2,2′-Bipyrimidine as a Building Block for the Design of Emissive Conjugated Polymers for Hybrid LED Lighting

This work introduces 2,2′-bipyrimidine as an acceptor unit in the design of donor-acceptor conjugated polymers. Regularly alternating this moiety with the electron-rich 2,7-dihexyl fluorene and 3,6-carbazole units lead to polymers P1 and P2, respectively, which both showed a red-shifted emission with respect to the parent polyfluorene and polycarbazole derivatives. Investigations on the thermal properties showed that P1 and P2 both possess decomposition temperatures higher than 250 °C under a mixture of N2 and O2. P1 was used as a representative example of this family of conjugated polymers to design white-emitting materials, both in solution and in the solid state. P1 and P2 were investigated as phosphors for LED lighting. Two composite films elaborated with P1 and P2 embedded into a PMMA matrix labeled P1c and P2c, respectively, were irradiated with a 375 nm-LED at a power as high as 48 W/m2. P1c was found to show a lower photostability. Two ways to improve the stability under UV exposure were investigated. On the one hand, replacing 2,7-dihexyl fluorene by 3,6-hexyl carbazole approximately improved the photostability by a factor of 2, while on the other hand, embedding the polymers into a liquid sol-gel hybrid matrix allowed an improvement of the stability by a factor of 3. We could obtain a stabilization of the intensity of P2c at ∼half of its initial intensity, thus showing an improvement of the photostability of more than five times with respect to pure P1.