Rotational mobility of guest molecules in o-terphenyl below T(g)

An optical anisotropy decay technique for measuring probe rotational times in glassy materials is presented. Rotational times from 10(1.4) to 10(5) s have been obtained for a molecule of 1-naphthyl-azomethoxybenzene (NAMB) in o-terphenyl (OTP) over a temperature range from T(g) +3.5 to T(g) -16.5 K. The rotational diffusion follows the temperature dependence of Debye-Stokes-Einstein down to T(g) -4 K with an activation energy of 320 +/- 30 kJ/mol. Below T(g) -9 K, the temperature dependence of rotation mobility was found to be much weaker with an activation energy of 70 +/- 15 kJ/mol.     

Probe dependence of spatially heterogeneous dynamics in supercooled glycerol as revealed by single molecule microscopy

Spatially heterogeneous dynamics in supercooled glycerol over the temperature range 198 K (1.04T(g))-212 K (1.12T(g)) is investigated using widefield single molecule (SM) fluorescence microscopy. Measurements are performed using three different perylenedicarboximide probes to investigate whether probe size and probe-host interactions affect breadth of heterogeneity reported in the glassy host by such SM experiments. Rotational relaxation times of single probe molecules are measured, and for all probes, log-normal distributions of relaxation times are found. No significant change in relaxation time distribution as a function of temperature is evident for a given probe. However, across probes, probe rotational relaxation time is correlated with breadth of heterogeneous dynamics reported. Molecules that undergo changes in dynamics are identified using two complementary approaches that interrogate time scales between 10(3) and 10(6) τ(α), with τ(α) the structural relaxation time of glycerol. Exchange is found on the shortest time scales probed (~30 τ(c), with τ(c) the rotational correlation time of the probe) and is relatively temperature and probe independent. No evidence is found for additional exchange occurring on the longest time scales interrogated. Taken together with the fact that probes that rotate the fastest report the greatest breadth of spatially heterogeneous dynamics in the system, this indicates that exchange times reported from analysis of SM linear dichroism trajectories as described here are upper bounds on the average exchange time in the system.     

Sub-Tg relaxations due to dipolar solutes in nonpolar glass-forming solvents

It is well known that rigid dipolar solutes (in smaller quantity) dispersed in a nonpolar glassy matrix exhibit a sub-T(g) (or beta(s)) relaxation due to the solute often designated as Johari-Goldstein (JG) relaxation, which is intermolecular in nature. In this article, we report the results of our study of such a sub-T(g) process in a wide variety of dipolar solutes in different glassy systems using dielectric spectroscopy over a frequency range of 20-10(6) Hz down to a temperature of 77 K. The T(g) of these solutions are determined using differential scanning calorimetry. The solvents used in this study are o-terphenyl (OTP), isopropylbenzene (IPB), and methylcyclohexane. In the case of rigid molecular solutes, like mono-halogen benzenes, the activation energy (DeltaE(beta)) of the beta(s) process is found to increase with decreasing T(g) of the solvent, with a corresponding decrease in the magnitude of the beta(s) process. In the case of more symmetrical molecular solute, for example, tert-butylchloride, the change in DeltaE(beta) is not very appreciable. These results emphasize the importance of the size of the cage of the host matrix in the relaxation of the solute molecules. We have also studied the sub-T(g) relaxation(s) due to some flexible molecular solutes, viz., 1butylbromide, 1hexylbromide, 1butylacetate, and benzylacetate. These solutes in IPB matrix exhibit only one relaxation, whereas in OTP matrix they exhibit an additional sub-T(g) process, which may be identified with a JG type of relaxation. These observations lead us to the conclusion that the beta process observed in the glassy states of these pure solutes is predominantly intramolecular in nature.     

Decomposition and isomerization of 1,2-benzisoxazole: single-pulse shock-tube experiments, quantum chemical and transition-state theory calculations

Isomerization and decomposition of 1,2-benzisoxazole were studied behind reflected shock waves in a pressurized driver, single-pulse shock tube. It isomerizes to o-hydroxybenzonitrile, and no fragmentation is observed up to a temperature where the isomerization is almost complete (approximately 1040 K at 2 ms reaction time). The isomerization experiments in this investigation covered the temperature range 900-1040 K. The lack of fragmentation is in complete contrast to the thermal behavior of isoxazole, where no isomerization was observed and the main decomposition products over the same temperature range were carbon monoxide and acetonitrile. In a series of experiments covering the temperature range 1190-1350 K, a plethora of fragmentation products appear in the post shock samples of 1,2-benzisoxazole. The product distribution is exactly the same regardless of whether the starting material is 1,2-benzisoxazole or o-hydroxybenzonitrile, indicating that over this temperature range the 1,2-benzisoxazole has completely isomerized to o-hydroxybenzonitrile prior to fragmentation. Two potential energy surfaces that lead to the isomerization were evaluated by quantum chemical calculations. One surface with one intermediate and two transition states has a high barrier and does not contribute to the process. The second surface is more complex. It has three intermediates and four transition states, but it has a lower overall barrier and yields the isomerization product o-hydroxybenzonitrile at a much higher rate. The unimolecular isomerization rate constants kinfinity at a number of temperatures in the range of 900-1040 K were calculated from the potential energy surface using transition-state theory and then expressed in an Arrhenius form. The value obtained is kfirst=4.15x10(14) exp(-51.7x10(3)/RT) s-1, where R is expressed in units of cal/(K mol). The calculated value is somewhat higher than the one obtained from the experimental results. When it is expressed in terms of energy difference it corresponds of ca. 2 kcal/mol.     

Orientation polarization from faster motions in the ultraviscous and glassy diethyl phthalate and its entropy

Dielectric spectra of the beta relaxation in glassy and ultraviscous liquid diethyl phthalate show that its relaxation strength Delta epsilon(beta), the distribution of times, and the relaxation rate are more sensitive to temperature T in the ultraviscous liquid than in the glassy state. The Delta epsilon(beta) against temperature plot has an elbow-shaped break near T(g) of approximately 181 K, which is remarkably similar to that observed in the entropy, enthalpy, and volume against temperature plots, and in the plot of Delta epsilon(beta) against the liquid's entropy minus its 0 K value. The ratio of Delta epsilon(beta) to diethyl phthalate's entropy, after subtracting the 0 K value, is 1.08 x 10(-3) mol K/J in the glassy state at 120.4 K, which decreases slowly to 0.81 x 10(-3) mol K/J at 176 K near T(g) and thereafter rapidly increases to 1.57 x 10(-3) mol K/J at 190 K. Variation in Delta epsilon(beta) parallels the variation of the entropy. A change in the activation energy of the beta process at T>T(g) indicates that its rate is also determined by the structure of the ultraviscous liquid. Features of beta relaxation are consistent with localized motions of molecules and may not involve small-angle motions of all molecules.     

Thermally induced magnetic anomalies in solvates of the bis(hexafluoroacetylacetonate)copper(II) complex with pyrazolyl-substituted nitronyl nitroxide

We succeeded in synthesizing of a whole family of isostructural solvates of the copper(II) hexafluoroacetylacetonate complex with pyrazolyl-substituted nitronyl nitroxide (L): Cu(hfac)2L x 0.Solv. The main feature inherent in nature of Cu(hfac)2L x 0.5 Solv single crystals is their incredible mechanical stability and ability to undergo reversible structural rearrangements with temperature variation, accompanied by anomalies on the mu(eff(T)) dependence. Structural investigation of the complexes over a wide temperature range before and after the structural transition and the ensuing magnetic phase transition showed that the spatial peculiarities of the solvent molecules incorporated into the solid govern the character of the mu(eff(T)) dependence and the temperature region of the magnetic anomaly. Thus, doping of crystals with definite solvent molecules could be used as an efficient method of control over the magnetic anomaly temperature (T(a)). The investigation of this special series of crystals has revealed the relationship between the chemical step and the magnetic properties. It was shown that "mild" modification of T(a) for Cu(hfac)2L x 0.5 Solv required a much smaller structural step than the typical change of one -CH2- fragment in a homologous series in organic chemistry. Quantum-chemical calculations with the use of X-ray diffraction data allowed us to trace the character of changes in the exchange interaction parameters in the range of the phase transition. In the temperature range of the phase transition, the exchange parameter changes substantially. The gradual decrease in the magnetic moment, observed in most experiments during sample cooling to T(a), is the result of the gradual increase in the fraction of the low-temperature phase in the high-temperature phase.     

Photochemical isomerization in the absence of a potential barrier

Different time resolved spectroscopic techniques have been used to investigate the photophysics of the isomerization reaction of 1,1′-diethyl-4,4′-cyanin. The molecule is characterized by a very short excited state lifetime, linear viscosity dependence over a wide viscosity range and no or negative temperature dependence of the reaction rate. The wavelength dependence of the ground state recovery experiment reported earlier (?kessonet al 1986,Chem. Phys. Lett. 126 385) has been shown to be the result of dependence mainly on the analyzing light. We believe that this molecule can be a representative of the barrierless reaction type (E₀ < 0) and that the probe wavelength dependence in the GSR experiment is due to the fact that different spectroscopic techniques may probe different physical events in the case of barrierless reactions, and suggest that it is a result of stimulated emission in combination with the resolution of the movement of the population on the excited state surface.     

Stable simple enols. Self-catalyzed E/Z-isomerization of the sterically crowded 2-(m-methoxymesityl)-1,2-dimesitylethenol

(E)-2-(m-methoxymesityl)-1,2-dimesitylethenol (3a) isomerizes in the absence of a catalyst in solution to a 1.0:0.9 E/Z (3a/3b) equilibrium mixture. In CDCl3 the isomerization is first order in 3a within a run, but the plot of the rate constant k(obs) vs the changing [3a]0 in different runs is a half-parabola, indicating self-catalysis by more than one enol molecule. At 0.09 M enol, the isotope effect k(3a)/k(3a)-OD = 2.1. In the presence of 0.025-0.25 M pyridine-d5, the k(obs) vs [pyridine-d5] plot displays a bell-shaped profile. The change in the shape of the OH signals of the 3a/3b mixture at 295-430 K in C6D5NO2 was followed by DNMR. The four signals of the diastereomeric 3a/3b mixture observed at 330 K coalesce at 350 K with barriers of 18.3 and 18.4 kcal x mol(-1) due to the diastereomerization of the vinyl propellers. The resulting two signals observed at >360 K further coalesce at 425 K with a barrier of 22.9 kcal x mol(-1) due either to oxygen-to-oxygen proton exchange or to E/Z isomerization. The estimated upper limit for the rate of proton exchange of k(ex) < or = (2-4) x 10(3) M(-1) x s(-1) at 425 K between the enol molecules is sufficiently slow to be a rate-controlling step in the isomerization. A process in which several enol molecules catalyze the isomerization is suggested, and several mechanistic routes are analyzed.     

Thermal conductivity and heat capacity of liquid and glassy poly(vinyl acetate) under pressure

The dependence of thermal conductivity λ and heat capacity per unit volume pc_p on temperature and pressure for poly(vinyl acetate) has been measured by a transient hot-wire probe technique. The measurements were made under pressures up to 0.5 GPa over a temperature range of 270–470 K. The temperature coefficient of thermal conductivity (? lnλ/?T)_p was found to increase with pressure for both the liquid and the glassy state. The change in heat capacity per unit volume in the region of the glass-transition temperature was found to decrease with increasing pressure. The Ehrenfest relation does not explain the variation of the pressure coefficient of the glass-transition temperature.     

Enhanced translational diffusion of rubrene in sucrose benzoate

The translational diffusion of rubrene in the fragile molecular glass former, sucrose benzoate (SB) (fragility index m approximately 94), has been studied from T(g)+6 K to T(g)+71 K(T(g)=337 K) by using the technique of holographic fluorescence recovery after photobleaching. In the temperature range of the measurements, the translational relaxation functions were observed to decay exponentially, indicating that Fick's law of diffusion governs the translational motion of rubrene in sucrose benzoate. The value of the translational diffusion coefficient D(T) obtained from the 1e time of the translational relaxation function varied from 5.3 x 10(-15) cm2 s(-1) at 343 K to 5.0x10(-9) cm2 s(-1) at 408 K. The temperature dependence of D(T) for diffusion of rubrene in SB is compared with that of the viscosity and the dielectric relaxation time tau(D) of SB. The temperature dependence of D(T) is weaker than that of Teta for T<1.2T(g) but tracks the reciprocal of the dielectric relaxation time 1tau(D) for 1.05T(g)相似文献   

Temperature dependence of solvation dynamics and anisotropy decay in a protein: ANS in bovine serum albumin

Temperature dependence of solvation dynamics and fluorescence anisotropy decay of 8-anilino-1-naphthalenesulfonate (ANS) bound to a protein, bovine serum albumin (BSA), are studied. Solvation dynamics of ANS bound to BSA displays a component (300 ps) which is independent of temperature in the range of 278-318 K and a long component which decreases from 5800 ps at 278 K to 3600 ps at 318 K. The temperature independent part is ascribed to a dynamic exchange of bound to free water with a low barrier. The temperature variation of the long component of solvation dynamics corresponds to an activation energy of 2.1 kcal mol(-1). The activation energy is ascribed to local segmental motion of the protein along with the associated water molecules and polar residues. The time scale of solvation dynamics is found to be very different from the time scale of anisotropy decay. The anisotropy decays are analyzed in terms of the wobbling motion of the probe (ANS) and the overall tumbling of the protein.     

Competitive isomerization and dimerization in co-crystals of 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol and sorbic acid: a new look at stereochemical requirements for [2+2] dimerization

Competitive [2+2] photodimerization and E-->Z isomerization reactions occur in a co-crystal of 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol upon irradiation with 325 nm light. At 90 K both reactions are observed, whereas at 280 K the dimerization reaction is very fast and inhibits isomerization as the nature of the chromophore is affected by the reaction. The temperature dependence of the stereospecificity of the dimerization reaction is related to the large sliding motion required to bring the reacting molecules into juxtaposition. The progress of the reactions has been monitored by photocrystallographic methods.     

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.     

Spin exchange in solutions of TEMPOL in n-octanol and 1-methyl-3-octylimidazolium hexafluorophosphate in the temperature range from 300 to 500 K

Comprehensive examinations of the motional properties (rotational correlation time τ(R)) and the spin exchange ω(SS) of the spin probe TEMPOL have been carried out using ESR spectroscopy in two different solvents. For the first time, the dynamic parameters τ(R) and ω(SS) have been determined simultaneously by simulation of spectra measured at three different ESR frequencies (L-, X-, and Q-band) between 293 and 500 K using a dynamic model based on a stochastic fitting program and, for comparison, two alternative models involving the shift of the hyperfine lines and considering the line broadening due to spin exchange in a wide range of conditions. Possibilities and limits of the used models are shown upon comparing the obtained results of the spin exchange. Moreover, the analysis of the ESR spectra gave evidence for the existence of cage effects that produce re-encounters of the spin probes. This has been done for the activation energies, which have been calculated from the temperature dependence of the rate constants of the spin exchange. From the ratio of the activation energies and the influence of the viscosities on the dynamics of the examined systems in n-octanol and an ionic liquid, conclusions can be drawn for the re-encounter effects in solvent cages. However, in contrast to n-octanol, the dynamics of the spin probe in the ionic liquid depends on specific and anisotropic interactions. The temperature dependence of the Q-band measurements required the development of a novel Q-band cavity.     

Fluorescence probing of temperature-dependent dynamics and friction in ionic liquid local environments

The solvation dynamics and local orientational friction for a series of four ionic liquids have been probed using coumarin 153 (C153) as a function of temperature. These ionic liquids are comprised of nonaromatic organic cations paired with a common anion, bis(trifluoromethylsulfonyl)imide (NTf(2)-). The specific liquids are as follows: N-methyl-tri-N-butylammonium NTf(2)- (N(1444)+/NTf(2)-), N-hexyl-tri-N-butylammonium NTf(2)- (N(6444)+/NTf(2)-), N-methyl-N-butylpyrrolidinium NTf(2)- (Pyrr(14)+/NTf(2)-), and N-methyl-N-ethoxyethylpyrrolidinium NTf(2)- (Pyrr(1(2O2))+/NTf(2)-). The observed solvation dynamics and fluorescence depolarization dynamics occur over a broad range of time scales that can only be adequately fit by functions including three or more exponential components. Stretched exponential distributions cannot adequately fit our data. The solvation and reorientational dynamics of the C153 probe are studied over a range of temperatures from 278.2 to 353.2 K. For both the solvation dynamics and the probe reorientational dynamics, the observed temperature dependence is well fit by a Vogel-Tammann-Fulcher law. To correlate the observed microscopic dynamics with macroscopic physical properties, temperature-dependent viscosities are also measured. Differential scanning calorimetry is used to study the thermodynamics of the phase transitions from the liquid to supercooled liquid to glassy states. For the two tetraalkylammonium liquids, the observed melting transitions occur near 300 K, so we are able to study the dynamics in a clearly supercooled regime. Very long time scale orientational relaxation time constants dynamics on the order of 100 ns are observed in the C153 fluorescence anisotropy. These are interpreted to arise from long-lived local structures in the environment surrounding the C153 probe.     

Specific features of molecular mobility of paramagnetic probe in chiral biomimetic solutions

Molecular mobility of a nitroxyl radical (as a paramagnetic probe) in methylcyclohexane (MCH) solutions of chiral biomimetic gelators—trifluoroacetylated amino alcohols (synthetic analogues of biological molecules)—has been studied by monitoring thermally induced changes in EPR spectra. The phase state of the systems has been examined by the low-temperature scanning calorimetry method. There has been found an unusual concentration and temperature dependence of probe species mobility in diluted solutions (10^–2–10^–3 M) of trifluoroacetylated amino alcohols with a molecular weight of no more than 200. From the temperature-induced changes in EPR spectra of the paramagnetic probe in the temperature range 170–290 K, the rotation activation energies of probe species in biomimetic solutions and in the neat solvent. The minimum radical rotation activation energy (2.1 kcal/mol) is observed in the most concentrated gelator solution, whereas in neat MCH, this energy is 3.7 kcal/mol.     

Water dynamics in n-propylene glycol aqueous solutions

The relaxation dynamics of dipropylene glycol and tripropylene glycol (nPG-n=2,3) water solutions on the nPG-rich side has been studied by broadband dielectric spectroscopy and differential scanning calorimetry in the temperature range of 130-280 K. Two relaxation processes are observed for all the hydration levels; the slower process (I) is related to the alpha relaxation of the solution whereas the faster one (II) is associated with the reorientation of water molecules in the mixture. Dielectric data for process (II) at temperatures between 150 and 200 K indicate the existence of a critical water concentration (x(c)) below which water mobility is highly restricted. Below x(c), nPG-water domains drive the dielectric signal whereas above x(c), water-water domains dominate the dielectric response at low temperatures. The results also show that process (II) at low temperatures is due to local motions of water molecules in the glassy frozen matrix. Additionally, we will show that the glass transition temperatures (T(g)) for aqueous PG, 2PG, and 3PG solutions do not extrapolate to approximately 136 K, regardless of the extrapolation method. Instead, we find that the extrapolated T(g) value for water from these solutions lies in the neighborhood of 165 K.     

New magnetoactive copper complexes with Schiff’s bases

Tridentate azomethine ligands with N₂O and N₃ donor atoms and their copper complexes have been synthesized and characterized. Magnetochemical measurements in the temperature range of 5–300 K show that the change in the coordination environment induces the change in the sign of exchange coupling in the molecules of complexes.     

Anisotropic pseudorotation of the photoexcited triplet state of fullerene C60 in molecular glasses studied by pulse EPR

Spin-polarized echo-detected electron paramagnetic resonance (EPR) spectra and the transversal relaxation rate T2(-1) of the photoexcited triplet state of fullerene C60 molecules were studied in o-terphenyl, 1-methylnaphthalene, and decalin glassy matrices. The model is composed of a fast (correlation time approximately 10(-12) s) pseudorotation of (3)C60 in a local anisotropic potential created by interaction of the fullerene molecule with the surrounding matrix molecules. In simulations, this potential is assumed to be axially symmetric around some axis of a preferable orientation in a matrix cage. The fitted value of the potential was found to depend on the type of glass and to decrease monotonically with a temperature increase. A sharp increase of the T2(-1) temperature dependence was found near 240 K in glassy o-terphenyl and near 100 K in glassy 1-methylnaphthalene and decalin. This increase probably is related to the influence on the pseudorotation of the onset of large-amplitude vibrational molecular motions (dynamical transition in glass) that are known for glasses from neutron scattering and molecular dynamics studies. The obtained results suggest that molecular and spin dynamics of the triplet fullerene are extremely sensitive to molecular motions in glassy materials.     

Influence of lattice interactions on the Jahn-Teller distortion of the [Cu(H2O)6]2+ ion: dependence of the crystal structure of K2[Cu(H2O)6](SO4)(2x)(SeO4)(2-2x) upon the sulfate/selenate ratio

The temperature dependence of the structure of the mixed-anion Tutton salt K2[Cu(H2O)6](SO4)(2x)(SeO4)(2-2x) has been determined for crystals with 0, 17, 25, 68, 78, and 100% sulfate over the temperature range of 85-320 K. In every case, the [Cu(H2O)6]2+ ion adopts a tetragonally elongated coordination geometry with an orthorhombic distortion. However, for the compounds with 0, 17, and 25% sulfate, the long and intermediate bonds occur on a different pair of water molecules from those with 68, 78, and 100% sulfate. A thermal equilibrium between the two forms is observed for each crystal, with this developing more readily as the proportions of the two counterions become more similar. Attempts to prepare a crystal with approximately equal amounts of sulfate and selenate were unsuccessful. The temperature dependence of the bond lengths has been analyzed using a model in which the Jahn-Teller potential surface of the [Cu(H2O)6]2+ ion is perturbed by a lattice-strain interaction. The magnitude and sign of the orthorhombic component of this strain interaction depends on the proportion of sulfate to selenate. Significant deviations from Boltzmann statistics are observed for those crystals exhibiting a large temperature dependence of the average bond lengths, and this may be explained by cooperative interactions between neighboring complexes.