Tetracene-doped Anthracene Crystals Research Articles

Desorption Dynamics of Tetracene Ion from Tetracene-Doped Anthracene Crystals Studied by Femtosecond Time-Resolved Mass Spectrometry

The desorption dynamics of tetracene ion ([tetracene]+) from tetracene-doped anthracene (TDA) crystals in which efficient energy transfer from anthracene to tetracene occurs was investigated by means of femtosecond time-resolved mass spectrometry. Using a 400 nm pump pulse, anthracene molecules in TDA crystals are initially excited to the S1 state. After a certain delay (<10 ps) of pumping, the excitation energy in the S1 anthracene is transferred to a tetracene molecule to produce the S1 state. Tetracene in the S1 state is then ionized by a time-delayed probe pulse (266 nm) and desorbed tetracene ions from the crystals are monitored with a time-of-flight mass spectrometer. On the time-resolved mass spectra, the peak intensity of the tetracene ions shows a gradual increase with increasing delay time. The rise time constant of tetracene ion, which is considered to the desorption time from the crystals, was determined to be 80.8 and 94.7 ps for TDA crystals with anthracene/tetracene concentration ratios of ...

Preparation and spectral characteristics of anthracene/tetracene mixed crystals

A series of tetracene-doped anthracene crystals with different doping concentrations (the highest molar ratio 100 1) are grown from solution. Crystal structures and optical characteristics of the above mixed crystals are investigated at room temperature. By changing the doping concentrations, the fluorescence can be adjusted from blue-green to green and even to yellow-green. The emission spectra of anthracene/tetracene (An/Te) mixed crystals reveal the sensitized fluorescence of tetracene and the partial quenching of anthracene emission. The data of transient photoluminescence (PL) decays illustrate that in An/Te mixed crystals, the decay of anthracene becomes faster, while the PL lifetime of tetracene is longer than that of the tetracene single crystals. All above experimental results suggest that there is excitation energy transfer from anthracene to tetracene in the mixed crystals.

Picosecond study of electronic energy transfer in tetracene-doped anthracene crystals

Picosecond time-resolved fluorescence spectroscopy measurements have been conducted on very thin tetracene-doped anthracene microcrystals. The observed time dependence of the fluorescence intensities of crystals with various concentrations of tetracene is consistent with predictions of the combined theory of exciton diffusion and long-range resonance energy transfer.

Polariton energy transfer to impurity in tetracene doped anthracene crystals

Polariton luminescence spectrum and kinetics of tetracene doped anthracene crystal have been studied. Deformation of the host luminescence spectrum due to polariton energy transfer to the guest molecules has been observed. The estimate of the polariton capture cross-section by guest molecules has been given.

Singlet-Exciton Energy Transfer in Tetracene-doped Anthracene Crystals as Studied by Time-Resolved Spectroscopy

Abstract The time-resolved sensitized fluorescence of tetracene-doped anthracene crystals following a two-photon picosecond-pulse excitation was investigated experimentally as a function of dopant concentration and temperature. Even at the optimum time resolution achieved in these experiments (about 5ps) the time dependence of the host fluorescence was always purely exponential, that of the guest fluorescence a superposition of two exponentials. Thus no time dependence of the energy-transfer rate could be identified. The temperature dependence of the energy-transfer rate is explained tentatively in terms of a "hopping model" (hopping time th ≈ 5.8·10-14s) and thermal reactivation from shallow exciton traps (trap depth 43 cm-1) in the vicinity of the guest molecules. In pure anthracene crystals the intrinsic exciton lifetime (observed at 20 K) is (5.0 ± 0.5) ns. At higher temperatures it increases to about 15.5 ns at 300 K due to reabsorption processes. Below 20 K it decreases to about 1.5 ns at 1.6 K because of exciton trapping at shallow X-traps (trap depth 23 cm -1). When using conventional one-photon excitation exciton-exciton annihilation was observed with an annihilation constant γss ≈ 1.0 × 10-8 cm3 s-1.

Matrix fluorescence due to optical pumping of upper excited states of tetracene doped anthracene crystals

Energy transfer from the higher excited states of tetracene to the lower excited states of host anthracene crystal has been found.

Magnetic Field Modulated Scintillation from Tetracene-Doped Anthracene Crystals under α-Particle Irradiation

en

Evidence for trapped-exciton fluorescence in anthracene crystals at room temperature

The fluorescence variations due to imposed exciton–charge carrier interactions in anthracene are examined as a function of emission wavelength. In the short wavelength region of the emission spectrum, the observed modulation of the fluorescence intensity due to the presence of trapped charge carriers is relatively small and, with the exception of the highly reabsorbed fluorescence at very short wavelengths, is independent of emission wavelength. At longer wavelengths, however, a series of maxima develop along with a tenfold increase in the modulation. These observations are interpreted in terms of emission from trapped singlet excitons. Though the results are consistent with those expected with a discrete exciton trap at 0.43 eV, they may also be interpreted as indicating a series of exciton traps with depths in the range 0.4–1.0 eV. Results are also presented for tetracene-doped anthracene crystals which support the hypothesis that the traps are perturbed molecules of the host crystals, rather than impurities.

Energy transfer in tetracene-doped anthracene crystals

Rise and decay curves of tetracene-doped anthracene crystals fluorescence have been investigated with single-photon counting technique. It has been shown that energy transfer is due to both free and trapped excitons.

Optical detrapping of charge carriers in undoped and in tetracene-doped anthracene crystals

The optical release of trapped charge-carriers in undoped and in tetracene-doped (10−6 mole/mole) anthracene crystals is investigated. There processes can be distinguished: 1) Release by mobile anthracene triplet excitons where the triplet absorption spectrum is observed. 2) Release by direct optical excitation of an allowed electronic transition of positive tetracene and anthracene ions (tetracene and distorted anthracene molecules charged with a trapped hole), at γ = 865 and 765 nm, respectively, and subsequent relaxation of the excited hole into the valence band. 3) Release in a broad IR spectral region showing a) negative peaks due to absorption losses into higher anthracene vibrational levels and b) a number of weak peaks with a mean separation of 144 cm−1 tentatively interpreted as phonon satellites. Die optische Befreiung getrappter Ladungsträger in undotierten und Tetrazen-dotierten (10−6 mol/mol) Anthracenkristallen wird untersucht. Drei Prozesse lassen sich unterscheiden: 1) Befreiung durch bewegliche Anthrazen-Triplett-Exzitonen, wobei das Triplett-Absorptionsspektrum auftritt. 2) Befreiung durch direkte optische Anregung eines erlaubten elektronischen Übergangs positiver Tetrazen- und Anthrazen-Ionen (mit einem getrappten Loch beladener Tetrazen- bzw, fehlorientierter Anthrazenmoleküle) bei γ = 865 bzw. 765 nm und anschließender Relaxation des angeregten Lochs in das Valenzband. 3) Befreiung in einem breiten IR-Spektralbereich, in dem a) negative Peaks auftreten, die von Absorptionsverlusten in höhere Anthrazen-Schwingungsniveaus herrühren, und b) eine Anzahl schwacher Peaks mit einem mittleren Abstand von 144 cm−1, welche Phononenseitenbanden zugeschrieben werden.

Magnetic field effects on exciton annihilation processes in tetracene doped anthracene crystals

The magnetic field dependence of host as well as of guest delayed fluorescence in tetracene-doped anthracene crystals was measured. The position of the high field level-crossing resonances for heterofusion indicate that tetracene molecules enter the anthracene lattice substitutionally. The changes in intensity of the resonance peaks with tetracene concentration indicate that the tetracene-anthracene triplet excitation annihilation gives rise to both tetracene and anthracene delayed fluorescence.

Magnetic field effects on exciton annihilation processes in tetracene doped anthracene crystals

Magnetic field effects on exciton annihilation processes in tetracene doped anthracene crystals

Delayed Fluorescence and Triplet Excition Kinetics in Tetracene Doped Anthracene Crystals

Abstract The triplet exciton kinectics in the host-guest system anthracene-tetracene is studied. Intensity and time dependence of the delayed fluorescence emission of both host and guest are investigated for guest concentrations between 10−8 and 10−5 mol/mol. The host triplet state is excited using a xenon are lamp or a krypton ion laser for steady state and a specially shaped xenon flash or a pulsed ruby laser for the transient measurements. The guest concentrations are measured down to 5 × 10−9 mol/mol with an accurcay of 20% by observing the prompt guest fluorescence excited directly with a suitable line of an argon ion laser. For relative guest concentrations smaller than 2 × 10−7 mol/mol the measurements can be described satisfactorily by assuming that the guest molecules act as traps with a lifetime of 0.8ms, a trapping rate for the host excitons of γ∗N with γ∗ = 1.8 × 10.11 cm3 s−1 (N = density of guest molecules), and a detrapping rate of about 100s−1. The analysis involves host-host as well as ho...

Doppelinjektion und Elektrolumineszenz in dotierten Anthracenkristallen

Abstract The current-voltage dependence and the current dependence of the electroluminescence at different wavelengths have been investigated in pure and in tetracene doped anthracene crystals. The influence of traps is shown to be very significant. The current-voltage dependence is calculated by dividing the crystal in a hole injecting region (density of electrons negligible) and in a space-charge-free region. The latter arises by emptying the traps by recombination, its thickness is proportional to the applied voltage. The temporal decay of the electroluminescence was measured after the contacts were shorted. The decay shows a fast component which had the fluorescence spectrum characteristic of tetracene and a delayed component which had the fluorescence spectrum characteristic of anthracene. The delayed component is explained by assuming that each recombination, including free- trapped carrier recombination, has a high probability of yielding a triplet exciton. This will lead to delayed fluorescence by...

Consequences of Symmetry Breaking in Triplet-Exciton Fusion

The magnetic field dependence of the fusion rate between inequivalent triplet excitons (free and trapped triplets in tetracene-doped anthracene crystals) is qualitatively different from that between equivalent excitons. Additional high-field level-crossing resonances and a new low-field dip appear in delayed fluorescence at room temperature.

On the question of singlet exciton diffusion in anthracene

The time evolution of anthracene and tetracene fluorescence intensities of tetracene doped anthracene crystals was monitored as a function of tetracene concentration and temperature for samples grown by different techniques and for different methods of excitation. Singlet exciton diffusion generally invoked to explain energy transfer in this system cannot explain the observed time dependence which indicates that energy transfer is more efficient at short times than at long times. Such a variation of energy transfer efficiency is characteristic of long range resonant interaction. A combined theory of long range interaction and exciton diffusion gives a good fit to the results only if the strength of the interaction is much greater than that determined from spectral considerations. The results are independent of the method of crystal growth and of the method of excitation. The temperature dependence of the energy transfer appears to be completely contained in the variation of the anthracene decay time as temperature is lowered to 10 °K.

Evidence for Long-Range Exciton-Impurity Interaction in Tetracene-Doped Anthracene Crystals

It is shown that diffusion of singlet excitons cannot explain the time dependence of the fluorescence intensity in tetracene-doped anthracene crystals; however, the predictions of a long-range energy-transfer mechanism can be made to agree with experimental results.