摘要:

Journal of Materials Chemistry Scientific Advisory Editor Professor Martin R. Bryce Department of Chemistry University of Durham South Road Durham DHl 3LE. IJK Associate Editor Profcssor Jean Etourneau ICMCB Avenue du Docteur Schweitzer 33600 Pessac Francc Editorial board Allan E. Underhill (Chairman) Btiripr Neil L. Allan Bristol Pctcr G. Bruce Sr. Andrew.\ Martin R. Bryce Durhim Michael J. Cook Norwich Managing Editor Janet L. Dean Deputy Editor Zoe G. Lcwin Assistant Editor Graham 6. McCann Editorial Secretary Miss D. J. Halls Jean Etourneau Bordeaux Wendy R. Flavell UMIST Colin Greaves Birrningham Philip Hodgc .Mmchesrer Stephen M. Kelly Hull International advisory editorial board K. Bechgaard Riso, Denmurh J.Y. Becker Beer-Shwu. Isruel A. J. Bruce Murriiy Hill. USA E. Chiellini P istr, Itulj, D. Coatcs Poole, UK P. Day Loridon, UK D. A. Dunmur Shfficld, UK B. Dunn Los Arigdes, USA W. J. Fcast Durhrrrn. UK R. H. Friend Cirmbririgc. UK A. Fu kuda Tokyo, .Jirpati D. Gattcschi Floruic~r,Ittilj. J. W. Goodby Hull, UK 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 tech n (1 I ogy. 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 including a top copy with figures c’tc. should be sent to the Managing Editor at the Cambridge address. Jourrirrl of hluteriirls Chemistry ( ISSN 0959-9428) is published monthly by The Royal Society of Chcmistry. Thomas Graham House. Science Park. Milton Road, Cambridge CB4 4WF, UK. 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Full details of the form of manuscripts for Articles and Materials Chemistry Communications, conditions for acceptance etc. are given in Issue 1 of Journul of’ Muteriuls Chemistry published in January of each year, on the world wide web (htpp://chemistry.rsc.org/rsc/) or may be obtained from the Managing Editor. There is no page charge for papers published in Journal of’ Materiuls Chemistry. Fifty reprints are supplied free of charge. Janet L. Dean, Managing Editor Tel.: Cambridge (01223) 420066 E-Mail (INTERNET): DEANJ@RSC.ORG Fax: (01223) 420247 Advertisement sales: Tel. +44 (0)171-287 3091; Fax f44 (0)171-494 1134 ~ ~ ~~~ ~!~~The Royal Society of Chemistry, 1996. 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.

ISSN:0959-9428    

DOI:10.1039/JM99606FX033

出版商:RSC    

年代:1996

数据来源: RSC

ISSN:0959-9428    

DOI:10.1039/JM99606BX035

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

ISSN 0959-9428 JMACEP( 10) 1605-1739 (1996) Synthesis, structures, properties and applications of materials, particularly those associated with advanced technology Feature Article bubble core(formation of rmdicabkxcited moleculcs) \-bubble vicinityDietmar Peters <prcssurchcmpersturc grmdient, electrical fields, jctshhock waves onto metrVsolid surfmca or other phrsc boundaria) \ bulk solution (solid or soluted reactants, expelled reactive intermediates) Articles 16 19 Ferroelectric and antiferroelectric liquid crystalline phases in some pyridine carbox ylic acid derivatives N. Kasthuraiah, B. K. Sadashiva, S. Krishnaprasad and Geetha G. Nair 1627 Characterization of novel TCNQ and TCNE 1 : 1 and 1 : 2 salts of the y....... /ytetrakis(dimethyamino)ethylene dication, ...,.. ......Y [{(CH,>2N>,C-C(N(CH,)2~2I2+ James R. Fox, Bruce M. Foxman, Donna Guarrera, Joel S. Miller, Joseph C. Calabrese and Arthur H. Reis, Jr. 1633 Syntheses, characterization and non-linear optical properties of nickel complexes of multi- sulfur 1,2-dithiolene with strong near-IR absorption Jing-Lin Zuo, Tian-Ming Yao, Fei You, Xiao-Zeng You, Hoong-Kun Fun and Boon- Chuan Yip i ii 1679 Chemically modified kaolinite. Grafting of methoxy groups on the interlamellar aluminol surface of kaolinite James J. Tunney and Christian Detellier 1687 Growth of emerald crystals by evaporation of a KZO-MOO, flux Shuji Oishi and Hirofumi Yamamoto 1693 Nucleation and crystal growth of analcime from clear aluminosilicate solutions Geoffrey S.Wiersema and Robert W. Thompson 1701 Hydrothermal conversion of amorphous NiFe, -xAlx(OH)8 into crystalline phases Emilia Wolska, Wlodzimierz Wolski, Janusz Kaczmarek, Pawel Piszora and Wojciech Szajda 1709 Structural study by energy dispersive X-ray diffraction of amorphous mixed hydroxycarbonates containing Co, Cu, Zn, Al Marilena Carbone, Ruggero Caminiti and C. Sadun 17 17 The structures of strontium teIlurite and strontium telluride aluminate sodalites studied by powder neutron diffraction, EXAF’S, IR and MAS NMR spectroscopies Sandra E. Dann and Mark T. Weller 1) X flny Tthe 2) Cellllllallon SHlS 3) S;ln@e Poailh 4) slcglo Cryctnl Solid SLIIU Energy Ssnslllvm Ik(Cel0c ... 111 iv

ISSN:0959-9428    

DOI:10.1039/JM99606FP091

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

i Joswig W, 1413 Joubert J-C , 1165 Juan A, 1433 Judeinstein P , 511 Jumas J-C, 41 Jungnickel G , 1649 Kaczmarek J , 1701 Kaczorowski D , 429 Kadokawa J-I , 1235 Kagawa S, 97 Kahn 0,1521 Kakkar A K, 1075 Kalfat R, 1673 Kamath P V, 1429 Kanamura K, 33 Kandon K, 1401 Kannan T S , 1395 Kamiainen T, 161, 983, Kano S, 1191 Karasu M, 1235 Kariuki B M, 1601 Kasthuraiah N, 1619 Katerski A, 377 Kaul A R, 623 Kawaguchi K , 117 Keana J F W, 1249 Keene M T J, 1567 Kelder E M, 765 Kemnitz E, 1731 Kennard C H L, 23, 137 Khomenko G E, 595 Kikuchi K, 501 Kilian D 935 Kim J H, 365 Kim S B, 365 Kinjo N, 727 Kitazawa T , 119 Klar G, 547 Klinowski J, 1391 Klissurski D G, 1035 Knowles J, 1135 Knowles J A, 89, 1567 Kochubey D I, 207 Kodenkandath T , 1575 Koehler K, 579 Kohler T, 1657 Kooli F, 1199 Koroglu A, 1031 Koto K, 459 Koudijs A, 1469 Koyama S , 1055 Kremer R K, 635 Kreuzer F-H , 935 Krishnaprasad S , 1619 Kristof J , 567 Krowczynski A, 733 Kruerke D, 927 Kruidhof H , 815 Kudnig J, 547 Kuroda K, 69, 1055 Kurosu H, 719 Kusumoto T , 753 Klonkowski A M, 579 Labes M M, 1 Lachowski E E, 1379 1497 Lagow R J, 917 Lai S-W, 469 Laine R M, 1441 Lambrecht W R L, Lanzetta R , 1473 Laureiro Y , 1517 Lavela P , 41, 861 Lebuis A-M , 1075 Lecante P , 1527 Leclercq D, 1665 Le Cras F, 1591 Lecuire J-M , 773 LeeC K, 331 LeeC Y, 1131 Lee E, 109, 871 Lee G R, 187 Lee M, 1079 Le Flem G, 381 899 Le Quesne J P, 1361 Lerner M M, 103 Leskela M, 161, 983, 1497 Leskela T, 781 Lezama L M, 421 YHentier P , 1165 Li S, 1207 Lin C L, 1 Lin J, 265 Linda11 C M, 1259 Lindroos S , Liu C S, 1131 Liu S, 305 Livage J, 505 Llavona R , 415 Loiseau TI 1073 Lorriaux-Rubbens A, 385 Lose D, 1297 Lougnot D J, 1595 Loukatnkou L A, 887 Loveday D C, 993 Lowendahl L, 213 Lunkwitz R, 1283 Lynch D E, 23 Machida M , 69, 455 MacKenzie K J D, 821, Macklin W J, 49 Madarasz J, 781 Madroiiero A, 1059 Magri P, 773 Mai S-M, 1099 Maignan A, 1245, 1549, Mairesse G , 1339 Maiti H S , 1169 Maksimov Y V, 207 Malandrino G , 1013 Malet P, 1419 Mancini N A, 1013 Maniero A L, 1723 Mann B E, 253 Manthiram A, 999 Marcelis A T M, 1469 Marcomini A, 1723 Marcos M, 349, 533 Manappan L , 1395 Marrot B, 789 Marson C M, 747 Martin C, 1245, 1585 Martinez E S, 547 Martinez J I, 533 Marucci A, 403 Marugan M M, 667 Marzotto A, 941 Matacotta F C, 1575 Mather G C, 1379, 1533 Matijevic E , 443 Matsui M, 1113 Matsuyama H , 501 Mattei G, 403 Maury F, 1501 McKeown N B, 315 McLendon G , 369 McMurdo J, 149 Meinhold R H , Menges B, 1319 Mercey B, 165, 1141 Merle-Mejean T , 595 Michel C, 175, 1549 Miller J S, 1521, 1627 Minami T, 459 Minceva-Sukarova B, 761 Misawa M, 1191 Mitchell G R, 1479 Mitov I G, 1035 Mittler-Neher S, 1319 Miura 0, 727 Miyachi K, 671 Miyasaka H , 705 Miyata F, 711 Miyazaki A, 119 Miyazaki K , 727 Moffat J B, 459 Li Y-J, 691 161, 983, 1497 833, 1225 1585 821, 833 Morales J, 37, 41, 861 Moran E, 1517 Mon T, 501 Monga T, 459 Monneau R, 505 Monoka H, 1235 Mosel B D, 635, 801 Mosset A, 789, 1527 Mueller B L, 1441 Mulley S, 661 Mullmann R, 635, 801 Muramatsu H, 1113 Mutin P H, 1665 Muto A, 1241 Nagase Y, 711 Nair G G, 1619 Naito H, 33 Najdoski M Z, 761 Nakano H, 117 Nakaya T, 691 Nakazumi H , 11 13 Narayana Rao D , 1119 Nasman J, 1309 Neat R J, 49 Needs R L, 1219 Nemoto N, 711 Neumann B, 1087 Newport R J, 337,449 Nickel K G, 595 Nieminen M , 27 Nii H, 97 Niinisto L, 27, 781 NionT, 1231 NomaN, 117 Nortier P, 653 Nowogrocki G, 1339 Nutz U, 1283 Nygren M, 97 O’Bnen P, 343, 1135 Oestreich S, 807 Ogata H T S, 1235 Ogawa K, 143 Ohashi M, 1191 Ohwaki K, 795 Ohyama T, 11 Oishi S, 1687 Oka Y, 1195 Okada A, 1487 Olbnch F, 547 Olivera-Pastor P , 247 Olivier-Fourcade J , 41 Omenat A, 349 Onakhi C 0,103 Orpen G A, 993 Ostrovski D , 1309 Otterstedt J-E, 213 Ouchi S, 1401 Ouyang J-m, 963 Pac C, 143 PaguraC, 567 Panda P K, 1395 Paprotny J, 1455, 1459 Parent C, 381 Park J W, 365 Parker M J, 911 Paronen M , 1309 Partndge R D, 183 Paschke R, 1283 Peacock R D, 1259 Pearson C, 699 Pedrini C, 381 Peeters K, 239 Pei Y, 1521 Pelizzi C , 1319 Pelloquin D , 175, 1549 Pelzl G, 1283 Peng B-X, 559, 1325 Peng Z-H, 559, 1325 Pereira-Ramos J-P , 37 Perez-Rodriguez J L , 1557 Perrin M-A, 653 Pertierra P , 415 Peters D , 1605 Petnc A, 1347 Oh N-K, 1079 Piccinllo C , 567 Pickett N L, 507 Piszora P, 1701 Pizarro J L, 421 Pohmer J, 957 Pola J, 155, 975 Pomonis P J, 887 Poojary D M, 639 Portemer F , 1543 Porzio W, 1319 Pottgen R , 63, 429, 801 635, Powell A V, 807, 1579 Predieri G , 1319 Prellier W , 165 Pringle P G, 993 Pyzuk W, 733 Qian M, 435 Qun L, 559 Radaev S F, 1413 Radhaknshnan T P , 1119 Ramachandra Rao C N, 1585 Ranjan R, 131 Rao K J, 391 Rao C N R, 1737 Rasheed R K, 277 Rasika Abeysinghe J , 155 Ratcliffe N M , 289, 295, Rauhala E, 27 Rautanen J, 781 Raveau B, 165, 175, 1141, 1245, 1549, 1585 Ravi M, 1119 Rawson J M, 1161 Rawson J 0, 253 Razafitnmo H , 369 Reis A H, 1627 Reynes A, 1501 Rice D A, 1639 Rigden J S, 337,449 Riley F L, 1175 Rio C d, 947 Risbud S HI 1643 Rives V, 1199, 1419 Rodnguez J , 415 Rodnguez M L, 415 Rodnguez-Castellon E , 247 Rohl A L, 653 Rohs S, 1591 Rojas R, 1517 ROJO T, 421 Ros M B, 1291 Rosseinsky M J, 1445 Ruiz-Amil A, 1557 Ruiz-Conde A, 1557 Russell D A, 149 Saadoune I, 193 Sadaoka Y, 953, 1355 Sadashiva B K, 1619 Sadun C, 1709 SaeedT, 1135 Saito K, 501 Saito Y, 1055 Sakamoto MI 1355 Sakata Y, 1241 Salvado M A, 415 Salvador S, 73 Samoylenkov S V, 623 Sanchez C , 511 Sanchez Escribano V, 879 Sanchez L, 37, 861 Sanchez-Soto P J , 1557 Sanchis M J, 547 Sanders G M, 357 Sano T, 605 Santos M, 975 Sasaki S, 501 Sat0 M , 1067, 1191 Sauer C, 1087 Sayle D C, 653 Scheler U, 1219 Schnelle W, 635 301 Ruiz-Hitzky E , 1005 Segal N, 395 Segre U, 1723 Seifert G, 1657 Sekine T, 1231 Senmaa R , 1309 Sermon P A, 1019, 1025 Serrano J L, 349, 533, Seshadri R, 1585 Sheldon T J, 1253 Shernngton D C, 719 Shibata K , 691, 1113 Shinton S, 667 Shirai Y, 711 Shirakawa Y , 1191 Shirota Y, 117 Shitara Y, 11 Silvert P-Y, 573 Simonin L , 1595 Singh N, 629 Sironi A, 661 Skjerhe K P, 595 Slade R C T, 73,629 Smart L E, 221 Smeulders J B A F, 871 Smith I K, 539 Smith J R, 295 Smith M E, 261, 337 Smith P C, 1639 Smith W, 1385 Smrcok L, 629 Soraru G D, 585 Southern J C, 73 Stefanis A De, 661 Steuernagel S, 261 Stoev M, 377 Stradling E P, 1211 Strobe1 P, 1591 Su Q, 265 Suarez M, 415 Subbanna G N, 1429 Subramanian M A, 867 Subrt J, 155 Sudholter E J R, 357, Sugahara Y, 69, 1055 Sugyama S, 459 Sun Y, 1019, 1025 Sundaresan A, 1549 Sundholm F , 1309 Sundholm G , 1309 Sung K, 917 Suzuki H, 501 Suzuki T, 671 Szajda w, 1701 Szepvolgyi J , 1175 Szydiowska J , 733 Tagliatesta P , 953 Tai Z, 963 Takahashi H , 795 Takahashi M , 119 Takanishi Y, 671, 753 Takashima M , 795 Takeda H, 1055 Takeda M, 119 Takehara Z-I, 33 Takezoe H , 753, 1231 Tamaki S, 1191 Tamaura Y, 605 Tanaka K, 953 Tanaka M, 459 Tang W, 963 Tatam R P, 131 Taylor R, 155 Teare G C, 301 ten Elshof J E, 815 Tendeloo G V, 1339 Teraoka Y, 97 Teunis C J, 357 Thatcher J H, 1099 Thiebaut B, 1379 Thompson D P , 1031 Thompson R W, 1693 Tirado J L, 37, 41, 861 1291 1469 Suzuki Y-1, 753 Lehtinen T , 1309 Mohai I, 1175 Petrunenko I A, 207 Schoonman J, 765 Toda K, 1067 Lemaire M , 1107 Lequan M, 5, 555 Lequan R M, 5, 555 Moine B, 381 Monk P M S, 183 Moon J H, 365 Petty M C, 699 Phillips B, 1643 Picard C.619 Schouten P G, 357 Schulz E , 1107 Sdoukos A T. 887 Tomkinson J, 449 Tomlinson A A G, 66 1 645, 11 iiiivvvi

ISSN:0959-9428    

DOI:10.1039/JM99606BP095

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Ultrasound in materials chemistry Dietmar Peters Universitat Rostock FB Chemie Buchbinderstr. 9 D-18051Rostock Germany Ultrasound has applications in materials organometallic polymer and synthetic chemistry as well as materials and waste degradation. After a brief description of the fundamentals of sonochemistry an overview about current uses and future prospects in these fields is given. The first steps in sonochemistry were taken about 70 years ago,’ but the extended application of ultrasound as a tool for synthetic and materials chemistry did not begin until the 1980s. Since then many papers have been published and sonochemis- try has become a versatile approach in several areas (for some related reviews see ref. 2-8).. The subject is quite large and one had to select from a large number of papers..Thus this article will focus on applications to materials chemistry (metals solids polymers biological materials) sonochemistry involving metals and non-metallic solids materials degradation and related industrial and laboratory applications in these fields. Even with such restrictions a personal choice was unavoidable and some fields are only described in general and not in detail. Therefore this article attempts to give an overview of the scope and trends in the application of ultrasound in materials science and related fields in chemistry. Fundamentals The part of the sonic spectrum which ranges from about 20 kHz to 10 MHz is called ultrasound and it can be sub- divided in three main regions power ultrasound (20-100 kHz) high-frequency power ultrasound (100 kHz-1 MHz) and diag- nostic ultrasound (1-10 MHz)..The latter range is also often called high-frequency ultrasound. Acoustic energy is mechanical energy i.e. it is not absorbed by molecules. Ultrasound is transmitted through a medium via waves by inducing vibrational motion of the molecules which alternately compress and stretch the molecular structure of the medium.. Therefore the distances between the molecules vary as the molecules oscillate about their mean position. If the intensity of ultrasound in a liquid is increased a point is reached at which the intramolecular forces are not able to hold the molecular structure intact. Consequently it breaks down and cavitation bubbles are created..This process is called cavitation and the point at which it starts is known as the cavitation threshold. Two forms of cavitation are known stable and transient. Stable cavitation means that the bubbles oscillate about their equilibrium position over several refraction-com- pression cycles while in transient cavitation the bubbles grow over one (sometimes two or three) acoustic cycle to double their initial size and finally collapse violently. There are three different theories about cavitation the hot- spot the electrical and the plasma theory. But according to each theory there is no doubt that the origin of sonochemical effects is cavitation. Furthermore it has been shown experimen- tally that cavitational collapse creates drastic conditions inside the medium temperatures of 2000-5000 K and pressures up to 1800 atm within the collapsing cavity..Thus from a practical point of view the parameters which influence cavitation are important (Table 1).However it should be noted that there is often no simple relationship and an optimum can generally be found for all parameters. For details of theoretical aspects of ultrasound and cavitation refer to the From a practical point of view there are three possible reaction sites of a collapsing bubble the cavity interior the bubble vicinity and the bulk solution (Fig. 1).. The following beneficial sono- chemical effects can be observed (i) ligand-metal bond cleavage in transition-metal complexes to give coordinatively Table 1 Ultrasound parameters influencing ~avitation~,~.’-~~ parameter frequency intensity solvent bubbled gas external temperature external pressure effects low long cycle large bubbles low amplitude required to induce cavitation high short cycle high amplitude necessary increased attenuation weak or no cavitation in the MHz range (rarefaction cycle is too short to create bubbles) considerably higher intensity at high frequency is necessary to maintain the same cavitation as at low frequency indefinite increase limited by the material stability of transducer decoupling with the medium and a large number of bubbles (transmission barrier) the higher the vapour pressure the less violent the collapse (increased penetration of vapour into the bubbles) induction of cavitation is more difficult in solvents with low vapour pressure cavitation is easier in solvents with low viscosity and surface tension y =CJC should be high as the collapse temperature is proportional to (y -1) the smaller the thermal conductivity of the gas the higher the local heating during the collapse the greater the amount of dissolved gas the smaller the intensity of the shock wave dissolved gas acts as cavitation nuclei and leads to more facile cavitation temperature rise increases vapour pressure and collapse is less violent less intensity necessary to induce cavitation temperature near the boiling point of the solvent dramatically increases the number of bubbles which can act as sound barrier pressure rise decreases vapour pressure and collapse is more violent; higher intensity is necessary to induce cavitation optimum depends on frequency J.Mater. Chem. 1996 6(lo) 1605-1618 1605 bubble core (formation of ladicalslexcitcd mo!e+g bubble vicinity (pressudtemperaturcgradient elarical fields jetdshock waves onto meWsolid I surfaces or other phase boundaries)I \ -- - - -' bulk solution (solid or toluted reactants erpelled reactive intermediates) Fig. 1 Reaction sites of a collapsing cavitation bubble unsaturated species or modified complexes as well as complete removal of ligands to produce amorphous metals; (ii) dis- ruption of the solvent structure altering the solvation of reactants; (iii) sonolysis of molecules (homolytic fragmentation to radicals rupture of polymers generation of excited states cell disrupture); (iv) mechanical effects by cavity collapse onto metals and solids (shear forces jets and shock waves resulting in rapid mass transfer surface cleaning particle size reduction crystal defects and metal activation); (v) effects in liquid-liquid systems (improved mass transfer emulsification increase of the effect of phase transfer catalysts or even their replacement); (vi) effects in gas-liquid systems (degassing of liquids or melts atomisation of liquids in air); (vii) single electron transfer (SET) steps in chemical reactions are accelerated and if an ionic and an electron transfer pathway are possible the latter is preferred ('sonochemical switching').Metals Powders and other Particles Reactions and processes involving metals powders or other solid particles are probably the most successfully investigated fields in the application of ultrasound in chemistry and mate- rials science. Table 2 gives an overview of such fields. Decomposition of metal carbonyls The effects of ultrasound on metal carbonyls were investigated initially by Suslick and co-workers" in 1981. Upon irradiation of a decane solution of Fe(CO) under argon with ultrasound (20 kHz 100 W cm-') they observed an unusual Fe3(CO)12 cluster formation together with the formation of fine powdered iron (Scheme 1).The unique nature of the ultrasonic treatment can be seen by comparison with the effects of light and heat.Table 2 Applications of ultrasound to metals and non-metallic solids material applications of ultrasound metals production of amorphous nanosized metal powders and nanocolloids preparation of activated and supported metals used as catalysts agglomeration of metals formation of metal carbides electroplating and spray pyrolysis to form metal layers crystallisation metal welding machining soldering casting sonocleaning organometallic sonochemistry solids impregnation of catalysts on solid supports preparation of fine particles and colloids particle size reduction cavitation erosion surface treatment layered solids dispersion dyeing sieving heterogeneous sonochemistry rFe Scheme 1 Sonolytic thermolytic and photolytic decomposition of Fe(CO) (ref.6) Photolysis of Fe(CO)5 yields Fe3(CO)9 via Fe(CO) which reacts with unconverted Fe(CO)5 (Scheme 1).. Thermolysis of Fe(CO) results in decomposition to iron clusters and CO. The formation of amorphous iron depends on the sonication parameters influencing cavitation (solvent vapour pressure. gas).. The influence of the reaction conditions on the measured properties (e.g. particle size) of sonochemically produced amorphous iron has been described recently.12 Sonochemically produced iron is an amorphous pyrophoric powder having a surface area of about 150m2 g-'. It has a coral-like structure which is built up from nanosized clusters. DSC measurements show an irreversible crystallisation exotherm at 350°C reflecting the formation of normal fully crystallised u-iroa6 Using sonoluminescence as a spectroscopic probe of iron pentacarbonyl decomposition Suslick6 estimated temperatures of >5000 K and pressures of about 1700 atm for the hot-spot parameters of the cavitation event during a period of less than 100 ns..Therefore cooling rates of more than 10" K s-' are deduced for this process.. These rates are sufficiently high that the material can be frozen before crystallisation occurs and amorphous metals can be formed. The initially formed amorphous nanoscale metal can be trapped by a solid support. Upon adding a polymer [e.g. poly(vinylpyrrolidone)] or silica to the sonicated solution nanocolloids and supported metal catalysts respectively are made..These materials have interesting magnetic properties. Nanophase amorphous iron is a very soft ferromagnet having a magnetic moment between those of crystalline and molten iron. Nanocolloidal iron is superparamagnetic and nanophase iron supported on silica has similar magnetic properties. Other metal nanopowders using different transition-metal carbonyls as precursors are also available. Amorphous metals on supports are interesting catalysts as will be shown below. Recently sonochemical iron pentacarbonyl decomposition has been carried out under an air atmosphere instead of under argon.. The X-ray diffraction pattern of the produced fine powder showed the formation of a non-crystalline phase of magnetite.Annealing of this powder for 2 h at 770 K resulted in a polycrystalline phase of magnetite with the same diffraction pattern as a sample produced by milling of a magnetite single ~rysta1.l~ Sonoelectrochemistry producing metal nanopowders and metal layers In recent years ultrasound has been introduced into electro- chemistry. Beneficial effects are among others acceleration of mass transport cleaning and degassing of the electrode surface or disturbing the diffusion layer.14-16 A pplications besides the use in electrochemical synthesis and electroanalytical methods are for example the electrodeposition of metals or alloys (e.g. Cu Zn Cu/Zn Ni/Fe; for reviews see ref. 7 17 and references therein) the production of catalytically active powders (e.g.Cu Co Zn)" and the preparation of Si films." Improved physical and mechanical properties such as better hardness brightness and adhesion as well as a higher depos- ition rate an increased plating current lower internal stress of 1606 J. Muter. Chern. 1996 6(lo) 1605-1618 the coatings and a more uniform microstructure are the benefits of using ultrasound in electroplating.. The effects of 1.2 MHz and 20 kHz ultrasound applied at an intensity of 4 W cm-' have been compared in a study of the electrodeposition of Cu by Drake.,' Using the high-frequency ultrasound the diffusion layer is reduced from 200 pm to about 20-30 pm whereas 20 kHz ultrasound reduces it to 3.4 pm.The properties of porous silicon thin films produced by open-circuit stain etching are better under sonication compared to those of samples generated in the absence of ultrasound. The differences induced by ultrasound in surface morphologies (rougher and thicker films) and chemical stability (greater stability on exposure to water and organoamines) proved that ultrasonic irradiation is a useful t00l.l~ Pulsed ultrasound has recently found more interest in chem- istry.,' Reisse and co-workers" described the pulsed sonored- uction of metal salts in aqueous solution which resulted in the formation of metals and other solid powders (Cu Co Zn Ni Cr Ag Co/Ni MnO CdTe) with particles sizes of about 100 nm.. The sonotrode acts as ultrasound emitter and also as the cathode.. The 20 kHz high-intensity ultrasound pulse (pulse duration 100 ms followed by a 200 ms switch off) is superposed by an electrical current (pulse duration 300 ms).During sonication the cathode surface is cleaned the metal particles are expelled (preventing the growth of large particles) and the double layer is replenished with metal cations.. The metals obtained are fine crystalline powders with high surface areas and chemical purity.. The electrical yields are about 75-90%. Using a 1 cm diameter sonoelectrode amounts of 1 g h-' can be produced. Sonoelectrochemically produced Zn has been used as an effective catalyst in the Barbier-type reaction (Scheme 2). Sonication of metal powders solids and supported metals used as catalysts or reagents Owing to cavitational collapse sonication of metals or solids leads to microjet and shock-wave impacts on the surface which can result in particle size reduction interparticle collisions depassivation surface cleaning defects and erosion..The extent of erosion depends on the type of metal. Sonication of soft metals with hard oxides (e.g. Al Na Li) results in metal deformation which will damage the oxide layer. Hard metals are not plastically deformed by ultrasound but the surface is activated by the above-mentioned cavitation effects owing to the low cohesion of the oxide coating. If the particles are very small and/or in close proximity (e.g. in slurries) they will impinge (interparticle collision) agglomerate and even fusion can occur owing to the high temperatures of the hot spot.6 A recent study' was devoted to the effects of the particle size morphology of Cu and Pb sonicated in dilute HCl using a 38 kHz cleaning bath.It was found that the cavitational modification of the metals depended on the initial size of the solids. Large copper turnings or lead foil are greatly depassiv- ated by microjets. Small particles <ca. 100-150 pm (in the study:" Cu<63 pm and Pb ca. 150 pm) are not subjected to microjet impacts because a collapsing bubble will have an average diameter of about 150 pm at 20 kHz. Consequently Zn powder yield sonoelectroproduced 82% (70% isolated) commercial 27% Scheme 2 Application of sonoelectrochemical produced Zn powder" shock-wave impacts and interparticle collisions are dominant and only small differences to a stirred control probe were found.There are in principle two main groups of applications of ultrasound to metals and solids in sonochemistry and they are described below. (1) Sonochemical preparation of catalysts used in non-son-icated reactions. Applications include the preparation of acti-vated metals by reduction of metal salts (e.g. reduction with Li in THF to Rieke-type powders or with formaldebvde to Pd or Pt); the generation of activated metals by sonicntion; the precipitation of metal (Cr Mn Co) oxide catalysts and the impregnation of metals or metal halides on supports. The reduction of metal halides with Li in THF in a low- intensity cleaning bath gives rise to active metal powders comparable to Rieke-type powders.Sonication for less than 1 h (the conventional Rieke procedure requires about 8 h and stirring for up to 26 h) yields powders of Mg Zn Cu Ni Cr Co Pt and Pd. Platinum black produced by reduction of platinum halides under ultrasound has increased activity in the hydrogenation of alkenes up to a factor of three.23 Furthermore increases in magnetic susceptibility (98%) and surface area (62%) has been measured.. The most catalytically active platinum and palladium blacks have been obtained at 20 kHz (Pt) and 3 MHz (Pd).24 Similar activity enhancements have been found for Ni and Co obtained after precipitation of metal oxalates under sonication and subsequent reduction to the meta1.25s26 The sonication of ordinary commercial Ni (known as a poor catalyst) to obtain highly active metal powder has been investigated extensi~ely.~~~"~~Irradiation with ultrasound increases the catalytic activity dramatically reaching that of Raney nickel.Marked changes in the surface morphology have been observed the surface is smoothed by removing the crystallites and the oxide layer.. The particle size is hardly reduced. If metal slurries containing different metals are son-icated melted necks between two different metal particles can be observed.. Thus S~slick~,~' found that for example in the cases of Fe and Sn the neck is an alloy between the two metals i.e. high velocity collisions with sufficient energy to melt the metals have occurred.If the particles experience glancing collisions smoothing occurs. From investigations of different metals one can estimate the local conditions in the slurry. If Fe and Cr (which melt at about 1500 and 18OO0C,respectively) are irradiated tremendous agglomeration is observed. Sonication of Mo (mp ca. 2600 "C)still results in agglomeration and practically no smoothing is observed. In the cme of W ultrasound has no effect.. Thus a temperature limit of ca. 3000°C can be estimated. In the cases of Mo and W the production of metal carbides (e.g. Mo2C) has also been reported re~ently.~' Sonication of a mixture of MoC1,-SiCl and sodium-potassium alloy followed by annealing at 900 "C gives nanocrystalline MoS~,.~ Sonochemically obtained MoSi shows 50-70% higher microhardness and compression strength than the conventional coarse-grained MoSi,.Sonochemically obtained Ni powder is a highly hetero- geneous catalyst for hydrocarbon reforming and CO hydrogen- ation. In the hydrogenation of alkenes it is comparable with Raney Ni; however it is more selective as C=O groups are ~naffected.~"~'Ultrasonic precipitation of chromium molyb- denum oxide yields a catalyst with 15-20% increased dispersity and better activity for the oxidation of methanol to f~rmaldehyde.~~ In 1973 the sonication of aqueous suspensions of Cr2O3 Co,03 MnOz and alumina giving supported metal oxide catalysts of higher activity and dispersity for H,OZ decomposition was rep~rted.~' Thus the surface area of sup- ported Cr,03/A1,03 is enhanced from 108 mz g-' (normal preparation) to 135 mz g-' (sonochemical preparation). Impregnation of Pt on silica gel produced by sonoreduction J.Muter. Chem. 1996 6(lo) 1605-1618 1607 of ammonium hexachloroplatinate at 440 kHz yields a metal catalyst with an 80% increased surface area compared to that .~prepared by mechanical Ragaini et ~2 reported ~ recently on the dispersion of Ru on alumina. RuC1 (in water) and Ru(acac) (in toluene) absorbed on A1203 were subjected to hydrazine reduction under ultrasound. At low Ru content (0.5%) a higher dispersion (66% with RuC1,j was found compared to a non-sonicated run (37% with RuCl,). With a higher Ru content (5%) the dispersion was relatively low in both runs and was comparable in value (about 20%).. The applied power is critical as at high power the alumina support is affected.A further study investigated the bromination of aromatics with CuBr,/A1203 .3 Sonication of the reaction mix- ture with non-impregnated reagents led to a substantial rate improvement for the bromination of naphthalene compared to a non-sonicated run. However even better results were obtained when CuBr was sonochemically preimpregnated on alumina followed by conventional procedures. Sonochemically supported CuBr on Al,O showed reduced particle sizes and pronounced changes in the surface morphology. Multicomponent catalysts used in chemical technology can also been activated or reactivated Ziegler-Natta Ni/Mo cracking Ti/V oxide or Pd-alumina deni trification catalysts (see ref.7 p. 54ff j. (2) Activation of solid reactants (metals and non-metallic solids) in heterogeneous chemical reactions by ultrasound. Applications include the preparation of activated metal solu- tions; the preparation of organometallic compounds from main group or transition metals; sonochemical reactions involving metals via in situ generated organoelement species; and reac- tions involving non-metallic solids. There are some obstacles to reactions between a solid and a liquid (or in a liquid-dissolved) reactant in a heterogeneous system e.g. the small surface area of a bulk solid; the solid surface may be coated by oxide layers or impurities; species have to diffuse to and away from the solid surface; and deposition of products may inhibit further reaction.These difficulties can be overcome by the effects of ultra- sound due to cavitation as has been mentioned already in part shock waves (causing plastic deformations on soft mate- rials break-up of coatings) microjets (causing surface erosion defects and deformations enhancing the surface area) and microstreaming (improving mass transfer removing eroded particles and deposited impurities or products).. There are Organotin fluorides are insoluble in most solvents owing to their polymeric structures.. Therefore their reactivities are extremely low. Sonication of derivatives R3SnF (R =Bu Ph) with metal salts NaX (X=Cl Br OCN SCN) leads to the rapid formation of monomeric compounds R3SnX.66 An interesting effect of sonication on the reaction pathway has been observed in the reaction of solid KCN/A1203 with benzyl bromide (Scheme 6).39767.The course of the sonochem- ical reaction is completely changed as under stirring a different product is observed.. This so-called sonochemical switching (see Scheme 6 for examples) is explained by different reaction mechanisms. Under sonication an electron-transfer mechanism (leading in this case to a different product) is preferred whereas under stirring an ionic pathway dominates. As this chapter covers mainly synthetic chemistry aspects some potential future trends and questions will be given. At present sonochemistry is an established field in laboratory preparations.However an as yet unsolved problem is the scale-up on the one hand the scale-up of a laboratory synthesis to a 20-50g scale and on the other hand chemical synthesis on an industrial scale.. There are already examples reported in these two areas e.g.the preparation of perfluoroalkyl aldehydes on a 40g scale6* and the introduction of ultrasound in three steps of a complex 14-step steroid synthesis to desogestrel yielding the intermediates on a kg scale.69. Thus the existing sonochemical equipment allows in principle the synthesis on a kg scale and the realisation of higher bulk amounts should not be a technical problem.70. The production of kg amounts of compounds seems to be economical in the preparation of highly valuable compounds especially drugs in the pharma- ceutical industry.Further bulk applications in synthesis are still limited because of cost concerns. In laboratory applications the following trends will be followed synthesis of complex molecules and natural products; introduction of sonochemical steps in multistep syntheses; increased use of ultrasound in established fields of chemistry i.e. the combination of sono- chemistry with electrochemistry photochemistry high-pressure chemistry polymer chemistry and biotechnology; use of ultra- sound in reaction mechanism studies; and study of frequency effects and reactor design dosimetry of the sound field. Other applications to metals and non-metallic solids The use of ultrasound in this field of materials science is already extensive which can be exemplified by some further excellent reviews on heterogeneous son~chemistry.~.~,~~-~' selected applications and related recent publications crystallis- Therefore some recent papers have been selected as examples to illustrate the wide range of metals and solids used in synthesis.Highly active reducing agents can be prepared by the sonication of Mg in the presence of anthra~ene,~' K in toluene and Na in ~ylene.~ The alkali metals are obtained in colloidal solution. Similarly highly dispersed Hg emulsions are a~ailable.~ Organometallic derivatives are useful reagents in organic synthesis.. They are either prepared or commercially available or generated in situ.Schemes 3-5 show examples of reactions involving different main group and transition metals. The application of ultrasound to non-metallic solids has been reviewed extensively by Ando and Kim~ra.,~ Thus some .~~recent examples will be given. Goh et ~1 described the effect of ultrasound on sulfur-metal systems. Sonication of Cu Fe Zn or Mg with elemental sulfur in the presence of 2 mol dm-3 HC1 yields the corresponding sulfides (Table 3).. The increase of sulfide formation in the order water <hexane <CS2 corre- sponds to the solubility of sulfur in these solvents. Surprisingly iron is an exception giving the highest yield in water and only poor results in CS,. A similar reaction of selenium with alkali metals (Li K Na) giving the dimetaldiselenides M2Se2 has been published by.Thompson and Bo~djouk.~~ 1608 J. Mater. Chem. 1996 6(10) 1605-1618 ation and precipitation of metals alloys zeolites and other solid^,^'-^^ agglomeration of degassing of melts;78 spray pyrolysis to form thin or fine particle^;*^-^' treatment of solid surfaces; 92-94 dispersion of solid^;^',^^ prep-aration of colloids (Ag Au Q-sized CdS j;97-99 ultrasonic sieving,"' filtration'" and micromanipulation (transportation concentration fractionation) of small particle^;^^^^'^^ intercal-ation of guest molecules into host inorganic layered solid^;^,^ ultrasonic-aided development in advanced lith~graphy;"~ and electroless plating.105-109 Some of the items of this list will be described in more detail as the number of papers discussing these aspects has increased significantly over the last decade.Crystallisation and precipitation. In the treatment of liquid metals or alloys during solidification and of saturated solu- tions the crystallisation process is affected by ultrasound as follows inhibition of crust formation; intensification of heat transfer; increase in the nucleation and the growth rate; and affecting growth morphology.. Thus ultrasound induces changes in the formation of InSb crystals altering the crystal diameter changing the width of the facet region and the inclined angle of non-facet interfaces near the facet region for the (111) plane. For BiSb single crystals a strong decrease in Iithium 1441 sodium PhSe-SePh 1)) (Na) w 2 PhSeNa THF P\C(O) 5 min quantitauve 14% probe high power 50% 33% probe low power 66% 0% cleaning bath 100% 1471 potassium 1481 quantitative(E,Z) (E,E) = 8 162 U 83% Scheme 3 Sonochemical reactions involving alkali metals (ref.numbers in square brackets) the density of growth striations due to ultrasonic-induced convection currents in the melt has been reported.73 Non-ferroelectric but polar-oriented ceramics and films showing excellent piezoelectric or pyroelectric properties (Li2B407 Ba2TiSi208 Ba2TiGe20s) have received much atten- tion.. These materials can be prepared by surface crystallisation of glasses. However it is difficult to obtain well oriented crystallites by conventional crystallisation.Enhanced nucle- ation precipitation and oriented growth of desired crystals have been found when the glass surface is treated ultrasonically in suspensions of crystalline particles.. Thus surface-crystallised dense thin films of Li,B407 P-BaB,O Ba,TiSi,O on glass have been ~repared.~~.~' Ultrasonic irradiation also improves the precipitation of ceramic powders from solutions (Pb-Zr-Ti oxalate Pb oxa- late mullite-composition powder) and the hydrolysis of metal (Si Al) alkoxides. Accelerated precipitation and improvement of the homogeneity in the precipitates were observed.76 Ultrasonic spray pyrolysis. One of the most extensively used applications in materials science is ultrasonic spray pyrolysis (USP) which is used to produce either fine particles or thin films.Particles prepared by this ultrasonic method have the following interesting features spherical shape; uniform size distribution; adjustable particle size from micron to submicron range; and high purity.. The process requires a short preparation time and is continuous. Furthermore the size distribution is controlled easily by changing the solution concentration. In general aqueous solutions of metal salts (e.g.chlorides nitrates alkoxides carboxylates) are sprayed and hence the products are metal oxide particles. Studies on oxide ceramic preparations have been intensified since the report of high-T superconductors. Exam- ples of sonochemically produced systems are Y-Ba-Cu-0 and Bi-Ca-Sr-Cu-0 materials (e.g. YBa2Cu307-x Bi2CaSr2Cu20x Bi2Ca,Sr,Cu30,) having the beneficial properties described above and showing similar characteristics to superconducting particles sintered from the powders by conventional solid-state reactions.88 Other oxide-type examples are the preparation of Ti0,-SnO powder89 used for stable humidity sensors and Pb(Zr,Ti)O (PZT)90 which is a widely used piezoelectric and electro-optic material.Recently the preparation of non-oxide particles has been reported fine ZnS particles were produced from Zn(N03)2thiourea complexes. Deposition at low temperature (400 "C) gave amorphous particles whereas at around 800 "C a hexagonal crystalline phase was observed.. The latter particles ranging from 0.5 to 1.3 pm are spherical with a smooth surface.ZnS particles which were rough and included a zinc oxide phase were found at high temperatures (900 0C).9' Ultrasonic spray pyrolysis to form thin films has many advantages very small droplets which can be transported without heating of the carrier gas narrow droplet size J. Mater. Chem. 1996 6(lo) 1605-1618 1609 magnesrum Y W*COOMe quantitative indium HOEx + tin 0 PhGH NH2 92% THF-H20(1 5),ry % 30min 98% Scheme 4 Sonochemical reactions involving group 11 I11 and IV metals (ref. numbers in square brackets) distribution; solvent vaporises as it reaches the substrate; deposition under atmospheric pressure fast deposition rate; easy control of film composition and thickness; pure deposits free of contaminants; and a variety of precursors inexpensive raw materials (such as nitrates or chlorides) metal-organic compounds precursors with low volatility or low stability may be used.At present not only USP-prepared films made from relatively simple compounds (e.g. TiO,," SnO,," ZnOE4),but also more complex systems such as Y-Fe gernet calcia- stabilized zirconia La _,Sr,MnO La -,M,CrO and super- conducting Y-Ba-Cu-0 films have been described." Even multilayer films such as PbTi0,(001)/LaNi03( loo)/ MgO( 100) have been prepared." Recently metal-organic chemical vapour deposition (MOCVD) using USP has been used to form ferroelectric BaTi0384 and oriented thin TiO films." For the latter an improved method using pulsed injection in conjunction with USP and allowing good control over the film deposition rate (growth rate achieved 2.5 monolayers per pulse) has been developed..The excellent control over film growth and proper- ties is also a key feature in the preparation of SnO films.. The USP technique allows the fast preparation (2 nm s-l) of a film having photoelectrochemical properties and a surface appro- priate for a solar. The combination of ultrasound with an inductively coupled plasma (spray-ICP) allows the use of 1610 J. Muter. Chem. 1996 6( lo) 1605-1618 nitrate solutions as precursors. Applying this method perov- skite-type oxides have been prepared LaCr0,,80381 PbTiO LaTiO LaNiO LaCoO LaA10 (see ref. 80 and refer- ences therein).Sonication has also been proved to enhance diamond forma- tion on various substrates.. Thus ultrasound has been used to increase nucleation by seeding with diamond dust in the deposition of diamond on a carbon-carbon composite.86387 Treatment of solid surfaces and particles. Ultrasonic surface treatment (UST) can have different effects on the structures of solids e.g. increase in hardness of metals cavitational erosion enhancement of nucleation sites and generating and affecting crystal defects. Some examples are given below. The impact of ultrasound on a solid depends amongst other factors on the irradiation time. As already mentioned in the electrodeposition of metals sonication can enhance the hard- ness of metals.. Thus brief irradiation (5 min) of various metals caused a hardness increase of about 27% (Ti.Nb) to 150% (CU)."~On the other hand prolonged sonication leads to erosion of metals and alloys due to cavitation. Data on the resistance of such materials are of interest for applications where hydrodynamic cavitation can occur. Sonication of these materials can easily give such data as has been shown for aluminium alloys. Ultrasonic irradiation effects directly affecting the properties of point and extended defects on semiconductors are due to PhCH2CNthe stimulation of different processes generation of Frenkel pairs dissociation of point defect complexes and enhanced gettering of point defects by sinks such as dislocation grain 70% 12%boundaries or precipitates.Point defect gettering in silicon- based materials such as silicon wafers and solar-grade polycrys- talline silicon are examples. For the latter an improved minority carrier length and enhanced dissociation of Fe-B pairs have been found.92 PhH-trace 60% Carbon nanotubes can be opened selectively and filled with Scheme 6 Examples of sonochemical switching3 certain metals or metal oxides. Sonication prior to the oxidative opening with nitric acid has been found to enhance the number of acidic groups formed in this oxidation step.. These groups on the surface of the carbon nanotubes can bind palladium ions strongly.. Thus the amount of nanosized palladium crystal- lites deposited on the inner and outer surfaces can be increased by sonication..This effect is due to the creation of small local defects in the tubes such as buckling bending and lattice dislocations on the surface.93 The dispersion and impregnation of particles by ultrasound is another important application.. Thus metal matrix com-posites have been prepared by dispersion of ceramic particles in liquid metals. Normally the poor wettability of the particles by the metal segregating and clustering are problems in this process.. The obtained particle size is > 10 pm.. Thus sonication has been applied successfully in the production of Si3N,- Al-Mg. First Si,N (1 pm) was presonicated in acetone and then the particles were irradiated directly in the melt. As a result the particles were well wetted by the A1-Mg matrix a homogeneous distribution was achieved and no agglomeration was observed resulting in particle sizes <5 The impregnation of solid particles can be exemplified by dyeing of leather9' and the preparation of dielectric polymer composites by impregnation of dielectric BaTiO gel into pores of microporous polyethylene membranes.In the latter process a Ba( TiOPr'),-Pr'OH gel with immersed microporous poly- ethylene is sonicated for 15 min.. The obtained dielectric mate- rial has a significantly higher relative permittivity and higher losses than a conventionally produced BaTi0,-polyethylene fiim.96 Polymers The sonochemistry of polymers consists of three main fields the degradation of polymers the ultrasonically assisted syn- thesis of polymers and the determination of the polymer structure (for reviews see ref. 7,38,112-114)..The last area has been reviewed recentlyll' and will not be discussed here. Polymer degradation It has long been known that the irradiation of polymer solutions reduces their viscosity e.g. sonication (960 kHz 6.8 W cm-*) of an air-saturated 1% solution of polystyrene in toluene reduces the flow time from 23.6 s to 18.3 s after 2 h.38 As the early investigators concluded the decrease in viscosity is due to the degradation of the polymer chains. In polymer chemistry not only the rate and the yield of the polymerisation reaction as well as the structure of the polymer but also particularly the molecular mass and its range and distribution in the polymeric material are of interest.From the experimental data available some general conclusions concern- ing the rate and the extent of the ultrasonic degradation process can be made.. The rate of depolymerisation decreases with decreasing molecular mass of the polymer but below a limiting molecular mass there is no further degradation. Furthermore the degradation is affected by the ultrasonic parameters (frequency intensity) solution conditions (solvent gas content polymer concentration initial molecular mass) as well as external temperature and pressure (see Table 4).. The results of the influence of frequency intensity solvent and external temperature are in accord with the effects of exper- imental parameters on cavitation (see Fundamentals section) i.e.degradation is improved if cavitation is favoured. The effects of gas and external pressure are somewhat more complicated.. There is no doubt that the degradation is higher in the presence of a gas. As would be predicted polyatomic gases show lower degradation rates than diatomic ones. However the rates in the presence of monoatomic gases are between those for polyatomic and diatomic gases. Attempts to explain this abnormal behaviour discuss the gas solubility and 1612 J. Muter. Chem. 1996 6(lo) 1605-1618 Table 4 Parameters influencing polymer degradation7s3' parameter effects on degradation frequency increasing the frequency reduces the extent of degradation intensity increasing the intensity increases the rate and the extent of degradation there is an upper intensity limit due to the material stability of the transducer decoupling with the medium and a large number of bubbles (transission barrier) solvent the higher the vapour pressure the less violent is the cavitation collapse and the less is the extent of degradation cavitation occurs more readily in solvents with low viscosity and surface tension bubbled gas the solution should be saturated with a noble gas which has a low thermal conductivity resulting in higher local heating more violent cavitation collapse and increasing polymer degradation the greater the amount of dissolved gas the smaller the intensity of the shock wave i.e.the lower the gas solubility the higher the degradation external increasing temperature increases vapour temperature pressure and collapse is less violent resulting in decreasing degradation temperature near the boiling point of the solvent increases dramatically the number of bubbles which can act as sound barriers concentration decreasing the concentration of the polymer in solution increases the degradation process molecular a larger initial molecular mass of the polymer mass increases the degree of degradation there is a molecular mass limit below which there is no degradation its thermal conductivity.Contradictory results were found for the effect of pressure (for details refer to ref. 7). A recent study by Price et a1.'16 on the ultrasonic degradation of polystyrene clearly showed that by suitable variation of reaction parameters (temperature solution concentration sol- vent vapour pressure ultrasound intensity) extensive control over the molecular mass and the polydispersity of the resulting material is possible.There have been several attempts to explain the degradation and to outline appropriate reaction mechanisms.. The following causes have been considered frictional forces shear gradients and impacts due to cavitational collapse; hydrodynamic forces caused by shock waves; and chemical reactions caused by reactive intermediates. Many of the observed trends in polymer degradation can be explained by these theories. However why the polymers are broken in the middle of the chains can only partially be explained as reported by Glynn and co-w~rkers,~~~,~~~ who found a Gaussian distribution of the scission around the middle of a chain.Direct thermal degradation by the high temperatures of the hot spot seems to play a minor or even infinitely small role as this is mainly random. Polymer synthesis There are two basic groups of polymerisation reactions to which ultrasound is applied (1) sonication of a solution already containing a homopolymer and either a second homopolymer or a monomer; and (2) sonication of a solution containing only monomers (with or without initiator). The first group is related to the macromolecular radicals formed as a consequence of the polymer chain cleavage. Sonication of a mixture of two polymers or a homopolymer and a different monomer results in the production of block or graft polymers.However only combinations of two homo- polymers having similar macroradical formation rates lead to block copolymers. 0therwise only degraded homopolymers are observed.ii9 Adding a radicophilic compound to a son-icated polymer solution leads to end-functionalized polymers or copolymers e.g.to the introduction of a fluorescent group.'l2 Following this strategy polymers with modified properties (solubility elasticity thermal behaviour etc.) or for special uses can be obtained. In the second group of polymerisation reactions several papers concerning the radically initiated polymerisation of vinyl monomers appeared.Conventionally the initiating rad- icals are generated by thermal or photochemical decomposition of the pure monomer or of an initiator (AIBN DBP etc.). One major aim of polymer chemists is control over the molecular mass structure and properties of the resulting polymer. As will be shown in the following section sonication of aqueous solutions generates high HO- and H- concen-trations. On the other hand initiators like AIBN or DPPH are cleaved readily to radicals by ultrasound.. The initiation reactions are accelerated by several orders of magnitude.. The propagation and termination reactions of growing radicals are affected only slightly or not at all by ultrasound in contrast to the radical production. At lower temperatures the micro- structure of the polymer can be influenced ultrasonically e.g.the portion of syndiotacticity increases from about 55% at 60 "C to 74% at -10"C in the peroxide-initiated polymeris- ation of methyl methacrylate (MMA).92. There are several factors influencing polymerisation cavitation-affecting param- eters (solvent temperature intensity) and other reaction con- ditions (e.g. concentration and monomer type). For details refer to the above-cited literature. That cavitation is essential for polymerisation has been shown for example by Price,"2 who investigated MMA polymerisation. At room temperature a final conversion of 12% was reached after 6 h. A pronounced sound change indicated that cavitation stopped at this point resulting in no further conversion.In general polymerisations do not exceed about 20% due to the rapid viscosity increase which prevents cavitation. Besides radical-initiated polymerisation ultrasound has also been applied successfully to emulsion and suspension poly- merisation.. There are also some papers on ring-opening and organometallic catalysis-based polymerisations under ultra- sound. Scheme 7 shows some examples of the latter. ring opening polperisation Application of ultrasound to non-radical catalysed polymerisationsMaterials degradation Practically no solvent is inert under ultrasonic cavitation Chemical effects are either a consequence of the sonolysis of solvent vapour inside the collapsing bubble due to the harsh reaction conditions (cf.fundamentals section) or can be caused by secondary reactions (at different sites see Fig.1) resulting from reactive species (mostly radicals or radical ions) generated during the collapse. Sonolysis in aqueous systems It has long been known (since 1929)l2O that H,O is a product of water sonolysis:'21 2H20 220H- +2H* +H,02+H2 Studies and spin-trapping experiments of Riesz et gave evidence for HO and H as intermediates. From recent results123 of such studies temperatures of 2000-4000 K were estimated for the cavitational collapse hot-spot [from spin trapping with N-(tert-buty1)-a-phenylnitroneof H and D atoms formed in argon-saturated 1:1 H20-D,O mixtures] con-firming the temperatures firstly reported by Suslick et ~21.l~~ Henglein et ~1.'~~demonstrated the trapping of HO* and H.by D2to form HDO and HD respectively. However approxi- mately 80% of the OH and H atoms recombine (under argon and in the absence of scavengers).. The average peroxide formation rate is about 10-50pmol 1-' min-' using a probe system at 20 kHz and applying about 50 W cm-2.121 Irradiation of water in the presence of non-inert gases such as 02,N H CH or mixtures of H,-CO and N,-CO gives a variety of products.12' The radicals produced by the sonolysis of water or the generated H202 can also be trapped by organic compounds dissolved or dispersed in water. Several oxidation hydroxyl- ation and/or decomposition products have been detected (Table 5). However the reaction mixtures are rather complex owing to several secondary reactions especially in the presence of gases.For details refer to the literature (ref. 121 and references therein). In recent years the sonolysis of organic pollutants in water has become a developing field of research in environmental technol- ogy. Amongst the investigated compounds are chlorinated com- pound~,'~~-'~~ phenol^'^^-'^^chlorofluorocarbons (CFCS),'~'-~~~ and pesti~ides.'~~,~~~ The utilisation of ultrasound to convert environmentally hazardous substances into more benign sub- strates or better still to mineralise organics into carbon dioxide has been described recently in some review^.'^^-'^^ Some examples are given in the following paragraphs. Table 5 Products of the aqueous sonolysis of organic compounds' substrate main sonolysis products halogen compounds.Nagata et a1 studied the decomposition of 10 ppm aque- ous solutions of chlorinated hydrocarbons ( l,l,l-trichloro-ethane 1,1,2,2-tetrachloroethene and 1,1,2-trichloroethene) using 200 kHz ultrasound (6 W cm-'). After 10min irradiation the content was lowered to about 2ppm. The decomposition rates under argon are CHCl= CCl zCH,CCl >CCl =CCl In the presence of air the results are comparable but oxidation occurs CHCl=CC1,+2H20 "".2CO+3HC1+H2 CHC1=CC12 +2H20+O2 %2C02+3HC1 The sonolysis of CFC 113 CClF,-CClF has been studied recently Aqueous solutions of 25-1000 ppm Freon 113 were sonicated in a closed vessel under air or argon 1))CClF2-CClF2 +3H2O -1 2C02 +0 8CO +3HCl+ 3HF The decomposition depends on the type of gas as well as the gas/liquid volume ratio eg the concentration of CFC 113 is reduced from 100 ppm to about 55 ppm in a 105 ml vessel having a gas/liquid volume ratio of 45/60 and to about 15 ppm with a 70 4ml vessel and a ratio of 104/60 (time 60 min).The experiments showed that the decomposition is due to high- temperature pyrolysis inside the bubbles and is not caused by OH radicals or by combustion with 0 or air The sonolysis of phenol gives dihydroxybenzenes and quin- ones as primary reaction products which are further degraded with time to yield low molecular mass carboxylic acids 133 134 Ultrasound has also been used in rubber chemistry eg the sonocleavage of styrene-butadiene rubber,14' the degradation of butyl rubber142 and the devulcanization of waste rubbers143 have been reported Earthy-musty odorous compounds (e g geosmin) have become indicators of deterioration of drinking water Geosmin and other odorous compounds with related bicyclic structures have a low odour threshold Consequently its decomposition is of interest for water improvement.Thus the sonochemical degradation of geosmin was studied under different gas atmos- pheres144 It was found that geosmin is readily decomposed within 60 min For a 33 pmol dmP3 solution initial decompo- sition rates in the order Ar >O >air >N were detected Addition of tert-butyl alcohol as a radical scavenger suppressed the decomposition by about 60% indicating that the sonolysis is due to radical-induced (60%) and thermolytic degradation (40yo) The sonolysis of three chlorophenols (2- 3- and 4-chloro- phenol) was examined under pulsed sonolytic conditions (20 kHz 50 W) in air-equilibrated aqueous media z e under oxidation conditions.These phenols were transformed com- pletely to dechlorinated hydroxylated intermediate prod-ucts 126. The sonochemical oxidation technique shows strong similarities with conventional photocatalytic oxidation pro- cesses (direct photolysis flash photolysis UV/peroxide and irradiated semiconductor particulates like Ti02 and ZnO) Ultrasound is also able to assist these conventional methods of waste treatment As a model compound pentachlorophenol in the presence of 02% Ti02 has been investigated Conventional ultraviolet irradiation yields after 50 min 40% decomposition as the final result Combination of ultraviolet and ultrasound irradiation resulted in 60% conversion after 50min and the decomposition is quantitative after 2 h treatment 70 Sonolysis in non-aqueous systems A wide range of organic liquids have been sonicated Suslick et al found first that the sonolysis of higher n-alkanes leads 1614 J Muter Chem 1996 6(10) 1605-1618 to lower alkanes and alkenes It has been shown that the degradation process is similar to high-temperature pyrolysis and involves radical species following the principles of the Rice mechanism.145 On irradiation of diesel fuels the sonolytic reactions are on the one hand similar to the degradation of alkanes including cracking of ClO-C components and radical-induced polym- erisations and on the other hand comparable to processes occurring during long-term storage of fuels involving sediment formation and breaking down of longer alkane chains Sediment analysis gave similar data to storage under ambient temperature and no sonication in terms of molecular mass distribution (1000-10 OOO) nitrogen content and UV and IR spectra Using ultrasound sediment production seems to result from the same processes but the degradation is accelerated Thus 6 h high-intensity sonication produced an amount of sediment which is comparable to a storage at ambient tempera- ture for 16 months 146 Ultrasound has been applied recently as a first processing step in reducing the heteroatom content in upgrading of coal syncrude A syncrude from direct liquefaction of sub-bitumi- nous coal was first deasphaltated by ultrasonic disaggregation in n-hexane removing 39% of N 43% of S and 47% of 0 14' Another interesting application is the sonoisomerisation of alkenes mediated by either halogen radicals'48 151 or metal carbonyls ls2 Carbon-halogen bonds are cleaved easily by ultrasound to give halogenoradicals X..Thus if alkenes are sonicated in the presence of halogenated hydrocarbons the halogenoradical can add reversibly to the double bond via a transition state allowing free rotation Sonoisomerisation are reported for the maleic/fumaric acid148 and ester'49 system trans-dichloro- ethene'" in the presence of CHBr and bromoalkanes respect- ively as well as czs-and trans-vinyl sulfones mediated by CBrC1 15' A wide range of metal carbonyls (M =Cr Mo W Fe Ru Co) catalyse the isomerisation of pent-l-ene to czs-and trans-pent-2-ene 15' Sonication enhances the reaction rate by a factor of about lo5 compared to thermal isomerisation.The used metal carbonyls are sonochemically as effective as in photo- chemical isomerisations except for Ru (CO)' which is more active under sonication Biological materials The effects of ultrasound on biological systems depend on the intensity and frequency applied. Thus the following appli- cations are known non-destructive sonication of biosystems where the cell membrane remains intact and cell rupture (disintegration) to release the contents due to cavitation Sonolysisof biological materials Using high-intensity and/or focused ultrasound and depending on the conditions employed destruction of biological materials occurs cell walls are destroyed and the interior is expelled (ultrasonic extraction see section on Industrial and laboratory applications) into the surrounding bulk solution Further effects and uses are enzyme dea~tivation,'~~ 154 bactericidal treatment and dispersal or destruction of bacteria (eg in view of milk paste~risation,'~~.The combination 156 or food pre~ervation'~~) of ultrasound with other decontamination techniques (heating chlorination or extreme pH) seems to be particularly effec- tive 15' High-intensity ultrasound has also been used to activate antitumour agents'" or to induce hypothermia in living tissues for oncological treatment At this point it should be noted that diagnostic ultrasound used in medicine is far below the energy densities and intensities which are applied under sono- lytic or sonochemical conditions.Thus the cavitation threshold is not reached Non-destructive sonication of biological materials There are several papers on the influence of ultrasound on enzymes microorganisms and living tissues (e.g. ref. 153,161-164). Just three recent papers will be discussed in more detail. An interesting application deals with the increase in the permeability of human skin resulting in the possibility of transdermal drug de1i~ery.l~~ By irradiating the skin with 1 MHz ultrasound the transport of hydrophobic drugs could be enhanced..This is explained by the disorganisation of the membrane bilayers and the resulting formation of transpor- tation channels due to ultrasonically induced air pockets in the keratiocytes. Insulin y-interferon and erythropoietin diffuse at therapeutically useful rates through the skin on exposure to 20kHz ultrasound.. The skin change is reversible as the skin reverted to its impermeable state after switching off the ultra- sound. In uiuo experiments showed that the blood glucose level of normal and diabetic rats can be reduced by transdermal insulin delivery to the same extent as by insulin injection.It should be noted that ultrasound has already been used to alter polymer-membrane permeability to stimulate the release of polymer-coated encapsulated dr~gs'~~?'~~ and to enhance the dialysis separation of electrolytes through a cellophane membrane.16* The influence of sonication conditions on enzymes can be exemplified by a porcine pancreas lipase catalysed hydrolysis which has been published recently153 (Table 6). Performing the reaction in an ultrasonic bath (40 kHz 375 W electrical power input) resulted in a seven-fold increase in the reaction rate and the stereoselectivity remained unaffected compared to a non-sonicated control experiment.. The rate enhancement was explained by the authors to be due to the locally high pressure and/or the increase in catalytic surface.Under ultrasonic probe conditions (20 kHz 600 W electrical power input) and at a higher energy density the rate and stereoselectivities decreased probably due to higher local temperatures causing denaturation. Bioleaching of metals has become of increasing importance. Recently ultrasound-assisted microbial nickel leaching using Aspergillus niger has been re~0rted.l~~ Ultrasound enhanced the leaching rate of Ni which reached 95% after 30min sonication while only 25% is obtained by conventional in situ bioleaching (incubation time 14 d). Increasing the ultrasound intensity leads to a maximum and further intensity increases decrease the leaching rate. Longer sonication decreases the leaching rate slightly to 81% after 60 min.Using different frequencies leaching rates of 95% (20 and 43 kHz) and 86% (720 kHz) are obtained. Furthermore indications of substantial selectivity for nickel over iron were reported (under optimum conditions 95% Ni and 0.16% Fe). Industrial and laboratory applications There are several industrial applications of ultrasound. However ultrasonic plastic welding and sonocleaning are by far the most important uses. Other well established areas are ultrasonic soldering spraying metal welding machining and sonocleaning in the field of metallurgy and materials sciences cell disruption in biological sciences as well as dental scaling and ultrasonic nebulizers in medical therapy (for overviews see e.g. ref. 72,162,169-172).Applications to solids and melts Introduced commercially in 1963 plastic ~eldingl~~,~~~ is now a well established industrial process which is suitable for almost all thermoplastics with low thermal conductivity and melting temperatures (100-200 "C).Using a sonotrode a stand- ing wave is generated with its maximum amplitude at the contact surface of the two components to be welded. Process parameters are the sonotrode shape the contact pressure amplitude/power and irradiation time.. The ability of ultra- sound to propagate through elastic media allows welds at some distance from the ultrasonic horn (far-field processing). It requires rigid materials which are able to transmit vibrations with low attenuation e.g. polystyrene polyoxymethylene or styrene-acrylonitrile polymers.. Thermoplastics with higher mechanical damping need a smaller distance (<6 mm) to the ultrasonic tool (near-field processing)..The welding of polymer films and sheets is a typical application.. Thus PTFE sheets were joined using a 50 kHz flexural-mode transducer system.175 In ultrasonic welding the heat is generated inside the material using internal friction.. Thus the energy is limited to the welding zone resulting in fast welding (welds are typically performed in 0.2-1.5 s) and low part distortion. Other advan- tages are lower welding temperatures resulting in less material degradation and higher yields highly reproducible weld-seam quality high energy efficiency no adhesives or solvents or other additives as well as automatic processing in mass production.Special applications are the embedding of metal inserts in thermoplastics and ultrasonic bonding of synthetic fibres (especially polypropylene).. The latter has the major advantage of substantially lower energy consumption com-pared to thermal body welding. Plastic welding requires of course high power intensities which are in the kW (about 1000 W cmP2) range at 20 kHz. Ultrasonic welding cannot of course replace conventional metal welding. However it is suitable for special appli- cation~.~~,~~~In contrast to plastic welding where ultrasound is usually applied vertically metal welding uses lateral oscillat- ing horns inducing frictional heating between the surfaces.Surface oxides (like on aluminium) and other contaminants are broken up absorbed by the weld and finally the exposed metal surfaces fuse together under pressure. Ultrasonic metal welding is a form of low-temperature diffusion welding. Therefore brittle problems resulting from recrystallisation and the formation of intermetallic compounds are avoided. Ultrasonic metal welding is used for delicate joining of metals such as electrical grade aluminium and copper musical instru- ments or tiny pieces.. Thus it is applied in the semiconductor manufacturing industry for producing miniature semicon-ductor leads and chips as well as microbonding. The localised use of ultrasound at some distance from the horn and the relatively cold conditions allow the machining of hard and brittle materials such as ceramics glasses gem- stones and ferrites.Ultrasonic impact grinding or rotary abras- ive machining are common applications.. The cleaning effect of ultrasonic cavitation on surfaces is also the main principle in fluxless soldering. Ultrasound erodes the oxide layer of molten solder and exposes clean metal to solder.. This is especially applied to aluminium which is attacked by common alkaline or acid fluxes. The termination of aluminium cables and the soldering of mirror frames or heat exchangers are typical applications Other uses are the soldering of hard metals eg nickel Applications to liquids or solutions Ultrasonic cleaning7’ 177 was the first industrial application of ultrasound and has been applied since the early 1950s Commercial ultrasonic cleaners work in a frequency range of 20-60 kHz and with a power of 25-2500 W.They consist of stainless-steel tanks of 2-200 1 capacity and are usually equipped with temperature-controlled heaters However there are also huge tanks of several hundreds or even thousands of litres which are equipped with numerous transducers. The overall power of such systems reaches several kW Typical objects include glassware jewellery lenses spec- tacles medical instruments semiconductors circuit boards engine blocks and other machining parts Practically all mate- rials which are sound-reflecting (glasses ceramics metals plastics) can be used whereas materials such as rubber or textiles are cleaned less efficiently.Thus the ultrasonic cleaning of precision mass standards in ethanol has been reported recently to be the most efficient method for cleaning polished stainless-steel surfaces 178 Sonocleaning in the 0 8-1 0 MHz range (called megasonic cleaning) has been applied successfully to the removal of particles from silicon wafers 179 In ultrasonic cleaning the conventional brush is ‘replaced’ by cavitational bubbles doing the job Cavitation effects (micro- streaming high temperature/pressure jets and shock waves) at or near the surface ‘brush off’ the contaminants dirt or oxide layers which are either dissolved or (if insoluble) dispersed in the solution In combination with the ultrasonic effects there are also external heating detergents and the use of special solvents or water of a given pH value as bath liquid.The advantages of sonocleaning are less cleaning solvents needed (sometimes water can be used) use of parts with complex and non-regular shapes as well as materials having blind holes crevices or inner surfaces easy scale-up for large parts up to several metres and application under clean-room conditions to com- puter equipment Besides ultrasonic cleaning the dispersion of solids or liquids in liquids (suspension and emulsification respectively) and of liquids in air (atomisation) are the most common applications of ultrasound to liquids The preparation of dye pigments insecticides and magnetic oxides are examples of industrial uses in solid/liquid systems Some attention has also been paid to coal dispersions Ultrasound is applied in the food industry to generate fine emulsions Ultrasonic homogenisation is for example used in the production of tomato sauce mayonnaise or yoghurt Stable emulsions of immiscible liquids can be obtained ultrasonically with less or no surfactant Despite the well developed conven- tional surfactant-based methods and cost concerns there is currently growing interest in the food industry 157 166 18’ 184 Many advances have been made in ultrasound food technology (fields of interest mixing blending extraction crystallisation foam destruction particle/aerosol precipitation oxidation pro- cesses influencing enzyme activity sterilisation) in the last decade A more intensive use of ultrasound will depend on the availability of low-cost instrumentation that is shown to have significant advantages over current technologies Furthermore more fundamental research is required on the relationship between ultrasonic treatment (duration intensity etc ) and the effects on the properties of food materials There are also several reports on the ultrasonic extraction of organic compounds (eg soybean protein,lB5 saponin from ginsengIB6) from mainly vegetable or other plant sources.The beneficial effects of ultrasound result in (as already mentioned in the last section about Biological materials) the facile release of plant substances after destroying the epidermis or even the inner cell membranes.The advantages are the easy process the low temperatures the reduced damage to the extracts the short extraction times and the reduced loss of volatile components The application of ultrasound has become a widespread tool in analytical chemistry in the extraction of organic compounds from liquid or solid environmental samples Some recent examples are the extraction of polycyclic aromatic compounds (PAH) from soil suspensions,187 the recovery of herbicide residues from milk by ultrasonically breaking up fat globules,ls8 the removal of elemental sulfur from environmental samples by means of different reagents,lB9 the extraction of pentachloro- phenol in soil wood or water sampledg0 and the solubilization of margarine in hexane for tocopherols analysis 19’ The fine dispersion of liquids in air or other gases is a common technique in medical nebulizers for inhalation ther- apy and in sample injection in mass and atomic emission spectrometry.The spray pyrolysis of solid particle suspensions has already been mentioned Application of ultrasound in the 1-1 5 MHz range to induc- tively coupled plasma-atomic emission spectrometry (ICP-AES) can enhance the detection limits by a factor of 10 The efficiency of nebulization is so high that the solvent loading of the aerosol needs to be reduced by thermal desolv- ation to prevent cooling of the plasma 192 Ultrasonically assisted electrospray spectrometry (ESI-MS) has been applied successfully in LC-MS coupled analysis of proteins 193 Conventional ESI-MS has some severe limi- tations as the interface after liquid chromatography LC mobile phases which have high flow rates (>5 yl min-I) conductivities or surface tensions are normally not applicable eg the required mobile phase gradient MeOH-H,O in nucleoside separations does not meet this criteria By the application of an ultrasonic nebulizer LC-MS determinations of proteins over a wide range of methanol-water mobile phase conditions (0-100%) with different flow rates could be performed easily For a review on applications of ultrasonics in analytical chemistry see ref 194,195 I would like to thank Prof Dr Ralf Miethchen (Rostock) for numerous critical discussions and helpful comments in the preparation of the manuscript.The BMBF (project 03D0018) and the DECHEMA are acknowledged for financial support This work was effected under the auspices of the EEC COST organisation (Programme Chemistry D6) References 1 W T Richards and A L Loomis J Am Chem SOC 1927 49 3086 2 C Einhorn J Einhorn and J L Luche Synthesis 1989,787 3 Current Trends in Sonochemistry ed G J Price,. The Royal Society of Chemistry Cambridge 1992 4 T J Mason Practical Sonochemistry Ellis Horwood Chichester 1991 5 Advances in Sonochemistry ed T Mason JAI Press London VO~1-4,1991-1994 6 K S Suslick MRS Bull 1995,20 29 7 T J Mason Chemistry with Ultrasound Elsevier London 1990 8 Ultrasound-Its Chemical Physical and Biological Eflects ed K S Suslick VCH Wemheim 1988 9 F R Young Cavitation McGraw-Hill London 1989 10 T J Leighton,. The Acoustic Bubble Academic New York 1994 11 K S Suslick P F Schubert and J W Goodale J Am Chem Soc 1981 103 7342 K S Suslick S -B Choe A A Chichowlas and M W Grinstaff Nature (London) 1991,353,414 12 X Cao Y Koltypin G Kataby R Prozorov and A Gedanken J Mater Res 1995,10,2952 13 P Diadoti L Mirri C Petrillo and F Sacchetti Solid State Commun 1994,92,433 1616 J Muter Chem 1996 6(10) 1605-1618 14 T J Mason J P Walton and D J Lorimer Ultrasonics 1990 61 A J Fry G S Ginsburg and R A Parente J Chem SOC Chem 28,333 Commun ,1978,1040 15 T J Mason J P Walton and D J Lonmer Ultrasonics 1990 62 K Burger and B Helmreich Chem Ztg ,1991,115,253 .28,251 63 M S F Lie Ken and W L K Lam J Chem Soc Chem 16 T J Mason Practical Sonoelectrochemistry Ellis Horwood Commun ,1987,1460 Chichester 1991 64 N K Goh A T Teoh and L S Chia Ultrason Sonochem 1994 17 S M Kochergin and G Y Vyaseleva Electrodeposition of Metals in Ultrasonic Fields Consultants Bureau New York 1966 65 1 S41 D P.Thompson and P Boudjouk J Org Chem ,1988,53,2109 18 19 A Durant J L Delplancke R Winand and J Reisse Tetrahedron Lett 1995 36 4257 J Reisse Homogeneous sono- chemistry From physical studies to nanopowder preparation pre-sented at the DECHEMA Annual Meeting 1996,Wiesbaden R R Chandlerhenderson J L Coffer and L A Filessesler 66 67 68 P D Lickiss and R Lucas J Inorg Organomet Polym 1995 5,247 T Ando T Kawate J Ichihara and T Hanafusa Chem Lett 1984,725 A Miller C Zur D Peters and R Miethchen J Fluorine Chem J Electrochem Soc 1994,141 L166 in press 20 M P Drake Trans Inst Met Finish 1980,58,67 69 S Ring G Weber B Erhart P Mollmann and S Schwarz 21 22 23 A Henglein Ultrason Sonochem 1995,2 S115 C C T Alex N K Goh and L S Chia J Chem Soc Chem Commun ,1995,201 L Wen-Chou A N Maltsev and N I Kobosev Russ J Phys Chem 1964,38,41 70 Ultrasound in steroid chemistry Applications in complex hormone synthesis presented at the DECHEMA Annual Meeting 1996 Wiesbaden T J Mason Sonochemistry Current Trends and Future Prospects in Current Trends in Sonochemistry ed G J Price,.The 24 25 26 27 A N Maltsev Russ J Phys Chem ,1976,50,995 E Kowalska and M Dziegelewska Ultrasonics 1976 14 73 B N Tyutyunnikov and I T Novitskaya Maslob Zhir Prom 1959,25,13 K S Suslick and D J Casadonte J Am Chem SOC 1987 109 3459 71 72 73 Royal Society of Chemistry Cambndge 1992 R Snnivasan I Z Shirgaonkar and A B Pandit Sep Sci Technol 1995,30,2239 L D Rosenberg Physical Principles of Ultrasonic Technology Plenum New York 1973 G N Kozhemyakin and L G Kolodyazhnaya J Cryst Growth 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 E A Cioffi W S Willis and L S Svit Langmuir 1988,4 692 K S Suslick Solid State Ionics 1989,32/3,447 K S Suslick Science 1990,247 1067 K S Suslick,T Hyeon M M FangandA A Cichowlas Muter Sci Eng A 1995,204,186 T J Trentler R Suryanarayanan S M L Sastry and W E Buhro Muter Sci Eng 1995,204 193 T Popov D G Kissvrski K Ivanov and J Pesheva Stud Surf Sci Catal 1987,31 191 R Ranganathan I Mathur N N Bakhshi and J F Mathews Ind Eng Chem Prod Res Dev 1973,12,155 V I Shekhobalova and L V Voronova Vestn Mosk Univ Ser Khim 1986,27,327 C L Bianchi R Carli S Lanzani D Lorenzetti G Vergani and V Ragaini Ultrason Sonochem ,1994,1 S47 P Boudjouk Heterogeneous Sonochemistry in Ultrasound-Its Chemical Physical and Biological Efects ed K S Suslick VCH Weinheim 1988 T J Mason and J P Lonmer Sonochemistry.Theory Applications and Uses of Ultrasound in Chemistry Ellis Horwood Chichester 1988 T Ando and T Kimura Adv Sonochem ,1991,2,211 S V Ley and C M R Low Ultrasound in Synthesis Springer Berlin 1989 H Bonnemann B Bogdanovic R Brinkmann D W He and B Spliethoff,Angew Chem ,Int Ed Engl 1983,22,728 J L Luche C Petner and C Dupuy Tetrahedron Lett 1984 25,753 A J Fry and D Herr Tetrahedron Lett 1978,19,1721 J Einhorn and J L Luche J Org Chem ,1987,52,4124 C M R Low Ultrason Sonochem ,1995,2 S153 L Xu F Tao and T Yu Tetrahedron Lett 1985,26,4231 G Hugel D Cartier and J Levy Tetrahedron Lett 1989 30 4513 T S Chou and M L You J Org Chem 1987,52,2224 J L Luche C Petrier and C Dupuy Tetrahedron Lett 1984 25,753 S E de Laszlo S V Ley and R A Porter J Chem SOC Chem Commun 1986,344 H C Brown and U S Racherla Tetrahedron Lett 1985 26 4311 N Ishikara M G Koh T Kitazume and S K Choi J Fluorine Chem 1984,24,419 Y T Lin J Organomet Chem ,1986,317,277 R W Fitch and F A Luzzio Tetrahedron Lett 1994,35,6013 W H Binder R H Prenner and W Schmid Tetrahedron 1994 50,749 C Einhorn and J L Luche J Organomet Chem ,1987,322,177 S Tsunoi I Ryu H Fukushima M Tanaka M Komatsu and N Sonoda Synlett 1995 1249 M S A Parker and C J Rizzo Synth Commun 1995,25,2781 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 1995,147,200 Y Ding Y Miura and A Osaka J Am Ceram SOC 1994 77 2905 Y Ding A Osaka and Y Miura J Am Ceram SOC 1994 77 749 H L Choi N Enomoto and Z E Nakagawa J Muter Sci 1994,29,3239 J Nyvlt and S Zacek Cryst Res Technol 1995,30,1055 G I Eskin Ultrason Sonochem ,1995,2 S137 V A Versteeg C T Avedisian and R Raj J Am Ceram SOC 1995,78,2763 H Ichinose H Katsuki and M Nagano J Cryst Growth 1994 144,59 A Furusaki H Konno and R Furuichi J Muter Sci 1995 30,2829 K H Yoon and D J Nam J Muter Sci 1995,30,3415 I T Kim C H Lee and S J Park Jpn J Appl Phys 1994 33,5125 Y Harima Y D Wang K Matsumoto and K Yamashita J Chem SOC Chem Commun 1994,2553 Y C Kang S B Park andY W Kang Nanostruct Muter 1995 5,777 J M Ting J Muter Sci 1995,30,4049 J M Ting J Muter Sci 1995,30,4095 K Okuyama T Seto M Shimada and N Tohge J Muter Sci Muter Electron 1994,5,210 J H Lee and S J Park J Muter Sci Mater Electron 1993 4,254 H B Kim J H Lee and S J Park J Muter Sci Muter Electron 1995,6,84 N Tohge S Tamaki and K Okuyama Jpn J Appl Phys 1995 34 L207 S S Ostapenko L Jastrzebski J Lagowski and B Sopori Appl Phys Lett 1994,65,1555 S S Ostapenko and R E Bell J Appl Phys 1995,77,5458 R M Lago S C Tsang K L Lu Y K Chen and M L H Green J Chem Soc Chem Commun 1995,1355 L Q Ma F Chen and G J Shu J Muter Sci Lett 1995,14,649 T J Mason Sonochemistry Cavitation in the Service of Science and Technology World Congress on Ultrasonics Berlin 1995 P Kathirgamanathan J Muter Sci Lett 1993 12 1810 S A Yeung R Hobson S Briggs and F Grieser J Chem SOC Chem Commun ,1993,378 Y Nagata Y Watanabe S Fujita T Dohmaru and S Taniguchi J Chem SOC Chem Commun ,1992,1620 R A Hobson P Mulvaney and F Grieser J Chem Soc Chem Commun ,1994,823 Y Fang,D K Agrawa1,D M RoyandR Roy Am Ceram SOC Bull 1995,74,70 L Bjoorno S Gram and P R Steenstrup Ultrasonics 1978 103 M Takeuchi and K Yamanouchi Jpn J Appl Phys 1994 33 3045 59 R S Bates and A H Wright J Chem SOC Chem Commun 1990,1129 103 K Yasuda S Umemura and K Takeda Jpn J Appl Phys ,1995 34,27 15 60 A Tai T Kikukawa Y Sugimura T Inone T Osawa and 104 T Iwamoto H Shimada S Shimomura M Onodera and J Fugii J Chem SOC Chem Commun 1991,795 T Ohmi Jpn J Appl Phys ,1994,33,491 105 Y Y Zhao C G Bao R Feng and Z H Chen Ultrason 149 T Lepoint F Mullie N Voglet D H Yang J Vandercammen .Sonochem ,1995,2 S99 and J Reisse Tetrahedron Lett 1992,33 1055 106 C T Walker and R Walker J Electrochem SOC ,1974,124,661 150 T P Caulier M Maeck and J Reisse J Org Chem 1995 60 107 M C Hsiao and C C Wan Plat Surf Finish 1989,76,46 272 108 A Chiba and W C Wu Plat Surf Finish 1992,79,62 151 D Peters F Pautet H El Fakih H Fillion and J L Luche 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 Shinko Electnc Jpn Kokai Tokkyo Koho JP 5970756,1984 R Walker and C T Walker Nature (London) 1974,250,410 W J Tomlinson and S J Matthews J Muter Sci ,1994,29,1101 G J Price Ultrasonically Assisted Polymer Synthesis in Current Trends in Sonochemistry ed G J Pnce,.The Royal Society of Chemistry Cambridge 1992 G J Price Adu Sonochem ,1990,1,231 A M Basedow and K Ebert Adu Polym Sci ,1977,22,83 R A Pethrick Adu Sonochem 1991,2,65 G J Price P J West and P F Smith Ultrason Sonochem ,1994 1 S51 P A R Glynn B M E van der Hoff and P M Reilly J Macromol Sci A 1973,6 1653,7 1695 B M E van der Hoff and P A R Glynn J Macromol Sci A 1972,6,1653 S L Malhotra and J M Gauthier J Macromol Sci Chem A 1982,18,783 F 0 Schmitt C H Johnson and R A Olson J Am Chem SOC 1929,51,370 K S Suslick Homogeneous sonochemistry in Ultrasound-Its Chemical Physical and Biological Efects ed K S Suslick VCH Weinheim 1988 P Riesz D Berdahl and C L Chnstman Environ Health Perspect ,1985,64,233 V Misik N Miyoshi and P Riesz J Phys Chem ,1995,99,3605 K S Suslick D A Hammerton and R E Cline J Am Chem SOC,1986,108,5641 C H Fischer E J Hart and A Henglein J Phys Chem 1986 90,222,1954,3059 N Serpone R Terzian H Hidaka and E Pelizzetti J Phys Chem 1994,98,2634 K Inazu Y Nagata and Y Maeda Chem Lett 1993,57 H M Cheung M Bhatnagar and G Lansen Enuiron Sci Technol 1991,25,1510 Proc Pacifichem Conference '95 Honolulu 1995 Y Nagata K Hirai K Okitsu T Dohmaru and Y Maeda 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 J Prakt Chem ,1995,337,363 K S Suslick J W Goodale P F Schubert and H H Wang J Am Chem SOC 1983,105,5781 G L Lin and H C Liu Tetrahedron Lett 1995,36,6067 R T Roberts Chem Ind 1993,15,119 S E Jacobs and M J.Thornley J Appl Bacteriol 1954,17,383 G R Rubleson T M Murray and M Pollard Appl Microbiol 1975,350 G W Gould New Methods in Food Preservation Chapman and Hall London 1994 H S Lillard Food Technol 1994,48,72 S Umemura Jpn J Appl Phys ,1994,33,2846 N N Amso Ultrason Sonochem 1994,1 S69 K M Swamy L B Sukla R N Narayana R N Kar and V V Panchanadikar Acoust Lett 1993,17,45 J V Sinisteria Ultrasonics 1992,30 180 G L Lin H C Liu and S H Liu J Chin Chem SOC 1995 42,957 K M Swamy L B Sukla R N Narayana R N Kar and V V Panchanadikar Ultrason Sonochem 1995,2 S5 S Mitragotri D Blankschtein and R Langer Science 1995 269 850 S Miyazaki C Yokouchi and M Takada J Pharm Pharmacol 1988,40,716 J Kost K Leong and R Langer J Macromol Sci Macromol Chem A 1985 22 455 J Kost and R Langer Trends Biotechnol ,1992,10,127 H Li E Ohdaira and M Ide Jpn J Appl Phys ,1995,34,2725 B Brown and J E Goodmann High Intensity Ultrasonic Industrial Applications Van Nostrand Princeton 1965 A Shoh Industrial Applications of Ultrasound in Ultrasound-Its Chemical Physical and Biological Eflects ed K S Suslick VCH Weinhem 1988 P I Polyukhin,.The Use of Ultrasonics in Extractiue Metallurgy Technicopy Stonehouse 1978 C Elbaum J Alloys Compd 1994,212,20 A Shoh Ultrasonics 1976,14,209 Chem Lett 1995,203 174 A Puskar Use of High Intensity Ultrasound Elsevier 131 H M Cheung and S Kurup Enuiron Sci Technol 1994 28 Amsterdam 1982 1619 175 Y Watanabe Y Tsuda and E Mori Jpn J Appl Phys 1995 132 E A Stolyarov N G Orlova and L V Vedernikova Rub 34,2730 Termodin Kinet Khim Protsessov 1974,1147 176 A Hulst Ultrasonics 1972 10,256 .133 J Berlan F Trabelsi H Delmas A M Wilhelm and 177 R Pohlmann B Werden and R Marziniak Ultrasonics 1972 134 135 136 137 138 139 140 141 142 143 144 145 J F Petrignani Ultrason Sonochem ,1994,1 S97 C Petrier M F Lamy A Francony A Benahcene B David V Renaudin and N Gondrexon J Phys Chem ,1994,98,10 514 C Petrier M Micolle and G Reverdy Enuiron Sci Technol 1992,26,1639 A Kotronarou G Mills and M R Hoffmann J Phys Chem 1991,95,3630 Proc 4th Meeting Eur SOC Sonochem 1994 Blankenberge Belgien P Colarusso and N Serpone Res Chem Intermed 1996,22,61 T J Mason and S S Phull Sonochemistry in waste minimization in Chemical waste minimization ed J H Clark Blackie Glasgow 1995 Y Maeda Sonolytic decomposition of harmful chemical substances 5th Meeting of the European Society of Sonochemistry Cambndge 1996 R S Mauler F G Martins and D Samios Polym Bull 1995 35,151 V Dubey S K Pandey and N B S N Rao J Anal Appl Pyrolysis 1995,34 11 1 A I Isayev J Chen and A Tukachinsky Rubber Chem Technol 1995,68,267 Y Yoo N Takenaka H Bandow Y Nagata and Y Maeda Chem Lett 1995,961 K S Suslick J J Gawlenowski P F Schubert and H H Wang 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 10,156 R Schwartz and M Glaser Meas Sci Technology 1994,5,1429 A A Busnaina I I Kashkoush and G W Gale J Electrochem SOC,1995,142,2812 W Kowalski and E Kowalska Ultrasonics 1978 16 84 T J Mason Applications of Sonochemistry in Food Technology Pacifichem Conference '95 Honolulu 1995 Proc Ultrasound in Food Processing 25/26 09 1996 Leeds to be published D J McClements Trends Food Sci Technol 1995,6,293 K J Moulton J Am Oil Chem SOC 1989,66,896 H Fukase E Ohdaira N Masuzawa and M Ide Jpn J Appl Phys ,1994,33,3088 H Li E Ohdaira and M Ide Jpn J Appl Phys 1994,33,3085 U Hechler J Fischer and S Plagemann Fresenius J Anal Chem ,1995,351,591 A Lagana G Fago A Marion and B Pardomartinez Chromatographia 1994,38,88 J T Anderson and U Holwitt Fresenius Z Anal Chem 1994 350,474 A Besner R Gilbert P Tetreault L Lepine and J F Archambault Anal Chem ,1995,67,442 G Micah F Lanuzza and P Curro HRC 1993,16,536 T S Conver and J A Koropchak Spectrochim Acta Part B 1995,50,341 J F Banks S Shen C M Whitehouse and J B Fenn Anal J Phys Chem 1983,87,2299 Chem ,1994,66,406 146 G J Pnce and M McCollom Ultrason Sonochem ,1995,2 S67 194 F A Chmilenko A N Baklanov L P Sidorova and Fuel 1995,74,1394 Y M Piskun J Anal Chem 1994,49,494 147 I Fernadez M T Martinez A Benito and J L Miranda Fuel 1995,74,32 195 P Linares F Lazaro M D Luque de Castro and M Valcarcel J Autom Chem 1988,10,88 148 I E Elpiner A V Sokolskayer and M A Margulis Nature (London) 1965,208,945 Paper 6/02549H Received 1lth April 1996

ISSN:0959-9428    

DOI:10.1039/JM9960601605

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

~~~~~ Ferroelectric and antiferroelectric liquid crystalline phases in some pyridine carboxylic acid derivatives N. Kasthuraiah,"B. K. Sadashiva,""S. Krishnaprasadb and Geetha G. Nairb "Raman Research Institute, C.V Raman Avenue, Bangalore-560 080, India bCentrefor Liquid Crystal Research, P.O. Box 1329, Bangalore-560 013, India The synthesis and mesomorphic properties of two series of compounds uiz. (S)-(+)-4-( l-methylheptyloxy)phenyl4'-(6"-alkoxypyridine-3-carbonyloxy) benzoates and (S)-(+)-1-methylheptyl4-[ 4'-( 6"-alkoxypyridine-3-carbonyloxy)benzoyloxyl-benzoates are reported; the homologues of the former series exhibit smectic A and smectic C* phases while the derivatives of the latter series show rich polymesomorphism including the antiferroelectric phase.The mesophases have been characterised by using optical polarising microscopy and differential scanning calorimetric methods. Some physical properties such as the spontaneous polarisation, helical pitch, tilt angle and relative permittivity of two derivatives have also been investigated. Since the discovery of the antiferroelectric chiral smectic phase (S,,) by Chandani et ul.,' and subsequent reports by Takezoe et uL,~a number of new materials exhibiting this phase have been ~ynthesised.~-' The compounds exhibiting the S,, phase generally show subphases such as smectic C,* (S,*a) and ferrielectric (SCsIand ScsII)phases which are being investigated by various groups. Although some molecular structural fea- tures have been identified for compounds which favour the formation of Sc, and other subphases, these have to be examined in a number different systems for a clear understand- ing of such behaviour.Subtle changes in the molecular struc- ture, including the nature and location of the chiral group, seems to play an important role in the appearance of these phases. Recently, the effect of the size of the lateral substituent has been investigated for some phenyl propiolates and benzo- atesg and a few other analogous compounds.lO.ll In this paper, we examine the influence of a substituted pyridine carboxylic acid moiety on the formation of these chiral smectic phases. Two series of compounds I and 11, which differ from one another in the way in which the chiral moiety is linked to the core unit, have been synthesised.I Some of the properties of the smectic C* phase, such as the spontaneous polarisation as a function of temperature, the helical pitch, tilt angle and the relative permittivity have been measured for one homologue of each series. The influence of the hetero nitrogen atom on the mesophases has been evaluated by comparing the mesomorphic behaviour of the correspond- ing carbon analogues. Experimental Synthesis The two series of compounds I and I1 were synthesised following the pathway shown in Scheme 1.4-Benzyloxybenzoic acid was prepared by refluxing ethyl 4-hydroxybenzoate with benzyl chloride in the presence of anhydrous potassium carbon- ate in butan-2-one and hydrolysing the resulting ethyl 4- benzyloxybenzoate with alkali.6-Alkoxypyridine-3-carboxylic acids were prepared following a procedure already described.I2 The final compounds were purified by column chromatography on silica gel using chloroform as eluent and repeated crystallis- ations using suitable solvents. The purities of all the compounds synthesised were checked by thin layer chromatography (Merck Kieselgel 6OFZ5, pre-coated plates) and by normal phase high performance liquid chromatography using p porasil column (3.9 mm x 300 mm, Waters Associates Inc.) and 2% ethyl acetate in heptane as the eluent. The purities were found to be greater than 99.5%. The yields of these compounds were in the range 60-70%. The chemical structures of all the compounds were confirmed by using a combination of nuclear magnetic resonance spectroscopy (Bruker WP8OSY spec-trometer), infrared spectroscopy (Shimadzu IR-435) and elemental analysis (Carlo-Erba 1106 analyser).The optical rotations of all the chiral compounds were determined in dichloromethane (Optical Activity AA 1000 polarimeter). (S)-(+)-4-( 1-Methylhepty1oxy)phenylbenzyl ether A This was prepared following a procedure described by Mitsunobu and Eguchi.', Thus, diethylazodicarboxylate (DEAD, 4.76g, 27.3 mmol) was added dropwise to a cold stirred solution of 4-benzyloxyphenol (5.0 g, 25 mmol), (R)-(-)-octan-2-01 (3.25 g, 24.9 mmol), triphenylphosphine (7.0 g, 26.3 mmol) and dichloromethane (56 ml) for 1 h. The reaction mixture was then stirred at room temperature for 4 h and the solid that formed was filtered off.The material obtained on removal of solvent from the filtrate was chromatographed on silica gel and eluted with a 3: 1 mixture of chloroform and light petroleum (bp 60-80 "C). The required compound was obtained as a viscous liquid (5.3 g, 76%); [a]L5 6.6; 'H NMR (CDC13) 6 0.7-2.0 (16H, m, 2 x CH,, 5 x CH2), 4.2 (lH, m, ArOCH), 4.1 (2H, s, ArCH,O), 6.83 (5H, s, ArH) and 7.29 (4H, s, ArH). (S)-(+)-4-( 1-Methylhepty1oxy)phenolB A mixture of (S)-(+)-4-( 1-methylhepty1oxy)phenylbenzyl ether (7.0 g, 22.43 mmol), ethanol (40 ml) and 5% Pd/C catalyst (1.Og) was stirred in an atmosphere of hydrogen until the calculated quantity of hydrogen was absorbed. The reaction mixture was then filtered and ethanol removed by distillation under reduced pressure.The viscous liquid thus obtained was dissolved in chloroform and filtered through silica gel. Removal J. Muter. Chern., 1996, 6(lo), 1619-1625 1619 I A I It B D 1Ill 1111 I II Scheme 1 Synthetic pathway used for the two series of compounds. Reagents and conditions: i, PPh,, DEAD, CH,Cl,; ii, H,, 5% Pd/C; iii, DCC, DMAP, CH,Cl,. of solvent afforded the required phenol (4.2 g, 84.3%); Calk5 7.6; 'H NMR (CDCI,) 6 0.7-2.0 (16H, m, 2 x CH,, 5 x CH,) 4.2 (lH, m, ArOCH), 6.78 (4H, s, ArH) and 7.2 (lH, s, ArOH). (S)-(+)-4-( l-Methylheptyloxy)phenyl4-benzyloxybenzoateC This was prepared following a procedure described by Hassner and A1e~anian.I~ Thus, a mixture of 4-benzyloxybenzoic acid (2.64 g, 11.6 mmol), (S)-(+)-4-(1-methylhepty1oxy)phenol (2.57 g, 11.6 mmol), N,N-dicyclohexylcarbodiimide (2.36 g, 11.6 mmol), 4-N,N-dimethylaminopyridine(0.14 g, 1.16 mmol) and anhydrous dichloromethane (20 ml) was stirred for 2 h at room temperature.The N,N-dicyclohexylurea formed was filtered off and the filtrate was washed successively with water (2 x 30 ml), 5% aqueous acetic acid (3 x 50 ml) and water (3x50ml), and dried (Na,SO,). The residue obtained on removal of solvent was chromatographed on silica gel using chloroform as eluent. Removal of solvent from the eluate afforded a white solid which was crystallised from ethanol (4.35 g, 87%); mp 118.5 "C; [a]h5 6; 'H NMR (CDCI,) 6 0.94-2.28 (16H, m, 2 x CH,, 5 x CH,), 4.35 (lH, m, ArOCH), 5.18 (2H, s, ArCH,OAr), 6.87 and 8.06 (4H, ABq, J 7.5 Hz, ArH), 6.83 (5H, s, ArH), 7.29 (4H, s, ArH).(27)-( +)-4-( 1-Methylheptyloxy) phenyl4-hydroxybenzoate D This was prepared as described above for compound B using the following quantities of reagents; (S)-(+)-4-( 1-Methylheptyl- oxy)phenyl 4-benzyloxybenzoate (4.7 g, 10.87 mmol), ethyl acetate (40ml) and 5% Pd/C catalyst (3.0g, 81.0%); mp 107°C; Cali5 7.1; 'H NMR (CDCI,) 6 0.75-2.0 (16H, m, 2 x CH3, 5 x CH2), 4.1 ( lH, m, ArOCH), 6.5-8.1 (7H, m, ArH), 7.2 (lH, s, ArOH). (S)-(+ )-4-(l-Methylheptyloxy)phenyl4-( 6"-heptyloxypyridine-3-carbonyloxy)benzoa te I (n=7) This was prepared following the same procedure described above for compound C using the following quantities of the reagents. 6-Heptyloxypyridine-3-carboxylic acid (138 mg, 0.46 mmol), (S)-(+)-4-( 1-methylhepty1oxy)phenyl4-hydroxy-benzoate ( 157 mg, 0.46 mmol), N,N-dicyclohexylcarbodiimide (94 mg, 0.46 mmol), 4-N,N-dimethylaminopyridine (5 mg, 0.046 mmol) and dry dichloromethane (10 ml).The product was crystallised from ethanol (283 mg, 87%); mp 89.3 "C; [a];' 4.08; vmax/cm-' 2950, 1740, 1720, 1605, 1490, 1270 and 1050; 'H NMR (CDC1,) 6 0.7-2.0 (29H, m, 3 x CH,, 10 x CH,), 5.0-5.34 (3H, m, ArOCH and ArOCH,), 6.82 (2H, d, J 9.7 Hz, ArH), 7.32 and 8.15 (4H, AB q, J 8.4Hz, ArH), 7.4 and 8.3 (4H, J 8.5 Hz, ArH), 9.0 (lH, d, J 2.0 Hz, ArH) (Found: C, 72.53; H, 7.85; N, 4.34. C34 H4, NO6 requires C, 72.85; H, 7.67; N, 4.49%).Measurements The transition temperatures were determined using a polarising microscope (Leitz Laborlux 12 POL) equipped with a heating stage and a controller (Mettler FP52 and FP5 respectively), and also from thermograms recorded on a differential scanning calorimeter (Perkin Elmer Model DSC-4 or Model DSC-7). The physical measurements were performed using samples sandwiched between ITO-coated glass plates. Mylar spacers were used to define the thickness of the cell (typically -10 pm for the polarisation and tilt angle measurements and 50 pm for pitch determination). For spontaneous polarisation (P) measurements the triangular wave meth~d'~,'~ was employed. To identify/confirm the presence of antiferroelectric and/or the 'sub phases' a low frequency (0.97 Hz) probing field had to be used.However, the measurements as a function of temperature were carried out at a higher frequency (9.7 Hz) to avoid conductivity problems associated with low frequency large magnitude fields. The tilt angle (8) data were obtained by applying a near DC (0.1 Hz) switching field. The pitch values were determined by the optical diffraction method.17 Dielectric measurements were carried out by using an impedance ana- lyser, the details of which are described elsewhere.'* Results and Discussion The transition temperatures together with the transition enthalpies for the two series of compounds I and I1 are summarised in Tables 1 and 2, respectively. All the compounds in both series are mesomorphic in nature.In series I, where the chiral tail is attached to the core through an ether linkage, only smectic A (S,) and chiral smectic C (Sp) phases were 1620 J. Mater. Chern., 1996, 6(lo), 1619-1625 Table 1 Phase sequences, transition temperaturesrc and enthalpies/kJ mol-' for the compounds of series I compound n C SC* SA I 1 7 0 89.3 0 0 100.0 0 25.42 3.75 2 8 0 76.2 0 0 108.0 0 33.98 4.24 3 9 0 52.7 0 80.5 0 100.5 0 33.16 0.023 4.1 7 4 10 0 56.7 0 86.1 0 101.3 0 37.38 0.048 4.75 5 11 0 65.7 0 91.6 0 100.6 0 46.0 0.098 4.92 6 12 0 65.8 0 94.0 0 100.5 0 43.65 0.15 5.1 observed. These two phases were identified from the character- istic optical textures exhibited by these compounds.On cooling the isotropic liquid, the transition from the focal-conic SA phase to the striated focal-conic texture of the S,* phase was quite clearly observable. Thermodynamically, although this is a weak transition, it could still be seen on a DSC thermogram with a low enthalpy value. A plot of the transition temperatures as a function of terminal alkyl chain length for this series is shown in Fig. 1. It is seen that there is a gradual decrease in the SA mesophase range while the Sc* mesophase range increases on ascending the series. As can be seen in Table 2, the compounds of series I1 exhibit a fairly rich polymesomorphism. The difference between series I and I1 is that there is an ester linkage between the core and the chiral group in the latter.Hence it is reasonable to assume that this is responsible for the appearance of S,z and other sub-phases in the latter series of compounds. The mesophases could be identified under a microscope in thin films by sandwiching a sample between a glass slide and cover slip. For example, on cooling the isotropic liquid of compound 11, the homeotropic and focal-conic texture of a S, phase was observed. The SC*.phase was hardly detectable by the above method because it's texture is similar to that of the SA phase. The ferroelectric Sc* phase appears with a striated fan-shaped or pseudo-homeotropic texture. Cooling further produces a transition to the ferrielectric phase the texture of which con- stantly moves, probably as a result of changes in helical pitch.As the temperature is decreased further, the antiferroelectric phase appears which looks like a ferroelectric phase. However, a homogeneous alignment of the sample can be used to distinguish the antiferroelectric phase from the ferroelectric I 90 9 80i= 70 60 50 1 I I I I 1 I 6 7 13 9 10 I1 12 1 number of carbon atoms in alkoxy chain Fig. 1 Plot of transition temperatures as a function of alkyl chain length for series I phase. A transition from S, to Spa phase could also be easily detected using this technique. In the SC*. phase the homo- geneous coloured regions of the SA phase becomes striated with thin, dark and clear bands parallel to the rubbing direction as observed by Cluzeau et aL7 At the transition to the S,* phase a texture reminiscent of ropes lying parallel to one another could be seen.The transition to the Scg phase was detected as a clear change of texture. The new texture appears with large stripes growing perpendicular to the rubbing direction. The phase behaviour of the compounds of series I1 as a function of alkoxy chain length is shown in Fig. 2. Here again, the clearing temperatures decrease gradually with no strong odd-even effect. The temperature range of existence of the SA phase decreases with increasing chain length, while that of the Sc* phase increases with increasing chain length. The S,-phase is injected into the series from compound 9 (n=8) as a monotropic phase.It is also seen that the Spa phase has a fairly wide range of temperature (2-3.5 "C). In order to confirm the existence of various mesophases observed in the compounds of series 11,miscibility studies were carried out between compound 11 and the standard material 4-(1-methylheptyloxycarbony1)phenyl 4'-octyloxybiphenyl-4-Table 2 Phase sequences, transition temperaturesrc and (in brackets) enthalpies/kJ mol- for the compounds of series 11" compound n C &** SC,* SC* SC. * SA I 7 6 0 76.5 0 0 0 0 0 112.5 0 36.56 4.7 8 7 0 80.3 40.1 6 0 0 (0 56.2) 0.015 (0 57.4) 0.013 0 107.5 4.23 0 9 8 0 72.2 (0 68.8) (0 71.0) 0 73.0 0 76.5 0 106.0 0 38.52 0.006 0.007 0.008 0.009 4.87 10 9 0 70.2 (0 66.7) 0 71.1 0 80.2 0 84.0 0 103.0 0 38.93 0.006 0.008 0.013 0.046 4.55 11 10 0 67.2 0 76.8 0 78.8 0 85.5 0 87.5 0 102.5 0 46.93 0.017 0.018 0.014 0.037 4.66 12 11 0 71.6 (0 60.1) (0 66.4) 0 89.4 0 90.1' 0 100.5 0 54.6 0.048 0.006 0.218 4.51 13 12 0 75.0 (0 64.6) (0 70.7) 0 91.0 0 0 99.0 0 55.66 0.01 4 0.009 0.335 4.54 " Key: C: Crystalline phase; Sc,*: Antiferroelectric phase; Sc,*: Ferrielectric phase; Sc*: Ferroelectric phase; Sc,*: Chiral smectic tl phase; S,: Smectic A phase; I: Isotropic phase.'Enthalpy could not be measured. J. Muter. Chem., 1996,6(lo), 1619-1625 1621 In the present series of compounds 11, a pyridine moiety has been chosen such that the nitrogen atom is ortho to the alkoxy lZO Ichain It is known that pyridine has a moment which is directed along a second order axis of symmetry in the direction of the unshared electron pair As a consequence any influence on the mesophase is due to dipolar effects and the steric effect 110' flis absolutely minimal A comparison of the effect on the 90 -80 -70-60-50 ! I I 1 I I I I 5 6 7 8 9 1011121 number of carbon atoms in alkoxy chain Fig.2 Plot of transition temperatures as a function of alkyl chain length for series I1 carboxylate (MHPOBC) The isobaric binary phase diagram thus obtained is shown in Fig 3 The mixtures were made as weight/weight ratio and mixed thoroughly in their isotropic states It can be clearly seen that there is continuous miscibility of all the phases over the whole composition range, confirming the optical observations From the available data it has been observed that the location of the transverse dipole on the phenyl ring containing the alkoxy chain has an influence on the mesophases Faye et ul have examined a number of fluoro substituted derivatives and concluded that a fluoro substituent ortho to the alkoxy chain does have an effect This steric effect is less pronounced on the cleanng temperatures They have inferred that the fluorine in the ortho position decreases the longitudinal moment without affecting the mesophase sequence Also, any substituent which increases the longitudinal moment also reduces the possibility of obtaining Sc* and Sc2 phases 9 120 i= 110 100 90 80 70 I I I I I I 1 I I 0 10 20 30 40 50 60 70 80 90 1 D % compound I1 clearing temperatures and the SA mesophase temperature range for three derivatives in each of the 4-( l-methylheptylcarbony1)-phenyl 4-(4-alkoxybenzoyloxy) benzoate (nHH8), 4-(l-methyl-heptylcarbony1)phenyl 4'-( 3-fluoro-4-alkoxybenzoyloxy) ben-zoate (nFH8) and series I1 has been made and are summarised in Tables 3 and 4 respectively X = Y = H nHHBBM7 (nHH series) X = F, Y = H nFHBBM7 (nFH series) For example, the clearing temperatures of the compounds of series JJ are about 30°C lower and those for the nFH series are about 8°C lower when compared with those for the corresponding nHH compounds However, as can be seen in Table 4, the thermal range of the SA phase of compounds of series I1 are between those for nHH and nFH compounds It is quite clear from the data shown in these two tables that the nitrogen atom of the pyridine moiety which is ortho to the Table 3 Clearing temperaturesrc for the three series nHH, nFH and I1 compounds n nHH8 nFH8 11 10 1355 127 1 101 5 11 131 4 124 6 1000 12 131 5 1230 99 8 Table4 Temperature rangePC of the S, phase for the three series nHH, nFH and I1 compounds n nHH8 nFH8 I1 10 20 2 13 8 14 8 11 140 10 3 110 12 12 3 75 93 60 40 N Eo C* s5 20 0 70 80 90 TI'C Fig. 3 Miscibility phase diagram (w/w ratio) between compound 11 and the standard compound MHPOBC Fig.4 Thermal variation of polarisation for compound 4 1622 J Muter Chem, 1996,6(10), 1619-1625 alkoxy chain does have an influence on the temperature without affecting the sequence and type of the mesophases. Polarisation Fig. 4 shows the temperature dependence of polarisation (P) for compound 4. The behaviour is typical for a compound exhibiting a second order SA-Sc* transition. The applied field was monitored and adjusted such that it was slightly higher than the field necessary to unwind the helix. Hence no induced polarisation is observed in the SAphase. Fig. 5 shows the current response to an applied triangular wave in different phases of compound 11.The switching profile in the Sc*. phase shows two peaks. As the sample is cooled to the Sc* phase one of the peaks vanishes. The profiles in the Sc-and SCea phases contain, in addition to a main peak, additional peaks, albeit of much smaller strengths. These oscilloscope traces are now accepted to be characteristic of the phases studied here.20 Notice that in the ferri- and antiferro- electric phases a strong buckJlow effect is also seen. The thermal variation of polarisation for compound 11 is shown in Fig. 6. As the frequency of the applied field was 9.7 Hz, the data represents saturation polarisation in all the phases. For this reason, one observes a smooth variation in P across the SC+-Sc*Y and Sc*rScX transitions.On the other half cycle I t Fig.5 Raw oscilloscope traces in the smectic Ca*, C*, C,* and C,* phases of compound 11. The arrows point to the subsidiary peaks. I20 cu ;E C*flo s 40 n-70 75 80 85 TI"C Fig. 6 Variation of spontaneous-induced polarisation as a function of temperature for compound 11. Notice the step in P at the smectic C,*-C* transition. hand, at the SA-SC*.transition there is a sharp but continuous increase in the value of P, perhaps suggesting a second order change. But remarkably, a step increase is also observed at the temperature corresponding to the SC*-Sc* transition. As we had ensured that the applied field was sufficient to unwind the helix, any effects due to the partial unwinding of the sample can be ruled out.To our knowledge, this is the first instance where a clear change in the P us. temperature plot has been seen at the SC*-Sc* transition. To understand the effect of finer variations of the molecular structure on the magnitude of P,let us compare the values obtained for compound 11, with those for compounds having a very similar structure. At 10°C away from the SA-Sc*= transition the value for compound 11 is cu. 120 nC cm-2 as against 60 nC cmd2 for 4-( 1-methylheptylcarbony1)phenyl4-(4-decyloxybenzoyloxy) benzoate ( 10HHBBM7), 70 nC cmU2 for 4-( 1 -methylhept ylcarbonyl )phenyl 4-(3-fluoro-4-decylox-Lh O 5 10 15 20 25rn 18 14 I 12 -0 2 4 6 5-T /"C Fig. 7 Plot of tilt angle us. reduced temperature for (a) compound 4 and (b) compound 11 J.Muter. Chem., 1996,6( lo), 1619-1625 1623 ybenzoyloxy)benzoate (lOFHBBM7), studied by Faye et ul (for details regarding the structural differences see an earlier section) The variation in the values of P indicates that the ring structure of the first phenyl ring, although very far from the chiral centre, plays a non-neghgible role 24 22 r 5 L I I I J E @ 5 10 15 20 0=4c1 3 2 0 2 4 6 c-T 1°C Fig. 8 Plot of helical pitch us reduced temperature for (a) compound 4and (b)compound 11 135 0 0 0 0 0 c; 0 0 0 045 0 0 0 0 0 n u~ 70 75 80 a5 90 85 86 87 88 T1"C Fig. 9 Temperature dependence of transverse static relative permit-tivity in the vanous phases of compound 11 The bottom panel shows the smectic A-C,* region on an enlarged scale 1624 J Muter Chern, 1996, 6(10), 1619-1625 Tilt angle and pitch Plots of the optical tilt angle 8 as a function of Tc-T (where Tc represents the temperature of transition from the Sc* phase) for compounds 4 and 11 are shown In Figs 7(u) and (b) respectively Although the value of the tilt angle far away from the transition is observed to be very similar in both the compounds, temperature variations are different Again com-paring with the studies carried out by Faye et u18 we notice that compound 11 shows a smaller tilt angle than 10HHBBM7 or 10FHBBM7 compounds mentioned above Combined with the fact that the polarisation values are high, this would mean that the presence of hetero nitrogen atom in the ring enhances the polarisation-tilt coupling The pitch values measured for a few temperatures in the Sc* phase of compounds 4 and 11 are shown in Figs 8(u) and (b)respectively The different types of temperature dependence may be associated with the fact that one compound has the SC*.phase intervening between the SA and S,* phases The pitch values observed are about 8 to 10 times higher than a thiobenzoate compound6 having a very similar structure Relative permittivity Fig 9 shows the temperature dependence of the real part of the relative permittivity measured at 105 Hz for compound 11 As the material cools from the S, to S,* phase, there is a large increase in the value, which is commonly observed In addition a small but clear step is seen at the SA-Sc*a transition Such a feature has been reported earlier by Gisse et ulZ1 In the S,* phase the main contribution to E~ comes from the Goldstone mode whose strength decreases as the material transforms from the Sc* to the Spa phase and completely vanishes in the Sc%phase This is reflected in the E values Application of a I 135 -(a) 00 0 oo 00 0 00 0 oo 90-0 001:: 0 0c; : 00 -0 0 0I 0 0 0 0 a0 I , I0 w+ 70 75 80 85 ! L I 85 86 87 88 T/"C Fig.10 (a)Static relative permittivity (E~)as a function of temperature for compound 11 Probing frequency f= 105 Hz with a DC bias of 125 V pm-l (b) Enlarged view, the arrow points to the smectic A-C,* transition I 1 Fig.11 Influence of the magnitude of the probing frequency on the thermal variation of cl for compound 11 DC bias used = 1 25 V pm The frequencies are (a) 0 105, (b) 10, (c) 5 0, (d) 100, (e) 20 0 and (f) 500 KHz DC bias field (1 25 V pm-') reduces the value in the SC*. transition (see Fig 10) Additionally, one clearly observes a small peak in the Scz phase, which exists for the zero bias case also but is very weak This peak vanishes as the frequency of the probing field is increased (Fig 11) Except for recalling that Gisse et a1 21 have also observed such a peak, we are not sure about the physical origin of this Finally, the change at the SA-Sc*n transition seems to be hardly affected by the application of a bias field Conclusions Two series of compounds incorporating 6-alkoxypyridine-3- carboxylic acids have been synthesised These two series differ from one another by the way in which the chiral group is linked to the core It is shown that the terminal ester group which has a strong transverse dipole is essential for obtaining the S,-and other sub-phases It is also shown that the dipole associated with the nitrogen of the pyridine moiety, which is ortho to the alkoxy chain, does have an influence on the clearing temperatures without affecting the sequence and type of mesophases We wish to thank Mr K Subramanya for recording the various spectra and for elemental analysis, and Dr A A Khan, Deputy Director, Indian Institute of Chemical Technology, Hyderabad for allowing us to use the Perkin-Elmer DSC-7 instrument References 1 A D L Chandani, E Gorecka, Y Ouchi, H Takezoe and A Fukuda, Jpn J Appl Phys ,1989,28, L1265 2 H Takezoe, J Lee, A D L Chandani, E Gorecka, Y Ouchi, A Fukuda, K Terashima and K Furukawa, Ferroelectrics, 1991, 114, 187, H Takezoe, A D L Chandani, J Lee, E Gorecka, Y Ouchi, A Fukuda, K Terashima, K Furukawa and A Kishi, Abstracts, 2nd International Symposium on Ferroelectric Liquid Crystals (Goteborg, 1989), p 26, H Takezoe, A D L Chandani, E Goreka, Y Ouchi and A Fukuda, Abstracts, 2nd International Symposium on Ferroelectric Liquid Crystals (Goteborg, 1989), p 108 3 J W Goodby, J S Pate1 and E Chin, J Muter Chem ,1992,2,197 4 S Inui, T Suzuki, N Iimura, H Iwane and H Nohira, Ferroelectrics, 1993, 148,79 5 S Inui, T Suzuki, N Oimura, H Iwane and H Nohira, Mol Cryst Liq Cryst, 1994,239,l 6 H T Nguyen, J C Rouillon, P Cluzeau, G Sigaud, C Destrade and N Isaert, Lq Cryst, 1994,17, 571 7 P Cluzeau, H T Nguyen, C Destrade, N Isaert, P Barois and A Babeau, Mol Cryst Liq Cryst, 1995,260,69 8 V Faye, J C Rouillon, C Destrade and H T Nguyen, Liq Cryst, 1995,19,47 9 I Nishiyama and J W Goodby, J Muter Chem ,1993,2,149 10 R P Tuffin,J W Goodby,D Bennemann,G Heppke,D Lotzsch and G Scherowsky, Mol Cryst Liq Cryst, 1995,260,51 11 Y Ouchi, Y Yoshioka, H Ishii, K Seki, M Kitamura, R Nayori, Y Takanishi and I Nishiyama, J Muter Chem ,1995,5 2297 12 A I Pavluchenko,N I Smirnova,V V Titov, E I Kovshevand K M Djumaev, Mol Cryst Liq Cryst, 1976,37,35 13 0 Mitsunobu and M Eguchi, Bull Chem SOC Jpn ,1971,44,3427 14 A Hassner and V Alexanian, Tetrahedron Lett, 1978,4475 15 K Miyasato, S Abe, H Takezoe, A Fukuda and E Kuze, Jpn J Appl Phys ,1983,22, L 661 16 S Krishnaprasad, Geetha G Nair and S Chandrasekhar, J Muter Chem ,1995,52253 17 S Krishnaprasad and Geetha G Nair, Mol Cryst Liq Cryst, 1991,202,91 18 S M Khened, S Knshnaprasad, B Shivkumar and B K Sadashiva J Phys II,1991,1, 171 19 V I Minkin, 0 A, Osipov and Y A Zhdanov, in Dipole Moments in Organic Chemistry, Plenum Press, New York, 1970,p 114 20 A Fukuda, Y Takanishi, J Isozaki, K Ishikawa and H Takezoe, J Muter Chem ,1994,4,997 21 P Gisse, J Pavel, H T Nguyen and V L Lorman, Ferroelectrics, 1993,147,27 Paper 6/02415G, Received 9th April 1996 J Mater Chem, 1996, 6(10), 1619-1625 1625

ISSN:0959-9428    

DOI:10.1039/JM9960601619

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Characterization of novel TCNQ and TCNE 1 :1 and 1 :2 salts of the tetrakis (dimethyamino) ethylene dication, [{ (CH3),N},C-C( N(CH,),},], + James R. Fox; Bruce M. Foxman,*,' Donna Guarrera,b Joel S. Miller,*.d Joseph C. Calabrese" and Arthur H. Reis, Jr.' "Du Pont, Central Research Department, Experimental Station, E228, Wilmington, DE 19880-0328 USA 'Department of Chemistry, Brandeis University, P. 0.Box 91 10, Waltham, MA 02254-9110 USA 'Department of Chemistry, Harvard University, Cambridge, MA 02138 USA 'Department of Chemistry, University of Utah, Salt Lake City, UT84112 USA Addition of TCNQ to a solution of tetrakis(dimethy1amino)ethylene in MeCN produces the salt [TDAE][TCNQ],; the dipositive cation, [{(CH3),N},C-C{N(CH,),},]2+,is nonplanar, with a flihedral angle of 63.9", while the TCNQ anions crystallize as [TCNQ]22- dimers with an interplanar separation of 3.16 A.The complex [TDAEICTCNE] was prepared in a similar manner; the [TDAEI2+ cation and [TCNEI2- anion are nonplanar, with dihedral angles of 71.3 and 76.6", respectively. The four CN groups in the [TCNEI2- anion each accept three C-H.-.N hydrogen bonds. In each case, one C-N...H angle is in the range 134-156", while the other two are near 90" (81-100"). Donor-acceptor (D-A) complexes have been studied for many decades.' Extension to studies of electron-transfer salt^^.^,^ led to the discovery (1960) of molecular-based, metal-like conduc- and the later discoveries of molecular-based materials exhibiting cooperative phenomena, e.g. superconductivity7 and ferr~magnetism.',~With the commercial uses of some of these materials already achieved,"." the anticipation of finding additional examples of materials exhibiting cooperative phen- omena, and the promise of improved physical properties including higher critical temperatures, new donors and acceptors continue to be the subject of intense study.Although many donors, principally those based on the tetrathiafulvalene framework, have been exhaustively studied, l2 electron-rich tetraaminoethylenes have received comparatively little attenti~n.'~-~'The prototypical tetraaminoethylene, tetrakis(dimethy1amino)ethylene(TDAE), is a strong electron donor with an ionization potential of 6.13 eV.13 TDAE undergoes two one-electron oxidations to [TDAE] and+ [TDAEI2+ at -0.53 and -0.68 V us.SCE in MeCN (Fig. l).13716A few electron-transfer salts of TDAE have been reported. The reaction of TDAE and TCNE (TCNE =tetracy-anoethylene) was reported to form only the violet-black 1:2 [TDAE]2+[TCNE]22-.'3*17 More recently, the 1 : 1 complex with c60 has been reported to exhibit unusual ferromagnetic coupling, albeit with a low saturation magnetization." However, the reported structure was obtained by using the results of molecular mechanics to obtain trial coordinates for neutral TDAE; these were then Fig. 1 Cyclic voltammogram of TDAE in MeCN used in a series of Rietveld refinements of the powder diffraction data. While the observed ferromagnetism indicated that TDAE had been oxidized, the structural consequences of that event were not discussed.When TDAE is oxidized from the neutral to the dipositive oxidation state, the twist angle about the central C-C bond changes from 28" to 67-76".14 In order to understand the structure of TDAE as a function of oxidation state and crystal environment, we have rein- vestigated the reaction of TDAE with TCNE and studied the reaction with TCNQ (TCNQ =7,7,8,8-tetracyano-p-quinodi-methane). Both acceptors react with TDAE to form both 1 :1 and 1 :2 electron transfer salts in high yield. The oxidation states of TDAE in these salts are uncertain, since 1: 1 [TDAEICTCNX] (X=E, Q) can be formulated, for example, as [TDAEI2+[TCNXI2- or [TDAE] + [TCNX] -, while 1 :2 [TDAE][TCNX], can be formulated as [TDAE]2+[TCNX]22-or [TDAE]+ ([TCNX]-}.[TCNX].Crystals of the 1:l salt with TCNE and the 1:2 salt with TCNQ were obtained, and have been studied by single crystal X-ray and infrared spectroscopic analyses. Experimental Microanalyses were carried out by Microanalytical Laboratory, Mountain View, CA. The purity and elemental analyses are typical of this class of electron-transfer salts. Detailed analyses of the purity were not undertaken. Conductivities of the TCNQ salts were measured by the four- probe method on compacted pellets; conductivities of the TCNQ salts are ca. lo-' S cm-l. Conductivity measurements were not carried out on the TCNE salts. Synthesis [TDAE] [TCNQ], 1 was prepared in an inert atmosphere glove box from TDAE (Aldrich, 68 mg; 0.34 mmol) dissolved in 2 ml of acetonitrile.To this was added a solution of TCNQ (140 mg; 0.69 mmol) in 20 ml acetonitrile; the resulting mixture became an emerald green colour. The solution was allowed to stand overnight. During this time purple crystals precipitated; these were collected by vacuum filtration (150 mg; 73% yield). Elemental analysis: Calc. for C34H32N12; C, 67.09; H, 5.30; N, 27.61. Found: C, 66.85,66.93; H, 5.09,4.89; N, 28.17,28.21%. [TDAE][TCNQ] 2 was prepared by analogy with the preparation of 1 using equimolar amounts of the donor and J. Muter. Chem., 1996, 6(lo), 1627-1631 1627 acceptor. Elemental analysis: Calc. for C2,H2,N8; c, 65.32; H, 6.98; N, 27.70.Found: C, 65.39; H, 7.07; N, 27.71%. [TDAE][TCNE], 3 was prepared by analogy with the preparation of 1 using a molar ratio of 1 :2 for the donor and acceptor, respectively. Elemental analysis: Calc. for C,,H,,N,, : C, 57.88; H, 5.30; N, 36.82. Found: C, 58.28; H, 5.53; N, 37.08%. [TDAEICTCNE] 4 was prepared by analogy with the preparation of 1 using equimolar amounts of the donor and acceptor. Elemental analysis: Calc. for CI6H2,N,: C, 58.52; H, 7.37; N, 34.12. Found: C, 58.50; H, 7.42; N, 34.61%. X-Ray diffraction Structure determination of 1. Routine operations were per- formed as described previously (Syntex P2, diffractometer at Brandeis University)." The structure was refined by using anisotropic displacement parameters for N and C atoms; H atoms were fixed at calculated positions.Crystallographic data are presented in Table 1 and selected bond distances and angles appear in Table 2. Structure determination of 4. Routine operations were per- formed as described previously ( Enraf-Nonius CAD4 diffractometer at Du Pont).8b The structure was refined by using anisotropic displacement parameters for N and C atoms and isotropic displacement parameters for H atoms. Crystallographic data appear in Table 1 and selected bond distances and angles are in Table 3. Atomic coordinates, ther- mal parameters and bond lengths and angles for structures 1 and 4 have been deposited at the Cambridge Crystallographic Data Centre (CCDC). See Instructions for Authors, 1996, Issue 1.Any request to the CCDC for this material should quote the full literature citation and the reference number 114518. Results and Discussion The molecular structure of the tet rakis (dimethy1amino)et hy- lene moiety is shown in Fig. 2. Bond lengths and angles, as well as torsion angles, are very near to the values found for the [TDAE],' cation in [TDAE]X2*2H,O (X=Cl, Br).14" For 1, the observed values for vCN (2186s, 2176s, 2158m and 2117vwcm-l) are comparable to those observed for [Cr(C6H6)2]2[TCNQ]2 (2182~, 2175~, 2156m CII-')~~" and [Fe(C,Me,),],[TCNQ], (2184s, 2176s, 2157s cm-1).19b On this basis we formulate the salt as [TDAE]2+[TCNQ]22-. Fig. 2 A view of the cation [{(CH,)2N>2C=C{N(CH3)2}2]2+in 1 (50% probability ellipsoids shown).Selected bond distances aqd angles include: C( 1C)-C(2C), 1.515(3); C(sp2)-N, 1.305-1.320( 3) A; dihedral angle between two CN, planes, 63.9'; average C-N-C-C torsion angle, 22.8'. The crystal structure (Fig. 3) lends additional support to this formulation, as the structure clearly contains [TCNQ]22- dimers. Bond lengths observed for each TCNQ anion in the dimer are in excellent agreement with those observed pre- viously.'9'~20 Further, the dihedral angles between the c6 and C(CN), moieties in the anionic TCNQ species range from 6.6 to 10.6", consistent with the assignment of each TCNQ as a monoar$on.20 The interplanar distance between pairs of anions is 3.16 A, with an interplanar angle of 0.9". Salt 2 is formulated as [TDAEI2+ [TCNQ12-, based upon Fig.3 Stereoscopic view of the crystal structure of 1 (a vertical, c horizontal) Table 1 Data for the X-ray diffraction studies of 1 and 2" 1 2 [CioH24N4l2+ [CI~H~N~I~~- [CioH24N4] + [C6N412-8.161 (2) 10.542( 2) 14.393( 3) 14.877( 3) 14.514(4) 1 1.670( 4) P ioi Y ('1VIA3 78.22( 3) 79.51(3) 82.34( 3) 1632.8 90 103.89( 1) 90 1776.7 Z 2 4 formula mass 6c8.71 328.42 space group P1 [C,';no.23 Cc [C,"; no. 91 T/"C 3, (Mo-Ka)/A p/cm-' (Mo-Ka) 20 range Pcalclg cm - 21(1)0.71073 1.238 0.74 2 I20 I46' -70 0.71073 1.228 0.75 3.6 I20 I52' no. of reflections measured 4804 1909 no. of refections used 3580 [I 2 1.96a(1)] 1203 [I 2 3a(I)] number of parameters transmission factors 416 0.879-1 .OO 311 - R 0.049 0.036 Rw 0.065 0.030 "R= ElIF0 I -IF, I I/Cl Fo I; R, = {Zw[IF, I -IF, Il"~Wl~,I2 1'',.1628 J. Mater. Chem., 1996, 6(10), 1627-1631 Table 2 Bond lengths (A)and angles (") for [CloH24N4]2+[C12H,N4]22-bond lengths C( 1A)-C( 2A) C( 1A)-C(6A) C( lA)-C(7A) C(2A)-C( 3A) C( 3A)-C( 4A) C(4A)-C( 5A) C(4A)-C (1 OA) C( 5A)-C( 6A) C( 7A)-C( 8A) C( 7A)-C( 9A) C( 8A)-N( 1A) C (9A)-N (2A) C( 10A)-C( 11A) C( 10A)-C( 12A) C( 1lA)-N( 3A) C(1 2A)-N (4A) C(4B)-C( 5B) C( 4B)-C( 3B) C (4B)-C( 10B) C( 5B)-C(6B) C(6B)-C( 1B) C( 1B)-C(2B) N (4C)-C ( 1OC) 1.417(3) 1.415(3) 1.408 (4) 1.357(4) 1.41 3 (3) 1.422( 3) 1.4 14( 4) 1.3 57 (4) 1.416( 4) 1.420( 3) 1.148( 4) 1.144( 3) 1.412( 3) 1.405(4) 1.152( 3) 1.143(4) 1.416( 3) 1.419( 3) 1.413 (4) 1.360(4) 1.409( 3) 1.416( 3) 1.469( 3) C( 1B)-C(7B) C( 2B)-C( 3B) C( 10B)-C( 12B) C( 10B)-C( 11B) C( 12B)-N( 4B) C( 11B)-N( 3B) C(7B)-C(9B) C( 7B)-C( 8B) C( 9B)-N( 2B) C(8B)-N( 1B) C( 1C)-N( 1C) C( 1C)-N( 2C) C( 1C)-C( 2C) N( 1C)-C(3C) N( 1C)-C(4C) N(2C)-C( 5C) N( 2C)-C( 6C) C( 2C)-N( 3C) C(2C)-N (4C) N( 3C)-C( 7C) N( 3C)-C( SC) N(4C)-C( 9C) 1.421 (4) 1.354( 4) 1.41 3( 4) 1.41 5( 3) 1.154(4) 1.147( 3) 1.41 l(3) 1.416(4) 1.146( 3) 1.142(4) 1.309(3) 1.318( 3) 1.515(3) 1.472( 3) 1.477( 3) 1.467( 3) 1.466( 3) 1.305 (3) 1.320( 3) 1.474( 3) 1.473( 3) 1.467(3) bond angles C(2A)-C( 1A)-C(6A) C( 2A)-C( 1A)-C( 7A) C( 6A)-C( 1A)-C( 7A) C( 1 A)-C (2A)-C (3A) C(2A)-C( 3A)-C(4A) C( 3A)-C(4A)-C( 5A) 116.7(2) 12 1.4( 2) 121.9(2) 121.8 (2) 121.5(2) 116.8(2) C( 3B)-C(4B)-C( 10B) C(4B)-C( 5B)-C(6B) C( 5B)-C( 6B)-C( 1B) C( 6B)-C( 1B)-C(2B) C(6B)-C( 1B)-C(7B) C( 2B)-C( 1B)-C( 7B) 121.9(2) 121.6(2) 121.5(2) 117.2( 2) 12 1.6( 2) 121.2(2) C( 3A)-C(4A)-C( 10A) C( 5A)-C(4A)-C( 10A) C(4A)-C( 5A)-C( 6A) C( 1A)-C( 6A)-C( 5A) C( 1A)-C( 7A)-C( 8A) C( 1A)-C( 7A)-C(9A) C( 8A)-C( 7A)-C( 9A) C(7A)-C(8A)-N( 1A) 120.8 (2) 122.4( 2) 121.5 (2) 121.7(2) 121.7(2) 122.4(2) 115.7(2) 179.4(3) C( 1B)-C (2B)-C (3B) C(4B)-C( 3B)-C(2B) C(4B)-C( 10B)-C( 12B) C(4B)-C( 10B)-C( 11B) C( 12B)-C( 10B)-C( 11B) C( 10B)-C( 12B)-N(4B) C( 1OB)-C( 11B)-N( 3B) C( lB)-C(7B)-C(9B) 12 1.4( 2) 121.7( 2) 121.6( 2) 115.8( 2) 179.7( 6) 179.1 (3) 120.6(2) 122.2( 2) C(7A)-C (9A)-N (2A) C (4A)-C ( 10A)-C( 1 1 A) C(4A)-C( 10A)-C( 12A) C( 11A)-C( 10A)-C( 12A) C( 10A)-C( 11A)-N(3A) C( 10A)-C( 12A)-N(4A) C( 5B)-C (4B)-C (3B) C( 5B)-C(4B)-C( 10B) C( 1 C)-N( 1C)-C( 4C) C( 3C)-N( 1C)-C(4C) C( lC)-N(2C)-C( 5C) C( lC)-N(2C)-C(6C) C(5C)-N (2C)-C (6C) C( lC)-C(2C)-N( 3C) C( 1 C)-C (2C)-N( 4C) 178.9( 3) 123.1 (2) 120.6(2) 116.3(2) 178.4( 3) 178.1 (3) 116.7(2) 121.5( 2) 123.1 (2) 115.1( 2) 121.3( 2) 123.2(2) 115.3( 2) 1 1 6.9 (2) 1170(2) C( 1B)-C( 7B)-C( 8B) C( 9B)-C( 7B)-C( 8B) C( 7B)-C( 9B)-N( 2B) C( 7B)-C( 8B)-N( 1B) N ( 1 C)-C ( 1 C)-N (2C) N( 1 C)-C ( 1C)-C( 2C) N( 2C)-C( 1C)-C(2C) C( 1C)-N( 1C)-C( 3C) N(3C)-C(2C)-N(4C) C(2C)-N( 3C)-C(7C) C(2C)-N( 3C)-C( 8C) C( 7C)-N( 3C)-C( 8C) C( 2C)-N( 4C)-C( 9C) C( 2C)-N(4C)-C( 1OC) C( 9C)-N(4C)-C( 1OC) 122.8 (2) 116.4(2) 178.3( 3) 177.7( 3) 126.0( 2) 117.0( 2) 117.0(2) 121.7(2) 126.2( 2) 121.9(2) 123.2 (2) 114.9(2) 123.2(2) 121.7(2) 1 15.0( 2) IR evidence.The observed vCN for 2 (2156s, 21 17m, 2105s cm-') in {[Co(C,Me,),] }2[TCNE]2-).23 Again, this salt is dia- + is suggestive of [TCNQI2-(cf. 2164s, 2096s cm-' for magnetic, which excludes a possible assignment as [TCNE'I- . Na2TCNQ2' and 2150s, 2105s cm-' for [Co Fig. 4 shows the dianion/dication pair. Bond lengths and angles (C5Me5),]2[TCNQ]19'). Similarly, the orange 1 :2 TCNE salt for [TDAE],' are near the values reported for 1 and other 3 is formulated as [TDAE]2+[TCNE]22-. The observed IR structures, while those for [TCNE]'- parallel those found (Nujol) has strong CGN absorptions at 2193m, 2174s, and previ~usly~~~~~for [TCNE]'-. Variations in the single bond 2163s cm-l, characteristic22 of z-[TCNE],~-(cf: 2190m, torsion angles are less than 13", and probably arise from 2169s, and 2160s cm-' for ([Cr'(C6Me,H6-,)2]+ 12-differences in packing or hydrogen bonding.Close inspection [TCNE]22-, where x=O, 3), but not isolated [TCNE 1-of the crystal structure of 4 reveals unusual hydrogen bonding (cf 2 184m, 21 54s cm -1).8b Further confirmation comes from patterns. The hydrogen bonding patterns, as delineated in the observed diamagnetism of the salt, which is only consistent Table 4 and Fig. 5, show that the anion receives an unusual with the dimer formulation. number of donor hydrogen bonds. For each N atom in the Similarly, the brown 1: 1 salt 4 is formulated as anion, there is one donor H atom which approaches the anion [TDAEI2+ [TCNEI2-.The observed IR (Nujol) has strong at a C-N...H angle within expected ranges (134-156", Table 4), C-N absorptions at 2143m and 2078s cm-', characteristic of plus two others which approach the CN-group at nearly [TCNEI2- (cf. 2140 and 2069s cm-' for isolated [TCNEI2- orthogonal positions. The hydrogen bonding pattern is related J. Muter. Chem., 1996, 6( lo), 1627-1631 1629 Fig. 4 A view of the anion and cation in 4 (50% probability ellipsoids shown) Selected bond distances and pngles (a) cation, C( 11)-C( 12), 1516(4), C(sp2)-N, 1311-1 323(5)A, dihedral angle between two CN, planes, 71 3", (b) anion, C( l)-C(2), 1 488(4) A, dihedral angle between two C(CN), planes, 76 6" to the C-H TC interactions observed in two ethynylgold complexes 25 However, the shortest approach of the hydrogen bond is not to the centre of the CEN n: bond, but rather to the more electronegative N atom Inspection of the other structures containing the [TCNEI2 ion does not reveal similar patterns It would appear that the relatively compact [TDAEI2+ has the correct combination of size and H-donor ability to produce this highly unusual structure 1630 J Muter Chem, 1996, 6(lo), 1627-1631 Table 3 Bond lengths (A)and angles (") for [CloH24N4]2+ [C6N4I2 bond lengths N(3)-C(3) N(41-C (4) N( 5 )-C( 5) N (6)-C( 6) N( 11)-C( 11) N( 11)-C( 17) N(11)-C( 18) N( 12)-C( 11) N( 12)-C( 19) N( 12)-C( 20) N( 13)-C( 12) 1163(5) 1161(5) 1162(5) 1150(5) 1323(4) 1 459( 5) 1476(5) 1311(4) 1483(5) 1470(5) 1315(5) N( 13)-C( 13) N( 13)-C( 14) N( 14)-C( 12) N( 14)-C( 15) N( 14)-C( 16) C(l)-C(2) C( 1 )-C(3) C( 1 )-C(4) C(2)-C( 5) C( 2)-C( 6) C( 11)-C( 12) 1 475( 5) 1473(5) 1316(5) 1471(5) 1468(5) 1488(4) 1 396( 6) 1 406( 6) 1 402( 5) 1391(5) 1516(4) bond angles C( 1 1 )-N( 11 )-C( 17) C(ll)-N( 11)-C( 18) C( 17)-N( 11)-C( 18) C( 11)-N( 12)-C( 19) C( 11)-N( 12)-C(20) C( 19)-N ( 12)-C (20) C(12)-N( 13) C( 13) C(12)-N( 13)-C( 14) C( 13)-N( 13)-C( 14) C( 12) N( 14)-C( 15) C(12)-N( 14)-C( 16) C( 15)-N( 14)-C( 16) N( 1 1 )-C( 1 1 )-N (12) N( 13)-C( 12)-N( 14) 121 4(3) 124 6(3) 114 l(4) 124 l(3) 121 7(3) 1142(3) 121 5(3) 1248(3) 113 8(3) 124 O(3) 122 4(3) 113 6(3) 126 5( 3) 126 3(3) N(3)-C(3)-C( 1) N(4)-C( 4)-C( 1 ) N(5)-C(5) C(2) N( 6)-C( 6)-C(2) N( 11)-C( 11)-C( 12) N( 12)-C( 11) C( 12) N( 13)-C( 12)-C( 11) N( 14)-C( 12)-C( 11) C( 2)-C( 1 )-C( 3) C(3)-C( 1)-c(4) C(l)-c(2)-C(5) C( 1)-C( 2)-C (6) C(5)-C (2)-C (6) C( 2)-C( 1 )-C( 4) 177 9(5) 178 3(4) 178 8(5) 1790(5) 116 3(3) 1172(3) 117 4(4) 116 3( 3) 121 2(4) 121 2(4) 1174(3) 121 l(4) 117 3(3) 121 7(3) Fig.5 A view of a portion of the crystal structure of 4, showing the orthogonal H-bonding pattern Three C-H N( 3) interactions are shown, the arrangement of hydrogen bonds about the other three cyano N atoms is similar (Table 4) The facile preparation of these TDAE salts suggests that many other interesting salts and/or D-A complexes of TDAE await discovery, in particular efforts are underway in our laboratories to isolate and characterize the elusive [TDAE] i-We are grateful to E Delawski for carrying out the electro- chemical measurements on MeCN solutions of TDAE B M F thanks the National Science Foundation for partial support of this work through grant DMR-9221487 Table 4” Hydrogen bonds in [CloH24N4]2f [C6N4I2- atoms N H-C H( 152)-C( 15)b H( 192)-C( 19)c H (202)-C( 20)d H( 142)-C( 14)’ H( 171)-C( 17)/ H( 181)-C( 18)c H( 133)-C( 13)’ H( 161)-C( 16)d H(203)-C( 20)d H( 131 )-C( 13)/ H(153)-C( 15)’ H( 163)-C( 16)’ dist ance/A dis tance/A distance1A angleldegrees angleldegrees NC NH C-H N H-C C-N H 3 404 2 45 0 99 164 4 155 5 3 352 2 71 105 119 3 95 1 3 417 2 45 1 04 154 5 92 7 3 461 2 80 0 98 125 5 150 5 3 328 2 75 101 116 8 91 5 3 473 2 75 100 129 5 100 2 3 426 2 45 101 161 9 133 5 3 407 2 51 103 145 2 93 2 3 443 2 93 0 91 117 3 80 8 3 280 2 44 0 92 153 1 99 6 3 469 2 61 104 139 9 92 8 3 410 2 50 0 93 167 0 136 5 “N-C distances haveo estimated standard deviations in the range 0 005-0 007 A,N-H and C-H distances have estimated standard deviations in the range 0 04-0 05 A, and are uncorrected for centroid errors M R Churchill, Inorg Chem , 1973, 12, 1213 ”-‘Symmetry operations (h)x, -y, z-l, (c)x++, y-+, z, (d)x,y, z, (e) x+ 1, -y, z+3, (f)x+ 1, y, z, (g) x++, -Y++, 2-3 References R S Mulliken and W B Person, Molecular Complexes A Lecture and Reprint Volume, Wiley, New York, 1969, Z G Soos, Annu Rev Phys Chem, 1974, 25, 121, R S Mulliken, J Phys Chem, 1952,56,801 D S Acker, R J Harder, W R Hertler, W Mahler, L R Melby, R E Benson and W E Mochel, J Am Chem Soc 1960,82,6408, R G Kepler, P E Bierstedt and R E Merrifield, Phys Rev Lett, 1960,5,503 L R Melby, R J Harder, W R Hertler, W Mahler, R E Benson and W E Mochel, J Am Chem SOC, 1962, 84, 3374, I F Shchegolev, L I Buravov, A V Zvarykina and R B Lyubovskii, JETP Lett (Engl Transl) 1968,8,218 See for example, Extended Linear Chain Compounds, vols 1-3, ed J S Miller, Plenum, New York, 1982, 1983, J Simon and J J Andre, Molecular Semiconductors, Spnnger Verlag, New York, 1985, R Roth, One-Dimensional Metals, VCH, New York, 1995 For a detailed overview, see the proceedings of the recent series of international conferences Synth Met 1995, 71, 72, 73, ed Y -W Park and H Lee, Synth Met 1993,55,56,57,ed S Stafstrom, W R Salaneck, 0 Inganas and T Hjertberg, Synth Met 1991,41, 42, 43, ed M Hanack, S Roth and H Schier, Synth Met, 1988, 27, 1989, 28, 29, ed M Aldissi, Mol Cryst Liq Cryst, 1985, 117-121, ed C Pecile, G Zerbi, R Bozio and A Girlando, J Phys (Paris) Collog , 1983,44-C3, ed ,R Comes, P Bernier, J J Andre and J Rouxel, Mol Cryst Liq Cryst 1981,77,79, 82,83,85, and 1982,86, ed A J Epstein and E M Conwell, Chem Scri 1981, 17, ed K Carneiro, Lecture Notes in Physics, 1979, 95 and 96, ed S Bartsic, A Bjehs, J R Cooper and B A Leontic, Ann N Y Acad Sci , 1978,313, ed J S Miller and A J Epstein A J Epstein and J S Miller, Scz Am, 1979,241,52, K Bechgaard and D Jerome, Sci Am, 1982,247,52 J M Williams and K Carnerio, Adv Inorg Chem Radiochem, 1985, 29, 249, P M Chaikin and R L Greene, Physics Today, 1986, 31, 24, D Jerome and K Bechgaard, Contemp Phys , 1982, 23, 583, J M Williams, Prog Inorg Chem, 1985, 33, 183, T Ishiguro and K Yamaji, Organic Superconductors, Springer Verlag, Berlin, 1990, J M Williams, J R Ferraro, R J Thorn, K D Carlson, U Geiser, H H Wang, A M Kini and M -H Wangbo, Organic Superconductors, Prentice Hall, Englewood Cliffs, NJ, 1992 (a) S Chittipeddi, K R Cromack, J S Miller and A J Epstein, Phys Rev Lett, 1987,58, 2695, J S Miller, J C Calabrese, R W Bigelow, A J Epstein, R W Zhang, and W M Reiff, J Chem Soc Chem Commun, 1986, 1026, (b)J S Miller, J C Calabrese, H Rommelmann, S Chittipeddi, J H Zhang, W M Reiff and A J Epstein, J Am Chem SOC ,1987,109,769 J S Miller and A J Epstein, Angew Chem ,1994,106,399, Angew Chem Int Ed Engl, 1994,33,385, D Gatteschi, Adv Muter, 1994, 6, 635, 0 Kahn, Molecular Magnetism, VCH, Weinheim, Germany, 1993, J S Miller, A J Epstein and W M Reiff, Chem Rev, 1988, 88,201, J S Miller and A J Epstein, NATO ASI Ser , Ser B, 1988, 168, 159, J S Miller, A J Epstein and W M Reiff, Ace Chem Res ,1988,21,114, J S Miller, A J Epstein and W M Reiff, Science, 1988, 240, 40, J S Miller and A J Epstein, in New Aspects of Organic Chemistry, ed, Z Yoshida, T Shiba and Y Ohsiro, VCH, New York, 1989, p 237, A L Buchachenko, Russ Chem Rev, 1990,59,307, Usp Khim ,1990,59,529 10 J S Miller, Adv Muter, 1993,5, 587, 671 11 R Roth, One-Dimensional Metals, VCH, New York, 1995, ch 10 12 M Narita and C U Pittman, Synthesis, 1976,8,489 13 N Wiberg, Angew Chem Int Ed Engl, 1968,7,766 14 (a) H Bock, K Ruppert, K Merzweiler, D Fenske and H Goesmann, Angew Chem, Int Ed Engl, 1989, 28, 1684, (b)H Bock, H Borrmann, Z Havlas, H Oberhammer, K Ruppert and A Simon, Angew Chem, Int Ed Engl, 1991,30,1678 15 P Allemand, K Khemani, A Koch, F Wudl, K Holczer, S Donovan, G, Gruner and J D Thompson, Science, 1991, 253, 301, K Tanaka, A A Zakhidov, K Yoshizawa, K Okahara, T Yamabe, K Yakushi, K Kikuchi, S Suzuki, I Ikemoto and Y Achiba, Phys Lett A, 1992, 164, 221, P W Stephens, D Cox, J W Lauher, L Mihary, J B Wiley, P Allemand, A Hirsch, K Holczer, Q Li, J D Thompson and F Wudl, Nature, 1992,355, 331, F Wudl and J D Thompson, J Phys Chem Sol, 1992, 53,1449 16 This observation is comparable to values of -0 61 and -0 75 V K Kuwata and D H Geske, J Am Chem SOC, 1964,86,2101 In DMF an unstructured two-electron oxidative wave between E, = -0 63 V and E,= -0 52 V was reported H Bock and D Jaculi, Angew Chem, Int Ed Engl, 1984, 23, 305 In our work cyclic voltammetry at 100 mV s-’ was performed in acetonitrile solution containing 0 1 M [Bu,N][ClO,] electrolyte in a conventional H cell with a platinum working electrode and Ag/AgCl reference electrode All reported potentials are vs SCE Voltammograms were recorded with a Pnnceton Applied Research 173,475 potentiostat/programmer 17 N Wiberg and J W Buchler, Angew Chem, Int Ed Engl, 1968, 7,406, Chem Ber ,1963,96,3223 18 B M Foxman, Inorg Chem, 1978, 17, 1932, B M Foxman and H Mazurek, Inorg Chem, 1979, 18, 113, MolEN An Interactive Structure Solution Procedure, Enraf-Nonius, Delft, The Netherlands, 1990, C K Johnson, ORTEP-II A Fortran Thermal Ellipsoid Plot Program for Crystal Structure Illustrations ORNL- 5138, 1976, R A Sparks et al, Operations Manual Syntex XTL Structure Determination System, Syntex Analytical Instruments, Cupertino, CA, 1976, D J Watkin, C K Prout and L J Pearce, CAMERON,Chemical Crystallography Laboratory, University of Oxford, Oxford, 1996 19 (a)D O’Hare, M D Ward and J S Miller, Chem Muter, 1990,2, 758, (b) J S Miller, unpublished observations, (c) J S Miller, W M Reiff, J H Zhang, L D Preston, A H Reis, Jr, E Gebert, M Extine, J Troup, D A Dixon, A J Epstein and M D Ward, J Phys Chem ,1987,91,4344 20 S Flandrois and D Chasseau, Acta Crystallogr Sect B, 1977, 33,2744 21 M S Khatkale and J P Devhn, J Chem Phys, 1979,70, 1851 22 J S Miller, D M O’Hare, A Chakraborty and A J Epstein, J Am Chem SOC, 1989,111,7853 23 D A Dixon and J S Miller, J Am Chem SOC, 1987,109,3656 24 G T Yee, J C Calabrese, C Vasquez and J S Miller, Inorg Chem, 1993,32,377 25 T E Muller, D M P Mingos and D J Williams, J Chem SOC Chem Commun , 1994,1787 Paper 6/019896, Received 21st March, 1996 J Mater Chem, 1996,6( lo), 1627-1631 1631

ISSN:0959-9428    

DOI:10.1039/JM9960601627

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Syntheses, characterization and non-linear optical properties of nickel complexes of multi-sulfur 1,2=dithiolene with strong near-IR absorption Jing-Lin ZUO,'Tian-Ming Yao,' Fei YOU,' Xiao-Zeng YOU,"'Hoong-Kun Funb and Boon-Chuan Yipb aCoordination Chemistry Institute and the State Key Laboratory of Coordination Chemistry, Nanjing University, Center for Advanced Studies in Science and Technology of Microstructure, Nanjing, 210093, P.R. China bX-Ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia A series of new nickel complexes with multi-sulfur 1,2-dithiolene ligands have been synthesized and characterized by electrochemical measurements, EPR, IR and UV-VIS spectroscopies. One of the typical structures for the neutral complex of [Ni( phdt),] 2 (phdt=5,6-dihydro-5-phenyl-l,4-dithiin-2,3-dithiolate)was obtained by X-ray structure determination.Electrical conductivities measured for compacted pellets range from 1.4 x lo-* to 1.4 x S cm-l. These complexes show intense absorptions in the near-IR region. The third-order non-linear optical susceptibilities, f3), for three of the complexes have been determined to be of the order of lo-'' esu. In 1972, Drexhage and co-workers discovered that transition- metal-dithiolene complexes are particularly suited for Q-switching lasers (such as Nd-YAG lasers, operating at 1064nm).' Their choice was the class of square-planar tran- sition-metal dithiolene complexes, especially those of Ni, because they possess delocalized n-electron systems and strong near-IR absorption bands, which can be tuned by altering the metal ion and substituents.The second important property of the dithiolenes is their ability to exist in several clearly defined oxidation states which are connected through reversible redox steps. The third useful characteristic is their high thermal and photochemical stabilities, even at their absorption maxima. An intense research effort is presently aimed at producing materials with low response times and favourable ratios of the non-linear refractive index (n,) to the linear (a) and the non- linear absorption coefficients (p).,-' In near-resonant systems tuning into the low-energy side of an electronic transition leads to an increase in the absorptive contribution to the linear loss, and also to a commensurate increase in the non-linear refrac- tive index.This would result in a figure of merit (W)increasing until the absorptive contribution was at least the size of other loss mechanisms. This suggests that one way to improve the performance of organic materials is to study near-resonant systems. For these reasons, a series of metal dithiolenes have featured in third-order non-linear optical studies and f3) values were found in the range 7.16~lo-', to 3.8~lo-'' esu.6 Polymeric metal-dithiolene complexes may be candidate mate- rials for all-optical signal-processing devices for their sub- picosecond response times and reasonable off-resonant non- linearity.'.' Metal complexes with sulfur-rich dithiolene ligands are also very interesting from the viewpoint of their conductive behav- iour, owing to the intermolecular sulfur-sulfur interactions of the ligands.8 Dmit-metal complexes (dmit = 1,3-dithiole-2-thione-4,Sdithiolate; C3S5,-) were reported to be molecular inorganic conductors and even superconductor^.^ Following this, researches aimed at the search for new multi-sulfur 1,2-dithiolene ligands have We report here the syntheses and properties of several new complexes based on the multi-sulfur dithiolene ligand.One of the typical structure of the series, the neutral salt of [Ni(phdt),] (phdt =5,6-dihydro-5-phenyl-1,4-dithiin-2,3-dithiolate) is2, presented. The third-order non-linear optical susceptibilities (x'~))for three of the complexes have also been measured using the Z-scan method.1 R = 4H(Ph)CHr n = 1 2 R = -CH(Ph)CHr n = 0 3 R = -CH(Me)CHr n= 1n=2 n= 15 R = S=C< 7 R = 4H2CH2-n= 1 4 R = -CH(Me)CHr n = 0n=l 6 R=S=Cc n=O8 R Z-CH~CHZ- 9 R = ~H~CH~CHT Experimental Reagents 2,3-Dihydro-2-phenyl- 173-dithiolo-[ 4,5-e]- 1,4-di thiin-6-one (phedo) was prepared by a literature method and recrystallized from ethanol-chloroform (3 :l).I4 All solvents were dried by standard techniques prior to use. [Bu,N][Ni(medt),] 3 (medt =5,6-dihydr0-6-methyl-1,4-dithiin-2,3-dithiolate),'~ [Bu,N], [Ni(dmit),] 5,16 [Bu,N][Ni(dmit),] 6,16 [Bu,N][Ni(dddt),] 7 (dddt=5,6-dihydro-1,4-dithiin-2,3-dithi-olate)," [Ni(dddt),] 8," [Bu,N][Ni(pddt),] 9 (pddt =6,7- dihydro-5H-1,4-dithiepin-2,3-dithiolate)13were prepared according to the literature methods. Syntheses All reactions were carried out under N,.Elemental analyses were performed using a Perkin-Elmer 240C analytical instru- ment. Ni element analysis was performed on a Jarrell-Ash ICP quantimeter. Preparation of [Bu,N][Ni(phdt),] 1. A mixture of ethanol (20 ml), potassium hydroxide (2.0 g) and phedo (1.0 g, 3.5 x lop3 mol) was stirred for 1 h at 40°C under N,. The resulting pure yellow microcrystals of potassium 5,6-dihydro- 5-phenyl-1,4-dithiin-2,3-dithiolate(K,phdt) were isolated by centrifugation. The salt was then dissolved instantly in meth- anol (20 ml). A solution of 0.39 g (1.7 x mol) NiC1, -6H20 in 20ml methanol was added dropwise to the solution under N,.After standing for 15 min, the solution was then stirred in air for 30 min. The colour of the solution changed from amber to dark green as the reaction proceeded. After filtration, to the filtrate was added 1 equiv. of tetrabutylammonium bromide, and dark green solids were precipitated immediately. The green J. Muter. Chew., 1996, 6(lo), 1633-1637 1633 solids were then collected by filtering and recrystallized from acetone. The yield was 0.99 g (70%). Anal. Calc. for C,,H,,NNiS8: C, 53.12; H, 6.44; N, 1.72; Ni, 7.21. Found: C, 52.92; H, 6.44; N, 1.58; Ni, 7.37. IR (KBr)~/cm-~:3026(w), 1468(m), 1450(m), 1365(s), 1179(m), 858(m), 696(m). Preparation of [Ni(phdt),] 2. The black powder of neutral [Ni(phdt),] was prepared by I,-oxidation of 1.Black opaque rectangular crystals were obtained by recrystallization from carbon disulfide. Anal. Calc. for C,,H,,S,Ni: C, 42.03; H, 2.82; Ni, 10.27. Found: C, 42.30; H, 3.12; Ni, 9.99. IR (KBr)v/cm-': 1234(s), 1174(s), 1134(m), 886(m), 469(m). Preparation of [Ni(medt),] 4. The preparation of the neutral complex 4 was similar to 2. The product was washed with acetonitrile and then recrystallized from benzene or carbon disulfide. Anal. Calc. for C,,H,,S,Ni: C, 26.85; H, 2.70; Ni, 13.12. Found: C, 27.02; H, 2.75; Ni, 13.07. IR (KBr)v/cm-': 1257(s), 1214(s), 1144(s), 1076(m), 873(m), 483(m). Physical measurements IR spectra were recorded on a Nicolet FTIR 170sx spectropho- tometer.UV-VIS spectra were recorded on a UV-3100 spectro- photometer. The cyclic voltammetry was performed by a model 79-1 V-A analyser with a electrochemical cell using a platinum wire as the working electrode, a platinum plate as the auxiliary electrode and an SCE as the reference electrode. Nitrogen was passed through the solution in the cell for 15 min prior to each measurement. The EPR spectrum was recorded on a Bruker ER 200-D-SRC spectrometer. Electrical conductivities for com- pacted pellets were measured on a ZC-43 high resistance meter by a two-probe technique at 25°C. Structure determination A representative crystal of [Ni(phdt),] was surveyed. A data set was collected using a Siemens P4 diffractometer eguipped with graphite-monochromated Mo-Ka (2=0.71073 A) radi-ation.The empirical (uia t,b scans) absorption correction was applied.18 The crystal data, data collection and structure refinement are summarized in Table 1. The structure was solved by direct methods using SHELXL86l'" and refined by full-matrix least-squares methods on F2 using SHELXL93.19' The H atoms were located from difference maps and refined isotropically. Atomic coordinates, thermal parameters, and bond lengths and angles have been deposited at the Cambridge Table 1 Crystal data and details of data collection and refinement formula M* crystal system monoclinic space group p2, /CTlF 293 (2) a14 8.9310( 10) bl4 6.9770(10) CIA 19.583( 3) PldFgrees 102.7 10( 10) VIA3 1190.3( 3) Z 2 DJg cmP3 1.595 p/cm-' 15.23 8 range for data collection 2.13-27.50 scan mode 8-28 maximum, minimum transmission 1.000, 0.641 reflections collected 3756 independent reflections 2724 data/restraints/parameters 2723/0/150 goodness of fit on F2 1.059 Crystallographic Data Centre (CCDC). See Information for Authors, J.Mater. Chem., 1996, Issue 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 1145/9. Results and Discussion Crystal structure of [Ni( phdt),] 2 Fig. 1 shows the structure and labelling scheme of [Ni(phdt),] 2. The interatomic distances and bond angles are given in Table 2. The four sulfurs surrounding the nickel atom yield a square- planar coordination with both S-Ni-S angles being 91.57( 3)".In the five-membered ring containing the nickel atom, the average S-Ni, S-C and C=C bond lengths are 2.126, 1.706 and 1.384 A, respectively. The correspondjng values in [Bu,N][Ni(dddt),] are 2.148, 1.735 and 1.339 A." The shorter C-S bond lengths are indicative of double-bond character. This electron delocalization in transition-metal dithi- olenes creates a difficulty in assignment of the oxidation state to the metal. The outer phenyl groups are almost perpendicular to the least-squares plane formed by the rest of the molecule. A stereoview of the unit-cell packing of complex 2 viewed down the c axis (Fig. 2) shows the molecules forming a layered structure in the ab plane. Fig. 3 shows that the molecules are arranged as two different oriented uniform 'stacks' approxi- mately aloFg the b axis.The closest Ni-..Ni distance is 6.9770( 10) A, which is the length of tee b axis. The shortest interstack S-..S contact is 3.6876( 14) A and occurs between S(3) and S(4), which is about the sum of the van der Waals radii (3.70 A). The large separation between stacks is due to the phenyl substitution on the external part of the ligand. This is consistent with the semiconductive property of the complex as described later. Fig. 1 ORTEP plot of the molecule of [Ni(phdt),] Table 2 Selected bond distances/A and bond angles/degrees in "i(Phdt),l 2 Ni-S( 1) 2.1246(8) Ni-S( la) 2.1246( 8) Ni-S(2) 2.1278(8) Ni-S( 2a) 2.1278( 8) S( 1 )-C( 1 ) 1.703(3) S(2)-C(2) 1.7 10( 3) S(3 )-C( 1) 1.736( 3) s(3 )-C (4) 1.756(4)s(4)-C( 2 ) 1.738(3) S(4)-C(3 1 1.765(4) C( 1)-C(2) 1.384( 4) C( 3 )-C(4) 1.285( 6) C(41-C ( 51 1SO8(5) S( 1)-Ni-S( la) 180.0 S( l)-Ni-S(2) 91.57(3) S( 1a)-Ni-S(2) 88.43( 3) S(l)-Ni-S(2a) 88.43( 3) S( 1a)-Ni-S (2a) 91.57(3) S(2)-Ni-S (2a) 180.0 C(1)-S( 1)-Ni 104.95( 11) C (2)-S (2)-Ni 105.06( 11) C( 1)-S(3)-C(4) 1043 2) c(2)-s (4)-c ( 3) 1033 2) C(2)-C( 1)-S( 1) 119.6(2) C(2)-C( 1)-s(3) 126.7 (2) S( 1)-C( 1)-S( 3) 113.7(2) c( 1)-c(2)-s (2) 118.7(2) C( l)-c(2)-s(4) 127.3 (2) S(2)-C( 2)-s(4) 113.9(2) C( 4)-C( 3)-S( 4) 127.7(4) c(3)-c (4)-c (5) 121.5 (4) C(3)-C(4)-S( 3) 127.7( 4) c(5)-C(4)-S( 3) 107.9( 3) C( 10)-C(5)-C(6) 118.0( 3) maximum shift error 0.001 Final R indices [1/20(1)] R(F)=0.0445, wR(~z)=0.1 190 Symmetry transformations used to generate equivalent atoms: a-x+l, --y, -z.1634 J. Mater. Chew., 1996, 6(lo), 1633-1637 Fig. 2 Packing diagram of the unit cell of [Ni(phdt),] looking down the c axis Fig. 3 Packing diagram of the unit cell of [Ni(phdt),] looking down the a axis IR, cyclic voltammetry, EPR and NIR spectra [Bu,N)[Ni(phdt),] 1 displays a very rich IR spectrum with the characteristic absorptions of monoanionic nickel dithiol- enes.20 vl, C=C at 1445cm-'; v2, C=S at 1179cm-l; v3, R-C (ring) at 858 cm-l, while the neutral complex 2 shows these absorptions at 1234, 1174 and 840 cm-l, respectively. Such a noticeable low-frequency shift of the C=C stretching band suggests that oxidation occurs essentially on the phdt ligand rather than on the metal centre.This is the same as for the complexes of [Bu,N][Ni(medt),] 3 and [Ni(medt),] 4, which show the C=C stretching band at 1443 and 1257 cm-', respectively. Cyclic voltammetry of [Bu,N] [Ni( phdt),] in acetonitrile (0.1 mol dmb3 Bu,NClO,, 250 mV s-l) reveals two reversible waves, 1/-2)= -0.65 V and E1,,(O/-1)=0.06 V. The corresponding values for [Bu,N][Ni(dddt),] are -0.69 and 0.17 V," and -0.70 and 0.05 V for [B~,N][Ni(medt),].'~ This trend follows the electron-withdrawing ability of Ph >H >Me. The greater the electron-attracting ability of the ligand, the more electron density the complex can accommo- date and the higher the redox potentials. The frozen-glass EPR spectrum of [Bu,N][Ni(phdt),] 1in DMF at 128 K shows three peaks with gl=2.104, g2=2.048 and g3=2.004.This spectrum is similar to those of [Ni(dddt),] -lo and [Ni(pddt),] -which show a rhombic g tensor. Fig. 4 shows the NIR spectra of complexes 1and 2. Complex 1 in acetonitrile solution shows a strong broad absorption at 1172 nm (E= 15000), while complex 2 in benzene solution exhibits this absorption at 1028 nm (E =43 000). Table 3 lists the near-IR absorption maxima and absorbances of the com- plexes. The intensity of the absorption is unmatched in any other transition-metal compound, where low-energy bands usually are of d+d character and thus considerably weaker. The EHMO calculations assign this strong electronic absorp- tion band to a n+n* transition (2A,, 2B,, 3B3,, 2B3,-+3B,,) of the delocalized ligand.,, The electron delocalization in the coordination ring and the central metal is made possible by the stronger overlap involving d-orbitals of sulfur and thus will lead to bathochromic shifts of the multi-sulfur 1,2-dithi- olene complexes.By inspection of the data in Table3, it can be noted that complexes 1, 3 and 7 exhibit NIR absorption bands at 1172, 1177 and 1175 nm, respectively. This is due to the weak donor property of the methyl group and the weak acceptor property of the phenyl group, which are in accordance with the results from the values of cyclic voltammetry discussed above. But this effect is not significant in [Ni(medt),] 4 and [Ni(phdt),] 2.Attachment of bulky substituents, such as phenyl and methyl A Inm Fig.4 Absorption spectra of 1 in acetonitrile (-) and 2 in benzene (---) (25°C) Table3 Absorbance and absorption maxima of the complexes of dithiolenes (25 "C) complex solvent I,,,/nm &/dm3 mol-' cm-' 1 [Bu,N][Ni(phdt),] acetonitrile 1172 15000 3 2 [Bu,N][Ni(medt),]"i(Phdt),l benzene acetonitrile 1028 1177 43000 15000 4 5 6 7 [Ni(medt),] [Bu,N],[Ni(dmit),] [Bu,N][Ni(dmit),] [Bu,N][Ni(dddt),] benzene acetonitrile acetonitrile acetonitrile 1029 1219 1137 1175 36000 6500 45000 15600 9 [Bu,N][Ni(pddt),] acetonitrile 938 11000 J. Muter. Chem., 1996,6(lo), 1633-1637 1635 groups, will increase solubility, which is favourable for use as -10 -5 0 5 10 candidates for Q-switching dyes 23 Complex 1, 3 and 7 with the outer six-membered ring show bathochromic shifts of cu 240 nm compared to that of complex 9 with the outer seven-membered ring, which has been used as an NIR absorbing dye 24 This means that structural factors 12-contribute to the absorption band shift significantly ...,...j::,1 0..*: =,....;..:..*..-...*....;-..,...,..-* a.Electrical properties Table 4 lists the electrical conductivities measured for the compacted pellets at 25°C Complexes 1, 3, 6 and 9 exhibit small conductivities due to the weak interactions between the anions However, in the neutral complexes 2 and 8, the interaction between the molecules may be strengthened as revealed in the crystal structures of [Ni(phdt),] and [Ni(dddt),] l7 The increase of conductivity is of the order of 10' Comparing 7 with 1 and 3, the face-to-face overlapping of the molecules may be prevented by the introduction of bulky phenyl and methyl groups into [Ni(dddt),]-, and thus decreased the conductivities significantly Non-linear optical properties The third-order susceptibility, x(~),at 532 nm, was determined by Z-scan techniques as described earlier 26 27 The experimental arrangement utilizes a M200 high power mode-locked Nd YAG laser with 200 ps pulse at a frequency of 5 Hz A CH3CN solution of the compound was placed in a 1 mm quartz cell and used for all the optical measurements The NLO properties of these complexes are dominated by non-linear refraction, as illustrated in Fig 5(u) The non-linear absorption is negligible (Fig 4) The valley-peak pattern of the normalized transmittance curve obtained under closed aperture configuration shows characteristic self-focusing behaviour of the progating light in the sample The valley and peak occur at equal distances from the focus with the valley-peak separation [Fig 5(b)] fitting eqn (l), where coo is the laser beam waist radius (33+5 pm) and A is the laser wavelength (532 nm) AZv-p = 1 72n.~r>,'/A (1) This result suggest that the observed optical non-linearity has a third-order dependence on the incident field The difference between normalized transmittance values at valley and peak portions, AT,,, is related to the non-linear refractive index n, (m2 W-') by eqn (2) and (3), where ASP, and I, are the on-axis phase shift and the on-axis irradiance, both at focus, respectively, and a, and L are the linear absorption coefficient and the optical path of the sample ATpp=0 406( 1-S)' 25 IASPO I (2) In our experiment, S=O 3, therefore A TvPp=0 3 7 1IASP0 I IAcDoI =(2n/A)I,[( 1-e-"oL)/a,]nz (3) By inserting the values I, =3 26 GW cm-', L =10 x m, then the n2 value can be calculated Experiments with varied Table 4 Electrical conductivities (u)of the complexes at 25 "C ~~ complex 01s cm-' ref 1 CBu,NICNl(Phdt),l 14x10 15 2 "l(Phdt),l 15x10 this work 3 [Bu,N][Ni(medt),] 2 6 x 15 6 [Bu4N][ Ni(dmit ),I 10 x 25 7 [B~4N][Ni(dddt),] 49x10 this work 8 [Ni(dddt),] 14x10 this work 9 cBu4N 1CWPddt),I 5 4 x this work "At 20 "C 1636 J Muter Chern, 1996, 6(10), 1633-1637 08-~ v)C -10 -5 0 5 10 c-l2 Zlmm Ua, -10 -5 0 5 10 ,.IT(.I 12 10 io 4 -10 -5 0 5 10 Z/mm Fig. 5 The Z-scan data of complex 1 (1 0 x lop3mol dm-') at 532 nm with 1,=3 26 GW cmP2 (a) Collected under the open aperture configuration showing very small NLO absorptions, (b) collected under the closed aperture configuratlon showing the self-confusing effect I, show that the n2 thus measured is indeed independent of I,, consistent with the notion that n =no+n,I, and the observed NLO phenomenon is third order in nature The samples are almost transparent in the green region of the spectrum and dispersion of n2 is expected to be negligible If we ignore the contribution of NLO absorption, the third- order NLO susceptibilities x(3) of the three complexes can be calculated from the n, value by using eqn (4) ~(~'(esu) =cno2nZ/8O~=x(~)~~ f3)(rn2V-') =c~,n,~n~ (4) The calculated figures of merit (W)for the complexes are based on the following formula W=Ansat/an =n21sat/an (5) where Ansat is the maximum change in the refractive index, which is the product of n2 and the damage intensity for most organic materials Isatis taken as 3 GW cm-' for the values given in Table 5 The measured susceptibilities for the three complexes are listed in Table 5, which are in the range 14x lo-'' to 1 6 x lo-'' esu This work was supported by grants for key research project in climbing program from the State Science and Technology Commission and National Nature Science Foundation of China H-K F and B-C Y would like to thank the Malaysia Government and Universiti Sains Malaysia for research grant R&D No 190-9609-2801 The authors also thank Mr Hai-Ming Wu from the National Laboratory of Molecular and Biomolecular Electronics, Southeast University of China, for help with the electrical conductivity measurements Table 5 Third-order non-linear optical susceptibilities f3) for some nickel dithiolene complexes ~ esucomplex i,,,/nm x/m-l x(3)/10-11 ~(~)/10-*Om2 v w ref 1 c Bum “l(Phdt),l 1172 56 16 35 7 6 this work 7 [B~,N][Ni(dddt)2] 1175 47 16 35 76 this work 9 cBu4NI “I( Pddt), 1 938 46 14 30 64 this work 1’“ bis [l-methyl-2-phenylethene-1,2-dithiolato(2-)-S,Y] nickel 770 9 14 90 3 9 bis [1,2-diphenylethene-l,2-dithiolato(2-)-S,S’] nickel 865 88 19 10 3 12’” bis [1,2-bis(4-methylphenyl)-1,2-ethenedithiolato(2-)-900 34 11 19 3 S,S’]nickel -)-13“ bis [1,2-bis(4-methylphenyl)-1,2-ethenedithiolato(2 935 216 34 09 3 S,S’]nickel Of3) was measured at 1064nm and used a degenerate four-wave mixing (DFWM) setup References 14 S Larsen, T Thorsteinsson, S Bowadt, T K Hansen, K S Varma, J Becher and A E Underhill, Acta Chem Scand ,1991,709 1 K H Drexhage and U T Muller-Westerhoff, IEEE J Quantum 15 T-M Yao, J-L Zuo, X-Z You and X-Y Huang, Polyhedron, 1995, Electron, QE-8, 1972, 759, K H Drexhage and U T Muller-14,1487 Westerhoff, US Pat, 3743 964, 1973 16 G Steimecke, J Sieler, R Kirmse, W Dietzsch and E Hoyer,2 P Calvert, Nature (London), 1991,350, 114 Phosphorus Sulfur, 1982,12,237 3 C S Winter, S N Oliver, R J Manning, J D Rush, C A S Hlll 17 H Kim, A Kobayashi, Y Sasaki, R Kato and H Kobayashi, Bull and A E Underhill, J Mater Chem ,1992,2,443 Chem SOC Jpn ,1988,61,5794 A E Underhill, C A S Hill, C S Winter, S N Oliver and 18 XSCANS Users Manual, version 21, Siemens Analytical X-ray J D Rush,Mol Cryst Lzq Cryst, 1993,217,7 Instruments Inc ,Madison, WI, USA, 1994 5 A E Underhill, C A S Hill, A Charlton, S Oliver and 19 (a) G M Sheldrick, Acta Crystallogr, Sect A, 1990, 46, 467, S Kershaw, Synth Met, 1995,71,1703 (b) G M Sheldrick, Program for crystal structure refinement,6 N J Long, Angew Chem, Int Ed Engl, 1995,34,1 University of Gottingen, Germany, 1993 7 C S Winter, C A S Hill and A E Underhill, Appl Phys Lett, 20 J A McCleverty, Prog Inorg Chem ,1968,10,971991,58,107 21 T-M Yao, X-Z You and Q-C Yang, Chin J Chem , 1994,12,2488 L Valade, P P Legros, M Bosseau, P Cassoux, M Garbauskas 22 Q Fang, Y-M Sun and X-Z You, Chin J Chem Phys, 1992,and L V Interrante, J Chem SOC, Dalton Trans, 1985,783 1,1299 M Bousseau, L Valade, J P Legros, P Cassoux, M Garbauskas 23 U T Muller-Westerhoff, D I Yoon and K Plourde, Mol Crystand L V Interrante, J Am Chem SOC,1986,108, 1908, Lzq Cryst, 1990,183,291A Kobayashi, H Kim, Y Sasaki, H Kobayashi, S Monyama, 24 T Hasegawa, Jpn Pat, 03155538,1991Y Nishino, K Kojita and W Sasaki, Chem Lett, 1987,1819 25 F Nusslein, R Peter and H Kisch, Chem Ber , 1989, 122, 1023 C T Vance, R D Bereman, J Bordner, W E Hatfield and 26 J-Y Niu, X-Z You, C-Y Duan, H-K Fun and Z-Y Zhou, InorgJ H Helms, Inorg Chem ,1985,24,2905 Chem ,in press J H Welch, R D Bereman, P Singh and C Moreland, Inorg 27 M Sheik-Bathe, A A Said and E W Van Stryland, Opt Lett, Chzm Acta, 1989,158, 17 J H Welch, R D Bereman and P Singh, Inorg Chem, 1988, 1989,17,955 27,2862 R D Bereman and H Lu, Inorg Chzm Acta, 1993,204,53 Paper 6/020111, Received 22nd March, 1996 J Mater Chem., 1996, 6(lo), 1633-1637 1637

ISSN:0959-9428    

DOI:10.1039/JM9960601633

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Carbonyl sulfide (OCS) as a sulfur-containing precursor in MOCVD: a study of mixtures of Me,Cd and OCS in the gas and solid phases and their use in MOCVD Matthew J. Almond," Brian Cockayne,b Sharon A. Cooke: David A. Rice," Philip C. Smithb and Peter J. Wrightb "Department of Chemistry, University of Reading, Whiteknights, Reading, Berks, UK RG6 6AD bDRA (Malvern), St. Andrews Road, Malvern, Worm., UK WR14 3PS Mixtures of dimethylcadmium (Me,Cd) and carbonyl sulfide (OCS) have been examined in the gas and solid phases over a wide range of temperatures. No interaction is observed between Me2Cd and OCS in a 1 :1molar ratio at room temperature in the gas phase, nor is any interaction detected in the solid phase at liquid-nitrogen temperature. On heating the 1:1mixtures of Me2Cd and OCS to 250 "C in a sealed vessel, gaseous products are formed.These consist of methane, carbon monoxide and ethane in an approximately 12 :2 :1 molar ratio, although a large excess of unreacted OCS is also present showing that this compound does not react fully with Me2Cd at 250 "C. In a flow system at 300 "C only methane and carbon monoxide are formed, in the molar ratio 6 :1, although the amount of reaction of the OCS is much less (as evidenced by a higher proportion of unreacted OCS). When the flow reaction is repeated at 450 "C more of the OCS reacts and the proportion of carbon monoxide in the gaseous reaction products is much higher. Using a commercial MOCVD apparatus, high-quality layers of cadmium sulfide are obtained from Me,Cd-OCS mixtures.Temperatures in the range 350-450 "C lead to somewhat slow growth rates which only reach 1 pm h-' when a 200-fold molar excess of OCS over Me2Cd is used. A small amount of prereaction is observed, but only when H, is used as the carrier gas. This is attributed to the formation of very small concentrations of H,S by reaction of OCS with H,. The resulting epitaxial layers have good thickness and electrical uniformity. These experiments confirm that OCS may be used as a precursor for the growth of thin layers of CdS by MOCVD. However, the large excess of OCS required here suggests that the compound might be more useful for doping than for the growth of pure layers of CdS. In order to exploit fully the technique of metal-organic chemi- cal vapour deposition (MOCVD) for the growth of thin layers of 11-VI semiconductor materials it is necessary to optimise the precursor compounds used.Such work is of considerable importance given the use of 11-VI compounds in blue-green light-emitting diodes (LEDs) and lasers.'-4 For many of these applications, controlled doping of the semiconductor layer is required. In this respect molecular beam epitaxy (MBE) has, to date, demonstrated better control than has MOCVD. It is only by careful selection of the most suitable precursor com- pounds that MOCVD will yield epitaxial layers of the relevant purity and uniformity for successful exploitation in devices. The subsequent problem of doping control can then be addressed. It is probably true that most of the possibilities for experi- ment within the rather limited range of group I1 precursors have been tried.5" Such experiments have included the use of adducts of Me2Cd with Lewis base donor molecules, e.g.TMEDA (Me,NCH2CH2NMe2), as the metal-containing pre- cursor, but these precursors are not successful in reducing prereaction when the metal is cadmium.5b In recent times attention has turned to the sulfur source where an extensive choice of reagents exists. The use of the hydride, hydrogen sulfide, provides good-quality layers in terms of optical proper- ties and purity with growth possible at low temperatures (300-400 "C). The problem with this precursor is that there is also an unwanted prereaction which gives rise to non-uniform- ities in the layer properties.The use of a number of alkyl group sources has been studied. These sources have included both monoalkyl compounds, e.g. methylthiol,6 and dialkyl compounds, e.g. diethyl~ulfide.~In addition, five- and six- membered heterocyclic ring compounds have been considered.* In the majority of cases the use of these types of compounds leads to a higher growth temperature (>4OO"C) and an associated degradation in the optical and electrical properties of the layers. However, the uniformity of the layer thickness and the elimination of unwanted prereaction are benefits for these compounds. Thus far, the best compromise in terms of optical and electrical properties and uniformity is the series of monosubs tit uted sulfur compounds (e.g.methyl t hiol ). Recent work has demonstrated the success of ButSH as the sulfur- containing precursor for the growth of thin layers of 11-VI compounds at temperatures around 325 OC9 In recent publi- cations we reported the use of the thiirane propylene sulfide as a precursor for both ZnS" and CdS" by MOCVD. In the current work we have turned our attention to the compound carbonyl sulfide, OCS. To our knowledge this compound has not been studied in MOCVD systems, although the related compound CS2 has been investigated in a preliminary series of experiments as a precursor, with Me,Zn, for ZnS forma- tion.12 There were three main reasons for our selection of OCS as a precursor. First is the well known behaviour of OCS to act as a source of sulfur atoms in a range of chemical reaction^.'^-'^ Second is that OCS is likely to generate sulfur in a very clean chemical reaction in which the only byproduct is carbon monoxide gas, so avoiding the problems associated with many organosulfur compounds whereby carbon may be incorporated as an impurity in the semiconductor layer.It is thought that the carbon impurity arises via the intermediacy of organic radicals which are formed upon decomposition of the organosulfur precursor. Third, an impetus was given to our research by our observations that OCS is often found as an impurity in commercially supplied samples of H2S. Thus IR spectra of gaseous samples of H2S obtained direct from the cylinder in which it was supplied invariably show the character- istic bands of OCS, of which the most intense is the feature arising from the v(C=O) vibration centred at around 2060cm-l.Since H,S is the most usual sulfur precursor for growth of thin layers of sulfide semiconductor materials it is of importance to ascertain the reactivity of OCS with Me,Cd under MOCVD conditions. Our experimental strategy has followed closely that employed when propylene sulfide was assessed as a precursor." First, we have looked for any evidence of molecular interactions between OCS and Me2Cd J. Muter. Chem., 1996, 6(lo), 1639-1642 1639 in the gas phase at room temperature and in the condensed solid phase at 77 K Secondly, we have studied the thermolysis of mixtures of OCS and Me,Cd in both static and flow systems over a range of temperatures Lastly, we have used OCS as the sulfur source in a commercial MOCVD reactor in order to grow layers of device-quality CdS Experimental An all-glass vacuum line was used for all the experiments at Reading except for the Grignard preparation of dimethyl- cadmium l6 l7 Gas chromatographic (GC) analyses were per- formed using a Perkin Elmer 8420 capillary gas chromatograph fitted with a 25 m Chrompack capillary column Detection was by flame ionisation detection (FID) Carbonyl sulfide (Aldrich, stated purity 96+ %) was used direct from the cylinder in which it was supplied Gas-phase IR measurements were made by containing samples of the vapours produced from the various reactions in an evacuated gas cell (path length ca 10cm) fitted with KBr windows A Perkin Elmer 1720-X Fourier Transform interferometer was used to record the IR spectra with a resolution of f2 cm-' 'Cold cell' experiments were executed as described in detail elsewhere Experiments were carried out in which mixtures of Me,Cd and OCS were heated under vacuum in static systems or under nitrogen in flow systems For static experiments an approxi- mately 1 1 mixture of Me,Cd and OCS (ca 1 5 mmol of each reagent) was sealed into a Pyrex vessel of volume ca 150 cm3 This vessel was placed in an oven and was held at a temperature of 250°C for 24 h The gaseous products were analysed by GC Similar sealed Pyrex vessels were also prepared containing only pure dimethyl cadmium or pure carbonyl sulfide Again these were heated and the evolved gases analysed by GC measurements In a separate experiment, Me,Cd and OCS were allowed to react under flow conditions A mixture of dimethylcadmium and a large excess of carbonyl sulfide was allowed to pass through a heated Pyrex tube, of length ca 40 cm and diameter ca 5 cm, under a flow of dry nitrogen The temperature, ca 300 "C, was controlled by a Eurotherm temperature controller Unreacted material, after passing through the reactor, was collected in a trap held at -78 "C The gases evolved during the course of the reaction, and which passed through the -78 "C trap, were collected and analysed by GC Because the degree of reaction of OCS in this experiment was small the run was repeated but the temperature of the reactor was increased to 450°C Growth of CdS by MOCVD The growth of the epitaxial layers was carried out at the Defence Research Agency (DRA), Malvern under atmospheric pressure in a conventional horizontal MOCVD reactor described previously for similar experiments l9 Palladium-diffused hydrogen was used as the carrier gas The sample of dimethyl cadmium was kept in a temperature-controlled bath and its flow rate into the reactor was varied by adjusting the temperature of the bath Carbonyl sulfide was admitted to the reactor direct from the cylinder in which it was supplied, its flow rate was adjusted by means of a mass-flow controller Growth was carried out in a water-cooled reactor cell with the substrates mounted on an RF heated graphite susceptor The substrates used were GaAs single-crystal wafers orien- tated on the (100) plane Prior to growth the wafers were polished and etched as described elsewhere l9 The prepared wafers were loaded into the reactor which was subsequently flushed with high-purity hydrogen Immediately before growth the wafers were heated to 500 "C for 10 min, after which the 1640 J Muter Chem , 1996, 6(lo), 1639-1642 temperature was reduced to the growth temperature and layer growth was initiated The layers were grown for 1 h After growth the reactor was flushed with hydrogen before the substrates and layers were allowed to cool to room temperature and removed Cleave-and-stain techniques in conjunction with an optical microscope were used to determine the layer thickness The surface morphologies of the layers were examined using both optical microscopy and a scanning electron microscope (SEM) In addition, one layer grown under optimum conditions was selected and its electrical properties were assessed using Hall effect measurements Secondary ion mass spectrometry (SIMS) was performed to determine the nature of any impurities present in the layer of CdS Results and Discussion A gaseous mixture of dimethylcadmium and carbonyl sulfide in a 1 1 molar ratio was examined by FTIR spectroscopy The spectrum of the mixture corresponded exactly to a super- position of the spectra of the two individual components Hence it is concluded that these reagents do not form any gaseous adduct at room temperature Moreover, as described in more detail elsewhere," there is no detectable interaction between these reagents in the condensed solid at 77 K This latter finding is in contrast to the results obtained from solid mixtures of dimethylcadmium and propylene sulfide at 77 K,1° where a weak interaction was observed Thus there is no evidence for the formation of any adduct between Me,Cd and OCS under the various conditions of our experiments The thermal decomposition of 1 1 molar ratio mixtures of Me,Cd and OCS was studied in static systems at 250°C As controls, the thermolyses of both pure compounds under identical conditions were determined individually On heating a sample of pure Me,Cd (2 5 mmol Me,Cd in a volume of ca 125 cm3) to 250 "C for 24 h, a range of gaseous alkanes and alkenes were formed (see Table 1) The data obtained are in accord with those obtained previously 2o Upon heating a sample of OCS (2 0 mmol in a volume of 217 cm3) under the same conditions, no reaction or decomposition occurred Having obtained data on the pure compounds, we proceeded to study the reactions of approximately 1 1 (ca 15 mmol of each reagent) molar mixtures of Me,Cd and OCS at 250°C After heating for 24 h, then allowing to cool, the gases produced were methane, carbon monoxide and ethane in an approxi- mately 12 2 1 molar ratio An excess of unreacted OCS was also present The appearance of the reactor had changed from colourless to orange with a layer of a solid material being deposited on the inside of the reactor The important finding from these experiments is that OCS itself does not decompose at a temperature as low as 250"C, but reacts in the presence of Me,Cd In an attempt to mimic more closely an MOCVD reactor we thus proceeded to study the 'flow' reaction between Me,Cd and OCS entrained in a flow of nitrogen gas A furnace was placed around the reactor and the temperature was raised to 300+ 5 "C The gas stream emerging from the reactor was passed through a trap held at -78 "C and the gases not trapped were collected and analysed by GC The flow system was held at 300+ 5 "C for ca 1 5 h during which time a very slight yellow colouration could be observed on the inside of the reactor in the heated zone only Only small quantities of gaseous products were evolved at this point of the reaction, such products consisted of CO and CH, However, a large quantity of unreacted OCS was collected from the reactor in the -78 "C trap The temperature was increased to 400 "C and another gas sample was taken for GC measurements This gave a similar result to that obtained at 300"C, but the presence of a small amount of C2H6 was noted as was an Table 1 Gas chromatographic analysis of the products obtained after heating samples of Me,Cd and carbonyl sulfide assignment" Me,Cd (static)b, 250 "c' mixture (static)b, 250 "Cd mixture (fl~w)~, 300 "c" mixture (fl~w)~, 450"Cf CH4 CH3 -CH3 51.7 28.7 80.9 6.4 85.7 17.2 20.7 CH, =CH, 12.3 CH,CH,CH3 0.5 CH, =CHCH, 6.8 co 12.8 14.3 62.1 "All values quoted are molar percentages of gases with an accuracy of +0.1%.bThe static systems were heated in an oven at the stated temperature for 24 h; in the flow experiments heating was continued for 1.5 h. 'The vessel was cooled to -78 "C before the measurements were taken. d21.8% of the OCS had reacted in this experiment. "1.6% of the OCS had reacted in this experiment. f3.3% of the OCS had reacted in this experiment: note that the degree of reaction of the OCS is, as expected, much less in the flow than in the static system but that it increases substantially with increasing temperature. increase in the relative amount of CO.Increasing the tempera- ture to 450°C produced a much higher concentration of CO (as evidenced by the GC measurement, see Table 1) and so it can be concluded that more OCS has reacted at the higher temperatures. A deeper yellow-orange solid was observed on the side of the reactor at this temperature. A sample of this yellow-orange coating was analysed by X-ray powder diffrac- tion. The d-spacings and the relative intensities obtained from the X-ray data were in accord with those of cubic CdS.,' It is noteworthy that the production of CO gas appears to be linked to the deposition of solid CdS, since greatly increased amounts of both of these products are observed when the temperature is raised to 450 "C. From these preliminary studies, it is apparent that the system merited further detailed investi- gation.Accordingly growth experiments were performed reacting Me2Cd and OCS in a conventional MOCVD reactor at DRA, Malvern. Growth experiments by MOCVD All growth experiments were performed using the (100)surfaces of GaAs crystals as substrates. The initial series of growth runs were carried out keeping the reagent concentrations fixed, and the growth time constant at 60min, but varying the growth temperature. Throughout these growth experiments the molar ratio, OCS :Me2Cd (VI :11), was maintained at 25 :1. In Fig. 1 the results obtained are illustrated graphically. The growth rate of cadmium sulfide is seen to increase with growth temperature from 300 to 400°C.In the temperature range 400-450 "C the growth rate remains approximately constant. In the second series of growth runs the growth temperature was fixed and the concentration of OCS in the reactor varied (see Fig. 2). The vapour-phase ratio of the reagents was increased from 25 : 1 to 200 :1. The growth rate is seen to rise 0.0 1 J 300 400 500 growth temperature/"C Fig. 1 Variation of CdS growth rate as a function of growth temperature. Dimethyl cadmium flow rate 25 cm3 min-'; the Me,Cd to OCS molar ratio is 1 :25. 0 50 100 flow rate of OCS/C~~min-' Fig. 2 Variation of CdS growth rate as a function of carbonyl sulfide flow. Growth temperature 350 "C; dimethyl cadmium flow rate 25 cm3 min-'. The Me2Cd to OCS molar ratio is varied from 1 :25 to 1:200.as the OCS to Me2Cd ratio is increased over this range. The quality of the layers was generally good. The best quality layers were obtained at growth temperatures of 375-400 "C and with an OCS flow rate of 80 cm3 min-' (i.e. OCS :Me2Cd ratios of ca. 200: 1). A small amount of prereaction was observed in each growth experiment when Me,Cd and OCS were used as precursors and H, as the carrier gas, although the extent of this was only very slight when compared with the amount of prereaction observed when using Me2Cd and H2S as precursors. The prereaction observed here is thought to arise from the reaction between OCS and H2, which forms a small amount of H,S. This is a similar reaction to that observed by Takata et al.when using CS2 and Me,Zn as precursors to grow layers of ZnS by M0CVD.l' The prereac- tion was not seen in the flow experiments carried out at Reading where N2 was used as the carrier gas. The findings suggest that He might be preferable to H, as the carrier gas when using a commercial MOCVD apparatus to grow layers of CdS using these precursors. Hall measurements show that the layers have a bulk carrier concentration of 3 x 1017 atoms ~m-~. This level of impurities, while acceptable, is somewhat high for a successful semicond- ucting device ( 1015-1017 atoms cm-3 is normally considered to be the desirable range for impurity levels). SIMS measure- ments provide information as to what these impurities are. First, there is evidence that gallium atoms (which here will act as an n-type dopant) have diffused into the layer from the GaAs substrate.Secondly, there is evidence for the presence of halogens (which will also act as n-type dopants) in the sample. These probably arise from the preparation of the OCS sample and it should be noted that no steps were taken in these experiments to purify the OCS. Doubtless these impurity levels could be reduced by more rigorous purification. J. Mater. Chem., 1996, 6(lo), 1639-1642 1641 Conclusions Growth of CdS at 350 "C using OCS and Me,Cd as precursors produces high-quality layers of CdS suitable for semiconductor device applications A flow rate of OCS of 80 cm3 min-' (I e a high OCS to Me2Cd molar ratio of ca 200 1) gives the optimum growth rate of CdS layers of ca 100 pm h-' The slow growth rate of CdS layers under these conditions means that OCS may have limited application in the growth of layers of pure CdS However, it could well find use in doping experiments where a precursor yielding low levels of sulfide would be desirable It is noteworthy that there is no sign of carbon incorporation in any of the layers of CdS grown using OCS as a precursor Lastly, it is apparent that OCS is much less reactive with Me,Cd than is H,S under MOCVD con- ditions This finding implies that the impurity of OCS com- monly encountered in commercial samples of H2S is unlikely to perturb significantly the MOCVD growth of layers of CdS when using H2S as the group 16 precursor It is of interest to compare briefly the results of growth experiments on CdS using propylene sulfide" or carbonyl sulfide as the sulfur source The former precursor gives much faster growth rates as it has a much higher reactivity with Me,Cd, as evidenced by the 'static' and 'flow' thermolysis experiments carried out in this laboratory Moreover, it is noteworthy that while neither Me,Cd and OCS nor Me,Cd and propylene sulfide show any interaction at room tempera- ture, the latter pair of reagents, unlike the former, does show a weak Lewis acid-base interaction in the solid phase at 77 K In summary, it is apparent that propylene sulfide is a more suitable sulfur source than is carbonyl sulfide in MOCVD, but that carbonyl sulfide might find specific appli- cations in doping experiments The authors wish to thank J Newey and D Osborne (DRA, Malvern) respectively for the SIMS and thickness and electrical measurements S A C gratefully acknowledges the Research Board of the University of Reading for providing a studentship OBritish Crown Copyright 1995/DRA Published with the permission of the Controller of Her Bntannic Majesty's Stationery Office References 1 M A Haase, J Qui, J M DuPuydt and H Cheng, Appl Phys Lett, 1991,59,1272 2 J M Gaines, R R Drenten, K W Haberern, T Marshall, P Mensz and J Petrozzello, Appl Phys Lett, 1993,62,2462 3 N Nakayama, S Itoh, H Okuyama, M Ozawa, T Ohata, K Nakano, M Ikeda, A Ishibashi and K Akimoto, Electron Lett, 1993,29,2194 4 J M DuPuydt,M A Haase,S Guha, J Qui,H Cheng,B J Wu, G E Hofler, G Meis-Haugen, M S Hagedorn and P F Baude, J Cryst Growth, 1994, 138,667 5 (a) P J Wright, B Cockayne, P J Parbrook, P E Oliver and A C Jones, J Cryst Growth, 1991, 108, 525, (b) M J Almond, M P Beer, K Hagen, D A Rice and P J Wnght, J Muter Chem, 1991,1,1065 6 S Fujlta, M Isemura, T Sakmoto and N Yoshimura, J Cryst Growth, 1988,87,581 7 P J Parbrook, A Kamata and T Uemoto, Jpn J Appl Phys, 1993,32,669 8 P J Wnght, R J M Gnffiths and B Cockayne, J Cryst Growth, 1984'66,525 9 D F Foster, I L Patterson, L D James, D J Cole-Hamilton, D N Armitage, H M Yates, A C Wright and J 0 Williams, Adv Muter Opt Electron, 1994,3, 163 10 M J Almond,M P Beer,S A Cooke,D A RiceandH M Yates, J Muter Chem, 1995,5,853 11 M J Almond, B Cockayne, S A Cooke, D A Rice, P C Smith and P J Wright, J Muter Chem, 1995,5, 1351 12 S Takata, T Minami, T Miyata and H Nanto, J Cryst Growth, 1988,86,257 13 T-L Tso and E K C Lee, J Phys Chem, 1984,88,2781 14 G Cook and 0 D Krogh, J Chem Phys, 1981, 74, 841 and refs therein 15 (a)M Hawkins and A J Downs, J Phys Chem, 1984, 88, 1527, 3042, (b)M Hawkins, M J Almond and A J Downs, J Phys Chem, 1985,89,3326 16 E Krause, Ber, 1917,50, 1813 17 P R Jacobs, E D Orrell, J B Mullin and D J Cole-Hamilton, Chemtronics, 1986, 1, 15 18 M J Almond,S A Cooke,D A RiceandL A Sheridan, J Phys Chem ,1995,99,14641 19 P J Wright and B Cockayne, J Cryst Growth, 1982,59,148 20 D A Rice, PhD Thesis, University of Exeter, UK, 1967 21 JCPDS, International Centre for Diffraction Data 1985, file number 10-454 Paper 6/03483G, Received 20th May, 1996 1642 J Muter Chem, 1996,6(10), 1639-1642

ISSN:0959-9428    

DOI:10.1039/JM9960601639

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Processing and optical properties of spin-coated polystyrene films containing CdS nanoparticles Andrk Chevreau, Brian Phillips, Brian G. Higgins and Subhash H. Risbud* Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA The processing, microstructure and optical properties of CdS semiconductor nanoparticles sequestered in spin-coated polymer films has been investigated. A simple processing protocol has been developed to form thin film structures consisting of CdS nanoparticles dispersed in the interstices created by a close-packed stacking of polystyrene spheres. We have decoupled the synthesis of the CdS nanoparticles from the polymer matrix by synthesizing a colloidal suspension of CdS nanoparticles in water that is compatible with a polymer latex water suspension.In this way we are able to vary the CdS concentration in the polymer film over a wide range and create concentrations of quantum dot particles two orders of magnitude larger than those reported in previous work. In contrast to the conventional reaction synthesis of nanoparticles in a polymer matrix, the present method is versatile enough to create several useful thin film polymer-semiconductor structures using a straightforward, environment-friendly spin-coating technology. The key to achieving quantum confinement effects in semicon- ductor nanoparticles lies in the chemical synthesis of stable suspensions of particles smaller than the exciton (electron-hole pair) diameter. Once these quantum dots' are formed they must be placed, isolated from each other, in a matrix of glass, polymer or crystalline hosts.Various approaches have been reported in the literature to achieve this Most of the nanocomposite structures containing quantum dots consist of a somewhat random dispersion of nanoparticles determined by, for example, arrested nucleation in glasses, precipitation from sol-gel solutions or porous sites in zeolite cavities. This work deals with the processing challenges associated with spin- coating a dispersion of polystyrene and CdS nanoparticles to create organized arrays of quantum dots in a polymer matrix. Using spin-coating, multilayered colloidal coatings of pre-scribed thickness and with a chosen composition of CdS nanoparticles have been deposited.Packing density, long range order and defects in the colloidal film were controlled through wetting properties, drying conditions and particle-particle interactions. We present results that demonstrate the viability and potential of this technique and draw attention to the versatility of this process for tailoring optoelectronic quantum devices by size-selection of the nanoparticles within each monolayer of the polymer thin film. Because more is under- stood about the surface chemistry and processing protocols for systems of nanoparticles dispersed in thin films, the practi- cability of obtaining tailored film properties through prescribed processing is enhanced. We also discuss the possibility of preparing organized arrays of quantum dots by the deliberate placement of nanocrystallites in the interstitial sites of a two- dimensional close-packed monolayer template created from colloidal polystyrene particles.Experimental Synthesis of CdS nanocrystallites The method of homogeneous nucleation and growth of CdS nanocrystallites in N,N-dimethylformamide (DMF), as reported by Chemseddine and Weller,' was used. In a 500ml three-necked flask, 308 ml DMF, 3.859 g cadmium acetate, 0.642 g thiourea (TU) and 1.428 ml thioglycerol(3-sulfanylpro-pane- 1,2-diol; TG) were combined and stirred under nitrogen for at least 1 h. The solution temperature was gradually raised to the dissociation temperature of thiourea (120"C), at which point the solution became yellow. Then, 92ml of water was added, the temperature was lowered to 100 "C, and the mixture was allowed to reflux for 15 h.The dissociation of thiourea initiates the burst of nucleation necessary to grow particles. Thioglycerol is used as the capping agent for the particle. As the particles grow, the sulfanyl group of thioglycerol competes with dissociated sulfur from TU for bonding sites with cad- mium ions on the surface of the particle. This competition for surface sites, as well as the quantitative ratio, [Cd2+]/[TU], and the type of counter-ion present, can be used to control the sizes of the nanoparticles during growth.8 To narrow the particle-size distribution resulting from the homogeneous nucleation step, the colloidal suspension was size-fractionated using size-selective precipitation (SSP).'*9 A beaker containing the CdS suspension was stirred continuously in a sealed dessicator, in the presence of acetone vapour.Although acetone is miscible with DMF, it is a 'poor solvent' for the capped CdS particles, and as a result, larger CdS particles precipitate preferentially as acetone diffuses into the DMF solution. The extent of precipitation is determined by the mole fraction of acetone present. The amount of acetone placed in the dessicator with the CdS-DMF suspension was chosen based on trial-and-error visual observation of the amount of solids precipitated. To avoid over-precipitation in any single step, the amount of acetone used was approximately 10-20% of the volume of the CdS-DMF suspension.During this process, there also may be agglomeration of smaller particles to form larger ones that subsequently precipitate. After precipitation was observed, the CdS suspension was centrifuged and the supernatant was decanted off. The process was then repeated using the supernatant solution until the desired particle-size distribution was achieved. When the size distribution of CdS nanocrystals in DMF was narrowed satisfactorily, it was necessary to resuspend the CdS particles in water, since DMF is not compatible with the aqueous latex suspension. This was achieved by adding ethanol (a poor solvent for the particles) dropwise to the suspension to cause precipitation of CdS nanoparticles.The solution was centrifuged, decanted and the precipitate redissolved in water. There are several non-solvents that can be added dropwise to precipitate the CdS from this solution, including acetone and cyclohexanone. In this study we chose ethanol because these solids redissolved in water interacted most favourably with the polystyrene nanosphere suspensions in water. J. Muter. Chem., 1996,6( lo), 1643-1647 1643 Optical, TEM and NMR characterization of CdS nanocr ystallites A Cary 3 UV-VIS spectrophotometer from Varian was used to measure optical absorbance in liquid samples and deposited film samples The liquid samples were placed in 3 ml quartz cuvettes with a path length of 1cm Optical absorbance measurements were also taken for film samples, which were prepared on 3 81 cm diameter, 0 159 cm thick quartz wafers (from G M Associates, Inc) Optical excitation experiments were performed on both the cast and the spin-coated films containing CdS nanoparticles dispersed in a polystyrene matrix The photoluminescence (PL) spectra were obtained using a 100 fs self-mode-locked Ti sapphire laser Excitation pulses varying from 355 to 400 nm at a 82 MHz repetition rate were generated by fre- quency doubling through a KDP crystal Room-temperature PL spectra were recorded using an optical multichannel ana- lyser and appropriate bandpass filters Photoluminescence experiments were also conducted with tuned light from a xenon arc lamp striking the film surface at about 45" from perpendicular, after which it was detected at about 15" from perpendicular Transmission electron microscopy (TEM) of the CdS nano- particles was conducted using a Phillips 400 transmission electron microscope with a point-to-point spatial resolution of O34nm Samples suspended in water were placed on TEM grids and the solvent was allowed to evaporate prior to insertion in the microscope Both bright-field imaging and selected area electron diffraction were performed to determine the size and crystallinity of the CdS nanoparticles Cadmium- 1 13 single-pulse magic angle spinning (SP MAS) and 113Cd('H} cross-polarization MAS (CP MAS) spectra were obtained under ambient conditions at 66 5 MHz with a Chemagnetics CMX-300 spectrometer The sample spinning frequency was 4 5-5 kHz We report the chemical shifts relative to 0 1 mol dm-3 Cd(C10,)2 (aq), but an external sample of cadmium acetate was used as a solid-state reference (6 51 6) For CP, the transverse 'H field was ramped approximately & 10 kHz about the Hartmann-Hahn match, which was meas- ured using a sample of cadmium acetate Thin film processing The final working suspensions were prepared by adding the CdS water suspension to the polystyrene (PS) nanosphere suspension Monodispersed PS latex suspensions containing 38 nm nanospheres were prepared by diluting with ultrapure water a stock suspension purchased from Interfacial Dynamics Corporation The suspensions as received were stabilized elec- trostatically through the dissociation of surface carboxylate terminal groups, with a surface charge density of 18 4 pC cm-2 The coefficient of variation for the particle diameter of the stock suspension was specified by the supplier to be 14 1% Solutions of 0 1mol dmW3 NaOH were used in preparations requiring adjustment of pH Cast films were prepared by depositing 225 ml of the working suspension with and without base onto 3 81 cm diameter quartz wafers, and then allowing the liquid to evapor- ate under vacuum The films were then annealed at 100°C The quality of the cast films was generally poor The films tended to crack and flake on drying To improve the film quality, spin-coating was used A desired film thickness was achieved by spin-coating multiple layers of thickness approxi- mately one monolayer (cu 38 nm) After each coating step, the film was heated to 100°C to prevent wash-off during spin- coating No cracks or defects were observed in the final film under an optical microscope at 50x magnification Thick-nesses of cast and spin-coated films were measured using a Dektak 3030 profilometer For each film, three thickness measurements were taken, and the average value was reported 1644 J Muter Chem , 1996, 6(lo), 1643-1647 Results and Discussion Characterizationof CdS nanocrystallites UV-VIS absorption spectra provide a semi-quantitative meas- ure of particle size, as well as the size distribution lo l2 The location of the excitonic peak identifies the average size of the nanocrystallites, and the shape of the curve (FWHM) indicates the width of the size distribution Fig 1 shows UV-VIS spectra for the suspension directly after reaction and for the super- natants of size-selective precipitations 1, 5 and 9 (Sl, S5 and S9, respectively) The spectrum for the suspension after reaction shows one main shoulder, attributed to the first excitonic transition (HOMO-LUMO transition), and a second shoulder attributable to higher order transitions The location of the absorption maximum for the spectra from the size fraction- ations (Sl-S9) was essentially constant Based on quantum mechanical calculations correlated with experimental evidence from transmission electron microscopy,1o the average size of the optically active core of the CdS nanocrystallites synthesized in this work was estimated to be 2 5 nm Fig 2 shows absorbance spectra for the reacted solution, the ninth size-selective precipitation and the nanocrystals redispersed in water The spectrum for CdS in water shows the re-emergence of the shoulder for higher excitonic transitions which may have been obscured owing to absorbance by DMF in the size-selected fractions Usually these higher transitions are not visible at room temperature or for polydisperse samples 'lo Our spectra suggest therefore that our samples are 14, 250 300 350 400 450 wave1ength/nm Fig.1 Evolution of CdS excitonic peak with size fractionation Suspension after reaction (R), after first size-selective precipitation (Sl), after fifth size-selective precipitation (SS) and after ninth size- selective precipitation (S9) 14 12 a10 gi06 04 02 0 250 300 350 400 450 wavelengthhm Fig.2 UV-VIS absorption of CdS nanocrystallites after reaction (-), after ninth size-selective precipitation (---) and after redispersion in water (--- ) monodisperse with a high degree of crystallinity for all CdS nanoparticles in the suspension. A further decrease in the FWHM from 40.1 nm for the final size-selected nanoparticles in DMF to 36.9nm for the absorption of CdS in H,O is evident in Fig. 2. Analysis of transmission electron micrographs revealed an average CdS nanocrystallite size of about 4 nm. This obser- vation is in accord with size estimates of the CdS core of the nanocrystallite based on optical absorption measurements plus the added size from the attached thioglycerate ligands at the surface of the nanoparticles.Analysis of electron diffraction patterns gives d spacings corresponding to a cubic crystal structure of CdS. This result is consistent with data in the literature showing that solution-precipitated CdS nanoparticles have a cubic structure,' while similar particles formed in reheated glasses have a hexagonal crystal structure;' nanopart- icles synthesized in situ in polymer matrices show both types of crystalline structures.* Solid-state NMR spectroscopic characterization Both the SP and CP MAS spectra of nanocrystalline CdS show a relatively broad central peak near 6 + 700 with spinning sidebands as shown in Fig.3. This chemical shift is very similar to that reported previously for bulk crystalline CdSi3,l4 and suggests that crystallite size does not greatly affect the average chemical shift. However, the peak widths (62 f5 ppm FWHM, for both SP and CP MAS) are considerably greater than for bulk CdS.'3,'4 The large MAS peak widths could arise from a distribution of chemical shifts, due to a range of structural environments or dispersion of Knight shifts (k,frequency shift arising from conduction-band electrons). Additional work is needed to confirm the source of the line-broadening, although spatial inhomogeneities in the conduction-band electron density might be expected from the small cluster sizes.Although we expect the CP MAS experiment to give a signal preferentially from the Cd atoms nearest the thioglycerol capping agents, we found only subtle differences between the CP MAS and SP (bulk) NMR spectra. The principal differences are the slightly more shielded peak position (cu. 10ppm) and broader spinning sideband envelope observed by CP MAS. Only a small chemical shift anisotropy has been reported for bulk CdS (54 ppm),I3 and the small spinning sideband envelope observed in the SP spectrum is consistent with this value. The I\ 1200 1000 800 600 400 200 6 Fig. 3 Solid-state '13Cd CP MAS (a) and single-pulse (b) spectra of CdS nanocrystals. (a) 2 ms contact time, 2 s recycle delay, 40000 transients; (b)5.5 ps pulses (90"),100 s recycle delay, 1600 acquisitions.larger envelope observed in the CP MAS spectrum implies that the Cd atoms nearest the thioglycerol capping moieties experience greater chemical shift or Knight shift anisotropy, which might be expected for Cd near the particle surface. Modifying suspension chemistry to reduce CdS-PS particle-particle interactions As the loading of CdS was increased relative to PS, the solution became noticeably turbid when the [PS]/[CdS] mass ratio was <30, with a solid precipitate eventually settling out after several hours. For mass ratios in the range lo< [PS]/[CdS] <30 the absorption peak was still present, but was reduced considerably, and the background absorption due to scattering was increased considerably (see Fig.4). It seems plausible that the thioglycerate groups used to cap the CdS nanoparticles destabilize the working suspension, and thereby cause flocculation. This flocculation may occur if the OH group on the thioglycerate is sufficiently acidic to pro- tonate a carboxylate group on the polystyrene. In addition, hydrogen bonding between surface thioglycerate groups on CdS and carboxylate groups on PS may cause enough CdS particles to adsorb to the PS particle surface to shield the surface charge and cause flocculation. If these scenarios are correct, then it is necessary to increase the number of dis- sociated carboxylate groups at the PS particle surface to ensure that the working suspension is stabilized at higher CdS concen- trations.This can be accomplished easily with the addition of base, such as NaOH. The amount of OH-required to ionize a certain percentage of surface carboxylate groups can be estimated from the surface charge density of the PS particles. The effects of NaOH addition on the absorption spectra of the working suspension are summarized in Fig. 5. Without the addition of base the absorption peak of the first excitonic transition for [PS]/[CdS] = 10 is barely visible above the background. With the addition of base, the turbidity was reduced considerably, and the absorption peak became visible. This method of colloidal film formation allows for the selection of specific film properties. Various sizes of optically active semiconductor material and polymer colloid may be chosen and stabilized in a wide range of concentrations.CdS quantum-dot concentrations increased by orders of magnitude over previous work have been achieved. Table 1 summarizes some ways to look at the CdS concentration in the present films compared to values for comparable systems found in the literature. The film from this work used in the comparison has [PS]/[CdS] = 10 on a mass/mass basis, and the precur- sor suspension was stabilized by 4 x mol dm-3 OH-. It should be noted that this concentration of CdS is not an upper limiting value, but was chosen here as the most concentrated in CdS. 1.8 1.6 1.4 0.4 0 1 250 300 350 400 450 wavelengthhm Fig. 4 Absorption spectra of suspensions with varying [PS]/[CdS] mass ratios.--, [PS]/[CdS] = 10; ---, [PS]/[CdS] =50. J. Muter. Chern., 1996, 6(lo), 1643-1647 1645 1.6 1.4 1.2 81 t ([I 0.8 sD ([I 0.6 0.4 0.2 4 0 300 350 400 450 wavelength/nm Fig.5 Effect of the addition of base to suspensions containing both thioglycerate-capped CdS nanocrystals and carboxylate-capped PS nanospheres. --, [PS]/[CdS] = 10,4 x mol dm-3 OH-.7 ---, [PS]/[CdS] = 10, water added. Table 1 CdS concentrations in the spin-coated films compared with similar systems reported previously spin-coated films previous results ref. mass% 9.67 1-7 15-17 vol% 9.61 0.092-1 4, 6, 8, 18 q-dots cm-3 4.28 x 1OI8 3 x 10l6 2 q-dots cm-3 2.04 x 1015 (5.1-5.9) x lOI3 19 Optical activity of CdS nanocrystallites in polystyrene films UV-VIS absorbance measurements on the dried cast films showed no characteristic excitonic absorbance.When these films were heated to 100°C, just above the glass-transition temperature for the PS nanospheres, the excitonic absorption peaks became visible. The spectra are shown in Fig. 6. In the absence of base, the absorption spectrum exhibits a broad plateau that begins near the absorption band. When base is added the absorption peak is revealed. UV-VIS absorption spectra of the spin-coated films were measured after deposition of each layer. As layers are built up, a characteristic excitonic peak becomes more pronounced. Note that there is no visible absorption peak after the first layer is coated, owing to the low quantity of optically active CdS nanocrystallites in a single coating.As additional layers 1.4 1, \ a,05 0.8 \ \ I42 \ 8 0.6a (d 0.4 0.2 I 250 300 350 400 450 wavelengthhm Fig. 6 UV-VIS absorption in cast films with and without charge stabilization after heating. --, [PS]/[CdS] = 10, 4 x mol dmP3 OH-(approx. 4.77 pm); ---, [PS]/[CdS] = 10, 0 mol dm-3 OH-(approx. 13.33 pm). 1646 J. Muter. Chern., 1996,6(lo), 1643-1647 0.05 0.04 a, 0.03 g2 0.02 (d 0.01 ol I 250 300 350 400 450 wavelengthhm Fig. 7 UV-VIS absorption in step-wise spin-coated PS/CdS composite film ([ PS]/[CdS] =20, 1000 rpm, approx. 142 nm). (a) after one coat- ing; (b)after five coatings. 0.006 -0.005 cnc.-t7 0.004 -:0.003 .-22 v)s 0.002 c..-c 0.00 1 0 400 500 600 700 800 wavelength/nm Fig. 8 Representative photoluminescence spectra for spin-coated PS/CdS composite films ([PS]/[CdS] =20) showing peak lumi-nescence at ca. 540 nm: (a) 200 rpm, (b)500 rpm, (c) 1000 rpm are deposited the absorption peak becomes evident, as shown in Fig. 7. Luminescence spectra for the spin-coated films are shown in Fig. 8. The spectra show a maximum intensity around 540 nm. Spectra from charge-stabilized suspensions show the narrowest emission linewidths. There was an order of magnitude difference in measured luminescence intensity for the spin-coated films, depending on the deposition protocol and on the amount of material laid down.The nature of the luminescence from spin-coated films made from charge-stabilized cosuspensions closely resembles 0.0007 , 0.0006 n.$ 0.0005 3$ 0.0004 v 0.0003 C Q,c1 .c 0.0002 0.0001 0 I-400 500 600 700 800 wavelength/nm Fig. 9 Detail of luminescence spectra for 500 and 1000 rpm films [(b) and (c) respectively from Fig. 81 showing the same emission linewidths that for cast films from charge-stabilized suspensions, indicat- ing retention of the narrow emission linewidth characteristics of a system which has uniformity of electro-optical properties in the quantum-confined CdS. Summary and Conclusions The viability of this processing scheme is exemplified by the ease with which film properties may be tailored, including CdS nanoparticle concentration and film height.The spin-coating of multiple layers presents a quick and reliable way to make durable, high-quality films in which the unagglomerated elec- tro-optical nature of the CdS nanoparticles is retained. Small chemical shift anisotropies observed in the CP MAS NMR spectra are consistent with the presence of hydroxy groups near surface groups on the CdS nanoparticles. These groups linked to the thioglycerol capping agents present a plausible explanation for the agglomerate-free nanoparticles synthesized in this work. These films have optical measurements which suggest that the CdS particles have not aggregated. It will be of interest to determine how the CdS nanoparticles are actually dispersed within the colloidal coating.The film thickness measurements for the 1000rpm film suggest that each layer is a single monolayer of particles. Previous work on PS particles by Steiner2' has shown that it is possible to sequester nanoparticles in the interstices of the two-dimensional close-packed monolayer template formed from colloidal PS. Electro-optical properties may be maximized by appropriate organization of semiconductor nanocrystallites into regular arrays. Our method also allows us to deposit multilayers of different PS particles with different CdS nanopar- ticles, and thereby to prepare stacked monolayers in which each layer has a different interstitial CdS nanoparticle size, allowing for the tuning of the optical properties of each layer.This work is based on Andre Chevreau's M. S. thesis and was supported by the MRSEC Program of the National Science Foundation under Award No. DMR-9400354. The NMR work was performed in laboratory facilities supported by the W. M. Keck Foundation. We are also grateful to Valerie Leppert (TEM work) and Christine Smith (luminescence work) for their assistance. References 1 A. Chemseddine and H. Weller, Ber. Bunsen-Ges. Phys. Chem., 1993,97,636. 2 K. Misawa, H. Yao, T. Hayashi and T. Kobayashi, J. Chem. Phys., 1991,94,4131. 3 V. Colvin, A. Goldstein and A. Alivisatos, J. Am. Chem. Soc., 1992, 114,5221. 4 Y. Wang, A. Suna, J. McHugh, E. Hilinski, P.Lucas and R. Johnson, J. Chem. Phys., 1990,92,6927. Y. Wang and W. Mahler, Opt. Commun., 1987,61,233. E. Hilinski, P. Lucas and Y. Wang, J. Chem. Phys., 1988,89,3435. L. C. Liu and S. Risbud, J. Appl. Phys., 1994,76,4576. Y. Wang, N. Herron, W. Mahler and A. Suna, J. Opt. SOC.Am. B, 1989, 6, 808. 9 C. B. Murray, D. J. Norris and M. G. Bawendi, J. Am. Chem. SOC., 1993,115,8706. 10 H. Weller, Angew. Chem., Int. Ed. Engl., 1993,32,41. 11 P. Lippens and M. Lannoo, Phys. Rev. B, 1989,39,10935. 12 H. Weller, H. Schmidt, U. Koch, A. Fojtik, S. Baral, A. Henglein, W. Kunath, K. Weiss and E. Dieman, Chem. Phys. Lett., 1986, 124, 557. 13 A. Nolle, Z. Naturforsch., Teil A, 1978,33,666. 14 P. DuBois Murphy and B. C. Gerstein, J. Am. Chem. Soc., 1981, 103,3282. 15 K. Choi and K. Shea, Chem. Muter., 1993,5,1067. 16 S. Yanagida, T. Enokida, A. Shindo, T. Shiragami, T. Ogata, T. Fukumi, T. Sakaguchi, H. Mori and T. Sakata, Chem. Lett., 1990,1773. 17 S. Yamazaki and Y. Kurokawa, Polym. Commun., 1991,32,524. 1s 0.Salata, P. Dobson, P. Hull and J. Hutchison, Thin Solid Films, 1994,251, 1. 19 R. Vogel, K. Pohl and H. Weller, Chem. Phys. Lett., 1990,174,241. 20 M. L. Steiner, PhD Thesis, University of California, Davis, 1996. Paper 6/03393H; Received 15th May, 1996 J. Mater. Chem., 1996, 6(lo), 1643-1647 1647

ISSN:0959-9428    

DOI:10.1039/JM9960601643

出版商:RSC    

年代:1996

数据来源: RSC