Solvents Of Medium Polarity Research Articles

Excited-State Symmetry Breaking and Localization in a Noncentrosymmetric Electron Donor-Acceptor-Donor Molecule.

Electronic excitation in quadrupolar conjugated molecules rapidly localizes on a single electron donor-acceptor (DA) branch when in polar environments. The loss of center of inversion upon this excited-state symmetry breaking (ES-SB) can be monitored by exploiting the relaxation of the exclusion rules for IR and Raman vibrational transitions. Here, we compare ES-SB in a right-angled (1) and a centrosymmetric (2) DAD dyes using time-resolved IR spectroscopy. We show that the localization of the excitation can also be identified with the bent molecule 1. We find that contrary to dye 2, subpopulations with localized and delocalized excitation coexist for 1 in weak to medium polar solvents. This difference originates from the torsional disorder present in the excited state of 1 but not of 2. Additionally, irreversible localization in a bent molecule is shown to require higher solvent polarity than in a centrosymmetric one.

Dendritic ZSM-5 zeolites as highly active catalysts for the valorization of monoterpene epoxides.

Dendritic ZSM-5 zeolites were investigated in the isomerization of monoterpene epoxides, including limonene-1,2-epoxide (LE), α-pinene epoxide, and β-pinene epoxide, which yields high-value compounds used in fragrances, cosmetics, and pharmaceuticals. The fresh catalysts were thoroughly characterized using XRD, Ar physisorption, pyridine-FTIR, TEM, FTIR/DTBPyr, and 27Al MAS NMR. In comparison with conventional and hierarchical ZSM-5 materials, the dendritic zeolite with a crystallization time of 4 days (d-ZSM-5/4d) was the most active material, with a turnover frequency value of 4.4 min-1 for LE isomerization. Likewise, remarkable yields of dihydrocarvone (DHC, 63%, 70 °C, 2 h), campholenic aldehyde (72.4%, 70 °C, 5 min), and myrtanal (47.7%, 50 °C, 5 min) were obtained with this material that exhibited the largest mesopore/external surface area (360 m2 g-1), showing also the narrowest mesopore size distribution. A direct relationship was observed between the TOF values and the concentration of external Brønsted acid sites, showing the presence of strong steric/diffusional limitations that are greatly overcome with the dendritic zeolites. The lower reactivity of trans-LE compared to cis-LE was attributed to the larger steric hindrance of the oxygen atom. Exploration of the solvent influence revealed that the reaction rate of LE was favored by non-polar solvents, while highly selective DHC formation occurred in the solvents of medium polarity. The d-ZSM-5/4d sample was shown to be robust because catalytic activity could be completely recovered by air calcination.

Solvent-polarity dependence of ultrafast excited-state dynamics of trans-4-nitrostilbene.

Ultrafast excited-state dynamics of the simplest nitrostilbenes, namely trans-4-nitrostilbene (t-NSB), was studied in solvents of various polarities with ultrafast broadband time-resolved fluorescence and transient absorption spectroscopies, and by quantum-chemical computations. The results revealed that the initially excited S1(ππ*) state deactivation dynamics is strongly influenced by the solvent polarity. Specifically, the t-NSB S1-state lifetime decreases by three orders of magnitude from ∼60 ps in high-polarity solvents to ∼60 fs in nonpolar solvents. The strong solvent-polarity dependence arises from the differences in dipole moments among the S1 and relevant states, including the major intersystem crossing (ISC) receiver triplet states, and therefore, the solvent polarity can modulate their relative energies and ISC rates. In nonpolar solvents, the sub-100 fs lifetime is due to a combination of efficient ISC and internal conversion. In medium-polarity solvents, the S1-state population decays via a competing ISC relaxation mechanism in a biphasic manner, and the ISC rates are found to obey the inverse energy gap law of the strong coupling case. In high-polarity solvents, the S1 state is stabilized to a much lower energy such that ISC becomes energetically infeasible, and the S1 state decays via barrier crossing along the torsion angle of the central ethylenic bond to the nonfluorescent perpendicular configuration. Regardless of the initial S1-state deactivation pathways in various solvents, the excited-state population is ultimately trapped in the metastable T1-state perpendicular configuration, at which a slower ISC occurs to bring the system to the ground state and bifurcate into either trans or cis form of NSB.

Free base porphyrin-cyanine dye conjugate: synthesis and optical properties.

The covalent combination of a cyanine dye (IR-783) with a tetraphenyl porphyrin unit through an ether linkage results in a photoactive system capable of producing singlet oxygen. The synthesis, characterization and photophysical properties of the resulting novel free base porphyrin-cyanine conjugate named TPPO-IR-783 (TOI) is reported. Excited state properties were studied in various solvents with differing polarity. The fluorescence is strongly solvent dependent, however this is not the case for singlet oxygen phosphorescence, which is only observed in tetrahydrofuran (THF), when comparing 8 different polar, non-polar and medium-polarity solvents. This novel type of porphyrin-cyanine photosensitizer has the ability to produce singlet oxygen and absorbs light at NIR wavelengths.

Anti-cancer activity of novel Schiff base copper (II) complex: synthesis and characterization

A novel thiosemicarbazone substituted schiff base ligand and its Cu(II) complex have been prepared and characterized. Schiff bases are considered as an important pro ligand in coordination chemistry because they easily form stable complexes with biologically active transition metal ions. Such as Cu 2+, Zn2+, Ni2+ etc. In this paper, novel Cu (II) complexes of thiosemicarbazone ligands were synthesised by refluxing thiosemicarbazone substituted pro ligands with copper salts in 1:1 molar ratio in absolute ethanol under thermal condition at 80o c for 5-6 hour. The resulting brown coloured copper complexes were filtered and recrystallized from petroleum ether. In this synthesis, we use absolute ethanol as a polar environment for the synthesis of copper complexes from schiff base ligands because the use of polar solvent medium in synthesis plays a very important role in reducing minimum possibilities of side reactions which hinders the proper conversion of ligands into copper complexes and also reducing the reaction time. All synthesized compounds were characterized through various spectroscopic and pharmacological techniques. FT-IR, UVVis, NMR, Mass, TGA-DTA, XRD spectra techniques were used to confirm the structures of copper complexes and ligands. All compounds are thermal stable up to 350o c. The good results of pharmacological activities of compounds like in-vitro anti -oxidant and anti - cancer activity against DPPH and cisplatin drug, explained the presence of biologically active functional groups are present in ligands as well as their copper complexes. Results found that the copper complexes were more active than the ligands. The synthesis of copper complexes from thiosemicarbazone schiff base ligands in proper stoichiometic ratio is an excellent method of preparing pharmacological active compounds which can be considered as good anti -cancer drug candidate for the treatment of cancer.

Improving dispersibility of lanthanide-doped upconversion nanoparticles for luminescence applications in medium polarity solvents

The dispersibility of lanthanide-doped upconversion nanoparticles (UCNPs) is related to the polarity of solvents. Typically, the as-synthesized UCNPs are poorly dispersed in medium and high polarity solvents. Here, we investigated the dispersibility of oleate-capped UCNPs in different solvents and improved the dispersibility of the nanoparticles in medium polarity solvents by a ligand exchange process. 2-naphthoxyacetic acid and α-hydroxyhippuric acid were selected as ligands to improve the dispersion of UCNPs in long-chain alcohol and ester solvents, respectively. The improvement of dispersibility was confirmed by dynamic light scattering measurements and transmission electron microscope images. In parallel, the luminescence features of the UCNPs dispersed in different solvents were measured to understand the influence of solvents on the luminescence lifetime, red-green emission ratio and power dependent properties of UCNPs. Additionally, the ligand exchange rendered an improved dispersibility of UCNPs in medium polarity media, which prevented aggregation of UCNPs in polydimethylsiloxane (PDMS) matrix improving transmittance of UCNPs-polydimethylsiloxane (PDMS). This study provides an effective method to improve the dispersion of UCNPs in organic polymer media expanding the applicability of nanoparticles in displays and anticounterfeiting.

Evaluation of antioxidant capacity, structure, and surface morphology of ginger (Zingiber officinale) using different extraction methods

The antioxidant capacity of ginger depends on the type of variety, growing conditions, postharvest, drying method, extraction, and measurement, among others. The objective of the research was to compare the efficiency of ultrasound (US), magnetic agitation (AM), maceration (M), and reflux (R) extraction methods. In the GFD (fresh air-dried ginger) extract, the contents of total phenolic content (TFC), 6-gingerol (6-G), and 6-shogaol (6-S) were evaluated; as well as the antioxidant capacity by FRAP (ferric reducing antioxidant power) and IC50 in DPPH (2,2-diphenyl-1-picrylhydrazyl radical). In addition, structural and morphological changes were evaluated with FTIR and SEM, respectively. The results for all extraction methods for TFC, 6-G, and 6-S were between 9.422 and 10.037 mg EAG/g dry matter (dm), 4.072–4.838, and 0.194–0.263 mg/g dm respectively, with the highest values for TFC and 6-G for M and 6-S for R. FRAP and IC50 in DPPH values were between 0.172 and 0.192 mmol Fe2+/g dm and 0.531–0.722 mg dm/mL respectively, presenting higher antioxidant capacity in M and R1, R2, and R3 (1, 2, and 5 h, respectively). Extracts from M and R1 methods presented lower FTIR transmittance values and greater changes in their surface morphology, with folds and breaks in the starch granules by SEM images. It is concluded that extracts with higher antioxidant capacity are obtained using medium polarity solvents such as methanol and with M and R1 methods. Because of the longer extraction time and moderate thermal stress, the structure and surface morphology of the extracted GFD sample showed greater changes on the surface of the starch granules and, consequently, greater extraction of bioactive compounds.

Hydrophile-lipophile balance solid phase extraction of surface water organics: Fluorescent elution preference and overlooked fractions

Fluorescent dissolved organic matter (FDOM) in surface water has broad implications on water quality research and operations. Solid phase extraction (SPE) is the most widely used technique to extract FDOM. However, fluorescent elution preferences by common solvents and content of quantifiable chromophores in waste fraction remain largely unknown, both quantitatively and qualitatively. In this work, the preferential selection of various types of FDOM captured by and lost from SPE as characterized by the fluorescence excitation-emission matrix (EEM) were investigated. Three elution solvents (methanol, acetone, and dichloromethane) were adopted to elute the DOM that was enriched on a typical SPE sorbent. Results revealed that high polarity (methanol) and medium polarity (acetone) solvents eluted the highest variety and quantity of humic acid-like substances (Region V), while the low polarity (dichloromethane) elution solvent was more suitable for eluting tyrosine (Region I) and tryptophan (Region II). Compared to eluting only with methanol, sequential elution and recombination using the three aforementioned solvents demonstrated a significant increase in not only DOC recovery (by 7%), but fluorescence integral values and fluorescence characteristics covering collectively much larger fluorescence regions that more closely resembled raw water. For the first time, the fluorescence EEM of waste after loading the sample revealed a previously overlooked FDOM loss of 20%, caused by ineffective adsorption onto the solid phase resin. Substantial carbonaceous and nitrogenous FDOM were present in this fraction (the fluorescence intensity of aromatic protein in waste exceeds 20% of that in raw water), indicating possible underestimations of FDOM-related research in areas such as disinfection byproduct and toxicity work. The results of this study provide both a qualitative and quantitative characterization of the elution and lost products of SPE in capturing FDOM.

Solvent Effect on Small-Molecule Thin Film Formation Deposited Using the Doctor Blade Technique

Small molecule thin films are the core of some organic optoelectronic devices. Their deposition by solution processes is an advantage for device fabrication and can be achieved via spin coating for small areas and slot-die or doctor blade for larger areas. Solution deposition of small molecule thin films is usually processed only with medium polarity solvents. Herein, the use and influence of solvents with several polarities and physicochemical properties to form small-molecule homogeneous thin films via the doctor blade technique over an area of 25 cm2 have been explored. Solvents with different polarity, heptane, chlorobenzene, N,N-dimethylformamide, acetonitrile, and methanol were used along with different deposition temperatures, from room temperature up to near the boiling temperature for each solvent. With heptane and chlorobenzene, smooth films with an average roughness of 3 nm and thickness of 100–120 nm were obtained. The film was homogeneous over the whole substrate for temperatures from room temperature to close to the boiling temperature of both solvents. On the other hand, with dimethylformamide, a film is observed when the deposition is conducted only at room temperature; when the deposition temperature increases, the formation of agglomerates of several sizes from 1 to 5 nm was observed. With acetonitrile, and methanol, no films were formed, and only nanoaggregates were created on the substrate due to the solvent high vapor pressure, and the agglomerate size depends on the deposition temperature. The measure of the contact angle of pure solvent and solutions indicated that wettability helps to film formation over the whole substrate. For heptane and chlorobenzene, a small angle was measured; meanwhile, the contact angle is large in acetonitrile leading to the formation of nanoaggregates. In the case of methanol solution, although it wets very well, no film is deposited because it has high volatility.

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.

Polarity-dependent Response of Phytochemical Extraction and Antioxidant Potential of Different Parts of Alcea rosea

Objectives: Alcea rosea is a good source of medicinlly importeant bioactive phytochemical compounds. The complete extraction of these biochemicals from plant material has remained a problem for the researchers and manufacturers. This study was planned to report the medicinal value of A. rosea and find out the suitable extraction solvent to enhance the extraction yield of its phytochemical compounds and their antioxidant potential. Materials and Methods: The powdered samples of the selected parts of A. rosea were extracted in solvents of varying dipole moments (hexane: 0 D, ethyl acetate: 2.8 D, and methanol: 5.1 D). The extracts were screened for the presence of important phytochemicals and analyzed for their phytochemical content and antioxidant potential. Results: The studied parts of A. rosea consisted of flavonoids, tannins, terpenoids, saponins, and cardiac glycosides. The regression analysis showed polarity-dependent significant positive effects (p<0.05) on the extract yield, phenolic content, and antioxidant activity in terms of Trolox equivalent total antioxidant activity, ferric reducing power, and nitrogen free radical scavenging activity of the extracts. However, a mixed response of total flavonoids and total tannins content and hydroxyl radical scavenging activity of the extracts was observed against solvent polarity. Conclusion: A polarity dependent increase in phytochemical content and free radical scavenging capacity was observed. The study suggests that the polar solvents are more suitable to achieve a good extract yield of phytochemicals possessing a strong antioxidant ability and the solvents of medium polarity may be suitable for the extraction of flavonoids and tannins from the studied parts of A. rosea.

Recent Advances in the Use of Dimethyl Sulfoxide as a Synthon in Organic Chemistry.

Dimethyl sulfoxide (DMSO), as extremely important aprotic polar solvent and reaction medium, has attracted widespread attention from chemists in recent years due to its wide range of uses and the multiple functions it displays in various chemical processes. Especially in the past decade, dimethyl sulfoxide has become increasingly favored as a synthon in organic chemistry, resulting in great progress in this research field. In this context, this review provides a comprehensive summary of the literature on the recent progress in organic synthesis using dimethyl sulfoxide as a synthon, covering all the reports from 1 January 2016 to 11 May 2022. This type of reaction is mainly performed by transferring one or more units of dimethyl sulfoxide, such as oxygen (-O-, =O), methyl (-CH3), methylene (-CH2-), methylidene (=CH2), methine (=CH-), donor of formylation (-CHO), sulfur (-S-), methylthio (-SMe), methyl sulfoxide (-SOMe), donor of methyl thiomethylation (-CH2SMe), or donor of methyl sulfoxide methylation (-CH2SOMe), to the target molecules. At the same time, we hope that this review will stimulate future studies and promote developments in this area.

Effect of polar solvents on the growth, morphology and thermoelectric properties of Bi2Te3 nano particles

Morphological tuning of nanostructures aims to enhance thermoelectric figure of merit (ZT) through a rise in the density of states near the fermi level and spatial confinement of acoustic phonons leading to reduction in thermal conductivity. In this work, we prepared Bi 2 Te 3 nanostructures with diverse morphology, by a facile and scalable single-pot hydro/solvothermal synthesis, and investigated the effect of different polar solvents on the morphology of the synthesized nanostructures. Single-phase and highly crystalline Bi 2 Te 3 with diverse morphologies including nanoparticles, nanosheets, nano and micro flakes, nanotubes and nanoflowers were grown in different polar solvent media. The Materials were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM) and selected area diffraction pattern (SAED). A possible growth mechanism for Bi 2 Te 3 nanostructures is proposed by considering the physical and chemical properties of the polar solvents, with special focus on their dielectric properties. The thermoelectric properties of the hot pressed samples were investigated in the temperature range of 300–480 K. The fermi level is found to be shifted by morphological tuning of nanostructures. The carrier concentration and transport parameters are found to be greatly influenced by the morphology of consolidated particles. A peak figure of merit of 0.46 was obtained for the in-situ fullerite coated sample synthesized with ethylene glycol as medium (BTEGEDTA) at a temperature of 474 K. • Bi 2 Te 3 nanoparticles with diverse morphology were grown in different solvents as media and the thermoelectric properties are found to be varied. • In-situ fullerite coated B i2 T e3 was obtained by using ethylene glycol as medium • A possible growth mechanism is suggested by considering the chemical and physical properties of the medium. • The fermi level is found to be shifted with morphological tuning.

Microwave-assisted extraction of sea buckthorn pomace and seed extracts as a proactive antioxidant to stabilize edible oils

Sea buckthorn, an ancient medicinal plant, is gaining attention because of its prodigious antioxidant activities. Microwave-assisted (MAE) and Soxhlet extraction techniques were attempted to obtain the extractives from sea buckthorn pomace and seed using low (n-hexane) and medium (ethanol) polar solvents aiming at higher extraction yield. The study's main purpose was to evaluate the efficacy of antioxidant activities (free radical scavenging activity) via electro paramagnetic resonance and oxidative stability through rancimat analysis for sea buckthorn seed and pomace extracts. The maximum extraction of 78.4 wt% and 59.9 wt% for pomace and seed, respectively, were obtained using a microwave reactor at temperature 80 °C and power 150 W with ethanol as solvent. A relative comparison of the MAE and conventional Soxhlet extraction yield revealed that MAE is effective in extracting more antioxidant compounds with higher yields. Further, ethanol extracted seed oil showed the highest (98%) 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical slaking at 5% (w/v) concentration. The oxidative stability of sea buckthorn pomace and seed extracts was found to be on par with butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA) synthetic antioxidants when the extracts (3000 ppm) were blended and tested with canola oil.

Three modes of interactions between anions and phenolic macrocycles: a comparative study.

Macrocyclic polyphenolic compounds such as resorcin[4]arenes can be considered as multidentate anion receptors. In the current work, we combine new experimental data and reports from the previous literature (solution data and deposited crystal structures from the CCDC) to systematically analyze binding motifs between resorcin[4]arene derivatives and anions, determine the role of supporting interactions from CH donors, ion pairing and estimate their relative strength. We have found that in medium polarity solvents (THF) anion binding is a main driving force for the formation of complexes between resorcinarenes and Alk4NX salts. Three binding modes have been detected using 1H NMR and DOSY, depending on the type of additional interactions. Mode I was observed for upper-rim unsubstituted resorcinarenes, which use OH groups and aromatic CH from the upper rim as hydrogen bond donors to form multidentate and multivalent binding sites at the upper rim. Mode II was observed for upper-rim halogenated resorcinarenes (tetrabromo- and tetraiodo-derivatives), which use OH groups and aliphatic CH atoms from the bridges to support the chelation of anions between aromatic units. This binding mode is also multidentate and multivalent, but weaker and more anion-selective than mode I (works effectively for chlorides but not for bromides). For O-substituted derivatives, mode III is observed, with anions bound in a nest formed by aromatic CH atoms in the lower rim (multidentate but monovalent binding). The relative strength of these three binding modes, their solvent-dependence, and emergence in the crystal structures (CCDC) have been evaluated.

In-vitro Antibacterial Study of Crude extract of Barringtonia asiatica (L.) Kurtz Against Salmonella typhi, Escherichia coli, Staphylococcus aureus, Klebsiella pneumonia

Introduction: The antibiotic-resistant pathogenic bacteria have been recognized as a serious problem for humans and animals. The aims of this study were to evaluate the in-vitro antibacterial study of crude extract of Barringtonia asiatica (L.) Kurtz against Salmonella typhi, Escherichia coli, Staphylococcus aureus, Klebsiella pneumonia. Materials and Methods: Extractions was carried out through conventional method, by soaking the ground, the plant material in non-polar, medium polar and polar solvents in the order of increasing polarity and the crude extracts were tested by the disc diffusion method on nutrient agar medium. Result: The growth inhibition was considered and compared based on the inhibition rate as reported and < 9 mm inhibition is considered inactive in this regards the methanol leaves extract was observed to exhibited higher activity at 500 µg/mL on Klebsiella pneumonia with growth inhibition zone of 16.23 ± 0.06 mm. And lower inhibition was observed for other organism tested at similar concentration. Conclusion: The focus of this study is to provide the prospects of Barringtonia asiatica different solvent from non-polar and polar crude extract as a good agent for combating antibacterial menace and disease resistance from modern medicine.

Composition and Antioxidant Properties of Pigments of Mediterranean Herbs and Spices as Affected by Different Extraction Methods.

This study examined the composition and properties of chlorophyll and carotenoid extracted from the leaves of several Mediterranean evergreen shrubs and subshrubs (Myrtus communis L., Pistacia lentiscus L., Thymus vulgaris L., Salvia officinalis L. and Laurus nobilis L.) commonly used as herbs and spices. In order to fully assess their composition over a wide polarity range, pigments were extracted by successive solvent extraction with hexane, 80% acetone and 96% ethanol. Agitation-assisted extraction (AAE), ultrasound-assisted extraction (UAE) and pressurized liquid extraction (PLE) were employed and compared regarding their effect on the pigments’ yield and composition. Individual chlorophylls and carotenoids were analyzed by HPLC-DAD, while the content of total pigments and the extracts’ antioxidant capacity were determined spectrophotometrically. Throughout the experiments, pheophytin a, b and b’ were dominant chlorophyll molecules, while lutein and β-carotene were dominant carotenoids. Overall, the extracted pigments were determined as being in the range of 73.84–127.60 mg 100 g−1 and were the lowest in T. vulgaris, with no significant differences between other species. M. communis and P. lentiscus had the highest antioxidant capacities, showing a moderate positive correlation with carotenoid and chlorophyll levels. Significant differences were found in the levels of individual pigments with most of them showing a medium level of polarity due to the dissolution in acetone as a medium polar solvent. AAE and PLE demonstrated similar efficacy in the extraction of both carotenoids and chlorophylls; however, preference can be given to PLE, being a novel method with numerous advantages, e.g., shorter extraction time and lower solvent consumption. The examined plant species certainly expressed great diversity and showed the potential for application in the production of various functional products.

Separation of 4C-Substituted Pyrrolidin-2-One Derivatives on Amylose-Based Chiral Stationary Phases

Our previous studies concerning the enantioseparation of 4C-substituted pyrrolidin-2-one derivatives on coated polysaccharide-based chiral stationary phases have shown that amylose-based chiral selectors, together with mobile phases consisting of ethanol and n-hexane mixtures, are the most effective. To extend this research, five commercially available immobilized amylose-based chiral stationary phases (3,5-dimethylphenylcarbamate, 3-chlorophenylcarbamate, 3,5-dichlorophenylcarbamate, 3-chloro-4-methylphenylcarbamate and 3-chloro-5-methylphenylcarbamate) were tested with ethanol-containing mobile phases and obtained results were compared with data obtained on coated phases (3,5-dimethylphenylcarbamate and 5-chloro-2-methylphenylcarbamate). The effect of the nature of chiral stationary phase and analytes studied on enantiomer elution order, retention and resolution was tested. Compared to immobilized phase based on amylose 3,5-dimethylphenylcarbamate with its coated version, a better enantioseparation is observed on coated column. However, in addition to the coated amylose-based phases, immobilized phases with both methyl- and chloro-substituents in phenylcarbamate moiety were found to best separate the 4C-substituted pyrrolidin-2-one derivatives. Non-typical eluents with medium-polarity solvents were tested for the separation of three 4C-phenyl-substituted racemic compounds on five immobilized phases. Enantiorecognition of amylose derivatives containing 3,5-dimethylphenylcarbamate and 3-chloro-5-methylphenylcarbamate improved when working with mobile phases containing ethyl acetate, methyl tert-butyl ether and dichloromethane.

Counterion Migration Driven by Light-Induced Intramolecular Charge Transfer.

A series of D−π–A+ pyridinium compounds, in which D = −NPh2 and A+ = −PyMe+ are linked by various amounts of linear phenyl spacers, were strategically designed and synthesized. Their characterization revealed the presence of excited-state intramolecular charge transfer (ESICT) that triggers a corresponding response from the counterion. In medium and strong polar solvents, the fast solvent relaxation occurring after ESICT overwhelms the counterion effect, showing typical emission solvatochromism. In weakly polar solvents, ESICT induces counteranion migration for electrostatic stabilization, the time scale of which is dependent on the radius of the counteranion, the length of the π-linker, and the viscosity of the solvent. In low-viscosity organic solvents such as toluene, counteranion migration occurs within several tens to hundreds of picoseconds, resulting in a time-dependent continuous emission that can be resolved from the spectral temporal evolution. Concrete evidence for this is provided by the chemical synthesis of a D−π–A+ pyridinium–sulfur trioxide– zwitterion, where anion migration is restricted due to its internally locked ion pair. As a result, only a single emission band can be observed. These comprehensive studies prove that the ion migration process may be significant for a wide range of ESICT-type ionic fluorophores. Such an ionic movement, triggered by optically pumped ESICT of the D−π–A+ dyad, is similar to the molecular machine driven by the redox reaction, but with a facile access and fast response.

TD-DFT Prediction of the Intermolecular Charge-Transfer UV-Vis Spectra of Viologen Salts in Solution

The absorption spectrum of viologen salts in a medium or low polar solvent is an essential feature that influences all its “chromic” applications, whether we are considering thermochromic, electrochromic, photochromic or chemochromic devices. The prediction by quantum chemical methods of such absorption bands, typically observed in the visible range and due to charge transfer (CT) phenomena, is a very challenging problem due to strong solvent effects influencing both the geometry and the electronic transitions. Here we present a computational protocol based on DFT to predict with very high accuracy the absorption maxima of the CT bands of a series of viologen salts in solvents of low and medium polarity. The calculations also allow a clear dissection of the solvent effects, direct and indirect, and orbital contributions to the CT band.