Trapping Of Triplet Excitons Research Articles

An intermediate model for fitting triplet–triplet annihilation in phosphorescent organic light emitting diode materials

Triplet–triplet annihilation (TTA) is one of the primary contributors to efficiency roll-off and permanent material degradation in phosphorescent organic light-emitting diodes. The two limiting case models typically used to quantify this quenching mechanism are multi-step Dexter and single-step Förster, which, respectively, assume ideal Fickian diffusion or perfect trapping of triplet excitons. For device-relevant guest doping levels (typically 5–12 vol. %), both significant diffusion of excitons and trapping due to spatial and energetic disorder exist, so neither conventional model fits experimental data well. We develop and validate an intermediate TTA model, which is a weighted average of the limiting cases of pure radiative decay (no TTA) and multi-step Dexter based TTA that returns an effective TTA rate constant and a parameter quantifying the portion of well-isolated excitons. Kinetic Monte-Carlo simulations and time-resolved photoluminescence measurements of an archetype host–guest system demonstrate that our intermediate model provides significantly improved fits with more realistic physical values, is more robust to variations in experimental conditions, and provides an analysis framework for the effects of trapping on TTA.

Peculiarities of the Energy Transfer of Electronic Excitation in Carbazolyle-Containing Polymers

Luminescence spectra of poly-N-vinylcarbazole (PVK), poly-N-epoxypropylcarbazole (PEPK), poly-N-vinyl-3-chlorocarbazole (Cl-PVK), and polystyrene (PS) films doped with bis[2-(2′-benzothienyl)-pyridinato-N,C3′](acetylacetonate) iridium [Btp2Ir(acac)] have been studied. The indicated carbazole-containing polymers are promising for the application in electroluminescence devices. The quantum yield of sensitized phosphorescence by Btp2Ir(acac) molecules in carbazole-containing polymer matrices is found to be lower than at their direct excitation in the PS matrix. The additional doping of the PVK-based composite with benzophenone, which transforms some of singlet excitons into triplet ones, diminishes the intensity of the sensitized phosphorescence of Btp2Ir(acac) molecules, which testifies that the efficiency of energy transfer via singlet excitons is higher than via triplet ones. The results obtained can be explained by a competition between the process of energy transfer onto the acceptor site, on the one hand, and the processes of exciton localization at the tail energy states and the trapping of triplet excitons by the products of polymer oxidation, on the other hand. The quantum yield of sensitized phosphorescence by Btp2Ir(acac) molecules in the PVK matrix is found to be almost 1.5 times lower than in the PEPK one. A conclusion is drawn that, in the PVKmatrix, the process of singlet exciton autolocalization at excimer-forming centers followed by the creation of sandwich-like excimers competes with the process of excitation energy transfer onto the acceptor.

Luminescence properties of N-isopropylcarbazole single crystal

A systematic study of photoluminescence properties of N-isopropylcarbazole (NIPC) single crystal is reported in this paper. The emission spectra and radiative lifetimes measured within the temperature range 1.7–300 K reveal short- and long-lived luminescence components, the first equivalent to prompt fluorescence (PF) and the second composed of delayed fluorescence (DF) and phosphorescence (PH). Whereas the PF intensity does not show a conclusive temperature dependence, the DF and PH intensities exhibit anomalous temperature dependences involving several distinct maxima. These experimental observations are interpreted in terms of exciton behavior in real organic solids containing exciton traps. It is shown that: (i) PF originates from Frenkel-type excitons (S 1) of energy E S 1 =3.53 eV; (ii) DF is due to emission from the S 1 states produced by triplet-triplet exciton annihilation with E T=3.0 eV; (iii) PHcharacterizes triplet exciton trapping levels with two discrete traps of depths about ϵ (s) t=6×10 -4 eV (shallow trap) and ϵ (d) t=0.3 eV (deep trap). Similar to tetracene single crystals the structural transition in NIPC, known to occur at T t ≅ 140 K, does not show up in the integral emission intensity, indicating its presence by a red shift in the spectra and a weak feature in the monotonic temperature dependence in the singlet exciton lifetime.

Zero-field ODMR studies of excited triplets in the B-TCNB crystal

Localized triplet excitations obtained by trapping of triplet excitons are investigated in the biphenyl (B)-tetracyanobenzene (TCNB) crystal by zero-field ODMR methods at low temperature (1.25 K). At high microwave power (1 W) the ODMR spectral lines are structured with new satellite lines appearing at the sides of the central peak due to forbidden transitions. Their analysis allowed us to obtain information on the quadrupole tensor components of the TCNB molecule in its lowest triplet state.

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.

Triplet exciton trapping in p‐terphenyl crystals studied by delayed fluorescence and phosphorescence

AbstractTriplet exciton trapping in p‐terphenyl crystals is investigated by delayed fluorescence and phosphorescence. Trap depth ((2800 ± 100) cm−1) and concentration (10−6 mol/mol) are deduced from the temporal behaviour of delayed fluorescence. The same trap is also found in the phosphorescence spectrum. The bimolecular annihilation rate constant between free and trapped excitons (1 × 10−13 cm3 s−1) is found to be orders of magnitude smaller than that for free—free annihilation.

On triplet exciton trapping in 1,4-dibromonaphthalene

From the study of triplet exciton trapping in 1,4-dibromonaphthalene it appears that several optically accessible excitons can exist with excitation energies within 1 cm −1 and lifetimes differing by two orders of magnitude. Furthermore trap-like states exist ≈ cm −1 below the exciton band. Delayed fluorescence arises from annihilation between an exciton and the very shallow trap-like state.

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.

Triplet Exciton Trapping by Dislocations in Anthracene Crystals

Trapping of triplet excitons at dislocations, produced in anthracene crystals by plastic bending, was investigated by means of the temperature and the radius-of-curvature dependence of the triplet decay rate. As a result of the kinetic analysis, the trapping depth of 0.3 eV was obtained. The trapping rate constant was found to be dependent on the square root of the curvature, the dependence being related to the diffusion of triplet excitons in the ab plane and to the distance between dislocations. This phenomenon was re-analyzed by applying a one dimensional random-walk model.

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.

Electron microscopic studies of extended defects in organic molecular crystals. Part 1.—p-Terphenyl

Transmission electron microscopy has been utilized to reveal directly the nature of the dislocations that occur in solution- and vapour-grown thin crystals of p-terphenyl. The identity of the linear defects was established chiefly by diffraction contrast using bright-field images, but it also proved possible to employ, to a limited degree, real-space crystallography. Many of the slip systems deduced to be present in simple aromatic solids by dislocation-etching and topographical studies have been more fully characterized in this study {e.g. (001)[010], (001)[110], (100)[010] and (100)[001]}. Of special interest is the identification of the hitherto undetected slip system (001)[12w](with w probably zero). At the cores of such dislocations, in aromatic solids crystallizing in the P21/a space-group, molecules within the structure adopt a space-symmetry which indicates that incipient dimers occur recurrently. This is relevant in the solid-state chemical behaviour of anthracene since such dislocations can rationally account both for the production of topochemically forbidden dianthracene upon u.v. irradiation of the cyrstalline monomer and te trapping of triplet excitons in single-crystals of anthracene.

Triplet excitons as probes for structural imperfections in crystalline anthracene

The phosphorescene and delayed fluorescence associated with triplet exciton trapping in crystalline anthracene have been used to demonstrate the physical nature of many of the traps and to determine their depth.

Triplet Exciton Trapping by Dislocations in Anthracene

Dislocations have been produced in anthracene crystals by plastic bending. The triplet exciton trapping characteristics have been obtained from a study of the temperature dependence of the triplet decay rate and delayed fluorescence intensity. The results indicate that only one kind of trap with depth 0.3 eV is introduced by the plastic bending. This trap can be removed by annealing the crystal for 1 h at 443°K. The trapping rate constant is three orders of magnitude smaller than expected for diffusion-limited trapping.

Temperaturabhängigkeit der Szintillationslichtausbeuten von p-Terphenyl-, Anthrazen- und Naphthalinkristallen bei Anregung mit α- und Β-Teilchen

The temperature dependence of the integral scintillation light yield of p-terphenyl, anthracene, and naphthalene single crystals excited byα- andΒ-particles was investigated betweenT=10 ‡K andT=293 ‡K. The variation of the scintillation anisotropy of p-terphenyl with temperature was measured as well. With decreasing temperature the scintillation light of anthracene and naphthalene is found to be emitted faster. The integral light yield of p-terphenyl shows a pronounced maximum atT=34 ‡K which is caused by the delayed component only. The maximum is explained by triplet exciton trapping.

Phosphorescence of Crystalline Pyrazine at 4.2°K

The high-resolution phosphorescence spectrum of pyrazine crystals has been recorded and analyzed. The fact that the crystal structure of pyrazine crystal is known and that the ground-state molecular and lattice vibrations are assigned, made it possible to accurately analyze and assign the observed spectrum. The comparison of the observed spectrum obtained from pyrazine crystals with that obtained from pyrazine in cyclohexane matrix at 4.2°K leaves no doubt that the observed emission from the crystals results from the B3u3←Ag1π*→n transition of the pyrazine molecule. The 0,0,0 band in emission is found to be 102 cm−1 lower in energy than the 0,0,0 band in absorption. This might suggest that the emission centers have slightly different environment and thus lower energy than those absorbing the radiation. This difference in environment also leads to the trapping of triplet excitons by pyrazine molecules located at physical defects. The different types of trapping mechanisms are briefly discussed in the light of the observed spectrum. There is a strong coupling between the electronic motion and the lattice modes of b3g symmetry (the most prominent ones have frequencies of 57 and 82 cm−1). The intensity of the emission of the phononic and phonon-vibronic bands is greater than that of the vibronic bands. This observation suggests that either the lattice parameters are different in the excited triplet state from those in the ground state or else there is a lattice-induced emission resulting from the coupling of the different molecular electronic states of pyrazine with lattice vibrations. These two mechanisms of phonon enhancement of the emission are discussed in terms of the site symmetry of the emitting pyrazine molecules at the trapping sites. Spectral evidence is given showing the coupling between the intramolecular C–H in-plane bending vibrations and the lattice modes. The b3g lattice modes are found to have lower frequencies when coupled to the C–H in-plane bending vibration than when it is coupled to the other vibrations. This observed phonon-vibronic coupling as well as the phonon-electronic coupling is consistent with the form of the force field proposed for the pyrazine crystal which consists of one term representing an intermolecular hydrogen bonding (C–H···:N) and two intermolecular hydrogen-hydrogen repulsion terms. A change in the distribution of the hydrogen atoms (as produced by C–H vibrations) or the lone-pair electron density (as produced in π*→n transition) can give rise to observed effects on lattice parameters and vice versa.

Trapping of Triplet Excitons and the Temperature Dependence of Delayed Fluorescence in Anthracene Crystals

Singh and Lipsett have observed that the intensity of fluorescence due to triplet—triplet annihilation in anthracene crystals exhibits an anomalous temperature dependence involving several distinct maxima. It is shown that this behavior can be understood on the basis of trapping of triplet excitons. A simple kinetic model leads to the conclusion that every trapping level with a triplet lifetime long compared to the lifetime of a ``free'' triplet exciton gives rise to such a maximum. In favorable cases these observations can be analyzed completely in terms of depths, concentrations, and triplet lifetimes of traps. An analysis of Singh and Lipsett's data is presented. It leads to a broad continuum of trapping levels with depths ranging from about 3000 cm−1 to very small values, and two more or less discrete traps with depths of about 650 and 130 cm−1, respectively. The shallower traps are shown to have the higher concentrations and the longer triplet lifetimes. Indirect evidence is presented for the existence of still shallower traps with a very short triplet lifetime. The nature of these traps is discussed.