1,3,4-Oxadiazole, electron transport

Scheme 1-14. Examples of a oxadiazole- and a quinoxatine-containing electron-transporting polymer.

Scheme 1-13. Examples of oxadiazole-containing electron-transporting materials.

Electron-hopping is the main charge-transport mechanism in ECHB materials. There is precedence in the photoconductivity Held for improved charge transport by incorporating a number of redox sites into the same molecule. A number of attempts to adapt this approach for ECHB materials have been documented. Many use the oxadiazole core as the electron-transport moiety and examples include radialene 40 and dendrimer 41. However, these newer systems do not offer significant improvements in electron injection over the parent PBD. 

Nonpolymeric amorphous dyes for electron transport, some of them containing an oxadiazole ring, were prepared and theoretically studied. It was concluded that reversible electron injections and ejection properties without impurity effects could be obtained for the symmetric and globular amorphous molecules <1997PCA2350>. Amplified spontaneous emission laser spikes were observed for some simple 2,5-diaryl oxadiazoles <1997PCA3260>. 

As we already mentioned, electron-transporting properties of PPV polymers can be adjusted by introduction of an oxadiazole moiety in the polymer structure. A variety of PPV copolymers... 

The tuning of electron injection and transport in PF has been undertaken by Shu s group [354], who introduced electron-deficient oxadiazole units as pendant groups in fluorene copolymer 257. The introduction of oxadiazole units into the PF can potentially improve the electron transport properties of the polymer, while their bulkiness can help to suppress aggregation effects (Chart 2.68). 

Due to relatively high electron affinity and very good PL efficiency, molecular materials based on oxadiazole, particularly, PBD (21) are among the most popular electron transport materials for OLEDs. The oxadiazole moieties, including PBD, were introduced as pendant... 

An interesting new approach to phosphorescent polymers has been reported by Thompson and coworkers [79]. Using a living polymerization reaction (with alkoxyamine catalyst), they have prepared the polymer 66 (Chart 4.22), which contains the electron transport oxadiazole... 

CHART 4.22 Chemical structure of the polymer that contains the electron transport oxadiazole units, hole-transporting triphenylamine units, and triplet-emitting Pt complex. 

Corresponding electron-transport materials have been made by replacing the amino substituents in the first shell by oxadiazole groups (29) [70, 61] or phenyl-quinoxalines (30) [60]. As in the case of the amines, dendrimer-shaped structures are obtained by repeating the substitution pattern in a second shell (31) [76], The Tg was increased from 142°C in 29 to 222° C in 31. 

The electron mobility of oxadiazoles have been measured in a polymer matrix, values of 10 7 up to 10 3 cm2/Vs have been obtained [262, 263], These values are exceeded by starburst phenylquinoxalines (30) that approach 10-4 cm2/Vs at 106 V/cm [264]. Other material classes that are very interesting candidates for electron-transport layers comprise naphtalene-, 60, and perylenetetracarboxylic diimides, 59 [265], as well as bathophenanthroline [266] with reported electron mobilities of 10 3 and 4.2 x 10 4cm2/Vs, respectively. 

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