Andrea Rosichini: A Dance of Electrons and Protons: Mechanistic investigation into excited state and proton-coupled electronic processes

Abstract

The movement of electrons is at the core of every chemical process, and is often accompanied by the movement of protons. The aim of this thesis is to shine light on the mechanism of some of these chemical processes by, well, shining light on them. Light is a powerful tool to trigger and investigate chemical reactions. Absorption of light, in particular visible light, by molecules brings them in excited, more reactive states. The different reactivity of excited states compared to their ground state counterparts is an interesting subject of study in itself, but can also be used to trigger ground state reactions in a controlled way, allowing researchers to study the time evolution of these reactions. In this thesis, both excited state reactions and photo-triggered reactions are discussed, and studied using steady-state and time resolved spectroscopic methods.

Paper I and Paper II examine the unexpected behaviour of Proton-Coupled Electron Transfer reactions from tryptophan in water when pH is varied. This behaviour has been observed for the first time almost two decades ago, and led to a series of studies aimed at identifying the source of it. However, to this day an explanation has not been found. In Paper I and Paper II, some existing hypotheses were disproven, and the source of the pH dependent behaviour was narrowed down to the role of the amine and carboxylic acid group of the amino acid. Paper III demonstrates the second known case of Proton-Coupled Energy Transfer, a recently discovered excited state proton-coupled reaction, investigates its mechanism and proposes rational design principles for new systems capable of undergoing Proton-Coupled Energy Transfer. Finally, Paper IV investigates Triplet-Triplet annihilation from benzo[ghi]perylene, adding a new element to the interesting and complex photophysics of this polycyclic aromatic hydrocarbon.

In this thesis, the results and conclusions from the four Papers above are rediscussed and framed in the context of their relevance in progressing fundamental scientific understanding.