Azo Dyes as Side Chains in Liquid Crystalline Oligomers for Holographic Application Oliver Haak,a Cheung-Bok Jeoung,a Andreas Pawlik,a Peter Boldt,*a Walter Grahn,*a Franz-Heinrich Kreuzer,b Horst Leigeberb and Hans-Peter Weitzelb aInstitute fu�â r Organische Chemie, Technische Universita�â t Braunschweig, D-38092 Braunschweig, Germany bConsortium fu�â r elektrochemische Industrie GmbH, Zielstattstra��e 11, D-81379 Mu�â nchen, Germany In a systematic study on materials for holographic data storage, a wide variety of azo dyes of different shapes and optical properties were covalently bound to cholesteric liquid crystalline oligosiloxanes; some of these materials exhibit high holographic efficiency and/or sensitivity but no general correlation between the holographic writing efficiency and/or the sensitivity with the structure was found.Most promising materials for optical storage are nematic or cholesteric liquid crystalline polymers that possess arylazo groups as side chains.2 The general structure of our liquid crystalline materials is shown in Scheme 1 with a simple azo dye as an example.They are composed of an octacyclo- siloxane backbone with diphenyl- and cholestanyl-p- hydroxybenzoates and the azo dyes as side chains. They were made by palladium-catalysed hydrosilylation of the appropriate alkenes with the oligosiloxane.5 A HeCd laser (442 nm) and a NdYAG laser (with fre- quency doubled emission at 532 nm) were used in the holo- graphic experiments as the writing beam.The holographic eciency was measured with a HeNe laser (633 nm, reading beam). The structural variations of the azo dyes were carried out for the following reasons: (1) To tune the position of the long wavelength absorption band by variation of the strength of the electron-acceptor group.6 (2) To generate a hypsochromic shift of the long wavelength absorption band by donor�}donor substitution, by acceptor�}acceptor substi- tution and by steric hindrance of the azo bridge.6 (3) So as to investigate the in�Puence of the length of the spacer on the liquid crystalline properties.(4) To study the e€ect of side-on instead of end-on binding on the mesogenic properties. (5) To get some information on the e€ect of the length of the photomechanically switched molecular part: the eciency of the writing process should be higher the more the liquid crystalline order is disturbed, i.e. the longer the free part of the switching azo molecule is at the photo- chemical isomerization.Simple azo dyes were made by the usual methods.8 Styryl and arylethynyl groups were introduced into the azo dyes by palladium-catalysed cross-coupling reactions.9 The results of the optical measurements, the long wave- length absorption bands, the optical densities (OD) of the siloxane Rlms, the eciencies (E€ ) and the sensitivities (S) of the holographic writing process at 442 and 532 nm are given in Table 1.Some of these materials exhibit high holographic e- ciency and/or sensitivity. But for a given azo dye/oligo- siloxane system no prediction of the holographic optical storage properties can be made: as expected, the systems absorbing at short wavelengths have low ODs at 532 nm J. Chem. Research (S), 1998, 630�}631 J. Chem. Research (M), 1998, 2701�}2735 Scheme 1 Table 1 Optical properties of the arylazo dyesa Dye lmax (nm, sol.) OD442 OD532 Eff442 Eff532 S442 S532 5c 464 >4 4 24 26 1 2.1 5d 476 >4 4 21 16 1.3 1.2 5e 486 2.6 2.7 22 29 1.2 6 5f 446 >4 0.9 17 40 0.8 1.4 5g 458 >4 4 9 23 1 1.7 6a 442 >4 2.7 16 30 0.5 0.3 6b 424 >4 0.8 5 9 0.2 0.2 6c 408 >4 0.25 11 12 0.3 0.03 7c 438 0.9 0.05 23 1 1.1 0.01 7d 438 0.5 0.02 24 0.2 2 0.05 7e 436 0.8 0.08 28 0.2 1.4 0.01 7f 436 0.33 0.03 10 0.2 0.3 0.03 7g 464 1.9 0.4 25 31 0.5 0.7 7h 470 0.35 0.13 21 15 0.8 0.4 7i 448 0.5 0.08 17 10 1.9 0.3 7j 446 0.6 0.08 20 15 3.3 0.6 7k 442 0.2 0.05 7 0.4 0.11 0.01 7l 458 0.2 0.08 4 7 0.07 0.06 8a 464 1.8 0.2 0.1 0.1 0.01 0.01 9a 436 4 0.55 21 18 1.9 1.3 9e 464 3.6 1.9 15 34 3.2 2.7 9f 464 4 3.2 22 32 2.8 3.2 9h 454 4 1.2 26 24 3.2 5.5 10a 450 >4 2.5 30 34 3.4 6.6 10b 450 >4 2.4 25 35 3 6.1 11c 380 3.8 0.12 15 5 1.8 0.06 13a 438 >4 0.9 18 26 3.1 3.5 13b 438 >4 0.06 19 0.1 1.6 0.01 13c 420 >4 0.07 31 17 1.4 0.1 13d 438 >4 0.2 34 20 1.4 0.4 13f 420 >4 0.05 15 0.1 1 0.01 aAbsorption wavelengths in solution, all other measurements in an oligosiloxane matrix.*To receive any correspondence. (E-mail: P.Boldt@tu-bs.de and W.Grahn@tu-bs.de). 630 J. CHEM. RESEARCH (S), 1998(connected with low eciencies and sensitivities). Never-theless some of these compounds exhibit high values. Nofurther relationship seems to exist between the structure orthe properties of the azo dye moieties and the E or S. It isnoteworthy that completely dierent molecules, i.e. extendedarylazostilbenes and arylazoimines and a simple azo dye,exhibit the best values.The nancial support of the Bundesministerium fu rBildung und Forschung (project no. 03 M 4059) is gratefullyacknowledged.Techniques used: IR, UV¡ÓVIS, 1H and 13C NMR, EI and FABmass spectrometryReferences: 17Schemes: 2Fig. 1: Absorption spectra of 5e, in methylcyclohexane (q), atmax=452 nm normalized to OD 1; after irradiation for 5 minwith a xenon lamp 250 W cm£¾2, Schott lter GG 385 nm (r);in siloxane matrix (w), at max1483 nm normalized to OD 1.The strong absorption below 350 nm is owing to the oligosiloxanemoietyFig. 2: Dependency of the eciency (E ) on the optical density. Eof 7c¡Óf, k; 8a; 13b, f<1Fig. 3: Dependency of the sensitivity (S) on the optical density. S of6a¡Óc; 7c¡Óf, h, j¡Ól; 11c; 13b¡Ód, fE0.6Fig. 4: Correlation of the sensitivity (S) with the eciency (E ) ofoligosiloxanes with arylazo dye side chainsTable 2: Method of preparation and analytical data of the arylazocompounds 5a¡Ói, 6a¡Óc, 7a¡Ól and 8a,bTable 3: Method of preparation and analytical data of the (E)-arylazo stilbenes 9a¡Ók, 11a¡Óc and arylazoimines 10a,bTable 4: Method of preparation and analytical data of the benzo-thiophenes 12a,b and the arylazo compounds 13a¡ÓfReceived, 25th March 1998; Accepted, 26th June 1998Paper E/8/02336KReferences cited in this synopsis2 (a) M.Eich and J. H. Wendor, Makromol. Chem., RapidCommun. 1987, 8, 59; (b) K. Anderle, R. Birenheide, M. Eich andJ. H. Wendor, Makromol. Chem., Rapid Commun. 1989, 10, 477;(c) R. Ortler, C. Bra uchle, A. Miller and G. Riepl, Makromol.Chem., Rapid Commun. 1989, 10, 189; K. Ichimura, Y. Suzuki,T. Seki and Y. Kawanishi, Makromol. Chem., Rapid Commun.1989, 10, 5; G. S. Kumar and D. C. Neckers, Chem. Rev., 1989,89, 1915; A. Shishido, O. Tsutsumi and T. Ikeda, Mater. Res.Soc. Symp. Proc. 1996, 425, 213; B. Fleck, D. A. Dowling andL. Wenke, J. Modern Opt., 1996, 43, 1485.5 G. Riepl, F.-H. Kreuzer and A. Miller, Consortium fu r elektro-chemische Industrie GmbH, EP 333022, 1989, Mu nchen.6 For a discussion of the inuence of the hindrance on theUV¡ÓVis-spectra see: (a) O. Haak, Dissertation, TechnischeUniversita t, Braunchweig, 1994; (b) C. B. Jeoung, Dissertation,Technische Universita t, Braunschweig, 1993.8 H. Zollinger, Color Chemistry, 2nd edn., VCH Verlagsgesell-schaft, Weinheim, 1991.9 C.-B. Jeoung, O. Haak, W. Grahn and P. Boldt, J. Prakt. Chem.,1993, 335, 521.J. CHEM.