Preliminary study on alternating thienylene/phenylene copolymers functionalized with an electron-donating or -withdrawin
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Three novel regioregular polymers substituted with electron-donating or -withdrawing groups, poly[1,4-bis(3-X-2,5-thienylene)phenylene-alt-2,5-dioctyl-1,4-phenylene] [PBT(X), X ⳱ OMe, H, CN], were synthesized and characterized. They are highly fluorescent, but the absorption wavelength of PBT(CN) and PBT(OMe) are shifted toward the blue and red regions, respectively, with reference to PBT(H). The onset reduction potential of PBT(CN) is −1.11 V versus SCE (saturated calomel electrode), which is higher than that of the cyano derivative of poly(phenylene vinylene), indicating it be a good candidate as electron transport layer. The onset oxidation and reduction potentials of PBT(OMe) (0.96 and −1.28 V versus SCE respectively) suggest it would perform well as emissive layer in light-emitting display applications.
Functionalization with electron-donating and/or -withdrawing groups on conjugated polymers has been an attractive area because of the possibility to produce materials with different electronic structures and, thus, the application potentials in light-emitting displays (LED)1,2 and nonlinear optics.3 The derivatives of poly(phenylene vinylene) (PPV) with alkoxy (OR) and with both OR and cyano (CN) groups, such as MEH-PPV and CN-PPV, have displayed greatly changed band structures as revealed by electrochemistry.4 The quantum efficiency of the LED with CN-PPV as an emissive layer and Al as cathode is noteworthily the same as that with Ca as the cathode.5 To further study the structure–property correlation6–8 and to produce polymers with special properties which are required in LED application, we have synthesized three regioregular polymers comprising alternating phenylene and thienylene repeating units but with electrondonating or -withdrawing groups attached on thienylene units, i.e., poly[1,4-bis(3-X-2,5-thienylene)phenylenealt-2,5-dioctyl-1,4-phenylene] [PBT(X), X ⳱ OMe, H, CN] (structures are shown in Scheme I). They were studied spectrally and electrochemically and the results of PBT(CN) and PBT(OMe) are compared to that of the polymer without electronic substitution groups [PBT(H)].
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 5, May 2001
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SCHEME I. Synthetic route to PBTX polymers.
The polymers were synthesized through Stille coupling (as shown in Scheme I), and the formation of polymers was monitored by 1H NMR spectra with reference to those of the corresponding monomers. Their structures were also confirmed by FTIR spectra, and the synthesis and full characterization aspects will be reported elsewhere. The polymers are partially soluble in CHCl3, THF, and toluene, and the soluble, low molecular weight fractions have moderate molecular weights (Mn ∼ 2000) as evaluated by GPC. PBT(H) and PBT(OMe) are dopable by I2 and FeCl3, while PBT(CN) is not. Table I summarizes the absorption peak wavelength (max) and emission peak wavelength (em) of the polymers in CHCl3 and as solid films, together w
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