Hydrogen bonding effects on the fluorescence properties of 4'-diethylamino-3-hydroxyflavone in water and water-acetone mixtures

Natural flavonols as probes for direct determination of borax: From conventional fluorescence analysis to paper-based smartphone sensing

Borax is strictly regulated in the food processing and pharmaceutical industry due to its physiological toxicity, and the development of a direct analytical method is essential for effectively monitoring the borax abuse. In this work, the fluorescence properties of flavonoids, including flavones, isoflavones and flavonols, were systematically investigated from aqueous to borax solutions, and it was found that the weak intrinsic fluorescence of flavonols could be pervasively sensitized by borax. A natural flavonol, morin, was subsequently chosen as a representative probe to develop a turn-on fluorescence sensing method for borax analysis, which achieved a linear response spanning four orders of magnitude with a detection limit of 1.07 μM (0.22 μg mL-1 in terms of Na2B4O7 content). Furthermore, a smartphone-assisted paper-based test device was designed and constructed by 3D printing technology. Using morin-impregnated test strips as the carrier, the borax could be visually detected by the RGB signals of the captured images, with a detection limit of 0.13 mM (27.05 μg mL-1 for Na2B4O7). Combining ion exchange treatment for food samples and sodium periodate oxidation for drug samples, the developed methods were successfully applied for the direct analysis of borax in various products with the recoveries of 86.9-106.3% for traditional fluorescence analysis and 82.7-108.8% for smartphone-assisted fluorescence sensing. The fluorescence property of the morin-borax system was studied using time-dependent density functional theory, and the sensing mechanism was discussed in conjunction with experimental research.

Tuning sensing efficacy of anthraimidazoledione-based charge transfer dyes: nitro group positioning impact

Anthraimidazoledione-based optical sensors have been designed by varying the position of the nitro functional group. All three positional isomers showed highly colored, photostable optical signals owing to intramolecular charge transfer interactions. Despite having the same anion-binding site (imidazole unit), the selectivity and sensitivity of the compounds depend on the positioning of the nitro group. The selectivity was fairly good for the meta isomer, followed by the ortho and para isomers, respectively. In contrast, the sensitivity towards anions followed a completely opposite trend, with the para isomer being the most sensitive one towards anions. Interestingly, the color changing response along the turn-on fluorescence signal was observed only with CN- ions in a semi-aqueous environment. Though the introduction of water as a co-solvent could improve the selectivity, the sensitivity was found to be slightly less than that observed in pure organic medium. Mechanistic studies indicated hydrogen bonding interactions between the imidazole -NH proton and cyanide, which further facilitated the extent of intramolecular charge transfer.

A Theoretical Study of Solvent Effects on the Structure and UV-vis Spectroscopy of 3-Hydroxyflavone (3-HF) and Some Simplified Molecular Models

Solvent effects on the UV-vis spectra of 3-hydroxyflavone and other structurally related molecules (3-hydroxychromen-4-one, 3-hydroxy-4-pyrone, and 4-pyrone) have been studied by combining time-dependent density functional theory (TDDFT) and the polarizable continuum method (PCM). Among the first five excited states of the four considered molecules, electronic states of n → π* and π → π* nature appear. In general, the stability of the n → π* states decreases as the π space becomes larger in such a way that only for 4-pyrone and 3-hydroxy-4-pyrone are they the first excited states. In addition, they become less stabilized in ethanol solution than the ground state, and this causes blueshift transitions in solution. The opposite trend is found for the π → π* excited states. They are less energetic with the π-system size and when passing from gas phase to solution. The solvent shift also depends strongly on the size of the π systems and on the formation of an intramolecular hydrogen bond; thus, it decreases when going from 4-pyrone to 3-hydroxyflavone. The performance of the three versions (cLR, cLR², and IBSF) of the specific-state PCM method in predicting transition energies are compared.

Photophysical properties of push-pull monocationic D-π-A+ thiophene based derivatives: Fluorosolvatochromism and pH studies

Photophysical properties of two thiophene salts of the form D-π-A⁺ are studied in several solvents and at various pH values of the aqueous solution. The studied compounds embrace methoxy group as electron donating moiety at one end and cationic amidine group with and without fluorine atom at the ortho position of the amidine group as the electron withdrawing group at the other end of the molecules and separated by thiophene ring. The two thiophene derivatives are 4-(5-(4-methoxyphenyl)thiophen-2-yl)benzamidine hydrochloride salt (MOTB) and 2-fluoro-4-(5-(4-methoxyphenyl) thiophen-2-yl)benzamidine hydrochloride salt (FMOTB). The observed changes in the fluorescence emission spectra with the nature of the solvent were found to be much more pronounced than the corresponding absorption spectra which signify an emission from the intramolecular charge transfer state. The higher bathochromic shift in the fluorescence emission spectra than the absorption spectra indicates that the excited state dipole moment is larger than that of the ground state. It has also been observed that the presence of the fluorine atom in the electron withdrawing part does not show any changes in the absorption spectra while a clear bathochromic shift is observed in the fluorescence emission spectra indicating an enhanced strength of the electron withdrawing ability in case of FMOTB. Effect of pH was also studied and pK_a values were evaluated. The observed photophysical properties were correlated to the normalized solvent polarity parameter (E_T^N) when solvents are classified to protic and aprotic solvents. This designates the importance of hydrogen bonding interactions. We have also applied a couple of linear solvation energy relationships for better understanding of the exact contribution of each solvent parameter on each photophysical property. We have found that both Catalán's and Laurence's treatments show that the photophysical properties are mainly controlled by the solvent's non-specific interactions. However, these models were not sufficient to interpret the observed data without the inclusion of the participation of the specific interactions.

A chemosensing approach for the colorimetric and spectroscopic detection of Cr3+, Cu2+, Fe3+, and Gd3+ metal ions

Metal cations are present in domestic and industrial wastewater and have adverse effects on human and aqueous life. The present study describes the development of the molecular probe 9-anthracen-9-ylmethylene)hydrazineylidene)methyl)-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-8-ol (AMHMPQ) to detect Cr³⁺, Cu²⁺, Fe³⁺, and Gd³⁺ ions by using UV-visible, fluorescence, colorimetric and excitation-emission matrix (EEM) spectroscopy techniques. The interaction of Cr³⁺, Cu²⁺, Fe³⁺, and Gd³⁺ can be observed by the absorption maxima shift, turn-off, colour changes, and EEM shifts. In addition, fluorescence limits of detection 17.66 × 10^-6 M, 6.44 × 10^-9 M, 28.87 × 10^-8 M, and 12.49 × 10^-6 M in wide linear ranges, low limits of quantifications, high values of Stern-Volmer constant, Job's plot and Benesi-Hildebrand plot justify the 1:1 association affinity with association constants of 1.46 × 10⁴ M^-1, 1.86 × 10⁷ M^-1, 2.69 × 10⁵ M^-1, 2.13 × 10⁴ M^-1 for AMHMPQ-metal ions (Cr³⁺, Cu²⁺, Fe³⁺, and Gd³⁺ ions), respectively. Paper- and mask-based kits are developed to explore the utility of the designed chemosensor. Additionally, AMHMPQ acts as a reusable sensor for two, seven, two, and zero cycles for Cr³⁺, Cu²⁺, Fe³⁺, and Gd³⁺ ions, respectively, when checked with EDTA.

A Rationally Designed Micellar Nanocarrier for the Delivery of Hydrophilic Methotrexate in Psoriasis Treatment

Methotrexate (MTX) is broadly applied in the clinic for the treatments of cancers and autoimmune diseases. Targeted delivery of MTX is attractive to improve its efficacy and reduce off-target toxicity. However, MTX encapsulation in nanoparticle is challenging due to its high water solubility. We rationally designed a well-defined telodendrimer (TD) nanocarrier based on MTX structure to sequester it in nanoparticles. Riboflavin (Rf) and positive charges groups were precisely conjugated on TD to form multivalent hydrogen bonds, π-π stacking and electrostatic interactions with MTX. A reverse micelle approach was developed to preset MTX and TD interactions in the core of micelles, which ensures the effective MTX loading upon dispersion into aqueous solution. As results, MTX loading capacity reaches over 20% (w/w) in the optimized nanocarrier with the particle size of 20-30 nm. The nanoformulations sustain the release of MTX in a controlled manner and exhibit excellent hemocompatibility. The in vitro cellular uptake of MTX was significantly improved by the nanoformulations. The potency of MTX nanoformulations is comparable to the free MTX in cytotoxicity. A psoriasis-like skin inflammation model was induced in mouse by imiquimod (IMQ) stimulation. MTX nanoformulations improved the psoriasis targeting and exhibited a superior long-lasting efficacy in reducing skin inflammation compared with the free MTX in psoriasis treatment.

Unraveling the effect of two different polar solvents on the excited-state intramolecular proton transfer of 4'-methoxy-3-hydroxyflavone fluorescent dye

The fluorescence properties of 4'-methoxy-3-hydroxyflavone (M3HF) dye in different solvents were investigated through experimental (Phys. Chem. Chem. Phys., 2018, 20, 7885) and theoretical (Org. Chem. Front., 2019, 6, 218) methods. However, the intermolecular hydrogen bonds between M3HF and solvents were ignored. In this work, we investigated the effect of methanol (MeOH) and N,N-dimethylformamide (DMF) solvents on the excited-state intramolecular proton transfer (ESIPT) of M3HF fluorescent dye. In excited state (S₁), the intramolecular hydrogen bonds are significantly strengthened, which can facilitate the ESIPT processes. The calculated absorption and fluorescence spectra agree well with the experimental date. The fluorescence spectra of M3HF and ESIPT tautomers (T^⁎) were found to be sensitive to the solvent polarity. Upon photo-excitation, the electron density of the M3HF molecular is redistributed, which can provide driving force for the ESIPT. The polar solvents MeOH (hydrogen bond donor) and DMF (hydrogen bond acceptor) can form different types of intermolecular hydrogen bonds with M3HF. The two different bonding modes of intermolecular hydrogen bonds are expected to weaken the intramolecular hydrogen bond of M3HF to varying degrees. The analysis of the potential energy curves indicate that the ESIPT processes of M3HF can be hindered by the intermolecular hydrogen bonds. The intermolecular hydrogen bond of M3HF-DMF complex is weaker than that of M3HF-MeOH complex, while the potential barrier of the ESIPT process in DMF solvent is higher than that of in the MeOH solvent. This is principally because, in DMF solvent, the hydroxyl group H₁ atom of M3HF can be captured by the O₃ atom of DMF and form O₃H₁ bond with O₃ atom in the intermediate process of ESIPT. There appears an energy barrier hopping point on the potential energy curve of M3HF in DMF solvent but does not appear in MeOH solvent.

Study on hydrogen bonding network in aqueous methanol solution by Raman spectroscopy

The Raman spectra of aqueous methanol solution with various concentrations were measured at room temperature and atmospheric pressure. We found that the CO stretching vibration mode of methanol showed a significant blue shift at V_m (V_m represents the volume fraction of methanol) >0.4, while the CH symmetric and asymmetric stretching vibration modes exhibited red shift under the same conditions. These results illustrate that the variation of hydrogen bond between methanol and water molecules lead to a phase transition of the methanol-water complex at V_m = 0.4. Furthermore, the red shift of the CH vibration mode indicates that there is no hydrogen bond formed between the CH₃ group of methanol and water molecules. In addition, we found that the frequency shift of C-H is affected by the hydrogen bond C-O…H-O formed between methanol and water molecules, and the corresponding theoretical discussion is given. Finally, the phase transition process of methanol-water complex in methanol-water binary solution was given by theoretical analysis.

Regular tuning of the ESIPT reaction of 3-hydroxychromone-based derivatives by substitution of functional groups

The electron-withdrawing ability of an atom and length of substitution groups would affect the ESIPT reaction and photophysical properties of 3-hydroxychromone-based derivatives.