The kinetics of the irreversible photoinduced switch in polarisation (p) observed in single crystals of a fullerene derivative possessing hydrogen-bonding barbiturate units were investigated using confocal fluorescence microscopy. In the samples investigated, it was found that the maximum luminescence polarisation (p = 0.78) is obtained for an orientation of ca. 60°from the long axis of the crystal. Upon irradiation at 385 nm, the maximum luminescence polarisation undergoes a rotation of ca. 70°with respect to the initial orientation and reaches a new value of p = 0.40. The results indicate that the process is not dependent on the orientation of the incident polarised excitation beam and that it is not accompanied by a noticeable change in the photophysical properties of the crystal. Based on these observations, a mechanism is proposed in which photoinduced dimerisation occurs from the lowest energy emissive excimer-like state that acts as a sink for the excitation energy. © 2012 the Owner Societies.