Quantum Chemical Study of Excited State Proton Transfer in Solvated Organic Molecules

The development of quantum chemical methods for the study of excited states had to major advancements in the ability to investigate the photochemistry of medium-sized to large organic molecules. In particular, tools for transition and difference density matrix analysis, allowing for the visualization of detachment/attachment, and difference density plots, along with natural transition orbitals, serve as compact descriptions of the excited state. Throughout this work, time-dependent density functional theory (TD-DFT) and the algebraic diagrammatic construction (ADC) scheme for the polarization propagator were used as the primary methods of investigation. An overview of the available quantum chemical methods for the study of excited states is given in Chapter 2. Several different molecular systems were studied, each presenting their own unique challenges, but unified under the theme of excited state proton transfer processes. Pigment Yellow 101 (PY101), a commercially available and highly photostable fluorescent yellow pigment, is the first system studied. Relaxed scans of the potential energy surfaces connecting the most stable conformers of the pigment were computed using TD-DFT. It was found that PY101 undergoes excited state intramolecular proton transfer (ESIPT) and trans-cis isomerization after photoexcitation to the bright first singlet electronically excited state (S 1 ). A simple kinetic rate model is presented for gaining a first look at the dynamics of the system, and information obtained from the potential surface scans and geometry optimizations of PY101 is used as input. Time-dependent quantum dynamics simulations are not yet feasible for systems larger than PY101, and therefore the development of such models is important. The results from the kinetic model agree well with those from time-resolved experiments, indicating that such models are promising new tools. The results of the PY101 project are presented in Chapter 3. The fluorescence quenching behavior of benzaldehyde in water is the primary subject of Chapter 4. TD-DFT calculations along the coordinate of proton transfer from an explicit water molecule to benzaldehyde show that photoexcitation is followed by ultra-fast decay from the bright S 3 (ππ ∗ ) state to the S 1 (nπ ∗ ) state, where the system then evolves. Along this coordinate, benzaldehyde is found to act not as a photobase but rather abstracts a hydrogen atom from the water, forming as a result a pair of radicals. Subsequent electron transfer to the hydroxyl radical, forming a hydroxide anion, is followed by proton back transfer and restoration of the initial scenario. For the elucidation of the fluorescence quenching mechanism of benzaldehyde in water, tools for detachment/attachment densities and Mulliken population analyses, as implemented for ADC, were employed. This study was then extended to chemical relatives of benzaldehyde, for example by increasing the number of aromatic rings. In Chapter 5, the photoacidic properties of a series of pyranine-based photoacids were studied using TD-DFT and a series of excited state descriptors based on the exciton wave function. Stronger photoacids exhibit higher lying states of charge transfer character from the substituents to the core, while these states are lower lying by about 1 eV in the weaker photoacids of the series. The stronger photoacids are characterized by more strongly electron-withdrawing substituents. In addition, single point calculations along the dissociation coordinate of neutral derivatives of pyranine reveals a second type of charge transfer state, going from the oxygen of the photoacid to the solvent molecule moeity, which crosses down over the course of the acid dissociation coordinate. It is suspected that this state may interfere with the excited state intermolecular (ESPT) process, as it does not cross down as rapidly in the case of a photoacid with more strongly deactivating substituents. More extensive study is necessary to fully describe the roles of these charge transfer states on the pyranine-based photoacids, and suggestions in this regard are made in detail at the end of Chapter 5. On the whole, the breadth of quantum chemical methods used to study ESPT processes in a range of organic systems were highly effective in this regard. This speaks not only to the effectiveness of currently available methods for the study of excited states, but also has allowed for the obtainment of detailed insights into these complex systems of industrial and biological relevance