ESR study of molecular orientation and dynamics of TEMPO derivatives in CLPOT 1D nanochannels

The molecular orientations and dynamics of 2,2,6,6‐tetramethyl‐1‐piperidinyloxyl (TEMPO) radical derivatives with large substituent groups at the 4‐position (4‐X‐TEMPO) in the organic one‐dimensional nanochannels within the nanosized molecular template 2,4,6‐tris(4‐chlorophenoxy)‐1,3,5‐triazine (CLPOT) were examined using ESR. The concentrations of guest radicals, including 4‐methoxy‐TEMPO (MeO‐TEMPO) or 4‐oxo‐TEMPO (TEMPONE), in the CLPOT nanochannels in each inclusion compound (IC) were reduced by co‐including 4‐substituted‐2,2,6,6‐tetramethylpiperidine (4‐R‐TEMP) compounds at a ratio of 1 : 30–1 : 600. At higher temperatures, the guest radicals in each IC underwent anisotropic rotational diffusion in the CLPOT nanochannels. The rotational diffusion activation energy, E_a, associated with MeO‐TEMPO or TEMPONE in the CLPOT nanochannels (6–7 kJ mol^?1), was independent of the size and type of substituent group and was similar to the E_a values obtained for TEMPO and 4‐ hydroxy‐TEMPO (TEMPOL) in our previous study. However, in the case in which TEMP was used as a guest compound for dilution (spacer), the tilt of the rotational axis to the principal axis system of the g ‐tensor, and the rotational diffusion correlation time, τ_R, of each guest radical in the CLPOT nanochannels were different from the case with other 4‐R‐TEMP. These results indicate the possibility of controlling molecular orientation and dynamics of guest radicals in CLPOT ICs through the appropriate choice of spacer. Copyright © 2016 John Wiley & Sons, Ltd.     

ESR study of the molecular orientation and dynamics of stable organic radicals included in the 1‐D organic nanochannels of 2,4,6‐tris‐4‐(chlorophenoxy)‐1,3,5‐triazine

The molecular orientation and dynamics of the organic stable radicals such as 2,2,6,6‐tetramethyl‐1‐piperidinyl‐1‐oxyl (TEMPO) or 4‐hydroxy‐TEMPO (TEMPOL) included in the one‐dimensional (1‐D) organic nanochannels of 2,4,6‐tris‐4‐(chlorophenoxy)‐1,3,5‐triazine (CLPOT) were investigated by examining the inclusion compounds (ICs) diluted by the co‐inclusion of non‐radicals using ESR spectroscopy. Spectral simulation showed that the axial rotation of TEMPO or TEMPOL molecules is excited in the nanochannels with activation energies of 8 and 7 kJ mol^?1, respectively. The rotation axis was estimated to be tilted towards the principal x direction in the axis system of the g ‐tensor of the respective radicals. This is quite different from that for similar ICs in the nanochannels of tris(o‐phenylenedioxy)cyclotriphosphazene (TPP), in which the radicals are axially rotating around the principal axis y of the g‐tensor. The difference is attributed to the larger nanospace of the CLPOT nanochannels. Copyright © 2012 John Wiley & Sons, Ltd.     

Nitroxide-mediated styrene polymerization initiated by an oxoaminium chloride

An oxoaminium chloride that is prepared by reacting 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) with chlorine in carbon tetrachloride initiates radical polymerization of styrene at 120°C. In the early stages of polymerization, a monomeric adduct, 2,2,6,6-tetramethyl-1-(2-chloro-1-phenylethoxy)piperidine, is formed. Thereafter, styrene polymerization exhibiting the characteristics of living polymerization proceeds. High molecular weight polymers with relatively narrow molecular weight distributions are obtained by this polymerization. ¹H-NMR spectra of the polymers reveal that a chlorine atom and a TEMPO group are present at the α- and ω-termini, respectively. The monomeric adduct was prepared by heating the oxoaminium chloride and styrene in carbon tetrachloride at 65–70°C, and was characterized by ¹H- and ¹³C-NMR spectroscopy. It was found to be suitable as an initiator for nitroxide-mediated radical polymerization of styrene to make polymers with chlorine on the chain end. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2555–2561, 1998     

Study on emission quenching by 2,2,6,6-tetramethyl-1-piperidinyloxy free radical

The emission quenching mechanism of four emissive compounds by 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO) was investigated using steady state and transient state spectroscopy. The quenching mechanism was considered to be the radical triplet pair mechanism (RTPM) from the experimental results of both the emission quenching study and the steady state and time resolved electron spin resonance (SSESR and TRESR) studies.     

甲基丙烯酸正丁酯正相微乳液可控/“活性”光聚合动力学研究

以末端含溴原子的光引发剂2-溴异丁酰氧基-2-甲基-1-苯基甲酮(HMPP-Br)为引发剂,2,2,6,6-四甲基哌啶-1-氧自由基(TEMPO)和2,2,6,6-四甲基哌啶醇(TMP)为调控剂,采用光聚合方法研究了甲基丙烯酸正丁酯(n-BMA)/十二烷基硫酸钠(SDS)/水/正丁醇 O/W型正相微乳液体系的光聚合反应动力学.结果表明,改性后的引发剂具有一定的引发活性,且聚合微乳液体系较稳定,聚合反应获得了良好的ln[M]₀/[M]与时间、数均分子量与转化率之间的线性动力学关系,制备了分子量分布较窄的Poly(n-BMA)均聚物.     

Controlled radical polymerization of styrene in the presence of nitronyl nitroxides

Bulk radical polymerization of styrene in the presence of nitronyl nitroxides (2-(4-substituted phenyl)-4,4,5,5-tetramethyl-4,5-dihydroimidazolyl-1-oxyl 3-oxide) was studied. All nitronyl nitroxides, like other nitroxyl radicals such as 2,2,6,6-tetramethylpiperidine 1-oxyl radical (TEMPO), act as reversible radical scavengers. The efficiency of controlling the polymerization is affected by the substituent at the 4′-position. The efficiency increases with electron donating strength of 4′-substituents, at least at the beginning of the reaction. However, the thermal stability of nitronyl nitroxides decreases in the same order. Thus, TEMPO is more suitable than nitronyl nitroxides for controlled/“living” radical polymerization of styrene.     

Theoretical Study on Electronic Gain-and-loss Properties of TEMPO and Its Derivates in Charge/Discharge Processes

Theoretical study on the electronic structures and related properties of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and its cationic lipid derivates in the charge/discharge processes has been carried out using the density functional theory (DFT) at the (U)B3LYP/6-31G(d,p) or 6-31+G(d,p) level. The changes and regularities of geometric and electronic properties of these compounds in the charge/discharge processes were revealed in detail. The compu-tational results show that the substitute group plays a very important role in the electronic structures and related properties of TEMPOs during the charge/discharge processes. It isvery interesting to find that after getting an electron, TEMPO is more stable in singlet state but the lipid is more stable in triplet state. For TEMPO, both the charge and the discharge processes greatly influence the electronic properties of N and O atoms of the radical part. For the cationic lipid, the discharge process mainly influences the pyridinium head and the charge process mainly influences the free radical head. Moreover, the solvent effect plays an important role in some bond lengths and the charge population of the free radical head. In addition, the UV-Vis absorption spectra of TEMPO and the lipid were calculated and simulated using TDDFT at the 6-31G(d,p) or 6-31+G(d,p) level, in satisfying agreement with the experimental ones.     

Catalytic Oxidation of Alcohols to Corresponding Aldehydes or Ketones with TEMPO-Mediated Iodosobenzene in Water in the Presence of a Surfactant

An efficient, facile, and rapid oxidation of alcohols to the corresponding aldehydes or ketones with a stoichiometric amount of iodosobenzene (PhIO) in the presence of catalytic amounts of 2,2,6,6-tetramethyl-1-piperidinyloxyl free radical (TEMPO), KBr, and a surfactant, such as SDS (sodium dodecylsulfate), was reported. The oxidation proceeded in water at room temperature to afford aldehydes or ketones in excellent yields and high selectivity without overoxidation to carboxylic acids. Selective oxidation of primary alcohols in the presence of secondary alcohols was also achieved with the catalytic system of PhIO/TEMPO/KBr/SDS. A possible mechanism for the oxidation was supposed.     

Homolysis of weak Ti-O bonds: experimental and theoretical studies of titanium oxygen bonds derived from stable nitroxyl radicals

Titanium-oxygen bonds derived from stable nitroxyl radicals are remarkably weak and can be homolyzed at 60 degrees C. The strength of these bonds depends sensitively on the ancillary ligation at titanium. Direct measurements of the rate of Ti-O bond homolysis in Ti-TEMPO complexes Cp2TiCl(TEMPO) (3) and Cp2TiCl(4-MeO-TEMPO) (4) (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl, 4-MeO-TEMPO = 2,2,6,6-tetramethyl-4-methoxypiperidine-N-oxyl) were conducted by nitroxyl radical exchange experiments. Eyring plots gave the activation parameters, deltaH++ = 27(+/- 1) kcal/mol, deltaS++ = 6.9(+/- 2.3) eu for 3 and deltaH++ = 28(+/- 1) kcal/mol, deltaS++ = 9.0(+/- 3.0) eu for 4, consistent with a process involving the homolysis of a weak Ti-O bond to generate the transient Cp2Ti(III)Cl and the nitroxyl radical. Thermolysis of the titanocene TEMPO complexes in the presence of epoxides leads to the Cp2Ti(III)Cl-mediated ring-opening of the epoxide followed by trapping by the nitroxyl radical. The X-ray crystal structure of the Ti-TEMPO derivative, Cp2TiCl(4-MeO-TEMPO) (4), is reported. DFT (B3LYP/6-31G*) calculations and experimental studies reveal that the strength of the Ti-O bond decreases dramatically with the number of cyclopentadienyl groups on titanium. The calculated Ti-O bond strength of the monocyclopentadienyl complex 2 is 43 kcal/mol, whereas that of the biscyclopentadienyl complex 3 is 17 kcal/mol, a difference of 26 kcal/mol. These studies reveal that the strength of these Ti-O bonds can be tuned over an interesting and experimentally accessible temperature range by appropriate ligation on titanium.     

ESR spin probes in ionic liquids.

The spin probes 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL), and 2,2,6,6-tetramethyl-4-trimethylammoniumpiperidine-1-oxylIodide (CAT-1) are examined in a number of ionic liquids based on substituted imidazolium cations and tetrafluoroborate and hexafluorophosphate anions, respectively. The reorientation correlation times tau(R) of the spin probes in these systems have been determined by complete spectra simulation and, for rapid reortientation, by analysis of the intensities of the hyperfine lines of the electron spin resonance (ESR) spectra. A comparison of the results with those from the model system glycerol/water and selected organic solvents is made. Additions of diamagnetic and paramagnetic ions allow the conclusion that salt effects and spin exchange are present, and that both are superimposed by motional effects. Specific interactions in the ionic liquids, as well as between the spin-probe molecules and the constituents of the ionic liquids are reflected in the spectra of the spin probes, depending on their molecular structure.     

Simultaneous production of superoxide radical and singlet oxygen by sulphonated chloroaluminum phthalocyanine incorporated in human low-density lipoproteins: implications for photodynamic therapy

Sulfonated chloroaluminum phthalocyanines have been studied for their use in the photodynamic therapy (PDT) of tumors. Plasma low-density lipoproteins (LDL) are important carriers of phthalocyanines in the blood, but on exposure to visible light, phthalocyanine-loaded LDL undergo an oxidation process that propagates to erythrocytes. We attempted to identify the reactive species involved in LDL and erythrocyte oxidation by means of electron paramagnetic resonance (EPR) spectroscopy in the presence of 2,2,6,6-tetramethyl-4-piperidone (TEMP) and the spin trap 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO). Irradiation of phthalocyanine-loaded LDL in the presence of DMPO resulted in the formation of a four-line EPR spectrum with relative intensity of 1:2:2:1 (a(N) = a(H) = 14.8 G), characteristic of DMPO-hydroxyl radical spin adduct. This signal was sensitive to superoxide dismutase and slightly sensitive to catalase, but a mixture of the two enzymatic activities was the most efficient in promoting a decrease in the intensity of the EPR signal. In the presence of erythrocytes, an increase in the quartet intensity for a hematocrit of 1% and 4% was observed, decreasing for higher erythrocyte concentrations. The irradiation of phthalocyanine-loaded LDL in the presence of TEMP resulted in the formation of a nitroxide radical, 2,2,6,6-tetramethyl-4-piperidone-N-oxyl radical, intensity of which was sensitive to histidine, a singlet oxygen ((1)O(2)) quencher. Under both incubation conditions, with DMPO and TEMP, the formation of the respective EPR signals required the sensitizer (phthalocyanine), light and oxygen. Overall, the results are compatible with the simultaneous formation of superoxide anion and (1)O(2), implying that Type-I and Type-II mechanisms of photochemistry are simultaneously operative in phthalocyanine-loaded LDL. However, for a constant LDL/phthalocyanine ratio, the formation of oxygen free radicals shows a biphasic behavior with the concentration of LDL increasing and reaching a plateau, whereas the formation of (1)O(2) increases linearly with LDL concentration. Erythrocytes at high (physiological) concentrations induced a decrease in the intensity of both EPR signals. The physiological relevance of these findings in the framework of PDT is briefly discussed.     

Synthesis and properties of nicotinic acid derivatives containing a 2,2,6,6- tetramethylpiperidine 1-oxyl residue

N¹-(Nicotinoyl)-N²-4-(2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl)hydraxine was obtained by condensation of nicotinoyl hydrazide with 2,2,6,6-tetramethyl-4-oxopiperidine 1-oxyl. Acylation of 2,2,6,6-tetramethyl-4-hydroxypiperidine 1-oxyl with nicotinoyl chloride gives nicotinic acid 2,2,6,6-tetramethyl-1-oxyl 4-piperidyl ester. A spin-labeled analog of nicotinamide was obtained by condensation of nicotinoyl azide with 4-amino-2,2,6,6-tetramethylpiperidine 1-oxyl. The synthesis of 1-N-(-D-ribofuranoside)-3'-N[4-(2,2,6,6-tetramethylpiperidine-1-oxyl)pyridinecarboxamide from 2,2,6,6-tetramethyl-4-nicotinoylaminopiperidine 1-oxyl and 2,3,5-tri-O-benzoyl--D-ribofuranosyl bromide proceeds without damage to the iminoxyl radical. The preparation of the corresponding spin-labeled nucleotide is hindered by destruction of the iminoxyl radical during ion-exchange chromatography.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 352–355, March, 1976.     

Synthesis of AB2 miktoarm star-shaped copolymers by combining stable free radical polymerization and atom transfer radical polymerization

A stable nitroxyl radical functionalized with two initiating groups for atom transfer radical polymerization (ATRP), 4-(2,2-bis-(methyl 2-bromo isobutyrate)-propionyloxy)-2,2,6,6-tetramethyl-1-piperidinyloxy (Br₂-TEMPO), was synthesized by reacting 4-hydroxyl-2,2,6,6-tetramethyl-1-piperidinyloxy with 2,2-bis-(methyl 2-bromo isobutyrate) propanoic acid. Stable free radical polymerization of styrene was then carried out using a conventional thermal initiator, dibenzoyl peroxide, along with Br₂-TEMPO. The obtained polystyrene had two active bromine atoms for ATRP at the ω-end of the chain and was further used as the macroinitiator for ATRP of methyl acrylate and ethyl acrylate to prepare AB₂-type miktoarm star-shaped copolymers. The molecular weights of the resulting miktoarm star-shaped copolymers at different monomer conversions shifted to higher molecular weights without any trace of the macroinitiator, and increased with monomer conversion.     

The nature of sites of absorption of low-molecular-mass compounds by butadiene–acrylonitrile copolymers

The rotational mobilities of the paramagnetic probes TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and BZONO (4-benzoate-2,2,6,6-tetramethylpiperidine-1-oxyl) are studied via EPR spectroscopy and the equilibrium swelling of random and multiblock butadiene–acrylonitrile copolymers of various compositions and stereostructures of butadiene units in n-heptane, methyl acetate, and toluene are studied. The nature of defective regions sorbing low-molecular-mass compounds is ascertained. The rotational mobilities of the probes in defects of various structures are estimated. The sites of sorption are found to be identical in crosslinked and noncrosslinked elastomers. It is shown that the sorption of n-heptane and methyl acetate appears in the same defective regions as those of the TEMPO radical, while the sorption of toluene emerges in sorption sites where the value of free volume is sufficient for the sorption of the bulky radical BZONO. A decay in local molecular mobility in structural defects (“holes”) does not hinder the absorption of lowmolecular- mass compounds if the free volume is sufficient.     

Dynamic coupling of a small rigid probe to viscous ortho-terphenyl

We have measured the dielectric relaxation of 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) as a rotational probe in supercooled ortho-terphenyl (OTP). Due to the significant dipole moment of TEMPO compared with OTP, its contribution to the loss spectra can be identified already at moderate concentrations. For time scales ranging from 10 μs to 1 s, it is found that the tumbling mode of TEMPO is a true replica of the structural relaxation of OTP regarding average time constant, relaxation time dispersion, and the temperature dependence. While the present dielectric results are consistent with a decoupling of a spinning mode (about the nitroxyl dipole axis) of TEMPO from viscosity, they do not agree with the strong decoupling of the tumbling mode derived from electron spin resonance experiments.     

A simple and highly practical oxidation of primary alcohols to acids mediated by 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)

Primary alcohols were quantitatively oxidized in one-pot to acids via 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) mediated oxidation. The reported method is simple and suitable for large scale synthesis.     

Determination of molecular orientation distribution of a stable paramagnetic probe in oriented 4-cyano-4’-<Emphasis Type="Italic">n</Emphasis>-pentylbiphenyl

The molecular orientation distribution function of a stable radical 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl in magnetic-field oriented 4-cyano-4’-n-pentylbiphenyl was determined from the angular dependence of the ESR spectra. The preferred molecular orientation of radical species in the liquid crystal matrix was determined. The temperature evolution of the orientation distribution function was studied.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 190–195, January, 2005.     

Synthesis and characterization of a persistent paramagnetic rotaxane based on α-cyclodextrin and α,ω-alkyl disulfides

A new synthetic strategy for the preparation of persistent paramagnetic cyclodextrin-based rotaxanes is described. The method consists in the formation of inclusion complexes between α-cyclodextrin (α-CD) and α,ω-dithiols containing an octamethylene chain covalently trapped by bulky stoppers composed of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical fragments. Interaction of α-CD (the bead) and 1,8-octanedithiol (the thread) occurs in aqueous alkaline media and encapsulation is obtained by nucleophilic substitution at both termini of the linear component with a bulky paramagnetic iodide [2,2,6,6-tetramethyl-4-(2-iodoacetamide)piperidine-N-oxyl]. Structure determination of the new [2]rotaxane by ¹H NMR is reported and the spectroscopic data are discussed.     

Gibbs monolayers at the air/water interface: surface partitioning and lateral mobility of an electrochemically active surfactant

Monolayer films of a water-soluble surfactant, 4-octaneamido-2,2,6,6-tetramethyl-1-piperidinyloxy (C8-TEMPO) were investigated at the air/water interface. An electrochemical, horizontal touch method was developed to measure the equilibrium surface concentrations (gamma) of C8TEMPO. The dependence of gamma on the solution concentration followed a Langmuir isotherm and yielded the partition constant K = (2.3+/-0.2) x 10(4) M(-1). These results were verified by surface tension measurements and Brewster angle microscopy. Within experimental error, the same K values were obtained. The lateral diffusion constants vs surface concentration of this molecule were measured by 2D voltammetry. In these experiments, the component of the oxidation current due to C8TEMPO in the bulk of the solution was subtracted from the total measured current to obtain the component due to the lateral surface diffusion. In the ange of mean molecular areas from 120 to 400 A2/molecule, the lateral diffusion constant of C8TEMPO increased from 1.0 x 10(-6) to 1.0 x 10(-5) cm2/s. The latter value is about 2.5 times larger than the C8TEMPO diffusion constant in bulk water. Comparison of the lateral mobilities of C8TEMPO and two longer alkane chain, water-insoluble homologues, C14TEMPO and C18TEMPO, showed no statistically significant differences.     

Controlled free radical ring-opening polymerization and chain extension of the “living” polymer

Free radical ring-opening polymerization of 2-methylene-1,3-dioxepane (MDP) in the presence of 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO) has been achieved to afford a chain polyester (PMDP) with di-t-butyl peroxide (DTBP) as an initiator at 125°C. The polydispersity of the polymers decreases as the concentration of TEMPO is increased. At high TEMPO concentrations, the polydispersity as low as 1.2 was obtained, which is below the theoretical lower limit for a conventional free radical polymerization. A linear relationship between the number-average molecular weight (M_n) and the monomer conversion was observed with the best-fit line passing very close to the origin of the M_n-conversion plot. The isolated and purified TEMPO-capped PMDP polymers have been employed to prepare chain extended polymers upon addition of more MDP monomer. These results are suggestive of the “living” polymerization process. A possible polymerization mechanism might involve thermal homolysis of the TEMPO-PMDP bonds followed by the addition of the monomers. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 761–771, 1998