Geometrical Isomerism of Ru^II Dye-Sensitized Solar Cell Sensitizers and Effects on Photophysical Properties and Device Performances

To supplement our study on thiocyanate-free ruthenium sensitizers (TFRS) for dye-sensitized solar cells (DSSCs), which belong to a class of Ru^II-based complexes coordinated by a single 4,4′-dicarboxylic acid-2,2′-bipyridine and two symmetrically arranged functionalized trans-azolate chelates, we carefully isolated and characterized the second and less-abundant stereoisomer, in which the two pyridyl azolate ancillaries are asymetrically cis-arranged to each other. Two distinctive ancillaries, namely: 5-[4-(5-hexyl-2-thienyl)-2-pyridinyl]-3-trifluoromethyl pyrazole and 5-(6-tert-butyl-1-isoquinolinyl)-3-trifluoromethyl pyrazole, were employed in this study, giving a total of four sensitizers, that is, thienyl substituted TFRS-2 a and 2 b, and isoquinolinyl substituted TFRS-52 a and 52 b, in which the suffix b indicates the cis-stereoisomers. To gain insight into their fundamental properties their photophysical, electrochemical, and spectroelectrochemical behavior was investigated by density functional theory. Upon comparison of the correspondingly fabricated DSSCs, the sensitizers TFRS-2 a and 52 a yielded significantly higher conversion efficiencies than their asymmetrical cis-counterparts, TFRS-2 b and 52 b. To rationalize the cell performances charge extraction/photovoltage decay and impedance spectroscopic measurements were carried out to compare the rates of interfacial electron recombination from the TiO₂ conduction band to the electrolyte.