Oxadiazoles mobilities

Triazines are well-known compounds with high thermal stability and higher EA than 1,3,4-oxadiazoles (PBD) and 1,2,4-triazoles (TAZ, 92). Schmidt et al. studied a series of dimeric 1,3,5-triazine ethers for application as ETMs for OLEDs [150], However, despite their high EA, the efficiency of the OLEDs improved only modestly. One possible explanation is due to their rather poor electron mobilities. 

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. 

Hole transport in polymers occurs by charge transfer between adjacent donor functionalities. The functionalities can be associated with a dopant molecule, pendant groups of a polymer, or the polymer main chain. Most literature references are of doped polymers. The more common donor molecules include various arylalkane, arylamine, enamine, hydrazone, oxadiazole, oxazole, and pyrazoline derivatives. Commonly used polymers are polycarbonates, polyesters, and poly(styrene)s. Transport processes in these materials are unipolar. The mobilities are very low, strongly field and temperature dependent, as well as dependent on the dopant molecule, dopant concentration, and the polymer host This chapter reviews hole transport in polymers and doped polymers of potential relevance to xerography. The organization is by chemical classification. The discussion mainly includes molecularly doped, pendant, and... 

Borsenberger et al. (1996e) measured hole mobilities of a series of donor glasses N,N/-diphenyl-N,N,-(2-naphthyl)-(p-terphenyl)-4,4/,-diamine (NDNTP), N,N -dipheny l-N,N -(2-naphthy l)-( 1,1 -phenyl)-4, -diamine (NDNPD), 9,9-bis 4-[N-(4-tolyl)-N-(2-naphthyl)amino]phenyl fluorene (BTNAF), and 2,5-bis(4-diethylaminophenyl)-l,3,4-oxadiazole (OXD). For OXD, the photocurient transients were dispersive. Nonetheless, transit times could readily be determined from the intersection of the two linear regions of the transients. In all glasses, the field and temperature dependencies were described as lo u /3E 2 and -(T0IT)2. The results are summarized in Table 6. 

Young and Fitzgerald (1995) analyzed the effects of dipole moments on the hole mobilities of a series of aiylalkane, aiylamine, carbazole, hydrazone, pyrazoline, and oxadiazole molecules doped into a PC, PS, and as vapor-deposited glasses. The results agreed with earlier work of Sugiuchi et al. (1991). With the exception of the aiylalkane derivatives, the mobilities decreased in a near exponential manner with increasing dipole moment. 

Crisa (1983) measured hole. mobilities of a mixture of 2,5-bis(4-diethylaminophenyl)-l,3,4-oxadiazole and a polyester. The time, thickness, and field dependencies of the photocurrent transients agreed with predictions of the Scher-Montroll model (1975). According to the model, the transit time scales with thickness and field as (L/E)l/a. The experimental value of a was 0.80. The study of Crisa, and later work by Bos and Burland (1987), are the only literature references to polymers where the scaling relationships predicted by Scher and Montroll have been observed over a range of thicknesses and fields. 

Hole mobilities of a series of pyrazoline, oxadiazole, and stilbene doped PC were described by Sugiuchi et al. (1991). The donor compounds were selected for differences in dipole moment and varied from 1.42 to 5.56 Debye. The results are shown in Fig. 96. The mobilities decrease with increasing dipole... 

Enokida et al. (1990) measured hole mobilities of 2,5-bis(p-diethylamino-pheny 1)-1,3,4-oxadiazole, 1 -(3 -methylpheny 1)-1,2,3,4-tetrahydroquinoline-6-carboxylaldehyde-1. l -diphenylhydra/.onc, and l,l-bis(p-diethylaminophenyl)-4,4-diphenyl-l,3-butadiene doped polymers. The polymer was PC. At 5.0 x 105 V/cm, the mobilities were 5.7 x 10-8,1.2 x 10-6, and 8.7 x 1(>6 cm2/Vs for the oxadiazole, hydrazone, and butadiene doped polymers, respectively. The authors attributed the differences in mobility to differences in ionization potential. The ionization potentials vary from 5.11 to 5.74 eV. 

There have been several attempts to relate mobilities to ionization or oxidation potentials (Enokida et al., 1990 Scott et al., 1990 Kanemitsu et al., 1991, 1992 Kitamura and Yokoyama, 1991). Scott et al. studied a series of hydrazone compounds in a polyarylate. The oxidation potentials of the donor compounds varied from 0.53 to 1.04 V. The results showed that the zero-field mobilities increase with increasing oxidation potential. In contrast, Enokida et al. investigated a series of butadiene, hydrazone, and oxadiazole compounds doped into a PC and concluded that the mobilities increased with decreasing ionization potential. Kitamura and Yokoyama studied a series of hydrazone compounds in a PC and concluded that the mobilities were independent of the ionization potential. While the reason for these discrepancies is not clear, it should be noted that the study of Scott and coworkers involved zero-field... 

Aromatic 1,3,4-oxadiazoles are known to be efficient electron transport materials. When incorporated by Heck coupling into the poly(phenylene) chain in polymers such as 38, the electron carrier mobilities are indeed increased, while the hole mobilities are unchanged [75]. Light-emitting diodes prepared from this polymer show increasingly metal-dominated emission as the %Ru content is increased. 

In blends of PVK with PBD and in random copolymers with carbaz-ole and oxadiazole groups attached as side chains, the active groups have different mobility, or are subjected to different topological constraints, respectively. In the blends, exciplexes emerge, and in the copolymers, electroplexes are effective. Both types of complexes shift the EL spectra to red in comparison to pure PVK homopolymer. The red-shift is significantly greater for the electroplex. 

Typical chemical compounds include oxadiazole derivatives [14], pyrazolines [ISIS], hydrazones [19-22], carbazole derivatives [23-26], triphenyhnethane (TPM) derivatives [27, 28], triphenylamine (TPA) derivatives [29, 30], and TAPC [31], which can be regarded as a dimer of TPA. The charge carrier mobilities at room temperature are typically in the range from 10 cmWs for N-isopropylcarbazole [25] to 10 cm A s for p-diethyl-aminobenzaldehyde diphenyl hydrazone (DEH) [22]. 

The time-of-flight electron mobility in a oxadiazole derivative namely, l,3,5-tris 5-[3,4,5-tris(octyloxy)phenyl]-l,3,4-oxadiazole-2-yl benzene,has recently been studied by Zhan et al. [211], The synthesis of this material is straightforward as shown in Scheme 4.38. Condensation of 1,3,5-benzenetricarbonyl trichloride with the hydrazide 182 yields the precursor 183, which can be converted to discotic oxadiazole derivative 184 by heating in POCI3 at 80 °C [211,212],... 

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