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In the past year, the PI s research on the fundamental properties and applications of nanoclusters has been centered around gold and silver, and many different models have been developed to address the core packing, electron configurations and to explain some of the observed optical properties. Gold and silver in their atomic state shares the same number of valence electrons and bulk packing distance. In this report, we summarize our recent work on silver based nanoclusters. Recent work produced stable silver nanoclusters in the condensed phase, purified by gel separation. The basic synthetic approach is similar to that of gold nanoclusters. However, the silver nanoclusters have not been optimized like some other nanoclusters, where high purity can be achieved in a single-pot synthesis. Moreover, the stability of silver nanoclusters is also a major concern. By utilizing gel separation methods in their synthesis, our coworkers have been able to purify the nanocluster mixtures to produce highly pure mono-disperse nanoclusters. The result of this work leads to the synthesis of stable silver nanoclusters in high purity, which allows for the identification of silver nanoclusters by mass spectrometry. Due to the very small size of nanocluster and the difficulty in obtaining crystal structures, mass spectrometry has become essential in the identification of nanocluster formula. In this report we focus on the Ag32(SG)19, cluster. The initial interested in silver based system was not solely based on synthetic exploration. One of the most interesting optical property of gold system is its emission and it has tremendous potential in imagining. The larger bandgap of silver (compare to gold) should leads to stronger emission, which would further demonstrate the possibility of using nanoclusters as imaging agents on a cellular level. Ag32(SG)19 also offers an unexplored area in the ultrafast spectroscopy of nanoclusters.

The PI has developed several design approaches for the creation and investigation of organic macromolecules with large 2PA cross-sections important to the goals of the Air Force for optical limiting and sensor applications. The PI s research laboratory has investigated the critical issues in novel macromolecular systems concerning excitation localization and delocalization, disorder and rigidity as well as the role of through space interactions and quinoidal character contribution in these systems. We have also investigated the intersystem crossing process in Pt acetylide complexes in a broad range of temperatures 4.2K-294K. Utilizing the quantum size effects in novel gold and silver nanoclusters will lead to cheap and efficient optical limiters as well as to the development of new generation of twophoton probes for imaging applapplications. Wetinued the systematic investigation of the extraordinary nonlinear optical properties and femtosecond optical excitation dynamics in large oligomeric macromolecules possessing a macrocyclic topology. For these systems we have demonstrated superior two-photon cross-sections per unit molecular weight with very small linear absorption in the transparency window. It paves the way to development of highly concentrated and inexpensive optical limiting materials., The original document contains color images.

The nature of one and two-photon absorption enhancement in a series of oligothiophene dendrimers, recently proposed for applications in entangled photon sensors and solar cells, has been analyzed using both theory (time dependent density functional theory calculations) and experiment (fluorescence upconversion measurements). The linear absorption spectra exhibit a red shift of the absorption maxima and broadening as a function of dendrimer generations. The two-photon absorption cross sections increase sharply with the number of thiophene units in the dendrimer. The cooperative enhancement in absorption two-photon cross sections is explained by (i) an increase in the excited-state density for larger molecules and (ii) delocalization of the low-lying excited states over extended thiophene chains. Fluorescence anisotropy measurements and examination of the calculated excited-state properties reveal that this delocalization is accompanied by a size-dependent decrease in excited-state symmetries. A substantial red shift of the emission maxima for larger dendrimers is explained through the vibronic planarization of the longest linear R-thiophene chain for the emitting excited state. For higher generations, the fluorescence quantum yield decreases due to increased nonradiative decay efficiency (e.g., intersystem crossing). The detailed information about the dendrimer 3D structure and excitations provides guidance for further optimizations of dendritic structures for nonlinear optical and opto-electronic applications., Pub. in J. Phys. Chem. B, v114 n48 p15808 15817, 2010.

Support from the Army Research Office (ARO) has provided the opportunity for studies encompassing a combined effort consisting of different experimental and synthetic methods to produce novel nonlinear optical (NLO) materials with enhanced properties. This included the development of new synthetic branched organic macromolecules with different central core groups such as benzene, nitrogen, phosphorous, carbon as well as adamantine. These investigations gave the opportunity to probe the importance of the electronic coupling of the core unit to the NLO enhanced effect. The synthesis of novel metal nanoparticle topologies for the purpose of enhanced nonlinear transmission effects as well as nano-sensing applications was successful. This will included the synthesis of dendrimer metal nanocomposites where the PAMAM dendrimer may be considered as a host or a stabilizer. Fabrication of transition metal nanocomposite systems such as copper, gold, silver, as well as iron were carried out. The synthesis of chromophore functionalized dendrimer metal nanocomposites was carried out for the purpose of enhancing the optical nonlinear effects. Metal topologies such as silver and gold metal rods and spheres as well as metal clusters were investigated. New procedures to move these novel materials in to the solid state were carried out, which included the fabrication of stable films of the dendrimer metal nanocomposite materials. Measurements with the novel materials were also carried out in conjunction with the design and fabrication of new branched, cyclic and two-dimensional structures and metal nanoparticle systems as to provide a direct feed back to the understanding of the structure/function relationships. The photo-physical measurements included investigations of the nonlinear transmission effect at a variety of wavelengths with use of an optical parametric oscillator system., The original document contains color images.