Synthesis and EL properties of fluorene-based copolymers containing electron-withdrawing thiazole moieties

Abstract

We have synthesized flourene-based copolymers PF-TTZT (2-(4-hexyl-5-(2-methyl-9,9-dioctyl-9H-fluoren-7-yl)thiophen-2-yl)-5-(4-hexyl-5-methyl-5-methylthiophen-2-yl)thiazole [5,4-d]thiazole) and PF-TBTT (5-(4-hexyl-5-(2-methyl-9,9-dioctyl-9H-fluoren-7-yl)thiophen-2-yl)-2-(5-(4-hexyl-5-methylthiophen-2-yl)thiazol-2yl)thiazole) containing electron-withdrawing moieties, thiazolothiazole and bithiazole, via the palladium-catalyzed Suzuki coupling reaction. The number-average molecular weight $(M_n)s$ of PF-TTZT and PF-TBTT were respectively found to be 26,000 and 70,400. These polymers dissolve in common organic solvents such as chloroform, chlorobenzene and toluene, etc. PL emission of PF-TBTT containing bithiazole moiety was more red-shifted than that of PF-TTZT containing thiazolothiazole moiety. Moreover both HOMO and LUMO energy levels of PF-TBTT were lower than those of PF-TTZT. The reason is that bithiazole moiety of the PF-TBTT has better electron accepting ability than thiazolothiazole moiety of the PF-TTZT. The thermal properties of these polymers were measured by TGA and DSC, and electrochemical properties were studied by cyclic voltammetry (CV). These two polymers show reversible oxidation (p-doping) and reduction (n-doping). The reversible oxidation peak is due to thiophene moiety having electron-donating property, and the reversible reduction peak is due to electron-withdrawing groups. OLED devices with the configuration of ITO / PEDOT (40nm) / Polymers (40 nm) / Balq (40 nm) / LiF (1 nm) / Al (70 nm) were fabricated. Because electron injection from cathode to PF-TBTT is much easier than that to PF-TTZT, EL properties of PF-TBTT are almost 3 times higher than those of PF-TTZT.