A new series of panchromatic ruthenium(II) sensitizers derived from carboxylated terpyridyl
complexes of tris-thiocyanato Ru(II) have been developed. Black dye containing different
degrees of protonation [(C(2)H(5))(3)NH][Ru(H(3)tcterpy)(NCS)(3)] 1, [(C(4)H(9))(4)N](2)[Ru(H(2)tcterpy)(NCS)(3)]
2, [(C(4)H(9))(4)N](3)[Ru(Htcterpy)(NCS)(3)] 3, and [(C(4)H(9))(4)N](4)[Ru(tcterpy)(NCS)(3)]
4 (tcterpy = 4,4',4' '-tricarboxy-2,2':6',2' '-terpyridine) have been synthesized
and fully characterized by UV-vis, emission, IR, Raman, NMR, cyclic voltammetry, and
X-ray diffraction studies. The crystal structure of complex 2 confirms the presence
of a Ru(II)N6 central core derived from the terpyridine ligand and three N-bonded
thiocyanates. Intermolecular H-bonding between carboxylates on neighboring terpyridines
gives rise to 2-D H-bonded arrays. The absorption and emission maxima of the black
dye show a bathochromic shift with decreasing pH and exhibit pH-dependent excited-state
lifetimes. The red-shift of the emission maxima is due to better pi-acceptor properties
of the acid form that lowers the energy of the CT excited state. The low-energy metal-to-ligand
charge-transfer absorption band showed marked solvatochromism due to the presence
of thiocyanate ligands. The Ru(II)/(III) oxidation potential of the black dye and
the ligand-based reduction potential shifted cathodically with decreasing number of
protons and showed more reversible character. The adsorption of complex 3 from methoxyacetonitrile
solution onto transparent TiO(2) films was interpreted by a Langmuir isotherm yielding
an adsorption equilibrium constant, K(ads), of (1.0 +/- 0.3) x 10(5) M(-1). The amount
of dye adsorbed at monolayer saturation was (n(alpha) = 6.9 +/- 0.3) x 10(-)(8) mol/mg
of TiO(2), which is around 30% less than that of the cis-di(thiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II)
complex. The black dye, when anchored to nanocrystalline TiO(2) films achieves very
efficient sensitization over the whole visible range extending into the near-IR region
up to 920 nm, yielding over 80% incident photon-to-current efficiencies (IPCE). Solar
cells containing the black dye were subjected to analysis by a photovoltaic calibration
laboratory (NREL, U.S.A.) to determine their solar-to-electric conversion efficiency
under standard AM 1.5 sunlight. A short circuit photocurrent density obtained was
20.5 mA/cm(2), and the open circuit voltage was 0.72 V corresponding to an overall
conversion efficiency of 10.4%.