Fluorene–Dithienothiophene-<i>S,S</i>-dioxide Copolymers. Fine-Tuning for OLED Applications

Three groups of fluorene–dithieno­[3,2-<i>b</i>;2′,3′-<i>d</i>]­thiophene-<i>S</i>,<i>S</i>-dioxides (DTT-<i>S</i>,<i>S</i>-dioxide) copolymers, each having four different ratios of DTT-<i>S</i>,<i>S</i>-dioxide (5, 15, 25, and 50%) were successfully synthesized through Suzuki coupling method. While the first group copolymers <b>P1</b> had direct connection of fluorene to the peripheral thiophenes of DTT-<i>S</i>,<i>S</i>-dioxide, second group copolymers <b>P2</b> had a thiophene extension between fluorene and DTT-<i>S</i>,<i>S</i>-dioxide, and in the third group, copolymers <b>P3</b>, fluorene had a connection with DTT-<i>S</i>,<i>S</i>-dioxide through the phenyl moiety of DTT. Absorbance and emission measurements of first two groups <b>P1</b> and <b>P2</b> displayed a regular bathochromic shift with increasing content of DTT-<i>S</i>,<i>S</i>-dioxide, which was more clearly observed in their solid state fluorescence measurements. Introduction of thiophene to the peripherals of the DTT-<i>S</i>,<i>S</i>-dioxide in copolymers <b>P2</b> caused even further bathochromic shift in absorbances and emissions. As the absorbance and emission of <b>P1</b> went up to 447 and 558 nm in solution, respectively, <b>P2</b> had them at 472 and 592 nm, respectively. In solid state, emissions of <b>P1</b> and <b>P2</b> even went further up to 585 and 646 nm, respectively. The bathochromic trend of <b>P1</b> and <b>P2</b> became opposite with absorbance and solid state emission of <b>P3</b>, which had a hypsochromic shift with increasing content of DTT-<i>S</i>,<i>S</i>-dioxide. Solid state emission of <b>P3</b>, particularly the copolymers having 5, 15 and 50% DTT-<i>S</i>,<i>S</i>-dioxide, covered a wide region between 400 and 675 nm. A spread of colors from light blue (border of white) to red through green and yellow was obtained with the OLED applications of the copolymers. Their optical and electronic band gaps varied between 2.17 and 2.99 eV and between 2.68 and 3.57 eV, respectively. While the highest quantum yield was obtained with <b>P2</b> (5%) as 0.66, the lowest was observed with <b>P2</b> (50%) as 0.03. Almost all of the polymers displayed good thermal stabilities. No weight loss was observed with the copolymers <b>P2</b> (5–15%) and <b>P3</b> up to 400 °C