Molecular modeling of 3,4-pyridinedicarbonitrile dye sensitizer for solar cells using quantum chemical calculations

The geometries, electronic structures, polarizabilities, and hyperpolarizabilities of organic dye sensitizer 3,4-pyridinedicarbonitrile was studied based on Hartree–Fock (HF) and density functional theory (DFT) using the hybrid functional B3LYP. Ultraviolet–visible (UV–Vis) spectrum was investigated by time dependent DFT (TD-DFT). Features of the electronic absorption spectrum in the visible and near-UV regions were assigned based on TD-DFT calculations. The absorption bands are assigned to π → π1 transitions. Calculated results suggest that the three lowest energy excited states are due to photoinduced electron transfer processes. The interfacial electron transfer between semiconductor TiO₂ electrode and 3,4-pyridinedicarbonitrile is due to electron injection process from excited dye to the semiconductor’s conduction band. The role of cyanine in 3,4-pyridinedicarbonitrile in geometries, electronic structures, and spectral properties were analyzed.     

Reactivity of molecular oxygen with ethoxycarbonyl derivatives of tetrathiatriarylmethyl radicals

Tetrathiatriarylmethyl (TAM) radicals are commonly used as oximetry probes for electron paramagnetic resonance imaging applications. In this study, the electronic properties and the thermodynamic preferences for O2 addition to various TAM-type triarylmethyl (trityl) radicals were theoretically investigated. The radicals' stability in the presence of O2 and biological milieu was also experimentally assessed using EPR spectroscopy. Results show that H substitution on the aromatic ring affects the trityl radical's stability (tricarboxylate salt 1-CO2Na > triester 1-CO2Et > diester 2-CO2Et > monoester 3-CO2Et) and may lead to substitution reactions in cellular systems. We propose that this degradation process involves an arylperoxyl radical that can further decompose to alcohol or quinone products. This study demonstrates how computational chemistry can be used as a tool to rationalize radical stability in the redox environment of biological systems and aid in the future design of more biostable trityl radicals.     

Michael addition of indoles to β-nitrostyrenes catalyzed by HY zeolite under solvent-free conditions

Michael addition of indoles to β-nitrostyrenes is reported using HY zeolite as catalyst under mild reaction conditions. This methodology allows the synthesis of various indole derivatives in good to high yields at 50 °C under solvent-free conditions. The short reaction time and achieving high yield of the desired products are the main advantages of the present work. The catalyst can be easily recovered and reused for six successive runs without considerable changes in yields. This Michael addition catalyzed by HY zeolite is operationally simple and can be considered as a greener protocol as it avoids the use of corrosive acids and toxic reagents.     

Highly Selective “Turn‐On” Fluorescent and Colorimetric Sensing of Fluoride Ion Using 2‐(2‐Hydroxyphenyl)‐2,3‐dihydroquinolin‐4(1 H)‐one based on Excited‐State Proton Transfer

A simple, highly selective and sensitive colorimetric system for the detection of fluoride ion in an aqueous medium has been developed using 2‐(2‐hydroxyphenyl)‐2,3‐dihydroquinolin‐4(1 H)‐one. This system allows selective “turn‐on” fluorescence detection of fluoride ion, which is found to be dependent upon guest basicity. An excited‐state proton transfer is proposed to be the signaling mechanism, which is rationalized by DFT and TD‐DFT calculations. The present sensor can also be applied to detect fluoride levels in real water samples.     

Stereoselective synthesis of the lichen metabolite, (+) montagnetol and its congeners as antimicrobial agents

Abstract

In view of structural diversity, (+) montagnetol, the major metabolite of the fruticose lichen, Roccella montagnei was synthesized along with three of its congeners by employing highly efficient protocols. (+) Montagnetol (2?R, 3S; 11) and (-) montagnetol (2S, 3R; 5) were synthesized in 7 and 9 steps, respectively, from L-ascorbic acid. The two new congeners 3 (2?R, 3R) and 6 (2S, 3S), which differ in configuration at C-2 and C-3 positions of the (+) montagnetol, were synthesized from (?) diethyl D-tartrate and (+) diethyl L-tartrate, respectively. The synthesized compounds were evaluated in vitro for antimicrobial activity against two Gram-positive (S. aureus and E. coli) and two Gram-negative (S. typhi and P. aeruginosa) bacteria and one fungal strain Candida albicans. Interestingly, the congener 3 showed promising anti-bacterial activity (MIC: 0.062?µg/ml) against P. aeruginosa, whereas the congener 6 displayed potent anti-fungal activity (MIC: 0.062?µg/ml) against C. Albicans.     

Chiral α-alkylation/arylation in 1-phenyl-2-(1-pyrrolidinyl)-1-propanol through Grignard reactions

Complete asymmetric induction has been achieved during Grignard alkylations/arylations resulting in (1S,2R)- and (1R,2R)-1-phenyl-1-alkyl/aryl-2-(1-pyrrolidinyl)-1-propanols which are isolated as hydrochlorides.     

In vivo EPR imaging of the distribution and metabolism of nitroxide radicals in human skin

While altered cellular free radical and redox metabolism are critical factors in many human diseases, it has not been previously possible to both measure and image these processes in humans. The development and application of electron paramagnetic resonance instrumentation capable of in vivo spectroscopy and imaging of free radicals in human skin are reported. The instrumentation uses a specially designed topical resonator and a 2.2-GHz microwave bridge. Noninvasive measurements of the distribution and metabolism of the topically applied nitroxide, (15)N-perdeuterated tempone (100 mM), in forearm skin were performed. A single broad peak due to the concentrated label at the skin surface was initially observed, followed by a sharp doublet from the diluted label that permeated the skin. The penetration of the label into the skin and its metabolic clearance were modeled using kinetic equations. It was observed that the penetration process from the skin surface into the dermis and subcutaneous regions, as well as its clearance from these regions, could be described by single exponential functions. Phantom imaging experiments using the nitroxide showed that a spatial resolution of up to 50 microm could be achieved. The skin imaging measurements showed two bands in the distribution of the label along the skin depth. The first band appeared in the outer 400 microm of the skin, the epidermis region, whereas the second band was centered at a depth of 1000 microm in the subcutaneous region with a thickness about 400 microm. These two bands decayed and merged into a single band with time. The results are important in the understanding of the permeability and metabolism of free radicals in human skin.     

Application of magnetic field over-modulation for improved EPR linewidth measurements using probes with Lorentzian lineshape

Magnetic field modulation in CW electron paramagnetic resonance (EPR) is used for signal detection. However, it can also distort signal lineshape. In experiments where the linewidth information is of particular importance, small modulation amplitude is usually used to limit the lineshape distortion. The use of small modulation amplitude, however, results in low signal-to-noise ratio and therefore affects the precision of linewidth measurements. Recently, a new spectral simulation model has been developed enabling accurate fitting of modulation-broadened EPR spectra in liquids. Since the use of large modulation amplitude (over-modulation) can significantly enhance the EPR signal, the precision of linewidth measurements is therefore greatly improved. We investigated the over-modulation technique in EPR oximetry experiments using the oxygen-sensing probe lithium octa-n-butoxy-substitued naphthalocyanine (LiNc-BuO). Modulation amplitudes 2-18 times the intrinsic linewidth of the probe were applied to increase the spectral signal-to-noise ratio. The intrinsic linewidth of the probe at different oxygen concentrations was accurately extracted through curve fitting from the enhanced spectra. Thus, we demonstrated that the over-modulation model is also applicable to particulate oxygen-sensing probes such as LiNc-BuO and that the lineshape broadening induced by oxygen is separable from that induced by over-modulation. Therefore, the over-modulation technique can be used to enhance sensitivity and improve linewidth measurements for EPR oximetry with particulate oxygen-sensing probes with Lorentzian lineshape. It should be particularly useful for in vivo oxygen measurements, in which direct linewidth measurements may not be feasible due to inadequate signal-to-noise ratio.     

EPR oximetry in three spatial dimensions using sparse spin distribution

A method is presented to use continuous wave electron paramagnetic resonance imaging for rapid measurement of oxygen partial pressure in three spatial dimensions. A particulate paramagnetic probe is employed to create a sparse distribution of spins in a volume of interest. Information encoding location and spectral linewidth is collected by varying the spatial orientation and strength of an applied magnetic gradient field. Data processing exploits the spatial sparseness of spins to detect voxels with nonzero spin and to estimate the spectral linewidth for those voxels. The parsimonious representation of spin locations and linewidths permits an order of magnitude reduction in data acquisition time, compared to four-dimensional tomographic reconstruction using traditional spectral-spatial imaging. The proposed oximetry method is experimentally demonstrated for a lithium octa-n-butoxy naphthalocyanine (LiNc–BuO) probe using an L-band EPR spectrometer.