Understanding the Photochemistry of Heptazines and Heptazinyl Radicals

Abstract

Heptazines are the molecular unit of carbon nitride, the most widely studied organic photocatalyst. In recent years, carbon nitride has shown potential as a low-cost, heavy metal free means to produce hydrogen from water with light without the need for a sacrificial electron donor. Carbon nitride is such a complex material, the atomistic mechanism by which it achieves this reactivity is still debated. In this thesis, the general approach has been to study the photochemical dynamics of the heptazine monomer to a new level of detail, to help gain greater understanding of carbon nitride as a whole.The molecular heptazine derivatives in this thesis, in particular trianisole heptazine (TAHz), have also proven to be excellent test systems on which to study excited-state proton coupled electron transfer (ES-PCET) and how it is affected by hydrogen bonding. TAHz also proved to be the first neutral molecule verified to violate Hund’s Rule by possessing a singlet-triplet inversion, a thorough discussion of which is provided in Chapter 3. A major shift in the present work was to focus on the intermediate heptazinyl radical TAHzH• which results from the H-atom abstraction that results from ES-PCET between the TAHz and hydroxylic substrates such as phenol derivatives and water. Computational predictions have identified the heptazinyl radical as the reactive intermediate for carbon nitride. In Chapter 4, we perform a series of experiments to tailor the hydrogen bonding environment around TAHz to extract the TAHzH• spectral fingerprint for the first time. Then in Chapter 5, this knowledge is used to explore the nature of the TAHzH• radical, unveiling experimental proof that heptazinyl radicals have dissociative πσ* states, a major prediction of computational models of carbon nitride reactivity. Chapter 6 explores ideas of how modifying the molecular structures of heptazines can enhance their reactivity or solubility, providing next steps for further research into heptazine photochemistry