New examples of Ru(II)-tetrazolato complexes as thiocyanate-free sensitizers for dye-sensitized solar cells

A set of three new Ru(II) polypyridyl complexes decorated with 5-aryl tetrazolato ligands (R-CN4)(-), (D series, namely D1, D3 and D4), is presented herein. Whereas complex D1 represents the pyrazinyl tetrazolato analogue of a previously reported Ru(II) complex (D2) with the general formula cis-[(dcbpy)(2)Ru ((NN)-N-boolean AND)](+), in which dcbpy is 2,2'-bipyridine-4,4'-dicarboxylic acid and (NN)-N-boolean AND is the chelating 2-pyridyl tetrazolato anion, the design of the unprecedented Ru(II) species D3 and D4 relied upon a completely different architecture. More specifically, the molecular structure of thiocyanate-based species cis-[(dcbpy)(2)Ru (NCS)(2)], that is typically found in benchmark Ru(II) dyes for dye sensitized solar cell (DSSCs), was modified with the replacement of two of the -NCS ligands in favour of the introduction of 5-aryl tetrazolato anions, such as the deprotonated form of 5-(4-bromophenyl)-1H-tetrazole, for complex D3 and 5-(4-cyanophenyl)-1H-tetrazole in the case of complex D4. To streamline the behavior of the D series of Ru(II) complexes as photosensitizers for DSSCs, an in-depth analysis of the excited state properties of D1, D3 and D4 was performed through TDDFT calculations and TDAS (nanosecond transient difference absorption spectroscopy). The obtained results highlight a trend that was confirmed once D1, D3 and D4 were tested as photosensitizers for DSSC under different conditions. Along the series of the Ru(II) complexes, the neutrally charged species D3 and D4 displayed the best photovoltaic performances.