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Global economies are transitioning towards net-zero emissions, but technological leaps are still needed to accelerate decarbonisation within the energy sector and beyond. Here, several novel material systems are studied to uncover physical properties which will dictate their suitability for use in state-of-the-art light-harvesting structures such as thin-film photovoltaics and photoelectrochemical fuel cells. Such materials offer promising avenues to cheap and efficient sustainable energy solutions. Metal halide perovskites excel in the pursuit of highly efficient thin film photovoltaics and light emitters. Substitution of the monovalent cations has advanced luminescence yields and device efficiencies. In this thesis, the change in photocarrier recombination behaviour caused by cation alloying is explored. Time-resolved optical spectroscopy and microscopy are used to reveal local charge accumulation in mixed cation perovskites, creating p- and n-type photodoped regions, unearthing a strategy for efficient light emission at low charge-injection in solar cells and light-emitting diodes. Operational stability of perovskite solar cells remains a barrier to their commercialisation, yet a fundamental understanding of degradation processes, including the specific sites at which failure mechanisms occur, is lacking. Here, multimodal microscopy techniques are utilised to show that nanoscale defect clusters, which are associated with phase impurities, are sites at which material degradation seeds. The trapping of charge carriers at sites associated with phase impurities, itself reducing performance, catalyses redox reactions that compromise device longevity. Importantly, this reveals that both performance losses and intrinsic degradation can be mitigated by eliminating these defective clusters. Carbon nanodots are an emergent material whose ease of fabrication and water solubility make them exciting candidates for photocatalytic processes. However, a full understanding of their excited charge carrier dynamics and interaction with common electron donors/acceptors is not yet established. This work identifies charge transfer processes in hybrid photocatalytic systems with carbon nanodot absorbers and builds bottom-up mechanistic insight.

This dissertation focuses on the interrogation of the photoacid sensitization (Förster) cycle to understand mechanisms for light-to-protonic energy conversion. I define protonic species as those capable of undergoing proton-transfer reactions, including H2O(l), H+(aq), OH-(aq), and Brønstead–Lowry (conjugate) acids/bases. The central theme of this work is relating operating principles between traditional electronic photovoltaics (electron/holes) and protonic photovoltaics (OH-/H+) including charge carrier generation and conduction with the goal of developing design strategies for efficient light-to-ionic power conversion. This is achieved by investigating ground-state proton transfer processes in the Förster cycle using electrochemical and spectroscopic techniques and leveraging this knowledge to understand photo-initiated ion transport in ion-exchange membranes.Transient absorption spectroscopy studies of aqueous photoacids provided insight into the protonic species generated during the Förster cycle and the inefficiencies of current state-of-the-art photoacids for generating large photoresponses in photoacid-modified ion-exchange membranes. An abundance of ultrafast spectroscopy data exist that revealed mechanistic details for excited-state proton transfer, but there was limited information regarding the mechanisms for the subsequent ground-state proton transfer. Proton acceptance in the excited-state and proton donation in the ground-state from H2O(l) in the Förster cycle results in the transient generation of H+ and OH-, analogous to photogeneration of electrons and holes in electronic photovoltaics. An obstacle for realizing this ideal mechanism is kinetics. These were analyzed via a systematic study using various photoacids by varying the concentration of deprotonated photoacids and protons, photoacid ground-state acidity, solution acidity, and total photoacid concentration. Photoacids were covalently bonded to ion-exchange membranes to assess the impact that their photochemical mechanisms had on their ability to exhibit photovoltaic action. Existing methodology for four-probe electrochemical measurements was revamped to produce reproducible signals and temporally stable baselines. The key factor was eliminating the introduction of external electrolyte from reference electrodes filled with saturated internal electrolyte by replacing those electrodes with reference electrodes immersed directly into the electrolyte in contact with the photoacid-modified ion-exchange membranes. This introduced experimental constraints that required physical modification of the experimental apparatus as well as deconvolution of the Nernst electrode potential from the membrane potential. Implementation of this new setup resulted in the observation of a “reverse” photovoltage, meaning that it was opposite in sign to photovoltages measured in traditional electronic photovoltaics and that observed in our group in all prior studies of these systems. A series of control experiments was performed, and a mechanism coined “electrolyte crossover induced bulk membrane polarization” was hypothesized to explain this reverse photovoltage.

Organic photochemistry is a powerful tool both for synthetic and application purposes. Photochemistry taking place in the solid-state is particularly powerful due to the selectivity induced by the crystal lattice. Along with increased reaction selectivity, we have shown that the cooperative nature of photochemical processes can result in mechanical processes such as bending or jumping in these crystals. In order to properly harness these reactions both for synthesis and application, it is important to understand the underlying reactive intermediates that are produced during the photoreaction process. This thesis is focused on the identification of reactive intermediates in solution, nanocrystalline suspensions and cryogenic matrices. By gather information on these intermediates in media, the power of photochemistry can be fully realized.

This dissertation presents a series of research projects designed and carried out to elucidate the physical chemistry and assess the atmospheric relevance of (1) carbonyl oxide radicals (i.e., Criegee intermediates) produced in alkene ozonolysis and (2) nitrous oxide (N2O) produced in lightning-induced corona discharges. The results provide UV absorption spectra and reaction rate coefficients for Criegee intermediates that will help constrain the formation and loss pathways of aerosol nucleation precursors such as H2SO4 and oxidized volatile organic compounds, and the isotopic signature of N2O formed in lightning that can help distinguish various N2O sources in atmospheric measurements.Criegee intermediates are byproducts of the reaction of alkenes with ozone. Bimolecular reactions of Criegee intermediates can lead to the production of low-volatility organic compounds and acids in the atmosphere, which in turn play a role in determining the concentration, size, and optical properties of aerosols. Recently, a novel method for producing measurable quantities of stabilized Criegee intermediates in the laboratory paved the way for the development of new experimental techniques to study their chemical properties and predict their importance in the atmosphere. For this dissertation, a unique apparatus combining time-resolved UV absorption in a flow cell with laser depletion in a molecular beam was adapted to obtain the absolute absorption spectrum of CH3CHOO with high resolution and accuracy relative to previous spectral measurements by other groups. The resulting absorption cross sections imply a photolysis lifetime of about seven seconds in the atmosphere, long enough for CH3CHOO to participate in unimolecular and bimolecular reactions. The broad absorption band with weak structure in the long-wavelength region of the spectrum represents a “spectral fingerprint” for identifying CH3CHOO in future studies, and the cross sections provide valuable benchmarks for theory to characterize electronically excited states of CH3CHOO.The fast reaction of CH2OO with water dimer is thought to dominate CH2OO removal in the atmosphere. However, reaction rates can vary considerably under different conditions of temperature, humidity, and pressure. A temperature-controlled flow cell was designed to measure the transient absorption of CH2OO and obtain rate coefficients for its reaction with water dimer from 283 to 324 K. The rate of the reaction of CH2OO with water dimer was found to exhibit a strong negative temperature dependence, pointing to the participation of a hydrogen-bonded pre-reactive complex between CH2OO and two water molecules. Due to the strong temperature dependence, and shifting competition between water dimer and water monomer (which has a positive temperature dependence), the effective loss rate of CH2OO by reaction with water vapor is highly sensitive to atmospheric conditions. The role played by the stable pre-reactive complex suggests that similar complexes could form between water dimer and other larger Criegee intermediates, and that the stability and relative energy of these complexes control the reaction rate with water and its temperature dependence.Effective loss rates of Criegee intermediates due to bimolecular reactions in the atmosphere are limited by their rates of unimolecular decomposition. The rates of decomposition depend strongly on the molecular geometry, which affects the accessible isomerization pathways and dissociation products. (CH3)2COO is the main product of tetramethylethylene ozonolysis, and has been found to react slowly with water dimer and rapidly with SO2. While CH2OO decomposes slowly via isomerization to dioxirane, (CH3)2COO may decompose faster via intramolecular hydrogen transfer to form vinyl hydroperoxide. Fast (CH3)2COO decomposition could affect the significance of the Criegee intermediate H2SO4 source, as well as the non-photolytic production of OH radicals. In this dissertation, measurements of the transient absorption of (CH3)2COO to obtain thermal decomposition rate coefficients from 283 to 323 K by extrapolating the observed loss rate to zero concentration are reported. The rate of unimolecular decomposition is ~400 s-1 at 298 K and varies by nearly an order of magnitude within the studied temperature range. The effective loss rate of (CH3)2COO in the atmosphere due to thermal decomposition is thus competitive with its loss due to reaction with water vapor and with SO2, suggesting that the unimolecular decomposition pathway is a significant sink for (CH3)2COO and possibly other di-substituted Criegee intermediates, and should be included in models of Criegee chemistry in the atmosphere as well as in kinetic models of tetramethylethylene ozonolysis.N2O is the third most important greenhouse gas after CO2 and methane, and is mainly emitted to the atmosphere as a byproduct of microbial activity in soils. The expanding use of nitrogen-containing fertilizers in agriculture has led to an increase in N2O atmospheric concentrations since preindustrial times. Isotopic measurements are a valuable tool to distinguish the influence of different sources of N2O, but the isotopic composition of N2O formed from corona discharge in lightning has not previously been measured. Here, a corona discharge cell apparatus was used to generate a corona discharge in flowing or static zero air, and the N2O formed at discharge cell pressures from ~0.1 to 10 Torr and discharge voltages from 0.25 to 5 kV was collected and measured with isotope ratio mass spectrometry to determine its isotopic composition. The results show enrichments in 15N of N2O up to 32‰ relative to the reactant N2, and even larger enrichments in 15N of up to 77‰ at the central nitrogen atom. Large depletions in 18O as large as -71‰ relative to reactant O2 were also measured. The isotopic composition measured here may help to elucidate the chemical mechanisms leading to N2O formation and destruction in a corona discharge. Furthermore, the isotope-isotope relationships of the N2O produced in the corona discharge experiments are distinct from those of N2O from other sources, implying that isotopic measurements can be used to determine whether local variations in the atmospheric concentration of N2O – e.g., the enhanced N2O levels recently measured in the upper tropical and subtropical troposphere – are due to lightning activity, soil emissions, or biomass burning.

Minerals and metals serve important roles in the organic geochemistry of natural environments. Mobility of organics, catalysis of degradation, and redox catalysis are among the processes affected by minerals. With the addition of ultraviolet light a new suite of photo-induced redox reactions is possible including reductive and oxidative ligand-to-metal/mineral charge transfer. Such reactions allow for novel chemistry that has relevance to the modern Earth as well as the pre-biotic origin of life. This thesis describes processes by which electrons transfer between minerals/metals and organic ligands relevant to natural systems as well as the origins of life. I present evidence of ultrafast electron transfer and the production of radical intermediates essential to deducing redox reaction mechanisms. I also present methods for communicating understanding of interfacial chemistry to the public that promote engagement in science. This thesis is broadly applicable to those interested in mineral organic photochemistry, electron transfer, the origin of life, and science teaching methods.I probed the chemistry between organic molecules and minerals/metals, using pump/probe transient absorption (TA) spectroscopy to observe the dynamics of electrons and vibrational modes at timescales ranging from picoseconds to nanoseconds. This technique can be conducted in solution and can be highly sensitive to intermediate reaction products. I examined the photolysis of the metal carboloto, ferric oxalate, under UV irradiation using mid-infrared TA spectroscopy in both D2O and H2O. Ferric oxalate is a model molecule for natural systems and is used to measure photo flux due to its well-characterized quantum efficiency. However, the mechanism of its photolysis is debated. This was the first time the intermediates of ferric oxalate photolysis were observed using techniques sensitive to the vibrational states of organic molecules. I observed the rapid intramolecular charge transfer and the production of CO2 and tentatively CO2•–. Additionally, we observed intermediate states that we interpret to be CO2 disassociating from ferrous iron, a signature never before reported. Investigations of photo-induced electron transfer were expanded to ZnS nanoparticles and fumarate. Fumarate is an intermediate metabolite in the tricarboxilic acid (TCA) cycle, which is a part of core metabolism in modern organisms. It undergoes a two-electron reduction to form succinate. Reductive versions of the TCA cycle may have been important for the origin of prebiotic metabolism. I measured the effect of adsorbed fumarate on the electronic states of photo-excited ZnS and observed electron transfer both at short (1 ns) timescales. Additionally, I observed an electronic signature tentatively attributed to fumarate radical, which persisted for at least 8 nanoseconds. The appearance of a long-lived radical intermediate product and the rapid initial electron transfer from the mineral to the organic suggests that ZnS could be a viable catalyst for prebiotic metabolism on the early Earth. To better educate students on the importance of mineral surface chemistry I designed and implemented a classroom experiment wherein students performed electrolysis of water using mineral electrodes. The experiment emphasized both the mineral catalysis and mineral redox chemistry, which occur at the solid/liquid interface. Concepts in interfacial chemistry are often difficult to exhibit, making this teaching tool unique and useful. Students were guided through a set of investigations and constructed their understanding through observations and sharing of ideas. The experiment was successfully implemented in a college level mineralogy course.

Phototriggered linkage isomerization of ruthenium-sulfoxide complexes is one of the few examples of molecular bistability, which is of interest for applications in information and energy storage. Sulfoxide complexes of the type, [M(bpy) 2(L2)] z+(where bpy is 2,2′-bipyridine; M is ruthenium or osmium, L2 is bis-dimethylsulfoxide (dmso), Chloride and dmso, methylsulfonylbenzoate (OSO), dimethylbis(methylsulfinylmethyl)silane (OSSO), or N-benzylidine-2-(ethylsulfinyl)ethanamine (PhNSO)) have been characterized by X-ray crystallography, 1H NMR spectroscopy, electrochemistry, steady-state UV-visible and emission spectroscopy, time-resolved emission, and transient absorption spectroscopy. The ground states of these complexes feature S-bonded sulfoxides. These complexes exhibit strong absorptions in visible region associated with metal-to-ligand charge transfer (MLCT) transitions. Irradiation of the S-bonded complexes within these absorption bands results in a subsequent red-shift in the absorption spectrum of the complex as well as a shift to a lower reduction potential in the electrochemistry. Typically, these values revert at room temperature to the original state. This behavior is interpreted as phototriggered Ru-S → Ru-O and thermal Ru-O → Ru-S intramolecular linkage isomerization. Transient data suggests that this excited-state transformation occurs from an initially formed excited state, prior to population of the 3MLCT excited state or thermal population of ligand field states. Picosecond transient-absorption studies have revealed excited-state O-to-S isomerization process occurring within two of these complexes. For the complex, [Ru(bpy) 2(OSSO)] 2+irradiation of yellow [S,O] with 400 nm light reveals both S-bonded and O-bonded excited states within the instrument response (~750 fs). Irradiation of red [O,O] with 532 nm light reveals both S-bonded an O-bonded excited states within the instrument response (~750 fs). These studies demonstrate two-color femtosecond photonic switching between two excited states. The excited-state reactivity, proposed mechanism of action and an electronic structural diagram of these complexes will be discussed.

The present thesis describes transient studies of the photodecomposition of organic peroxyesters. The successful implementation of transient techniques over wide temporal (picosecond to microsecond) and spectral (UV-Vis to mid-IR) windows allows characterization of the transient species involved in the decomposition. The first part of the thesis illustrates a model approach for transient studies of coumarin based peroxyesters. Direct observation of transient species yielded the rates of decays of singlet and triplet excited states of the coumarin chromophore as well as provided spectra of the radical intermediates. Particularly, to the best of our knowledge, this is the first observation of an aroyloxyl radical by time-resolved infrared spectroscopy. Transient studies of a series of peroxyesters containing the 1,4-bis(phenylethynyl)benzene chromophore elucidated the photodecomposition mechanism together with providing information about the electronic structure of radical intermediates involved therein. The fission of the peroxy bond leads to formation of an aroyloxyl radical possessing 2B1 molecular orbital symmetry with the unpaired electron delocalized through the entire phenylene-ethynylene chormophore. This delocalization causes a decrease in the acetylene bond order that can be observed experimentally as a red shift of IR vibration conveying cumulene-like character to the structure of the radical. Studies of two-photon absorption (TPA) properties of phenylene-ethynylene peroxyesters reveal them promising candidates for two photon microfabrication. The compounds possess relatively high TPA cross-sections (110 GM) and are capable of initiating the polymerization of acrylic monomers. Model structures with 10 micrometers resolution were fabricated in two-photon process using phenylene-ethynylene peroxyesters as photoinitiators.