We report on the synthesis and spectroscopic characterization of well-defined hybrid structures that consist of a gold core overcoated with a silica shell, followed by a dense monolayer of CdSe nanocrystal quantum dots (QDs). The dielectric silica spacer of a controlled thickness provides a simple means for tuning interactions between the QD emitters and the metal core. To illustrate this tunability, we demonstrate switching between QD emission quenching and enhancement by varying the silica shell thickness. Synthetic procedures developed here employ a final step of self-assembly of QDs onto the silica shell performed via simple titration of the QD solution with prefabricated core/shell Au/SiO2 particles. This approach allows us to perform an accurate quantitative analysis of the effect of the metal on the QD emission intensity. One important result of this analysis is that nonuniformity of nonradiative rates across the QD ensemble has a significant effect on both the magnitude and the shell-thickness dependence of the emission enhancement/quenching factors.

Three Pulse Photon Echo Peak Shift data are reported for the B850 band of the peripheral light-harvesting complex (LH2) of Rb. sphaeroides at room temperature. The data are obtained for both the detergent solubilized and purified complex, and the native membrane samples containing LH2 as the sole bacteriochlorophyll-protein complex. Both samples reveal an ultrafast decay on a 100−200 fs time scale that is attributed to exciton relaxation dynamics within individual aggregates. The samples in native membranes also contain a ∼5 ps decay component which is assigned to inter-complex, LH2−LH2 energy transfer. The observation of the 5 ps component in the membrane samples strongly suggests the presence of two levels of energetic disorder in the system.

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