T. K. Razumova

Effect of the thickness of a dicarbocyanine dye layer on the conformational composition and spatial orientation of layer components

The conformational and aggregate structure and the spatial orientation of polyatomic organic molecules on solid substrates are investigated by the example of molecular layers of symmetric and asymmetric dicarbocyanine dyes. It is shown that the structure of a dicarbocyanine dye layer is determined by two mechanisms: (i) the change in the symmetry of the intramolecular charge distribution in the monomeric components of the layer as a result of the intermolecular interaction with the substrate, which leads to closer values of the equilibrium concentrations of two monomers with different spatial structure (rotational stereoisomers), and (ii) the association of monomer molecules with formation of dimers and J aggregates. The relative position of the spectra of the layer components, the spatial orientation of the components, and the concentration ratio of different monomeric forms depend on the structure and the electron-donating ability of terminal groups. In going from a monomolecular layer to multilayered structures, the asymmetry of the intramolecular charge distribution induced by the substrate and the ratio of different stereoisomeric forms first change and then become stabilized.

Step photoisomerization of dicarbocyanine dyes

Step photostereoisomerization of solutions of dicarbocyanine dyes was studied by laser photolysis. It is shown that, due to excitation, primary mono-cis-photoisomers are formed (a one-step process), whose excitation results in the formation of a γβ-di-cis-isomer (a two-step process). It was proved that the γβ-di-cis-isomer is formed mainly from a γ-mono-cis-isomer. The relaxation of the ground state of the γβ-di-cis-isomer to the ground state of a stable all-trans-isomer occurs in steps via two channels, in which γ-and β-mono-cis-isomers arise as intermediate products. The relaxation rates of both channels are different. The shift of the wavelength related to the maximum of absorption in the spectra of the primary and secondary isomers relative to that of the all-trans-isomer depends on the electron-donor ability of terminal groups. Quantum-chemical calculations of changes in the energies of the ground and first excited singlet states of a molecule, caused by the rotation around several bonds in the methine chain, were carried out.

Absorption and Fluorescent Properties of Pyrylium Compounds: I. The Nature of Electronic Transitions and Structural Rearrangement in the Excited State

The localization of molecular orbitals in 2,4,6-substituted derivatives of pyrylium is studied. The conformation of three asymmetrical molecules with oxyethyl substituents in positions 2 and 4 and different substituents in position 6 of the pyrylium ring is calculated by the AM1 method. The localization of the four upper occupied and two lower unoccupied MOs is determined, the fragment localization numbers are found, and the energies of five optical transitions, localization numbers, and the numbers of charge transfer between fragments are calculated. The conformation analysis of molecules in the S0 and S1 states is performed. Solid and liquid pyrylium solutions of different viscosity and polarity are experimentally investigated. The absorption spectra are recorded and absorption cross sections are measured, as well as fluorescence spectra and fluorescence anisotropy spectra. The following conclusions are made. In nonplanar molecules of pyrylium salts, four absorption transitions are localized at different parts of the molecule containing the pyrylium ring and one of the substituents. Upon excitation of molecules with complex substituents in position 6, the molecular fragment in position 2 turns around. This results in a flattening of the molecular fragment containing the pyrylium ring and substituents 2 and 6 on which the fluorescence transition is localized. The rearrangement involves the lowamplitude motion; it occurs almost without a loss of the excitation energy and only slightly affects the localization of molecular orbitals. As a result, two excited conformers are formed that possess close absorption and fluorescence properties. The radiative transitions in these conformers completely determine fluorescence of liquid solutions of any viscosity, including glycerol solutions. Strong solvatochromism is related to the nonplanar structure of stable pyrylium molecules, whereas the weak solvatochromism of liquid solutions is caused by localization of radiative transitions on a planar fragment of unstable fluorescing conformers.

Thermally induced variations in the conformational composition and spatial orientation of molecular components of a dicarbocyanine dye layer

The effect of heating on the spatial orientation and concentration of the components of dicarbocyanine dye layers on a glass substrate is studied experimentally. Layers of two types differing in the spatial orientation angles of all their components (monomeric all-trans and β-mono-cis isomers, dimers, and J aggregates) are considered. It is shown that storing of samples at temperatures of 150–200°C causes partial thermal destruction but does not change the absorption spectra of the molecular components. Heating of the layer of the first type changes neither the orientation nor the relative concentration of the components. Heating of the layer of the second type leads to irreversible changes in the orientation angles of all the molecular components and causes the β-mono-cis isomer and the dimer to transform into the all-trans isomer and the J aggregate. The dependences of the orientation angle on the heating time have the form of saturating functions. The rate of variation of the orientation angle and its limiting value in the saturation region increase with increasing temperature of heating. At high temperatures, the layer of the second type transforms completely into the layer with the first type of spatial orientation. A mechanism of the changes in the angles of the spatial orientation of the layer components, which includes the stages of thermal isomerization and reorientation of each molecular component, as well as rearrangement of its environment, is proposed. Energetic and kinetic parameters of all the stages of layer reconstruction satisfying the experimental results obtained in the region from 150 to 230°C are determined.

Changing the spatial orientation of the components of a molecular layer under the action of laser radiation

This paper discusses the spectral properties and spatial orientation of the absorbing molecular components in layers of polymethine dyes on a glass substrate. Depending on the thickness of the layers, from one to four components that differ in spectra and in orientation angles are observed. One type of angular distribution of the components (the first) is observed in submonolayers, while two types (the first and second) are observed in multilayer coatings. Optical excitation of the layers converts the angular distribution of the second type into that of the first type. The relative yield of this process increases with increasing energy density per radiation pulse. It can be assumed that the reorientation is associated with an increase of the vibrational-energy content in the radiation-absorbing components and its transport to the nonabsorbing components of the layer.

How temperature affects the stereoisomerization variations of the optical properties of solutions of polymethine dyes

The influence of temperature on the absorption and fluorescence properties of solutions of dicarbocyanine dyes and on stereoisomerization processes is studied in the range from −60 to 60 °C. It is shown that the optical density and the absorption spectrum vary as a result of temperature redistribution of the concentration of four stereoisomeric forms. The fluorescence-intensity variations are determined by variations of both the concentration of the isomers and the quenching rates associated with photoisomerization of the molecules in the excited state. It is concluded that recording the changes of the optical density and fluorescence intensity makes it possible to investigate the temperature distribution in a solution. It is shown that the fluorescence technique of investigation is several times more sensitive than the absorption technique.

Rearrangement of spatial orientation of molecular layer components by laser radiation and heating

Changes in the spatial orientation of molecular components of a dicarbocyanine layer under the action of temperature and resonance laser excitation are studied experimentally. The rate and the limiting value of the orientation change increase with increasing radiation power density and temperature. The model adequately describing the experimental dependences is proposed.

Change in spatial orientation of components of molecular layer under the action of laser radiation

The spectral characteristics and the orientation of absorbing centers in the polymethine dye molecular layers on solid dielectric substrates were studied. Depending on thickness, the layers have from 1 to 4 molecular components, differing in the absorption spectra and title angles. The components of submonolayers always possess one type of angular distribution (the first type). In multilayers two types of the angular distribution of components was observed. The action of laser light does not affect the component orientation in layers characterized by the first type of angular distribution. In layers with the second type of angular distribution the reorientation of components was observed under the action of radiation. The relative yield of this process increases with increasing single-pulse energy density. It was suggested that the photostimulated reorientation of components results from an increase in the vibration energy, which is stored by absorbing component of the layer and transferred to its other components not absorbing the radiation.

Photochemistry of pyrylium compounds: excitation-induced rearrangement of a molecule-solvent complex

Photochemical rearrangement of the complex pyrylium molecule - solvent is studied in solvents of various polarity and nucleophilicity. Upon photoexcitation two types of transient fluorescent excited-state complexes are formed due to intramolecular charge transfer and geometrical rearrangement of the complex.

Absorption and Fluorescent Properties of Pyrylium Compounds: II. Spectra and Cross Sections for Absorption from the Ground and Excited States of the Structurally Rearranged Form

The nonstationary absorption of solutions of 2,4,6-substituted pyrylium salt is studied by the pump-probe method. The solutions in methylene chloride are studied upon picosecond pumping and probing with different time delays. The solutions in methyl ethyl ketone are studied upon pumping and synchronous probing by nanosecond pulses. It is shown that excitation of the initially nonplanar molecule causes its structural rearrangement. As a result, substituent 2 enters into the pyrylium ring plane and two nonstationary conformers are formed that differ in the position of substituent 6. The motion upon structural rearrangement has a small amplitude, and localization of molecular orbitals changes only slightly. The temperature dependence of the nonstationary absorption is studied in the range from 293 to 223 K. The spectra and cross sections are measured for transitions from the S0 and S1 states. In addition, the activation energies and kinetic parameters of the rearrangement in the ground and excited states are determined. Experimental data are compared with theoretical calculations of the transitions from the S1 state. It is shown that a partial flattening of conformer molecules results in the long-wavelength shift of the bands and an increase in the cross sections for transitions localized on a flat part of the molecule and does not change parameters of the band localized on the molecular part that did not change upon the rearrangement but remained nonplanar.