Heteroleptic dyes

An analogous behavior extends to other species having small reorganization energies and appropriate potentials such as the iron(II) complexes Fe(DMB)32 + and Fe(DTB)32 + (Ey2 0.95 V versus SCE). When used in the presence of an excess of Co(DTB)32 + and in conjunction with suitable sensitizers like the heteroleptic dye Ru(dnbpy)(H2DCB)22+ (Em = 1.25 V versus SCE) (Fig. 17.28), the iron(II) comediators clearly enhance the performance of the Co(DTB)32+ and outperform the I /I3 redox couple, at least in terms of monochromatic photon to current conversion efficiency, with maximum values close to 85%. 

In general, the modest efficiencies of Cu(I)/(II) redox couples compared to the iodide/triiodide can be explained by a slow dye reduction, which could be reasonably anticipated, given the slow kinetics that are associated to the Cu(I)/(H) redox chemistry. On the other hand, using a suitably built photoanode equipped with a compact Ti02 underlayer and an appropriate heteroleptic dye like Z907, the detrimental electronic... 

Moreover, unexpectedly, experiments have shown that Vqc for DSCs employing heteroleptic dyes is significantly lower compared to that observed using homoleptic sensitizers containing the same number of protons [46]. This difference is likely to... 

The low efficiencies could be due to lack of intimate contact (interface) between the sensitizer (which is hydrophilic) and the spirobifluorene (which is hydrophobic). Moreover, the surface charge also plays a significant role in the regeneration of the dye by the electrolyte.98 In an effort to reduce the charge of the sensitizer and improve the interfacial properties between the surface-bound sensitizer and the spirobifluorene hole-carrier, amphiphilic heteroleptic ruthenium(II) complexes ((48)-(53)) have been used as sensitizers. These complexes show excellent stability and good interfacial properties with hole-transport materials, resulting in improved efficiencies for the solar cells. 

Buryak and Severin have described the use of dynamic libraries of Cu(II) and Ni(II) complexes as sensors for tripeptides [61]. A notable aspect of this work is that as isolation of the metal complexes is not necessary (sensing is accomplished by observing changes in the UV-vis spectrum), potential concerns over the lability of coordination complexes do not apply. Specifically, three common dyes [Arsenazo I (41), Methyl Calcein Blue (42), and Glycine Cresol Red (43), Fig. 1.18] were mixed with varying ratios and total concentrations of Cu(II) and Ni(II) salts in a 4X5 array. Previous work had demonstrated that these conditions produced equilibrating mixtures of 1 1 and 2 1 homo- and heteroleptic complexes [62], These arrays were able to clearly and unambiguously differentiate tripeptides based on the differential pattern of response. The Severin laboratory has... 

Kuang D, Ito S, Wenger B, Klein C, Moser JE, Baker RH, Zakeeruddin SM, Gratzel M (2006). High molar extinction coefficient heteroleptic ruthenium complxes for thin-film dye sensitized solar cells. J Am Chem Soc 128 4146-4154... 

Keywords Dyes / Solar cells / Ruthenium / Heteroleptic complexes / Charge recombination / Electron transfer... 

Scheme 1 shows the structnre of a heteroleptic rutheninm complex coded K-19, which dne to the extension of the 7t-system in one of its ligands has an enhanced absorption coefQcient. An analogne of this dye with long alkyl side chains on the bipyridyl gronp, named Z-907, showed excellent light conversion performance and cell stability [16]. These dyes of Z-series have proved themselves to be very use-fnl to sohd-state DSC, where their hydrophobic nature indeed became a helpful factor. Subsequently, they enhanced the performance of systems containing ionic electrolytes and hole conductors. The K-19 dye also exhibits excellent conversion yield and stability [17,18]. 

The heteroleptic ruthenium dye (347) which was utilized both as a sensitizer component and a molecular bridge to connect metal oxide particles in a metal oxide semiconductor has been designed by Youngblood and co-workers. Phosphonates have been found chemically selective for Ti02 and the malonic groups selective for Ir02 H20. 

The other important aspect in dye-sensitized solar cells is water-induced desorption of the sensitizer from the surface. Extensive efforts have been made in our laboratory to overcome this problem by introducing hydrophobic properties in the ligand. The heteroleptic complexes containing hydrophobic ligands of the type [Ru(dcbpy)(mhdbpy)(NCS)2] 1, [Ru(dcbpy)(dtdbpy)(NCS)2] 2 [Ru(dcbpy) (mddbpy)(NCS)2] 3 (dcbpy = 4,4 -dicar-boxy-2,2 -bipyridine, mhdbpy = 4-methyl-4 -hexadecyl-2,2 -bipyridine and dtdbpy = 4,4 -ditridecyl-2,2 -bipyridine, mddbpy = 4-methyl-4 -didodecyl-2,2 bipyridine) have been synthesized (Fig. 6). The photo-current action spectra of these complexes show broad features covering a large part of visible spectrum and displays a maxima around 550 run, where the incident monochromatic IPCE exceeds 80%. The performance of these hydrophobic complexes as CT photosensitizers in nanocrystaUine Ti02-based solar cell shows excellent stabdity toward water-induced desorption [55]. 

Work on solid-state DSCs has clearly shown a >100 mV Ti02 conduction band shift between a heteroleptic ruthenium dye and an organic dye, which was interpreted in terms of a dipole-induced Ti02 CB shift of different sign [50], Such shifts are generally more difficult to observe in DSCs based on a liquid electrolyte [31], in which the high ion strength and the effect of thermal motion may hinder the role of interface dipoles. Nevertheless, Kusama et al. reported a combined experimental and theoretical study which showed a clear correlation between the dipole moment of electrolyte additives and their DSC yoc-[48]. 

During the last two to three years the advent of heteroleptic ruthenium complexes furnished with an antenna function has raised the performance of the DSSC to a new level. Two examples of these dyes are ClOl and Z991 (Figure 3.16). 

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