Bis-Cyclometalated Iridium Complexes Containing 4,4'-Bis(phosphonomethyl)-2,2'-bipyridine Ligands: Photophysics, Electrochemistry, and High-Voltage Dye-Sensitized Solar Cells.

In this report, the synthesis and characterization of two bis-cyclometalated iridium(III) complexes are presented. Single-crystal X-ray diffraction shows that [Ir(ppy)2(4,4'-bis(diethylphosphonomethyl)-2,2'-bipyridine)]PF6 adopts a pseudooctahedral geometry. The complexes have an absorption feature in the near-visible-UV region and emit green light with excited-state lifetimes in hundreds of nanoseconds. The redox properties of these complexes show reversible behavior for both oxidative and reductive events. [Ir(ppy)2(4,4'-bis(phosphonomethyl)-2,2'-bipyridine)]PF6 readily binds to metal oxide supports, like nanostructured SnIV-doped In2O3 and TiO2, while still retaining reversible redox chemistry. When incorporated as the photoanode in dye-sensitized solar cells, the devices exhibit open-circuit voltages of >1 V, which is a testament to their strength of these iridium(III) complexes as photochemical oxidants.

Sterically demanding methoxy and methyl groups in ruthenium complexes lead to enhanced quantum yields for blue light triggered photodissociation.

Ruthenium complexes containing a sterically congested metal center can serve as light activated prodrugs through photo-activated chemotherapy (PACT). In this work, we modified PACT agents containing 6,6'-dihydroxybipyridine (6,6'-dhbp) (Papish et al., Inorg. Chem., 2017, 56, 7519) by replacing it with a sterically bulky isoelectronic ligand, 6,6'-dimethoxybipyridine (6,6'-dmbp). The resulting complexes, [(phen)2Ru(6,6'-dmbp)]Cl2 (2OMe, phen = 1,10-phenanthroline) and [(dop)2Ru(6,6'-dmbp)]Cl2 (3OMe, dop = 2,3-dihydro-[1,4]dioxino[2,3-f][1,10]phenanthroline), have been fully characterized and display enhanced quantum yields for blue light triggered photodissociation of 0.024(6) and 0.0030(2), respectively. We have also synthesized 4OH = [(dmphen)2Ru(4,4'-dhbp)]Cl2 wherein dmphen = 2,9-dimethyl-1,10-phenanthroline and 4,4'-dhbp = 4,4'-dihydroxybipyridine. These ligands enhance steric bulk near the metal center and move the hydroxy groups further from the metal center, respectively. Complex 4OH displays a relatively low quantum yield of 0.0014(2). All of the new complexes (2OMe, 3OMe, 4OH) were tested in breast cancer cells (MDA-MB-231) and were non-toxic (IC50 > 100 µM). This has been interpreted in terms of unfavorable log(Do/w) values and furthermore photodissociation alone is insufficient for cytotoxicity. We also report the crystal structures of 4OH and 2OMe, the thermodynamic acidity of complex 4OH, and the redox potentials for all new complexes.