Excitation transport

Barzykin A V, Barzykina N S and Fox M A 1992 Electronic excitation transport and trapping in micellar systems— Monte-Carlo simulations and density expansion approximation Chem. Rhys. 163 1-12... 

When the process can repeat itself so that the excitation migrates over several molecules, it is called excitation transport or energy migration. 

Homotransfer does not cause additional de-excitation of the donor molecules, i.e. does not result in fluorescence quenching. In fact, the probability of de-excitation of a donor molecule does not depend on the fact that this molecule was initially excited by absorption of a photon or by transfer of excitation from another donor molecule. Therefore, the fluorescence decay of a population of donor molecules is not perturbed by possible excitation transport among donors. Because the transition dipole moments of the molecules are not parallel (except in very rare cases), the polarization of the emitted fluorescence is affected by homotransfer and information on the kinetics of excitation transport is provided by the decay of emission anisotropy. 

The relationship between excitation transport and fluorescence depolarization in two and three dimensional disordered systems has been discussed by Anfinrud and Struve . In the usual discussion of excitation transport by dipole-interaction it is conventional to assume that excitation is completely depolarized after a single hop. This supposition has been critically examined and a theory formulated suitable for application to Langmuir Blodgett films and absorbed species. 

Spectroscopy is a sensitive probe of the interactions between chromophores that can be very important at distances characteristic of condensed phases. Excitation transfer between chromophores is a simple example. In pure phases of excitable molecules we observe coherent many-body effects. The interesting excitation transport, localization and dephasing phenomena that take place in such systems are largely beyond the scope of this text but they are obviously important manifestations of optical response of condensed phase molecular systems. 

The application of this technique as a morphological tool requites that there be a close coupling between polymer photophysics and polymer physics. In the photophysical studies described in this paper emphasis will be placed on the development of analytical models for electronic excitation transport (EET). The areas of polymer physics that we will consider involve the configurational statistics of Isolated chains and phase separation in multicomponent polymer systems. The polymer system of primary interest is the blend of polystyrene (PS) with poly(vinyl methyl ether) (PVME). 

We now decided to extend our measurements to low temperatures, in the hope of slowing down excitation transport to an accessible time regime. Emission spectra for pure polystyrene at low temperatures are shown in Figure 6. At the lowest temperatures a very strong monomer-like emission is evident. As the temperature is increased, this emission rapidly decreases, while excimer emission increases, resulting in a clean isoemissive point at 327 nm (13). 

A method for calculating observables resulting from incoherent excitation transport among chromophores randomly tagged in low concentration on isolated, flexible polymer chains is described. The theory relates the ensemble average root-mean-square radius of gyration ) of a polymer coil to the rate... 

Analysis of experiments which monitor the rate of excitation transport among naphthyl chromophores in low concentration on isolated coils of poly-(2-vinylnaphthalene-co-methyl methacrylate) in poly-(methyl methacrylate) (PMMA) host by time-resolved fluorescence depolarization spectroscopy allows the quantitative detemination of the copolymer... 

There has been increasing interest in recent years in using incoherent electronic excitation transport as a probe of molecular interactions in solid state polymer systems. The macroscopic properties of such systems arise from the microscopic interaction of the individual polymer chains. The bulk properties of polymer blends are critically dependent on the mixing of blend components on a molecular level. Through the careful adjustment of the composition of blends technological advances in the engineering of polymer materials have been made. In order to understand these systems more fully, it is desirable to investigate the interactions... 

The dependence of excitation transport on local chromophore concentration has been used to provide qualitative information on the characteristics of polymers in blends. Excimer fluorescence resulting from excitation transport has been employed to characterize polymer miscibility, phase separation and the kinetics of spinodal decomposition (1-31. Qualitative characterization of phase separation in blends (4.51 and the degree of chain entanglement as a function of sample preparation and history (6.71 has also been investigated through transport with trapping experiments. In these experiments one polymer in the blend contains donor chromophores and the second contains acceptors. Selective excitation of the former and detection of the latter provides a qualitative measure of interpenetration of the two components. 

Due to the sensitivity of electronic excitation transport to the separation and orientation of chromophores, techniques which monitor the rate of excitation transport among chromophores on polymer chains are direct probes of the ensemble average conformation (S). It is straightforward to understand qualitatively the relationship between excitation transport dynamics and the size of an isolated polymer coil which is randomly tagged in low concentration with chromophores. An ensemble of tagged coils in a polymer blend will have some ensemble averaged root-mean-squared radius of gyration,... 

Previous experiments measuring fluorescence depolarization arising from excitation transport among chromophores on isolated guest colls in solid polymer blends demonstrated the feasibility of determining the relative size of individual chains in various host environments (18). The ability of these experiments to... 

Although both theories correctly predicted the shape of the time-dependent anisotropy, the EF theory predicted values for theta-condition PMMA which were lower than expected (as compared to determinations by light scattering and neutron scattering) while the FAF theory resulted in values which were too high. The reason for the disparities in the excitation transport size determinations has been shown to be due to Inadequate models of the spatial distribution of chromophores on a polymer chain (18). 

The accurate description of excitation transport on Isolated polymer colls is an Interesting and difficult problem. 

Chromophores attached to a polymer present an Inhomogeneous medium for excitation transport. Rather than being randomly distributed, as in a solution, the positions of the chromophores are correlated through the covalent bonds of the polymer. Also, the finite size of the polymer limits the number of sites the excitation can sample. This inhomogeneity in the chromophore distribution resulting from the requirements of polymer chain structure can... 

If the transition dipoles of the chromophores in a solid polymer matrix are randomly oriented, the main source of depolarization in these experiments will be due to excitation transport. The initially excited ensemble is polarized along the direction of the excitation E field and gives rise to polarized fluorescence. Transport occurs into an ensemble of chromophores with randomly distributed dipole directions and the fluorescence becomes unpolarlzed. The random distribution is assured by the low concentration of the chromophores. To a slight extent, on the time scale of interest, depolarization also occurs as a result of chromophore motion. In this case the fluorescence anisotropy is approximately... 

In order to obtain G (t) for a given polymer, experiments on two different samples must be performed. These samples differ only in that the guest copolymers have a different fraction of chromophore containing monomers. Copolymer A is the polymer of Interest and has an appreciable number of chromophores, such that excitation transport will occur. Its fluorescence anisotropy, r (t), is given by Equation 10. Copolymer B has such a small ntmiber of chromophores that excitation transport is negligible (G (t) - 1) and only chromophore motion contributes to the anisotropy. 

G (t) arising from the excitation transport on copolymer A can be calculated from the two experimental anisotropies ... 

Earlier experiments have shown the utility of excitation transport measurements in providing relative information regarding coll size in pol)rmer blends (18). Here, we will summarize the results of recent experiments (28) which demonstrate that monitoring excitation transport on isolated colls in solid blends through time-resolved fluorescence depolarization techniques provides a quantitative measure of for the guest pol3mier. 

A and 64 7 A, respectively. The results from the excitation transport experiments are in excellent agreement with these values. 

Copolymer (R 2) /2 Determined by Excitation Transport R 2)1/2 for equivalent tf-condition PUMA... 

These experiments, in conjunction with the theory described here, demonstrate the utility of excitation transport induced fluorescence depolarization in the study of pol3rmer blends. The technique allows quantitative determination of for... 

Excitation transport experiments have already provided useful qualitative information on polymer systems. However, the... 

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