Fluorescence Quenching by Nitro Compounds within a Hydrophobic Deep Eutectic Solvent

Revealing Solvation within [l(-)-Menthol : Thymol] Deep Eutectic Solvents via Microfluidity Assessment

Properties of deep eutectic solvents (DESs) can be effectively fine-tuned by judicious selection of the constituents and their relative amounts. The DESs prepared by mixing l(-)-menthol (Men) and thymol (Thy), two nonionic constituents, offer a nonpolar solvation environment desired in many chemical applications. These DESs are shown to exhibit temperature-dependent nonideality and possess complex intermolecular interactions dominated by sterically hindered H-bonded clusters. Key insights into solute solvation within the (Men : Thy) DES system are obtained by employing structurally different fluorescence microfluidity probes that operate based on different mechanisms. The (Men : Thy) DES system at nine (9) different molar ratios, from (5 : 1) to (1 : 5), in the temperature (T) range of 293 to 363 K is investigated using an intramolecular excimer-forming probe, two fluorescence anisotropy probes, and a fluorescence intensity-based microfluidity probe. The logarithm of microviscosity (ln ημ) estimated from the response of the excimer-forming probe varies linearly with the logarithm of the bulk viscosity (ln η) across all nine compositions in the entire temperature range. Further, excellent linear correlation is observed between the rate constant (ka) of the intramolecular excimer formation and T/η implying adherence to the Stokes-Einstein relationship. Rotational reorientation times (θ) obtained from the excited-state fluorescence anisotropy decay of the two structurally different probes follow the Perrin formulation (θ varying linearly with η/T) indicating the absence of significant microheterogeneity toward solute rotational diffusion within the (Men : Thy) DES system. The response of the fluorescence intensity (IF) probe also exhibits a simple Arrhenius-type T dependence with IF versus η for all DES compositions, fitting well to exponential growth-to-maxima. The solute solvation behavior observed from the responses of different fluorophores indicates a homogeneous solubilization environment afforded by the (Men : Thy) DES system that is independent of the composition and temperature. These findings have significant implications in chemical synthesis and analysis where such nonpolar DES systems have the potential to be effectively employed.

Study of fluconazole drug behavior in deep eutectic solvents: thermodynamic properties, solubility measurement, and fluorescence spectroscopy

Fluconazole is a crucial antifungal medication with a broad spectrum of activity against various fungal infections. This study thermodynamic properties, solubility measurements and spectrofluorometric method were used for investigating the interactions between fluconazole (FCZ) and deep eutectic solvents (DESs). Five choline chloride-based deep eutectic solvents (DESs) were synthesized. Each DES was prepared by combining choline chloride (a hydrogen bond acceptor, HBA) with a different hydrogen bond donor (HBD): oxalic acid (OX), malonic acid (MA), ethylene glycol (EG), glycerol (G), or urea (U). Subsequently, the interactions between fluconazole (FCZ) and these synthesized DESs were investigated using fluorescence spectroscopy at a temperature of 298.15 K. Fluorescence spectroscopy revealed a strong interaction between fluconazole (FCZ) and deep eutectic solvents (DESs). This was evident from the significant quenching of FCZ's intrinsic fluorescence upon DES addition. The association constant and binding sites were determined. Among the tested DESs, the choline chloride-oxalic acid mixture exhibited the strongest interaction with FCZ. Furthermore, the solubility of FCZ in DES-water mixtures studied at a temperature range of (298.15 to 313.15) K was found to increase with increasing DES concentration. The solubility data were accurately fitted using the e-NRTL and Wilson thermodynamic models. To gain deeper insights, conductor-like screening model (COSMO) calculations were performed on the studied systems. The obtained surface cavity volume and dielectric solvation energy provide valuable information about the intermolecular interactions. Finally, thermodynamic analysis using Gibbs and van't Hoff equations indicated that the dissolution of FCZ in these systems is an endothermic process.

Anomalous Fluorescence Quenching in Fluorous Solvent-Added Media

Due to the inherent high electronegativity of fluorine, perfluorocarbons have the potential to exhibit unusual characteristics. Fluorous solvents, in this context, may afford an anomalous solubilizing behavior compared to their hydrocarbon analogues. Addition of perfluorodecalin (PFD) to n-hexane results in unusual fluorescence quenching of polycyclic aromatic hydrocarbons (PAHs) by the quencher nitromethane. As more viscous PFD is added to less viscous n-hexane, the dynamic viscosity (η) of the media increases. The bimolecular quenching rate constants (kq) of the PAHs, instead of decreasing, increase as PFD is added to n-hexane until an equimolar mixture composition is obtained; kq exhibits an expected decrease only in the PFD-rich region of the mixture. The expected decrease in kq in the hydrocarbon analogue decalin added to n-hexane is observed across all compositions. It is proposed that highly electronegative fluorines on PFD stabilize the partial positive charge (δ+) that develops on excited PAHs during electron/charge transfer to the quencher nitromethane, facilitating quenching in the process. In the PFD-rich region, however, increased η starts to dominate the quenching, resulting in the expected decrease in kq. A monotonic decrease in kq is observed in the PFD-added n-hexane system for fluoranthene quenching by triethylamine (TEA) as TEA acts as the electron/charge donor and a partial negative charge (δ-) develops on excited fluoranthene during the quenching process. No such stabilization by PFD is observed when nitrobenzene is employed as the quencher. This is attributed to the significantly higher quenching (KS > 104 M-1) of PAH fluorescence by nitrobenzene due to the presence of aromatic π-π interactions between PAH and nitrobenzene, further facilitated by the higher electron affinity of nitrobenzene as compared to nitromethane. The role of fluorous media in facilitating electron/charge transfer processes is clearly established.

Dissolution mechanism of cellulose in a benzyltriethylammonium/urea deep eutectic solvent (DES): DFT-quantum modeling, molecular dynamics and experimental investigation

In this paper, a benzyltriethylammonium/urea DES was investigated as a new green and eco-friendly medium for the progress of organic chemical reactions, particularly the dissolution and the functionalization of cellulose. In this regard, the viscosity-average molecular weight of cellulose (M̄w) during the dissolution/regeneration process was investigated, showing no significant degradation of the polymer chains. Moreover, X-ray diffraction patterns indicated that the cellulose dissolution process in the BTEAB/urea DES decreased the crystallinity index from 87% to 75%, and there was no effect on type I cellulose polymorphism. However, a drastic impact of the cosolvents (water and DMSO) on the melting point of the DES was observed. Besides, to understand the evolution of cellulose-DES interactions, the formation mechanism of the system was studied in terms of H-bond density and radial distribution function (RDF) using molecular dynamics modeling. Furthermore, density functional theory (DFT) was used to evaluate the topological characteristics of the polymeric system such as potential energy density (PED), laplacian electron density (LED), energy density, and kinetic energy density (KED) at bond critical points (BCPs) between the cellulose and the DES. The quantum theory of atoms in molecules (AIM), Bader's quantum theory (BQT), and reduced density gradient (RDG) scatter plots have been exploited to estimate and locate non-covalent interactions (NCIs). The results revealed that the dissolution process is attributed to the physical interactions, mainly the strong H-bond interactions.

Enhanced luminescence sensing performance and increased intrachain order in blended films of P3HT and cellulose nanocrystals

Blended films comprising poly(butyl acrylate) (PBA)-grafted cellulose nanocrystals (CNCs) and poly(3-hexylthiophene) (P3HT), exhibited more intense photoluminescence (PL) and longer PL emission lifetimes compared to pristine P3HT films. Optical absorption and photoluminescence spectra indicated reduced torsional disorder i.e. enhanced backbone planarity in the P3HT@CNC blended composites compared to the bare P3HT. Such molecule-level geometrical modification resulted in both smaller interchain and higher intrachain exciton bandwidth in the blended composites compared to the bare P3HT, because of reduced interchain interactions and enhanced intrachain order. These results indicate a potential switch of the aggregation behavior from dominant H-aggregates to J-aggregates, supported by Raman spectroscopy. The reorganization of micromolecular structure and concomitant macroscopic aggregation of the conjugated polymer chains resulted in a longer conjugation length for the P3HT@CNC blended composites compared to the bare P3HT. Additionally, this nanoscale morphological change produced a reduction in the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gap of the blends, evidenced from optical absorption spectra. Classical molecular dynamics simulation studies predicted the probability of enhanced planarity in the polymer backbone following interactions with CNC surfaces. Theoretical results from density functional theory calculations corroborate the experimentally observed reduction of optical bandgap in the blends compared to bare P3HT. The blended composite outperformed the bare P3HT in nitro-group PL sensing tests with a pronounced difference in the reaction kinetics. While the PL quenching dynamics for bare P3HT followed Stern-Volmer kinetics, the P3HT@CNC blended composite exhibited a drastic deviation from the same. This work shows the potential of a functionalized rod-like biopolymer in tuning the optoelectronic properties of a technologically important polymeric organic semiconductor through control of the nanoscale morphology.

Ionic Rigid Organic Dual-State Emission Compound With Rod-Shaped and Conjugated Structure for Sensitive Al3+ Detection

Dual-state emission (DSE) luminogens, a type of luminescent material which can effectively emit light in both dilute solution and solid states, have attracted tremendous attention, due to their widespread applications in chemical sensing, biological imaging, organic electronic devices, and so on. They overcome the shortcomings of aggregation-induced emission (AIE)-type compounds that do not emit light in dilute solutions and aggregation-caused quenching (ACQ)-type compounds that do not emit light in a concentrated or aggregated state. This work reports a novel ionic DSE material based on rigid rod-shaped organic conjugated structure using 4,4′-bis(2-sulfonatostyryl) biphenyl disodium salt (BSBDS); the ion repulsion effect can reduce the strong π–π interaction in aggregation and achieve high-efficiency luminescence in solution and solid states. In addition to excellent DSE characteristics, BSBDS also exhibits a mechanochromic nature and sensitive detection performance for aluminum ion (Al³⁺).

Selective sensing of the nucleoside analogue, trifluridine and tipiracil in dosage form and biological matrices

A new fluorescent sensor is introduced to analyze nucleoside analogue, trifluridine and tipiracil in tablets and biological fluids. The synthesized fluorophore exhibits good fluorescence at 446 nm after excitation at 257 nm. The interaction between the studied drugs and the reagent was a quenching effect. Different experimental parameters and the mechanism of quenching were discussed. The present method was utilized to analyze trifluridine and tipiracil raw materials and tablets over the concentration range of 20-1000 ng/mL and spiked biological fluids over the range of 30-1000 ng/mL. The method is selective, specific, and possesses good accuracy and high precision. The method is highly sensitive, with detection limits of 5.8 and 6.0 ng/mL for trifluridine and tipiracil, respectively, and quantitation limits of 17.7 and 18.1 ng/mL for trifluridine and tipiracil, respectively. In vivo analysis of trifluridine was achieved selectively and the mean pharmacokinetic parameters were studied.

Effect of Water on a Hydrophobic Deep Eutectic Solvent

Deep eutectic solvents (DESs) formed by hydrogen bond donors and acceptors are a promising new class of solvents. Both hydrophilic and hydrophobic binary DESs readily absorb water, making them ternary mixtures, and a small water content is always inevitable under ambient conditions. We present a thorough study of a typical hydrophobic DES formed by a 1:2 mole ratio of tetrabutyl ammonium chloride and decanoic acid, focusing on the effects of a low water content caused by absorbed water vapor, using multinuclear NMR techniques, molecular modeling, and several other physicochemical techniques. Already very low water contents cause dynamic nanoscale phase segregation, reduce solvent viscosity and fragility, increase self-diffusion coefficients and conductivity, and enhance local dynamics. Water interferes with the hydrogen-bonding network between the chloride ions and carboxylic acid groups by solvating them, which enhances carboxylic acid self-correlation and ion pair formation between tetrabutyl ammonium and chloride. Simulations show that the component molar ratio can be varied, with an effect on the internal structure. The water-induced changes in the physical properties are beneficial for most prospective applications but water creates an acidic aqueous nanophase with a high halide ion concentration, which may have chemically adverse effects.

Facile synthesis of an aggregation-induced emission (AIE) active imidazoles for sensitive detection of trifluralin

A novel imidazoles fluorescent probe (2) was synthesized from vanillin, o-phenylenediamine, and N,N-diphenylcarbamyl chloride. Its structure was characterized by fluorescence spectra, UV-Vis spectra, ¹H NMR, ¹³C NMR, and high-resolution mass spectrometry (HRMS). Moreover, its aggregation-induced emission (AIE) feature was investigated in THF/MeOH solution. Furthermore, the fluorescence quenching experimental results suggest that compound 2 is the potential fluorescent probe of small organic molecules showing high selectivity and sensitivity for nitroaromatic compounds. In addition, the probe could be applied in the determination of trifluralin with fast response and stability. The fluorescence response of the probe exhibited a good linear correlation with the concentration of trifluralin ranging from 10 to 100 μM, and the limit of detection (LOD) was as low as 5.066 μM. Finally, the probe was successfully utilized to determine the amount of trifluralin in real samples, and the recoveries were 91.1% to 111.2%, indicating the applicability and reliability of the probe.

Fluorescent sensing for some nitric oxide donors in dosage forms and biological matrices

New fluorescent sensing of some nitric oxide donors, nitroglycerin and isosorbide dinitrate was developed in our laboratories. Two fluorescent reagents, 2-(2-hydroxyethylamino)-4,6-dimethylpyridine-3-carbonitrile, 3a and 2-(3-chloro-phenylamino)-4,6-dimethylpyridine-3-carbonitrile, 3b were synthesized in our laboratories and a comparative study was performed between them from the point of fluorescence intensity. The fluorophore, 3a, was selected for the analytical study as it exhibit higher quantum yield value. The interaction between the selected drugs and the fluorophore was noticed to be quenching. The mechanism of quenching was studied and it was supposed to be collisional quenching through photo induced electron transfer process. The proposed sensing method was applied successfully for the analysis of nitroglycerin and isosorbide dinitrate in dosage forms within concentration range of (0.05-0.5 µg/mL) with percentage recoveries of 99.9 ± 0.5 and 99.9 ± 0.7 respectively. The studied drugs, nitroglycerin and isosorbide dinitrate, were also probed in spiked biological matrices such as plasma samples with percentage recoveries of 99.1 ± 1.97 and 100.7 ± 1.96 and urine samples with percentage recoveries of 100.4 ± 1.8 and 100.3 ± 1.7 respectively. In vivo analysis of both drugs in real plasma was also investigated. The sensing method exhibit well intra-day and inter day precision with %RSD < 2%.

Multiple evidences of dynamic heterogeneity in hydrophobic deep eutectic solvents

Hydrophobic deep eutectic solvents (HDESs) have gained immense popularity because of their promising applications in extraction processes. Herein, we employ atomistic molecular dynamics simulations to unveil the dynamics of DL-menthol (DLM) based HDESs with hexanoic (C6), octanoic (C8), and decanoic (C10) acids as hydrogen bond donors. The particular focus is on understanding the nature of dynamics with changing acid tail length. For all three HDESs, two modes of hydrogen bond relaxations are observed. We observe longer hydrogen bond lifetimes of the inter-molecular hydrogen bonding interactions between the carbonyl oxygen of the acid and hydroxyl oxygen of menthol with hydroxyl hydrogen of both acids and menthol. We infer strong hydrogen bonding between them compared to that between hydroxyl oxygen of acids and hydroxyl hydrogens of menthol and acids, marked by a faster decay rate and shorter hydrogen bond lifetime. The translational dynamics of the species in the HDES becomes slower with increasing tail length of the organic acid. Slightly enhanced caging is also observed for the HDES with a longer tail length of the acids. The evidence of dynamic heterogeneity in the displacements of the component molecules is observed in all the HDESs. From the values of the α-relaxation time scale, we observe that the molecular displacements become random in a shorter time scale for DLM-C6. The analysis of the self-van Hove function reveals that the overall distance covered by DLM and acid molecules in the respective HDES is more than what is expected from ideal diffusion. As marked by the shorter time scale associated with hole filling, the diffusion of the oxygen atom of menthol and the carbonyl oxygen of acid from one site to the other is fastest for hexanoic acid containing HDES.

Formation of water-in-oil microemulsions within a hydrophobic deep eutectic solvent

Hydrophobic deep eutectic solvents (DESs) as neoteric, non-toxic, and inexpensive media have the potential to replace organic solvents in various aggregation processes. Conventional water-in-oil microemulsions are formed using mostly environmentally unfavorable toxic organic solvents as the bulk oil phase. Evidence of formation of water-in-DES microemulsions is presented. These novel assemblies are formed using a hydrophobic DES constituted of n-decanoic acid (DA) and tetra-n-butylammonium chloride (TBAC) in 2 : 1 mole ratio, termed TBAC-DA, as the bulk oil phase. It is observed that in the presence of a common and popular non-ionic surfactant Triton X-100 (TX-100), water pools are formed within TBAC-DA under ambient conditions with maximum water loading (w0 = [water]/[TX-100]) of 60 ± 3 for [TX-100] = 300 mM. The formation of the microemulsions is established by using fluorescence probe pyranine, which exhibited the appearance of a band characterizing the un-protonated form of the probe clearly implying onset of water-in-TBAC-DA microemulsion formation. The UV-vis absorbance of CoII further corroborates TX-100-assisted water pool formation within TBAC-DA via the appearance of the band that is assigned to the response of the probe in water. Dynamic light scattering (DLS) measurement suggests average aggregate sizes to be in the range of 72(±4) to 122(±7) nm. These unprecedented water-in-DES microemulsions may have far reaching implications due to their benign nature.

Donor-acceptor complex formation in tetra-n-butylammonium chloride: n-decanoic acid deep eutectic solvent

Complex formation between pyrene (Py) and N,N-dimethylaniline (DMA) is presented in a deep eutectic solvent constituting of tetra-n-butylammonium chloride (TBAC) and n-decanoic acid (DA) in a 1:2 mol ratio, respectively, named TBAC:DA. The addition of DMA to a Py solution of TBAC:DA results in the formation of a fluorescent Py-DMA charge-transfer complex, which is manifested via a broad structureless bathochromically shifted band centered at 550(±2) nm. The solvatochromic nature of the Py-DMA fluorescent complex indicates the solvent polarity of TBAC:DA to be higher than that of methanol. The absence of a negative pre-exponential factor in the intensity decay at 550 nm combined with the excitation scans implies the presence of weak interaction between Py and DMA in the ground-state, leading to the rapid formation of a Py-DMA complex possibly at a sub-nanosecond time scale. The Stern-Volmer quenching constant (K_SV) varies from 53(±2) to 96(±1) M^-1, and the bimolecular quenching rate constant (k_q) varies from 3.0(±0.4) × 10⁸ to 8.8(±1.3) × 10⁸ M^-1 s^-1 by increasing the temperature (T) from 283.15 to 313.15 K, implying efficient deactivation of electron-acceptor Py in the excited-state induced effectively by the electron-donor DMA within TBAC:DA. ln k_q varies linearly with 1/T with an activation energy (E_a) of 26.4(±0.4) kJ mol^-1. The linear behavior between k_q and 1/η suggests conformity to the Stokes-Einstein relationship within TBAC:DA. The Py-DMA complex formation efficiency increases with an increase in T and reaches maxima at 298.15 K before decreasing with a further increase in T. The initial reduction in η favors Py-DMA complex formation; this effect is overcome by preferential thermal deactivation of the Py-DMA fluorescent complex as compared to that of pyrene.

Synthesis, and the optical and electrochemical properties of a series of push–pull dyes based on the 4-(9-ethyl-9H-carbazol-3-yl)-4-phenylbuta-1,3-dienyl donor

A series of twelve dyes based on the 4-(9-ethyl-9H-carbazol-3-yl)-4-phenylbuta-1,3-dienyl donor were prepared with electron acceptors varying in their structures but also in their electron-withdrawing ability.