Spontaneous switching of permanent dipoles in single conjugated polymer molecules

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.