Oxadiazoles electron injection

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>. 

Several groups introduced an oxadiazole moiety as a part of the PPV backbone (polymers 139a [169,170], 139b [171], 140 [172], 141 [169], and 142 [170]). Not unexpected, the oxadiazole moieties lowered the LUMO energy of these polymers (as demonstrated by CV measurements). The decreased electron injection barrier is manifested by lowered turn-on voltage (6 V for ITO/139b/Al) [171]. However, relatively low efficiency (0.15% for 139b [171]) was reported for these copolymers (Chart 2.28). 

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). 

Q. Pei and Y. Yang, 1,3,4-Oxadiazole-containing polymers as electron-injection and blue electroluminescent materials in polymer light-emitting diodes, Chem. Mater., 7 1568-1575, 1995. 

Compared to Alqa, the oxadiazoles have a lower tendency toward reduction and thus a higher barrier for electron injection. Spiro-PBD (140), for example, can accept four electrons, the first electron transfer (merged wave for two electrons) taking place at -2.46 eV vs. Fc/Fc" " (Fig. 3.29) [87]. The oxadiazoles 17a, 18, and 29 exhibit reversible reduction waves at -2.39 and -2.18 eV and an irreversible reduction, respectively [238]. Since the HOMO-LUMO gap is >1 eV larger than for Alqs, the hole blocking properties are better for the oxadiazoles. 

Kambe [3] prepared electroluminescent devices containing two or more stacked organic layers, one of which consisted of an electron injecting an organic layer of conjugated poly(aryl-oxadiazole) derivatives, (111) and (IV). 

Thin films containing electron injecting layers of low crystalline oxadiazole derivatives, (V) and (VI), were prepared by Saitoh [4] and used in electroluminescence device. 

Oxadiazole derivatives found application for the production of electron transporting material for blended-layer organic light-emitting diodes. Compound 209 was used to enhance electron injection in an emissive polymeric material <05JMAC194>. 

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... 

The copolymethacrylates produced LEDs having better stability and brighter blue light emission than those obtained from the corresponding homopolymers, which may be attributed to the better electron injection by the oxadiazole units. 

Scheme 10. Synthesis of a ternary copolymethacrylate 36 bearing a blue-emitting chromophore, an electron injection oxadiazole and a photocrosslinkable unit...

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. 

Within the last years, oxadiazoles like 2-(biphenyl)-5-(4-tert.butylphenyl)-1,3,4-oxadiazole (PBD) 2 have been frequently applied in organic light emitting diodes [3]. Here the electron withdrawing oxadiazole unit dominates the electronic properties and the oxadiazole compounds act as electron injection and transport layers. Furthermore, 2,5-diphenyloxa-diazoles have been used as building blocks in thermostable polymers and they are highly fluorescent as well. 

The starburst oxadiazole compoimds are now being tested as electron injection and transport layer in organic LEDs and as photoconductors. First tests of two layer LEDs with PPV show that the novel materials possess properties comparable to 2 but have the great advantage to show no recrystallization if thin films were made by spin-coating. We will report on these measurements in the near future. 

The functioning of such a device is illustrated schematically in Fig. 16-6. Here, holes injected from the ITO electrode are blocked at the interface with the electrontransporting polymer layer, which comprises a 1 1 blend of PMMA with 2-(4 biphenylyl)-5-(4-ter-butylphenyl)-l,3,4-oxadiazole)(calledbutyl-PBD).Thisblocking causes increased electron injection from the other electrode, forcing a balance in electron and hole currents. Additionally, excitons formed at the PPV/PBD-PMMA interface are kept away from the other electrode. Such tailoring allows the use of less reactive metal cathodes, such as Mg, in place of Ca, and yields quantum efficiencies as high as 0.4%. 

Bipolar Molecular Glasses. Recently, bipolar molecular glasses have been described that allow both injection of holes and electrons (Fig. 3.30). 2- 4-[bis(4-methylphenyl)amino]phenyl -5-(dimesitylboryl)thiophene (PhAMB-lT, 68) and 2- 4-[bis(9,9-dimethylfluorenyl)amino]phenyl -5-(dimesitylboryl)thiophene (F1AMB-1T, 69) show oxidation potentials of 0.62 and 0.58 V, and reduction potentials of —2.13 and —2.01 V vs. Ag/0.01 Ag+, respectively [145]. Oxidation as well as reduction leads to stable radical ions. With the conversion rules given above, the HOMO and LUMO levels can be estimated to be approximately at —5.3 and —2.8 eV. In comparison, for the bipolar compound 70, consisting of triarylamine and oxadiazole moieties, the values are —5.5 and — 2.7eV [129]. However, in this case no data on the stability of the radical ions are available. 

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