Showing papers in "Molecular Crystals and Liquid Crystals in 1968"
TL;DR: In this article, a review of triplet excitons in anthracene is presented, focusing on the work at room temperature where the triplets are essentially free and where understanding of their properties has progressed the farthest.
Abstract: The investigation of optically generated triplet excitons in anthracene crystals began in 19633 with the discovery that red light from a ruby laser can generate detectable concentrations of triplet excitons in spite of the low probability for this transition. Following this discovery, experimental and theoretical studies of triplet excitons became a much more active area of research in teh field of the organic sold state. Anthracene has been chosen as the organic solid state. Anthracene has been chosen as the model system by the vast majority of the investigators. For this reason this review deals with anthracene almost exclusively. We, furthermore, emphasize the work at room temperature where the triplet excitons are essentially free and where understanding of their properties has progressed the farthest. Only brief mention will be made of low temperature experiments where complicated trapping effects play an essential role, and a great deal of additional research is necessary for gaining a clea...
198 citations
TL;DR: In this article, the high-temperature modifications are dynamically disordered, in agreement with the results of n.m.r. and dielectric studies, and the crystal structure of the phase of t-butyl chloride which is stable between −54 and −90° was determined.
Abstract: Polymorphism of the methylchloromethane compounds, (CH3)n CCI4-n, where n varies from 0 to 4, has been studied using low-temperature X-ray diffraction techniques. Single crystals of both high and low-temperature phases were grown and crystal data were obtained for the various phases. The crystal structure of the phase of t-butyl chloride which is stable between −54 and −90° was determined. On the basis of the present investigation, the conclusion is reached that the high-temperature modifications are dynamically disordered, in agreement with the results of n.m.r. and dielectric studies.
68 citations
TL;DR: In this article, the triplet lifetime was used as a monitor to follow the progress of purification during the zone-refining of natural and synthetic anthracene, and it was found to be a satisfactory initial purification process if a highly pure solvent, e.g. benzene, is used.
Abstract: The triplet lifetime has been used as a monitor to follow the progress of purification during the zone-refining of natural and synthetic anthracene. Chromatography has been found to be a satisfactory initial purification process if a highly pure solvent, e.g. benzene, is used.
36 citations
TL;DR: In this paper, the difference between the ionization energies of various organic molecules both in the vapour and in various crystal lattices was examined, and it was shown that photoelectric emission thresholds of organic crystals, as well as being of intrinsic interest, provide information on the energies of a molecule in the gas (IG ) and in a molecular crystal (Ic ) measures 1 the calculable polarization energy P of the crystal, a key quantity in the donor-acceptor theory of organic semiconductors.
Abstract: This paper examines the difference between the ionization energies of various organic molecules both in the vapour and in various crystal lattices. Photoelectric emission thresholds of organic crystals, as well as being of intrinsic interest, provide information on the energies of the ionization energy of a molecule in the gas (IG ) and in a molecular crystal (Ic ) measures1 the calculable polarization energy P of the crystal, a key quantity in the donor-acceptor theory of organic semiconductors.
36 citations
TL;DR: In this article, the nuclear magnetic resonance linewidths of 23Na have been determined at several compositions in two isotropic solution phases and five mesomorphous phases in the sodium caprylate-decanol-water system.
Abstract: Nuclear magnetic resonance line-widths of 23Na have been determined at several compositions in two isotropic solution phases and five mesomorphous phases in the sodium caprylate-decanol-water system. The results obtained demonstrate that the rate of nuclear magnetic relaxation of 23Na+ is primarily determined by the number of water molecules per sodium ion. this indicates that in all the phases investigated, the sodium ions are predominantly coupled to water molecules. A comparison with 81Br nuclear magnetic relaxation data shows that the bonding of sodium ions is markedly different from that of bromide ions in systems containing association colloids and water. This diversity is found to be due to differences in ion-water coupling.
36 citations
TL;DR: In this paper, it was shown that a low temperature phase of s-triazine exists which conforms with this rule and the transition occurs over a large temperature range below −60° C.
Abstract: s-Triazine has been counted an exception to the fairly general rule taht aromatic compounds with threefold molecular symmetry occupy general positions in the crystal. It is shown that a low temperature phase of s-triazine exists which conforms with this rule. The transition occurs over a large temperature range below −60° C.
31 citations
TL;DR: The real path of the polarized radiation in passing throughout an anisotropic crystal essentially depends on the orientation of the principal dielectric tensor axes with respect to the cleavage plane of the sample.
Abstract: The real path of the polarized radiation in passing throughout an anisotropic crystal essentially depends on the orientation of the principal dielectric tensor axes with respect to the cleavage plane of the sample. Proper elucidation of this fact leads to new expressions for the dichroie ratio of a molecular crystal which contains terms connected both with the orientation of molecular transition moments and with the dielectric tensor axes as well. The agreement of the theory with experiment is illustrated in several examples.
27 citations
TL;DR: Theoretical calculations for the splittings, caused by intermolecular forces, of vibrational spectroscopic transitions in crystalline benzene and naphthalene are presented in this article.
Abstract: Theoretical calculations are presented for the splittings, caused by intermolecular forces, of vibrational spectroscopic transitions in crystalline benzene and naphthalene. Semi-empirical potential functions for the interactions between pairs of molecules are assumed. The interaction between two molecules is further assumed to be composed of central forces acting between pairs of atoms, one on each molecule. Several semi-empirical potentials for hydrogen-hydrogen, hydrogen-carbon and carbon-carbon interactions are used in the computations. The infrared spectra of single crystals of naphthalene have been remeasured, and the observed splittings are compared with the theoretically calcualated values; comparisons are also made for benzene. It is concluded that short-range potentials similar to the ones assumed are primarily responsible for the observed splittings, that both hydrogen-hydrogen and hydrogen-carbon interactions are important, and that carbon-carbon interactions are only of marginal impor...
23 citations
TL;DR: In this paper, the authors considered the problem of recombination in molecular crystals and liquids from the point of view of continuity of current and showed that the probability for escape of an electron from its parent molecule depends exponentially upon how far it travels before thermalization.
Abstract: The problem of recombination in molecular crystals and liquids is considered from the point of view of continuity of current. The probability for escape of an electron from its parent molecule depends exponentially upon how far it travels before thermalization. When account is taken of the temperature dependence of this distance, good agreement with experiment is obtained. Comparison between β particle and photo excitation suggests that initial back recombination between the emitted electron and the parent molecule may not be important in photoconductivity.
21 citations
TL;DR: In this article, Blum, Mattern, Arndt, and Damask1 reported the production of both optically absorbing and paramagnetic species in the gamma radiolysis of crystals of anthracene.
Abstract: Recently Blum, Mattern, Arndt, and Damask1 reported the production of both optically absorbing and paramagnetic species in the gamma radiolysis of crystals of anthracene. They found no such behavior on exposure of this material of 50 Ke V X-rays.
18 citations
TL;DR: In this article, a new technique for evaluating the shape of trap level distribution was proposed, and the current as a function of voltage and temperature indicated in layers examined the existence of an exponential trap-level distribution.
Abstract: Measurements of space-charge limited currents have been made on evaporated thin films of organic compound p-quaterphenyl using metal electrodes. A study of current as a function of voltage and temperature indicated in layers examined the existence of an exponential trap level distribution. The new technique for evaluating the shape of trap level distribution was proposed.
TL;DR: In this article, the authors compared the experimental X-ray diffraction intensities and the radial distribution functions calculated from the intensities at different temperatures for the nematic and isotropic liquid states of the compound p-methoxybenzylidene-p'-cyanoaniline.
Abstract: The nemotogenic compound p-methoxybenzylidene-p'-cyanoaniline in the nematic and isotropic liquid states has been studied by X-ray diffraction. A structure is proposed by comparing calculated intensities of a model with experimental diffraction intensities. The structure is comparable to the “herringbone” packing observed in many organic crystals. The procedure presented in this paper illustrates the practicability of using the method of comparison of experimental intensities with those calculated for a particular structural model. Comparisons of the experimental X-ray diffraction intensities and the radial distribution functions calculated from the intensities at different temperatures indicate an essentially unchanged structure throughout the nematic range but a gross structural change upon passing to the isotropic liquid state.
TL;DR: In this paper, a procedure for growing relatively large anthracene crystals with tetracene uniformly distributed throughout is reported, which is not suited to the task of growing large and uniformly doped crystals.
Abstract: Molecular crystals in general are receiving increased attention and anthracene crystals in particular are in demand for use in various studies. Consequently, several techniques have been developed for growing anthracene crystals–most of which have involved growth from the melt.1–3 In those common method is to use a cooled collector plate.3,4 The principal disadvantage of the latter method is the limited thickness of the crystal that can be obtained, owing to the low thermal conductivity of anthracene. In connection with our various studies of the interaction of energy with anthracene it is often necessary to be able to grow large and uniformly doped crystals. The techniques referred to above are not suited to the task. A procedure for growing relatively large anthracene crystals with tetracene uniformly distributed throughout is reported here.
TL;DR: The second harmonic generation of light (SHG) is used as a probe to study the structure of the nematic, cholesteric or smectic mesophases of various liquid crystals, with non centro-symmetric molecules.
Abstract: Second Harmonic Generation of light (SHG) is used as a probe to study the structure of the nematic, cholesteric or smectic mesophases of various liquid crystals, with non centro-symmetric molecules. In one case (cholesteryl 2-(2-ethoxy ethoxy) ethyl carbonate) where SHG has already been reported, it is shown that SHG occurs in the crystalline rather than the mesomorphic phase. In all cases, no evidence is found of any SHG in any mesophase. Consequently, these are probably centrosymmetric by compensation, as is the normal liquid phase.
TL;DR: In this paper, experimental and conceptual problems associated with the identification of thermotropic mesophases are reviewed and discussed, and the requirements on temperature scanning thermal analysis necessary to insure detection of all mesophase of a given sample are specified.
Abstract: Experimental and conceptual problems associated with the identification of thermotropic mesophases are reviewed and discussed. Theories linking the observed optical properties of mesophases with their molecular building principle have considerable shortcomings. Therefore the identification of mesophases is mainly based on empirical rules pertaining to the optical behavior of characteristic textures. Certain ambiguities of a strictly optical approach may only challenge the ingenuity of the investigator, while other difficulties can prevent the determination of mesomesophase and liquid-mesophase transition temperatures. Since phase transitions are defined by variations of more general thermodynamic quantities, thermal analysis is suggested as a remedy. The requirements on temperature scanning thermal analysis necessary to insure detection of all mesophases of a given sample are specified. The significance and the determination of sample impurities by themal analysis are discussed. Comments are made...
TL;DR: In this article, the authors measured the temperatures of transition for five pure compounds which were known or expected to form mesophases or liquid crystals using differential scanning calorimetry.
Abstract: The heats and temperatures of transitions were measured for five pure compounds which were known or expected to form mesophases or liquid crystals. The five compounds were N-(p-cyanobenzal)-p-anisidine, p-(4-cyanobenzalamino)-cinnamic acid-active amyl ester, N, N'-tereph-thalylidene-di-p-toluidine, p-acetoxy-cinnamic acid, and p-[N-(p-methoxy-benzylidine)-amino]-phenyl acetate. Temperatures of transition for the compounds are reported here for the first time using a well-calibrated differential thermal analyzer. Heats of transitions for the compounds have not been previously reported. They were measured here by differential scanning calorimetry. Tests were made for phase reversibility of the multiple transitions and the possible changes in heats and temperatures of transition with sample origin, i.e., solvent or melt crystallized.
TL;DR: Ashida et al. as discussed by the authors investigated the crystal structures of several metal phthalocyanine thin films and found that the structures in the two types of thin films may have different and have different space groups.
Abstract: In a recent series of papers, Ashida et al.1 investigated the crystal structures of several metal phthalocyanine thin films. Ashida obtained two types of films which he assumed had identical crystal structures but in different orientations. A comparison of Ashida's data with other published data (Honigman2) suggest that the structures in the two types of thin films may be different and have different space groups.
TL;DR: In this paper, the Hartree-Fock wave function was used to calculate the mobility tensor for electron and hole in the tight binding approximation for 95°K and 290°K in anthracene crystal.
Abstract: Components of the mobility tensor for electron and hole have been calculated in the tight binding approximation for 95°K and 290°K in anthracene crystal. The results for the temperature dependence of mobilities are in fair agreement with experiment. It follows from the performed calculations that, considering exchange effects, the application of the Hartree-Fock wave functions instead of those of the STO's (and possibly the potential of neutral hydrogens) should give good agreement with experiment for the anisotropy of the mobilities in both crystallographic planes.
TL;DR: The polymorphism of cyclooctanone and cyclonononanone has been investigated using X-ray diffraction, nuclear magnetic resonance spectroscopy and differential thermal analysis as discussed by the authors.
Abstract: The polymorphism of crystalline cyclooctanone and cyclononanone has been investigated using X-ray diffraction, nuclear magnetic resonance spectroscopy and differential thermal analysis Both compounds undergo phase transitions below room temperature; the crystalline phases stable at room temperature are highly disordered and are isostructural with corresponding phases of several widely differing compounds, including cyclooctane, cycloheptatriene, β-fluorine and γ-oxygen
TL;DR: In this paper, an analysis of the spectrum of deuterated anthracene (C14D9H) and C14D10 powders has been carried out up to a dose of 7 × 108 rad.
Abstract: EPR Studies of radiation damage in anthracene C14H10 and deuterated anthracene (C14D10) powders have been made up to a dose of 7 × 108 rad. Interpretation of the spectra is consistent with that resulting from the hyperfine interaction of three equivalent protons. Excessive breadth of the spectrum of the radical produced in C14D10 is attributed to a high concentration of incompletely deuterated radical resulting from the presence of C14D9H.
TL;DR: In this article, the principal molecular polarizabilities of some molecules of the 4, 4′-di-n-alkyloxy-azobenzenes and of the four, 4´-di]-n-alkyloxoxy-benzenes are calculated of the high frequency dielectric constants e1∞ and e2∞ of the magnetically oriented nematic liquid crystals.
Abstract: The principal molecular polarizabilities of some molecules of the 4, 4′-di-n-alkyloxy-azobenzenes and of the 4, 4′-di-n-alkyloxy-azoxy-benzenes are calculated of the high frequency dielectric constants e1∞ and e2∞ of the magnetically oriented nematic liquid crystals. The increases of the principal polarizabilities of such molecules in complete trans-forms by adding CH2 groups to the alkyl chains are known from an estimation. By comparing these values to the differences between the principal polarizabilities of the investigated molecules it follows that the molecules in the liquid crystals are in a stretched form.
TL;DR: Shubin et al. as mentioned in this paper used pulse radiolysis to map the absorption spectra of short-lived species (typical lifetimes of 10−3-10−5 sec) in ice, and they attrib...
Abstract: Since the successful recording of the transient absorption spectrum of the hydrated electron by Keene1 and Hart and Boag2 using pulse radiolysis, an enormous amount of experimental and theoretical data3 has been presented confirming the existence of the hydrated electron in water (at room temperature). In comparison, however, teh solvated electron in pure ice was identified only recently by Eiben and Taub.4 They irradiated pure ice samples at 77°K with 60Co gamma radiation, recorded and analyzed the absorption spectra of the irradiated samples, and subsequently assigned the observed absorption maximum at 6400 A (1.9 eV) to the hydrated electron. The observed absorption was thermally stable at 77°K and could be photobleached using visible radiation. Working with pure ice samples in the temperature range of 200°K to 260°K, V. N. Shubin et al.5 employed the techniques of pulse radiolysis to map the absorption spectra of short-lived species (typical lifetimes of 10−3-10−5 sec) in ice, and they attrib...
TL;DR: In this paper, the rotational diffusion rates of molecules and of mobile molecule segments in crystalline solids are expressed as relaxation rates relative to the corresponding torsional oscillation rate of the molecule or molecule segment.
Abstract: The rotational diffusion rates of molecules and of mobile molecule segments in crystalline solids are expressed as relaxation rates relative to the corresponding torsional oscillation rate of the molecule or molecule segment. The resulting dimensionless relaxation time τi(ΔH s/I i )1/2 turns out to be a function of packing density (φ∗) of the crystal and of the characteristic temperature fRT/ΔHs , where f is the number of external degrees of freedom per molecule. The dimensionless activation energy ΔE/ΔHs is a uniform function of the packing density for crystals composed of rigid molecules and another function of φ∗ for crystals composed of flexible molecules. The rate determining factor for molecular rotation in crystals containing hydrogen bonds appears to be the breaking energy of the hydrogen bond.
TL;DR: In this paper, the authors proposed a multipole expansion for the singlet exciton states of naphthalene and anthracene and evaluated the results by the Ewald Kornfeld procedure or by direct summation over a sphere of 20-30 A.
Abstract: Calculations of singlet exciton states of the molecular crystals naphthalene and anthracene have been performed by Craig and Walsh,1 Craig and Hobbins,2 and more recently by Silbey, Jortner, Vala and Rice3 and Silbey, Jortner and Rice.4 Theoretical treatments follow the orignial work of Davydov for the interaction of a molecule and the weak crystal field. Since intermolecular overlap in a molecular crystal is small, the interaction between molecules may be represented as a multipole expansion. For an intense transition (f > 1.0) the Craig method retains only dipole terms in the expansion. These dipole sums are evaluated by the Ewald Kornfeld procedure or by direct summation over a sphere of 20–30 A. Silbey et al., however, compute the intermolecular interactions to 55 A using π-electron wavefunctions or dipole summation, and beyond 55 A using dipole sums. The main differences in the results of the treatments are due to the contributions of these long range interactions.
TL;DR: In this paper, the authors make use of the Robinson and Frosch proposals together with the harmonic oscillator representation of the exciton problem to show that damping at a particular site leads to damping of a molecular exciton.
Abstract: The problem of the damping of molecular exciton states in a crystal is considered with respect to the trapping of the exciton at a particular site, and the subsequent radiationless transition within a molecule at the site. Damping considered in this manner has a close connection with the proposals concerning radiationless transitions put forward by Robinson and Frosch. In this paper we make use of the Robinson and Frosch proposals together with the harmonic oscillator representation of the exciton problem to show that damping at a particular site leads to damping of the exciton. The problem we consider is analogous to the damping of the electromagnetic field in a cavity which has been considered by Senitzky. The success of the analogy in this case suggests its further extention to more complicated damping and rate problems.
TL;DR: In this article, the doublet structure in both charge transfer bands of naphthalene-TCNE can be observed at 1 atm and at higher pressures, and the importance of the relative sizes of donor and acceptor in environmental interactions are discussed.
Abstract: : Five aromatic hydrocarbon-tetracyanoethylene (TCNE) complexes were dispersed in sodium chloride pellets and subjected to high pressures (0-25 kbar). The charge transfer absorption was enhanced and displaced toward lower energies at the highest pressure. An exception is the crystalline complex HMB-TCNE which exhibits a blue shift at the highest pressure. The doublet structure in both charge transfer bands of naphthalene-TCNE can be observed at 1 atm and at at higher pressures. The importance of the relative sizes of donor and acceptor in environmental interactions are discussed. (Author)
TL;DR: Two processes for photogeneration of carriers in Anthracene crystals have been proposed, among others, are the interaction of two singlet excitons created by light, and the ionization of one singlet or triplet exciton by absorption of a photon as discussed by the authors.
Abstract: Two processes for photogeneration of carriers in Anthracene crystals, which have been proposed, among others, are the interaction of two singlet excitons created by light, and the ionization of one singlet or triplet exciton by absorption of a photon. Their relative importance is still controversial.
TL;DR: In this article, a spectroscopic and photoelectric study is made of matrix isolated crystallites of phenanthrene in rigid 3-methylpentane (3-MP) at 77°K.
Abstract: A spectroscopic and photoelectric study is made of matrix isolated crystallites of phenanthrene in rigid 3-methylpentane (3-MP) at 77°K. The crystallites appear to be optically thin and exhibit persistent internal polarizaton (PIP) following photosensitization in the phenanthrene absorption region. A variety of studies including lux-ampere experiments strongly support the picture of a bi-excitonic primary charge generation. The charges exhibit lengthy storage properties. They may be directly excited in the near infrared or they may also be excited in the phenanthrene absorption region, the latter probably by an exciton collision. In either case typical polarization and polarization release curves are generated. A small steady photocurrent is also seen which corresponds to electrons passing through the 3-MP matrix. Changes in viscosity of the matrix alters the mobility of the carriers associated with the crystallites. Lux-ampere studies of the peak of the polarization curve show good agreement wit...