Benzimidazole-functionalized ancillary ligands for heteroleptic Ru(ii) complexes: synthesis, characterization and dye-sensitized solar cell applications

Abstract

We have designed and synthesized heteroleptic Ru(II) complexes with a pyridine-benzimidazole ligand (PYBI) for dye-sensitized solar cell (DSSC) applications. The PYBI ligand has major advantages by having the flexibility to introduce appropriate substituents at the four readily available positions through molecular engineering (Chart 1) compared to other ancillary bipyridyl-based ligands. We have substituted position A of the PYBI ligand with either electron-releasing triphenylamine (1) or pyrene (2). We have also introduced 2-hexylthiophene at position A and 3,5-di tert-butyl phenyl group at position B of the PYBI ligand (3). All three heteroleptic Ru(II) complexes have been characterized by mass spectrometry, ¹H NMR, and absorption and emission spectroscopies as well as electrochemical methods. The absorption spectrum of complex 3 is red-shifted and the emission spectrum is blue-shifted, when compared to the standard N719 sensitizer. Testing of these newly designed heteroleptic Ru(II) sensitizers has revealed that complex 3 exhibits an efficiency of 7.88% using an I⁻/I₃⁻ redox electrolyte. Experimental observations corroborated by computational calculations have elucidated the high efficiency of complex 3, primarily due to the fact that the substituents at position A are more influential than those at position of B of the PYBI ligand.