Triarylamine compound

Copolymerization of fluorene with triarylamine compounds was shown to increase the hole-transport properties of the polymers. Several copolymers of triarylamine and fluorene (246-250) synthesized by Suzuki coupling were reported by Bradley et al. [347,348], The hole s... 

The most commonly used HTL materials are triarylamine compounds. These compounds were developed as HTMs for photoconductive applications such as xerography [69]. They naturally have been selected as HTMs for OLED applications largely because of their ready availability and their good electrochemical and thermal stabilities. The hole mobilities of these materials are also adequate for OLED applications. In addition, high purity, so as to ensure low hole-trap contamination, is believed necessary for long-lived OLED performance and such materials may often be train sublimed to very high purity. 

Further developments in this area have included the neparation of several additional N,N -diaryl indolo[3,2-h]carbazoles with substituents such as m-tolyl, ffi-anisoyl, or triarylamine-containing species. Like 221, these compounds, possessing excellent hole-transport properties, also occurred in stable amorphous states and displayed high glass-transition temperatures. LED devices involving these systems were also constructed and showed promising characteristics [OOSMO11-112)42]]. 

A very productive strategy for the synthesis of glass-forming materials is the use of highly branched rigid structures. As a suitable center for starburst molecules with a threefold symmetry, triarylamine or benzene are used most frequently. Due to the large number of starburst molecules described in the literature, we divide this class into two subgroups, compounds based on the triarylamine and the benzene centers. 

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. 

This process is more efficient when being redox-catalyzed [110, 111]. Thus the use of electrogenerated cation radicals of some triarylamines (Ar = Tol, p-Br-C6H4 [112]) as mediators provides even higher yields of the corresponding carbonyl compounds. 

For oxidations, the cation radicals of aromatic compounds like 9,10-diphenyl-antracene, thiantrene, phenoxathiine, or dibenzodioxine are likely candidates. Their reactivity towards nucleophiles, however, limits their application to media of low nucleophilicity. Sometimes the stability of such cation radicals can be enhanced through blocking the reactive positions by substituents. For example, para-substituted triarylamines deliver cation radicals with often excellent stability even in methanol. The stability is further increased by incorporation of urzAu-substituents. Other mediators which have been applied in indirect electrosyntheses are those which are able to abstract hydrogen atoms or hydride atoms. 

Table 10. Standard potentials of triarylamines and related compounds...

As for halogens as oxidizing reagents, bromine has proved more useful than its homologs chlorine and iodine. It was employed as early as 1879 on di- and tetra-methyl-p-phenylenediamine [27-29] and early in this century, Wieland used it to generate the aminium salts of triarylamines and tetraarylhydrazines [30, 31]. Since bromine adds readily to unsaturated as well as to some strained ring compounds, it is not expected to be very useful in the context of the radical cations discussed here. 

Cyclic voltammetry revealed that the N-atoms of 86a, 88a and 91a are oxidized at lower potentials than the trivalent P atoms. Comparison of these data with those observed with model compounds shows a very weak electronic delocalization via the P centers for copolymers 86a and 91a. In contrast, the low first oxidation potential observed for 88a (Table 4.5) is assumed to result from an electronic communication between the N moieties through the connecting P centers [59b]. The equivalence of the oxidation potentials for the oxidized polymers (Table 4.5) suggests the presence of electronically isolated triarylamine fragments in these derivatives. Note that the involvement of the P lone pair in jt-delocalization... 

Figure 6.34 Variation of the diffusion coefficient, Dapp, following a potential step from 0.2 to 1.0 V for the oxidation of a triarylamine in a mixed monolayer with a redox-inactive compound as a function of the mole fraction x1. The electrolyte is l-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Reprinted with permission from P. Bonhote, E. Gogniat, S. Tingry, C. Barb , N. Vlachopoulos, F. Lenzmann, P. Comte and M. Gratzel, /. Phys. Chem., B, 102, 1498 (1998). Copyright (1998) American Chemical Society...

R SR Gem-dithioacetals C / R SR 9,10-Diphenylanthracene and triarylamines Rapid decomposition of radical cation by cleavage (R = aromatic) or nucleophilic attack (R = aliphalic) (oxidative deprotection of carbonyl compounds) [117]... 

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