A new fluorescent pH probe for extremely acidic conditions

A novel turn-off fluorescent probe based on coumarin and imidazole moiety for extremely acidic conditions was designed and developed. The probe with pK_a = 2.1 is able to respond to very low pH value (below 3.5) with high sensitivity relying on fluorescence quenching at 460 nm in fluorescence spectra or the ratios of absorbance maximum at 380 nm to that at 450 nm in UV–vis spectra. It can quantitatively detect pH value based on equilibrium equation, pH = pK_a − log[(I_x − I_b)/(I_a − I_x)]. It had very short response time that was less than 1 min, good reversibility and nearly no interference from common metal ions. Moreover, using ¹H NMR analysis and theoretical calculation of molecular orbital, we verified that a two-step protonation process of two N atoms of the probe leaded to photoinduced electron transfer (PET), which was actually the mechanism of the fluorescence quenching phenomenon under strongly acidic conditions. Furthermore, the probe was also applied to imaging strong acidity in bacteria, E.coli and had good effect. This work illustrates that the new probe could be a practical and ideal pH indicator for strongly acidic conditions with good biological significance.     

A highly selective and ratiometric fluorescence probe for the detection of Hg and pH change based on coumarin in aqueous solution

A simple but highly selective coumarin-based fluorescence probe 1, 8-(1,3-dithiane)-7-hydroxycoumarin was designed and synthesized for both the ratiometric detection of Hg²⁺ and the on–off response to pH change in aqueous solution. The sensor detected Hg²⁺ selectively via Hg²⁺-promoted thioacetal deprotection reaction within five minutes and reflected pH in the range from 7.8 to 11.9 as a result of the equilibrium between weak-fluorescent acid form and strong-fluorescent base form. In addition, the probe has an excellent selectivity towards Hg²⁺ over other competitive metal ions for biomedical and environmental applications. The sensing behavior of our probe was studied by UV–visible absorption spectra and fluorescence spectra.     

Recent Advances in the Synthesis of Coumarin Derivatives via Knoevenagel Condensation: A Review

The synthesis of coumarins through Knoevenagel condensation is one of the most important processes in synthetic organic chemistry and medicinal chemistry. Compounds including a coumarin (2-oxo-2H-1-benzopyran) backbone have a wide range of application in the pharmaceutical field. Thus, many methodologies have been developed for the synthesis of this important class of compounds. However, some methods are always associated with toxic and corrosive catalysts, longer reaction time, poor yield, less purity, and by-products along with the desired product. Furthermore, some of these processes are not efficient and environmentally friendly. Therefore, mild, efficient, and environmentally friendly protocols have been developed recently by many scientists for the synthesis of coumarin derivatives via Knoevenagel condensation with good yield and purity. In this review, we have summarized various methods for the synthesis of coumarins via Knoevenagel condensation.     

高效可回收催化剂In(OTf)_3用于无溶剂条件下Pechmann缩合反应(英文)

In(OTf)3 plays the role of a Lewis acid catalyst in the Pechmann condensation of phenols with β-ketoesters under solvent-free conditions to give coumarin derivatives. This novel and inexpensive method has advantages such as short reaction time, excellent product yields, and avoids the use of organic solvents in agreement with green chemistry principles. Catalyst loadings can be as low as 1 mol% to give high yields of the corresponding coumarins at 80 °C. The catalyst can be recovered after the reaction, and reused with only a slight decrease in the yield.     

Experimental investigations on energy-transfer characteristics and performance of some laser dye mixtures

Energy transfer from both Coumarin 120 (C120) and p-Bis(o-methylstyryl)-benzene (Bis-MSB), energy donors, to coumarin 7(C7) (acceptor) individually has been studied by steady-state emission measurements in methanol. The dye laser characteristics of the above-mentioned dyes have also been studied with respect to the energy transfer mechanism in the same solvent using a pulsed nitrogen laser as a pumping source. The large values of the critical transfer distances, R₀, indicate that the dominant mechanism responsible for energy transfer is due to long-range dipole–dipole interaction between the excited donor and ground state acceptor molecule. Both photophysical and photochemical stability of the donors has been examined under the effect of N₂ laser pumping.     

BrMOZPhC, a novel coumarin type reagent for the fluorescent derivatisation of carboxylic acids

A new fluorescent reagent was synthesized in order to be used as a fluorescent probe in analytical HPLC derivatisation reactions. Briefly, 2,4-dimethoxyaniline was acylated and demethylated and the resulting diphenol reacted with α-bromoacetoacetate in the presence of conc. sulfuric acid (the Von Pechmann reaction). The resulting acetamido coumarin was hydrolyzed and the aminohydroxy derivative cycled to form the corresponding bromo-coumarinoxazine. The analytical reaction was performed with butyric acid as model compound and catalyzed by 18-crown-6 ether. The calibration curve constructed was characterized by good linearity whereas the precision of the reaction gave an R.S.D. values below 3%. The reaction applied to the separation of five aliphatic acids.     

Lifetime determination of materials that exhibit a stretched exponential luminescent decay

We report frequency- and time-domain luminescent lifetime measurements for materials that exhibit a broad spectrum of lifetimes, specifically, those for which the observed decay dynamics can be described by a stretched exponential function. The spectrum of lifetimes of such materials can be characterized by an average lifetime, and in principle this average can be extracted from either time- or frequency-domain measurements. In practice, this requires some care, because the extent to which the various states are excited depends on how long their lifetime is relative to the excitation pulse width or period. For the complex luminescent materials ZnS:Cu,Al and CdS quantum dots we compare the average lifetime obtained from frequency-domain data and from time-domain data under both pulsed laser excitation and steady-state, dc excitation. The agreement between the average measured lifetimes is good, but not perfect, showing that quoted average lifetimes for complex materials can be dependent on the measurement technique. Finally, a spectrum of lifetimes is given that gives rise to stretched exponential relaxation and this spectrum is used to compute frequency-domain data for a stretched exponential material.     

An efficient one-pot synthesis of coumarins mediated by propylphosphonic anhydride (T3P) via the Perkin condensation

An efficient one-pot synthesis of coumarins mediated by T3P, a mild and low toxic peptide coupling agent, via the Perkin condensation has been demonstrated.     

Neutral cholic acid–coumarin conjugate exhibit excellent anion binding properties by cooperative aryl CH and amide NH segments

A neutral cholic acid–coumarin conjugate was developed for anion recognition. It is revealed by the experimental and theoretical results that the coumarin group can provide CH segments as hydrogen bond donors by cooperation with the adjoining amide NH segments. With excellent biocompatibility, this receptor with coumarin as fluorescence sensors also have the potential to be used as an efficient and non-destructive probe for anion detection in living cells. This work displays a new insight into the importance of coumarin group as anion recognition group, which is not well presented so far.     

Recent advances in the microwave- and ultrasound-assisted green synthesis of coumarin-heterocycles

Given the importance of the highly reactive nature and attractiveness of coumarins as a precursor of natural products and pharmacological agents, tremendous efforts have been dedicated to their synthesis in the last decades. Starting from the Knoevenagel, Perkin, Kostanecki-Robinson, Pechmann, Reformansky, Baylis Hillman reactions to several new one-pot multicomponent, sequential tandem reactions have been devised. After the emergence of non-conventional energy sources like microwaves (MW) and ultrasound irradiation, the field of organic synthesis has reached an outstanding level in this century in terms of synthetic efficiency as well as green chemistry viewpoint. Enlightened by this, a great deal of attention has been paid to the synthesis of bioactive coumarin-heterocycles by employing the non-conventional approach and a vast array of synthetic procedures has been established. Therefore, a time to time investigation is required for the synthesis and biological aspects of these hybrid molecules. The present review aims to highlight the current progress achieved in the synthesis of coumarins either linked or fused with diverse bioactive five- and six-membered heterocycles by making the utilization of microwaves and ultrasound-assisted strategies from 2014 to date. Besides highlighting the development achieved in this field, we have attempted to point out the drawbacks and challenges associated with the reaction discovery, which would hopefully provide the impetus for future exploration.