Rotation of Aryl Groups in 9,10-Diarylphenanthrenes: Does the Rotational Barrier Become Lower as the Backbone Becomes More Crowded?

A series of 9,10-diarylphenanthrenes was prepared, and their structures and dynamic behaviors were both theoretically and experimentally investigated. A more twisted and crowded phenanthrene backbone was associated with a lower barrier to the rotation of an aryl group.