Isotropic reorientations of fullerene C70 triplet molecules in solid glassy matrices revealed by light-induced electron paramagnetic resonance

Continuous-wave X-band electron paramagnetic resonance (EPR) of fullerene C(70) molecules excited to a triplet state by continuous light illumination was studied in molecular glasses of o-terphenyl and cis/trans-decaline and in the glassy polymers polymethylmethacrylate (PMMA) and polystyrene (PS). Above ～100 K, a distinct narrowing of EPR lineshape of the triplet was observed, which was very similar for all systems studied. EPR lineshape was simulated reasonably well within a framework of a simple model of random jumps, which implies that the C(70) molecule performs isotropic orientational motion by sudden jumps of arbitrary angles. In simulations, a single correlation time τ(c) was used, varying in the range of 10(-7)-10(-8) s. Near and below 100 K electron spin echo (ESE) signals were also obtained which were found to decay exponentially. Correlation times τ(c) obtained from simulation of the EPR spectra in the slow-motion limit (τ(c) close to 10(-7) s) turned out to be in good agreement with the phase memory times T(M) of the ESE decay, which additionally supports the employed simple model. The observed motional effects provide evidence that the nanostructure of the solid glassy media of different origins is soft enough to allow a large asymmetric C(70) molecule to reorient rapidly. Except for the EPR spectra of the triplet, in the center of the spectra, a small admixture of a narrow line was also observed; its possible nature is briefly discussed.     

Self-diffusion of supercooled o-terphenyl near the glass transition temperature

Self-diffusion coefficients for the low molecular weight glass former o-terphenyl have been measured near Tg by isothermally desorbing thin film bilayers of deuterio and protio o-terphenyl in a vacuum chamber. We observe translational diffusion that is about 100 times faster at Tg + 3 K than the Stokes-Einstein prediction. Predictions from random first order transition theory and a dynamic facilitation approach are in reasonable agreement with our results; in these approaches, enhanced translational diffusion is associated with spatially heterogeneous dynamics. Self-diffusion controls crystallization in o-terphenyl for most of the supercooled liquid regime, but at temperatures below Tg + 10 K, the reported crystallization rate increases suddenly while the self-diffusion coefficient does not. This work and previous work on trisnaphthylbenzene both find a self-diffusion-controlled crystal growth regime and an enhancement in self-diffusion near Tg, suggesting that these phenomena are general characteristics of fragile low molecular weight glass formers. We discuss the width of the relaxation time distributions of o-terphenyl and trisnaphthylbenzene as they relate to the observation of enhanced translational diffusion.     

Nitrogen nuclear spin flips in nitroxide spin probes of different sizes in glassy o-terphenyl: possible relation with α- and β-relaxations

The pulsed electron-electron double resonance (ELDOR) technique was employed to study nitroxide spin probes of three different sizes dissolved in glassy o-terphenyl. A microwave pulse applied to the central hyperfine structure (hfs) component of the nitroxide electron paramagnetic resonance spectrum was followed by two echo-detecting pulses of different microwave frequency to probe the magnetization transfer (MT) to the low-field hfs component. The MT between hfs components is readily related to flips in the nitrogen nuclear spin, which in turn are induced by molecular motion. The MT on the time scale of tens of microseconds was observed over a wide temperature range, including temperatures near and well below the glass transition. For a bulky nitroxide, it was found that MT rates approach dielectric α (primary) relaxation frequencies reported for o-terphenyl in the literature. For small nitroxides, MT rates were found to match the frequencies of dielectric β (secondary) Johari-Goldstein relaxation. The most probable motional mechanism inducing the nitrogen nuclear spin flips is large-angle angular jumps, between some orientations of unequal occupation probabilities. The pulsed ELDOR of nitroxide spin probes may provide additional insight into the nature of Johari-Goldstein relaxation in glassy media and may serve as a tool for studying this relaxation in substances consisting of non-rigid molecules (such as branched polymers) and in heterogeneous and non-polar systems (such as a core of biological membranes).     

Nuclear magnetic resonance and dielectric investigations of molecular motions in a glassy crystal: the mixed compound (CN-adm)0.75(Cl-adm)0.25

The dynamic properties of plastic crystalline mixed adamantane's derivatives namely cyanoadamantane (75%) and chloroadamantane (25%) were investigated by dielectric and nuclear magnetic resonance (NMR) spectroscopy, covering a spectral range of 12 decades in the temperature range 110-420 K. Phase transformations were studied and dynamical parameters of the plastic (I), glassy (Ig), and ordered (III) phases were determined and compared with those of pure compounds. The dynamics of the supercooled plastic phase is characterized by an alpha-process exhibiting an Arrhenius behavior which classified the mixed compound as a strong glass former. In the plastic phase, NMR relaxation times were interpreted by using a Frenkel model, which takes into account structural equilibrium positions. This model explains adequately the experimental results by considering two molecular motions. In both the glassy state and plastic phase the motional parameters agree with those of 1-cyanoadamantane. On the contrary, in the ordered phase, the motional parameters related to the uniaxial rotation of chloroadamantane molecules indicate an accelerated motion.     

Inertial suppression of protein dynamics in a binary glycerol-trehalose glass

The traditional approach used to predict the ability of a glassy matrix to maximally preserve the activity of a protein solute is the glass transition temperature (T(g)) of the glass. Recently it has been shown that the addition of a low T(g) diluent (glycerol) can rigidify the structure of a high T(g) glassy matrix in binary glycerol-trehalose glasses. The optimal density of glycerol in trehalose minimizes the average mean square displacements of non-exchangeable protons in the glass samples. The amount of glycerol added to a trehalose glass coincides with the maximal recovery of biological activity in a separate study using similar binary glass samples. In this study, we use molecular dynamics (MD) simulations to investigate the dynamics of a hydrated protein encased in glycerol, unary trehalose and binary glycerol-trehalose glasses. We have found that we are able to reproduce the rigidification of the glycerol-trehalose glassy matrix and that there is a direct correlation between bulk glass dynamics and the extent of atomic fluctuation of protein atoms. The detailed microscopic picture that emerges is that protein dynamics are suppressed mainly by inertia of the bulk glass and to a lesser extent specific interactions at the protein-solvent interface. Thus, the inertia of the glassy matrix may be an influential factor in the determination of pharmaceutically relevant formulations.     

Rotational dynamics and polymerization of C60 in C60-cubane crystals: a molecular dynamics study

We report classical and tight-binding molecular dynamics simulations of the C(60) fullerene and cubane molecular crystal in order to investigate the intermolecular dynamics and polymerization processes. Our results show that, for 200 and 400 K, cubane molecules remain basically fixed, presenting only thermal vibrations, while C(60) fullerenes show rotational motions. Fullerenes perform "free" rotational motions at short times (approximately < 1 ps), small amplitude hindered rotational motions (librations) at intermediate times, and rotational diffusive dynamics at long times (approximately > 10 ps). The mechanisms underlying these dynamics are presented. Random copolymerizations among cubanes and fullerenes were observed when temperature is increased, leading to the formation of a disordered structure. Changes in the radial distribution function and electronic density of states indicate the coexistence of amorphous and crystalline phases. The different conformational phases that cubanes and fullerenes undergo during the copolymerization process are discussed.     

Isothermal desorption measurements of self-diffusion in supercooled o-terphenyl

Isothermal desorption of o-terphenyl thin-film bilayers was used to measure self-diffusion coefficients of supercooled o-terphenyl near the glass transition temperature (Tg=243 K). Diffusion coefficients from 10(-15.5) to 10(-12) cm2 s(-1) were obtained between 246 and 265 K. Protio and deuterio o-terphenyl were sequentially vapor deposited, then annealed to simultaneously diffuse and desorb the sample in a vacuum chamber. During the desorption of the bilayer, the concentration of each isotope was detected by a mass spectrometer, which revealed the extent of interfacial broadening. In these experiments, isotopic interdiffusion is indistinguishable from self-diffusion and the measured interfacial broadening is consistent with Fickian diffusion. The samples prepared under several different deposition conditions yielded the same self-diffusion coefficients, indicating that the experiments were conducted in the equilibrium supercooled liquid state.     

Glass transition of polymers confined to nanoporous glasses

The glassy dynamics of poly(propylene glycol) (PPG) and poly(methyl phenyl siloxane) (PMPS) confined to nanoporous glasses (pore sizes 2.5–20 nm) investigated by dielectric spectroscopy, temperature modulated DSC and neutron scattering is compared. For both systems the relaxation rates estimated from dielectric spectroscopy and temperature modulated DSC agree quantitatively indicating that both experiments sense the glass transition.For PPG the glassy dynamics in nanopores is determined by a counterbalance of an adsorption and a confinement effect where the temperature dependence of the relaxation times obeys the Vogel/Fulcher/Tammann (VFT-) equation. The former effect results from an interaction of the confined macromolecules with the internal surfaces which in general slows down the molecular dynamics. A confinement effect leads to an acceleration of the segmental dynamics compared to the bulk state and points to an inherent length scale on which the glassy dynamics takes place. The step of the specific heat capacity c_p at the glass transition vanishes at a finite length scale of 1.8 nm. This result supports further the conception that a characteristic length scale is relevant for glassy dynamics.For PMPS down to a pore size of 7.5 nm the temperature dependence of the relaxation times follows the VFT-dependence and a confinement effect is observed like for PPG. At a pore size of 5 nm this changes to an Arrhenius-like behavior with a low activation energy. At the same pore size c_p vanishes for PMPS. This points to a dramatic change in the character of molecular motions responsible for glassy dynamics and supports further the relevance of a characteristic length scale on which it takes place.Quasielastic neutron scattering experiments on PMPS reveal that the microscopic dynamics characterized by the mean square displacement depends on confinement above the glass transition. The diffusive character of the relevant molecular motions seems to disappear at a length scale of about 1.6 nm.     

Dynamical heterogeneity in glassy o-terphenyl: manifestation of environment changes in photoisomerization kinetics of probe molecules

A new method for the investigation of dynamical heterogeneity in glassy matrixes is presented and illustrated by the example of o-terphenyl (OTP). UV-vis absorption spectroscopy has been used to monitor the cis-trans isomerization kinetics of probe molecules in glassy OTP. The dependence of isomerization quantum yield on light intensity has been established. This dependence is shown to be due to the change in the local environment of the probe molecules. The simple model is suggested to estimate the time required for the environment to change, tauex. The tauex values from 2.6 x 10(2) to 1.9 x 10(5) s have been obtained for environments of molecules of 1-naphthylazomethoxybenzene (NAMB) in OTP over a temperature range from 244 to 204 K (Tg+1 to Tg-39 K). The temperature dependence of exchange time has a non-Arrhenius character. As the temperature decreases, an increase in exchange time slows down. The activation energy of the relaxation process is 54 kJ/mol over the range of 224-239 K.     

Glass transition line in C60: a mode-coupling/molecular-dynamics study

We report a study of the mode-coupling theory (MCT) glass transition line for the Girifalco model of C60 fullerene. The equilibrium static structure factor of the model, the only required input for the MCT calculations, is provided by molecular dynamics simulations. The glass transition line develops inside the metastable liquid-solid coexistence region and extends down in temperature, terminating on the liquid side of the metastable portion of the liquid-vapor binodal. The vitrification locus does not show re-entrant behavior. A comparison with previous computer simulation estimates of the location of the glass line suggests that the theory accurately reproduces the shape of the arrest line in the density-temperature plane. The theoretical HNC and MHNC structure factors (and consequently the corresponding MCT glass line) compare well with the numerical counterpart. Our results confirm the conclusion drawn in previous works about the existence of a glassy phase for the fullerene model at issue.     

Interfacial effects on vitrification of confined glass-forming liquids

Mesoporous silica phases, with uniform pores of dimensions in the 2-30 nm range, offer a uniquely well-defined environment for the study of the effects of two-dimensional spatial confinement on the properties of glass-forming liquids. We report observations by differential scanning calorimetry of the vitrification of o-terphenyl (OTP), salol, and glycerol in hexagonal mesoporous silica (MCM-41 and SBA-15) in a wide range of pore sizes from 2.6 to 26.4 nm. In agreement with previous studies, where a controlled porous glass is used as a solid matrix, the glass transition temperature for o-terphenyl diminishes with decreasing pore size. In contrast to OTP, glycerol shows a gradual increase in glass transition temperature, while in salol a slight reduction of glass transition temperature is observed, followed by an increase, which results in glass transition temperature indistinguishable from that of the bulk for the smallest pores. These results are discussed in terms of liquid-surface interactions in an interfacial layer, monitored by Fourier-transformed infrared spectroscopy in the study. The hydrogen bonding with silica surface silanols dominates the glass transition trends observed in salol and glycerol.     

Physical gelation processes revealed by fluorescence technique

The structure of an isotactic polystyrene (iPS) chain in its thermoreversible gel formed with trans‐ and cis‐decalin was determined to be of 3₁‐helical form or near 3₁‐helical form by monitoring the fluorescence behavior of iPS. The fluorescence depolarization measurements of five fluorescent molecules having different molecular sizes doped into iPS in decalin clearly showed that there is enough space for decalin molecules to intercalate among the iPS helical chains. Our fluorescence data support the idea that a polymer‐solvent compound that can be described by a ladder‐like model is formed in iPS‐decalin gels.     

Temperature dependence of ESR spectra of nitroxide spin probe in glassy polymers

Temperature dependencies of ESR spectra of nitroxide spin probes in glassy polymers near and below glass transition temperature were examined in detail. Three temperature ranges, each characterized by specific changes in spectral shape, were defined. (i) In the low temperature range, ESR spectra of nitroxide radical in glassy polymer matrix weakly depend on temperature and remain qualitatively the same. (ii) In the intermediate temperature range, significant changes in the shape of spectra are observed. (iii) A new phenomenon was revealed near and below glass transition temperature: narrowing of linewidths occurs while the ratio of amplitudes of different components varies insignificantly. Analysis of molecular rotational mobility was carried out by means of commonly used empirical approaches. It was shown that widely used formulas and empirical approaches are not applicable for characterization of molecular mobility in glassy polymers. Mechanisms of rotational molecular movements in glassy polymers are discussed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 563–575, 2009     

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.     

Perturbation of phospholipid bilayer properties by ethanol at a high concentration

Atomistic molecular dynamics (MD) simulations have been carried out at 30 degrees C on a fully hydrated liquid crystalline lamellar phase of dimyrystoylphosphatidylcholine (DMPC) lipid bilayer with embedded ethanol molecules at 1:1 composition, as well as on the pure bilayer phase. The ethanol molecules are found to exhibit a preference to occupy regions near the upper part of the lipid acyl chains and the phosphocholine headgroups. The calculations revealed that the phosphocholine headgroup dipoles (P- --> N+) of the lipids prefer to orient more toward the aqueous layer in the presence of ethanol. It is noticed that the ethanol molecules modify the dynamic properties of both lipids as well as the water molecules in the hydration layer of the lipid headgroups. Both the in-plane "rattling" and out-of-plane "protrusion" motions of the lipids have been found to increase in the presence of ethanol. Most importantly, it is observed that the water molecules within the hydration layer of the lipid headgroups exhibit faster translational and rotational motions in the presence of ethanol. This arises due to faster dynamics of hydrogen bonds between lipid headgroups and water in the presence of ethanol.     

Slow dynamics in glassy methyl alpha-l-rhamnopyranoside studied by 1D NMR exchange experiments

It is widely known that the ability of sugar glasses to preserve anhydrobiotic systems in nature is important but the process is not yet fully understood. Molecular motions in the glassy state are likely to be important in the process but until now have remained largely uncharacterized. Here we describe the use of 1D 13C NMR exchange experiments using CODEX (centreband only detection of exchange) methods to study the dynamics of the well characterised model glassy monosaccharide, methyl alpha-l-rhamnopyranoside. The glass was prepared by fast cooling of a melt inside an NMR rotor. Molecular motions in the range of seconds to milliseconds were observed in the glass, whereas identical experiments using the crystalline material displayed no observable motions in the time-scales covered by the experiment. At 13 to 14 K above Tg the nature of the motion in the glass changed probably due to the onset of larger scale reorientation. A bimodal distribution of jump angles combined with a broad distribution of correlation times was found to best represent the observed motions.     

Charge shift and triplet state formation in the 9-mesityl-10-methylacridinium cation

The target donor-acceptor compound forms an acridinium-like, locally excited (LE) singlet state on illumination with blue or near-UV light. This LE state undergoes rapid charge transfer from the acridinium ion to the orthogonally sited mesityl group in polar solution. The resultant charge-transfer (CT) state fluoresces in modest yield and decays on the nanosecond time scale. The LE and CT states reside in thermal equilibrium at ambient temperature; decay of both states is weakly activated in fluid solution, but decay of the CT state is activationless in a glassy matrix. Analysis of the fluorescence spectrum allows precise location of the relevant energy levels. Intersystem crossing competes with radiative and nonradiative decay of the CT state such that an acridinium-like, locally excited triplet state is formed in both fluid solution and a glassy matrix. Phosphorescence spectra position the triplet energy well below that of the CT state. The triplet decays via first-order kinetics with a lifetime of ca. 30 micros at room temperature in the absence of oxygen but survives for ca. 5 ms in an ethanol glass at 77 K. The quantum yield for formation of the LE triplet state is 0.38 but increases by a factor of 2.3-fold in the presence of iodomethane. The triplet reacts with molecular oxygen to produce singlet molecular oxygen in high quantum yield. In sharp contradiction to a recent literature report, there is no spectroscopic evidence to indicate the presence of an unusually long-lived CT state.     

Exponential probe rotation in glass-forming liquids

Using time resolved optical depolarization, we have studied the rotational behavior of molecular probes in supercooled liquids near the glass transition temperature T(g). Simultaneously, the dynamics of the liquid immediately surrounding these rigid probes is measured by triplet state solvation experiments. This direct comparison of solute and solvent dynamics is particularly suited for assessing the origin of exponential orientational correlation functions of probe molecules embedded in liquids which exhibit highly nonexponential structural relaxation. Polarization angle dependent Stokes shift correlation functions demonstrate that probe rotation time and solvent response time are locally correlated quantities in the case of smaller probe molecules. Varying the size of both guest and host molecules shows that the size ratio determines the rotational behavior of the probes. The results are indicative of time averaging being at the origin of exponential rotation of probes whose rotational time constant is slower than solvent relaxation by a factor of 20 or more.     

Molecular dynamics simulations of melting and the glass transition of nitromethane

Molecular dynamics simulations have been used to investigate the thermodynamic melting point of the crystalline nitromethane, the melting mechanism of superheated crystalline nitromethane, and the physical properties of crystalline and glassy nitromethane. The maximum superheating and glass transition temperatures of nitromethane are calculated to be 316 and 160 K, respectively, for heating and cooling rates of 8.9 x 10(9) Ks. Using the hysteresis method [Luo et al., J. Chem. Phys. 120, 11640 (2004)] and by taking the glass transition temperature as the supercooling temperature, we calculate a value of 251.1 K for the thermodynamic melting point, which is in excellent agreement with the two-phase result [Agrawal et al., J. Chem. Phys. 119, 9617 (2003)] of 255.5 K and measured value of 244.73 K. In the melting process, the nitromethane molecules begin to rotate about their lattice positions in the crystal, followed by translational freedom of the molecules. A nucleation mechanism for the melting is illustrated by the distribution of the local translational order parameter. The critical values of the Lindemann index for the C and N atoms immediately prior to melting (the Lindemann criterion) are found to be around 0.155 at 1 atm. The intramolecular motions and molecular structure of nitromethane undergo no abrupt changes upon melting, indicating that the intramolecular degrees of freedom have little effect on the melting. The thermal expansion coefficient and bulk modulus are predicted to be about two or three times larger in crystalline nitromethane than in glassy nitromethane. The vibrational density of states is almost identical in both phases.     

Characterizing molecular motion in H2O and H3O+ with dynamical instability statistics

Sets of finite-time Lyapunov exponents characterize the stability and instability of classically chaotic dynamical trajectories. Here we show that their sample distributions can contain subpopulations identifying different types of dynamics. In small isolated molecules these dynamics correspond to distinct elementary motions, such as isomerizations. Exponents are calculated from constant total energy molecular dynamics simulations of H(2)O and H(3)O(+), modelled with a classical, reactive, all-atom potential. Over a range of total energy, exponent distributions for these systems reveal that phase space exploration is more chaotic near saddles corresponding to isomerization and less chaotic near potential energy minima. This finding contrasts with previous results for Lennard-Jones clusters, and is explained in terms of the potential energy landscape.