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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 photochemical properties of two basic easily synthesized isatin N(2)-phenylhydrazones were investigated. Contrary to the corresponding isatin N(2)-diphenylhydrazones, only Z-isomers were isolated from the reaction mixtures during the synthesis due to their stabilization by intramolecular hydrogen bonding. Although the presence of the C=N double bond creates conditions for the formation of a simple on-off photoswitch, the low photochemical quantum yield and particularly the low switching amplitude in absorbance hamper their photochromic applications. However, the addition of strongly basic anions to phenylhydrazone solutions leads to isatin NH group deprotonation and creates a new diazene T-type Vis-Vis photochromic system with sufficiently separated absorption maxima. Interestingly, although the thermally stable A-form is also photostable in ambient light, its irradiation with a stronger LED source leads to thermally unstable B-form formation which rapidly isomerizes back to the corresponding A-form. The process is reversible and switching cycles can be repeated in both directions. The important advantages of this two-component organic chromophore-inorganic anion photochromic system are its easy synthesis, easy handling due to its insensitivity to room light, easy further structural modification and reversibility. The corresponding photochemical quantum yield, however, remains relatively low (Φ ∼ 0.001). The theoretically calculated properties are in agreement with the obtained experimental results and support the proposed reaction mechanism.

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 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.

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.

The self-association and tautomerism of (E)-isatin-3-4-phenyl(semicarbazone) and (E)-N-methylisatin-3-4-phenyl(semicarbazone) were investigated in solvents of various polarity. In weakly interacting non-polar solvents, such as CHCl3 and benzene, phenylsemicarbazone concentrations above 1×10−5 mol dm−3 result in the formation of dimers or higher aggregates of E-isomers and . This aggregate formation prevents room temperature E–Z isomerization of and to more stable Z-isomers. In contrast to the situation in non-polar solvents, E–Z isomerization from the monomeric form of phenylsemicarbazone and E-isomers occurs in highly interactive polar solvents including MeOH and DMF only at temperatures above 70 °C. Moreover, decrease in phenylsemicarbazone concentration below 1×10−4 mol dm−3 in these highly solute–solvent interacting systems leads to aggregate dissociation, and a new hydrazonol tautomeric form with a high degree of conjugation predominates in these solutions. Theoretical calculations confirm obtained experimental results. Copyright © 2013 John Wiley & Sons, Ltd.

The photophysical properties of 7-(dimethylamino)coumarin-3-carbaldehyde 3 and its phenylsemicarbazone 4 were investigated in solvents of various polarity and in differing solvent mixtures. The different fluorescent quantum yield (ΦF) behavior of 3 and 4 in highly polar solvents is discussed in terms of Twisted Intramolecular Charge-Tranfer (TICT) state formation and the specific solute-solvent interactions. Because of the weak intermolecular hydrogen bonding ability of both the radiative ICT and nonradiative TICT excited state of 3 and the linear steep decrease in ΦF from a medium to high polarity region, coumarin 3 could be a useful polarity probe for microenvironments containing hydrogen bonding groups. Compared to 3, coumarin 4 exhibits the highest ΦF values in highly polar solvents with strong hydrogen bond acceptor ability. The high quantum yield of fluorescence in DMSO, DMF, and alcohols qualifies coumarin 4 as a laser dye in the given medium, with kF higher than k(nr). Contrary to previous reports that many H-aggregates are nonfluorescent in nature, coumarin 3 forms highly fluorescent H-aggregates in MeOH and EtOH. On the basis of the restrictions of the Kasha-exciton theory model, we assume that the formation of fluorescent H-dimer aggregates of 3 is driven by π(+)-π(-) interactions. To the best of our knowledge, this is the first report on aggregation of coumarin dye in alcoholic solutions. In addition, restrictions in the fitting procedure relating to determination of the solvation number, n, using the Covington-Newman model of preferential solvation and also the solvent nonideality parameter, h', are discussed in this article.