Electric fields applied in low temperature single molecule spectroscopy of conjugated polymers open a direct window to the microscopic charge distribution within the individual molecule. We typically observe a linear Stark effect, characteristic of either an internal electric field within the molecule or a difference in polarisation of ground and excited state. Here, we report evidence for spontaneous switching of the permanent dipole responsible for the shift in emission with field in a ladder-type polymer. This switching most likely results from photoinduced radical formation and annihilation and can be linked to blinking in single molecules and nanocrystal quantum dots. (c) 2006 Elsevier B.V. All rights reserved.