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Ruthenium(II) sensitizers with heteroleptic tridentate chelates for dye-sensitized solar cells.

Dye-sensitized solar cells (DSSCs) are a promising technology with the potential to harvest sunlight at low cost. Specifically, DSSCs based on either organometallic dyes or organic push-and-pull dyes adsorbed on nanocrystalline TiO2 photoanodes have attracted intensive research interest in the past two decades. The use of ruthenium(II)-based sensitizers is still the most attractive approach, because their photophysical and electrochemical properties can be systematically fine-tuned to achieve optimal material characteristics. Key examples of Ru sensitizers for DSSCs include the wellestablished N3 and N719 dyes and their functionalized derivatives. 6] These dyes are commonly assembled by incorporation of at least one 4,4’-dicarboxy-2,2’-bipyridine chelate (Scheme 1), on which the carboxy anchors allow efficient electron injection into the TiO2 nanoparticles. The accompanying thiocyanate ancillary ligands are pointed away from the TiO2 surface, thus providing intimate contact to the I /I3 redox couple in the electrolyte and subsequently triggering rapid regeneration of the oxidized sensitizer. The lower-lying absorption bands are attributed to metal-toligand charge transfer (MLCT) and display absorption thresholds close to 800 nm, unmatched by the majority of organic dyes known to date. For optimizing Ru sensitizers, it has been reported that, upon introduction of 4,4’,4’’-tricarboxy-2,2’:6,2’’-terpyridine (H3tctpy), the lowest energy transition could be extended toward the near-IR region, thus affording the panchromatic sensitizer known as N749, or black dye [nBu4N]3[Ru(Htctpy)(NCS)3]. [7] Although this design seems to show satisfactory sensitization up to 900 nm, it still possesses two major deficiencies. One is the inferior absorptivity in the visible region, attributed to the lack of effective auxochrome. The second is the presence of three thiocyanate ligands, which not only reduce the synthetic yield owing to coordination isomerism but also deteriorate the lifespan of the as-fabricated cell units, because the weak Ru NCS dative bonding causes notable dye decomposition during operation. In conjunction with current endeavors, we described one panchromatic dye PRT4, which possesses a styrene-substituted pyridyl pyrazolate (pypz) auxochrome. Its recorded solar-cell performance supersedes our best reference cells using N749, thus providing the first success for our research initiative. Herein we present a breakthrough design employing both the H3tctpy anchor and a newly designed tridentate ancillary ligand; the latter is also targeted for replacing the last remaining thiocyanate in our PRT4 dye as well as all three thiocyanates in N749 sensitizer. Thiophene derivatives or other auxochrome units were tethered to the central pyridyl group in an attempt to increase the light-harvesting capability. In this synthetic protocol (Scheme 2), the functionalized 2,6-bis(5-pyrazolyl)pyridine chelate was first prepared using a multistep process starting from chlorination of chelidamic acid, except for the synthesis of parent 2,6-bis(5-pyrazolyl)pyridine, which employed the commercially available 2,6diacetylpyridine. Bis-tridentate Ru complexes were then obtained from addition of the relevant 2,6-dipyrazolylpyridine derivative with source complex [Ru(tectpy)Cl3] in ethanol solution (tectpy = 4,4’,4’’-triethoxycarbonyl2,2’:6’,2’’-terpyridine). The crude ethoxycarbonyl Ru products were purified on a silica gel column and eluted with a mixture of hexane and ethyl acetate. To confirm their molecular structure, single-crystal X-ray analysis of one derivative (TF-2OEt; TF = thiocyanate-free) was carried Scheme 1. Ru sensitizers N3, N719, N749, and PRT4.