Energy Transfer in Organic Dendrimer Antenna Funnel and Anti-Funnel Supermolecules
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Mat. Res. Soc. Symp. Proc. Vol. 543 ©1999 Materials Research Society
and solvent. In addition, anomalously large excimer formation is observed under particular conditions, and is seen to be strongly dependent on excitation wavelength. This unusual behavior is proposed to be the result of solvated molecular geometry and intermolecular interactions. EXPERIMENT Two types of dendrimers, both composed of repeat phenylacetylene units, have been studied using steady state and time-dependent fluorescence lifetime techniques. The first is a series of molecules, known as 'compacts,' which have all molecular units composed of diphenylacetylene chains (Fig. IA). The second dendrimer type is referred to as 'extended,' (Fig. 1B) and is identical except that the generational units are composed of increasing length phenylacetylene (PA) chains toward the molecular locus. Both types have been derivatized with a perylenic moiety at the center, which acts as an exciton trap (vide infra.) It has been shown previously [12,13] that due to the metaposition branching at each node, each PA chain is conjugatively delocalized from neighboring units. The result is that following photoexcitation, excitons are localized on specific chromophores, rather than being delocalized over the molecular backbone. Each unit of the compact dendrimers, being composed of identical length units, has an identical frequency absorption spectrum, regardless of generation number and molecular size. This pinning of the excitation energy, combined with a monotonic increase in optical absorption power, sheds some interesting light on photon harvesting antenna funnels and "anti-funnels." The truly fractal (Bethe lattice or Cayley tree) dendrimers (Fig. IA) form exciton "anti-funnels" instead of "funnels" (Fig. 2). A simple hopping model [14] shows that the larger the size (generation number) of the dendrimers, the larger the "anti" effect. Quantitatively, the mean passage time from the absorbing canopy to the trapping center grows exponentially with size. In contrast, for a linear, square or three-dimensional lattice antenna, it only grows algebraically. However, with an "extended" fractal design (Fig. 1B) an energy funnel is achieved (Fig. 2A), and now the mean passage time grows only linearly with size (while the absorption power grows nearly exponentially). Samples were prepared in spectroscopic grade dichloromethane and n-hexane (Fisher), and degassed by bubbling with N 2. Serial dilution was used to prepare extremely low concentration samples (10- 7 M to 10-9M) in order to avoid significant intermolecular interaction. Steady state fluorescence measurements have been done on an ISS Fluorolog spectrometer. Excited state fluorescence lifetime studies were done using time-correlated single photon counting [15]. The tunable laser excitation source was an argon-ion pumped mode locked Ti:S (Spectra Physics Tsunami, 82MHz), followed by frequency doubling and tripling (GWU-23FS). Excitation pulse repetition (4 MHz) was determined with a pulse picker (Conoptics 350-1
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