摘要:

THE ROYAL SOCIETY OF CHEMISTRY Journal of Materials Chemistry Scientific Advisory Editor Editorial Manager Dr. Martin R. Bryce Dr. Robert J. Parker Department of Chemistry The Royal Society of Chemistry University of Durham Thomas Graham House South Road Science Park Durham DH1 3LE, UK Cambridge CB4 4WF, UK Staff Editor: Mrs. Janet M. Leader Senior Assistant Editor: Mrs. S. Shah Assistant Editor: Mrs. S. Youens Editorial Secretary: Miss D. J. Halls Graphics Designer: Ms. C. Taylor-Reid Materials Chemistry Editorial Board Allan E. Underhill (Bangor) (Chairman) Peter G. Bruce (St. Andrews) John W. Goodby (Hull) Martin R. Bryce (Durham) Klaus Praefcke (Berlin) David A. Dunmur (Sheffield) Brian J. Tighe (Aston) Jean Etourneau (Bordeaux) Anthony R.West (Aberdeen) Wendy R. Flavell (UMIST) John D. Wright (Canterbury) Robert J. Parker (Secretary) International Advisory Editorial Board K. Bechgaard (Risa, Denmark) J. S. Miller (Salt Lake City, UT, USA) J. Y. Becker (Beer-Sheva, Israel) K. Mullen (Mainz, Germany) J. D. Birchall (Runcorn, UK) M. Nygren (Stockholm, Sweden) A. J. Bruce (Murray Hill, USA) Y. W. Park (Seoul, Korea) A. K. Cheetham (Santa Barbara, USA) V. Percec (Cleveland, OH, USA) E. Chiellini (Pisa, Italy) N. Plate (Moscow, Russia) D. Coates (Poole, UK) M. Prato (Trieste, Italy) P. Day (London, UK) C. N. R. Rao (Bangalore, India) B. Dunn (Los Angeles, USA) J. Rouxel (Nantes, France) W. J. Feast (Durham, UK) R. Roy (University Park, PA, USA) A. Fukuda (Tokyo, Japan) J. L.Serrano (Zaragoza, Spain) D. Gatteschi (Florence, Italy) J. N. Sherwood (Glasgow, UK) J. B. Goodenough (Austin, TX, USA) J. Simon (Paris, France) A. C. Griffin (Hattiesburg, USA) J. F. Stoddart (Birmingham, UK) S-i. Hirano (Nagoya, Japan) S. Takahashi (Osaka, Japan) P. Hodge (Manchester, UK) J. 0.Thomas (Uppsala, Sweden) H. lnokuchi (Okazaki, Japan) G. J. T. Tiddy (Bebington and Salford, UK) W. Jeitschko (Munster, Germany) Yu. D. Tretyakov (Moscow, Russia) 0. Kahn (Orsay, France] J. W. White (Canberra, Austra4ia) R. McCullough (Pittsburgh, USA) R. Xu (Changchun, China) Y. Yamashita (Okazaki, Japan) Journal of Materials Chemistry (ISSN 0959-9428) is published monthly by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB44WF, UK.All orders accompanied with payment should be sent directly to The Royal Society of Chemistry, Turpin Distribution Services Ltd., Blackhorse Road, Letchworth, Herts SG6 lHN, UK. NB Turpin Distribution Services Ltd., distributors, is wholly owned by The Royal Society of Chemistry. 1994 Annual subscription rate EC (inc. UK) f381.00, USA $718.00, Canada f431.00 (plus GST), Rest of World f410.00. Customers should make payments by cheque in sterling payable on a UK clearing bank or in US dollars payable on a US clearing bank. Air freight and mailing in the USA by Publications Expediting Inc., 200 Meacham Avenue, Elmont, NY 11003. USA POSTMASTER: send address changes to Journal of Materials Chemistry, Publications Expediting Inc., 200 Meacham Avenue, Elmont, NY 11003.Second Class postage paid at Jamaica, NY 11431. All other dispatches outside the UK by Bulk Airmail within Europe, Accelerated Surface Post outside Europe. PRINTED IN THE UK. 0 The Royal Society of Chemistry, 1994. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form, or by any means, electronic, mechanical, photographic, recording, or otherwise, without the prior permission of the publishers. Dr. R. J. Parker, Editorial Manager Tel.: Cambridge (01223) 420066 E-Mail (INTERNET): RSCl@RSC.ORG Fax: (01223) 426017, 420247 or 423623 Advertisement sales: Tel. +44 (0171-287 3091; Fax +44 (0171-494 1134 INFORMATION FOR AUTHORS The Royal Society of Chemistry welcomes submission of manuscripts intended for publication in two forms, Articles and Materials Chemistry Communications.These should describe original work of high quality dealing with the synthesis, structures, properties and applications of materials, particularly those associated with advanced technology. Articles Full papers contain original scientific work that has not been published previously. However, work that has appeared in print in a short form such as a Materials Chemistry Communication is normally acceptable. Four copies of Articles includ- ing a top copy with figures etc. should be sent to The Editor, Journal of Materials chemistry, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK.Materials Chemistry Communications Materials Chemistry Communications con- tain novel scientific work in short form and of such importance that rapid publication is warranted. The total length is normally restricted to two pages of the double-column A4 format. For a Communication consisting entirely of text and ten refer-ences, with no figures, equations or tables, this corresponds to approximately 1600 words plus an abstract of up to 40 words. However, special consideration will be given to communications with a large amount of essential diagramatic information. Submission of a Materials Chemistry Communication can be made either to The Editor, Journal of Materials Chemistry, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK, or via a member of the International Advisory Editorial Board. In the latter case, the top copy of the manuscript including any figures etc., together with the name of the person to whom the Communication is being submit- ted, should be sent simultaneously to the Editor at the Cambridge address.Authors may wish to contact the Board member to ensure that he is available to arrange review of the manuscript within reasonable time. In order to avoid delay in publication, proofs of Communications are not sent to authors unless this is specifically requested. Full details of the form of manuscripts for Articles and Materials Chemistry Communications, conditions for accept-ance etc. are given in issue number one of Journal of Materials Chemistry published in January of each year, or may be ob- tained from the Staff Editor. There is no page charge for papers pub- lished in Journal of Materials Chemistry. Fifty reprints are supplied free of charge. Any author who is publishing in Journal of Materials Chemistry is entitled to a free copy of the issue in which the paper appears.

ISSN:0959-9428    

DOI:10.1039/JM99404FX033

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Each lssue with Subject-, Author- and Materials DWECT nnnIndexes Additional 10- Volume lndexes 12 volumes per year Annual Subscription Rate: SFr 1320.00 Postage/Hanci/ing: SFr 120.00 Agency Discount: 10% ISSN 0377-6883 EDITORS: Professor G.E. Murch Department of Mechancal Engineering, The University of Newcastle, NSW 2308, Australia H. Neber-Aeschbacher Scitec Publications Untermuehleweg 11 CH-6300 Zug, Switzerland Dr. Fred H. Wohlbier Trans Tech Publications Hardstrasse 13 CH-4714 Aedermannsdorf Switzerland ABSTRACT EDITOR: Dr. David J. Fisher Cardiff, United Kingdom The International Journal of the Defect Solid State Pt. A of Diffusion and Defect Data efect and Diffusion Forum is an international journal serving the advanced materials research community as a permanent record of significant developments in the general area of the defect solid state. Started in 1967 as Diffusion Data, the material pre- sented today encompasses the well-known ex-tended abstract section which has made this series a standard in its field, as well as critical reviews, data collections and original contributions.Complete spe- cial issues regularly focus on topics of current inter- est. Particular emphasis is placed on atomic and ionic transport, solid-state defect properties (both structural and electronic), radiation damage and de- fect production. In order to shorten publication time, correspondence can be sent electronically, and manuscripts can be accepted in electronic form. IDistributed by: Scitec Publications Member of the Trans Tech Group of Publishers Trans Tech Materials Science Solid State Physics Engineering Untermuehleweg 11 CH-6300 Zug Switzerland Fax: ++41 -42 32 52 12 E-Mail: ddf@scitec.ch Publications Ltd U Trottenstr. 20 / CH-8037 Zurich / Switzerland Fax: (++41) 12 72 I0 92 E-Mail: ddf@transtech.ch Write us for: -Detailed scope of coverage -Instructions for authors -Information on further Materials Science titles

ISSN:0959-9428    

DOI:10.1039/JM99404BX035

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

ISSN 0959-9428 JMACEP(9) 7 359-7 509 ( 1994) Journal of Materials Chemistry Synthesis, structures, properties and applications of materials, particularly those associated with advanced technology CONTENTS 359 Synthesis of high refractive index acrylic copolymers T. P. Davis, M. J. Gallagher, M. G. Ranasinghe and M. D. Zarnmit 365 Inversion of chirality-dependent properties in helical liquid crystals: effects of structural modification P. Styring, J. D. Vuijk, S. A. Wright, K. Takatoh and C. Dong 377 Magneto-structural correlation in a series of iodide salts of p-N-alkylpyridinium nitronyl nitroxides: Dependence of tht: iodide- pyridinium ring interaction on the length of the N-alkyl chain K. Awaga, A. Yamaguchi, T. Okuno, T. Inabe, T. Nakarnura, M. Matsumoto and Y.Maruyarna 387 Polymer-mediated crystallisation of inorganic solids: Calcite nucleation on the surfaces of inorganic polymers K. K. W. Wong, B. J. Brisdon, B. R.Heywood, A. G. W. Hodson and S. Mann 393 MOCVD of high-quality YBazCu,07-, films: in situ preparation of fluorine-free layers from a fluorinated barium source I. M. Watson, M. P. Atwood, D. A. Cardwell and T. J. Cumberbatch 1403 Growth of TiO, overlayers by chemical vapour deposition on a single-crystal copper substrate Y. M. Wu and R. M. Nix 1409 Growth of PbS thin films from novel precursors by atomic layer epitaxy E. Nykanen, J. Laine-Ylijoki, P. Soininen, L. Yinistii, M. Leskela and L. G. Hubert-Pfalzgraf 1413 Thermodynamic and kinetic properties of lithium insertion into titanium misfit layer sulfides P.Lavela, J. Morales and J. L. Tirado 1419 Intercalation of large cluster cations in TaS, L. F. Nazar and A. J. Jacobson 1427 Selectivity and composition dependence of response of gas-sensitive resistors. Part 1.-Propane-carbon monoxide selectivity of Ba,Fe,Nb,,-,O,, (1 dx d2) G. S. Henshaw, L. J. Gellman and D. E. Williams 1433 Influence of chlorine-oxygen substitution on the electrical properties of some oxychloride tellurite glasses J. M. Rojo, P. Herrero, R. M. Rojas, J. Sanz, J. M. RCau, S. Rossignol, B. Tanguy and J. Portier Uo.4Pro.6)02+x1437 Electrode kinetic behaviour of (Uo.4Pro.6)02~JYSZ/( cells S. P. S. Badwal 1441 Phase diagrams and stoichiometries of the solid electrolytes, Bi4V201, :M, M =Co,Cu, Zn, Ca, Sr C.K. Lee, G. S. Lirn and A. R.West 1445 Reinterpretation of the magnetic structures of the perovskites SrFeO,,,,, and Sr,LaFe,O,.,,, T. C. Gibb 1451 Synthesis under high pressure and characterisation by Mossbauer spectroscopy of non-stoichiometric CazFezO,,,z T. C. Gibb, A. J. Herod, D. C. Munro and N. Peng 1457 YMoO, revisited: The crystal structure of Y5MO,O,, N. J. Stedrnan, A. K. Cheetham and P. D. Battle 1463 Neutron diffraction structural study of the Nasicon-related phases Li,M1l,M1ll, -,(SO4),-y( Se04),( MI1= Mg, Ni, Zn; Mi'' =PI, Cr) P. R. Slater and C. Greaves 1469 Powder neutron diffraction study of the Nasicon-related phases Na,Mn,M'112 --x (SO,), -,( SO,),: MI1= Mg, M" =Fe, In P. R. Slater and C. Greaves 1475 Single-crystal study of topotactic changes between NH4V03 and V205 A.Shimizu, T. Watanabe and M. Inagaki 1479 Synthesis, characterization, chemisorption and thermodynamic data of urea immobilized on silica C. Airoldi and M. R. M. C. Santos 1487 Electrogeneration of base by cathodic reduction of anions: novel one-step route to unary and layered double hydrc )xides (LDHs) L. Indira and P. V. Kamath 1491 Effect of thermal treatment on microporous accessibility in aluminium pillared clays A. Gil and M. Montes 1497 High-speed preparation of metal oxide fine powders by microwave cold plasma heating K. Sugiyama, Y. Nakano, H. Aoki, Y. Takeuchi and T. Matsuda 1503 Catalytic activity of aluminas obtained by the thermal decomposition of mechanically ground alumina monohydrates, u-and P-Al,03.Hz0 T.Tsuchida, S. Ohta and K. Horigorne 1507 Book reviews: C. Greaves; C. Booth; N. C. Billingham; G. S. Simpson; G. J. Gittens i Cumulative Author Index ... 111 Conference Diary Note: Where an asterisk appears against the name of one or more authors, it is included with the authors’ approval to indicate that correspondence may be addressed to this person. COPIES OF CITED ARTICLES The Royal Society of Chemistry Library can usually supply copies of cited articles. For further details contact: The Library, Royal Society of Chemistry, Burlington House, Piccadilly, London W1V OBN, UK. Tel: +44 (0)71-437 8656, Fax: +44 (0)71-287 9798, Telecom Gold 84: BUR210, Electronic Mailbox (Internet) LIBRARY@RSC.ORG. If the material is not available from the Society’s Library, the staff will be pleased to advise on its availability from other sources. Please note that copies are not available from the RSC at Thomas Graham House, Cambridge.

ISSN:0959-9428    

DOI:10.1039/JM99404FP079

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index Aarik J., 1239 Abrahams I., 185, 775 Abser M. N., 1173 Agullo J. M., 695 Ahmet M. T., 1201 Ahn S-K., 949 Aidla A., 1239 Ainslie B. J., 1233 Airoldi C., 1479 Aka G., 907 Akhtar M. J., 1081 Akhtar Z.-u.-N., 1081 Akimoto H., 61 Aksay I. A., 353 Alagna L., 943 Ali-Adib Z., 1 Aliev A. E., 35 Allan N. L., 817 A1 Raihani H., 1331 Alves 0. L., 389, 529 An Y., 985 Ando M., 631 Andreani F., 1035 Angeloni A. S., 429, 437 Angeloni L., 1047 Annila A.. 585 Aoki H., 1497 ap Kendrick D., 399 AraK., 551 Arai H., 653 Arai K., 275 Aranha N., 529 Armelao L., 407 Armes S. P., 935 Armigliato A,, 361 Arnold Jr. F. E., 105 Aruga Katori H., 915 Asaka N., 291 Aspin I. P., 385 Attfield J. P., 475, 575 Atwood M. P., 1393 Auld J., 1245, 1249 Auroux A,, 125 Awaga K., 1377 Azuma K..139 Baba A., 51 Babu G. P., 331 Babushkin O., 413 Bach S., 133, 875 Bachir S., 139 Badwal S. P. S., 257, 1437 Badyal J. P. S., 1055 Bae M-K., 991 Baetzold R. C., 299 Baffier N., 133, 875 Bagshaw S. A., 557 Baiios L., 445 Baram P. S., 817 Barbieri A., 1255 Barbosa L. C., 529 Barker C. P., 1055 Barriga C., 11 17 Barton J. M., 379, 385 Bashall A., 1201 Battaglin G., 407 Battle P. D., 421, 641, 707, 831, 1457 Batyuk V. A., 761 Bautista F. M., 311 Bazin D., 1101 Bechgaard K., 675 Bedioui F., 1215 Bedson J., 571 Beguin F., 669 Bell R. G., 781 Bellwood M., 1173 Benzi P., 1067 Bertoncello R., 407 Beveridge M., 119 Big S., 361 Bignozzi M. C., 429 Billingham N. C., 1508 Bjrarnholm T., 675 Blasse G., 1349 Bonanos N., 899 Bonardi A., 713 Bond S. E., 23 Booth C., 591, 1507 Booth C.J., 747 Botto L. I., 541 Bowden K., 1201 Bradley R. H., 487, 1157, Branitsky G. A., 373 Branton P. J., 1309 Braybrook J. H., 1157, 1357 Brewis D. M., 487, 683 Brisdon B. J., 1387 Britt S., 161 Brock T., 229 Brodsky C. J., 651 BrownT., 771 Bruce D. W., 479, 1017 Bruce P. G., 167 Bryant G. C., 209 Buckley C. M., 1173 Buist G. J., 379, 385 Bujanowski V. J., 1181 Bujoli B., 1319 Bulmer G., 1149 Burnell G., 1309 Busca G., 965, 1123 Bush T. S., 831 Cairns J. A., 393 Campelo J. M., 311 Caneschi A., 319, 1047 Cao X., 417 Capelletti R., 713 Cardwell D. A., 1393 Carlino S., 99 Carr S. W., 421 Carrazan S. R. G., 47 Carruthers B., 805 Carvalho A., 515 Casciola M., 1313 Cassagneau T., 189 Castellanos M., 1303 Castighoni M., 1067 Castillo R., 903 Catlow C. R.A,, 781, 831, Causa M., 825 Cellucci F., 579 Cervini R., 87 Cesar C. L., 529 Chaair H., 765 Challier T., 367 Chang S-H., 1271 Charlton A., 1233 Chassagneux F., 1331 Cheetham A. K., 641, 707, Chehimi M. M., 305, 741 Chen C., 469 Chen Q., 327 Cheng S. Z. D., Chernyaev S. V., 1107 Chevalier B., 463 Chiba K., 551 Chiellini E., 429, 437 Choisnet J., 895 Chu P., 719 Ciacchi F. T., 257 Clegg W., 891 Colbourn E. A., 805 Cole-Hamilton D. J., 657 Coles G. S. V., 23 Coles H., 869 Colque S., 1343 Connell J. E., 399 Conroy M., 1 Conway L. J., 337 Cook M. J., 209, 1205 Cook S. L., 81 Cooney R. P., 557 Copplestone F. A., 421 Corriu R. J. P., 987 1189 1081 1457 105, 719 Choy J-H., 1271 Costa Bizzarri P., 1035 Costa F. M. A., 515 Cox P. A,, 805 Craig S. R., 977 Crayston J. A., 1093 Crespin M., 895 Critchlow G.W., 1245, Cumberbatch T. J., 1393 Dan M., 1195 Daolio S., 1255 Darriet B., 463 David L., 1047 Davidson I. M. T., 13 Davies A., 113 Davies M. J., 813 Davis T. P., 1359 Deazle A. S., 385 De Battisti A., 1255 Dekker J. P., 689 del Arc0 M., 47 del Carmen Prieto M., 1123 Della Casa C., 1035 Delmon B., 903 Dennison S., 41 Depaoli G., 407 Deschenaux R., 679, 1351 De Stefanis A., 959 Devynck J., 1215 Dhas N. A., 491 Diamond D., 145,217 Dissanayake M. A. K. L., Dong C., 1365 Douglas W. E., 1167 Drabik M., 265, 271 Drennan J., 245 Dunmur D. A,, 747 Durand B., 1331 Eda K., 205, 775 Eguchi K., 653 Ekstrand A., 615 Eldred W. K., 305 Ellis A. M., 13 Elsegood M. R. J., 891 Endregard M., 943 Ericsson T., 1101 Errington R. J., 891 Etourneau J., 463 Fabretti A., 1047 Facchin B., 1255 Faguy P.W., 771 Fau-Canillac F., 695 Feast W. J., 1159 Feng S., 985 Fernandez J. M., 1117 Ferraro F., 1047 Fettis G. C., 1157, 1357 Fisher G. A., 891 Fitzmaurice J. C., 285 Fitzpatrick A. D., 1055 Fleming R. J., 87 Fletcher J. G., 1303 Flint S. D., 509 Folkerts H. F., 1349 Forsyth M., 1149 Foster D. F., 657 Fragala 1. L., 1061 Fraoua K., 305 Freakley P. K., 1189 Frederiksen P., 675 Friend R. H., 1227 Frfalova M., 271 Fujii T., 635 Fujimoto T., 61, 533, 537 Fujita T., 955 Fujiwara Y., 1219 Fukuda A., 237,997 Gaillon L., 1215 Gale J. D., 781, 831 Galikova i., 265, 271 Gallagher M. J., 1359 Gallardo Amores J. M., 965, 1123 1249 1075, 1307 Galli G., 429, 437 Ganguli P., 331 Garcia A., 311 Garcia-Martin S., 1307 Garcia-Martinez O., 611 Gatteschi D., 319, 1047 Gee M.B., 337 Gellman L. J., 1427 Gibb T. C., 1445, 1451 Gibson R. A. G., 393 Gier T. E., 1111 Gil A., 1491 Gil-Llambias F-J., 47 Gittens G. J., 1508 Glomm B., 55 Godinho M. M., 515 Goodby J. W., 71, 747 Gopalakrishnan J., 703 Gormezano A., 817 Goto T., 915 Gozzi D., 579 Grange P., 1343 Granozzi G., 407 Gravereau P., 463 Greaves C., 931, 1463, 1469, Gregory D. H., 921 Grins J., 445, 1293 Guillon D., 679, 1351 Guo Z., 327 Gutierrez M. P., 1303 Hall P. G., 1309 Hamerton I., 379, 385 Hamstra M. A., 1349 Han Y-S., 1271 Hannington J., 869 Harris F. W., 105 Harris K. D. M., 35 Harris S. J., 145, 217 Hamson W. T. A,, 11 11 Haslam S. D., 209, 1205 Hastie G. P., 977 Hatayama F., 205, 775 Hayashi A,, 915 Heath K. D., 825 Heath R. J., 487, 683 Hector A.L., 279 Heinrich B., 679 Henshaw G. S., 1427 Hermansson L., 413 Herod A. J., 1451 Herrero P., 1433 Hervieu M., 1353 Heughebaert J-C., 765 Heughebaert M., 765 Heywood B. R., 1387 Hickey E., 463 Higuchi A., 171 Hill C. A. S., 1233 Hinds B. J., 1061 Hirose N., 9 Hitchman M. L., 81 Hix G. B., 189 Hobson R. J., 113 HochiK., 599 Hodby J. W., 469 Hodge P., 1, 869 Hodson A. G. W., 1387 Holmes M. C., 1173 Holmes P. A., 365 Holmgren A,, 413 Hong L., 1041 Hopkins J., 1055 Horigome K., 1503 Hosokoshi Y., 1219 Houlton D. J., 1245, 1249 Hourd A. C., 393 Howlin B. J., 379, 385 Hu Y., 469 Hubert- Pfalzgraf L. G., 1409 Hudson M. J., Hudson S. A., 479 Hughes A. E., 257 1507 99, 113, 1337 Huxham 1. M., 253 Ibanez A., 1101 Ibn-Elhaj M., 1351 Ichimura K., 883 Ikemoto H., 537 Imanishi N., 19 Imayoshi K., 19 Inabe T., 1377 Inada H., 171 Inagaki M., 1475 Indira L., 1487 Inman D., 1331 Irvine J.T. S., 995 Ishikawa K., 997 Islam M. S., 299 Ismail H., 1189 Isoda S., 291 Isozaki T., 237, 997 Ivanovskaya M. I., 373 Iyer R. M., 1077 Jacobson A. J., 1419 Jaek A., 1239 James M., 575 Janes R., 1071 Jennings R. A., 931 Jimenez R., 5 JimCnez-L6pez A., 179 Jin-Hua C., 1041 Joachimi D., 1021 Jones A. C., 1245, 1249 Jones D. J., 189 Jones J. R., 379, 3x5 Jones P. J. V., 805 Jouanneaux A., 13 19 Jung K., 161 Jung W-S., 949 Kadokawa J-i., 551 Kaharu T., 859 Kahn-Harari A., 907 Kakkar A. K., 1227 Kamath P. V., 1487 Kang J. S., 747 Karasu M., 551 Kareiva A,, 1267 Karppinen M., 1267 Kassabov S., 153 Kato C., 519 Kato R., 915, 1219 Katsoulis D. E., 337, 1181 Kawamura I., 237 Kennedy B.J., 87 Kerridge D. H., 13 11 Kershaw S., 1233 Khan M. S., 1227 King T., 1 Kinoshita M., 915, 1219 Kiyozumi Y., 585 Klein M. L., 793 Klissurski D., 153 Knight K. S., 899 Knowles J. C., 185, 775 KO E. I., 651 Kobayashi T., 291 Koch B., 903 Kohmoto T., 205, 775 Komatsu T., 533, 5 17 Komppa V., 585 Kossanyi J., 139 Kosztics I., 1351 KouyatC D., 139 Krist6 J., 1255 Kriitofik M., 271 Kubono K., 291 Kubranova M., 265 Kunitomo M., 205, 775 KunouI., 955 Kuramoto N., 1195 Kuroda K., 519 Kuwano J., 9, 973 Labajos F. M., 1115 Lacey D., 1029 Lahti P. M., 161 Kall P-O., 1293 Kim H-B., 883 i Laine-Ylijoki J., 1409 Landee C., 161 Laus M., 429,437 Lavela P., 1413 Lawrence L. W., 571 Lawrenson B., 393 Lea M. S., 1017 Le Bideau J., 1319 Lee C.K., 525, 1441 Lee G. R., 1093 Lee S., 991 Lee S-I., 991 Leece C. F., 393 Lefebvre F., 125 Le Goff P., 133, 875 le Lirzin A., 319, 1047 Leskela M., 1239, 1409 LetouzC F., 1353 Le van Mao R., Lewis A. L., 729 Lewis J., 1227 Li J., 413 Li R, 773 Li X., 657 Lightfoot P., 167 Lim G. S., 1441 Linda11 C. M., 657 Lindback T., 413 Lindgren M., 223 Lindqvist O., 1101 Little F. J., 167 605, 1143 Meakin P., 1149 Mellen R. S., 421 Mendonqa M. H., 515 Merkelbach P., 615 Metcalfe K., 331 Michel C., 1353 Miles D. A., 1205 Miller J. D., 729 Miller J. R., 1201 Mills G. P., 13 Minelli G., 541 Min-Hua J., 1041 Mirtcheva E., 611 Mishima S., 853 Miura N., 631, 1259 Miyamae N., 955 Mizukami F., 585, 1131 Mohanty D. K., 623 Monk P. M. S., 1071 Montes M., 1491 Morales J., 1413 Moreau J.J. E., 987 Moretti G., 541 Morpurgo S., 197 Mouron P., 895 Mozhaev A. P., 1107 Mueller J., 623 Muller W. F., 895 Mun M-O., 991 Munn R. W., 849 Munro D. C., 1451 Pelizzi C., 713 Peluau S., 1353 Peng N., 1451 Pennington M., 13 Peraio A., 1313 Percec V., 719 Pereira-Ramos J-P., 133, Perez G., 959 Perez-Jimenez C., 145 Petrov K., 611 Pettiti I., 541 Philippot E., 1101 Pic0 C., 547 Picone P. J., 571 Pigois-Landureau E., 741 Porta P., 197, 541 Portier J., 1433 Pottgen R., 463 Povey I. M., 13 Poynter R. H., 1205 Predieri G., 361 Pressman H. A,, 501, 1313 Prosperi T., 943 Qi F., 1041 Qiu S., 735 Rahmat S., 1201 Raithby P. R., 1227 Ramsaran A., 605, 1143 Ramsden J. J., 1263 Ranasinghe M. G., 1359 875 Shamlian S. H., 81 SharmaV., 703 Shen D., 105 Shen P., 1289 Sheng E., 487, 683, 1189 Sheridan P., 161 Sherrington D.C., 229, 253 Sherwood J. N., 977 Shimizu A., 1475 Shimokawatoko T., 51 Shiomi D., 915 Shrota Y., 171, 599 Shoji H., 1131 Shukla A. K., 703 Silver J., 1201 Simmons J. M., 1205 Simon M., 305 Simpson G. S., 1508 Sinclair D. C., 445 Singh N., 509 Slade R. C. T., 265, 367, 501, 509, 1313 Slater P. R., 1463, 1469 Smart S. P., 35 Smith E. G., 331 Smith J. M., 337 Smith M. E., 245 SnCtivy D., 55 Soininen P., 1409 Solano Reynoso V. C., 529 Solzi M, 361 Tirado J. L., 1413 Toba M., 585, 1131 Tomellini M., 579 Tomkinson J., 1309 Tomlinson A. A. G., 943, Tondello E., 407 Torres-Martinez L. M., 5 Toyne K. J.. 747 Trigg M. B., 245 Trotter J., 1201 Tschierske C., 1021 Tseung A. C. C., 1289 Tsuchida T., 631, 1503 Ueda M., 883 Ueda Y., 915, 1219 Ulibarri M.-A., 11 17 Underhill A.E., 1233 Ungar G., 719 Urbana M. R., 311 Uzunova E,., 153 Vaillant M , 765 van Aken P. A., 895 van der Put P. J., 689 Van Grieken R., 499 Vancso G. J., 55 Veiga M. la., 547 Veringa H. J., 689 Viana B., 907 Vidgeon E A., 399 Vivien D., 907 959 Liu C-W., 393 Liu S., 379 Liu-Cai F. X., 125 Lo Jacono M., 197 Murray K. S., 87 Nagae S-i., 591 Nakajima H., 1325 Nakajima T., 853 Ranlsv J., 867 Ratcliffe P. J., 1055 Raveau B., 1353 Raynor J. B., 13 Song S-W., 1271 Sotani N., 205, 775 Spagna A,, 437 Sprik M., 793 Volpe P., 1067 Vuijk J. 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G., 995 Ross A., 119 Rossignol S., 1433 Rothlisberger U., 793 Rourke J. P., 1017 Rowatt B., 253 TagaT., 291 Tagaya H., 551 Tajbakhsh A. R., 1017 Takahashi M., 519 Takahashi S., 859 Takanishi Y., 997 Wolf M., 839 Wong Chi Man M., 987 Wong K. K. W., 1387 Workman A. D., 13, 1337 Wright S. A,, 1365 WuY. M.. 1403 Mann S., 1387 Manning R. J., 1233 Marcos M. D., 475 Nygren M., 615, 1275 Nykanen E., 1409 O’Brien P., 565, 1249 Rowley A. T., 285 Rozikre J., 189 Ruiz P., 903 Takano M., 19 Takatoh K., 1365 Takebe Y., 599 Xiao F-S..735 Xiao S., 605, 1143 Xu R., 735, 985 Marder T. B., 1227 Marinas J. M., 311 Marks G., 399 Marks T. J., 1061 Marsden J. R., 1017 Martin C., 1353 Martin T. L., 623 Maruyama Y., 1377 Mather G. C., 1303 Mathieson I, 1157 Matsuba T., 599 Matsubayashi G-e., 1325 Matsuda H., 51 Matsuda T., 955, 1497 Matsumoto M., 1377 Matsuzaki I., 853 Maury F., 695 Maza-Rodriguez J., 179 McCabe R. W., 1173 McCarrick M., 217 McGhee L., 29, 119 McKeown N. B., 1153 McMeekin S. 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I., 761 1249 1123 1479 647, 1077 TakedaY., 19 Takeuchi M., 955 Takeuchi Y., 1497 Takezoe H., 237,997 Tamaki J., 1259 Tamura M., 915, 1219 Tan M.P., 525 Tanabe K., 853 Tanaka T., 859 Tanguy B., 1433 Tarasconi P., 713 Tetley L., 253 Thanapprapasr K., 591 Thanh Vu N., 1143 Thatcher J. H., 591 Thebault J., 669 Thepot P., 987 ThCry J., 907 Thomas H. J., 541 Thomas J. O., 839 Thomas M. J. K., 399 Thomson J. B., 167 Thorne A. J., 209 Thorne J. R. G., 1157, 1357 Tian M., 327 Xu W., 135 Xu Y., 985 Yakhmi J. V., 1077 Yamaguchi A., 1377 Yamamoto H., 635 Yamamoto I., 61, 533, 537 Yamamoto O., 19 Yamazoe N., 631, 1259 Yang H., 55 Yao J., 605 Yarovoy Y. K., 761 Yogo T., 353 Yokoyama A., 983 Yoon Y., 719 Yoshizawa A., 449, 983 Yu H., 327 Yue Y., 985 Zammit M. D., 1359 Zarbin A. J. G., 389 Zhang W-r., 161 Zhao L., 623 Zheng Q.. 1041 Zhuang Z., 1041 Ziemelis M. J., 1181 Zotov N., 611 11 Conference Diary 1994 Denotes a new or amended entry ths month September 10-14 11th European Conference on Biomaterials Pisa, Italy Secretariat 11th ESB Conference on Biomaterials, Dipartimento di Ingegneria Chimica, Univ.di Pisa, via Diotisalvi, 2-56126 Pisa, Italy. Tel: +39 50 511 277/287; Fax: +39 50 511 266 September 11-14 Ceramic Processing Science and Technology Friedrichshafen (Bodensee), Germany Deutsche Keramische Gesellschaft e.V., Frankfurter Strasse 196, D-51147 Koln, Germany Tel: +49 2203 69069; Fax: +49 2203 69301 September 11-14 1 lth European Conference on Biomaterials Pisa, Italy Professor Paolo Giusti, 11th European Conference on Biomaterials, Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali, Via Diotisalvi, 2-56126 Pisa, Italy September 11-17 1st Euroconference on Solid State Ionics Ionian Sea, Greece Professor Dr.W. Weppner, Christian Albrechts University, Chair for Sensors and Solid State Ionics, Kaiserstrasse 2, D-24098 Kiel, Germany September 18-22 21st International Symposium on Compound Semiconductors San Diego, CA, USA James P. Harbison, Bellcore, NVC 32-175, 331 Newman Springs Road, Red Bank, NJ 07701-5699, USA Tel: +1 908 758 3386; Fax:+1 908 758 4372 September 19-21 8th UK Conference on High-Tc Superconductors Birmingham, UK Dr. C. Greaves, School of Chemistry, University of Birmingham, Birmingham, UK, B15 2TT Tel: +44 121 414 4397; Fax: +44 121 414 4403 September 25-30 ICMEBC: International Conference on Molecular Electronics and Biocomputing Goa, India Dr.Ratna S. Phadke, Scientific Secretary, ICMEBC '94, Chemical Physics Group, Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400 005, India. Tel: +91 22 215 2971; Fax: +91 22 215 2110/2181 September 28-30 Fluorine in Coatings Salford, UK Mrs. Avril Henn, Conference Secretary, Paint Research Association, Waldegrave Road, Teddington, Middlesex, UK, TW11 8LD. Tel: +44 181977 4427; Fax: +44 181 943 4705 September 29 Grouts and Grouting London, UK SCI Conference Secretariat, 14/15 Belgrave Square, London, UK, SWlX 8PS Tel: +44 171 235 3681; Fax: +44 171 823 1698 October 2-6 66th Annual Meeting of the Society of Rheology Philadelphia, PA, USA Professor Norman Wagner, Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA Tel: +1302 831 8079; Fax: +1 302 831 10 October 3-7 First European Solid Oxide Fuel Cell Forum Lucerne, Switzerland European SOFC Forum, Secretariat, P.O.Box 1929, CH-5401 Baden, Switzerland Fax: +41 56 218466 October 10-12 3rd International Symposium on Structural and Functional Gradient Materials Lausanne, Switzerland FGM '94, Swiss Federal Institute of Technology of Lausanne, Materials Department, LMM, CH-1015 Lausanne, Switzerland. Tel: +41 21 693 29 15/50; Fax: +41 21 693 46 64 October 10-13 1994 International Display Research Conference and Materials Workshop Monterey, USA SID do Palisades Institute for Research Services, 201 Varick Street, New York, NY 10014, USA October 11-13 10th Optical Fibre Sensors Conference Glasgow, UK Mrs.Aileen Mitchell, University of Strathclyde, Royal College Building, Glasgow, LTK, G1 1XW Tel: +44 141 552 4400 ext.2543 October 1621 Molecule-Based Magnets Salt Lake City, UT,USA Joel S. Miller, Department of Chemistry, University of Utah, Henry Eyring Bldg 2124, Salt Lake City, UT 84112, USA Tel: +I 801 585 5455; Fax: +1801 581 8433; E-mail (preferred) JSMILLER@chemistry.chem.utah.edu ... 111 October 17-19 1 st International Conference on Materials For Microelectronics Barcelona, Spain Ms M. Peacock, Conference Department (C421), The Institute of Materials, 1Carlton House Terrace, London, UK, SWlY 5DB Tel: +44 171 839 4071; Fax: +44 171 823 1638 October 21-22 Displays Seminar Zakopane, Poland Professor Dr.Josef Zmija, Instytut Fizyki Technicznej ul Kaliskiego 2, 00908 Warzawa, Poland Fax: +48 22 362254 October 24-25 International Polypropylene Conference London, UK Ms M. Peacock, Conference Department ((34461, The Institute of Materials, 1Carlton House Terrace, London, UK, SWlY 5DB. Tel: +44 171 839 4071; Fax: +44 171 823 1638 October 24-27 11th Conference on Solid and Liquid Crystals: Materials Science and Applications Zakopane, Poland Professor Dr. Josef Zmija, Instytut Fizyki Technicznej ul Kaliskiego 2, 00908 Warsawa, Poland Fax: +48 22 3622254 October 24-28 Fourth Post-Doctoral Course on Degradation and Stabilization of Polymeric Materials Clermont-Ferrand, France Professor J.Lemaire, Laboratoire de Photochimie, URA CNRS 433, Universite Blaise Pascal, 63 177 Aubiere Cedex, France October 25-27 Electronic Information Displays Exhibition and SID Conference Sandown, UK Association Exhibitions, 6 Foldyard Close, Walmley, Sutton Coldfield, West Midlands, UK, B76 8QZ 0 October 26-28 International Symposium onGuided-Wave Optoelectronics; Device Characterization, Analysis and Design Brooklyn, NY,USA IEEE/LEOS, 445 Hoes Lane, P.O. Box 1331, Piscataway, NJ 08855-1331, USA Tel: +1 908 562 3893; Fax: +1 908 562 8434 October 27 The Fundamental Properties of Bituminous Materials London, UK SCI Conference Secretariat, 14/15 Belgrave Square, London, UK, SWlX 8PS Tel: +44 171 235 3681; Fax: +44 171 823 1698 0 November 3-8 ILOPE '94: The 2nd International Laser and Optoelectronic Products Exhibition Beijing, China CIFC Exhibition Company (HK) Ltd.39/F, China Reources Building, 26 Harbour Road, Hong Kong Tel: +852 827 5078; Fax: +852 834 5535 November 10 Calorimetry and Thermal Methods Applied to Construction Materials London, UK SCI Conference Secretariat, 14/15 Belgrave Square, London, UK, SWlX 8PS Tel: +44 171 235 3681; Fax: +44 171 823 1698 November 10-12 Second Annual Meeting on Biomaterials of GIB (Interdivisional Group on Biomaterials of the Italian Chemical Society) Italy Ledi Menabue, Executive Committee of GIB, Universita Degli Studi di Modena, Dipartimento di Chimica, via Campi, 183-41100 Modena, Italy. Tel: +39 59 378402; Fax: +39 59 373543 November 14-19 IRaP94: Ionizing Radiation and Polymers Guadeloupe, France Ing. Alai? LeMoel, SRSIMLPI, CEA CEN Saclay, BBtiment 466, F-91191 Gif Sur Yvette C&dex, France Tel: +33 16908 5485; Fax: +33 16908 9600 November 15-18 2nd Color Imaging Conference: Color Science, Systems and Applications Scottsdale, AZ,USA Pam Forness, The Society for Imaging Science and Technology, 7003 Kilworth Lane, Springfield, VA 22151, USA Tel: +1 703 642 9090; Fax: +1 703 642 9094 0 November 29-1994 MRS Fall Meeting December 1 Boston, MA, USA Mary E.Kaufold, Materials Research Society, 9800 McKnight Road, Suite 327, Pittsburgh, PA 15237, USA Tel: +1 412 367 3036; Fax: +I 412 367 4373 December 1 Waste Immobilisation London, UK SCI Conference Secretariat, 14/15 Belgrave Square, London, UK, SWlX 8PS Tel: +44 171 235 3681; Fax: +44 171 823 1698 December 8-9 European Workshop on Chemical Sensors for Metallurgical Processes Mol, Belgium F.De Schutter, Energy Division, VITO, Boeretang 200, B-2400 Mol, Belgium Fax: +32 14 32 1185 December 15-16 C-MRS and E-MRS Joint Symposium on Electronic and Optoelectronic Materials Beijing, China Professor Yang Xiongfeng, Secretary General of the Symposium, Institute of Semiconductors, Chmese Academy of Sciences, P.O. Box 912, Beijing 100083, China. Tel: +86 01 255 8131 ext. 321; Fax: +86 1256 2389 iv December 19-22 1994 International Conference on Electronic Materials (ICEM'94) 8i 1994 IulMRsInternational Conference in Asia (IUMRS-ICA) Hsinchu, Taiwan C/o Materials Research Laboratories, ITRI, Conference Department, IUMRS-ICEM/ICA'94, Bldg 77,195 Chung-king Rd,Sec.4, Chutung, Hsinchu, 3105, Taiwan, ROC. Tel: +886 35 820064/916801; Fax: +886 35 8202471820262 Conference Diary 1995 January 5-6 Plastics for Portable Electronics Las Vegas, NV, USA Dr. hoop Agrawal, Donneuy Corporation, 4545 E. 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Fogarassy, CNRS, Laboratoire Phase, BP 20, 67037 Strasbourg Cedex 2, France Tel: +33 88 10 62 57; Fax:+33 88 10 62 93 March 13-15 Low- and No-VOC Coating Technologies: 2nd Biennial International Conference Durham, NC, USA Ms. Coleen M.Northeim, Research Triangle Institute, P.O. Box 12194, Research Triangle Park, NC 27709-2194. USA Tel: +1 919 541 5816; Fax: +1 919 541 7155 April 2-6 Seventh Biennial Workshop on Organometallic Vapor Phase Epitaxy Fort Meyers, FL, USA TMS Meeting Services Department, 420 Commonwealth Drive, Warrendale, PA 15086, USA Tel: +1 412 776 9000 ext. 241; Fax: +1 412 776 3770; E-mail: wilson@tms.org April 3-6 ECIO '95: 7th European Conference on Integrated Optics Delft, The Netherlands ECI0'95 Secretariat, P.O. 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Kennedy, F-68200 Mulhouse, France Tel: +33 89 42 01 55; Fax: +33 89 32 09 96 September 11-15 Electrochem '95 Bangor, Wales Dr. Maher Kalaji, Department of Chemistry, University of Wales, Bangor, Gwynedd, Wales, UK, LL57 2UW Tel: +44 1248 351151 ext.2516; Fax: +44 1248 370528 0 September 11-16 LB7: The Seventh International Conference on Organized Molecular Films Numana, Ancona, Italy Dr. M. G. Ponzi Bossi, Scientific Secretary LB7, Istituto di Scienze Fisiche, Facolta' di Medicina e Ancona, via Ranieri 65, 60131 Ancona, Italy. Tel: +39 71 220 4606; Fax: +39 71 220 4605; E-mail: fismed@anfisi.cineca.it September 13-15 EuroOMet '95 incorporating the 29th Metallographie-Tagung Friedrichshafen, Germany Deutsche Gesellschafi fur Materialkunde e.v., Adenauerallee 21, D-61440 Oberursel, Germany Tel: +49 6171 4081; Fax: +49 6171 52554 0 September 17-21 2-ICPEPA 2nd International Conference on Photo-Excited Processes and Their Applications Jerusalem, Israel Organizing Committee: ICPEPA-2, Professor A.Peled, Chairman, CTEH aff. Tau, 52 Golomb St., Holon 58102, Israel Tel: +972 3 502 8902; Fax: +972 3 502 8967; E-mail: photoexe@ilctehol.bitnet September 24-28 6th International Topical Meeting on Optics of Liquid Crystals Le Touquet, France Dr. M. Warenghem, Universitd des Sciences et Technologies de Lille, UFR de Physique, Batiment P5,59655 Villeneuve D'Ascq Cedex, France 0 September 25-29 Workshop on Non-Equilibrium Phenomena in Supercooled Fluids, Glasses and Amorphous Materials Pisa, Italy Dr. Din0 Leporini, Dipartimento di Fisica, Universita di Pisa, Piazza Torricelli, 2 1-56100 Pisa, Italy Tel: +39 50 911284; Fax: +39 50 48277; E-mail: leporini@ipifidpt.difi.unipi.it 0 September 25-29 OLC '95: VIth International Topical Meeting on Optics of Liquid Crystals Le Touquet, France Prof.M. Warenghem, OLC '95, C.R.U.A.L., Facultd Jean Perrin, Rue J. Souvraz, S.P. 18, F 62307 Lens Cedex. France Tel: +33 20 43 48 12; Fax: +33 20 43 40 84; E-mail: warenghem@lip5nx.decnet.citilille.fr October 18-20 MOC '95: 5th Microoptics Conference Hiroshima, Japan K. Iga, Representative of the Microoptics Group, Optical Society of Japan (JSAP), Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama, Japan 227. Tel: +8145 922 1111ext.2064; Fax: +8145 821 0898 December 10th International Conference on Solid State Ionics Singapore B. V. R. Chowdari, Department of Physics, National University of Singapore, Singapore-0511 Tel: +65 772 2956; Fax: +65 777 6126 Conference Diary 1996 0 June 24-28 ILCC: 16th International Liquid Crystal Conference Kent, OH, USA 16th International Liquid Crystal Conference, Liquid Crystal Institute, Kent State University, P.O. Box 5190, Kent, OH 44242-0001, USA. Tel: +1 216 672 2654; Fax: +1 216 672 2796; E-mail: ILCC16@alice.kent.edu 0 August4-9 IUPAC MACRO SEOUL '96: 36th IUPAC International Symposium on Macromolecules Seoul, Korea Dr. Kwang Ung Kim, Secretariat of IUPAC MACRO SEOUL '96, Division of Polymers, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea Tel: +82 2 957 6104; Fax: +82 2 957 6105; E-mail: iupac@kistmail.kist.re.kr Entries in the Conference Diary are published free of charge. 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ISSN:0959-9428    

DOI:10.1039/JM99404BP081

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(9), 1359-1363 Synthesis of High Refractive Index Acrylic Copolymers Thomas P. Davis,*a Michael J. Gallagher,b Millagahamada G. Ranasingheb and Michael D. Zammit? a School of Chemical Engineering and Industrial Chemistry and bSchool of Chemistry, The University of New South Wales, P.O. Box 7, Kensington, New South Wales 2033, Australia A series of brominated carbazole-containing monomers with a polymerizable acrylic or methacrylic functionality were synthesized and characterized. These monomers were polymerized thermally and characterized by differential scanning calorimetry (DSC) and refractive index via Brewster's angle. The monomer 4-(1,3,6,8-tetrabrom0-9-carbazolyl)-l -butyl acrylate (4b) was copolymerized with methyl methacrylate (MJ yielding the reactivity ratios r, =1.08 and r, =0.93; these values were used in the Skeist procedure to predict compositional drift with conversion. The Alfrey-Price Q and e values for (4b) were 0.86 and 0.46,respectively. The copolymers were all colourless and transparent, despite the absence of an azeotrope for this comonomer pair, and their refractive indices were found to lie between those of the constituent homopolymers.Copolymers rich in methyl methacrylate (MMA) were found to be soluble, yet those rich in 4b were found to be intractable. The attainment of high refractive index (nD)in polymeric materials has been an important goal for several years. One commercial application is that of spectacle lens manufacture where a high refractive index allows the design of lighter weight products.The preferred monomer for many years has been ally1 diglycol carbonate (ADC) with a homopolymer refractive index of 1.51, although alternative materials are currently being developed. Other applications include wave- guides, optical fibres and adhesives for optical components where refractive-index matching is of critical importance. Poly(N-vinyl carbazole) (PVK) has a very high refractive index of 1.68 and also possesses excellent thermal stability and useful mechanical properties. However, it is extremely brittle and is relatively intractable.' The aim of the work reported here was to enhance the tractability of PVK whilst retaining the large refractive index value that the carbazole moiety induces.A study by Minns and Gaudiana2 suggested that this might be achieved by maintaining the carbazole chromophore in the side-chain but at a greater distance from the polymer backbone. The insertion of a straight-chain alkylene group between the nitrogen atom and the polymerizable group was proposed to achieve this end. The inclusion of a spacer chain lowers the refractive index, so this is compensated by halogenating the carbazole rings, with bromination or iodination expected to yield the greatest effect. Homopolymers of these monomers were prepared by Minns and Gaudiana' and found to be fairly intractable and insoluble except in 1-chloro-, 1-bromo- and 1-methyl-naphthalene. One of the goals of this work was to improve the tractability of the materials by copolymerization and to provide a range of different copolymers with different refractive indices which would enable the refractive index matching for adhesive applications.The large refractive index between the co-monomers indicated that the ideal situation would be to locate the azeotrope composition for copolymerization, thereby enabling the production of homogeneous and there- fore transparent copolymers. Experimental and Results 300 MHz H1 NMR spectra were recorded on a Bruker AC-300F spectrometer at 300 K, and are reported in ppm from internal tetramethylsilane. Data are reported as follows: chemical shift [multiplicity (s, singlet; d, doublet; q, quartet; qu, quintet; m, multiplet), coupling constant in Hz, integra- tion, interpretation].All solid-state C13 NMR spectra were recorded on a Bruker MSL300 spectrometer at 300 K and a rotational speed of 4-9 kHz in 4 mm zirconia rotors. Thin-layer chromatography (TLC) was carried out on Kieselgel 60 F-254 precoated silica-gel TLC plates obtained from Merck. The plates were run on the eluting solvents stated. All plates were detected by observation under UV light. All DSC thermograms were performed on a DuPont 910 DSC with nitrogen flow rate of 40ml min-I. All samples were performed at a heating rate of 20 "C rnin-l from 20 to 400 "C with a sample size of 10mg. The only distinctive feature of the DSC thermograms was the glass-transition temperature, Tg,which was taken as the temperature at which the midpoint of the heat capacity change at transition was achieved.All reaction temperatures refer to those of the reaction mixture. Reactions requiring an inert atmosphere were carried out under a blanket of argon with a positive pressure. Monomer Synthesis The synthetic methods used were based on those reported by Minns and Gaudiana.2 As several modifications were clevel- oped, the full experimental details are reported. Preparation of 1,3,6,8-Tetrabromocarbazole(2) 1,3,6,8-Tetrabromocarbazolewas prepared from carbazole 1 by the method of Pielich~wski.~ Into a 11 round-bottamed flask were placed carbazole (12 g, 72 mmol) and glacial acetic acid (150ml). The flask was fitted with a condenser and topped with a calcium chloride drying tube leading to a water trap to remove evolved HBr.The mixture was magnetically stirred at room temperature and a solution of bromine (16 ml, 0.31 mol) in acetic acid (200 ml) was added dropwise. The mixture was then heated in an oil bath at 95 "C for 21 h. The reaction was monitored by TLC using a mixture of hexane and ethyl acetate in a ratio of 5 : 1 as the developing solvent. The Rf values for 1 and 2 were 0.51 and 0.71, respectively. After the suspension had been cooled to ambient temperature it was filtered, recrystallized from toluene (ca. 400 ml) and dried to yield 2 (26.6 g, 76% yield); mp 233-235 "C, (lit.2 mp 233-235 "C). 'H NMR (CDC13) 6 7.75 (s, 2H, H2, H7), 8.05 (s, 2H, H4, H'), 11.51 (s, lH, NH). Preparation of 9-(4-Bromobutyl)-1,3,6,8-tetrabromocarbazole (3)The following reagents were placed in a reaction flask fitted with a condenser; product 2 (10 g, 41 mmol), powdered anhy- drous potassium carbonate (16 g, 0.12 mol), anhydrous aceto- nitrile (200 ml) and 1,4-dibromobutane (50 ml, 0.83 mol).The reaction vessel was purged with argon and this atmosphere was maintained throughout the reaction. The reaction mixture was stirred and heated at reflux in an oil bath at 95°C for 19 h. The reaction was monitored as before by a 7 :1 mixture of hexane and ethyl acetate. Rf values for 2 and 3 were 0.49 and 0.66, respectively. Acetonitrile was removed by evaporation and the remaining yellow suspension was subjected to liquid-liquid extraction using dichloromethane and distilled water. The organic com- ponent was dried over sodium sulfate, filtered and concen- trated.The product was precipitated by the addition of methanol, filtered and dried in a desiccator to yield 3 (10.6 g, yield 82%), mp 142-144 "C, (lit.2 mp 142-144 "C). 'H NMR (CDC1,) 6: 1.93 (m, 4H), 3.41 (t, J 7, 2H, CH,Br), 5.12 (t, J 7, 2H, CH,N), 7.76 (d, J 2, 2H), 8.00 (d, J 2, 2H). Preparation of4-( 1,3,6,8-Tetrabromo-9-carbazolyl)-1-butyl Methacrylate (4a) A solution of tetrabutylammonium methacrylate was prepared in a 100ml round-bottomed flask by the addition of meth- acrylic acid (0.6 ml, 7.2 mmol), methanol (60 ml) and 7.2 ml of a 1 moll-' solution of tetrabutylammonium hydroxide in methanol.The pH of the solution was then adjusted by the addition of the ammonium salt until basic conditions pre- vailed. The translucent solution was then acidified by the addition of a few drops of the acid. The solvents were then removed and acetonitrile (30ml) was then added and removed by rotary evaporation. The resulting oil was then dissolved in acetonitrile (60ml), and a solution of 3 (4.1 g, 6.6 mmol) in warm toluene (60ml) was added. The solution was stirred under an argon atmosphere at 45°C for 12 h. TLC was used to monitor this reaction, with the eluting solvent being a 1:1 dichloromethane-hexane mixture. Rfvalues for 3 and 4a were 0.82 and 0.43, respectively. The solvents were removed and the residue was extracted with ether (l00ml) and water (200ml).The organic phase 7 -w& 1 Br 2 8r I mC(CH3)CH2I Bf I * (cp)4Br Br@q*Br Bf (Cy4 Br Br@Q Br 4a 3 OCOCHCHz /I Br Br 4b Scheme 1 Preparation of monomers 4a and 4b J. MATER. C'HEM., 1994, VOL. 4 was then dried with anhydrous sodium sulfate and the solid product was isolated from the ether. Further purification was achieved by reprecipitating the solid from dichloromethane solution by the addition of an excess of methanol. The white crystals were dried to yield 4a (3.15 g, 76%) mp 152-153 "C. (lit.2 mp 152-153 "C). 'H NMR (CDCl,) 6: 1.75 (m, 4H), 1.95 (m, H, CH,), 4.15 (t, J 6 H, 2H, CH20), 5.15 (t, J 8, 2H, CH,N), 5.55 (d, J 2, lH, vinyl), 6.05 (d, J 2, lH, vinyl) 7.8 (d, J 2, 2H), 8.05 (d, J 2, 2H).Preparation of 4-( 1,3,6,8-Tetrabromo-9-carhazolyl)-l-but~l Acrylate (4b) This monomer was prepared by a procedure analogous to that described in the preparation of monomer 4a. The reaction was monitored by TLC with the eluting solvent being a 1:1 dichloromethane-hexane mixture. The R, values for 3 and 4b were 0.82 and 0.39, respectively. Yield 85%, mp 130-132 "C. 'H NMR (CDCl,) 6: 1.25 (t, J 4, lH, -HC=), 1.75 (m, 4H), 4.2 (t, J 4, 2H, CHZO), 5.15 (t, J 8, 2H, CH,N), 5.8 (d, J 8, lH, vinyl), 6.1 (d, J 8, lH, vinyl), 6.4 (d, J 16.5, lH, vinyl), 7.8 (s, 2H), 8.05 (s, 2H). Homo polymerization* The homopolymers were prepared from their monomers via thermal polymerization in the absence of any initiator. The reaction was effected under reduced pressure at a temperature sufficient to melt the monomers, above 160"C, for 1h.The resulting polymers were dissolved in 1-chloronapthalene (15 ml for 1 g polymer) at 100"C. After dissolution had occurred, the solution was further diluted by the addition of toluene (8 ml). The polymer was then isolated from the solution by precipitation in cold dichloromethane. The product was washed with dichloromethane and dried to yield conversions in excess of 75%. Copolymerization Copolymer systems were prepared that encompassed the entire feed ratio system between monomers MMA and 4b. Two situations arose, that of an intractable copolymer in the high 4b feed ratio set, and soluble copolymers in the high MMA feed ratio set.Precipitation Copolymerization A mixture of monomer 4b (l.Og, 0.16mmol), MMA (0.16 mmol), toluene and lauroyl peroxide (0.0025 g) was purged with argon and heated to 70°C. The reaction was allowed to run for 4 h. During this time the solid polymer precipitated from the solution. This was isolated, dried and weighed to yield a conversion of 10%. Solid-state NMR confirmed that the precipitate was a copolymer. Solution Copolymerization Homogenous copolymerization was successfully attempted by maintaining high feed ratios of MMA. The same procedure as detailed above was followed, and no precipitation occurred. The copolymer was precipitated from the reaction mixture using diethyl ether. The composition of the copolymers was determined by 'H NMR.The feed compositions, copolymer compositions and yields are given in Table 1. Also see Fig. 1 for the 'H NMR spectrum of the soluble copolymer (5 :95% feed ratio 4b :MMA) and Fig. 2 for the DSC thermogram. J. MATER. CHEM., 1994, VOL. 4 Table 1 Table of feed composition, copolymer composition, (determined by 'H NMR) and yield; note: (1 ~4b) fi (feed comp.) F, (copoly. comp.) yield (YO) 4.87 3.8 6.17 4.98 3.09 13.8 9.49 5.93 10.4 10 4.78 5.04 10.36 4.67 4.90 13.97 7.74 4.5 14.8 8.17 4.37 15.15 8.77 15 20 11.47 9.4 20.4 13.7 3.7 20.48 8.47 3.48 20.73 11.26 13.7 24.86 13.5 5.0 Refractive Index Measurement The refractive indices of the carbazole homopolymers were beyond the measurement range of the Abbk refractometer.Consequently a Brewster's angle technique was employed. This procedure utilises a vertically polarised He-Ne laser (1= 633 nm). Transparent polymer films were prepared by a hot-melt procedure and placed in the laser beam. By starting with the sample reflecting the laser light back onto the source, then rotating the sample, on a rotatable angle table, and observing when the reflected light passes through a minimum of intensity, the Brewster angle can be read. The refractive index (nD) is then the tangent of that angle. iI 1 --LA 1 0.8~ 227.63"C -110.11 "C 2 Om4'3 g 0.2 CI Qa,x 0.0. -0.2' 304.25 'C -0.4$0 iio iio zio 260 3io-TIOC Fig. 2 DSC thermogram of copolymer from feed ratio 5 :95%, 4b :MMA.Tg= 110"C and showing the two thermal degradation characteristics of the copolymer at 227.6 and 304.2 "C Discussion Homopolymerization The primary difficulty encountered in this work m'as the intractability of the polymers. The carbazole-con taining homopolymers are soluble in the reported solvents, with no additional solvents being found. Unfortunately the corre-sponding carbazole-containing monomers were fount1 to be -7I -3 8.08.0 7.07.0 6.06.0 5.05.0 4.04.0 3.03.0 2.02.0 11.o.o Fig. 1 'H NMR of copolymer from feed ratio 5 :95%, 4b :MMA only sparingly soluble in the naphthalenes; in fact no common solvent could be found for both the monomers and polymers. Copolymerization The reactivity ratios were determined for the copolymerization of 4b (M,) with MMA (M2) using the Fineman-Ross4 and Kelen-Tudos' procedures.An attempt was also made at using the non-linear error-in-all-variables (EVM) approach,6 but this would not converge. This may well be because the data were collected over a limited range because of the difficulty in maintaining a homogenous polymerisation. The average reactivity ratio values obtained were rl = 1.08& 0.005 and r2= 0.93& 0.004. Thus there is no azeotrope for this ideal copolym- erisation. The introduction of the alkyl spacer group removed any reactivity the halogenated carbazole moiety would have induced in the reacting acrylate unit. Thus we see that the reactivity ratios are virtually the same with a slight preference for the synthesized monomer.The acrylate functionality of the synthesized monomer has significantly altered the reactiv- ity of the carbazole compared with N-vinyl carbazole. Also the Alfrey-Price7 Q and e values for 4b are 0.86 and 0.46, respectively. These reactivity ratio values were used with the integrated form of the Skeist equation to predict compositional drift.' Here one calculates C (conversion) when fl changes during this conversion from a value fli to a value flj. The most convenient form of this equation is the integrated form given by Meyer and Lowry.' where a=r2/(1-r2); /3 =rl/( 1-rl); y =(1-rlr2)/[( 1-rl) x ( 1-4; 6 =( 1-r2)/(2 -r1 -r2), A plot of conversion us. comonomer feed composition is shown in Fig.3. Despite the absence of an azeotrope it is clear that compositional drift is minimal across the composi- tional range. The only significant drift occurs at conversions >95%. The compositions of the copolymers were deduced from NMR data. The MMA monomer '-0-CH,' peak (3.6 ppm) was used, along with the aromatic peaks (7.7-8.2ppm), for the synthesized monomer 4b. All the copolymers, from both the precipitation copoly- merization and the homogenous copolymerization yielded colourless transparent films. This is despite the large refractive index differences between the constituent comonomers. This can be attributed to the production of virtually homogenous -J 0.2 0.4 0.6 0.8 1.o f, (feed composition) Fig.3 Comonomer feed composition uersus conversion J. MATER. CHEM., 1994, VOL. 4 copolymers (with respect to composition), as indicated by the compositional drift equation. It is thus possible to synthesize a range of transparent copolymers for refractive index matching applications. The Tgsand refractive indices of the copolymers are given in Table 2. There is not a simple linear relationship between the feed composition and a weighted average of the two homopolymer refractive indices. One possible explanation for this is that there may be a strong solvent effect on the copolymerization, so that the two different copolymerization approaches may have yielded significantly different compositions. There is strong evidence for this from work by Ledwith et al.," who found a 'bootstrap'-type effect for the copolymerisation of NVK with MMA.This offers the possibility of manipulating the copolymer composition (and therefore the refractive index of the copolymers) not by just controlling the monomer feed ratio but also by manipulating medium effects. The goal of improving the tractability of the carbazole by copolymerization was not generally attained. In fact the copolymers with a high carbazole content were more intractable than the homocarbazole polymer. The solubility parameter of a 50 :50 mol% copolymer was calculated to be 10.9 (cal cm3)0.' mol-'. However, no solvents could be found which would dissolve this copolymer, including chloronaph- thalene. The paper from which the monomer synthesis was derived2 also provided a method for the heptahalogenation of the carbazole unit.The heptabrominated monomers were synthesized, but no copolymerization was performed. The increase in bromination had an increase in refractive index for the homopolymer, and it is expected that copolymers would display the same reactivity as the monomer reported here, with an according increase in Tgand refractive index. Finally, there are only three transitions that occur on the DSC traces. The first transition is the glass-transition tempera- ture, which is shown in Fig. 2 to be 110.1 "C.The other two transitions on this thermogram can be explained by attributing those peaks to the thermal degradation of acrylate materials." Those methyl methacrylate oligomers containing unsaturated end groups degrade at ca.255 "C (227.6 "C), whilst acrylate chains containing saturated terminal units degrade at tempera- tures in excess of 300 "C (304.2 "C). The DSC thermograms indicate that the copolymers are amorphous and exhibit the degradation features of PMMA. Conclusions (1) The bromination of the carbazole rings does- compensate for the potential refractive index reduction caused by intac- ing a spacer chain between the nitrogen atom and the polymerizable group. (2) Copolymers of monomer 4b with MMA formed from feed compositions 40-95% 4b were insoluble in all common solvents. Therefore copolymerization with MMA in high 4b feed did not improve the tractability of these carbazole polymers.However, copolymers rich in MMA were found to be soluble in many common solvents and hence commercially interesting. (3) All the copolymers of 4b with MMA were colourless Table 2 Glass-transition temperatures and refractive indices of soluble copolymers fi (copoly. comp. 4b) glass-transition temp. (TJC) n, 0.0309 110.71 1.502 0.0593 125.87 1.520 0.0877 135.78 1.540 0.1126 128.45 1.541 J. MATER. CHEM., 1994, VOL. 4 1363 and transparent with refractive indices between those of the 2 R. A. Minns and R. A. Gaudiana, J.M.S. Pure Appl. Chem, 1992, constituent homopolymers. A29, 19. J. Pielichowski and J. Kyziol, Monatsh. Chem., 1974, 105, 1306. M. Fineman and S. D. Ross, J. Polym. Sci., 1949,5259. T. Kelen and F. Tudos, J. Macromol. Sci. Chem., 1975, A9, 1.We thank Professor M. Gal and Mr. John Tann for their P. M. Rielly and H. Patino-Leal, Technometrics,1981,23 221. assistance with the refractive index determinations, and Mrs T. Alfrey Jr. and C. C. Price, J. Polym. Sci., 1946,2, 101. Hilda Stender, Mrs Than Vo Ngoc and Dr. Jim Hook for I. Skeist,J. Am. Chem. SOC.,1946,68, 1781. processing the NMR samples. V. E. Meyer and G. G. Lowry, J. Polym. Sci., Polym. Chern., 1965, 3,2843. 10 A. Ledwith, G. Galli, E. Chiellini and R. Solaro, Poljm. Bull., 1979, 1,491. 11 P. Cacioli, G. Moad, E. Rizzardo, A. K. Serclis andReferences D. H. Solomon, Polym. Bull., 1984,11,325. 1 D. Bailey, D. Tirrell and 0.Vogel, J. Macromol. Sci. Chem., 1978, A12, 661. Paper 4/011045; Received 23rd February, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401359

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(9), 1365-1375 Inversion of Chirality-dependent Properties in Helical Liquid Crystals: Effects of Structural Modificationt Peter Styring,* Jelle D. Vuijk, Sharron A. Wright, Kohki Takatoh and Chuchuan Dong Liquid Crystals and Advanced Organic Materials Research Group, School of Chemistry, The University of Hull, Hull, UK HU6 7RX The effects of structural modification on the inversion of chirality-dependent properties in the helical mesophases of a number of terphenyls have been investigated. The occurrence of inversion phenomena are very sensitive to small changes in the molecular structure and subtle changes can often produce profound effects. Structural modification of the non-chiral or chiral terminal chains has been shown to destabilize the inversion of chirality-dependent properties; however, a stable room temperature ferroelectric smectic C*phase has been prepared.The inclusion of lateral fluoro- substituents in the aromatic core causes a large reduction in the melting points of the terphenyl materials and the stabilization of inversion phenomena in the cholesteric phase. Changes in the fluoro-substitution pattern results in a change in the direction of the molecular dipole and hence a change in the sign of the spontaneous polarization. Helical mesophases and therefore also inversion phenomena are suppressed in the non-fluorinated material. The unusual liquid-crystalline phase behaviour of a number of materials that undergo inversions in chirality-dependent properties in their helical mesophases, and in unwound cells at infinite pitch, as a function of temperature has been reported previously.'-'' In particular, work has centred on the changes in the helical twist sense, spontaneous polarization (P,j and 1 apparent optical tilt angle (8,) in the chiral smectic C* (Sc*j phase and changes in the helical twist sense in the cholesteric CQH1QO-10 \/(N*) phase.\/ yIf only the helical twist sense and spontaneous polarization in the (S,* phase and the helical twist sense in the N* phase are considered, it is possible to predict a number of combi- 2 nations that might be expected to be observed (Table 1). In addition, when the polarization inverts it can be accompanied by an inversion of 8, in the (Sc* phase, as the tilt drives the spontaneous polarization.An inversion of helical twist sense in either phase is necessarily accompanied by an inversion in the pitch of the helix because, in order for the handedness to change, the pitch of the helix must first increase and then pass through a region of infinite pitch before a helix of opposite twist sense can reform. Several materials have been identified that exhibit inversion phenomena and these can be classified according to the types listed in Table 1. Compound 1 shows inversion phenomena Table 1 Possible combinations of properties that might show inversion 5 smectic C* cholesteric of type 1, i.e. inversion of helical twist sense in both phases accompanied by an inversion in the spontaneous polerization.' inversion of helical inversion of inversion of helical This is the only material known to display type 1 behaviour.tY Pe twist sense ps twist sense The chiral chloro-ester 2,7 derived from the amino acid (S)-1 J J J 2 J X J 3 X J J 4 X X J 5 J X X 6 X J X 7 J J X ~~~ t This paper was submitted in association with the 1st International Conference on Materials Chemistry, July, 1993.alanine, and the chiral fluoro-ester 3,' show inversions of the helical twist sense in the cholesteric phase only and are therefore classified as type 4 inversions. Compound 4, which has a core structure similar to that of 2, shows an inversion in the helical twist sense but no inversion in spontaneous polarization in the (Sc* phase with no inversion in the N* phase and is therefore classified as type 5.6 Finally, the biphenylcarboxylic acid ester of (Sj-2-methylbutm-1-01 (5) shows only an inversion of spontaneous polarization in the (Sc* phase without any helix inversions (type 6).?Although one compound is known (PACMB),’ which shows only a helix inversion in the Sc* phase (type 5), it is a diastereoisomer and contains two independent chiral centres, which are believed to compensate for each other.Thus, the competition between the chiral centres in the diastereoisomers produces the temperature dependent inversion in the helical pitch. In this work, we have concentrated on materials containing a chiral oxirane ring, which possesses two sequential chiral centres.However, these are constrained in a cyclic structure and therefore behave as a single chiral entity relative to the mesogenic core. These materials produce a number of chiral property combinations, however we do not yet have materials possessing a single chiral centre that show inversions of types 2, 3 and 7. We have explained the origins of these inversion phenomena in materials possessing single chiral centres in terms of interconversions between competing rotational conformers, which involve the chiral centre relative to the rigid mesogenic core of the mole~ule.~-l~ We have proposed that at least two competing rotational conformers exist that possess approxi- mately equal potential energies and that these conformers are separated by an energy barrier to rotation (AE), which is accessible over the temperature regime of the mesophase.Demus and co-workers” have also observed an inversion of spontaneous polarization in a number of chiral phenylpy- rimidine liquid crystals. This has been explained in terms of changes in the interaction between molecular steric dipoles and is based on Pikin and Osipov molecular statistical theory.’, We have recently undertaken an investigation of both our own conformer theory and the molecular statistical theory in relation to our materials in order to clarify the situation. In this paper we report the results of our studies into the effects of changes in the structure of (i) the chiral terminal group, (ii) the non-chiral terminal group and (iii) the meso- genic core on helical inversion phenomena.In order to achieve this we have taken the basic structure of 1 and made the appropriate modifications to the terphenyl core and terminal substituents while retaining the oxirane ring as the chiral centre as shown in Fig. 1. Experimental Analysis of Materials The structures of all intermediates and final products were elucidated by a variety of analytical techniques. Proton nuclear magnetic resonance (‘H NMR) spectra were recorded using a JEOL JNM-GX 270 FT NMR spectrometer. Infrared (IR) spectra were recorded as either KBr disks or liquid films using a Perkin-Elmer 783 IR spectrometer. Mass spectra were recorded using a Finnigan Matt 1020 Automate GC/MS (gas chromatography/mass spectrometry) spectrometer.Satis- factory analyses were achieved in all instances. The purities of the final products were determined by high-performance liquid chromatography (HPLC) in both normal and reversed modes. Normal phase analysis was performed over silica gel (5 pm pore size, 25 cm x 0.46 cm, Dynamax scout column) and reversed-phase analysis was performed over octadecylsi- loxane (5 pm pore size, 25 cm x 0.46 cm, ODS Microsorb Dynamax 18 column). Methanol was used as eluent in both cases. Optical rotations were measured using an AA-10 auto- Fig. 1 General structure of the materials under investigation J. MATER. CHEM., 1994, VOL. 4 matic polarimeter at the sodium D line.Satisfactory analyses were achieved in all instances. Elemental analyses were per- formed on a Carlo Erba 1106 CHN analyser using cyclohexanone-2,4-dinitrophenylhydrazoneas the reference standard. The mesomorphic phase sequence of each compound, sand- wiched between glass slides, was determined by thermal optical microscopy using a Zeiss Universal polarizing microscope equipped with either a Mettler FP82HT microfurnace and FP90 temperature controller or a Mettler FPX2 microfurnace and FP80 temperature controller. Optical results were confirmed by differcntial scanning calorimetry (DSC) using a Perkin-Elmer DSC‘7-PC equipped with an intracooler. Enthalpies of transition, in J g-l, are shown in parentheses below the transition temperatures.Samples were encapsulated in standard aluminium pans and the mesophase ranges scanned at rates of 10 and 2 K min-’. The accuracy of the data derived from the DSC experiments was confirmed by measuring the enthalpy of fusion and melting temperature of pure indium metal. The melting enthalpy of 29.8 J 8-l and melting temperature of 156.7 C compared well with the literature values of 28.5 J g-’ and 156.6“C, respectively. Electro-optic studies were performed on the materials con- tained in 0.25 cm2 ITO-coated active area test cells which were obtained from the Electronics Chemicals High Tech- nology Group. The inner surfaces of the cells were coated with a polyimide alignment layer that had been unidirection- ally buffed and assembled with parallel rubbing directions.The absence of interference fringes in the unfilled cell indicated a homogeneous cell thickness across the active area. Ac voltages were applied in sine-wave mode using an Advance Electronics AF signal generator J2C and dc voltages using a Farnell Instruments LT30-2 stabilized power supply. Applied voltages were determined accurately using a Beckman Industrial DM78 multimeter. When determining the magni- tude of the spontaneous polarization, the hysteresis loop was observed on a Dartron Instruments dual trace oscilloscope D17 and the P, determined using a Diamant bridge.’, Synthesis Six compounds of the general structure shown in Fig. 1 were prepared according to the procedures detailed in Scheme 1. 4-Bromophenylmethano1 was alkylated under standard con- ditions to give l-bromo-4-(propoxymethyl)benzene, 6 (R’= C,H7, R”=H; n=l), which was transformed into boronic acid 8 by treatment with n-butyllithium followed by triisopro- pyl borate in tetrahydrofuran (THF) at -78 ’C.A similar method was used to prepare l-bromo-4-(2-ethoxyethyl) ben-zene, 9 (R=C,H,; n =l), from 2-( 4-bromopheny1)ethanol. Three chiral derivatives of 4’-bromo-2’-fluorobiphenol were prepared14 from (2S73S)-3-propyloxiranemethanol( R’ =C,H,, R”=H) to give 10, (2S,3S)-3-methyl-3-[ 5-( 2-methylpent-2- enyl)] oxiranemethanol [R’ =CH,CH,CH =C(CH,), , R”= CH,] to give 11 and (2&3R)-3-methyl-3-[ 5-( 2-methylpent-2-enyl)] oxiranemethanol [R” =CH,CH,CH =C( CH,),, R’= CH,]to give 12. In order to examine the effect of changes in the mesogenic core, the (2S,3S)-3-propyloxiranemethanol (R’= C,H,, R”=H) derivatives of 4’-bromo-3-fluorobiphenol and 4’-bromobiphenol were prepared using similar procedures to give 13 and 14, respectively.Intermediates 11-14 were coupled with boronic acid, 8 to give the target compounds 15-17 and 20, respectively. Standard cross-coupling procedure^'^ were employed, using [tetrakis(triphenylphosphine)palladium(o)] as the catalyst and 1,2-dimethoxyethane (DME) as the solvent. Similar pro- cedures were used to obtain cross-coupled products from the J. MATER. CHEM., 1994, VOL. 4 6 I 7 0 10-14 9 15,16,17, 20 10,lQ Scheme 1 reaction between boronic acid 9 and intermediates 10 and 11 to give the target materials 18 and 19, respectively.Each of the final products 15-20 were recrystallized a number of times, until constant transition temperatures and HPLC purit- ies of greater than 99.5% were achieved. 1-Bromo-4-( propoxymethyl )benzene (6) A mixture of sodium hydride (80% dispersion in oil, 1.15 g, 40 mmol) and 1-bromoethane (6.0 g, 40 mmol) was added to a solution of 4-bromophenylmethano1(6.0g, 30 mmol) in dry N,N-dimethylformamide (DMF; 50 cm3), and the resulting mixture stirred at room temperature (20 h) under an atmos- phere of dry nitrogen. Water was added carefully to destroy unreacted sodium hydride and the mixture diluted with water (200 cm3) and extracted with diethyl ether (50 cm3). The ethereal layer was washed with water (50 cm3), dried (MgSO,) and the solvent removed in uucuo. The product was purified by distillation under reduced pressure then passed through a short gravity column (silica gel; dichloromethane) to afford a colourless oil.Yield =4.35 g (61 Yo);bp =80 "C (0.5 mmHg). 'H NMR 6, (CDC13,270 MHz, TMS) 0.95 (3 H, t, CHZCH,), 1.65 (2 H, sex, CH,CH,), 3.45 (2 H, t, OCH,CH,), 4.45 (2 H, s, OCH2Ar), 7.24 (2 H, AA'XX', Ar-H), 7.48 (2 H, AA'XX', Ar-H). IR (liquid film): vrnax/cm-l 2980 (CH), 1485 (Ar), 1100,1010,800. MS: m/z 230/228 (M)+, 202/200(M-C2H4)+, 171/169, 107, 91, 58. 1-Bromo-4-( 2-ethoxyethy1)benzene (7) 2-( 4-Bromopheny1)ethanol (5.6 g, 30 mmol) was alkylated with 1-bromoethane using the method described above to afford a colourless oil.Yield =1.80 g (47%); bp =58 "C (0.5 mmHg). 'H NMR 6, (CDCl,, 270 MHz, TMS) 1.20 (3 H, t, J 7.0 Hz, CH,CH,), 2.83 (2 H, t, J 7.0 Hz, OCH,CH, -Ar) 3.47 (2 H, q, J 7.0 Hz, OCH,CH,), 3.60 (2 H, t, J 7.0 Hz, OCH,CH2-Ar), 7.10 (2 H, AA'XX, Ar-H), 7.4 (2 H, AA'XX', Ar-H). IR (liquid film): vrnax/cm-l 2980 (CH),1485 (Ar), 1100, 1010, 800. MS: m/z 230/228 (M)', 186/184 (M-OC,H,)+, 171/169, 89, 59. 4-Propoxymethylphenylboronicacid (8) A solution of 1-bromo-4-(propoxymethyl) benzene (4.0 g, 17.5 mmol) in THF (40 cm3) was cooled to -78 'C in an atmosphere of dry nitrogen and a solution of n-but yllithium (1.6 mol dme3 in hexane, 11.8 cm3, 17.5 mmol) was added slowly. After stirring the mixture at -78 "C (2 h), triisoprop- ylborate (6.6 g, 35 mmol) was added slowly and stirring was continued (1 h) at -78 "C.The mixture was allowed to come to room temperature and stirred (20 h). Hydrochloric acid (lo%, 20 cm3) was added to the stirred mixture, which was then extracted with diethyl ether (3 x 25 cm3). The Gombined ethereal layers were washed with water (25 cm3) md dried (MgSOJ. The solvent was removed in uucuo, to afford a light- yellow oil that solidified on standing. Yield =3.20 g (94%). 'H NMR 6, (C2H,]DMS0, 270 MHz, TMS) 0.75 (3 H, t, CH,CH,), 1.5 (2 H, sex, CH,CH,CH,), 3.3 (2 H, t, OCH,CH,CH,), 4.4 (2 H, s, OCH,-Ar), 7.3 (2 H, AA'XX', Ar-H), 8.05 (2 H, AA'XX', Ar-H). IR (liquid film): vrnax/cm-' 3350 (br, OH), 1610 (Ar) 1350, 700. MS: m/z 484 (3M-44)+, 176, 116, 91, 58.442-Ethoxyethy1)phenylboronicacid (9) The boronic acid derived from 1-bromo-4-( 2- ethoxyethy1)benzene (7)(4.0 g, 17.5 mmol) was prepared using the method described above to afford a yellow oil that solidified on standing. Yield= 3.58 g (76%). 'H NMR 6, ([2H6]DMS0, 270 MHz, TMS) 1.25 (3 H, t, CH3CH,), 3.00 (2 H, t, Ar-CH,CH,O), 3.50 (2 H, q, OCH,CH3), 3.57 (2 H, t, OCH,CH,Ar), 7.4 (2 H, AAXX, Ar-H), 8.25 (2 11, AA'XX', Ar-H). IR (liquid film): vmax/cm-' 3300 (br, BOH), 2940 (CH), 1610, 1370 (br), 1100 (br). MS: m/z 529 (.JM+H)+, 468, 162, 103. 4-Bromo-2-fluoro-4’-[(2S,3S)-3-propyloxiran-2-ylmethoxy] biphenyl (10) (2S,3S)-3-Propyloxiranemethanol( 1.45 g, 12.5 mmol) and diethyl azodicarboxylate (DEAD) (2.18 g, 12.5 mmol) were added successively to a solution of 4-bromo-2’-fluoro-4-hydroxybiphenyl (3.34 g, 12.5 mmol) and triphenylphosphine (3.28 g, 12.5 mmol) in THF (40 cm3).The reaction mixture was stirred at room temperature (20 h), the solvent removed in uucuo and the crude product purified by flash column chromatography [silica gel; dichloromethane: light petroleum (bp 60-80°C) (4: l)], to afford a white solid. Yield= 3.58 g, (78%); mp= 76-78 “C. ‘H NMR 6, (CDCl,, 270 MHz, TMS) 0.99 (3 H, m, CH,CH,), 1.5 (4 H, m, CH,CH,CH,), 2.98 (1 H, m, CH-O-CH-C,H7) 3.12 (1 H, m, CH-O-CH- C3H7), 4.03 (1 H, ABX, 0-CH,-CH-0-CH), 4.22 ( 1 H, ABX, 0-CH2 -CH-0-CH), 6.99 (2 H, AA’XX’, Ar-H), 7.25 (1 H, m, Ar-H), 7.32 (1 H, m, Ar-H), 7.44 (2 H, AA’XX’, Ar-H).IR (liquid film): v,ax/cm-l 2980 (CH), 1800, 1470 (Ar), 1250 (C-F), 1000, 800. MS: m/z 366/364 (M)’, 268/266, 170, 99. 4-Bromo-2-fluoro-4’[( 2S,3S)-3-methyl-3-[ 542-methylpent-2-en-1-yl)] oxiran-Zylmethoxy] biphenyl (1 1 ) 4-Bromo-2-fluoro-4’-hydroxybiphenyl (0.67 g, 2.5 mmol) was alk yla ted with (2S,3S)-3-met hyl- 3- [5-(2-me t hylpen t-2-en- 1-yl)] oxiranemethanol using the method described above to afford a colourless oil. Yield =0.94 g (90%). ’H NMR 6, (CDCl,, 270MHz, TMS) 1.36 (3 H, s, CH-O-C-CH3), 1.62 (3 H, S, C=C-CH3), 1.69 (3 H, S, C=C-CH3), 1.7 (2 H, m, C=CH-CH2-CH,), 2.13 (2 H, q, C= CH--23,-CH,), 3.17 (1 H, t, CH,-CH-0-CMe), 4.11, ( 1 H, ABX, 0-CCH, -CH-0-CMe), 4.19 ( 1 H, ABX, 0-CH,-CH-0-CMe), 5.11 (1 H, m, CH=CMe,), 7.01 (2 H, AA’XX’, Ar-H), 7.26 (1 H, AA’XX’, Ar-H), 7.33 (2 H, AA’XX’, Ar-H), 7.46 (2 H, AAXX‘, Ar-H).IR (liquid film): vmaX/cm-’ 2960 (CH), 1600, 1480 (Ar), 1240, 870, 810. MS: m/z 420/418 (M)’, 268/266, 170, 109. 4-Bromo-2-fluoro-4-[(2S,3R)-3-methyl-3-[ 5-(2-methylpent-2-en-1-yl )] oxiran-2-ylmethoxy] bipohenyl(l2) 4-Bromo-2-fluoro-4’-hydroxybiphenyl ( 1.34 g, 5 mmol) was oxiranemethanol using the method described above to afford a colourless oil. Yield= 1.75 g (83%). ‘H NMR 6, (CDCl,, 270MHz, TMS) 1.41 (3 H, S, CH-0-C-CH,), 1.59 (3 H, S, C=C-CH+,), 1.69 (3 H, S, C=C-CH,), 1.7 (2 H, m, C=CH-CH,-CCH,), 2.15 (2 H, q, C=CH-CH,-CH,), 3.16 (1 H, t, CH,-CH-0-CMe), 4.10 (1 H, ABX, 0-CH, -CH-0-CMe), 4.18 (1 H, ABX, 0-CH,-CH-0-CMe), 5.12 (1 H, m, CH=CMe,), 7.00 (2 H, AA’XX’, Ar-H), 7.27 (1 H, AA’XX’, Ar-H), 7.34 (2 H, AA’XX’, Ar-H), 7.46 (2 H, AAXX’, Ar-H).IR (liquid film): vmaX/cm-’ 2960 (CH), 1600, 1480 (Ar), 1240, 870, 810. MS: m/z 420/418 (M)’, 268/266, 170, 109. 4-Bromo-2-fluoro-4-[(2S,3S)-3-propyloxiran-2-ylmethoxy] biphenyl (13) 4-Bromo-2’-fluoro-4’-hydroxybiphenyl(1.37 g, 5.0 mmol) was alkylated with (2S,3S)-3-propyloxiranemethanol(0.58 g, 5.0mmol) using the method described above to afford a colourless oil. Yield =0.84 g, (53%); mp =66-68 “C. ‘H NMR 6, (CDC13,270 MHz, TMS) 1.00 (3 H, t, CHZCH,), 1.62 (4 H, m, CH2CH,CH,), 2.97 (1 H, ABX, OCH2 -CH-0-CH- C3H7), 3.13 (1 H, ABX, OCH,--CH-O-CH-C3H7), 4.03 alkylated with (2S,3R)-3-methyl-3-(4-methylpent-3-en-l-y1)-white solid that was recrystallized several times from meth- J.MATER. CHEM., 1994, VOL. 4 (1 H, ABX, OCH,-CH-0-CH-), 4.22 (1 H, ABX, OCH2-CH-0-CH-), 6.98 (2 H, AA’XX’, Ar--H), 7.3 (3 H, m, Ar-H), 7.44 (2 H, AA’XX’, Ar-H). IR (KBr): vmaX/cm-’ 2980 (CH), 1800, 1470 (Ar), 1250 (C-F), 1000, 800 cm-l. MS: m/z 366/364 (M)’, 268/266, 170, 99. 4-Bromo-4-[ (2S,3S)-3-propyloxiran-2-ylmethoxy]biphenyl (14) 4-Bromo-4-biphenol (0.75 g, 3.01 mmol) was alkylated with (2S,3S)-3-propyloxiranemethanol(0.35 g, 3.00 mmol) using the method described above to afford a colourless oil. Yield = 0.80 g (94%); mp= 110.3 “C. ‘H NMR 6, (CDCl,, 270 MHz, TMS) 0.99 (3 H, t, J 8.0Hz, CH,), 1.56 (2 H, m, OCH,CH,CH,), 1.60 (2 H, m, CH-0-CH-CH,CH,CH,), 2.96 (1 H, ABX, J 5.0, 3.0 Hz, OCH2-CH-0-CH-), 3.13 (1 H, ABX, J 5.0, 3.0, 2.5Hz, OCH2-CH-0-CH-), 4.03 [l H, ABX, J 11.5 (gem), 5.0 (anti)Hz, OCH,-CH-0-CH-],4.22 [l H, ABX, J 11.5 (gem), 3.0 (syn)Hz) OCH2-CH-0-CH--1, 6.98 (2 H, AA’XX’, J 8.5 Hz, Ar-H), 7.40 (2 H, m, J 8.5 Hz, Ar-H), 7.48 (2 H, AAXX’, J 8.5 Hz, Ar-H), 7.53 (2 H, AA’XX’, J 8.5 Hz, Ar-H).IR (KBr): vmax/cm-’ 2950, 2920 (CH), 1600, 1480 (Ar), 1285,1250(CH-OCH), 825, 805 cm-’. MS: m/z 348/346 (M)’, 250/248, 99, 55. 2’-Fluoro-4-[( 2S,3S)-3-met h yl-345-( 2-methylpen t-2-en- 1-yl )] oxiran-2-ylmethoxy [-4”-propoxymethyl-l ,1‘:4’,1’’- terphenyl(l5) An aqueous solution of sodium carbonate (2 mol dm-3, 8 cm3) and the coupling catalyst [tetrakis( triphenylphos- phine)palladium(o)] (30 mg, 30 pmol) were added to a solution of 4-bromo-2-fluoro-4’-(2S,3S)-3-methyl-3-[5-(2-methylpent-2-en- 1-yl)] oxiran-2-ylmethoxy] biphenyl (0.45 g, 1.07mmol) in DME (8 cm3).4-Propoxymethylphenylboronic acid (0.19 g, 1.07 mmol) was added and the mixture stirred at 90 “C (24 h). The dark-brown mixture was diluted with diethyl ether (100 cm3) and water (50 cm3). The separated aqueous layer was washed with diethyl ether (2 x 25 cm3) and the combined organic layers were washed with water (50 cm3) and dried (MgSO,). The solvent was removed in uucuo and the crude product was purified by flash column chromatography (silica gel; dichloromethane) to afford a anol. Yield =140 mg (30%); mp = -8.0 ”C: for meso-morphism see Results section.[x]”’~ = -9.8 O; HPLC > 99.5%. Calc. for C32H37F03; C, 78.58; H, 7.62%. Found: C, 78.63; H, 7.68%. ‘H NMR 6, (CDCl,, 270MHz, TMS) 0.97 (3 H, t, J 7.2 Hz, CH,CH,), 1.33 (3 H, s, CH-0-C-CH,), 1.55 (2 H, m, C=CH-CH, -CH,), 1.65 (3 H, s, C=C-CH,), 1.68 (2 H, m, CH,CH,CH,), 1.69 (3 H, S, C=C-CH3), 2.14 (2 H, q, J 7.1 Hz, C=CH-CCH2-CH2), 3.18 (1 H, t, J 5.4Hz, CH,-CH-0-CMe), 3.48 (2 H, t, J 7.2 Hz, -OCH,CH,CH,), 4.13 [l H, ABX, J 11.0 (gem), 5.6 (anti)Hz, 0-CH,-CH-0-CMe], 4.19 [l H, ABX, J 11.0 (gem), 5.0 (syn)Hz, 0-CH,-CH-0-CMe], 4.55 (2 H, s, Ar-CH,-0), 5.11 (1 H, m, J 9.2, 1.8 Hz, CH=CMe,), 7.05 (2 H, AA’XX’, J 9.0Hz, Ar-H), 7.34-7.63 (9 H, m, Ar-H). IR (liquid film): vmax/cm-l 2980 (CH), 1600, 1485 (Ar), 1230 (C-F), 1240, 800.MS: m/z 488 (M)’, 336, 277. 248. 2’-Fluoro-4-[( 2S,3R)-3-methyl-3- [5-(2-methylpen t-2-en-l- yl )] oxiran-2-ylmethoxy]-4”-propoxymethyI-l,1’:4‘,l’’-terphenyl(16) 4-Bromo-2-fluoro-4’-[( 2S,3R)-3-methyl-3-[ 5-( 2-methylpent-2-en- 1-yl-)] oxiran-2-ylmethoxy] biphenyl (0.45 g, 1.07 mmol ) J. MATER. CHEM., 1994, VOL. 4 was coupled with 4-propoxymethylphenylboronic acid (0.19 g, 1.07mmol) using the method described above to afford a white solid, which was recrystallized several times from light petroleum (bp 30-40 “C). Yield =100 mg (19”/0); mp =56.6 “C; for mesomorphism see Results section. = -3.1”; HPLC>99.5%. Calc. for C,,H,,FO,: C, 78.58; H, 7.62%. Found: C, 78.70; H, 7.65%. ‘H NMR 6, (CDCl,, 270 MHz, TMS) 0.96 (3 H, t, J 7.4 Hz, CH2CH3), 1.41 (3 H, s, CH-0-C-CH,), 1.57 (2 H, m, C=CH-CH2-CH2), 1.62 (3 H, s, C=C-CH,), 1.68 (2 H, m, CH,CH,CH,), 1.70 (3 H, S, C=C-CH,), 2.19 (2 H, q, J 7.7 Hz, C=CH-CH,-CH,), 3.18 (1 H, t, J 5.2Hz, CH,-CH-0-CMe), 3.48 (2 H, t, J 6.7 Hz, -OCH,CH,CH,), 4.12 [1 H, ABX, J 10.8 (gem), 6.0 (anti)Hz, 0-CH,-CH-0-CMe], 4.21 [l H, ABX, J 10.8 (gem), 5.2 (syn)Hz, 0-CH,-CH-0-CMe], 4.56 (2 H, s, Ar-CH,O), 5.13 (1 H, m, J 7.1, 1.6 Hz, CH=CMe,), 7.02 (2 H, AA’XX’, J 8.8 Hz, Ar-H), 7.35-7.61 (2 H, m, Ar-H).IR (liquid film): vma,/cm-l 2980 (CH), 1600, 1485 (Ar), 1235 (C-F). 1240, 800. MS: m/z 488 (M)+, 336, 277, 248. 2’-Fluoro-4-[(2S,3S)-3-propyloxiran-2-ylmethoxy]-4-propoxymethy1-1,1’:4’1”-terpheny1(17) 4-Bromo-3-fluoro-4’-[(2S, 3S)-3-propyloxiran-2-ylmethoxy]-biphenyl (0.80 g, 2.2 mmol) was coupled with 4-propoxy- methylphenylboronic acid (0.5 g, 2.8 mmol) using the method described above to afford a white solid, which was recrystallized several times from methanol.Yield =270 mg (28%); mp =66.5 “C; for mesomorphism see Results section. [cY],~= -10.5”; HPLC >99.5%. Calc. for C28H,1F0,: C, 77.39; H, 7.19%. Found: C, 77.37; H, 7.23%. ‘H NMR 6, (CDCI,, 270 MHz, TMS) 0.96 (3 H, t, J 7.2 Hz, CH,CH,), 0.99 (3 H, t, J 7.2Hz, CH,CH,), 1.4-1.7 (6 H, m, 3xCH,), 2.98 (2 H, ABX, J 7.2Hz, OCH,CHOCH), 3.13 (1 H, m, J 5.5, 3.0 Hz, OCH,CHOCH), 3.48 (2 H, t, J 7.1 Hz, ArCH,OCH,CH,), 4.01 (2 H, ABX, J 11.0, 5.5 Hz, CH2-CH-0-CH), 4.23 (2 H, ABX, J 11.0, 3.0Hz, CH,-CH-0-CH), 4.56 (2 H, s, OCH,Ar), 7.0 (2 H, AA‘XX‘, J 8.9Hz, Ar-H), 6.78 (2 H, m, Ar-H), 7.35 (3 H, m, Ar- H), 7.62 (4 H, m, Ar-H).IR (KBr): vmax/cm-l 2940 (C-H), 1620 (C-C), 1485 (C-H), 1310 (C-0), 1230 (C-F), 1170, 1100, 820. MS: m/z 434 (M)’, 277, 149, 99, 43. 2’-Fluoro-4-[( 2S,3S)-3-propyloxiran-2-ylmethoxy]-4”-ethoxyethyl-l,l’:4‘,1’’-terphenyl(18) 4-Bromo-2-fluoro-4’-[( 2S, 3S)-3-propyloxiran-2-ylmethoxy]-biphenyl (0.45 g, 1.07 mmol) was coupled with 4-ethoxy- ethylphenylboronic acid (0.19 g, 1.07 mmol) using the method described above to afford a white solid, which was recryst- allized several times from methanol. Yield= 108 mg ( 18%); mp =101.5 “C; for mesomorphism see Results section. = -13.0’; HPLC >99.5YO.Calc. for C,,H,,FO, : c, 77.39; H, 7.19%. Found: C, 77.45; H, 7.25%. ‘H NMR hH (CDCI,, 270 MHz, TMS) 0.99 (3 H, t, J 7.2 Hz, CH,CH,), 1.22 (3 H, t, J 7.0 Hz, OCH,CH,), 1.57 (4 H, m, CH,CH,CH,), 2.95 (2 H, t, J 7.3Hz, Ar-CH,CH,O), 3.00 (1 H, ABX, J 7.2Hz, OCH,CH-0-CCH-), 3.14 [l H, ABX, J 5.4 (anti), 3.8 (syn)Hz, OCH,CH-0-CH-1, 3.53 (2 H, q, J 7.0Hz, OCH,CH,), 3.68 (3 H, t, J 7.3 Hz, Ar-CH,CH,O), 4.05 [1 H, ABX, J 11.1 (gem), 5.4 (anti)Hz, OCH,CH-0-CH-1, 4.22 [l H, ABX, J 11.1 (gem), 3.8 (syn)Hz, OCH,CH-O-CH-],7.01 (2 H, AA’XX’, J 8.9 Hz, Ar-H), 7.32-7.45 (5 H, m, Ar-H), 7.54 (4 H, m, Ar-H). IR (liquid film): vmax/cm-’ 2’380 (Ar), 1600, 1490 (Ar), 1250 (C-F), 1060, 810. MS: m/z 434 (M)+,375, 335, 277, 246. 2’-Fluoro-4-[( 2S,3S)-3-methyl-3-[ 5-(2-methylpent-2-en-1-yl )]oxiran-2-ylmethoxy]-4”-ethoxyethyl-l,1’:4,1“-terphen yl (19) 4-Bromo-2-fluoro-4’[(2S, 3S)-3-methyl-3-[ 5-( 2-methj Ipent-2- en-1-yl)] oxiran-2-ylmethoxy] biphenyl (0.45 g, l.07 mmol) was coupled with 4-(2-ethoxyethyl)phenylboronicacid (0.19 g, 1.07mmol) using the method described above to .ifford a white solid, which was recrystallized several times from meth- anol.Yield =200 mg (55%); mp =45.7 “C; for mesomarphism see Results section. = -10.7”; HPLC >99.5%. Calc. for C,,H3,F0,: C, 78.58; H, 7.62%. Found: C, 78.71; H 7.66%. ‘H NMR 6, (CDC1, 270 MHz, TMS) 1.23 (3 H, t, J 7.2 Hz, OCH,CH,), 1.37 (3 H, s, CH-0-C-CH,), 1.58 (2 H, m, C=CH-CH,-CH,), 1.63 (3 H, S, C=C-CH,), 1.70 (3 H, S, C=C--CH,), 2.13 (2 H, q, J 7.1 Hz, C=CH-CH, -CH,), 2.95 (2 H, t, J 7.2 Hz, Ar-CH,CH,O), 3.18 (1 H, t, f 5.4 Hz, CH,-CH-0-CMe), 3.54 (2 H, q, J 7.1 Hz, -OCH,CH,), 3.68 (2 H, t, J, 7.2 Hz, Ar-CH,CH,O), 4.13 [l 31, ABX, J 11.0 (gem), 5.6 (anti)Hz, 0-CCH, -CH-0--CMe], 4.19 [l H, ABX, J 11.0 (gem), 5.0 lsyn) Hz, 0-CH,-CH-0-CMe], 5.12 (1 H, m, J 9.2, 1.8Hz, CH=CMe2), 7.03 (2 H, AA’XX’, J 8.9 Hz, Ar-H), 7.32-7.46 (5 H, m, Ar-H), 7.54 (4 H, AA’XX’, J 8.1 Hz, Ar--H).IR (liquid film): vmax/cm-’ 2980 (CH), 1600, 1490 (Ar), 1250 (C-F), 1060, 810. MS: m/z 488 (M)+,336, 277, 153 4-[(2S,3S)-3-propyloxiran-2-ylmethoxy]-4-propoxymethyl-l,1’:4,l”-terphenyl (20) 4-Bromo-4‘- [(2S,3S)- 3 -propyloxiran-2-ylmethoxy]biphenyl (97.5 mg, 0.282 mmol) was coupled with 4-ethoxyethyl-phenylboronic acid (80.0 mg, 0.412 mmol) using the method described above to afford a white solid, which was recryst- allized several times from methanol.Yield =69.2 mg (59%); mp =2 10 “C; for mesomorphism see Results section. [a]24= -28.0”; HPLC>99.5%. Calc. for C28H3203: C, 80.73; H, 7.74%. Found. C, 80.88; H, 7.86%. ‘H NMR 6, (CDCI,, 270 MHz, TMS) 0.96 (3 H, t, J 8.8 Hz, CH,CII,), 0.99 (3 H, t, J 7.6 Hz, CH,CH,), 1.56 (4 H, m, 2 x CH,CH,CH,), 1.66 (2 H, m, CH,CH,-CH-0-CH-), 2.99 (1 H, m, J 6.0, 4.8, 3.7Hz, CH,-CH-0-CH-CH,-0), 3.14 (1 H, m, J 6.0, 3.7 Hz, CH,-CH-OO--CH-CI~~-O]), 3.47 (2 H, t, J 6.6Hz, OCH2CH2-), 4.05 [l H, dd, J 11.4 (gem), 6.0 (anti)Hz, CH-0-CH-CH2-01, 4.23 [l H, dd, J 11.4 (gem), 3.8 (syn)Hz, CH-O-CH- CH,-01, 4.55 (2 H, s, 0-CH,-Ar), 7.01 (2 H, AA’XX’, J 8.5 Hz, Ar-H), 7.43 (2 H, AA‘XX‘, .I 8.2 Hz, Ar-If), 7.57 (2 H, AA’XX’, J 8.5Hz, Ar-H), 7.59 (2 H, AA’XX’, J 8.2Hz, Ar-H), 7.73 (4 H, AA’XX’, Ar-H).IR (liquid film): v,a,/cm-l 2980 (Ar), 1600, 1490 (Ar), 1250 (C-F), 1060, 810. MS: m/z 434 (M)’, 375, 335, 27’7,246. Results Optical and Electro-optical Studies EfSeect of Structural Changes in the Chiral Terminal Chain While retaining the oxirane ring as the source of chirality, the nature of the hydrocarbon component of the chain N as varied. This was achieved by using the chiral oxirane.; derived from (2E)-3,7-dimethylocta-2,6-dien-l-ol(geraniol) and (22)- 3,7-dimethylocta-2,6-dien-l-o1 (nerol)14 to give 15 and 16, respectively. The remaining molecular structure was main- tained as for 1, which has been reported previously as showing the following phase sequence.8 J. MATER.CHEM., 1994, VOL. 4 F H on a Silicon Graphics XS24 4000 UNIX-based workstation. foqJqJ+o&-LAll structures were minimized using CHARMm (MSI) rou-H 1 K 59 Sc: 46.2 S,--46.8 S,: 103.3 Ng 106.3 NZ 112.1 NZ 158.8 BPI 162.9 BPI1 164.6 "C Is0 It should be noted that we have recently reassessed16 the helical twist sense in the Sc* and cholesteric phases of 1. We have found that the higher temperature Sc* and N* phases both possess left-handed helices whereas the lower tempera- ture phases have right-handed helices. However, the inversion temperatures are confirmed to be as reported previously. The problem experienced in the initial assignment arose because measurements were made close to the inversion points and this made it difficult to assign the twist sense correctly owing to the subtle colour changes observed. All the compounds reported here will be discussed relative to 1, which will act as the reference standard.Compound 15 is a low melting solid (mp= -8 "C) that possesses a room temperature Sc* phase. The full phase sequence has been determined as: F ti 15 K -8 S,: 44.3 Ng 76.9 Is0 "C (4.2) (0.13) (0.62) (J g-7 No inversions are observed in either phase, nor is there an inversion in the helical twist sense at the phase transition, both the Sc* and N* phases possess right-handed helices (Zaevorotation of plane-polarized light).In both phases, the helix direction is the same as that observed in the lower temperature regions of each of the corresponding helical phases observed for 1. Compound 16 on the other hand shows no mesomorphism, simply melting from the crystal to the isotropic liquid (K 60 "C Iso, AH=34.0 J g-'). Although supercooling occurs down to room temperature, there is no evidence of a monotropic phase and shearing the sample only induces crystallization. H 16 The inclusion of a methyl group (R") and long hydrocarbon group (R') at the chiral C-3 carbon in 15 produces a substantial decrease in the transition temperatures, particularly with respect to the melting temperature. Furthermore, the increased steric bulk of the methyl group at the chiral centre is detrimen- tal to helix inversion.In addition, the long hydrocarbon substituent (R') would be expected to provide considerable rotational damping, which also leads to a reduction in liquid- crystalline properties because of the increased steric repulsive effects. Modelling studies have been carried out to determine the effect of the methyl substituent and R' on the torsional energy profile of 15 relative to that observed in 1 (Fig. 2). The initial calculations on 1 were performed using Microsoft EXCEL 4.0 interfaced via Dynamic Data Exchange (DDE) to the molecular modelling package HYPERCHEM 3, operating in the Microsoft Windows environment. These studies were extended to the more sophisticated package QUANTA 3.3 [Molecular Simulations Inc.(MSI)] running tines (conjugate gradients method) prior to the calculations. The loss of mesomorphism in 16 can be explained in terms of the unfavourable geometry about the C-3 chiral centre. Whereas the long hydrocarbon chain in 15 lies within a rotational cone of fairly small diameter relative to the meso- genic core, the diameter of the cone is much larger in 16. This is because the restricted geometry at C-3, imposed by the cyclic chiral moiety, results in lowest energy conformations where the long substituent lies well off-axis. This effective molecular broadening disrupts the packing of the molecules and destabilizes mesophase formation. The large degree of disorder resulting from the molecular broadening is manifested in the large cooling hysteresis effect observed on crystalliz- ation.Plate 1 shows equivalent lowest energy conformers of 1, 15 and 16 in order to emphasize the broadening effect. EfSect of Structural Changes in the Non-chiral Tev-mind Chain Compounds 18 and 19, which possess ethoxyethyl terminal substituents, were investigated in order to determine if the position of the oxygen atom in the non-chiral chain had an effect on helix inversion. It has been demonstrated previously* that increasing the length of the terminal hydrocarbon sub- stituent from C,H, to C5H,, and C7H,, while retaining an oxymethylene link to the aromatic core destabilizes inversion phenomena. This may be attributable to an increase in rotational damping brought about by the increased chain length or to a lengthening of the 'zig-zag' shape.Compounds 18 and 19 both show orthogonal smectic A* (SA*)and N* phases but no Sc* phases. Compound 19 shows an additional blue phase I (BPI). The helical twist sense in the N* phases was investigated in both materials, however no inversions were observed. The full phase sequences are shown below: F H4Yx50 0m-L\ / \ / \ / , H 18 K 101.5 S,* 123.2 Ng 152.4 Is0 C (28.0) (0.43) (0.66) (J g I) F H 19 K 46 SA*85.3N; 87.1 BPI 87.2 Is0 C (52.0) (2.3) (0.18) (-) (J g-') The orthogonal SA* phase is characterized by its typical focal conic texture and high degree of fluidity. The change in position of the oxygen link therefore suppresses helix inver- sions and tilted smectic phases while stabilizing the ortho- gonal SA*phase. The melting points are increased significantly relative to the oxymethylene-linked analogues, while the clear- ing temperatures are reduced.Overall, the introduction of an oxygen in the 3-position relative to the core serves to destabil- ize mesophase formation relative to the oxymethylene deriva- tives. This is possibly due to an odd-even effect relating to the position of the oxygen relative to the core and it is therefore conceivable that an oxytrimethylene spacer would stabilize tilted mesophase formation and inversions in helical properties. J. MATER. CHEM.. 1994. VOL. 4 0 36 108 180 252 324 0 36 108 180 252 324 torsion angle/degrees Fig. 2 Calculated relative potential energies as a function of torsion angle [O-C, -C,-C,] for 1 and 15 Plate 1 Lowest-energy conformers (QUANTAICHARMm) for 1, 15 and 16 Efect of Structural Changes in the Mesogenic Core The possibility that the lateral aromatic fluoro-substituent aids the inversion of helical twist sense and chirality-dependent properties is of major interest. We have looked at the effect of changing the substitution pattern whilst keeping the overall structure as similar as possible.In this respect 17, a structural isomer of 1, and 20, which does not possess lateral fluorine atoms, were prepared. For 17, the fluorine atom was simply omitted from the 2'-position in the terphenyl core but a fluorine atom was included at the 2-position on the phenyl ring carrying the chiral substituent.Only the chiral products derived from (2S,3S)-3-propyloxiranemethanolwere synthe- sized as this chiral moiety had given inversion properties in 1. In this way the overall molecular dipole would be shifted while still retaining an approximately equal dihedral angle between the first and second phenyl rings, relative to the chiral substituent. The dihedral angles in question have been calculated as 42" for 1 and 43" for 17 (Fig. 3) using QUANTA/ CHARMm. All structures were minimized using CHARMm routines (conjugate gradient methods) prior to the calculations. Compound 17 shows an inversion in the helical twist sense of the N* phase but not in the Sc* phase.The full phase sequence is given below: H 17 K 53 K' 66.5 Sq 97.5 NR104.5NZ 106.0Nt150.21~0 "C (26.1) (37.1) (0.7) (-) (-) (1.6) (J g-') Optical microscopy shows that in the upper temperature N* phase the pitch increases as the sample is cooled. At 106.0"C the pitch diverges suddenly to infinity. This is charac- terized by a change in the optical texture from a characteristic cholesteric Grandjean plane texture in the N*L phase to a schlieren texture containing two- and four-brushed singularit- ies (i.e. a normal, non-helical nematic phase) in the N*, phase. On further cooling, a Grandjean plane texture reforms. The helical twist sense in the mesophases of 17 was determined optically relative to fixed, crossed polarizers.The upper tem- perature cholesteric phase was determined to have a left-handed helix as it caused a dextro (D) rotation of plane polarized light. However, as the chiral group is composed of two sequential chiral centres within a three-membered hetero- cycle, it is not possible to relate the Gray-McDonnell rules17 to this material. In the lower temperature cholesteric phase a laevo (L) rotation of light was observed, thereby confirming the helix to be right-handed. No inversion of helical twist sense was observed in the Sc* phase, the phase causing D-F += 42" 1 #= 43" 17 Fig. 3 Calculated torsion angles (QUANTA/CHARMm) for 1 and 17 J. MATER. CHEM., 1994, VOL. 4 rotation of plane polarized light throughout the complete mesophase range, thereby proving the helix to be left-handed. In order to further investigate the Sc* phase, we determined the temperature dependence of 6, and P, of 17.When all lateral fluorines were removed. to give 20, a dramatic increase in the melting point was observed. Whereas the monofluoro derivatives 1 and 17 melted at 59 and 66.5 "C, respectively, 20 did not melt until 213 "C, giving a chiral, orthogonal E* phase which was characterized by its optical texture. A transition to an orthogonal SA*phase was observed at 231 "C, which persisted to 257 "C when clearing was observed. H 20 K 210 E* 227.8 SA*257.3 Is0 "C (22.7) (26.7) (-) (kJ rnol-' Electro-optic Studies on Compound 17 Temperature Dependence of the Apparent Opticrrl Tilt Angle The apparent optical tilt angle was measured using a field reversal method.A positive dc field of 3.2 V pm -' was applied across the electrode area, which had a cell spacing of 4.8 pm, and the crossed polarizers were rotated until extinction was achieved. Homogeneous planar alignment was achieved by overlaying a 1 kHz, 30 V (peak-to-peak) ac signal on the dc field to cause molecular fribulation. Once alignment was satisfactory, and prior to the switching studies, the ac signal was removed. The angle on the Vernier scale of the microscope stage was noted and the polarity of the applied voltage was reversed to give the transmission state. The microscope stage was then rotated to regain extinction and the angle on the Vernier scale was again noted.The difference between the two readings corresponds to the angle 28,. The results of these studies are shown graphically in Fig. 4. Compound 1 has 8, approaching 0' close to the S,*-N* transition, which increases with decreasing temperature to a maximum of 22.5". On further cooling, 8, falls to 0" and then increases in a negative sense to a maximum of -22.5" prior to crystallization. In contrast, 17 has 6, approaching 23" just below the Sc*-N* transition and this decreases almost linearly when the temperature is reduced. An increase in 8, is observed prior to crystallization in a 1.4 pm cell, however, this could be attributed to pre-transitional or surface latching effects. It should be noted that the transition temperatures in the 4.8 pm cell are approximately 3 "C lower (e.g.Sc* 95.2 N*) than as a thick film on an untreated glass slide. This is an artifact of the reduced cell thickness and interactions with the polyimide surface coating and is particularly evident in a 1.4 pm cell in ** 0. t -I 1OL 45 55 65 75 a5 95 TI'C Fig. 4 Temperature dependence of the apparent optical tilt angle in 17 J. MATER. CHEM., 1994, VOL. 4 ..*** *.*. ** ** . 0 40 50 60 70 80 90 100 TIT Fig. 5 Temperature dependence of the spontaneous polarization in 17 which the transition is observed at 94.1 "C. A similar linear decrease in 8, has been observed in the Sc* phase of another material,' which exhibits an inversion of helical twist sense in the N* phase.Temperature Dependence of the Spontaneous Polarization The spontaneous polarisation of 17 was determined using an applied ac voltage of 68 V (14.2 V pm-') peak-to-peak at a frequency of 100 Hz in a 4.8 pm cell from a temperature just below the Sc,*-N*Rphase transition. The results are shown graphically in Fig. 5. In contrast to 1, the maximum value of P,observed is almost double, peaking at 31.2 nC cm-2. The direction of the spontaneous polarization was determined according to reported procedures." A dc voltage of -20 V was applied, making the top plate of the cell the cathode. The polarizers were rotated to give extinction. The applied field was reversed to make the top plate of the cell the anode and the direction in which the optical stage had to be rotated to regain extinction gave the polarization direction.The ScT phase has a negative polarization, Ps(-), in contrast with 1 which shows the opposite polarization direction in the S,,* phase, formed likewise from the cholesteric phase. The optical tilt angle was found to vary almost linearly with the spontaneous polarization throughout the temperature range of the study. This is in good agreement with theory" and shows the material to be behaving normally. It would appear from the trend in the curve of spontaneous polarization uersus temperature that the chirality-dependent properties in the S,* phase are beginning to diverge towards inversion and that the inversion phenomena are not observed simply because the material crystallizes.This would suggest that if crystalliz- ation could be suppressed then inversion would occur. That is to say, there is a 'virtual' inversion point. Discussion A number of structural modifications have been made to the basic terphenyl structure shown in Fig. 1. The effect of these changes on the chirality-dependent properties of the resulting materials has been investigated. It has been shown previously' that inversion phenomena are sensitive to the length of the terminal non-chiral hydrocarbon chain (R). We have extended this by demonstrating that the position of the oxygen link in the five-atom non-chiral substituent, relative to the mesogenic core, is also important in the stabilization of inversion phen- omena.While inversions are observed in materials of the same general structure when n= 1, they are absent when n=2. We intend to extend this study further by examining selected examples where n =0 or 3, while maintaining the total number of atoms in the chain at five. Compounds with n=2 possess stable orthogonal SA* and cholesteric phases but Sc* phases are destabilized and the melting point is increased significantly; typically by 50 "C relative to materials with n =1. The nature of the substituents around the chiral centre was also found to be very important. Inversion phenomena were only observed in compounds where R" =H and R' is ;i simple unsubsituted hydrocarbon. When a long, substituted hydro- carbon is introduced as R' and a methyl group as R", the mesophase stability is considerably decreased but resiilts in a material exhibiting a room temperature Sc* phase If the substituents R' and R" are inverted however, all mesomorph- ism is lost; this has been attributed to the unfavour:ible off-axis arrangement of the long hydrocarbon chain.which destabilizes the packing of the molecules and consthquently mesophase formation. For 15 the broadening effect i < princi-pally due to the methyl substituent, which serves to destroy a degree of the molecular packing, resulting in a decrease in the melting point. However, in this isomer there is only one high-energy conformation (see Fig. 2) with a torsion mgle for C-C, -C2 -C, approaching 0".This gives an insurmount- able energy barrier to rotation so that the available confor- mations are confined within a potential energy valley. Fig. 6 shows three conformers that are contained within that valley and would be expected to be readily accessible over the temperature regime of the experiment. In terms of mcsophase formation, conformer (a) is the most favoured as the terminal chain lies along the molecular long axis. However, the other two conformers, (b) and (c), are not conducive to mcsophase formation even though they are low-energy structure <.Hence, conversion between such conformers would not be expected to give inversion of liquid-crystalline properties but would lead to destabilization of the mesophase, as was Observed.The most interesting effects are observed when the fluorine- substitution pattern in the aromatic core is chan;ed. The torsion angles between the first and second rings rrdative to the chiral centre (see Fig. 3) in 1 and 17 have been cdlculated and have been found to be essentially the same. The torsion angle has the same relative sign in both cases and the only difference between the two materials is the direction of the overall molecular dipole. The properties observed fix 17 are in many ways similar to those of 1 although a nJmber of significant differences have been observed. In particular, the loss of the inversion phenomena in the Sc* phase. However, it is proposed that this is related to the increa:,e in the crystallization temperature of 17 relative to 1 and it would appear that the trend is towards inversion, if the spcmtaneous polarization is extrapolated to lower temperature. rherefore one might consider that 17 has a 'virtual' inversion point at ca.35 "C. It has been noted that the direction of spontaneous polarization in the Sc* phase of 17 is opposite to triat in the higher temperature S,: phase of 1. This can be explained in terms of the shift in molecular dipole caused by changing the fluorination position from 2'-(ring two) in 1 to 3-(ring one) in 17. This has been demonstrated in computer simulations using the QUANTA/CHARMm molecular model1 ing pack- age. The inversion of helical twist sense in the csholesteric phase of 17 is identical in nature to that observed in 1, with a slight decrease in the transition temperatures.Fig. 7 shows a schematic representation of the overall molecul-ir dipoles calculated for 1, 17 and 20. By using conventional notation to predict the direction of the spontaneous polarization of 1 and 17, relative to the molecular shape and dipole direction, we predict that 1 should have a positive spontaneous polariz- ation, P,(+j, whereas 17 should have a negative value, Ps(-), at the same temperature. We have found from our studies that this is the case and we are now looking into prediction of spontaneous polarization in other systems using modelling techniques. Compound 20 shows orthogonal, non-helical E* and SA* phases at vastly increased temperatures relative to 1 and 17, and substantial decomposition is observed.This shows the importance of lateral fluoro-substitution on the formation of J. MATER. CHEM., 1994, VOL. 4 R OAr predict Ps(+) R on-axis: pro-mesogenic 0 F H R R highly off-axis: disfavoured H F H R R highly off-axis: disfavoured Fig. 6 Examination of the structures of the three lowest-energy conformers of compound 15.For explanation see text. compound side elevation front elevation 1 /flcalcS = 2.15 D predict f,(+) 17 kal$= 1.61 D predict fs(-) peal: = 2.15 D predict fs(-) Fig. 7 Calculated molecular dipoles (using CINDO calculations in QUANTAICHARMm) and their relationship to the spontaneous polarization in the S,, phase for 1, 17 and 20 thermally stable liquid-crystal phases.Whereas lateral fluor- ination has been shown to reduce dramatically the melting point relative to a non-fluorinated system, the melting point of 1 is more than 150"C less than that of 20. Further studies are being carried out to investigate the effects of lateral fluorination and the direction of the overall dipole on the occurrence of inversion phenomena. Conclusions A number of structural changes have been related to the occurrence of inversion phenomena in chiral liquid-crystal systems. We have shown that the occurrence of inversion phenomena are very sensitive to small changes in the molecu- lar structure and that subtle changes can often produce profound effects. We are grateful to Thorn EM1 Central Research Laboratories and Bell Northern Research (Europe) for funding the lec- tureship to P.S.and the Erasmus Scheme for support in the form of a studentship to J.D.V. We also thank Mr. R. Knight, Mr. A. D. Roberts and Mrs. B. Worthington for the spectro- scopic analysis of the materials, and Mrs. J. Haley for assist- ance in the electro-optical studies. References 1 Ph. Martinot-Lagarde, R. Duke and D. Durand, Mol. Cryst. Liq. Cryst., 1981,75,249. J. MATER. CHEM., 1994, VOL. 4 1375 2 3 4 L. Komitov, S. T. Lagerwall, B. Stebler, G. Andersson and K. Flatischler, Ferroelectrics, 1991, 114, 167. (a)J. S. Patel and J. W. Goodby, Philos. Mug. Lett., 1987,55, 283; (b)J. S. Patel and J. W. Goodby, J. Phys. Chern., 1987,91, 5838.H. Stegemeyer, K. Siemensmeyer, W. Sucrow and L. Appel, 2.Nuturforsck, A. Phys. Sci., 1989,44A, 1127. 13 14 Cryst. Liq. Cryst., 1988,158,3; M. A. Osipov and S. A. Pibin, Mol. Cryst. Liq. Cryst., 1983, 103, 57. H. Diamant, K. Drenck and R. Pepinsky, Rec. Sci. Instrurn.,1957, 28, 30. The chiral oxiranes were prepared according to the procedure described in: J. G. Hill, K. B. Sharples, C. M. Exon and 5 L. Komitov, K. Flatischler, G. Andersson, S. T. Lagerwall and R. Regenye, Org. Synth., 1984,63,66. B. Stebler, Ferroelectrics, 1991, 114, 151. 15 S. Gronowitz, A.-B. Hornfeldt and Y.-H. Yang, Chem. Scr., 1986, 6 I. Dierking, F. GeiDelmann, P. Zugenmaier, W. Kuczynski, 26, 311. 7 S. T. Lagerwall and B. Stebler, Liq. Cryst., 1993,13,45. A. J. Slaney, I. Nishiyama, P. Styring and J. W. Goodby, J. Muter. 16 J. W. Goodby, P. Styring, A. J. Slaney, J. D. Vuijk, J. ‘5. Patel, C. Loubser and P. L. Wessels, Ferroelectrics, 1994, 147-9 291. 8 Chem., 1992,2, 805. P. Styring, J. D. Vuijk, I. Nishiyama, A. J. Slaney and 17 G. W. Gray and D. G. McDonnell, Mol. Cryst. Liq. Cry t., 1977, 34, 21 1. 9 10 11 J. W. Goodby, J. Muter. Chem., 1993,3, 399. C. Loubser, P. L. Wessels, P. Styring and J. W. Goodby, J. Muter. Chem., 1994, 4, 71. J. W. Goodby, A. J. Slaney, C. J. Booth, I. Nishiyama, J. D. Vuijk, P. Styring and K. J. Toyne, Mol. Cryst. Liq. Cryst., 1994,243,231. S. Saito, K. Murashiro, M. Kikuchi, T. Inukai, D. Demus, 18 19 J. W. Goodby, E. Chin, T. M. Leslie, J. M. Geary and J. 4.Patel, J. Am. Chem. SOC., 1986,108,4729. J. W. Goodby, R. Blinc, N. A. Clark, S. T. Lagerwal,, M. A. Osipov, S. A. Pikin, T. Sakura, K. Yoshino and 13. Zeks, Ferroelectric Liquid Crystals, Gordon and Breach, Philadelphia, PA. 1991. M. Neundorf and S. Diele, Ferroelectrics, submitted for 12 publication. L. A. Beresnev, L. M. Blinov, M. A. Osipov and S. A. Pikin, Mol. Paper 4/02014F; Receiued 5th April, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401365

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(9), 1377-1385 Magneto-Structural Correlation in a Series of Iodide Salts of p-N-Alkylpyridinium Nitronyl Nitroxided Dependence of the lodide- Pyridinium Ring Interaction on the Length of the N-Alkyl Chain Kunio Awaga,a Akira Yamaguchi," Tsunehisa Okuno: Tamotsu Inabe," Takayoshi Nakamura,c Mutsuyoshi Matsumoto" and Yusei Maruyamad a Department of Pure and Applied Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo 753, Japan Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan National Institute of Materials and Chemical Research, Tsukuba, lbaraki 305, Japan Institute for Molecular Science, Myodaiji, Okazaki 444, Japan Magnetic measurements and X-ray crystal analyses were carried out on iodide salts of p-N-alkylpyridinium a-nitronyl nitroxides [4-(4,4,5,5-tetramethyl-l -oxido-3-oxyl-4,5-dihydro-3H-imidazol-2'-yl)-l -R-pyridinium, with R =methyl (1+), ethyl (2+),n-propyl (3+)and n-butyl (4+)].The strongly antiferromagnetic crystal of 1+.I-consists of a radical dimer and the iodide ion is out of the plane of the pyridinium ring.2+4-,which is weakly antiferromagnetic, includes two crystallographically independent molecules, 2A+ and 2B', each of which forms a centrosymmetric dimer. In the pyridinium ring of 2A+ the iodides are 'out-of-plane' while for 26' they are 'in-plane'. The ferromagnetic 3+.1-and 4'V have similar structures: the crystal consists of a two-dimensional (2D) layer formed by a contact between the pyridinium ring and in-plane iodides.In this series, the iodide ion changes position from out-of-plane to in-plane and the magnetism varies from antiferromagnetic to ferromagnetic. It is found that the nitronyl nitroxide with an out-of- plane iodide has a short intermolecular contact between the NO groups (type I), while that with an in-plane iodide forms a contact between the NO group and the pyridinium ring (type 11). The observed magnetic behaviour can be interpreted in terms of an antiferromagnetic interaction for the type I contact and a ferromagnetic interaction for type II. There is currently rapid development in the field of molecular- based magnetic materials involving pure organic crystals, organic/organometallic polymers, metal-organic radical sys- tems and transition-metal complexes.' Recent advances include reports of bulk ferromagnetism in some pure organic crystal^,"^ a carbene with the largest spin quantum number of S=9,6 multi-dimensional crystal structures of some transition-metal complexes with T,> 10 K,7,8 T,>300 K in V(TCNE),.xCH,Cl, .9 It is also notable that experimental and theoretical reports on the ferromagnetic properties of nitronyl nitroxide radicals now appear quite freq~ently."-'~ The electronic structure of the nitronyl nitroxide has been examined with EPR,20 ultraviolet photoelectron spectroscopy (UVPS)21 and neutron diffraction.,, This radical family pos- sesses a strong spin polarization effect, mainly because of the proximity of the unpaired rc electron and the non-bonding electrons (n-n exchange interaction).The spin polarization effect stabilizes triplet charge transfer (CT) excited states, and the admixture of these states results in a ferromagnetic inter- molecular intera~tion.,~ This mechanism was originally pro- andposed by M~Connell,~~ the importance of the spin polarization effect has been illustrated theoretically by Yamaguchi et aLZ5 From reviewing the ferromagnetic nitronyl nitroxides that have been studied so far,''-'* we note that there is always an aromatic substituent at the a-position and that, in the crystal, there is a short intermolecular distance between the NO group and the aromatic substituent or the a-carbon, which is a bridge between the two NO groups. These observations can be understood as follows.The magnetic orbital (SOMO) of the nitronyl nitroxide is localized on the two NO groups, making a node on the central a-carbon, and has little popu- lation in the aromatic substituent, while the other frontier non-magnetic orbitals (NLUMO, NHOMO, etc.) are distrib- uted on both the nitronyl nitroxide group and the substitu- t The use of the term 'nitroxide' is discouraged by IUPAC; the preferred term is 'aminoxyl'. ent.26 Therefore, a short contact between the NO groups usually means an overlap between the magnetic orbitals, which always makes the intermolecular interaction antiferro- magnetic. An intermolecular contact between the NO group and the aromatic substituent or the r-carbon, on the other hand, means an interaction between the magnetic orbital and the non-magnetic orbitals.The non-magnetic orbi? als are naturally orthogonal to the magnetic orbital in the adjacent molecule. Ferromagnetic coupling can be expected -hrough the [magnetic or bi tall -[non-magnetic or bit all -[n iagnetic orbital] superexchange pathway. Recently, we initiated a study of N-alkylpyridinium nitronyl nitroxide cation radicals. They are designed to produce an intermolecular arrangement which satisfies the empirical con- dition for ferromagnetic coupling. Since the oxygen dtom in the NO group is equipped with a large negative charge, resulting from an electronic polarization in the NO bond, namely Ndf06-, a shorter intermolecular contact between the NO group and the pyridinium ring is expected in the solid state, owing to the Coulombic attraction force between the negative charge on the oxygen and the positive charge on the pyridinium ring (see Scheme 1).In this paper we describe the crystal structures and the magnetic properties of iodide salts of p-N-alkylpyridinium a-nitronyl nitroxides in detail. Scheme 2 shows the atom numberings. Both the mag- netic and the structural properties depend largely on the Q. R-,N+%;+x 0-Scheme 1 2 0-13 14 15 16 -CH2CH2CH2CH3 (4*) Scheme 2 length of the N-alkyl chain. Although we have reported the crystal structures and magnetic properties of 1+.I- and 2+.I- as short communication^,^^^^^ a new interpretation of their properties is given in this work from a systematic point of view in terms of the magneto-structural correlation in the + +series 1 .I --4 .I-.Experimental Materials The nitronyl nitroxide cation radicals, 1+-4+, were prepared by N-alkylation of p-pyridyl nitronyl nitroxide, as reported in ref. 27: the iodide salts of l+-4+ were precipitated in the corresponding alkyl iodide solutions of p-pyridyl nitronyl nitroxide, in the form of crystals (2+-1-) or microcrystalline powder (lf.I-, 3'01-and 4+.I-). 2+.I- crystallized with water, which could be from the air and/or the solvent, as (2+.I-),.Hz0. Single crystals of l+-I-, 3+.I-and 4+.I- were obtained by slow evaporation of their acetone solutions. Elemental analyses (Found: C, 41.35; H, 5.00; N, 11.29.Calc. for l+-I-: C, 41.50; H, 5.09; N, 11.17). [Found: C, 42.51; H, 5.32; N, 10.59. Calc. for (2+.T-),.H,O: C, 42.12; H, 5.55; N, 10.52.1 (Found: C, 44.82; H, 5.69; N, 10.53. Calc. for 3+-I-: C, 44.57; H, 5.73;N, 10.39.) (Found: C, 45.69; H, 5.94; N, 10.00. Calc. for 4+.I-: C, 45.94; H, 6.02; N, 10.05.) X-Ray Structure Determination X-Ray diffraction data were collected on a Rigaku AFC-5 (l+.I-, 3+.I- and 4+.I-) or an Enraf Nonius CAD4 (2+.1-) automatic four-circle diffractometer with graphite monochro- matized Mo-Kr radiation at room temperature. Unit-cell dimensions were obtained by a least-squares refinement using 25 reflections with 20 <2Q/degrees<25. During data collection the intensities of three representative reflections were measured as a check on crystal stability, and no loss was shown.The intensities for l+.I-, 2+.1- and 3+.1-were corrected for absorption, but not those for 4+.I-, because the influence of the absorption was found to be negligibly small. The crystal structures of l+.I-, 2+-I- and 4+.I- were solved by direct methods and the positions of hydrogen atoms were obtained by subsequent difference Fourier syntheses or by calculations. A block-diagonal least-squares technique (UNICS 111) was employed for the structure refinement, in which the positions of the non-hydrogen atoms were treated with anisotropic thermal parameters and those of the hydrogens were treated with isotropic parameters. In the analysis of the structure of 3+.I-, the iodide positions were obtained from a Patterson function and the other atoms were found via subsequent Fourier syntheses.The unit cell of 3+.1-was found to include four independent molecules, and the structure refinement with anisotropic parameters was carried out for all atoms except the hydrogens, in order to avoid excessive parametrization. The difference Fourier synthesis aft?r the refinement indicated no peak with an intensity >0.74 e AP3. Details of the crystal- J. MATER. CHEM., 1994, VOL. 4 lographic parameters are given in Table 1. Final positional parameters for 1+.1--4+-1-are listed in Tables 2-5, respectively.? Magnetic Measurements Static magnetic susceptibility and magnetization were meas- ured with a Faraday balance whose details were described previou~ly.~~The temperature dependence of the magnetic susceptibility was examined in the range 3-250K in a field of 1 T.Corrections for the diamagnetic contribution were carried out, using diamagnetic susceptibilities evaluated by assuming that paramagnetic susceptibilities follow the Curie law at high temperatures. Molecular Structures Two and four molecules are crystallographically independent in the crystals of 2+.I- and 3+.I-, respectively, while there is only one in l+.I- or 4+-I-. There are no signiiicant differences in the bond lengths and angles of the nitronyl nitroxide group or in those of the pyridinium ring, among the molecules 1+-4+. Furthermore, the N-alkyl chains in 1+-4+ take all- trans conformations. However, the dihedral angle between the nitronyl nitroxide group, 0-N-C-N-0, and the pyridin- ium ring tends to decrease with the extension of the N-alkyl chain, as shown in Table 6.The angle is known26 to be much affected by intermolecular contacts of the aromatic substituent, and is considered to be determined by the relative position of the iodide ion to the pyridinium ring, as is discussed later. MO calculations were performed on 1' -4+ with their atomic coordinates determined experimentally. There is little difference in either spin or atomic charge distribution among the molecules: most of the spin densities are distributed in the nitronyl nitroxide group, in contrast to the positive charge, which is localized in the pyridinium ring.Spin-charge separ- ation is characteristic of the N-alkylpyridinium nitronyl nitroxide cation radical. The electronic structure is negligibly affected by the length of the N-alkyl chain. Crystal structures We define two kinds of intermolecular arrangement between a pyridinium ring and an iodide ion, which are actually observed in crystals of N-methylpyridinium iodide.30 In this crystal, the N-methylpyridinium molecule interacts with two iodide ions, one of which lies in the plane of the pyridinium ring with short distances to the hydrogens on the ring ('in- plane' structure), and the other of which is located just above the pyridinium ring with a short distance to the nitrogen on it ('out-of-plane' structure). The positive charge in an N-alkylpyridinium ring is known to be distributed mainly on the nitrogen and the hydrogens of the pyridinium ring.30 Therefore, both the in-plane and the out-of-plane structure could be caused by Coulombic attraction forces.The former can be thought of as a CH--.I-hydrogen bond. Interestingly, the positions of the iodide ions in the crystals of 1+.1--4+.1- can be classified as in-plane or out-of-plane. 1+.I-The structure crystallizes into the triclinic Pi space group with 2=2. Fig. 1 (a)shows a projection of the structure along the [liO] direction. The iodide ions I(A)- and I(B)-occupy Further crystallographic data (atomic coordinates, hydrogen atom coordinates, bond lengths and angles, isotropic and aniso- tropic thermal parameters) are deposited with the Cambridge Crystallographic Data Centre.Details available from the Editorial Office. J. MATER. CHEM., 1994, VOL. 4 Table 1 Crystal data and experimental conditions for 1' -IP-4+-I 1 + -1-2+ -1-3+-1-4+-1 C13H1902N311 formula weight 376.22 399.25 404.27 418.30 crystal system triclinic triclinic triclinic monclclinic spcce group Pi pi P1 p2 L In a/'+ 11.843( 7) 12.582(6) 13.644( 3) 12.3I 1(2) biA 12.695( 7) 13.633(8) 14.798( 1) 13.6-13(2) CIA 9.532( 2) 11.120( 6) 9.873(2) 11.7J9( 2) rldegrees 95.53( 5) 93.31(4) 91.56( 1) Pldegrees 90.55( 5) 115.47(2) 113.11(1) 100. '5(2) ?/degrees 146.89(2) 88.91(4) 92.23( 1) VIA3 768.3(8) 1719(2) 1830.3( 5) 1945.3(6) Z 2 4 4 4 D(calc)/g cm-3 1.626 1.543 1.467 1.428 radiation Mo-Kz (jb=0.71073 A)graphite monochromator 20 range degrees 4.0-55.0 2.0-60.0 4.0-55.0 4.0- 55.0 no.collected 3852 5241 9238 4988 no. obsd 3056 4866 5571 2$65w0i>3.04~~1)R 0.0346 0.086 0.06 16 0.0552 Rw 0.0344 0.085 0.06 16 0.0548 Table 2 Atomic cpordinates ( x lo4)and equivalent isotropic thermal parameters ( lo2 A) for 1+ -1-Table 3 Atomic coordinates (x lo4) and equivalent isotropic thermal parameters ( lo2 A)for 2+.1--0.5H20O(0) O(0) O(0) 4.0 O(0) O(0) 5000(0) 4.0 4174(5) 9038( 5) 7615( 4) 3.0 816(5) 6040(5) 7007 (4) 2.8 4155(6) 5030( 5) 7620( 4) 3.2 9272( 1) 2258( 1) 5265(1) 5.4 6151(5) 10486( 4) 7991 (4) 4.5 4973( 1) 2788( 1 j 9746( 1) 6.3 -886(4) 41 84(4) 6550(4) 4.1 940( 7) 2979(5) 1469(8) 4.2 2756(6) 7134( 6) 7276(4) 2.7 1490( 7) 4528(5) 1751 (9) 4.6 3192(6) 9342(6) 7426( 5) 2.9 -2100( 9) 4661(8) 2793( 11) 7.1 861(6) 7236(6) 7451(5) 2.8 424( 7) 2191(5) 1465(9) 6.7 4196(8) 10920(7) 8628( 6) 4.6 1460(7) 5466( 5) 1907( 10) 7.3 3604( 7) 9973 (7) 5987( 5) 3.8 701(8) 3892(6) 1823 (9) 3.7 278(8) 6903 (7) 8948(5) 4.1 l882(8) 2982( 6) 973(9) 3.8 -719(7) 6552(7) 6419(6) 4.1 2457(8) 4009(6) 1555( 10) 3.8 3205(6) 6369( 6) 7326 (4) 2.7 1203( 11) 2893(8) -550( 11) 5.5 5138(7) 7484(6) 7077( 5) 3.3 271 3( 9) 2110(7) 1493( 11) 5.2 1749(6) 4532(6) 7671(5) 3.0 2794( 10) 4582(7) 618(11) 5.1 5567(7) 6778(7) 7215(5) 3.5 3498(9) 3989(7) 2928( 10) 5.3 2262( 7) 3901 (6) 7820( 5) 3.3 -242(8) 4155(6) 2179(8) 3.6 -1219(9) 3527(7) 1817( 11) 5.04708( 8) 4355(8) 7830(7) 4.6 -218(9) 5052( 7) 2931 (10) 4.7 -2119(10j 3814(8) 2139( 12) 5.8 -11 64( 1 1) 5283(8) 3192( 12) 6.1 the special positions (O,O,O) and (0,0,1/2), respectively.Each -3322( 14) 5070( 12) 2892( 15) 9.2 -3183( 17) 4639( 13) 3996( 19) 11.9pyridinium ring makes contacts with four out-of-plane iodide ions, resulting in a 2D network parallel to the o(liO) 6078(6) 879( 5 j 7793 (7) 3.7 4761 (6) 1591(5) 6075 (7) 3.7plane, where the shortest djstances are 3.606(5) A for 2225(7) -221(5) 7791(8) 4.1C(ll)(i).;.I(A)-(ii), 3.984(5) A for C(12)(i).-.I(A)-(iii), 6557(6) 389(5) 8832( 7) 5.3 3.829(5) A for C(ll)(i)-.-I(B)-(iv) and 3.742(5) A for 3766(6) 1847(5) 5149( 7) 5.5 C(12)(i).,.I(B)-(i) [symmetry operations; (i) x, y, z;(ii) x+ 1, 4907(7) 1012(6) 7068(8) 3.2 y+ 1, z + 1; (iii) x, y, z+ 1; (iv) x+ 1, y+ 1, 21.Fig. l(b) shows 6801(8) 13 12( 7) 7169(9) 3.9 the structure projected along the [1lo] direction; it consists 5899(8) 1948(7) 6116(9) 3.9 7825( 10) 1860( 9) 8208(12 6.7of a radical dimer whose top view is shown in Fig. 2(u). The 721 1 ( 11) 428(8) 6556( 13 5.9 two molecules are related by an inversion symmetry yith a 5976( 10) 3038( 7) 6525( 12 5.8 short distance between the NO groups; 3.383(8) A for 5829( 10) 1806(9) 4717(11 6.0 0(2)(i)...N(2)(v) [symmetry operation; (v) -x, -y+ 1, 3997(8) 599( 6) 7348(9) 3.7 4200(8) -203(6) 8115(9) 3.8-z + 11, which could be due to the intermolecular Coulombic attraction of N6+ 06-,reflecting a large charge polarization 2867(9) 983(8) 6805( 12) 5.7 3291(8) -617(7) 8319(10) 4.1on the NO bond.In this arrangement, the overlap between 1999( 9) 567(8) 7058( 12) 5.7 the TC orbitals on the NO groups appears very large. The 1257(10) -695( 7) 8027( 12) 5.4 interdimer arrangemept also has a short distance between the 614( 12) -10( 9) 8521( 16) 7.7 NO groups: 3.159(8) A for O(l)(i)-.-O(2)(vi) [symmetry oper- 1721(9) 3057( 7) 4701( 10) 9.3 ation; (vi) x + 1, y+ 1, z] (not shown). This is shorter than that in the intradimer arrangement, but the interdimer inter- J. MATER. CHEM., 1994, VOL. 4 Table 4 Atomic coordinates (x lo4) and equivalent isotropic thermal parameters (lo2 A)for 3' .I-atom x Y Z B eq atom X Jl Z B eq 408( 1) 2738( 1) 1916(2) 7.0 8721(19) 10294( 17) 8?33( 30) 9.5 9587( 1) 7268( 1 ) 8090(2) 7.6 9727( 16) 10629( 16) 7979(29) 9.6 3469(1) 7784( 1) 3198(2) 5.7 7588( 13) 7399( 12) 2214(20) 8.1 6529( 1) 2216( 1) 6802(2) 7.1 7554( 12) 6166( 11) 3466( 18) 6.5 6009( 15) 2510(11) 2318( 17) 7.2 10963( 11) 5644( 10) 2617( 16) 5.8 6647( 11) 1188(9) 2961( 17) 5.6 7900(9) 7874(9) 1293( 14) 6.7 2679( 12) 765( 10) 1990( 19) 6.7 7816(9) 5469(8) 41 15( 13) 5.6 5394(9) 3177(7) 1942( 14) 6.1 8140( 15) 6523( 12) 2753(22) 6.3 6680( 12) 355(9) 3346( 19) 8.8 6810( 14) 7653( 11 ) 2700( 19) 5.2 5731( 15) 1705(11) 2674( 17) 5.5 6492(14) 6572( 13) 3 143( 23) 6.6 7204( 13) 2687( 12) 2529( 18) 5.3 5878( 13) 8009(15) 1m7( 22) 6.9 7511( 16) 1639( 13) 2612( 25) 7.3 7370( 16) 8252( 14) 4011(22) 6.9 7224( 21 ) 3070( 16) 1087(28) 9.6 5675( 14) 6079( 12) 1746( 23) 6.7 7819( 17) 3207( 12) 4003 (21) 6.6 6409(17) 6683( 14) 4654( 22) 6.9 7273( 19) 1182( 15) 1022( 24) 7.7 9109( 12) 6245( 12) 2687(20) 5.5 8578( 13) 1385( 14) 3842( 25) 7.6 9757( 14) 6808( 10) 2O65( 20) 5.4 4743( 16) 1377( 13) 2604( 22) 6.4 9374( 17) 5386( 12) 31 21(21) 6.5 3940( 16) 1984( 17) 2224( 27) 8.7 10730( 17) 6622( 14) 2043 (24) 7.6 4515(16) 483( 13) 2630( 3 1) 8.6 10388( 14) 5106( 13) 3006(23) 6.4 2914( 16) 1563( 14) 2188(25) 7.3 11994( 14) 5420( 13) 2525(21) 5.7 3472( 16) 161( 14) 2297( 30) 8.6 11746( 18) 5083( 17) 929( 25) 8.0 1571( 16) 351( 15) 1630( 26) 7.5 12843( 17) 4737( 15) 891 (26) 7.7 1368( 18) 98(20) 2950( 3 1 ) 9.9 2345(9) 2654( 8) 7843( 12) 3.7 190( 18) -347( 20) 2421 (3 1) 10.0 2433 (9) 3797(8) 6617( 13) 4.0 3977(8) 7461 (7) 7670( 12) 3.4 -963 (9) 4242(8) 7504( 14) 4.2 3389( 11) 8783(9) 7002( 14) 5.O 2118( 11) 2046( 10) 8538(17) 8.0 7135( 12) 9317( 10) 7211(20) 7.1 2213( 12) 4542( 10) 5841( 18) 8.5 4595( 11) 6808( 9) 8000(15) 7.O 1880( 10) 3356(9) 7274(15) 3.4 3278 (9) 9597(8) 6572( 15) 6.5 3265( 13) 2496( 11) 7250( 18) 5.0 4197( 11) 8315( 10) 7320( 16) 4.0 3450( 13) 3381( 12) 6854( 17) 5.2 2830( 14) 7370( 11) 7437( 19) 5.2 4251 (15) 2052( 13) 8569(20) 6.2 2501( 12) 8343(11) 7376( 16) 4.6 2826( 18) 1747( 14) 5884( 21) 7.5 2768(13) 6801( 11) 8721( 15) 4.6 4360( 15) 4045( 15) 8077 (22 ) 6.8 2294( 14) 6755( 14) 5969(20) 6.4 3780( 15) 3538( 15) 5843( 21 ) 6.7 2293( 15) 8754( 12) 8797( 21) 6.2 901(11) 3659( 9) 7364( 16) 4.0 1420( 14) 8427( 13) 6 102( 20) 6.0 320( 11) 3114(11) 7830( 18) 4.8 5255( 11) 8623(10) 7347( 16) 3.9 533( 12) 4556( 10) 6934( 20) 5.1 6161( 12) 8134( 11) 7817( 18) 4.7 -609( 10) 3493(9) 7913( 16) 3.8 5292( 18) 9477( 14) 6550( 30) 8.6 -360( 14) 4821( 12) 69S1( 19) 5.5 7148( 17) 8422( 12) 8053( 19) 6.2 -2003( 15) 4651( 13) 7444(21) 6.0 6256( 15) 9777( 13) 6508( 26) 7.1 -1872( 13) 4945( 12) 8977( 19) 5.4 8177( 14) 9710( 13) 7013( 19) 5.9 -2873( 16) 5450( 15) 8838(22) 6.9 Table 6 Dihedral angles (degrees) between the pyridinium ring and Table 5 Atomic coordinates (x lo4)and equivalent isotropic thermal the nitronyl nitroxide group for 1' -4' parameters (lo2A)for 4+ .I-1+ 2+ 3+ 4i atom x V Z B eq 3 1.7( 3) A 22.1(4) A 16.9(14) 8.7( 2) 7934(0) 1324(0) 6041(0) 5.9 B 20.2(3) B 13.2(6) 4755(4) -23 10( 4) 2905 (4) 5.3 C 14.9(10) 4030( 5) -1945(4) 1129( 5) 5.9 D 10.5(6) 5080(4) 1419 (4) 2983(5) 5.4 5202(4) -2252( 4) 3970(4) 7.2 3761(7) -1469(4) 181(4) 9.5 action seems smaller than the intradimer one, because the 4491(5) -1565(4) 2167(5) 4.9 4554( 6) -3300( 5) 2335 (6) 5.8 two molecules, l+(i) and l+(iv), are arranged side by side 3760( 6) -3018( 5) 1195(6) 5.6 without a large n-orbital overlap.5684(8) -3662( 7) 2166(9) 8.8 4057(8) -3971 (6) 3126(7) 7.4 +3989(9) -3532(6) 127(7) 8.2 2 -1-.0.5H20 2550( 7) -3095( 7) 1260( 7) 7.3 The iodide salt of 2' takes an intermediate structure between 4697(5) -536( 5) 2444( 5) 4.9 5306(7) -238(5) 3500(7) 6.5 that of l+.I-and that of 3+.I-.The structure crystallizes in 4294( 6) 198(5) 1671(6) 6.2 the triclinic Pi space group with Z=4, consisting of two 548 l(6) 717(6) 3747(7) 6.7 crystallographically independent molecules, 2A and 2Bf .+ 4489(6) 1162(5) 1945( 6) 6.1 Fig. 3 shows a view of the crystal structure. The molecular 5321(7) 2456(6) 3316(7) 7.1 planes of 2Af(i) and 2B+(i) are oriented nearly perpendicular 6427( 7) 2747(6) 3 109 (8) 7.2 to each other [symmetry operation; (i) x,y,z].The pyridinium 6663(9) 3819(7) 3512( 12) 10.4 7725( 10) 4172( 10) 3390( 13) 12.4 ring of 2A+ makes contact with two out-of-plane iodide ions, while that of 2B+ does so with two in-plane iodide ions. This crystal includes both out-of-plane and in-plane iodide ions. The intermolecular, interatomic distances between the mol- J.MATER. CHEM., 1994, VOL. 4 :u + 0U" ,? -Fig. 1 Projection of the crystal structure of l+.I-:(a)along the [lTO] direction; (b) along the [1103 direction. For the symmetry operations, see text. ecule 2A+ and the two out-of-planeo iodide ions are 3.89( 1) A for C(llA)(i)-a-(A)(ii) and 3.72( 1)A for C( llA)(i)...I(B)(iii) [symmetry operations: (ii) x-1, y, z; (iii) x-1, y, z-11 and those between theomolecule 2B+ and the two in-plane i2dide ions are 3.822(9) A for C( llB)(i)...I(B)(iv) and 3.93( 1)A for C( 12B)(i)...I(A)(ii) [symmetry operation; (iv) --x+ 1, -y, -z+2]. The unit cell includes the two radical dimers, 2A+(i)...2A+(v) (dimer a) and 2B+(i)...2B+(iv) (dimer b) whose arrangements are shown in Fig.2(b) [symmetry oper- ation; (v) -x, -y+ 1, -z]. The intermolecular overlap in the dimer a is not so large, but tbe NO groups are not arranged far from each other; 4.16(1) A for 0(2A)(i)---N(2A)(v). The dimer b is formed by a contact between the NO group and the pyridinium ring with shqrt distances; 3.09( 1)A for O(lB)(i)-.-C( 11B)(iv), 3.40( 1)A for O(lB)(i)..-C( 13B)(iv) and 3.41( 1) A for O(lB)(i).--N(3B)(iv). 3+*I-The crystal of 3+.I- belongs to the triclinic P1 space group. The unit cell consists of four crystallographically independent molecules, 3A+-3D+, although 3A' and 3B+, and 3C+and 3D + are related by a pseudo-inversion symmetry. The analysis that assumes the Pi space group in which two molecules are independent, leads to a structure involving orientational dis- order of the pyridinium ring, in contrast to the analysis with the P1 space group which resulted in a structure without the disorder. If the orientational disorder is intrinsic, the latter structure should also include it.For these, we adopted the P1 space group. The large standard deviations of the atomic coordinates and thermal parameters given in Table 4 are presumably due to the pseudo-centrosymmetry. Q 6 dimer a dimer b , D Fig. 2 Intermolecular arrangements in the crystals of l+-I- -4'01~. For the symmetry operations and the labelling, see text. Fig.3 View of the crystal structure of 2+.I-. For the symmetry operations, see text. Fig. 4(u) shows a 2D layer parallel to the ub plane, consisting of 3A+, 3Cf, I(A)- and I(C)- .The molecular planes of the organic radicals are oriented parallel to the layer and the iodide ions lie in plane with respect to the pyridinium rings. The shortest distances bFtween the iodide ions and the pyridiq- ium rings are 3.65(3) A for C(l?A)(i)...I(C)-(ii), 3.98(3) A for C( 13A)(i)...I(A)-(i), 3.82(3) A for C( llA)(i)...IO(A)-(i), 3.87(2)A for C( 13C)(i)...I(S)-(iii), 3.78(2) A for C( llC)(i)...I(C)'-(iii) and 3.64(2) A for C( 12C)(i)...I(A)-(iii) [symmetry operations; (i) x, y, z; (ii) x,y-1, z; (iv) x+ 1, y,21. The molecules, 3B' and 3D+, and iodide ions form a similar 2D layer parallel to the ub plane. Fig. 4(b) shows a projection of the structure along the a axis.The two layers appear alternately along the c axis. There are short contacts between 3Af and 3B+, and between 3C+ and 3D+, in the interlayer molecular arrangement. Their arrangements shown in Fig. 2(c) are formed by short contacts between the NO group and the pyridinium ring. The sbortest distances between 3Af(i) oand 3B+(ii) are 3.57(3) A for 0(2A)...C( 13B), 3.51(3) A for 0(2A)...C(12B), 3.98(3) A for 0(2A)...N(3B) and 3.62(3) A for C( 1tA).-.0(2B) .Those between 3C+(i] and 3D+(iii) are 3.45(2) A fvr 0(2C)-..C( 13D), 3.98(2) A for 0(2C).-.C( 10D), 3.13(2)b for 0(2C)...C( 12D), 3.67(2) A for 0(2C).-.N(3D), 3.46(3) A for C(13C)-.-0(2D), 3.09(3) A for C( 12C)...0(2D) and 3.46(2) A for N(3C).-.0(2D). In the interdimer arrangements, interactions of the NO groups are protected by the methyl groups in the nitronyl nitroxide (not shown) .J. MATER. CHEM., 1994, VOL. 4 4+-I-The iodide salt of 4+ crystallizes into the monoclinic P&/n space group with Z=4. The crystal consists of a 2D layer parallel to the (lOi) plane, which resembles those in 3+.I-. Fig. 5(u) shows a projection of the layer along the [lOi] direction. The iodide ion in 4+-I- stands in plane with respect to the pyridinium ring, where the stortest distances are 3.751(8) A for C( ll)(i)a.-I-(i), 3.975(8) A for C( 12)(i)e.-I-(ii) and 3.957(9) A for C( 13)(i)-*.I-(ii) [symmetry operations; (i) x, y, z; (ii) x-1/2, -y+1/2, 2-1/21. Fig. 5(b) shows a projection of the structure along the b axis. The nitronyl nitroxide 4' exhibits a 1D alternating stacking along the c axis, whose geometry is shown in Fig.2(4. The radical molecules are arranged head to tail, and are connected by short contacts between the NO group and the pyridinium ring. The intermolecular, interatomic distances are 3.63( 1)A for O(l)(i)...C( ll)(iii), 3.39( 1)A for O(l)(i)...C( 13)(iii), C 3B+(ii) Fig. 4 Crystal structure of 3+.I-:(a) 2D layer projected along the c Fig,5 Crystal structure of 4+.I-:(a) 2D layer projected along the axis; (b)projection along the a axis (side view of the layers). For the [loll direction; (b) projection along the b axis (side view of the symmetry operations, see text. layers). For the symmetry operations, see text. J. MATER. CHEM., 1994, VOL.4 3.62(1)A for C(9)(i)...C( ll)(iii) and 3.62(1)A for 0(2)(i)..-C( 12)(iv) [symmetry operations; (iii) --x+ 1, -y, -z + 1; (iv) -x + 1, -y, -z].There is little n-orbital overlap between the neighbouring chains. Summary of the Solid-state Structures The position of the iodide ion in 1+.1--4+.1-changes from out-of-plane to in-plane with the length of the N-alkyl chain, which could cause the sequential decrease in the dihedral angle seen in Table 4. Analogously, the crystal of N-methyl- pyridinium iodide includes both in-plane and out-of-plane iodide ions,30 while that of N-butylpyridinium chloride includes only in-plane chloride ions.31 This may be because in the process of crystallization, free rotation of a longer N-alkyl chain prevents the iodide ions from approaching the nitrogen with a large share of the positive charge on the pyridinium ring.In this case, the iodide ions prefer the in-plane structure in which a Coulombic stabilization energy can be gained by making contacts with the hydrogens on the pyridin- ium ring. Fig. 2 shows the nearest-neighbour molecular arrangements observed in the crystals of 1'.1--4+*I-. The intermolecular contacts in this figure can be classified into the following two groups. The arrangements of 1 and 2+ (dimer a) are formed + by a contact between the NO groups (type I), while those of 2+ (dimer b), 3+ and 4+ are formed by a contact between the NO group and the pyridinium ring (type 11). It is the type I1 contact that we expected in the crystal of the N-alkylpyridin- ium nitronyl nitroxide.Note that the nitronyl nitroxide with an out-of-plane iodide ion has a type I nearest-neighbour arrangement, while that with an in-plane iodide ion has a type I1 arrangement. Fig. 6 shows space-filling views of the nearest-neighbour arrangement of 1 with the out-of-plane + iodide ions and 4+ with the in-plane iodide ions. The out-of- plane iodide ion appears to block the nitrogen on the pyridin- ium ring from approach by the neighbouring molecule. This could be why the out-of-plane iodide ion is involved in type I rather than type I1 intermolecular interactions. According to the discussion above, an antiferromagnetic interaction is expected for type I contacts, and ferromagnetic for type 11.Magnetic Properties Fig. 7(u) shows the temperature dependence of the paramag- netic susceptibilities, xp,of 1+-1--4+.1-, where xpT is plotted as a function of temperature. Fig. 7(b) shows the low-temperature behaviour of 3+-I-and 4+-I-on an enlarged OC N Fig. 6 Space-filling views of the intermolecular arrangements of l+-I-(a) and 4+.I-(b) I I I ' (a) I I I I I L O 10 20 30 40 500 0.1 I/ 0.0 100 150 200 250 0 50 TIK Fig. 7 (a)Temperature dependence of the paramagnetic susceptibility of: A, l+.I-; 0, 2+*I-;e, 3+-1-; 0, 4+.I-. (b) Low-temperature behaviour of 3+-I-and 4+*I-on an enlarged scale scale. The xpT values of l+*I-and 2+.I-decrease with decreasing temperature, indicating antiferromagnetic inter- molecular interactions, although the antiferromagnetic inter- action in 2+.I-is much weaker than that in l+.I-.xpT of 3+.I-shows an increase with decreasing temperature down to cu. 10K, and then a rapid decrease after passing through a maximum. This behaviour indicates the coexistence of a stronger ferromagnetic interaction and a weaker antiferromag- netic coupling between the ferromagnetic units. xpTof 4+.1-increases with decreasing temperature over the range 3-250K. In conclusion, the magnetic interaction of the p-N-alkylpyridinium nitronyl nitroxide changes from anti- ferromagnetic to ferromagnetic, as the N-alkyl chain length increases. Hereafter, we describe quantitative analyses of the magnetic properties of lf.I--4+.I-, in terms of an antiferromagnetic interaction in the type I contact and a ferromagnetic inter- action in the type I1 contact.1+*1-The crystal consists of a dimer whose molecular arrangement is of type I, suggesting an antiferromagnetic coupling. An antiferromagnetic interaction is also predicted for the inter- dimer arrangement, because it has a short contact between NO groups. In fact, the magnetic behaviour is well interpreted in terms of the modified singlet-triplet model, 4c xp = T[3+exp(-2J/k,T)] -6 where J is the intradimer coupling constant, 6 is the Weiss constant caused by the weak interdimer interaction, C is the Curie constant and kB is the Boltzmann constant. The deri- vation of eqn. (1)is described elsewhere.32 When J is positive, the ground state is a triplet, while a negative J means a ground singlet state.The solid curve fitted to the plots for l+-I-in Fig. 7 is the theoretical best fit of eqn. (1) with the parameters, J/kB= -74 K, 6= -4.7 K and C=0.376 emu 1384 K mol-' (1emu=4n x lop5m3). The intradimer strong anti- ferromagnetic coupling is caused by the large overlap between the SOMOs in the intradimer molecular arrangement shown in Fig. 2(a). 2+-1-*0.5H20 The crystal includes both type I and type I1 dimers. The observed weak antiferromagnetic interaction could result from cancellation of the antiferromagnetic contribution from the type I dimer by the ferromagnetic one from the type I1 dimer. The temperature dependence of xp is hence interpreted, assuming the two kinds of magnetic dimers, using 2c 2c xp = T[3 +exp(-2Jl/kBT)] + T[3+exp(-2J2/kBT)] (2) where J1 is the antiferromagnetic coupling constant for the type I dimer a, and J2is the ferromagnetic coupling constant for the type I1 dimer b.The theoretical best fit is obtained with J1/kB= -6.0 K, J2/kB =1.7 K and C = 0.376 emu K mol-'. Ferromagnetic interactions are predictable in the two type I1 dimers in Fig. 2(c), which have a very similar geometry. The rapid decrease of xpT below 10K indicates an interdimer antiferromagnetic interaction, although the origin in the crys- tal structure is not clear. We interpret the observed tempera- ture dependence using eqn. (1)with a positive J and a negative 8. The best fit is obtained with J/kB=2.4K, 8= -0.7 K and C =0.376 emu K mol-'.The radical moleculer 4' forms 1D alternating stacking chains, in which the two intermolecular arrangements are both of type 11, suggesting ferromagnetic interactions. Since the alternation appears very weak, the temperature depen- dence of xp is interpreted in terms of a 1D ferromagnetic chain,33 using xp= (C/T)[( 1+5.7979916K+16.902653K2 +29.37688~~+29.832959K4 +14.036981K5)/(1+2.7979916K+7.0086780K2 +8.6538644K3+4.57431 14K4)I2l3 (3) with K = J/2kBT). The theoretical best fit to the experimen- tal data is obtained with J/kB=0.30 K and C= 0.374 emu K mol-l. Table 7 shows the shortest intermolecular, interatomic distances in the type I1 arrangements of 2+, 3' and 4+, and the ferromagnetic coupling constants obtained.Nitronyl nitroxides 2+ and 3+, with distances of ca. 3.1 A, have ferromagnetic coupling constants of J/kBz2 K, while 4+, in which the distance is longer, has a much smaller constant. Comparison with m-N-Alkylpyridinium Nitronyl Nitroxides We have already reported that the magnetic properties of the iodide salts of m-N-R-pyridinium nitronyl nitroxides, with R=methyl, ethyl and n-propyl, can be interpreted in terms of a ferromagnetic intradimer interaction and an antiferromag- netic interdimer intera~tion.~~ The magnetic behaviour of the meta derivatives depends little on the length of the N-alkyl chain, in contrast to the variety of dependences exhibited by J. MATER. CHEM., 1994, VOL.4 Fig. 8 Intermolecular arrangement of rn-N-methylpyridinium nitronyl nitroxide (from ref. 17) the para derivatives. Fig. 8 shows the intradimer molecular arrangement of the rn-N-methyl derivative, which belongs to type I1 and, in fact, results in a ferromagnetic interaction such that J/kB z10 K.17 The intermolecular overlap of the meta derivatives appears much larger than those of the type I1 arrangements of the para derivatives shown in Fig. 2. If the para derivatives take an intermolecular arrangement such as that in Fig. 8, there should be a steric repulsion between the N-alkyl chain and the methyl groups of the nitronyl nitroxide. This implies that the type I1 arrangement, which is expected to be realized in the N-alkylpyridinium nitronyl nitroxide, is intrinsically unstable in the para derivatives, because of steric repulsion.In other words, the variety observed in the structure and magnetism of the para derivatives originated from a balance between the steric repulsion and the Coulombic attraction in the expected type I1 arrangement. Conclusion We have described the crystal structures and magnetic proper- ties of iodide salts of p-N-alkylpyridinium a-nitronyl nitrox- ides. We found interesting correlations between the length of the N-alkyl chain, the position of the iodide ion, the nearest- neighbour molecular arrangement and the magnetic inter- action. The iodide ion in the crystal of l+.I-is located out of the plane of the pyridinium ring, while the iodide ions in 3+.I-and 4+.I-are located in the plane of their rings.Both out-of-plane and in-plane iodide ions are observed in 2+.T-. The iodide ion in 1+.1--4+-1-changes position from out-of- plane to in-plane with increasing N-alkyl chain length, which can be understood in terms of a steric effect of the N-alkyl chain. The dihedral angle between the pyridinium ring and the nitronyl nitroxide group decreases with length of the chain, presumably reflecting the positions of the iodide ions. The out-of-plane iodide ion causes a type I nearest-neighbour molecular arrangement of the nitronyl nitroxide in which there is a short distance between NO groups, while the in-plane iodide ion produces a type I1 arrangement in which there is a contact between the NO group and the pyridinium ring.The former situation means that there is an overlap between the SOMOs, while latter results in an overlap between the SOMO and the other frontier orbitals. The magnetic behaviour of 1 +-1--4+-1- can be quantitatively interpreted Table 7 Shortest intermolecular, interatomic distances in type I1 contacts and ferromagnetic coupling constants 2' 3+ (dimer b) 3.09(1) 3.09(3) 1.7 2.4 4+ 3.39(1) 0.3 J. MATER. CHEM., 1994, VOL. 4 in terms of an antiferromagnetic intermolecular interaction in the type I contact and a ferromagnetic interaction in the type I1 contact. This work was supported by a Grant-in-aid for Scientific Research (No. 05453051) on Priority Area ‘Molecular Magnetism’ (area no.228/04242103) from the Ministry of Education, Science and Culture, Japan. Support from New Energy and Industrial Technology Development Organization (NEDO)is also acknowledged. References 1 Research Frontiers in Magnetochemistry, ed. Charles J. O’Connor, World Scientific, Singapore, 1993. 2 M. Kinoshita, P. Turek, M. Tamura, K. Nozawa, D. Shiomi, Y. Nakazawa, M. Ishikawa, M. Takahashi, K. Awaga, T. Inabe and Y. Maruyama, Chem. Lett., 1991, 1225; M. Takahashi, P. Turek, Y. Nakazawa, M. Tamura, K. Nozawa, D. Shiomi, M. Ishikawa and M. Kinoshita, Phys. Rev. Lett., 1991, 67, 746; M. Tamura, Y. Nakazawa, D. Shiomi, K. Nozawa, Y. Hosokoshi, M. Ishikawa, M. Takahashi and M. Kinoshita, Chem. Phys. Lett., 1991, 186, 401; L. P. Le, A.Keren, G. M. Luke, W. D. Wu, Y. J. Uemura, M. Tamura, M. Ishikawa and M. Kinoshita, Chem. Phys. Lett., 1993, 206,405. 3 P-M. Allemand, K. C. Khemani, A. Foch, F. Wudl, K. Holczer, S. Donovan, G. Gruner and J. Thompson, Science, 1991,253,301; K. Tanaka, A. A. Zakhidov, K. Yoshizawa, K. Okahara and T. Yamabe, Phys. Rev. B, 1993,47,7554. 4 R. Chiarelli, M. A. Novak, A. Rassat and J. L. Tholence, Nature (London), 1993,363,147. 5 T. Nogami, K. Tomioka, T. Ishida, H. Yoshikawa, M. Yasui, F. Iwasaki, H. Iwamura, N. Takeda and M. Ishikawa, Chem. Lett., in the press. 6 N. Nakamura, K. Inoue and H. Iwamura, Angew. Chem., Int. Ed. Engl., 1993,32,872. 7 H. 0.Stumpf, L. Ouahab, U. Pei, D. Grandjean and 0. Kahn, Science, 1993, 261, 447. 8 H. Tamaki, Z.J. Zhong, N. Matsumoto, S. Kida, M. Koikawa, K. Achiwa, Y. Hashimoto and H. Okawa, J. Am. Chem. SOC., 1992, 114,6974. 9 J. M. Manriquez, G. T. Yee, R. S. McLean, A. Epstein and J. S. Miller, Science, 1991, 252, 1415; P. Zhou, B. G. Morin, J. S. Miller and A. Epstein, Phys. Rev. B, 1993,48, 1325. 10 K. Awaga, T. Inabe and Y. Maruyama, Chem. Phys. Lett., 1992, 190, 349. 11 P. Turek, K. Nozawa, D. Shiomi, K. Awaga, T. Inabe, Y. Maruyama and M. Kinoshita, Chem. Phys. Lett., 1991, 180, 327. 12 P-M. Allemand, C. Fite, P. Canfield, G. Srdanov, N. Keder and F. Wudl, Synth. Met., 1991,41-43, 3291. 13 T. Sugano, M. Tamura, M. Kinoshita, Y. Sakai and Y Ohashi, Chem. Phys. Lett., 1992,200,235. 14 E. Hernandez, M. Mas, E. Molins, C. Rovira and J.Veciana, Angew. Chem., Int. Ed. Engl., 1993,32,882. 15 F. L. Panthou, D. Luneau, J. Laugier and P. Rey, J. Am. Chem. SOC.,1993, 115, 9095. 16 K. Awaga, T. Inabe, T. Nakamura, M. Matsumoto and Y. Maruyama, Chem. Phys. Lett., 1992,195,21. 17 M. Tamura, D. Shiomi, Y. Hosokoshi, N. Iwasawa, K. Vozawa, M. Kinoshita, H. Sawa and R. Kato, Mol. Cryst. Liq. Crj yt., 1993, 232, 45. 18 K. Inoue and H. Iwamura, Chem. Phys. Lett., 1993,207, :!51. 19 M. Okumura, K. Yamaguchi, M. Nakano and W. Moii, Chem. Phys. Lett., 1993, 207, 1. 20 D. G. B. Boocock and E. F. Ullman, J. Am. Chem. SOC., 1968,90, 6873; E. F. Ullman, J. H. Osiecki, D. G. B. Boocock and K. Darcy, J. Am. Chem. SOC.,1972,194,7049. 21 K. Awaga, T. Yokoyama, T. Fukuda, S. Masuda, Y. Harada and Y. Maruyama, Mol. Cryst. Liq. Cryst., 1993,232,27. 22 E. Ressouche, J-X. Boucherle, B. Gillon, P. Rey and J. Sc hweizer, J. Am. Chem. SOC.,1993,115,3610. 23 K. Awaga, T. Sugano and M. Kinoshita, Chem. Phys. Lett., 1987, 141, 540. 24 H. M. McConnell, J. Chem. Phys., 1963,39,1910. 25 K. Yamaguchi, T. Fueno, K. Nakasuji and I. Murata, Cht m. Lett., 1986,629. 26 K. Awaga, T. Inabe, U. Nagashima and Y. Maruyama, I. Chem. SOC., Chem. Commun., 1989,1617; 1990,520. 27 K. Awaga, T. Inabe, U. Nagashima, T. Na kamura, M. Matsumoto, Y. Kawabata and Y. Maruyama, Chew. Lett., 1991,1777. 28 A. Yamaguchi, K. Awaga, T. Inabe, T. Nakamura, M. Ma tsumoto and Y. Maruyama, Chem. Lett., 1993,1443. 29 K. Awaga and Y. Maruyama, Chem. Muter., 1990,2,535. 30 R. A. Lalancette, W. Furey, J. N. Costanzo, P. R. Hemines and F. Jordan, Acta Crystallogr., Sect. B, 1978,34,2950. 31 D. L. Ward, R. R. Rhinebarger and A. Popov, Acta Cry:,tallogr., Sect. C, 1986,42, 1771. 32 K. Awaga, T. Okuno, A. Yamaguchi, M. Hasegawa, I.Inabe, Y. Maruyama and N. Wada, Phys. Rev. B, 1994,49,3975. 33 D. D. Swank, C. P. Landee and R. D. Willett, Phys. Rev. B, 1979, 20,2154. 34 K. Awaga, T. Inabe, T. Nakamura, M. Matsumcto and Y. Maruyama, Mol. Cryst. Liq. Cryst., 1993,232,69. Paper 4/01395F; Received 9th March, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401377

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(9), 1387-1392 Polymer-mediated Crystallisation of Inorganic Solids: Calcite Nucleation on the Surfaces of Inorganic Polymers Kim K. W. Wong," Brian J. Brisdon,*a Brigid R. Heywood,bAnnabelle G. W. Hodson"and Stephen Manna a School of Chemistry, University of Bath, Bath, UK BA2 7AY Department of Chemistry, University of Salford, Salford, UK M5 4WT Department of Chemical and Physical Sciences, University of the West of England, Bristol, UK BS16 IQY Nucleation of calcite from supersaturated calcium hydrogencarbonate solution has been achieved on the surfaces of poly(dimethylsi1oxane)films formed by cross-linking in the presence of catalytic quantities of dibutyltindilaurate or zinc octanoate. Films containing high concentrations of the diorganotin catalyst produced, in addition to calcite, amorphous calcium silicate and calcium silanolate under similar conditions.Polar functional groups incorporated into the polysilox-ane network served to promote calcite growth, but prolonged heating of all films at 90°C rendered them inactive. The controlled deposition of inorganic crystalline solids upon polymeric surfaces is of considerable interest because of the potential technological' and biological applications2 of such processes, particularly in the area of nanoscale synthesis, crystal engineering, and microstructural fabrication., Mineral nucleation and growth upon the surfaces of organic polymers has been widely but to date we are not aware of analogous studies conducted on the controlled growth of inorganic materials upon polymers with an inorganic back-bone, with the exception of a single preliminary publication concerning poly(organosi1oxane) films.' Both the polarity of the Si-0 link, and the ease with which a range of organofunc- tional substituents can be introduced in a controlled and reproducible manner into polysiloxane membranes9-'' pro-vides a combination of highly desirable properties which may be useful in the study of crystal engineering and fabrication of hybrid composites.Polysiloxane membrane films, produced by cross-linking Me,SiO [MeSi(H)O],SiMe, (xz40) with a,co-dihydroxypoly-(dimethylsiloxane), in the presence of traces of tetraethoxysil-ane, using a dibutyltindilaurate catalyst, are formed as elastic, transparent, permeable membranes with a random three-dimensional network of polysiloxane chains.A variety of organofunctional groups can be introduced into these films by reactions leading in effect to partial replacement of the Me substituents on silicon, and as reported previously,' both functionalised and unfunctionalised membranes promote the growth of inorganic solids from a supersaturated calcium hydrogencarbonate solution. The solid deposits were shown to contain crystalline calcite and amorphous calcium silicate, but the role of the substituent groups on Si in promoting these effects was unclear. In this paper we define in more detail the nature of the inorganic solids nucleated from aqueous calcium hydrogen-carbonate on poly(organosi1oxane) surfaces, and elucidate possible mechanisms by which they are formed.Experimental Preparation of SupersaturatedCalcium Hydrogencarbonate Solution Kitano Method12 Scrubbed carbon dioxide gas was bubbled into a stirred, aqueous suspension of calcium carbonate (10 g calcite in 4 dm3) at a rate of 0.18 m3 h-l for ca. 70 min. The suspension was filtered and the filtrate purged with C02 gas for ca. 35 min to dissolve any residual crystal nuclei. The resultant solution had a pH of 5.8-6.2. The total dissolved calcium (ca. 9 mmol drnp3)was determined by EDTA titration. Slow loss of C02 gas from the solution resulted in CaCO, crystallihation. CaCO,(s) +C02(g)+H20(1)SCa2+(aq)+2HCO,( hq) Metastable Method13 Equal volumes of sodium hydrogencarbonate and calcium chloride dihydrate, solutions (8 mmol dmP3 in 0.1 mmol dm-3 NaCl) were mixed at ambient temperature (ca.25 "C) and the pH raised to 8.3 with NaOH (as) (0.1mol drnp3), whilst stirring, to give a metastable solution of calcium hydrogen-carbonate. 2NaHC03(aq)+CaCl,(aq) g2NaCl (as)+Ca(HCO,),(aq)I1 CaCO,(s)+ CO,(g)+H,O(l) Preparation of Polysiloxane Membrane Films Unfunctionalised poly(dimethylsi1oxane) membrane films were prepared by the addition of dibutyltindilaurate (0.016-0.36 g; 0.025-0.57 mmol drnp3) to a thoroughly stirred mixture of poly(methylhydrosi1oxane) Me,SiO [MeSi(H)O],SiMe, (xz 40; 0.60 g; 0.23 mmol dm-,), a,o-dihydroxypoly (dimethylsiloxane) (Mwz 74 000; 6.0 g; 0.08 mmol drn-,) and tetraethoxysilane (0.60g; 2.9 rnmol drn-,).The resulting gel was held between Cellophane hheets and pressed under ca. 12.5kg cm-2 to produce large trans- parent films with a thickness of ca. 0.1 mm. Unfunctionalised membrane films cross-linked with dibutyltindiacetate (0.001-0.60 g; 0.003-1.7 mmol drn-,), and dibutyltindiineth- oxide (0.15-0.92 g; 0.56-3.5 mmol dm-,) or zinc octanoate (0.18 g; 0.51 mmol drnp3) dispersed in 1.97 g polydimethyl-siloxane, were prepared similarly. Unfunctionalised poly(dimethylsi1oxane) membrane films containing only two silicon-containing components were also prepared in a similar manner by the addition of dibutyl- tindilaurate (ca. 0.2g; 0.3 mmol dm-,) to a thoroughly stirred mixture of a, o-dihydroxypoly(dimethy1silouane) 6.57 g; 0.014 mmol dm-,) with either pol!;(me-(M, ~74000; thylhydrosiloxane) Me3Si0[MeSi( H)O],SiMe, (x z40; 0.66 g; 0.26 mmol drn-,), or tetraethoxysilane (0.66g; 3.2 mmol drn-,).Synthesis of Zinc Octanoate Zinc octanoate was synthesized by dissolving sodium octano- ate (24.40 g, 0.15 mol drn-,) in warm EtOH (250 cm3), and adding anhydrous ZnC1, (10 g, 0.073 mol drn-,). The reaction mixture was heated under reflux for 24 h, then allowed to cool to room temperature and treated with water (100cm3). The insoluble solid was filtered, washed thoroughly with water, and dried over P,05 in uucuo to afford a white crystalline product (18.8 g, 73%). mp 134-135 "C; v,,,(Nujol)/cm-l; 1533 (C=O str.).'H NMR (DMSO; 400MHz; 120°C) SH: 0.88 (6 H, q, 2 x -CH,, J 7.0), 1.29 [16 H, s, 2 x CH,fCH,),CH,], 1.52 (4 H, d of t, 2x -O-CH2-CH2-, J 7.0), 2.14 (4 H, t, 2 x -O-CCH,-CH,-, J 7.0). Found: C, 54.30; H, 8.86%; C16H3,Zn0, requires: C, 54.63; H, 8.60%. Preparation of Functionalised Polysiloxane Films9-11 A series of linear polymers of general formula Me,SiO [MeSi( H)0], { MeSi[( CH,),L] 0},%Me, containing side-chain functionalities -(CH,),L (L =-C,H,, -C02Me and -C6F5) were prepared by the platinum-catalysed addition of the appropriate l-alkenyl derivative to the poly (methylhydrosiloxane) Me,SiO [MeSi( H)O],,SiMe, . The molar concentrations of reagents were adjusted to produce materials with a high degree of functionalisation.The remaining Si-H groups in the linear fluids Me,SiO [Me%( H)O], { MeSi [(CH,),L]O),SiMe, were cross- linked with the same a,o-disilanol in the presence of minimum tetraethoxysilane and dibutyltindilaurate catalyst, to produce membranes containing up to 60 mol%t of -C3H7, -CO,Me or -C,F, groups (Fig. 1). Alternatively, functional groups were introduced into the polysiloxane membrane films by the reaction of commercially available alkoxysilanes (RO),Si (CH,),L (R =Me, Et), which t Mol% functionality expressed as a percentage of the total moles of Si atoms bearing functionalities. Me3SiO[MeSi( H)O],+iMe3 + CH2=CH(CH2),-2L n = ca.40 L = functional group Me3SiO[MeSi(H)O]fleSi(CH2),&(0)],,SiMe3 HO(Me$iO),HI cross-linked membrane film Me Me Me3Si-0 Si-0 I I L L Fig.1 Idealised structure of a functionalised polysiloxane membrane film prepared by method A J. MATER. CHEM., 1994, VOL. 4 contain a side-arm functionality L [L= -NMe,, -C5H4N, -CN, CH,=C(CH,)CO,-and -NHCO,Et] with an a,o-dihydroxypoly(dimethylsi1oxane)of molecular weight 16000, to yield pressed membranes containing up to 30mol% of functionalised Si atoms (Fig. 2). Crystallisation Experiments All experiments were conducted at ambient temperature (ca. 20-25 "C). Membrane film samples (ca. 18 mm x 18 mm) were cut from the polysiloxane membrane film, mounted onto glass slips and immersed at the bottom of clean crystallisation dishes containing supersaturated calcium hydrogencarbonate solu- tions.A glass slip was used as control surface. Mineral deposition was allowed to occur for ca. 24 h under still conditions. The calcium hydrogencarbonate solution was removed and the samples removed for optical microscopy, X-ray diffraction, transmission infrared and FTIR spec-troscopy, and scanning and transmission electron microscopy. Treated Films Crystallization experiments were also undertaken with mem- brane films cured at 80-90°C for 7 days. Some of these cured films were immersed in dilute acid (HC1,2 mol dm-,, 50 cm3) or base (NaOH, 2mol drn-,, 50 cm3) for 24 h, then thor- oughly rinsed in distilled water and immersed in water for 24 h before use. For comparison uncured samples of film were also immersed in acid (1mol dmP3 H,SO,; 2 mol dmP3 HNO,) or base [2mol dmP3 NH, (as)] for either 1 h or for 24 h at 40°C, washed thoroughly in water and then soaked in water for a minimum of 1h.The water was changed 0.5 h before the membranes were immersed in a calcium hydrogen- carbonate solution. Individual Components and Functionalised Glass Surfaces Separate samples of a,o-dihydroxypoly(dimethylsi1oxane) (a,co-disilanol; M, z 74OOO), poly(methylhydridosi1oxane) Me,SiO [MeSi( H)O],SiMe, (xG40) and tetraethoxysilane (ca. 0.25 cm3 each) were spread onto the surface of a solution of calcium hydrogencarbonate solution and a glass slip allowed to fall vertically through the surface film to the bottom of the crystallisation dish. Additionally, samples of a,o-disilanol (6.0 g) mixed with dibutyltindilaurate (ca.0.1 g), and a,o-disilanol (10g) mixed with lauric acid (0.05 g), which is liberated during the cross-linking process, were separately smeared onto glass slips before immersion in a supersaturated calcium hydrogencarbonate solution. Glass (RO)3Si(CH2),,L + HO(Me2SiO)J-l cross-linked membrane I I0 0 0 0 I I Fig. 2 Idealised structure of a functionalised polysiloxane membrane film prepared by method B J. MATER. CHEM., 1994, VOL. 4 slips (10 mm diameter) were silanised in a refluxing toluene- trialkoxysilane (RO),Si( CH,),L’ (R =Me, Et;) [L’ = -CN, -CH,NMe,, -(CH,),CH,, -C02CZH5, -CH,C,H,N,, -CH,NHC0,C2H, or -CH,CO,C(CH,)=CH,] mixture for 24 h. The glass slips were recovered, washed thoroughly in toluene and air dried before immersion in supersaturated calcium hydrogencarbonate solution.Results and Discussion Unfunctionalised Poly (dimethylsiloxane) Membrane Films Efect oj Cutulyst Concentration As noted previously,8 uncured polysiloxane films, prepared as above, promote the growth of inorganic solids on immersion in a supersaturated calcium hydrogencarbonate solution. The cross-linking catalyst, dibutyltindilaurate, used in the fabri- cation of unfunctionalised polysiloxane membrane films, had a profound effect on the rate of formation and composition of the mineral growth. At low concentration (0.2 wt.%) only calcite growth was observed [Fig. 3(a)], with the crystals being discrete and partially embedded in the membrane. Crystals were clustered, with sizes ranging from ca.0.5 to 3 pm, the larger crystals having well defined rhombohedra1 calcite morphology, and with no preferred crystal orientation. This habit is characteristic of the equilibrium form of calcite which was often observed in the control experiments, suggest- ing that the crystals, once nucleated on the membrane, grow unperturbed in the supersaturated solution. In contrast, at higher catalyst concentrations (up to 4 wt.%) the calcite growth was overlaid with a filamentous amorphous ‘coral’- like growth [Fig. 3(b)],this growth becoming more profuse and elaborate as the cross-linking catalyst concentration was increased [Fig. 3(c)].Changing the degree of supersaturation of the calcium hydrogencarbonate solution over the range 8.5-1.1 mmol dm-, had no significant effect on the growth of the amorphous solid.SEM and XRD analyses confirmed that calcite was present at a level of 5-10wt.% in the amorphous solid, which was shown by FTIR, TEM and energy dispersive X-rays (EDXA), to be amorphous calcium silicate together with a calcium salt or salts of one or more organosiloxane species. Attempts to separate the individual components have not been successful to date, but analytical and IR data on the amorphous material are fully consistent with it being a calcium salt of one or more oligomeric dimethylsilanolates of the type known for various other metal ions. Thus the solid exhibited strong IR absorption in the C-H, Si-Me (sym.and asym.) and Si-0-Si stretch regions, and on thermal decomposition, cyclo-Me6Si,0, was identified in the products from its mass spectrum. These products are presumably formed by a diffusion-limited process at the polymer/water interface from silaceous materials eman- ating from the solid matrix into a CaZf-rich solution. A similar growth pattern was also observed when membrane films were immersed in calcium hydrogencarbonate solutions prepared by mixing chemical solutions rather than CO, gassing. As precipitation in the latter is driven by the evolution of C02 gas, it seems unlikely that promotion of crystal growth in the presence of the membranes is due to the dissolution of carbon dioxide in the films with concomitant crystal forma- tion.It seems probable therefore that calcite growth results from calcium-ion clustering around polar sites on or within the surface of the membrane film, resulting in a lowering of the activation energy to calcite nucleation. These polar sites may be unreacted silanol groups, which IR measurements reveal to be present in uncured films and which arise from hydrolysis of residual Si- H and/or Si-OR groups. Moreover, these polar residues may be segregated into Fig. 3 Scanning electron micrographs of (a)calcite, (b)and (c ) amor-phous overgrowth, deposited on the surface of unfunctic malised polysiloxane membranes. Scale bar = 10 pm. ‘domains’ of exposed Si-0-H linkages within the three- dimensional network of cross-linked polysiloxane chains such that they act as regiospecific nucleation sites.EfSects of Membrane Composition A study of the individual components from which the film was prepared was performed in order to determine R hether the activity of the membrane film might be related dirrctly to only one of them. None of the individual components pro- moted ‘coral’ growth or calcite formation while the m:iterials were immersed in supersaturated calcium hydrogencar bonate solution. Membrane films were also fabricated from the u,m- disilanol plus either tetraethoxysilane or the polyhydride, and were cross-linked using a dibutyltindilaurate catalyst, in order to ascertain whether a combination of only two components was responsible for the calcium silicate overgrowth.Growth occurred on all such membrane films on immersion in calcium hydrogencarbonate solution. However, those fabricated with (EtO),Si appeared to be the more active, probably due to a higher final concentration of silanol groups after immersion in the aqueous phase. Membranes containing DifSerent Catalysts The effect of two other tin-containing cross-linking catalysts, dibutyltindiacetate and dibutyltindimethoxide, were investi- gated in order to determine the effect of the catalyst composi- tion on mineral growth. Catalyst concentrations were varied from 0.01 to 7.6% (ca. 0.001-0.60 g) and 1.2-11.3% (0.15-0.92 g), respectively, of the total film weight. Unlike membranes cross-linked with dibutyltindilaurate, films cross-linked with dibutyltindiacetate and dibutyltindime- thoxide did not promote the growth of amorphous material, but crystalline calcite was deposited.The dibutyltindiacetate catalyst [Fig. 4(a)] was the more effective, producing discrete crystals, with sizes ranging from ca. 1 to 3 pm, which exhibited well defined rhombohedra1 morphology, and with the majority of crystals possessing secondary crystal growth. Crystals grown on the membrane films cross-linked with dibutyltindi- methoxide were of similar size, discrete but less well defined, and with a nucleation density ca. 20% of that found on membranes containing the former catalyst at a comparable -Fig. 4 Scanning electron micrographs of calcite deposited on the surface of unfunctionalised polysiloxane membranes cross-linked with (a)dibutyltindiacetate and (b)zinc octanoate.Scale bar = 10 pm. J. MATER. CHFM., 1994, VOL. 4 Sn concentration. The crystals had the appearance of being embedded in the surface of the membrane film in both instances. Varying the catalyst concentration did not greatly enhance the activity of the films. Membranes were also prepared using zinc octanoate? as a 9% dispersion in poly(dimethy1siloxane) as catalyst in order to ascertain whether mineral growth promotion was a prop- erty innate only to tin catalysts. Such films promoted calcite growth only, and it was found that zinc octanoate was more active in this respect than dibutyltindimethoxide. Crystals were discrete, with evidence of some secondary growth, and nucleation densities were similar to those on films containing an equivalent concentration of dibutyltindiacetate catalyst. Crystal size ranged from 1 to 10 pm, with the smaller crystals (<3 pm) possessing well defined calcite morphology [Fig.4(b)].Attempts to use high zinc catalyst concentrations were unsuccessful due to the highly insoluble nature of the catalyst in suitable solvents of low volatility. We conclude that unfunctionalised membranes containing tin or zinc catalysts promote calcite growth on the membranes, and that tin does not therefore play a unique role in determin- ing the nucleation and growth of this mineral. although some catalysts are more effective in producing active films than others. Of the three tin catalysts investigated only dibutyltindi- laurate containing films generated calcium silicate/calcium silanolates.The effect of laurate appears to be very subtle in that its addition as the free acid to dibutyltindiacetate cross- linked membrane films resulted in increased sizes (ca.5-20 pm) of calcium carbonate crystals. Its role in the promotion of other amorphous calcium salts is not completely clear, but as siloxane linkages are acid-sensitive, both calcium silicate and calcium silanolates may arise from fragmentation of the polymer matrix at its SiO,,, and R,SiO,/, centres induced by free lauric acid from the catalyst. Alternatively, disruption of the polysiloxane framework, when immersed in the calcium hydrogencarbonate solution, may be initiated by the acid at heterosiloxane linkages involving the metal catalyst, which for Sn are known to be particularly hydrolytically unstable14 t The cross-linking process when zinc octanoate, as a 9% dispersion in poly(dimethylsiloxane), is used as a catalyst is very slow compared to other catalysts in this study.R2’Sn(OCOR2)2t H20 ,0COR2 R2’Sn, R,Si(OR), ROH f%’Sn,OSi(OR),&,,0COR2OH 1 OCOP R,’Sn’I + HOSi(RO),3R, OH Jca-Ca2[0Si(R0),&L Fig. 5 Catalytic cycle of the SiOH/SiOR condensation illustrating a possible pathway for the formation of silanolates J. MATER. CHEM., 1994, VOL. 4 and easily cleaved by protic medial5 (Fig. 5). We note that a sample of a di butyltindilaurate-containing membrane film on immersion in a calcium chloride solution (9mmol dm-3) yielded after 5 days maturation, a similar amorphous deposit to that described above, although the coverage on the mem- brane’s surface was not extensive.Treated Membranes Curing the dibutyltindilaurate membrane films by prolonged heating rendered them inactive. A timed study showed that very little effect was evident on the rate of mineral growth after 8 h at 80-90°C, but a marked decrease was observed after 24 h curing, with complete inhibition of mineral growth on membranes after 3-4 days at this temperature, by which time no silanol groups were detectable by IR measurements. Such conditions are also likely to lead to rearrangement of Fig. 6 Scanning electron micrographs of growth deposited on the surface of unfunctionalised polysiloxane membranes: (a) amorphousovergrowth, cross-linked with dibutyltindilaurate and treated with dilute HNO,; (b) calcite, cross-linked with dibutyltindiacetate and treated with dilute aqueous NH,; and (c) calcite, cross-linked with dibutyltindilaurate, cured and treated with dilute HC1.Scale bar = 10 pm. Si-0-Sn linkages to more stable Si -0-Si and Sn -0-Sn combinations. Acid treatment of uncured dibutyltindilaurate cross-linked films enhanced solid formation, compared to untreated membrane films. The crystal morphology of the calcite formed was poorly defined [Fig. 6(a)], and crystals were extensively overgrown by amorphous calcium salts. Base treatment resulted in the growth of calcite (ca.1 pm) with well defined crystal morphology and with a very low nucleation density. Uncured dibutyltindiacetate-containing membranes yielded significantly greater quantities of calcite after treatment with NH3 (as) [Fig. 6(b)].Crystals were discrete, possessing well defined calcite morphology, were large in size (ca. 10 pm) with a narrow size distribution, and showed no preferred crystal orientation. Treatment of such films with dilute H,S04 resulted in calcite only with a poorly defined morphology and low crystal nucleation density. Treatment with HNO, inhibited mineral growth completely. Treatment of cured, inactive dibutyltindilaurate-cont aining membrane films with 2mol dmd3 HC1 caused reactivation towards calcite growth only [Fig. 6(c)].Crystals were discrete with a high nucleation density approaching that produced by analogous uncured films, and they were significantly larger (ca. 10 pm) than previously observed, with no preferred orien- tation. Curing and then treatment with dilute H2S04 or HN03 rendered the membrane films very much less active. Treatment of such films with 2mol dm-3 NaOH, dld not reactivate them, and this may be due to neutralisation of free lauric acid and Na’ ions capping the available nucleation sites required for mineral growth. Organo-functionalised Poly(dimethylsi1oxane) Membranes Uncured functionalised membrane films promoted, to varying degrees, mineral growth of a similar nature to that above, H Fig.7 Optical micrographs illustrating the effect of organo-functionalities on mineral deposition: (a) CH, =C(CH3)C ‘02-; (h) -C,F,; (c) -NHC02Et; and (d) -CN. Scale bar= 500 prn. (a ) H Fig. 8 Optical micrographs illustrating the effect of increasing mol% organo-functionality -(CH,),L (L= -OMe) on the amorphous overgrowth: (a) 15, (h)30, (c)40 and (d) 50 molyo. Scale bar =500 pm. with calcite as a minor component (Fig. 7). The activity of the functionalities increased in the order: -NMe, z -C,H,N zz -C7HI3 < CH,=C(CH,)CO,-< -OMe< < -C,F, < < -CN d -NHCO,Et. A series of uncured and cured/acid-treated membrane films with 15-50 mol% methyl ester loadings were also studied. The quantity of amorphous calcium salts deposited appeared to increase per unit time on uncured films with increasing methyl ester loading (Fig.8). SEM analysis confirmed that calcite growth had also occurred, with crystals often branching off from a central calcite crystal on the membrane film surface (cf unfunctionalised membrane), but these centres were surrounded by ‘islands’ of amorphous material. Curing the films rendered them completely inactive, but subsequent treatment with 2 mol dmP3 HC1 partially reactivated the membrane films towards calcite and calcium silicate/silanolate formation. Glass slips, treated with organo- functional alkoxysilanes, were not found to promote mineral growth. Thus the more polar functionalities serve only to enhance the inherent ability of uncured, cross-linked poly- (dimethylsiloxane) membranes to promote calcite growth.Conclusions We have shown that it is possible to promote and control the growth of the inorganic mineral calcite on unfunctionalised J. MATER. CHEM., 1994, VOL. 4 polysiloxane membrane film surfaces. This can be achieved by adjusting the concentration of the cross-linking catalyst, dibutyltindilaurate, used to fabricate the membrane films. In this way the growth of amorphous Ca/Si-containing materials can be inhibited, permitting the growth of calcium carbonate to predominate. The growth activity is not restricted to tin- containing films, since a zinc catalyst can also be used to form membranes which promote the growth of calcium carbonate. In addition we have shown that all salt deposits can be completely inhibited by curing procedures and that these films can be re-activated by acid treatment to induce calcium carbonate growth only.We have also shown that the presence of different organo- functional groups incorporated into the polysiloxane network may increase or inhibit the nucleation activity of the mem- brane films. Films with accelerated growth activity show growth patterns similar to those of the unfunctionalised membrane films. We conclude that clusters of polar functionalities at the surface of the film (silanol or organofunctional) provide the effective nucleation sites, and that the polar backbone of the polymer does not influence the crystal growth of the inorganic material calcite.The authors thank SERC for their support in this work. References 1 P. D. Calvert and S. Mann, J. Muter. Sci., 1988,23, 3801. 2 L. Addadi, J. Moradian, E. Shay, N. G. Maroudas and S. Weiner, Proc. Natl. Acad. Sci. USA, 1987,84,2732. 3 S. Mann, Nature (London), 1993,365,499. 4 E. Dalas, J. Muter. Chern., 1991, 1,473. 5 E. Dalas, J. Kallitsis and P. G. Koutsoukos. J. Crystal Growth, 1988,89, 287. 6 P. A. Bianconi, J. Lin and R. Strzelecki, Nafure (London), 1991, 349, 3 15. 7 P. D. Calvert and A. Broad, Materials Synthesis Utilising Biological Processes, ed. P. C. Rieke, P. D. Calvert and M. Alper, MRS Symp. 1990, vol. 17, pp. 61-67. 8 B. J. Brisdon, B. R. Heywood, A. G. W. Hodson, S. Mann and K. K. W. Wong, Adv. Muter., 1993,549. 9 A. J. Ashworth, B. J. Brisdon, R. England, B. S. R. Reddy and I. Zafar, Br. Polym. J., 1989,21,491. 10 A. J. Ashworth, B. J. Brisdon, R. England, B. S. R. Reddy and I. Zafar, J. Membr. Sci., 1991,56, 217. 11 A. J. Ashworth, B. J. Brisdon, R. England and A. G. W. Hodson, unpublished results. 12 S. Mann, J. M. Didymus, N. P. Sanderson, B. R. Heywood and E. J. A. Samper, J. Chem. SOC.,Faraday Trans., 1990,86, 1873. 13 J. B. A. Walker, B. R. Heywood and S. Mann, J. Muter. Chem., 1991, 1,889. 14 V. Gouran, B. Joussume, M. Pereyre, J-B. Verlhac, J-M. Frances, in Chemistry and Technology of Silicon and Tin, ed. V. G. Kumar Das, N. S. Wang and M. Gielen, Oxford University Press, Oxford, 1992, p. 239. 15 F. W. van der Weij, Makromol. Chem., 1980,181,2541. Paper 4/00032C; Received 5th January, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401387

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(9), 1393-1401 1393 MOCVD of High-quality YBa,Cu,O, Films: in situ Preparation of Fluorine-free layers from a Fluorinated Barium Source Ian M. Watson,* Matthew P. Atwood, David A. Cardwell and Toby J. Cumberbatch IRC in Superconductivity, Cambridge University West Site, Madingley Road, Cambridge, UU CB3OHE An MOCVD process is reported that employs the thermally stable barium 1,1,1,5,5,5-hexafluoropentane-2,4-dionate adduct Ba(hfac),.(l8-crown-6). This compound was used in combination with non-fluorinated copper and yttrium sources to deposit superconducting YBa,Cu,O,-, films in a computer-controlled MOCVD reactor specially developed for use with solid metal sources requiring vaporisation at 120-200 "C. The characterisation of films grown on MgO and LaAIO, substrates by X-ray diffraction, electron microscopy, compositional analyses and electrical measurements is described.Optimised films were grown at 690 "C under 0, partial pressures of 0.19 Torr. By depositing in the presence of an excess of water vapour, at a partial pressure of 3.5 Torr, fluorine-free layers were obtained by an in situ method not involving any post-deposition annealing. Results on films grown under other conditions suggest that hydrolysis reactions are extremely important in this deposition process. The YBa,Cu,O, -,growth technique employed can produce films with crystallographic and electrical properties similar to the best films grown by any current method. This is 11illustrated by data on a film on LaAIO,, characterised by the epitaxial relationships YB~,CU,O,-~ (001)LaAIO, (001) and YBa, Cu3O7-, [lOO]IILaAIO,[loo], which showed zero resistance at 91.8 K and a critical current density in self- field at 77 K of ca.1.5x 1 O6 A cm-2. The fabrication of electronic devices using high- T, supercon-L is a polyether ligand, show much more promise as precur- although fluorinated b-diketonate ligands seem to ducting oxides depends on their preparation as highly oriented sor~,'~-'~ Thethin films,'-4 which are also useful for some fundamental be required for adequate thermal ~tability.~~~'' Methods including metal organic chemical mononuclear nature of two Ba( hfac),-L complexes has been physical ~tudies.~ vapour deposition (MOCVD), pulsed laser ablation, metal established by X-ray structure determination.; on co-evaporation, and sputtering are widely used to grow such Ba( hfac),*( tetragl~me)'~ and the analogous 18-crown-6 films.Most current work is aimed at preparing the 93 K complex14 used in the work we discuss here. superconductor YBa2Cu307 -,,and films are usually grown Most attempts to prepare YBa,Cu,O, -,films via M( )CVD on insulating, monocrystalline metal oxide substrates. have used non-fluorinated barium sources of nominal com- Certain highly developed MOCVD processes now provide position 'Ba( thd),'.3 However, the limited volatility and ther- YBa2Cu3O7-films with structural and electrical properties mal stability of these materials has motivated previous matching the best films grown by any other method, and offer experimentation with fluorinated precursors, including Ba(tdfnd),-(tetragl~me)'~as well particularly attractive opportunities for large area gr~wth.~,~ Ba(hfa~),.(tetraglyme),'~-~' andSo far, metal P-diketonate precursors have been used almost as the presumably oligomeric materials 'Ba(f~d),'~~,'~ exclusively in such work.However, the characteristics of these 'Ba( hfa~),'.'~ In oxide MOCVD processes using fluorinated metal sources have posed significant problems, prompting precursors, the tendency to form metal fluoride phases poses interest in reactors with unorthodox methods of precursor a problem. One technique for preparing YBa2Cu30,- films vap~risation.~.~ involves initially depositing non-superconducting films, typi- In most current MOCVD processes for YBa,Cu307 -, cally containing crystalline CuO, Y203 and BaF,, with overall growth, Cu(thd), and Y (thd), are employed as precursor^.^ Y :Ba :Cu ratios close to 1 :2 : 3.Such films can be defluori- (Table 1 summarises the ligand names and abbreviations used nated by heating them in an H,O-0, atm~sphere,'~,~~-~~ and Y(thd),8 are mononuclear where fluorine is presumably removed as gaseous HF.in this paper.) C~(thd),~ complexes, which attain acceptable vapour pressures for film However, methods for high- T, film preparation involving deposition if heated to over 1200C.4,9 Although thermally post-deposition treatments are generally less attractive than stable, these precursors remain solid at practical vaporisa- those allowing in situ growth of the target phase as epitaxial tion temperatures, unlike the liquid metal sources generally films.The most obvious method for in situ MOCVD of used in commercial MOCVD processes. Barium complexes YBa,Cu,07 -,using fluorinated barium sources involvvs con- containing the thd ligand have much less satisfactory proper- ducting film growth in the presence of water vapour, and we ties for use as precursors, as summarised previously.10-12 have successfully employed such a technique in the work Multinuclear species including Bas( thd),(OH)( H20)310and discussed here. Previous routine in situ growth of optimised Ba,( thd)*ll have been characterised by X-ray crystallography. YBa2Cu307-,films using a fluorinated barium source appears Barium P-diketonate adducts of the type Ba(dike),.L, where to have been achieved by only one group, using low-pressure, plasma-assisted MOCVD.19-21 The YBa2Cu30, -,deposition process we discuss involves film growth in a specially developed MOCVD system of Table 1 Ligand abbreviations and systematic names traditional layout, using Cu(thd),, Y(thd), and Ba( hfac),-( 18-dike general abbreviation for P-diketonate crown-6) as precursors.To our knowledge, this particular thd 2,2,6,6-tetramethylheptane-3,5-dionate barium source has been used by no other MOCVD group. hfac 1,1,1,5,5,5-hexafluoropentane-2,4-dionate The high thermal stability of the 18-crown-6 complex is tdfnd 1,1,1,2,2,3,3,7,7,8,8,9,9,9-tetradecafluorononane-4,6-dionate advantageous in ensuring run-to-run reproducibility in deposi- fod 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyloctane-3,5-dionate tion, and quantitative vapour pressure data will be re ported 18-crown-6 1,4,7,10,13,16-hexaoxacyclooctadecane el~ewhere.,~We have characterised recent superconducting tet raglyme 2,5,8,11,14-pentaoxapen tadecane films by a range of standard techniques, demonstrating that J.MATER. CHEM., 1994, VOL. 4 they have properties similar to the best YBa2Cu307-, films grown by other methods, and superior to earlier films we deposited at higher 0, partial pressure [p(02)] value^.^^,^^ The in situ growth of YBa,Cu,O,_, films containing no obvious BaF, impurity phase is a significant feature of this work, and we have therefore attempted to detect low levels of residual fluorine in our optimised films.We also stress the critical effects of the water vapour partial pressure [p(H,O)] during film growth, which are not clear from earlier related st~dies.'*-~~,~~Finally, we highlight the potential of our MOCVD system for depositing uniform, high-quality YBa,Cu30, -,films over areas considerably larger than 1 cm2, a practical upper limit for many physical deposition systems. Experimental Precursor Preparation and Characterisation Previously published methods were used to prepare the pre- cursors C~(thd)~,~~Y(thd)330 and Ba( hfac),.( 18-crown-6)15 on scales of 5-log. The compounds were purified by subli- mation at pressures below 0.05 Torr. Visual observations of melting and decomposition were made in N,-filled capillaries. Samples were also characterised by combustion microanalysis (Perkin-Elmer 240), and other standard techniques.MOCVD Apparatus and General Operating Procedures Films were grown in a reconfigured Special Gas Controls MO 1000 system, originally used for compound semiconductor deposition. The modified reactor retains features including vent-run operation, control of gas streams by mass-flow controllers and flow switching by pneumatic valves. The system operates under computer control, using software devel- oped by author T.J.C. Precursor vaporisation is by conven- tional means, i.e.for YBa,Cu,O,_, growth three metal sources are placed in heated vaporisers ('bubblers') and entrained into initially separate streams of carrier gas.Developing heated gas-handling systems compatible with metal P-diketonate precursors has provided a general challenge for high- T, MOCVD groups, and we will therefore briefly describe our hardware. The overall layout of the MOCVD system is similar to that illustrated in ref. 18. Gas-handling hardware upstream of the film deposition cell is predominantly stainless steel. Working gas pressures are directly controlled within the cell by a feedback arrangement, but are not governed in upstream parts of the system. Details of the cell, which has a rectangular cross-section, are shown in Fig. 1. This is a cold-wall design, featuring induction heating of a susceptor machined from high-conductivity silicon.The susceptor top surface measures 8.6 cm x 18 cm, providing considerable scope for large-area growth. Philips Thermocoax metal-sheathed resistance cable is used to heat the precursor bubblers, plus the pipes, valves and manifolds downstream of these. The latter items are heated to set temperatures 1"Cabove the maximum bubbler tempera- ture. We employ a Gardsman controller (West Instruments) for temperature management of 16 independently heated zones, and can reliably maintain temperatures within f0.5 "C of set points. The precursor bubblers are cooled to room temperature between runs to minimise thermal degradation of the metal sources. Much experimentation was required to find a sealing material for use in the heated valves, in which pistons tipped with compliant material are seated against metal orifices.Carbon-filled PEEK polymer eventually proved to be a satisfactory tip material. Boil-off N2 is used as a carrier gas for the precursors, which susceptor heating coil door pressure sensor port silica inner cell \ I exhaust port I n \ II \o I outer silica cell thermocouple to pump, via throttling valve Fig. 1 Longitudinal cross-section of the MOCVD reactor cell are vaporised from bubblers of ca. 500 ml capacity. To achieve reproducible transport rates from solid metal sources, we load each bubbler with 6 mm diameter stainless steel balls, inter- spersed with two layers of powdered precursor. Initial oper- ation of the reactor after a precursor loading results in the balls becoming evenly coated with metal complex, providing a large surface area for vapour uptake.To commence a series of growth runs, new 10 g batches of each precursor are loaded, and these are used for a series of 10 or so depositions. At the start of each run series, small adjustments to the film cation stoichiometry may be made by manipulating precursor vapor- isation temperatures and carrier gas flow rates, guided mainly by characterisation of films using electron probe microanalysis (EPMA).26,27After these initial stoichiometry adjustments, we generally aim to make as many runs as possible without further alterations to growth conditions. At the end of a set of runs, the precursor bubblers are rinsed with dichloro- methane. This procedure allows the estimation of average precursor transport rates by weighing the amounts of residual precursor extracted into solution, and also observation of any precursor degradation.The design of our MOCVD system allows the oxidant gas used in film growth to flow through a heated bubbler contain- ing water, via a similar arrangement to that described else- where." Estimates of p(H20)inside the reactor cell are based on preliminary calibrations of the water transport rates obtained with bubbler temperatures between 20 and 70 "C. During film growth, the three precursor-laden gas streams are mixed together and diluted with N, buffer gas in a manifold ca. 75 cm upstream of the deposition cell, but water-saturated oxidant gas is added only ca.5 cm from the cell inlet port. YBa,Cu,O,_, films are always cooled from their growth temperature under a dry 0,-rich atmosphere, without the oxidant gas passing through the water bubbler. The optim- ised deposition conditions, which involve the growth of YBa2Cu307-, at low p(0,) values, are described fully in Table 2. These were realised by using 1% 0, in N, as the oxidant gas during film growth. and switching to pure O2for the cool-down sequence. Monocrystalline metal oxide substrates used in film depos- ition are obtained from various commercial suppliers and are solvent-cleaned immediately before use. Substrates are usually ca. 1cm square and cu. 0.5 mm thick. Film Growth We make detailed comparisons between films from four deposition runs, numbered 190, 193, 194 and 195.The only 3. MATER. CHEM., 1994, VOL. 4 Table 2 Optimised film growth conditions, runs 193 and 194 Cu(thd), temperature: 126 "C carrier gas flow rate: 30 sccm molar transport rate: ca. 1 pmol min-l Y(thd), temperature: 128"C carrier gas flow rate: 30 sccm molar transport rate: ca. 1pmol min-l Ba( hfac),.( 18-crown-6) temperature: 189 "C carrier gas flow rate: 240 sccm molar transport rate: ca. 2 pmol min-' water bubbler temperature: 55 "C oxidant gas flow rate: 3000 sccm total cell pressure: 25 Torr total gas flow: 4000 sccm p(0,): 0.19 Torr p( H,O): 3.5 Torr substrate temperature: 690 "C deposition period: 9 h YBa,Cu,O,-, growth rate: 0.024 pm h-' Gas flows are in units of standard cm3 min-' (sccm), referred to 760 Torr.parameter varied was p(H,O). Runs 193 and 194 were conduc- ted under conditions optimised for in situ growth of high- quality YBa2Cu307-d films, as summarised in Table 2. The cooling sequence included an isothermal hold of 5 h at 450 "C, with p(02)= 130 Torr. Runs 190 and 195 were deliberate experiments which did not yield superconducting films. In the latter experiment, introduction of water into the deposition cell was omitted, while in run 190 a water bubbler temperature of 35 "C resulted in an sub-optimal p(H,O) value of 1.2 Torr. The design of our MOCVD reactor allows us to perform routine depositions with multiple 1 cm2 substrates, and we make some comparisons between films deposited simul-taneously onto different substrates in run 194.The substrates used were cleaved and polished MgO squares with (001) orientation and a polished LaA10, substrate with similar orientation. The codes used for these film samples are 194A-l94F, where the suffix letters correspond to the different substrate positions illustrated in Fig. 2. The films discussed from runs 190, 193 and 195 were all deposited in position A. gas flow H1 1cm Fig. 2 Labelling scheme for different growth positions on the susceptor surface. In run 194, positions A, D and F were occupied by cleaved MgO, position B by polished MgO, and position C by polished LaAlO, Film Characterisa tion We have previously described in detail the X-ray difTraction (XRD) and scanning electron microscopy (SEM) procedures used to examine film phase compositions and microstruc- ture~.~~The present study also included preliminary investi- gations of film 194C by transmission electron micl-oscopy (TEM). Cross-sectional samples were prepared by standard thinning methods31 and were examined in a JEM-4000EX I1 microscope operating at 400 kV.The sub-micron film thicknesses and rough surface morpho- logies prevented EPMA studies giving accurate quar: titative results. Routine cation analyses were made using d Link AN1000 energy-dispersive analyser, with an electron beam accelerating voltage of 10 kV. Fluorine analyses were per- formed with a custom wavelength-dispersive instrumen t, fitted with two monochromators.32 The beam accelerating voltage was 20 kV, and the analysis areas were 30 pm square.Background count rates were measured for each individual sample position by setting the beam off-axis. The films and the BaF, standard were sputter-coated with gold to avoid insulator charging effects. Accurate cation analysis of film 194F was performed at Pascher Laboratories, Remagen, Germany. This film was dissolved by immersion in 10/0 nitric acid solution for 10min and the resulting solution was analysed by inductively coupled plasma optical emission spectroscopy. Electrical measurements were made on custom rigs, with automated data logging. Resistance versus temperature (R-T) characteristics were determined by a four-point method, using silver paint contacts.These measurements were made with low-frequency ac test signals (typically 68 Hz), and were prone to temperature calibration errors of <0.5 K. Transport critical current density (J,) measurements on sample 194C werc made by photolithographically patterning the film into 20 pin wide tracks, using aqueous ethylenediaminetetracetic arid for etching, and standard interpretation of current-voltage char- acteristic~.~~No attempt was made to re-oxygenate the film after patterning, a common practice with lower-qualit y films that are susceptible to degradation. Results and Discussion YBa,Cu,O, Growth Conditions Before describing the superconducting films grown under conditions summarised in Table2, we will discuss the back- ground to the selection of this deposition regime.Although p(H20) is a particularly important parameter, it is most convenient to consider this later, after describing the character- istics of optimised films. Data on the precursor compounds are summarised in Table 3. Although Cu( thd), and Ba( hfac),-( 18-crown-6) are air-stable, combustion microanalyses suggested that freshly sublimed Y(thd), is hygroscopic, and forms the hydrate Y(thd),.H,08 on exposure to air. Consequently, after loading a bubbler with this material, it was dehydrated by gradual heating to 120°C under flowing N,. Each precursor was vaporised well below its melting onset temperature. The high melting point of Ba( hfac),-( 18-crown-6), which is ca.110 "C above that of the analogous tetraglyme complex,13 is advanta- geous in allowing it to be handled by procedures similar to those used for Cu(thd), and Y(thd),. In contrast, many YBa2Cu307-deposition processes combine evaporation of molten barium sources of nominal composition 'Ba( thd)2'12 with vaporisation of C~(thd)~ and Y(thd), as solids. Our preference for Ba( hfac),-( 18-crown-6) is also based on its excellent thermal stability, though we recognise that some of its close analogues like Ba( hfa~)~.( tetraglyme) are consider- J. MATER. CHEM., 1994, VOL. 4 Table 3 Data on precursors used in film growth Cu ( t hd), Y(thd13 Ba(hlac)2.( 18-crown-6) melting range/'CU sublimation temperature used for purification/"C 195- 198 (d) 140 173-175 160 260-266(d) 170 vapour pressure under Table 2 conditions/mTorrb 37 38 73 Cu(thd), and Ba( hfac),.( 18-crown-6) decompose on melting.* Estimated using equations derived from measurements on sublimed samples, as described in ref. 9. The values for Y (thd), and Ba( hfac),.( 18-crown-6) are averages from two datasets. ably more ~olatile.'~*~~Nearly all our work has involved vaporising Ba( hfac),.( 18-crown-6) at 189 "C, at which tem- perature the compound sublimes with minimal decomposition. Ba( hfac),.( 18-crown-6) transport rates remain sufficiently con- stant to deposit high-quality YBa,Cu,O, -,films for at least nine consecutive runs under conditions similar to those in Table 2. The precursor transport rates in Table 2 illustrate that the gas-phase Y: Ba: Cu ratio differs markedly from the 1:2 : 3 cation stoichiometry in YBa2Cu,07 -,.Although equilibrium vapour pressures for each precursor are noted in Table 2, comparisons between the transport rates we observe and those expected assuming saturation of the carrier gas streams require a knowledge of the total pressures inside the bubblers under working conditions, which we have not investigated. The reactor geometry results in rather low precursor utilis- ation efficiency; over 80% of the total precursor flux entering the deposition cell is probably not involved in reactions depositing solid material on the substrates or elsewhere. Although our films are thus deposited more slowly than in some optimised MOCVD proces~es,~,~,~~ this has not been a great concern because of the fully automated nature of the reactor.We have not systematically investigated the dependence of film properties on growth temperature, although that used is YBa,Cu,O, -,phase occurs with minimal cation rearrange- ment during cooling. Composition and Structure of Superconducting Films In this and the following section we describe the properties of YB~,CU,O,-~ films from runs 193 and 194. We have subsequently obtained other films of comparable quality under very similar deposition conditions, demonstrating the repro- ducibility of our process, Data are presented on films grown simultaneously on different types of substrate in run 194.Regardless of deposition technique, substrates strongly affect the properties of YBa,Cu,O,-, films,'-, and the multi-sub- strate capacity of our MOCVD system is therefore valuable for making comparisons between films grown on different substrates under identical conditions. The (001 )-oriented LaA10, and MgO substrates used were expected to induce orientation of the orthorhombic YBa,Cu,O, phase with the crystallographic c axis perpendicular to the substrate surface, i.e. YBa,Cu,O,-, (0Ol)llMgO or LaA10, (001). The close lattice match between LaAIO, and the YBa,Cu,O,-, a-b plane, and the greater lattice misfit between MgO and the high-T, phase2,, are therefore of importance in under- standing our results. Fig. 3 shows a 8-28 XRD trace obtained from film 194A, typical of recent practice in YBa,Cu,O,-, depositi~n.~.~,~~,~~grown on cleaved MgO (001).No substrate reflections were The estimated substrate surface temperature of 690 "C is based on measurements with a foil thermocouple bonded to a substrate placed in growth position A (Fig. 2) and heated under simulated film deposition conditions. The p(0,) value of 0.19Torr used in runs 190-194 was chosen with reference to empirical optimisations of oxidant gas partial pressures and deposition temperatures discussed but merits further comment. It is important to relate oxidant partial pressures for YBa,Cu,O, -,growth by MOCVD to the extensive experience gained with physical deposition techniques. Workers depositing YBa2Cu,0, -films by such methods realised that the optimum conditions for in situ growth almost coincide with the decomposition line of YBa,Cu,O,-, on a p(0,) us.temperature phase diagram.' This effect is attributed to an increase in cation mobilities as the optimum deposition conditions are approached. The critical p(0,)value for YBa,Cu,O,-, decomposition at 690 "C is actually ca. 0.02 Torr. However, with oxide MOCVD pro- cesses there is a competing requirement for a large stoichio- metric excess of oxidant gas, to prevent any deposition of carbonaceous material. In our work, the partial pressures of the individual precursors in gas entering the reactor cell are within the range 0.1-0.3 mTorr. Our preferred p(0,) values of ca. 0.2 Torr are several hundred times larger, so that there is no realistic possibility of ligand oxidation creating 02-deficient conditions.The cooling regime applied to our films involves p(0,) values much greater than during growth, as per standard It is usually assumed the phase composition and crystallographic quality of high- T, films are determined by their original growth conditions, and that oxygenation of the expected in the range scanned, and the only prominent diffraction peaks are attributable to 002, 003, 004 and 005 reflections of the YBa,Cu,O, -,phase.,, The very high inten- sities of these peaks confirm the anticipated orientation of YBa,Cu,O,-, (001)I/Mg0 (001). YBa,Cu,O,-, films grown on MgO (001) often contain some material oriented with YBa,Cu,O,-, (1OO)llMgO (OOl), which can be quantified by 8-20 XRD scans over the YBa2Cu,07_, 006/200 region.35 However, such measurements provided no evidence for any high-T, material with this second orientation in films on MgO substrates.Only very weak peaks attributable to impurity r 28ldegrees Fig.3 8-28 XRD data from film 194A, plotted with square root intensity scaling. Peaks labelled with indices are attributed to YBa2Cu,07-, reflections. p( H20)= 3.5 Torr. J. MATER. CHEM., 1994, VOL. 4 phases can be seen in Fig. 3, suggesting that these are randomly oriented. Essentially identical XRD results to those described so far were also obtained from film 194C, grown on LaAlO, (001). 8-28 XRD measurements can also be used to determine the lattice parameter c of (001)-oriented YBa,Cu30,-, films accurately.We made such measurements on three films on cleaved MgO substrates, by slowly scanning the YBa,Cu30, -, 00 13 reflections at ca. 118" 28. The values of c found for films 193A, b94A and 194D were, respectively, 11.691, 11.694 and 11.695A. Tt is well known that in bulk YBa,Cu,O,-, c depends on oxygen content, $creasing as 6 increases, with a minimum c value of ca. 11.68 A for fully oxygenated material with 6 =0.05.38However, in YBa,Cu,O, -,films, cation dis-order can also affect unit-cell dimension^^.^^ and some fully oxygenated films with excellentoelectrical properties show c values at least as high as 11.74A.35XRD and other data on our films are consistent with them having 6 values below ca.0.1, as required for optimised electrical properties5 The crystallographic quality of oriented YBa,Cu30, films is routinely assessed by the o-scan XRD technique, involving scanning peaks in fixed 26' mode. The Aco,,, data in Table4 are FWHM values for YBa2Cu,07-, 005 reflections. These measurements were prone to an instrumental broadening of ca. 0.1", such that substrate 002 reflections showed FWHM values of 0.14-0.27'. Nonetheless, the A00,35 values we quote for our samples compare favourably with data on other YBa,Cu30, -,films grown by various in situ Data in Table 4 also illustrate that our earlier films, grown at relatively high p(0,) value^,^^,^^ do not show A0005 values systematically larger than those grown under better optimised conditions.The XRD measurements discussed so far do not provide information on in-plane film orientation, which we examined by the #-scan technique. Data obtained from films 194B and 194C, by scanning YBa,Cu30, -,103/013 reflections, are shown in Fig. 4. In both plots, zero degrees 4 corresponds to an arbitrary substrate (100) direction. The 4 scan data from film 194C, grown on LaAIO, (OOl), are straightforward to interpret. There are sharp diffraction peaks at 0, 90, 180 and 270" 4, and low intermediate count rates. These features demonstrate that the film is triaxially oriented with respect to its substrate, and shows the orien-tation relationship YBa,Cu,O, -6 [1001IILaA10, [1001 pre-dicted by simple lattice match consideration^.^,^ The count rates observed away from the peaks are similar to those produced by scattering from a bare substrate, ruling out the presence of much high-T, material with random in-plane orientation.Although MgO remains an important substrate material in YBa,Cu,O, -,film deposition work, nucleation-and-growth processes are complex compared to more closely lattice matched substrates such as SrTi0, and LaAlO,.,' Our 4 scan results on film 194B, grown on a polished MgO (001) Table 4 Summary of w-scan XRD and R-T data on films 193A cleaved MgO 0.60 88.8 85.5 194A cleaved MgO 0.30 90.8 87.0 194B polished MgO 0.43 88.2 84.4 194C polished LaAlO, 0.33 92.4 91.8 194D cleaved MgO 0.31 87.4 84.6 139E" cleaved MgO 0.40 85.5 59.0 153A" cleaved MgO 0.53 86.8 75.6 "Examples of our earlier films grown at higher p(0,) values, and described in ref.27 under the codes 139.4 and 153.1. T,,ovalues were rechecked during this study. 1397 5 4. 3-2. h 1-I v) 0 c c 2 0-\ -v 5-0 -4-3-2. d0 160 2$0 4 /degrees Fig.4 &Scan XRD data from films 194C (a)and 194B (b),plotted with logarithmic intensity scaling. The peaks in the top trace only contain minor contributions from substrate 110 reflections substrate, indicate the presence of grains aligned with YBa,Cu,O,-, (001)IIMgO (OOl), but a continuous range of in-plane orientations. The data show major peaks attributable to material aligned with YBa2Cu30, [100111 MgO [1001 and [llO].These orientations, differing in azimuth by 45", have often been observed by other ~orkers.~,~~,~'However, other sets of weaker peaks are also present in our data, and the minimum count rates are a decade higher than those produced by scattering from bare MgO substrates, Films 193A and 194A, grown on cleaved MgO substrates, gave rather similar # scan results to 194B, but with different intensity envelopes. These observations concur with previous reports that YBa,Cu,O,-, growth on MgO is very sensitive to the exact condition of the substrate surface.40 SEM and element-mapping EPMA provided valuable infor-mation on the occurrence of phases other than YBa2Cu307-6 in our earlier high-T, film^.'^,^^ Fig.5 shows an SEM plan view of film 194C on LaAlO, (001). All our superconducting films show basically similar morphologies, with a reaskinably smooth, continuous layer of the high-T, phase covering the substrate surface. Discrete particles of other phases are always visible, as a consequence of the overall Y :Ba: Cu ratios in the films deviating from the ideal 1:2 : 3. Fig. 5 shows several protruding particles with lateral dimensions of 0.5-1.5 pm, which consist of the insulating phase CuO. Cross-sectional TEM on film 194C confirmed that many CuO particles nucleated at the substrate surface, and penetrate right through the YB~,CU,O,_~film. Fig. 5 also shows pits in the YBa,Cu,O, -,layer, containing needle or lath-like particles, which could not be positively identified by XRD or TEM.However, TEM examination of sample 194C did reveal numerous Y203 platelets, only a few unit cells in thickness, J. MATER. CHEM., 1994, VOL. 4 Fig. 5 Plan-view SEM micrograph of film 194C. The YB~,CU,O,-~ thickness is ca. 0.2 pm buried within the film. These appear similar to inclusions previously found in YBa,Cu,O,-, films grown by MOCVD and sputtering, and thought to be significant in enhancing J, values in applied magnetic fields.41 As accurate determinations of the cation stoichiometries of our high-T, films by EPMA are problematic, we obtained a wet chemical analysis on film 194F, grown on a 1.3 cm x 1.3 cm cleaved MgO substrate. This indicated the presence of 21 yg of yttrium, 49 pg of barium, and 61 pg of copper, corresponding to a nominal Y: Ba: Cu mole ratio of 1.00:1.51:4.07.Hence the presence of micron-sized CuO particles and nanoscale Y203 inclusions in films from run 194 is readily understandable. We assume that the amount of YBa2Cu307-, present in film 194F is limited by its barium content. The substrate area and the YBa,Cu,O,-, bulk density of ca. 6.4 g cm-, (ref. 37) then lead to an estimated average thickness of 0.22 pm for the high-T, layer. The growth rate in Table 2 is based on this value, which is likely to be a slight underestimate because of the volume fraction of the YBa,Cu,O, -,film occupied by secondary phase particles. Cross-sectional SEM on film 194A confirmed a thickness of ca.0.2 ym. Electrical Properties of YBa,Cu,O,-, Films Data on the R-T characteristics of films from runs 193 and 194 are summarised in Table4. For each sample we give the temperature of the transition midpoint (T,,m),corresponding to the inflection point of the R-T curve in the transition region, and also the temperature at which the resistance became immeasurably small (Tc,o).A full R-T dataset for film 194C on LaA10, is plotted in Fig. 6. Here the linearity from 300 to 120 K and the extrapolation through the origin are significant, as high-quality YBa,Cu,O, -,single crystals show similar R-T characteristic^.^, A lack of curvature in the region well above T, is thought to indicate optimum carrier doping level^.^ The z,mand Tc,ovalues tabulated show that even our optimised films on MgO substrates show broader transitions, at lower temperatures, than film 194C on LaA10,.However, such differences between YBa,Cu,O, -,films grown on MgO and closely lattice matched substrates are well d~cumented.~,,The potential of our process for large area film deposition is illustrated by data for films 194A and 194D, which were grown ca. 2.5 cm apart. Our earlier films on cleaved MgO substrates have Tc,mvalues only slightly lower than optimised films deposited in comparable growth pos- OY I I I I I 0 50 100 150 200 250 300 TIK Fig. 6 R us. T characteristic of film 194C, showing z,oof 91.8 K, and the extrapolation of the high-temperature portion of the plotthrough the origin itions, but their R-T curves show pronounced low-tempera- ture tails, resulting in lower values for T,,o. The optimised Tc,ovalues we measured match or exceed published data on high-quality YBa,Cu,O, -, films grown by various method^.'-^,^,^^ For example, maximum Tc,ovalues of 89.5 K were recently quoted for YB~,CU,O,_~ films deposited on large LaAlO, (001) substrates by a highly developed MOCVD pro~ess.~ Transport J, measurements are also widely used to assess film quality, and XRD results on our film 194C justified performing such investigations. The acid-etched tracks showed zero resistance at ca.91.5 K, confirming minimal degradation of the film during patterning. Typical J, values, measured at 77 K with no applied magnetic field, were ca.1.5 x lo6 A cm-,, exceeding most comparable values reported previously for MOCVD films., Further experiments elucidated the magnetic field and temperature dependence of Jc.33 Significanceofp(H,O) for in situ Growth of Superconducting Films To show the importance of p(H20) in successful YBa,Cu,O, -growth by our process, we compare data obtained from films 194A, 190A and 195A, all grown on cleaved MgO (001) substrates. We also discuss attempts to detect residual fluorine in films in which no BaF, could be detected by XRD. One motivation for this effort was the previous characterisation of oxide fluoride phases such as YBa2C~30tj.7F0.18,43 which would be extremely difficult to distinguish from fluorine-free YB~,CU,O,-~ by the XRD techniques applied to thin films.Fig.7 shows 8-20 XRD traces obtained from the non-superconducting films 190A and 195A, for comparison with data on 194A presented in Fig. 3. XRD data from film 190A, grown with p( H,O) =1.2 Torr, show sharp peaks matching two BaF, reflection^.,^ Two other sharp peaks correspond to intense CuO powder pattern reflection^.^, However, the broad feature at ca. 21.5" 28 suggests that this film also contains much poorly crystalline material. Film 195A was grown without any introduction of water vapour to the gas stream entering the deposition cell. Only weak diffraction peaks were observed from this sample, which are attributable to BaF, 11 1, BaF, 200, and CuO 11 1 or 200 reflections.,, SEM and EPMA showed further striking differences between films 194A, 190A and 195A.The morphology of film 194A is similar to that of film 194C, illustrated in Fig. 5. Micrographs of films 190A (Fig. 8) and 19SA (Fig. 9) show rougher morphologies, and the absence of a continuous YBa,Cu,O,-, base layer. EPMA showed that film 190A is yttrium-deficient compared to 194A, but has a similar copper and barium content, while film 195A apparently contains no yttrium, and much less copper and barium per unit area than J. MATER. CHEM., 1994, VOL. 4 900-F I v) v)c t 3 90 0. Y.-2 144. a,Y .-C 0 2#degrees Fig.7 8-28 XRD data from films 190A (a) and 195A (b), showing BaF, and CuO reflections. Both plots have square root intensity scaling, but the vertical scales are different.V, BaF, peaks; 0,CuO peaks. (a)p( H,O) =1.2 Torr, (b)no water added. H1 pm Fig. 8 Plan-view SEM micrograph of film 190A H1 pm Fig. 9 Plan-view SEM micrograph of film 195A the other two samples. These results are consistent with hydrolysis reactions playing a major role in film deposition, and the behaviour of Y (thd), being particularly dependent on P(H20).Light-element EPMA studies involved simultaneous moni- toring of X-rays emitted in barium La and fluorine Ka channels. Representative results are summarised in Table 5. To assist interpretation, we analysed two standard samples in addition to our films: a piece of BaF, bulk crystal, and a YBa2Cu30,-, film ca.0.3 pm in thickness from Emcore Inc., prepared using an MOCVD process involving only non-fluorinated precursor^.^ Films 194A and 194C gave net fluor- ine counts over 1400 times lower than the BaF, crystal, and similar to that from the fluorine-free film EM387. We conclude that any residual fluorine content in the films from run 194 is too low to detect reliably by our EPMA method. The much higher fluorine count from film 190A correlates with the reasonably intense BaF, peaks in 0-28 XRD data. Film 195A, in which XRD again indicated the presence of BaF,, produced a much lower barium count than the other films, consistent with other EPMA results. However, differences in crystallinity and thickness make it difficult to relate the XRD and fluorine count data on this film to other samples.In particular, small amounts of BaF, occurring as an impurity phase in YBa,Cu30,-d films would almost certainly be more difficult to detect by XRD than in film 195A. As commented in the introduction to this paper, there has been extensive earlier work on YBa2Cu30,-, film preparation via MOCVD processes using fluorinated barium sources. Apart from ourselves, only Chang's group appears to have described routine preparation of superconducting layers with- out any post-deposition defluorination in work employing Ba( hfac)2-( tetraglyme). The self-adopted name for their deposition technique is pulsed organometallic beam Table 5 Summary of EPMA data on film fluorine contents ~~ sample BaF, detected Ba La counts (50 s) F Kr counts (50s) by XRD? gross net gross net 190A194A Yes no 8375 7256 7968 6872 1382 77 1304 6 194C no 7947 7390 109 10 195AEM387" Yes no 685 11935 345 11166 116 103 22 5 BaF, crystal - 43359 42394 15600 15405 " Fluorine-free YBa,Cu,O,.d film on LaAIO,, 0.3 pm thick, prepared as described in ref.4. epitaxy (POMBE). Published XRD data suggest that early YB~,CU,O~-~films grown on LaAlO, (001) by this method contained crystalline BaF,, and reported T,,o, Amoo5 and J, value^'^ are inferior to our data on film 194C. Less comprehen- sive data have been published on later POMBE films, contain- ing no impurity phases detectable by XRD.20,21 The reports on the POMBE technique do not elaborate greatly on the optimisation of deposition conditions, although, for example, data in ref.20 imply a standard value of 0.1 mTorr for p(H,O), at a total operating pressure of 5 mTorr. In more conventional MOCVD work involving Ba( hfac),-(tetraglyme), phase-pure BaTiO, films have been deposited by an in situ process.28 These were shown to be free from BaF, by XRD.and Auger electron spectroscopy, but there was again no discussion of the p(H,O) requirements for successful growth of the target phase. Our data on the influence of p(H,O) are therefore especially significant. The threefold increase in p(H20) between deposition runs 190 and 193/194 resulted in dramati- cally different film compositions and properties.Because the lower p(H20) value used in run 190 still represents a very large stoichiometric excess of H20 over the metal organic precursors, we suggest that this effect is of thermodynamic, rather than kinetic, origin. Conclusions We have shown that the barium P-diketonate adduct Ba( hfac),-( 18-crown-6) has excellent long-term thermal stab- ility, in contrast to most barium sources tested in MOCVD application^.^^'^-'^ At our standard vaporisation temperature of 189 "C Ba( hfac),-( 18-crown-6) has a moderate but useful vapour pressure of ca. 0.07T0rr.~~In common with the precursors Cu(thd), and Y(thd), we and most other MOCVD groups use for growing YBa,Cu,O, films,3 the 18-crown-6 complex is a solid at practical vaporisation temperatures.The efforts we describe in developing MOCVD hardware and vaporisation procedures compatible with solid metal P-dike- tonate precursors have allowed us to grow superconducting films of excellent quality using a reactor of traditional design. The potential of this system for large-area film deposition, and simultaneously growing films on different types of substrate for comparative studies, is also evident. Our recent YBa,Cu,O, -films show electrical properties superior to earlier samples we prepared using Ba( hfac),.( 18-cro~n-6),~~,~~ which we attribute to growth at a p(0,) value better matched to the deposition temperature. We have also shown that successful in situ growth of supercon- ducting films depends critically on the p(H20) value during deposition, suggesting that hydrolysis reactions are important in our MOCVD process.No residual BaF, was detected by XRD in films grown under optimised conditions, and careful EPMA studies also indicated minimal levels of fluorine in such samples. In common with other in situ growth techniques, our deposition process produces highly oriented YBa,Cu,O, films on appropriate single-crystal substrate^.',^ All the super- conducting films discussed here are oriented with the YBa2Cu,07 [OOl] direction perpendicular to the substrate surface. However, those deposited onto MgO are not charac- terised by a unique type of in-plane orientation relationship, as found for the film on LaAlO,. Our results confirm that continuous YBa2C~307-6 films can be obtained without very precise cation stoichiometry control, and that inclusions of phases such as CuO do not prevent the manifestation of excellent electrical properties.The structural and electrical characteristics of YBa2Cu30,-films grown under our optimised conditions resemble those of the best films grown by any technique, including various diverse MOCVD pro- J. MATER. CHEM., 1994, VOL. 4 ce~ses.~'~,~,~~After the growth of the films discussed here in detail, we have obtained further samples with similar crystallo- graphic quality, T, values and J, characteristics, demonstrat- ing the reproducibility of our deposition process. Further work has included a demonstration that films with Tc,o >90.5 K can be deposited on LaAlO, over an area of at least 40 cm2.We thank SERC for principal funding, British Aerospace for a CASE award to M.P.A., and Air Products for initial supplies of Ba( hfac),.( 18-crown-6). The Cambridge University MOCVD system is based on equipment donated by Special Gas Controls and PA Technology, as negotiated by W. I. Milne of the Engineering Department. R. T. Parsons provided excellent technical assistance during the reconfiguration of this equipment. We are grateful to colleagues from the IRC, P. A. Pullan, R. A. Doyle, P. D. Hunneyball and Y. Yan, for film characterisation. EPMA standards were supplied by J. Zhao (Emcore Inc.) and A. V. Chadwick (University of Kent). References 1 M. Schieber,J. Crystal Growth, 1991, 109,401.2 F. C. Wellstood, J. J. Kingston and J. Clarke, J. Appl. Phys., 1994, 75,683. 3 M. Leskela,H. Molsa and L. Niinisto, Supercond. Sci. Tech., 1993, 6,627. 4 C. S. 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Chem., 37 JCPDS Powder Difraction File-Inorganic Phases, Intvrnational Center for Powder Diffraction Data, Swarthmore, Penrisylvania, 1991; relevant entries include: YBa,Cu,O, 38-1433, BbF, 4-452, CUO5-661. 30 31 32 33 34 35 1973,35,4061. K. E. Eisentraut and R. E. Sievers, Inorg. Synth., 1968, XI, 94. M. J. Casanove, A. Alimoussa, C. Roucau, C. Escribe-Filippini, P. L. Reydet and P. Marcus, Physica C, 1991,175,285. P. D. Hunneyball, M. H. Jacobs, T. J. Law and R. B. Newberry, Electron Microsc., 1980,3, 36. P. A. Pullan, PhD thesis, University of Cambridge, 1994. J. Hudner, 0.Thomas, E. Mossang, M. Ostling, P. Chaudouet, A. Gaskov, F. Weiss, D. Boursier and J. P. Senateur, J. Appl. Phys., 1993,74,4631. M. Hein, S. Hensen, G. Muller, S. Orbach, H. Piel, M. Strupp, N. G. Chew, J. A. Edwards, S. W. Goodyear, J. S. Satchel1 and R. G. Humphreys, in High-T, Superconductor Thin Films, ed. L. Correra, Elsevier, Amsterdam, 1992, pp. 95-100. 38 39 40 41 42 43 A. Manthiram, J. S. Swinnea, Z. T. Sui, H. Steinfink and J. B. Goodenough, J. Am. Chem. SOC.,1987,109,6667. S. J. Pennycook, M. F. Chisholm, D. E. Jesson, R. Feenstra, S. Zhu, X. Y. Zheng and D. J. Lowndes, Physica C, 1992,202,l. B. H. Moeckly, S. E. Russell, D. K. Lathrop, R. A. Buhrman and Jian Li, Appl. Phys. Lett., 1990,57, 1687. T. I. Selinder, U. Helmersson, Z. Zan and L. R. Wallenburg, Thin Solid Films, 1993, 229, 237. T. Ito, K. Takenka and S. Uchida, Phys. Rev. Lett., 1993. 70,3995. C. Perrin, A. Dinia, 0.Pefia, M. Sergent, P. Burlet and .I. Rossat-Mignod, Solid State Commun., 1990,76,401. Paper 4/03171G; Received 27th May, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401393

出版商:RSC    

年代:1994

数据来源: RSC

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J. MATER. CHEM., 1994, 4(9), 1403-1407 Growth of TiO, Overlayers by Chemical Vapour Deposition on a Single-crystal Copper Substrate Yuan Min Wu and Roger M. Nix* Department of Chemistry, Queen Mary and Westfield College, University of London, Mile End Road, London, UK El 4NS The deposition of ultrathin titania films by chemical vapour deposition from titanium tetraisopropoxide has been studied on a Cu(lO0) substrate using the surface analytical techniques of X-ray photoelectron spectroscopy (XPS) arid low- energy electron diffraction (LEED). The titania initially grows as a near-uniform layer with a well defined hexagonal structure, possibly related to the rutile structure. There is no evidence for a strong electronic interaction between the oxide overlayer and substrate.At higher exposures further growth at 600 K proceeds by 3D crystallite nucleation. Chemical vapour deposition has established itself as one of the most important techniques for the production of thin films of a wide range of materials. Titanium dioxide itself may be deposited using either the volatile chloride, TiC1,',2 or various titanium alkoxides3-" as the source of titanium. The most commonly employed alkoxide precursor is the isoprop- oxide and this precursor contains sufficient oxygen for depos- ition of TiO, even in the absence of an oxidising carrier gas. Although single-crystal surfaces of TiO, have been studied extensively over the last two decades,''*l2 relatively little detailed work has been carried out on characterising the surface geometrical and electronic structure of titania overlay- ers grown by vapour deposition techniques.Previous work in the area has instead concentrated on investigating the mor- phology and crystallographic structure of the oxide films grown, as a function of the growth parameters and substrate, using bulk analytical techniques. As with the epitaxial growth of semiconductor films, it is clear, however, that the initial film structure may be strongly influenced by the structure of the underlying substrate. This effect may then be propagated to the rest of the film structure, as the initial layers effectively act as nucleation centres for further growth. Even in the case of growth on the surface of a polycrystalline material, single- crystal grains of the substrate will act as local templates for the initial growth of the oxide.The substrate may also influence the initial deposition mechanism and, in the case of metallic substrates, metal/oxide interactions can substantially modify the properties of ultrathin oxide films. In this work, therefore, we have started to investigate the growth of titanium oxides on macroscopic single-crystal substrates under well defined high vacuum conditions, using surface science tech- niques to characterise the electronic and geometric structure of the ultrathin films initially formed during growth from titanium tetraisopropoxide. Experimental These experiments were carried out in a UHV system con- sisting of an analytical chamber, a deposition chamber and a fast-entry lock for the introduction of samples. The analytical chamber is equipped for LEED and AES/XPS using a VG-CLAM2 100 mm hemispherical analyser.The XPS data reported below were all obtained using Mg-Ka radiation from a standard, non-monochromated X-ray source with the ana- lyser operated in a fixed pass energy mode for a given spectral region (50 eV for 0 1s and C 1s; 20 eV for Ti 2p and Cu 2p). Spectra were acquired at normal emission angles under PC control and are presented in the as-collected form, without background subtraction or digital manipulation, unless specifically mentioned. The quoted binding energies arc taken directly from the intensity maxima of the raw daia and calibrated by reference to the Cu 2p3,, peak from the clean Cu(100) surface at 932.65 eV.The Cu(100) crystal used in this work was prepared by standard techniques and polished with successively finer grades of diamond paste prior to insertion in the vacuum system. The crystal was then mounted on a sample carrier which could be moved between the different sections of the system using a magnetic translator, and transferred onto either one of two precision manipulators in each of the two main chambers. For heating purposes the crystal was sup- ported on the gold-coated sample carrier between 0.75 mm Ta heating wires passing through spark-eroded holes in the corners of the crystal. Temperature measurement was by a chromel/alumel thermocouple attached to the edge of the crystal. In situ cleaning was by Ar+-ion bombardment (1000 eV, 300 K), with subsequent annealing at 900 K for 10min.The titanium tetraisopropoxide (TTIP, 99.9%) was pur- chased from Aldrich. A small quantity of the precursor was introduced into a glass sample vial under a nitrogen atmos- phere and this vial then attached to the deposition chamber via a UHV leak valve. Facilities for pumping on the precursor permitted purification by degassing and vacuum distillation prior to use. During dosing the whole dosing assembly was maintained at around 50°C and the precursor was delivered directly onto the front face of the crystal through the use of a directional doser. A differentially pumped quadrupole mass spectrometer situated in the deposition chamber was employed to monitor the flux and purity of the TTIP.After the required period of deposition the sample was moved into the analytical chamber for analysis by XPS and LEED at ambient temperatures. Results XPS Studiesof Growth at 600 K In the initial experiments the clean Cu(100) crystal was exposed directly to a flux of TTIP at elevated temperatures. Under these conditions XPS showed only a very slow initial uptake at the standard growth temperature of 600K. These observations reflect the very low reactivity of the clean Cu(100) surface and in all subsequent experiments, therefore, the Cu(100) crystal was preoxidised by exposure to 300 Lf 0, (research grade) at 400 K. This yielded the c(2 x 2) LEED pattern characteristic of the oxidised Cu( 100) surface.t 1L (langmuir)= Torr s. J. MATER. CHEM., 1994, VOL. 4 Thin TiO, overlayers were then grown on the Cu(100) c(2 x 2)-0 surface by exposing it to various amounts of TTIP at 600 K. For the purposes of presentation we have categorised the TTIP exposure on the basis of the uncorrected ion gauge reading for the deposition chamber. This is unlikely to be an accurate representation of the true exposure owing to the opposing effects of (i) the directional nature of the dosing and Clean(ii) the fragmentation of the precursor that occurs both on Oxithe hot substrate and within the ion gauge itself. The values 5reported do, however, provide an internally consistent measure of the exposure experienced by the crystal.20 A set of spectra recorded during the initial exposure process 40 at 600 K are shown in Fig. 1. In particular, it may be noted 60 that Fig. l(u) shows a monotonic decrease in the Cu 2p3,, 90 peak, with no observable shifts (<0.1 eV) or change in FWHM 120 (1.25 eV), consistent with a gradual growth of the titanium 200 oxide overlayer. The attenuation of this peak may be used to provide an estimate of the oxide film thickness [Fig. 2(a)]: 925 935 945 this calculation is based on the assumption of a laterally uniform overlayer and uses the inelastic mean free path data for inorganic compounds tabulated by Seah and DenchI3 which give a value of 2 of ca. 7.5 monolayer (ML) for the Cu 2p3,, photoelectrons.The gradual growth of the titania film is clearly evident from the development of the Ti 2p spectra of Fig. l(b). During the very early stages of growth it 200 also appears that the surface Ti species exhibit a marginally higher (cu. 0.3 eV) binding energy than that which character- 120 ises the subsequent growth. 90 In Fig. l(c) it can be seen that the pre-deposited oxygen of the Cu( 100) c(2 x 2)-0 structure gives rise to an 0 1s peak 60 at a clearly distinct binding energy (529.9 eV) from that which 40 characterises the growing oxide (530.5 eV). Since there is no evidence for the 529.9eV peak once growth of titania has 20 been initiated [see, for example, the 5L TTIP trace in Fig. 1(c)], ACleans5 it would appear that the pre-adsorbed oxygen readily reacts with and becomes incorporated into the growing oxide film.I I 1 460 470Some incorporation of carbon (Eb=284.5 eV) into the film 450 was also observed. Since the measured C level was found to increase with film thickness, the origin of this carbon is taken to be the precursor itself as opposed to segregation from the copper substrate. In Fig. 2(b) the intensity of the Ti 2p and 0 1s signals are plotted against the estimated film thickness. From the data in this figure it is immediately clear that the oxide stoichiometry shows no major variations with film thickness. Both this plot and the plot of the estimated film thickness versus TTIP exposure [Fig. 2(u) (inset)] do, however, show a noticeable 200 break (change in slope) at around 7 ML thickness which may 120 be indicative of a decrease in growth rate or change in growth 90 characteristics.This is discussed further below. 60 40 20Effects of Post-annealing 5 OxiPost-deposition annealing experiments were carried out by Clean heating the titania films to 900 K, i.e. 300 K above the growth temperature. The most extensive examination was carried out 525 530 535 5 .O binding energylevon a film produced by exposure of the Cu( 100) c(2 x 2)-0 surface to 1200 L TTIP at 600 K. After 60 min annealing at Fig. 1 XP spectra as a function of TTIP exposure at 600K: ~ 900 K in vacuum a marked shift to higher binding energy was (a) Cu 2 ~ (b)~Ti 2p; (c)~ 0 ;1s. Curves are labelled with valuesevident in the Ti 2p3,, spectra (Fig.3), but no decrease in the expressed in langmuir.0:Ti ratio was observed. An increase in the residual Cu 2p3,2 signal might be taken to be indicative of the partial aggre- gation of the film, but since there was a slight increase also inelastic mean free paths and an increase of all the signals as observed in the 0 Is and Ti 2p3,, signals, we propose that is observed. More extensive annealing did lead to a small that is actually due to a change in electron transmission decrease in the apparent 0:Ti ratio but also resulted in a characteristics of the film. Annealing will promote ordering gradual accumulation of surface carbon, presumably byof the oxide which may reduce inelastic electron scattering of segregation from the bulk of the Cu( 100) substrate.photoelectrons in the film, leading to an increase in the Further annealing of the same film in an oxidising atmos- J. MATER. CHEM., 1994, VOL. 4 exposurellI. */' I L I I 1 I 0 250 500 750 1000 12 io exposurell Fig.2 (a) Variation of Cu 2p,,, peak intensity with TTIP exposure, including the line of best fit to the low coverage data for uniform layer-by-layer growth, with inset showing the variation of film thickness, derived from Cu peak intensity, with exposure. (b)Variation of 0 Is (+) and Ti 2p (H) signal intensities with exposure. 1 459.1 eV , I I I I I 1 I 4k5 457 459 461 463 465 binding energylev phere (2 x lop7Torr 0,)did not induce any further shifts in the Ti 2p,,, spectra, but did result in a significant reduction of the C level that had built up during the previous vacuum annealing.Surface Structural Studies by LEED The Cu( 100) surface itself exhibited a sharp (1 x 1) LEED pattern, following the standard ion-bombardment/annealing procedure used to clean the crystal. Following the preoxi- dation treatment (300 L 0, at 400 K) a clear c(2 x 2) was observed, as reported by many other workers. Upon exposing the Cu( 100) c(2 x 2)-0 surface 10 TTIP (>5 L) at 600 K, a pattern of apparent 12-fold symmetry was obtained. This same pattern was retained up to high TTIP exposures and the pattern obtained after 100 L exposure is shown in Fig. 4. This pattern is most obviously attributed to a combination of two equivalent hexagonal patterns related by a rotation of 90°, corresponding to two domains of hexagonal structure aligned along the principal symmetry directions of the (100) surface.The unit-cell size of the hexagonal structure was determined by measuring the recipro- cal mesh of the oxide and comparing it with the clean and pre-oxidised Cu( 100) LEED pattern. The hexagonal super- structure lattice constant deriyed by this method for the pattern shown in Fig. 4 is 2.82 A. With further exposures of TTIP at 600K, however, the LEED pattern gradually became more diffuse and was no longer observable at TTIP exposures over 200 L. This implies Fig.3 Ti 2p,,, XP spectra recorded after: (a) 1200L exposure of Fig.4 LEED patterns from a TiO, film grown on Cu(100) by TTIP at 600 K; (b) additional annealing at 900 K under vacuum; exposing the preoxidised Cu( 100) c(2 x 2)-0 surface to 10(l L TTIP (c) further annealing at 900 K under 2 x lo-' Torr O2 at 600 K (a) 58 eV; (b)40 eV J.MATER. CHEM., 1994, VOL. 4 that the surface becomes more disordered at higher TTTP exposures; this apparent disordering could, however, be related to a build-up of surface carbon and/or a decrease in the average domain size. The disordered surface obtained after exposure to 1200 L TTIP was subjected to various annealing sequences in an attempt to order the film, but no LEED pattern could be observed even after post-annealing at 900K in vacuum for cu. 2 h. As noted earlier, however, such extended annealing at these very elevated temperatures does lead to a gradual build- up of surface carbon.Annealing in a low pressure oxygen atmosphere at 900 K (known to reduce the surface carbon) did result in the reappearance of the pseudo-12-fold LEED pattern, albeit very diffuse and weak. Discussion From the XPS data for this system, it would appear that there is only a relatively weak interaction between the titanium oxide film and the Cu(100) substrate. More specifically, no layer dependent shifts are evident in the 0 Is XP spectra and only a minor perturbation of the Ti 2p spectra is seen at very low coverages. This latter effect may be associated with a difference in the local titanium ion coordination. This proposal about the nature of the bonding is broadly consistent with previous studies of the reverse system in which it has been concluded that there is only a weak interaction between the two phases when copper is deposited on the Ti0,(110) ~urface.~~”~ In the growth of transition-metal oxides, the oxide stoichi- ometry can strongly influence both the electronic and struc- tural properties of the oxide film.Previous studies of CVD growth from TTIP in the absence of oxygen, however, have concluded that the deposited film is close in composition to the stoichiometric dioxide TiO, .779 The fact that the pre- adsorbed oxygen is scavenged by the first titanium to be deposited [Fig. l(c)], certainly reflects the fact that the inter- action between oxygen and the copper substrate is compara- tively weak compared to the affinity between titanium and oxygen.The oxidation of Ti metal itself has been studied by several workers using a variety of techniques including high- resolution AES, UPS, work function measurements, ELS and XPS.16-23 There is general agreement that sub-oxides, includ- ing Ti0 and Ti,O,, are initially formed on the Ti surface upon exposure to 0, and may persist at the metal/oxide interface. In the case of growth from TTIP, however, it must be recognised that the titanium is formally already present in the 4+ oxidation state in the precursor and in earlier work we have already shown how the distribution of gas-phase products is entirely consistent with formation of TiO, in the deposition process.” Distinction between the different oxi- dation states of titanium is aided by a relatively large variation in binding energy with oxidation state, and the binding energy of the Ti 2p3,, peak observed in this work is in good agreement with that reported elsewhere.22 There are some substantial variations in the absolute binding energy values reported in the literature, however, and we have therefore also compared the data for the oxide films grown from TTIP with data obtained from a Ti foil sample which was first bombarded and then oxidised in 760 Torr 0, at 500 “C (Table 1). This comparison conclusively demonstrates that the main Ti 2p,,, peak from the films is attributable to the presence of Ti4+.The oxygen stoichiometry is also compared for the two oxide samples in the table and shows remarkably good agreement.Thus, whilst it must be noted that the experimen- tally determined 0:Ti ratio from the films did show some variation between individual measurements, it is still clear that the grown films have a stoichiometry close to TiO,. Nevertheless, we would expect some loss of oxygen may occur in the reducing vacuum environment at either the growth temperature itself or upon subsequent annealing at 900 K. Somewhat surprisingly, however, annealing in vacuum initially caused an increase in the Ti 2p3,, binding energy (Fig. 3). Since this cannot possibly represent an increase in the oxi- dation state, this indicates that the binding energy of the titanium is also sensitive to the oxide structure, and that the shift to higher binding energy is caused by the enhanced ordering of the oxide induced by such annealing. The observation of some carbon incorporation may be a feature of the relatively low growth temperature employed in this study.Higher temperatures will favour the decomposition of any carbonate species and the oxidative removal of hydro- carbon fragments. This also points to another role of the use of oxygen as a carrier gas since, in addition to ensuring TiO, stoichiometry is achieved, it will also facilitate the removal of any carbon deposited. The growth mode of the oxide is difficult to discern purely from the variation of the XPS signals obtained, but on the basis of the LEED and XPS data we propose the following model.On the pre-oxidised surface the initial growth appears to give a uniform oxide film (i.e. layer-by-layer or simultaneous multilayer growth) which gives rise to the observed LEED pattern that is discussed further below. Above a coverage of around 7 ML, however, the Cu 2p3,, XPS signal is attenuated less rapidly than would be expected for continued uniform growth [Fig. 2(u)] and the progressive weakening of the LEED pattern indicates less ordered growth on top of the existing ordered oxide film. This suggests that 3D crystallite growth begins to occur in this exposure regime, ultimately leading to a film consisting of an intergrowth of crystallites which, once formed, is difficult to order even by high- temperature annealing.The apparent decrease in growth rate suggested by a plot such as Fig. 2(a) is thus believed to be associated with a change in the growth mode and film morphology rather than any real decrease in the growth kinetics, which appear to be relatively insensitive to the surface structure. The ordered oxide film observed at lower exposures has been attributed above to two dqmains of hexagonal symmetry with a 2D unit cell size of 2.82 A. The copper substrate clearly therefore exerts an orientating influence on the oxide, but the interaction between substrate and oxide film is not sufficiently strong for the substrate to cause growth of a truly epitaxial oxide orientated to expose a face of rectangular (square) symmetry. In this respect the LEED observations confirm the conclusions based on the XPS data about the strength of the interaction.The size of the unit cell also strongly suggests that the oxide structure is basedo upon layers of hexagonal close packed 0,-ions (r= 1.40 A). This would point to a structure most closely related to the rutile phase of TiO, which can be considered to be based upon hexagonal close packed oxygen with titanium ions in alternate rows of octa-hedral holes. Note, however, that this is not recognised to be the most stable face of the bulk oxide [which is the (110) face12] and, furthermore, also leads to the formation of a permanent dipole proportional to the film thickness. Such a situation is considered unsustainable for the bulk oxide unless the dipole can be offset by surface reconstruction and/or vacancy formation, but in the case of such a thin film the dipole can also be stabilised by an image charge interaction with the metal.In general terms, there is no obvious reason for good alignment of an incommensurate hexagonal overlayer with the principal symmetry directions of the substrate. In this instance, however, there appears to be a near coincidence relationship with the underlying substrate structure that might J. MATER. CHEM., 1994, VOL. 4 Table 1 A comparison of XPS data for CVD films with standard samples Ti 2P3/2 0 1s sample Ti foil (bombarded) Ti foil (oxidised) 200 L TTIP/Cu( 100) peak Eb/eV FWHM/ eV peak Eb/ eV FWHM/ eV 10 ll,i 454.49 1.71 - - - 459.17 1.33 530.33 1.86 7.3 459.05 1.60 530.49 1.88 7.3 Data obtained by peak fitting to spectral regions after background subtraction.Intensity ratio based on measurements of the iniegrated intensities of the Ti 2~3,~and 0 1s peaks. 4.88 AI T I 5.11 AI 5.11 AI Fig. 5 Possible models for the interfacial relationship between the oxide layer and the Cu( l00)surface (a)a perfect hexagonal overlayer structure with JbJ=2.82 A; (b)an expanded hexagonal overlayer (161= 2.95 A), showing ‘( 2 x n)’ coincidence with the substrate; (c) a distorted hexagonal overlayer exhibiting coincidence in one dimension, and a 0-0 separation of 2.69 A in the second strongly favour such an alignment. The inter-row spacing of a perfect hexagonalo mesh of the experimeatally determined unit cell size is 2.44 A, within 5% of the 2.56 A Cu-Cu spacing of the Cu( 100) surface.Fig. 5 compares the oxi$e/substrate interfacial relationships for an undistorted 2.82 A hexagonal mesh and two possible ‘(2 x n)’ 1D coincidence meshes, one of which simply shows a uniform 5% expansion and retains hexagonal symmetry, whilst the other retains the original 02-packing density. In the last two cases, there also exists the possibility of a large coincidence parameter for the second dimension through further minor distortions. In summary, it would thus seem that the distortion of the oxide in at least one dimension is both feasible and would provide a reason for the good domain alignment observed.Some of the additional diffraction spots evident at various beam energies would be consistent with such a coincidence lattice, but it should be emphasised that we have no definitive experimental evidence for this effect. Conclusions Titania films are readily deposited from titanium isopropoxide under high-vacuum CVD conditions. On the Cu( 100)surface, the initial growth is greatly facilitated by the preadsorption of oxygen and proceeds to give an ordered thin film of a hexagonal structure. The XPS data show little interaction between the substrate and oxide, although influences of the substrate surface geometry on the oxide layer structure are apparent. At coverages above ca. 7 ML, the growth mode switches to 3D crystallite growth.Financial support from the Materials Science and Engineering Commission of SERC and the British Council (a Sino-British Scholarship for Y.M.W.) is gratefully acknowledged. References 1 S. Hayashi and T. Hirai, J. Crystal Growth, 1976,36, 157. 2 D. J. Cheung, W. P. Sun and M. H. Hon, Thin Solid Films, 1987, 109,45. 3 M. Yokozawa, H. Iwasa and I. Taramoto, Jpn. J. Appl. Phys., 1968,7,96. 4 E. T. Fitzgibbons, K. J. Sladek and W. H. Hartwig, J. Elect,*ochern. Soc., 1972, 119,735. 5 K. J. Sladek and H. M. Herron, Ind. Eng. Chem. Prod. Rt s. Dev., 1972, 11,92. 6 H. Komiyama, T. Kanai and H. Inoue, Chem. Lett., 1984, 1283. 7 Y. Takahashi, H. Suzuki and M. Nasu, J. Chem. SOC., lhraday Trans. 1, 1985,81,3117. 8 T. Fukuyi, T. Kobayashi and H. Matsunami, J. Electrochem. SOC., 1988,135,248. 9 K. L. Siefering and G. L. Griffin, J. Electrochem. SOC., 1990, 137, 814. 10 Y-M. Wu, D. C. Bradley and R. M. Nix, Appl. SurJ Sci., 1993, 64,21. 11 V. E. Henrich, Prog. Sug.Sci.,1983,14,21. 12 V. E. Henrich, Rep. Prog. Phys., 1985,48, 1481. 13 M. P. Seah and W. A. Dench, Surf. Interface Anal., 1979,l 2. 14 M. C. Wu and P. J. Merller, Sug. Sci., 1989,224, 250. 15 U. Diebold, J.-M. Pan and T. E. Madey, Phys. Rev. B, 1993, 47,250. 16 D. E. Eastmann, Solid State Commun., 1972,10,933. 17 N. R. Armstrong and R. K. Quinn, Surf. Sci., 1977,67,451 18 J. B. Bignolas, M. Bujor and J. Bardolle, Surf. Sci., 1981, 108, L453. 19 M. C. Burrell and N. R. Armstrong, J. Vac. Sci. Technol. A, 1983, 1, 1831. 20 E. Bertel, R. Stockbauer and T. E. Madey, Surf Sci., 19&4, 141, 355. 21 B. M. Biwer and S. L. Bernasek, Surf. Sci., 1986,167,207. 22 A. F. Carley, P. R. Chalker, J. C. Riviere and M. W. Roberts, J. Chem. Soc., Faraday Trans. I, 1987,83,351. 23 C. Ocal and S. Ferrer, SurJ Sci., 1987,191, 147. Paper 4/024541(; Received 26th April, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401403

出版商:RSC    

年代:1994

数据来源: RSC