Slow dyes

Slow dyes that respond via a redistribution across the entire membrane (sometimes called Nemstain dyes) do so because of a change in the transmembrane electrical potential. As such, they can only be used as probes of the transmembrane potential and not as probes of the surface potential or the dipole potential. Dyes whose electric field sensing mechanism involves a movement between the aqueous medium and its adjacent membrane interface on one side of the membrane can, in principle, respond to changes in both the transmembrane electrical potential and the surface potential. Fast dyes that remain totally in the membrane phase (e.g., styrylpyridinium, annellated hemicyanine, and 3-hydroxyflavone dyes) respond to their local electric field strength, whatever its origin. Therefore, these dyes can, in principle, be used as probes of the transmembrane electrical potential, the surface potential, or the dipole potential. 

In general, the modest efficiencies of Cu(I)/(II) redox couples compared to the iodide/triiodide can be explained by a slow dye reduction, which could be reasonably anticipated, given the slow kinetics that are associated to the Cu(I)/(H) redox chemistry. On the other hand, using a suitably built photoanode equipped with a compact Ti02 underlayer and an appropriate heteroleptic dye like Z907, the detrimental electronic... 

No polymer formation occurred in any film containing only MB (though slow dye bleaching was observed in Elvamide films). Relative rates of polymer formation were determined in PVA films containing equimolar amounts of the activators NPG, TBBS, or TEOA. The rates obtained for freshly formed film samples and for films stored one day in the dark are shown in Table 6. The results parallel the solution studies described earlier. NPG is superior to the tin compound (TBBS) in photospeed. Both are better than the amine TEOA. However, after only one day of "aging," both the NPG and TEOA are inferior to TBBS, which does not suffer any speed loss on storage. 

Slow Dyes. Slow dyes generally operate by a potential-dependent partitioning between the extracellular medium and either the membrane or the cytoplasm. This redistribution of dye molecules is effected via the interaction of the voltage with the ionic charge on the dye. Unlike fast potentiometric indicators, slow redistribution dyes must be charged. Three chromophore types have yielded useful slow dyes cyanines, oxonols, and rhodamines. Each of these chromophores has special features that suit different kinds of experimental requirements. A set of important slow dyes is depicted in Chart III. 

Chart III. A representative sampling of good slow dyes. 

The slow dyes that change their spectral characteristics as a result of potential-dependent accumulation are well suited for cells in suspension. However, the complexity of these mechanisms makes it very difficult to interpret the change in dye fluorescence within a single cell. Accordingly, we set out to find dyes that display a rapid, reversible, potential-dependent intracellular accumulation with no accompanying spectral perturbation (44). As previously discussed, the dyes TMRE (Chart III) and TMRM emerged from this investigation and have proven useful in a wide variety of cultured cells. 

FIGURE 11.9 Slow dyeing, high-temperature dyes. Relative rates of color exhaustion from 1% dyeings at 30 1 liquor ratio using 1 h at 180°F as standard rate. (From F. Fortess and V.S. Salvin, Tex. Res. J., 28, 1009 (1958).)... 

Single molecules also have promise as probes for local stmcture when doped into materials tliat are tliemselves nonfluorescent. Rlrodamine dyes in botli silicate and polymer tliin films exliibit a distribution of fluorescence maxima indicative of considerable heterogeneity in local environments, particularly for the silicate material [159]. A bimodal distribution of fluorescence intensities observed for single molecules of crystal violet in a PMMA film has been suggested to result from high and low viscosity local sites witliin tire polymer tliat give rise to slow and fast internal conversion, respectively [160]. 

Synthesis by high-dilution techniques requires slow admixture of reagents ( 8-24 hrs) or very large volumes of solvents 100 1/mmol). Fast reactions can also be carried out in suitable flow cells (J.L. Dye, 1973). High dilution conditions have been used in the dilactam formation from l,8-diamino-3,6-dioxaoctane and 3,6-dioxaoctanedioyl dichloride in benzene. The amide groups were reduced with lithium aluminum hydride, and a second cyclization with the same dichloride was then carried out. The new bicyclic compound was reduced with diborane. This ligand envelops metal ions completely and is therefore called a cryptand (B. Dietrich, 1969). 

At HOY speeds, the rate of increase in orientation levels off but the rate of crystallization increases dramatically. Air drag and inertial contributions to the threadline stress become large. Under these conditions, crystallization occurs very rapidly over a small filament length and a phenomenon called neck-draw occurs (68,75,76). The molecular stmcture is stable, fiber tensde strength is adequate for many uses, thermal shrinkage is low, and dye rates are higher than traditional slow speed spun, drawn, and heat-set products (77). 

The mechanism of oxidative dyeing involves a complex system of consecutive, competing, and autocatalytic reactions in which the final color depends on the efficiency with which the various couplers compete with one another for the available diimine. In addition, hydrolysis, oxidation, or polymerization of diimine may take place. Therefore, the color of a mixture caimot readily be predicted and involves trial and error. Though oxidation dyes produce fast colors, some off-shade fading does occur, particularly the development of a red tinge by the slow transformation of the blue indamine dye to a red phenazine dye. 

Metallic Dyes. MetaUic dyes are among the older hair color materials known. Commercial products are based on a 1% solution of lead acetate in an aqueous, slightly acidic, alcohoHc medium. Precipitated sulfur appears to be essential. The convenience aspect is stressed by the leave-in appHcation method. Actually, the color development is so slow, taking about a week to ten days, that there is no alternative to this technique. Daily appHcation is needed at first. 

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

For most color photographic systems, development is the rate determining step, and within that step the formation of semiquinone is the slow process (37). The fate of the highly reactive QDI is deterrnined by the relative rates of a number of competing processes (38). The desired outcome is reaction with ionized coupler to produce dye (eq. 3). Typically, the second-order rate constant for this process with ionized coupler is about 10 to 10 ... 

The relatively low pX values seen for the benzoyl acetanilides, especiaHy as two-equivalent couplers, minimize concerns over slow ionization rates and contribute to the couplers overaH reactivity. But this same property often results in slow reprotonation in the acidic bleach, where developer carried over from the previous step can be oxidized and react with the stiH ionized coupler to produce unwanted dye in a nonimage related fashion. This problem can be eliminated by an acidic stop bath between the developer and the bleach steps or minimized by careful choice of coupling-off group, coupler solvent, or dispersion additives. 

Polyester (Textured or Filament) Dyed Under Pressure. The dyebath (50°C) is set with water conditioning chemicals as required, acetic acid to ca 5 pH, properly prepared disperse dyes, and 1—3 g carrier/L. The bath is mn for 10 minutes, then the temperature is raised at 2°C/min to 88°C and the equipment is sealed. Temperature is raised at l°C/min to 130°C, and the maximum temperature held for 1/2—1 h according to the fabric and depth of shade required. Cooling to 82°C is done at 1—2°C/min, the machine is depressurized, and the color sampled. The shade is corrected if needed. Slow cooling avoids shocking and setting creases into the fabric. Afterscour is done as needed. 

Isotherms. When a fibei is immersed, in a dyebath, dye moves fiom the external phase into the fibei. Initially the late is quick but with time this slows and eventually an equiUbrium is reached between the concentration of dye in the fiber and the concentration of dye in the dyebath. For a given initial dyebath concentration of a dye under given dyebath conditions, eg, temperature, pH, and conductivity, there is an equiUbrium concentration of dye in fiber, Dj and dye in the dyebath external solution, D. Three models describe this relationship simple partition isotherm, Freundhch isotherm, and Langmuir isotherm. 

If diffusion through the fiber is not carried out efficiendy then not only will the rate of dyeing be slow, with a chance that equihbrium between dye and fiber is not reached, but also the fibers will be dyed unevenly and possibly be ring dyed leading to poor fastness properties. Diffusion through the fiber is dependent on the actual dye and fiber chain molecular stmcture and configuration, and also, especially with hydrophobic fibers, the mobiUty of the chemical chain (7). 

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