V. P. Krutyakova

Formation mechanisms of equilibrium component composition of molecular layers of polymethine dyes

We study the concentration dependences for the absorption spectra and component composition of molecular layers from three homologous series of symmetric polymethine dyes of different electron-donating ability of their terminal heterocyclic groups. We find that a change in the layer thickness leads to a change in the width and position of the spectrum due to a change in the number of absorption bands. The number of bands of monomers increases with increasing chain length and electron-donating ability. The concentration ratio of monomers and associated forms depends on the spatial orientation of molecules in the layer. The electron-donating ability of terminal groups affects the angle between the chromophores of molecules that form a dimer and the intensity ratio between the short- and long-wavelength absorption bands of dimers. We conclude that the effect of the thickness of the layer on its spectral parameters is determined by the degree of intramolecular electron asymmetry that arises as a result of the interaction of chemically symmetric molecules with charges of the substrate surface and upon intermolecular interaction. This asymmetry leads to changes in free energies of ground states of monomeric molecules and, as a consequence, to an increase in equilibrium concentrations of cis-stereoisomeric forms in the layer.

The Influence of the Molecular Structure of Cyanine Dye on the Component Composition of Molecular Layers

The formation of the component composition of symmetric cationic cyanine dyes on glass is studied. The absorption spectra of layers of three homologous series of dyes with end heterocyclic groups of different spatial and chemical compositions are measured, and the absorption spectra of monomer components and aggregates are separated. The component compositions of layers of different thicknesses are compared. It is shown that the widening of the absorption spectra of molecular layers against the spectra of ethanol solutions of these compounds is caused mainly by the formation of various monomer stereoisomers and molecular aggregates and their interaction with the substrate surface and the neighborhood. The number of isomer forms and their relative concentrations depend on the layer thickness, the electron donor ability and spatial structure of end groups, and the cation conjugation chain length. The influence of the anion manifests itself only in the concentration ratio of the formed monomers and a small shift of the maxima of their absorption bands. The increase in the number of monomer forms produced in the layer corresponds to the increase in the conjugation chain length. Spatial obstacles created by heterocyclic groups inhibit the formation of definite stereoisomers, which reduces the number of components of the layer.

Photostimulated modification of the structure and optical properties of a molecular layer of a polymethine dye

This paper discusses the structural modification of a molecular layer of polymethine dye under the action of laser radiation. Resonance excitation of the all-trans isomer irreversibly alters the spatial orientations and relative concentrations of all the isomeric and aggregated species of the molecules present in the composition of the layer. The influence of the total excitation-energy density on the orientational variation and relative concentrations of the species depends on the lasing regime and the energy density in a single pulse, on the concentration of the all-trans isomer, which determines the amount of absorbed energy in each pulse, and on the total concentration of molecules in the composition of all the species, which affects the heating temperature. A model is proposed for the photostimulated structural modification of a layer of polymethine dye.

Mechanisms of the changes in the conformational composition and spatial reorientation of molecular components of a dicarbocyanine dye layer induced by heating or resonance photoexcitation

The effect exerted by temperature and time of heating and by a resonance photoexcitation on the relative concentration of the molecular components of a dicarbocyanine polymethine dye layer, the rate of variation of the orientation angle of components and its limiting value was studied. The model of the thermally and photo-induced rearrangement of the layer is developed.

Excitation transfer between components of molecular layers of cyanine compounds

Using the example of layers of symmetric polymethine dyes, we have studied the optical excitation energy transfer processes between nanocomponents of a molecular layer. The fluorescence yields and life-times of excited states of monomeric stereoisomers have been estimated, and the stereoisomerization yields have been determined. We show that the excitation transfer yield between the S1 states of monomeric stereoisomers (tens of percent) is considerably higher than the fluorescence yield (several percent) and depends on the structure of terminal groups of the molecule.

Studies of thermally stimulated reversible restructuring and variation of the optical parameters of molecular layers of polymethine dyes

This paper discusses the thermally stimulated restructuring of the molecular components of layers of symmetric polymethine dicarbocyanine dyes. Heating the layer to a temperature above 100°C causes a partially reversible conversion of the dimers into monomeric stereoisomers (the all-trans isomer and the β-cis isomer) and into J aggregates, as well as restructuring the β-cis isomer into the all-trans isomer. The restructuring time is tens of minutes. The rate of such conversions depends on the components present in the layer, the structure of the heterocyclic end groups of the monomeric molecules, and the temperature.

The optical properties of thin films of cyan dyes with silver nanoparticles and how they change when they are photostimulated

Hybrid materials consisting of silver nanoparticles and thin films of cyan dyes have been investigated by means of scanning electron microscopy and absorption and fluorescence spectroscopy. It is detected that the molecular absorption and fluorescence of the dye become stronger in the presence of Ag nanoparticles, and that the plasmon absorption band in the silver nanoparticles shifts. The mutual modification of the optical properties of the components of the hybrid materials is explained by the dependence of the plasmon frequency on the permittivity of the surrounding substance and by the influence of the near fields of the silver nanoparticles on the dye molecules. Photoinduced transformations have been studied in organic thin films in the presence of Ag nanoparticles. It is established that the change of the optical density of a dye layer with nanoparticles during irradiation exceeds the analogous changes in a dye layer without nanoparticles by a factor of 10.

Influence of molecule-substrate interaction on component composition of cyanine thin films

It was found that the absorption spectra of thin films of cyanine molecules are considerably broader than spectra of their solutions. This is because the equilibrium solutions of molecules contain all-trans isomers as major component, whereas their layers contain several monomeric (all-trans and cis isomers) and associated (dimers and J aggregates) components. Presence of molecular components in layer depends on the electron-donor ability of end groups and surface molecular concentration. The asymmetry in the electron density distribution along conjugated chain affects the equilibrium concentration of isomers in a molecular layer.

Variations of the equilibrium make-up and spatial orientation of a molecular layer of polymethine dyes

Molecular layers of polymethine dyes on glass substrates have been investigated. As the surface concentration of molecules increases, the number of cis isomers of various structures decreases, and the relative concentration of all-trans isomers increases. The increased electron-donorability of the end groups and the lengthening of the conjugation chain increases the number of cis isomers of various structures. The equilibrium stereoisomer composition of the molecular layer is determined by the degree of asymmetry of the intramolecular electron-density distribution induced by its interaction with the substrate. A model is proposed for the reconstruction of the structure of the layer under the action of heating and photoexcitation. Reversible changes in the make-up of the layer are caused by stereoisomerization when a segment of a molecule rotates around the second bond of the conjugation chain; irreversible changes are caused by transformations in the asymmetry of the electron-density distribution in the molecule due to irreversible spatial reorientation of the components of the layer.

Influence of the molecular orientation on the optical properties and photomodification of cyanine thin film

The structural photomodification of the multicomponent cyanine films was studied. The spectrum of the film shows different bands of components, formed by dye molecules. Optical properties of the films were determined by the relative concentrations of the molecular forms and their orientation angles. Laser-induced changes in the absorption spectra of film associated with the variation of the relative concentrations of the forms and their orientation angles. The dependence of angle on the total photoexcitation energy had the form of saturating functions. Maximum limiting angles depend on the initial orientation angles of the component.