Synthesis of poly(spirosilabifluorene) copolymers and their improved stability in blue emitting polymer LEDs over non-spiro analogs

Abstract

We report herein a unique deep blue emitting copolymer, poly(3,6-dimethoxy-9.9′-dihexylsilafluorene-co-3.6-dimethoxy-2′,3′,6′,7′-tetrahexyloxy-9,9-spiro-9-silabifluorene) (PHSSF-co-PDHSF), which exhibits brilliant solid state blue luminescence, high quantum efficiency, excellent solubility and thermal stability. We have found that using PHSSF-co-PDHSF copolymers with large volume fractions of spirosilabifluorene as the emissive layer in OLEDs correlates to more stable EL intensity and have improved lifetimes over non-spiro poly(silafluorene) devices. The HSSF monomer is prepared via a two-part procedure, with each part resulting in one of two biphenyl segments which combines in a final cyclization step involving tetrachlorosilane. One segment possesses two bromide groups necessary for the polymerization. We utilized an efficient nickel catalyzed polymerization based on diarylmagnesate monomers to create PHSSF-co-PDHSF in good yield with number average molecular weights exceeding 50 kg mol⁻¹ with a PDI = 1.8. The polymerization was complete in less than 30 min. For PHSSF-co-PDHSF OLED devices, the maximum irradiance of the device was 40 W m⁻² at a current density of 60 mA cm⁻² and EL maximum centered at 410 nm. The maximum device external quantum efficiency was 2.9% when operating at 38 mA cm⁻². To measure OLED stability, we monitored normalized EL intensity for both PHSSF-co-PDHSF and PDHSF devices. Over the course of 10 h, the EL intensity of the PDHSF device drops 20% more than the PHSSF-co-PDHSF device when operating at 6 mA cm⁻²