Juraj Filo

Semiconducting Organic Molecular Materials

Semiconducting Organic Molecular Materials This feature article gives an overview of recent advances in development of high performance molecular organic semiconductors for field-effect transistors with emphasis on the structure of molecular materials and requirements for high-performance.

Organocatalytic diastereoselective synthesis of spirooxindoles via [3+2] cycloadditions of azomethine ylides with α,β-unsaturated esters

An efficient three-component organocatalytic cascade 1,3-dipolar cycloaddition reaction of in situ generated azomethine ylides with α,β-unsaturated esters is described. This reaction afforded a small library comprised of 20 new highly substituted spirooxindole derivatives. The products were obtained in yields up to 76%, usually as single diastereomers. A range of different hydrogen bond donor, Bronsted basic or acidic organocatalysts were tested. Bifunctional thioureas and squaramides were identified as competent catalysts for this transformation. The effect of microwave irradiation, as well as solvent-free conditions, was also examined.

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.

Synthesis and Free Radical Scavenging Activity of New Hydroxybenzylidene Hydrazines

Hydroxybenzylidene hydrazines exhibit a wide spectrum of biological activities. Here, we report synthesis and free radical scavenging activity of nine new N-(hydroxybenzylidene)-N′-[2,6-dinitro-4-(trifluoromethyl)]phenylhydrazines. The chemical structures of these compounds were confirmed by 1H-NMR, 13C-NMR, 19F-NMR, IR spectroscopy, LC-MS, and elemental analysis. The prepared compounds were tested for their activity to scavenge 2,2-diphenyl-1-picrylhydrazyl (DPPH), galvinoxyl radical (GOR), and 2,2′-azino-bis(3-ethylbenzothiazoline)-6-sulphonic acid (ABTS) radicals. The free radical scavenging activity expressed as SC50 values of these compounds varied in a wide range, from a strong to no radical scavenging effect. The most effective radical scavengers were hydroxybenzylidene hydrazines containing three hydroxyl groups in the benzylidene part of their molecules. The prepared compounds were also tested for their activity to inhibit photosynthetic electron transport in spinach chloroplasts. IC50 values of these compounds varied in wide range, from an intermediate to no inhibitory effect.

Photoswitching Behavior of 5-Phenylazopyrimidines: In Situ Irradiation NMR and Optical Spectroscopy Combined with Theoretical Methods

The photoswitching behavior of 5-phenylazopyrimidines was investigated by optical methods and NMR spectroscopy with in situ irradiation sustained by mathematical modeling and DFT calculations. Irradiation of various compounds with electron-donating groups on the pyrimidine ring and substituents with electron-withdrawing as well as electron-donating substituent in the para-position of the phenyl ring were examined. All compounds could be successfully converted to the cis isomer; this isomerization and the subsequent thermal fading were studied. Switching cycles can be repeated without signs of photodegradation for most of the compounds, which makes them adept to molecular photoswitches. Interestingly, the chloro and cyano derivatives can be switched without UV light, which makes them vis(π → π*)-vis(n → π*) photoswitches. Surprisingly equal trans-to- cis photoisomerization quantum yields for π → π* and n → π* excitation indicate the blocking of the inversion pathway following π → π* excitation. In contrast to that, DFT computations suggest the inversion mechanism for the reverse thermal cis-to- trans isomerization of 5-phenylazopyrimidines.

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.