Investigation of charge localization and charge delocalization in model molecules by multiphoton ionization photoelectron spectroscopy and DFT calculations

In this work we focus on the question to which degree a surplus charge is localized or delocalized in extended molecular systems. Molecules consisting of a flexible tail and the benzene chromophore, such as n-propylbenzene, 2-phenylethyl alcohol and 2-phenylethylamine, are used as model molecules. Their S0-S1 resonance enhanced multiphoton ionization (MPI) spectra containing origin transitions of different conformers appear at similar wavelengths. This shows, that in the neutral the electronic excitation is localized at the benzene chromophore. Geometry differences between the neutral and the cation can be qualitatively derived from intensities of vibrational transitions or the onset behavior in MPI high-resolution photoelectron (MPI-PE) spectra. We identify two possible reasons for structural changes: Charge-dipole interaction and charge delocalization. Whereas both effects can be active for the folded gauche conformers, the charge-dipole interaction is expected to be small for the extended anti conformers and geometry changes are attributed to charge delocalization. Density functional calculations of structures and energies qualitatively confirm the experimental results for all molecules and their conformers. They predict charge delocalization into the end group of below 20% for n-propylbenzene and 2-phenylethyl alcohol. In the case of 2-phenylethylamine the charge is equally shared by the near-isoenergetic charge sites of the benzene chromophore and the amine group.     

Electron delocalization and aromaticity in linear polyacenes: atoms in molecules multicenter delocalization index

Molecular aromaticity in the linear polyacenes is investigated using an atoms in molecules based six center index (SCI-AIM) which measures the electron delocalization. SCI-AIM values for the linear polyacenes indicate decreasing aromaticity going from outer to inner rings in the polyacene series. The SCI-AIM approach is compared to a Mulliken-like approach, and a critical comparison to the PDI index is made.     

Charge transfer in organic molecules for solar cells: theoretical perspective

This tutorial review primarily illustrates rate theories for charge transfer and separation in organic molecules for solar cells. Starting from the Fermi's golden rule for weak electronic coupling, we display the microcanonical and canonical rates, as well as the relationship with the Marcus formula. The fluctuation effect of bridges on the rate is further emphasized. Then, several rate approaches beyond the perturbation limit are revealed. Finally, we discuss the electronic structure theory for calculations of the electronic coupling and reorganization energy that are two key parameters in charge transfer, and show several applications.     

Electron localization and delocalization in open-shell molecules

Localization and delocalization indices derived in the framework of the quantum Atoms in Molecules theory have recently been used to analyze the electron-pair structure of closed-shell molecules. Here we report calculations of localization and delocalization indices for open-shell molecules at the Hartree-Fock (HF) level. Several simple doublet and triplet radical molecules are studied. In general, interatomic delocalization between bonded atoms is heavily dependent on the order and polarity of the bond. Unpaired electrons also have a significant effect on the interatomic delocalization indices. Indeed, for many radicals, the analysis of the spin components reveals that the interatomic delocalization is very different for alpha and beta spin electrons in many cases. In general, at the HF level, the results can be rationalized in terms of orbital contributions. However, the definition of localization and delocalization indices is completely general, and they could be calculated at any level of theory, provided that the one- and two-electron densities are available.     

Tunable charge delocalization in dinickel quinonoid complexes

When a 2,5-diamino-1,4-benzoquinonediimine C6H2(=NR)2(NHR)2 (2) is used as a bridging ligand, new dinickel(II) complexes [(acac)Ni[mu-C6H2(=NPh)4]Ni(acac)] (3a: R=Ph) and [(acac)Ni[mu-C6H2(=NCH2tBu)4]Ni(acac)] (3b: R=CH2tBu) are obtained; upon one-electron oxidation of these complexes delocalized mixed-valence compounds are formed. An X-ray diffraction study on 3b reveals equalization of the bond lengths within each of the ligand 6 systems and a lack of conjugation between them. The oxidized state in 3b+ involves both the bridging quinonoid ligand and the metal centers, with a major contribution coming from the bridging ligand. Electrochemical and spectroscopic methods were used to study the influence of the N-substituents of the tetranitrogen donor ligands 2. In this combined experimental and theoretical (DFT) study, it is also shown that the electronic structure within the dinickel system can be altered by addition of a coordinating ligand such as pyridine. The latter favors the high-spin configuration with semi-occupied metal-centered orbitals, leading to a metal-metal interaction in the mixed-valence Ni(II)-Ni(III) 3b+ system.     

Radial exchange density and electron delocalization in molecules

The six-dimensional exchange density Gamma(X)(r1,r2) is a measure of electron delocalization at the Hartree-Fock level. Fixation of r1 to a constant point results in a three-dimensional function, which displays electron delocalization that originates from r1 in position space. In this work, the dimensionality of Gamma(X)(r1,r2) is lowered from six to four by integration with regard to r2 over sphere surfaces of radius d, centered at r1 = r. The resulting radial exchange density Gamma(X)(d,r) is visualized for constant d values as a function of r. This approach indicates regions of position space which are origins of delocalization over a certain distance d. The shape of these regions very strongly depends on d. Structures similar to pi orbital densities are observed at large d values (4.5 au) in unsaturated carbon compounds, while smaller values (1.5 au) can result in structures, which resemble the Laplacian of the electron density. The abundance of different spatial structures inherent in the radial exchange density implies interesting capabilities for the orbital-independent interpretation of electronic structures in position space.     

Investigation and comparison of the electrochemical behavior of some organic and biological molecules at various conducting polymer electrodes

Electrodes modified by the electrodepozition of conducting organic polymers such as poly(3-methylthiophene)(PMT), polypyrrole (PPY) and polyaniline (PAN) were used as chemical sensors for voltammetric analysis and flow injection detection of some organic and biological molecules. The electrochemical behaviors of catechol, ascorbic acid, hydroquinone, dopamine, epinephrine, acetaminophen and p-aminophenol were examined by differential pulse voltammetry. The electrochemical behavior of these molecules at different electrodes was compared and the effects on behavior of electrolyte type and its pH and the film thickness were systematically examined. The results showed that the proposed modified surface catalyzes the oxidation of these compounds. Electrocatalytic 'efficiency' decreases in order of poly-3-methylthiophene, polypyrrole and polyaniline. Voltammetric peak positions were affected by the nature of the electrolyte and its pH. Also, the effect of increasing film thickness was to observe increased peak heights. Polymer coated electrodes were also used in an amperometric detector for flow injection analysis of most of the these compounds. The responses of the polymer electrode were 5-15 times larger as compared with those of bare platinum. PMT showed improved performance as an amperometric detector for flow injection analysis systems over other types of polymer electrodes. Detection limits as low as 10(-8)-10(-9) M were achieved using the PMT, compared with 10(-6)-10(-8) M using platinum electrodes. In the flow injection analysis, with increasing molecular weight of analyte molecules was to observe decreased peak heights.     

Interplay between charge and vibrational delocalization in cationic helium clusters

The stable structures and low temperature thermodynamics of cationic helium clusters are investigated theoretically using a diatomics-in-molecules model for the potential energy surfaces and a computational framework in which both electronic and nuclear degrees of freedom are treated on a quantum mechanical footing. While the charge is generally carried by two atoms, vibrational delocalization significantly spreads out the charge over multiple isomers for clusters containing five or more helium atoms. Our calculations indicate that large clusters are essentially fluid with a well-defined solvation shell around the charged core.     

Excess number and charge functions in liquids

Abstract

Definitions for the excess number and charge functions are given. They are used to show how the structure of a liquid about a reference particle differs from the avarage values. Some features of the structure are ilustrated more clearly by this representation than by the radial distribution functions.     

Adsorption of small organic molecules on anatase and rutile surfaces: a theoretical study

The adsorption of several small organic molecules on rutile (110) and (100) as well as on anatase (101) surfaces was investigated by Car-Parrinello molecular dynamics in aqueous solution and a new approach to the calculation of adsorption energies is proposed, taking into account the potential energy fluctuation of larger systems. Acetylene and ethylene insert into twin oxygen vacancies in the surface and form polarized covalent Ti-C bonds. In one case spontaneous coupling of two acetylene molecules to a C(4)H(3) molecule with a structure similar to trans-butadiene was observed. Neutral catechol and the singly charged anion were not reactive on any titanium dioxide surface, but the twofold-charged anion attained stable mono- and bidentated geometries on anatase. Methanol, ethanol, formaldehyde and acetaldehyde adsorbed with their functional groups. Very stable geometries provide a Ti-O bond and have adsorption energies of 60-200 kJ/mol. The adsorbates compete with water molecules for similar adsorption sites in point defects as well as on perfect surfaces.     

Electrical and magnetic properties of some organic charge transfer complexes

Two 7,7,8,8-tetracyanoquinodimethane (TCNQ) complexes with cystein and guanine have been investigated. EPR spin signals of these charge transfer complexes were recorded and the electron spin lattice relaxation time (T₁) temperature dependence was investigated. The electron scattering time, calculated from T₁, agreed reasonably well with that from the current carrier effective mass (as determined through the Hall mobility measurements).     

Semiempirical calculations on some organic molecules containing heteroatoms

The -electron structure of the ground and excited states of formamide, thioformamide, urea, thiourea, and acrolein have been calculated by the Pople-Pariser-Parr method. The attribution of the frequencies in the electronic absorption spectra has been discussed. The results are in satisfactory agreement with the experimental results.