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Roaming mechanisms, involving the brief generation of a neutral atom or molecule that stays in the vicinity before reacting with the remaining atoms of the precursor, are providing valuable insights into previously unexplained chemical reactions. Here, the mechanistic details and femtosecond time-resolved dynamics of H 3 + formation from a series of alcohols with varying primary carbon chain lengths are obtained through a combination of strong-field laser excitation studies and ab initio molecular dynamics calculations. For small alcohols, four distinct pathways involving hydrogen migration and H 2 roaming prior to H 3 + formation are uncovered. Despite the increased number of hydrogens and possible combinations leading to H 3 + formation, the yield decreases as the carbon chain length increases. The fundamental mechanistic findings presented here explore the formation of H 3 + , the most important ion in interstellar chemistry, through H 2 roaming occurring in ionic species. H 2 roaming is associated with H 3 + formation when certain organic molecules are exposed to strong laser fields. Here, the mechanistic details and time-resolved dynamics of H 3 + formation from a series of alcohols were obtained and found that the product yield decreases as the carbon chain length increases.

Design-specific control over the transitions between excited electronic states with different spin multiplicities is of the utmost importance in molecular and materials chemistry 1 – 3 . Previous studies have indicated that the combination of spin–orbit and vibronic effects, collectively termed the spin–vibronic effect, can accelerate quantum-mechanically forbidden transitions at non-adiabatic crossings 4 , 5 . However, it has been difficult to identify precise experimental manifestations of the spin–vibronic mechanism. Here we present coherence spectroscopy experiments that reveal the interplay between the spin, electronic and vibrational degrees of freedom that drive efficient singlet–triplet conversion in four structurally related dinuclear Pt(II) metal–metal-to-ligand charge-transfer (MMLCT) complexes. Photoexcitation activates the formation of a Pt–Pt bond, launching a stretching vibrational wavepacket. The molecular-structure-dependent decoherence and recoherence dynamics of this wavepacket resolve the spin–vibronic mechanism. We find that vectorial motion along the Pt–Pt stretching coordinates tunes the singlet and intermediate-state energy gap irreversibly towards the conical intersection and subsequently drives formation of the lowest stable triplet state in a ratcheting fashion. This work demonstrates the viability of using vibronic coherences as probes 6 – 9 to clarify the interplay among spin, electronic and nuclear dynamics in spin-conversion processes, and this could inspire new modular designs to tailor the properties of excited states. Many aspects of materials chemistry rely on singlet–triplet spin conversion, but spin–vibronic effects are shown to accelerate the process when vibronic coupling causes the quantum-mechanical mixing of spin states.

Roaming mechanisms, involving the brief generation of a neutral atom or molecule that stays in the vicinity before reacting with the remaining atoms of the precursor, are providing valuable insights into previously unexplained chemical reactions. Here, the mechanistic details and femtosecond time-resolved dynamics of H formation from a series of alcohols with varying primary carbon chain lengths are obtained through a combination of strong-field laser excitation studies and ab initio molecular dynamics calculations. For small alcohols, four distinct pathways involving hydrogen migration and H roaming prior to H formation are uncovered. Despite the increased number of hydrogens and possible combinations leading to H formation, the yield decreases as the carbon chain length increases. The fundamental mechanistic findings presented here explore the formation of H , the most important ion in interstellar chemistry, through H roaming occurring in ionic species.

Design-specific control over the transitions between excited electronic states with different spin multiplicities is of the utmost importance in molecular and materials chemistry(1-3). Previous studies have indicated that the combination of spin-orbit and vibronic effects, collectively termed the spin-vibronic effect, can accelerate quantum-mechanically forbidden transitions at non-adiabatic crossings(4,5). However, it has been difficult to identify precise experimental manifestations of the spin-vibronic mechanism. Here we present coherence spectroscopy experiments that reveal the interplay between the spin, electronic and vibrational degrees of freedom that drive efficient singlet-triplet conversion in four structurally related dinuclear Pt(II) metal-metal-to-ligand charge-transfer (MMLCT) complexes. Photoexcitation activates the formation of a Pt-Pt bond, launching a stretching vibrational wavepacket. The molecular-structure-dependent decoherence and recoherence dynamics of this wavepacket resolve the spin-vibronic mechanism. We find that vectorial motion along the Pt-Pt stretching coordinates tunes the singlet and intermediate-state energy gap irreversibly towards the conical intersection and subsequently drives formation of the lowest stable triplet state in a ratcheting fashion. This work demonstrates the viability of using vibronic coherences as probes(6-9) to clarify the interplay among spin, electronic and nuclear dynamics in spin-conversion processes, and this could inspire new modular designs to tailor the properties of excited states.

To better engineer molecules for ideal photochemical outcomes, we require a deeper understanding of how nuclear parameters such as structure and quantum mechanical vibrations impact electronic behavior. This dissertation aims to further our understanding of these parameters through use of a tunable platinum(II) dimeric framework and a combination of ultrafast optical and x-ray spectroscopic techniques. Chapter 3 utilizes X-ray Transient Absorption (XTA) to track transient bond formation between the Pt(II) dimer centers or lack thereof. The study incorporates optical transient absorption (OTA) to track the excited state dynamics of the two systems studied and provide a correlation between optical and X-ray observations. This study provided the first structural evidence of the lack of Pt-Pt bond formation in the charge transfer band of a Pt(II) dimer with sufficiently separated metal centers while confirming the oxidation of the Pt(II) centers. Chapter 4 examines the nuclear wave packet dynamics along the Pt-Pt mode in a Pt(II) dimer donor acceptor complex using ultrafast transient x-ray solution scattering (TRXSS) to give a purely nuclear picture to the vibration and the bond contraction. This is supplemented with OTA to detail the excited electronic states of the complex. This study shows that Pt-Pt contraction is sufficiently slow enough to potentially modulate the rate of electron transfer during the contraction, as the transient bond formation moves the Pt centers closer together thus possibly altering the energy of the electronic states and the electron transfer driving force. Chapter 5 is a study of the electron transfer (ET) dynamics in two different Pt(II) dimer donor acceptors. Utilizing OTA, this study shows that the Pt(II) dimer offers a framework in which the optically resonant MLCT transitions can be tuned by bridge group bulkiness without altering ET dynamics. Chapter 6 utilizes broadband transient absorption (BBTA) two-dimensional electronic spectroscopy (2DES) to probe vibronic wave packets in the Pt(II) dimers. This study showed a unique result of decoherence and subsequent recoherence of the Pt-Pt stretching mode as the system undergoes intersystem crossing (ISC), shedding new light on the processes of spin-vibronic coupling through a short-lived intermediate triplet state en route to the lowest energy triplet state.

The utilization of metal-organic frameworks (MOFs) in photocatalysis applications requires light-responsive architectures with tunable optical bandgaps. Here, we demonstrate a facile approach to optical bandgap tuning via post-synthetic modifica-tions of pbz-MOF-1, a Zr-based MOF with polyphenylene ligands. A simple reaction of pbz-MOF-1 with FeCl3 was shown to induce three different chemical reactions of the ligands: oxidative dehydrogenation, chlorination and one/two electron oxi-dation of the ligands. The result of these reactions was a gradual decrease in the optical bandgap from 2.95 eV to as little as 0.69 eV. Time-resolved optical spectroscopy and electron paramagnetic resonance spectroscopy, coupled with density functional theory calculations provide insights into the mechanisms of bandgap tuning using chemical oxidation methods. The facile bandgap tuning report here has promising application in the utilization of photo-responsive MOFs in photocatalysis, sensing and other light-triggered applications.