Triplet Exciton Dynamics

Dexter-type triplet energy transfer intrinsically requires a significant orbital overlap of the donor and acceptor molecules and thus exhibits a pronounced distance dependence, which is expressed in an exponential dependence of the transfer rate on the molecular distance between donor and acceptor  

A prominent and widely appUed model of electron transfer in organic molecules is Marcus theory. It is thoroughly introduced in Section 4.3.1.1 and hence we concentrate here on its applicabiUty to triplet energy transfer supported by recently reported experimental studies of the temperature dependence of the triplet diffu-sivity and of the effect of disorder on triplet transfer in organic molecules. We would also like to refer the interested reader to a recent overview on experimental and theoretical work concerning a unified description of triplet energy transfer by Kohler and Bassler [30]. 

In classical Marcus theory (see below), the parameters that determine the rate of an electron transfer process between a donor and an acceptor are the 

Having discussed the triplet generation and the microscopic mechanism of triplet motion in organic molecules, we will now turn to have a closer look at the dynamics of triplet states in ordered and disordered systems. In general, the 

At low triplet densities, the bimolecular term can be neglected and thus the intensity of the phosphorescence, which is given by 7(t)ph = kr[T](f)]. follows  

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Another interesting example concerning exciton dynamics was also described by Kim and Weissman (83,84). They applied the transient magnetization technique to the study of the photoexcitation of phenazine doped with anthracene. In this system photo-excitation in the phenazine absorption region leads to a selectively populated phenazine triplet exciton which transfers the excitation to anthracene with conservation of polarization (27). The authors emphasized that the time dependence of the transient response requires further analysis, which must include an adequate quantum theoretical treatment on the evolution of the system shortly after excitation. One can expect that future refinement of this technique will lead to more exciting studies of the dynamics of triplet states and excitons in solid state. 

Mohwald, H., Erdle, E., and Thaer, A. Orientational phase transition in a charge-transfer crystal triplet excitons as probes for lattice dynamics. Chemical Physics 27, 79-87 (1978). 

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... 

In Chap. 6, we discussed low-energy optical excitation states, the singlet and triplet excitons and energy transfer. The primary experimental method applied there was optical spectroscopy in the visible, in the near IR and in the UV spectral ranges. In the present chapter, we treat the structure and the dynamics of localised triplet states, of triplet mini-excitons, and of triplet excitons in molecular crystals. The primary experimental method for the investigation of the lowest-energy triplet level Ti is electron-spin resonance (ESR) (Fig. 7.1). 

From the analysis of the ESR line shape and width using the model of motional narrowing, one obtains the following results for the dynamics of the quasi-one-dimensional triplet excitons in DBN ... 

Montes, VA., Perez-Bolivar, C., Estrada, L.A., Shinar, J., Anzenbacher, R, 2007. Ultrafast dynamics of triplet excitons in Alq3-Bridge-Pt (If) porphyrin electroluminescent materials. J. Am. Chem. Soc. 129,... 

Better resolution of an excimer intermediate in fission was achieved in a more recent study of TIPS-tetracene solutions (Fig. 7). In this work, the diffusion-limited dynamics, endothermic energetics and unusually sharp triplet exciton absorption features enabled identification of a spectroscopically distinct intermediate state in transient absorption and photoluminescence. In contrast to TIPS-pentacene, TIPS-tetracene represents a typical tetracene system where singlet fission is endothermic by 200 meV. This endothermicity is well known to have a drastic effect on the rate of fission in the solid state where triplet formation occurs three orders of magnitude slower in films of tetracene than in pentacene. Recent work suggests that fission in tetracene may not require thermal activation. Notably, the decay of singlet excitons and the rise of triplet exciton absorption have been shown to occur independent of temperature. A low-lying, dark intermediate state in tetracene was invoked to explain these observations, however it had been difficult to isolate such a state experimentally. ... 

In general, the PA2 dynamics differ from the SE or PAj dynamics due to contributions from other species, such as triplets, polarons, and PPs [34,55]. Specifically, in the case of pristine PPVDO films PA2 appears to have a long-lived component, which is also observed in DOO-PPV [34]. The long-lived component was attributed to triplet excitons produced from the singlet excitons by intersystem crossing [62]. There are also discrepancies between the PA and PA2 dynamics on the subpicosecond timescale. The match between the PAi and SE dynamics, on the other hand is almost perfect. Their ps decays are virtually identical, and the rise-time dynamics are also very close to each other, as shown in Figure 22.9b. 

The improved order in films cast from toluene may also be induced by prolonged illumination at low temperatures, as recently discovered at the Technion [155]. Delocalized photoexcitations among adjacent chains may lead to several characteristic properties that are not common in isolated chains. These are [71] (i) reduced PL quantum efficiency, (ii) PL redshift, (iii) relatively large generation of PP excitations, rather than intrachain excitons, (iv) more substantial delayed PL due to PP recombination, (v) reduced formation efficiency of triplet excitons since the intersystem process is hampered by the interchain delocalization [156], and (vi) increased exciton dissociation efficiency in Ceo" doped polymer films. It is thus important to review the ultrafast excitation dynamics in MEH-PPV in comparison with those of DOO-PPV. 

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