Research
Research is being carried out in three areas of chemistry: photochemistry/photophysics, catalysis, and bioinorganic chemistry.Photochemistry and photohysics of metal-metal bonded exciplexes
Photochemical-photophysical studies are being carried out on metal-metal bonded excimers and exciplexes. In solution or in the solid state, the d10 dicyanoaurate(I) and dicyanoargentate ions form luminescent oligomers.1 These systems can be tuned with the luminescence energy varying over 18000 cm-1. We have reported2 the successful synthesis and characterization of lanthanum tris(dicyanoaurate) trihydrate, La[Au(CN)2]3·3H2O, lanthanum tris(dicyanoargentate) trihydrate, La[Ag(CN)2]3·3H2O, and the mixed metal system La[AgxAu1-x(CN)2]3·3H2O (x = 0.83, 0.55, 0.33, 0.19). Single crystal X-ray diffraction analysis revealed that the complexes are isostructural and the lanthanum ion provides a bridging network and charge balance for the systems. The mixed metal complexes were found to be strongly luminescent at ambient temperature and that the luminescence spectra were dependent on the Au/Ag stoichiometric ratio (Figure, below). That is, the position of the emission and excitation peak maxima can be tuned by varying the Ag/Au ratio. We assign the emission in mixed metal crystals to arise from delocalized mixed metal clusters with different Ag/Au stoichiometric ratios.
We have also reported the study of the series of heterobimetallic complexes Eu[AgxAu1-x(CN)2]3 (x = 0, 0.25, 0.50, 0.75, 0.90, 1.0).3 The Ag/Au ratios in the mixed metal systems were computed by integration of the CN stretch peak areas taken from Raman measurement---a technique that allowed us to determine the Ag/Au loading for samples where suitable single crystals for X-ray structure analysis cannot be grown. We found that energy transfer occurs from the tunable [AgxAu1-x(CN)2]- donor excited states to the luminescent Eu3+ acceptor ions. The energy tunability of the donor ion is dependent on the Ag/Au stoichiometric ratio. The mixed metal systems with higher Ag loading (Eu[Ag0.85Au0.15(CN)2]3 and Eu[Ag0.66Au0.34(CN)2]3) have better energy transfer than mixed metal systems with less Ag (Eu[Ag0.35Au0.65(CN)2]3 and Eu[Ag0.14Au0.86(CN)2]3). This is due to the greater extent of overlap between donor emission and acceptor absorption and in more acceptor states being available for the Eu3+ ion involved in the energy transfer processes.
Inorganic complexes with this unusual photochemical behavior can be used to build solid state photonic systems. There are also applications in sensors and chemical assays using luminescence detection as well as nanoscale science and engineering. Funding for this project is provided by the National Science Foundation (NSF) and the American Chemical Society's Petroleum Research Fund (ACS-PRF).
References
1. Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H., J. Am. Chem. Soc. 2000, 122, 10371-10380; Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H.; Fackler, J. P., Jr., J. Am. Chem. Soc. 2001, 123, 11237-11247. Rawashdeh-Omary, M. A.; Larochelle, C. L.; Patterson, H. H., Inorg. Chem. 2000, 39, 4527-4534.
2. Colis, J. C. F.; Larochelle, C.; Staples, R.; Herbst-Irmer, R.; Patterson, H., Dalton Trans. 2005, 675-679.
3. Colis, J. C. F.; Staples, R.; Tripp, C.; Labrecque, D.; Patterson, H., J. Phys. Chem. B 2005, 109, 102-109.
Photodecomposition and Fuel Cell Catalysis
Research is also being carried out in the areas of photodecomposition of pesticides and nerve agents and removal of CO from hydrogen fuel for use in Polymer Electrolyte Membrane (PEM) fuel cells. Metals such as Ag, Fe, Mo and Co are being doped either individually or simultaneously into porous alumminosilicate nanostructures called zeolites. These zeolite-supported homometallic or heterometallic systems, when in the presence of specific wavelengths of UV light, exhibit photodecomposition of pesticides and catalyze reactivity between CO and O2 to create CO2. To conduct this research, unique characterization experiments using techniques such as low-temperature solid state luminescence spectroscopy and resonance Raman spectroscopy, kinetics experiments using an OPOTEK Opollette UV tunable laser source and a state-of-the-art GC-MS with a Gerstel Thermal Desorption System, and ab initio calculations using Gaussian 03 software are all being completed. Collaborators on this research include the Laboratory for Surface Science and Technology at the University of Maine, the High Temperature Materials Laboratory at the Oak Ridge National Laboratory in Tennessee, and the Department of Chemical Engineering at Osaka Prefecture University in Japan.
Water Quality and Pollution Monitoring
We are also working on an EPA grant focusing on source water analysis and warning technology (SWWAT) using real-time multi-contaminant detectors to protect drinking water supplies and homeland security. The primary goal is to use algae fluorescence as a monitor for drinking water supplies. To accomplish this, "indicator" monitors sensitive to multiple contaminants in solution are used to detect chemical, biological, and radiological contaminants which are infinite in number. We are using a sensor which monitors algae fluorescence to determine potential changes and contamination of water supplies. Algae are particularly useful because of their rapid response to environmental changes. Toxic contaminants introduced to water supplies will cause changes in algae chlorophyll chemistry which is monitored using fluorescence technologies or sensors. These sensors will monitor a water supply and send an alert to shut down the water supply if contamination occurs.
In order for the sensor to be applicable, we need to determine that algae fluorescence is changing as a function of stress (Figure, below). In the lab we will determine if algae fluorescence of specific cultured algae respond to different stressors. We want to determine how different stressors affect algae fluorescence intensity and if these affects are similar for different types of algae and different types of stressors. To do this we will use cultured algae as well as field samples containing multiple unknown algae species. The algae currently being cultured is Ankistrodesmus falcatus, a Chlorophyceae or green algae. Potential stressors include: carbaryl, copper sulfate, cadmium, diazinon, cyanide, toluene, and atrazine. We hope to compare the cultured and field results to indicate if cultured algae respond in similar ways to stressors as communities of algae do in the natural environment. This will also allow us to link lab results to real time data obtained from the fluorescence field sensor.
This work is being done with John Peckenham (UMaine), Collin Roesler (Bigelow Laboratory for Ocean Sciences), and Todd Crane (Sewell Engineering Company).