Fluorescent Sensor Based on 1H-Pyrazolo[3,4-b]quinoline Derivative for Detecting Zn2+ Cations.

The photophysical and sensory properties of the donor-acceptor pyrazoloquinoline derivative (PQPc) were investigated using absorption, steady-state, and time-resolved fluorescence measurements. The compound synthesized from commercial, readily available substrates exhibited absorptions in the UV-Vis range, with a maximum of the longwave band around 390 nm. The maximum fluorescence was around 460-480 nm, depending on the solvent. The quantum yield was between 12.87% (for n-hexane) and 0.75% (for acetonitrile) and decreased with increasing solvent polarity. The PET mechanism was implicated as the cause of fluorescence quenching. Divalent ions such as Zn2+, Pb2+, Cd2+, Ca2+, Mg2+, Co2+, Ni2+, and Cu2+ were introduced to study the fluorescent response of PQPc. A 13-times increase in fluorescence quantum yield was observed after the addition of Zn2+ ions. Detailed research was carried out for the PQPc-Zn2+ system in order to check the possibility of analytical applications of PQPc as a fluorescent sensor. A detection limit of Zn2+ was set at the value level 1.93 × 10-7 M. PQPc-Zn2+ complexes had a stoichiometry of 1:1 with a binding constant of 859 M-1. Biological studies showed that the sensor was localized in cells near the membrane and cytoplasm and may be used to detect zinc ions in eukaryotic cells.

Multicomponent Synthesis of 4-Aryl-4,9-dihydro-1H-pyrazolo[3,4-b]quinolines Using L-Proline as a Catalyst-Does It Really Proceed?

Looking for effective synthetic methods for 1H-pyrazolo[3,4-b]quinolines preparation, we came across a procedure where, in a three-component reaction catalysed by L-proline, 4-aryl-4,9-dihydro-1H-pyrazolo[3,4-b]quinolines are formed. These compounds can be easily oxidised to a fully aromatic system, which gives hope for a synthetic method that could replace, e.g., Friedländer condensation, often used for this purpose, even though severely limited by the availability of suitable substrates. However, after careful repetition of the procedures described in the publication, it turned out that the compounds described therein do not form at all. The actual compounds turned out to be 4,4-(phenyl-methylene)-bis-(3-methyl-1-phenylpyrazol-5-oles). Therefore, 4-Aryl-4,9-dihydro-1H-pyrazolo[3,4-b]quinolines were prepared by another method and used as standards to compare the products formed in the original procedure.

Photoinduced charge transfer in push-pull pyrazoline-based chromophores - Relationship between molecular structure and photophysical, photovoltaic properties.

The manuscript describes the effect of molecular structure on the photophysical and photovoltaic properties of the pyrazoline-based donor-branched-π-system-acceptor compounds decorated with two end groups: phenyl or thiophene. Although the absorption to the first singlet excited state is strongly allowed, the emission quantum yield is low in all studied solvents. This behaviour was explained by the existence of two non-radiative deactivation channels: the back electron transfer process, especially operated in polar solvents, and internal conversion realized as the rotation of flexible rotors (cyano, keto phenyl or thiophene). The feasibility of the photoinduced electron transfer process was corroborated by electrochemical, spectroelectrochemical measurements as well as DFT calculations. DFT calculations also support the existence of multiple conformations in the ground state, which differ from one another in terms of charge distribution and the values of ground state dipole moment. Finally, the mechanism of the singlet excited state deactivation of the studied compounds was determined by ultrafast pump-probe measurements. Our studies revealed that charge/electron transfer process may undergo over carbonyl bridge, included in branched π-system. Moreover, the thiophene decorated pyrazoline is characterized by a better photovoltaic power conversion efficiency, while the phenyl-ended pyrazoline can be applied as a viscosity sensor.

1H-Pyrazolo[3,4-b]quinolines: Synthesis and Properties over 100 Years of Research.

This paper summarises a little over 100 years of research on the synthesis and the photophysical and biological properties of 1H-pyrazolo[3,4-b]quinolines that was published in the years 1911-2021. The main methods of synthesis are described, which include Friedländer condensation, synthesis from anthranilic acid derivatives, multicomponent synthesis and others. The use of this class of compounds as potential fluorescent sensors and biologically active compounds is shown. This review intends to summarize the abovementioned aspects of 1H-pyrazolo[3,4-b]quinoline chemistry. Some of the results that are presented in this publication come from the laboratories of the authors of this review.

Fleeting Beauty-The World of Plant Fragrances and Their Application.

This article is devoted to some aspects of the fragrant substances of plant origin applied in the food industry and perfumery as well. Since antiquity many extractive techniques have been developed to obtain essential oils. Some of them are still applied, but new ones, like microwave or ultrasound-assisted extractions, are more and more popular and they save time and cost. Independently of the procedure, the resulting essential oils are the source of many so-called isolates. These can be applied as food additives, medicines, or can be used as starting materials for organic synthesis. Some substances exist in very small amounts in plant material so the extraction is not economically profitable but, after their chemical structures were established and synthetic procedures were developed, in some cases they are prepared on an industrial scale. The substances described below are only a small fraction of the 2000-3000 fragrant molecules used to make our life more enjoyable, either in food or perfumes. Additionally, a few examples of allelopathic fragrant compounds, present in their natural state, will be denoted and some of their biocidal features will be mentioned as an arising "green" knowledge in agriculture.

Spectral Properties of Highly Emissive Derivative of Coumarin with N,N-Diethylamino, Nitrile and Tiophenecarbonyl Moieties in Water-Methanol Mixture.

The new derivative of coumarin (E)-3-[7-(diethyloamino)-2-oxo-chromen-3yl]-2-(tiophene-2-carbonyl)prop-2-enenitrile (NOSQ) was easy synthesized with commercial substrates as a result of the search of new Michael type addition sensors based on coumarins. Spectral properties of highly emissive NOSQ were investigated by steady state analysis (absorption and fluorescence measurements) and time-resolved analysis (fluorescence lifetime measurements). The effect of water-methanol mixture on the photophysical properties of the NOSQ molecule was analyzed. With increasing of volumetric fraction of water the intensity of absorbance and fluorescence was strongly reduced. The NOSQ quantum yield in methanol was quite high and the first portions of water caused a significant increase in this value. Water, which is usually a quencher, in this case caused the increase in the quantum yield. The fluorescence lifetimes had second-order decay and the values of fluorescence lifetime increased with increasing alcohol content. Density functional theory (DFT) calculations and experimental data remained in agreement and showed that the interaction between the NOSQ molecule and the solvent affects the appearance of the new conformer.

Stationary and time-resolved spectra analysis of pyrazoloquinoline derivatives with pyridyl moiety.

Two derivatives of pyrazoloquinoline with pyridyl moiety: 6-N,N-dimethyl-3-phenyl-1-(2-pyridyl)-1H-pyrazolo[3,4-b]quinoline (DMA-1PPhPQ) and 6-N,N-dimethyl-1,3-(di-2-pyridyl)-1H-pyrazolo[3,4-b]quinoline (DMA-1,3PPQ) were synthesized with commercial substrates. The theoretical characterization of both compounds was done. Geometry optimizations give not flat structure with the first absorption band at the wavelength about 390nm for both compounds. Several electro-optical parameters were also calculated. The optical properties of DMA-1PPHPQ and DMA-1,3PPQ were investigated by ultraviolet-visible spectroscopy and stationary as well as time-resolved fluorescence. The fluorescence maximum and fluorescence quantum yield are strongly dependent on solvent polarity function. Results indicate CT fluorescence for both compounds. Because of high emission the investigated pyrazoloquinoline derivatives can be potential candidates for fabrications of electroluminescent devices.

Novel scintillating material 2-(4-styrylphenyl)benzoxazole for the fully digital and MRI compatible J-PET tomograph based on plastic scintillators.

A novel plastic scintillator is developed for the application in the digital positron emission tomography (PET). The novelty of the concept lies in application of the 2-(4-styrylphenyl)benzoxazole as a wavelength shifter. The substance has not been used as scintillator dopant before. A dopant shifts the scintillation spectrum towards longer wavelengths making it more suitable for applications in scintillators of long strips geometry and light detection with digital silicon photomultipliers. These features open perspectives for the construction of the cost-effective and MRI-compatible PET scanner with the large field of view. In this article we present the synthesis method and characterize performance of the elaborated scintillator by determining its light emission spectrum, light emission efficiency, rising and decay time of the scintillation pulses and resulting timing resolution when applied in the positron emission tomography. The optimal concentration of the novel wavelength shifter was established by maximizing the light output and it was found to be 0.05 ‰ for cuboidal scintillator with dimensions of 14 mm x 14 mm x 20 mm.

Applications of fluorescent sensor based on 1H-pyrazolo[3,4-b]quinoline in analytical chemistry.

Fluorescent dye 2-[(2-Hydroxyethyl)-(1,3-diphenyl-1H-pyrazolo[3,4-b]quinolin-6-ylmethyl)-amino]ethanol (LL1) was examined for its efficiency in the detection of small inorganic cations (lithium, sodium, barium, calcium, magnesium, cadmium, lead and zinc). The dye was synthesized in the laboratory and investigated by means of both, steady-state and time-resolved fluorescence techniques. This compound acts as a fluorescent sensor suitable for detection of small inorganic cations (lithium, sodium, barium, calcium, magnesium, cadmium, lead and zinc) in strongly polar solvent (acetonitrile). An electron transfer from the electro-donative part (receptor) of the molecule to the acceptor part (fluorophore) is thought to be the main mechanism that underlies functionality of the compound as a sensor. This process can be retarded upon complexation of the receptor moiety by inorganic cations. Relatively high sensitivity but poor selectivity of the amino alcohol that contains indicator towards the two-valued cations was observed. However, upon addition of some amounts of water the selectivity of this sensor has been enhanced (especially towards lead cation). The preliminary results in analytical application of the sensor are discussed.

A new fluorescent sensor based on 1H-pyrazolo[3,4-b]quinoline skeleton. Part 2.

A novel fluorescent dye bis-(pyridin-2-yl-methyl)-(1,3,4-triphenyl-1H-pyrazolo[3,4-b]quinolin-6-ylmethyl)-amine (P1) has been synthesized and investigated by means of steady state and time-resolved fluorescence techniques. This compound acts as sensor for fluorescence detection of small inorganic cations (lithium, sodium, barium, magnesium, calcium, and zinc) in highly polar solvents such as acetonitrile. The mechanism which allows application of this compound as sensor is an electron transfer from the electron-donative part of molecule (amine) to the acceptor part (pyrazoloquinoline derivative), which is retarded upon complexation of the electro-donative part by inorganic cations. The binding constants are strongly dependent on the charge density of the analyzed cations. The 2/1 complexes of P1 with Zn(++) and Mg(++) cations posses large binding constants. Moreover, in the presence of these cations a significant bathochromic shift of fluorescence is observed. The most probable explanation of such behaviour is the formation of intramolecular excimer. This is partially supported by the quantum chemical calculations.

Exploring reversible quenching of fluorescence from a pyrazolo[3,4-b]quinoline derivative by protonation.

Pyrazolo[3,4-b]quinoline derivatives are reported to be highly efficient organic fluorescent materials suitable for applications in light-emitting devices. Although their fluorescence remains stable in organic solvents or in aqueous solution even in the presence of H(2)O, halide salts (LiCl), alkali (NaOH) and weak acid (acetic acid), it suffers an efficient quenching process in the presence of protic acid (HCl) in aqueous or ethanolic solution. This quenching process is accompanied by a change in the UV spectrum, but it is reversible and can be fully recovered. Both steady-state and transient fluorescence spectra of 1-phenyl-3,4-dimethyl-1H-pyrazolo-[3,4-b]quinoline (PAQ5) during quenching are measured and analyzed. It is found that a combined dynamic and static quenching mechanism is responsible for the quenching processes. The ground-state proton-transfer complex [PAQ5H(+)] is responsible for static quenching. It changes linearly with proton concentration [H(+)] with a bimolecular association constant K(S)=1.95 M(-1) controlled by the equilibrium dissociation of HCl in ethanol. A dynamic quenching constant K(D)=22.4 M(-1) is obtained by fitting to the Stern-Volmer equation, with a bimolecular dynamic quenching rate constant k(d)=1.03x10(9) s(-1) M(-1) under ambient conditions. A change in electron distribution is simulated and explains the experiment results.

Investigation of the photoisomerisation process in four p-benzoxazoyl-substituted stilbenes.

Fluorescence properties and trans-cis photoisomerisation of the benzoxazole derivatives 2-[4-(E)-(styryl)phenyl]benzoxazole (I), 2-{4-[(E)-2-(4-methoxyphenyl)vinyl]phenyl}benzoxazole (II), {4-[(E)-2-(4-benzoxazol-2-yl-phenyl)vinyl]phenyl}dimethylamine (III) and {4-[(E)-2-(4-benzoxazol-2-yl-phenyl)vinyl]phenyl}diphenylamine (IV) have been investigated in solvents of different polarities. It was found that these compounds exhibit efficient fluorescence with quantum yields and lifetimes strongly dependent on solvent polarity, although only compounds III and IV possess a significant charge transfer character in solvents of medium and high polarities. In addition, the photoisomerisation quantum efficiency depends strongly on the substitution of the phenyl ring in the electron donor moiety. A strong dependence of the quantum efficiency of the photoisomerisation on solvent was established. That quantity depends linearly on the non-radiative quantum yield of the deactivation of the excited singlet state for all investigated compounds. These results are consistent with a singlet state mechanism of the photoprocess. For compounds III and IV, with strong electron donors (N,N-dimethylaniline and triphenylamine), the molecule in the excited state trans configuration is more stabilized by solvent polarity than in the perpendicular form which causes more efficient isomerisation in nonpolar solvents. For compounds I and II the energy of the perpendicular configuration decreases more rapidly than that of the trans configuration when solvent polarity increases. In this case the energy barrier decreases with increasing solvent polarity. This makes the photoisomerisation process easier in polar solvents.

New fluorescent sensors based on 1H-pyrazolo[3,4-b] quinoline skeleton.

Novel fluorescing dyes 1,3,4-triphenyl-6-(1,4,7,10-tetraoxa-13-aza-cyclopentadec-13-ylmethyl)-1H-pyrazolo [3,4-b]quinoline (K1) and 2-[(2-hydroxyethyl)-(1,3,4-triphenyl-1H-pyrazolo[3,4-b]quinolin-6-ylmethyl)-amino] ethanol (L1) have been synthesized and investigated by the means of steady state and time-resolved fluorescence techniques. These compounds act as sensors for the fluorescence detection of small inorganic cations (lithium, sodium, barium, magnesium and calcium) in solvents of different polarities (THF and acetonitrile). The mechanism, which allows application of these compounds as sensors, is an electron transfer from the electro-donative part of molecule to the acceptor part (fluorophore), which is retarded upon complexation of the electro-donative part by inorganic cations. We found that crown ether-containing compound is very sensitive to the addition of any investigated ions but amino alcohol-containing one exhibits better selectivity to the addition of two-valued cations. Two kinds of the complexes (LM(+) and L(2)M(+)) were found in the investigated systems. In addition, the dyes may be used as fluorescence indicators in solvents of lower polarity like tetrahydrofuran.

3-Methyl-1,4-diphenyl-1H-pyrazolo-[3,4-b]quinoline.

In the title mol-ecule, C(23)H(17)N(3), the phenyl substituents at positions 1 and 4 are twisted relative to the central core by 27.09 (5) and 66.62 (4)°, respectively. In the crystal, mol-ecules are assembled into centrosymmetric dimers via π-π stacking inter-actions between the 1H-pyrazolo-[3,4-b]quinoline -units, with an inter-planar distance of 3.601 (2) Šand by weak inter-molecular C-H⋯N inter-actions.

Excited-state double proton transfer in 1H-pyrazolo[3,4-b]quinoline dimers.

Pyrazoloquinolines are highly fluorescent, both in liquid solutions and in the solid state, which makes them good candidates for various optical devices. The aim of the current work is to understand the photochemical behavior of pyrazolo[3,4-b]quinoline (PQ), which is quite complicated since in n-alkane solvents PQ tends to form strong complexes with protic solvent constituents (often present as minor impurities), as well as dimers. Both types of H-bond complexes were studied systematically by temperature-dependent conventional absorption and fluorescence spectroscopy; the effect of protic solvent constituents was mimicked by varying the ethanol concentration in n-octane in the range from 0.0 to 0.8%. At room temperature the PQ:ethanol association constant was estimated at 80 M(-1) and the dimerization constant at 2 x 10(3) M(-1). Dimer formation is enhanced upon lowering the temperature in pure n-alkane down to 220 K, and the fluorescence is strongly reduced since the dimer is nonfluorescent. Surprisingly, when irradiating a frozen sample for several minutes at very low temperatures (<40 K), a narrow-banded Shpol'skii-type fluorescence spectrum gradually appears. To explain this unusual photochemical behavior, PQ and its deuterated analogue were studied using low-temperature absorption and fluorescence spectroscopy over the 300-5 K temperature range. In the case of normal (protonated) PQ, very fast excited-state intermolecular double proton transfer is responsible for the efficient quenching of PQ dimer fluorescence. Deuteration significantly slows down this proton transfer process, and in that case under cryogenic conditions a fluorescent dimer is observed. Photoirradiation under cryogenic conditions leads to molecular rearrangement of the dimers and the appearance of monomer spectra. For both H-PQ and D-PQ, these processes were found to be reversible. A simplified reaction scheme, in which the excited tautomeric dimer plays a crucial role, is presented to explain the observations.

Intramolecular exciplexes based on benzoxazole: photophysics and applications as fluorescent cation sensors.

Exciplex behaviour of three benzoxazole derivatives has been detected and intensively investigated by means of steady-state and time-resolved fluorescence techniques and transient absorption spectroscopy. The fluorescence of these compounds shows the properties which are typical for the excited state charge transfer complexes (exciplexes). Besides of the short wavelength fluorescence, which is similar in spectral distribution to the fluorescence of the electron acceptor (2-p-tolyl-benzoxazole), the red shifted, broad and structureless emission band is observed in solvents of low and medium polarity. The detailed analysis of the fluorescence data shows that the ratio of the CT and LE fluorescence initially increases with increasing solvent polarity, achieves a maximum, and drops for more polar solvents (epsilon(s) = 7). Similar behaviour is observed for the exciplex fluorescence lifetimes. The overall fluorescence and the relative intersystem crossing quantum yields show the decrease of these values with increasing solvent polarity. These observations have been explained on the basis of Marcus-type theory for nonradiative charge transfer rate constants. Increasing solvent polarity strongly accelerates the back electron transfer process which recovers the whole molecule in the ground state. The probability of the compact exciplex formation (i.e. sandwich-type structures) depends on solvent viscosity and degree of freedom of the bending of the saturated linker. The compound containing crown ether as a donor subunit may be used as a fluorescent indicator of inorganic cations (barium and lithium). We found an effective complexation of the compound in the ground state with barium and lithium cations. The complex is also stable in the excited state which manifests itself in strong increase of the fluorescence intensity.

Probing the photochemical mechanism in photoactive yellow protein.

Selectively bridged model compounds related to the chromophore in photoactive yellow protein have been synthesized where the single bond adjacent to the benzene ring (bond 1) and where both bond 1 and the adjacent double bond (bond 2) are bridged. They were compared to the nonbridged reference compound regarding their photophysical properties using steady-state and time-resolved fluorescence at various temperatures. Quantum chemical calculations were additionally performed and showed that several conformers are populated in the ground state. The neutral model compounds show that the nonradiative deactivation channel is linked to both single- and double-bond twisting. The relative importance of single-bond twisting is increased for the corresponding deprotonated hydroxy compounds with an enhanced donor character. The simultaneous photochemical activity of both single and double bonds explains the ease of photochemical isomerization in the confined environment of the natural PYP protein and also of the primary step in the vision process in rhodopsin.

1,3-Diphenyl-1H-pyrazolo[3,4-b]quinoline: a versatile fluorophore for the design of brightly emissive molecular sensors.

[structure: see text] The 1,3-diphenyl-1H-pyrazolo[3,4-b]-quinoline chromophore is a versatile building block for the construction of brightly fluorescent molecular sensors. Facile synthetic procedures allow integration of the chromophore into fluorophore-spacer-receptor systems as well as fluoroionophores operating via intramolecular charge transfer. Whereas the former photoinduced electron-transfer probes show strong analyte-induced fluorescence enhancement, the latter exhibit bright ratiometric dual emission. Employing prototype macrocyclic receptors, the favorable signaling features for metal ion recognition are demonstrated.