Photo-assisted catalysis using Ni complexes is an emerging field for cross-coupling reactions in organic synthesis. However, the mechanism by which light enables and enhances reactivity of these complexes often remains elusive. Although optical techniques have been widely used to study the ground and excited states of photocatalysts, they lack the specificity to interrogate the electronic and structural changes at specific atoms. Herein we report metal-specific studies using transient Ni L- and K-edge X-ray absorption spectroscopy of a prototypical Ni photocatalyst, (dtbbpy)Ni(o-tol)Cl (dtb = 4,4'-di-tert-butyl, o-tol = ortho-tolyl), in solution. We unambiguously confirm via direct, metal-centered evidence, that the long-lived ( 5ns) excited state is a tetrahedral metal-centered triplet state.

Photophysics and photochemistry of nucleic acid constituents are of fundamental interest in understanding how genetic codes are protected from ultraviolet (UV) light irradiation. The underlying mechanisms of photostability, i.e., the non-destructive dissipation of the excess energy imposed by the UV light absorption, remain elusive even at the single nucleobase level. In this study, femtosecond extreme ultraviolet time-resolved photoelectron spectroscopy (XUV-TRPES) was employed to investigate the relaxation dynamics of adenine (Ade), adenosine (Ado), and adenosine-5-monophosphate (AMP) in aqueous solution. An XUV probe pulse at 21.7 eV can ionize all excited states of a molecule, allowing for full relaxation pathways to be addressed after excitation at 4.66 eV. We also incorporated a gas-dynamic flat liquid jet, which significantly enhanced the pump-probe signal intensity due to the large exposure area (200 μm) of liquid to the incident laser beams compared to a conventionally used cylindrical jet (ca. 30 μm). The obtained TRPE spectra in all systems exhibited signals between 3 and 7 eV electron binding energies. The ultrafast decays within 1 ps were accurately reproduced by the global lifetime analysis under a bi-exponential function with time constants around 100 fs and 500 fs. The decay-associated spectra with a ~500 fs time constant showed distinct peaks 1-eV separated from those at ~100 fs, suggesting that the different electronic state, namely the nπ* state, was involved in the relaxation from the initially populated ππ* state. At the conference site, the ultrafast stepwise relaxation mechanisms depending on the sugar and phosphate groups will be discussed in greater detail.

Bis-μ-oxo Fe(III) porphyrin systems have been of interest to the bioinorganic and inorganic chemistry community for decades because of their ability to perform photocatalytic biomimetic oxidation chemistry using green oxidants. However, the quantum yield of these reactions is historically on the order of Φ=10⁻⁴. Decades of research and optimization of these photocatalysts has only improved the quantum yields of these reactions to Φ=10⁻². We present time-resolved Nitrogen K-edge spectroscopy of a bis-μ-oxo Fe(III) porphyrin system to probe the formation of possible ligand-centered states that are more favorable than the catalytically active Fe(IV)-oxo species.

Femtosecond electron diffraction technique has been extensively used to study ultrafast dynamics of molecules in the gas phase. In this presentation, I will talk about our recent study of real-time reaction coordinate measurement on the isomerization of trans azobenzene to its cis isomer. Isomerization dynamics of the azobenzene is a paradigmatic case to benchmark quantum chemistry calculations. From the excitation of the molecule to the S₂(π,π*) state at 300 nm, our preliminary results suggest that the excited state population evolves through the S₂/S₁ planar conical intersection (CI) geometry before reaching the reactive isomerization located at S₁/S₀ point. An evident delayed appearance of approximately 4 to 5 Å bond distance at a time scale of approximately 500 fs agrees qualitatively with simulations. In addition, this new sample delivery system for solid compounds to the gas phase will be offered to the users at SLAC MeV-UED facility in the near future.

The photodissociation pathways of small organic molecules is intensely interesting for their involvement in atmospheric chemistry. Ultrafast gas-phase spectroscopy can be employed to understand intramolecular proton transfer, electronic energy dissipation through vibrations, and to directly observe molecular rearrangements for fragmentation. Rearrangement mechanisms such as the McLafferty rearrangement and Norrish type II reaction are well known to occur in pentanones. I will present recent measurements using femtosecond pump-probe spectroscopy coupled with time-of-flight mass spectrometry, where we demonstrate such rearrangements also occur in primary alcohols. Primary alcohols with up to six carbon atoms were investigated to highlight how the length of the carbon backbone affects the rearrangement dynamics. Due to the limited chain length of propanol, it undergoes a one-step dehydration mechanism on the fs timescale upon ultraviolet photoexcitation. However, longer chain alcohols, including both 1-butanol and 1-pentanol, exhibit a two-step dehydration mechanism. A fast step with a lifetime of 0.5 ps corresponds to the backbone rotation forming an intermediate stable ring structure which facilitates δ-hydrogen transfer, and a subsequent step with a longer 18 ps lifetime aligning with Norrish type II cyclization and water release. Furthermore, 1-pentanol also undergoes McLafferty rearrangement on a faster timescale as recorded through the β- and γ-carbon bond cleavage of the cyclic intermediate.

Indigo is a blue dye of immense cultural significance, having been used in art and textiles around the world for centuries. Part of its popularity is owed to its high stability under light, which has been previously posited as a result of indigo’s tendency to, when excited, follow a preferential decay pathway involving fast double proton transfer over more destructive or emissive processes. In contrast, indirubin and isoindigo are isomers of indigo in which this same double proton transfer may be structurally constrained or altered, leading to changes in their response to irradiation and overall stability. In order to further elucidate the mechanism of this proton-transfer process, we here characterize the excited-state dynamics of indirubin and isoindigo, obtained by a combination of resonance-enhanced multi-photon ionization (REMPI) spectroscopy, pump-probe spectroscopy in the picosecond and nanosecond regimes, and IR-visible hole burning. With accompanying computational studies, these data serve to provide insight on a decay pathway highly characteristic of the indigoid family.

3d transition metal complexes often have short excited-state lifetimes due to rapid relaxation into high-spin metal-centered states. Typical design approaches focus on using strong ligand fields to destabilize the unoccupied d orbitals and raise the energy of those states. fac-Co(ppy)₃ (where ppy = 2-[2-(pyridyl)phenyl]) combines strong-field carbene ligands with a Co(III) metal center to further raise the energy of e_g* orbitals. Despite the strong ligand field, we found the MLCT lifetime to be only 10 ps via optical transient absorption spectroscopy. Density functional theory calculations suggest this arises from a mixed ³MC/MLCT state undergoing Jahn-Teller distortion, which ruptures a weaker Co-N bond to form a five-coordinate complex. This work provides guidance for future design of Co(III) chromophores: more symmetrical ligand fields and more rigid scaffolds are likely required to prevent deactivation through distortion.

Nonadiabatic interaction of adsorbate nuclear motion with the continuum of electronic states is known to affect the dynamics of chemical reactions at metal surfaces. A large body of work has probed the fundamental mechanisms of such interactions for atomic and diatomic molecules at surfaces. In polyatomic molecules, the possibility of mode-specific damping of vibrational motion due to the effects of electronic friction raises the question of whether such interactions could profoundly affect the outcome of chemistry at surfaces by selectively removing energy from a particular intramolecular adsorbate mode. However, to date there have not been any fundamental experiments demonstrating nonadiabatic electron-vibration coupling in a polyatomic molecule at a surface. In this work, we scatter excited metastable formaldehyde and formaldehyde-d₂ from a low work function surface and detect ejected exoelectrons that accompany molecular relaxation. The exoelectron ejection efficiency exhibits a strong dependence on the vibrational mode that is excited: out-of-plane bending excitation (ν₄) leads to significantly more exoelectrons than CO stretching excitation (ν₂). The results provide clear evidence for mode-specific energy transfer from vibration to surface electrons.

Excited-state relaxation processes in two shortwave infrared (SWIR) polymethine dyes used for bioimaging applications, heptamethine chromenylium Chrom7 and flavylium Flav7, are studied via femtosecond transient absorption spectroscopy with broadband ultraviolet-to-SWIR probing supplemented by steady-state and time-resolved fluorescence and phosphorescence measurements. The relaxation processes of these dyes in dichloromethane are resolved with a sub-100 fs temporal resolution using excitation at different (from SWIR to visible) wavelengths. Population components of the ground-state inhomogeneous ensemble are found to equilibrate predominantly through deformational changes of the solute on a timescale of 90 fs and either 230 fs (Chrom7) and 350 fs (Flav7) as well as diffusive solvation dynamics (~1 ps timescale). Vibrational relaxation in the first excited singlet S₁-state of Chrom7 and Flav7 proceeds with rate constants of 350- and 800-fs for an excess vibrational energy of 1000- and 10000-cm⁻¹accumulated in S₁ following near-IR excitation and relaxation after visible excitation, respectively. The molecules promoted into the highly excited electronic states by visible excitation are observed to repopulate the corresponding S₁ state with time constants of 400 fs for Chrom7 and 450 fs for Flav7 followed by decay of S₁ on a several hundreds of picoseconds timescale in dichloromethane and chloroform. Two competing radiationless relaxation processes are found to be present in S₁: i) temperature-independent direct internal conversion governed by the energy gap law, and ii) thermally activated, mostly non-reactive twisting that is substantial at room temperature and leads to minimal isomer product formation on the ground-state. Intersystem crossing in S₁ is also minor. Intense absorption from the excited S₁ state is observed and its importance for high-intensity excitation applications is discussed.

In today's technological landscape, modern applications incorporate highly concentrated electrolytes to enhance energy density, safety measures, and overall performance of aqueous ion batteries. However, optimizing these applications requires a deep fundamental understanding of such systems. Despite being exceptionally accurate for dilute solutions, classical kinetic theory can no longer describe dynamics nor properties arising from the interactions and collective motions of densely packed, partially solvated ions. To address it, we utilized optical heterodyne detected optical Kerr effect (OHD-OKE) measurements to study the overall structural dynamics of the solution, i.e., the complex, transient system of cations, anions, and water molecules. By comparing multiple concentrated salts and acids (LiCl, LiBr, NaCl, and HCl), we could identify two distinct types of dynamics: bulk water-like relaxation and significantly slower kinetics associated with the formation of transient ion pairs and larger ion-water clusters. We demonstrated that the correlation time of slower decaying components is directly related to the viscosity of salt solutions across all concentrations and temperatures studied, suggesting that the relative fraction of local ion-water clusters and their dynamics determine the bulk viscosity. However, measurements also revealed a significant difference between concentrated salts and acids: the viscosity of HCl solutions is determined by the structural dynamics of the ion-water clusters that relax with the slowest decaying time constant, independent of the relative fraction of the dynamics that relax on faster time scales. Furthermore, a comparison to molecular dynamics simulations and ultrafast 2D IR experiments revealed that the same structural relaxation is also responsible for the hydronium hydrogens' potential fluctuations that cause proton hopping. As a result, the viscosities of the concentrated acid solutions are determined by the temporal evolution of ion-water clusters: structural fluctuations that also enable proton hopping. These results provide insights into solvation and proton hopping, yielding a method to characterize highly concentrated ionic solutions in terms of structure and dynamics.