Apolar Solvents Research Articles

Insights into Hildebrand Solubility Parameters - Contributions from Cohesive Energies or Electrophilicity Densities?

We introduce certain concepts and expressions from conceptual density functional theory (DFT) to study the properties of the Hildebrand solubility parameter. The original form of the Hildebrand solubility parameter is used to qualitatively estimate solubilities for various apolar and aprotic substances and solvents and is based on the square root of the cohesive energy density. Our results show that a revised expression allows the replacement of cohesive energy densities by electrophilicity densities, which are numerically accessible by simple DFT calculations. As an extension, the reformulated expression provides a deeper interpretation of the main contributions and, in particular, emphasizes the importance of charge transfer mechanisms. All calculated values of the Hildebrand parameters for a large number of common solvents are compared with experimental values and show good agreement for non- or moderately polar aprotic solvents in agreement with the original formulation of the Hildebrand solubility parameters. The observed deviations for more polar and protic solvents define robust limits from the original formulation which remain valid. Likewise, we show that the use of machine learning methods leads to only slightly better predictability.

Addressing Challenges of Macrocyclic Conformational Sampling in Polar and Apolar Solvents: Lessons for Chameleonicity.

We evaluated a workflow to reliably sample the conformational space of a set of 47 peptidic macrocycles. Starting from SMILES strings, we use accelerated molecular dynamics simulations to overcome high energy barriers, in particular, the cis-trans isomerization of peptide bonds. We find that our approach performs very well in polar solvents like water and dimethyl sulfoxide. Interestingly, the protonation state of a secondary amine in the ring only slightly influences the conformational ensembles of our test systems. For several of the macrocycles, determining the conformational distribution in chloroform turns out to be considerably more challenging. Especially, the choice of partial charges crucially influences the ensembles in chloroform. We address these challenges by modifying initial structures and the choice of partial charges. Our results suggest that special care has to be taken to understand the configurational distribution in apolar solvents, which is a key step toward a reliable prediction of membrane permeation of macrocycles and their chameleonic properties.

Efficient selective hydrogenation of terminal alkynes over Pd–Ni nanoclusters encapsulated inside S-1 zeolite

Selective hydrogenation of alkynes is a challenging reaction that requires selective and efficient catalysts to achieve the desired alkenes. The development of general and robust catalysts that can operate under mild conditions and tolerate a wide range of functional groups is still an active area of research. Here, Pd-M bimetallic clusters (M = Ni, Cu, Mn, Fe) encapsulated inside S-1 zeolite were prepared and applied in the selective hydrogenation of substituted terminal alkynes. Among them, the Pd0.6Ni@S-1 catalyst displayed 91 % selectivity to styrene at the full conversion of phenylacetylene. Derivatives of phenylacetylene containing either electron withdrawing/donating or sensitive functional groups were effectively converted into the corresponding alkenes. The confinement effect of zeolite effectively inhibited the over-hydrogenation reaction and enhanced the stability of the catalysts. Furthermore, the product distribution was solvent-dependent by correlating the reaction selectivity with solvent properties. Apolar solvent showed the highest selectivity to styrene in the selective hydrogenation of phenylacetylene due to the competitive adsorption of apolar solvent and styrene on catalysts.

Solvent Acts as the Referee in a Match-Up Between Charged and Preorganized Receptors.

The prevalence of anion-cation contacts in biomolecular recognition under aqueous conditions suggests that ionic interactions should dominate the binding of anions in solvents across both high and low polarities. Investigations of this idea using titrations in low polarity solvents are impaired by interferences from ion pairing that prevent a clear picture of binding. To address this limitation and test the impact of ion-ion interactions across multiple solvents, we quantified chloride binding to a cationic receptor after accounting for ion pairing. In these studies, we created a chelate receptor using aryl-triazole CH donors and a quinolinium unit that directs its cationic methyl inside the binding pocket. In low-polarity dichloromethane, the 1 : 1 complex (log K1 : 1 ~ 7.3) is more stable than neutral chelates, but fortuitously comparable to a preorganized macrocycle (log K1 : 1 ~ 6.9). Polar acetonitrile and DMSO diminish stabilities of the charged receptor (log K1 : 1 ~ 3.7 and 1.9) but surprisingly 100-fold more than the macrocycle. While both receptors lose stability by dielectric screening of electrostatic stability, the cationic receptor also pays additional costs of organization. Thus even though the charged receptor has stronger binding in apolar solvents, the uncharged receptor has more anion affinity in polar solvents.

Fluorescent Nanoassembly of Tetrazole-Based Dyes with Amphoteric Surfactants: Investigation of Cyanide Sensing and Antitubercular Activity.

Tetrazole-based easily synthesizable fluorogenic probes have been developed that can form self-assembled nanostructures in the aqueous medium. Though the compounds could achieve detection of cyanide ions in apolar solvents, such as, THF, significant interference was observed from other basic anions, such as F-, AcO-, H2PO4-, etc. On the other hand, a highly specific response was observed for CN- ions in the aqueous medium. However, the sensitivity was so poor that it could hardly be useful for real-life sample analysis. Interestingly, the co-assembly of such probe molecules with hydroxyethyl-anchored amphoteric surfactants could drastically improve the sensitivity toward CN- ions in water without dampening their excellent selectivity. Also, it was observed that the degree of fluorescence response for CN- ions depends on the nature of the polyaromatic scaffolds (naphthyl vs anthracenyl), the nature of the surfactant assembly (micelle vs vesicle), etc. The mechanistic investigation indicates the hydrogen bonding interaction between the tetrazole -NH group and cyanide ions in the aqueous medium, which can effectively change the electronics of the tetrazole unit, resulting in alteration in the extent of charge transfer interaction. Then, the biocompatible composite materials (dye-surfactant assemblies at different ratios) were tested for antituberculosis activity. Fortunately, in a few cases, the compositions were found to be as effective as the commercially available antituberculosis drug, ethambutol.

Charge-driven liquid-crystalline behavior of ligand-functionalized nanorods in apolar solvent.

Concentrated colloidal suspensions of nanorods often exhibit liquid-crystalline (LC) behavior. The transition to a nematic LC phase, with long-range orientational order of the particles, is usually well-captured by Onsager's theory for hard rods, at least qualitatively. The theory shows how the volume fraction at the transition decreases with increasing aspect ratio of the rods. It also explains that the long-range electrostatic repulsive interaction occurring between rods stabilized by their surface charge can significantly increase their effective diameter, resulting in a decrease in the volume fraction at the transition, as compared to sterically stabilized rods. Here, we report on a system of ligand-stabilized LaPO4 nanorods, of aspect ratio ≈11, dispersed in apolar medium exhibiting the counter-intuitive observation that the onset of nematic self-assembly occurs at an extremely low volume fraction of ≈0.25%, which is lower than observed (≈3%) with the same particles when charged-stabilized in polar solvent. Furthermore, the nanorod volume fraction at the transition increases with increasing concentration of ligands, in a similar way as in polar media where increasing the ionic strength leads to surface charge screening. This peculiar system was investigated by dynamic light scattering, Fourier-transform infrared spectroscopy, zetametry, electron microscopy, polarized light microscopy, photoluminescence measurements, and X-ray scattering. Based on these experimental data, we formulate several tentative scenarios that might explain this unexpected phase behavior. However, at this stage, its full understanding remains a pending theoretical challenge. Nevertheless, this study shows that dispersing anisotropic nanoparticles in an apolar solvent may sometimes lead to spontaneous ordering events that defy our intuitive ideas about colloidal systems.

Photoinduced Ring-Opening and Phototoxicity of an Indolin-3-one Derivative.

The study of a fluorescent indolin-3-one derivative is reported that, as opposed to its previously described congeners, selectively undergoes photoactivated ring-opening in apolar solvents. The excited state involved in this photoisomerization was partially deactivated by the formation of singlet oxygen. Cell studies revealed lipid droplet accumulation and efficient light-induced cytotoxicity.

One-Pot Synthesis and Excited-State Dynamics of Null Exciton-Coupled Diketopyrrolopyrroles Oligo-Grids.

Exciton coupling in molecular aggregates plays a vital role in impacting and fine-tuning optoelectronic materials and their efficiencies in devices. A versatile platform to decipher aggregation-property relationships is built around multichromophoric architectures. Here, a series of cyclic diketopyrrolopyrrole (DPP) oligomers featuring nanoscale gridarene structures and rigid bifluorenyl spacers are designed and synthesized via one-pot Friedel-Crafts reaction. DPP dimer [2]Grid and trimer [3]Grid, which are cyclic rigid nanoarchitectures of rather different sizes, are further characterized via steady-state and time-resolved absorption and fluorescence spectroscopies. They exhibit monomer-like spectroscopic signatures in the steady-state measurements, from which null exciton couplings are derived. Moreover, in an apolar solvent, high fluorescence quantum yields and excited-state dynamics that resembled DPP monomer are gathered. In a polar solvent, the localized singlet excited state on a single DPP dissociates into the adjacent null coupling DPP with charge transfer characteristics. This pathway facilitates the evolution ofthe symmetry-broken charge-separated state (SB-CS). Notable is the fact that the SB-CS of [2]Grid is, on one hand, in equilibrium with the singlet excited state and promotes, on the other hand, the formation of the triplet excited state with a yield of 32% via charge recombination.

Electrohydrodynamic convection instabilities observed in suspensions of cellulose nanocrystals

Cellulose nanocrystals are slender, negatively charged nanoparticles that spontaneously form a cholesteric liquid crystal in aqueous suspension above a critical concentration. When they are suspended in apolar solvents such as toluene using surfactants, the application of an AC electric field leads to the reorientation and then distortion of the cholesteric order until the cholesteric structure completely unwinds into a nematic-like order, typically above 0.4–0.6 kV/cm at 1kHz. In this work, we show that at much higher electric fields (ge 4.6 kV/cm at 1 kHz) the sample develops a periodic pattern that varies with the field amplitude. We ascribed this pattern to electrohydrodynamic convection instabilities. These instabilities usually present complex regimes varying with the field, the voltage, the frequency and the geometry. However, the typical geometry where these instabilities were most documented across the literature differs from the geometry used in this work. This work concludes with possible future experimental investigations to clarify the exact regime of instability responsible for these observations.

Fast Visualized Conformational Transition of Polyphenylacetylenes with Stepwise Tunable Circularly Polarized Luminescence.

Two novel poly(3,5-diamide substituted phenylacetylene)s bearing L- or D-alanine residues with a long alkyl chain were synthesized. In low polar or apolar solvents, the polymers adopted contracted cis-cisoid helix showing strong yellow fluorescence, which was transferred to stretched cis-transoid helix by adding polar solvents or decreasing temperature, accompanied with the disappearance of fluorescence. The critical conformational transition temperature could be readily tuned in a wide range by altering the content of polar solvent, and the conformational variation at molecular level could be directly read out via the change in fluorescent property. The synergistical participation of amide and ester groups in the formation of intramolecular hydrogen bonds accelerated the conformational transition rate, resulting in remarkable reduction of hysteresis. Moreover, the polymers displayed switchable CPL performance and the luminescent dissymmetry factor could be stepwise amplified by forming chiral aggregates or liquid crystalline structure. This study may open a new avenue for the development of controllable helical structure-based smart CPL materials.

Continuous Making, Stretch Breaking, and Exciton Dynamics of a Red-Emissive Organic Submicrometer-Sized Triangular Pyramid

Highly photoluminescent, single-component self-assembly being scalable and sustainable has a profound commercial significance. In this article, the challenges that we address in development of self-assembly of π-conjugated molecules are ─(i) feeble photoluminescence caused by poor molecular orientational ordering and (ii) costly fluorescent monomer owing to lack of an atom/step/energy economical synthetic method. We have discovered that a bright yellow fluorescent xanthene analog capable of instantly forming self-assembly can be synthesized by a home-built coil-in-spiral reactor using an inexpensive precursor, pyrogallol. Stimuli such as temperature (35 °C), acid fumes, and apolar solvent can trigger the formation of red-emissive self-assembly. In solution orientation of about 13 molecules yielding hydrogen-bonded self-assembly has a direct resemblance with a Coulomb-coupled J-aggregate highlighted in the Kasha model. After photoexcitation, the excited state dynamics of the monomer progress via three pathways: (i) relaxation (2 ± 0.3 ps), (ii) solvation (19.5 ± 3 ps), and (iii) decay (2.5 ns). On the other hand, self-assembly exciton evolves via two pathways: (i) relaxation (81 ± 25 ps) and (ii) decay (∼1 ns). In the solid state, the self-assembly retains the submicron-sized trigonal pyramidal structures by layer-by-layer stacking of triangular plates. This self-assembly doped in polymer even exhibits mechanofluorescence. Our study will pave way the use of this photoluminescent self-assembly for improved performance of organic/sustainable electronics and stretchable electronics.

The effect of energy level degeneracy on symmetry-breaking charge transfer: Excited octupolar dyes.

A three-level model of symmetry-breaking charge transfer (SBCT) in excited octupolar molecules is developed. The model describes the joint dynamics of the solvent and the dye in the excited state. For this, a distribution function in the space of two reaction coordinates is introduced. An evolution equation of this function is derived. A strict definition of the reaction coordinates is given, and its dynamic characteristics are determined. The free energy surface in the space of these coordinates is calculated. To quantify the symmetry-breaking degree, a two-dimensional dissymmetry vector is introduced. The model predicts the absence of SBCT in apolar solvents and an abrupt increase in its degree to half the maximum value in weakly polar solvents. The dye dipole moment is revealed to be directed along a molecular arm independently of the direction and the strength of the electric field of the solvent created by its orientational polarization. The conditions for the occurrence and nature of this effect are discussed. The effect of the degeneracy of excited states, which is inherent in octupolar dyes in the excited state, on SBCT is revealed. Degeneracy of energy levels is shown to lead to a significant increase in the symmetry-breaking degree. The effect of SBCT on the dependence of the Stokes on the solvent polarity is calculated and compared with the available experimental data.

Classical Force Field Parameters for InP and InAs Quantum Dots with Various Surface Passivations.

Classical molecular dynamics (MD) simulations on realistic colloidal quantum dot (QD) systems are often hampered by missing force field (FF) parameters for an accurate description of the QD-ligand interface. However, such calculations are of major interest, specifically for studying the surface chemistry of colloidal nanocrystals. In this work, we have utilized a previously published stochastic optimization algorithm to obtain FF parameters for InP and InAs QDs capped by Cl, amine, carboxylate, and thiolate ligands. Our FF parameters are interfaced with well-established FFs for organic molecules, allowing for the simulation of InP and InAs QDs with a broad range of organic ligands in explicit apolar solvents. The quality of our FF parameters was assessed by comparing properties of the classical MD simulations with ab initio MD simulations and experimental and theoretical values from the literature.

Straightforward adsorption-based formulation of mesoporous silica nanoparticles for drug delivery applications

Mesoporous silica nanoparticles (MSNs) have emerged as a very promising drug delivery platform. However, multi-step synthesis and surface functionalization protocols rise the hurdle for translation of this promising drug delivery platform to the clinic. Furthermore, surface functionalization aiming at enhancing the blood circulation time, typically through surface functionalization with poly(ethylene glycol) (PEG) (PEGylation), has repeatedly been shown to be detrimental for the drug loading levels that can be achieved. Here, we present results related to sequential adsorptive drug loading and adsorptive PEGylation, where the conditions can be chosen so that the drug desorption during PEGylation is minimized. At the heart of the approach is the high solubility of PEG both in water and in apolar solvents, which makes it possible to use a solvent for PEGylation in which the drug exhibits a low solubility, as demonstrated here for two model drugs, one being water soluble and the other not. Analysis of the influence of PEGylation on the extent of serum protein adsorption underline the promise of the approach, and the results also allow the adsorption mechanisms to be elaborated. Detailed analysis of the adsorption isotherms enables determination of the fractions of PEG residing on the outer particle surfaces in comparison to inside the mesopore systems, and also makes it possible to determine the PEG conformation on the outer particle surfaces. Both parameters are directly reflected in the extent of protein adsorption to the particles. Finally, the PEG coating is shown to be stable on time-scales compatible with intravenous drug administration, which is why we are convinced that the presented approach or modifications thereof will pave the way for faster translation of this drug delivery platform to the clinic.

A combinatorial effect of TICT and AIE on bisulfate detection using a pyrenylated charge-transfer luminogen

A donor-acceptor based fluorogenic probe, 4-(2-pyren-1-yl-vinyl) pyridine, 1 is synthesized that can form pH-sensitive nanoscopic aggregates in the aqueous medium. The fluorescence response of 1 in apolar solvents originates from LE state, while in polar solvents, it is dominated by TICT state. On the contrary, emissions from both TICT and AIE states is observed in water. A change in solution color from pale yellow to deep yellow is observed specifically upon the addition of HSO4¯ ions with a concomitant change in fluorescence color. The mechanistic investigations indicate the hydrogen bonding interaction of HSO4¯ ions with the pyridine nitrogen end, which can effectively alter the extent of charge transfer. Moreover, the interaction with HSO4¯, leading to larger aggregate formation. Further, the current system is involved in the analysis of water samples and screening care-kit products. Finally, the dye-coated paper strips are designed for cost-effective, on-location detection of HSO4¯.

Molecular Dynamics Simulation Study of Organic Solvents Confined in PIM-1 and P84 Polyimide Membranes

Organic solvent nanofiltration (OSN) has recently proved to be a promising separation process thanks to the development of membrane materials with suitable resistance toward organic solvents. Among those materials, P84 polyimide membranes are currently the most used in OSN while PIM-1 membranes have recently attracted attention due to their high permeance in apolar solvents and alcohols. Both P84 and PIM-1 membranes have nanosized free volumes, and their separation performance is finely connected to polymer/solvent interactions. Consequently, modeling OSN membranes at the molecular scale is highly desirable in order to rationalize experimental observations and gain a deeper insight into the molecular mechanisms ruling solvent and solute permeation. A prerequisite for understanding solvent transport through OSN membranes is therefore to characterize the membrane/solvent interactions at the molecular level. For that purpose, we carried out molecular simulations of three different solvents, acetone, methanol, and toluene in contact with P84 and PIM-1 membranes. The solvent uptake by both membranes was found to be correlated to the degree of confinement of the solvent, the polymer swelling ability and polymer/solvent interactions. The translational dynamics of the solvent molecules in the PIM-1 membrane was found to be correlated with the solvent viscosity due to the relatively large pores of this membrane. That was not the case with the P84 membrane, which has a much denser structure than the PIM-1 membrane and for which it was observed that the translational dynamics of the confined solvent molecules was directly correlated to the affinity between the P84 polymer and the solvent.

Low-Molecular-Weight Organogelators Based on N-dodecanoyl-L-amino Acids-Energy Frameworks and Supramolecular Synthons.

Lauric acid was used to synthesize the low-molecular-weight organogelators (LMOGs), derivatives of two endogenous (L)-alanine, (L)-leucine, and three exogenous (L)-valine, (L)-phenylalanine, and (L)-proline amino acids. The nature of processes responsible for the gel formation both in polar and in apolar solvents of such compounds is still under investigation. Knowing that the organization of surfactant molecules affects the properties of nano scale materials and gels, we decided to elucidate this problem using crystallographic diffraction and energy frameworks analysis. The single crystals of the mentioned compounds were produced successfully from heptane/tBuOMe mixture. The compounds form lamellar self-assemblies in crystals. The energetic landscapes of single crystals of a series of studied amphiphilic gelators have been analyzed to explore the gelling properties. The presented results may be used as model systems to understand which supramolecular interactions observed in the solid state and what energy contributions are desired in the designing of new low-molecular-weight organic gelators.

Poly(ε-caprolactones) Initiated by Chiral Compounds: A New Protocol to Support Organocatalysts

This work investigates the support of organocatalysts in polyesters, a class of polymers seldom used for this purpose. The proposal is to use the hydroxyl groups present in the structure of the chosen chiral compounds to promote the polymerization of ε-caprolactone, generating the support and anchoring the organocatalyst in a single step. A very simple method, with acid catalysis, was employed, that showed versatility in generating supported catalysts with different structures and functional groups and modulating the mass of the materials to generate specific solubility characteristics. In this way, the catalysts can be solubilized in some organic solvents, such as dichloromethane, but at the end of the reaction, they can be recovered in a heterogeneous way, through precipitation in more apolar solvents. The materials were applied as organocatalysts on an aldol addition test reaction and the product could be obtained in excellent yields and good stereoselectivity. The polymer did not show signs of degradation after the reaction, proving to be robust and suitable for use in catalysis; however, a recycling process appears to be necessary for its reuse.

Bis-porphyrin tetragonal prisms assembled through Watson-Crick hydrogen bonding between complementary nucleobases

We report here on the self-assembly of two components, a Zn(II) porphyrin substituted with four lipophilic cytidines and a linear linker substituted with two guanosines, into a Watson-Crick hydrogen-bonded tetragonal prism. The corresponding ensemble, comprising two cofacial porphyrin macrocycles, exhibited similar supramolecular behavior as a reference cyclic tetramer previously studied by us. The establishment of strong G:C interactions in apolar chlorinated solvents was demonstrated by the chemical shift recorded for the relevant H-bonded protons. Most importantly, the hexacomponent prism assembly, when partially dissociated by the addition of the competing DMSO solvent, exhibited a slow exchange on the NMR timescale with the individual constituents, which is characteristic of kinetically stabilized discrete assemblies. Absorption, emission, and CD spectroscopy experiments revealed that the spectral features of the tetragonal prism were totally different from the ones recorded for the porphyrin and linker components, which also showed strong self-aggregation in apolar solvents. Temperature-dependent experiments, monitored by NMR and optical spectroscopy techniques, revealed remarkable thermodynamic stability for the prism assembly.

Effects of donor and acceptor substituents on the photophysics of 4-ethynyl-2,1,3-benzothiadiazole derivatives.

The present work explores the photophysical, electrochemical, and fluorescence polarization properties of a group of π-conjugated phenylethynyl-2,1,3-benzothiadiazole derivatives (BTDs) bearing different electron-donating (ED) or electron-withdrawing (EW) substituents at the para position of the phenylethynyl moiety. The BTDs were synthesized through the Sonogashira cross-coupling reaction between 4-bromo-2,1,3-benzothiadiazole and the respective para-substituted phenylethynyl derivatives. The BTDs with the EW-substituents show relatively weak solvatochromic behavior, while the BTDs with the strong ED-substituents like methoxy and N,N-dimethylamino-based substituents (BTDPhOMe and BTDPhNMe2) exhibit a pronounced solvatochromic behavior. The change in dipole moments in the excited states of the derivatives was calculated using Lippert-Mataga plots. The conclusions drawn on the spectral behavior of the molecules could be rationalized by TD-DFT calculations involving electron density difference (EDD) maps that correlate with the ICT characteristics of the molecules. The experimental and theoretical calculations reveal that the BTDs with the strong ED-substituents (strong push-pull type BTDs) have a strong ICT character in the excited state. These strong push-pull type BTDs show high fluorescence quantum yield (ΦF) in apolar solvents and low ΦF in polar solvents. In contrast, the BTDs with the weak ED-substituents (weak push-pull type BTDs) and EW-substituents (pull-pull type BTDs) have a weaker ICT character with low ΦF in apolar and high ΦF in polar solvent media. There is good a agreement among the HOMO-LUMO band gaps obtained from absorption spectroscopy and electrochemical studies and theoretical calculations. The fluorescence anisotropy measurement in the glycerol medium shows that the studied BTDs generally exhibit higher sensitivity towards microviscosity than the traditional DPH fluorescence anisotropy probe.