Phenanthrene Crystals Research Articles

Heat Capacity Anomaly in Chrysene

Abstract Single crystals of phenanthrene were found to exhibit a heat capacity anomaly and anomalous electrical effect in the vicinity of 72°C.1,2 A heat capacity anomaly was also found in this material in powder form. As a result of calculations3 and other measurements4 it is quite possible that the source of the anomalous behavior of phenanthrene is the crowded hydrogen atoms at the 4,5 positions of the molecule. A calorimetric survey was made of some other hydrocarbons with crowded hydrogens in the hope of finding materials which exhibit similar behavior.

Fluorescence of Doped Crystals of Anthracene, Naphthalene, and Phenanthrene under High Pressures: The Role of Excimers in Energy Transfer to the Guest Molecules

Fluorescence spectra of crystals of anthracene, naphthalene, and phenanthrene doped with other polycyclic aromatic hydrocarbons have been studied at pressures as great as 50 kbar. The most significant effects of pressure on the spectra of the host crystals are a loss of intensity of the normal fluorescence with increasing pressure and the almost simultaneous appearance of a broad, featureless emission at energies about 3000–6000 cm−1 lower than that of the normal fluorescence. This emission is assigned as the fluorescence of excimers formed upon optical excitation of the crystals under high pressure. The growth of the intensity of the excimer emission with increasing pressure, relative to the intensity of the normal fluorescence, is the same in doped crystals as in pure crystals. The intensities of the guest molecule fluorescence relative to the intensity of the normal host fluorescence in the various doped crystals increase with pressure at about the same rate as the intensity of the excimer emission. These observations are interpreted as indicating that excimers are the lowest-energy excited singlet species of aromatic molecular crystals at these pressures and that when the excimer concentration is appreciable, the path of energy transfer is from the host via excimers to the guest.

Excimer Emission of Naphthalene, Anthracene, and Phenanthrene Crystals Produced by Very High Pressures

Fluorescence spectra of ultrapure crystals of naphthalene, anthracene, and phenanthrene have been studied at pressures as great as 50 kbar. These spectra exhibit complex irreversible effects superimposed upon the expected reversible shifts in the energies of the normal fluorescence spectra. The irreversible effects include a loss of the intensity of the normal fluorescence with increasing pressure and the almost simultaneous appearance of a broad, featureless emission at energies about 3000–6000 cm−1 lower than that of the normal fluorescence. This emission is assigned as the fluorescence of excimers formed upon optical excitation of the crystals under high pressure. The crystals continue to exhibit this excimer fluorescence after the pressure is reduced to atmospheric pressure, but the irreversible effects can be removed by thermal annealing of the crystal at atmospheric pressure. The irreversibility is attributed to trapping of pairs of molecules in an excimer-like orientation as a crystal defect after they lose the excitation energy of the excimer. These defects then act as traps for excitation energy in the crystal. Other possible interpretations of the irreversible effects are discussed.

Growth and Decay of Delayed Luminescence

We have investigated the rise lifetimes and decay lifetimes of the long-lived luminescences of the following systems: a naphthalene-ds, a phenanthrene, and a triphenylene solution in a rigid-glass matrix; a phenanthrene-d10, a phenanthrene, a naphthalene-ds, and a chrysene solution in a biphenyl crystal; and chrysene in solution in a phenanthrene crystal. It has been found that the luminescence rise time is invariably shorter than the luminescence decay time. This phenomenon is satisfactorily explicable on the basis of the following attitudes: In rigid glasses, triplet-triplet absorption events contribute to depopulation of the triplet state and decrease the rise time; in mixed crystals, on the other hand, triplet-triplet annihilation events are dominantly responsible for abbreviating the rise time. The effects of excitation intensity and excitation wavelength on the delayed luminescence rise times of rigid-glass solutions, and the effects of excitation intensity, concentration of guest species, and temperature on the delayed luminescence rise times of mixed-crystal systems are discussed. The activation energies for the annihilation process were obtained from a first-order analysis of the luminescence decay and rise times. The agreement between calculated activation energies and T1H − T1G spectroscopic energy gaps is satisfactory and validates the postulated kinetic processes.

Unusual Behaviors in Electrical Conductivity of Phenanthrene Crystal

Abstract Two kinds of unusual behavior in the temperature dependence of electrical conductivity has been observed. One is the anomaly that the resistance begins to increase remarkably with the rise in the temperature around 64°C; the other is a bending-phenomenon in the linear temperature-dependence of the resistance about 54°C. The observed activation energies for conduction are 3.0–3.3 eV below and 4.5–4.8 eV above the anomaly, the energies of which are almost compatible with the intrinsic band-gap energies for conduction, 3.5 and 4.6 eV, estimated, respectively, from the optical absorption and from the values of the ionization potential and the electron affinity. As for below the bending-phenomenon, the observed activation energy is 1.4–1.6 eV, which is quite small compared with the intrinsic band-gap energies estimated. The anomaly beginning around 64°C is interpreted as due to the crystal-phase transition of a higher-order at 60–70°C; the remarkable increase in resistance may be connected with the changes in the intermolecular interaction in the crystal due to the rearrangement of molecules caused by the phase transition. The bending-phenomenon at about 54°C seems to be attributable to an impurity-effect by oxygen.

An X-ray Study on the Phase Transition of Phenanthrene Crystal

Abstract The presence of a higher-order phase transition at around 60–70°C in phenanthrene crystal, previously suggested by the anomalies in electrical conductivity and in thermal expansion, and by the endothermic phenomenon, has been confirmed through an X-ray powder diffraction study. The results disclose the following facts: (1) the phenomena of the transition begin about 45°C and gradually become remarkable with a rise in the temperature, until the phenomena practically cease to occur at 68–69°C; (2) the lattice constants for the monoclinic cell increase conspicuously in the temperature range from about 50°C to 68°C, while both the crystal system, which is monoclinic, and the space group, P21, remain unaltered on passing through the transition temperature, and (3) the unit cell dimensions of the high-temperature modification determined at 80°C are: a=8.54, b=6.25, c=9.55 Å and β=99.0°.

Conductivity and Polarization Phenomena in Phenanthrene Crystals

Dark conductivity and photoconductivity perpendicular to the a—b plane in phenanthrene single crystals were investigated. The dark conductivity shows anomalous behavior in the region of 72°C, the same temperature at which an anomaly in the heat capacity appears. An Arrhenius plot of equilibrium resistivity yields an activation energy of 1.5 eV below 72°C and 1.1 eV above 72°C. When a crystal is cooled from above 72°C in an electric field, a polarization charge is detected which is shown to be a volume rather than a surface phenomenon. When a crystal is illuminated and heated above 72°C in an electric field, it becomes permanently polarized. Electrical-conductivity measurements show that there can be photo-carrier motion in addition to the polarization current. No anomalous behavior was found in x-ray, thermal-expansion, or dielectric measurements.

A Phase Transition in Phenanthrene Crystal

A Phase Transition in Phenanthrene Crystal

The Absorption Spectrum of Phenanthrene Crystals Beyond 3000 Å

Abstract The polarized absorption spectra of phenanthrene and phenanthrene-d10 crystals have been measured at 4°K, 77°K, and room temperature from 33,000 cm−1 to a cutoff at 37,500 cm−1. The absorption, which rises steeply towards shorter wavelengths and is twice as intense in the b-as in the a-polarized spectrum, corresponds to the moderately weak 3000 A system superimposed on the tail of the intense 2500 A system. The former is assigned to a B2—A1, and the latter to an A1—A1 transition. No measurable Davydov splitting was observed in the 3000 A system.

Fluoreszenz und Energieübertragung in Phenanthren-Kristallen

The results of fluorescence studies on phenanthrene single crystals and on mixed crystals of phenanthrene with anthracene or tetracene as guest are the following: Even after extreme zone refining phenanthrene crystals contain impurities which have a strong influence on the fluorescence spectra. The intrinsic fluorescence spectrum of phenanthrene is analyzed with the O.O-band at 28 590 cm-1. The quantum yield is of the order 0.1 and temperature dependent. The quantum ratios for the guest and host emission in the mixed crystals can be explained only partially by an exciton diffusion mechanism. There is a partial quenching of energy transfer around 60°K. The low quantum efficiency and its dependence on temperature and concentration prevent a complete analysis of the energy transfer measurements.

Phosphorescence Lifetime Studies in Some Organic Crystals at Low Temperatures

Triplet-singlet decay curves form naphthalene, phenanthrene, chrysene, durene, and hexamethylbenzene have been studied in the crystalline phase at temperatures of 4°K and higher, and for comparison in rigid glass solutions at 77°K. The decay from true dilute solutions was found to follow an exponential law. At 4°K, the phosphorescence emission from crystals of naphthalene, phenanthrene, and chrysene followed an exponential law, while that from durene and hexamethylbenzene was always nonexponential. Various theories concerning the causes of the nonexponential decay and the change in the general shape of the decay curves with different modes of sample preparation and with temperature are discussed.

First Singlet-Singlet Electronic Transition in the Phenanthrene Molecule and the Band Structure of the Phenanthrene Crystal

The absorption and emission spectra of single crystals of phenanthrene have been studied at 20°K. The data show that the transition moment in the molecule is short-axis polarized, and that the factor group splitting in the crystal is 47 cm—1. The emission originates in a region of very weak absorption at a frequency below the main absorption in analogy to several other molecular crystals whose spectra have been studied. It is suggested that the emitting level corresponds to a distorted lattice site.

Electronic and Vibrational States of Anthracene

The polarized absorption and fluorescence spectra of anthracene have been measured in dilute solid solution in single crystals of naphthalene and phenanthrene at 20°K. The lowest singlet-singlet transition is proven to be polarized along the short molecular axis, and hence 1B2u—1Ag(1La—1A), in agreement with theoretical predictions. Vibrational frequencies are compared in the ground state and in the first excited state. The polarization of the fluorescence of anthracene in phenanthrene has also been studied as a function of temperature. The fluorescence is partially polarized at 300°K, but is depolarized at 20°K.

Polarized Fluorescence of Molecules of Some Single Organic Crystals

The polarized fluorescence of single crystals of pyrene, phenanthrene, and benzil have been studied. It has been observed that, whether the incident light is polarized or not, the fluorescent light is polarized in all three cases. From the study of the intensity of fluorescence light the directions of oscillators, and so the favored condition of excitation of fluorescence with particular reference to the crystal structure, have been determined, and an attempt has been made to find the orientation of the molecules with reference to the crystal axis where the structure is not known.

Indian Association for the Cultivation of Science

THE annual report for the year 1947-48 of the Indian Association for the Cultivation of Science covers the year ending March 31, 1948, and in addition to the usual financial statement and balance sheet there are appended lists of books added to the library during the year, periodicals received by sub scription or exchange, members of the Council and of the Committee of Management and a report on the scientific work of the Association. In the latter Prof. K. Banerjee reports on R. K. Sen's continued study of the extra Laue reflexions of benzil, while over-exposed Laue photographs of pyrene have been taken by M. N. Dutta at 30-100° C. which show that the intensities invariably increase with temperature for all the different spots. M. Ganguly has found that very clear reflexions of the low-angle scattered X-radiation can be obtained by a crystal which is very slightly mis-set from the position for reflecting the direct beam. B. S. Basak has undertaken the complete determination of the structure of phenanthrene crystals by the Fourier analysis method, and, with A. K. Rai Choudhury, has made X-ray diffraction studies of beryllium - aluminium and tin - aluminium systems. X-ray studies of samples of Indian coals have been made by N. N. Gupta, and the mechanism of the formation of jutellite, a plastic from jute waste, is being studied by J. C. Maitra. B. K. Banerjee has continued to study the dispersion of metals and salts in glass, and Dr. A. Bose has undertaken a systematic investigation of crystal paramagnetism to determine whether group structure formed by the internal fields in the solid state persists in solution. A. Dutta has undertaken a systematic study of the magnetic and electric properties of semiconductors, such as tungstenite.