Molecular Glass Research Articles

Invariant Fast Diffusion on the Surfaces of Ultrastable and Aged Molecular Glasses.

Surface diffusion of molecular glasses is found to be orders of magnitude faster than bulk diffusion, with a stronger dependence on the molecular size and intermolecular interactions. In this study, we investigate the effect of variations in bulk dynamics on the surface diffusion of molecular glasses. Using the tobacco mosaic virus as a probe particle, we measure the surface diffusion on glasses of the same composition but with orders of magnitude of variations in bulk relaxation dynamics, produced by physical vapor deposition, physical aging, and liquid quenching. The bulk fictive temperatures of these glasses span over 35K, indicating 13 to 20 orders of magnitude changes in bulk relaxation times. However, the surface diffusion coefficients on these glasses are measured to be identical at two temperatures below the bulk glass transition temperature T_{g}. These results suggest that surface diffusion has no dependence on the bulk relaxation dynamics when measured below T_{g}.

Tensile Fracture of Molecular Glasses Studied by Differential Scanning Calorimetry: Reduction of Heat Capacity by Lateral Constraint.

Molecular glasses indomethacin and ortho-terphenyl were formed and fractured by cooling a liquid on a less thermally expansive substrate. In-plane tension was created by the mismatch of thermal expansion coefficients and accumulated to cause catastrophic network fracture. Differential scanning calorimetry was used to characterize the process. The heat of fracture exceeds by 10 times the strain energy released, and matches the excess enthalpy stored by an elastic film that is cooled under lateral constraint. The constrained film has a smaller heat capacity than a free-standing film, by approximately 0.01 J/g/K or 1%. This allows the constrained film to reach higher enthalpy on cooling and the excess enthalpy is released at fracture.

Long-term aging behaviors in a model soft colloidal system.

Colloidal and molecular systems share similar behaviors near to the glass transition volume fraction or temperature. Here, aging behaviors after volume fraction up-jump (induced by performing temperature down-jumps) conditions for a PS-PNIPAM/AA soft colloidal system were investigated using light scattering (diffusing wave spectroscopy, DWS). Both aging responses and equilibrium dynamics were investigated. For the aging responses, long-term experiments (100 000 s) were performed, and both equilibrium and non-equilibrium behaviors of the system were obtained. In the equilibrium state, as effective volume fraction increases (or temperature decreases), the colloidal dispersion displays a transition from the liquid to a glassy state. The equilibrium α-relaxation dynamics strongly depend on both the effective volume fraction and the initial mass concentration for the studied colloidal systems. Compared with prior results from our lab [X. Di, X. Peng and G. B. McKenna, J. Chem. Phys., 2014, 140, 054903], the effective volume fractions investigated spanned a wider range, to deeper into the glassy domain. The results show that the α-relaxation time τα of the samples aged into equilibrium deviate from the classical Vogel-Fulcher-Tammann (VFT)-type expectations and the super-Arrhenius signature disappears above the glass transition volume fraction. The non-equilibrium aging response shows that the time for the structural evolution into equilibrium and the α-relaxation time are decoupled. The DWS investigation of the aging behavior after different volume fraction jumps reveals a different non-equilibrium or aging behavior for the considered colloidal systems compared with either molecular glasses or the macroscopic rheology of a similar colloidal dispersions.

Properties of kinetic transition networks for atomic clusters and glassy solids.

A database of minima and transition states corresponds to a network where the minima represent nodes and the transition states correspond to edges between the pairs of minima they connect via steepest-descent paths. Here we construct networks for small clusters bound by the Morse potential for a selection of physically relevant parameters, in two and three dimensions. The properties of these unweighted and undirected networks are analysed to examine two features: whether they are small-world, where the shortest path between nodes involves only a small number or edges; and whether they are scale-free, having a degree distribution that follows a power law. Small-world character is present, but statistical tests show that a power law is not a good fit, so the networks are not scale-free. These results for clusters are compared with the corresponding properties for the molecular and atomic structural glass formers ortho-terphenyl and binary Lennard-Jones. These glassy systems do not show small-world properties, suggesting that such behaviour is linked to the structure-seeking landscapes of the Morse clusters.

Structure-property relationship of blue solid state emissive phenanthroimidazole derivatives.

Seven new derivatives of phenanthro[9,10-d]imidazole having differenet substituents at the 1st and the 2nd positions of the phenanthroimidazole moiety were synthesized and characterized. The comparative study of their properties was performed employing thermal, optical, electrochemical and photoelectrical measurements. The properties of the newly synthesized compounds were compared with those of earlier reported derivatives of phenanthroimidazole and several interesting new findings were disclosed. Density functional theory calculations accompanied by optical spectroscopy measurements have shown the possibility of tuning the emission properties (excited-stated decay rate, fluorescence quantum yield, etc.) of phenanthro[9,10-d]imidazole derivatives via attachment of different substituents to the 1st and the 2nd positions. The most polar and bulky substituents linked to the 2nd position were found to have the greatest impact on the emissive properties of compounds causing (i) fluorescence quantum yield enhancement of dilute liquid and solid solutions (up to 97%), (ii) suppression of intramolecular torsion-induced nonradiative excited-state relaxation in rigid polymer films as well as (iii) inhibition of aggregation-promoted emission quenching in the neat films. Most of the studied compunds exhibited ambipolar charge transport character with comparable drift mobilities of holes and electrons. The highest hole and electron mobilities approaching 10-4 cm2 V-1 s-1 were observed for the derivative having a triphenylamino group at the 1st position of the imidazole ring and the phenyl group at the 2nd position. The estimated triplet energies of phenanthro[9,10-d]imidazole compounds were found to be in the range of 2.4-2.6 eV, which is sufficiently high to ensure effective energy transfer to yellow/red emitters.

Elastic properties of the hydrogen-bonded liquid and glassy glycerol under high pressure: comparison with propylene carbonate

We compare elastic properties of the liquid and glassy glycerol and propylene carbonate as the archetypal molecular glass formers with and without hydrogen bonding.

Why is surface diffusion the same in ultrastable, ordinary, aged, and ultrathin molecular glasses?

Recently Fakhraai and coworkers measured surface diffusion in ultrastable glass produced by vapor deposition, ordinary glass with and without physical aging, and ultrathin films of the same molecular glass-former, N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD). Diffusion on the surfaces of all these glasses is greatly enhanced compared with the bulk diffusion similar to that previously found by others, but remarkably the surface diffusion coefficients DS measured are practically the same. The observed independence of DS from changes of structural α-relaxation due to densification or finite-size effect has an impact on the current understanding of the physical origin of enhanced surface diffusion. We have demonstrated before and also here that the primitive relaxation time τ0 of the coupling model, or its analogue τβ, the Johari-Goldstein β-relaxation, can explain quantitatively the enhancement found in ordinary glasses. In this paper, we assemble together considerable experimental evidence to show that the changes in τβ and τ0 of ultrastable glasses, aged ordinary glasses, and ultrathin-films are all insignificant when compared with ordinary glasses. Thus, in the context of the explanation of the enhanced surface diffusion given by the coupling model, these collective experimental facts on τβ and τ0 further explain approximately the same DS in the different glasses of TPD as found by Fakhraai and coworkers.

Magnetic-field effects in ambipolar transistors based on a bipolar molecular glass

Magnetoresistance effects in ambipolar transistors based on a single amorphous organic thin-film.

Photoactive/Passive Molecular Glass Blends: An Efficient Strategy to Optimize Azomaterials for Surface Relief Grating Inscription.

Irradiation of azomaterials causes various photophysical and photomechanical effects that can be exploited for the preparation of functional materials such as surface relief gratings (SRGs). Herein, we develop and apply an efficient strategy to optimize the SRG inscription process by decoupling, for the first time, the important effects of the azo content and glass transition temperature (Tg). We prepare blends of a photoactive molecular glass functionalized with the azo Disperse Red 1 (gDR1) with a series of analogous photopassive molecular glasses. Blends with 10 and 40 mol % of gDR1 are completely miscible, present very similar optical properties, and cover a wide range of Tg from below to well above ambient temperature. SRG inscription experiments show that the diffraction efficiency (DE), residual DE, and initial inscription rate reach a maximum when Tg is 25-40 °C above ambient temperature for low to high azo content, respectively. Indeed, for a fixed 40 mol % azo content, choosing the optimal Tg enables doubling the SRG inscription rate and increasing DE 6-fold. Moreover, a higher azo content enables higher DE for a similar Tg. Spectroscopy measurements indicate that the photo-orientation of DR1 and its thermal stability are maximal with Tg around 70 °C, independent of the azo content. We conclude that the SRG potential of azomaterials depends on their capability to photo-orient but that the matrix rigidity eventually limits the inscription kinetics, leading to an optimal Tg that depends on the azo content. This study exposes clear material design guidelines to optimize the SRG inscription process and the photoactivity of azomaterials.

X-ray Diffraction Study on Simple Molecular Glasses Created by Low-Temperature Vapor Deposition

The structure of simple molecular glasses has been one of the central issues in the research of glasses. We tried to form the glasses of carbon dioxide (CO2), carbon monoxide (CO), and nitrogen (N2) molecules by slow vapor deposition at 3 K (deposition rate: ca. 10 nm/h). Glassy CO2 was successfully obtained and it crystallized on heating at 25 K, which is lower than the hypothetical glass transition temperature of CO2. The CO2 glass is the simplest molecular glass ever created. The X-ray diffraction patterns and derived pair distribution functions revealed that the glassy CO2 has a structure in which the intermolecular distance is shorter and the intermolecular correlation is much stronger than those of liquid CO2. Orientational glasses, which are positionally ordered and orientationally disordered, of CO and N2 were obtained and they were gradually ordered as the annealing temperature is increased.

A new secondary relaxation in the rigid and planar 1-methylindole: Evidence from binary mixture studies.

Found in our recent dielectric study of a planar and rigid glass-former, 1-methylindole (1MID), is an unusual secondary relaxation unrelated in its dynamic properties to the structural α-relaxation. We speculated that it originates from the in-plane motion of the molecules, and the supposedly universal Johari-Goldstein (JG) β-relaxation with strong connection to the structural α-relaxation in rigid glass-formers is not resolved [X. Q. Li et al. J. Chem. Phys. 143, 104505 (2015)]. In this work, dielectric measurements are performed in binary mixtures of 1MID with two aromatics of weak polarity, ethylbenzene (EB) and triphenylethylene (TPE), in the highly viscous regimes near glass transition. EB and TPE have smaller and larger molecular sizes and glass transition temperatures Tg than 1MID, respectively. Strikingly, the results show that the resolved secondary relaxations of 1MID in the two mixtures share the same relaxation time and their temperature dependence as pure 1MID, independent of the mode and degree of dilution. The results indicate that the unusual secondary relaxation is not directly coupled with the α-relaxation, and support the in-plane-rotation interpretation of its origin. On the other hand, the supposedly universal and intermolecular JG β-relaxation coming from the out-of-plane motion of the planar molecule has weaker dielectric strength, and it cannot be resolved from the more intense in-plane-rotation secondary relaxation because the dipole moment of 1MID lies on the plane.

Indolyl-substituted carbazole derivatives: Electrochemical and photophysical properties and computational studies

ABSTRACTA series of new indolyl-substituted carbazole derivatives were synthesized and their properties were studied. All the final compounds form molecular glasses. They absorb electromagnetic radiation in the range of 200–400 nm and emit in the range of 420–600 nm. The electrochemical properties of the derivatives were found to be determined by the nitro group attached to indole moiety. Relaxation dynamics of transient absorption spectra clearly indicates charge transfer state formation in the solid sample of 5-nitro substituted indolylcarbazole derivative.

Bulk Free‐Radical Copolymerization of n‐Butyl Acrylate and n‐Butyl Methacrylate: Reactivity Ratio Estimation

The reactivity ratios for the bulk free‐radical copolymerization of n‐butyl acrylate (BA)/n‐butyl methacrylate (BMA) are estimated at 80 °C. By performing a series of low conversion runs including replicate runs, the reactivity ratios are estimated as rBA = 0.460 and rBMA = 2.008. Runs to high conversions are then conducted at three different feed compositions (fBMA = 0.2, 0.5, and 0.8) to validate the reactivity ratios. The composition data from the high conversion experiments show good agreement with the estimated reactivity ratios in the integrated form of the Mayo–Lewis model. The molecular weight, gel content, and glass transition temperature of BA/BMA copolymers are also determined. image

Effect of adding nanometre-sized heterogeneities on the structural dynamics and the excess wing of a molecular glass former.

We present the relaxation dynamics of glass-forming glycerol mixed with 1.1 nm sized polyhedral oligomeric silsesquioxane (POSS) molecules using dielectric spectroscopy (DS) and two different neutron scattering (NS) techniques. Both, the reorientational dynamics as measured by DS and the density fluctuations detected by NS reveal a broadening of the α relaxation when POSS molecules are added. Moreover, we find a significant slowing down of the α-relaxation time. These effects are in accord with the heterogeneity scenario considered for the dynamics of glasses and supercooled liquids. The addition of POSS also affects the excess wing in glycerol arising from a secondary relaxation process, which seems to exhibit a dramatic increase in relative strength compared to the α relaxation.

Ultrastable glasses portray similar behaviour to ordinary glasses at high pressure.

Pressure experiments provide a unique opportunity to unravel new insights into glass-forming liquids by exploring its effect on the dynamics of viscous liquids and on the evolution of the glass transition temperature. Here we compare the pressure dependence of the onset of devitrification, Ton, between two molecular glasses prepared from the same material but with extremely different ambient-pressure kinetic and thermodynamic stabilities. Our data clearly reveal that, while both glasses exhibit different dTon/dP values at low pressures, they evolve towards closer calorimetric devitrification temperature and pressure dependence as pressure increases. We tentatively interpret these results from the different densities of the starting materials at room temperature and pressure. Our data shows that at the probed pressures, the relaxation time of the glass into the supercooled liquid is determined by temperature and pressure similarly to the behaviour of liquids, but using stability-dependent parameters.

Derivatives of 2-phenylindole and carbazole as host materials for phosphorescent organic light emitting diodes

Synthesis and thermal, electrochemical, photophysical and charge-transporting properties of the derivatives of carbazole and 2-phenylindole are reported. Compounds with reactive functional groups are emphasized. Two compounds form molecular glasses with the glass transition temperatures of 57 °C and 134 °C. The synthesized compounds absorb electromagnetic radiation in the range of 200–375 nm and emit in the range of 350–550 nm. Their solutions exhibit Stokes shifts up to 81 nm. Their triplet energy levels were found to be in the range of 2.88–3.04 eV. The ionization potentials of the synthesized compounds were found to range from 5.45 eV to 5.88 eV. The ability of photopolymerization in the solid state of the synthesized monomers was demonstrated by ATR-FTIR spectroscopy. The charge transporting properties were studied by the space-charge-limited current (SCLC) method. The zero-field hole mobilities reaching 1.97 × 10−5 cm2 V−1s−1 were observed. According to the results of characterization, two compounds were selected for studying them as the hosts in blue and green phosphorescent organic light emitting diodes. The best fabricated device consisting of indium tin oxide anode, hole-transporting layer, emitting layer with 3-((1-(4-vinylbenzyl)-2-phenyl-1H-indol-3-yl)methyl)-9-ethyl-9H-carbazole as a host and the green emitter, electron transporting layer and calcium layer topped with aluminum layer as cathode, exhibited the maximum current, power, and external quantum efficiencies of 10.3 cd/A, 7.2 lm/W, 2.9%, respectively, in the absence of light out-coupling enhancement. The devices based on the twin derivatives of 2-phenylindolylcarbazolylmethane demonstrated the relatively low values of the turn-on voltages of 3.7 and 3.1 V as well as the efficiency roll-offs of 12.5 and 22.7%.

Long-range correlated dynamics in ultra-thin molecular glass films

It has been previously shown that the free surface of molecular glasses has enhanced surface diffusion compared to the bulk. However, the degree by which the glass dynamics are affected by the free surface remains unexplored. Here, we measure enhanced dynamics in ultra-thin molecular glass films as a function of film thickness. We demonstrate that these films exhibit a sharp transition from glassy solid to liquid-like behavior when the thickness is reduced below 30 nm. This liquid-like behavior persists even at temperatures well below the glass transition temperature, Tg. The enhanced dynamics in these films can produce large scale morphological features during physical vapor deposition and lead to a dewetting instability in films held at temperatures as low as Tg − 35 K. The effective viscosity of these films are measured by monitoring the dewetting kinetics. These measurements combined with cooling-rate dependent Tg measurements show that the apparent activation barrier for rearrangement decreases sharply in films thinner than 30 nm. This sharp transition in the dynamics suggests that long-range correlated dynamics exists in these films such that the enhancement induced by the free surface can strongly affect the dynamics of the film over a length scale that is ten times larger than the size of the molecules.

Granular core phenomenon induced by convection in a vertically vibrated cylindrical container.

A mixture of 13X molecular sieve (13XMS) particles and glass particles with identical diameters is placed in a cylindrical container. Under vertical vibration, heavier glass particles tend to cluster and are wrapped inside the convection of 13XMS particles, resulting in the granular core phenomenon. The vibration frequency f strongly influences particle convection and particle cluster modes. By contrast, the effect of the dimensionless acceleration amplitude Γ can be neglected. For different f ranges, the granular core is classified as center-type and ring-type cores. For the center-type core, heavy particles are distributed as an approximate zeroth-order Bessel function of the first kind in the radial direction and an exponential function in the height direction. For the ring-type core, the concentration of heavy particles follows the power-series function in the radial direction. A granular transport model is then established based on heavy-particle movements under steady state to analyze the effect of vibration parameters and granular convection on density segregation.

Surface diffusion and surface crystal growth of tris-naphthyl benzene glasses

Surface self-diffusion coefficients of α,α,β-tris-naphthyl benzene (TNB) glasses have been measured using the method of surface grating decay. For 1000 nm wavelength gratings, the decay occurs by viscous flow at temperatures above Tg + 15 K, where Tg is the glass transition temperature (347 K), and by surface diffusion at lower temperatures. Surface diffusion of TNB is vastly faster than bulk diffusion, by a factor of 107 at Tg. Comparing TNB with other molecular glasses, each evaluated at its own Tg, we find that surface diffusion has a greater system-to-system variation than bulk diffusion, slowing down with increasing molecular size and intermolecular hydrogen bonding. Experimentally determined surface diffusion coefficients are in reasonable agreement with those from simulations and theoretical predictions. TNB and other molecular glasses show fast crystal growth on the free surface and the growth velocity is nearly proportional to the surface diffusion coefficient, indicating that the process is supported by surface mobility.

Second-order nonlinear optical properties of mexylaminotriazine-functionalized glass-forming azobenzene derivatives

The second-order nonlinear optical coefficients of thin films of mexylaminotriazine-functionalized azobenzene molecular glass derivatives were measured using second harmonic generation. The thin films were poled using a custom corona poling set-up and the second harmonic light from a pulsed 1064-nm laser was detected. Four out of the six tested compounds showed optical nonlinearity and a maximum coefficient of 75 pm/V was obtained. The time dependence of the nonlinear coefficients was studied under ambient light and under dark; the second harmonic generation intensity stayed constant for thiazole-containing derivatives while a significant decay was measured for the other compounds.