Solvent-dependent structure of molecular iodine probed by picosecond X-ray solution scattering

Reaction dynamics studied via femtosecond X-ray liquidography at X-ray free-electron lasers

X-ray free-electron lasers (XFELs) provide femtosecond X-ray pulses suitable for pump–probe time-resolved studies with a femtosecond time resolution. Since the advent of the first XFEL in 2009, recent years have witnessed a great number of applications with various pump–probe techniques at XFELs. Among these, time-resolved X-ray liquidography (TRXL) is a powerful method for visualizing structural dynamics in the liquid solution phase. Here, we classify various chemical and biological molecular systems studied via femtosecond TRXL (fs-TRXL) at XFELs, depending on the focus of the studied process, into (i) bond cleavage and formation, (ii) charge distribution and electron transfer, (iii) orientational dynamics, (iv) solvation dynamics, (v) coherent nuclear wavepacket dynamics, and (vi) protein structural dynamics, and provide a brief review on each category. We also lay out a plausible roadmap for future fs-TRXL studies for areas that have not been explored yet.

Femtosecond X-ray liquidography using X-ray free-electron lasers (XFELs) visualizes various aspects of reaction dynamics.

Structural Dynamics of C2F4I2 in Cyclohexane Studied via Time-Resolved X-ray Liquidography

The halogen elimination of 1,2-diiodoethane (C2H4I2) and 1,2-diiodotetrafluoroethane (C2F4I2) serves as a model reaction for investigating the influence of fluorination on reaction dynamics and solute-solvent interactions in solution-phase reactions. While the kinetics and reaction pathways of the halogen elimination reaction of C2H4I2 were reported to vary substantially depending on the solvent, the solvent effects on the photodissociation of C2F4I2 remain to be explored, as its reaction dynamics have only been studied in methanol. Here, to investigate the solvent dependence, we conducted a time-resolved X-ray liquidography (TRXL) experiment on C2F4I2 in cyclohexane. The data revealed that (ⅰ) the solvent dependence of the photoreaction of C2F4I2 is not as strong as that observed for C2H4I2, and (ⅱ) the nongeminate recombination leading to the formation of I2 is slower in cyclohexane than in methanol. We also show that the molecular structures of the relevant species determined from the structural analysis of TRXL data provide an excellent benchmark for DFT calculations, especially for investigating the relevance of exchange-correlation functionals used for the structural optimization of haloalkanes. This study demonstrates that TRXL is a powerful technique to study solvent dependence in the solution phase.

Optical Kerr Effect of Liquid Acetonitrile Probed by Femtosecond Time-Resolved X-ray Liquidography

Optical Kerr effect (OKE) spectroscopy is a method that measures the time-dependent change of the birefringence induced by an optical laser pulse using another optical laser pulse and has been used often to study the ultrafast dynamics of molecular liquids. Here we demonstrate an alternative method, femtosecond time-resolved X-ray liquidography (fs-TRXL), where the microscopic structural motions related to the OKE response can be monitored using a different type of probe, i.e., X-ray solution scattering. By applying fs-TRXL to acetonitrile and a dye solution in acetonitrile, we demonstrate that different types of molecular motions around photoaligned molecules can be resolved selectively, even without any theoretical modeling, based on the anisotropy of two-dimensional scattering patterns and extra structural information contained in the q-space scattering data. Specifically, the dynamics of reorientational (libration and orientational diffusion) and translational (interaction-induced motion) motions are captured separately by anisotropic and isotropic scattering signals, respectively. Furthermore, the two different types of reorientational motions are distinguished from each other by their own characteristic scattering patterns and time scales. The measured time-resolved scattering signals are in excellent agreement with the simulated scattering signals based on a molecular dynamics simulation for plausible molecular configurations, providing the detailed structural description of the OKE response in liquid acetonitrile.

Solvent-dependent complex reaction pathways of bromoform revealed by time-resolved X-ray solution scattering and X-ray transient absorption spectroscopy

The photochemical reaction pathways of CHBr3 in solution were unveiled using two complementary X-ray techniques, time-resolved X-ray solution scattering (TRXSS) and X-ray transient absorption spectroscopy, in a wide temporal range from 100 ps to tens of microseconds. By performing comparative measurements in protic (methanol) and aprotic (methylcyclohexane) solvents, we found that the reaction pathways depend significantly on the solvent properties. In methanol, the major photoproducts are CH3OCHBr2 and HBr generated by rapid solvolysis of iso-CHBr2-Br, an isomer of CHBr3. In contrast, in methylcyclohexane, iso-CHBr2-Br returns to CHBr3 without solvolysis. In both solvents, the formation of CHBr2 and Br is a competing reaction channel. From the structural analysis of TRXSS data, we determined the structures of key intermediate species, CH3OCHBr2 and iso-CHBr2-Br in methanol and methylcyclohexane, respectively, which are consistent with the structures from density functional theory calculations.

Fate of transient isomer of CH2I2: Mechanism and origin of ionic photoproducts formation unveiled by time-resolved x-ray liquidography

Diiodomethane, CH₂I₂, in a polar solvent undergoes a unique photoinduced reaction whereby I₂ ^- and I₃ ^- are produced from its photodissociation, unlike for other iodine-containing haloalkanes. While previous studies proposed that homolysis, heterolysis, or solvolysis of iso-CH₂I-I, which is a major intermediate of the photodissociation, can account for the formation of I₂ ^- and I₃ ^-, there has been no consensus on its mechanism and no clue for the reason why those negative ionic species are not observed in the photodissociation of other iodine-containing chemicals in the same polar solvent, for example, CHI₃, C₂H₄I₂, C₂F₄I₂, I₃ ^-, and I₂. Here, using time-resolved X-ray liquidography, we revisit the photodissociation mechanism of CH₂I₂ in methanol and determine the structures of all transient species and photoproducts involved in its photodissociation and reveal that I₂ ^- and I₃ ^- are formed via heterolysis of iso-CH₂I-I in the photodissociation of CH₂I₂ in methanol. In addition, we demonstrate that the high polarity of iso-CH₂I-I is responsible for the unique photochemistry of CH₂I₂.

Halogen bonding and host-guest chemistry between N-alkylammonium resorcinarene halides, diiodoperfluorobutane and neutral guests

Single crystal X-ray structures of halogen-bonded assemblies formed between host N-hexylammonium resorcinarene bromide (1) or N-cyclohexylammonium resorcinarene chloride (2), and 1,4-diiodooctafluorobutane and accompanying small solvent guests (methanol, acetonitrile and water) are presented. The guests’ inclusion affects the geometry of the cavity of the receptors 1 and 2, while the divalent halogen bond donor 1,4-diiodooctafluorobutane determines the overall nature of the halogen bond assembly. The crystal lattice of 1 contains two structurally different dimeric assemblies A and B, formally resulting in the mixture of a capsular dimer and a dimeric pseudo-capsule. ¹H and ¹⁹F NMR analyses supports the existence of these halogen-bonded complexes and enhanced guest inclusion in solution.

Hexakis(2,3,6-tri-O-methyl)-α-cyclodextrin-I5 - complex in aqueous I-/I3 - thermocells and enhancement in the Seebeck coefficient

Supramolecular thermocell composed of I^– (yellow balls), I₃^– (trio of red balls), I₅^– (five connected dark red balls) and Me₁₈-α-CD (gray cone-shaped cylinder).

A large Seebeck coefficient (S_e) of 1.9 mV K^–1 was recorded for the I^–/I₃^– thermocell by utilizing the host-guest complexation of hexakis(2,3,6-tri-O-methyl)-α-cyclodextrin (Me₁₈-α-CD) with the oxidized iodide species. The thermocell measurement and UV-vis spectroscopy unveiled the formation of an Me₁₈-α-CD–pentaiodide (I₅^–) complex, which is in remarkable contrast to the triiodide complex α-CD–I₃^– previously reported. Although the precipitation of the α-CD–I₃^– complex in the presence of an electrolyte such as potassium chloride is a problem in thermocells, this issue was solved by using Me₁₈-α-CD as a host compound. The absence of precipitation in the Me₁₈-α-CD and I^–/I₃^– system containing potassium chloride not only improved the S_e of the I^–/I₃^– thermocell, but also significantly enhanced the temporal stability of its power output. This is the first observation that I₅^– species is formed in aqueous solution in a thermocell. Furthermore, the solution equilibrium of the redox couples was controlled by tuning the chemical structure of the host compounds. Thus, the integration of host-guest chemistry with redox couples extends the application of thermocells.

Transient isomers in the photodissociation of bromoiodomethane

The photochemistry of halomethanes is fascinating for the complex cascade reactions toward either the parent or newly synthesized molecules. Here, we address the structural rearrangement of photodissociated CH₂IBr in methanol and cyclohexane, probed by time-resolved X-ray scattering in liquid solution. Upon selective laser cleavage of the C-I bond, we follow the reaction cascade of the two geminate geometrical isomers, CH₂I-Br and CH₂Br-I. Both meta-stable isomers decay on different time scales, mediated by solvent interaction, toward the original parent molecule. We observe the internal rearrangement of CH₂Br-I to CH₂I-Br in cyclohexane by extending the time window up to 3 μs. We track the photoproduct kinetics of CH₂Br-I in methanol solution where only one isomer is observed. The effect of the polarity of solvent on the geminate recombination pathways is discussed.

Tracking the picosecond deactivation dynamics of a photoexcited iron carbene complex by time-resolved X-ray scattering

Recent years have seen the development of new iron-centered N-heterocyclic carbene (NHC) complexes for solar energy applications. Compared to typical ligand systems, the NHC ligands provide Fe complexes with longer-lived metal-to-ligand charge transfer (MLCT) states. This increased lifetime is ascribed to strong ligand field splitting provided by the NHC ligands that raises the energy levels of the metal centered (MC) states and therefore reduces the deactivation efficiency of MLCT states. Among currently known NHC systems, [Fe(btbip)2]2+ (btbip = 2,6-bis(3-tert-butyl-imidazol-1-ylidene)pyridine) is a unique complex as it exhibits a short-lived MC state with a lifetime on the scale of a few hundreds of picoseconds. Hence, this complex allows for a detailed investigation, using 100 ps X-ray pulses from a synchrotron, of strong ligand field effects on the intermediate MC state in an NHC complex. Here, we use time-resolved wide angle X-ray scattering (TRWAXS) aided by density functional theory (DFT) to investigate the molecular structure, energetics and lifetime of the high-energy MC state in the Fe-NHC complex [Fe(btbip)2]2+ after excitation to the MLCT manifold. We identify it as a 260 ps metal-centered quintet (5MC) state, and we refine the molecular structure of the excited-state complex verifying the DFT results. Using information about the hydrodynamic state of the solvent, we also determine, for the first time, the energy of the 5MC state as 0.75 ± 0.15 eV. Our results demonstrate that due to the increased ligand field strength caused by NHC ligands, upon transition from the ground state to the 5MC state, the metal to ligand bonds extend by unusually large values: by 0.29 Å in the axial and 0.21 Å in the equatorial direction. These results imply that the transition in the photochemical properties from typical Fe complexes to novel NHC compounds is manifested not only in the destabilization of the MC states, but also in structural distortion of these states.

SVD-aided pseudo principal-component analysis: A new method to speed up and improve determination of the optimum kinetic model from time-resolved data

Determination of the optimum kinetic model is an essential prerequisite for characterizing dynamics and mechanism of a reaction. Here, we propose a simple method, termed as singular value decomposition-aided pseudo principal-component analysis (SAPPA), to facilitate determination of the optimum kinetic model from time-resolved data by bypassing any need to examine candidate kinetic models. We demonstrate the wide applicability of SAPPA by examining three different sets of experimental time-resolved data and show that SAPPA can efficiently determine the optimum kinetic model. In addition, the results of SAPPA for both time-resolved X-ray solution scattering (TRXSS) and transient absorption (TA) data of the same protein reveal that global structural changes of protein, which is probed by TRXSS, may occur more slowly than local structural changes around the chromophore, which is probed by TA spectroscopy.

Ultrafast X-Ray Crystallography and Liquidography

Time-resolved X-ray diffraction provides direct information on three-dimensional structures of reacting molecules and thus can be used to elucidate structural dynamics of chemical and biological reactions. In this review, we discuss time-resolved X-ray diffraction on small molecules and proteins with particular emphasis on its application to crystalline (crystallography) and liquid-solution (liquidography) samples. Time-resolved X-ray diffraction has been used to study picosecond and slower dynamics at synchrotrons and can now access even femtosecond dynamics with the recent arrival of X-ray free-electron lasers.

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.

Tracking reaction dynamics in solution by pump–probe X-ray absorption spectroscopy and X-ray liquidography (solution scattering)

TRXL and TRXAS are powerful techniques for real-time probing of structural and electronic dynamics of photoinduced reactions in solution phase.