Alia Tadjer

Sphere-to-Rod Transitions of Nonionic Surfactant Micelles in Aqueous Solution Modeled by Molecular Dynamics Simulations

Control of the size and agglomeration of micellar systems is important for pharmaceutical applications such as drug delivery. Although shape-related transitions in surfactant solutions are studied experimentally, their molecular mechanisms are still not well understood. In this study, we use coarse-grained molecular dynamics simulations to describe micellar assemblies of pentaethylene glycol monododecyl ether (C(12)E(5)) in aqueous solution at different concentrations. The obtained size and aggregation numbers of the aggregates formed are in very good agreement with the available experimental data. Importantly, increase of the concentration leads to a second critical micelle concentration where a transition to rod-like aggregates is observed. This transition is quantified in terms of shape anisotropy, together with a detailed structural analysis of the micelles as a function of aggregation number.

Unprecedented Route to Ordered Polyaniline: Direct Synthesis of Highly Crystalline Fibrillar Films with Strong π-π Stacking Alignment.

Films of polyaniline (PANI) featuring about 80% crystallinity and characterised with strong π-π stacking alignment parallel to the film surface have been obtained directly after the original synthesis upon simple drying of the aqueous PANI suspension. A strong anisotropy in the growth of the nano-sized crystals produced during the synthesis results in the formation of micrometer-length fibrils perpendicular to the film surface in the course of water evaporation. The regular intercalation of water molecules between the PANI chains seems to be crucial for their ordering throughout the synthesis and film formation.

Melatonin: Quantum-chemical and biochemical investigation of antioxidant activity.

Experimental and theoretical investigation of the antioxidant activity of melatonin is carried out. The theoretical approach comprises the evaluation of several appropriate descriptors of scavenging activity with the help of quantum-chemistry methods. The values obtained are compared with available data for substances with established antioxidant properties. One of the most widely used markers for in vivo free radical oxidation processes is malondialdehyde (MDA) as an end product of membrane lipid peroxidation. Experimental support of the computed scavenging parameters is provided by estimation of the effect of supplementary melatonin therapy on the plasma levels of MDA in CRF patients on maintenance HD therapy. Different reaction paths have been considered and related to the obtained data, allowing speculations about the reaction mechanism and the antioxidant potential of melatonin for practical purposes.

Fully Doped Oligomers of Emeraldine Salt: Polaronic versus Bipolaronic Configuration

Calculations for model oligomers of the emeraldine salt with UBLYP/6-31G*/PCM are performed. The models differ in number of monomers, in the position of the counterions (Cl(-)), and in multiplicity. The molecular features affected most prominently by the protonation, namely, structure, energetics, and electron and spin density partitioning are analyzed. The results show unequivocally that the studied molecular characteristics are essentially size independent. The octamer profiles of all parameters are repeated in the dodecamer and the hexadecamer. The bipolaronic forms are energetically more favorable than the polaronic ones within the chosen protocol. The electronic structure in the intermediate multiplicities differs from the bipolaronic and polaronic periodicity. The geometrical changes and electron density redistribution upon increase of multiplicity illustrate the pathway of intramolecular bipolaron-polaron conversion. The orbital analysis rationalizes the observed behavior of the oligomers.

Evidence for Generation of Delocalized Polarons in Conducting Polyaniline: A Raman Scattering Spectroscopy Approach

Abstract Photoexcitation as a novel route for obtaining delocalized polarons in conducting polyaniline is predicted theoretically and confirmed experimentally. Original model experiments and theoretical calculations on protonated emeraldine species with various chain alignments as well as of isolated chains were carried out. In addition, experimental models matching theoretical models of isolated and bulk polyaniline (PANI) chains were synthesized and their absorption and Raman spectra recorded. The detection of the photoexcited states was performed by conventional Raman scattering spectroscopy (generally employed for studying ground states), which is an attractive alternative for generating and characterizing photoexcited delocalized polarons in polyaniline. The occurrence of photoexcited delocalized polaronic species is favored by chain stacking.

A new approach to the study of oxidative polymerization of aniline and transformations of polyaniline. Support by means of the Hueckel method

An interrelationship between the mechanism of aniline polymerization and the direction of redox processes with participation of polyaniline chains on the one hand, and the π-electron energy of oligoanilines with various numbers of units, degrees of oxidation and protonation on the other hand, has been found for the first time. Calculations performed using the Hueckel method revealed that the difference in the π-electron energy of the oxidized and reduced forms of the chain correlated very well with the experimentally determined electrochemical potential for the respective redox transition. It is suggested that the Hueckel method can be used for investigation of the electrochemical activity of conjugated polymers obtained by oxidative polymerization, as well as for studying the mechanism of the latter.

Is 2D graphite an ultimate large hydrocarbon?: III. Structure and energy spectra of large polybenzenoid hydrocarbons with different edge structures☆

Abstract The structure and energy spectra of very large polybenzenoid alternant hydrocarbons (with up to N ∼10 3 C-atoms) with different edge structures and defects were investigated theoretically. In all the cases, the calculated values of the energy gap width tend monotonically to a value different from zero when the electron correlation is taken into account. Typical surface (Tamm) states, namely non-bonding MOs, appear in the cases of non-Kekule, open-shell hydrocarbons featuring point defects with a definite topology. The electrons occupying the non-bonding MOs are ferromagnetically coupled. These hydrocarbons are in fact polyradicals with a substantial value of the specific π-electron energy, which determines their relative stability.

Calculated and experimental spectra of some 1,8-naphthalimide derivatives

The influence of different electron donating substituens on the absorption of 1,8-naphthalimide derivatives was studied and compared with the theoretically (AM1,PPP-SCF-CI) calculated predictions. It was shown that the experimental results and the calculated electronic transitions are in good agreement, assuming that in relation to the solvent polarity the possible mesomeric forms contribute to a different extent. In cases where a labile hydrogen atom is present, tautomerism takes place.

Understanding the Fluorescence of TADF Light-Emitting Dyes

In order to afford in a controlled fashion fine-tuning of the color and the intensity of the emitted light of potential fluorophores for organic light-emitting diodes (OLED), directed molecular design based on a donor-spacer-acceptor model is undertaken. One way of increasing emission efficiency is triplet harvesting. This can be achieved by thermally activated delayed fluorescence (TADF) when triplet and singlet excited states are quasi degenerate. Molecular building units are selected and bound in a specific pattern to allow for increase in emission performance, also due to TADF. Using time-dependent density functional theory, the relevant singlet-singlet and triplet-singlet energy gaps corresponding to absorption or emission transitions of the compounds are computed to simulate the electroluminescent spectrum. The results are analyzed in depth and relations between some spectral and structural properties are proposed. The best suited molecules are delineated as potential OLED building blocks. Guidelines for systematic improvement of the molecular characteristics are outlined.

A theoretical study of molecular structure in the excited state and molecular luminescence part II: An SCF—CI calculation of luminescence and molecular structure in equilibrium excited states of some compounds with strong bond alternation

Abstract It is shown that the iterative procedure SCF—CI-SC previously suggested [1] can be successfully used for calculation of the Stokes shift, FC luminescent maxima and equilibrium molecular geometry in excited states of compounds which have strong bond alterna- tion in their ground state. The influence, in the excited state, of molecular geometry on the positions and intensities of the luminescent transitions is discussed. The calculated molecular geometries show that there is a strong conjugation effect in some excited states of compounds with strong, moderate and weak bond alternation; this latter factor probably determines the luminescent properties of these organic compounds.