Migration exciton

An even longer-range transfer, showing a 1/r3 dependence, may occur in crystals, solid solutions, and some fluids, as a result of exciton migration. The concept of the exciton was introduced by Frenckel to interpret certain crystal spectra an electron-hole pair was looked upon as an entity that could move about the crystal as a result of interactions between lattice sites. For the present purposes, the electronic excitation in an irradiated species can be regarded as an exciton that is free to wander over a considerable number of lattice sites. 

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The nature of the light emissions is influenced by the way in which the absorbed energy is transferred through the polymer matrix. In crystalline polymers, exciton migration is possible as all molecules lose their energetic individuality and all electronic and oscillation levels are coupled [20]. Thus, new exciton absorption and emission bands are formed and the excitation energy can move along the chain ... 

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. 

Fluorescence techniques have been demonstrated in recent reviews 1( to be powerful methods for obtaining detailed information on the molecular structure of biopolymers and synthetic polymers. The objective of the present review is to concentrate on two aspects of the photophysics of synthetic polymers — excimer formation and singlet exciton migration. Both topics have been considered recently 11 but in less detail. 

One point which can be settled qualitatively, however, is whether singlet exciton migration does in fact occur in the aryl vinyl polymers. It will be shown that available evidence supports energy migration as an important feature of the photophysics of polystyrene (PS), poly(l-vinyl naphthalene) (P1VN), and poly(2-vinyl naphthalene) (P2VN), the homopolymers which are the subject of the majority of the review. 

Scheme 3 summarizes this problem with a minimum number of sites and competing processes. In this scheme, two sites, square-well type (X) and spherical-well type (Y), are available for the residence of reactant molecules (A). For the sake of convenience, molecules residing at sites X and Y are labeled Ax and AY. Excitation of these molecules gives rise to A and A. Photoreactivity of molecules excited in each site will be identical if they equilibrate between X and Y before becoming photoproducts. In media with time-independent structures, such as crystals, equilibration requires diffusion of molecules of A in media with time-dependent structures, such as micelles and liquid crystals, equilibration can be accomplished via fluctuations in the microstructure of the reaction cavities as well as translational motion of A (Scheme 4). An additional mechanism for site selective reactions or equilibration of A and A molecules can be achieved via energy migration (e.g., energy hopping, exciton migration, or Forster energy transfer).

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... 

The exciton migration within aggregates of cyanine dyes and the possibility of oxygen diffusion into the porous dye film result in a bulk generation of photocurrent [80]. Photoholes produced due to the oxidation of excitons by molecular oxygen diffuse to the back contact. The diffusion coefficient of charge carriers in dye layer (Dc) can be evaluated from the potential-step chronoamperometric measurements in the indifferent electrolyte. Considering dye film as a thin-layer cell, the current vs. time dependence can be described as follows [81] ... 

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