Synthesis and Properties of Carbazole Main Chain Copolymers with Oxadiazole Pendant toward Bipolar Polymer Host: Tuning the HOMO/LUMO Level and Triplet Energy

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Abstract

Three new carbazole copolymers, poly(9-(2,5-diarene-[1,3,4]oxadiazole)-carbazole-alt-9-(2-ethylhexyl)-carbazole-3,6-diyl)s (P1), poly(9-(2,5-diarene-[1,3,4]oxadiazole)-2, 7-carbazole-alt-9-(2-ethylhexyl)-3, 6-carbazole-diyl)s (P2), and poly(9-(2,5-diarene-[1,3,4]oxadiazole)-carbazole-alt-9-(2-ethylhexyl)-carbazole-2,7-diyl)s (P3), were synthesized by the Suzuki coupling reaction. The copolymers were characterized by 1H NMR, 13C NMR, and elements analysis, and their molecular weights were estimated using gel permeation chromatography. The TGA and DSC results revealed their good thermal stability with high glass-transition temperatures (Tg) at 211 °C (P1), 194 °C (P2), and 208 °C (P3), respectively. The copolymers exhibited blue emission with significantly improved fluorescence quantum efficiencies compared to their analogous polymers. The triplet energies of P1, P2, and P3 were determined to be 2.52, 2.42, and 2.32 eV, respectively, from their phosphorescent spectra at 77 K. The HOMO/LUMO levels of the carbazole copolymers can be tuned by different coupling positions and substitution at the 9-position of carbazole. P1 by connecting carbazole units via their 3 (6) positions shifts the HOMO/LUMO levels to higher energy compared to P2 via 2 (7) positions, whereas replacing alkyl groups at the 9-position of carbazole with electron-withdrawing diaryl-1,3,4-oxadiazole group shifts the HOMO/LUMO levels to lower energy. Finally, polymer light-emitting diodes employing the P1−3 as host and bis(2,4-diphenylquinolinato-N,C2′)iridium(acetylacetonate) (Ir(ppq)2(acac)) as guest were constructed and characterized electrically.

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