Electroluminescent polymers chemical structures

FIGURE 4.7 (Left) Chemical structures of PPLED materials. (Right) The device structure and the electroluminescence spectra of the PPLEDs prepared with PYK (4) host polymer and the above dopants (5wt%). (From Kawamura, Y., Yanagida, S., and Forrest, S.R., J. Appl. Phys., 92, 87, 2002. With permission.)... 

FIGURE 5.4 Chemical structures of photo- and electroluminescent polymers employed for polarized LEDs poly(2-methoxy-5-(2 -ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) poly[2,5-dioctyloxy-l, 4-diethynyl-phenylene-a/t-2,5,-bis(2 -ethylhexyloxy)-l,4-phenylene] (EHO-OPPE) poly(p-phenylene), PPP poly(3-(4-octylphenyl)-2,2 -bithiophene), PTOPT poly(p-phenylene vinylene), PPV poly(3-alkylthio-phene vinylene), P3AT Acetoxy-PPY PPV-polyester, poly(9,9-dialkyl fluorene), PF. 

Abstract. This article reviews mainly the results of our recent research on the relationship between the structure and the luminescence properties of PPV derivatives. PPV derivatives are particularly useful in an effort toward the establishment of such relationship because their chemical structures can be manipulated very systematically. Attachment of a wide variety of substituents, inclusion of kinky structural units, modification of main chain structures by inclusion of hole- and/or electron-transferring structures, and blending of polymers having different optical and electronic properties are representative approaches. The device characteristics of the light-emitting diodes (LEDs) fabricated from these polymers are discussed in relation to their structures. In certain cases, their photoluminescence (PL) properties are compared with their electroluminescence (EL) properties. 

FIGURE 7.11 Chemical structures of electroluminescent polymers (copolymers) with ion-transporting side groups. 

Interestingly, polymer 18 was found to emit blue-purple light. Aromatic oxadiazole compounds are known to show blue electroluminescence (40-42), Main chain oxadiazole polymers can also be made fluorescent by proper modification of the chemical structure. Pei and Yang (43,44) have reported a new oxadiazole polymer 25 with both a flexible linkage and solubilizing alkoxy side-chains (Scheme 7). An LED of structure ITO/polyaniline/polymer 25/Al has an external quantum efficiency close to 0.1 % and a tum-on voltage around 4.5 V. 

Although poly(para-phenylene vinylene), or PPV, was the first phenyl-based polymer to exhibit electroluminescence its slightly more complicated chemical structure than PPP means that PPP is a more convenient model system to study theoretically. Nonetheless, the remarkable similarities in the optical spectroscopy of the two systems means that we should seek a common description of their excited states. Indeed, as we explain in this section, the theoretical description of the excited states of PPV, apart from overall energy differences, is very similar to PPP. 

The chemical structures of a number of mainchain, highly conjugated polymers used in OLEDs are shown in Fig. 7.6. The electroluminescence of the polymer poly(l,4-phenylenevinylene) (PPV) was reported in 1990 [10], Since PPV itself is completely insoluble and thin films are only accessible by thermal conversion of a soluble polyelectrolyte precursor, a number of soluble PPV-derivatives have been developed. Among these are MeH-PPV [20] and a number of soluble PPVs... 

Fig. 7.23 Tcp Chemical structures of the liquid crystalline emitters used for the white emitting polymer network [73]. Bottom left Electroluminescence spectra of the white crossiinked mixture measured parallel (a) and perpendicular (b) to the rubbing direction. Bottom right Device characteristics of the WOLED [73]...

An alternative approach is to dissolve an efficient electroluminescent chro-mophore in an inert polymer host in a guest-host system or polymer blend. Unfortunately, phase separation and demixing often limits the amount of low-molar-mass electroluminescent chromophores that can be dissolved in polymer hosts. Therefore, the relative brightness is lower for such guest-host systems. This problem can be overcome by fixing the chromophore chemically to the polymer itself as a pendent group on a side-chain polymer separated by spacer units. Bulk phase separation is then impossible, although microphase separation may still take place. This is illustrated by the structures collated in Table 6.10 for some typical side-chain polymers 53-55. ... 

All electronic and electro-optic applications of poly-conjugated systems require the preparation of polymers with high chemical and structural homogeneity. Several optical and electrical properties of conjugated polymers such as their quantum efficiency of electroluminescence or maximum conductivity after doping can be correlated with the concentration of conjugation breaking defects introduced to the polymer upon its preparation. 

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