V. G. Avakyan

Synthesis, Structure, Spectroscopic Studies, and Complexation of Novel Crown Ether Butadienyl Dyes

Butadienyl dyes of the benzothiazole series with various fragments of benzocrown ethers 1a-c were synthesized for the first time. The structures and spectral properties of crown-containing butadien ...

Very low pressure pyrolysis (VLPP) of monosilacyclobutanes. Infrared absorptions of 1,1-dimethyl-1-silaethylene, (CH3)2SiCH2, and 1,1-dideuteriomethyl-1-silaethylene, (CD3)2SiCH2 isolated in argon matrices

Abstract The pyrolysis products of 1,1-dimethyl-1-silacyclobutane (DMSCB) and 1,1,3-trimethyl-1-silacyclobutane (TMSCB), and also of the Si-deuteriomethyl analogs (DMSCB-d6 and TMSCB-d6) isolated in argon matrices at 10 K have been studied by IR spectroscopy. Pyrolysis of DMSCB and TMSCB gives rise to an identical set of bands: 644, 696, 817, 824, 932, 992, 1001 and 1253 cm-1 which permanently vanish with the increase in temperature. Correspondingly, the bands at 543, 580, 683, 718, 722, 768, 891, 929, 985 and 1012 cm-1 are observed in the spectra of pyrolysis products of DMSCB-d6 and TMSCB-d6. The appearance of an identical set of bands is attributed to 1,1-dimethyl-1-silaethylene (DMSE) isolated in the argon matrix, but in the case of deuterio analogs, to DMSE-d6. Based on normal coordinate treatment, the above mentioned bands have been preliminary assigned to certain modes of vibrations of DMSE and DMSE-d6.

Infrared spectroscopic study of very low pressure pyrolysis products of cyclosilthianes and 3,3-dimethyl-3-silathietane isolated in argon matrices. A new source of 1,1-dimethyl-1-silaethylene (Me2siCH2), thioformaldehyde (H2CS) and dimethylsilanthione (Me2SiS)

The products of very low pressure pyrolysis (VLPP) of hexamethylcyclotri-silthiane (I), tetramethylcyclodisilthiane (II) and 3,3-dimethyl-3-silathietane (III) were isolated in Ar matrices and were studied by IR spectroscopy. The only pyrolysis product of I was cyclosilthiane II, a dimer of transient dimethylsilanthione (Me2SiS) (IV). The starting material was recovered on pyrolysis of II. Thermal decomposition of III involves three intermediate unsaturated compounds: dimethylsilaethylene (Me2 SiCH2) (V) and thioformaldehyde (H2CS) (VI), both isolated in Ar matrix at 10 K, as well as silanthione IV fixed in the matrix in a form of the cyclic dimer II. The latter was also observed in the study of copyrolysis of 1,1-dimethyl-1-silacyclobutane and thietane, being authentic sources of intermediates V and VI. IR spectra of starting compounds I, II and III isolated in Ar matrices were obtained. The theoretical structure of IV and force constant F(SiS) were determined by the CNDO/2 method. With regard to CNDO/2 errors, SiS bond distance and F(SiS) are equal to 1.993 A and 4.72 mdyn/A, respectively. Calculation of normal vibrations resulted in the following values of vibrational frequencies of dimethylsilanthione (cm−1): 884 νs(SiC2) (A1), 735 νas(SiC2) (B2), 626 νs(SiS) (A1), 200 ⪯ CSiC (A1).

Long-lived room temperature phosphorescence of a naphthalene-β-cyclodextrin-adamantane complex in the presence of oxygen

Naphthalene-d8—β-cyclodextrin—adamantane triple complexes were prepared in an aqueous solution at room temperature. Irradiation (λ = 285 nm) of the solution in the presence of molecular oxygen results in the long-lived (τ = 10.3 s) room temperature phosphorescence (RTP). The removal of oxygen from the solution increases the RTP intensity and phosphorescence lifetime by 1.5 times. The RTP spectrum contains a well-resolved vibrational structure, whose bands are assigned to full symmetric vibrations of naphthalene, their overtones, and the combination tones of full symmetric vibrations. The quantum-chemical calculation of the triple complex structure confirms that both naphthalene and adamantane can simultaneously be included into the β-cyclodextrin cavity and suggests that the role of the latter as the third component is the more efficient shielding of naphthalene from the oxygen effect due to both the formation of three-component complexes and their aggregation to form submicronic particles.

Classical planar doubly bonded Si-substituted silenes—a boundary system between olefins and heavier Group 14 analogs: Geometric and electronic structures, rotational barriers about the SiC double bond, and comparison with the analogs and isoelectronic phosphenes, further evidence against C-ylides of silicon

Abstract An ab initio study of a number of isostructural ethenes, silenes, and germenes at the MP4/6-311G(d)//MP2/6-31G(d)+ZPE level of theory showed that R 2 SiCH 2 silenes are the last classical planar doubly bonded system because unlike the heavier Group 14 analogs electronegative substituents do not disturb a planar geometry, shorten and weaken the SiC double bond. The calculations of the potential energy profiles and the rotational barriers of isoelectronic silene and phosphene as well as phosphorane are in favor of silenes to be more like phosphenes rather than phosphoranes. The rotational barriers decrease as more electronegative substituents are attached to the Group 14 atom. For ethenes, silenes, and germenes the maximal effect is observed for fluorine substitution. Fluorine does not affect the rotational barrier in phosphenes. A thermochemical approach based on the strain energies and 2+2 cycloreversion enthalpies was used to estimate the difference between the EC (E=C, Si, Ge, P) σ- and π-bond energies in elementaalkenes. The Bader analysis of the electron density distribution results in a covalent and highly polar double bonds whose polarity decreases in the order: silenes>phosphenes>germenes.

Synthesis and structure of bis-crown-containing stilbenes

An improved procedure was proposed for the synthesis of stilbenes fused to two crown ether fragments at both benzene rings. The structure of new homologous symmetric bis-crown-containing stilbenes was determined by X-ray analysis. Relations were revealed between the size of the crown ether moiety and stilbene conformation in crystal and the mode of crystal packing. Conformational analysis of the prepared stilbenes in solution and in the solid state was performed by 1H and 13C NMR spectroscopy and by DFT quantum-chemical calculations.

Structure of guest-host complexes of β-cyclodextrin with arenes: a quantum-chemical study

The structure of β-cyclodextrin (β-CD), as well as the structure and energetics of β-CD-naphthalene, β-CD-fluorene, β-CD-phenanthrene, β-CD-cyclohexane (1:1), and β-CD-naphthalene (2:2) inclusion complexes was studied by the semiempirical MNDO/PM3 method. Calculations of a β-CD-naphthalene-cyclohexane (1:1:1) complex were also performed. The minimum heat of formation was found for the symmetric β-CD conformation withC7 symmetry axis. The structure is stabilized by the ring of interunit H-bonds formed by the protons of the 2-OH groups and the O atoms of the 3′-OH groups of the glucose units. Preferableness of this orientation of interunit H-bonds was confirmed byab initio calculations of the molecule of α-(1–4)-glucobiose (maltose) in the MP2/6-31G(d,p)//6-31G(d,p) approximation. The formation of any inclusion compounds of β-CD with arenes is energetically favorable: the complexation energy varies in the range −9 to −12 kcal mol−1. Among complexes with naphthalene, that of composition 2:2 is the most energetically favorable, which is in agreement with experimental data. In this complex, β-CD exists as a dimer of the “head-to-head” type, in which both partners are linked by a system of H-bonds. The structure of the “head-to-head” dimer of β-CD was simulated byab initio calculations of the H-bonded dimer of α-d-glucose in the RHF/6-31G(d,p) approximation. In the dimer, both components are linked by a pair of H-bonds formed by the protons of the 3-OH groups and the O atoms of the, 2-OH groups. The dimerization energies obtained fromab initio and semiempirical MNDO/PM3 and AM1 calculations differ by about 2.5 times (8.6vs 3.2 and 3.8 kcal mol−1, respectively).

Effect of cucurbituril on the primary photoprocesses in indocarbocyanine dyes in water

The effect of cucurbit[7,8]urils (CB7, CB8) on photoprocesses in indocarbocyanine dyes has been studied. Complexation with CB7 and CB8 reveals bathochromic shifts of the maxima in the absorption and fluorescence spectra. Fluorescence enhancement and decreasing have been observed for the complexes with CB7 and CB8, respectively. The dyes and their complexes are capable of trans–cis photoisomerization and hardly undergo intersystem crossing to the triplet state. The rate of the thermal cis–trans isomerization reaction is decreased due to complexation.

Specificity of photonics of 3,3′-diethyl-5,5′-dichloro-9-ethylthiacarbocyanine dimers in the presence of cucurbit[7]uril

Molecules of 3,3′-diethyl-5,5′-dichloro-9-ethylthiacarbocyanine form dimers in aqueous solutions, which are capable of fluorescence and intersystem crossing to the triplet state. In the presence of cucurbit[7]uril and alkali metals or ammonium cations, dye dimer complexes are formed, which exhibit phosphorescence and thermally activated delayed (E-type) fluorescence in air-saturated solutions at room temperature. With the use of quantum-chemical calculations, the structure of dimeric dye complexes with cucurbit[7]uril is suggested.

Excimer fluorescence and structures of the inclusion complexes of β-cyclodextrin with naphthalene and its derivatives

Fluorescence of the inclusion complexes with different compositions formed by naphthalene-h8, naphthalene-d8, 2,7-dimethylnaphthalene (DMN), and 2-benzylnaphthalene (BN) with β-cyclodextrin (β-CD) in water was studied. Two types of fluorescence are observed, monomer (MF) and excimer (EF_ fluorescence. The excimer fluorescence of the 2∶2 complex emitted by aggregated light-dispersing crystals forming a precipitate, whereas is the MF is concentrations, EF predominates for the resulting complexes. A proposed structure of the inclusion complexes was derived from MNDO/PM3 semiempirical quantum-chemical calculations. The EF is caused by the structure of the complex, in which both naphthalene molecules are separated by a distance of 4.7 Å: they lie in parallel orientation to each other, whereby one ring is displaced from the other by one-fourth of the length of the naphthalene ring. The complexes of 2,7-DMN and 2-benzylnaphthalene with β-CD do not exhibit EF. For the 2∶2 complex of 2,7-DMN with β-CD, this is due to the fact that the aromatic fragments are removed too far from one another 2-Benzylnaphthalene is unable to form an inclusion complex with β-CD, in whose structure the aromatic fragments inside the cavity could be arranged in parallel planes; instead, it forms a 1∶2 complex with β-CD.