Multi-functional ratiometric detection based on dual-emitting N-doped carbon dots

Ratiometric fluorescence probes based on multi-emission carbon dots improve accuracy and sensitivity on detecting various environment issues. Herein, a novel dual-emitting N-doped carbon dots (N-CDs) was synthesized from citric acid and urea via a solvothermal method in N,N-dimethylformamide (DMF). The blue and orange emissions of N-CDs in water were modulated, and pure white light-emitting with Commission Internationale de L'Eclairage (CIE) coordinates of (0.33, 0.33) was achieved. The two PL centers behaved differently for Fe3+, Cu2+ and Ag+ ions, with the limit of detection (LOD) of ppm as fluorescence probes. Additionally, N-CDs displayed unique solvatochromism phenomenon. A new green emission appeared in organic solvents and gradually quenched with the increase of solvent polarity. The ratiometric PL displayed an excellent linear response for detecting water, and the LOD was between 0.003 % and 0.3 % in DMF, ethanol, isopropanol and N-methylpyrrolidone. Furthermore, N-CDs exhibited pH-sensitive response in the range of 4.0-7.0 and temperature-dependent response during heating-cooling cycles between 15 and 70 °C. A simple, efficient and reliable multi-functional ratiometric probe for detecting metal ions, water content, pH and temperature simultaneously was realized. However, there is a need for future application research to overcome the limitation imposed by the excitation wavelength of 330 nm.

Solvatochromism in Mixtures of Hydrogen Bond Acceptor (HBA) Solvents with Water

The UV/Vis absorption energies νmax of Reichardt's dye B30 with respect to ET(30) and 4-nitroaniline (NA) are investigated as a function of the solvent composition Nav,z. in co-solvent/water mixtures. Nav,z. is the average molar concentration of the solvent mixture at a given solvent fraction z. The z can be the mole, the volume or the mass fraction. The co-solvents considered were acetonitrile, acetone, tetrahydrofuran, pyridine, piperidine and 2-(diethylamino)-ethanol. Acetone and acetonitrile can be expected to slightly enhance the water structure at low co-solvent concentrations. This interpretation is supported by the analysis of the refractive index as a function of the solvent composition. In general, it can be stated that the structural complexity of the binary solvent mixtures is mainly responsible for the evolution of the absorption energies ET(30) or νmax(NA) as a function of the mixture composition. In particular, the endothermic solvation of NA in co-solvent/water mixtures and its effect on the νmax(NA) is highlighted.

Synthesis of 1,3-Dibromopyrene as Precursor of 1-, 3-, 6-, and 8-Substituted Long-Axially Symmetric Pyrene Derivatives

Pyrene derivatives bearing substituents at positions 1, 3, 6, and 8 find numerous applications, as exemplified by their use in lasers, sensors, and bioimaging probes. However, these derivatives typically have point-symmetric or short-axially symmetric structures, whereas long-axially symmetric derivatives remain underexplored because of the difficulty in obtaining their precursor, 1,3-dibromopyrene. To address this problem, we herein synthesized 1,3-dibromopyrene from 1-methoxypyrene in an overall yield (71 % over four steps) considerably exceeding those of existing methods. 1,3-Dibromopyrene was converted into 13OPA, a long-axially symmetric pyrene dye with electron-donor (alkoxy) groups at positions 1 and 3 and electron-acceptor (formyl) groups at positions 6 and 8. 13OPA exhibited photophysical properties distinct from those of its point-symmetric and short-axially symmetric isomers, featuring a broad and strongly redshifted absorption, strong fluorescence with reduced sensitivity to protic solvents, and small dipole moment change upon photoexcitation. The derivatization of 13OPA into a Schiff base and its functionalization via Lewis acid-base pairing were also demonstrated. Thus, our work expands the design scope of pyrene-based molecules, particularly those used as emitters.

Studies About the Effect of Halogenated Solvents on the Fluorescence Properties of 9-Aryl-Substituted Isoquinolinium Derivatives - A Case Study

It was demonstrated that 9-aryl-substituted isoquinolinium derivatives have significantly increased fluorescence quantum yields in halogenated solvents, mostly pronounced in chloroalkanes, which appears to be specific for this type of solvents. Further analysis with selected halogenated solvents revealed that the type and number of halogen substituents and the dielectric constant of the solvent have a distinct impact on the emission quantum yield. The solvent effect is explained by a solvation of the charge shift (CS) state by attractive halogen-π interactions (halogen bond), which impedes the torsional relaxation of the excited state.

Solvatochromic and Proton-Responsive characteristics of Bi-1,3,4-Oxadiazole derivatives with symmetric dimethylamino substitution

D-A molecules find extensive use in intelligent stimulus-response systems due to their exceptional attributes, including high sensitivity, rapid response, wide compatibility, and structural adaptability. The strength of Intramolecular Charge Transfer (ICT) plays a pivotal role in determining the performance of these devices. To enhance the ICT strength and explore new applications for D-A molecules, we meticulously designed a pair of symmetric dimethylamino-substituted bi-1,3,4-oxadiazole derivatives (DMAOXD and DMAOXDBEN). These symmetric D-A-A-D molecules, with strong electron donor terminals, displayed a modest redshift of less than 25 nm in the UV-vis absorption spectra. However, there was a significant redshift in the emission spectra (140 nm for DMAOXD and 170 nm for DMAOXDBEN) when transitioning from cyclohexane to dimethyl sulfoxide, indicating a pronounced ICT characteristic. Theoretical calculations support the idea that the dimethylaminophenyl unit serves as an electron donor in both DMAOXD and DMAOXDBEN, while the 1,3,4-oxadiazole and central benzene ring act as acceptors. The pronounced ICT characteristic observed in DMAOXD and DMAOXDBEN can be attributed to long-distance electron transfer. Additionally, it's noteworthy that the emission of DMAOXD and DMAOXDBEN solution samples can be quenched by adding trifluoroacetic acid (TFA) and restored by the addition of triethylamine (TEA). Inspired by this, a pattern created with ink samples containing DMAOXD and DMAOXDBEN can be concealed through fumigation with TFA and subsequently revealed by treating them with TEA, suggesting their potential use in data encryption.

Unique structural effect on the fluorosolvatochromism and dual fluorescence emission of D-π-A+ cationic chromophores with furyl bridge. An approach to white light emitters

Photophysical behavior of two D - π - A+ cationic compounds with the same furyl bridge and nicotinamidine group as an electron acceptor moiety and two electron donating groups, namely methoxy (I) and N,N-dimethylamino (II) groups was examined using steady-state and time-resolved techniques in variety of solvents. Time-dependent density functional theory (TDDFT) calculations were performed in some representative solvents and compared with the experimental results. Steady state and time-resolved studies in different solvents reveal that fluorescence emission of (I) is ascribed to an emission from an excited state (ICT) with higher dipole moment than the ground state while the emission of (II) is a dual emission from a state with high charge transfer nature (ICT) in addition to the locally excited state (LE). The fluorescence emission spectra of (II) were found to depend on the excitation wavelength and an increase in the excitation wavelength led to the formation of a longer wavelength emission band with lower quantum yield. It has also been found that the fluorescence excitation spectra were dependent on the emission wavelength. The effect of solvent on the nature of dual emission was examined. Correlation of the photophysical properties of the excited states of (I) and (II) with the solvent polarity, ε, reveals the charge transfer nature of (I) and the long wavelength emission band of (II), while their correlation with the solvent polarity parameter (ETN) shows two different trends when the solvents are divided to aprotic and protic solvents. For precise investigation of the impact of each solvent parameter on each photophysical property, Catalán's and Laurence's four parametric linear solvation energy relationships were studied. We have found that the non-specific interactions of the solvent are primarily responsible for controlling the photophysical properties, as demonstrated by Catalán's and Laurence's treatments. DFT and TDDFT calculations were used to anticipate the dipole moments in the ground and excited states and geometry of both states.

Synthesis, Solvatochromism and Estimation of Ground and Excited State Dipole Moments of Silylated Benzothiazole Dyes

Dyes derived from benzothiazoles are an important class of heterocycles which have remarkable photophysical properties. New photoluminescent 2-phenylbenzothiazole derivatives containing different functional groups were synthesized in high yields and used for silylated derivatives synthesis. The new photoactive compounds were fully characterized and their photophysical properties were investigated. The absorption and fluorescence spectra of the benzothiazoles and their silylated derivatives were evaluated in a series of organic solvents. The results showed that the benzothiazoles present absorption in the ultraviolet range and emission in the blue region with moderate quantum yields and large Stokes shift. The solvatochromism of these compounds was investigated by using Lippert and ET(30) Dimroth-Reichardt empirical solvent polarity scales. The dipole moments obtained by Bakshiev and Kawaski-Chamma-Viallet equations revealed that the excited states were more polar than the ground states.

Design and Photonics of Merocyanine Dyes

Merocyanines, thanks to their easily adjustable electronic structure, appear to be the most versatile and promising functional dyes. Their D-π-A framework offers ample opportunities for custom design through variations in both donor/acceptor end-groups and the π-conjugated polymethine chain, and leads to a broad range of practical properties, including noticeable solvatochromism, high polarizability/hyperpolarizabilities, and the ability to sensitize various physicochemical processes. Accordingly, merocyanines are applied and extensively studied in various fields, such as light-converting materials for optoelectronics, nonlinear optics, optical storage, solar cells, fluorescent probes, and antitumor agents in photodynamic therapy. This review encompasses both classical and novel more important publications on the structure-property relationships in merocyanines, with particular emphasis on the results by A.  I. Kiprianov and his followers in Institute of Organic Chemistry in Kyiv, Ukraine.

Luminescent Properties of β-(hydroxyaryl)-butenolides and Fluorescence Quenching in Water

This work describes the luminescent properties of the new compound β-(hydroxyaryl)-butenolides recently discovered. The compounds were subjected to UV-Vis absorption and fluorescence analyzes when diluted in different solvents. Through the results, it was possible to observe that the β-hydroxyarylutenolides have two absorption bands, one at 289-291 nm and the other with higher intensity at 328-354 nm. The emission band between 385-422 nm is observed under excitation at 324-327 nm. The compounds showed solvatochromism as a function of the analyzed solvent. In water, fluorescence quenching of all compounds occurs. Therefore, studies with compound containing the methylenedioxy group attached in phenyl ring were carried at different concentrations of water in DMSO. The decrease in the fluorescence intensity of this compound is linearly proportional to the increase in the amount of water in the DMSO, with a minimum detection volume of 0.028%. Quantum yields of three compounds were evaluated in different solvents, showing that the relationship between the structure of the compound and the solvent is essential for a high value. The fluorescence quantum yield was also measured by Thermal Lens Spectroscopy (TLS) using DMSO as the solvent, confirming the high value for the analyzed samples. Despite being preliminary, the studies revealed that these compounds have luminescent properties that could be applied in the development of chemical sensors for detecting water in DMSO.

Exciplex luminescence of difluoroboron meta- and para-Nitrodibenzoylmethanates in solutions and polymer matrix

Molecular systems with intense exciplex luminescence are promising for the creation of OLEDs and light-transforming materials. The luminescent properties of difluoroboron meta- and para-nitrodibenzoylmethanates (1 and 2) were studied by the steady-state and time-resolved luminescence spectroscopy methods and quantum chemistry simulation. The influence of the position of the nitro group on the spectral properties of the complexes was revealed. The nitro group withdraws the electron density upon photoexcitation in the para-isomer in contrast to the meta-isomer. The bond order of the CC bond between the phenyl and chelate rings decreases in the excited relaxed state of 2, which leads to rotation of the substituent and a low quantum yield. Formation of exciplexes of 1 and 2 in benzene, toluene and polystyrene was found. The formation of the ground and excited-state electron donor-acceptor complexes based on difluoroboron nitrodibenzoylmethanates and benzene was revealed. The TDDFT method showed that the complexes are characterized by the presence of short contacts between the fluorine atom of the dye molecule and the hydrogen atoms of the aromatic hydrocarbon molecule. These contacts shorten during the transition from the ground state to the excited state. Polymeric compositions based on polystyrene (PS) and poly(methyl methacrylate) (PMMA) doped with 1 and 2 were obtained. The luminescence intensity of dyes 1 and 2 increases during the transition from PMMA to PS. A significant bathochromic shift of the luminescence maximum of 1 is observed, which is associated with the formation of exciplexes and triplexes. The obtained polymeric materials are promising for the development of optical smart materials.

Solvent-Dependent Photoluminescence Emission and Colloidal Stability of Carbon Quantum dots from Watermelon Peels

Waste peels are considered an environmental burden and typically disposed in landfills. The aim of this study was to investigate the effects of various solvents on the luminescence properties of carbon quantum dots (CQDs). Watermelon peels were recycled and reuse as precursors for the synthesis of biomass CQDs via a green carbonization method. The colloidal stability, surface charge, and particle size were characterized using zeta potential and dynamic light scattering (DLS). DLS revealed that the size of the CQDs was approximately 5.80 ± 0.4 nm to 9.74 ± 0.8 nm. The high-resolution transmission electron microscopy (HRTEM) results demonstrated a correlation with the DLS results. The optical properties were characterized by photoluminescence (PL) and UV-Visible (UV-Vis) spectroscopy. PL measurements at different excitation wavelengths revealed that the CQDs emissions were influenced by the polarity of the solvents. Meanwhile, the Fourier transform infra-red (FTIR) results showed the presence of oxygen-containing groups on the surface of the CQDs. These results deepen our understanding of the solvent-dependent behavior and colloidal stability of the CQDs.

Direct evaluation of polarity of the ligand binding pocket in retinoid X receptor using a fluorescent solvatochromic agonist

High selectivity of small-molecule drug candidates for their target molecule is important to minimize potential side effects. One factor that contributes to the selectivity is the internal polarity of the ligand-binding pocket (LBP) in the target molecule, but this is difficult to measure. Here, we first confirmed that the retinoid X receptor (RXR) agonist 6-(ethyl(1-isobutyl-2-oxo-4-(trifluoromethyl)-1,2-dihydroquinolin-7-yl)amino)nicotinic acid (NEt-iFQ, 1) exhibits fluorescence solvatochromism, i.e., its Stokes shift depends on the polarity of the solvent, and then we utilized this property to directly measure the internal polarity of the RXRα-LBP. The Stokes shift of 1 when bound to the RXRα-LBP corresponded to that of 1 in chloroform solution. This finding is expected to be helpful for designing RXR-selective ligands. A similar approach should be appliable to evaluate the internal polarity of the LBPs of other receptors.

The Shift in the Behavior of Methylene Blue Toward the Sensitivity of Medium: Solvatochromism, Solvent Parameters, Regression Analysis and Investigation of Cosolvent on the Acidity Constants

The influence solvents from different polarity and hydrogen bonding ability on electronic absorption spectrum of Methylene blue was investigated. The visible absorption spectra were recorded in eleven neat solvents in the range 400-700 nm. Methylene blue has two absorption maxima, the first band assigned to n-π* from amino groups and the second band assigned to weakly forbidden n-π* transition with charge transfer. The charge transfer band of Methylene blue showed red shift with increasing the relative permittivity of neat solvents. The red shift in wavelength(λmax) for the charge transfer band of Methylene blue was observed when proceeding from dioxane (λmax = 650 nm) into methanol (λmax = 655 nm) into cyclohexanone (λmax = 660 nm) into dimethylsulfoxide (λmax = 665 nm) as well as water (λmax = 665 nm), this shift not agree with the polarity of solvents but due to combination of several parameters. The absorption of charge transfer band in methanol and ethanol as hydrogen bonding donating solvents (HBD) showed the highest intensity than the absorption band in dimethylsulfoxide and dimethylformamide as hydrogen bonding accepting solvents (HBA) due to non-electrostatic interaction between the amino groups and solvents. The charge transfer band in neat solvents were correlated with several parameters using linear solvation energy relationships. The results showed that electrostatic interactions of the solvents play an important role in the shift of absorption maxima of Methylene blue in neat solvents. The acidity constants (pKa) of Methylene blue were estimated by using absorbance measurements in different media. The acidity constants (pKa) values of Methylene blue were affected by cosolvent, which the pKa values increasing in the order propanol < methanol < dioxane, this order not agreement with increasing the relative permittivity of the medium.

Exploration of Coumarin Derivative: Experimental and Computational Modeling for Dipole Moment Estimation and Thermal Sensing Application

The Optical properties of the FBTC (1-((4-((5-chlorobenzo[d]oxazol-2-ylthio)methyl)-1H-1,2,3-triazol-1-yl)methyl)-3H-benzo[f]chromen-3-one) molecule were studied experimentally and theoretically. The spectra of absorption and fluorescence were recorded in various solvents to explore their Solvatochromic behavior and dipole moment at room temperature. To determine the ground and excited state of dipole moment experimentally and theoretically, we employed different Solvatochromic techniques, including microscopic solvent polarity functions developed by Lippert, Bakhshiev, Kawaski-Chamma-Viallet, and Reichardt's, as well as density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The stability of the excited state dipole moment in FBTC is higher. Using prime functional, FBTC was optimized in its ground state, and its HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital), energies were estimated. These values were then compared with those obtained through cyclic voltammetry. Based on the HOMO and LUMO values given, we calculated the global reactivity parameter and energy gap, which was found to be low at 3.77 eV. This study also includes an estimation of electron absorption energies and oscillator strength. Natural population analysis (NPA), Milliken atomic charge, and molecular electrostatic potential (MESP) map are correlated. In addition, FBTC exhibited exceptional physiological temperature sensing behaviour from 20 °C -65 °C with high relative sensitivity and firm stability. Hence these results confirm that FBTC is a potential candidate for photonic devices and it's also applicable in optical temperature sensing.

Regulating donor-acceptor system toward highly efficient dual-state emission for sensitive response of nitroaromatic explosives

Dual-state emission luminogens (DSEgens) as fluorophores emit efficiently in solution and solid forms have gained increasing concern in the field of chemical sensing. Recent efforts by our group led to the identification of DSEgens as an easy-to-visualize nitroaromatic explosives (NAEs) detection platform. However, none of the previously studied NAEs probes show effective improvement in sensitivity. Here, we designed a series of benzoxazole-based DSEgens through multiple strategies driven by theoretical calculations, revealing their improved detecting performance on NAEs. Compounds 4a-4e exhibit thermal- and photo-stability, large Stokes shift as well as sensitivity solvatochromism (except for 4a and 4b). A subtle balance between rigid conjugation and distorted conformation endows these D-A type fluorophores 4a-4e with DSE properties. Furthermore, 4d and 4e show aggregation-induced emission phenomenon caused by distorted molecular conformation and restricted intramolecular rotation. Interestingly, DSEgen 4e displays anti-interference and sensitivity towards NAEs with a detection limit of 10^-8 M. It can be applied for expedient and distinct visual identification of NAEs not only in solution but also on filter paper and film, supporting this new DSEgen as reliable NAEs chemoprobe.

Solute - solvent repulsion effects on the absorption spectra of anthracene in n-hexane investigated under high pressure

The band positions in the UV-VIS absorption spectra of compressed solution of anthracene in n-hexane significantly depend not only on the dispersive but also on the repulsive solute-solvent interactions, what has so far been omitted. Their strength is determined not only by the solvent polarity but also by Onsager cavity radius changing with pressure. The results obtained for anthracene show that repulsive interactions should be included in the interpretation of barochromic and solvatochromic results of aromatic compounds. We show that the barochromic studies in the liquid solvent can be an alternative to solvatochromic studies, e.g. to determine the polarizability of organic molecules in the electronic excited state. The pressure-induced polarity change in n-hexane exceeds that induced by the exchange of n-alkane solvents between n-pentane and n-hexadecane.

A novel fluorophore with multichromic effect and its application in LED light for the volatile vapor detection

Successfully designed and synthesized diaryl maleimide DAM, and their structure was confirmed by mass spectroscopy and NMR techniques. They investigated their photophysical properties, such as solvatochromic and the aggregation effect of AIE/ACQ on the water/DMF ratio, and other studies of solid-state mechanofluorochromic, such as grinding, exposure to solvent fumes, hydrostatic pressures, and vapochromic. Interestingly, the solvent methods gave very similar results in both the dissolving phase and the vapor phase. For the solution state from hexane to CH₂Cl_2, a greenish-yellow to orange emission was observed. From hexane to dichloromethane, naked-eye colorimetric changes (from yellow to orange in a solid state) were observed. Very interesting results were obtained, the yellow solid was ground for 1 min, it turned into an orange color, and its wavelengths were red-shifted in both absorption and emission. Subsequently, the ground sample was exposed to diethyl ether vapors, which returned to the original green-yellow emission and absorption. The applied hydrostatic pressures (0-4Mpa) in DAM gave excellent red-shifted emission, and then their hydrostatic pressures were increased to 4-16 Mpa, and the red-shifted emission gradually decreased.

Solvatochromism and solution π-stacking of N-(4-pyridyl)-1,8-naphthalimide and its corresponding triruthenium coordination complex

Due to the scarcity of spectroscopic studies on metal-coordinated naphthalimides, and aiming to investigate fundamental spectroscopic aspects, we have described here the aggregates of N-(4-pyridyl)-1,8-naphthalimide (NI-py) in solution as well as solvatochromism displayed by it and by the coordination compounds [Ru₃O(CH₃COO)₆(NI-py)₃]ⁿ, n = +1 or 0. Based both on theoretical calculations and luminescence spectra, we demonstrated that in aqueous media, the NI-py π-stacking is thermodynamically favored, suggesting a preferable conformation where the pyridine and naphthalene moieties of two NI-py molecules are parallel to each other, but are not co-planar within an individual molecule, due to steric hindrance. The NI-py ππ* band displayed positive solvatochromism, to which the major contribution was the Catalan's SP parameter (solvent polarizability). This observation is fully consistent with the extended π-electron cloud of the NI-py naphthalene ring. However, a secondary contribution of the SA (solvent acidity) was also observed, owing to the electron pairs available at the N-heteroatom of the pyridine rings and at the carbonyl-group oxygen atoms. Finally, the multiparametric solvent effect analysis indicated that the electronic coupling between coordinated NI-py and the metallic core is modulated by the charge of the [Ru₃O(CH₃COO)₆] core, being higher for the reduced species [Ru₃O(CH₃COO)₆(NI-py)]⁰. In addition, in this reduced species, there is no overlap between NI-py ππ* and the [Ru₃O(CH₃COO)₆] charge transfer (CT) transitions, leading to the observation of the dependence of the CT energy with the SdP parameter (solvent dipolarity) since the CT transition implies in a charge-separation state.

Synthesis, structure and mechanofluorochromic properties of phenothiazine-S-oxide and phenothiazine-S,S-dioxide derivatives

In this work, two novel luminescent molecules containing distorted phenothiazine-S-oxide and phenothiazine-S,S dioxide skeletons were synthesized by oxidation reactions using different oxidants (m-chloroperoxybenzoic acid, acetic acid /hydrogen peroxide). The target compounds were all confirmed by ¹H NMR, ¹³C NMR and EI-MS. Combined with the results of UV-vis absorption spectra and fluorescence emission spectra, we found that the different oxidation states of S-atom, from sulfide (+2) to sulfoxide (+4) and sulfone (+6), led to the blue, yellow-green and yellowish fluorescence of these compounds in the solid states. Subsequent studies showed that the molecule containing the phenothiazine-S-oxide skeleton exhibited obvious solvatochromism, and the increase of solvent polarity induced a red-shift in the emission wavelength. Moreover, this molecule also exhibited a rare self-recovery mechanochromatic behavior. In addition, these properties were further confirmed by theoretical calculations and X-ray single-crystal diffraction analysis.

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 novel spectroscopic technique for studying metal-organic frameworks based on Mie scattering

Metal-organic frameworks (MOFs) are promising candidates for a wide range of applications, and spectroscopic techniques are important tools for analyzing their structures and properties. Here, we propose a novel and general scattering spectroscopic approach to study various MOFs such as zeolitic imidazolate frameworks (ZIF-67 and ZIF-8), HKUST-1, Co-based MOF (Co-MOF), and Ni-based MOF (Ni-MOF) based on their inherent Mie scattering properties. We show that by using a dark-field microscope, the inherent scattering colors and spectra can be obtained, which are mainly from the high-order magnetic and electric resonant modes. The scattering capacities are dependent on the chemical structures for producing polarized charges and internal circular displacement currents. Additionally, all the MOFs are capable of responding to solvent guests due to their high porosity, and the scattering peaks are in a linear correlation with solvent refractive indices, displaying scattering solvatochromic behaviors. Our results open up a powerful and universal avenue for visually studying the host-guest interactions in MOFs.

Determination of water content in dimethyl sulfoxide/N,N-dimethyl formamide and methanol content in ethanol by solvatochromism of azo dye, 2-(tert-butyl)-4-methoxy-6-(naphthalen-1-yldiazenyl) phenol

A novel 2-(tert-butyl)-4-methoxy-6-(naphthalen-1-yldiazenyl)phenol (NAP) was synthesized by coupling reaction of 2-tert-butyl-4-methoxyphenol with diazotized naphthylamine as diazo component. The azo dye was characterized by NMR, FT-IR and UV-vis spectroscopic techniques. The visible spectrum of NAP was recorded in different solvents and at different pHs. NAP exhibited a large wavelength shift with increasing solvent polarity, showing significant color change over a wide range in different solvents. The determination of water content in organic solvents miscible with water such as dimethyl sulfoxide (DMSO)/N,N-dimethyl formamide (DMF) and methanol content in ethanol, which is also a common mixture were investigated with NAP which is azo dye. The present reported solvatochromic compound for the determination of water content in DMSO/DMF and methanol content in ethanol showed a fairly wide linear range compared to some previously reported solvatochromic compounds in the literature. In addition, the solvatochromism of NAP allows the determination of methanol content in ethanol, which has caused many deaths, with a fast, cheap and easy method.

Empirical Hydrogen Bonding Donor (HBD) Parameters of Organic Solvents Using Solvatochromic Probes - A Critical Evaluation

The solvatochromicity of established solvatochromic UV/Vis probes, which appear to be sensitive to the so-called hydrogen bond donor (HBD) property of the solvent, is analysed using the hydroxyl group density of alcoholic solvents D_HBD as a physical parameter in comparison to the pKa, the chemical benchmark for acidity. Reichardt's dye B30, Kosowers Z-indicator 1-ethyl-4-(methoxycarbonyl) pyridinium iodide (K), Kamlet-Tafts α, Dragos S parameter, Catalans SA scale, the cis-dicyano-bis (1,10-phenanthroline) iron II complex (Schilt's Ferrocyphen dye, Fe) and Gutmann's acceptor number (AN) have been investigated. The observed dependencies of the empirical polarity parameters as a function of D_HBD for several alcoholic solvent families requires a ompletely new physicochemical understanding of these established HBD parameters. Only the AN scale (or Fe) is able to bridge the gap between global polarity and acidity, provided the values are interpreted correctly and applied accordingly.

Insight into the release mechanisms of diflunisal and salicylic acid from poly(vinyl alcohol). The role of hydrogen bonding interactions

Diflunisal (5-(2,4-Difluorophenyl)salicylic acid, DIF), salicylic acid (SAL) derivative, which, on the one hand, is active pharmaceutical ingredient, on the other hand, belongs to the compounds exhibiting excited-state intramolecular proton transfer (ESIPT) behaviour was used to study the drug interactions with poly(vinyl alcohol) (PVA) matrix. For clarifying the nature and mechanisms of the drug-matrix interactions the salicylic acid (SAL) molecule was selected as the model active ESIPT compound, whose physicochemical properties in different media are well understood. The solute-solvent interactions (non-specific (dipole-dipole) versus specific (hydrogen bonding)) of DIF and SAL with different neat solvents were investigated using the steady-state spectroscopic technique. The solvent effect on spectral behaviours of DIF and SAL was analyzed based on the parametric solvent scales. In order to identify functional groups in the PVA matrices, determine the structure present in the studied molecule-PVA system and thus obtain information about the potential interactions between PVA and the studied molecules, the Raman spectra of pure PVA, SAL-PVA and DIF-PVA systems were measured. It has been shown that the molecular structure of the active substance entrapped in the polymer matrix affects the structure of the polymer, i.e., isotactic (SAL-PVA) versus syndiotactic (DIF-PVA) structure. The analysis of drug release kinetics revealed that the DIF is more strongly bound to PVA in comparison to SAL, which confirms conclusions drawn from the analysis of the Raman spectra i.e., the isotactic structure of SAL-PVA material results in a faster initial release process of weakly bound, located on the surface of the polymer SAL molecules.

Experimental and Theoretical Spectroscopic Investigation on Coumarin Based Derivatives for Non-Linear Optoelectronics Application

The spectral properties of MBTC (4-((4-((Benzo[d]oxazol-2-ylthio)methyl)-1H-1,2,3-triazol-1-yl)methyl)-7-methoxy-2H-chromen-2-one), CBTC (4-((4(((5Chlorobenzo[d]oxazol-2-yl)thio)methyl)-1H-1,2,3-triazol-1-yl)methyl)-2H-benzo[h]chromen-2-one) and TBTC (4-((4-((Benzo[d]oxazol-2-ylthio)methyl)-1H-1,2,3-triazol-1-yl)methyl)6(tertbutyl)2Hchromen-2-one) were studied in series of solvents with different polarity at room temperature to explore their solvatochromic effect and dipole moment. Stokes shift revealed a bathochromic shift with varying solvent polarity for all molecules which implies π-π^*transition. The ground state and excited state dipole moment of the molecules are calculated experimentally using salvatochromic methods like Lippert-Mataga, Bakhshiev, Kawaski-chamma-viallet, and Reichardt's microscopic solvent polarity functions and computationally by density functional theory (DFT) method. It is observed that the excited state dipole moment is higher than the ground state so synthesized molecules are more polar in the excited state than in the ground state. Using the DFT method HOMO and LUMO energy values were obtained and compared with values obtained by the cyclic voltammetry. Using the values of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) we have estimated energy gap, chemical hardness (ɳ), chemical softness (s), ionization potential (IP), electron affinity (EA), electronegativity (χ), electrophilicity (ω), and chemical potential (μ) of the molecules were estimated. The energy gap of MBTC, CBTC, and TBTC were found to be low, that is 3.861 eV, 3.822 eV, and 3.801 eV respectively, this indicates molecules are more reactive and it has the easiest π-π^* transition. Further electrophilic and nucleophilic sites were figured out using molecular electrostatic potential (MESP) which is useful in photochemical reactions. Hence the quantum chemical calculation and spectroscopic properties of the molecules can give a better understanding of their use in an optoelectronic device.

Effect of Hydroxyl Group on Photo-Physical Properties and Dipole Moments of Fluorescent Dyes: An Experimental and Computational Approach

In this work, structurally similar, (E)-N'-(2-hydroxybenzylidene)-3,5-di-tert-butyl-2-hydroxybenzohydrazide (A) and (E)-N'-(2-4-dihydroxybenzylidene)-3,5-di-tert-butyl-2-hydroxybenzohydrazide (A-OH) dyes dissolved in general solvents have been studied to explore photo-physical properties, employing solvatochromic shift method, thereby determining their dipole moments in the ground (μ_g) and excited (μ_e) states. The molecule A shows a bathochromic shift of fluorescence emission maxima in aprotic solvents whereas a hypsochromic shift in protic solvents. Interestingly, A-OH follows a hypsochromic shift in both protic and aprotic solvents with increasing solvent polarity. The effect of hydroxyl substituent on UV-Visible absorption, fluorescence emission, and dipole moment of the titled organic molecules was explained. Theoretical methods such as Bilot-Kawski method for determination of μ_g and μ_e and Bakshiev, Kawski-Chamma-Viallet, Lippert-Mataga equations for μ_e, and Reichardt method for the difference between μ_g and μ_e were employed. It is observed that μ_e is higher than that of μ_g for both the molecules, and interestingly, upon substituting an additional hydroxyl group the value of μ_g has increased while μ_e is decreased. The DFT calculations have been performed to support experimental results by employing DFT/B3LYP/6-311G + (d) and TD-DFT/B3LYP/6-311G + (d) method using Gaussian09 software. The electrophilic and nucleophilic sites on the molecules were studied with the help of MEP. The NBO analysis results show that the interaction N24 (σ) → C22-O23 (π*) is found to be stronger in both the molecules with energy 68.90 kJ/mol and the effect of hydroxyl group is also discussed on the basis of HOMO and LUMO.

4-(1H-Imidazo[4,5-f][1,10]phenanthrolin-2-yl)benzaldehyde as a probe in pure solvents: Solvatochromism, electric dipole moment and pH influence

The spectral properties of 4-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)benzaldehyde (1) in eleven organic solvents of different polarity have been studied. In order to determine the contributions of specific and non-specific interactions between the considered compound and the solvents, the solvatochromic Lippert-Mataga, McRae, Bakhshiev methods have been applied. The compound demonstrates positive solvatochromism. The dipole moment of the excited state of 1 obtained using the Reichardt method is equal to 10.56/7.08 D for trans- and cis-conformers, respectively, and agrees well with the theoretically calculated value. The influence of the polarizability of 1 on changes in the dipole moments has been analyzed using the Bilot-Kawski method. The multiple linear regression analysis in the framework of the Kamlet-Abboud-Taft and Catalán models has highlighted that the main properties which determine the Stokes shift of 1 are the acidity and dipolarity of the solvent. The variation of pH by additions of acid or base to solution 1 leads to significant changes in absorption and fluorescence spectra, therefore, 1 can be of interest as a solvatochromic probe, being sensitive to acidic/base properties of the environment. It has also been found out that the anion form of 1 is present in the DMSO solution. An addition of N,N-dimethylcyclohexylamine intensifies the dissociation of the considered compound in the DMSO solution and suppresses the fluorescence at a large amine excess.

The negative solvatochromism of Reichardt`s dye B30 - a complementary study

The UV/Vis spectra of a hypothetical negative solvatochromic dye in a solvent are theoretically calculated assuming the classical damped harmonic oscillator model and the Lorentz‐Lorenz relation. For the simulations, the oscillator strength of the solvent was varied, while for the solute all oscillator parameters were kept constant. As a result, a simple change of the oscillator strength of the solute can explain the redshift and intensity increase of the UV/Vis band of the solute. Simulated results are compared with measured UV/Vis spectroscopic data of 2,6‐diphenyl‐4‐(2,4,6‐triphenylpyridinium‐1‐yl) phenolate B30 (Reichardt‘s dye) Significant correlations of the absorption energy (1/λ_max) with the molar absorption coefficient ϵ as function of solvent polarity are demonstrated for several derivatives of B30. The approach presented is only applicable to negative solvatochromism. Therefore, while the approach is vital to fully understand solvatochromism, it needs to be complemented by other approaches, e. g., to describe the changes of the chemical interactions based on the nature of the solvent, to explain all its various aspects.

Improvement of theoretical UV-Vis spectra calculations by empirical solvatochromic parameters: Case study of 5-arylazo-3-cyano-1-ethyl-6-hydroxy-4-methyl-2-pyridones

In order to improve the performance of theoretical UV-Vis spectra predictions, a theoretical and experimental study of solvatochromic properties of ten azo pyridone dyes has been performed. For quantitative estimation of intermolecular solvent-solute interactions, a concept of the linear solvation energy relationships has been applied using Kamlet-Taft and Catalán models. Theoretical UV-Vis spectra for all dyes have been calculated using four TD-DFT methods in nine different solvents with the aim to define the most reliable model. Finally, new polylinear equations for more accurate theoretical prediction of UV-Vis maxima are developed using empirical Kamlet-Taft and Catalán solvent parameters as additive corrections for specific and nonspecific solvent-solute interactions.

Solvent Effects on the Photophysical Properties of a Donor-acceptor Based Schiff Base

In this work, a donor-acceptor substituted aromatic system ((E)-N-((E)-3-(4 (dimethylamino)phenyl) allylidene)-4-(trifluoromethyl) benzenamine (DPATB) has been synthesized and its detailed photophysics of intramolecular charge transfer process have been explored on the basis of steady state absorption, fluorescence and time resolved spectroscopy in combination with density functional theory calculations. Large solvent dependency fluorescence spectral shift and the calculated large excited state dipole moment clearly indicate an efficient charge transfer occurring from the donor group to the acceptor moiety in the excited state. Effect on addition of acid and pH on steady state spectral properties further reveals excited state charge transfer character. Quantum chemical calculations were performed in order to study the conformation and polarity of DPATB at their ground as well as excited electronic states. The HOMO and LUMO molecular orbital pictures are obtained at DFT level using B3LYP functional and 6-311 + g(d,p) basis set which clearly support excited state intramolecular charge transfer process. The molecular electrostatic potential maps for the optimized ground state, donor twisted and acceptor twisted geometry shed insight on the electrostatic potential and charge distribution in a system which gives information about the reacting site of the probe and nature of the reaction. In this work, detailed photophysics of excited state intramolecular charge transfer process in donor-acceptor system (DPATB) was evaluated using steady state and time-resolved fluorescence spectroscopy in combination with density functional theory calculations. Large solvent dependency fluorescence spectral shift and the calculated large excited state dipole moment clearly indicate an efficient charge transfer occurring in DPATB. Molecular orbital pictures as obtained from DFT based computational analysis reveals a significant change in the distribution of electron density upon transition from HOMO to LUMO which confirms an ICT process occurring from the donor group to the acceptor moiety in the excited state.

BF2 group chelated AIE fluorescent probe for polarity mapping of lipid droplets in cells and in vivo

Lipid droplets (LDs), are multi-functional organelles with the storage of neutral lipids and proteins, participating in various of physiological processes. However, abnormal of LDs in morphology and numbers always lead to multiple diseases, including cancer, viral infection, obesity, inflammation. To better understand the physiological function of LDs in living cells, we designed two new fluorescent probes LDs-CA and LDs-BCA based on the triphenylamine and coumarin fluorophores to monitor LDs polarity and numbers variation in this work. The one-step strategy for the regulation of BF₂ group realized a gratifying in emission wavelengths from orange fluorescence of LDs-CA to the red fluorescence of LDs-BCA, surprisingly. The two novel probes showed strong positive solvatochromism effect in different solvents and exhibited the aggregation-induced emission (AIE) effect. Based on the above excellent optical properties, LDs-CA and LDs-BCA were applied for imaging of the LDs with high overlap coefficient when co-stained with commercial dyes, respectively. The probes of LDs-CA and LDs-BCA provided an intuitive method to visualize the dynamic changes of LDs in morphology, size, and numbers under nutritionalstimulation, affording a powerful tool for fluorescence visualization of LDs related biological processes. Notably, the near-infrared emissive probe LDs-BCA successfully imaged the gastric fat in living obese mouse, which may provide a new idea for medical diagnostics.

Coumarin-pyridine push-pull fluorophores: Synthesis and photophysical studies

A series of coumarin-pyridine-based push-pull fluorophores were prepared starting from 1,2,4-triazines by using direct C-H functionalization (S_N^H-reaction)-Diels-Alder-retro Diels-Alder domino reaction sequence. This efficient synthetic strategy allowed to obtain a series of 19 coumarin-pyridine fluorophores. Their photophysical properties were studied. While pyridine-substituted derivatives of 4-alkylcoumarins may be considered as alternative to coumarin dyes characterized by emission maxima mainly in a visible region with wavelengths of 402-415 nm, absorption in the UV range at 210-307 nm, and good photoluminescence quantum yields of 6-19%, all the derivatives of 4-phenylcoumarin did not exhibit any noticeable fluorescence. More detailed photophysical studies were carried out for two the most representative derivatives of 4-alkyl-coumarin-pyridines to demonstrate their positive solvatochromism, and the collected data were analyzed by using Lippert-Mataga equation, as well as Kosower and Dimroth/Reichardt scales. The obtained results demonstrate that the combining two chromophore systems, such as 2,5-diarylpyridine and coumarin ones, is promising in terms of improving the photophysical properties of the new coumarin-pyridine hybrid compounds.

Carbon dots with aggregation induced quenching effect and solvatochromism for the detection of H2O in organic solvents

In this work, a type of carbon dots (CDs) which can be well dispersed in organic solvents but precipitated in neutral aqueous solution was synthesized by using one-step hydrothermal heating treatment of o-phenylenediamine in H₂SO₄ solution. Although the emissions of the CDs in different organic solvents are excitation independent, solvatochromism is observed for the CDs since the emission wavelength is red shifted with an increase in the polarity of solvents. The optical properties suggest that the emission of the CDs is controlled by molecular state. According to aggregation induced quenching effect and solvatochromism caused by an increase in the solvent polarity with the content of H₂O increasing, detection methods of H₂O in the organic solvents were developed by using the CDs as a probe. The spiked H₂O in acetone and methanol could be recovered in the range of 98.2% to 101.7%, which indicates that the as-proposed method has a high potential for the determination of H₂O in organic solvents.

Solvatochromism and estimation of ground and excited state dipole moments of 6-aminoquinoline

The spectral behaviour of 6AQ was investigated using fluorescence spectroscopy in several polar and non-polar solvents. Both the absorption and fluorescence spectra displayed solvatochromism. The Stokes shift increased significantly with increasing solvent polarity and signifies a more polar excited state with possible change in the excited state (ES) geometry. The involvement of π→π^∗ transition was observed. The ground state (GS) and excited state (ES) dipole moments were determined by the solvatochromic shift method using Bilot-Kawaski, Lippert-Mataga, Kawski-Chamma-Viallet, and Reichardt equations. The experimental value of GS dipole moment matches closely with the theoretical value computed using DFT/B3LYP/6-311G(d,p). The ES dipole moment is higher than the GS dipole moment. Besides, the solvatochromic study reveals that the ES of 6AQ is more polarized than the GS due to intramolecular charge transfer (ICT), possibly aided by a change in the geometry of the molecule in the ES. The influence of the non-specific and specific interactions in the photophysical properties of the titled molecule was analyzed using the Catalan scale. The study shows that 6AQ has reasonable band-gap energy and good CIE chromaticity coordinate in the blue region close to the national television standard committee system (NTSC) for the ideal blue CIE coordinate. Therefore, future research into 6AQ as a source of light-emitting diodes and fluorescent sensors may have potential applications in the field of optoelectronics.

Polymer films doped with boron difluoride ortho-hydroxydibenzoylmethanates: Solvatochromism and solvent-induced exciplex formation

The spectral properties and solvatochromic behavior of boron difluoride ortho-hydroxydibenzoylmethanates in polymer matrices with different polarity (polystyrene, polyvinyl chloride, polycarbonate, poly(methyl methacrylate)) were investigated. The specific interaction of polystyrene, polycarbonate and poly(methyl methacrylate) with the investigated fluorophores was revealed. It was shown that the dyes are capable to form exciplexes with phenyl rings of polystyrene. The spectral properties of polystyrene films doped with boron difluoride ortho-hydroxydibenzoylmethanates, which were obtained by the pouring method, significantly depend on the solvent from which they were formed. This makes it possible to control the luminescence color of the films at the manufacturing stage.

Emission characteristics of carbon films in comparison with solvatochromic effects of carbon nanoparticles

Carbon nanoparticles (CNPs) are getting wide attention due to their fluorescence and low level of toxicity compared to other semiconducting photoluminescent materials. CNPs show strong 'solvatochromism', and the emission mechanism is still under discussion. Florescent carbon in the form of films would tremendously increase its potential for applications. In this work, we report for the first time the fluorescent emission characteristics of carbon films formed by aggregation of CNPs. Films of carbon were grown on glass substrates by using a novelCold Vapour Deposition System. We have performed a detailed comparative study of the emission spectra of film and CNPs (prepared using the microwave synthesis method) in various solvents. A qualitative model based on solvatochromism of CNPs is used to understand the emission pathways in the film.

The relationship between solvatochromic properties and in silico ADME parameters of new chloroethylnitrosourea derivatives with potential anticancer activity and their β-Cyclodextrin complexes

In view of the anticancer effect of nitrosoureas a set of four new N-(2-chloroethyl)-N-nitrosourea (CENU) derivatives was synthesized. An in silico absorption, distribution, metabolism, excretion and toxicity (ADME/Tox) prediction study revealed that the CENU derivatives satisfied all the required criteria for oral administration and introduced them as remarkable anticancer candidates in the central nervous system (CNS). A comparative solvatochromic study including the Kamlet-Taft, Catalán and Laurence models indicated that the solvatochromic behavior of the CENUs depended on both, unspecific and specific solvent-solute interactions. In detail, the solvatochromic effect of the solvent polarity on the absorption and emission maxima was significant for all CENUs, whereas the solvatochromic effect of the solvent's ability to donate or accept hydrogen bonds on the absorption and emission maxima was critically dependent on the electron density of the N'-aryl group. From the solvatochromic comparison method, excellent correlations (r ≥ 0.890) were obtained between the ADME parameters and the solvatochromic regression coefficients obtained by the Catalán model. As potential stabilizers, inclusion complexes of the investigated CENU derivatives with β-cyclodextrin (β-CD) were also explored. The spectrofluorimetric host-guest experiments included double-reciprocal Benesi-Hildebrand plots as well as the molar ratio and continuous variation plots (Job's plots), which established a 1:1 β-CD to CENU binding stoichiometry and relatively high affinities of β-CD for CENU derivatives.

Low temperature photoluminescence and solvatochromic studies of organic hybrid 4-methoxybenzylammonium chloride (4-MBACl)

We report temperature dependent photoluminescence (PL) and solvatochromic studies of 4-methoxybenzylammonium chloride (4-MBACl). The single crystals of 4-MBACl has been grown using slow solvent evaporation method under ambient conditions and have been confirmed using X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopic studies. Thermal stability of the material has been analyzed by thermogravimetric (TG) and differential thermal analyzes (DTA). 4-MBACl crystal is photoluminescent with emission wavelength maximum at 300 nm. Exciton-phonon coupling in 4-MBACl is realized from temperature dependent PL studies. Changes in full width at half maximum (FWHM) of PL emission peak, intensity maximum (I_max) and wavelength maximum (λ_max) have been analyzed systematically between temperatures 290-10 K in order to understand the photophysical phenomenon behind the emission. Optical band gap (E_g) of 4-MBACl has been varied using solvent polarity, proticity and solution concentration. Interactions like OH---O, OH---π and π-π stacking are explained to reason the notable shifts.

Complexes of carboxylato pillar[6]arene with Brooker-type merocyanines: Spectral properties, pKa shifts and the design of a displacement assay for trimethyl lysine

The supramolecular complexes of three strongly solvatochromic dyes, Brooker's merocyanine (M1) and its two derivatives (M2, M3) with carboxylato pillar[6]arene (WP6) were studied in aqueous solutions. The dye-WP6 mixtures were described in terms of four equilibrium reactions: the acidic dissociations of the pyridinium phenols into the zwitterionic phenolates, the acidic dissociations of the complexed phenols, the bindings of the phenol form dyes to WP6 and the bindings of the phenolates to WP6. The equilibrium constants were determined by an analysis of the absorption spectra. It was found that the acidity of the phenol form merocyanines were largely reduced on complexation, pK_a shifts of 1.1-1.6 units were observed. In neutral solutions, the complexes of the phenol forms of M1 and M2 were dominant, in contrast to the more acidic M3 (a dibromo derivative), of which the phenolate complex was more stable. Comparing the spectral properties, the binding constants and the pK_a-s of the dye-WP6 complexes, the complex M3⋅WP6 was chosen to be tested as a displacement assay. It was demonstrated that this complex functioned as a colorimetric indicator displacement assay which discriminated trimethyl lysine from other lysine derivatives.

Solvatochromy and symmetry breaking in two quadrupolar oligophenylenevinylenes

Electrooptical absorption measurements (EOAM), solvatochromic dependences and quantum chemical simulations testify to large dipole moments change of two quadrupolar oligophenylenevinylenes upon transition to Franck-Condon excited state μ_e^FC. The values of the dipole moments μ_g and μ_e^FC are in the range [(4.2 - 4.9)10³⁰] C m and (30.8 - 47.0)10³⁰C m, respectively. The relations of dipole moments in the ground and excited states determined by EOAM correlate well with results obtained via the solvatochromic method. Calculations carried out by density functional theory (DFT) show that optimized configuration of the ground state of these molecules is not planar. The results from all methods applied unequivocally show the structural symmetry breaking in the studied compounds.

Spectroscopic and electrochemical properties of thiophene-phenylene based Shiff-bases with alkoxy side groups, towards photovoltaic applications

The influence of presence and elongation of alkoxy side chains in the π-conjugated Schiff-bases has been considered on the basis of UV-Vis absorption and photoluminescence spectra of model compounds and polymers solutions in chloroform and binary solvents of different polarity. The results of these investigations have been supported by electrochemical data. It has been demonstrated that introduction of electron donating methoxy side groups decreases the energy gap, however the elongation of alkyl chains only slightly affects the electronic structure of model compounds. In the case of polymers, such octyloxy side chains improves the solubility, enabling formation of longer polymer chains, with the enhanced effective π-conjugation length and narrower energy gap, however the intensity of emission band clearly decreased. Positive solvatochromism has been observed in both absorbance and photoluminescence spectra for all investigated compounds. As the concluding task, bulk-heterojunction (BHJ) photovoltaic (PV) structures, consisting of polyazomethines blended with the fullerene derivative, [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) have been prepared and tested in the context of potential application in solar cells. All investigated polymers have shown the photovoltaic effect, but the best power conversion efficiency and other PV parameters have been obtained for polyazomthine with octyloxy side chains.

Reinvestigation of the photophysics of 3-aminobenzoic acid in neat and mixed binary solvents

The present study elucidates the reinvestigation of the photophysical behavior of 3-aminobenzoic acid (3ABA) in solvents of different polarities using the steady-state spectroscopic techniques. Kamlet-Taft and Catalan solvatochromic models have been used to analyze the solvatochromic changes in neat solvents. The hydrogen bond donating ability of the solvent was found to be the main parameter affecting the spectral behavior of 3ABA. The solvatochromic characteristics of 3ABA have also been examined in binary solvent mixtures viz. acetonitrile (ACN)-methanol (MeOH) and benzene (BEN)-MeOH using the concept of preferential solvation. The preferential solvation of 3ABA shows unusual behavior for BEN-MeOH binary mixture and described unnoticed sigmoidal behavior in the ground state and synergistic impact in the excited state. Besides, the 3ABA was studied theoretically by quantum chemical calculations using (HF) Hartree-Fock and (DFT/B3LYP) density functional theories and its electronic absorption bands have been assigned by time-dependent density functional theory (TD-DFT). The effect of solvents on 3ABA was considered using a IEF-PCM-TDDFT (integral equation formalism of the polarizable continuum model- TDDFT) method. Thus, the theoretical results were found to be closer to the experimental results.

Exploiting the unique specialty of hydrazone functionality: Synthesis of a highly sensitive UV-Vis active solvatochromic probe

The unique physico-chemical attributes of the hydrazone functionality have been extensively studied for a diverse range of chemical, biological and analytical applications. The synthesis of a highly sensitive hydrazone based UV-Vis active solvatochromic probe that exhibits excellent sensitivity toward sensing of solvent polarity, microstructural changes and onset of micellization in aqueous systems was carried out. Specifically, synthesis of 2,4-dinitrophenyl-2-(2-nitrobenzylidene)hydrazone (DNPNBH), through an easy to carry, atom economical, one-pot single step approach via use of low-cost precursors viz. ortho-nitrobenzaldehyde and 2,4-dinitrophenyl hydrazine is presented. The UV-Vis absorption features of the synthesized hydrazone exhibit excellent sensitivity toward the polarity of its immediate microenvironment. The microenvironment polarity sensing potential of DNPNBH is demonstrated for some single solvent systems and DMF-Water mixture as a model binary solvent system and the results are supported by quantum mechanical calculations. Use of the DNPNBH as a probe (at concentrations many orders lower than required for conventional probes) to precisely reflect the onset of micellization and estimation of critical micelle concentration (CMC) of amphiphilic molecules (5.25 mM for SDS, 1.53 mM for CTAB and 0.055 mM for Brij56) in aqueous solutions is also demonstrated. The results clearly qualify the synthesized hydrazone as a highly sensitive UV-Vis probe that can be employed for reliable sensing of solvent polarity, composition dependence of physicochemical attributes in mixed solvent systems and the estimation of CMC of surfactant systems via spectrophotometry.

Linear and nonlinear optical properties of human hemoglobin

In this study, we investigated the possibility of interactions between the solvent molecules with the Heme group in the human hemoglobin. The results of this study answer a key question: whether the interactions of the Heme unit with its surroundings are interdependent or independent of the protein units of human hemoglobin. Contributions of the intermolecular interactions were determined by exploiting the solvatochromism spectroscopic data by Kamlet-Taft (KAT) polarity functions. Solvent polarity effects on the nonlinear properties of the Heme's groups in the human hemoglobin (Hb) were investigated via the Z-scan method. The experimental results obtained with spectroscopic and nonlinear optical parameters (absorption coefficient and refractive index) show that the mechanism of solvation and the interactions of Heme are controlled by suitable configuration of the protein units of hemoglobin. In other words, interactions of the Heme with α- and β-globins are an effective factor in controlling the optical behavior of Heme.

Modulation of benzofuran structure as a fluorescent probe to optimize linear and nonlinear optical properties and biological activities

The study presents the influence of structure modulation by introducing selected donor and acceptor substituents on optical properties of benzofuran used in biological imaging. As the starting form, 2-(5-formylbenzofuran-2-yl)acetamide described experimentally was used. This molecule contains an aldehyde group as reactive site, through which conjugation with protein occurs. Structure modulation was carried out by attaching additional electron-donating and electron-withdrawing substituents to the amino group, namely -NH2, -NHCH3, -NO2, -OH, and -OCH3. Studies have shown that the -NH2, -NHCH3, -OH, and -OCH3 substituents do not induce a significant change in the position of maximum absorption and fluorescence relative to each other. They also do not change the parameters describing the nonlinear response. Only the presence of the -NO2 substituent results in significant solvatochromic shifts. Changing substituents also does not significantly affect the LD50 value, and all tested fluorescent probes should not be considered toxic to humans. Modulation of the benzofuran derivative structure also does not change the active center in which the biocomplex with the protein is formed. In each case, the conjugation takes place via LYS114. In addition, the study was prompted to analyze the linear and nonlinear optical properties of conjugates formed after the reaction with Concanavalin A.Graphical abstract.

Solvatochromic characteristics of dansyl molecular probes bearing alkyl diamine chains

A series of dansyl-based fluorescent probes bearing linear alkyl-1,n-diamine chains of different length (DA_1.n, n = 2-8, 10, 12) was characterized in terms of the absorptive and emissive features in solvents of different polarity and hydrogen bond donor/hydrogen bond acceptor character. The probes show solvent-dependent absorption, a feature that is uncommon among dansyl derivatives. The dual emission of DA_1.n probes is strong in non-aqueous solvents and is influenced by the chain length and interactions with the solvent. Solvent effects on the spectral parameters were rationalized on the basis of the Kamlet-Taft and Catalán solvatochromic models, in order to quantify the degree of polarity-driven and hydrogen bonding interactions. A comparative discussion of the results predicted by the two models was made. In ground state, the DA_1.n probes act as hydrogen bond acceptors. In excited state, hydrogen bonding is less favoured, the solute-solvent interactions being governed by the increasing polarity of the solvent that results in a large bathochromic shift of the emission. A comparison was made with the spectral features previously reported for the corresponding series of bis-dansyl fluorescent probes (2DA_1.n).

A spectrophotometric study of impact of solvent, substituent and cross-conjugation in some 4-aminoantipyrine based Schiff bases

The influences of substituent and solvent polarity on the electronic transition of seven different 4-aminoantipyrine based Schiff bases have been investigated in 14 solvents of different polarity. Reichardt'sE_T(30) scale has been used to propose a quantitative approach towards the relative stability of the electronic ground and excited state species. The Schiff bases can sense the polarity changes of the solvents and accordingly there appear three categories of solvents: non-polar, dipolar aprotic and polar protic. The first two categories of solvents induce negative solvatochromism and the third one induces positive solvatochromism in the molecules. Cross conjugation and geometrical configuration have been proposed to play an important role in the extent of electronic transition of the Schiff bases in various solvent media. Out of various substituent effects, field effect and resonance significantly influence the electronic environment of the compounds, as evidenced from the regression models involving change in UV-Vis absorption wavelengths ¹HNMR chemical shifts and IR frequencies.

Quantifying protein-protein interactions of the acyl carrier protein with solvatochromic probes

Protein-protein interactions (PPIs) are universal to life and their study and understanding is critical to drug discovery and bioengineering efforts. Historically, X-ray crystallography, isothermal titration calorimetry and other biophysical methods have been used to study PPIs, but can be costly and are low throughput, hindering progress towards rapid evaluation of these interactions. Recent interest in targeting PPIs and in engineering biosynthetic pathways in which PPIs play a critical role has driven innovation in their evaluation but a universal screen is still needed. One of the best characterized systems relying upon PPIs is Escherichia coli type II fatty acid biosynthesis in which the central acyl carrier protein (EcACP) shuttles substrates to a series of partner enzymes. Here we present a method by which EcACP is labeled with a solvatochromic dye, 4-DMN, and then allowed to interact with its various partner enzymes. Upon interaction, there is a large increase in fluorescence intensity which is easily monitored via fluorometer or plate reader. This method is useful in the study of known PPI, hypothetical PPI and in evaluation of inhibitors of both partner enzyme active site and of the PPI itself.

Deciphering the interaction of solvents with dual emissive carbon dots: A photoluminescence study and its response for different metal ions

Carbon-based fluorescent quantum dots exhibited tunable photoluminescence (PL) property which is mainly dependent on its excitation wavelength which is an important factor required for optoelectronic and sensing applications. Here, we have established a microwave-based synthesis of dual emissive carbon quantum dots (CDs) using mixed-acid (H₃PO₄ + H₂SO₄) and mono/disaccharides as carbon precursor. In aqueous medium, CDs showed dual emission peaks at 434 nm and 518 nm, which exhibited excitation independent fluorescence property with particle size in the range of 4-7 nm. Furthermore, its photophysical properties were explored in different solvents. The astonishing bathochromic shift was observed in its emission wavelengths with the decrease in polarity of the solvents. Moreover, the metal sensing efficacy of CDs was explored in these solvent systems. It exhibited utmost selectivity for Fe(III) ions in both the organic and aqueous medium with maximum and minimum detecting limit of 1 μM and 0.4 μM respectively. These experimental results were also validated through real samples (Ferric citrate tablets 200 mg) and it showed excellent metal ion sensing accuracy for CDs in different media. The fundamental motive of this work was to explain the role of the mixed acid for the development of dual emissive CDs using a single precursor. The solvatochromic and sensing properties of CDs were explored in various solvents, which could be useful for sensing applications.

Optical Properties of Azo-Benzothiazole Side Chain Liquid Crystalline Polymers: Effect of Solvents, Substituents and Temperatures

The optical properties of a series of side chain liquid crystalline polymers (P1-P3) containing azo-benzothiazole mesogen with different terminal substituents (-H, -CH₃ and -OCH₂CH₃) in four organic solvents of varying polarity have been investigated by absorption and fluorescence spectral analysis. Solvatochromic studies of P1-P3 did not show any regular variation on the absorption and emission intensities with changing the polarity of solvent. Theoretical studies were performed based on different solvent correlation methods such as Dimroth-Reichardt and Kamlet-Taft methods to investigate the solute-solvent interactions. Both absorption and emission maxima of investigated polymers were bathochromically shifted with the replacement of sixth position hydrogen atom by electron donating groups in benzothiazole moiety. The emission intensities of the studied polymers showed decreasing trend with increasing temperature.