Tuning the optical properties of 2-Thienylpyridyl iridium complexes through carboranes and anions

Thienylpyridyl iridium(III) complexes containing an o-, m-, or p-carboranylvinyl-2,2′-bipyridine ligand and various counteranions (denoted o-PF₆, m-BF₄, m-PF₆ m- SbF₆, m-ClO₄, m- OTf, m-NO₃, m-BPh₄, m-F, m-Cl, and p-PF₆) were synthesized by using C-formyl carboranes as starting materials. The solid-state structures of o-PF₆, m- PF₆, m-ClO₄, and m-BF₄ showed that the cations form twisted cavities in which the anions are fixed by multiple hydrogen bonds. Anion-hydrogen interactions were investigated for nine m-carborane-based complexes with different counteranions. All carborane-based iridium(III) complexes show similar phosphorescence yields in solution but significantly different emission in the solid state. Anion-exchange titration and theoretical calculations revealed the relationships between structures and optical properties. The size of the anion and C-H••• X anion-hydrogen bonds strongly influence the phosphorescence quantum yield in the solid state. In particular, the C_car-H •••X hydrogen bonds between the carboranyl unit and the anion play an important role in solid-state phosphorescence. Complex p -PF₆ was successfully applied in phosphorescence-lifetime bioimaging owing to its low toxicity and near-infrared emission.