The magnetic field effects on photochemical reactions in ionic liquids

The magnetic field effects (MFEs) on photoinduced hydrogen abstraction reactions of benzophenone (BP) with thiophenol (PhSH) in the ionic liquids (ILs) N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl) imide (TMPA TFSI), N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl) imide (P13 TFSI), and N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl) imide (PP13 TFSI) were investigated at 296 K by using a nanosecond laser flash photolysis technique under magnetic fields of 0-1.7 T. Large MFEs were observed for the first time in the ILs. In TMPA TFSI, the yield of the benzophenone ketyl radical gradually decreased with increasing magnetic field strength from 0 to 1.7 T, producing a 20% decrease at 1.7 T.     

Characterizing ionic liquids as reaction media through a chemical probe

The triplet N,N-dimethylaminophenyl cation, a highly reactive but chemospecific electrophile, has been used as a probe for characterizing the properties of reaction media for a series of imidazolium ILs. With the N-hexyl-N-methyl imidazolium derivatives (not with the N-butyl analogues), hydrogen transfer leading to the aniline was the main process. Trapping by iodide occurred with an inverse dependence on viscosity. Trapping by pi nucleophiles exhibited a more complex behavior. This was explained by the effect of both the bulk viscosity and the structure of the IL cation on both steps of the reaction, namely, initial electrophilic attack and ensuing cation elimination or nucleophile addition. However, with an excellent nucleophile, such as thiophene, or when the latter step was intramolecular, as with 4-pentenol, the difference was obliterated and trapping became uniform. Incorporation of the probe into the IL cation (through insertion into the C--H bond alpha to the imidazolium ring) was demonstrated, while no addition to the anion tested (including bis(trifluoromethanesulfonimide)) took place.     

Electrochromic and magnetic ionic liquids

Ionic liquids can be made intrinsically electrochromic and magnetic through the appropriate combination of electrochromic and magnetic anions based on ethylenediaminetetraacetic metal complexes, combined with several organic cations. These novel and highly multi-functional materials encompass the peculiar properties of ionic liquids together with the characteristics of electrochromic and magnetic materials.     

Determination of the polarities of some ionic liquids using 2-nitrocyclohexanone as the probe

Solute-solvent interactions on the keto-enol tautomerism of 2-nitrocyclohexanone in several organic solvents and room-temperature ionic liquids (RTILs) have been analyzed in terms of multiparameter equations. Permittivity and cohesive pressure values of the RTILs, unavailable by direct measurements, have been derived.     

Synthesis of bis-ANS fluorescence probe in ionic liquids

A new application of ionic liquids in the preparation of fluorescence probe 5,5'-bis-8-phenylamino-l-naphthylsulfonate (bis-ANS) is represented. The method for the preparation of fluorescence probe bis-ANS in alkyl imidazolinm cationic liquids under acidic conditions is described. The effects on reaction yields under different concentration of sodium nitrite and different ionic liquid were studied and good yields were achieved.     

Rapid ligand substitution reactions in ionic liquids studied by stopped-flow technique

Detailed kinetic studies on ligand substitution reactions of [M(II)(terpy)Cl](+) complexes (M = Pt, Pd; terpy = 2,2':6',2'-terpyridine) with thiourea as entering nucleophile were for the first time performed in the imidazolium based ionic liquid [emim][NTf(2)] using stopped-flow techniques, opening the route to study fast reactions of transition metal complexes in ionic liquids.     

Structures of ionic liquids with different anions studied by infrared vibration spectroscopy

We investigated the structures of ionic liquids (1-butyl-3-methylimidazolium iodide [BMIM][I] and 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4]) and their aqueous mixtures using attenuated total reflection (ATR) infrared absorption and Raman spectroscopy. The ATR spectrum in the CHx (x = 1, 2, 3) vibration region from 2800 to 3200 cm-1 was very different between [BMIM][BF4] and [BMIM][I] even though all the spectral features in this region were from the butyl chain and the imidazolium ring of the same cation. The spectrum did not change appreciably irrespective of the water concentration for [BMIM][BF4], whereas the spectrum from [BMIM][I] showed significant changes as the water concentration was increased, especially in CH-vibration modes from the imidazolium ring. For very diluted solutions both aqueous mixtures of [BMIM][I] and [BMIM][BF4] showed very similar spectra. Mixing of [BMIM][I] with heavy water (D2O) facilitated the isotopic exchange of the proton attached to the most acidic carbon of the imidazolium ring into deuterium from D2O, whereas even prolonged exposure to D2O did not induce any isotopic exchange for [BMIM][BF4]. Raman spectra around 600 cm(-1) indicative of the butyl chain conformation also changed differently as the water concentration was increased between [BMIM][I] and [BMIM][BF4]. These differences are considered to come from the variation in the position of the anion, where I- is expected to be closer to the C(2) hydrogen of the imidazolium cation and interacting more specifically as compared to BF(4-).     

Exploring 12'-apo-beta-carotenoic-12'-acid as an ultrafast polarity probe for ionic liquids

The ultrafast excited-state dynamics of the carbonyl-containing carotenoid 12'-apo-beta-carotenoic-12'-acid (12'CA) have been used for probing the microscopic environment in various ionic liquids (ILs). The following IL cations were investigated: 1,3-di-n-alkyl-imidazolium featuring different n-alkyl chain lengths and also additional methylation at the C2 position, triethylsulfonium, as well as two tetraalkylammonium ions. These were combined with different anions: [BF4]-, [PF6]-, ethyl sulfate ([EtOSO3]-), and bis(trifluoromethylsulfonyl)amide ([Tf2N]-). The probe molecule was excited via the S0 --> S2 transition at 425 or 430 nm, and the characteristic stimulated emission decay of the low-lying excited electronic S1/ICT (intramolecular charge transfer) state of 12'CA was monitored in the near IR (850 or 860 nm). Its lifetime tau1 is sensitive to the micropolarity-induced stabilization of S1/ICT relative to S0. The lifetime tau1 of the S1/ICT state varies only moderately in all ionic liquids studied here ( approximately 40-110 ps), which lies in the range between ethanol (109 ps) and methanol (49 ps). While organic solvents show an excellent correlation of tau1 with the solvent polarity function Deltaf = (epsilon - 1)/(epsilon + 2) - (n2 - 1)/(n2 + 2), where epsilon and n are the static dielectric constant and the refractive index of the solvent, respectively, this is not the case for ILs. This is due to dominant local electrostatic probe-cation interactions which cannot be easily quantified by macroscopic quantities. Methylation at the C2 position of 1,3-di-n-alkyl-imidazolium reduces the accessibility of the cation and therefore the electrostatic stabilization of the probe, resulting in an increase of tau1. A similar increase is observed upon extension of one of the n-alkyl chains from ethyl to n-decyl. Tetraalkylammonium ILs show an increased tau1 probably due to their more delocalized positive charge which cannot interact so favorably with the probe, in contrast to trialkylsulfonium ILs where the charge is more localized on the sulfur atom. The dependence of tau1 on the IL anion is much weaker, the only notable exception being [EtOSO3]-, where 12'CA experiences a less polar local environment than expected on the basis of extrapolated static dielectric constants. This is possibly due to the competition of the anion and probe for the cation interaction sites. Considerable electrostatic probe-cation interactions can be also introduced by addition of large amounts of LiClO4 salt to ethanol and diethyl ether. In this case, tau1 also strongly decreases, indicating an efficient coordination of Li+ cation(s) with the carbonyl oxygen at the negative end of the probe molecule. The S1/ICT --> S0 internal conversion of the 12'CA probe in ILs accelerates with increasing temperature, which can be characterized by an apparent activation energy of a few kJ mol-1, which is expected for energy-dependent nonradiative processes.     

Triaminocyclopropenium salts as ionic liquids

Salts of the charge-delocalised cations of the triaminocyclopropenium (tac) family bearing alkyl substituents have been prepared and shown to be air- and water-stable ionic liquids.     

A metallacage encapsulating chloride as a probe for a solvation scale in ionic liquids

With the purpose of assessing the reactivity of chloride ions dissolved in ionic liquids (ILs), a relative scale for the solvation of chloride is given for a series of ILs based on the bis(trifluoromethane)sulfonimide ([Tf(2)N]) anion and different cations, 1-butyl-3-methylimidazolium ([bmim]), 1-butyl-2,3-dimethylimidazolium ([bdmim]), 1-butyl-1-methylpyrrolidinium ([bmpy]), 1-butylpyridinium ([bpy]), 1-pentyl-1,1,1-triethylammonium ([C(5)e(3)am]), and 1-(2-hydroxy)ethyl-3-methylimidazolium ([mimeOH]). Insights into the solvation of chloride are achieved by the thermodynamic study of the reaction of dissociation of a chloride-templated nickel(II) metallacage performed at various temperatures by UV-visible spectroscopy in each IL. The order of chloride solvation [C(5)e(3)am][Tf(2)N] < [bmpy][Tf(2)N] < [bmim][Tf(2)N] 相似文献   

Determination of the Lewis acidity of ionic liquids by means of an IR spectroscopic probe

Pyridine and ethanenitrile can be used as molecular probes to measure the Lewis acidities of ionic liquids by monitoring the shift of IR absorption bands near 1450 cm(-1) for pyridine and in the range 2250-2340 cm(-1) for ethanenitrile.     

Spin dynamics of the radical pair in a low magnetic field studied by the transient absorption detected magnetic field effect on the reaction yield and switched external magnetic field

The spin dynamics of the radical pair generated from the photocleavage reaction of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TMDPO) in micellar solutions was studied by the time-resolved magnetic field effect (MFE) on the transient absorption (TA) and by a novel technique, absorption detected switched external magnetic field (AD-SEMF). Thanks to the large hyperfine coupling constant (A = 38 mT), a characteristic negative MFE on the radical yield was observed at a magnetic field lower than 60 mT whereas a positive effect due to the conventional hyperfine (HFM) and relaxation mechanisms (RM) was observed at higher magnetic field. The negative effect can be assigned to the mechanism "so-called" low field effect (LFE) mechanism and has been analyzed thoroughly using a model calculation incorporating a fast spin dephasing process. The time scale of the spin mixing process of LFE studied by AD-SEMF is shorter than the lifetime of the recombination kinetics of the radical pair. These results indicate that the LFE originates from the coherent spin motion. This can be interfered from the fast spin dephasing caused by electron spin interaction fluctuations.     

Photoinduced electron transfer in ionic liquids: use of 2,4,6-triphenylthiapyrylium as a photosensitizer probe

The photochemistry of 2,4,6-triphenylthiapyrylium, TPTP+, in an ionic liquid, bmim-PF6, and in zeolite Y has been investigated and compared. Fluorescence spectroscopy was used to characterize the singlet excited state and to demonstrate singlet quenching by dicyclopentadiene, DCP, as an electron donor. Time-resolved laser spectroscopy documents generation of the triplet excited state, 3TPTP+, and reduction by DCP, generating the corresponding radical, TPTP*, and radical ion, DCP+*. The highly polar media stabilize the organic intermediates, causing them to be long lived.     

Spectral characteristics of ethylene sorbed by silver-containing ionic liquids studied by in situ ATR-FTIR spectroscopy

The spectral characteristics of ethylene absorbed by ionic liquids (ILs) containing silver ions were studied by in situ ATR-FTIR spectroscopy at high pressure. Three different states of ethylene were observed based on the band shift of out-of-plane bending vibrations (ωCH₂) due to van der Waals interactions, hydrogen bonds, and π-complex formation with silver ions. Thus, ethylene can be used as an IR-sensitive probe molecule for characterizing interactions with ILs and other substances.     

Micelle-polymer complexes as studied by the ESR spin probe technique

Complexes between sodium dodecyl sulfate micelles and the water-soluble polymers poly(N-vinylpyrrolidone), poly(ethylene oxide) and a copolymer of poly (vinyl alcohol) and poly(vinyl acetate) have been studied in aqueous solution by the electron spin resonance (ESR) technique using di-tert-butyl nitroxide as a spin probe. The effective rotational correlation times reveal lowering of the critical micelle concentration and decreased headgroup packing in the micelle upon interaction with the polymers.     

Synthesis of cyano-bridged magnetic nanoparticles using room-temperature ionic liquids

A principally new exploit of ionic liquids as an alternative reaction medium in the synthesis of cyano-bridged coordination-polymer nanoparticles is reported. Stable colloid solutions containing nanoparticles of cyano-bridged molecule-based magnets, M)[Fe(CN)6]2/[RMIM][BF4] (M2+=Ni, Cu, Co) and Fe4[Fe(CN)6]3/[RMIM][BF4] (R=1-butyl (BMIM), 1-decyl (DMIM)), were prepared in the corresponding 1-R-3-methylimidazolium tetrafluoroborate [RMIM][BF4], which acts as both a stabilising agent and a solvent. By varying the length of the N-alkyl chain on the imidazolium cation of [RMIM]+ and the temperature, the growing process can be controlled to produce nanoparticles of different sizes. By studying the magnetic properties of frozen colloids it is shown that the relaxation of magnetisation is strongly influenced by interparticle interactions, which leads to the appearance of spin-glass-like dynamics in these systems.     

Longitudinal electron spin relaxation induced by degenerate electron exchange as studied by time-resolved magnetic field effects

T(1) paramagnetic relaxation of radical ions induced by degenerate electron exchange (DEE) reactions is studied theoretically and experimentally. Our theoretical analysis shows that T(1) relaxation time is well described by the Redfield theory at arbitrary values of the characteristic DEE time tau. Longitudinal relaxation of norbornane (NB) radical cation is studied by means of the time-resolved magnetic field effects (TR-MFE) technique; the rate constant of DEE involving NB(*+) radical cation and NB neutral molecule is obtained. Advantages of the TR-MFE technique and its potential for measuring the short DEE times are discussed in detail.     

Kemp elimination: a probe reaction to study ionic liquids properties

The amino induced elimination of benzisoxazole into the relevant o-cyanophenolate ion (Kemp elimination) has been studied in [bmim][BF 4] solution at 298 K. To have information about the interactions between reactants and ionic liquid, the reaction has been carried out at different temperatures (293-313 K). Several primary, secondary, and tertiary amines have been used to study the effect of amine structure on the reaction rate. The collected data show that the amine structure seems to have a crucial role in determining the reaction rate. Furthermore, as different cation or anion structures of an ionic liquid can significantly affect its properties, the title reaction has been performed in four different ionic liquids ([bmim][PF6], [bmim][NTf 2], [bm 2im][NTf 2], and [bmpyrr][NTf 2]), using pyrrolidine and piperidine as model amines. An H-donor negative solvent (MeOH and [bmim][NTf 2]) effect on reaction rate was detected. Finally, a narrow range of activation parameters was calculated both for the reaction induced by different amines and for pyrrolidine and piperidine, in the presence of different ILs. This fact suggests the occurrence of an "early" transition state.     

Electrode screening by ionic liquids

In this work we are concerned with the short-range screening provided by the ionic liquid dimethylimidazolium chloride near a charged wall. We study the free energy profiles (or potentials of mean force) for charged and neutral solutes as a function of distance from a charged wall. Four different wall charge densities are used in addition to a wall with zero charge. The highest magnitude of the charge densities is ±1 e nm(-2) which is close to the maximum limit of charge densities accessible in experiments, while the intermediate charges ±0.5 e nm(-2) are in the range of densities typically used in most of the experimental studies. Positively and negatively charged solutes of approximately the size of a BF ion and a Cl(-) ion are used as probes. We find that the ionic liquid provides excellent electrostatic screening at a distance of 1-2 nm. The free energy profiles show minima which are due to layering in the ionic liquid near the electrodes. This indicates that the solute ions tend to displace ionic liquid ions in the layers when approaching the electrode. The important role of non-electrostatic forces is demonstrated by the oscillations in the free energy profiles of uncharged solutes as a function of distance from the wall.