Azobenzene-Appended Bis-Cyclometalated Iridium(III) Bipyridyl Complexes

A new synthetic route toward photochromic azobenzene-appended bipyridyl ligands has been developed, and 20 new cationic iridium­(III) bis-cyclometalated complexes containing up to four azobenzene fragments on their structure have been synthesized and characterized electronically and spectroscopically. These compounds incorporate photochromic azobenzene units in their neutral pyridyl ligands <b>1</b>–<b>5</b>, and in four of them azobenzenes have been also introduced <i>a posteriori</i> on their phenylpyridyl ligands by palladium-catalyzed cross-coupling. UV–vis-monitored light-triggered <i>trans</i>-to-<i>cis</i> isomerization of the azobenzene-appended ligands revealed a strong inhibition of this process upon coordination to the d<sup>6</sup>-metal ion. TD-DTF calculations revealed that this inhibition could be a consequence of a metal to ligand charge transfer relaxation process.