Francesco Tarantelli

Influence of the dye molecular structure on the TiO2 conduction band in dye-sensitized solar cells: disentangling charge transfer and electrostatic effects

We report a thorough theoretical and computational investigation of the effect of dye adsorption on the TiO2 conduction band energy in dye-sensitized solar cells that is aimed at assessing the origin of the shifts induced by surface adsorbed species in the position of the TiO2 conduction band. We thus investigate a series of working dye sensitizers and prototypical surface adsorbers and apply an innovative approach to disentangle electrostatic and charge-transfer effects occurring at the crucial dye–TiO2 interface. We clearly demonstrate that an extensive charge rearrangement accompanies the dye–TiO2 interaction, which amounts to transfer of up to 0.3–0.4 electrons from the dyes bound in a dissociative mode to the semiconductor. Molecular monodentate adsorption leads to a much smaller CT. We also find that the amount of CT is modulated by the dye donor groups, with the coumarin dyes showing a stronger CT. A subtle modulation of the semiconductor conduction band edge energy is found by varying the nature of the dye, in line with the experimental data from the literature obtained by capacitance and open circuit voltage measurements. We then decompose the total conduction band shift into contributions directly related to the sensitizer properties, considering the effect of the electric field generated by the dye on the semiconductor conduction band. This effect, which amounts to ca. 40% of the total shift, shows a linear correlation with the TiO2 conduction band shifts. A direct correlation between the dye dipole and the observed conduction band shift is retrieved only for dyes of similar structure and dimensions. We finally found a near-exact proportionality between the amount of charge transfer and the residual contribution to the conduction band shift, which may be as large as 60% of the total shift. The present findings constitute the basis for obtaining a deeper understanding of the crucial interactions taking place at the dye–semiconductor interface, and establish new design rules for dyes with improved DSC functionality.

Ion Pairing in Cationic Olefin#Gold(I) Complexes

The relative anion-cation orientation in [(PPh(3))Au(4-Me-styrene)]BF(4) (1BF(4)) and [(NHC)Au(4-Me-styrene)]BF(4) [2BF(4); NHC = 1,3-bis(di-iso-propylphenyl)-imidazol-2-ylidene] has been investigated by combining (19)F,(1)H-HOESY NMR spectroscopy and Density Functional Theory (DFT) calculations incorporating solvent and relativistic effects. It has been found that BF(4)(-) locates on the side of 4-Me-styrene, close to the olefin region that is opposite to the 4-Me-Ph moiety in 1BF(4). In 2BF(4), the counterion approaches the cation from the side of the NHC ligand and is mainly located close to the imidazole ring. In both cases, the counterion resides far away from the gold site, the latter carrying only a small fraction of the positive charge. This indicates that the preferential position of the counterion is tunable through the choice of the ancillary ligand, and this opens the way to greater control over the properties and activity of these catalysts.

Double Core-Hole Production in N2: Beating the Auger Clock

We investigate the creation of double K-shell holes in N2 molecules via sequential absorption of two photons on a time scale shorter than the core-hole lifetime by using intense x-ray pulses from the Linac Coherent Light Source free electron laser. The production and decay of these states is characterized by photoelectron spectroscopy and Auger electron spectroscopy. In molecules, two types of double core holes are expected, the first with two core holes on the same N atom, and the second with one core hole on each N atom. We report the first direct observations of the former type of core hole in a molecule, in good agreement with theory, and provide an experimental upper bound for the relative contribution of the latter type.

Block Lanczos and many-body theory: Application to the one-particle Green's function

The importance of the block or band Lanczos method for many-body Green’s function calculations of atomic and molecular systems is discussed. The usual computation schemes for determining the Green’s function involve the diagonalization of Hermitian secular matrices. Considerable numerical difficulties arise, on the one hand, from the size of these matrices and, on the other hand, from the large number of eigenvalues and eigenvectors which often need to be computed in practice. In the case of the one-particle Green’s function it is shown how the computational effort of the diagonalization process can be substantially reduced using block Lanczos. The proposed procedure which consists of a block Lanczos ‘‘prediagonalization’’ and a subsequent diagonalization of the resulting smaller secular matrices quite naturally exploits the specific structure of the secular problems encountered. Its computational performance is demonstrated in a model application to the benzene molecule. The calculation of the complete valence-shell ionization spectra of the systems BeF42 , BeF3 , and BeF2 is devised as a further application of the method in the particular case where the treatment of the full secular problem is computationally prohibitively expensive.

On double vacancies in the core

The energies needed to create different types of double core vacancies as well as the resulting redistribution of the valence electrons are analyzed in comparison with single core vacancies. Numerical results are presented for CH4 and in particular for the molecules C2H2, C2H4, and C2H6. A detailed perturbation theory analysis of the relaxation energies in terms of localized and delocalized molecular orbital pictures is presented. It is shown that the binding energies associated with double core vacancies where each of the two core holes is at a different atomic site sensitively probe the chemical environment of the atoms.

Theoretical investigation of many dicationic states and the Auger spectrum of benzene

The outer valence double ionization transitions in the benzene molecule have been computed using Green’s functions and the results are discussed in connection with the Auger spectrum of this molecule. It is found that already at low energy the double ionization transitions are characterized by strong correlation effects and the appearance of a very large number of satellite states. The 226 computed dicationic states are analyzed in terms of their energy distribution weighted by their two‐hole components. It is shown that the relative energies of the maxima in this distribution agree with the experimental Auger peaks to within 0.3 eV. These results emphasize the extreme usefulness of the method in the investigation of double ionization spectra of large molecules, which are practically beyond the reach of conventional ab initio approaches.

Nuclear dynamics of decaying states: A time‐dependent formulation

The wave packet dynamics accompanying the excitation to a decaying electronic state and the subsequent decay to final electronic states are discussed. The cross sections for the excitation and for the production of final states are related to the corresponding wave packets. The time‐dependent formulation adds insight into the process and is amenable to semiclassical approximations and interpretations. It can also be used to compute the gross features of the observed spectra via a spectral moment expansion. An illustrative application demonstrates the usefulness of the expansion.

On the Dewar–Chatt–Duncanson Model for Catalytic Gold(I) Complexes

We provide a rigorous model-free definition and a detailed theoretical analysis of the electron-charge displacements making up the donation and back-donation components of the Dewar-Chatt-Duncanson model in some realistic catalytic intermediates of formula L-Au(I)-S in which L is an N-heterocyclic carbene or Cl(-) and S is an eta(2)-coordinated substrate containing a C-C multiple bond. We thus show, contrary to a widely held view, that the gold-substrate bond is characterized by a large pi back-donation component that is comparable to, and often as large as, the sigma donation. The back-donation is found to be a highly tunable bond component and we analyze its relationship with the nature of the auxiliary ligand L and with structural (interdependent) factors such as metal-substrate bond lengths and carbon pyramidalization.

Ultrafast X-ray Auger probing of photoexcited molecular dynamics

Molecules can efficiently and selectively convert light energy into other degrees of freedom. Disentangling the underlying ultrafast motion of electrons and nuclei of the photoexcited molecule presents a challenge to current spectroscopic approaches. Here we explore the photoexcited dynamics of molecules by an interaction with an ultrafast X-ray pulse creating a highly localized core hole that decays via Auger emission. We discover that the Auger spectrum as a function of photoexcitation--X-ray-probe delay contains valuable information about the nuclear and electronic degrees of freedom from an element-specific point of view. For the nucleobase thymine, the oxygen Auger spectrum shifts towards high kinetic energies, resulting from a particular C-O bond stretch in the ππ* photoexcited state. A subsequent shift of the Auger spectrum towards lower kinetic energies displays the electronic relaxation of the initial photoexcited state within 200 fs. Ab-initio simulations reinforce our interpretation and indicate an electronic decay to the nπ* state.

Charge-Transfer Energy in the Water-Hydrogen Molecular Aggregate Revealed by Molecular-Beam Scattering Experiments, Charge Displacement Analysis, and ab Initio Calculations

Integral cross-section measurements for the system water-H(2) in molecular-beam scattering experiments are reported. Their analysis demonstrates that the average attractive component of the water-H(2) intermolecular potential in the well region is about 30% stronger than dispersion and induction forces would imply. An extensive and detailed theoretical analysis of the electron charge displacement accompanying the interaction, over several crucial sections of the potential energy surface (PES), shows that water-H(2) interaction is accompanied by charge transfer (CT) and that the observed stabilization energy correlates quantitatively with CT magnitude at all distances. Based on the experimentally determined potential and the calculated CT, a general theoretical model is devised which reproduces very accurately PES sections obtained at the CCSD(T) level with large basis sets. The energy stabilization associated with CT is calculated to be 2.5 eV per electron transferred. Thus, CT is shown to be a significant, strongly stereospecific component of the interaction, with water functioning as electron donor or acceptor in different orientations. The general relevance of these findings for waters chemistry is discussed.