On the origin of green emission bands in fluorene-based conjugated polymers

Blue-light-emitting diodes made of polyfluorenes have low stability and, under operation, rapidly degrade and produce undesirable low-energy emission bands (green or g-bands). A spectroelectrochemical study of the degradation process suffered by polyfluorenes is reported here. These polymers lose their electronic properties by electrochemical oxidation and reduction 1 through a-bond breaking. In addition, upon electrochemical reduction, the development of a structured green emission band at 485 nm is observed. The position and shape of this band is different from the usual featureless band at 535 nm assigned to fluorenone defects. The green-light-emitting product is isolated and analyzed by Fourier-transform IR spectroscopy; fluorenone formation is excluded. The isolated product is crosslinked; its green emission is probably related to the formation of an intramolecular excimer.