Energy transfer in unsymmetrical phenylene ethynylene dendrimers

We have applied the fluorescence upconversion technique to explore the electronic excitation energy transfer in unsymmetrical phenylene ethynylene dendrimers. Steady-state emission spectra show that the energy transfer from the dendrons to the core is highly efficient. Ultrafast time-resolved fluorescence measurements are performed at various excitation wavelengths to explore the possibility of assigning absorption band structures to exciton localizations. We propose a kinetic model to describe the time-resolved data. Independent of the excitation wavelength, a typical rise-time value of 500 fs is measured for the fluorescence in the dendrimer without an energy trap, indicating initial delocalized excitation. While absorption is into delocalized exciton states, emission occurs from localized states. When an energy trap such as perylene is introduced on the dendrimer, varying the excitation wavelength yields different energy-transfer rates, and the excitation energy migrates to the trap through two channels. The interaction energy between the dendrimer backbone and the trap is estimated to be 75 cm(-1). This value is small compared to the vibronic bandwidth of the dendrimer, indicating that the monodendrons and the energy trap are weakly coupled.     

Synthesis and characterization of colloidal ternary ZnCdSe semiconductor nanorods

For the synthesis of colloidal ternary ZnCdSe nanorods, CdSe nanorods were first prepared under a mixture of tetradecylphosphonic acid/trioctylphosphine oxide surfactants at 250 degrees C, and then ZnSe shell layer was grown onto CdSe nanorods at 180 degrees C, forming CdSeZnSe core/shell nanorods. Green-yellow emitting ternary ZnCdSe nanorods were obtained by a subsequent alloying process at 270 degrees C for 1-3 h through the diffusion of Zn ions into CdSe nanorods. The photoluminescence quantum yield (QY) of ZnCdSe nanorods was 5%-10%, which is higher than that from pristine CdSe nanorods (0.6%). The QY of these alloy nanorods depends on the alloying time and is discussed in terms of compositional disorders and defects produced by the alloying process. The Raman and time resolved photoluminescence spectroscopies were used to understand the detailed alloying process from CdSeZnSe core/shell to ZnCdSe alloy nanorods.     

The role of exciton hopping and direct energy transfer in the efficient quenching of conjugated polyelectrolytes

The dynamics of fluorescence quenching of a conjugated polyelectrolyte by a cyanine dye are investigated by femtosecond fluorescence up-conversion and polarization resolved transient absorption. The data are analyzed with a model based on the random walk of the exciton within the polymer chain and a long-range direct energy transfer between polymer and dye. We find that rapid intrachain energy migration toward complex sites with the dye leads to the highly efficient energy transfer, whereas the contribution from direct, long-range energy transfer is negligible. We determine the actual density of complexes with the dye along the polymer chain. A clear deviation from calculations based on a constant complex association constant is found and explained by a reduced effective polymer concentration due to aggregation. Altogether, the quenching efficiency is found to be limited by (i) the energetic disorder within the polymer chain and (ii) the formation of loose polymer aggregates.