Density Functional Study on the Intercalation of Fullerenes into AnE-PV Copolymer Layers.

We investigated the intercalation of C into poly(p-anthracene-ethynylene)-alt-poly(p-phenylene-vinylene) copolymers layers by density functional theory calculations in respect of crystal structures and electronic band structures. Based on the experimental observations, we found that the copolymer with branched side chains substituted next to the anthracene units and the linear side chains substituted to the vinylene units has a better tendency to intercalate with C than the reversely substituted copolymer. The calculated electronic band structures of the intercalated phase, featured by flat in-gap states resulting from C molecules, explain the experimentally observed variations of the photocurrent, photoluminescence, and electroluminescence yields with different ratio between PCBM and the two types of copolymers in the ternary blend.

Density Functional Study of Stacking Structures and Electronic Behaviors of AnE-PV Copolymer.

In this work, we report an in-depth investigation on the π-stacking and interdigitating structures of poly(p-anthracene-ethynylene)-alt-poly(p-phenylene-vinylene) copolymer with octyl and ethyl-hexyl side chains and the resulting electronic band structures using density functional theory calculations. We found that in the π-stacking direction, the preferred stacking structure, determined by the steric effect of the branched ethyl-hexyl side chains, is featured by the anthracene-ethynylene units stacking on the phenylene-vinylene units of the neighboring chains and vice versa. This stacking structure, combined with the interdigitating structure where the branched side chains of the laterally neighboring chains are isolated, defines the energetically favorable structure of the ordered copolymer phase, which provides a good compromise between light absorption and charge-carrier transport.

Effect of Side Chains on Molecular Conformation of Anthracene-Ethynylene-Phenylene-Vinylene Oligomers: A Comparative Density Functional Study With and Without Dispersion Interaction.

Using density functional calculations with and without dispersion interaction, we studied the effects of linear octyl and branched 2-ethylhexyl side chains on the oligomer conformation of the conjugated copolymer poly(p-anthracene-ethynylene)-alt-poly(p-phenylene-vinylene). With dispersion included, the branched side chains can cause significant bending of the oligomer backbone, while without dispersion they induce mainly torsional disorder. The oligomers with mainly linear side chains keep good planarity when optimized with and without dispersion.

Quantum chemical insights into the dependence of porphyrin basicity on the meso-aryl substituents: thermodynamics, buckling, reaction sites and molecular flexibility.

The chemical and sensing properties of porphyrins are frequently tuned via the introduction of peripheral substituents. In the context of the exceptionally fast second protonation step in the case of 5,10,15,20-tetraphenylporphyrin (TPP), as compared to porphin and 5,10,15,20-tetramesitylporphyrin (TMesP), we investigated the macrocycle-substituent interactions of these three porphyrin derivatives in detail. Using quantum chemical thermodynamics calculations, the analysis of geometric structures, torsional profiles, electrostatic potential distributions, and particularly the analysis of molecular flexibilities via ab initio molecular dynamics simulations, we obtained a comprehensive picture of the reactivities of the studied porphyrins and how these are influenced by the meso-substituents.

Sub-bandgap absorption in organic solar cells: experiment and theory.

Most high-performance organic solar cells involve bulk-heterojunctions in order to increase the active donor-acceptor interface area. The power conversion efficiency depends critically on the nano-morphology of the blend and the interface. Spectroscopy of the sub-bandgap region, i.

Surface electronic structure of [XMIm]Cl probed by surface-sensitive spectroscopy.

We apply electron spectroscopy methods with different surface sensitivities to elucidate the DOS of the surface and the near-surface region of [XMIm]Cl (X=octyl, hexyl, butyl, and ethyl alkyl chain) ionic liquids. Using metastable induced electron spectroscopy (MIES) we are able to detect the density of states in front of the outermost surface, whereas ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) measurements provide lower surface sensitivity. The assignment of certain structures in the valence band spectra to particular atoms/functional groups of the ionic liquid based on DFT calculations and the reconstruction of PES spectra enables us to obtain information on the dominating groups at the surface, or in other words, on the molecular/ionic arrangement and orientation at the surface.

Theoretical reconstruction and elementwise analysis of photoelectron spectra for imidazolium-based ionic liquids.

We have recently measured core level and valence band XPS, UPS, and MIES spectra of two room temperature ionic liquids composed of bis(trifluoromethylsulfonyl)imide anions ([Tf(2)N](-)) and either 1-ethyl-3-methyl-imidazolium ([EMIm](+)) or 1-octyl-3-methyl-imidazolium cations ([OMIm](+)). [T. Ikari, A.

Spectroscopic units in conjugated polymers: a quantum chemically founded concept?

In conjugated polymers the concept of spectroscopic units belonging to different spatial segments of the chain, which are responsible for the spectroscopic properties of the polymer, has been used to explain the spectral heterogeneity and the excitation migration by (Förster type) hopping transfer. In the present work we study the possible mechanism of segmentation of polythiophene into spectroscopic units by using quantum-chemical methods (ZINDO). We found that static geometric defects such as kinks or torsions do not result in a significant localization of the excited states to a certain segment.

Anomalous energy transfer dynamics due to torsional relaxation in a conjugated polymer.

In isolated conjugated polymers two explanations are in discussion for the redshift of the emission on a picosecond time scale-exciton energy transfer (EET) between conjugated segments along the chains and conformational changes of these segments themselves, i.e., torsional relaxation.

Conformational disorder of conjugated polymers.

Conformational disorder of conjugated polymers is an important issue to be understood and quantified. In this paper we present a new method to assess the chain conformation of conjugated polymers based on measurements of intrachain energy transfer. The chain conformation is modeled on the basis of monomer-monomer interactions, such as torsion, bending, and stretching of the connecting bond.

Control of emission by intermolecular fluorescence resonance energy transfer and intermolecular charge transfer.

Control of emission by intermolecular fluorescence resonant energy transfer (IFRET) and intermolecular charge transfer (ICT) is investigated with the quantum-chemistry method using two-dimensional (2D) and three-dimensional (3D) real space analysis methods. The work is based on the experiment of tunable emission from doped 1,3,5-triphenyl-2-pyrazoline (TPP) organic nanoparticles (Peng, A. D.

Spectral broadening and diffusion by torsional motion in biphenyl.

We have studied biphenyl by time-dependent density-functional theory. In particular, we have analyzed the dependence of singlet excitation energies and transition dipoles on the torsional angle between the phenyl groups. The torsional spectrum has been computed quantum mechanically as well as semiclassically in order to understand how this influences the broadening of absorption and luminescence spectra.

The electronic states of polyfluorene copolymers with alternating donor-acceptor units.

We calculate the electronic states of the low bandgap polyfluorene-based copolymer DiO-PFDTBT, which consists of alternating 9,9-dioctyl-9H-fluorene and 4,7-di-thiophen-2-ylbenzo[1,2,5]thiadiazole (TBT) units, and compare with the steady-state absorption, emission, and excitation spectrum. Using the semiempirical quantum-chemical (ZINDO) method we can assign the characteristic bands of the "camel-back" absorption spectrum to one charge transfer state at lower energy localized on the TBT unit, and one delocalized excitonic state at higher energy corresponding to the pi-conjugated electron system. Additional "dark" charge transfer states in the gap between these bands have been revealed.

Excitonic coupling in polythiophenes: comparison of different calculation methods.

In conjugated polymers the optical excitation energy transfer is usually described as Forster-type hopping between so-called spectroscopic units. In the simplest approach using the point-dipole approximation the transfer rate is calculated based on the interaction between the transition dipoles of two spectroscopic units. In the present work we compare this approach with three others: The line-dipole approximation, the Coulomb integral between the transition densities, and a quantum-chemical calculation of the interacting dimer as entity.

In vitro self-assembly of the light harvesting pigment-protein LH2 revealed by ultrafast spectroscopy and electron microscopy.

Controlled ensemble formation of protein-surfactant systems provides a fundamental concept for the realization of nanoscale devices with self-organizing capability. In this context, spectroscopic monitoring of pigment-containing proteins yields detailed structural information. Here we have studied the association behavior of the bacterial light-harvesting protein LH2 from Rhodobacter spheroides in an n,n-dimethyldodecylamine-n-oxide/water environment.

Excitonic coupling of chlorophylls in the plant light-harvesting complex LHC-II.

Manifestation and extent of excitonic interactions in the red Chl-absorption region (Q(y) band) of trimeric LHC-II were investigated using two complementary nonlinear laser-spectroscopic techniques. Nonlinear absorption of 120-fs pulses indicates an increased absorption cross section in the red wing of the Q(y) band as compared to monomeric Chl a in organic solution. Additionally, the dependence of a nonlinear polarization response on the pump-field intensity was investigated.