Cyanine chromophores

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... 

Kiyose K, Kojima H, Urano Y et al (2006) Development of a ratiometric fluorescent zinc ion probe in near-infrared region, based on tricarbo-cyanine chromophore. J Am Chem Soc 128 6548-6549... 

A single crystal structure analysis of 56a revealed the molecule as a symmetric mesoionic betaine having a plane of symmetry (6 2w) with all delocalized C-N bonds being approximately the same length (1.34 A). In this respect, the molecule can be considered as a cyanine chromophore <2004CC1860>. 

The absorption maximum of the cyanine dye can be changed by altering the number of conjugated alkene units linking the cyanine chromophores. This makes the cyanine borate photo-redox pair a so-called tunable photoinitiator, in that compounds which absorb throughout the visible and infrared spectrum can be obtained. Recently, Kabatc et al. [35] described the important features of cyanine borate photo-redox pairs (Table 2). The structures of dyes tested are shown in Figure 5. 

The investigation of the spectral properties of unfolded phytochrome65) revealed that the Pr chromophore has a A-dihydrobiliverdin structure (like phycocyanobilin in situ). The red shift of the phytochrome chromophore compared with the phyco-cyanin chromophore (Table 4) can be explained by the presence of a vinyl group in the former and a ethyl group in the latter. The A-dihydrohiliverdin structure has been confirmed by comparison with the synthetic model compound /7158), (cf. Table 5). Structure 18 has been derived from these data — together with the results of the degradation experiments — for the Pr chromophore in situ. 

Reaction of VI with Benzyltrimethylammonium Hydroxide. The hydroxide, 0.125 mmol, (60 mg of a 35% methanol solution) was placed in a tube and the methanol evaporated by warming at reduced pressure. Subsequent addition of 175 mg (0.5 mmol) of VI immediately resulted in the formation of a green color at the contact surface of VI and the solid hydroxide. Warming of the solid mixture to 100°C over a 10 minute period appeared to give complete reaction and heating was dlscontined. The i r spectrum showed greater than 95% decrease in cyano absorbtion and the visible spectrum showed no absorbance characteristic of phthalo-cyanine chromophore. 

The type and location of donor and acceptor groups on this chromophore that will maximize second order NLO properties is not obvious, however, because the ends of the cyanine chromophore are coordinated to the boron atom and therefore not available for direct modification. A noncentrosymmetric substitution pattern must provide for both a charge transfer transition that is related to the strong absorption in the symmetric molecule and a ground state dipole moment which is substantially parallel to this transition. Candidate structures were therefore evaluated computationally with MOPAC using the AM 1 basis for geometry optimization. Spectroscopic INDO/S methods with configuration interaction (ZINDO) were used for electronic spectra estimation (6). 

Compound A s absorbance maximum (A,max) is 555 nm in benzene with a molar absorptivity (s) of 86 x 103 L/mol-cm. The large molar absorptivity indicates that the oscillator strength of the initial cyanine chromophore is not lost in the asymmetric system. The absorption maxima are only slightly solvatochromic. 

Rhodacyanines possess two chromophoric systems. They are at the same time neutrocyanine derivatives, which involves position 5 of the ketomethylene, and methine cyanine, which involves position 2. Following lUPAC s standard nomenclature rules, structure 7 is named 3-ethyl-4-phenyl-2- 4-oxo-3-ethyl-5-[2-(3-ethy]-2,3-dihydro-benzo-l,3-thiazo-lylidene)ethylidene]-tetrahydro-l,3-thiazolylidene-methyl -1.3-thiazolium iodide (Scheme 5). It implies that the 4-phenyl thiazole ring having the... 

These dyes possess two independent chromophoric chains of even methine (neutro) and uneven methine (cyanine) fixed on a central ketometbylene nucleus. The methylene reactive group is first used for the neutrocyanine synthesis in position 5. the, quaternization of which can ensure a subsequent polymethine synthesis in position 2 of a cationic dye by ordinary means (Scheme 58). As indicated, this quaternized neutrocyanine (37) may as well give another neutrocyanine. 

In tetranudear dyes such as in Scheme 69 with two independent chromophoric chains, providing the chains are of different length, the absorptions maxima lie at the same A as each of the component cyanine but chains of equal lengths produce a shift of the two A, one toward longer... 

By varying the molecular stmcture, it is possible to synthesize dye initiators with the requited characteristics. The synthesis of polymethine dyes with different chain length, end groups, and substituents, or with other variations of the chromophore, has been summarized (3,4,9,21,73,74) (see also Cyanine dyes). 

Additional Chromophores. Other types of dyes that have been studied as chromophores in dye developers include rhodamine dyes, azamethine dyes, indophenol dyes, and naphthazarin dyes (21). Cyanine dyes, although not generally stable enough for use as image dyes, have also been incorporated in dye developers (31). 

The color and constitution of cyanine dyes may be understood through detailed consideration of their component parts, ie, chromophoric systems, terminal groups, and solvent sensitivity of the dyes. Resonance theories have been developed to accommodate significant trends very successfully. For an experienced dye chemist, these are useful in the design of dyes with a specified color, band shape, or solvent sensitivity. More recendy, quantitative values for reversible oxidation—reduction potentials have allowed more complete correlation of these dye properties with organic substituent constants. 

Several examples of the chromophoric systems (A), (B), and (C) are shown ia Figure 1. The early dyes were single chromophore stmctures of the type (A) WiUiams cyanine [862-57-7] Piaacyanol [605-91-4], and thiacarbocyanine [905-97-5]. The more compHcated dye stmctures ia Figure 1 stUl contain these chromophoric systems. 

Fig. 1. Chromophoric systems. Note that (A) = amidinium-ion system (a cyanine), (B) = carboxyl-ion system (an oxonol), and (C)...

More recent research provides reversible oxidation-reduction potential data (17). These allow the derivation of better stmcture-activity relationships in both photographic sensitization and other systems where electron-transfer sensitizers are important (see Dyes, sensitizing). Data for an extensive series of cyanine dyes are pubflshed, as obtained by second harmonic a-c voltammetry (17). A recent "quantitative stmcture-activity relationship" (QSAR) (34) shows that Brooker deviations for the heterocycHc nuclei (discussed above) can provide estimates of the oxidation potentials within 0.05 V. An oxidation potential plus a dye s absorption energy provide reduction potential estimates. Different regression equations were used for dyes with one-, three-, five-methine carbons in the chromophore. Also noted in Ref. 34 are previous correlations relating Brooker deviations for many heterocycHc nuclei to the piC (for protonation/decolorization) for carbocyanine dyes the piC is thus inversely related to oxidation potential values. 

Detailed studies on this line are in progress in our laboratory in an attempt to reach equally clear conclusions for more complex cyanines characterized by the same (pentamethine) chromophore as BMPC (e.g. DOC and DTC). 

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