Simple light emitting diodes can be constructed using fluorescent organic materials. Conjugated polymers can be used both for charge transport and for light emission. It is considered necessary for maximum device efficiency to balance the rates of electron and hole injection. We report the synthesis of a poly(cyanoterephthalylidenene) that was designed to exhibit an increased electron affinity. Electrochemical measurements showed a significant shift in the oxidation and reduction potentials due to the cyano functionality. The use of this polymer in a range of electroluminescent devices is described. Internal quantum efficiencies of up to 4% can be achieved in a bilayer device using stable electrode materials. The route used to synthesize this polymer is amenable to considerable variation in the subunits employed. This allows tuning of both the band-gap and the electron affinity of the resulting polymer.