Ultrafast Optical Study of Small Gold Monolayer Protected Clusters: A Closer Look at Emission

Monolayer-protected metal nanoclusters (MPCs) were investigated to probe their fundamental excitation and emission properties. In particular, gold MPCs were probed by steady-state and time-resolved spectroscopic measurements; the results were used to examine the mechanism of emission in relation to the excited states in these systems. In steady-state measurements, the photoluminescence of gold clusters in the range of 25 to 140 atoms was considerably stronger relative to larger particle analogues. The increase in emission efficiency (for Au25, Au55, and Au140 on the order of 10-5) over bulk gold may arise from a different mechanism of photoluminescence, as suggested by measurements on larger gold spheres and rods. Results of fluorescence upconversion found considerably longer lifetimes for smaller gold particles than for larger particles. Measurements of the femtosecond transient absorption of the smaller clusters suggested dramatically different behavior than what was observed for larger particles. These results, combined with the result of a new bleach band in the transient absorption signal (which is presumably due to an unforeseen ground state absorption), suggest that quantum size effects and associated discrete molecular-like state structure play a key role in enhanced visible fluorescence of small clusters

Origin of spectral broadening in pi-conjugated amorphous semiconductors

We present a study of the picosecond fluorescence dynamics of pi-conjugated semiconducting organic dendrimers in the solid state. By varying the degree of branching within the dendrons, referred to as the dendrimer generation, a control of intermolecular spacing of the emissive core and therefore of the lattice parameter for Forster-type energy transfer is achieved. This allows a distinction between spectral diffusion and excimer formation as the two main sources of spectral broadening in organic semiconductors. Whereas Forster-type dispersive spectral relaxation is independent of temperature but strongly dependent on the interchromophore distance, excimer formation is also strongly thermally activated due to temperature-dependent conformational changes and the influence of thermally activated dynamic disorder. The rapid spectral diffusion allows a determination of the excimer rise in the emission, which is shown to have a profound impact on the steady state luminescence properties of dendrimer films. We show that the dendrimer generation not only allows a microscopic control of intermolecular interactions but also a direct control of the rate of spectral diffusion. Implications for the design of novel materials for optoelectronic devices are discussed

Femtosecond luminescence dynamics in a nonlinear optical organic dendrimer

The ultrafast intrinsic dynamics of an organic dendrimer in solution and in a thin film is reported using fluorescence upconversion spectroscopy. Femtosecond decay is detected at higher emission energies, while at lower energies a fluorescence rise time (∼3 ps) was observed that is dependent on the solvent’s polarity. A strong excitation energy dependence of the decay pattern was also observed. Different synthetic functional groups that comprise the macromolecular dendrimer structure were investigated. The mechanism, which describes the complex dynamics in the dendrimer system, was found to be associated with the excitation of the attached chromophore nitroaminostilbene. These results indicate the absence of excited-state interactions of functional groups within the dendrimer macromolecule. A model, which includes the existence of an intermediate nonradiative state, is proposed to describe the complex ultrafast fluorescence dynamics in the dendrimer system