Ultrafast Photochemical Reactivity of Heptazine-Based Materials
MetadataShow full item record
Solar energy conversion and storage is one key aspect in addressing and mitigating the global energy crisis. Photon conversion efficiencies and production rates of photovoltaics have dramatically increased over the last few decades, yet intermittent availability of solar energy will continue to limit the environmental and economic benefits of this technology unless storage becomes more cost effective and available. One avenue that is gaining traction for all-in-one solar energy conversion and storage is photocatalytic water splitting. Graphitic carbon nitride, an organic photocatalyst for hydrogen evolution, has become popular to study for the ease of synthesis and low cost due to the abundance of precursor materials. However, high structural ambiguity and low solubility causes characterization of charge transport in carbon nitride-like materials to be nearly unattainable, leaving many unanswered photophysical questions. Work enclosed in this dissertation is the first to spectroscopically demonstrate an electron transfer between two carbon nitride co-catalysts for increased hydrogen evolution. This finding suggests electron quenching happens on the order of the diffusion rate for small molecules, which led us to study a model heptazine-based molecule for hydrogen evolution. Additionally, we are interested in studying the water oxidation mechanism to eliminate the need for sacrificial electron scavengers. Interestingly, we discovered the first excited-state proton-coupled electron transfer (ES-PCET) mechanism from water to heptazine resulting in heptazinyl and hydroxyl radicals, which validates theoretical predictions. In efforts to better understand the ES-PCET mechanism, we used a series of phenol derivatives, R-PhOH, to tune the excited state energy landscape. As the electron donating ability of the substituent on phenol increases, the luminescence quenching increases, and the kinetic isotope effect decreases. This suggests as the oxidation potential of R-PhOH shifts cathodically, the charge transfer curve is easier to access and thus the electron transfer becomes more favorable. This is supported by increased radical generation rates seen for more electron donating substituents on phenol. Furthermore, we studied the role of branching ratios post-excitation on accessing the charge transfer state using ultrafast pump-push-probe spectroscopy. We found that the excited- state landscape is heavily influenced by an additional “push” pulse for electron donating phenols compared to electron withdrawing phenols, confirming again that the barrier height for reaching the charge transfer state decreases with more electron donating phenols. From these studies we can outline a set of design rules for chemists and materials scientists to synthesize heptazine-based molecules for increased water oxidation. Specifically, our results suggest functionalizing the heptazine core with electron withdrawing groups to shift the charge transfer curve toward the lowest hydrogen bound excited state for increased ES-PCET with water.
- Chemistry