Artificial Photosynthesis

Photosynthesis is the most important biological process on Earth, and directly or indirectly it fulfills all our food requirements and energy needs. Biological photosynthetic systems convert and store energy from sunlight into energy-rich compounds that produce biomass and over geological time, fossil fuels. Currently, much of the energy that we are burning is coming from fossil fuels. However, the fossil fuel production rate is much slower than our current consumption rates; consequently, these energy reserves are progressively decreasing. Moreover, their continued use produces pollution that threatens human health and the environment. Thus, there is increasing demand and interest into the search for alternative energy sources that are cost effective and environmentally benign. One such source is the Sun, if we could trap and convert it into useful form of energy that would be a major breakthrough for our future energy needs. One way to harvest solar energy is to build an artificial photosynthetic system with concepts of natural photosynthesis. In this regard, my research is aimed at gaining detailed understanding on potential artificial photosynthetic systems that may lead to the development of more cost-effective and efficient solar cells and photocatalytic cells for solar energy conversion and storage.

Photocatalytic Cell. In photosynthesis, the oxidaton of water by the oxygen evolving complex (OEC) occurs as a result of a highly oxidizing chlorophyll radical cation P₆₈₀^+·, which is generated by light-induced electron transfer from P₆₈₀ to a pheophytin. The challenge in designing artificial systems is to find an appropriate model for P₆₈₀^+·. In this respect we are using phosphorus(V) or antimony(V) porphyrin (PPor or SbPor) as they are robust, easily modifiable with light-harvesting and photoredox properties. The high oxidation number makes these porphyrins very strong oxidants when they are photo-excited. Using these properties we are designing new dye sensitized solar cells and photocatalytic cells in combination with water oxidation catalyst.

Reaction center models: Among the porphyrins, aluminum(III) porphyrin (AlPor) is unique, since it can form two different axial bonds that allow two different types of molecular components to be attached. Carboxylic acids react with the axial hydroxide of AlPor to form covalent ester linkages while Lewis bases such as pyridine and imidazole form coordination bonds to the Al center which is a Lewis acid. AlPor also has moderate oxidation and reduction potentials, and therefore could be used as a primary electron donor (D) and/or acceptor (A). This combination of properties makes AlPor an ideal candidate for constructing multi-component D-A systems as model compounds for artificial photosynthetic reaction center.