Exploring design strategies to tune the electronic structure and ultrafast dynamics of iron(II) polypyridyl chromophores

The overall goal of this research project is to develop low cost sensitizers for dye-sensitized solar cells (DSSCs) by moving to first row transition metal-based chromophores, specifically iron(II) polypyridyl complexes. To be an effective sensitizer the MLCT state of a chromophore must be sufficiently long lived for injection into the semiconductor to occur. Unfortunately, iron(II) polypyridyl complexes intrinsically have short lived MLCT states due to ultrafast deactivation to lower lying ligand field states. The research has been developed to gain more understanding of the fundamental causes underlying the ultrafast processes. This study is highly collaborative within our group and the aim here has been along the synthetic front, providing the means through which we may begin to tackle this complicated problem.;Iron(II) polypyridyl complexes commonly possess a strained coordination environment through which orbital degeneracy is removed. The dense ligand field manifold that arises from the deviation from octahedral symmetry may be mediating the charge transfer to ligand field excited state deactivation. We set out to develop a highly symmetric iron(II) chromophore through which we may begin to investigate this theory. Our first efforts yielded a complex with a nearly perfect octahedral coordination environment that exhibits some exciting properties along with a significantly altered electronic structure.;A series of symmetric iron(II) polypyridyl complexes were prepared to evaluate the impact of symmetry and substituent effects on the electronic structure and the synthesis and characterization will be presented. We also made the first step toward implemeting such a chromophore in a DSSC by introducing anchoring groups to the periphery of the ligands through which it can bind to a metal oxide semiconductor. We have not yet developed a viable sensitizer for a TiO2, however we have uncovered some promising possibilities and the future design strategies that are proposed that may bring us one step closer.