Cyanines absorption

Convergent series, in resonance theory. 70 Cyanines, absorption of, in relation with pKa, 73... 

The spectra of linear polyenes are modelled well as one-dimensional free-electron systems. The cyanine dyes are a classical example. They constitute a class of long chain conjugated systems with an even number n of 7r-electrons distributed over an odd number N = n — 1 of chain atoms. The cyanine absorption of longest wavelength corresponds to promotion of an electron from the highest occupied energy level, En/2 to the lowest unoccupied level, such that in terms of a free-electron model... 

The more basic benzimidazole-phosphamethin-cyanines can be easily protonated. In acetonitrile with 50% ethereal HBF4 the cyanine absorption bands at 440—421 nm and 347—335 nm(Table 1) of bis-[l,3-dimethyl-benzimidazole-2]-phosphacyanine disappear completely, A new sharp absorption at 300 nm (e = 30000) can be observed. Addition of tert-butanol restores 9C of the starting material, as can be seen by the spectrum. If the protonation is carried out in ethylene chloride with an excess of ethereal HBF4, the product can be isolated as a colorless, stable, but not analytically pure salt. Ethyldiisopropylamine or more ether will regenerate the cyanine. 

Figure 2. Absorption spectra of three cyanine sensitizing dyes thiacyanine (CH), thiacarbocyanine (CH)., and thiadicarbocyanine (CH) and illustrates the progression of cyanine absorption bands by lOOnm for each addition of (CH)2, a...

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... 

Nitro-substituted indolino spiroben2opyrans or indolino spironaphthopyrans are photochromic when dissolved in organic solvents or polymer matrices (27). Absorption of uv radiation results in the colorless spiro compound [1498-88-0], C22H2gN202, being transformed into the colored, ring-opened species. This colored species is often called a photomerocyanine because of its stmctural similarity to the merocyanine dyes (see Cyanine dyes). Removal of the ultraviolet light source results in thermal reversion to the spiro compound. 

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. 

Most cyanines show prominent, hypsochromic vibrational shoulders at shorter wavelength associated with the long-wavelength electronic transition (Fig. 2). These vibrational shoulders include one or two vibrational quanta (0 — 1 0 — 2 ), in addition to the absorption energy at Intensity... 

Data on duorescence, phosphorescence, excited-state lifetimes, transient absorption spectra, and dye lasers are tabulated in Ref. 16. The main nonduorescent process in cyanine dyes is the radiationless deactivation Sj — Sg. Maximum singlet-triplet interconversion ( 52 ) methanol for carbocyanines is about 3% (maxLgrp > 0.03), and the sum [Lpj + st] I than 0.10. 

Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

Sensitivity of the proposed method correlates with molai absorptivity of the cyanine dye. Mixed POMs PMeMo O j (Me=TP+, Sb +, BP+) were chosen as analytical form because of its higher stability as compared with 12-molybdophosphate. Only 2-3T0 concentration of molybdate is enough for complete formation of POM avoiding in this way formation of lA with polymolybdate ions. In addition, mixed POMs are stable in wider interval of pH. Increasing of anion chai ge from 3 to 5(6) is also favorable. Constant absorbance of lA is observed in the acidity range of 0.12-0.28 M. 

Araki, G., and Araki, H., Progr. Theoret. Phys. [Kyoto) 11, 20, Interaction between electrons in one-dimensional free-electron model with application to absorption spectra of cyanine dyes. ... 

Steady-state experiments can also be designed within the same kind of strategy. As an example, we can cite recent works [25,45], where the results of a quantitative analysis of the resolved absorption spectra of a number of trans and cis isomers of cyanine dyes were eompared with calculated oscillator strengths and transition energies so as to propose the identification of the observed phototropic species as well defined cis isomers. 

There has been some interest in extending the absorption range of cyanine dyes to longer wavelengths into the near-infrared region of the spectrum. Consideration of the spectral data for thiazole derivatives 118-120 is of some interest in this respect. Cyanine dye 118 shows the characteristic visible absorption spectrum for a dye of this type, giving a... 

Figure 6.17. Absorption spectra of l,r-diethyI-2,2 -cyanine bromide in ethylene glycol water (1 1) at room temperature and at 173°K, showing the //-aggregate states.(84> Reprinted by permission of Chemical Physics Letters.

Figure 6.18. Absorption and fluorescence spectra of l,r-diethyl-2,2 -cyanine bromide at 298 and 77°K. Fluorescence excitation at 498 nm and 565 nm, as indicated by arrows/941 Reprinted by permission of Chemical Physics Letters.

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