Adsorption sensitizing dyes

Sensitizers as well as desensitizers form a reversal oxidoreduction system with silver halides, according to both pH and pAg of the photographic emulsion. But besides the specific influence of the emulsion, the efficiency of a sensitizing dye depends on many other factors such as its adsorption, its spectral absorption, the energetic transfer yield, the dye aggregate to the silver halide, and finally on its desensitizing property in... 

There are several requirements for a good sensitizing dye. A good dye is adsorbed strongly to silver haUde. The dye molecules attach themselves to the surface of the silver haUde crystals, usually up to monolayer coverage. This amount can be deterrnined by measuring the adsorption isotherm for the... 

Davey, E. P. Optical sensitization and adsorption of dyes on silver halide The state of the adsorbed dye. Trans. Faraday Soc. 35, 323 (1940). 

Some effects of gelatin on the adsorption of sensitizing dyes on silver... 

Fiber optical sensors are popular devices for the design of optical chemosen-sors. They are based on the change of optical properties (such as adsorption or luminescence) of particular chemical indicators. For example, fiber optical oxygen sensors are produced by the immobilization of oxygen sensitive dyes on the tip of an optical fiber and in an appropriate matrix. 

Dye-sensitized solar cells (DSSCs) are photoelectrochemical solar devices, currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. DSSCs are based upon the sensitization of mesoporous nanocrystalline metal oxide films to visible light by the adsorption of molecular dyes.5"7 Photoinduced electron injection from the sensitizer dye (D) into the metal oxide conduction band initiates charge separation. Subsequently, the injected electrons are transported through the metal oxide film to a transparent electrode, while a redox-active electrolyte, such as I /I , is employed to reduce the dye cation and transport the resulting positive charge to a counter electrode (Fig. 17.4). 

The applicability of this in situ method for the determination of surface areas depends not only on knowledge of the dye s molecular area in the adsorbed state but also on the assumption that the chosen spectral parameter measures the surface concentration of the dye. In order to test the relation between adsorption of dye to silver halide and its spectral characteristics in the bound state, the behavior of Pseudocyanine in a coarse silver halide suspension (Dispersion D) was studied. This particular dispersion was chosen because some of its relevant adsorption characteristics had already been examined (22, 23). Moreover, observations by Boyer and Cappelaere with Pseudocyanine adsorbed on AgBr powders (5) indicated that /-band intensity varied with the amount of adsorbed dye and was not sensitive to the concentration of Ag+ or Br" ions in the range pAg 3.3-8.7. 

Adsorption of dye to the surface increased the local concentration, enhancing the TIRF sensitivity. 

Adsorption to the silver surface is one of the most important requirements for a sensitizing dye. Intimate contact with the silver surface facilitates electron transfer. To make stable films with predictable photographic properties it is also... 

Spectroscopic methods can be used to specify the position of donors and acceptors before photoexcitation [50]. This spatial arrangement can obviously influence the equilibrium eomplexation in charge transfer complexes, and hence, the optical transitions accessible to such species [51]. This ordered environment also allows for effective separation of a sensitizing dye from the location of subsequent chemical reactions [52], For example, the efficiency of cis-trans isomerization of A -methyl-4-(p-styryl)pyridinium halides via electron transfer sensitization by Ru(bpy) + was markedly enhanced in the presence of anionic surfactants (about 100-fold) [53], The authors postulate the operation of an electron-relay chain on the anionic surface for the sensitization of ions attached electrostatically. High adsorptivity of the salt on the anionic micelle could also be adduced from salt effects [53, 54]. The micellar order also influenced the attainable electron transfer rates for intramolecular and intermolecular reactions of analogous molecules (pyrene-viologen and pyrene-ferrocene) solubilized within a cationic micelle because the difference in location of the solubilized substances affects the effective distance separating the units [55]. 

The classic sensitizer dye employed in DSC is a Ru(II) bipyridyl dye, cis-bis(isothiocyanato)-bis(2,2/-bipyridyl-4,4/-dicarboxylato)-Ru(II), often referred to as N3 , or in its partially deprotonated form (a di-tetrabutyl-ammonium salt) as N719. The structure of these dyes are shown in 2 and 26. The incorporation of carboxylate groups allows immobilization of sensitizer to the film surface via the formation of bidendate coordination and ester linkages, whilst the (- NCS) groups enhance the visible light absorption. Adsorption of the dye to the mesoporous film is achieved by simple immersion of the film in a solution of dye, which results in the adsorption of a dye monolayer to the film surface. The counter electrode is fabricated from FTO-coated glass, with the addition of a Pt catalyst to catalyze the reduc-... 

The photoelectrochemical activity inherent in thin films of aggregated cyanine dyes permits them to act as the spectral sensitizers of wide bandgap semiconductors [69]. It is seen from Fig. 4.14 that the photoelectrochemical behaviour of semiconductor/dye film heterojunctions fabricated by deposition of 200 nm-thick films of cyanine dyes on the surface of TiC>2 and WO3 electrodes, bears close similarity to that of semiconductor electrodes sensitized by the adsorption of dye aggregates. Thus, both anodic and cathodic photocurrents can be generated under actinic illumination, the efficiency of the photoanodic and photocathodic processes and the potential at which photocurrent changes its direction being dependent on dye and semiconductor substrate [69]. 

Simultaneous electrodeposition of ZnO and dye molecules from aqueous solution was reported as well to achieve a deeper penetration of the dye molecules into the inorganic film, as compared with those obtained by adsorption of dye molecules by the pre-assembled particles (mostly limited to the top layers). Yoshida and coworkers (2000) conducted such deposition process in an aqueous solution of Zn (N03)2 and eosin Y at 70°C and obtained ZnO/eosin Y dye-sensitized semiconductor films without high temperature annealing. The eosin Y molecules condensed at a sufficiently high concentration at both inside and on the surface of the film, which improved light absorption and IPCE. 

Adsorption of dyes onto a semiconductor surface allows for another mode of photoactivation [44-48]. The dye adsorbs a photon, generating an excited state in which a sufficiently higher-energy orbital is populated to allow direct injection of an electron into the conduction band edge [1]. The dye thus becomes oxidized and can either react chemically with nucleophiles by bond formation or can be restored to its original oxidation level by electron transfer. In the latter case, the reaction partner is oxidized, regenerating the ground state of the sensitizer ready to participate in... 

Figure 3. Adsorption isotherms for some cyanine and merocyanine sensitizing dyes.

In this paper we have attempted to explore useful tools and unfinished business in the surface science of silver halide photographic systems. The sensitizing dye/AgBr system has been explored as a compliant model system for persons who might wish to carry out fundamental investigations in modes of adsorption and surface phase transitions. Many of the experimental and conceptual tools developed within the photographic industry are... 

Herz, A. Adsorption of Sensitizing Dyes to Silver Halides, Chapter 9 of "The Theory of the Photographic Process", 4th ed., James, T.H., editor, MacMillan Co., New York, 1977. 

The interface between two immiscible liquids is used as a characteristic boundary for study of charge equilibrium, adsorption, and transport. Interfacial potential differences across the liquid-liquid boundary are explained theoretically and documented in experimental studies with fluorescent, potential-sensitive dyes. The results show that the presence of an inert salt or a physiological electrolyte is essential for the function of the dyes. Impedance measurements are used for studies of bovine serum albumin (BSA) adsorption on the interface. Methods for determination of liquid-liquid capacitance influenced by the presence of BSA are shown. The potential of zero charge of the interface was obtained for 0-200 ppm of BSA. The impedance behavior is also discussed as a function of pH. A recent new approach, using a microinterface for interfacial ion transport, is outlined. 

A. Fillinger, B. A. Parkinson, The adsorption behavior of a ruthenium-based sensitizing dye to nanocrystalline Ti02 - coverage effects on the external and internal sensitization quantum yields, J. Electrochem. Soc. 1999, 146(12), 4559-4564. 

Adsorption isotherms can conveniently be measured for sensitizing dyes on silver halides. The formation of a J-aggregate indicates a close-packed condensed state that corresponds to a certain limiting area per dye molecule. Such areas are conveniently measured by performing adsorption measurements on monodisperse emulsions of a given silver halide type. Limiting areas can then be used to estimate surface areas of polydis-perse emulsions of the same halide type. 

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