Effect of side chains on color purities of mono-triphenylamine-functionalized polyspirobifluorenes for pure blue polymer light-emitting diodes

Abstract

In this paper, we report soluble mono-triphenylamine-functionalized polyspirobifluorenes (C8T-PsF, C8T-alt-mO-PsF and T-alt-mO-PsF) and compare their electroluminescent properties with a simple polyspirobifluorene containing the 2-ethylhexyloxy substituent (mO-PsF). C8T-PsF is a soluble homopolymer consisting of mono-triphenylamine-functionalized spirobifluorene with two octyl groups (C8T-sF). C8T-alt-mO-PsF is an alternating copolymer of C8T-sF and 2-ethylhexyloxy spirobifluorene (mO-sF) and can be compared to its homopolymer C8T-PsF. Mono-triphenylamine-functionalized spirobifluorene (T-sF) is a monomer without alkyl groups which was copolymerized with mO-sF to form T-alt-mO-PsF. These polymers are excellent blue and/or deep-blue light emitting polymers according to their photoluminescence (PL) spectra. However, they have similar EL emission maxima from 430 nm to 450 nm and different low-energy emissions from 520 to 580 nm. Since the synthetic route to polyspirobifluorenes produces them free of keto-defects, these low-energy emissions should be due to the interchain excimer and/or exciplex. Among them, mO-PsF shows the strongest green emission band around 520–550 nm. C8T-PsF with a larger substituent exhibits better color purity (CIE: 0.20, 0.11) than mO-PsF (CIE: 0.25, 0.28), which indicates that the substituents play an important role in excimer emission. Copolymerization in C8T-alt-mO-PsF did not improve the color purity compared to C8T-PsF. However, T-alt-mO-PsF exhibits the best color purity (CIE: 0.19, 0.15) and luminous efficiency of 0.81 cd A⁻¹, which may be due to the less flexible side chains. These results indicate that large and rigid substituents may facilitate the color purity of polyfluorenes. It is necessary to ensure both appropriate solubility and color purity when we design the side chains of the molecules for PLEDs.