Ruthenium(II) Charge-Transfer Sensitizers Containing 4,4‘-Dicarboxy-2,2‘-bipyridine. Synthesis, Properties, and Bonding Mode of Coordinated Thio- and Selenocyanates

Abstract

The synthesis and properties of several complexes of Ru(II) containing 4,4′-dicarboxy-2,2′-bipyridine (dcbpyH 2 ), 2,6-bis(1-methylbenzimidazol-2-yl)pyridine (bmipy), or 2,6-bis(1-methylbenzimidazol-2-yl)-4-phenylpyridine (phbmipy), and monodentate ligands (X -) Cl -, I -, NCS -, NCSe -, CN -) are reported. The introduction of the ambident ligands X -) NCS -, NCSe -, and CNinto the coordination sphere of [Ru(bmipy)(dcbpy)I] -and cis-Ru(dcbpyH 2 ) 2 Cl 2 has been studied in situ via 1 H and 13 C NMR spectroscopy using 13 C-enriched ligands X -. Introduction of thiocyanate and selenocyanate initially yields the two possible linkage isomers in comparable amounts; prolonged reaction time converts the S-bound isomer and the Se-bound isomer to the N-bound isomers. The isoselenocyanate complex decomposes rapidly, yielding the cyano complex under loss of Se. The N-bound isothiocyanato complex K[Ru(bmipy)(dcbpy)(NCS)] was found to be an efficient sensitizer for nanocrystalline TiO 2 ; the incident monochromatic photon-to-current efficiency (IPCE) is nearly quantitative at 520 nm. Introduction of a phenyl group in the 4-position of the 2,6-bis(1-methylbenzimidazol-2-yl)pyridine ligand gives a red-shifted absorption maximum for the corresponding phenylated K[Ru(ph-bmipy)(dcbpy)(NCS)] complex with an increased molar absorption coefficient for the MLCT maximum at 508 nm. At longer wavelengths above 620 nm, phenyl substitution does not enhance the absorption coefficients of the complex. Compared to that of K[Ru(bmipy)-(dcbpy)(NCS)], the performance of the phenylated complex is reduced in a solar cell due to lower IPCE values. The visible spectra of the halide complexes K[Ru(bmipy)(dcbpy)X] (X -) Cl -, I -) show enhanced red response, but the complexes exhibit strongly reduced overall IPCE values. A comparison of the complexes to cis-Ru-(dcbpyH 2 ) 2 (NCS) 2 is presented. Possible strategies for the design of more efficient sensitizers are discussed.

Review of Recent Progress in Dye-Sensitized Solar Cells

“…Moreover, these bulky sensitizers require more space on the TiO 2 surface and penetrate less easily in the small cavities of the nanocrystalline TiO 2 than the mononuclear complexes. Hence, for polynuclear complexes, the increased absorption coefficients in solution do not necessarily lead to the enhanced light absorption on the TiO 2 electrode due to the reduced surface concentration of the bulkier sensitizer molecules on the nanoporous TiO 2 [51].…”

Dye–Sensitized Solar Cells: An Overview

“…41- 44 An example of high-performance ruthenium polypyridine complexes is the cis-dithiocyanatobis (4, 4 -dicarboxilic acid -2, 2 -bipyridine)ruthenium(II), commonly known as the N3 dye. The carboxylate groups in N3 dye confer a strong binding to the TiO 2 surface, explaining its excellent stability, while DYE-SENSITIZED SOLAR CELLS: AN OVERVIEW 5 the thiocyanate groups (NCS) enhance its response to visible light.…”

Supramolecular chemistry and technology

“…Transition metal polypyridine complexes have been extensively investigated from the perspectives of developing useful supramolecular devices, exploiting their remarkable chemical and photochemical properties (Bignozzi et al 1997, Balzani et al 1996, Kohle et al 1996, Sauvage et al 1994. A particularly interesting system, is the tris(bipyrazine)ruthenium(II) complex, [Ru(bpz) 3 ] 2+ .…”