Judicious Design of Transition Metal Complexes for Photochemical Energy Conversion

In order for society to have a smooth transition away from carbonaceous fuels that continue to put undue stress on the environment, robust research into alternative, sustainable energy sources are of paramount importance. In particular, solar is a prime candidate given its abundance and minimal environmental consequences through its use compared to fossil fuels. While intermittent sunlight can diminish the value of using photovoltaics for a full 24-hour day, it is possible to store that energy into what can be considered a “solar fuel”, with hydrogen being an ideal target. Similarly, the use of light energy can drive numerous reactions that are valuable to fields such as industrial synthesis, medicine, and sanitation, at a fraction of their current energy inputs.
This work reports on the design of new Ir(III) based photosensitizers that address concerns from previously established complex structures. Replacement of a labile
bipyridine with an electron rich terpyridine imparts greater stability and photocatalytic abilities in highly coordinating solvent environments, which are known to degrade
previously established photosensitizers. These complexes were assessed photophysically, electrochemically, computationally, as well as for photocatalytic hydrogen generation from water. The photosensitizers were also used for photoredox catalysis, a burgeoning area of research that uses light energy to drive synthetic transformations.
In addition, this work reports on the switching of a terpyridine ligand to a bisquinolylpyridine for Ir(III) photosensitizers. This new ligand allows for improved
coordination ability to the Ir center, and imparts an enhancement to several properties when compared to the corresponding bipyridine analogue. The complexes see excited state lifetimes as high as 30 microseconds, which is notable for Ir(III) complexes given that there are no additional organic dye molecules tethered to the complexes, a common strategy employed to extend excited state lifetimes. The complexes were assessed as
photosensitizers not only for photocatalytic hydrogen generation and photoredox catalysis, but also for singlet oxygen generation, where a longer excited state lifetime is
advantageous. Finally, this work reports the synthesis of Fe(II), Zn(II) and Ru(II) based hemicaged complexes that utilize the more rigid phenanthroline ligand in place of the
traditional bipyridine. The ligand synthesis required adaptation for the phenanthroline moiety but also allowed for synthesis of a corresponding mesityl-capped hemicage as
well. The complexes were evaluated photophysically and electrochemically in order to determine the effects of hemicaging on the ground state and excited state properties when compared to the corresponding phenanthroline analogues.