Triplet exciton fusion

It is a competing process to Si So emission and is partly responsible for the the low fluorescence quantum yield from crystalline tetracene. Fission is also a biex-citonic process, which, like triplet exciton fusion, can be modulated by an applied magnetic field with a similar dependence as shown in Fig. 6.24 for fusion (see also Fig. 6.26). The fluorescence intensity of tetracene is therefore magnetic-field dependent [51]. 

Colliding triplet excitons are said to undergo triplet-triplet fusion. If they belong to the same species, the process is called homofusion. The collision of triplets belonging to different species is called heterofusion. The final products of the triplet-triplet interaction process are preceded by intermediate complex pair states ... 

The fusion of two triplets may give rise to autoionization as observed in the case of singlet exciton fusion (67)... 

Case I The triplet exciton concentration is too low to give rise to triplet-triplet fusion and the T2 terms in Eqs. (331) can be dropped. Then,... 

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 films and LEDs also exhibited PL-, EL-, and a-detected triplet exciton resonances. The primary PL-detected patterns were attributed to triplet excitons localized on the thiophene and phenylene rings, and only slightly larger. However, patterns due to other distinct triplets were observed in some films and LEDs. The nature of the triplet exciton ODMR was discussed in relation to the role of triplets as Si quenching sites, ground state recovery , and triplet-triplet fusion to S,. ... 

Excitons can, as we have seen in Sect. 6.9.2, react with one another and also with other excitation states. The most important processes are exciton annihilation or fusion and exciton splitting or fission. The annihilation of two triplet excitons, which leads to delayed fluorescence, takes place more precisely via the alternate reaction mechanism... 

Additional experimental evidence for triplet state involvement is provided by the quenching of both phosphorescence and delayed fluorescence by piperylene, a well known triplet quencher. In fact a study of its quenching effects on the delayed emission demonstrates that at least one of the triplet excitons involved in the fusion process is mobile. This is evident from the phosphoresence quenching curves (Stern-Volmer graphs) of PIVN and 1-ethylnaphthalene illustrated in Fig. 3. 

To explain the held dependence, we have to assume that the applied held has an influence on v hether the spin selection rules can be obeyed in the fusion process. A kinetic explanation by Merrifleld and Johnson [49, 50] assumes in the reaction Ti + Ti (Ti Ti) Si + So the existence of an intermediate pair state (Ti Ti), in v hich the tv o excitons repeatedly collide before they react. The possible spin correlations in this pair state have both triplet as well as singlet character. The triplet fraction in the pair state is also influenced by an applied magnetic field via the Zeeman interaction of the coupled individual spins with the field. The strength and direction of the field thus determine the relative fraction of triplet and singlet states in the pair. The singlet fraction leads to the states Si and So, and thus to delayed emission. Therefore, the intensity and lifetime of the emission can be modulated by an applied magnetic field. This holds for all biexcitonic processes in which two triplet states participate. 

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