Triarylamines, hole-transporting

SCHEME 3.1 Chemical structures of anthracene, a hole transport triarylamine, an electron transport and a green emitter Alq3, and a phosphorescent dopant PtOEP. 

Scheme 24 Copolymers of fluorene with hole-transporting triarylamines...

The design of such polymers with controlled chromophore length is quite a large area, as chromophores and spacers can be combined in an almost infinite way. Chromophores can be hole-transporting triarylamine or electron-transporting ones such as oxadiazoles, with ethers, esters [113], amines, amides, imides [114-116], silanes, fluorenylidene as possible conjugation interrupters [117,118],... 

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]]. 

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... 

S. Liu, X. Jiang, H. Ma, M.S. Liu, and A.K.-Y. Jen, Triarylamine-containing poly(perfluorocy-clobutane) as hole-transporting materials for polymer light-emitting diodes, Macromolecules, 33 3514-3517, 2000. 

By introducing the hole transport arylamine as an end cap for an anthracene backbone, Lin et al. designed a series of novel materials (207-212) (Scheme 3.65) [247]. The aim of these dual function materials is to combine the emitting property of the blue anthracene lumino-phore with the hole transport property of the triarylamine to simplify the device fabrication steps. Though the introduction of the arylamino moieties produces moderate QE (f 20%) for these materials, the OLEDs using them as emitters as well as HTMs demonstrate only moderate EL performance with a maximum luminance of 12,922 cd/m2 and 1.8 lm/W with CIE (0.15, 0.15). 

Arylamines display electronic properties that are favorable for materials science. In particular, arylamines are readily oxidized to the aminium form, and this leads to conductivity in polyanilines, hole-transport properties in triarylamines, stable polyradicals with low energy or ground-state, high-spin structures, and the potential to conduct electrochemical sensing. The high yields of the palladium-catalyzed formation of di- and triarylamines has allowed for ready access to these materials as both small molecules and discrete oligomeric or polymeric macromolecules. 

The current organic photoreceptors are triarylamines, triarylmethanes, hy-drazones, oxadiazoles, pyrazolines, oxazoles, and more recently, stilbene derivatives. The polymer matrix, on the other hand, is constituted by polyesters and polycarbonates (Fig. 5). The common presence of aromatic amines as substituents in all these materials contributes to efficient hole transport [44]. The nonbonding electron pair on the nitrogen atom, in fact, confers on these molecules a low oxidation potential, and consequently, the production of a chemically stable radical cation with the possibility of an effective overlap of nonbonding molecular orbitals between neighboring molecules. 

Figure 41. Correlation of mobilities (//), in the limit of zero-field strength, with the net dipole moment of hole-transporting molecules doped into polystyrene. The hole-transporting unit is essentially the same in each case, a substituted triphenylamine, and it is present at the same molar concentration. The mobility is much reduced for the molecules whose dipole moments are larger because they contain polar substituents (ESTER, BROMO). It is essentially independent of dipole moment for the molecules whose dipole moments are larger just because several triarylamine groups with small dipole moments are chemically linked (TTA, TAPC, TETRA). (Reprinted with permission from Ref [55b].)...

It should be noted here for triarylamine networks and dendrimers [104], respectively, the radical cations have interesting properties like the formation of high-spin polyradicals with ferromagnetic coupling [105] or conducting polymers [106]. Very often, triarylamines have been used as the hole-transport layer in electroluminescent devices [107]. 

Hole-transporting materials (HTM) have relatively low ionization potentials (1P). 2 The IP is defined as the energy required to remove an electron from the highest occupied molecular orbital (HOMO) of a substance. It can be measured, for instance, by photoelectron spectroscopy or obtained from electrochemical oxidation potentials in solution. It is also preferable that the HTM have sufficiently high hole drift mobilities. Various classes of materials have been used in the HTL, for example, starburst amorphous molecules, spirocompounds, triarylamines, and tetraarylbenzidines are representative classes of well-known HTMs. The structure of the most commonly used HTM, 4,4 -bis[N-(l-naphthyl)-N-phenylamino]biphenyl (NPB), is shown in Figure 14.2. 

Triarylamine and carbazole-based low-molar-mass compounds and polymers have been extensively studied for the different applications due to their good hole transport and luminescent properties. ... 

C.iii.c. Preparation of Small Molecules for Materials Science. Pd-catalyzed amination has also been used to prepare small molecules that are useful as hole-transport materials, selective metal-cation detection systems, and dyestuffs. As mentioned briefly in the section on reacting diarylamines with aryl halides, Marder and co-workers used palladium chemistry to form triarylamines, which are useful as hole-transport layers. Reactions of primary arylamines with aryl halides using DPPF-hgated palladium as catalyst allows for the selective addition of one aryl halide, followed by the addition of a second aryl halide to form mixed triarylamines, as shown in Eq. 42. This procedure has been used to generate unsymmetrical triarylamines that are analogs of TPD, as shown in Eq. 43. hi addition, they have used aminoferrocene as a substrate to conduct diarylations to form N, A-diarylaminoferrocenes. ... 

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