Triplet excitons quenching

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

Reineke, S., Walzer, K., and Leo, K. 2007. Triplet-exciton quenching in organic phosphorescent light-emitting diodes with Ir-based emitters. Phys. Rev. B 75 125328. 

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 inclusion of 5 wt% PCBM [l-(3-methoxycarbonyl)propyl-l-phe-nyl-[6,6]C6i] in the spin-coating solutions resulted in efficient polymer emission quenching for all the polythiophenes studied. The transient absorption spectra of the amorphous poly(541)/PCBM blend film. At 10 ps exhibited an absorption peak around 700 nm, similar to that observed for the poly(541) pristine film. The shape of the transient spectrum varied with time, with the absorption peak shifting from 700 nm at 10 is to 900 nm for time delays >100 (is, demonstrating the formation of two distinct transient species in the blend film. The monoexponential lifetime was t = 8 (is under Ar atmosphere and significantly shortened under 02 atmosphere. Monoexponential phase is therefore assigned to the decay of poly(541) triplet excitons. 

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 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... 

Em and Merrifield [3] found that paramagnetic defects in anthracene crystals quenched triplet excitons and changed the field dependence from a negative to a positive gradient. They interpreted these MFEs in terms of the triplet-doublet quenching expressed by... 

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],... 

A distinction between the different possibilities can be achieved by time-resolved fluorescence and quenching experiments which have not yet been performed for aromatic polymers in solid solution. A more detailed discussion on exciton trapping in pseudo one-dimensional systems is presented in the next section, since much more information la available about phosphorescence quenching and triplet excitons than about fluorescence. 

There remain, however, some questions about the real meaning of these data and the correctness of the evaluation procedure. First of all, the data presented in Table II suggest efficient trapping of the triplet excitons by intrinsic, not exclmeric traps of unknown structure. This is corroborated by reports of all authors who performed quenching and phosphoresence decay measurements that the phosphorescence lifetime is not shortened by adding the quencher. This is a clear proof that it is primarily not the free triplet which is observed in phosphorescence but rather a trapped species, since otherwise Equation (5) should apply. 

A more discriminating picture of the role of traps in triplet exciton mobility can be drawn from the work by Webber (64, 65) who studied the kinetics of triplet quenching using a quencher molecule with a short lived triplet state (biacetyl) as a probe of the triplet exciton lifetime. In a study on P2VN (64), PACN (64) and PVCA (65) it has been shown that trapping is more efficient in PVCA compared to P2VN where triplet excitons exist long after the decay of delayed fluorescence (next section). 

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