Triplet Exciton Lifetime Research Articles

Purity and Perfection of Anthracene Crystals

Crystals grown from the melt using synthetic Kodak `H 480' anthracene subjected to intensive purification efforts, including liquid chromatography, sublimation and multiple zone-refining, have been examined for purity and perfection by measuring triplet exciton lifetime from an analysis of the decay of the delayed fluorescence. In the best crystals, the triplet exciton lifetimes were ∼ 23 ms in agreement with earlier published values. The values for drift mobilities and their temperature coefficients agree well with other published values. However, no obvious correlation between the triplet exciton lifetimes and the lifetime measured under electron bombardment for these crystals has been detected.

ChemInform Abstract: STRUCTURAL IMPERFECTIONS AND THE DELAYED FLUORESCENCE OF ANTHRACENE CRYSTALS

Steady-state and temporal dependences of delayed fluorescence as a function of temperature have been measured for a range of high purity anthracene crystals with the aim of investigating the triplet exciton trapping characteristics. Direct excitation of triplets with weakly absorbed red light of low and high intensity has been employed and the response of the crystals to (a) low frequency (≈5–20 Hz) chopped-light and (b) a δ-function excitation has been recorded.For high intensity excitation there is an inflection in the decay for both modes of excitation, revealing the existence of trap saturation over the entire temperature range, 350–4 K. Our study indicates that even at high temperatures the triplet exciton is localised in deep structural traps of low concentration (≈10–11 M/M). Even in the purest samples available the triplet exciton lifetimes reflect the fact that the routes controlling triplet exciton decay are extrinsic via trapping sites.The highest technical lifetime measured from delayed fluorescence is ≈27 ms for sealed melt-grow crystals; this value shows an appreciable reduction upon cleavage. All observations may be interpreted in terms of the perturbed anthracene molecules, both as separate entities and in pairs (“incipient dimers”), associated with dislocations and a polymorphic modification, that yield a quasi-continuum of triplet trapping levels extending downwards from ≈4,700 cm–1. The deepest traps are removed upon prolonged u.v. irradiation of the crystals under conditions that yield the photodimer, whereupon other shallower traps appear.

Structural imperfections and the delayed fluorescence of anthracene crystals

Steady-state and temporal dependences of delayed fluorescence as a function of temperature have been measured for a range of high purity anthracene crystals with the aim of investigating the triplet exciton trapping characteristics. Direct excitation of triplets with weakly absorbed red light of low and high intensity has been employed and the response of the crystals to (a) low frequency (≈5–20 Hz) chopped-light and (b) a δ-function excitation has been recorded.For high intensity excitation there is an inflection in the decay for both modes of excitation, revealing the existence of trap saturation over the entire temperature range, 350–4 K. Our study indicates that even at high temperatures the triplet exciton is localised in deep structural traps of low concentration (≈10–11 M/M). Even in the purest samples available the triplet exciton lifetimes reflect the fact that the routes controlling triplet exciton decay are extrinsic via trapping sites.The highest technical lifetime measured from delayed fluorescence is ≈27 ms for sealed melt-grow crystals; this value shows an appreciable reduction upon cleavage. All observations may be interpreted in terms of the perturbed anthracene molecules, both as separate entities and in pairs (“incipient dimers”), associated with dislocations and a polymorphic modification, that yield a quasi-continuum of triplet trapping levels extending downwards from ≈4,700 cm–1. The deepest traps are removed upon prolonged u.v. irradiation of the crystals under conditions that yield the photodimer, whereupon other shallower traps appear.

Amplitude-normalized decay integral: A method for measuring low-level luminescence lifetimes

A method which utilizes photon counting is developed for measuring low-level luminescence lifetimes. The technique requires two counters and a pulse sequence generator. The method is applied to the measurement of the delayed fluorescence lifetimes in anthracene and tetracene single crystals. The lifetime of triplet excitons in vapor-grown tetracene crystals is found to be 43 μsec.

Exciton Sounding of α-Particle Induced Radiation Defects in Anthracene

Abstract Anthracene crystals have been irradiated ∥ to c by α-particles at 28.3, 30.00 and 32.26 MeV. A laser beam focused to a line ⊥ to the c direction is used to generate triplet excitons at 50 micron intervals in this direction. The spatial distribution of residual damage is determined from the variations in the delayed fluorescence and triplet exciton lifetime. The modulation of the triplet exciton lifetime in a magnetic field indicates that the defects are principally paramagnetic. The α-particle ranges at 30.00 and 32.26 MeV are 580 ± 20μ and 660 ± 20μ in agreement with the Bragg additivity rule. The G value for the production of the paramagnetic defects is 0.02.

Determination of the Singlet-Triplet Absorption Coefficient in Anthracene

Abstract Measurements of the delayed-fluorescence excitation spectrum in anthracene could so far yield only the relative variation of the singlet-triplet absorption coefficient α with wavelength. In the present work the absolute magnitude of α is derived from studies involving the interaction of photo-generated triplet excitons with trapped electrons introduced into the sample by contact injection. Such an interaction results in the liberation of trapped electrons on the one hand and in the quenching of the triplet-exciton lifetime on the other. The former process is studied by photocurrent measurements while the latter is monitored by triplet lifetime measurements. The value of α so determined is (1.2, ± 0.2) × 10−3 cm−1 at the first singlet-triplet absorption peak (6780 A).

Effects of polishing anthracene single crystals on the triplet exciton lifetime

Polishing anthracene single crystals creates defects which reduce the triplet lifetime not only at the surface but also throughout the bulk. By annealing the crystal, the triplet lifetime can be essentially restored to its initial value.

Triplet exciton lifetime in crystalline pyrene

The triplet exciton lifetime in crystalline pyrene is found to be 140 ± 10 msec at room temperature, over 500 times longer than previously reported values. The temperature dependence of the triplet lifetime has been measured over the range 80–300°K. The rate of bimolecular annihilation of triplet excitons in pyrene is found to be independent of temperature over the range 150–300°K. It is concluded that the transfer of triplet energy within the crystal may be described using the monomer exciton model.

Delayed Fluorescence and Triplet Lifetime of Crystalline Chrysene

Abstract The delayed fluorescence emission spectrum of pure crystalline chrysene at temperatures down to 80° K has been measured. Careful purification is required in order to remove traces of impurities which trap triplet excitons in chrysene giving rise to impurity fluorescence. The triplet exciton lifetime in crystalline chrysene at room temperature was found to be 30 ± 2 msec.

Carrier-Exciton Interactions in Crystalline Anthracene

Quenching of delayed fluorescence by injected holes and electrons is observed when either monomolecular or bimolecular exciton processes afford the dominant exciton decay mechanism. With small triplet exciton densities, injected carriers reduce the triplet exciton lifetime, and hence the delayed fluorescence intensity. With high triplet exciton densities this effect is unimportant; however, the over-all triplet-triplet annihilation rate constant is found to increase while the singlet component of the annihilation rate constant decreases in the presence of charge carriers. These effects are discussed in terms of triplet pair formation and annihilation.

Quenching of Triplet Excitons in Anthracene Crystals by Internal Beta-Irradiation

Abstract We wish to report effects of bulk irradiation of anthracene by low energy electrons, tritium β-particles, on triplet exciton lifetime in anthracene crystals doped with small concentrations of randomly tritiated carbazole.

Oxygen quenching of delayed fluorescence in the solid state

A gradual increase in delayed fluorescence intensity is observed in crystalline 2-chloroanthracene and 9,10-diphenylanthracene in air under irradiation with light of constant intensity in the first triplet band. Detailed measurements in 2-chloroanthracene of delayed fluorescence intensity and triplet exciton lifetime show that the behavior is consistent with a triplet exciton-oxygen interaction which results in a depletion of the ground-state molecular oxygen concentration in the lattice and leads to a time dependence of the triplet exciton lifetime and of the effective S 0 → T 1 absorption coefficient of the material.

Verzögerte Fluoreszenz und Phosphoreszenz in reinen Naphthalin-Kristallen

In extremely purified naphthalene crystals triplet exciton lifetimes up to 500 msec at room temperature were measured. The triplet annihilation coefficient is measured from an analysis of the light intensity dependence of delayed fluorescence as γ = 3.5 · IO-12 cm3 sec-1 at 300 °K and at 135 °K. The quantum yields in the weak excitation limit at room temperature are ηDF = (3 ± 0.6) · 10-3 for delayed fluorescence and ηp = (1.2 ± 0.3) • 1O-3 for prompt phosphorescence, both after triplet excitation. The phosphorescence spectra of naphthalene and perdeuteronaphthalene crystals are analyzed between 300 and 50 °K. The line width decreases from 270 cm-1 at 300 °K down to 42 cm-1 at 60 °K.

Calculation of the diffusion length, diffusion coefficient and lifetime of triplet excitons in crystalline tetracene

The recombination, or fusion, of triplet excitons (which are efficiently produced by singlet exciton fission at 300 °K) leads to an enhancement in the fluorescence quantum efficiency Φ at excitation light intensities I ≳ 1015 quanta-cm-2-sec-1 (366 and 436 nm wavelength). It is possible to fit a calculated Φ (I) curve to the experimental data if values of the triplet exciton lifetime τ = 4.6 × 10-6 sec, diffusion coefficient perpendicular to ab plane D⊥ab = 2.0 × 10-4cm2-sec-1, and diffusion length l⊥ >ab = 4.2 × 10-5 cm are used.

Triplet Exciton Decay in Phenanthrene Single Crystals

Abstract Delayed fluorescence in single crystals of high purity phenanthrene has been studied. The lifetime of triplet excitons has been obtained as a function of temperature and estimates have been made of the rate constant for triplet-triplet annihilation and of the triplet diffusion constant. The results have been interpreted in terms of impurity trapping of the triplet excitions.

Diffusion Length and Lifetime of Triplet Excitons and Crystal Absorption Coefficient in Tetracene Determined from Photocurrent Measurements

AbstractBy comparing the decrease of the i+ and i− currents as function of the exciting wavelength and from the maximum of the i− current the crystal absorption coefficient of tetracene can be determined. With the penetration depth of the radiation thus known the diffusion length and lifetime of triplet excitons are determined from the dependence of the currents on the wavelengths. The diffusion length is found to be (0.4 ± 0.2) μm, the lifetime about 100 μs.

Temperature dependence of the phosphorescence of pure and impure benzophenone crystals

The spectrum and quantum yield of the phosphorescence of pure and impure benzophenone crystals have been examined from 77 to 293 °K. The molecular quantum yield has a negative temperature coefficient, as does the transfer of triplet energy. The latter effect is due mainly to reduction in the lifetime of triplet excitons.

Diffusion of Triplet Excitons in Crystalline Anthracene

In this work we present an experimental study of triplet-exciton diffusion in crystalline anthracene. Triplet-exciton lifetime was determined by the time intermittency method, while the diffusion coefficients and the components of the diffusion tensor were determined by the space intermittency technique. From the comparison of the experimental data with the results of the theoretical analysis we conclude that: (a) The magnitude of the diffusion coefficient for triplet excitons [D=(2.0±0.5)×10−4cm2/sec] yields a strong support for the strong scattering random-walk model for triplet-exciton migration in crystalline anthracene. (b) The diffusion tensor of triplet excitons in crystalline anthracene was found to be isotropic. (c) The isotropy of the diffusion tensor in the ab plane can be properly accounted for in terms of the calculated intermolecular electron exchange and charge-transfer interactions.