Solvent Dependence of the Electronic Structure of I– and I3–

Valence and Core Photoelectron Spectra of Aqueous I3- from Multi-Reference Quantum Chemistry

The I3- molecule is known to undergo substantial structural reorganization upon solvation by a protic solvent, e.g., water. However, the details of this process are still controversially discussed in the literature. In the present study, we combined experimental and theoretical efforts to disentangle this controversy. The valence (5p), N4,5 (4d), and M4,5 (3d) edge photoelectron spectra were measured in an aqueous solution and computed using high-level multi-reference methods. Our previous publication mainly focused on obtaining reliable experimental evidence, whereas in the present article, we focused primarily on theoretical aspects. The complex electronic structure of I3- requires the inclusion of both static and dynamic correlation, e.g., via the multi-configurational perturbation theory treatment. However, the resulting photoelectron spectra appear to be very sensitive to problems with variational stability and intruder states. We attempted to obtain artifact-free spectra, allowing for a more reliable interpretation of experiments. Finally, we concluded that the 3d Photoelectron Spectrum (PES) is particularly informative, evidencing an almost linear structure with a smaller degree of bond asymmetry than previously reported.

Solvent-Dependent Structural Dynamics in the Ultrafast Photodissociation Reaction of Triiodide Observed with Time-Resolved X-ray Solution Scattering

Resolving the structural dynamics of bond breaking, bond formation, and solvation is required for a deeper understanding of solution-phase chemical reactions. In this work, we investigate the photodissociation of triiodide in four solvents using femtosecond time-resolved X-ray solution scattering following 400 nm photoexcitation. Structural analysis of the scattering data resolves the solvent-dependent structural evolution during the bond cleavage, internal rearrangements, solvent-cage escape, and bond reformation in real time. The nature and structure of the reaction intermediates during the recombination are determined, elucidating the full mechanism of photodissociation and recombination on ultrafast time scales. We resolve the structure of the precursor state for recombination as a geminate pair. Further, we determine the size of the solvent cages from the refined structures of the radical pair. The observed structural dynamics present a comprehensive picture of the solvent influence on structure and dynamics of dissociation reactions.

Probing the molecular structure of aqueous triiodide via X-ray photoelectron spectroscopy and correlated electron phenomena

Liquid-microjet-based X-ray photoelectron spectroscopy was applied to aqueous triiodide solutions, I₃^-_(aq.), to investigate the anion's valence- and core-level electronic structure, ionization dynamics, associated electron-correlation effects, and nuclear geometric structure. The roles of multi-active-electron (shake-up) ionization processes - with noted sensitivity to the solute geometric structure - were investigated through I₃^-_(aq.) solution valence, I 4d, and I 3d core-level measurements. The experimental spectra were interpreted with the aid of simulated photoelectron spectra, built upon multi-reference ab initio electronic structure calculations associated with different I₃^-_(aq.) molecular geometries. A comparison of the single-to-multi-active-electron ionization signal ratios extracted from the experimental and theoretical core-level photoemission spectra suggests that the ground state of the solute adopts a near-linear average geometry in aqueous solutions. This contrasts with the interpretation of time-resolved X-ray solution scattering studies, but is found to be fully consistent with the rest of the solution-phase I₃^-_(aq.) literature. Comparing the results of low- and high-photon-energy photoemission measurements, we further suggest that the aqueous anion adopts a more asymmetric geometry at the aqueous-solution-gas interface than in the aqueous bulk.

Simulating core electron binding energies of halogenated species adsorbed on ice surfaces and in solution via relativistic quantum embedding calculations

In this work, we investigate the effects of the environment on the X-ray photoelectron spectra of hydrogen chloride and chloride ions adsorbed on ice surfaces, as well as of chloride ions in water droplets. In our approach, we combine a density functional theory (DFT) description of the ice surface with that of halogen species using the recently developed relativistic core-valence separation equation of motion coupled cluster (CVS-EOM-IP-CCSD) via the frozen density embedding formalism (FDE), to determine the K and L_1,2,3 edges of chlorine. Our calculations, which incorporate temperature effects through snapshots from classical molecular dynamics simulations, are shown to reproduce experimental trends in the change of the core binding energies of Cl^- upon moving from a liquid (water droplets) to an interfacial (ice quasi-liquid layer) environment. Our simulations yield water valence band binding energies in good agreement with experiment, which vary little between the droplets and the ice surface. For halide core binding energies there is an overall trend for overestimating experimental values, though good agreement between theory and experiment is found for Cl^- in water droplets and on ice. For HCl on the other hand there are significant discrepancies between experimental and calculated core binding energies when we consider structural models that maintain the H-Cl bond more or less intact. An analysis of models that allow for pre-dissociated and dissociated structures suggests that experimentally observed chemical shifts in binding energies between Cl^- and HCl would require that H⁺ (in the form of H₃O⁺) and Cl^- are separated by roughly 4-6 Å.

Adsorption Dynamics of Redox Active Species onto Polarized Surfaces of Sensitized NiO

Mesoporous NiO films were deposited by means of a screen printing technique onto fluorine-doped tin oxide transparent electrodes and consequently sensitized with Erythrosin B (EryB) dye. The obtained colored NiO material was used as a working electrode in a three-electrode cell to study the evolution of the triple semiconductor/dye/electrolyte interface upon electrochemical polarization in dark conditions. The electrolyte was a solution of I₃ ^-/I^- in acetonitrile, with the redox couple representing the typical redox shuttle of dye-sensitized solar cells (DSCs). The adopted electrochemical conditions were devised in order to simulate the actual electrical environment of the NiO/dye photocathode in a light-soaked DSC. The use of a benchmark sensitizer EryB and of the most widely used redox mediator I₃ ^-/I^- is particularly meaningful for the study of the adsorption dynamics and the determination of possible degradative phenomena on the basis of the behavior of numerous analogue systems. Therefore, for the first time, the evolution of the NiO/EryB/I₃ ^-/I^- multiple interface was investigated combining the electrochemical characterization with ex situ spectroscopic analysis by means of X-ray photoelectron spectroscopy. The resulting picture shows that EryB in the immobilized state promotes the redox processes based on the I₃ ^-/I^- couple. Moreover, the EryB sensitizer inhibits the phenomena of recombination between the metal oxide semiconductor and the redox couple.

Photoelectron shake-ups as a probe of molecular symmetry: 4d XPS analysis of I3− in solution

Accurate XPS simulations reveals the connection between solvent-induced nuclear asymmetry and shake-up intensity in the 4d spectra of I₃⁻.

Adsorption Behavior of I3- and I- Ions at a Nanoporous NiO/Acetonitrile Interface Studied by X-ray Photoelectron Spectroscopy

The adsorption of I^- and I₃^- anions, i.e., the two species constituting the most common redox couple of dye-sensitized solar cells (DSCs), onto the surface of screen-printed nanoporous NiO was studied by means of X-ray photoelectron spectroscopy (XPS). Nanoporous NiO films were deposited on transparent metallic fluorine-doped tin oxide (FTO) and polarized as working electrodes in a three-electrode cell with differently concentrated I^-/I₃^- electrolytes to simulate the different conditions experienced by the NiO cathodes during the lifecycle of a p-type DSC (p-DSC) at those atomic sites not passivated by the dye. Bare NiO films were tested also as photocathodes of nonsensitized p-DSCs. The ex situ XPS analysis of I 4d ionization region of both reference and electrochemically treated NiO films showed that the presence of native and electrochemically generated Ni³⁺ and Ni⁴⁺ centers induces fast adsorption/desorption of I^- ions and catalyzes their oxidation to I₃^- ions. The adsorption phenomena generated by I^- and I₃^- species on nanoporous NiO electrodes can also induce an effect of electrochemical passivation toward a fraction of charged Ni sites. Such an effect would render these sites inactive for the further realization of those photoelectrochemical processes at the basis of the operation of a p-DSC.

Identification of photofragmentation patterns in trihalide anions by global analysis of vibrational wavepacket dynamics in broadband transient absorption data

Photodissociation pathways of a trihalide series are systematically investigated by globally fitting vibrational wavepacket signals in broadband transient absorption spectra.