Electronic energy migration

Since the properties of the modular components are known and different modules can be located in the desired positions of the dendrimer array, synthetic control of the various properties can be obtained. It is therefore possible, as schematically shown in Figure 2, to construct arrays where the electronic energy migration pattern can be predetermined, so as to channel the energy created by light absorption on the various components towards a selected module (antenna effect). 

Electronic Energy Migration and Bichromophoric Processes in Aryl Vinyl... 

Electronic energy migration (or Hopping) The movement of electronic excitation energy from one molecular entity to another of the same species, or from one part of a molecular entity to another of the same kind (e.g. excitation migration between the chromophores of an aromatic polymer). The migration can happen via radiative or radiationless processes. 

It has been suggested that in the solid state electronic energy migrates rapidly from the initially excited chromophore to excimer tr sites by either exciton diffusion or single-step mechanisms. 

The challenge which these naturally occurring processes presents, of course, involves the laboratory construction of organizational units which also are able to perform some pre-determined, photo-initiated task. It seems to be widely accepted that a necessary first step to this end requires a clear understanding of the interaction between the mechanism of electronic energy migration and the role played by the positioning of... 

The photophysical behavior of poly(vinyl aromatics) has stimulated much interest from both practical and theoretical points of view. The proposal that such molecules exhibit electronic energy migration, e.e.m., resulted in, among other things, development of the concept of synthetic antennae macromolecules (1 ). The object of this paper is to introduce a note of caution to interpretation of observed photophysical phenomena. The bulk of the discussion is concerned with polystyrene, although the behaviour of poly(vinyl naphthalene) will also be considered. 

The great sensitivity of fluorescence spectral, intensity, decay and anisotropy measurements has led to their widespread use in synthetic polymer systems, where interpretations of results are based upon order, molecular motion, and electronic energy migration (1). Time-resolved methods down to picosecond time-resolution using a variety of detection methods but principally that of time-correlated single photon counting, can in principle, probe these processes in much finer detail than steady-state techniques, but the complexity of most synthetic polymers poses severe problems in interpretation of results. 

ENERGY TRANSFER AND ELECTRONIC ENERGY MIGRATION PROCESSES... 

Excimer formation has been studied in polystyrene and poly(a-alkylstyrenes)189 (PS), poly(vinylcarbazole),139>140 poly-(2-vinylnaphthalene), and poly-(4-vinyl-biphenyl).141 For polystyrene films, David et a/.189 showed that the fluorescence yield increased with increasing crystallinity, at both ambient temperature and 77 K. The contribution of excimer fluorescence yield increased in the sequence atactic (0.7) < atactic oriented (0.60) < isotactic amorphous (0.28) < isotactic crystallized (0.01), with normal yields relative to excimer given in parentheses. Similar results were obtained for poly(vinylcarbazole), PVCZ, although the contribution of excimer fluorescence at 77 was independent of crystallinity. The results can be interpreted in terms of electronic energy migration to low-energy defect sites from which excimer emission can occur. In PVCZ copolymers with fumaronitrile (10), diethyl fumarate (11), and diethyl maleate (12) (Scheme 6),... 

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