Anton A. Starovoytov

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.

Optical properties of polymethine molecules incorporated into a macroscopic set of parallel nanotubes

The structural properties of polymethine molecules incorporated into nanofibers of chrysotile asbestos have been studied by absorption-spectroscopy methods. These experiments have shown that, in chrysotile nanotubes, monomeric, all-trans- and cis-isomers, dimers and J aggregates can be observed. Upon incorporation of a dye from weakly concentrated solutions, monomeric forms of polymethine molecules, extended chromophores of which are oriented along parallel asbestos nanotubes, are predominantly observed. The hybrid material under investigation may be of interest for applied problems.

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.

Optical properties of silver nanoparticles coated by cyanine dyes molecular overlayers

Optical properties of supported silver nanoparticles with cyanine dyes overlayers were investigated. The spectrum of the hybrid material was treated as a result of mutual interactions between the plasmon oscillations in the metal nanoparticles and resonance absorption and refraction of dye molecules. The spectral positions of the plasmon resonances are shifted due to the anomalous refraction of dye molecules while the absorption of dye molecules is enhanced due to the incident field amplification in the near field of metal nanoparticles. The photoinduced transformations of hybrid material were also observed.

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.

Plasmon-exciton interaction in the thin film of inhomogeneous ensemble of silver nanoparticles and cyanine J-aggregates

The optical properties of organometallic films of silver nanoparticles and J-aggregates of pseudoisocyanine dye have been studied to observation of the plasmon-exciton interaction. The original method for obtaining J-aggregates on inhomogeneous island silver films without use of salt and water was developed. Owing to the broad bandwidth of inhomogeneous plasmon resonances; it is possible to study the interaction of nanoparticles and J-aggregates, obtained in the spin-coated layer on the silver island film. The absorption spectrum of the organometallic film is not a simple sum of the spectra composing its components. The absorption of dye molecules increases several times in the presence of silver nanoparticles, which is due to the influence of the near fields of the latter. The spectral dip in the absorption maximum of the J-aggregate of pseudoisocyanine was observed; it became more symmetrical with an increasing equivalent thickness of the island film.

Optical properties of pseudoisocyanine molecular clusters embedded in a nanoporous alumina

The molecular clusters, so called J-aggregates of pseudoisocyanine dye, were obtained in ordered cylindrical nanopores of anodic aluminum oxide. The absorption and luminescence of the samples were studied by the VIS-spectroscopy and laser confocal microscopy. The band of J-aggregates has the same shape, but is inhomogeneous broadened in comparison with solution. The luminescence maximum of J-aggregates was observed at 578 nm upon excitation at 543 nm as well as at 405 nm. Non-resonant luminescence excitation occurred due to energy transfer from oxygen vacancy of alumina to molecular nanoclusters. This is also confirmed by time-resolved luminescence spectroscopy, which shows the increase of luminescence decay time of J-aggregates placed in alumina up to the luminescence time of the clean alumina in comparison with J-aggregates coated on glass substrate.

Optical properties of cyanine dyes in nanotubes of chrysotile asbestos

Optical properties of cyanine dye molecules incorporated in nanotubes of natural chrysotile asbestos are studied. The absorption and fluorescence spectra of dye in asbestos have the similar shapes as in the ethanol solution, apart from small blue shift of the maxima. The Stokes shift in asbestos is smaller than in the ethanol solution. The fluorescence decay times of the dyes in asbestos nanotubes are found to be larger than that in the case of thin films of the same dyes formed on the transparent dielectric supports. This observation is rationalized in terms of the stereoisomerization hindrance in the excited electronic state of dye molecules. At the same time linear dichroism and fluorescence anisotropy observed in the experiment indicate that the embedded dye molecules are well-isolated monomer oriented predominantly along asbestos nanotubes.

Optical properties of cyanine dyes in the nanoporous chrysotile asbestos

The optical properties of the cyanine dye in nanotubes of the chrysotile asbestos are studied. Absorption and fluorescence spectra have blue shift; the Stokes shift for asbestos is smaller than for solution. The fluorescence decay lifetime of dye in asbestos is longer that in films, due to hindrance of stereoisomerization in the excited state. Observed linear dichroism and fluorescence anisotropy indicate that embedded dye molecules are well-isolated monomer oriented predominantly along asbestos nanotubes.