Book Description

Photosynthetic organisms efficiently absorb photons and transfer electrons over long distances, through a series of spatially ordered donor and acceptor molecules that are arranged within a complex architecture to create charge separated states capable of catalysis. However, the design and synthesis of artificial photosynthetic systems with similarly complex arrangements of donors and acceptors remains synthetically challenging. In our lab, inorganic coordination paired with peptide coupling chemistry provides a potential solution for synthesizing complex supramolecular structures that link donors and acceptors via self-assembly. Using a [Ru(bpy)3]2+ chromophore linked to various donors and acceptors via an aminoethylglycine (aeg) backbone, we are able to systematically study the structural features that lead to long lived charge separation in an inorganic supramolecular structure.Tris(bipyridine)ruthenium(II) complexes were functionalized with an aeg backbone functionalized with either a phenothiazine (PTZ) ligand or a phenyl-terpyridine (-tpy) ligand. The PTZ-functionalized aeg backbone covalently binds the [Ru(bpy)3]2+ acceptor to the PTZ donor by forming an amide bond between the [Ru(bpy)32+] and the aminoethylglycine. Addition of Zn2+ causes the linkage of the -tpy-functionalized aeg [Ru(bpy)3]2+ to pyridine-functionalized aeg derivatized with PTZ, creating a donor/acceptor complex via metal coordination. These donor/acceptor complexes provide a facile method to study through space electron transfer and excited state relaxation dynamics, which can include the formation of a charge separated state. We examine these properties by measuring and comparing the emission quantum yields, lifetimes, and rates of electron transfer of these compounds. Our study shows an increase in the nonradiative decay rate when the donor-acceptor pair is linked, either directly or via formation of the Zn coordinative complex, i.e. when the PTZ is attached to [Ru(bpy)3]2+. An increase in the nonradiative decay rate is observed as the distance between the Ru and PTZ donor/acceptor decreases. Both of these observations are consistent with excited state relaxation via an electron transfer from the Ru excited state to the bound PTZ acceptor. To further test our hypothesis, the aeg -tpy-functionalized [Ru(bpy)3]2+ was coordinated to an aeg pyridine-functionalized methyl viologen (MV2+) by addition of Zn2+. As before, the excited state dynamics of the Ru-MV2+ compound were observed: linking the [Ru(bpy)3]2+ compound via Zn coordination to the viologen results in a decrease in the [Ru(bpy)3]2+ excited state lifetime, and increased rates of radiative and nonradiative decay. These rates however are slower in comparison to the rates observed in the Ru-PTZ complexes. The decrease in decay rates may be attributed to conformational flexibility in the Zn coordinated complexes and the electrostatic repulsion between the positively charged Ru and MV2+ in the Ru-MV2+ complex potentially keeping the donor and acceptor at a further distance than in the Ru-PTZ complexes. Based on literature precedence and our observations of the energy levels of excited state [Ru(bpy)3]2+*, PTZ, and MV2+ along with the observed increase in the rates of radiative and nonradiative decay upon formation of either the Ru-PTZ or Ru-MV2+ complexes leads us to conclude that the formation of a charge separated state was achieved. Ongoing investigations aim to directly observe the charge separated state via transient absorption spectroscopy.