(26) The oligomers prepared for this study include poly(2,5-didecyloxy-1,4-phenylene vinylene) ( PDOPV) and poly(2,5-didecyl-1,4-phenylene vinylene) ( PDDPV) and the new PPV derivatives poly(2,5-bis(3-(heptyloxy)propyl)-1,4-phenylene vinylene) ( PHOPPV), poly(2,5-bis(3-(2-butoxyethoxy)propyl)-1,4-phenylene vinylene) ( PBEPPV), poly(2,5-bis(3-(heptyloxy)propyl)-1,4-phenylene vinylene- alt-2,5-didecyl-1,4-phenylene vinylene) ( PHOP- alt-DDPV), and poly(2,5-bis(3-(heptyloxy)propyl)-1,4-phenylene vinylene- alt-2,5-didecyloxy-1,4-phenylene vinylene) ( PHOP- alt-DOPV) the molecular structures of the oligomers used in this study are shown in Figure 2. Meanwhile, because of the improved performance of the alternating copolymer poly(2,5-didecyl- p-phenylene vinylene- alt-2,5-didecyloxy- p-phenylene vinylene) ( PDDPV- alt-DOPV) relative to the homopolymers in a previous study, the alternating copolymer incorporating the new heptyloxypropyl moiety was also studied. Therefore, herein we report the synthesis and characterization of new PPV derivatives in which the solubilizing chains are varied to study the effect on performance in organic light-emitting diode (OLED) devices. However, when the solubility-enhancing oxygen was not present, or in a position to donate electron density through resonance, the optical properties were less red-shifted, which is more amenable to realizing blue emitters. Apparent from the previous study, the presence of oxygen, directly attached to the aromatic ring, significantly affected the optical properties of the polymer.
While the oxygen present in PDOPV is detrimental to device lifetime, it appears to be necessary for improved device efficiency based on the role of solubility in device preparation.
(25,29) The decrease in solubility complicated comparisons in device performance because of the inability of PDDPV to form films of adequate thickness. However, in a previous study, removal of the oxygen atoms attached to the aromatic ring not only had the expected consequence of producing a blue shift in the emission spectrum (from orange to green), because of the replacement of the strong electron-donating oxygen with the weakly electron-donating alkyl group, but also produced a marked reduction in solubility from the dialkoxy PPV derivative-poly(2,5-didecyloxy-1,4-phenylene vinylene) ( PDOPV)-to the new dialkyl PPV derivative-poly(2,5-didecyl-1,4-phenylene vinylene) ( PDDPV), Figure 2. The increased acidity of the phenolic hydrogen facilitates alkylation of the aromatic ring, opening avenues for the synthesis of a variety of functionalized PPV derivatives. (16−25) Synthetically, the solubilizing moieties can easily be attached through a phenoxy precursor. (11−15) One of the most common methods used to improve PPV solubility is the incorporation of solubilizing long alkyl chains-linear, branched, or alkyl chains with polar functional groups-onto the phenyl rings, see Figure 1 for examples. Methods to improve solubility include the introduction of solubilizing side groups on the PPV structure, introduction of soluble polymers to yield PPV copolymers, and introduction of flexible blocks. (6−10) However, unsubstituted PPV is known to be insoluble and infusible, and because the oft-cited advantage of polymeric materials over small molecules in electronic devices is ease of processability, the search for soluble conjugated polymers has been extensive. (1−5) Since then, many modifications have been made to improve polymer processability, device efficiency, emission wavelength, and turn-on voltage. Devices were fabricated to analyze the electroluminescent characteristics of the oligomers in organic light-emitting diodes.Ĭonjugated semiconducting polymers have been the focus of much research since the first green electroluminescent (EL) device, generated from poly(phenylene vinylene) (PPV), was presented. Dialkyl and dialkoxy PPV oligomers were also prepared and characterized following the same instrumental methods used for the ether-substituted oligomers, providing a known reference system to judge the performance of the new conjugated oligomers. Estimation of the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels and other electrochemical characteristics of the oligomers were investigated by cyclic voltammetry. Measurements of UV–vis absorption and fluorescence were used to characterize the optical properties of the oligomers. The molecular weights of the oligomers were characterized by gel permeation chromatography, giving the number-average and weight-average molecular weights and the corresponding polydispersity indices. The structures of the monomers and the resultant oligomers were confirmed by 1H and 13C NMR spectroscopies.
New ether-substituted poly(1,4-phenylene vinylene) (PPV) derivatives were synthesized via Horner–Emmons coupling.
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