A series of bidentate cyclometalated Ru(II) complexes of general formula [Ru(N^N)(N^N)(C^N)]+, where N^N = polypyridyl ligand and C^N = cyclometalating ligand, have been synthesized, characterized and tested in dye-sensitized solar cells (DSSCs). Cyclometalated Ru(II) complexes, in general, exhibited broader absorption profiles and cathodically shifted electrochemical potentials compared to their polypyridyl analogues. The prototypical cycloruthenated compound, [Ru(bpy)2(ppy)]+ (bpy = 2,2'-bipyridine; Hppy = 2-phenylpyridine), displayed comparable UV-vis spectral coverage to the standard DSSC dye, N3. Molar extinction coefficients were enhanced and the absorption profile was red-shifted through substitution of the molecular periphery. Molecules from the [Ru(bpy)2(ppy)]+ family displayed HOMO and excited-state energy levels properly aligned for use in the DSSC. Anchoring –CO2H groups were ideally located on the bidentate polypyridyl ligands (e.g., H2dcbpy; H2dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) because this arrangement localized excited-state electron density proximate to TiO2.
Increased molecular light absorption was accomplished by installing conjugated substituents (e.g., -NO2, -phenyl, -pyridyl, -2-thiophene-carbaldehyde) on the anionic ring of molecules with general formula [Ru(H2dcbpy)2(C^N)]+. Aromatic substituents were superior to –NO2 because of an ideally positioned lowest excited-state (i.e., localized to H2dcbpy instead of –NO2). Substitution of the anionic ring with 2-thiophene-5-carbaldehyde para to the Ru-C bond resulted in a superior absorption profile enabling a modest cell power conversion efficiency (PCE) of 3.3%.
Replacement of one H2dcbpy ligand with bpy generated tris-heteroleptic cyclometalated Ru(II) dyes with general formula [Ru(H2dcbpy)(bpy)(C^N)]+. The use of electron-rich cyclometalating ligands, however, led to poor PCEs because of incompatible Ru electrochemical potentials for dye regeneration. Strong electron withdrawing groups (e.g., –CF3) were required on the C^N ligand to overcome this problem. The combination of electron-rich aromatic (e.g., thiophene, triarylamine) groups on the ancillary bpy and –CF3 groups on the C^N ligand enabled the best light-absorption properties of any dyes examined in this dissertation, while also maintaining properly aligned HOMO and excited-state energy levels for use in the DSSC. Power conversion efficiencies of 7.3% at 1 Sun were attained with these dyes, exceeding the 6.3% achieved by the paradigmatic N3 dye under the same conditions.