1,2,4-Triazole, electron transport

Another large band-gap electron transport host is 3-phenyl-4-(l -naphthyl)-5-phenyl-1,2,4-triazole (TAZ), which has a HOMO (-6.6 eV) and LUMO (-2.6 eV). Using TAZ1 (109) as the host, a maximum EQE (ext) of 15.5% and a luminous power efficiency of 40 lm/W can be achieved in a phosphorescent OLED the value of phosphorescent decay lifetime of 7% Ir(ppy)3 in the TAZ (t-650 ns) is longer than that in CBP (t-380 ns) and the phosphorescence efficiency is approximately proportional to the excited state lifetime [174]. 

Other structural motifs for designing molecular glasses have been exploited (Fig. 3.16). Some of them are based on five-membered rings as the central part of the molecule, like 1,2,4-triazoles (48) [94] or the electron-transporting material NAPOXA (49) [95]. Pentaphenylcyclopentane (50) exhibits a Tg of 57°C [27]. Braun et al. [96] synthesized some fulvenes with Tg ranging from 74 to 120°C (51). 

Reaction of 232 with 4-substituted l,3-oxazol-5(4/7)-one 247 led to diacylhydrazines 248 or to imidazole derivatives 249 depending on the reaction temperature (Scheme 24). l,2,4-Triazole-3-thione 250 was obtained by a two-step sequence from 232 with phenyl isothiocyanate and subsequent base-catalyzed cyclization of thiosemicarbazide 251. The effects of hydrazones 241-246 on inhibition of photosynthetic electron transport in spinach chloroplasts and chlorophyll content in the antialgal suspensions of Chlorella vulgaris were investigated <2005CEC622>. 

Other widely-used electron-transporting materials include 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-l,3,4-oxadiazole (butyl-PBD), which is essentially nonemis-sive and often introduced between the cathode and the emitting layer precisely for that reason, and 3-(4-Biphenylyl)-5-(4-tert-butylphenyl) l-phenyl-l,2,4-triazole (TAZ-1).6,74... 

As compared with hole-transporting materials, fewer electron-transporting materials have been reported. A well-known green emitter, Alq3, has been used as a good electron transporter. Other reported electron-transporting materials include oxadiazole derivatives, a dendrimer-type oxadiazole, a triazole derivative, tris(phe-nylquinoxaline), silole derivatives, benzimidazole derivative, and boron-containing compounds (Table 7.3). 

Most common electron transporters are derived from heterocyclic aromatic compounds, either as pure organic materials or as metal coordination complexes. Pure organic molecules such as phenanthrolines, oxidiazoles, and triazoles can have high electron mobilities (Figure 25). Some of the highest carrier mobilities for amorphous molecular materials have been reported for oligofluorenes (lO -lO cm V j-i) 305,313 inefficient carrier... 

The 4-(4-(hexyloxy)phenyl)-3,5-diphenyl-4H-1,2,4-triazole moiety can be used in electron transporting polymeric segments and shows electroluminescence in poly(p-phenylene vinylene) (PPV) derivatives. 

S. H. Chen and Y. Chen. Luminescent copoly(aryl ether)s with new electron transporting bis(3-(trifluoromethyl)phenyl)-l,3,4-oxadiazole or bis(3-(tri-fluoromethyl)phenyl)-4-(4-hexyloxyphenyl)4h-1,2,4-triazole segments. J. Polym. Set, Part A Polym. Chem., 42(23) 5900-5910, December 2004. 

S. H. Chen and Y. Chen. Poly(p-phenylenevinylene) derivatives containing electron-transporting aromatic triazole or oxadiazole segments. Macromolecules, 38(l) 53-60, Jannary 2005. 

J. Kido, C. Ohtaki, K. Hongawa, K. Okuyama, and K. Nagai. 1,2,4-triazole derivative as an electron transport layer in organic electroluminescent devices. Jpn. J. Appl. Phys., Part 2, 32(7A) L917-L920, July 1993. 

For electron transport/hole blocking purposes, a wide variety of electron-deficient moieties are well known, e.g., 1,3,4-oxadiazoles [12], 1,2,4-triazoles [13], 1,3-oxazoles, pyridines and quinoxalines [14] (see Scheme 3). Materials with conjugated 7i-electron system (e.g., styrylarylenes, arylenes, stilbenes, oligo- and poly(thiophene)s — see Scheme 3) are widely used as combined charge transport and luminescence layers as well [12,15]. 

Polymers with triazole groups act to enhance the electroluminescent efficiency when used in two layer devices with PPV as a hole-transporting emitter. PPV-hased copolymers bearing an electron-withdrawing triazole unit in the main chain can be synthesized by the Wittig reaction between triazole diphosphonium salt and the corresponding dialdehyde monomers, respectively. ... 

An even better balance in the injection of the opposite charges was found when another layer more suitable for hole transport, such as unsubstituted PPV, is used [70]. A doublelayer LED, formed by ITO/PPV/cyano-PPV/Al has an internal quantum efficiency as high as 4% with emission at 610 nm [70]. The cyano-PPV approach was extremely useful and other withdrawing substituents were introduced in the PPV backbone with the aim of increasing the electron affinity other electron-deficient nitrogen containing groups such as oxadiazole [102,103], triazole [94], pyridine [104,105] and quinoxaline [106] were also introduced in the polymer structure. 

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