solvent properties of the IL-water mixtures were found to be influenced by IL type and IL mole fraction (x_IL). All these studied systems showed the non-ideal behavior. The maximum deviation to ideality for the solvatochromic parameters can be obtained in the xIL range from 0.1 to 0.3. For most of the binary mixtures, the π^* values showed the synergistic effects instead of the E_T^N, α and β values. The observed synergy extent was dependent on the studied systems, such as the dye indicator and IL type. A preferential solvation model was utilized to gather information on the molecular interactions in the mixtures. The dye indicator was preferentially solvated on the following trend: IL >IL-water complex >water.     

EPR,the X-ray Structure and DFT Calculations of the Nitroxide Biradical with One Acetylene Group in the Bridge

Two short nitroxide biradicals of similar composition, R6′–R6′ (B0) and R6′–(C ≡ C)–R6′ (B1), where R6′ is 1-oxyl-2,2,6,6-tetramethyl-3,4-ene-nitroxide ring, have been investigated by electron paramagnetic resonance spectroscopy and X-ray structural analysis. Quantum chemical calculations at UDFT/B3LYP/cc-pVDZ level were also performed and compared with the X-ray structural data. Zero-field splitting parameters D for B0 and B1 were found to be equal ?0.0048 and ?0.0022 cm^?1, respectively, in good agreement with quantum chemical prediction. Potential energy surface scans corresponding to pathways of the conformational rearrangements were calculated; rotation barriers for B0 and B1 were found to be 19.2 and 4.0 kJ/mol, respectively; and structural rigidity and probable differences in biradicals behavior are discussed. Calculations of spin density distribution in biradicals B0 and B1 were also carried out.     

离子液体催化苯与环己烯的Friedel-Crafts烷基化反应

室温离子液体是由特定的阳离子和阴离子构成,具有独特的性质和功能。将路易斯酸型离子液体应用于苯和环己烯的烷基化反应,考察了苯与环己烯摩尔比、反应温度、反应时间对烷基化反应产物收率的影响。结果表明以制备的咪唑盐离子液体为催化剂,在反应温度80℃、反应时间1.0h、苯烯摩尔比8：1的条件下,所得环己基苯的产物收率最高,为86.75%。离子液体可重复使用,活性基本没有降低。     

有机溶剂中C_(60)荧光的研究

本文研究了C6 0 溶于有机溶剂时 ,荧光能级的变化和荧光强度的大幅度增强。通过拉曼光谱结果显示 ,发现C6 0 分子与溶剂分子之间没有发生显著的化学反应。     

炔功能化的芘衍生物在有机场效应晶体管的应用

利用Sonogashira偶联反应在芘的1,3,6,8位引入炔键合成了一个X型化合物(SiPy)．它的光物理性质、热学性质、薄膜形态和有机场效应晶体管性质被研究．热重分析结果表明SiPy具有高的稳定 性．原子力显微镜图像显示SiPy的薄膜形态依赖于衬底的温度．基于SiPy的薄膜OFET器件被制作,SiPy呈现p型半导体特征．     

Anomalous Luminescence of Some Organic Solvents

Luminescence from some organic solvents has been observed when irradiated by infrared (1060nm) laser light. This effect is unusual in that the luminescence occurs in the visible spectrum while the excitation is in the infrared. Furthermore, a precondition is required for such an effect to occur, i.e. the sample must first be broken down (creation of a spark in the sample at the focuss of the laser) permanent.     

紫外光谱法检测水中咪唑类离子液体的含量

介绍了用紫外光谱测定水中离子液体含量的方法。测定了7种离子液体([C4mim][Cl]、[C4mim][BF4]、[C4mim][PF6]、[C5mim][Cl]、[C6mim][BF4]、[C8mim][Cl]和[C8mim][BF4])的最大吸收波长,并且绘制了校准曲线。结果表明,在实验试剂用量范围内,离子液体在波长211nm附近的吸光度与浓度呈线性关系,其线性相关系数均大于0.999。说明用紫外光谱法检测水中离子液体含量简单、快速且测定结果准确可靠。     

Exploring Spectroscopic and Physicochemical Properties of New Fluorescent Ionic Liquids

In the current study, spectroscopic and physicochemical properties of newly prepared ionic liquids were investigated. Ionic liquids were synthesized via a simple and straightforward route using a metathesis reaction of either N,N-diethyl-p-phenylenediamine monohydrochloride or N-phenacylpyridinium bromide with bis(trifluoromethane)sulfonimide lithium in water. High yield and purity were obtained for the resultant ionic liquids. Data acquired by use of ¹H NMR and FT-IR measurements were consistent with the chemical structures of newly prepared ionic liquids. Results of thermal gravimetric analysis also implied that these ionic liquids have good thermal stability. In addition, UV–vis and fluorescence spectroscopy measurements provided that new ionic liquids are good absorbent and fluorescent. Time-based fluorescence steady-state measurements showed that ionic liquids have high photostability against photobleaching. For a deeper mechanistic understanding of the analytical potential of newly synthesized ionic liquids, spectroscopic and physicochemical parameters, including singlet absorption, extinction coefficient, fluorescence quantum yield, Stokes shift, oscillator strength and dipole moment, were also investigated.     

Fluorescence Characteristics of Bisphenol A in Room Temperature Ionic Liquids

Room temperature ionic liquids (RTILs) are emerging as a new class of ‘green’ solvents for use in a wide range of chemical processes. RTILs can sensitize or quench fluorescence of organic chemicals and their interactions still remain unclear, especially for weakly fluorescent chemicals. Herein, we report the effects of six RTILs on the fluorescence behavior of bisphenol A (BPA). The fluorescence intensities (FIs) of BPA in a RTIL-acetonitrile system were significantly quenched compared to acetonitrile. The quenching effect was stronger for [C_nMIM]BF₄ than [C_nMIM]PF₆. A decreasing trend of fluorescence lifetime (FL) of BPA was observed for [C₆MIM]PF₆ (4.26 to 3.86 ns) and [C₁₄MIM]PF₆ (4.15 to 3.78 ns) with increasing RTIL concentrations in the range of 1–10 mM. The quenching mode was consistent with a static quenching mechanism based on the consistency of FL and FI results. The emission bands of BPA and RTILs did not interfere with each other when RTILs were used as the solvent. The investigated RTILs enhanced the FIs of strongly fluorescing chemicals (12.0?~?19.0-fold for norfloxacin and 6.1?~?8.5-fold for dansyl chloride), but quenched those of weakly fluorescing chemicals (BPA). These results demonstrate that RTILs have different fluorescent effects on organic chemicals with different fluorophores. The interactions between RTILs and BPA result from many factors in addition to viscosity, such as solvent electrostatic dielectric constant, refractive index, density, polarization and molecular interaction. These results provide a theoretical foundation for application of RTILs in the analysis of weakly fluorescing chemical.     

离子液体在大气压等离子体中稳定性研究

采用发射光谱、紫外可见吸收光谱、红外吸收光谱和核磁共振技术分析1-丁基-3-甲基咪唑硫酸氢盐（[Bmim]HSO₄）, 1-丁基吡啶硫酸氢盐（HSO₄）和1-丁基-3-甲基咪唑四氟化硼（[Bmim]BF₄）三种离子液体在大气压介质阻挡放电氩等离子体体系中的稳定性,并分别以上述三种离子液体为辅助液采用大气压介质阻挡放电等离子体技术制备TiO₂,进一步研究三种离子液体在等离子体中的稳定性对所制备的TiO₂晶相结构的影响。结果表明：向大气压介质阻挡放电氩等离子体中分别引入[Bmim]HSO₄,HSO₄和[Bmim]BF₄离子液体后并未改变氩等离子体放电光谱谱峰的位置和数量且没有新的谱峰生成,但谱峰强度都明显降低,说明上述三种离子液体没有在等离子体区蒸发形成激发态物种;[Bmim]HSO₄和HSO₄放电前后的红外吸收光谱基本一致,表明离子液体在放电后的化学键未发生改变;[Bmim]HSO₄和HSO₄的紫外可见吸收光谱显示其吸收峰的位置和强度未发生改变,说明两种离子液体在等离子体作用后的结构是稳定的;[Bmim]BF₄放电前后的红外吸收光谱各个特征峰并无明显差异,但其紫外可见吸收光谱图谱吸收峰的位置却发生较大的偏移,进一步对放电前后的[Bmim]BF₄离子液体进行核磁共振分析,两者的1H NMR峰数相同,但放电后的离子液体化学位移向高位偏移大约0.2单位,说明其化学环境发生了变化,表明有部分[Bmim]BF₄结构发生改变。光谱和核磁共振技术分析表明离子液体[Bmim]BF₄在等离子体作用后结构发生了改变。采用三种离子液体辅助大气压介质阻挡放电等离子体技术制备TiO₂样品的X-射线衍射分析结果表明 [Bmim]HSO₄和HSO₄辅助制备的HSO₄-TiO₂和[Bmim]HSO₄-TiO₂,谱图与锐钛矿相TiO₂标准谱图基本一致,表明所制备的TiO₂为纯锐钛矿型。而[Bmim]BF₄辅助制备的[Bmim]BF₄-TiO₂在2θ=24.1°处的衍射峰向小角度偏移,2θ=48°处的衍射峰向大角度偏移,说明[Bmim]BF₄在辅助制备TiO₂过程中,F进入TiO₂的晶格,破坏了TiO₂原子间的平衡状态,生成了F掺杂TiO₂光催化材料。F掺杂TiO₂光催化材料的形成也间接证明了离子液体[Bmim]BF₄在大气压等离子体中的不稳定性,此结果与核磁共振及紫外可见光的检测结果相一致。同时说明离子液体在等离子体的作用下对于纯锐钛矿晶格的形成和促进高活性掺杂型的光催化材料具有重要作用。为等离子体辅助离子液体制备高性能纳米材料提供重要的实验和理论依据。