Exciton migration model

The role of disorder in the photophysics of conjugated polymers has been extensively described by the work carried out in Marburg by H. Bassler and coworkers. Based on ultrafast photoluminescence (PL) (15], field-induced luminescence quenching [16J and site-selective PL excitation [17], a model for excited state thermalizalion was proposed, which considers interchain exciton migration within the inhomogenously broadened density of states. We will base part of the interpretation of our results in m-LPPP on this model, which will be discussed in some detail in Sections 8.4 and 8.6. 

In thin films of electroluminescent conjugated polymers, Kanner et al. [12] and Yan et al. [13] explained the quenching of PL by invoking an exciton migration mechanism to nonradiative traps based on the Balagurov-Vaks model [14], The survival probability of an exciton on a 1-D lattice (fi- = 1/3) is characterized by a diffusion time t(i. For times t t,i. the PL intensity follows... 

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...

For concentrated Bi compounds another model yields a similar temperature dependence, viz. mobile excitons with concentration nj and self-trapped excitons with concentration n and an energy difference AE, representing the thermal activation energy for exciton migration through the lattice. Unfortunately it is seldom checked whether the temperature dependence of the decay time in concentrated systems refers to an intrinsic property of an isolated luminescent centre or to an activation energy for migration. This, by the way, holds also for other compounds which have been discussed above. ... 

Hennebicq E et al (2005) Exciton migration in rigid-rod conjugated polymers an improved Forster model. J Am Chem Soc 127 47 - 762... 

For the band model phonons are not needed to cause energy migration as in the hopping model and instead exciton-phonon scattering limits the mean free path of the excitons and, thus, inhibits the migration at high temperatures. The time between scattering events for diffusion in the ith direction can be calculated from... 

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