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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.

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.

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).

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.

The light and thermally initiated mutual E- and Z-isomer transformations of two efficient isatin N-phenylsemicarbazone colorimetric sensors for strongly basic anions differing in sensitivity, selectivity and sensing mechanism were investigated in solvents of different polarity. The reaction mechanism of photochemically and thermally initiated E–Z isomerization and the absence of back Z–E isomerization is discussed in terms of self-aggregate formation, hydrazide-hydrazonol tautomeric equilibrium, the role of intra- and inter-molecular hydrogen bonds, excited-state proton transfer, solvent polarity and concentration effects. The effect of the activation energies related to conformational and geometric changes of the isomers and the possible analytical application were investigated. Theoretically calculated properties are in good agreement with obtained experimental results and support the proposed reaction mechanism.

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.