Pair Of Triplet Excitons Research Articles

Evidence for triplet inter-chain polaron pairs in poly(phenylene vinylene) and their transformations

Photoluminescence of films of poly(phenylene vinylene) (PPV), and changes of its intensity under conditions of electron spin-resonance as a function of temperature and light intensity were studied. Three types of resonant signals were found in magnetic resonance spectrum:(i) a narrow (17mT), (ii) a broad (130mT) enhancement signals at g=2, and (iii) the signal at g=4. The results permitted to conclude that Coulomb bound polaron pairs are produced in PPV under photoexcitation. The narrow signal was shown, to appear due to a microwave induced resonant transition in triplet polaron pairs. The resonant transitions changed the rate of geminate recombination of the pairs that was revealed to lead to formation of triplet intra-chain excitons. Those excitons annihilated in the second order reactions showing themselves as delayed fluorescence. The annihilation rate was found to be influenced by resonant transitions in triplet exciton pairs as well. The lifetime of triplet intra-chain excitons was estimated from microwave modulation frequency dependence. The results showed that energy level of the lowest polaron pair state situated below that of singlet intra-chain exciton can act as a sink of the excitation energy influencing the quantum yields of the photoluminescence, electroluminescence and photoconductivity

Yields of triplet exciton pairs in polydiacetylene films

Triplet-triplet-induced absorption has been measured in pump-probe experiments on polydiacetylene DCH films. The excitation spectrum of the induced absorption has been obtained for pump photon energies in the range 1.95–3.40 eV. Above 2.15 eV the induced absorption depends linearly on the pump intensity. Below this energy a quadratic law is valid showing that triplet excitons are produced in a two-step process for the lower photon energies.

Delayed-fluorescence ODMR and EPR studies of triplet excitons in charge-transfer molecular crystals

Triplet excitons in electron donor—acceptor charge-transfer (CT) molecular crystals are generated through the intersystem crossing process by excitation in the CT visible band and give rise to delayed fluorescence. Delayed-fluorescence optically detected magnetic resonance (DF ODMR) in magnetic field is analyzed in terms of microwave-induced transitions between energy levels of either the isolated triplet excitons or the annihilating triplet exciton pair. The spin polarization of the triplet excitons plays an important role in the described phenomena. A comparison between DF ODMR and EPR spectra of the anthracene—tetracyanobenzene and biphenyl—tetracyanobenzene systems is presented. In the former case the microwave transitions occurring between free exciton sublevels are predominantly responsible of the DF ODMR signal, whereas the transitions between energy levels of the exciton pair are the most important for biphenyl—TCNB.

Production of pairs of singlet excitons and triplet excitons in anthracene crystals

In anthracene crystals excited by high-energy protons (10–50cV) there is enough energy to yield pairs of triplet excitons and singlet excitons. The efficiencies of these processes are measured as a function of the incident photon energy.

Excitation Spectra of Exciton Fission in Organic Crystals

AbstractIn a sensitive experimental arrangement the excitation spectra of singlet exciton fission into a pair of triplet excitons are measured via the magnetic field effect on the prompt fluorescence. For monomeric crystals as anthracene and β‐perylene the excitation spectra show similar structure which in the case of anthracene does not change remarkably upon perdeuteration. Excimer forming crystals as α‐perylene, however, exhibit a blue shift of the fission threshold of ≈ 3500 cm−1. This effect is explained by the competition between fission and excimer formation.

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.

Fission of singlet excitons into triplet-exciton pairs in molecular crystals

A kinetic model of singlet-exciton fission into pairs of triplet excitons in aromatic hydrocarbon crystals is presented. The model is based on Suna's hopping model for triplet-exciton fusion. The assumptions made in this description of fission and the relationship between fusion and fission are analyzed in detail. According to this theoretical model, the magnetic field modulation of anthracene hot singlet fission is the reverse of the modulation of triplet fusion. Accurate observations of fission, however, have shown there are some consistent differences, and these differences are discussed in terms of the fission process.

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.

Triplet exciton caging in two dimensions

It is proposed that the bimolecular process of triplet exciton fusion to form singlet excitons can be enhanced by reducing the size of the domain in which the triplet exciton pair is free to move. These small domains, or exciton cages, are much more effective when the host material is highly anisotropic, and the triplet excitons are constrained to move in one or two dimensions. In the present case, the host material is tetracene crystal in which triplet exciton diffusion is essentially two dimensional. The exciton caging is produced by introducing relatively high concentrations of a guest molecule that can reflect the triplet exciton rather than trap it; in the present case, 2,3-benzocarbazole (BC) is used. Polycrystalline mixtures of BC in tetracene were prepared in the mol fraction range 0%–50%. In tetracene, the mode of decay of the first excited singlet state is by fission into two neighboring triplet excitons that can undergo geminate recombination. The introduction of BC is found to increase the tetracene fluorescence lifetime at room temperature from 100 psec at 0% BC to 360 psec at 50% BC, while the lifetime at 77 °K is relatively constant. In addition, the characteristic increase in the tetracene fluorescence quantum efficiency with increasing excitation intensity is found to diminish as the BC concentration is increased. The lifetime and the relative fluorescence efficiency experiments are interpreted in part by invoking the concept of exciton caging, in which the geminate recombination of the original triplet exciton fission pair is enhanced by the presence of exciton reflectors that reduce the size of the domain in which the excitons can freely move. A computer simulation is made of the fission and fusion process in an ideal mixed crystal containing randomly distributed exciton reflectors, and the response of the model agrees well with the observed results.

Pressure Modulation of Exciton Fission in Tetracene

AbstractThe fluorescence quantum efficiency of tetracene crystals is found to increase rapidly with pressure. This behavior is interpreted as a reduction in the singlet exciton fission rate due to an increase in the energy separation between the singlet exciton state and the triplet exciton pair state. A red shift in the energy of the S0 → T1 transition by 15 ± 7 cm−1/kbar is used to explain the experimental results. This shift is discussed in terms of the interactions which control the transition energy.

Contributions to the theory of singlet exciton decay into pairs of triplet excitons in organic molecular crystals

Probability of singlet exciton decay into pairs of triplet excitons in organic molecular crystals is calculated. The dependence of the calculated probability on the separation of created pairs of triplet excitons, on temperature as well as on variations of equilibrium positions of molecules caused by changes of the state of electron excitations in the crystal is discussed. In conclusion there is given the estimation of the order of the probability value.

Magnetic effects on triplet exciton interactions

Abstract

Fission of singlet excitons into pairs of triplet excitons in tetracene crystals

Fission or dissociation of singlet excitions into pairs of triplet excitons in tetracene crystals is demonstrated by investigation of the magnetic field dependence of the prompt fluorescence intensity.

Fission of singlet excitons into pairs of triplet excitons in tetracene crystals

Fission or dissociation of singlet excitons into pairs of triplet excitons in tetracene crystals is demonstrated by investigation of the magnetic field dependence of the prompt fluorescence intensity.

Triplet Sampled Radiation damage

Abstract The emission produced by mutual annihilation of a pair of triplet excitons in single crystalline anthracene was observed to be an indicator of low level gamma-ray dose. Under steady red light illumination the blue emission intensiv Fr is related to gamm-ray dose D by F−1/2 = a + bD in the range 300R to 104R. Independent measurements show the effect cannot be due to changes in absorption in the red region, changes in quantum yield of fluorescence, nor to changes in the specific rate of triplet interaction. An interpretation based on dose induced quenchers and the long range of triplet excitons is given. By use of mulitple techniques an individual single crystal of anthracene can be used as a wide range dosimeter; triplet-triplet annihilatin in the range 102 to 104 R, space charge limited photo-current in the range 103 to 106 R, fluorescence quenching in the range 105 to 108 R, and absorption spectroscopy above 107 R.

Effects of Magnetic Fields on the Mutual Annihilation of Triplet Excitons in Molecular Crystals

The intensity of triplet-exciton annihilation luminescence in anthracene crystals at room temperature increases in weak magnetic fields up to a maximum increase of 5% at 350 Oe. For stronger fields the intensity decreases, finally leveling off at 80% of the zero-field value for $H\ensuremath{\gtrsim}5$ kOe. Sharp dips are found in the high-field intensity for field orientations at which a particular kind of level crossing occurs for pairs of triplet excitons. These effects result solely from field-induced changes in the rate constant for triplet-triplet annihilation.