Charge delocalization in self-assembled mixed-valence aromatic cation radicals

The spontaneous assembly of aromatic cation radicals (D(+?)) with their neutral counterpart (D) affords dimer cation radicals (D(2)(+?)). The intermolecular dimeric cation radicals are readily characterized by the appearance of an intervalence charge-resonance transition in the NIR region of their electronic spectra and by ESR spectroscopy. The X-ray crystal structure analysis and DFT calculations of a representative dimer cation radical (i.e., the octamethylbiphenylene dimer cation radical) have established that a hole (or single positive charge) is completely delocalized over both aromatic moieties. The energetics and the geometrical considerations for the formation of dimer cation radicals is deliberated with the aid of a series of cyclophane-like bichromophoric donors with drastically varied interplanar angles between the cofacially arranged aryl moieties. X-ray crystallography of a number of mixed-valence cation radicals derived from monochromophoric benzenoid donors established that they generally assemble in 1D stacks in the solid state. However, the use of polychromophoric intervalence cation radicals, where a single charge is effectively delocalized among all of the chromophores, can lead to higher-order assemblies with potential applications in long-range charge transport. As a proof of concept, we show that a single charge in the cation radical of a triptycene derivative is evenly distributed on all three benzenoid rings and this triptycene cation radical forms a 2D electronically coupled assembly, as established by X-ray crystallography.     

Sensing of Micellar Microenvironment with Dual Fluorescent Probe, Triazolylpyrene (TNDMBPy)

We report a dual fluorescent triazolylpyrene ( ^TNDMB Py) as an efficient fluorescent light-up probe of various micellar microenvironments. The absorption spectra of ^TNDMB Py in an aqueous solution of varying surfactant concentration, CTAB, SDS and TX-100 showed that as the surfactant concentration was increased the absorbance increased with no shift in wavelength maxima. The increase of absorbance in each surfactant solution with increase in surfactant concentration was due to the enhanced solubilization of ^TNDMB Py in surfactant solutions. Our investigations based on steady state and time resolved fluorescence techniques showed that the probe reports the microenvironment of ionic surfactant solutions (CTAB and SDS) via dual emission (LE and ICT) at low surfactant concentration. The ICT band showed a blue shifting pattern with enhanced intensity that disappeared as the concentration of surfactant increases (> 1 mM for CTAB and > 3 mM for SDS). In non-ionic surfactant (Triton X-100) solution, the fluorophore showed dual emission with dominant ICT behaviour over LE emission at low concentration (up to 0.35 mM). In reverse micelle we observed a blue shifted ICT band with no LE band with increasing molar concentration of water. We found 100 nm blue shifting when we moved from R?=?0 to R?=?7, where R is the molar ratio of water to TX-100 (R?=?[H2O]/[TX-100]). The blue shifting of ICT band is because of the movement of the probe from hydrophilic core to hydrophobic core (surface) of the reverse micelle. Thus from the steady-state fluorescence study it was observed that the ICT band of the probe, ^TNDMB Py was more influenced by the micellar environment in comparison to the LE band. This difference in behaviour of the fluorophore is probably because of varying extent of hydrophobic/hydrogen bonding interactions experienced by the probe and its relative disposition inside the various micellar nanocores.      

Identification of novel S-adenosyl-L-homocysteine hydrolase inhibitors through homology-model-based virtual screening, synthesis, and biological evaluation

The present study describes a successful application of computational approaches to identify novel Leishmania donovani (Ld) AdoHcyase inhibitors utilizing the differences for Ld AdoHcyase NAD(+) binding between human and Ld parasite. The development and validation of the three-dimensional (3D) structures of Ld AdoHcyase using the L. major AdoHcyase as template has been carried out. At the same time, cloning of the Ld AdoHcyase gene from clinical strains, its overexpression and purification have been performed. Further, the model was used in combined docking and molecular dynamics studies to validate the binding site of NAD in Ld. The hierarchical structure based virtual screening followed by the synthesis of five active hits and enzyme inhibition assay has resulted in the identification of novel Ld AdoHcyase inhibitors. The most potent inhibitor, compound 5, may serve as a "lead" for developing more potent Ld AdoHcy hydrolase inhibitors as potential antileishmanial agents.     

Sequential oxidative transformation of tetraarylethylenes to 9,10-diarylphenanthrenes and dibenzo[g,p]chrysenes using DDQ as an oxidant

Tetraarylethylenes can be sequentially transformed into 9,10-diarylphenanthrenes and dibenzo[g,p]chrysenes using 1 and 2 equiv of DDQ, respectively, in CH(2)Cl(2) containing methanesulfonic acid, in excellent yields. Efficient access to substituted dibenzochrysenes from tetraarylethylenes establishes the versatility of this procedure over the existing multistep syntheses of dibenzochrysenes. Moreover, the ready regeneration of DDQ from easily recovered reduced DDQ-H(2) continues to advance the use of DDQ/H(+) for the oxidative C-C bond forming reactions.     

Installation/modulation of the emission response via click reaction

We have demonstrated the installation of a fluorescence property into a nonfluorescent precursor and modulation of an emission response of a pyrene fluorophore via click reaction. The synthesized fluorophores show different solvatochromicity and/or intramolecular charge transfer (ICT) feature as is revealed from the UV-visible, fluorescence photophysical properties of these fluorophores, and DFT/TDDFT calculation. We observed that some of the synthesized fluorophores showed purely ICT character while emission from some of them arose from the LE state. A structureless and solvent polarity-sensitive dual emission behavior was observed for one of the triazolylpyrene fluorophores that contains an electron-donating -NMe(2) substituent (fluorophore, 7a). Conversely, triazolylpyrene with an electron-withdrawing -CN group (fluorophore, 7b) showed a solvent polarity-independent vibronic emission. The effect of ICT on the photophysical properties of these fluorophores was studied by fluorescence emission spectra and DFT/TDDFT calculations. Fluorescence lifetimes were also measured in different solvents. All of our findings revealed the delicate interplay of structure and emission properties and thus having broader general utility. As the CT to LE intensity ratio can be employed as a sensing index, the dual emissive fluorophore can be utilized in designing the molecular recognition system too. We envisage that our investigation is of importance for the development of new fluorophores with predetermined photophysical properties that may find a wide range of applications in chemistry, biology, and material sciences.     

Substoichiometric extraction of chromium(III) with 2-picolinic acid into n-butanol

A substoichiometric radiochemical extraction procedure has been designed for the estimation of trace amounts of chromium(III). The method employs the extraction of spiked (⁵¹Cr) chromium by chelation with 2-picolinic acid in n-butanol solvent at pH 4. The interference effects of various closely related foreign ions on the extraction efficiency were found to be negligible, thus establishing the selectivity of the proposed technique over other standard analytical procedures available for the estimation of chromium(III). The validity of this method has been established by successful estimation of chromium present in various certified alloys. Investigations have been made to understand the parameters controlling the extraction of chromium(III).     

Automated separation of 99Tc using plastic scintillation resin PSresin and openview automated modular separation system (OPENVIEW-AMSS)

Automated methods for the analysis of radionuclides potentially increase laboratory productivity by reducing operator intervention and increasing the number of samples that can be treated in a given time. To this end, here we report a new openview automated modular separation system which can be used in combination with PSresin, in this case, for the analysis of ⁹⁹Tc. Quality parameters of this method using the automated system were comparable to those obtained manually and quantification of water samples spiked with low levels of ⁹⁹Tc resulted in deviations lower than 10% for all the samples analysed.

     

Homoleptic Ti[ONO]2 type complexes of amino-acid-tethered phenolato Schiff-base ligands: Synthesis,characterization, time-resolved fluorescence spectroscopy,and cytotoxicity against ovarian and colon cancer cells

Six homoleptic Ti (IV) compounds of dianionic tridentate ONO Schiff base ligands with various substitutions were prepared from chiral amino acids, 2-hydroxybenzaldehyde, which were generated in situ, and Ti (OⁱPr)₄. The compounds were spectroscopically characterized and the molecular geometries of five complexes were established by X-ray crystallography; for two structures, two independent molecules compose the asymmetric units. The di-anionic, tridentate ligands coordinate the titanium centers via the carboxylate-O-, imine-N- and phenoxide-O atoms to form five- and six-membered chelate rings. The molecules are based on a trans-N₂O₄ donor with each carboxylate-O atom trans to a phenoxide-O atom. For the smallest derivative with Me substitution, racemization occurs during repeated recrystallization. Photophysical profiles of the titanium compounds in the solid phase and in solution are discussed. Marked cytotoxicities were recorded toward human ovarian A2780 and colon HT-29 cancer cells with IC₅₀ values ranging between 23 ± 2 and 103 ± 3 μM. Comparative hydrolytic stability of the complexes were studied by NMR in 10% D₂O solutions which provided t_1/2 values of up to 15 ± 2 hr.