Fluorene–fluorenone copolymer: Stable and efficient yellow-emitting material for electroluminescent devices

We have synthesized and characterized a new fluorene copolymer exhibiting bright yellow luminescence. In order to ensure a complete π-stacking of the active layer, a 9-fluorenone monomeric unit (FOne) has been used as comonomer in conjunction with the more classical 9,9-di-n-nonylfluorene unit. As expected with fluorene-based materials, when excited at 370 nm, the corresponding dilute copolymer solution photoluminescence spectra exhibit a main peak centered at 450 nm in the blue part of the visible spectrum. However, in the solid state, immediate structural reorganization of the layer occurs, leading to a red-shifted emission (bright yellow emission) centered at 550 nm. The origin of the emitted light has been attributed to excimers and/or aggregates based on short FOne segments and involves mainly exciton transfer between nonaggregated fluorene segments and aggregated ones. It is noteworthy that organic light-emitting devices based on these new materials exhibit no spectral evolution upon device operatio...