Unsaturated ketones are of great significance in synthetic organic chemistry as well as a subject for theoretical investigations. a,b­Enones with halogen atoms in vinylic fragment show high reactivity towards nucleophilic reagents and are suitable objects to study halogen atom influence on structure and transmission of the electronic effects in conjugated systems. Fluorine containing compounds are of particular interest because of the small size and unique electronic properties of the fluorine atom in conjugated systems. Fluorinated unsaturated ketones were almost unavailable at the beginning of this study. Fluorine containing 1,3­diaryl-1­propene-3­ones (chalcones) seemed to be the most interesting objects because their properties could be widely modified by substitutients in benzene rings. Therefore the investigation of fluorine influence upon structure and reactivity of unsaturated ketones based on chalcone as a model seemed important from both practical and theoretical points of view.

Conclusions:

• We developed a convenient general method for the synthesis aryl substituted trans-a,b­difluoroenones based on low temperature (-80 -100C) interaction between a,b­difluorostyryllithium derivatives and dimethylformamide or other carboxylic acid dimethylamides [1,2]:

• By the methods X-ray [3], dipole moments, NMR and IR [2-4] it was found that a,b-difluorochalcones exist in non-planar sterically hindered S-trans conformation (Figure 1) unlike hydrogen analogous which are S-cis structure in solid state. Such a structure is evidently caused by a non-valent fluorine atom p,p-interaction between F^b and benzene ring of aroyl fragment of molecule. This assumption corresponds to results of NMR ¹H, ¹³C and ¹⁹F investigation of fluorinated and non-fluorinated chalcones. a­Fluorochalcones can exist in both S-cis and S-trans conformations in solid state (Figure 2) and position of conformational equilibrium in solutions strongly depend on solvent polarity (Figure 3). Stability of S-trans conformers can be the result of p,p-interaction between F^a and O orbitals (similar effect was observed in a-halogen ketones).
• It was found that difluorovinylic alcohols, which are obtained from a,b­difluoroenones, quantitatively transform into correspondent a­fluoroenones. This new rearrangement was in a basis of the in situ method of synthesis a­fluoroketones [2,4]:

• It was shown that difluoroenones could be a source for obtaining various types of fluorine containing organic compounds. It was found that LiBH₄ quantitatively reduces the carbonyl group and difluorovinylic group is a p-orientant in electrophilic reactions of aromatic rings. Vinylic group quantitatively adds halogens (Cl₂, Br₂) forming a mixture of diastereomers. Reactions with C­nucleophiles (malonic acid dinitrile, Wittig reagent, and quaternary salts of 2­methyl-benzothiazole or 1,2,3,3­tetramethylindolenine) involve carbonyl group forming corresponding unsaturated compounds [2]. S- and O­nucleophiles react with b­carbon of difluoroenones substituting F^b. N­nucleophiles can react with carbonyl group and b­carbon atom:

• The basicity of fluoroenones has been studied because many reactions of enones with nucleophilic reagents proceed under acidic catalysis [2,5]. An essential decrease of fluorochalcones basicity as compared to non-fluorinated chalcones makes them considerably less reactive in acid-catalyzed hydrolysis. It was shown that pre-equilibrium (first stage) has a substantial influence on the general reaction rate. Data obtained (pK_BH+ and k₀) can be used for prediction of reactivity fluoroenones depend on reaction condition.

Coumarin, iminocoumarin and carbostyril derivatives are considered as perspective laser dyes in the near UV and visible region. These compounds show effective fluorescence strongly dependent on solvent acidity. In particular well-known laser dye 4­methyl-7­hydroxycoumarin has intensive fluorescence of neutral, anionic and tautomeric forms in a wide spectral region. It was considered that tautomeric form of the dye is generated from anionic form as a result of an excited state proton transfer (ESPT). However, our preliminary studies have shown that carbonyl group bacisity of 7-substituted coumarins increases significantly under electronic excitation [6]. Moreover, it was found that the tautomerism efficiency strongly depends on the nature of proton donor reagent and structure of H­bonded complexes [7]. We studied absorbance and fluorescence of these complexes including time-resolved nanosecond fluorometry technique, obtained bacisity constants in ground and excited states and H-complexes formation constants in non-aqueous systems for coumarin [6-8], iminocoumarin [9] and carbostyril derivatives (see Table).

Table. Constants of acid-base equilibria (pK_BH+ and pK*), H-complex formation (Dn_OH and K_AH) and constants of hydrolysis (k)

Note: pK_BH+ and pK* - constants of acid-base equilibria in S₀ and S₁* states respectively; Dn_OH -spectral shifts in phenol complexes (1:1), K_AH - constants of H-complex formation with trifluoroacetic acid (TFA), k - constants of hydrolysis.

• Tautomerism efficiency in the singlet excited state depends on the structure of H-bonded complex and proton-donor properties of acidic reagent. Detail analysis UV and fluorescence spectra combined with fluorescence quantum yields makes it possible to obtain H-bond formation constants, structure of complexes and their ability to ESPT (Figure 4).
• Pathways of ESPT depend on proton mobility in the system (Figure 5). For example, tautomeric form can be generated from cationic form or synchronously in non-aqueous media (proton transfer can occur only in H-complexes).
• In the case of high proton acceptor ability of compounds tautomeric forms can form in the ground state. It was found that the position of tautomeric equilibria of 7­hydroxyiminocoumarins in S₀-state depends on water concentration in the system. Constants of tautomeric equilibria in ground state were obtained for system EtOH-H₂O:

• Stability of tautomeric forms in ground state depends on electronic structure of compounds. For example, it was found that salt of 2-hydroxy-7-aminobenzopirilium (analog of 7­hydroxycoumarin tautomeric form) is stable in non-aqueous systems though only using photochemical reaction can generate it.

The ability of chiral organic compounds to induce a helical superstructure in the nematic mesophase (i.e. "twisting power") and the dependence of this ability on the molecular structure as well, is an important field of study in the physical chemistry of liquid crystal systems (LCS). This research was concerned with the creation approach to structural chemical problems in prediction of twisting power of chiral organic compounds.

Structure of chiral diastereomeric 1R,4R and 1R,4S E-isomers p­menthane-3­ones [10,11], (­)S­a­phenyl and (­)S­a­benzylethylamines [12], and (1R,4R)2­aroyloxymethylene menthane derivatives [13] have been studied by using X­ray, NMR and molecular mechanics methods. It was shown that the good correlation between X­ray and molecular mechanics data gives the possibility to use structural chemical calculations for analysis of various types of chiral structures for both understanding the phenomenon of nematic phase twisting and searching for new chiral components of induced LCS. High twisting power of these compounds have been analyzed by using dissymmetry functions, axes of polarizability and inertia, anysometry etc. compared to experimental data.

Quantitative investigations of E,Z-photoisomerization of chiral dopants (-)­2­arylidene-p­menthanones were carried out using solvents of different nature and various LC matrices [14,15]. An abnormal high efficiency E,Z-photoisomerization in comparison to backward reaction were interpreted by using quantum chemical calculations (MNDO, AM1) and molecular mechanics. It was found that the twisting power in nematic LCS is essentially lower for the chiral Z-isomers than for the corresponding E forms. The change in the sign of the twisting power and the temperature dependence of the helix step have been determined [16].

It was shown that LCS with helical supermolecular structures induced by chiral photosensitive dopants present interesting perspectives for optical data processing.

Structural and dynamical studies of optically induced effects in Smectic A and C* liquid crystals shown that smectic materials (for example, octylcyanobiphenyl) with photosensitive molecules p,p'-diheptylazobenzene are capable to the formation of light-induced strongly scattering textures which can be stabilized by polymeric network [17]. The written information can be erased by another beam with shorter wavelength.

The prediction of photostability is the essential part of directed search of new effective fluorescent dyes. For this purpose we developed an experimental technique and theoretical approach.

A comparative study of photostability under laser pulsed (XeCl, 308 nm) and lamp (Hg, 313 and 365 nm) excitation has been carried out for the series of 2,5-diarylsubstituted oxazoles, oxadiazoles and 1,3,4­triazoles, which are effective sensitizers and laser dyes.

The transition density matrix technique was used for theoretical analysis of pathways and efficiencies of 5-membered heterocycle photoreactions. We have calculated the indices, which describe photoisomerization probability: excitation localization on the heterocycle atoms and that for the bonds, the indices of bond formation/destruction probability. The indices have been obtained on the base of perturbation theory for excited states. Experimentally observed tendencies for changes of azoles photostability have been interpreted as follows:

- the increase of N atom number in heterocycle molecule leads to the increase of photostability of this molecule. This is explained as a result of the decrease of probability for both the triplet reaction proceeding via bicyclic intermediates (2-5 bond formation) and the singlet isomerization through azirine intermediates (2-4 bond formation);

- the photostability of heterocycles grows considerably with the increase of phenyl group number;

- the quantum yields of 2,5-diaryloxazoles photodestruction are highly influenced by substituent electronic effects compared with 2,5-diaryloxadiazoles and 1,3,4-triazoles.

Application of index approach to similar photoreactions gives the possibility to describe the tendencies in photostability changes as it has been shown for these compounds.

The structure of some sterically hindered fluorescent compounds was studied using X­ray, UV and IR methods (Figure 6). It was shown that an abnormal Stokes shift is caused by the structural relaxation of the excited states [18,19].

Related works:

• Laboratory methods for analysis of chemical equilibria, kinetics of chemical and photochemical reactions (UV, IR and fluorescence).
• Correlations between properties and parameters of organic compounds (UV, IR and NMR spectroscopy, constants of chemical reactions and equilibria).
• Quantum-chemical calculations of spectral and energetic properties of organic compounds (methods PPP, MNDO, AM1)
• Experimental technique for time-resolved nanosecond fluorometry and laser flash photolysis.
• Various aspects of fluorescent analysis (spectral correction of reabsorption, analysis of equipment performance, 3D-spectra of fluorescence).
• Developed equipment (hardware & software) for photochemical and fluorescent experiments. Applications included software support for standard and unique techniques: spectral analysis, kinetics, chromatography, laboratory spectral database, and photochemistry, pre- and postprocessing for quantum chemical and molecular mechanics calculations.

Equipment:

Spectrophotometers: Specord M40, Specord M80, Specord M85, Hitachi-3210
Spectrofluorimeters: Hitachi-850, Hitachi-4010.
Chromatographs: HP-1030, Milichrom-4, Chrom-5.
NMR spectrometers: Bruker-200, Varian-300.
Lasers: XeCl excimer laser (8 ns, 10 -100 Hz)
Nonstandard equipment: Nanosecond spectrofluorometer and photochemical equipment (lamp and laser)

Selected bibliography

1. Yarmolenko S.N., Fialkov Yu.A., Kremlyov M.M., Yagupol'skii L.M. " Fluorine-Containing chalcones. I. Synthesis of a,b­Difluorochalcones", Journal of organic chemistry of the USSR, 1988, Vol. 24, N 12, Part. 2, pp. 2334 - 2339.
2. Yarmolenko S.N., Cherednichenko P.G., Kremlyov M.M., Fialkov Y.A., Yagupol'skii L.M. "Aryl substituted a,b-difluorovinylcarbonyl compounds", Proceedings of the Second Soviet-West German Symposium on Fluorine Chemistry, Novosibirsk, USSR, 1988, pp. 1 - 18.
3. Tret'yak S.M., Yarmolenko S.N., Fialkov Yu.A., Yagupol'skii L.M. " Fluorine-Containing Chalcones. II. Conformations of a,b­Difluorochalcones and Structure of 4'-Methyl- a,b­difluorochalcone", Journal of organic chemistry of the USSR, 1989, Vol. 25, N 9, part. 2, pp. 1762 - 1766.
4. Yarmolenko S.N., Fialkov Yu.A., Yagupol'skii L.M. " Fluorine-Containing Chalcones. III. Rearrangement of 1,3-Diaryl-1-2-difluoro-1-propen-3-ols to a­Fluorochalcones. ", Journal of organic chemistry of the USSR, 1989, Vol. 25, N 9, Part. 2, pp. 1767 - 1774.
5. Yarmolenko S.N., Semenkova G.P., Fialkov Yu.A., Yagupol'skii L.M. " Fluorine-Containing Chalcones IV. Investigation of the Basicity of 4- and 4'-Substituted a­Fluoro- and a,b­Difluorochalcones. Mechanism of Hydrolysis of a,b­Difluorovinylcarbonyl Compounds in Sulfuric Acid-Water System", Journal of organic chemistry of the USSR, 1990, Vol. 26, No 4, Part 2, pp. 726 - 737.
6. Ponomaryov O.A., Vasina E.R., Yarmolenko S.N., Mitina V.G. "Basicity of coumarin derivatives in the ground and excited states", Journal of general chemistry of the U.S.S.R., 1985, Vol. 55, N 1, Part. 2, pp. 158 - 161.
7. Ponomaryov O.A., Vasina E.R., Yarmolenko S.N., Mitina V.G. "Prototropic transformations of 7­hydroxy-4-methylcoumarin in non-aqueous media", Journal of general chemistry of the U.S.S.R., 1988, Vol. 58, N 2, Part 2, pp. 381 - 388.
8. Ponomaryov O.A., Vasina E.R., Yarmolenko S.N., Mitina V.G., Pivnenko N.S. "Acid-base and spectral luminescence properties of derivatives of coumarin and carbostyril", Journal of general chemistry of the U.S.S.R., 1990, Vol. 60, N 5, part 2., pp. 1035 - 1042.
9. Karasev A.A., Lukatskaya L.L., Rubtsov M.I., Zhykol E.K., Yarmolenko S.N., Ponomaryov O.A. "Synthesis, protlytic equilibria and stability of 2-amino-3-(2-benzimidazolyl)-1'-benzopyrilium salts in alchohol-water media", Russian Journal of General Chemistry, 1995, Vol. 65, N 9, pp. 1547 - 1557 (in Russian issue).
10. Kulishov V.I., Kutulya L.A., Tolochko A.S., Vaschenko V.V., Yarmolenko S.N., Mitkevich V.V., Tret'yak S.M. "Molecular and crystal structures of diastereomeric (­)-2[p-chlorbenzoyl)-oxymethylene]-p-menthane-3-ones - effective chiral components of induced cholesteric systems. 1. cis-Diasteromer", Soviet Physics, Crystallography, 1991, Vol. 36, N 5, pp. 669 - 674.
11. Kutulya L.A., Kulishov V.I., Tolochko A.S., Yarmolenko S.N., Vaschenko V.V., Novikova I.E. "Molecular and crystal structures of diastereomeric (-)-2[p-chlorbenzoyl)-oxymethylene]-p-menthane-3-ones - effective chiral components of induced cholesteric systems. II. trans-Diastereomer", Soviet Physics, Crystallography, 1992, Vol. 37, N 1, pp. 52 - 58.
12. Kutulya L.A., Semenkova G.P., Yarmolenko S.N., Fedorkova S., Novikova I.E., Patsenker L.D. "New chiral imines on basis S-a-phenyl- and S-a-benzylethylamines in induced cholesteric and smectic mesophases I. Structure and twisting power of chiral dopants in induced cholesteric mesophases 4-alkyl-4'-cyanobiphenyles", Kristallographiya, 1993, Vol. 38, N 1, pp. 183 - 194.
13. Shapiro Yu.E., Kutulya L.A., Kalyusky A.P., Pivnenko N.S., Vaschenko V.V. "Conformational analysis of diastereomeric (-)-2-aroyloxymethylene-n-menthane-3-ones derivatives by the PMR method I. Trans-diastereomers", Russian Journal of General Chemistry, 1993, Vol. 63, N 6, pp. 1381 - 1390 (in Russian issue).
14. Yarmolenko S.N., Kutulya L.A., Vashchenko V.V., Chepeleva L.V. "Photosensitive chiral dopants with high twisting power" Liquid Crystals, 1994, Vol. 16, N 5, pp. 877 - 882.
15. Yarmolenko S.N., Chepeleva L.V., Kutulya L.A., Vaschenko V.V., Drushlyak T.G., Ponomaryov O.A. "Photochemical properties of chiral 2-arylmethylene-p-menthane-3-ones", Russian Journal of General Chemistry, 1995, Vol. 65, N 1, part 2., pp. 127 - 136.
16. Kutulya L.A., Yarmolenko S.N., Vashchenko V.V., Chepeleva L.V., Patsenker L.D., Ponomarev O.A. "Molecular structure of isomeric 2-arylidene derivatives of p­menthanones and p-(4-menthen)ones and their ability to coil the nematic mesophase", Russian Journal of Physical Chemistry, 1995, Vol. 69, N 1, pp. 80 - 86.
17. Folks W.R., Reznikov Yu.A., Yarmolenko S.N., Lavrentovich O.D. "Light-Induced Periodic Lattice of Defects in Smectic A and C Liquid Crystals: Structural and Dynamical Aspects", Mol. Cryst. Liq. Cryst., 1997, Vol. 292, pp. 183-197.
18. Doroshenko A.O., Patsenker L.D., Baumer V.N., Chepeleva L.V., Van'kevich A.V., Shilo I.P., Yarmolenko S.N., Schershukov V.M., Mitina V.G., Ponomaryov O.A. "Synthesis and structure of sterically crowded polynuclear aryl derivatives of five-membered N-heterocycles, orto-analogs of POPOP", Russian Journal of General Chemistry, 1994, Vol. 64, N 4, Part 2, pp. 588 - 593.
19. Doroshenko O.A., Patsenker L.D., Baumer V.N., Chepeleva L.V., Van'kevich A.V., Kirichenko A.V., Yarmolenko S.N., Scherschukov V.M., Mitina V.G., Ponomaryov O.A. "Structure of sterically hindered aryl derivatives of five-membered nitrogen containing heterocyclic orto-analogs of POPOP", Molecular Engineering, 1994, Vol. 3, pp. 353 - 363.