Oldřich Pytela

N,N′-Dibutylbarbituric acid as an acceptor moiety in push–pull chromophores

Twelve novel D–π–A chromophores with the N,N′-dibutylbarbituric acid acceptor, the N,N-dimethylamino donor and a systematically extended π-linker were synthesized. The extent of intramolecular charge-transfer, structure–property relationships and nonlinear optical properties were further investigated by X-ray analysis, electrochemistry, UV/Vis absorption spectra, calculations and EFISH experiments.

Dicyanopyrazine-derived push–pull chromophores for highly efficient photoredox catalysis

Here, we report dicyanopyrazine (DPZ)-derived push–pull chromophores, easily prepared and tunable organic compounds, as new kinds of photoredox catalysts. In particular, the DPZ derivative H, containing 2-methoxythienyl as electron-donating moiety, exhibits a broad absorption of visible light with an absorption edge up to 500 nm and excellent redox properties, and has been demonstrated as a desirably active and efficient photoredox catalyst in four challenging kinds of photoredox reactions. The amount of catalyst in most reactions is less than 0.1 mol% and even 0.01 mol%, representing the lowest catalyst loading in the current photoredox organocatalysis.

Solvent and branching effect on the two-photon absorption properties of push–pull triphenylamine derivatives

The photophysical and two-photon absorption (2PA) properties of two tri-podal molecules and of their quadrupolar and dipolar counterparts are reported for a series of solvents with varying polarity. The molecules possess a tri-phenylamine electron donating group and mono-cyano acceptors while olefinic and acetylenic π-linkers have been used. Branching led to an increase of the molar extinction coefficient and to a slight bathochromic shift of the absorption spectra while the fluorescence quantum yields decrease but they are maintained to relatively high values. Solvatochromic measurements in the tri-podal molecules revealed an emitting state with a polar nature. The 2PA cross sections in general increase upon branching but the observed behaviour strongly depends on the type of solvent. The highest 2PA cross sections are obtained in solvents of medium polarity and values as high as 1420 GM are reported.

Modulation of (non)linear optical properties in tripodal molecules by variation of the peripheral cyano acceptor moieties and the π-spacer

A series of twelve tripodal push-pull molecules with a central triphenylamine donor and peripheral cyano substituted acceptors has been prepared. These molecules possess systematically altered π-linkers as well as cyano acceptors. Based on the experimental properties measured by differential scanning calorimetry, electrochemistry, one and two photon absorption/emission spectroscopy, supported by the DFT calculations, thorough structure–property relationships were elucidated.

Effect of intercalation and chromophore arrangement on the linear and nonlinear optical properties of model aminopyridine push–pull molecules

Three push–pull aminopyridine derivatives having D–π–A, D–(π–A)2, and D–(π–A)3 arrangements were examined as model organic chromophores capable of intercalation into inorganic layered materials (alpha modification of zirconium hydrogen phosphate, zirconium 4-sulfophenylphosphonate, and gamma modification of titanium hydrogen phosphate). The fundamental properties of these dyes, their methylated analogues as well as their intercalates were studied by X-ray analysis, electrochemistry, UV/Vis absorption spectra, TGA, IR spectra, SHG, and were completed by DFT calculations. The synthesis of tripodal tris(pyridin-4-yl)amine is given for the first time. The HOMO–LUMO gap, the position of the longest-wavelength absorption maxima, and the dipole moment of aminopyridines can easily be tuned by attaching/removing pyridin-4-yl electron withdrawing units and their quaternization (pyridine vs. pyridinium acceptors). Their intercalation proved to be feasible affording novel inorganic–organic hybrid materials. The intercalation is accompanied by protonation of the guest, which enhances its ICT and strongly anchors the aminopyridines into the confined space of the layered host. Moreover, this process results in ordering of the organic chromophores and also brings improved thermal and chemical robustness. As a result, the measured SHG efficiencies of the intercalates are larger than those observed for the pure organic push–pull chromophores. Hence, the methodology of intercalation turned out to be very useful strategy for property tuning of NLO-active organic molecules.

Dipolar NLO Chromophores Bearing Diazine Rings as π-Conjugated Linkers

The synthesis of a series of push-pull derivatives bearing triphenylamine electron-donating group, cyclopenta[c]thiophen-4,6-dione electron acceptor and various π-linkers including (hetero)aromatic fragments is reported. All target chromophores with systematically varied π-linker structure were further investigated by electrochemistry, absorption measurements, and EFISH experiments in conjunction with DFT calculations. Based on electrochemical and photophysical measurements, when a polarizable 2,5-thienylene moiety is embedded into the chromophore π-backbone the highest intramolecular charge transfer (ICT) is observed. Benzene, pyrimidine, and pyridazine derivatives exhibit lower polarizability and extent of the ICT across these π-linkers. The elongation of the π-conjugated system via additional ethenylene linker results in a significant reduction of the HOMO-LUMO gap and an enhancement of the NLO response. Whereas it does not significantly influence electrochemical and linear optical properties, the orientation of the pyrimidine ring seems to be a key parameter on the μβ value due to significant variation of the dipolar moment (μ) value. In 2a and 2c, pyrimidine is oriented to behave as an acceptor and thus generate dipolar molecule with μ above 5 D, whereas in 2b and 2d ground state dipole moment is significantly reduced. This study seems to indicate a high aromaticity of pyrimidine and pyridazine derivatives, close to the benzene analogues and significantly higher than thiophene analogues.

Imidazole as a Donor/Acceptor Unit in Charge‐Transfer Chromophores with Extended π‐Linkers

Eleven new, stable, push-pull systems that feature 4,5-bis[4-(N,N-dimethylamino)phenyl]imidazole and 4,5-dicyanoimidazole as the donor and acceptor moieties and the systematically extended and varied π-linker were prepared and investigated. Evaluation of the measured UV/Vis spectra, electrochemical data (cyclic voltammetry (CV), rotating-disc voltammetry (RDV), and polarography) and calculated β and γ polarizabilities showed efficient charge transfer (CT) in biimidazole-type chromophores. Push-pull system 27, which features a planar thiophene-derived π-linker, was revealed to be the most efficient chromophore within the studied series. This chromophore possessed the most bathochromically shifted CT band, the lowest electrochemical gap, and highest β and γ polarizabilities. The CT transition was most significantly affected by structural features such as π-linker length, planarity, conjugating arrangement, and the presence of olefinic/acetylenic or 1,4-phenylene/thiophene subunits in the π-linker.

Imidazole-based Potential Bi- and Tridentate Nitrogen Ligands: Synthesis, Characterization and Application in Asymmetric Catalysis

Twelve new imidazole-based potential bi- and tridentate ligands were synthesized and characterized. Whereas in the first series the α-amino acid and imidazole moieties were linked by an amino bond, in the second series the tridentate ligands, containing two imidazole groups, were separated by an amide bond. The first series was obtained by the reductive amination of 2-phenylimidazole-4-carboxaldehyde with α‑amino acid esters. The tridentate ligands were prepared from 2‑phenylimidazole-4-carboxylic acid and chiral amines. In the Henry reaction, the amines were revealed as a more reactive species than the less nucleophilic amides, however the enantiomeric excesses were generally poor.

Multipodal arrangement of push–pull chromophores: a fundamental parameter affecting their electronic and optical properties

A series of model push–pull molecules with linear, quadrupolar, and tripodal arrangements, a varyingly substituted amino donor, two acceptors, and a partially extended π-system has been prepared. Two peripheral electron acceptors, namely N,N′-dibutylbarbituric acid and dicyanovinyl, were employed. The fundamental properties of 24 push–pull chromophores were investigated by differential scanning calorimetry, electrochemistry, one-photon absorption spectroscopy, photoinduced piezooptics, and were supported by DFT calculations. Thorough structure–property relationships were elucidated, while a significant influence of the structural arrangement/branching on the electronic and optical properties has been revealed. The fundamental optoelectronic properties of push–pull molecules are affected by their arrangement (linear/quadrupolar/tripodal), the peripheral acceptor attached, extension and planarization of the π-system, and also by the type of auxiliary N-substituent.

Relative ionization cross-sections of oxygenated C(4) molecules

Abstract A method is presented for the determination of relative electron impact ionization cross-sections for polar and less volatile compounds. The compound is generated in the gas phase by flash-vacuum pyrolysis of a norbornene precursor, whereby the cyclopentadiene co-formed in a stoichiometric amount serves as internal standard. Cross-sections of 31 oxygenated C (4) molecules have been measured and tested for correlation with several molecular and bulk parameters