Aromatic β‐Diketone as a Novel Anchoring Ligand in Iridium(III) Complexes for Dye‐Sensitized Solar Cells

Abstract

A novel β‐diketone ligand bearing a thiophene moiety and an anchoring Ph‐COOCH3 unit has been designed, prepared and used in synthesis of cyclometalated benzimidazole‐based iridium(III) complexes for application in dye‐sensitized solar cells (DSSC). The replacement of traditional 4,4'‐dicarboxy‐2,2'‐bipyridine by this aromatic β‐diketone results in excellent tuning the redox potentials and excited state properties of these IrIII complexes, one of which exhibits good efficiency when used as a dye in DSSC. All the complexes demonstrate reversible redox behavior, with oxidation potentials (Ir4+/Ir3+) strongly depending on the electron‐donor/withdrawing nature of the substituents in the cyclometalated ligands. Surprisingly, the latter has just a little effect on their luminescence spectra, in which structured emission bands are observed. Time‐resolved spectroscopic studies in combination with DFT calculations show that the complexes emit light by mixed 3MLCT–3LC excited states predominantly composed of the diketone ligand‐centered triplet state.

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

“…Unique photophysical properties of octahedral d 6 cyclometalated iridium complexes in combination with their high stability determine current and potential application of these complexes [1][2][3][4][5] in organic light emitting diodes (OLEDs), [6][7][8] photodynamic therapy and bioimaging, [9][10][11][12][13] as homogeneous photocatalysts, 14,15 biosensors, [16][17][18] building blocks for supramolecular architectures [19][20][21] and photosensitizers in dyesensitized solar cells (DSSC). [22][23][24][25][26][27] The latter technology is experiencing a renaissance, 28 being highly attractive for several niche applications (mostly indoors) for which diffuse light performance, transparency and flexibility are more important factors than the high efficiency. As the DSSC panels should in general withstand high temperatures, thermally and photostable cyclometalated iridium(III) complexes are considered as potential alternatives to the ruthenium(II) and organic dyes in the long term.…”

“…[34][35][36][37] In fact, such modifications of the ligands result in a dramatic drop of the metal d-orbitals contribution to the highest occupied molecular orbital (HOMO) of the complexes decreasing the intensity of the metal-to-ligand charge transfer (MLCT) bands. 24 Still, the narrowing of the HOMO-LUMO gap with simultaneous maintaining the significant contribution of the metal d-orbitals to the HOMO is not sufficient for creation of an ideal metal photosensitizer for DSSC. The localization of the frontier molecular orbitals enabling the charge transfer directionality from the cyclometalated fragment to the "anchoring" ligand (necessary for binding a dye to the semiconductor surface) is also very important.…”

Recent Developments in the Synthesis of β-Diketones

“…An aromatic 1,3-diketone bearing a thiophene moiety has been recently described, also employing soft enolization techniques [84]. This compound has further been employed as a ligand in iridium complexes with interesting photochemical properties.…”

A Panchromatic Cyclometalated Iridium Dye Based on 2-Thienyl-Perimidine

“…Cyclometalated iridium(III) complexes have proved to be the best emitters in phosphorescent organic light-emitting diodes (PHOLED) due to their bright luminescence, appropriate excited state lifetimes, high thermodynamic and kinetic stability, and fine tuneability of the emission color by simple ligand variation [ 1 , 2 ]. These complexes are also considered as effective photocatalysts [ 3 , 4 ] and promising photosensitizers in solar cells [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ]. Still, for the latter application, the light-harvesting properties of iridium(III) complexes need to be improved by judicious selection of cyclometalating and/or ancillary ligands.…”