Cyanine dimers

Visible Absorption Spectroscopy and Structure of Cyanine Dimers in Aqueous Solution 176... 

Cell membrane-impermeant dyes, inch stains for dead cells (SYTOX Dyes), high affinity stains (the cyanine dimers TOTO, YOYO etc.), and counterstains (cyanine monomers, YO-PRO, TO-PRO e.o.),... 

Substituents on the methine chain can stabilize the dye radical cation if the substituent (like methyl) is located on the high electron density carbons. However, no significant stabilization occurs when alkyl groups are on the alternate positions (like 9, 11 for the dication in Fig. 9). Current results for several dyes including die arbo cyanines and carbocyanines indicate that electronic stabilization of the dication radical lengthens the radical lifetime and also enhances the reversibiUty of the dimerization process (37). 

Optical properties of cyanines can be usefiil for both chiral substituents/environments and also third-order nonlinear optical properties in polymer films. Methine-chain substituted die arbo cyanines have been prepared from a chiral dialdehyde (S)-(+)-2-j -butylmalonaldehyde [127473-57-8] (79), where the chiral properties are introduced via the chiral j -butyl group on the central methine carbon of the pentamethine (die arbo cyanine) chromophore. For a nonchiral oxadicarbocyanine, the dimeric aggregate form of the dye shows circular dichroism when trapped in y-cyclodextrin (80). Attempts to prepare polymers with carbocyanine repeat units (linked by flexible chains) gave oligomers with only two or three repeat units (81). However, these materials... 

With l,3-dimethyl-2,l-benzisoxazolium salts, however, considerable reactivity has been reported. Condensation occurs readily with aldehydes, ketones, orthoesters and diazonium salts to yield styryl, cyanine and azo compounds, respectively (78JOC1233). In the presence of triethylamine, dimerization was observed, and the reactions of the cation were considered to involve the intermediacy of the anhydro base (77JOC3929). 

Many multiple copper containing proteins (e.g., laccase, ascorbate oxidase, hemo-cyanin, tyrosinase) contain so-called type III copper centers, which is a historical name (cf. Section 5.8 for type I and type II copper) for strongly exchange-coupled Cu(II) dimers. In sharp contrast to the ease with which 5=1 spectra from copper acetate are obtained, half a century of EPR studies on biological type III copper has not produced a single triplet spectrum. Why all type III centers have thus far remained EPR silent is not understood. 

West W, Pearce S (1965) The dimeric state of cyanine dyes. J Phys Chem 69 1894-1903... 

Cyanine dye-CD complexes were first reported by Kasatani et al. in 1984 using 3,3 -diethyloxadicarbocyanine iodide (DODC) [22]. It was found that this dye formed complexes with (3- and y-CDs but not with a-CD. Later, they demonstrated the same tendency with cyanines 1 and 2 [23]. It was shown that inclusion of cyanine dyes in [i-CD and Me-(3-CD helps to inhibit dimer formation as well as to enhance the photostability of these cyanines, thereby enhancing the dyes utility as a fluorescent probe [7, 24],... 

Kasatani K, Ohashi M, Kawasaki M, Sato H (1987) Cyanine dye-cyclodextrin system. Enhanced dimerization of the dye. Chem Lett 16 1633-1636... 

Nafion is thought to form dimers giving rise to a biexponential decay. 44,46) Using diode laser excitation at 670 nm, the fluorescence of oxazine in Nafion and its quenching by copper ions has been shown to give rise to a complex fluorescence decay.(47) Despite such complications there is still room for optimism. For example, Zen and Patonay(48) have demonstrated a pH sensor based on cyanine dye fluorescence intensity in Nafion excited with 30 mW diode laser excitation at 780 nm. 

An interesting equilibrium study was performed by Kasatani and coworkers on the interaction of a series of cyanine dyes with beta and gamma cyclodextrin, using u.v.-visible spectrophotometric measurements. The results were consistent with the enhancement of dimer formation by inclusion of the dye molecules within the cyclodextrin. As expected from the work of others, such geometrical factors " as the size of the aromatic groups and the lengths of the central methine chains strongly influence the occurrence of dimer formation within the cyclodextrin cavity. 

Good evidence for the formation of dimeric 4,4, 4",4 "-tetrasulphophthalo-cyanine oxovanadiumtiv) species in HjO-DMF solutions has been obtained from e.s.r. spectra, and the metal centres are estimated to be 450 pm apart. In aqueous solution, although dimers are present, most of the oxovanadium(iv) complexes are polymerized, and in DMF solution substantial amounts of monomer exist. Stability constants for the formation of 1 1 complexes between pyridine and some oxovanadium(iv)-2,4-disubstituted deuteriopor-phyrin dimethyl esters have been determined in chloroform solution at 25 °C. Magnetic and spectroscopic evidence suggest the presence of an electronic interaction between centres in these compounds. 

It is unlikely in such cases where different species exist on the surface that the photocurrent spectrum coincides with the absorption spectrum, since the efficiency of charge transfer will be different for different species. Memming 52> has concluded that for a cyanine dye adsorbed on a SnO 2-electrode the monomer seems to be more effective for charge injection than the dimer or higher aggregates. Hauffe and co-worker S3> have found that chelating dyes are especially efficient for sensitized electron injection into ZnO-electrodes which is seen in the variation of the photocurrent spectrum. 

Linear-dichroic spectra of SA monolayers prepared from mixtures of OTS and a cyanine-dye surfactant established the absence of dimerization and the orientation of the chromophore parallel to the substrate [183]. In contrast, the same cyanine dye underwent sandwich-type dimer formation in LB films and had its chromophore oriented perpendicular to the water surface [192]. These results highlight an important difference between LB and SA monolayers. Parameters which determine monolayer formation on an aqueous subphase are also responsible for the orientation and organization of the surfactants therein. Furthermore, the configuration of the surfactants is retained regardless of the structure of the substrate to which the floating monolayer was subsequently transferred to by the LB technique. Conversely, in SA monolayers, surfactant organization is primarily dependent upon the nature of the substrate [183]. 

One such reaction that has been studied is the electrocatalytic reduction of oxygen directly to water.25,27 The electrocatalysts for this process are often based on metal porphyrins and phthalo-cyanins. Thus a graphite electrode whose surface was modified by the irreversible adsorption of a cofacial dicobalt porphyrin dimer was able to reduce oxygen under conditions where the reduction did not occur on the bare electrode itself. Similarly, a catalytic chemically modified electrode for the oxidation of chloride to chlorine has been prepared28 where the active catalyst was reported to be a ruthenium dimer, [(bipy)2(OH)RulvORuvO(bipy)2]4+, which was reduced to the corresponding [Rum-RuIV] dimer during the reaction. 

The activity is strongly influenced by the state of aggregation of the phthalo-cyanines and porphyrins. The aggregation of phthalocyanines can conveniently be investigated with the help of visible electronic spectra. Not only monomeric species but also dimers seem to have a high activity (148,157). Consequently, the immobilization method and the loading with complex strongly affect the rates. 

The doughnut geometry of the host was taken a step further by use of a functionally active spacer group. Instead of simple aromatic spacers previously used, phthalo cyanines were employed [155]. The geometry of the resulting dimer (65)2 is similar to that of the doughnut, and dimerisation is observed to be preferable in aromatic solvents which will fit the cavity and template the dimerisation process (Fig. 57). The use of the phthalo cyanine introduces a potential catalytic site within the capsule. To date, no work has been published investigating the catalytic potential of this dimer. 

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