Iridium(III) cyclometalates (<b>1c</b> and <b>2c</b>) 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 <i>o</i>-carborane
on phosphorescence efficiency. In comparison with the unlinked complex,
tethering the <i>o</i>-carboranes to the 5-positions of
ppy ligands (<b>2c</b>) enhanced phosphorescence efficiency
by over 30-fold in polar medium (Φ<sub>PL</sub> = 0.37 vs 0.011
in THF), while restricting the rotation of <i>o</i>-carborane
at the 4-positions (<b>1c</b>) negatively affected the phosphorescence
efficiency. The different effects of restricted rotation of <i>o</i>-carborane on phosphorescence efficiency were likely a
result of the different variations of the carboranyl C–C bond
distances in the excited state
