Tetraphenylmethane-Based 1,3,4-Oxadiazole as Electron Transporting Materials in Organic Light-Emitting Devices
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multiple-branch structure can effectively raise the melting point and hence the possible glass transition temperature (Tg) of the material. Among oxadiazole compounds suitable for ETL, one most often used is 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD).14-17 In spite of frequent application for ETL, PBD has a major drawback of low melting point around 138 oC, which makes it vulnerable to the heat. It was shown recently that a spin-coated PBD film is prone to crystallize even at room temperature .18 Similar multiple-branch design for triarylamine-based HTL has been applied to oxadiazole -based ETL to change the unfavorable thermal properties of PBD. Up to date, reports about oxadiazole ETL materials with starburst shape (multiple-branch design) are limited to planar or pseudoplanar triangular configuration of trimers.18-22 Accordingly, we utilize tetraphenylmethane as a nonplanar tetrameric molecular framework connecting four 5-aryl-1,3,4-oxadiazole moieties in order to increase molecular weight and hence melting point of the material. By varying peripheral substituent of tetraphenylmethane-based 1,3,4-oxadiazole , stable amorphous glass phase can be obtained. We will compare electroluminescence properties of OLEDs with tetraphenylmethane-based 1,3,4-oxadiazole as ETL to those with PBD as ETL to justify our approaches . EXPERIMENT
NC
H N N N N
CN
N NNH N
NH4 Cl
H3 CO H3 CO H3 CO
OCH3 OCH3 NN
O N N
O
COCl
H3 CO
NaN3 NC
CN
1
N HN N N
2
N N NN H
N N O F3 C COCl
COCl
3
O NN
H3 CO H3 CO
CF3
F3 C N
N
N N O
Scheme 1
NN
O N N
O
4
OCH3 OCH3
O N N
O
O NN
N N O
5
O NN CF3
F3 C
Tetraphenylmethane-based 1,3,4-oxadiazole compounds 3-5 were all prepared similarly by reacting tetrazole derivative 2 with benzoyl chloride carrying appropriate substituents (Scheme 1) by modified Huisgen reaction with good yields (50-92%).23 We chose commercially available 3,4-dimethoxybenzoyl chloride and 3-(trifluoromethyl)benzoyl chloride as well as readily synthesized 3,5-di-tert-butylbenzoyl chloride 24 for an illustrative purpose. The previously unknown
4,4’,4”,4’’’-tetrazolyltetraphenylmethane 2 was synthesized in quantitative yield by the reaction of sodium azide and ammonium chloride with known 4,4’,4”,4’’’-tetracyanotetraphenylmethane 1.20,25 All new compounds were fully characterized by elemental analysis , FAB-M S, 1H, and 13C NMR spectroscopies . RESULTS AND DIS CUSSION Compounds 3-5 are either white or off-white solids with absorption maximum(λa) at 315, 296, and 292 nm in ethyl acetate solution, respectively. The observed λa of 3-5 follows a general trend that the molecule with donor substituent (such as OCH3 of 3) exhibits longer absorption wavelength than molecule with acceptor substitutents (such as 5). On the other hand, among 3-5, the smallest shift (blue shifted with 7 nm) of λa compared to that of PDB was found for 4, which has a tert-butyl substituent on the meta position that hardly affects the electronic state of oxadiazole ring. These spectroscopic observation indica
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