Cis-trans isomerization of glyoxal

The CNDO/2 method with the original parameter set predicts the conformation with the mutually perpendicular CH=O groups to be the most stable isomer of glyoxal; the cis form is favoured over the trans form. The order of stabilities is not changed upon the full optimization of coordinates of the isomers. Calculations fail to reproduce the observed order of isomer stabilities unless allowance is made for limited configuration interaction with the lowest ππ* doubly excited state and for geometry optimization. Doubly excited configurations of the ππ* and σσ* types have negligible effect on the energies of isomers.     

Cis-trans isomerization of styrylpyrroles by stray light

A series of substituted N-methylstyrylpyrroles (H, p-Br, p-CN, o,p-diCl, m-CH₃, m-CN, m-NO₂) were prepared via the Wittig reaction of 1-methylpyrrole-2-carboxaldehyde and substituted benzyltriphenylphosphonium bromides. Both cis and trans isomers were found to be present in the reaction mixture and they were separable by column chromatography in a few cases (H, p-Br, m-CN, m-NO₂). Photochemical isomerizations of cis-styrylpyrroles to trans isomers were observed when the substituents were o,p-diCl and m-NO₂, while the opposite was the case with compounds having H, p-Br, m-CH₃, m-CN. It was difficult to separate the cis and trans mixture of p-cyanostyrylpyrroles and the equilibrium ratio did not change under similar photochemical reaction conditions.     

Photochemistry of anionic polymethine dyes

Spectral and kinetic properties of photoisomers and triplet molecules of a number of anionic polymethine dyes, namely, differential absorption spectra, rate constants (k _i ) and activation parameters of reversecis-trans-isomerization, and triplet-state-decay rate constantsk _T are studied by flash photolysis. CINDP/2 calculations of electron density in anionic dyes show thatk _i andk _T in polar solvents are determined primarily by the electronic effect of the substituents in themeso-position of the polymethine chain of a dye. The increase ink _i due to the formation of ion pairs consisting of a dye with a counterion is observed in nonpolar solvents.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 713–718, April, 1993.     

Mass spectrometry of polymethine dyes

The mass spectra of some unsubstituted (in the heteroresidues and polymethine chain) thiacarbo- and polycarbocyanines were investigated. It was ascertained that the first act of disintegration of the dye molecules is splitting out of an alkyl halide ion to give an anhydro base ion. Some regularities between the structure of the dyes and the fragmentation of the anhydro ions obtained from them are exposed. The peculiarities of the dissociative ionization of the dye molecules as the polymethine chain is lengthened are shown.     

Cis-trans-isomerization of cationic-anionic polymethine dyes

The effect of solvent polarity and ion pair formation oncis-trans-isomerization of cationic-anionic polymethine dyes and of related simple cationic and anionic dyes is studied by flash photolysis. The change in the rate constant for reversecis-trans- isomerization of photoisomers due to the interaction of a cation with an anion in an ion pair is observed in nonpolar solvents. A drop in the yield of the photoisomers to zero is observed for a number of cationic-anionic dyes in weakly polar and nonpolar solvents which is possibly due to steric hindrances in the photoisomerization process in ion pairs.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 719–724, April, 1993.     

Polymethine-polyene transition parameter in polymethine dyes

Within the framework of the HMO approximation and an equal bond model, quantitative characteristics are proposed for the alternation of electron density on the atoms and bonds of an unsubstituted chain of polymethine dyes with arbitrary end-groups. These characteristics can be used, depending on the type of electron distribution, to classify a compound as a polymethine or polyene. A structural parameter determined by the nature of the end-groups has been found, this parameter being responsible for realization of one type of electron density distribution or the other in the polymethine chain.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 2, pp. 129–138, March–April, 1988.     

Fluorescent properties of cationic-anionic polymethine dyes

The fluorescence spectra were studied and the quantum yields of the fluorescence of a number of cationic-anionic polymethine dyes were measured in polar, low-polarity, and nonpolar solvents. It was shown that the fluorescence spectra of cationic-anionic dyes in polar solvents, like the absorption spectra, represent the sum of the fluorescence spectra of the corresponding cationic and anionic dyes. For dyes in which the absorption bands of the anion and cation are close and a new short-wave band arises in the ion pairs, excitation into this band virtually does not lead to fluorescence, which is a consequence of the forbidden nature of the long-wave transition that arises as a result of the interaction of the chromophores. For a number of cationic-anionic dyes in ion pairs an energy transfer is observed: When an ion possessing short-wave absorption is excited, an ion with long-wave absorption fluoresces.N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117977 Moscow. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 11, pp. 2532–2539, November, 1992.     

Photoisomerization of sterically hindered polymethine dyes

Conclusions In sterically hindered polymethine dyes the photoisomerization process, with the formation of sterically hindered trans and di-cis forms, successfully competes with the nonradiative degradation of the energy on the higher vibrational sublevels of the ground state.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1926–1930, August, 1977.     

Cis-trans isomerization and 13C-NMR chemical shift of polyphenylacetylene

Before and after cis-trans isomerization, the observed ¹³C-NMR chemical shifts of poly(phenylacetylene) (PPA) in the solid state were investigated on the basis of ¹³C-NMR chemical shift calculations within AM1 for the cis-transoidal and deflected trans-transoidal forms. Two ¹³C-resonance peaks in the observed CP/MAS ¹³C-spectrum were assigned theoretically by the ¹³C chemical shifts of the main and side chains. After thermal isomerization, the ¹³C peak of the main chain for PPA shifted upfield by 3.5 ppm, in contrast to the downfield shift of the ¹³C peak for polyacetylene. This upfield shift of trans-PPA largely was attributed to the increases of the excitation energy from the ground state to the lowest φ_π–π* state in the paramagnetic terms of ¹³C chemical shift on the main chain carbons with the increase in deflected angle τ of 0 to 80°. The ±80° deflected conformation of the trans-transoidal chain due to the cis-trans isomerization was confirmed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1657–1664, 1999     

Cis-trans isomerization of polybutadiene double bonds during olefin metathesis

The cis-trans isomerization of polybutadiene double bonds during metathesis degradation using WCl₆/(CH₃)₄Sn catalyst system has been estimated kinetically along with productive metathesis. The isomerization was followed both for noncrosslinked and for crosslinked polybutadiene. Ninety-six percent cis-1, 4 units are found to isomerize into ca. 75% trans-1, 4 units. The rate of stereomutation is found to be different in the presence and absence of a low-molecular-weight olefin. The results are explained with the help of a stereo model originally proposed by Katz (Advances in Organometallic Chemistry, Academic, New York, 1977, Vol. 16.)     

Study of fluorescent aggregates of polymethine dyes

Absorption, fluorescence, and fluorescence excitation spectra have been studied for a number of anionic, cationic, and cationic-anionic polymethine dyes in low-polar and nonpolar solvents, as well as in binary mixtures of solvents differing in polarity. For most of the dyes studied, fluorescent aggregates have been found to form. Their broad fluorescence bands are located in the long-wave region with respect to those of the initial dyes. The quantum yield of the aggregate fluorescence is normally higher than that of the initial dyes. Fluorescence excitation spectra of some cationic-anionic dyes in nonpolar solvents disagree with their absorption spectra because of contact and solvent-separated ion pairs simultaneously present in the solution.Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 69–75, January, 1993.     

Quantum-chemical investigation of bond orders in symmetric polymethine dyes

The behavior of the bond orders in symmetric model polymethine dyes has been investigated by the HMO, PPP, and CNDO/2 methods. It has been established that the greatest equalization of the bonds is achieved in the case of dyes with moderately basic terminal groups. In other cases, alternation of the bond orders, which diminishes with increasing length of the polymethine chain and attenuates with increasing distance from the end, has been discovered at the ends of the polymethine chain. The amplitude of the alternation of the bond orders in a polymethine chain is greater, the more the basicity of the terminal groups, as characterized by the electron-donor parameter _o (0 _o 90°), deviates in either direction from the mean (_o = 45°). It is considerably smaller in the first excited state than in the ground state. The -electronic approximation is a correct method for describing the electronic structure of a polymethine chain. The neglect of the electrons can distort the bond orders of the terminal groups.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 2, pp. 138–146, March–April, 1985.     

Symmetry breaking in cationic and anionic polymethine dyes

The electron density distribution and equilibrium molecular geometry of ionic polymethine dyes containing both the simplest terminal groups and carbo and heterocyclic residues were investigated by ab initio and AM1 methods. It is shown that charge waves and wave of the carbon–carbon bond lengths in the conjugated chain are of solitonic nature. The dimensions of the charge (electron or hole) soliton and geometrical (topological) soliton are practically unsensitive to the chain lengths, while the wave shape depends on the donor/acceptor strength of the terminal groups. If the length of the chromophore exceeds the dimension of the solitons, then the soliton center in the substituted ionic conjugated systems shifted to one of the chain ends, what leads to an appearance of the asymmetrical form. The conditions of this symmetry breaking (crucial number of the vinylene groups in the chain, effective length and donor/acceptor strength of the terminal residues, total charge) was found and was established that no moving of the solitons occurs in the unsubstituted conjugated ions.     

Effective length of benzannellated terminal groups in polymethine dyes

The effect of benzannellation on the effective length of the terminal groups was studied by the method of asymptotic nonbonding MOs in the approximation of dyes with an extremely long polymethine chain. Both alternant and nonalternant conjugated hydrocarbon systems were examined. The variation of the effective length in relation to the number of added benzene rings was investigated. It was shown that the effect of annellation can be assessed by using the concept of equivalent terminal groups and the theory of perturbations in the asymptotic nonbonding MOs of polymethine dyes. Examples of the extension of the obtained relationships to heterocyclic terminal groups are given. The proposed approach makes it possible to interpret the cases where benzannelation is accompanied by a hypsochromic effect.Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 26, No. 2, pp. 142–148, March–April, 1990.