O-terphenyl Research Articles

High throughput optimization procedure to characterize vitrification kinetics

We present VITRIFAST, a high throughput optimization procedure to characterize the vitrification kinetics based on calorimetric measurements. By analyzing the temperature dependence of specific heat capacity, the method determines the fictive temperature, Tf, and the enthalpy change during physical aging, ΔH, within only a few seconds. We tested VITRIFAST on the low molecular weight glass-former o−terphenyl (OTP) and on an archetypal glass forming polymer, polystyrene (PS), by analyzing the outcome of two classical sets of experiments. By means of fast scanning calorimetry (FSC), we characterized the vitrification kinetics in a wide range of cooling rates and the isothermal physical aging after vitrification at a given rate. In less than 3 minutes, our method could process 18 different calorimetric scans and provided values of Tf and ΔH in excellent agreement with those reported in the literature. VITRIFAST can be employed in the analysis of the temperature dependence of any type of second order derivative of free energy and represents a tremendous advance in the data analysis of calorimetric scans. The method is particularly helpful for fast scanning calorimetry users, considering the extremely large number of heat capacity scans recorded by this technique within a few minutes.

Accessing the non-ergodicity factor of o-terphenyl via multi-line nuclear γ-resonance time-domain interferometry

ABSTRACT Nuclear γ-resonance time-domain interferometry (TDI) was used to investigate the dynamics of the model glass-former o-terphenyl (OTP) at the timescale and close to the peak of the static structure factor () employing a recently introduced 3-lines scheme. The measured relaxation times (τ) agree with those available in the literature and confirm that in OTP the dynamics are dominated by the structural relaxation at the inter-molecular length-scale and by the Johari–Goldstein () relaxation at the intra-molecular one. The experimental scheme here employed presents the distinct advantage of providing access to the non-ergodicity factor () on an absolute scale. The extracted values are qualitatively consistent with those derived in the previous neutron scattering experiments, with a very similar wavevector, q, and temperature dependence. Our data thus further establish the capability of multi-line TDI to measure the absolute strength of relaxation processes.

Estrogenic activity of multicyclic aromatic hydrocarbons in rainbow trout (Oncorhynchus mykiss) in vitro assays

A representative group of multicyclic aromatic hydrocarbons (MAHC) which can be further classified as bridged-ring (bridged-MAHC) or fused-ring (fused-MAHC) were examined for their ability to interact with the estrogen receptor of rainbow trout (rtER) in a hepatic cytosolic estrogen receptor competitive binding assay (cyto rtERαβ) and the vitellogenin (Vtg) mRNA gene activation liver slice assay. All five fused-MAHCs; naphthalene (NAFT), fluorene (FE), Fluoranthene (FAT), pyrene (PY), and 9,10-dihydroanthracene (DAC) had no estrogenic activity in the in vitro assays used. Five of the eight bridged-MAHCs; triphenylethylene (3PE), o-terphenyl (OTP), triphenylmethane (TPM), 1,1-diphenylethylene (DPE), and cis-stilbene (CSB) were positive in the rtER-binding assay. The additional three bridged-MAHC’s; trans-stilbene (TSB), tetraphenylethylene (4PE), and 4,4-di-tertbutylphenyl (DtBB) were determined to be non-binders due to isomeric configuration, solubility limitation, and possible steric hinderance. It is possible that the bridged-MAHCs bind to the rtER through a proposed aromatic-aromatic stacking (π-π interaction) facilitated by perpendicular ring orientation achieved through free rotation of the bridged rings. The fused-ring structures are locked in a planar configuration which doesn’t allow for rotation of rings perpendicular to one another. This first report of the rtER-binding of bridged-MAHCs in fish demonstrates binding for a class of chemicals normally not thought of as having an affinity for the estrogen receptor and further supports the versatility or promiscuity of ER ligand selectivity.

Effect of Low-Concentration Polymers on Crystal Growth in Molecular Glasses: A Controlling Role for Polymer Segmental Mobility Relative to Host Dynamics.

Low-concentration polymers can strongly influence crystal growth in small-molecule glasses, a phenomenon important for improving physical stability against crystallization. We measured the velocity of crystal growth in two molecular glasses, nifedipine (NIF) and o-terphenyl (OTP), each doped with four or five different polymers. For each polymer, the concentration was fixed at 1 wt % and a wide range of molecular weights was tested. We find that a polymer additive can strongly alter the rate of crystal growth, from a 10-fold reduction to a 10-fold increase. For a given polymer, increasing molecular weight slows down crystal growth and the effect saturates around DP = 100, where DP is the degree of polymerization. For all the systems studied, the polymer effect on crystal growth rate forms a master curve in the variable (Tg,polymer - Tg,host)/Tcryst, where Tg is the glass transition temperature and Tcryst is the crystallization temperature. These results support the view that a polymer's effect on crystal growth is controlled by its segmental mobility relative to the host-molecule dynamics. In the proposed model, crystal growth rejects impurities and creates local polymer-rich regions, which must be traversed by host molecules to sustain crystal growth at rates determined by polymer segmental mobility. Our results do not support the view that host-polymer hydrogen bonding plays a controlling role in crystal growth inhibition.

Surface Mobility of Amorphous o-Terphenyl: A Strong Inhibitory Effect of Low-Concentration Polystyrene.

Previous work has shown that a surface wave on amorphous o-terphenyl (OTP) decays by viscous flow at high temperatures and by surface diffusion at low temperatures. We report that the surface mass transport can be efficiently suppressed by low-concentration polymers. Surface-grating decay has been measured for OTP containing 1 wt % polystyrene (PS, Mw = 1-8 kg/mol), which is miscible with OTP. The additive has no significant effect on the decay kinetics in the viscous-flow regime, but a significant effect in the surface-diffusion regime. In the latter case, surface evolution slows down and becomes nonexponential (decelerating over time). The effect increases with falling temperature and the molecular weight of PS. These results are attributed to the very different mobility of PS (slow) and OTP (fast) and their segregation during surface evolution, and relevant for understanding the surface mobility of multicomponent amorphous materials.

Structural and solvent control of nonadiabatic photochemical bond formation: photocyclization of o-terphenyl in solution.

Elucidating the molecular dynamics that underlie photoinduced electrocyclization is a critical step toward controlling nonadiabatic photochemistry that enables bond formation. Here we present a comprehensive examination of the photochemical dynamics of o-terphenyl (OTP) in solution. Ultrafast transient absorption measurements demonstrate that OTP cyclizes upon 266 nm photoexcitation to form 4a,4b-dihydrotriphenylene (DHT) on a solvent-dependent time scale of 1.5-4 ps, considerably slower than the nonadiabatic cyclization of related diarylethenes. Correlations in these time scales versus bulk solvent properties reveal that mechanical rather than electrostatic solvent-solute interactions impact the excited-state relaxation rate, impeding nuclear dynamics leading toward the conical intersection for cyclization. In contrast, solvent-dependent mechanical interactions are observed to facilitate vibrational relaxation of DHT on time scales of 10-25 ps. DHT decays via thermally activated ring-opening with a lifetime of ∼46 ns in tetrahydrofuran, 12 orders of magnitude faster than dihydrophenanthrenes. We conclude that the differences in excited-state dynamics of OTP and diarylethenes and the relative stability of their cyclized products are determined by the relative strain induced by twisting the central carbon-carbon bond that bridges the terminal phenyl rings in each to enable bond formation. We relate these structure-dynamics relationships to the feasibility of photoinduced cyclodehydrogenation of o-arenes and design considerations for molecular photoswitches.

Ultrafast Excited-State Dynamics of ortho-Terphenyl and 1,2-Diphenylcyclohexene: The Role of "Ethylenic Twisting" in the Nonadiabatic Photocyclization of Stilbene Analogs.

Nonadiabatic photocyclization is the fundamental step underlying photoswitching and light-assisted bond formation within diarylethylenes, yet the details of the nuclear dynamics leading to cyclization remain unclear. We have examined the ultrafast excited-state dynamics of o-terphenyl (OTP) and 1,2-diphenylcyclohexene (DPCH) in solution to determine how variation in structural constraints impacts the course of nonadiabatic photocyclization specifically in stilbenoids. Measured spectral dynamics reflect cyclization through a S1-to-S0 transition for both systems on picosecond time scales, with excited-state decay appreciably faster for DPCH versus OTP. Supportive ab initio calculations reveal a higher energetic penalty in OTP versus DPCH for reaching the lowest-energy conical intersection from the S1 minimum; this penalty is associated primarily with twisting about the carbon-carbon bond that bridges terminal phenyl groups, a structural change that has a critical role in nonadiabatic cis-trans isomerization of diarylethylenes. Findings provide a new experimental perspective on the elusive nuclear dynamics underlying cis-stilbene photocyclization.

NVU dynamics. III. Simulating molecules at constant potential energy

This is the final paper in a series that introduces geodesic molecular dynamics at constant potential energy. This dynamics is entitled NVU dynamics in analogy to standard energy-conserving Newtonian NVE dynamics. In the first two papers [T. S. Ingebrigtsen, S. Toxvaerd, O. J. Heilmann, T. B. Schrøder, and J. C. Dyre, J. Chem. Phys. 135, 104101 (2011)10.1063/1.3623585; T. S. Ingebrigtsen, S. Toxvaerd, T. B. Schrøder, and J. C. Dyre, J. Chem. Phys. 135, 104102 (2011)10.1063/1.3623586], a numerical algorithm for simulating geodesic motion of atomic systems was developed and tested against standard algorithms. The conclusion was that the NVU algorithm has the same desirable properties as the Verlet algorithm for Newtonian NVE dynamics, i.e., it is time-reversible and symplectic. Additionally, it was concluded that NVU dynamics becomes equivalent to NVE dynamics in the thermodynamic limit. In this paper, the NVU algorithm for atomic systems is extended to be able to simulate the geodesic motion of molecules at constant potential energy. We derive an algorithm for simulating rigid bonds and test this algorithm on three different systems: an asymmetric dumbbell model, Lewis-Wahnström o-terphenyl (OTP) and rigid SPC/E water. The rigid bonds introduce additional constraints beyond that of constant potential energy for atomic systems. The rigid-bond NVU algorithm conserves potential energy, bond lengths, and step length for indefinitely long runs. The quantities probed in simulations give results identical to those of Nosé-Hoover NVT dynamics. Since Nosé-Hoover NVT dynamics is known to give results equivalent to those of NVE dynamics, the latter results show that NVU dynamics becomes equivalent to NVE dynamics in the thermodynamic limit also for molecular systems.

Effects of lowering temperature and raising pressure on the spatially heterogeneous dynamics of glass-forming van der Waals liquids

Here we study how changes in temperature and pressure affect the dynamic heterogeneity in glass formers by using dielectric, volumetric, and heat capacity experimental data for two typical supercooled van der Waals liquids, 1,1\ensuremath{'}-bis ($p$-methoxyphenyl) cyclohexane (BMPC) and $o$-terphenyl (OTP). We demonstrate that these thermodynamic variables are not equivalent. We find that changes in temperature exert a stronger influence on heterogeneous dynamics than those in pressure, and the degree of heterogeneity in different thermodynamic conditions is mirrored in some way in corresponding structural relaxation times.

Isomorphs in Model Molecular Liquids

Isomorphs are curves in the phase diagram along which a number of static and dynamic quantities are invariant in reduced units (Gnan, N.; et al. J. Chem. Phys.2009, 131, 234504). A liquid has good isomorphs if and only if it is strongly correlating, i.e., if the equilibrium virial/potential energy fluctuations are more than 90% correlated in the NVT ensemble. Isomorphs were previously discussed with a focus on atomic systems. This paper generalizes isomorphs to liquids composed of rigid molecules and study the isomorphs of systems of small rigid molecules: the asymmetric dumbbell model, a symmetric inverse power-law dumbbell, and the Lewis–Wahnström o-terphenyl (OTP) model. For all model systems, the following quantities are found to a good approximation to be invariant along an isomorph: the isochoric heat capacity, the excess entropy, the reduced molecular center-of-mass self-part of the intermediate scattering function, and the reduced molecular center-of-mass radial distribution function. In agreement with theory, we also find that an instantaneous change of temperature and density from an equilibrated state point to an isomorphic state point leads to no relaxation. The isomorphs of the Lewis–Wahnström OTP model were found to be more approximative than those of the asymmetric dumbbell model; this is consistent with the OTP model being less strongly correlating. The asymmetric dumbbell and Lewis–Wahnström OTP models each have a “master isomorph”; i.e., the isomorphs have identical shape in the virial/potential energy phase diagram.

Universal scaling between structural relaxation and caged dynamics in glass-forming systems: Free volume and time scales

It is shown that the Debye–Waller factor (DW), a measure of the cage dynamics, is contributed by free-volume in o-terphenyl (OTP) and glycerol. An elementary ansatz provides an alternative way to get the reduced DW from Positron Annihilation Lifetime Spectroscopy (PALS). The ansatz supports the scaling of the slow relaxation with the fast caged dynamics over about ten decades in relaxation times in OTP and glycerol. Both PALS and neutron scattering experiments show that, in order to evidence the scaling, the observation times must be shorter than the time scales of the relaxation processes.

Does Brillouin light scattering probe the primary glass transition process at temperatures well above glass transition?

The primary alpha-relaxation time (tau(alpha)) for molecular and polymeric glass formers probed by dielectric spectroscopy and two light scattering techniques (depolarized light scattering and photon correlation spectroscopy) relates to the decay of the torsional autocorrelation function computed by molecular dynamics simulation. It is well known that Brillouin light scattering spectroscopy (BLS) operating in gigahertz frequencies probes a fast (10-100 ps) relaxation of the longitudinal modulus M*. The characteristic relaxation time, irrespective of the fitting procedure, is faster than the alpha-relaxation which obeys the non-Arrhenius Vogel-Fulcher-Tammann equation. Albeit, this has been noticed, it remains a puzzling finding in glass forming systems. The available knowledge is based only on temperature dependent BLS experiments performed, however, at a single wave vector (frequency). Using a new BLS spectrometer, we studied the phonon dispersion at gigahertz frequencies in molecular [o-terphenyl (OTP)] and polymeric [polyisoprene (PI) and polypropylene (PP)] glass formers. We found that the hypersonic dispersion does relate to the glass transition dynamics but the disparity between the BLS-relaxation times and tau(alpha) is system dependent. In PI and PP, the former is more than one order of magnitude faster than tau(alpha), whereas the two relaxation times become comparable in the case of OTP. The difference between the two relaxation times appears to relate to the "breadth" of the relaxation time distribution function. In OTP the alpha-relaxation process assumes a virtually single exponential decay at high temperatures well above the glass transition temperature, in clear contrast with the case of the amorphous bulk polymers.

Comment on “Hyperquenched Glassy Water and Hyperquenched Glassy Ethanol Probed by Single Molecule Spectroscopy”

Comment on “Hyperquenched Glassy Water and Hyperquenched Glassy Ethanol Probed by Single Molecule Spectroscopy”

Dynamical Heterogeneity in Glassy o-Terphenyl. 2. Measurement of Environment Structure Lifetime Using Reversible Reactions

A new method is proposed for measuring the lifetime of heterogeneities in a glassy matrix. UV-vis absorption spectroscopy has been used to monitor the kinetics of photoinduced cis-->trans isomerization of 1-naphthyl-azomethoxybenzene (NAMB) in o-terphenyl (OTP) below T(g). The dependence of isomerization rate on the duration of dark interval after generation of cis molecules was established; an increase in the dark interval causes a decrease in isomerization rate. This dependence is shown to be due to the change in the local environment of NAMB molecules. The time required for the environment to change was estimated over a temperature range of 234 (T(g) - 9 K) to 241.5 K (T(g) - 1.5 K). The change in the environment of the guest molecules has been interpreted in terms of the exchange processes. The values obtained for the exchange time coincide with the rotation times of the NAMB molecule. Also, it is shown that the time of structural rearrangement of the environment near the fast reacting molecules is of the same order of magnitude as the time of structural rearrangement of the environment near the slow reacting ones.

Possible origin of enhanced crystal growth in a glass

Contrary to the common belief that a glassy state is stable against crystallization, Oguni and co-workers discovered unusual enhancement of the crystal growth rate of a few molecular liquids below the glass transition temperature ${T}_{g}$. We studied this phenomenon using $o$-terphenyl (OTP) and phenyl salicylate (salol), focusing on the roles of volume contraction $\ensuremath{\Delta}V$ upon crystallization. We confirmed enhancement of crystal growth below ${T}_{g}$ for OTP. For salol, which has two kinds of crystal, the crystal growth rate below ${T}_{g}$ is faster for a crystal of larger $\ensuremath{\Delta}V$ than for another crystal of smaller $\ensuremath{\Delta}V$. Our results suggest the following physical scenario for the phenomenon: for a material having large $\ensuremath{\Delta}V$, the volume contraction upon crystallization provides a crystal-glass interface with large excess free volume, which results in the mobility increase at the growth front and leads to enhancement of the crystal growth. This mechanism may be effective only below ${T}_{g}$, where density fluctuations cannot be quickly relaxed by hydrodynamic transport.

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.

Entropy basis for the thermodynamic scaling of the dynamics ofo-terphenyl

Structural relaxation times and viscosities for non-associated liquidsand polymers are a unique function of the product of temperature,T, times specific volume,V, with the latterraised to a constant, γτ.Similarly, for both neat o-terphenyl (OTP) and a mixture, the entropy for differentT andpressure, P, collapses to a single curve when expressed versusTVγS, with the scaling exponent for the entropy essentially equal to the thermodynamicGrüneisen parameter. Since the entropy includes contributions from motions such asvibrations and secondary relaxations, which do not affect structural relaxation,γS<γτ.We show herein that removal of these contributions gives a satisfactory account of the magnitude ofγτ. Moreover, the relaxation times of OTP are found to be uniquelydefined by the entropy, after subtraction from the latter of aV-independent component.

Heterogeneities at short and long time scales of the dynamics of a probe molecule in different model glass formers

The correlations, namely dynamic heterogeneity, and different relaxation time and length scales were investigated in two model glass formers. Decoupling phenomena were observed on the transport coefficients, which would correspond to different averages over these heterogeneities. The temperature dependence of the rotational dynamics of a nearly cylindrical molecular tracer in phenyl salicylate (SALOL) was investigated over the short time scale of the Longitudinal Detected Electron Spin Resonance spectroscopy (LODESR). A comparison with the dynamics of the same probe dissolved in either o-terphenyl (OTP) or SALOL is also reported on the short and long time scales of the rotational relaxation, the latter being provided by linear Electron Spin Resonance (ESR) studies. The rotational dynamics, at short and long times, exhibited a similar behavior in both glass formers with exponential relaxation and coupling to the viscous flow at higher temperatures, and with selective decouplings on cooling. Interestingly, by proper scaling, the dynamics at long times and the viscosity of the host matrices collapse in a single curve in a wide temperature range. The scaling does not work at short times.

Response to “Comment on ‘On the dielectric susceptibility spectra of supercooled o-terphenyl’” [J. Chem. Phys. 123, 154502 (2005)

The recent claim of time-temperature superposition (TTS) for o-terphenyl (OTP) in its viscous regime is revisited in order to clarify whether peak versus area normalization of the dielectric loss profiles impacts the results derived from the master plot. In the case of OTP, noise is the main source of differences between the two options. Using synthetic noisy data, it is shown that an area normalized master plot is more sensitive to deviations from TTS than the peak normalized counterpart. As a result, OTP is characterized by a stretching exponent β=0.50±0.03 in the 248–284K temperature range.

Rotational Mobility of Guest Molecules in o-Terphenyl below Tg

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.