Probing the Transition from Hydrophilic to Hydrophobic Solvation with Atomic Scale Resolution

Picosecond and femtosecond X-ray absorption spectroscopy is used to probe the changes of the solvent shell structure upon electron abstraction of aqueous iodide using an ultrashort laser pulse. The transient L-1,L-3 edge EXAFS at 50 ps time delay points to the formation of an expanded water cavity around the iodine atom, in good agreement with classical and quantum mechanical/molecular mechanics (QM/MM) molecular dynamics (MD) simulations. These also show that while the hydrogen atoms pointed toward iodide, they predominantly point toward the bulk solvent in the case of iodine, suggesting a hydrophobic behavior. This is further confirmed by quantum chemical (QC) calculations of I-/I-0(H2O)(n=1-4) clusters. The L-1 edge sub-picosecond spectra point to the existence of a transient species that is not present at SO ps. The QC calculations and the QM/MM MD simulations identify this transient species as an I-0(OH2) complex inside the cavity. The simulations show that upon electron abstraction most of the water molecules move away from iodine, while one comes closer to form the complex that lives for 3-4 Ps. This time is governed by the reorganization of the main solvation shell, basically the time it takes for the water molecules to reform an H-bond network Only then is the interaction with the solvation shell strong enough to pull the water molecule of the complex toward the bulk solvent. Overall, much of the behavior at early times is determined by the reorientational dynamics of water molecules and the formation of a complete network of hydrogen bonded molecules in the first solvation shell.

Hydrophobicity with atomic resolution: Steady-state and ultrafast X-ray absorption and molecular dynamics studies

Majed Chergui, Thomas James Penfold, Ivano Tavernelli

Static and time-resolved X-ray absorption spectroscopy (XAS) is used to probe the solvent shell structure around iodide and iodine. In particular, we characterize the changes ob served upon electron abstraction of aqueous iodide, which reflects the transition from hydrophilic to hydrophobic solvation after impulsive electron abstraction from iodide. The static spectrum of aqueous iodide, which is analyzed using quantum mechanical/molecular mechanics (QM/MM) molecular dynamics (MD) simulations, indicates that the hydrogens of the closest water molecules point toward the iodide, as expected for hydrophilic solvation. In addition, these simulations demonstrate a small anisotropy in the solvent shell. Following electron abstraction, most of the water molecules move away from iodine, while one comes closer to form a complex with it that survives 3-4 ps. This lifetime is governed by the reorganization of the main solvation shell, basically the time it takes for the water molecules to reform a hydrogen bond network in the hydrophobic solvation shell.

De Gruyter2013

Explicit solvent simulations of the aqueous oxidation potential and reorganization energy for neutral molecules: Gas phase, linear solvent response, and non-linear response contributions

Jeremy Samuel Arey, Jennifer Guerard, Marianne Seijo, Peter Rudolf Tentscher

First principles simulations were used to predict aqueous one-electron oxidation potentials (E-ox) and associated half-cell reorganization energies (lambda(aq)) for aniline, phenol, methoxybenzene, imidazole, and dimethylsulfide. We employed quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations of the oxidized and reduced species in an explicit aqueous solvent, followed by EOM-IP-CCSD computations with effective fragment potentials for diabatic energy gaps of solvated clusters, and finally thermodynamic integration of the non-linear solvent response contribution using classical MD. A priori predicted E-ox and lambda(aq) values exhibit mean absolute errors of 0.17 V and 0.06 eV, respectively, compared to experiment. We also disaggregate E-ox into several well-defined free energy properties, including the gas phase adiabatic free energy of ionization (7.73 to 8.82 eV), the solvent-induced shift in the free energy of ionization due to linear solvent response (-2.01 to -2.73 eV), and the contribution from non-linear solvent response (-0.07 to -0.14 eV). The linear solvent response component is further apportioned into contributions from the solvent-induced shift in vertical ionization energy of the reduced species (Delta VIEaq) and the solvent-induced shift in negative vertical electron affinity of the ionized species (Delta NVEA(aq)). The simulated Delta VIEaq and Delta NVEA(aq) are found to contribute the principal sources of uncertainty in computational estimates of E-ox and lambda(aq). Trends in the magnitudes of disaggregated solvation properties are found to correlate with trends in structural and electronic features of the solute. Finally, conflicting approaches for evaluating the aqueous reorganization energy are contrasted and discussed, and concluding recommendations are given.

Royal Soc Chemistry2015

Computational studies for the characterization and design of dye sensitized solar cells (DSSCs) with improved efficiencies

Negar Ashari Astani

During millions of years of evolution, all creatures have found their ways of harvesting, storing and utilizing the energy coming from the sun. Plants use photosynthesis to convert carbon dioxide to glucose. With a low efficiency of only 3%, plants still capture enough energy to grow their roots, fruits and leaves. Animals regulate their body temperature by basking in the sun or seeking a shadow when needed. Human beings are the only species that is not satisfied with this natural share. With the exponential population growth and a gross domestic product per capita that doubles every fifty years, finding a clean, secure, and sustainable source of energy is inevitable. Photovoltaic solar cells together with a proper energy storage technology, like water splitting and a fuel cell to burn the stored hydrogen, can form the promising green cycle that solves the energy problem. Among different photovoltaic technologies, hybrid organic-inorganic solar cells, such as dye sensitized solar cells (DSSCs) or perovskite solar cells, are subject of active research due to their low-fabrication costs and easy production. In order to become commercially viable, these modules need to reach their theoretical Shockley-Queisser efficiency limit while acquiring enough durability and stability. In this context ab initio first-principles studies are of great help in order to give a fundamental insight of the chemistry and physics going on inside the cell. The main purpose of this dissertation is to make use of computational methods for a characterization of the fundamental processes in dye-sensitized and perovskite solar cells and the use of this knowledge for a rational design of solar cells with higher efficiencies. Starting from a systematic investigation of bio-inspired porphyrin-based dyes focused on their light harvesting properties, we were able to outline some key guidelines for designing efficient dye sensitizers. The outcome of this study was an efficient light harvester, which when integrated in an optimized device achieved the record-breaking 13% efficiency porphyrin- based dye sensitized solar cell. While having good sensitizers is a necessary condition for a high performance DSSC, it is often not enough. It is essential to know the interactions of different components of a DSSC and how they work together. For this reason, we performed atomistic molecular dynamics simulations of a comprehensive model of a DSSC consisting of dye molecules, TiO2 substrate, solvent and electrolyte. In particular, in these simulations, we studied the accessibility of oxidized electrolyte to the TiO2 substrate for different dye coverages and show that at medium to high coverage, peripheral alkyl groups of the dyes are highly efficient in shielding the TiO2 substrate from direct contact with the oxidized electrolyte preventing recombination. In a next chapter, and as a proof of principles study, we investigated the importance of solvation effects on the stability of a carbocation (as a prototype for the cationic dye species after electron injection). The last two parts of this thesis focus on perovskite solar cells where we show the origins of the highly tunable band gap and propose a simple model in terms of orbital overlap that can be used to rationalize and predict variations of the band gap as a function of crystal structure and chemical composition of the perovskite. Finally, we study the efficient carrier transport properties of these materials and show that there is a direct correlation between the efficiency of carrier transport and the band gap.