Showing papers in "Molecular Crystals and Liquid Crystals in 1967"

Controlled Transfer of Excitation Energy Through Thin Layers

Abstract: In biological systems the molecules are in a distinct order and entirely new properties are caused by this organization at the molecular level. The synthesis of molecular aggregates of a planned order is an important new task for chemists. The chemist has methods for the synthesis of very complicated molecules but he has no methods for synthesizing aggregates of molecules of a given order or organization, of such systems which we may call organizates. He needs a kind of pincette for picking out single molecules and arranging them in a planned manner. The task of synthesizing organizates may be seen in two aspects: One may try to imitate simple biological structures or one may try to build any planned arrangement of molecules that has useful properties depending on the details of the arrangement of the molecules.

ESR-Untersuchungen an Naphthalin-d8: Naphthalin-h8-Mischkristallen in deren Triplett-Zustand

Abstract: Mixed crystals of perdeuterated naphthalene (N-d8) and normal naphthalene (N-h8) were investigated by epr-absorption during the UV-irradiation at 4.2°K. Besides the well-known epr-lines of the isolated N-h8 molecules in their lowest triplet state one observes in this isotopically dilute system new epr-lines. We call them M-lines. They are due to pairs of N-h8 molecules as nearest neighbours. Caused by exchange interaction the triplet excitation oscillates quickly between the two molecules. The triplet exchange energy of two nearest neighboured N-h8 molecules within the unit cell is found from the line width of the M-lines to be J/hc - (5±0.7) cm−1. Furthermore one observes epr-lines of N-h8 trimers in teh triplet state. Preliminary results are reported concerning the concentration dependence of the relaxation and of the lifetime of the metastable triplet state.

Charge Transfer Sensitization of Some Organic Photoconductors Based on Carbazole

Abstract: The photoresponse of thin layers of vinyl carbazole (VK), dissolved in an acrylic binder, and of polyvinyl carbazole (PVK) was measured in a quasi-xerographic mode. Complexing with strong electron acceptors gives rise to visible charge transfer bands, which are found to be efficient at exciting photoresponse in this part of the spectrum. In the case of the binder layers, complexing greatly enhances UV response, as well. The observed photoresponse is tentatively accounted for by the photosensitized injection of carriers into the dielectric layer, rather than by bulk photoconductivity.

The Orientational Freedom of Molecules in Crystals: I. Joint Interpretation of Thermal and N.M.R. Data in an “All or Nothing” Correlation of Orientation Theory

Abstract: It is shown quantitatively that the Frenkel model (step reorientations among potential wells) is, in most cases, a very good approximation to the motion in crystals presenting a molecular orientational freedom. In contrast to n.m.r., thermodynamics is not sensitive to the dynamies of the motion, but rather to the appearance of the orientational disorder, whatever its rate of rearrangement. A classification accounting for the different observable temperature relations between n.m.r. line width narrowing and thermal data is proposed. This classification is based on the comparison between the symmetry of the molecule and the symmetry of its allowed orientations (potential wells). It applies to crystals of rigid molecules, in thermodynamic equilibrium, in which the correlation of orientation of the molecules is either very large (low temperature) or very small (high temperature).

Kinetics of Crystallization of Anthracene from the Vapor

Abstract: Anthracene was purified by zone melting, chromatography and sublimation. It was again zone melted in a closed system within which it was transferred to a crystal-growth tube. A sublimation apparatus was constructed allowing direct, automatic control of temperature-difference δT between sublimand and crystallization site as well as photography of the growing crystal. In evacuated tubes and in the presence of propane at 1 torr, it was not possible to control nucleation at 120°C. In the presence of N2 at 1 torr, single nuclei were formed at δ T=9.5° ± 0.5° and these were grown to 1 cm diameter crystals of good optical quality. The basal plane of the crystals was the ab plane. Cinematographic observation of growth and evaporation at under-saturations and supersaturations up to 6% showed that growth rate R varies linearly with supersaturation σ. According to the Frank theory, R should be proportional to σ2 for small σ when growth takes place by a single dislocation. Evidently this very unlikely condit...

The Polymorphism of Cholesteryl Esters: Differential Thermal and Microscopic Measurements on Cholesteryl Myristate

Abstract: The techniques of differential thermal analysis and photomicroscopy have been combined to eludicate the complex phase relationships and crystallography of cholesteryl myristate. Also, the amount of polarized light rotated by the samples as a function of temperature has been measured at a fixed sample thickness. Cholesteryl myristate exhibits a complex liquid crystal mesophase system on heating. In addition to the phase transformations obtainable on heating, photomicrographs were obtained of the phases which result on subjecting the smectic and cholesteric mesophases to a very low shear. Excellent correlation was obtained between the present work and earlier studies of the myristate ester. The combined instrumental and microscopic method appears to be a valuable tool in the study of subtle phase changes in complex systems.

γ-Radiation Induced Color Centers in Anthracene

Abstract: The effects of γ-irradiation on pure anthracene have been investigated by optical absorption and electron paramagnetic resonance (EPR) measurements. It is found that the optical absorption peaks which are produced by the radiation at 30°C are stable at 30°C but increase upon annealing up to 140°C; the most prominent of these being at 6060 A and 5350 A. EPR signals arising from irradiation are stable to 140°C but do not grow between 30°C and 140°C. Both optical density and EPR signals decrease upon annealing above 190°C. A model is proposed to explain the nature of the EPR signals based upon cross-linking of the anthracene molecules.

Etude par Résonance Paramagnétique Electronique des Radicaux Libres Produits par Irradiation γ à 77°K de Dérivés Globulaires du Méthane et du Silane

Abstract: In compounds of general formula (CH3)nCCl4-n, γ irradiation at 77°K creates free radicals by C-Cl bond rupture but for (CH3)nSiCl4-n compounds both C-H, Si-C and Si-Cl breakages occur. A study of the e.s.r. spectra of ĉ(CH3)3 in (CH3)3CCl, (CH3)2ĉCl in (CH3)2CCl2 and ĉH2(CH3)2SiCl in (CH3)3SiCl shows the influence of the matrix upon the structure of the trapped free radical. The hyperfine coupling constant found for the 29Si interaction (α29si=416 gauss) in the 29S[icaron]l3 free radical shows that S[icaron]l3 is not a π-planar radical.

The Equivalence of the Two Theories of the Nematic Mesophase When Applied to Magnetic Resonance Experiments

Abstract: The use of nematic liquid crystals as solvents in magnetic resonance experiments provides a potential source of information about the anisotropic interactions. To utilize this potential it is necessary to employ some model of the nematic mesophase in order to describe the ordering of the solute. Both the swarm and the distortion theory have been applied to the problem and in this paper it is shown that they lead to isomathematical conclusions.

Photoconductivity in p-diiodobenzene

Abstract: Castro and Hochstrasser1 have measured the singlet-triplet absorption spectrum of crystalline p-diiodobenzene and interpreted their findings as evidence of strong vibronic coupling between molecular states. Within the band-theory model of excess charge transport this implies a wide conduction band in comparison to other molecular crystals and hence a relatively long mean-free path and high drift mobility of the excess charge. In light of these implications we have undertaken a preliminary study of p-diiodobenzene photoconductivity primarily to measure the drift mobilities. Such data are particularly important at this time because of the uncertain state of charge transport theory in molecular crystals as discussed recently by Glaeser and Berry.2 Our study has indeed revealed relatively high drift mobilities. In addition we have found (1) that the action spectrum for charge generation bears no apparent relation to the known portion of the absorption spectrum of crystalline p-diiodobenzene, and (2) ...

Autoionization and Exciton Annihilation in Anthracene

Abstract: Evidence is presented that the intrinsic carrier generation process in anthracene is one of autoionization (AI) rather than a band-to-band transition. These results are also consistent with the assertion that at least at 6.3 eV, the AI intermediate state is the same, regardless of whether the energy is delivered by a single photon, or by an exciton-exciton process. We have shown that in many cases of “pure” anthracene crystals, there exist impurities that have an effective surface concentration of ∼5 × 10−5 mol fraction, as indicated by exciton quenching properties. These impurities are presumed to be oxygen. The effective exciton lifetime in the surface region is about 5 × 10−9 sec. We have also indicated how the Millikan condenser technique can be used to measure a variety of properties of the exciton.

The Paracrystal as a Model for Liquid Crystals

Abstract: It is emphasized, that the loss of long range order does not necessarily destroy three-dimensional lattices. Nematic or smectic mesophases as well as special types of liquid crystals, many crystalline or amorphous high polymers, crystals near the melting point, molten substances and mixed crystals are examples in nature. The theory of paracrystals describes such lattices quantitatively in a most convenient mathematical way. A very short survey of the main formulas is given, which are needed for a practical application of the theory to X-ray, electron and neutron diffraction problems. The derivations are given in standard texts.1,2 This publication is confined to the discussion of the integral widths of the reflections. Two reasons for line broadening are discussed: paracrystalline distortions and particle size effects. It will be shown how one can separate these two effects and also the effect of thermal oscillations. These results are applied in a separate section to the analysis of paracrystall...

Tetracyanoquinodimethan Derivatives of Cyanine Dyes: A Model for Organic Superconductors?

Abstract: Recently, Little suggested that certain organic materials could act as superconductors at temperatures much higher than the conventional inorganic superconductors. In view of the complicated nature of the proposed compounds it was felt that this idea could only be tested directly if a simpler system could be found. For that purpose a number of TCNQ derivatives of cyanine dyes were prepared and their electrical conductivities were measured in a simple way. It turned out that their conductivities (2.5 × 10−3 to 8 × 10−11 ω−1cm−1) are comparable to those of other TCNQ derivatives. There were no indications of an enhanced conductivity due to the presence of the highly polarizable cyanine cations.

Photoconductivity of Carotenoids

Abstract: Further studies have been made of the photoconductivity of single crystals of all-trans β-carotene. Photoconductivity spectra have also been determined for the related molecules 15–15′ cis β-carotene, lycopene, β-apo-8′-carotenal, β-apo-8′ -apocarotenoic acid ethyl ester and canthaxanthin, as well as a β-carotene glass. The results strongly indicate that a photoconductivity band which occurs well below the absorption edge in these materials is an intrinsic property of this type of molecule. The UV photoconductivity spectrum of 15–15′ cis β-carotene throws some doubt on previous theories concerning the production of free carriers in β-carotene following illumination by strongly absorbed light.

Molecular Solid Solution of Tetraphenyl-porphin and Silver Tetraphenylporphin

Abstract: Molecular solid solutions have received surprisingly little attention in the past, yet we must study them before we can hope to understand why their natural occurrence appears to be so much more limited than that of their inorganic counterparts. Examples might conceivably have escaped attention because single-crystal X-ray work has not been carried out on “impure” molecular crystals. Porphyrins are well suited for solid solution study. Because of their biological importance, their molecular structure is known in detail from numerous refined crystal structures that have appeared in recent years.1 Methods of porphyrin synthesis have been perfected over many years in the laboratory of Professor A. H. Corwin at The Johns Hopkins University.

Des Ondes Électromagnétiques qui ne Satisfont Pas aux Conditions d'Airy Peuvent se Propager dans les Lames Nématiques Hélicoïdales

Abstract: G. B. Airy, in the year 1830, conceived a hypothesis for light propagation in an anisotropic and active medium: two privileged vibrations orthogonally polarized exist. Rigorously resolved, the electromagnetic equations of theories of active anisotropie media show two privileged waves polarized non orthogonally. All the crystalline rotatory powers known are too small for experimental observation. In twisted nematic films the twisting angle is limited to 90°, but the thickness can be arbitrarily fixed. For very small thicknesses the calculation gives a difference betwen ellipticities of two privileged waves attaining a few degrees. Thin layers of nematic phase of p[pacute] azoxyanisole are oriented between two edges of potassium chloride cleavage (100). Some zones with twisting angle of 90° exist. Between elliptical polarizer and analyser in a polarizing microscope with heating stage we can point privileged waves for sensitive tint. Significant differences of a few degrees between ellipticities are...

Generation of Electrons and Holes in Anthracene by Ruby Laser Light

Abstract: The number of carriers generated in an anthracene crystal by a pulse of light from a Q-spoiled ruby laser has been investigated. The number observed far exceeds the number expected on the basis of existing theoretical predictions, varies approximately as the intensity of the laser light cubed and increases exponentially with increasing temperature as e −To/T where T 0=2 × 103 °K. The carriers are generated uniformly throughout the volume of the crystal. A neodymium laser pulse of the same energy density produced no detectable signal so it is concluded that the carriers are not produced by the electric field associated with the light pulse.

The D. C. Conductivities of Some N-Salicylideneanilines

Abstract: A good deal of spectroscopic and structural data are now available on crystalline N-salicylideneanilines. The crystals fall into two groups which are markedly different in both molecular and crystal structures, on the one hand, and in photochemical and spectroscopic properties, on the other. It was considered that these differences should be reflected in a difference in the conduction characteristics of the two groups. A number of members of each group were selected for this study. Conductivity measurements were performed on single crystals in different crystallographic directions. In general, all crystals studied were found to have similar specific resistivities and activation energies for conduction. However, in one case appreciably lower values of these properties were observed. This is believed to be due to suitable orientation of the molecules with respect to the crystal faces to which the electrodes were attached. Overlap calculations for this system are described in an Appendix, and the fa...

The Heat Capacity Anomaly in Phenanthrene; Effects of Deuteration, Purification and Dissolution

Abstract: Arndt and Damask1 reported a heat capacity anomaly of 380 cal/mole in phenanthrene around 70°C. An electrical anomaly occurs at the same temperature and has also been described in detail.2 No explanation as yet exists for these phenomena. Further studies being conducted involve the use of fully deuterated phenanthrene but, since this is available only in limited quantities, some of the purification procedures had to be omitted. It was of interest to see if these omitted procedures suppressed the heat capacity anomaly to any extent, since it has been reported that impurities and radiation suppress the electrical anomaly.2 Accordingly, the heat capacity anomaly of phenanthrene was remeasured under different conditions.

On the Interpretation of the Factor Group Splitting in Naphthalene Crystal

Abstract: In a recent note Sarti-Fantoni1 has discussed the effect of deuteration on the factor group splitting in the lowest excited singlet state of crystalline naphthalene. There are two tentative explanations for the origin of this splitting, one favoring2 higher transition multipole moment couplings, the other suggesting the interaction of nearby charge transfer states with the lowest excited singlet.3 Good agreement is obtained with experiment in both cases. In the first case, both octupole moment parameters are used to fit the spectrum, while in the second case, the position of the first charge transfer state relative to the excited singlet state is used to fit the spectrum. Because in perdeuteronaphthalene the difference in the excitation energy from the ground state to the first excited state (αband) is 118 cm−1 larger than the corresponding transition in the perhydro compound,4 and since the explanation of the splitting invoking charge transfer states requires close proximity of the charge transf...

The Structure of Neat Mesomorphic Phase

Abstract: Optical microscopy, low-angle X-ray diffraction, and electron microscopy have been used to study the lamellar mesomorphic phase (neat phase) given by the sodium 2-ethyl hept-6-enyl sulphate + water system. Good agreement for the interlayer spacing has been obtained from X-ray diffraction measurements and the electron microscopy techniques of surface replication and thin sectioning. Comparable surface replicas have been obtained from anhydrous neat phase and from the quench-frozen mesomorphic phase containing 20% water.

Energy Transfer in Polyacene Solid Solutions VIII: A Bibliography for 1968

Abstract: This bibliography is the seventh in a series. Earlier issues were listed in the preceding issue (VI), which appeared in Molec. Cryst. 3, 1–101 (1967). An outline of the energy transfer process and remarks on the scope of the bibliographies were given in issue IV. Copies of earlier issues are available.

Effect of Ultraviolet Irradiation on Phase Transformations in HMX Polymorphs

Abstract: 1,3,5, 7-Tetranitro-1,3,5,7-Tetrazacyclooctane, commonly known as HMX in the explosives field, exists in four polymorphic forms.1 During a study of solid phase transitions in these polymorphs of HMX, utilizing Differential Thermal Analysis, we have found that prior irradiation with 2537 A light causes the β to δ transformation to occur approximately 15°C below the transition point of unirradiated material. Also, a similar but smaller effect, i.e. a decrease of 5°, is observed after irradiation of β-HMX with 3100–3400 A light which is in the region of the apparent absorption edge. Previous reports of similar effects were not found in the literature.

Photoeletric Characterization of X-Ray Damage in Anthrancene Crystals

Abstract: Most physical effects of moderate levels of ionizing radiation on aromatic organic crystals are not readily observable. Thus anthracene, with a total G-yield for X-rays of less than 1, can be used as a scintillation detector with no decrease of fluorescence efficiency except after extreme exposure to ionizing radiation. The electrical properties of anthracene crystals, however, are markedly affected by thermal neutron bombardment at exposures which are too weak to lower the fluorescence efficiency.1,2 The photocurrent excitation spectrum is altered and shifted to longer wavelengths, photocurrent gain is reduced, and the peaks in thermal-conduction “glow curves” are intensified.1

A Packing Relationship Between 1, 4-Dibenzyloxybenzene and 4-Benzyloxyphenol

Abstract: Molecular compounds of a given homologous series often assume closely related modes of packing in the solid state. Kitaigorodskii1 has called this phenomenon “homologous isomorphism”, and it is readily detectable from unit cell parameters. The molecules must be aligned with, or tilted only slightly from, one of the cell directions. Then two cell dimensions of the members of the homologous series are nearly equivalent in size, while the third reflects a change in the length of the molecule; the space groups of the members of the series are often identical. An example of such a series of compounds is biphenyl, terphenyl and quarterphenyl.2 All crystallize in space group P21/c with both the b and c dimensions within ± 0.1 A of each other; and a, the long axis, measured as 9.39 A, 13.7 A and 17.8 A, respectively.

On the Fluorescence Decay Curve of Anthracene

Abstract: Recent contributions in Phys. Rev. Letters indicate that the interpretation of fluorescence decay experiments in anthracene is by no means clear-cut.1,2 The basic dilemma is how to reconcile the fluorescence experiments of Bergman et al.1 with the photogeneration of free carriers experiments of Silver et al.3 Bimolecular rate constants are derived from both of these experiments which differ by a factor of about 104. Thus, if these experiments are measuring the same basic processes a serious discrepancy exists.