Research in the Wamser Group
at Portland State University
Artificial Photosynthesis
Solar Energy Conversion

Useful background reading ( .pdf files ):

Donald Aitken (white paper for the International Solar Energy Society),
Transitioning to a Renewable Energy Future

U. S. Department of Energy Workshop,
Solar Energy Utilization

Greg Smestad and Michael Grätzel (Journal of Chemical Education),
Demonstrating Electron Transfer and Nanotechnology: A Natural Dye-Sensitized Nanocrystalline Energy Converter

Selected research publications ( .pdf files ):

Suman Cherian and Carl Wamser (Journal of Physical Chemistry),
Adsorption and Photoactivity of Tetra(4-carboxyphenyl)porphyrin (TCPP) on Nanoparticulate TiO2

James R. Weinkauf, Sharon W. Cooper, Aaron Schweiger, and Carl C. Wamser (Journal of Physical Chemistry A),
Substituent and Solvent Effects on the Hyperporphyrin Spectra of Diprotonated Tetraphenylporphyrins

Hooi-Sung (Brian) Kim and Carl Wamser (Photochemical & Photobiological Sciences),
Photoelectropolymerization of Aniline in a Dye-Sensitized Solar Cell

Michael G. Walter, Carl C. Wamser, Joseph Ruwitch, Yinping Zhao, Dale Braden, Matt Stevens, Al Denman, Rick Pi, Alexander Rudine, and Peter J. Pessiki
(Journal of Porphyrins and Phthalocyanines),
Syntheses and optoelectronic properties of amino/carboxyphenylporphyrins for potential use in dye-sensitized TiO2 solar cells

Michael G. Walter and Carl C. Wamser (Journal of Physical Chemistry C),
Synthesis and Characterization of Electropolymerized Nanostructured Aminophenylporphyrin Films

Alexander B. Rudine, Michael G. Walter, and Carl C. Wamser (Journal of Organic Chemistry),
Reaction of Dichloromethane with Pyridine Derivatives under Ambient Conditions

Michael G. Walter, Alexander B. Rudine, and Carl C. Wamser (Journal of Porphyrins and Phthalocyanines), cover article, September 2010
Porphyrins and Phthalocyanines in Photovoltaic Solar Cells

Specific project areas:

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Research in the Wamser lab is focused on solar energy conversion, using an approach called artificial photosynthesis. The long-term goal is development of a solar cell that efficiently collects solar energy and converts it to a useful form of chemical energy, such as the decomposition of water into hydrogen and oxygen using the energy of sunlight. Many of the design strategies are roughly based on natural membrane systems as used in photosynthesis. For example, the light-absorbing molecules used in the research are porphyrins, structural analogs of chlorophyll. Porphyrins are specifically organized in various ways to enhance their ability to collect solar energy, transfer their excitation energy to a reactive site, and initiate electron transfer reactions. Currently there are three main lines of research active in the Wamser group.

Thin films of polymeric porphyrins are created by the technique of interfacial polymerization. Two reactive porphyrin monomers are dissolved separately in immiscible liquids; rapid reaction occurs only at the interface between these two solutions, creating a thin polymer film. In the case of acid chloride and amine derivatives, a polyamide film is created. As the polymerization reaction proceeds, the interfacial film becomes a barrier that slows further reaction; hence interfacial polymer films can be exquisitely thin (10 - 100 nm). Nevertheless, these films absorb visible light well and undergo photoinduced charge transfer reactions that are directional, analogous to the charge transport membranes of natural photosynthesis. The directionality of these films is caused by an asymmetry of functional groups on the porphyrin units within the polymer film; specifically, excess amine groups appear on the surface of the film that was made in contact with the porphyrin amine derivative and excess carboxylic acid groups appear on the surface of the film that was made in contact with the porphyrin acid chloride derivative (acid chlorides are hydrolyzed to carboxylic acids during the workup of the finished film). This structural asymmetry of the thin films is a novel feature that is still under study both experimentally and theoretically. The structural asymmetry leads to the photochemical asymmetry because the redox potentials of the different porphyrins are such that electron transfer is favored from aminoporphyrins to carboxy-porphyrins.

The second approach under active investigation is sensitization of high surface area TiO2 semiconductor electrodes using porphyrin derivatives and porphyrin polymers. The use of high surface area semiconductors has recently led to remarkable improvements in the efficiency of solar cells and allowed the use of simple and inexpensive semiconductors such as TiO2. Since TiO2 is white (i.e., it absorbs in the ultraviolet but not in the visible), efficient collection of the solar spectrum requires sensitization by a molecule absorbing in the visible. We have demonstrated that our carboxyporphyrin derivative is an excellent sensitizer for TiO2, rapidly injecting an electron into TiO2 with high quantum efficiency. We are actively investigating various ways of attaching a series of porphyrins to TiO2 electrodes. This approach also uses the concept of directional electron transfer between porphyrins, with an electron transfer gradient from aminoporphyrin to carboxyporphyrin to TiO2.

We are also investigating the oxidative electropolymerizations of aminoporphyrins, which form conductive polymers apparently analogous to polyaniline. These polymers are being studied with respect to their structure, conductivity, acid-base, and redox properties. We have also started investigating copolymerizations with aniline. In particular, we would like to use these conductive polymers as components of a TiO2 solar cell. Aminoporphyrins and the related hydroxyporphyrins have an interesting set of spectroscopic properties, chracteristic of what are called hyperporphyrins, and we are also investigating the uv-visible and fluorescence spectroscopy of these compounds, in particular correlating their redox chemistry with their acid-base chemistry.

Individual research projects in the Wamser group can involve a great variety of possible areas, including synthesis, spectroscopy, computer modeling, electrochemistry, photochemistry, polymer chemistry, materials science, and combinations of any or all of the above. Additional new techniques and collaborations with colleagues at other institutions are always being sought to further characterize the novel properties of the polymers and thin films being developed.