On the S1 potential energy surface, conformer 1b of our molecular motor is 0.23 eV lower in energy than conformer 1a. This lowering in energy can be rationalised in terms of the increased delocalisation of electron density across both the rotor and the stator (Fig. 4; this delocalisation is also present in 1c and 1d) and is the driving force for the initial rotation around the axle of the molecular motor following S0 to S1 photoexcitation. The energy difference between the 1a and 1b conformers on the S1 potential energy surface is almost identical to the equivalent energy difference calculated for the neutral, fluorene-based molecular motor, using SA-REBH&HLYP and TD-BH&HLYP methods.26