Cyanine quantum efficiency

Excited-state properties of the cyanine and related dyes are complex. Most cyanine dyes exhibit small Stokes shifts for duorescence maxima. Typical carbocyanines (1) with n = 1 show 14- to 16-nm shifts in methanol solution with low quantum efficiencies for duorescence (Op ) of less than 0.05. The diearbocyanine analogues also show small Stokes shifts but higher quantum yields (Lpj = 0.3-0.5). 

Muenter and Cooper (30) measured the room-temperature fluorescence of two J-aggregated dyes, l,l -diethyl-2,2 -quino-cyanine and 1,1, 3,3 -tetraethyl-5,5, 6,6 -tetrachlorobenzimid-azolocarbocyanine, adsorbed on cubic AgBr grains. The quantum efficiency of spectral sensitization was inversely related to the relative fluorescence. The fluorescence by the dyes in the molecular state was low compared to that for the aggregated dyes. Addition of a styryl or thiohydantoin dye as a supersensitizer for the quinocyanine quenched its fluorescence and increased the relative efficiency from 0.06 to nearly 1.0. 

The quantum efficiency of these cyanine borates when they are used as photoinitiators depends strongly on structure (Figure 6). However, in general, cyanine borates are not particularly efficient photoinitiators. For example the RBAX-iV-phenylglycine photo-redox pair [36] is much more efficient. The main value of the cyanine borates is that they give substantial wavelength flexibility so they can be used in cases where polychromaticity is required (Figure 7). 

Spiro-functionalized polyfluorenes have been used for the fabrication of blue polymer LEDs [27,153]. A device was reported using ITO/PDOFSBF 132/Ca that exhibited a maximum external quantum efficiency of 0.12%. Turn-on voltage and efficiency were improved by inserting copper phthalo-cyanine (CuPc) between anode and polymer (external quantum efficiency 0.54%). 

Fig. 2 Photocurrent spectrum for Ti02 (101) anatase sensitized with a mixture of the three cyanine dyes shown on the right of the figure. The solution spectrum is shown for comparison. IPCE stands for incident photon to current efficiency, i.e., it is the external quantum efficiency (Reproduced with permission from Ref [11])...

An efficient combinatorial solid-phase synthesis of asymmetric cyanine dyes was developed by Isacsson and Westman [17] using a Rink amide polystyrene resin. The picolinium and lepidinium salts 6 were linked to the solid-phase resin by amide coupling, then the benzothiazole derivatives 7 were subsequently condensed with the coupled picoline and lepidine moieties to give the yellow to blue (Amax ahs = 420-590 nm) asymmetric, fluorescent (Amax em = 480-650 nm) cyanine dyes 8 (Scheme 5.2, Fig. 5.3). As a consequence of restricted rotation upon intercalation, the fluorescence quantum yields increase significantly when these dyes are bound to DNA. 

Much larger RISC values were found for the cyanine dyes 56-58 [38]. A similar two-color technique was used to measure these yields with the exception that meyo-tetraphenylporphyrin was used in place of Aberchrome 540 as the two-laser actinometer. As Table 4 shows, there is an inverse relationship between the triplet depletion (bleaching) quantum yield, 4>B1, and the 5 —> T ISC yield, 1SC. Thus a large value for 4>B1 was accompanied by a small 4>ISC. This reflects the cyclic flow of energy following excitation of Tx to Tn. Once RISC has occurred, decay of the S, state partitions between ISC and fluorescence. If 4>ISC is small, relatively few Sj states will be cycled back to T and 4>B1 will be large. This effect masks the actual efficiency of RISC. Thus the ratio < B1/(1 — isc) s giyen as 311 indicator of RISC efficiency. 

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