In particular, the overcrowded alkene-based motors (abbreviated here as alkene motors) generate robust unidirectional rotation, which has been harnessed in several demonstration applications . The alkene motors have a central double bond between two carbons (Fig ), which divides the molecule into a lower part, referred to as the stator, and an upper part, referred to as the rotor. If one views the motor, in the stereochemistry shown, along the central bond, from the rotor to the stator, the rotor turns clockwise relative to the stator. In the first step, the central double bond undergoes Z-E photoisomerization, converting the stable form (A) to the metastable form (B). In the second step, thermal helix inversion (THI) converts B to another stable form (C). Here, the change in stator pucker from B to C resembles a flap of butterfly wings, putting the stator benzene rings from the back to the front. Another, similar, 180 • rotation then converts C to A via D. Because the motors shown here have symmetric stators, A and C have identical energy, as do B and D. In contrast, A and Figure : Chemical structures and rotation processes of the molecular motors studied in this paper. The rate constants for their THI steps (vertical arrows) all are on the microsecond timescale (8 × 10 6 s -1 , 2 × 10 6 s -1 , 3 × 10 5 s -1 for Motors S, N, O respectively ). Although the metastable forms have multiple substates, only the lowest energy metastable substate is drawn here. The corresponding 3D structures provide atom numberings used throughout this study. D always have different stabilities, as do B and C, due to difference in ring pucker. Thus, the pucker of the rotor 5-member ring puts the methyl group in either axial (A, C) or equatorial (B, D) position, the latter giving larger repulsion with the stator, and hence higher energy . As previously reported , the metastable forms, B and D, are actually mixtures of metastable substates with distinct conformations, and hence are more complex than shown in Fig .