Intrinsic Properties of Two Benzodithiophene-Based Donor--Acceptor Copolymers Used in Organic Solar Cells: A Quantum-Chemical Approach.

Conjugated donor-acceptor (D-A) copolymers show tremendous promise as active components in thin-film organic bulk heterojunction solar cells and transistors, as appropriate combinations of D-A units enable regulation of the intrinsic electronic and optical properties of the polymer. Here, the structural, electronic, and optical properties of two D-A copolymers that make use of thieno[3,4-c]pyrrole-4,6-dione as the acceptor and differ by their donor unit-benzo[1,2-b:4,5-b']dithiophene (BDT) vs the ladder-type heptacyclic benzodi(cyclopentadithiophene)-are compared using density functional theory methods. Our calculations predict some general similarities, although the differences in the donor structures lead also to clear differences. The extended conjugation of the stiff ladder-type donor destabilizes both the highest occupied and lowest unoccupied molecular orbital energies of the ladder copolymer and results in smaller gap energies compared to its smaller counterpart. However, more significant charge transfer nature is predicted for the smaller BDT-based copolymer by natural transition orbitals than for the ladder copolymer. That is, the influence of the acceptor on the copolymer properties is "diluted" to some extent by the already extended conjugation of the ladder-type donor. Thus, the use of stronger acceptor units with the ladder-type donors would benefit the future design of new D-A copolymers.

Porphyrin adsorbed on the (101[combining macron]0) surface of the wurtzite structure of ZnO--conformation induced effects on the electron transfer characteristics.

Electron transfer at the adsorbate-surface interface is crucial in many applications but the steps taking place prior to and during the electron transfer are not always thoroughly understood. In this work a model system of 4-(porphyrin-5-yl)benzoic acid adsorbed as a corresponding benzoate on the ZnO wurtzite (101[combining macron]0) surface is studied using density functional theory (DFT) and time-dependent DFT. Emphasis is on the initial photoexcitation of porphyrin and on the strength of coupling between the porphyrin LUMO or LUMO + 1 and the ZnO conduction band that plays a role in the electron transfer. Firstly, ZnO wurtzite bulk is optimized to minimum energy geometry and the properties of the isolated ZnO (101[combining macron]0) surface model and the porphyrin model are discussed to gain insight into the combined system. Secondly, various orientations of the model porphyrin on the ZnO surface are studied: the porphyrin model standing perpendicularly to the surface and gradually brought close to the surface by tilting the linker in a few steps. The porphyrin model approaches the surface either sideways with hydrogen atoms of the porphyrin ring coming down first or twisted in a ca. 45° angle, giving rise to π-interactions of the porphyrin ring with ZnO. Because porphyrins are closely packed and near the surface, emerging van der Waals (vdW) interactions are examined using Grimme's D2 method. While the orientation affects the initial excitation of porphyrin only slightly, the coupling between the LUMO and LUMO + 1 of porphyrin and the conduction band of ZnO increases considerably if porphyrin is close to the surface, especially if the π-electrons are interacting with the surface. Based on the results of coupling studies, not only the distance between porphyrin and the ZnO surface but also the orientation of porphyrin can greatly affect the electron transfer.

The effect of N-ligands on the geometry, bonding, and electronic absorption properties of ruthenium carbonyl chains.

The stability and structural properties of linear chain compounds [Ru(L)(CO)(1-2)](n) (L = substituted or unsubstituted 2,2'-biimidazole, 2,2'-bipyridine, 1,10-phenantroline, or 2,2':6',2''-terpyridine) were investigated by computational DFT methods. The favored torsional orientation of the neighboring units in the chain was studied using dinuclear (n = 2) models. Models up to 8 [Ru(L)(CO)(1-2)] units were used to analyze Ru-Ru interactions by Mayer bond order and charge density analysis. Compared to the related polymeric compound [Ru(CO)(4)](n), the presence of chelating nitrogen ligands led to slightly weaker Ru-Ru bonds. Electronic structures and corresponding excitations were investigated by TD-DFT methods. The studied nitrogen ligands, especially biimidazole compared to polypyridines, were found to produce differences in the absorption characteristics of the chains. Moreover, as the chain length was increased, a red-shift in excitations was observed. The results show that even with rather subtle structural changes in the surrounding ligands, we can produce significant effect on the properties of the materials.

Metal-metal interactions in linear tri-, penta-, hepta-, and nona-nuclear ruthenium string complexes.

Density functional theory (DFT) methodology was used to examine the structural properties of linear metal string complexes: [Ru(3)(dpa)(4)X(2)] (X = Cl(-), CN(-), NCS(-), dpa = dipyridylamine(-)), [Ru(5)(tpda)(4)Cl(2)], and hypothetical, not yet synthesized complexes [Ru(7)(tpta)(4)Cl(2)] and [Ru(9)(ppta)(4)Cl(2)] (tpda = tri-α-pyridyldiamine(2-), tpta = tetra-α-pyridyltriamine(3-), ppta = penta-α-pyridyltetraamine(4-)). Our specific focus was on the two longest structures and on comparison of the string complexes and unsupported ruthenium backboned chain complexes, which have weaker ruthenium-ruthenium interactions. The electronic structures were studied with the aid of visualized frontier molecular orbitals, and Bader's quantum theory of atoms in molecules (QTAIM) was used to study the interactions between ruthenium atoms. The electron density was found to be highest and distributed most evenly between the ruthenium atoms in the hypothetical [Ru(7)(tpta)(4)Cl(2)] and [Ru(9)(ppta)(4)Cl(2)] string complexes.

Concerted halogen and hydrogen bonding in [RuI2(H2dcbpy)(CO)2]···I2···(CH3OH)···I2···[RuI2(H2dcbpy)(CO)2].

A new type of concerted halogen bond-hydrogen bond interaction was found in the solid state structure of [RuI(2)(H(2)dcbpy)(CO)(2)]···I(2)···(MeOH)···I(2)···[RuI(2)(H(2)dcbpy)(CO)(2)]. The iodine atoms of the two I(2) molecules interact simultaneously with each other and with the OH group of methanol of crystallization. The interaction was characterized by single crystal X-ray measurements and by computational charge density analysis based on DFT calculations.

Computational DFT Study of Ruthenium Tetracarbonyl Polymer.

Ruthenium tetracarbonyl polymer, [Ru(CO)4]n, a chainlike compound formed by metal-metal interactions, was studied computationally. We first performed tests with selected pure and hybrid GGA density functionals and ab initio methods at HF and MP2 levels of theory to find the most suitable method. Calculated geometries and molecular orbitals were compared to see effectiveness and possible differences of the methods. Hybrid functionals, especially PBE1PBE and MPW1K, were found to produce accurate geometrical parameters compared to the experimental structure, with reasonable computational cost. Bonding in [Ru(CO)4]n chains was studied by calculation of Mayer bond order and theoretical structure factors followed by multipole refinement to get bond critical points according to the quantum theory of atoms in molecules. Ruthenium-ruthenium bonding comparable to that in a Ru3(CO)12 cluster was found with both methods.