Nine new molecular motors, consisting of a 2,3-dihydro-2-methylnaphtho[2,1-b]thiopyran
or 2,3-dihydro-3-methylphenanthrene upper part and a (thio)xanthene, 10,10-dimethylanthracene,
or dibenzocycloheptene lower part, connected by a central double bond, were synthesized.
A single stereogenic center, bearing a methyl substituent, is present in each of the
motors. MOPAC93-AM1 calculations, NMR studies, and X-ray analysis revealed that these
compounds have stable isomers with pseudoaxial orientation of the methyl substituent
and less-stable isomers with pseudoequatorial orientation of the methyl substituent.
The photochemical and thermal isomerization processes of the motors were studied by
NMR and CD spectroscopy. The new molecular motors all show two cis-trans isomerizations
upon irradiation, each followed by a thermal helix inversion, resulting in a 360 degrees
rotation around the central double bond of the upper part with respect to the lower
part. The direction of rotation is controlled by a single stereogenic center created
by the methyl substituent at the upper part. The speed of rotation, governed by the
two thermal steps, was adjusted to a great extent by structural modifications, with
half-lives for the thermal isomerization steps ranging from t(1/2)(theta) 233-0.67
h. The photochemical conversions of two new motors proceeded with near-perfect photoequilibria
of 1:99.