With the rapid progress in information and telecommunication technologies ("multimedia") faster methods for transmitting, storing, and processing data are requiredj'] While fast optical data transmission through glass fibers is already well established. there is a need to develop high-speed storage and switching devices based on photons instead of electrons. A new promising method for reversible data storage is based on the photorefractive (PR) effect, whose "parallel character" allows optical processing of large amounts of data by holography.Photorefractivity has been demonstrated only recently in amorphous organic materials.[2] Here, research is confronted with the difficult task of developing a complex functional material that provides photoconductivity and refractive-index modulation in films of high optical quality and stability against dielectric breakdown. The materials investigated so far are mostly composites consisting of a charge-transport agent (e. g. triarylamine or carbazole-based photoconductorsI3I), an electrooptic (EO) chromophore with high secondorder polarizability pJ41 small amounts of a sensitizer, and variable amounts of additives (plasticizer, polymeric binder).Among the E O chromophores the azo dye 2,5-dimethyl-(4-nitropheny1azo)anisole (DMNPAA) dominates the scene C"3 DMNPAA because of its outstanding compatibility with carbazole charge-transport With DMNPAA-doped polymers holographic gratings with almost 100 YO diffraction efficiency were achieved for the first time. To get such a strong PR effect, however, DMNPAA concentrations as high as 50 wt% and high electric fields of 50-100 V p-' are required. For applications improved chromophores are needed to reduce the chromophore content and, thus, avoid problems with phase separation and to operate the devices at lower field strength. Here we introduce a conceptionally new class of dyes which represent a significant step towards this goal.Our work was triggered by the fact that the refractive-index modulation in organic materials with a low glass-transition temperature is predominantly due to the Kerr and not the Pockels effect as in inorganic This difference is because in these composites the dipolar chromophores are able to reorient in the light-induced space-charge field.r61 As a result, a new figure-of-merit F, for photorefractive chromophores was derived which is approximated fairly well by Equation (1).17a*8] Here M is the molecular weight, pLg is the dipole moment, and a. and Po are two-level approximations for the static first-and second-order polarizabilities of the chrornophore.r8I For "one-dimensional'' chromophores consisting of a n system with donor and acceptor substitutents these polarizabilities can be reasonably approximated by considering the transition dipole pag, the transition wavelength lag, and the difference in dipole hp between the ground (g) and excited states (a) [Eqs. (2) and (3)

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 ...

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Reachability testing is an approach to verifying concurrent programs. During reachability testing, every partially ordered synchronization sequence of a program with a given input is exercised exactly once. In this paper, we present the design and implementation of a distributed reachability testing algorithm for a cluster of workstations. This algorithm allows different test sequences to be exercised concurrently by different workstations without any synchronization, and without any duplication of sequences among workstations. Dynamic load balancing is performed using a work‐stealing scheme. A novel aspect of this scheme is that work‐stealing requests progress in rounds. This round‐based structure identifies overloaded workstations to target for work stealing. Empirical studies show good speedup for four benchmark Java programs and one Lotos specification. Copyright © 2010 John Wiley & Sons, Ltd.