Formation Of Molecular Aggregates Research Articles

Experimental and DFT analysis of the concentration dependency of corrosion inhibition by a pyridine Schiff base for mild steel in HCl solution

In the present article, we studied the corrosion inhibition of the pyridine Schiff base N, N´-bis(4-dimethylaminobenzaldehyde)-2, 6-pyridinediamine (4-DMBPy) during the corrosion of mild steel AISI 1020 in a 0.1 M HCl solution. The electrochemical analysis by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) revealed that the Schiff base promotes a protective efficiency up to 81 % at 25 ppm, followed by a decrease at higher concentrations.The Schiff base – surface interactions follow a Freundlich isotherm, indicating a physical adsorption mechanism. The theoretical calculus by Fukui function and dual reactivity descriptors confirmed the physical interaction with mild steel and the low reactivity of the adsorbed species that led to corrosion protection affecting the cathodic and anodic reactions. The quantum calculations by density functional theory (DFT) indicated that protective efficiency decreases due to the tendency to interact with each other. These interactions led to the formation of molecular aggregates and a desorption process. Molecular dynamics (MD) simulations exhibited that the 4-DMBPy molecule preferred to arrange parallel to the Fe (110) surface through physical adsorption. The results were complemented by the scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and atomic force microscopy (AFM) analyses to reveal the further information about the corrosion process and the effect of the 4-DMBPy Schiff base as a corrosion inhibitor.

Swift and quantitative naked-eye detection of cyanide: A dual-mode approach using an ICT-based colorimetric and ratiometric fluorescent chemodosimeter

A chemodosimeter (1) based on dicyanovinyl with a strong intramolecular charge transfer (ICT) mechanism has been devised for the selective and sensitive detection of cyanide (CN−) in a tetrahydrofuran aqueous solution. In polar solvents, chemodosimeter 1 exhibits robust ICT-induced emission; however, in the solid state, its fluorescence weakens owing to the formation of molecular aggregates prompted by intermolecular π–π interactions. The colorimetric and ratiometric fluorescent changes observed in the chemodosimeter upon exposure to CN− result from the incorporation of CN− at the β-conjugated site of the dicyanovinyl moiety of chemodosimeter 1, effectively hindering the ICT process. This alteration is supported by 1H NMR titration experiments and density functional theory calculations, which corroborate the cyanide-induced ICT on–off switching mechanism. The detection limit of chemodosimeter 1 (5.8 × 10−7 M) for CN− falls below the maximum allowable level of cyanide in drinking water (1.9 × 10−6 M), as established by the World Health Organization. Test strips utilizing chemodosimeter 1 offer a potential solution for swift and quantitative naked-eye detection of CN− in real-time scenarios. Moreover, chemodosimeter 1 was effectively employed for detecting cyanide in actual environmental water samples.

Self-aggregation of stilbazolium ion pairs in liquid chloroform. A molecular dynamics study

Molecular aggregation plays an important role on optical phenomena, and optimizing the latter requires understanding the aggregation behavior as a function of the molecular structures. The formation of molecular aggregates composed of stilbazolium chromophores and small to medium-size anions is studied by carrying out classical molecular dynamics (MD) simulations, shedding light on the dynamics of spontaneous self-aggregation of anion-cation (AC) pairs and estimating their equilibrium aggregation constants. First, analyses based on atom-atom distances and on contact mapping (Voronoi tessellation) highlight that the interactions occur at the level of the methylpyridinium groups of the cations, which enabled monitoring the clustering dynamics. Then, simulations show that clusters containing up to 5 AC pairs dominate the populations. Among the AC pairs, two of them (where the stilbazolium bears a strong donor substituent and when the anion is SCN- or pTS-) aggregate into small clusters even at high concentration, corroborating related experimental results [ChemPhysChem. 11 (2010) 495–507]. Still, these MD simulations do not reveal strict relationships between the aggregate size and the nature of the anion. Nevertheless, AC pairs bearing iodide anions require longer MD simulation times to reach an asymptotic equilibrium tendency of the average number of clusters. Finally, the aggregates are shown to adopt a preferential Λ-shape with contributions from head-to-head and stacked forms. Complementarily, the analyses of the isoperimetric quotient reinforce the AC pairs are not forming spherical aggregates even for clusters including more than 10 AC pairs. So, when the stilbazolium bears a strong donor substituent and when the anion is pTS-, stacked aggregates are formed, at both low and high concentrations, resulting in excellent agreement with the measured aggregation equilibrium constant.

The impact of polymers’ supramolecular structure on water vapour sorption and drug release from films on the basis of some polysaccharide

The article establishes the relationship between supramolecular film structure of some polysaccharides and their diffusion characteristics. The differences in the supramolecular film structures, in their turn, are associated with the differences in the structural-physical state of the solutions the films are made from and the time of the film dry out. It was established that in a definite area of concentrations of polymer solutions there was a sharp increase in viscosity due to the interaction of the macromolecular chains and mass scale formation of molecular aggregates in the solution. As a result, the polysaccharide solutions of different concentration differ from the structural-physical point of view. It was proved that the polymer concentration in the initial solution contributes greatly to the structure of the films formed, and the changes in the supramolecular structure of the film samples have an impact on the diffusion characteristics. It was established that the values of the diffusion coefficients determined in the experiments depend on the polymer concentration in the initial solution and the thickness of the film samples. It was also showed that the diffusion processes do not follow the Fick's law and occur in an abnormal diffusion mode.

Aggregation-Induced Emission: From Small Molecules to Polymers-Historical Background, Mechanisms and Photophysics.

The enhancement of photoluminescence through formation of molecular aggregates in organic oligomers and conjugated organic polymers is reviewed. A historical contextualization of aggregation-induced emission (AIE) phenomena is presented. This includes the loose bolt or free rotor effect and J-aggregation phenomena, and discusses their characteristic features, including structures and mechanisms. The basis of both effects is examined in key molecules, with a particular emphasis on the AIE effect occurring in conjugated organic polymers with a polythiophene (PT) skeleton with triphenylethylene (TPE) units. Rigidification of the excited state structure is one of the defining conditions required to obtain AIE, and thus, by changing from a flexible ground state to rigid (quinoidal-like) structures, oligo and PTs are among the most promising emerging molecules alongsidewith the more extensively used TPE derivatives. Molecular structures moving away from the domination of aggregation-caused quenching to AIE are presented. Future perspectives for the rational design of AIEgen structures are discussed.

Understanding the photophysics of stercobilin-Zn(II) and urobilin-Zn(II) complexes towards faecal pigment analysis

A detailed photophysical study of two faecal pigments (FPs), Urobilin (UB) and Stercobilin (SB), and their zinc complexes [FP-Zn(II)] was carried out. The enhancement of UB and SB fluorescence resulting from the formation of their Zn(II) complexes was attributed to the complexation-induced rigidity of the chromophoric units, and the corresponding decrease of nonradiative decay rate constants of the excited singlet states (knr). The effect of various physicochemical environments was also studied in detail in order to understand the fluorescence behaviour of the Zn(II) complexes. FP-Zn(II) complexes have a lower solubility in water that results in the formation of molecular aggregates. The aggregation-induced loss of fluorescence of FP-Zn(II) complexes could be overcome by using the appropriate mixture of ethanol and water (70:30). Molecular orbital calculations on the FP-Zn(II) complexes provided a good idea of the geometry of the complexes and helped rationalise the enhancement of fluorescence after complexation. This study could pave the way towards developing a convenient non-extraction aqueous phase analytical procedure for detection of FPs using Zn(II) complexation method.

Determination of a critical separation concentration for associative polymers in porous media based on quantification of dilute and semi-dilute concentration regimes

Hydrophobic interactions are an inherent property of associative polymers which results in the formation of molecular aggregates. The loss of hydrophobic interactions under reservoir conditions have been reported, and this results in increased fluid – rock interaction effect such as adsorption. However, existing adsorption studies only reports the interaction of individual molecules with rock surface without taking into account mechanically retained molecular aggregates in narrow pores. The implication of this is a reduction in associative effect between polymer molecules during transport in a porous media. In this work, the minimization of this phenomenon and sustenance of the interaction network was studied through a theoretically defined dimensionless parameter for the quantification of molecular interactions in the various polymer concentration regimes. It was established in this work that associative polymers exhibit a critical separation concentration during propagation in a porous media under given reservoir conditions. This critical separation concentration marks the concentration beyond which large molecular aggregates constituting the hydrophobic network is sterically excluded and retained in narrow pore spaces. This separation phenomenon was predicted to occur when the proportion of molecular interactions arising from hydrophobic and intramolecular interactions are equal. Thus, the balance in the molecular interactions in the semi – dilute concentration regime was identified as key in minimizing the loss of hydrophobic interactions to aggregate retention and optimizing its sustainability in a porous media. Consequently, a novel approach was developed based on this knowledge for the transport of associative polymers at which these hydrophobic interactions are sustained. It was demonstrated that propagating associative polymers using this approach ensured the sustainability at concentrations below the critical separation concentration tend to maximize the hydrophobic interaction network with minimal loss of polymer chains retention mechanisms.

A Computational Predictive Approach for Controlling the Morphology of Functional Molecular Aggregates on Substrates

AbstractFull control of the growth dynamics and morphology of nanoscale molecular aggregates on substrates is a crucial factor for the development of novel materials and devices for technological applications. A novel computational approach based on molecular dynamics, which is able to predict the structure of nanoscale aggregates of organic small molecules on substrates as a function of growth and processing conditions, is presented. In the simulation protocol proposed, molecules are progressively added to the substrate, one by one, reproducing vacuum thermal deposition processes. The simulation parameters affect the structure of the configuration, and formation of molecular aggregates is spontaneously observed upon reaching a critical molecular density at the interface. Suitable simulation parameters allow to access different molecular aggregation morphologies on substrates, including crystalline phases, matching the structural variability observed in real fabrication conditions. This approach is benchmarked by simulating the morphology of aggregates of N,N′‐ditridecylperylene‐3,4,9,10‐tetracarboxylic diimide (PTCDI‐C13), a prototypical n‐type semiconductor for organic electronics, on graphene, in different growth conditions. Simulations predict structural features of aggregates of PTCDI‐C13 on graphene that are in excellent agreement with experiments and, at the same time, provide a rationale and quantitative relationship between molecular structure and observed aggregation morphologies.

Excited state dynamics of organic semiconductors measured with shot-to-shot correction of scatter and photoluminescence

Molecular aggregates and crystallites that emerge during the casting of semiconducting organic films from solution are highly sensitive to deposition conditions. Their structure determines the electronic properties, and ultimately the utility, of these materials as semiconductors. We report the development of a single-shot transient absorption (SSTA) spectrometer to measure the evolving excited state dynamics of organic molecules and polymers in situ during their casting into films from solution. The formation of molecular aggregates and crystallites changes the electronic structure, excited state dynamics, and concomitantly, the photoluminescence (PL) yield and structural heterogeneity. When these latter contributions are dynamic, they hinder accurate SSTA measurements of excited state dynamics. In this work, an additional optical chopper in the probe beam path is shown to correct for dynamic scatter and PL from the pump pulse. The importance of the correction provided by this optical chopping scheme to the measurement of photoluminescent and/or scattering samples is demonstrated using SSTA measurements of a sulforhodamine solution with high PL, a drop-cast sulforhodamine film with less PL, and a film of a prototypical organic semiconductor, poly(3-hexylthiophene-2,5-diyl).

Relevance of the Pharmacokinetic and Pharmacodynamic Profiles of Puerariae lobatae Radix to Aggregation of Multi-Component Molecules in Aqueous Decoctions.

The complexity of traditional Chinese medicines (TCMs) is related to their multi-component system. TCM aqueous decoction is a common clinical oral formulation. Between molecules in solution, there exist intermolecular strong interactions to form chemical bonds or weak non-bonding interactions such as hydrogen bonds and Van der Waals forces, which hold molecules together to form “molecular aggregates”. Taking the TCM Puerariae lobatae Radix (Gegen) as an example, we explored four Gegen decoctions of different concentration of 0.019, 0.038, 0.075, and 0.30 g/mL, named G-1, G-2, G-3, and G-4. In order of molecular aggregate size (diameter) the four kinds of solution were ranked G-1 < G-2 < G-3 < G-4 by Flow Cell 200S IPAC image analysis. A rabbit vertebrobasilar artery insufficiency (VBI) model was set up and they were given Gegen decoction (GGD) at a clinical dosage of 0.82 g/kg (achieved by adjusting the gastric perfusion volume depending on the concentration). The HPLC fingerprint of rabbit plasma showed that the chemical component absorption into blood in order of peak area values was G-1 < G-2 > G-3 > G-4. Puerarin and daidzin are the major constituents of Gegen, and the pharmacokinetics of G-1 and G-2 puerarin conformed with the two compartment open model, while for G-3 and G-4, they conformed to a one compartment open model. For all four GGDs the pharmacokinetics of daidzin complied with a one compartment open model. FQ-PCR assays of rabbits’ vertebrobasilar arterial tissue were performed to determine the pharmacodynamic profiles of the four GGDs. GGD markedly lowered the level of AT1R mRNA, while the AT2R mRNA level was increased significantly vs. the VBI model, and G-2 was the most effective. In theory the dosage was equal to the blood drug concentration and should be consistent; however, the formation of molecular aggregates affects drug absorption and metabolism, and therefore influences drugs’ effects. Our data provided references for the rational use of Chinese medicines in the clinic, such as the best oral preparation and decoction concentration.

Free Energy of Ligand Removal in the Metal-Organic Framework UiO-66.

We report an investigation of the “missing-linker phenomenon” in the Zr-based metal–organic framework UiO-66 using atomistic force field and quantum chemical methods. For a vacant benzene dicarboxylate ligand, the lowest energy charge-capping mechanism involves acetic acid or Cl–/H2O. The calculated defect free energy of formation is remarkably low, consistent with the high defect concentrations reported experimentally. A dynamic structural instability is identified for certain higher defect concentrations. In addition to the changes in material properties upon defect formation, we assess the formation of molecular aggregates, which provide an additional driving force for ligand loss. These results are expected to be of relevance to a wide range of metal–organic frameworks.

Effect of Solvent Additives on the Solution Aggregation of Phenyl-C61–Butyl Acid Methyl Ester (PCBM)

High-boiling-point solvent additives, employed during the solution processing of active-layer formulations, impact the efficiency of bulk heterojunction (BHJ) organic solar cells by influencing the morphological/topological features of the multicomponent thin film. Here, we aim at a better understanding of how these additives change the aggregation landscape in the casting solution prior to film deposition via a multiscale computational study of the aggregation phenomena of phenyl-C61-butyric-acid methyl ester (PCBM) in various solutions. The energetic landscape of PCBM-solvent/solvent-additive intermolecular interactions is evaluated at the electronic-structure level through symmetry-adapted perturbation theory to determine the nature and strength of noncovalent forces important to aggregation. Molecular dynamics simulations highlight how the choice of solvent and solvent additives control the formation of molecular aggregates. Our results indicate that high-boiling point solvent additives change the eff...

The formation of molecular aggregates of sulfophthalocyanine in complexes with semiconductor nanocrystals

In this study, complexes of CdSe/ZnS quantum dots and quantum rods with sulfophthalocyanine molecules have been formed. Analysis of spectral and luminescent properties of solutions of the complexes has revealed that an increase in the number of molecules per one nanocrystal in a mixed solution results in a noticeable decrease in the intensity of the luminescence of the quantum dots and quantum rods. In addition, it has been found that, upon an increase in the concentration of sulfophthalocyanine molecules, the absorption spectra of the samples in the region of their first absorption band have signs of formation of nonluminiscent aggregates of sulfophthalocyanine molecules. Analysis of the absorption spectra of the mixed solutions has made it possible to demonstrate that the complexes with the quantum rods have a content of the sulfophthalocyanine aggregates significantly lower than the complexes with the quantum dots.

Catalyst-Free, Atom-Economic, Multicomponent Polymerizations of Aromatic Diynes, Elemental Sulfur, and Aliphatic Diamines toward Luminescent Polythioamides

Sulfur-containing polymers have been widely studied because of their high refractivity and low dispersion, but the efficient synthetic approach of them is quite limited. In this work, we use the abundantly existed elemental sulfur as monomer to prepare polythioamide directly and efficiently through a facile multicomponent polymerization (MCP) of aromatic diynes, sulfur, and aliphatic diamines. This MCP can proceed smoothly in a catalyst-free manner with high atom utilization to afford polythioamide with well-defined structure, high molecular weight, and high yield. It demonstrates a convenient approach to convert elemental sulfur into functional polythioamide. Fluorescence is observed from the polythioamide, despite the absence of typical fluorophores, owing to the “heterodox clusters” composed of a large number of lone-pair-containing electron-rich heteroatoms. The emission maxima and efficiencies of the polymers depend on the formation of molecular aggregates through intrachain and intermolecular intera...

Excitation and emission properties of Zn(II) Schiff base complex by combined crystallographic, spectroscopic and DFT studies

Abstract The structure and photophysical properties of the Zn(II) complex with the novel fluorescent Schiff base ligand L: (E)-N′-((2-hydroxynaphthalen-1-yl) ethylene)acetohydrazide have been investigated. X-ray crystallography has proved the neutral, mononuclear complex: [ZnL2]·H2O·DMF, ZnL. The crystal packing proves intramolecular-interactions between the alternating naphthalene rings. Photophysical measurements of the complex in solvents of varying polarity (DCM, THF and DMSO) provide evidence for the formation of molecular aggregates in non-polar solvents that enhance the energy of the S0–S1 level. In the solid states, the complex exhibits unusual blue-shifted emission with respect to the ligand, due to excited state intermolecular proton transfer (ESIPT) within the ZnL molecules in the lattice. The experimental measurements are also complimented by DFT and TD-DFT calculations in the gas-phase and solution states.

Phenothiazine decorated carbazoles: effect of substitution pattern on the optical and electroluminescent characteristics.

A series of thienylphenothiazine decorated carbazoles were synthesized and characterized by optical, electrochemical, thermal, and theoretical investigations. Absorption spectra of the compounds are influenced by the substitution pattern and chromophore number density. Compounds containing 2,7-substitution exhibited red-shifted absorption, while the chromophore loading on the other positions led to the increment in molar extinction coefficients due to the increase in the chromophore density. Multiple substitutions resulted in twisting of chromophores and affected the conjugative delocalization of the π-electrons, which produced shorter wavelength absorption for the 2,3,6,7-tetrasubstituted derivative. Interestingly, the compounds exhibited excited-state solvatochromism attributable to the structural reorganization-induced electronic perturbations. The solvatochromic data are supportive of a general solvent effect, which is further confirmed by Lippert-Mataga correlation. End-capping with butterfly shaped phenothiazine restrained the formation of molecular aggregates in the solid state. All of the compounds displayed exceptional thermal stability attributable to the rigid carbazole building block. Solution processed OLED fabricated using the new materials as emitting dopants in 4,4'-bis(9H-carbazol-9-yl)biphenyl host exhibited bluish green electroluminescence.

Secondary Crystal Nucleation: Nuclei Breeding Factory Uncovered

AbstractSecondary nucleation, wherein crystal seeds are used to induce crystallization, is widely employed in industrial crystallizations. Despite its significance, our understanding of the process, particularly at the molecular level, remains rudimentary. An outstanding question is why do a few seeds give rise to a many‐fold increase in new crystals? Using molecular simulation coupled with experiments we have uncovered the molecular processes that give rise to this autocatalytic behavior. The simulations reveal formation of molecular aggregates in solution, which on coming in contact with the surface of the seed undergo nucleation to form new crystallites. These crystallites are weakly bound to the crystal surface and can be readily sheared by fluid, making the seed surfaces available again to repeat the process. Further, the new crystallites on development can in turn serve as seeds. This mechanistic insight will enable better control in engineering crystalline products to design.

Secondary Crystal Nucleation: Nuclei Breeding Factory Uncovered.

Secondary nucleation, wherein crystal seeds are used to induce crystallization, is widely employed in industrial crystallizations. Despite its significance, our understanding of the process, particularly at the molecular level, remains rudimentary. An outstanding question is why do a few seeds give rise to a many-fold increase in new crystals? Using molecular simulation coupled with experiments we have uncovered the molecular processes that give rise to this autocatalytic behavior. The simulations reveal formation of molecular aggregates in solution, which on coming in contact with the surface of the seed undergo nucleation to form new crystallites. These crystallites are weakly bound to the crystal surface and can be readily sheared by fluid, making the seed surfaces available again to repeat the process. Further, the new crystallites on development can in turn serve as seeds. This mechanistic insight will enable better control in engineering crystalline products to design.

Mixing thermodynamic properties of ester-containing solutions: A study on the ternary (methyl alkanoate (pentanoate and methanoate) + methanol) and the corresponding binaries. New contributions to the (ester + ester) interactions

This work studies the volumetric (VmE) and energetic (HmE) properties resulting from the mixing processes of binary systems and the corresponding ternary of two methyl esters (methanoate and pentanoate) with methanol. The three binaries produce net endothermic mixing effects, with important energetic interactions, with maximum values of HmE≅400J⋅mol−1, for the (ester+ester) system. This produces expansive effects VmE>0, but the binaries of the (methyl esters+methanol) give rise to contractions VmE<0, due to the formation of molecular aggregates. The endothermicity in the mixing processes is a net effect which is justified by interactions of different nature, especially dipolar interactions and hydrogen bonds of the substances involved. The overall results in the ternary respond to the individual contributions of the binaries and the increment due to the simultaneous presence of the three compounds in the solution. The experimental results are correlated with our own model that gives a good representation of the properties of the solutions studied. The analysis of the behavior of the solutions is reinforced by spectral data obtained by 1H NMR, supporting the structural model established. The application of UNIFAC to estimate the HmE in the ternary improves when one takes into account the individual contributions of the (ester+ester) interactions whose parameter are predetermined.

Influence on Crystallisation Process of Amino Acids from Solution

The famous Miller experiment to model the primordial soup demonstrated that amino acids can form spontaneously as the essential building blocks of life in solutions. It is, however, still an open question how self-recognition processes influence the transformation of these spontaneously formed amino acids in solvents into higher ordered structures in the solid state, thereby creating chiral materials and catalytically competent structures. The understanding of the first steps of molecular self-assembly processes in such environments will thus give important clues towards the understanding of biological evolution. Most of intermolecular interactions are not very strong and their formation is related to and affected by small changes in the molecular structure and the crystallisation conditions. Continuing our investigations on aggregation of substituted aromatic molecules in the solid state, we studied the influence and boundaries of weak directing substituents like deuterium on the aggregation of small molecules. Hydrogen/deuterium (H/D)-exchange, the smallest possible modification of a molecule, is generally seen as a non dominating parameter in the formation of crystal structures of chemical compounds. On the other hand, it could already be shown that the aggregation of molecules in the solid state of polymorphic N-heterocycle systems like pyridine-N-oxide or acridine can be very sensitive to small changes of the isotopic substitution pattern of the selected molecules. Within our project, the molecular aggregation of amino acids in solution with the formation of molecular aggregates and pre-nucleation clusters in deuterated and non-deuterated systems, and in particular the role of the solvent in these processes, will be studied in both experiment and theory.