Charge‐carrier transport in organic materials is the fundamental physical process behind devices such as laser printers. Charge‐carrier mobility data are presented for hexabutyloxytriphenylene (H4T), which exhibits an unusually high charge‐carrier mobility, which can be traced to the formation of a plastic discotic phase. The Figure shows the normal discotic hexagonal texture with six‐fold symmetry observed for H4T at 144°C; this symmetry disappears at lower temperatures (see also the cover). magnified image

Electrooptical (EO) materials are able to change their refractive index under the influence of an externally applied electric field, an effect that is of great technological importance as a means to control the phase of laser light. [1] In recent years, a variety of cheap and easily processable organic nonlinear optical (NLO) materials [2] were designed for this purpose. Poled polymers for high-frequency modulation of optical signals were developed, and photorefractive (PR) polymers [3] emerged as promising materials for widespread holographic optical applications. In both types of material, the EO response is provided by chromophores that are usually incorporated into the material as guests or as side-groups in functionalized polymers. In this communication, we analyze the microscopic mechanism leading to the refractive index modulation and define appropriate molecular figures-of-merit (FOMs) for EO chromophores, F 0 Pockels and F 0 Kerr . In order to investigate optimization strategies for both FOMs, we synthesized a series of dyes based on quinonoid carbocyclic and heterocyclic acceptor units, which exhibit significantly higher acceptor strength and better thermal stability than common open-chain acceptor groups.All chromophores investigated in this study are displayed in Figure 1. The synthetic procedures for the new EO dyes 4±11 are shown in Schemes 1 and 2. Accordingly, EO dyes 4 and 10 were prepared by Knoevenagel condensations of p-N,N-dimethylamino cinnamaldehyde (13a) with benzylidene malononitrile 15 [4] and 1-naphthylmalononitrile (16) [5] in acetic acid anhydride in yields of 72 and 43 %, respectively (Scheme 1). Similarly, dye 3 (DCM, a commercial laser dye) was obtained according to the literature from 14. [6] For dye 11, the polyenic chain of 13b was extended via a Wittig oxopropenylation [7] using the protected formylmethylenetriphenylphosphorane 17 to give pentadienal 18 in 60 % yield. The following condensation of 18 with 16 gave 11 in 10 % yield. The EO dyes 5±9, bearing the new powerful thiazolidenemalononitrile acceptor unit, were synthesized according to Scheme 2. Reaction of stoichiometric amounts of a-thiocyanatoketones 19a and b with malononitrile afforded 2-dicyanomethylthiazoles 20a and b in almost quantitative yields. [8] Because of an equilibrium between various tautomeric forms, these heterocycles may react easily in condensation reactions with aldehydes 13a and b, 21, and 22 to give dyes 5, 6, and 9a and b in yields of 60±80 %. The corresponding aldehydes 21 and 22 were obtained from N,N-dibutylaniline and N-alkylated 3,3-dimethyl-2-methyleneindoline by Vilsmeier formylations. For the synthesis of EO dyes 7 and 8 the reaction conditions had to be changed because of the high basicity of the methylene bases of 23 and 24. In a one-pot sequence in acetic acid anhydride, 3ethyl-2-methylbenzoxazolium iodide (23) and 1-hexyl-4methylpyridinium bromide (24) were first converted to the anils by N,N-diphenylformamidine (DPFA). They were then in-situ N-acetylated at 150 C, [9] before being ...

Processible organic materials exhibiting high charge carrier mobilities are a step closer with the development of liquid crystalline photoconducting materials. The mobilities have been measured in various phase regions over the temperature range from – 100 to 165°C and the influence of different annealing conditions assessed, indicating that these materials could soon compete with amorphous semiconductors in electronics applications.

A highly efficient multicomponent reaction based on the little-known formylating agent dimethylformamide in acetic anhydride led to a multitude of dyes of the indoline (1) and of the thiazole type (2), whose systematic evaluation revealed desired and surprising solid-state properties.

The synthesis of polymerizable triphenylene derivatives with acrylate groups is described. The thermal properties and the photoconductive properties of the discotic monomers as well as of the polymers is investigated. In the mesophase typically photoinduced hole conductivity with a charge carrier mobility exceeding a value of 10-4 cm2Ns is observed with conventional time-of-flight (TOF) techniques. Different conditions for the photopolymerization of the acrylates in the mesophase are compared. The influence of the photoinitiators and the polymerization on the mesomorphic behavior and on the hole conductivity is described. Already the presence of small amounts of photoinitiator influences the packing within the mesophase. The polymerization further decreases the order and creates defects. This results in an at least one order of magnitude reduced charge carrier mobility at room temperature, compared to the monomer.

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Liquid-crystalline copolymers with covalently attached dichroic dyes were synthesized and characterized. Copolymers with high anthraquinone and trisazo-dye content were obtained by copolymerization of dye monomers with 4-cyanophenyl4-(6-acryloyloxyhexyloxy)benzoate (1) and 6-acryloyloxyhexyl 4-(trans-4-propylcyclohexyl)benzoate (2). Particular attention is given to the influence of the dye content and the dye structure on the liquid-crystalline behavior of the copolymers.

A number of novel nonlinear optically (NLO) active polymethacrylates were prepared from the NLO active methacrylates 2a-d with azobenzene side groups and the bulky comonomer 1-adamantyl methacrylate. The polymers exhibit unusually high glass transition temperatures between 160 "C and 190°C. The copolymerization parameters of the monomer pair 1-adamantyl methacrylate (1)lDisperse red methacrylate 2 b (r, = 1,l t 0,2, r2 = 0,s f 0,2) show that the two monomers are incorporated almost statistically into the polymer chain. Polymers 3a-d are soluble in common organic solvents and excellent films can be obtained by spin coating. After poling in an electric field of 120 V/pm polymer 3b shows a large electrooptic (EO) coefficient (r33) of 25 pm/V at 633 nm. Within two weeks, only a negligible decay of 7% of the EO coefficient was observed at room temperature. On-line monitoring of the second harmonic generation (SHG) at 100 O C showed a fast initial drop (10%) of the SHG signal and subsequently a slow decay of 20% within 10 h. Afterwards, the signal remained almost constant for further 5 h at 100°C. The novel polymers can thus be considered as easy processible NLO materials with a high thermal stability of the chromophore orientation obtained by poling.