Variable temperature UV/Vis spectrophotometer for study of NHC-stabilized gold nanoclusters

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Metal nanoparticles and nanoclusters are often highly coloured, making UV-vis spectroscopy a powerful technique for their analysis. Metallic nanoparticles were initially developed for their colour. In a technique dating back to the 15th century, artisans added gold chloride to molten glass, which resulted in bright red glasses, which were actually gold nanoparticles trapped in glass. Similarly yellow glasses resulted from the generation of silver nanoparticles. These red and yellow glasses were used in the preparation of stained-glass windows. Metal nanoclusters are also coloured but are substantially different from nanoparticles in several important ways. Firstly, they are typically smaller, with sizes ranging from 3 to 300 gold atoms. Secondly, they are prepared and isolated as single molecular entities, where nanoparticles are still isolated as mixtures with varying levels of polydispersity. Finally, because of these two properties, nanoclusters do not have plasmonic signals in their optical spectra, they have discrete molecular transitions. Because of this, UV-visible spectroscopy is a highly valuable method for the analysis of metal nanoclusters. Decomposition to nanoparticles is characterized by the appearance of plasmonic peaks and loss of individual transitions. The change from small clusters to larger clusters is also easily observed by UV-visible, and, in combination with computational studies, is highly effective at structural identification. *** Although they are fascinating molecules, metallic nanoclusters are still prepared using a black box approach that makes control of and prediction of structure largely a thermodynamic issue: namely, the most stable structures are isolated either by control of synthesis or by selective etching/crystallization or purification methods. Remarkably, despite the fact that these materials have been known for decades, a balanced equation cannot be written for their preparation. The infrastructure requested in this proposal is critical in righting this wrong. The use of N-heterocyclic carbenes as ligands is a key part in simplifying this process, since the starting material for the nanocluster synthesis is a simple organometallic NHC-Au-X species, rather than an unknown [RS-Au-] polymer. However, without the ability to analyze reactions as they progress by UV-visible spectroscopy and understand the process, it will be virtually impossible to gain the insight we need to understand and optimize the synthesis of these interesting materials. Similarly, understanding the stability of the novel NHC-functionalized nanoclusters we are preparing will be exceptionally difficult without temperature-controlled UV/vis analysis. Developing applications of these unique molecular materials, namely in solar cells, bio-imaging and catalysis, will be impossible without this instrument. HQP in the Crudden and Stamplecoskie groups will have hands on use of this instrument on a daily basis. **