Polypyridyl ligands for nanostructured materials - dye-sensitized solar cells and coordination polymers

Burning of fossil fuels in this and the last century lead to global warming and the climate crisis. It is utmost important to prevent further global warming and reverse the increases of the past few decades. A reasonable approach is to replace fossil fuel burning power plants with renewable energy producing technologies. The most promising renewable energy source is the sun. The sun delivers 23 000 TW of sun-light energy every year to the earth, which is a ~1000 times more than the energy consumption of the earth.
One technology to harvest sun light energy and produce electricity is the solar cell. Different solar cell technologies exist, and the approach presented in this thesis is concerning dye-sensitized solar cells. Dye-sensitized solar cells (DSSCs) have the advantage of a relatively easy fabrication and the incorporation of cheap light harvesting materials is possible. In DSSCs a metal oxide (TiO2) semiconductor is sensitized with an organic or inorganic dye, enabling photon absorption and electron injection into the semiconductor, which generates electric current. The dyes presented in this thesis are based on the relatively cheap and abundant metal copper. Copper in its +1 oxidation state can form heteroleptic complexes with two bisdiimine ligands. One of these ligands (the anchoring ligand) bears anchoring groups, that allow attachment of the complex on TiO2. The emergence of new anchoring ligands for heteroleptic bisdiimine copper(I) dyes was the aim of my PhD studies and the results are presented in this thesis. In four chapters a systematic investigation of bisdiimine ligands as anchors and capping ligand is described. Various spacers and anchoring groups were incorporated into the anchoring ligand and the performance of these compounds was investigated in solar cell devices. In the last chapter of the main part the synthesis of compound with potential application in p-type DSSCs is presented.
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The careful choice of building blocks, the consideration of their geometry, topology and their influence on the crystal packing interactions enables control of the self-assembly processes in coordination polymer synthesis. When these aspects are well understood the specific construction of various multidimensional motifs, by self-assembly of building blocks can be achieved. The building blocks are often easily accessible in the form of metal salts and organic ligands, that often require few synthetic steps to synthesize. Screening of building blocks for the creation of multidimensional motifs on the nano to micro scale is an important research field.
The building blocks self-assemble and form these structures without further influence of the scientist. With the right selection of building blocks and by careful consideration of conditions, like temperature and concentration it is possible to predefine the outcome of the self-assembly process.
Organic ligands based on 4,2':6',4''- and 3,2':6',3''-terpyridine were synthesized and the control of the self-assembly process of these linkers with various metal centres was investigated.
Rigid ditopic 4,2':6',4''-terpyridines proved to be the most reliable ligand in this respect. In combination with linear two-connecting metal centres, 1D zig-zag chains were the dominant motif. With metal centres, like Co(II) (and to some extent Cd(II)), that have a connectivity of four, the dominant motif was the formation of 2D (4,4) nets.
Tetratopic bis(4,2':6',4''-terpyridines) and bis(3,2':6',3''-terpyridines) were used to assemble 2D networks, 3D frameworks and MOFs. 3D frameworks are known for their potential applications in gas-sorption, guest exchange, drug-delivery etc. Their formation was not predictable, but the change from ditopic linkers to tetratopic linkers increased the probability to obtain these 3D motifs. A 2-fold interpenetrating MOF with ~65% void space, relative to the total volume, was obtained with a tetratopic bis(4,2':6',4''-terpyridine) ligand in combination with ZnBr2, which highlights the great potential of this ligand class.
A collection of ~30 multidimensional structures is presented in this thesis and should give an indication how one might be able to control self-assembly processes in coordination polymers with 4,2':6',4''- and 3,2':6',3''-terpyridines as linkers.