Anthracene Single Crystals Research Articles

Effect of crystal defects on triplet-triplet annihilation in anthracene

The rate constants of triplet-triplet annihilation were measured in anthracene single crystals with various degrees of damage. It was found that, as with the single-excitonic triplet decay rate, the double-excitonic triplet decay rate is severely affected by the presence of crystal defects. The experimental results were analyzed kinetically in the framework of triplet exciton trapping at crystal defects.

Temperature and Wavelength Dependence of the Fluorescence Lifetime of Anthracene Crystals

The effect of temperature (from 10 K to 300 K), crystal thickness, and fluorescence wavelength (at 10 and 300 K), on fluoresence lifetime for anthracene single crystal is studied. The effect was attributed to 1) reabsorption due to change of lifetime with thickness and 2) two guest fluorescing levels in thermal equilibriumn with the singlet exciton. This is in agreement with some of the reported results. The effect of fluorescence wavelength was interpreted in terms of the suggested time dependent reabsorption model. This model has been confirmed with the results obtained at two different temperatures.

Observation of Anthracene Single Crystals with Delayed Fluorescence Tomography

Abstract Anthracene single crystals are studied with delayed fluorescence tomography. This method allows the observation of the interior of thick ‘as grown’ crystals. It is shown that the spatial distribution of impurities and inclusions can be observed. Limitations and possible improvements are discussed.

Photoelectrochemically deposited silver clusters on the ab surface of anthracene single crystals

Electron micrographs and size distribution curves are presented for silver clusters formed by the photoelectrochemical deposition of silver atoms onto the surface of pure single crystals of anthracene. The clusters were resolved to a diameter of 2 nm. For comparison clusters were also deposited using a cluster source in vacuum.

The reduction of grown-in dislocation density in anthracene crystals due to heat treatment and growth conditions

Two kinds of experiments were carried out to improve the perfection of anthracene single crystals: (1) dependence of the grown-in dislocation density on crystal size and (2) heat treatment. Non-basal dislocation densities were measured by an etch-pit technique. The results are discussed in the light of the motion of non-basal dislocations.

Picosecond transient grating measurements of singlet exciton transport in anthracene single crystals

Picosecond transient grating experiments are used to measure the rate of exciton transport in anthracene single crystals. The method permits a direct measurement of the transport by introducing an accurate distance scale into the sample. Results from transient grating experiments at 10 K are presented, and the diffusion constant along the â crystal axis for the anthracene singlet exciton is reported. Preliminary studies of the temperature dependence of the exciton diffusion constant are also discussed.

On the microhardness of some organic molecular solids

AbstractMicrohardness measurements have been carried out on the (001) cleavage surfaces of anthracene, Phenanthrene and benzoic acid single crystals. The variation of the Vicker's Hardness Number (VHN) with load (P) shows a decrease in hardness with increasing load. Plots of load (P) vs the square of the diagonal (d2) and logarithm of load (log P) vs logarithm of diagonal (log d) are found to be linear. These observations are being explained in terms of the geometrical packing of molecules in these crystals which play an important role in causing slip.

AC modulation of the recombination electroluminescence in anthracene single crystal

The recombination electroluminescence (REL) in an anthracene single crystal has been modulated by ac voltage at different dc biasing voltages. The frequency and ac voltage dependences observed are interpreted in terms of the presented model of sinusoidal current wave. The dc-voltage-induced changes in the modulation are explained by the voltage variation of the triplet exciton lifetime and wavelength of the current wave based on concomitant evolution of the triplet-to-singlet exciton concentration ratio responsible for the contribution of delayed and prompt components of the REL.

Phonon‐assisted decay of excitons in anthracene crystals

AbstractThe reflection spectra, R(ω), are investigated from the (201) face of anthracene single crystals at low temperatures (4.2 to 60 K). Absence of surface effects makes possible to judge from the reflection curve on the spectral dependence of the decay parameter, γ, due to exciton‐phonon interaction. The calculation of γ(ω) is made by use of a nomogram graphically representing the functional relation between the values R, ω, and γ. It is shown that the decay parameter is essentially frequency dependent at low temperatures. On the basis of the calculated γ(ω) the mean value is estimated of the interaction energy of excitons with acoustic and optical phonons in anthracene crystals.

Zone growth: A simple procedure for growing organic crystals from the melt of from the vapour phase

Two variants of a simple procedure for growing ultrapure organic crystals within a zone-refining tube are described. The method is based upon zone growth from the melt or vapour, in an inert atmosphere. Single crystal of anthracene and 1-naphthol have been prepared.

Nature of the Electron‐Hole Pair in the System Anthracene Crystal/Rhodamine B Adsorbed from Aqueous Solution Part II: Dependence of Kinetics and Spin‐Dynamics on Shallow Surface Traps

AbstractThe dependence of the rhodamine B sensitized hole current (SHC) and of its magnetic field modulation (ΔSHC) on the applied electric field has been measured in anthracene single crystals with different concentrations of shallow surface traps. Our theoretical model for the influence of the magnetic hyperfine effect (HF effect) on the charge carrier kinetics has been extended to account for the effect of shallow traps. The model predicts in agreement with the experimental finding as effect of shallow surface traps, firstly an increase in the magnitude and secondly an increase in the halfwidth of the magnetic field modulation of the SHC, and thirdly a weakened influence of the electric field on these two signals. The smallest concentration of shallow surface traps was found on naturally grown surfaces of sublimation crystals, whereas solution grown crystals showed in general a higher concentration of such traps. It was found that shallow hole traps can be induced with carbazol molecules.

Time-resolved measurement of the escape of charge carriers from a Coulombic potential well by diffusional motion

The irreversible escape of charge carriers from the bottom of a coulombic potential well over the barrier by diffusional motion has been measured, for the first time, directly as a rise time of 1 ns time-resolved current transients in anthracene single crystals with electrolytic contacts. The observed steep decrease in the characteristic escape time down to 1 ns, with rising electric field up to 1×105 V cm−1, is in excellent agreement with the predictions of a stationary one-dimensional charge carrier injection model with generalized boundary conditions. The electric field dependence of the rate constant for escape cannot be approximated simply by an exponential dependence on the barrier height, as in the familiar Schottky correction for the case of electron emission or in the Poole–Frenkel formula for change carrier escape from a charged trap. As a side result, the hole mobility in the c′ direction of anthracene single crystals has been found constant μ = 0.85±0.05 cm2 V s−1 up to an electric field strength of 5±105 V cm−1 at room temperature.

Etching characteristics of anthracene single crystals in sulphuric acid-acetic acid solutions

Etching characteristics of anthracene single crystals in sulphuric acid-acetic acid solutions

Excitation spectra of single crystals of anthracene on various substrates

Excitation spectra of single crystals of anthracene on various substrates

Generation of hot and complex charge carriers due to the interaction between excitons and free charge carriers in molecular crystals

Applying microscopic many-body theory, the consequences of interaction between a free Frenkel exciton and free electron present in molecular crystal are investigated. Two kinds of reactions are possible: (a) a hot electron is created and (b) a complex charge carrier (exciton+electron) is formed. The rate of reaction (a) between singlet exciton and electron in anthracene single crystal is obtained as ∼7×109 sec−1, in agreement with the rate calculated from experiments. The possibility of reaction (b) is attributed to the net attractive potential of interaction between the exciton and electron. Such a potential of interaction is rigorously derived and its quantitative nature is calculated in pure as well as in mixed crystals. It is shown that the complex charge carrier moves throughout the crystal via hopping from one molecule to another. Competitive importance of both reactions (a) and (b) is discussed.

ChemInform Abstract: EFFECT OF PRESSURE ON THE FIRST SINGLET EXCITON IN CRYSTALLINE ANTHRACENE

The first singlet exciton state of anthracene single crystals has been studied by optical reflection spectroscopy under hydrostatic pressures up to 30 kbar. The Davydov splittings of the 0–0, 0–1, and 0–2 vibrational bands increase much faster than expected on the basis of first order dipolar resonance interaction. The average pressure coefficients are 16.6, 13.6, and 9.0 cm−1/kbar for the 0–0, 0–1, and 0–2 bands, respectively. The pressure effect on the exciton shift is small compared to the strong pressure dependence of the Davydov splitting. The pressure induced shifts of the excitonic transition energies are discussed in terms of molecular exciton theory.

Radical Pairs in Irradiated Anthracene Single Crystals

Abstract Angular dependent EPR spectra of anthracene single crystals irradiated with γ rays at 77° K have been measured and two kinds of septet side lines with 7.5 G splitting and maximum separations by 237.5 and 206.3 G were found. From the spectra the presence of two kinds of radical pairs were concluded, whose pair axes lie within ac-plane and directed in 90° and 45° from the a-axis. Similar septet side lines were also found in the crystals which were annealed at room temperature for one hour. Radicals forming a pair were considered to be 9-dibenzo-cyclohexadienyl radical judging from the septet line of 7.5 G splitting and the stability of radicals. The directions of pair axes were discussed on the base of the migration of hydrogen atom within the ab-plane.

REDUCTION OF THE EXCITED SINGLET STATE OF CHLOROPHYLL AND PHEOPHYTIN AT THE ORGANIC CRYSTAL/AQUEOUS SOLUTION INTERFACE

— Reduction of the excited singlet state of chlorophyll a, chlorophyll b, and pheophytin a at the surface of perylene, anthracene, and chrysene single crystals has been measured as hole injection current. The dependence of the quantum yield on the standard free-energy difference of the reaction was in accordance with theoretical expectations without correcting for an interaction energy. The maximum quantum yield of only about 10−2 holes per absorbed photon is ascribed to a very small effective lifetime of the excited singlet state due to concentration quenching of the excited singlet state in the adsorbed dye layer.

Effect of pressure on the first singlet exciton in crystalline anthracene

The first singlet exciton state of anthracene single crystals has been studied by optical reflection spectroscopy under hydrostatic pressures up to 30 kbar. The Davydov splittings of the 0–0, 0–1, and 0–2 vibrational bands increase much faster than expected on the basis of first order dipolar resonance interaction. The average pressure coefficients are 16.6, 13.6, and 9.0 cm−1/kbar for the 0–0, 0–1, and 0–2 bands, respectively. The pressure effect on the exciton shift is small compared to the strong pressure dependence of the Davydov splitting. The pressure induced shifts of the excitonic transition energies are discussed in terms of molecular exciton theory.

Photoplastic effect in anthracene crystals

The photoplastic effect is observed in anthracene single crystals under illumination with near-ultraviolet light. The overall features of the action spectrum of hardening bear a close resemblance to the absorption spectrum. The maximum hardening due to illumination reaches as much as 10% of yield stress at 120 W/m2.