Tetracene Crystals Research Articles

Intermediate Exciton-Phonon Coupling in Tetracene

Polarized absorption spectra of tetracene (naphthacene) single crystals have been investigated between 300 and 13 K, with particular emphasis on the b -polarized spectrum. The low energy tails of the a - and b -polarized first absorption bands are expressed by the Urbach rule with the steepness coefficients σ a ′ =1.37 ±0.12, σ b ′ =1.58 ±0.13, with which possibility of self-trapping of excitons is discussed along with the dimensionality of tetracene crystals. The exciton-phonon coupling constants are found to be g a =1.09 ±0.1 and g b =0.95 ±0.08, consistent with luminescence experiments which show that excitons in tetracene are self-trapped very shallowly. The line halfwidth, the oscillator strength of the first and the second absorption bands, refractive indices, and surface exciton structure are briefly discussed.

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.

Lattice-dynamical calculations for tetracene and pentacene

Lattice-dynamical, calculations for evidently non-rigid molecules of aromatic hydrocarbons have been carried out on tetracene and pentacene. In these substances, “out-of plane” vibrations mix extensively with lattice vibrations, and significant differences can be noted between results from a “rigid-body” and a “non-rigid” treatment. For tetracene crystals, whose Raman spectral data are given in the literature, the agreement with experiment is satisfactory. This confirms the validity of such procedures for interpreting and/or predicting spectroscopic behaviour, starting from empirical atom—atom potentials and valence force fields.

Exciton‐Enhanced Double Injection Currents in Tetracene Crystals

Exciton‐Enhanced Double Injection Currents in Tetracene Crystals

Quantum beats of prompt fluorescence in tetracene crystals

We have measured the quantum beats in the decay of pulse-excited “prompt” fluorescence in tetracene crystals as a function of magnetic field. We have demonstrated that counting the beats is a very simple and reliable method to study a possible influence of triplet-triplet interactions. In all so far performed experiments in tetracene crystals no influence has been found.

Fluorescence spectra of anthracene and tetracene crystals corrected for spatial distribution of excitons

The fluorescence spectra of anthracene and tetracene single crystals have been measured with the application of a special procedure for correction for reabsorption and to independently determine the spatial distribution of excitons. The corrected spectra show the short-wavelength fluorescence maximum to be blue shifted and over an order of magnitude greater than those for the remaining emission bands.

Trapping effects on emission-limited current flow in narrow-band insulators

The effect of charge carrier traps on emission-limited injected currents in narrow-band insulators is studied. At low voltages the physical mechanism responsible for the current is the trapping of injected carriers in the crystal image force potential well between the emitter and the potential maximum; the Schottky emission is the dominant conduction mechanism at high voltages. A theoretical model is presented and the voltage dependence of currents is derived including the effects of trapping and barrier lowering. It is shown, in agreement with the model, that in injection experiments on anthracene and tetracene crystals the voltage (U) dependence of current (j) can be described by j ≈ j0 exp (aU1/2 — bU−1/2), where a and b are constants characterizing the crystals. The constant b dependent on the trap concentration, determines at what voltage the conductivity changes from trapping to Schottky barrier lowering mechanism. The hole diffusion length in five different anthracene and tetracene crystals is found to be within the range 19 to 310 A and the trap concentration in the vicinity of the injection contact, within the range 6 × 1017 to 9 × 1020 cm−3. [Russian Text Ignored]

Double quantum external photoelectric effect in crystalline tetracene

AbstractThe subject effect (DQEPE) is confirmed in tetracene crystal and attributed to the interaction of a free electron and a trapped charge‐transfer (CT) state. The energy of the trapped CT state is 2.85 eV. The light intensity (I) dependence of the DQEPE yield Y is Y ∝︁ I1.5. Greatly improved experimental techniques for measuring photoemission rates and retardation potentials in a gaseous environment are described.

Rate constants of singlet exciton fission in a tetracene crystal determined from the rydmr spectral linewidth

The lifetime τ of a correlated pair of triplet excitons has been determined from the linewidth of the RYDMR spectrum. The value τ = 7 × 10 −9 s obtained has been used to calculate the rate constants of the singlet exciton fission into a pair of triplet excitons in a tetracene single crystal. The fission was shown to go via three steps: the first one is contact pair formation, the second step is its transformation into a diffusive pair, and the third one is the dissociation of the diffusive pair into free triplet excitons. Both the values of the rate constants of every step and the entropy growth in the transition from contact pair to diffusive pair have been evaluated.

Applicability of the kinematic theory to singlet exciton fission in tetracene

It is shown that the magnetic field dependence of singlet-exciton fission in tetracene crystals can be correctly accounted for with the kinematic theory of Suna [Phys. Rev. B 1, 1716 (1970)] including the two-dimensional character of the motion of the resulting pair of triplet excitons.

Kinematics of triplet pairs in anthracene and tetracene crystals

The diffusion and annihilation of a pair of correlated triplet excitons in anthracene and tetracene crystals has been studied by solving the diffusion equation by the prescribed diffusion methods. These results were used to calculated the magnetic field effect on the yield of fission of singlet excitons into a triplet pair in anthracene crystals. This description of the triplet pair kinematics was also used to explain the presence of a slow component in the prompt fluorescence decay of tetracene crystals.

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 dependence of singlet exciton fission and fluorescence in crystalline tetracene at 300 K

The magnetic field dependence of singlet exciton fission and fluorescence was studied at 300 K in tetracene crystals in several crystal planes for different field intensities and orientations. The positions of the resonances at high field correspond to those predicted by the zero-field splitting tensor as obtained by EPR, indicating that the outcome of the singlet fission is a pair of free migrating triplet excitons. It is shown that the field dependence and the resonance lineshapes can only be fitted, to within the experimental error, with the kinematic theory of Suna assuming a nearly two-dimensional motion of triplet excitons, restricted to the (a'b) plane. With the assumption of an isotropic triplet exciton diffusion in the (a'b) plane reasonable sets of values can be deduced from the fit for the in-plane hopping rate, for the anisotropy of the diffusion, and for the nearest neighbour triplet-triplet annihilation rate. In many respects, triplet exciton dynamics in tetracene appears to be similar to that in anthracene, as expected from the calculation of the transfer matrix elements and the similarity of the crystal structures. The dependence of the effects on the exciting light intensity are also well accounted for using the usual kinetic equations for the different processes involved during the lifetime of the triplet excitons.

Pressure dependence of the Davydov splitting in tetracene single crystals

The reflection spectrum of the OO component of the first singlet transition of tetracene single crystal has been measured as a function of pressure in the range 1 atm–6.5 kbar. A discontinuous change in the Davydov splitting occurs near 3 kbar confirming the existence of a pressure-induced first order phase transition discovered recently by measurements of the magnetic field anisotropy in the single crystal tetracene fluorescence. The pressure-induced spectral red-shifts are larger in the low-pressure (LP) phase than those in the high-pressure (HP) crystal structure. The Davydov splitting of the OO band however increases with increasing pressure at a larger rate of 57 cm −1/kbar for the HP phase as compared with 46 cm −1/kbar for the LP phase.

Pressure-Induced Phase Transition in Pentacene-Doped Tetracene Single Crystals

Abstract From the pressure dependence of the spatial anisotropy of the magnetic field effect on the guest fluorescence of pentacene-doped tetracene single crystals it is concluded that a phase transition occurs near 3 kbar. The result is briefly discussed in comparison with a similar phenomenon in neat tetracene crystals.

Picosecond measurements of exciton migration in tetracene crystals doped with pentacene

The kinetics of electronic energy transfer from host to guest have been measured for a tetracene crystal doped with pentacene. With single picosecond pulse excitation and streak camera diagnostics, the fluorescence risetime of the guest is measured to be the same as the decay time of the host. For low pentacene concentrations the exponential decay of the host at 170 K is consistent with a diffusion model for singlet migration from host to guest. At high intensities, no evidence was found for guest saturation because of the dominating effect of bimolecular exciton annihilation in the host.

Magnetic resonance of short-lived triplet exciton pairs detected by fluorescence modulation at room temperature

The possibility of resonant electromagnetic field modulation of the rate of reactions between paramagnetic particles in a condensed medium lasting for 10 −8–10 −10 sand shorter than the spin-lattice relaxation time has been demonstrated experimentally. The EPR spectra of triplet exciton pairs in tetracene and charge transfer crystals have been recorded at room temperature.

Exciton fission and annihilation in crystalline tetracene

The time-resolved fluorescence of tetracene crystals excited by a single 7-ps pulse of 530-nm light has been measured with a streak camera/OMA system. The singlet exciton lifetime was found to be 300�30 ps at 293 K. Singlet-singlet exciton annihilation was observed, and the rate constant found to be 5 x 10-9 cm3 s-1. In order to fit the observed decay curves, an upper limit of c. 1 x 10-9 cm3 s-1 must be set on the singlet-triplet exciton annihilation rate. The measured singlet-singlet exciton annihilation rate is consistent with a hopping model for singlet exciton motion at room temperature.

Pressure-induced phase transition in tetracene crystals

The magnetic field dependence of the fluorescence of tetracene single crystals has been measured at high pressures of up to ≈ 5 kbar. A discontinuous change in the spatial anisotropy of this magnetic field effect occurs near 3 kbar signifying the occurence of a phase transition.

Fluorescence decay time measurements in tetracene crystals excited with synchrotron radiation

Fluorescence decay times from tetracene single crystals excited at room temperature with synchrotron radiation have been recorded as a function of the excitation wavelength (in the 400–500 nm range). A non-exponential decay with two decay rates is observed. The analysis of our data shows that the first singlet exciton level of tetracene (single crystal) decays radiatively mainly through, as we call it, channel 1, with a lifetime of 0.200 ± 0.020 ns. About 10% of the emitted fluorescence transits through channel 2 with a lifetime of 1.7 ± 0.2 ns. These results do not agree with previously published decay data obtained when tetracene is excited by means of powerful lasers. Thus there is experimental evidence to believe that the decay properties of condensed materials can be very dependent on the excitation density. Because synchrotron radiation compared to lasers is a very weak source, and therefore secondary effects are minimized in our experimental conditions, the decay values reported in the present work are the true lifetimes of the tetracene single crystal.