Aromatic oxadiazole

H. Meng, W. Yu, and W. Huang, Facile synthetic route to a novel electroluminescent polymer — poly(p-phenylene vinylene) containing a fully conjugated aromatic oxadiazole side chain, Macromolecules, 32 8841-8847, 1999. 

Kim TH, Kim HJ, Kwak CG, Park WH, Lee TS (2006) Aromatic oxadiazole-based conjugated polymers with excited-state intramolecular proton transfer their synthesis and sensing ability for explosive nitroaromatic compounds. J Polym Sci A Polym Chem 44 2059-2068... 

Aromatic oxadiazoles have been isolated from the reaction mixture resulting from the oxidation of some oxidized nitrogen compounds such as aromatic aldoximes and amidoximes. 

In a subsequent investigation by the author [2] additional poly(aromatic-oxadiazole) agents were prepared using [l,2-d 4,5-d ]bisoxazole, (I), and derivatives. 

Fiber Properties of Aromatic Oxadiazole/N-methyl Hydrazide Copolymers with Partial m-Phenylene Orientation... 

During the polymerization of aromatic diacids and diesters with hydrazine in fuming sulfuric acid, aromatic oxadiazole units are formed first which are subsequently methylated by monomethyl sulfate or its homologues (derived from reaction of the diester with H2SO4) forming an aromatic N-methyl oxadiazolium polymer. This polymer is hydrolyzed during the spinning operation to the fiber polymer, i.e., p-/m-phenylene oxadiazole/N-methyl hydrazide copolymer. 

As shown in this paper, aromatic oxadiazole/N-methyl hydrazide copolymers with partial m-phenylene orientation give yarn with increased elongation-to-break and tire cord with improved fatigue resistance when compared to the all-para analogue. 

Whether ultimately the same high strength (18-21 gpd) as demonstrated for the latter (1) could be obtained with aromatic oxadiazole/N-methyl hydrazide copolymers of partial meta-orientation remains to be proven. 

A typical preparation of an aromatic oxadiazole/N-methyl hydrazide copolymer and fiber was conducted as follows ... 

Aromatic oxadiazole compounds such as 2-(4-biphenyl)-5-(4-teft-butyl-phenyl)-1,3,4-oxadiazole (PBD) have been used as electron transporting materials in organic light emitting diodes (15-17). Multi-layered polymeric LEDs with improved... 

The potential of poly(aromatic oxadiazole)s for electron injection has been studied by cyclic voltamme. When swept anodically, the polymer 14b-l was found to exhibit an irreversible oxidation peak at 1.67 V (with onset of 1.54 V) this value is... 

Processible Main Chain Aromatic Oxadiazole Polymers with Flexible Linkage. 

A wide range of polyoxadiazole polymers with hexafluoropropylidene linkages have been synthesized in Cambridge (3. As illustrated in Scheme 5, aromatic oxadiazole polymers 18 and 20 have been synthesized by a one pot reaction of the diacid 15 with tiie dicarboxylic hydrazide 16 and hydrazine sulphate 17 respectively at 100 °C in the presence of P2O5/CH3SO3H which serves both as solvent and cyclodehydration agent. 

Scheme 5. Synthesis of aromatic oxadiazole polymers with hexafluoropropylidene linkage 18-20 (adapted from 36)...

Table II. Optical properties and solubilities of poly(aromatic oxadiazoles) 18-20...

Different two layer light emitting diodes (LEDs) have been tested using the aromatic oxadiazole polymers as electron transporting layer, with PPV as emissive layer. It was found that the use of the aromatic oxadiazole polymers can realise efficient light emission using stable aluminum metal as cathode. No definite enhancement of electroluminescence efficiency has yet been observed. This can probably be explained by the fact that PPV and the oxadiazole polymers 18-20 have similar reduction potentials, and therefore by analogy similar LLMO levels. 

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. 

Soluble main chain oxadiazole polymers can be realised using solubilizing side groups such as tert-bu y and long flexible alkoxy or alkyl groups. Alternatively, flexible spacers can be incorporated between aromatic oxadiazole units, and finally a soluble precursor route may be employed. 

All the polymers containing aromatic oxadiazoles were found to be easily n-doped whereas they were difficult to p-dope as revealed by cyclic voltamme, indicating that they possess electron injection and hole blocking properties which make them suitable for use as charge transporting layers in multilayer polymer LEDs. It was also shown that oxadiazole polymers can used as emissive materials. 

Aromatic oxadiazoles. See Oxadiazole-based polymers Arylene-vinylene copolymers alternating and random copolymerization, 346-348... 

C. W. Spencer, F. Cacialli, J. Griiner, and R. H. Friend, Synthesis and optoelectronic properties of aromatic oxadiazole polymers, J. Chem. Soc. Chem. Commun. 7995 2211. 

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