The rotary cycle of a fluorene-based molecular motor is illustrated in Fig. 1. In its lowest energy conformation (1a), the methyl substituent at the stereogenic centre adopts an axial orientation in which steric hindrance is minimised. Following absorption of ultraviolet (UV) light, excited state photoisomerisation causes the molecule to rotate around the axle (1b) and in this conformation the methyl substituent is forced to adopt an equatorial orientation. This step is sometimes referred to as the photochemical power stroke and determines the photochemical conversion efficiency of the motor. Following relaxation back to the ground electronic state, increased steric interactions between the rotor and the stator create tension in the molecule that is subsequently released as the rotor and stator blades snap over one another in a thermal helix inversion (1b–1c) and the methyl group re-adopts its thermodynamically favourable axial orientation. Importantly, the steric barrier to reverse rotation on the ground electronic state locks the molecular motor in this conformation until it can absorb another photon. A full unidirectional rotary cycle is then completed following a second UV photoisomerisation and ground-state thermal helix inversion (1c–1d–1a).