Dicyanovinyl-unit-induced absorption enhancement of iridium(III) complexes in long-wavelength range and potential application in dye-sensitized solar cells

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

Iridium(iii) complexes with the dithieno[3,2-b:2',3'-d]phosphole oxide group and their high optical power limiting performances

A new 2-phenylpyridine-type (ppy-type) ligand with the dithieno[3,2-b:2',3'-d]phosphole oxide (DTPO) group has been successfully synthesized. Based on this novel ligand, three cyclometalated iridium(iii) complexes (P-Ir-P, P-Ir-T and P-Ir-C) are synthesized with symmetrical and unsymmetrical structures. Photophysical results reveal that these cyclometalated iridium(iii) complexes can show weak near-infrared (NIR) phosphorescence emission with wavelengths of 739 nm for P-Ir-P, 750 nm for P-Ir-T and 746 nm for P-Ir-C. Importantly, transient absorption characterization shows that these cyclometalated iridium(iii) complexes can exhibit strong excited state absorption in the range of ca. 520 to 700 nm, indicating their optical power limiting (OPL) potential in this wavelength range. Open-aperture Z-scan against a 532 nm laser shows their OPL ability in the order of P-Ir-P > P-Ir-C > P-Ir-T. Complex P-Ir-P shows an even better OPL ability than the state-of-the-art OPL material C60, indicating the important potential application of these cyclometalated iridium(iii) complexes as new OPL materials.