Potential energy surfaces of biphenyl and 9-phenylanthracene

While the biphenyl molecule has a twisted geometry in the S₀ state, it has a planar conformation in the S₁ state and its potential energy surface is almost harmonic along the torsional motion coordinate. The interference between the two phenyl groups in biphenyl is too weak to form an effective energy barrier in the S₁ state.

On the other hand, in the S₀ and S₁ states of 9-phenylanthracene, the potential energy surfaces along the torsion angle between the phenyl and anthracene planes have an almost single and a double minimum, respectively, due to the strong steric hindrance.