Cyanine dyes are widely used in studies of various molecularly organized and biological systems [1][2][3][4]. Examples of such organized systems are microemulsions, normal and reverse micelles, lipid vesicles, and others. In these systems, individual stages of photosynthesis are modeled, and supramolecular systems for solar energy conversion are designed [3]. The formation of J aggregates in vesicles was observed in [5], when positively charged dye molecules were adsorbed on the negatively charged vesicle interface. In this case, strong Coulomb interaction between dye molecules and the vesicle interface can lead to the formation of J aggregates only at a very high concentration of dye molecules, and the minimum number of dye molecules necessary for J aggregate formation is 60 [6]. It has been shown that dye molecules are preferentially localized in the vesicles that already contain dye molecules rather than are uniformly distributed among all vesicles [3].The behavior of cyanines in solutions was intensely studied in the presence of surfactants [7][8][9], in normal [10-12] and reverse micelles [8,9,13]. It has been shown on the basis of spectral data that the decomposition of the dimer and the conversion of the cis-monomer to the trans -isomer occurs in reverse AOT micelles [14]. This cis -trans isomerization is due to strong electrostatic interaction of the dye with hydrophilic AOT groups at the interface between the organic phase and water, whereas the isomerization effect is much weaker in reverse micelles of neutral Triton X-100. Electrostatic interaction results in polarization and alignment of charged subunits in the dye molecule along the electric field, leading to conversion of the cis -to the transisomer. Hence, the absence of dye dimers in reverse micelles, as noted in [14], is due to the fact that the trans -isomer, unlike the cis -isomer, does not tend to form dimers in an aqueous medium.The preparation of monodispersed J aggregates of a given size, including those of an extremely small size, is a fundamental problem of great practical importance.One of possible ways of its solving seems to be the use of reverse micelles.Information on possible aggregated states of cyanine dyes in reverse micelles is rather controversial. For example, it has been established [13] that one dye molecule at the most can be present in the water pool of a single micelle, and the probability of occurrence of two dye molecules is of the order of 10 -5 . On the other hand, it is stated [14,15] that in water pools of reverse micelles with W ≥ 5 ( W = [H 2 O]/[AOT]), dye J aggregates are formed even at average dye concentrations much lower than one dye molecule per micelle. Zhang and Liu [13] believe that a dye molecule in micelles with a small water-pool radius is compressed by the surfactant shell, which results in distortion of planarity of the molecule, and molecule transforms into the nonfluorescent twisted monomeric state. In micelles of a greater size, the dye molecule exists in the planar fluorescent state in the water ...
1 Spectral sensitization of AgHal grains by dyes was discovered by Vogel in 1873 [1]. This phenomenon has been studied rather well and is widely applied in conventional photographic materials, providing their photosensitivity over the entire visible range and the near IR region of the spectrum [2][3][4]. A necessary condition for spectral sensitization is the adsorption of a sensitizing dye onto the grain surface. Cyanine dyes in the form of adsorbed J aggregates are known to be the most effective sensitizers for AgHal grains.Interplay between the nature and structure of the AgHal-grain surface and the absorption spectra of adsorbed sensitizer dyes was studied by many authors [5][6][7]. An analysis of published data leads to the conclusion that this factor can have a significant effect on the absorption spectra of an adsorbed dye.The addition of an ethanolic dye solution to an AgBr emulsion produces a significant bathochromic shift of the monomer absorption band, which is believed to be due to the formation of J aggregates [3]. A cyanine dye adsorbed from an aqueous methanolic solution (20% MeOH) onto the surface of AgBr and AgCl grains with different faceting was shown to have different absorption maximums of the aggregate depending on the nature of the substrate [7]. Meso -ethylsubstituted carbocyanines are adsorbed onto the {111} AgBr surface in the form of J aggregates with the packing angles in the structures of 30° and 19° .Cationic and anionic dyes are readily adsorbed on cubic and octahedral AgHal grains. However, the same dye has different abilities to form J aggregates on the {100} and {111} faces. Furthermore, the absorption spectra of dyes of the cationic and anionic types adsorbed onto the grains of the same form are different [8]. 1 e-mail: brichkin@icp.ac.ru Meso -ethyl-and meso -phenylsubstituted carbocyanines are adsorbed in the all-trans form on the {111} faces of AgBr but in the all-cis form on the {100} faces [7]. Boyer and Cappelaere [6], and Markocki [9] believe that J states appear more readily on grains that have the cubic form with the {100} faces, with the energy of adsorption of the dyes on these faces being higher than on the {111} faces [10].In contrast to this, Eggers et al. [11] and Philippaerts et al. [12] claim that J aggregation is favored by the crystallographic structure and electrostatic properties of {111} faces, and only those dyes whose aggregation is significantly affected by the aqueous medium form J aggregates on the surface of cubic grains [12]. Breslav et al. [8] concluded that cyanine dyes form more diverse structures in an emulsion with octahedral grains, whereas J aggregates in an emulsion with cubic grains have a more uniform structure and are stronger adsorbed on the silver halide surface.The state of an adsorbed dye substantially depends on its concentration. At low surface concentrations of cyanine dyes on AgHal and other adsorbents, their absorption spectra correspond to the molecular form of the dye ( å ‡ ) with a bathochromic shift (~30 nm on AgHal) whose val...
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