Book Description

To meet the increasing global demand for energy we need to innovate new ways to harness and convert renewable energy sources. Solar energy provides a huge potential to meet these demands, but there is still a plethora of unanswered questions surrounding how photons interact with organic molecules and materials, particularly in the field of photovoltaics and photochemistry. The low dielectric constant of these organic materials causes them to behave differently than their inorganic counterparts. Chapter 2 provides background for the variety of applications of organic optoelectronic materials and common techniques used to study them. In recent years, the aza-aromatic material carbon nitride, and its monomer unit, heptazine, has seen a surge in popularity to mediate photochemical transformations, particularly hydrogen evolution. Yet despite the thousands of publications in this field, there are few fundamental photophysical studies, which are critical to enable new reaction pathways. This work describes a series of photophysical explorations into a molecular heptazine derivative, 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz). Chapter 3 provides an overview of the photophysical and electrochemical properties of TAHz, including the unusual inversion of the lowest singlet and triplet state. Chapter 4 provides the first experimental evidence for excited-state proton-coupled electron transfer (ES-PCET) from water to a heptazine chromophore using time-resolved photoluminescence and radical scavenging. This first step of water oxidation opens a new range of possible photochemical reactions to harness solar energy. Chapter 5 proposes design rules for increasing reaction efficiency of ES-PCET with heptazine chromophores by studying reaction rates with a series of phenol derivatives. By adding electron-donating groups on phenol, increased reactivity and support the corollary: adding electron-withdrawing groups to heptazine could increase reaction efficiency with water. Chapter 6 considers another aspect of chromophore design by studying hydrogen bonding and how the local excited-state landscape is modulated by hydrogen-bond strength. Together, this work presents a holistic picture of heptazine0́9s photophysics with implications for tailoring organic chromophores to meet unique photochemical demands.