S. L. Selektor

Peculiarities of the reflection-absorption and transmission spectra of ultrathin films under normal incidence of light

This paper analyzes the prospects of studying optically anisotropic monolayers on a liquid substrate using linearly polarized light. For quantification of the electronic transmission spectra of the studied monolayers on the transparent subphase and Langmuir-Blodgett films transferred onto transparent plates as well, we obtained formulas for optical densities of transmission spectra in a triple system: surrounding medium-ultrathin film-substrate (liquid or solid). The expressions derived in this work are compared with previously obtained formulas for optical densities of spectra measured in the reflection mode for an analogous system. It is shown that, with all other conditions being equal, employment of this measurement technique allows one to obtain spectra the peak intensities of which (measured in absorbance units) are almost an order of magnitude higher than the corresponding values for transmission spectra. The regularities revealed in this work are confirmed by experimental measurements carried out for monolayers of certain conjugated organic compounds of various structures: amphiphilic chromoionofores, photosensitive oligomeric organic conductors, and electrochemically multistable rigid macrocyclic compounds. A good quantitative agreement between theory and experiment was obtained.

Orientation-induced redox transformations in Langmuir monolayers of double-decker cerium bis[tetra-(15-crown-5)-phthalocyaninate] and multistability of its Langmuir-Blodgett films

The spectral, electrochemical, and optical properties of Langmuir-Blodgett films (LBFs) and cast films from a solution of new double-decker cerium bis[tetra-(15-crown-5)-phthalocyaninate] (Ce(R4Pc)2) are studied. Based on analysis of compression isotherms and quantum-chemical calculations, schemes of the organization of Ce(R4Pc)2 molecules at different states of its monolayers are proposed. Correlation dependences are determined in order to relate the optical and electrochemical characteristics of monolayers and LBFs of sandwich-type lanthanide phthalocyaninates to the ionic radii of their metal centers. The valent state of Ce ions in a monolayer-forming complex is determined, and a sequence of redox transformations occurring in LBF uppon appliance of a potential is proposed, one of the transformations being associated with the Ce3+/Ce4+ redox transition. Orientation-induced intramolecular electron transfer is revealed in the planar supramolecular system. It is shown that, during the formation of a monolayer from a Ce(R4Pc)2 solution, a tetravalent metal center passes to a trivalent state. Monolayer compression to a high surface pressure reverts the complex to the electronic state typical of the solution. The reversible transformations observed upon the monolayer compression result from intramolecular electron transfer from the 4f-orbital of Ce to the phthalocyanine ring and backwards. The high operation rate and the reversibility of switching between the stable states, which are determined by means of the surface plasmon resonance technique, upon a stepwise change in the electrode potential within the range of 200–850 mV may underlie the development of optoelectronic systems. With a large number of molecules in a stacking aggregate, changes in the distance between the decks of the complex that occur with changes in the oxidation level of the metal center can substantially modulate the sizes of molecular ensembles. A supramolecular device capable of performing mechanical work can be developed based on this effect.

Physicochemical properties of solutions and ultrathin films of triple-decker gadolinium tetra-15-crown-5-phthalocyaninate

The ability of the triple-decker gadolinium complex with tetra-15-crown-5-phthalocyanine Gd2(R4Pc)3, (R = 15-crown-5) (1) to form monolayers and Langmuir-Blodgett films (LBFs) was studied for the first time. The charge characteristics of molecules of a triple-decker phthalocyaninate in monolayer, as well as their orientation and adhesion to the water subphase, were controlled by changing the surface pressure, pH, and subphase composition (aqueous solutions containing triethylenetetramine (TETA) and metal cations). It was shown that the presence of Na+ and protonated TETA aminogroups in resulted in an increasing limiting monolayer area and significant decreasing of the monolayer liquid state region. It is proven that the observed effects are caused by the conformational and charge transitions of peripheral crown ethers induced by their interaction with cations. A comparison of the differential reflectance spectra of the complex monolayer on the deionized water surface with UV-Vis absorption spectra of three-layer Gd2(R4Pc)3 LBF and complex solution in chloroform shows that partial complex oxidation and intensive stacking formation occurs already at the stage of monolayer formation on the subphase surface. Electrochemical studies of three-layer LBFs performed at the indium-tin-oxide electrodes (ITO-electrodes) using cyclic voltammetry (CV) showed three reversible redox waves in the potential range −200 to +1100 mV (vs. Ag+/AgCl). All registered peaks remain the same position and intensity upon multiple cycling. plasmon resonance (SPR) measurements allow to register three stable redox states of studied LBF upon applying the external potential. Such behavior shows the possibility to use multistep redox transformations of studied complex LBF for developing of stable and reproducible switchable optoelectronic systems.

Conjugated Compounds in Supramolecular Informational Systems: A Review

1. Features of physico chemical properties of conju gated compounds from the standpoint of application in informational systems 2. Inter and intramolecular interactions that deter mine the chemical structure of materials based on con jugated compounds (donor–acceptor interactions, hydrogen bonds, redox isomerism) 3. Absorbance and transformation of light by solu tions of conjugated compounds upon supramolecular organization 3.1. Spectral Manifestations of Coordination Interac tions 3.2. Spectral Manifestations of Aggregation 3.3. The Influence of Aggregation on Energy Transfer in Solutions of Conjugated Compounds 4. Langmuir monolayers and langmuir—blodgett films (LBF)—basic ideas and definitions, principles of self organization of conjugated molecules 5. Features of physico chemical properties of monolayers of conjugated compounds. functional materials on their basis 5.1. Absorbance and Emission of Light in 2D Systems, the Role of Aggregation 5.2. Light Collecting Systems, Irradiation Energy Transfer 5.3. Electric Conductivity and Magnetic Properties of LBF 5.4. LBF As Semiconductor Elements for Molecular Electronics

Photosensitized Degradation of Model Lipid Membranes based on 1-palmitoyl-2-oleyl-phosphatidylcholine (POPC)

In this work, we study the interaction of a well-known photosensitizer, MePha, with models of biological membrane (Langmuir monolayers and Langmuir–Schaeffer planar bilayers) based on one of the most important natural lipid, POPC, for the subsequent investigation of photodestruction processes in a context of photodynamic therapy treatment. Changes of macroscopic properties and morphology of POPC/MePha model membranes upon irradiation by visible light are recorded by means of contact angle measurements and atomic force microscopy, demonstrating clearly the possibility to use these methods for the study of photodestruction of artificial lipid membranes on solid substrates, but also for a comparative study of the efficiency of novel photosensitizers.

Factors Affecting the Structural Organization of Hemicyanine Chromoionophores in Langmuir Monolayers

The present work deals with processes of structure-formation and aggregation behavior of hemicyanine chromoionophores in Langmuir monolayers and Langmuir-Blodgett films. It is revealed that the appearance of excimers upon compression of monolayers of these compounds is associated with cation-ionophore group interaction. The key role of the organic solvent in the preorganization of such monolayers on subphases containing indifferent cations is demonstrated. The extreme character of the dependence of the excimer formation efficiency on concentration of the above ions is shown, and optimum concentrations are identified.