Amorphous Organic Solids Research Articles

Aliphatic/Aromatic Systems under Irradiation: Influence of the Irradiation Temperature and of the Molecular Organization

With the aim of understanding the electronic excitation, charge or reactive species transfers occurring during irradiation, we studied the role of the aromatic content on ethylene/styrene random copolymers (PES) and on cyclohexane/benzene glasses (amorphous organic solids). Radiation-induced modifications were monitored in situ, at the molecular level, using Fourier transform infrared spectroscopy (FTIR). Irradiations were performed under a vacuum, and thanks to in situ measurements, oxidation was avoided. We followed both the C═C bond creation in the aliphatic moiety and the destruction of the aromatic moiety. The influence of the irradiation temperature was investigated by irradiating samples at room temperature and at 11 K. At such a low temperature, long-range migration hardly occurs and its influence is considerably reduced or could even vanish. Therefore, low temperature irradiation gives insight on the relative influence of reactive species transport and electronic excitation and charge transport. We found that the effect of lowering the PES irradiation temperature from room temperature to 11 K is small, indicating a minor role for the reactive species transport. Moreover, the two chosen systems allow the examination of the relative magnitude of intra- and intermolecular transfers. We demonstrate that, under conditions where reactive species are almost frozen, intermolecular transfers are very efficient.

Stability of Amorphous Pharmaceutical Solids: Crystal Growth Mechanisms and Effect of Polymer Additives

We review recent progress toward understanding and enhancing the stability of amorphous pharmaceutical solids against crystallization. As organic liquids are cooled to become glasses, fast modes of crystal growth can emerge. One such growth mode, the glass-to-crystal or GC mode, occurs in the bulk, and another exists at the free surface, both leading to crystal growth much faster than predicted by theories that assume diffusion defines the kinetic barrier of crystallization. These phenomena have received different explanations, and we propose that GC growth is a solid-state transformation enabled by local mobility in glasses and that fast surface crystal growth is facilitated by surface molecular mobility. In the second part, we review recent findings concerning the effect of polymer additives on crystallization in organic glasses. Low-concentration polymer additives can strongly inhibit crystal growth in the bulk of organic glasses, while having weaker effect on surface crystal growth. Ultra-thin polymer coatings can inhibit surface crystallization. Recent work has shown the importance of molecular weight for crystallization inhibitors of organic glasses, besides "direct intermolecular interactions" such as hydrogen bonding. Relative to polyvinylpyrrolidone, the VP dimer is far less effective in inhibiting crystal growth in amorphous nifedipine. Further work is suggested for better understanding of crystallization of amorphous organic solids and the prediction of their stability.

Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features

Unidentified infrared emission bands at wavelengths of 3-20 micrometres are widely observed in a range of environments in our Galaxy and in others. Some features have been identified as the stretching and bending modes of aromatic compounds, and are commonly attributed to polycyclic aromatic hydrocarbon molecules. The central argument supporting this attribution is that single-photon excitation of the molecule can account for the unidentified infrared emission features observed in 'cirrus' clouds in the diffuse interstellar medium. Of the more than 160 molecules identified in the circumstellar and interstellar environments, however, not one is a polycyclic aromatic hydrocarbon molecule. The detections of discrete and broad aliphatic spectral features suggest that the carrier of the unidentified infrared emission features cannot be a pure aromatic compound. Here we report an analysis of archival spectroscopic observations and demonstrate that the data are most consistent with the carriers being amorphous organic solids with a mixed aromatic-aliphatic structure. This structure is similar to that of the organic materials found in meteorites, as would be expected if the Solar System had inherited these organic materials from interstellar sources.

Amorphous organic solids as a component of interstellar dust

The discovery of rapid synthesis of complex organic solids in the late stages of stellar evolution has led to a new realization that carbonaceous compounds can be a major significant component of interstellar dust. Signatures of aromatic and aliphatic solids are seen in interstellar clouds as well as the diffuse interstellar medium. Similar features are also seen in the integrated spectrum of galaxies. This has raised the possibilities that many of the unidentified astronomical phenomena such as the diffuse interstellar bands, the 217 nm feature, the extended red emission, the 21 and 30 μm emission features, could also arise from complex organics. In this paper, we discuss the possible chemical structures of these organic solids and the relationships between circumsmtellar and interstellar dust with the organics found in meteorites, asteroids, comets and planetary satellites. The possibility that all these organics share a common origin is discussed.

Effects of Aluminum Sulfate and Polyelectrolyte Solutions on the Geotechnical Properties of Organic Clay

ABSTRACT This paper presents the effects of dilute polyelectrolyte solution used in combination with aluminum sulfate on the geotechnical properties of a homogeneous high-plasticity organic clay derived from the purification of turbid water. Apart from the chemical additives, the alum-polyelectrolyte clay (APC) and untreated clay were similar in composition, comprising 41% micro-fibrous amorphous organic solids by dry mass. However, the geomechanical behavior of the APC material was enhanced by polymer-particle bridging phenomenon, which contributed to higher stress-strain modulus, 10-20% greater shear strength in undrained triaxial compression, a higher effective angle of shearing resistance value, reduced compressibility and a slower rate of consolidation compared to the untreated clay. Hence, this study raises the possibility of using dilute polyelectrolyte solution in combination with aluminum sulfate as an alternative soil improvement measure for organic residues and ultra-soft organic clay deposits.

Polarization Energies in Oligoacene Semiconductor Crystals

Characterization of the electronically polarized environment and the nuclear relaxation that accompanies charge carriers is fundamental to charge transport in crystalline, polycrystalline, and amorphous organic solids. To study the polarization effects of localized charged carriers, we use quantum/classical QM/MM approaches with charge redistribution and polarizable force field schemes and apply them to crystals of naphthalene through pentacene. We describe the results of a comprehensive investigation of the electronic polarization energies in molecular crystal structures of these oligoacenes and discuss as well the evolution of the nuclear relaxation energies calculated for model oligoacene systems.

Photoconduction in Amorphous Organic Solids

Herein, we focus on the principles of photoconduction in random semiconductors-the key processes being optical generation of charge carriers and their subsequent transport. This is not an overview of the current work in this area, but rather a highlight of elementary processes, their involvement in modern devices and a summary of recent developments and achievements. Experimental results and models are discussed briefly to visualize the mechanism of optical charge generation in pure and doped organic solids. We show current limits of models based on the Onsager theory of charge generation. After the introduction of experimental techniques to characterize charge transport, the hopping concept for transport in organic semiconductors is outlined. The peculiarities of the transport of excitons and charges in disorderd organic semiconductors are highlighted. Finally, a short discussion of ultrafast transport and single chain transport completes the review.

Exciton energy disorder in polar amorphous organic thin films: Monte Carlo calculations

We report on Monte Carlo calculations of the exciton energy level disorder in polar amorphous organic solids arising from electrostatic interactions between polarizable molecular charge distributions. In contrast to previous reports in which the discrete molecular polarizabilities are modeled as a dielectric continuum having dielectric constant $ϵ$, our calculations take account of the molecular-scale polarization inhomogeneity. We demonstrate that exciton energy level disorder is only weakly dependent on $ϵ$. For a range of $ϵ$ typical of organic solids, we report an empirical expression for the energy level disorder, and show that conventional dielectric continuum models are not consistent with the observed results. Results of our calculations are shown to be consistent with experimental measurements of exciton energy disorder in aluminum tris-(8-hydroxyquinoline).

Charge Carrier Energy Disorder in Polar Amorphous Organic Thin Films

We report Monte Carlo calculations of the charge carrier energy level disorder in polar amorphous organic solids arising from electrostatic interactions between polarizable molecular charge distributions. In contrast with previous reports in which the discrete molecular polarizibilities are modeled as a dielectric continuum having dielectric constant epsilon, our calculations take account of the molecular-scale polarization inhomogeneity. We find that the previously reported 1/epsilon scaling rule underestimates the charge carrier energy level disorder by more than a factor of 2 for typical values of epsilon. Such an underestimation strongly impacts the analysis of charge carrier transport in polar amorphous organic thin films, due to the strong dependence of the charge carrier mobility on the energy disorder.

Charge-Transfer Complexes and Energetic Disorder in Amorphous Organic Solids

Charge-Transfer Complexes and Energetic Disorder in Amorphous Organic Solids

Trap-controlled charge carrier transport in organopolysilanes doped with trap-forming organic compounds

Charge carrier transport in the poly(phenylmethylsilane), (PhMeSi) x films doped with trap-forming organic compounds with different ionization potentials has been investigated. The carrier transport is discussed on the basis of trapping events in the transport-active (PhMeSi) x host polymer. The field and temperature dependencies of the mobilities have been analyzed in the framework of the Gill's expression, which is widely accepted for the hopping conduction in amorphous organic solids. Its characteristic parameter T o at which the field dependence disappears, has been newly found to depend on both trap depth and concentration. An explicit expression to determine T o has been experimentally derived.

Optical dephasing of impurities in amorphous organic solids down to 0.3 K

The homogenous linewidths of the 0-0 S 1 ← S 0 transitions of various organic molecules in a series of polymers and glasses have been studied by photochemical holeburning. They follow a T ⊥.3±0.1 dependence, which in some systems crosses over to a linear T-dependence at lower temperatures. The lifetime-limited value is actually reached at T≃ 0.3 K for two impurities in a polyethylene host. The results are briefly discussed in relation to theoretical models.

Urbach rule in the optical spectra of crystalline and amorphous organic-solids

In this paper we explore the effects of microscopic structural disorder on the low energy optical absorption edges in some crystalline and amorphous organic solids. We advance a theoretical model which incorporates the simultaneous effects of static disorder and of exciton—phonon coupling on Frenkel exciton states. Explicit theoretical expressions were derived for the self energy and for the absorption lineshapes by the introduction of a two-step effective-field approximation, defining an effective exciton—phonon hamiltonian which was subsequently utilized within the framework of a single site average t-matrix approximation for disorder scattering. Numerical results for the low energy lineshapes over a broad temperature range demonstrate that exciton scattering by a static disorder field results in Urbach type low energy absorption tails, where the Urbach slope is temperature independent.