Cyanine dyes absorption spectra

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... 

The close correspondence between the absorption spectrum in solution and the photocurrent spectrum of the adsorbed dye is by no means found in all cases. The adsorbed state can be different in structure from the solution state which is seen in a different photocurrent spectrum. It has been found e. g. that polymers are formed in the adsorbed state. This is a well known phenomenon for cyanine dyes 50-5b where polymer bands are found in the absorption spectrum of the adsorbed molecules. An example is given in the photocurrent spectra of Fig. 15. One sees that with increasing amount of adsorbed dye — no equilibrium adsorption was reached during this experiment — the polymer absorption band appears in the photocurrent. 

Fig. 15. Photocurrent spectra for -ZnO-electrode (at pH = 5) with different amounts of adsorbed cyanine dye, increasing in concentration from 1 to 3. For comparison the light absorption spectrum of the adsorbed dye on ZnO (dashed curve)...

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. 

Direct labeling of a biomolecule involves the introduction of a covalently linked fluorophore in the nucleic acid sequence or in the amino acid sequence of a protein or antibody. Fluorescein, rhodamine derivatives, the Alexa, and BODIPY dyes (Molecular Probes [92]) as well as the cyanine dyes (Amersham Biosciences [134]) are widely used labels. These probe families show different absorption and emission wavelengths and span the whole visible spectrum (e.g., Alexa Fluor dyes show UV excitation at 350 nm to far red excitation at 633 nm). Furthermore, for differential expression analysis, probe families with similar chemical structures but different spectroscopic properties are desirable, for example the cyanine dyes Cy3 and Cy5 (excitation at 548 and 646 nm, respectively). The design of fluorescent labels is still an active area of research, and various new dyes have been reported that differ in terms of decay times, wavelength, conjugatibility, and quantum yields before and after conjugation [135]. New ruthenium markers have been reported as well [136]. 

Figure 2. a) Transient absorption spectra recorded after excitation of di- -butyltetramethylindo-carbocyanine benzyltriphenylborate in benzene solution with a 18 ps laser pulse. Negative absorbance related to the bleaching of the dye absorption. The band at Amax = 430 nm is attributed to the presence of the cyanine dye radical b) transient absorption spectra recorded after excitation of naphthyltriphenyl borate in benzene solution with a 18 ps laser pulse. The new feature is attributed to the presence of the (2-naphthyl)methyl radical c) the difference spectrum obtained by subtracting spectrum A from spectrum B the maximum at 385 nm is characteristic for the 2-methylnaphthyl radical. Data are taken from [25],... 

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. 

Figure 1.15 shows the absorption spectrum of a cyanine dye embedded in a stretched poly(vinyl alcohol) sheet, measured with linearly polarized light. The... 

Fig. 10.10 Photocurrent aclion spectra for an n-ZnO electrode in aqueous solution (pH 5) with different amounts of adsorbed cyanine dye curves 1-3, increasing concentration of adsorbed dye dotted curve, absorption spectrum.

Figure 4 shows the photocurrent action spectra of three typical cells under short circuit conditions [20]. Three different dyes were used to sensitize the heterojunction. In all three cases the action spectrum matches closely the absorptivity spectrum of the sensitizing dye. Solid-state heterojunctions sensitized with the mero-cyanine dyes (structure see Fig. 4) used in this study showed higher peak IPCE values when compared to Ru(lI)L2(SCN)2-sensitized junctions. Strongly improved... 

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