Jiří Kulhánek

Imidazole as a parent π-conjugated backbone in charge-transfer chromophores

Summary Research activities in the field of imidazole-derived push–pull systems featuring intramolecular charge transfer (ICT) are reviewed. Design, synthetic pathways, linear and nonlinear optical properties, electrochemistry, structure–property relationships, and the prospective application of such D-π-A organic materials are described. This review focuses on Y-shaped imidazoles, bi- and diimidazoles, benzimidazoles, bis(benzimidazoles), imidazole-4,5-dicarbonitriles, and imidazole-derived chromophores chemically bound to a polymer chain.

Ferrocene-Donor and 4,5-Dicyanoimidazole-Acceptor Moieties in Charge-Transfer Chromophores with π Linkers Tailored for Second-Order Nonlinear Optics

A series of new nonlinear optical chromophores (1-15) that were comprised of ferrocene-donor and 4,5-dicyanoimidazole-acceptor moieties and various π linkers of different length were synthesized. Support for the presence of significant DA interactions in these NLO-phores was obtained from the evaluation of the quinoid character of the 1,4-phenylene moieties and their electronic absorption spectra, which featured intense high-energy (HE) bands that were accompanied by less-intense low-energy (LE) bands. The redox behavior of these compounds was investigated by cyclic voltammetry (CV) and by rotating-disc voltammetry (RDV); their electrochemical gaps decreased steadily from 2.64 to 2.09 V. In addition to the experimentally obtained data, DFT calculations of their absorption spectra, HOMO/LUMO levels, and second-order polarizabilities (β) (-2ω,ω,ω) were performed. A structure-property relationship study that was performed by systematically altering the π linker revealed that the intramolecular charge-transfer and nonlinear optical properties of these inorganic-organic hybrid D-π-A systems (1-15) were primarily affected by: 1) The presence of olefinic/acetylenic subunits; 2) the length of the π linker; and 3) the spatial arrangement (planarity) of the π linker.

Optical operation by chromophores featuring 4,5-dicyanoimidazole embedded within poly(methyl methacrylate) matrices.

We have studied photoinduced absorption, birefringence, and optical second-harmonic generation in poly(methyl methacrylate) (PMMA) films doped by organic chromophores featuring 4,5-dicyanoimidazole in the weight content equal to 5%. The chromophores indicated as IM1-IM6 were synthesized from 2-bromo-1-methylimidazole-4,5-dicarbonitrile by either nucleophilic substitution or Suzuki-Miyaura cross-coupling reaction. The samples were obtained as films of several micrometers thickness by the spin-coating method on a quartz substrate. Measurements of the optically induced birefringence were done by the Senarmont method at wavelength 1150 nm, and photoinduced absorption was studied in the spectral range 250-700 nm under optical treatment by 300 mW cw 532 nm laser. Photoinduced optical effects were studied by bicolor 1064 and 532 nm coherent laser pulses. The maximal changes were observed for the ratio between fundamental and writing beam intensities equal to about 7:1. To interpret the observed experimental measurements, theoretical simulations of photoinduced optical properties were performed by quantum chemical computational methods.

Acidity of ortho-substituted benzoic acids: an infrared and theoretical study of the intramolecular hydrogen bonds.

The structures of ortho-substituted benzoic acids with substituents bearing hydrogen atoms (OH, NH2, COOH and SO2NH2) were investigated by means of IR spectroscopy and of density functional theory at the B3LYP/6-311 + G(d,p) level. All possible conformations, hydrogen bonds, tautomeric forms and zwitterions were taken into consideration and particular attention was given to intramolecular H-bonds and their effect on acidity. Strong H-bonds in the anions of all four acids, were revealed by calculations. In three cases they were confirmed by the IR spectra of the tetrabutylammonium salts in tetrachloromethane solution, while the salt of 1,2-benzenedicarboxylic acid was not sufficiently soluble. The H-bonds are of different strengths but in all cases they are the main cause of the strengthened acidity of these acids in the gas phase and also in solution, although their effect is opposed by weaker H-bonds present in the undissociated acid molecules. The substituent effect on the acidity was evaluated in terms of isodesmic reactions, separately in the acid molecules and in the anions. While the acidity of the 2-OH and 2-NH2 acids is determined essentially by the H-bonds, that of the 2-COOH and 2-SO2NH2 acids is strengthened by the polar effect operating in the undissociated molecule in addition to the H-bond in the anion. The steric inhibition of resonance (SIR), estimated from model conformations with fixed torsional angles, is of little importance. This analysis goes significantly beyond the classical explanation obtained from the acidities in solution but essentially conforms with it.

Peculiarities of the Environmental Influence on the Optical Properties of Push–Pull Nonlinear Optical Molecules: A Theoretical Study

Gas-phase geometry optimization of NLO-active molecules is one of the standard approaches in the first principle computational methodology, whereas the important role of the environment is usually not considered during the evaluation of structural parameters. With a wide variety of environmentally influenced models in most cases only the high quality single point calculations are prepared. Among different approaches, the most used polarizable continuum model (PCM) seems to be promising. In this study, we have compared the electronic properties of gas-phase optimized geometries of imidazole-derived push-pull compounds with those optimized using PCM solvation approach including CH(2)Cl(2) and PMMA as media. We have focused particularly on the linear optical properties of investigated molecules, namely on the UV-vis absorption spectra. The analysis of presented results shows the applicability of the different quantum chemical (QC) methods for the UV-vis spectra calculations of linear NLO molecules. Herein we also present the need of molecule geometry optimization affected by the environment. Following the performed calculations, the electronic properties of gas-phase optimized molecules give conformable results with respect to those obtained by more time-consuming continuum optimizations. All computational data are supported by experimental investigations.

Convenient methods for preparing π-conjugated linkers as building blocks for modular chemistry

Summary Simple, straightforward and optimized procedures for preparing extended π-conjugated linkers are described. Either unsubstituted or 4-donor substituted π-linkers bearing a styryl, biphenyl, phenylethenylphenyl, and phenylethynylphenyl π-conjugated backbone are functionalized with boronic pinacol esters as well as with terminal acetylene moieties allowing their further use as building blocks in Suzuki–Miyaura or Sonogashira coupling reactions.

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.

STERIC EFFECTS AND STERIC HINDRANCE TO RESONANCE IN TERT-BUTYLBENZOIC ACIDS IN THE GAS PHASE AND IN SOLUTION

Substituent effects of the tert-butyl group in isomeric tert-butylbenzoic acids have been investigated on the basis of enthalpies of formation, gas-phase acidities, acidities in methanol and in dimethyl sulfoxide, and the IR spectra in tetrachloromethane. In contrast to 2-methylbenzoic acid, 2-tert-butylbenzoic acid must adopt a non-planar conformation, as is confirmed by its IR spectrum. Enhanced acidity in the gas phase has been found for all isomers and attributed to a pole-induced dipole interaction in the anion (polarizability effect); in the ortho isomer the steric inhibition of resonance is only responsible for a minor part of the observed effect. In solution, the electrostatic interaction is attenuated, but remains strong in the case of the ortho isomer.

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.

Solvation and steric hindrance in methyl-substituted benzoic acids

Dissociation constants of all isomers of methyl-substituted benzoic acids have been measured in methanol and in dimethyl sulfoxide. From the pK values, the substituent effects of the methyl groups are calculated and tentatively divided into polar and steric effects. Also, in the case of polymethyl derivatives, the buttressing effect is calculated with reference to monomethyl derivatives. These quantities are processed within the framework of correlation analysis using as reference the corresponding quantities in the gas phase. The steric effects may be classified as steric hindrance to resonance—observed only in derivatives with two ortho methyl groups, and electrostatic induction in the deprotonated molecules—observed in all derivatives. Both effects make the acids stronger and both are attenuated in solution, in methanol more than in DMSO. The electrostatic effect is attenuated in a specific way: the effect of more distant methyl groups is practically cancelled and only that of the ortho groups remains. As a result, the overall substituent effects are rather different in the two solvents and in the gas phase.