Light-Driven Molecular Motors: Stepwise Thermal Helix Inversion during Unidirectional Rotation of Sterically Overcrowded Biphenanthrylidenes

To investigate the unidirectional rotation of chiral overcrowded biphenanthrylidenes in more detail,
the size of the substituent next to the double bond responsible for the unidirectionality of rotation was
varied. The thermal and photochemical isomerization of three sterically overcrowded alkenes is described.
The behavior of the biphenanthrylidenes with methyl and ethyl substituents is rather similar, and these
compounds undergo a unidirectional 360° rotation around the central double bond in a four-step sequence
involving two photochemical cis−trans isomerizations and two thermal helix inversions. The only difference
between these two true molecular motors was a small entropic effect, which causes the ethyl substituted
molecular motor to rotate slightly faster. The behavior of the i-propyl substituted compound differs significantly
from that of the other two. Although not all different isomers of the i-propyl substituted molecular motor
were detected spectroscopically, experimental data led to the conclusion that this compound can also be
considered as a molecular motor and is capable of performing a 360° unidirectional rotation. 1H NMR and
X-ray analysis show a meso-like form as an intermediate in the unidirectional rotation, which proves that
the thermal helix inversion is a stepwise process.