A. D. Kachkovskii

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 Φo (0 ⩽ Φo ⩽ 90°), deviates in either direction from the mean (Φo = 45°). It is considerably smaller in the first excited state than in the ground state. The π-electronic approximation is a correct method for describing the electronic structure of a polymethine chain. The neglect of the σ electrons can distort the bond orders of the terminal groups.

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.

Quantum-chemical investigation of the distribution of the electron density in asymmetric polymethine dyes

It has been shown with the use of the HMO, PPP, and CNDO/2 approximations that the distribution of the electron density in asymmetric polymethine dyes at the ends of the polymethine chain is determined mainly by the electron-donor ability of the nearby terminal groups, whereas in the middle their influence becomes weaker as the chain becomes longer. The asymmetry of the dyes causes a decrease in the alternation of the charges on the atoms and an increase in the alternation of the bond populations. These phenomena are most clearly displayed in the SCF approximations, which always emphasize alternation. The closer are the electron-donor properties of the terminal groups and the longer is the polymethine chain, the smaller is the difference in the distribution of the electron density near the terminal residues in asymmetric and symmetric dyes.

Length of Soliton Wave in the Excited State in the Cations of Symmetrical Pyridocyanines and Their Carbo and Hetero Analogs

Quantum-chemical calculations in the AM1 approximation were undertaken for the optimized geometry of the cations of polymethine dyes [R+—(CH=CH)n—CH=R], where R represents phenyl substituents or pyridinium, pyrylium, and thiopyrylium groups and n = 1-6, in the ground state and for the electron density distribution in the Frank – Condon excited state. It was found that excitation of the molecule by a light quantum leads to a decrease in the length of the charge wave (soliton). It was shown that the length of the soliton depends on the electron-donating character of the terminal groups R and on the length of the conjugated chain, while shortening of the soliton in the thiopyrylocyanines leads to disruption of its symmetry.

Fluorescence Properties of Thiacarbocyanine Dyes at Low Temperatures

Measurements were carried out for time-resolved fluorescence spectra of dye solutions containing thiacarbocyanine and its derivatives with ethylene and vinylene bridges in the polymethine chain. At low temperature, an additional band appears in the spectrum related to an intramolecular electronic transition. Analysis of the transition lifetimes and a quantum-chemical model indicated that this transition should be attributed to a short-lived asymmetrical component formed along with the symmetrical component upon relaxation of the excited state but not appearing at room temperature due to conformational transformations.

Charge distribution and 13C NMR spectra of cyanine dyes with bridging groupings in the chromophore

The electronic structure of cationoid and anionoid cyanine dyes was calculated by the CNDO/2 method. The calculation results were compared with the data of the 13C NMR spectra. It was shown that the anionoid dyes of the malonocyanine class have all the inherent properties of a polymethine state, i.e., alternation of charges and leveling of bonds, and these are also characteristic of cationic streptopolymethines. According to the calculation data, the introduction of bridging groupings causes considerable changes in the electronic structure of the cyanine dyes, as confirmed by the results of the study of their 13C NMR spectra.

Perturbation theory of nonbonding orbitals in polymeric dyes

In terms of the approximation of an extremely long polymethine chain a relation was obtained between the electron-donating power Φ0 and the effective length L of the terminal groups in cyanine dyes, on the one hand, and the form of their asymptotic nonbonding orbital, on the other. A simple general procedure for determining the coefficients of such orbitals is indicated. Expressions were obtained through these coefficients for the derivatives of Φ0 and L with respect to the elements of the topological Hamiltonian of the terminal group, and this makes it possible to assess the effect of structural perturbations of the terminal groups on Φ0 and L. The possibility of refinement of the absorption region of the dyes with a short polymethine chain is considered.

Electronic structure and absorption spectra of verdazyl radicals, the corresponding cations, and leuco compounds

The wavelengths of transitions in electronic absorption spectra of verdazyl radicals, the corresponding cations, and leuco compounds were calculated in valence and π-electron approximations with allowance for configuration interaction. It was shown that the longest-wave bands in the investigated compounds are due to ππ* electron transitions. The effect of the nature of the substituents on the position of the bands in the spectrum is discussed.

Specific search for heterocycles for cyanine dyes by means of a computer

A method for the selection by means of a computer of terminal heterocyclic groups for cyanine dyes that absorb over a predesignated spectral range is described. New monoazaheterocycles, the introduction of which into the compositions of cyanine dyes as terminal groups should ensure absorption in the near-IR region of the spectrum, are presented as examples.