ALGORITMOS PARA LA VIGILANCIA EPIDEMIOLÓGICA Y

A primary advantage of molecular solar-to-fuels catalysts is the ability to identify and characterize the components of the catalytic cycle. While inorganic chemistry enjoys a diverse array of physical techniques for characterizing chemical species, most of these methods are only useful for characterization of isolable species and are incompatible with studying the transient intermediates which are critical to the mechanism of chemical transformations. The work detailed in this dissertation endeavors to apply the atypical techniques outlined below to elucidate the fundamental properties of molecular solar-to- fuels systems. We hope that the insights gleaned can be applied in the development of solar-to-fuels cycles that can free us from our dependence on fossil fuels by providing an efficient mechanism to store solar energy.

1.6.1. Transient Absorption

Studies leveraging transient absorption (TA) will dominate the bulk of this dissertation. Fundamentally, TA leverages electronic absorption spectroscopy in the ultra-violet and visible (UV-Vis) range of the electromagnetic spectrum. The underlying principle of TA is the use of a short duration light pulse to excite the system and monitor its evolution using UV-Vis absorption spectroscopy. Figure 1.12 depicts the two TA experimental

configurations used in this dissertation. The vastly different timescales of TA experiments lead to different methods of synchronization: ultrafast experiments must rely on the characteristic speed of light while nanosecond experiments can be electronically timed. The high sensitivity of TA can be partially attributed to the use of well-developed CCD and PMT technologies as the primary methods of detection.

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TA has been a powerful tool for studying the kinetics of chemical systems due to the sensitivity and high time resolution of the technique. TA has found extensive use in the domain of studying photo-induced charge transfer, enabling the observation of chemical kinetics on timescales that would be impossible to study by other means. Due to TA’s high

sensitivity and time resolution there are numerous reports leveraging its ability to identify photochemical intermediates. A recent example is a study of the oxidation of bromide by ruthenium(II) excited states.109 _{This study found evidence for the formation of bromine}

radicals which subsequently react with free Br– _{in solution to form Br2}•–_{, demonstrating the}

photo-induced formation of a Br–Br bond. Another interesting example is a study of the

Figure 1.12. Diagram of typical TA experimental configurations. (a) Nanosecond TA often consists of a continuous probe source and a pulsed laser, commonly a Nd:YAG. Timing is performed electronically using a gated CCD camera or recorded on a ADC, typically an oscilloscope. (b) Picosecond and Femtosecond experimental configuration consists of a time-of-flight delay scheme controlled by a translation stage. Both pump and probe are pulses generated from the same Ti:Sapphire oscillator.

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photochemistry of tellurium bromide complexes which yielded a proposed mechanism for Br2 addition and photoelimination based on evidence obtained by TA.110–112

While there have been many studies which demonstrate halogen photoelimintation through characterization of photochemical products, 91,92,113–116 _{studies probing the}

mechanism and intermediates of photo-halogen elimination are relatively rare. The Nocera group has led the field in applying TA to the study of HX slitting to better understand the mechanisms underpinning the observed photochemistry. Spectroscopic evidence for a photointermediate in the photolysis of bimetallic systems was first reported by Cook et. al. for Pt-Rh complexes.117 _{Utilizing TA spectroscopy, a new species was observed}

immediately following laser flash photolysis which decays over the course of tens of

microseconds. It was proposed that this intermediate was either a complex with a chloride

in a μ-bridging position or a Cl2 adduct. Despite the uncertainty as to the nature of this

intermediate, the observation of a transient species in photochemical HX splitting process is an exciting step forward for the field.

1.6.2. Photocrystallography

X-ray crystallography is a ubiquitous technique of the inorganic community due to its ability to elucidate structure on the molecular scale. Since the first reported structures determined by X-ray diffraction techniques reported in 1913 by W. L. Bragg118 _(Figure

1.13) X-ray crystallography has been a mainstay of the inorganic chemist’s analytical

toolbox. Initially only simple structures were achievable but with the advance of structural solving tools,119 _{X-ray sources, and detectors X-ray crystallography of molecules as complex}

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Photocrystallography, the study of crystal structures in excited states, is a relatively recent development. The first reported example of steady state photocrystallography was in 1991 showing the photoinduced changes in the structure of Na2[Fe(CN)5(NO)] 2H2O

using neutron diffraction.121,122 _{This system was revisited by Coppens and coworkers}123,124

using X-ray crystallography to report the first electron density study of an excited state species. Since that time the field has expanded to include time resolved

photocrystallography, enabling the analysis of molecular excited states on time scales as short as picoseconds.125–127

Figure 1.14 contains a depiction of a typical photocrystallography experiment.

Photocrystallography is typically performed using a UV or visible radiation source to excite the molecules in a crystal and subsequently collecting the X-ray diffraction pattern in order to get structural information about the excited state. 128 _{Synchrotron radiation sources are}

commonly used due to their higher X-ray brilliance. The experiment can be either steady

Figure 1.13. Diffraction pattern obtained from Fluorspar crystals and the predicted diffraction pattern. Figures adapted with permission from Bragg, W. L. The Structure of

Some Crystals as Indicated by Their Diffraction of X-Rays. Proceedings of the Royal Society

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state, using continuous excitation sources such as diode laser, or time resolved by using a pulsed laser and a method of gating the X-ray source. Gating of the X-ray source typically

consists of a series of beam ‘choppers’ synchronized with the electron bunches circling the synchrotron ring. The resultant X-ray diffraction pattern consists of a superposition of structures of the ground state and excited state geometries which can be modeled as a form of disorder.

Photocrystallography presents some unique challenges and possibilities in the context of studying photochemical processes. Like TA, photocrystallography allows the study of intermediates in a photochemical reaction but unlike TA, photocrystallography provides direct evidence for structural changes associated with the photochemical process. Photocrystallography is not without its challenges as samples most be able to maintain their crystallinity for the duration of the experiment.

Figure 1.14. Typical photocrystallography experimental apparatus. If the experiment is using continuous irradiation the X-ray chopper is absent or stopped in the ‘open’ position.

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