Fluorescence and Photophysical Properties of Anthracene and Phenanthrene in Water

Polyaromatic hydrocarbons (PAHs) are widely spread pollutants in the environment, including soil and water. Anthracene (anth) and phenanthrene (phen) pose severe health impacts on human lives due to their carcinogenic nature by increasing cancer risk to the skin, lungs, and bladder. Fluorescence spectroscopy is a promising , efficient and straightforward tool for characterizing these trace PAHs in water. Therefore, the current work provides a detailed insight into the fluorescence properties of anth and phen in water. The fluorescence EEMs (excitation-emission matrices) of anth showed emissions at 380 nm, 400 nm, and 425 nm with single excitation at 250 nm, whereas phen showed two emissions < 380 nm, at 350 nm and 365 with single excitation at 250 nm. Then the theoretical EX/EM wavelengths were calculated by DFT and CIS-B3LYP for these compounds in water. The environmental effect of pH variation on fluorescence EEM shows a significant difference in fluorescence intensity without changing in peak locations, with highest fluorescence intensity at neutral pH than acidic and alkaline. Furthermore, the theoretical pH effect was described for the first time by simulating the protonated (+ 1), deprotonated (-1) and neutral molecules in water at the DFT level of theory. The variation in simulated oscillator strengths was similar in trend with the experimental fluorescence intensity of these compounds. The HOMO-LUMO were calculated to obtain the energy gap, molecular softness, molecular hardness, electronic potential and electrophilicity of anth and phen. To find the fluorophore contribution, the fluorescence of homogeneous mixture of both isomers was analyzed, which showed an enhanced fluorescence intensity of anth by 12-20%, whereas a decrease of 9-14% was observed in phen. This study describes that the fluorescence technique could be a fast and easy method to distinguish and identify PAHs isomers (anth and phen) in water.

Enantioseparation of planar chiral ferrocenes on cellulose-based chiral stationary phases: Benzoate versus carbamate pendant groups

In this study, the enantioseparation of 14 planar chiral ferrocenes containing halogen atoms, and methyl, iodoethynyl, phenyl, and 2-naphthyl groups, as substituents, was explored with a cellulose tris(4-methylbenzoate) (CMB)-based chiral column under multimodal elution conditions. n-Hexane/2-propanol (2-PrOH) 95:5 v/v, pure methanol (MeOH), and MeOH/water 90:10 v/v were used as mobile phases (MPs). With CMB, baseline enantioseparations were achieved for nine analytes with separation factors (α) ranging from 1.24 to 1.77, whereas only three analytes could be enantioseparated with 1.14 ≤ α ≤ 1.51 on a cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC)-based column, used as a reference for comparison, under the same elution conditions. Pendant group-dependent reversal of the enantiomer elution order was observed in several cases by changing CMB to CDMPC. The impact of analyte and chiral stationary phase (CSP) structure, and MP polarity on the enantioseparation, was evaluated. The two cellulose-based CSPs featured by different pendant groups were also compared in terms of thermodynamics. For this purpose, enthalpy (ΔΔH°), entropy (ΔΔS°) and free energy (ΔΔG°) differences, isoenantioselective temperatures (T_iso ), and enthalpy/entropy ratios (Q), associated with the enantioseparations, were derived from van 't Hoff plots by using n-hexane/2-PrOH 95:5 v/v and methanol/water 90:10 v/v as MPs. With the aim to disclose the functions of the different substituents in mechanisms and noncovalent interactions underlying analyte-selector complex formation at molecular level, electrostatic potential (V) analysis and molecular dynamics simulations were used as computational techniques. On this basis, enantioseparations and related mechanisms were investigated by integrating theoretical and experimental data.

Recognition in the Domain of Molecular Chirality: From Noncovalent Interactions to Separation of Enantiomers

It is not a coincidence that both chirality and noncovalent interactions are ubiquitous in nature and synthetic molecular systems. Noncovalent interactivity between chiral molecules underlies enantioselective recognition as a fundamental phenomenon regulating life and human activities. Thus, noncovalent interactions represent the narrative thread of a fascinating story which goes across several disciplines of medical, chemical, physical, biological, and other natural sciences. This review has been conceived with the awareness that a modern attitude toward molecular chirality and its consequences needs to be founded on multidisciplinary approaches to disclose the molecular basis of essential enantioselective phenomena in the domain of chemical, physical, and life sciences. With the primary aim of discussing this topic in an integrated way, a comprehensive pool of rational and systematic multidisciplinary information is provided, which concerns the fundamentals of chirality, a description of noncovalent interactions, and their implications in enantioselective processes occurring in different contexts. A specific focus is devoted to enantioselection in chromatography and electromigration techniques because of their unique feature as "multistep" processes. A second motivation for writing this review is to make a clear statement about the state of the art, the tools we have at our disposal, and what is still missing to fully understand the mechanisms underlying enantioselective recognition.

A review on fluorescence spectroscopic analysis of water and wastewater

In recent years, the application of fluorescence spectroscopy has been widely recognized in water environment studies. The sensitiveness, simplicity, and efficiency of fluorescence spectroscopy are proved to be a promising tool for effective monitoring of water and wastewater. The fluorescence excitation-emission matrix (EEMs) and synchronous fluorescence spectra have been widely used analysis techniques of fluorescence measurement. The presence of organic matter in water and wastewater defines the degree and type of pollution in water. The application of fluorescence spectroscopy to characterize dissolved organic matter (DOM) has made the water quality assessment simple and easy. With the recent advances in this technology, components of DOM are identified by employing parallel factor analysis (PARAFAC), a mathematical trilinear data modeling with EEMs. The majority of wastewater studies indicated that the fluorescence peak of EX/EM at 275 nm/340 nm is referred to tryptophan region (Peak T1). However, some researchers identified another fluorescence peak in the region of EX/EM at 225-237 nm/340-381 nm, which described the tryptophan region and labeled it as Peak T2. Generally, peak T is a protein-like component in the water sample, where T1 and T2 signals were derived from the <0.20μm fraction of pollution. Therefore, a more advanced approach, such as an online fluorescence spectrofluorometer, can be used for the online monitoring of water. The results of various waters studied by fluorescence spectroscopy indicate that changes in peak T intensity could be used for real-time wastewater quality assessment and process control of wastewater treatment works. Finally, due to its effective use in water quality assessment, the fluorescence technique is proved to be a surrogate online monitoring tool and early warning equipment.

Enantioseparations of polyhalogenated 4,4'-bipyridines on polysaccharide-based chiral stationary phases and molecular dynamics simulations of selector-selectand interactions

2'-(4-Pyridyl)- and 2'-(4-hydroxyphenyl)-TCIBPs (TCIBP = 3,3',5,5'-tetrachloro-2-iodo-4,4'-bipyridyl) are chiral compounds that showed interesting inhibition activity against transthyretin fibrillation in vitro. We became interested in their enantioseparation since we noticed that the M-stereoisomer is more effective than the P-enantiomer. Based thereon, we recently reported the enantioseparation of 2'-substituted TCIBP derivatives with amylose-based chiral columns. Following this study, herein we describe the comparative enantioseparation of both 2'-(4-pyridyl)- and 2'-(4-hydroxyphenyl)-TCIBPs on four cellulose phenylcarbamate-based chiral columns aiming to explore the effect of the polymer backbone, as well as the nature and position of substituents on the side groups on the enantioseparability of these compounds. In the frame of this project, the impact of subtle variations of analyte and polysaccharide structures, and mobile phase (MP) polarity on retention and selectivity was evaluated. The effect of temperature on retention and selectivity was also considered, and overall thermodynamic parameters associated with the analyte adsorption onto the CSP surface were derived from van 't Hoff plots. Interesting cases of enantiomer elution order (EEO) reversal were observed. In particular, the EEO was shown to be dependent on polysaccharide backbone, the elution sequence of the two analytes being P-M and M-P on cellulose and amylose tris(3,5-dimethylphenylcarbamate), respectively. In this regard, a theoretical investigation based on molecular dynamics (MD) simulations was performed by using amylose and cellulose tris(3,5-dimethylphenylcarbamate) nonamers as virtual models of the polysaccharide-based selectors. This exploration at the molecular level shed light on the origin of the enantiodiscrimination processes.

Enantiomeric separation of prothioconazole and prothioconazole-desthio on chiral stationary phases

Prothioconazole is a type of broad-spectrum triazole thione fungicide developed by the Bayer Company. Prothioconazole-desthio is the main metabolite of prothioconazole in the environment. In our study, enantiomeric separation of prothioconazole and prothioconazole-desthio was performed on various chiral stationary phases (CSPs) by high-performance liquid chromatography (HPLC). It was found that polysaccharide CSPs showed better ability than brushing CSPs in enantiomeric separation. The successful chiral separation of prothioconazole could be achieved on self-made Chiralcel OD, commercialized Chiralcel OJ-H and Lux Cellulose-1. Chiralpak IA, Chiralpak IB, Chiralpak IC, Chiralcel OD, Chiralpak AY-H, Chiralpak AZ-H, and Lux Cellulose-1 realized the baseline separation of prothioconazole-desthio enantiomers. Simultaneous enantiomeric separation of prothioconazole and prothioconazole-desthio was performed on Lux Cellulose-1 using acetonitrile (ACN) and water as mobile phase. In most cases, low temperature favored the separation of two compounds. The influence of the mobile phase ratio or type was deeply discussed. We obtained larger Rs and longer analysis time with a smaller proportion of isopropanol (IPA) or ethanol and more water content at the same temperature. The ratio of ACN and water had influences on the outflow orders of prothioconazole-desthio enantiomers. This work provides a new approach for chiral separation of prothioconazole and prothioconazole-desthio with a discussion of chiral separation mechanism on different CSPs.

Enantioseparation of fluorinated 3-arylthio-4,4'-bipyridines: Insights into chalcogen and π-hole bonds in high-performance liquid chromatography

A chalcogen bond (ChB) is a σ-hole-based noncovalent interaction between a Lewis base and an electrophilic element of Group VI (O, S, Se, Te), which behaves as a Lewis acid. Recently, we demonstrated that halogen bond, the more familiar σ-hole-based interaction, is able to promote the enantioseparation of chiral compounds in HPLC environment. On this basis, an investigation to detect ChBs, functioning as stereoselective secondary interactions for HPLC enantioseparations, was started off and the results of this study are described herein. Our investigation also focused on the impact of the perfluorinated aromatic ring as a π-hole donor recognition site. For these purposes, seven atropisomeric fluorinated 3-arylthio-4,4'-bipyridines were designed, synthesized and used as potential ChB donors (ChBDs) with two cellulose-based chiral stationary phases (CSPs) containing carbonyl groups as ChB acceptors (ChBAs). In addition, one and two analogues lacking fluorine and sulphur, respectively, were prepared as terms of comparison. The design of the test analytes was computationally guided. In this regard, electrostatic potentials (EPs) associated with σ- and π-holes were computed and the atomic contributions to the sulphur EP maxima were derived using a molecular space partitioning in terms of Bader's atomic basins. This procedure is akin to the Bader-Gatti electron density source function (SF) decomposition, yet suitably extended to the EP field. For five 3-substituted-4,4'-bipyridines, thermodynamic parameters were derived from van't Hoff plots. Finally, the use of molecular dynamic (MD) simulation to model ChB in cellulose-analyte complexes was explored. Evidences that σ-hole and π-hole interactions can jointly drive HPLC enantioseparations through recognition sites generated by electronic charge depletion emerged from both experimental results and theoretical data.

Halogen bond in high-performance liquid chromatography enantioseparations: Description, features and modelling

Halogen bond (XB)-driven enantioseparations involve halogen-centred regions of electronic charge depletion (σ-hole) as electrophilic recognition sites. The knowledge in this field is still in its infancy. Indeed, although the influence of halogens on enantioseparation have been often considered, only recently the function of electrophilic halogens (Cl, Br, I) as enantioseparations 'drivers' has been demonstrated by our groups. Further to these studies, in this paper we focus on some unexplored issues. First, as XB-driven chiral recognition mechanisms are at an early stage of comprehension, a theoretical investigation based on a series of 32 molecular dynamic (MD) simulations was performed by using polyhalogenated 4,4'-bipyridines and polysaccharide-based polymers as ligands and receptors, respectively. Enantiomer elution orders (EEOs) were derived from calculations and the theoretical model accounted for some analyte- and chiral stationary phase (CSP)-dependent experimental EEO inversions. Then, the function of halogen-centred σ-holes in competitive systems, presenting also hydrogen bond (HB) centres as recognition sites, was considered. In this regard, Pirkle's enantioseparations of halogenated compounds performed on Whelk-O1 were theoretically re-examined and electrostatic potentials (EPs) associated with both σ-holes on halogens and HB centres were computed and compared. Then, the enantioseparation of halogenated 2-nitro-1-arylethanols was performed on cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC) and the influence of halogen substituents on the chromatographic results was evaluated by correlating theoretical and experimental data.

High performance chiral separation materials based on chitosan bis(3,5-dimethylphenylcarbamate)-(alkyl urea)s

Enantioseparation plays an important role for many fields and for pharmaceutical industry in particular. Chiral stationary phase (CSP) is the core of chiral liquid chromatography that effectively implements enantioseparation. In order to develop coated type CSPs with excellent enantioseparation capability and high tolerance against mobile phases, in this work, a series of chitosan bis(3,5-dimethylphenylcarbamate)-(alkyl urea)s were synthesized, which were coated on 3-aminopropyl silica gel to afford new CSPs. Owing to strong hydrogen bonds formed among the synthesized derivatives, the supra-structure of the derivatives should be highly ordered. Hence, these CSPs could provide excellent separation capability and could tolerate common organic solvents that are usually prohibited for coated type CSPs of cellulose and amylose derivatives. Therefore the newly prepared CSPs exhibited promising prospects for enantioseparation of chiral compounds.

Insights into halogen bond-driven enantioseparations

Although the halogen bond (XB) has been so far mainly studied in silico and in the solid state, its potential impact in solution is yet to be fully understood. In this study, we describe the first systematic investigation on the halogen bond in solvated environment by high-performance liquid chromatography (HPLC). Thirty three atropisomeric polyhalogenated-4,4'-bipyridines (HBipys), containing Cl, Br and I as substituents, were selected and used as potential XB donors (XBDs) on two cellulose-based chiral stationary phases (CSPs) containing potential XB acceptors (XBAs). The impact of the halogens on the enantiodiscrimination mechanism was investigated and iodine showed a pivotal role on the enantioseparation in non-polar medium. Electrostatic potentials (EPs) were computed to understand the electrostatic component of CSP-analyte interaction. Moreover, van't Hoff studies for ten HBipys were performed and the thermodynamic parameters governing the halogen-dependent enantioseparations are discussed. Finally, a molecular dynamic (MD) simulation is proposed to model halogen bond in polysaccharide-analyte complexes by inclusion of a charged extra point to represent the positive 'σ-hole' on the halogen atom. On the basis of both experimental results and theoretical data, we have profiled the halogen bond as a chemo-, regio-, site- and stereoselective interaction which can work in HPLC environment besides other known interactions based on the complementarity between selector and selectand.

Enantioseparation of aldols by high-performance liquid chromatography on polysaccharide-based chiral stationary phases that bear chlorinated substituents

Two families of aldols, obtained from the condensation of aromatic aldehydes with cyclohexanone or acetone (ten examples in each group), were analyzed by high-performance liquid chromatography in normal phase elution mode on three polysaccharide-based chiral stationary phases of the Lux series, namely, Lux Cellulose-2, Lux Cellulose-4 and Lux Amylose-2, which share the common feature of chlorinated substituents in the chiral selectors. Following simple optimization steps, the enantioseparation of all aldols derived from cyclohexanone was achieved and the highest values of separation factor (α, 1.32 < α < 2.20) and resolution (Rs , 4.5 < Rs <17.2) were observed on Lux Cellulose-2, with the only exception of the 4-nitro-substituted derivative that was better resolved on Lux Cellulose-4. On the contrary, Lux Amylose-2 was the best choice for aldols derived from acetone and only specific analytes in this group were resolved on the cellulose-based supports. A variable-temperature study of selected compounds allowed us to determine thermodynamic parameters of the enantioseparation process, which was enthalpy-controlled in all the cases except one.