Annihilation, triplet excitons

Kolubayev T, Geacintov N E, Paillotin G and Breton J 1985 Domain sizes in chloroplasts and chlorophyll-protein complexes probed by fluorescence yield quenching induced by singlet-triplet exciton annihilation Biochimica Biophys. Acta 808 66-76... 

The impurities may capture this migrating exciton and lose its excess energy. The mutual annihilation of two or more triplet excitons occurs in the same polymer chain and delayed fluorescence is observed. 

In pure crystals, singlet excitons can be created by mutual annihilation of triplet excitons. The intensity of the singlet exciton fluorescence depends quadratically on the triplet exciton concentration and is therefore proportional to the square of the singlet-triplet extinction coefficient. It is interesting to compare such a delayed fluorescence excitation spectrum, observed by Avakian et cd. 52) on naphthalene, with a corresponding phosphorescence excitation spectrum (Fig. 22). 

At present it is universally acknowledged that TTA as triplet-triplet energy transfer is caused by exchange interaction of electrons in bimolecular complexes which takes place during molecular diffusion encounters in solution (in gas phase -molecular collisions are examined in crystals - triplet exciton diffusion is the responsible annihilation process (8-10)). No doubt, interaction of molecular partners in a diffusion complex may lead to the change of probabilities of fluorescent state radiative and nonradiative deactivation. Nevertheless, it is normally considered that as a result of TTA the energy of two triplet partners is accumulated in one molecule which emits the ADF (11). Interaction with the second deactivated partner is not taken into account, i.e. it is assumed that the ADF is of monomer nature and its spectrum coincides with the PF spectrum. Apparently the latter may be true when the ADF takes place from Si state the lifetime of which ( Tst 10-8 - 10-9 s) is much longer than the lifetime of diffusion encounter complex ( 10-10 - lO-H s in liquid solutions). As a matter of fact we have not observed considerable ADF and PF spectral difference when Sj metal lo-... 

The RYDMR spectrum of the TBPDA (C6o)2 crystals is interpreted by a model, which considers external-field modulation of a triplet-triplet exciton annihilation rate constant [6], The kinetic model of triplet-triplet exciton annihilation can be presented as follows ... 

The migration of triplet energy in pure or mixed crystals by an exciton mechanism has been well established very recently.86 Thus the occurrence of delayed fluorescence in organic crystals is explained by triplet-triplet annihilation following triplet exciton migration.86 However, as yet, no conclusive evidence for the occurrence of triplet exciton migration in fluid media has been forthcoming. 

The relatively low values of yss and Ds in quasi-amorphous solids might be underlain by disorder (see Sec. 2.4.3) and/or a contribution of triplet excitons in quenching of fluorescent singlets (cf. Sec. 2.5.1.2). The diffusion coefficient of triplets is expected to be lower than of singlets since both energy donor and acceptor transitions are disallowed. A low value of yss has been found for the triplet-triplet annihilation rate constant from biexcitonic quenching... 

The pre-exponential factor depends on the definition of y and can amount 9/2 if in the triplet—triplet annihilation pathway for decay of triplet excitons a factor of 1/2 is introduced [192],... 

The triplet exciton quenching by doublet species is well established. Annihilation of triplet excitons has been observed on... 

The total concentration of holes nh is a sum of the concentration of trapped (nht) and free (nhf) carriers. However, often rihf/nht —> 0, nh nht due to a large concentration of traps. Then, the excitons are quenched by trapped carriers and the annihilation rate constant yTq is equivalent to the mobile exciton-immo-bile (trapped) charge carrier interaction rate constant yxq. Under space-charge-limited conditions, the concentration of charge is simply proportional to the applied voltage (U), nht = (3/2) o U/ed2, where d is the sample thickness, e is the electronic charge, s is the dielectric constant of the sample material, and s0 is the permittivity of free space. Thus, it may be seen that the fractional change in the triplet exciton decay rate... 

In 1967, Johnson et al. found that the delayed fluorescence of anthracene crystal could be modulated by external magnetic fields below 2 T [11]. The magnitude of the MFEs was dependent on both the field strength and the orientation of its single crystal. These MFEs were successfully interpreted in terms of triplet exciton spin Hamiltonian [12]. The delayed fluorescence is induced from the lowest excited singlet state ( Si) by the annihilation of two lowest triplet excited states ( T ). 

In 1967 Johnson et al. [1] discovered that the intensity of triplet-exciton annihilation luminescence in anthracene cryatal at room temperature increased in weak magnetic fields up to a maximum increase of 5 % at 35 mT and that the intensity decreased in higher fields. The intensity was found to level off at 80 % of the zero-field value at fl > 0.5 T. Their typical result is shown in Fig. 13-1. 

A triplet exciton annihilation mechanism has been proposed for charge generation in molecularly doped polymers initiated with very fast, high-intensity, excitation [25b,c]. In this mechanism it is proposed that both charge separation, associated with the Onsager model, and the fusion process are very strongly field-dependent. This model has not been invoked in recent years. 

The maximum value for the production of singlet excitons is assumed to be 25%, or 40% when we take into account T-T annihilation, so we should make full use of triplet excitons. 

In order to arrive at meaningful (exponential) decay rates in such experiments, non-linear triplet quenching by diffusion, has to be avoided. For example, the initial accelerated decay in Fig. 7 is caused by bimolecular triplet-triplet annihilation dominating the decay of the triplets. Similarly, a faster decay is observed at higher temperature, where triplet exciton diffusion to quenching sites is faster than monomolecular decay. Nevertheless, by using low temperatures and low excitation doses exponential decay kinetics are observed yielding radiative decay rates as low as 1 s x, which sets an upper limit for the triplet excited state lifetime [28,34],... 

Zaushitsyn Y, Jespersen KG, Valkunas L, Sundstrom V, Yartsev A (2007) Ultrafast dynamics of singlet-singlet and singlet-triplet exciton annihilation in poly(3-2 -methoxy-5 -octylphenyl)thiophene films. Phys Rev B 75(19) 195201-1—195201-7... 

List EJW, Scherf U, Mullen K, Graupner W, Kim CH, Shinar J (2002) Direct evidence for singlet-triplet exciton annihilation in pi-conjugated polymers. Phys Rev B 66(23) 235203-1-235203-5... 

A study of the comparative rates of triplet exciton migration has been carried out on molecularly doped polymers and on vinyl aromatic polymers. In both cases specific rate constants for triplet exciton migration were estimated from rate constants for triplet-triplet annihilation. The rate data were obtained by using a laser pulse-optical probe method to determine triplet concentrations directly by triplet-triplet absorption. It is found that triplet exciton migration rates for polymers are ten-fold to one-hundred-fold larger than those for doped polymer matrices probably due to the more dense local chromophore concentrations in the former. 

A triplet exciton localized at such a pair would be expected to spend, on average, fifty percent of its time at the molecule which originally absorbed the exciting photon and fifty percent at the partner site. Thus, triplet-triplet annihilation involving interaction of a second exciton with this pair-trapped species will, with equal probability, produce a DF photon nearly co-1 inear with the excitation polarization or else randomly oriented with respect to it. 

Polymers in Rigid Solution. The emission spectrum of PCVA in 2-methy1tetrahydrofuran (MTHF) at 77 K consists of prominent delayed fluorescence and phosphorescence bands(19). For this reason it was decided to investigate the rate of triplet exciton decay in these rigid solutions and to treat the data in terms of concurrent first and second order processes. For systems in which an equilibrium distribution of potential reactants may be assumed, eq 1 may be employed for data analysis. It is not clear, however, that such a distribution is valid for polymer solutions especially in light of evidence suggesting that T-T annihilations occur principally by intra-coil processes(14-15). 

It is the intermediate range which is not yet fully understood. Clearly, the distribution of the triplets eimong the chains should be governed by Poisson statistics which qualitatively predict the M- and intensity effects observed. Quantitatively, however, Poisson statistics fail to explain the effects, since M influences the DF and P intensity even in a region where there should be simultaneously many excitons per chain so that, statistically, there should be little differences between the individual macromolecules. Yokoyama et al. proposed (59) that triplet traps at the chain ends, supposed not to contribute to T-T annihilation, could explain the M dependence of DF. If this is true, there remains the question, why heterofusion, so common in other systems, should not work between triplet excitons moving along the chain and trapped triplets at the chain ends. 

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