Triplet exciton migration

Strong coupling would not be expected for many fluid solvents but weak coupling could lead to energy transfer from solvent to solvent molecule at rates in excess of diffusion rates. This could occur as a result of singlet or even triplet exciton migration. 

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. 

Table 2 Various Singlet and Triplet Exciton Migration Parameters in Some Organic Solids...

Rothe C, Monkman AP (2003) Triplet exciton migration in a conjugated polyfluorene. Phys Rev B 68(7) 075208... 

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. 

These results may well be typical of most polymer/dopant systems. Certainly the behavior of these systems with respect to triplet exciton migration seems quite similar in spite of their rather distinctive molecular architecture. Let us now turn to an examination of systems in which chromophore units are regularly arranged by virtue of their being bonded to the backbone of a polymer chain. In this way, it may be possible to assess the effects of chromophore organization on the mechanism of triplet exciton migration and decay. 

Furthermore, the diffusion coefficients for triplet exciton migration extracted from this three-dimensional intramolecular model were nearly the same as those obtained using the conventional kinetic equation (i.e. eq 1). The hopping frequencies for triplet exciton migration in PVCA for these three models are summarized in Table 111(20). Neither the electron exchange mechanism(21) nor the Forster... 

Table III. Frequencies of Triplet Exciton Migration for Rigid Solutions of PVCA in MTHF at 77 K Using Various Models for...

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. 

As well known, so-called excimer sites exist in poly-N-vinylcarbazole. It is well established that these excimer sites are the efficient traps for the singlet and triplet excitons, which migrate along the polymer chain. The structure of these sites are thought to be a special conformation having a pair of carbazolyl groups arranged parallel each other. 

Triplet Decay and Exciton Migration. The rate of triplet state decay following a photoexcitation pulse is conveniently followed by monitoring the time dependence of the delayed fluorescence. A limitation of this approach is that absolute triplet concentrations cannot usually be evaluated and so rate constants for processes having a kinetic order greater than unity cannot be determined. 

Energy transfer is a ubiquitous phenomenon in molecular crystals in which singlet and triplet excitons with lifetimes of nanoseconds and up to 10-100 milliseconds, respectively, migrate incoherently via either dipole-dipole or exchange interaction. Typical intermolecular jump times are 1 ps for singlet excitons and 10 ps for triplets [2], This implies that a singlet (triplet) exciton can visit some... 

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