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.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 3, pp. 347–351, May–June, 1986.     

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.     

Principles of molecular design of binuclear polymethine systems

The electronic nature of reaction centers of reactants in the formation of polymethine compounds has been analyzed, and it has been established that the polymethine structures, depending on the combination of parity of the external polymethine chain and the signs of the electron-donor strength of the terminal heterocyclic nuclei, can be divided into two classes: classical polymethine dyes and generalized Hünig systems. In the generalized Hünig systems, the dependence of the charge distribution and bond orders in the polymethine chain on the difference in basicities of the terminal nuclei should be the opposite of the known relationships in a number of classical polymethine dyes.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 3, pp. 307–314, May–June, 1990.     

Deeply colored anionic polymethine dyes derived from bis(2,2,3,3,4,4,5,5-octafluoropentyl) 9 H-fluorene-2,7-disulfonate

Anionic symmetric polymethine dyes have been synthesized based on bis(2,2,3,3,4,4,5,5-octafluoropentyl) 9H-fluorene-2,7-disulfonate, which exhibit intensive absorption in the near IR region of the spectrum: pentamethine with an open polymethine chain (⋉_max = 790 nm) and nonamethine with six- and five-membered saturated rings at δ-positions of the chain (@#@ ⋉_max = 1044 and 1086 nm, respectively). To make comparisons, anionic dyes with analogous structure of the polymethine chain, viz., 1,3-dimethylbarbituric and 1,3-diethylthiobarbituric acid derivatives, have been also synthesized. Quantum chemical analysis of the electron structure of the dyes has been performed. Their solvatochromism in polar solvents depends weakly on electrophilicity and nucleophilicity of the medium and is primarily determined by universal interactions. This is the reason that they, in contrast to cationic dyes, retain a high selectivity of absorption (sharp absorption bands) and universal contour of electronic bands in strong polar solvents even with the long polymethine chain. Special solvatochromic effects of the anionic dyes in weakly polar media have been revealed and interpreted. The role of a counterion in these effects has been analyzed.     

Effect of Hydrophobic Chain of Dye Molecule and Surface of Active Crystal on the Organized Assemblies Formation of Dyes

Four derivatives of 5,5',6,6'-tetrachlorobenzimidazolocarbocyanine iodide with different long alkyl chain substituents in N-position of polymethine chromophore have been used for investigating the influence of hydrophobicity of dyes on the aggregation.It has been found that all dyes formed monomeric species in methyl alcohol. However, after addition of water to CH_3OH to change the polarity of the solvent, difference between dyes appeared.Addition of inorganic salt facilitated the J-aggregation of easy soluble dye, but it seems useless for the sparely soluble dye.Platelets cut from AgBr polycrystal or pressed AgBr powder have been used as substrate for adsorbing dyes. After addition of hexanoic acid, eventually the J-aggregate on AgBr surface could be destroyed.Voltammetry is a useful tool to investigate the interaction between dyes and AgBr. Experimental results showed that the longer the carbon chain substituents is, the stronger the interaction between AgBr and dyes would be.     

The homonuclear Overhauser effect and stereochemistry of a cyanine dye containing an orthophenylene bridge in the chromophore

Differential NMR spectroscopy has been used to derive the homonuclear Overhauser effect for protons in cyanine dyes, where the chromophore in each case contains an orthophenylene bridge. The data serve to define the polymethine chain conformation. A substituent in the meso position in the polymethine chain can alter the conformation.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 4, pp. 505–507, July–August, 1988.     

Structure and fluorescence properties of indole cyanine and merocyanine dyes with partially locked polymethine chain

The fluorescent properties of a series of symmetric cationic cyanine dyes and merocyanines with trimethylene bridge connecting N-atom of the indole residue with α-position of the polymethine chain have been explored in detail. It is shown that the constraining group influences their fluorescent properties both by steric and electronic effects. It revealed an essential decrease of the fluorescence quantum yields of the constrained symmetric cyanine dyes in comparison with those of their analogues without bridge group in their chromophore. This effect diminishes essentially at polymethine chain lengthening. In the case of merocyanines the bridge group can cause a decrease as well a substantial increase of their fluorescence quantum yields depending on their type of solvatochromism and solvent polarity. The conclusions made on the basis of the experimental data are in good agreement with the results of semiempirical quantum-chemical calculations.     

Merocyanine dyes with improved photostability

Merocyanine dyes have proven valuable for live cell fluorescence imaging applications, but many structures have been limited by rapid photobleaching. We show that photostability is substantially enhanced for merocyanines having a cyano group at a specific position in the central polymethine chain. Evidence is presented that this is due to reduction in reactivity of the dyes with singlet oxygen. These results point toward cyano-substitution as a general strategy for improving dye performance in imaging applications.     

Reaction of 3-formylchromone with methyl- and methylene-active azocycles

The reaction of 3-formylchromone with quaternary salts of nitrogen heterocycles containing active methyl or methylene groups was investigated. It is shown that the compounds obtained can be used in the synthesis of merocyanine dyes containing an o-hydroxybenzoyl substituent in the polymethine chain. Dicarbocyanines with such a substituent were not isolated, evidently because of the high electrophilicity of the polymethine chain and the ease of formation of a six-membered heteroring.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1271–1275, September, 1990.     

The Effects of Substituents and Solvents on the Ground‐State π‐Electronic Structure and Electronic Absorption Spectra of a Series of Model Merocyanine Dyes and Their Theoretical Interpretation

Two series of new merocyanine dyes have been synthesised and the dependence of their electronic structure on substituents and solvents has been studied by NMR spectroscopy, by using both the NMR ¹³C chemical shifts between adjacent C atoms in the polymethine chain and the ³J(H,H) coupling constants for trans‐vicinal protons. The widely used valence bond (VB) model based on two contributing structures cannot account theoretically for the observed alternating π‐electron density in the polymethine chain. In addition, the prediction of zero‐π‐bond order alternation (or zero‐bond length alternation) by this model is also incorrect. However, the results are consistent with the predictions of a qualitative VB model which considers the resonance of a positive charge throughout the whole polymethine chain. Based on this model and the Franck‐Condon principle the effect of substituents and solvents on the fine structure of the electronic spectra of these dyes can be explained as vibronic transitions from the vibrational state v=0 to v′, where v is the vibrational quantum number of the totally symmetric C?C valence vibration of the polymethine chain in the electronic ground state and v′ is that in the electronic excited state. In contrast, neither the effects of substituents or solvents on the electronic structure of merocyanines and their electronic spectra can be accounted for by the simple two state VB model.     

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.     

Fluorescence lifetime properties of near-infrared cyanine dyes in relation to their structures

Structurally diverse near-infrared (NIR) absorbing polymethine dyes were prepared and their fluorescence lifetimes (FLT) were evaluated in relation to their structural features. Comparative FLT analysis based on the modification of methine chain length and heterocyclic system showed that indolium or benzo[e]indolium heptamethine dyes exhibited longer FLT than the benzo[c,d]indolium trimethine dye. Modification of heterocyclic system alone with an intact chain length showed that indolium-based heptamethine dyes showed approximately 30% longer FLT than the benzo[e]indolium-based dyes. In general, the FLT of polymethine dyes increased from polar to non-polar solvents. In addition, correlation study between the theoretical and the experimental FLT for indocyanine green (ICG) suggests that the lack of structural rigidity for these cyanine dyes is primarily responsible for the loss of the excited state energy via non-radiative pathway.     

The effects of substituents and solvents on the ground-state π-electronic structure and electronic absorption spectra of a series of model merocyanine dyes and their theoretical interpretation

Two series of new merocyanine dyes have been synthesised and the dependence of their electronic structure on substituents and solvents has been studied by NMR spectroscopy, by using both the NMR (13)C chemical shifts between adjacent C atoms in the polymethine chain and the (3)J(H,H) coupling constants for trans-vicinal protons. The widely used valence bond (VB) model based on two contributing structures cannot account theoretically for the observed alternating π-electron density in the polymethine chain. In addition, the prediction of zero-π-bond order alternation (or zero-bond length alternation) by this model is also incorrect. However, the results are consistent with the predictions of a qualitative VB model which considers the resonance of a positive charge throughout the whole polymethine chain. Based on this model and the Franck-Condon principle the effect of substituents and solvents on the fine structure of the electronic spectra of these dyes can be explained as vibronic transitions from the vibrational state v = 0 to v', where v is the vibrational quantum number of the totally symmetric C=C valence vibration of the polymethine chain in the electronic ground state and v' is that in the electronic excited state. In contrast, neither the effects of substituents or solvents on the electronic structure of merocyanines and their electronic spectra can be accounted for by the simple two state VB model.     

Synthesis of thiapenta- and thiahexacarbocyanine dyes from alkoxy-substituted polymethine salts with dimethylamine residues

The possibility of the use of alkoxypolymethine salts with dimethylamine residues to obtain thiapenta- and thiahexacarbocyanine dyes is demonstrated. The synthesis of a number of meso-alkoxythiapentacarbocyanines that contain various condensed rings in the 12 and 14 positions of the polymethine chain was realized, and their photographic properties were investigated. It was established that the dyes obtained are effective infrachromatic sensitizers. The electronic absorption spectra of the synthesized compounds were measured.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1631–1636, December, 1981.     

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 ₀ 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 ₀ 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 ₀ and L. The possibility of refinement of the absorption region of the dyes with a short polymethine chain is considered.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 3, pp. 257–262, May–June, 1988.     

Ratiometric analysis of fluorescence lifetime for probing binding sites in albumin with near-infrared fluorescent molecular probes

A number of diseases have been linked to abnormal conformation of albumin, a major extracellular protein in blood. Current protein structural analysis requires pure isolated samples, thereby limiting their use for albumin analysis in blood. In this study, we report a new approach for high-throughput structure-related analysis of albumin by using the fluorescence lifetime properties of near-infrared (NIR) polymethine dyes. Based on molecular modeling, polymethine dyes are bound to two binding sites with different polarities on albumin. As a result, an NIR molecular probe exhibits two distinct lifetimes with two corresponding fluorescent fractional contributions. The distribution of fractional contributions along with individual fluorescence lifetimes represents unique parameters for characterizing albumin architecture by ratiometric analysis. After screening a small library of NIR polymethine dyes, we identified and used a polymethine dye with optimal fluorescence lifetime properties to assess structure-related differences in commercially available bovine serum albumin as model systems. The results show that changes in the lifetime of NIR dyes reflect the perturbation of the tertiary structures of albumin and that albumin prepared by different methods has slightly altered tertiary structures. Because of the reduced absorption of light by blood in the NIR region, the method developed can be used to determine structural changes in albumin in whole blood without prior isolation of the pure protein.     

近红外吸收菁染料分子链结构对其光氧化稳定性能的影响

本文对具有不同分子链结构及不同链长,而母核结构分别为吲哚类及喹啉类的六种菁染料在溶液中的光氧化稳定性能进行了研究。结果表明菁染料的光褪色主要是由光氧化反应所致,当在分子链上引入不饱和环体结构时,可以使菁染料分子的光稳定性能增加;而随着分子链长的增加菁染料的光氧化稳定性能则明显下降。通过顺磁共振谱测定结果表明,在菁染料的自敏光氧化反应原初过程中,既存在单重态氧过程又存在超氧负离子过程。     

Effect of electron-donating ability of terminal groups of cationic polymethine dyes on thermal polymerization of methyl methacrylate in solution

Thermal polymerizability of methyl methacrylate solution was examined in dimethylformamide in the presence of cationic symmetric and asymmetric polymethine dyes with different terminal electron-donating groups by dilatometric method. It was found that the polymethine dyes according to the electronic structure can initiate or inhibit the polymerization of methyl methacrylate both in the presence of standard initiator, azoisobutyronitrile, and in its absence by a radical mechanism. It was shown that the initiating ability was weakened with altering an electron-donating activating ability of terminal groups of dyes from the average level in both directions of its increasing or decreasing. The transition from symmetric to asymmetric dyes in the same structural type is accompanied by a loss of initiating ability. Based on the comparison of experimental kinetic data of polymerization of methyl methacrylate in the presence of dyes and quantum chemical calculations of electronic structure for dyes it was demonstrated that a HOMO energy value of an initiating dye must be higher and that of an inhibiting dye, lower than the methyl methacrylate value.     

Energy structures and electronic-transition wavelengths for trinuclear polymethine dyes

A study has been made on the energy structures and long-wave electronic transitions for linear trinuclear polymethine dyes by the method of additive generating functions. The necessary and sufficient conditions have been established for the occurrence of local levels related to the central nucleus. It is found that the bisvinylene shift is dependent on the structure of the central nucleus in this class of dyes. The relationship fits the available measurements, and it is predicted that there is a positive bridge effect, namely an increase in the bisvinylene shift in trinuclear dyes by comparison with binuclear ones.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 1, pp. 10–21, January–February, 1987.     

The length of the polymethine chain and the spectral-luminescent properties of symmetrical cyanine dyes

The general characteristics that relate the length of the polymethine chain of symmetrical cyanine dyes to their spectral-luminescent properties depending on the electron-donor character of the heterocycles and the nature of the solvent are formulated. For various types of symmetrical cyanines, the Stokes shifts decrease with the elongation of the polymethine chain due to weakening of the vibronic interactions. The vinylene shifts of the band maxima are essentially constant and fall within the range 100 to 130 nm depending on the nature of the heterocycles and the solvent. When the polymethine chain elongates the fluorescence quantum yields first increase and then decrease. The greater the effective length of the heterocycle the stronger the decrease. The fluorescence decay occurring when the polymethine chain gets longer is associated with intensification of the internal conversion. For symmetrical cyanines, the changes in the shapes of the electronic bands (their width, asymmetry, excess, and fine structure) as the chain elongates are governed by the competing effects of the vibronic and intermolecular interactions. The former decrease as the chain lengthens, causing the narrowing of the absorption bands for the lower vinylogs. On the other hand, the latter increase as chain lengthens, which leads to broadening of the bands for the higher vinylogs. The higher the solvent nucleophilicity and the greater the deviation of the electron-donor ability of the heterocycle from the average value the greater the broadening. Any elongation of the polymethine chain of symmetrical cyanines causes only narrowing of the bands and an increase in the asymmetry, excess, and structuring in the fluorescence spectra, which, unlike the absorption spectra, is independent of the electron-donor character of the heterocycles and the nature of the solvent. These effects are caused by the fact that, in contrast to absorption, changes in the shape of emission bands with increasing chain length are governed predominantly by vibronic rather than by intermolecular interactions.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1227–1239, July, 1994.