Photophysical and anion sensing properties of a triphenylamine-dioxaborinine trimeric compound.

Herein, we report the synthesis and photophysical characterization of the novel tris(4-(2,2-difluoro-6-methyl-2H-1λ3,3,2λ4-dioxaborinin-4-yl)phenyl)amine trimeric probe (A2) via the reaction between triphenylamine (1), acetic anhydride, and BF3·OEt2 implying the twelve new bond formation in a one-pot manner. This highly fluorescent compound in solution (φ up to 0.91 at 572 nm) and solid state (φ = 0.24 at 571 nm) showed a better solvatofluorochromism than its analog monomeric A1 due to symmetry-broken charge transfer, which is consistent with high solvent dipolarity (SdP) response in Catalán's multiparametric regression. Notably, A2 had a high sensibility and selectivity for CN- or F- in solution (LODCN-/F- = 0.18/0.70 µM), and CN- can be discriminated from F- by the reaction of A2 with 3.0 equiv. of CN-. In addition, A2 was impregnated on filter paper to prepare test strips that were applied to naked-eye qualitative sensing of CN- or F-. Finally, the octupolar system in A2 allows for better action of two-photon excitation cross-section values when compared with that of the dipolar structure in A1. These findings provide further information for the design of new efficient two-photon absorption dyes.

BF3-Mediated Acetylation of Pyrazolo[1,5-a]pyrimidines and Other π-Excedent (N-Hetero)arenes.

An operably simple microwave-assisted BF3-mediated acetylation reaction of pyrazolo[1,5-a]pyrimidines and a plausible mechanism based on density functional theory (DFT) theoretical calculations for this transformation are reported. Remarkably, and to the best of our knowledge, this is the first example of the direct acetylation for the functional pyrazolo[1,5-a]pyrimidine (PP) core. The synthesis of this essential building block is reported in high yields using mild reaction conditions, inexpensive reagents, and even substrates with electron-deficient or highly hindered groups. In addition, one of the new methyl ketones was successfully used as a substrate for producing novel and valuable bis-electrophilic compounds with yields of up to 90%. Notably, the discovered acetylation method was successfully applied in other π-excedent (N-hetero)aromatic substrates.

Cyanide chemosensors based on 3-dicyanovinylpyrazolo[1,5-a]pyrimidines: Effects of peripheral 4-anisyl group substitution on the photophysical properties.

Novel dual-mode colorimetric/fluorometric probes based on 3-dicyanovinylpyrazolo[1,5-a]pyrimidines for cyanide (CN-) sensing have been developed (DPPa-c). These probes displayed high selectivity and sensitivity toward CN- over other interfering anions, with a detection limit (LOD) as low as 610/170 nmol L-1 (absorption/emission) for some of the prepared probes. After a reaction with CN-, low-fluorescent DPPa-c showed a significant decrease of the intramolecular charge transfer (ICT) bands at approximately 390 nm (color changes from yellow to colorless) and exhibited up to an 82-fold fluorescence enhancement at approximately 465 nm (strong blue-light emission). The successive introduction of 4-anisyl (4-MeOPh) groups on periphery of the heterocyclic core had a dramatic influence on both the photophysical properties and CN- detection capability. The number of channels for CN- quantification in the absorption spectra increased from 1 in DPPa to 3 in DPPc. Moreover, the fluorescence emission LOD decreased from 300 nmol L-1 in DPPa to 170 nmol L-1 in DPPc. Finally, the selectivity toward CN- demonstrated a notable improvement when the probe had three 4-anisyl groups in its periphery (i.e., DPPc).

Pyrazolo[1,5-a]pyrimidines-based fluorophores: a comprehensive theoretical-experimental study.

Fluorescent molecules are crucial tools for studying the dynamics of intracellular processes, chemosensors, and the progress of organic materials. In this study, a family of pyrazolo[1,5-a]pyrimidines (PPs) 4a-g has been identified as strategic compounds for optical applications due to several key characteristics such as their simpler and greener synthetic methodology (RME: 40-53%) as compared to those of BODIPYS (RME: 1.31-17.9%), and their tunable photophysical properties (going from ε = 3320 M-1 cm-1 and ϕ F = 0.01 to ε = 20 593 M-1 cm-1 and ϕ F = 0.97), in which electron-donating groups (EDGs) at position 7 on the fused ring improve both the absorption and emission behaviors. The PPs bearing simple aryl groups such as 4a (4-Py), 4b (2,4-Cl2Ph), 4d (Ph) and 4e (4-MeOPh), allow good solid-state emission intensities (QYSS = 0.18 to 0.63) in these compounds and thus, solid-state emitters can be designed by proper structural selection. The properties and stability found in 4a-g are comparable to commercial probes such as coumarin-153, prodan and rhodamine 6G. Ultimately, the electronic structure analysis based on DFT and TD-DFT calculations revealed that EDGs at position 7 on the fused ring favor large absorption/emission intensities as a result of the ICT to/from this ring; however, these intensities remain low with electron-withdrawing groups (EWGs), which is in line with the experimental data and allows us to understand the optical properties of this fluorophore family.

Recent progress in chemosensors based on pyrazole derivatives.

Colorimetric and fluorescent probes based on small organic molecules have become important tools in modern biology because they provide dynamic information concerning the localization and quantity of the molecules and ions of interest without the need for genetic engineering of the sample. In the past five years, these probes for ions and molecules have attracted great attention because of their biological, environmental and industrial significance combined with the simplicity and high sensitivity of absorption and fluorescence techniques. Moreover, pyrazole derivatives display a number of remarkable photophysical properties and wide synthetic versatility superior to those of other broadly used scaffolds. This review provides an overview of the recent (2016-2020) findings on chemosensors containing pyrazole derivatives (pyrazoles, pyrazolines and fused pyrazoles). The discussion focuses on the design and physicochemical properties of chemosensors in order to realize their full potential for practical applications in environmental and biological monitoring (sensing of metal ions, anions, explosives, and biomolecules). We also present our conclusions and outlook for the future.

Synthesis of Pyrrolo[2,3-c]isoquinolines via the Cycloaddition of Benzyne with Arylideneaminopyrroles: Photophysical and Crystallographic Study.

An efficient and quick access toward a series of (E)-2-arylideneaminopyrroles 6 and to their benzyne-promoted aza-Diels-Alder cycloaddition products is provided. These products are three pyrrolo[2,3-c]isoquinolines 8a-c substituted in position 5 with different electron-acceptor (A) or electron-donor (D) aryl groups. Intermediates and products were obtained in good yields (up to 78 and 84%, respectively), and their structures were determined on the basis of NMR measurements and HRMS analysis. Photophysical properties of 8a-c were investigated, finding good Stokes shift in different solvents, but only the product 8c showed appreciable fluorescence intensity since its 5-aryl group (2,4-Cl2Ph) could favor the twisted intramolecular charge transfer effect. In addition, a riveting relationship between solvent viscosity and fluorescence intensity was found. Structures of 6 and 8 were studied and confirmed by single-crystal X-ray diffraction, observing that their electronic distributions effect the supramolecular assembly but with only long-distance hydrophobic interactions. A CE-B3LYP model was used to study the energetic topology and understand the crystal architecture of compounds as well as find a connection with both the synthetic and photophysical results.

Integrated pyrazolo[1,5-a]pyrimidine-hemicyanine system as a colorimetric and fluorometric chemosensor for cyanide recognition in water.

A new probe for cyanide detection based on the integrated pyrazolo[1,5-a]pyrimidine-hemicyanine (PpHe) system was synthesized in an efficient and straightforward manner using microwave-assisted heating. Photophysical studies in a 100% aqueous solution demonstrated high cyanide selectivity and detection limits as low as 600 and 86 nmol L-1 for UV-vis absorption and fluorescence emission, respectively. Both values are well below 1900 nmol L-1, which is the maximum concentration permitted for drinking water by the World Health Organization (WHO). HRMS analysis and NMR experiments were performed to confirm the mechanism of detection based on blocking the ICT phenomenon via nucleophilic addition of CN- on the C˭N+ bond (iminium salt moiety) of the probe.

3-Formylpyrazolo[1,5- a]pyrimidines as Key Intermediates for the Preparation of Functional Fluorophores.

A one-pot route for the regioselective synthesis of 3-formylpyrazolo[1,5- a]pyrimidines 4a-k in good yields through a microwave-assisted process is provided. The synthesis proceeds via a cyclocondensation reaction between ß-enaminones 1 with NH-3-aminopyrazoles 2, followed by formylation with an iminium salt moiety (Vilsmeyer-Haack reagent). These N-heteroaryl aldehydes 4 were successfully used as strategic intermediates for the preparation of novel functional fluorophores with yields up to 98%. The structures of the products obtained and regioselectivity of the reactions were determined on the basis of NMR measurements and X-ray diffraction analysis. Since pyrazolo[1,5- a]pyrimidines (PPs) 3 have shown an important fluorescence, photophysical properties of four 2-methylderivatives substituted at position 7 with different acceptor (A) or donor (D) groups were investigated. The compounds evaluated exhibited large Stokes shift in different solvents, but only the substituted p-methoxyphenyl (4-An) showed a strong fluorescence intensity with quantum yields up to 44% due to its greater ICT. Therefore, hybrid systems based on pyrazolo[1,5- a]pyrimidines could be used as fluorescent probes to detect biologically or environmentally relevant species.

Geometric influence on intramolecular photoinduced electron transfer in platinum(II) acetylide-linked donor-acceptor assemblies.

A new donor-acceptor system, in which the electron donor triphenylamine (TPA) and the electron acceptor C60 are bridged through a cis- or trans-platinum(II) acetylide spacer have been prepared. Ground-state studies were conducted using electrochemistry and UV/Vis spectroscopy. Fluorescence studies suggested that charge transfer is the deactivation mechanism for the singlet excited state, and this was verified by transient absorption spectroscopy. Selective photoexcitation of 1 and 2 at 387 nm leads to a fast charge transfer between the TPA and C60, which gives rise to a radical ion-pair state (TPA(·+)-Pt-C60(·-)). Our results suggest that charge transfer is favored for the cis configuration while the presence of the trans configuration in the Pt(II) diacetylide results in a longer-lived charge separated states.

Synthesis of 1-substituted 3-aryl-5-aryl(hetaryl)-2-pyrazolines and study of their antitumor activity.

Three series of novel 1,3,5-trisubstituted 2-pyrazoline derivatives containing thiophene and benzodioxol moieties as potential antitumor agents were synthesized. The in vitro antitumor activity of the obtained compounds was determined at the National Cancer Institute (NCI). The 5-(benzo[d][1,3]dioxol-5-yl)-3-(4-methoxyphenyl)-4,5-dihydro-1H-pyrazole-1-carbothioamide (9a) is the most prominent of the compounds due to its remarkable activity toward leukemia (RPMI-8226), renal cancer (UO-31) and prostate cancer (DU-145) cell lines with GI(50) values of 1.88, 1.91 and 1.94 µM, respectively.

Synthesis of novel pyrazolic analogues of chalcones and their 3-aryl-4-(3-aryl-4,5-dihydro-1H-pyrazol-5-yl)-1-phenyl-1H-pyrazole derivatives as potential antitumor agents.

Novel (E)-1-aryl-3-(3-aryl-1-phenyl-1H-pyrazol-4-yl)prop-2-en-1-ones 5/6 (pyrazolic chalcones) were synthesized from a Claisen-Schmidt reaction of 3-aryl-1-phenylpyrazol-4-carboxaldehydes 4 with several acetophenone derivatives 1. Subsequently, the microwave-assisted cyclocondensation reaction of chalcones 5/6 with hydrazine afforded the new racemic 3-aryl-4-(3-aryl-4,5-dihydro-1H-pyrazol-5-yl)-1-phenyl-1H-pyrazoles 7 or their N-acetyl derivatives 8 and 9 when reactions where carried out in DMF or acetic acid, respectively. Several of these compounds were screened by the US National Cancer Institute (NCI) for their ability to inhibit 60 different human tumor cell lines, where 5c and 9g showed remarkable activity mainly against leukemia (K-562 and SR), renal cancer (UO-31) and non-small cell lung cancer (HOP-92) cell lines, with the most important GI50 values ranging from 0.04 to 11.4 microM, from the in vitro assays.