Iridium Cyclometalates with Tethered o-Carboranes: Impact of Restricted Rotation of o-Carborane on Phosphorescence Efficiency

Abstract

Iridium(III) cyclometalates (<bold>1c</bold> and <bold>2c</bold>) in which the two carborane units on the 4- or 5-positions of 2-phenylpyridine (ppy) ligands were tethered by an alkylene linker were prepared to investigate the effect of free rotation of o-carborane on phosphorescence efficiency. In comparison with the unlinked complex, tethering the o-carboranes to the 5-positions of ppy ligands (<bold>2c</bold>) enhanced phosphorescence efficiency by over 30-fold in polar medium (Phi(PL) = 0.37 vs 0.011 in THF), while restricting the rotation of o-carborane at the 4-positions (<bold>1c</bold>) negatively affected the phosphorescence efficiency. The different effects of restricted rotation of o-carborane on phosphorescence efficiency were likely a result of the different variations of the carboranyl C-C bond distances in the excited state.