Tailoring the π-system of benzimidazole ligands towards stable light-harvesting cyclometalated iridium() complexes

The synthesis, structure, optical and redox properties as well as photovoltaic studies of iridium( iii ) complexes with cyclometalated 2-arylbenzimidazoles decorated with various polyaromatic fragments and an ancillary aromatic β-diketone are reported. Despite the strong preference of the iridium( iii ) ion to form bis- or tris-cyclometalated complexes in which the metal participates in five-membered metallacycles, the cyclometalation of the benzimidazole ligands containing rigid π-extended systems yields dimeric complexes containing strained five- or six-membered metallacycles and allows for generating an extremely rare monocyclometalated complex. X-ray crystallography shows that the steric strain observed in the dimers is retained in heteroleptic diketonate complexes which is also corroborated by gas-phase DFT calculations. While emission maxima and redox potentials of the heteroleptic complexes exhibit just a moderate variation upon the change of the cyclometalated ligands, the extension of the π-system of the benzimidazole ligands give the complexes remarkable light absorption in the visible spectral range, which meets the requirements for application in dye-sensitized solar cells. At the titania photoanodes, these iridium dyes retain their optical properties and exhibit power conversion efficiencies under standard AM 1.5 G conditions comparable to those of other iridium-based sensitizers. These results demonstrate that the size and position of the π-extended fragment in cyclometalated ligands can modulate not only the electronic structure of the corresponding iridium( iii ) complexes, but also affect their composition, structure and reactivity that may find implications in future design of emerging iridium dyes, emitters and catalysts. Cyclometalation of 2-arylbenzimidazoles decorated with various polyaromatic fragments leads to formation of strongly absorbing iridium dyes applicable in DSSC and enables access to structurally diverse iridium( iii ) complexes.