Vertical excitation energies (VEEs) of the first few singlet excited states of anions 1a–1d (R = COO–) were calculated at DFT-optimised geometries using the state-averaged complete active space self consistent field (SA-CASSCF) method45 with equal weighting given to the first four states (SA(4)). The active space consisted of six pairs of π and π* orbitals (12, 12) on the stator, rotor and axle, selected by inspection of the natural orbital occupations (see ESI†). Orbitals localised predominantly on the carboxylic acid group were excluded, since this group was included for experimental reasons and does not contribute significantly to the photochemistry (Section 3.2).