Klaudia Jakusová

Isatin N-phenylsemicarbazone: effect of substituents and concentration on anion sensing selectivity and sensitivity

The effect of substituent and concentration on anion sensing selectivity and sensitivity of nine new easily synthesized isatin N-phenylsemicarbazone E-isomer sensors IIIa–XIa was investigated. The substantial difference between the association constants for IIIa–XIa interaction with strongly and weakly basic anions allows detection of F− or CH3COO− anions even at high weakly basic anion excess. Substitution in position 5- of the isatin ring and para-substitution on the phenyl ring in the phenylsemicarbazide chain influence sensor:anion complex stoichiometry and thus also sensor sensitivity. Detection limits of 3–4 × 10−7 mol dm−3 for F− and CH3COO− anions by sensors IVa and Va bearing electron-withdrawing substituents are among the lowest published detection limits for these anions in organic solvents. The high selectivity and sensitivity of sensor VIa allows confident F− detection at tolerated fluoride drinking-water level. The solution dilution leads to a dramatic change in sensor selectivity. Consequently, one E-isomer can be used to sense both strongly and weakly basic anions. On the other side, higher sensor solution concentrations increase the F− and CH3COO− anion detection range, similar to the additional Z-isomer utilization.

Effect of reactants’ concentration on the ratio and yield of E,Z isomers of isatin-3-(4-phenyl)semicarbazone and N-methylisatin-3-(4-phenyl)semicarbazone

In this work, the effect of inter- and intramolecular interactions of reactants and products, reactants concentration as well as the solvent effect on the ratio of E and Z isomers of isatinphenylsemicarbazones in the reaction mixture is examined. Theoretical calculations proved that Z isomers are more stable than E isomers. Experimental results confirmed the noncovalent intermolecular donor-acceptor interactions of the reactants in the reaction mixture at concentrations above 0.1 mol L−1. The E/Z isomer ratio of isatin-3-(4-phenyl)semicarbazone (I) and N-methylisatin-3-(4-phenyl)semicarbazone (II) depends on the initial concentrations of 3-amino-1-phenylurea (phenylsemicarbazide; V) and 1H-indole-2,3-dione (isatin; III), or 3-methylindol-2,3(1H)-dion (3-methylisatin; IV), respectively. Both isomers exhibit high thermal stability. Thermal E-Z isomerization takes place at temperatures above 70°C in N,N-dimethylformamide.

Coumarin phenylsemicarbazones: sensitive colorimetric and fluorescent “turn-on” chemosensors for low-level water content in aprotic organic solvents

The water sensing properties of four new efficient two-component colorimetric and fluorescent “turn-on” chemosensors based on the coumarin phenylsemicarbazone skeleton bearing different acceptor/donor functional groups on aniline structural subunits were investigated and are discussed in terms of their photophysical properties, interaction mechanism, sensitivity and substituent effects. Due to the sensing mechanism based on reversible acid–base keto/enolate (hydrazide/hydrazonolate) equilibrium, all the studied coumarin phenylsemicarbazone chemosensors provided rapid and reversible response to low-level water content in polar aprotic solvents. To the best of our knowledge, the determined detection limits for water by studied chemosensors are among the lowest published detection limits in the literature and have competitive sensitivity with chemodosimeters. Their deficiency is, however, the necessity of F− base presence.

Isatin N2-diphenylhydrazones: new easily synthesized Vis-Vis molecular photoswitches

The photochromic properties of isatin N2-diphenylhydrazones as a new type of molecular switch were investigated. Interestingly, dimerization stabilizes the formation of only E-isomers in the synthesis, even without a Z-isomer presence in the mother liquor. The irradiation of E-isomers in solution at their absorption maxima leads to the formation of the corresponding Z-isomers. Although the shift in the absorption maximum is not large and photochemical quantum yields are lower compared to in commonly used photoswitches, the absorbance changes are sufficient for potential practical use of these compounds as On-Off switches when the readout wavelength is optimally selected. The process is reversible and switching cycles can be repeated many times in both directions. The determined thermodynamic parameters and calculated geometry of the transition state clearly indicate the inverse mechanism of the thermally initiated slow back Z–E isomerization. Due to the Vis-Vis character of molecular photoswitching and significant geometric changes during the switching process, isatin N2-diphenylhydrazones are eminently suitable for molecular switching in biological/biotechnological applications. Moreover, the interaction of isatin N2-diphenylhydrazones with strongly basic anions increases their functionality and creates a four-state On-Off switch in one molecule. This four-state switch includes the unique E–Z isomerization induced photochemical switching between two stable tautomeric forms of organic amide anions. Simple synthesis of these derivatives and their sufficiently sensitive response to external stimuli promote isatin N2-diphenylhydrazones as suitable candidates for both discussed photochemistry areas (molecular switching and actinometry).

7-Dialkylaminocoumarin Oximates: Small Molecule Fluorescent “Turn-On” Chemosensors for Low-Level Water Content in Aprotic Organic Solvents

The water sensing properties of two efficient two-component fluorescent “turn-on” chemo-sensors based on the 7-dialkylaminocoumarin oxime acid-base equilibrium were investigated. Interestingly, although simple frontier orbital analysis predicts an intramolecular photoinduced electron transfer quenching pathway in conjugated oximates, TD-DFT (Time-dependent density functional theory) quantum chemical calculations support non-radiative dark S1 excited state deactivation as a fluorescence quenching mechanism. Due to the acid-base sensing mechanism and sensitive “turn-on” fluorescent response, both studied coumarin aldoxime chemosensors exhibit rapid response to low-level water content in polar aprotic solvents, with detection limits comparable to chemodosimeters or chemosensors based on interpolymer π-stacking aggregation.

Effect of Structure on Charge Distribution in the Isatin Anions in Aprotic Environment: Spectral Study

Five isatin anions were prepared by deprotonation of initial isatins in aprotic solvents using basic fluoride and acetate anions (F− and CH3COO−). The F− basicity is sufficient to deprotonate isatin NH hydrogen from all the studied compounds. This process is reversible. In the presence of proton donor solvents, the anions form the corresponding isatins. The isatin hydrogen acidity depends on the overall structure of the isatin derivatives. The anions were characterized by ultraviolet–visible (UV–Vis), Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy. Interestingly, the anions form aggregates at concentrations above 10−3 mol·dm−3. Further, the effect of cations on the UV–Vis spectra of the studied anions was studied. Charge transfer and its distribution in the anion depends on the radius and the cation electron configuration. The alkali metal cations, tetrabutylammonium (TBA+), Mg2+ and Ag+, interact with the C-2 carbonyl oxygen of the isatin anion. The interaction has a coulombic character. On the other hand, Cd2+, Zn2+, Hg2+, Co2+, and Cu+ cations form a coordinate bond with the isatin nitrogen.

Isatin pentafluorophenylhydrazones: interesting conformational change during anion sensing

The anion sensing properties of two new easily synthesized isatin pentafluorophenylhydrazone reversible colorimetric chemosensors are studied herein. The F− or CH3COO− anion addition to isatin pentafluorophenylhydrazone solutions in aprotic organic solvents results in hydrazone NH group deprotonation of the initial Z-hydrazo form (acid–base keto/enolate reaction), with significant equilibrium shift to the more conjugated E-azo enolate side. Initial solutions thus turn orange. This fast equilibrium is followed by slow conformational change to the more stable E-azo enolate conformer with the configuration of the E-hydrazo isomer. Interestingly, the F− or CH3COO− detection in semi-aqueous media leads directly to slow formation of the second E-azo enolate conformer, without the observation of an initial fast equilibrium between Z-hydrazo and the corresponding first E-azo enolate conformer. Although the reaction time in semi-aqueous media thus increases to several minutes (tens of minutes), the advantage of isatin pentafluorophenylhydrazone chemosensors still remains their easy synthesis and their reversibility. The determined sensitivity towards F− and CH3COO− anions is among the highest published sensitivity to these anions in organic solvents and allows confident F− detection at the tolerated drinking-water fluoride level in semi-aqueous media.