摘要:

THE ROYAL SOCIETY OF CHEMISTRY Journal of the Chemical Society Faraday Transactions Scientific Editor Dr. Peter J. Sarre Department of Chemistry University of Nottingham University Park Nottingham NG7 2RD, UK Faraday Editorial Board Prof. I.W. M. Smith (Birmingham) (Chairman) Prof. M. N. R. Ashfold (Bristol) Dr. B. E. Hayden (Southampton) Dr. D. C. Clary (Cambridge) Prof. A. R. Hillman (Leicester) Dr. L. R. Fisher (Bristol) Prof. J. Holzwarth (Berlin) Prof. H. M. Frey (Reading) Dr. P. J. Sarre (Nottingham) Dr. R. K. Thomas (Oxford) Editorial Manager and Secretary to Faraday Editorial Board Dr. Robert J. Parker The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF, UK Senior Assistant Editors: Mrs.S. Shah, Dr. R. A. Whitelock Assistant Editor: Mrs. C. J. Seeley Editorial Secretary: Mrs. J. E. Gibbs International Advisory Editorial Board R. S. Berry (Chicago) Y. Marcus (Jerusalem) A. M. Bradshaw (Berlin) B. J. Orr (North Ryde) A. Carrington (Southampton) R. H. Ottewill (Bristol) M. Che (Paris) R. Parsons (Southampton) M. S. Child (Oxford) S. L. Price (London) B. E. Conway (Ottawa) F. Rondelez (Paris) G. R. Fleming (Chicago) J. P. Simons (Oxford) R. Freeman (Cambridge) S. Stolte (Amsterdam) H. L. Friedman (Stony Brook) J. Troe (Gottingen) H. lnokuchi (Okazaki) J. Wolfe (Kensington, NSW) J. N. lsraelachvili (Santa Barbai ?a) C. Zannoni (Bologna) M. L. Klein (Philadelphia) A. Zecchina (Turin) R. A. Marcus (Pasadena) C. Zhang (Dalian) Journal of the Chemical Society, Faraday Transactions (ISSN 0956-5000) is published twice monthly by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK.All orders accompanied with payment should be sent directly to The Royal Society of Chemistry, Turpin Distribution Services Ltd., Black- horse Road, Letchworth, Herts. SG6 1 HN, UK. NB Turpin Distribution Services Ltd., dis- tributors, is wholly owned by the Royal Society of Chemistry. 1994 Annual subscription rate EC f744.00, Rest of World f800.00, USA $1400.00, Canada f840 (excl. GST). Customers should make payments by cheque in sterling payable on a UK clearing bank or in US dollars payable on a US clearing bank. Second class postage is paid at Rahway, NJ.Airfreight and mailing in the USA by Mercury Airfreight International Ltd. Inc., 2323 Randolph Avenue, Avenel, NJ 07001, USA and at additional mailing offices. USA Postmaster: send address changes to Journal of the Chemical Society, Faraday Trans- actions, c/o Mercury Airfreight International Ltd. Inc., 2323 Randolph Avenue, Avenel, NJ 07001. All despatches outside the UK by consolidated Airfreight. PRINTED IN THE UK. @ The Royal Society of Chemistry, 1994. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form, or by any means, electronic, mechanical, photographic, recording, or otherwise, without the prior permission of the publishers. Advertisement sales: tel.+44(0)71-287-3091 ; fax. +44(0)71-494-1134. INFORMATION FOR AUTHORS The Royal Society of Chemistry welcomes submission of manuscripts intended for pub- lication in two forms, Research papers and Faraday Communications. These should describe original work of high quality in the sciences lying between chemistry, physics and biology, and particularly in the areas of physical chemistry, biophysical chemistry and chemical physics. Research Papers Full papers contain original scientific work which has not been published previously. However, work which has appeared in print in a short farm such as a Faraday Communi- cation is normally acceptable. Four copies including a top copy with figures erc. should be sent to The Editor, Faraday Transactions, at the Editorial Office in Cambridge.Authors may, if they wish, suggest the names (with addresses) of up to three possible referees. Faraday Communications Faraday Communications contain novel scientific work in short form and of such importance that rapid publication is war-ranted. The total length is rigorously restricted to two pages of the double-column A4 format. For a Communication consisting entirely of text and ten references, with no figures, equations or tables, this cor- responds to approximately 1600 words plus an abstract of up to 40 words. Submission of a Faraday Communication can be made either to The Editor, Faraday Transactions, at the Editorial Office in Cam- bridge or via a member of the International Advisory Editorial Board, who will arrange for the manuscript to be reviewed. In the latter case, the top copy of the manuscript including any figures etc., together with the name of the person through whom the Com- munication is being submitted, should be sent simultaneously to the Editor at the Cambridge address.Proofs of Communications are not normally sent to authors unless this is specifically requested. Faraday Research Articles Faraday Research Articles are occasional invited articles which are published follow- ing review. They are designed to be topical articles of interest to a wide range of research scientists in the areas of Physical Chemistry, Biophysical Chemistry and Chemical Physics. Full details of the form of manuscripts for Articles and Faraday Communications, con-ditions for acceptance erc. are given in issue number one of Faraday Transactions, published in January of each year, or may be obtained from the Editorial Manager. There is no page charge for papers published in Faraday Transactions. Fifty reprints are supplied free of charge. Dr. P. J. Sarre, Scientific Editor. Tel.: Nottingham (0602) 51 3465 (24 hours) E-MaiI (JANET):PCZ PS F@ UK.AC. N 0TT.VAX Fax: (0602) 513466 Telex: 37346 UNINOT G Dr. R. J. Parker, Editorial Manager. Tel. : Cambridge (0223) 420066 E-Mail (INTERNET): RSCl @RSC.ORG (For access from JANET use RSCl %RSC.ORG@UK.AC.NSF NET- RELAY) Fax: (0223) 423623 or 420247 Telex: 81 8293 ROYAL G

ISSN:0956-5000    

DOI:10.1039/FT99490FX025

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

HAZARDS IN THE CHEMlCAL LABORATORY 5th Edition ‘. easy to read, an excellent reference text, and a worthwhile investment.’ Journal of the American Chemical Society reviewing the 4th Edition. The new edition of this essential laboratory handbook is the ‘key’ requirement for all research, development, production, analytical and teaching laboratories worldwide. The 5th Edition provides: New features include: 0 a quick guide to the hazardous properties of expanded ‘Yellow Pages’ section on 1339 substances (over 800 more than were hazardous substances, providing immediate covered in the previous edition) information on hazardous properties, details of the latest UK and EC regulations recommended control procedures and safety measuresan extremely useful emergency action check complete guide to labelling requirements to list -users can fill in their own key contacts for hospitals, fire etc.comply with EC directives and UK legislation, including the risk and safety phrases that must handy tables, symbols and statistics for ease appearof reference chapter on electrical hazards a description of the American scene, including index to ‘Yellow Pages’ section, with US legislation and safety practices -synonyms of compounds highlighting differences between the UWEC index to CAS Registry Numbers and USA PVC Protective Binding xx + 676 pages ISBN 0 85186 229 2 (1992) Price €45.00 If you have not yet ordered your copy of the NEW edition, do so now! Why take chances? Be informed and safe. kI To order, please contact: ROYAL Royal Society of Chemistry, Turpin Distribution Services Ltd, Blackhorse Road, Letchworth, InformationHerts SG6 lHN, United Kingdom. Services Telephone: +44 (0)462 672555 Fax: +44 (0)462 486947. 0956-5000(199417;1-1

ISSN:0956-5000    

DOI:10.1039/FT99490BX027

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

ISSN 0956-5000 JCFTEV(7) 935-1 054 (1 994) JOURNAL OF THE CHEMICAL SOCTETY Faraday Transactions Physical Chemistry & Chemical Physics CONTENTS 935 Appearance energies of small cluster ions and their fragments B. R. Cameron, C.G. Aitken and P. W. Harland 941 Protonated carbamic acid. Collisional activation and unimolecular dissociation of CH,NO; H. Egsgaard and L. Carlsen 947 Intramolecular photodimerization of 2-naphthoates : Successful application of hydrophobic forces in the preparation of large-ring compounds C-H. Tung, Y. Li and 2-Q.Yang 953 Photophysical properties of Merocyanine 540 derivatives A. C. Beaniston, A. Harriman and K. S. Gulliya 963 Triplet of cyclooctatetraene: Reactivity and properties T. N.Das and K. I. Priyadarsini 969 Formation and structure of Langmuir-Blodgett films of C,, and arachidic acid C. Ewins and B.Stewart 973 Statistical thermodynamics of hard spheres in a narrow cylindrical pore Q. Xin,I. Hiyane and P. Siders 979 Enzymatic reaction in water-in-oil microemulsions. Part 2.-Rate of hydrolysis in a hydrophobic substrate, 2-naphthyl acetate Y. Miyake, T.Owari, F. Ishiga and M. Teramoto 987 Enzyme catalysis at hydrogel-modified electrodes with soluble redox mediator F. Battaglini and E. J. Calvo 997 Solid-state hydrolysis of aspirin M. C. Ball 1003 Morphology and polymorphism in molecular crystals: Terephthalic acid R. J. Davey, S. J. Maginn, S. J. Andrews, S. N.Black, A. M. Buckley, D. Cottier, P. Dempsey, R. Plowman, J. E.Rout, D. R. Stanley and A. Taylor 101 1 Enhancement in the optical CO sensitivity of NiO film by the deposition of ultrafine gold particles M.Ando, T. Kobayashi and M. Haruta 1015 Adsorption and decomposition of methanol on TiO, ,SrTiO, and SrO N.Aas, T. J. Pringle and M. Bowker 1023 FTIR study of carbon monoxide adsorption on ceria: COi- carbonite dianion absorbed species C. Binet, A. Badri, M. Boutonnet-Kizfing and J-C. Lavalley 1029 FTIR study of adsorption and transformation of methanethiol and dimethyl sulfide on zirconia M. Ziolek, 0.Saur, J. Lamotte and J-C.Lavalley 1033 CoAPO molecular sieve acidity investigated by adsorption calorimetry and IR spectroscopy J. Janchen, M. P. J. Peeters, J. H.M. C. van Wolput, J. P. Wolthuizen, J. H. C. van Hooff and U.Lobse 1041 Comparison of thermal stability, acidity, catalytic properties and deactivation behaviour of novel aluminophosphate- based molecular sieves of type 36 D.B. Akolekar 1047 Brnrnsted acid sites in zeolites. FTIR study of molecular hydrogen as a probe for acidity testing M. A. Makarova, V. L. Zbolobenko, K. M. Al-Ghefaili, N.E. Thompson, J. Dewing and J. Dwyer Note: Where an asterisk appears against the name of one or more of the authors, it is included with the authors' approval to indicate that correspondence may be addressed to this person. COPIES OF CITED ARTICLES The Royal Society of Chemistry Library can usually supply copies of cited articles. For further details contact: The Library, Royal Society of Chemistry, Burlington House, Piccadilly, London W1V OBN, UK Tel: +44 (0)71-437 8656 Fax: +44 (0)71-287 9798 Telecom Gold 84: BUR210 Electronic Mailbox (Internet) LIBRARY @RSC.ORG. If the material is not available from the Society’s Library, the staff will be pleased to advise on its availability from other sources. Please note that copies are not available from the RSC at Thomas Graham House, Cambridge.

ISSN:0956-5000    

DOI:10.1039/FT99490FP067

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index 1994 Aas, N., 1015 Afanasiev, P., 193 Aikawa, M., 911 Aitken, C. G., 935 Akolekar, D. B., 1041 Aldaz,A., 609 Alfimov, M.V., 109 Al-Ghefaili, K. M., 383, Ali, V., 579, 583 Allegrini, P., 333 Allen, N. S., 83 A1 Rawi, J. M. A., Amorim da Costa, A. M., Amoskov, V. M., 889 Ando, M., 1011 Andrews, S. J., 1003 Aragno, A., 787 Aramaki, K., 321 Aravindakumar, C. T., 597 Asai,Y., 797 Avila, V., 69 Baba, T., 187 Badri, A., 1023 Ball, M. C., 997 Ball, S. M., 523 Barbaux, Y., 895 Barthomeuf, D., 667,675 Basini, L., 787 Bassoli, M., 363 Battaglini, F., 987 Bauer, C., 5 17 Bell, A. J., 17, 817 Bendig, J., 287 Bengtsson, L. A., 559 Benko, J., 855 Benniston, A. C., 953 Bensalem, A., 653 Bkrces, T., 41 1 Bergeret, G., 773 Bickelhaupt, F., 327 Biczok, L., 41 1 Binet, C., 1023 Black, S.N., 1003 Blackett, P. M., 845 Blandamer, M. J., 727 Blower, C., 919,931 Boggis, S. A., 17 Borisenko, V. N., I09 Boutonnet-Kizling, M., Bowker, M., 1015 Bozon-Verduraz, F., 653 Bradley, C. D., 239 Bradshaw, A. M., 403 Braun, B. M., 849 1047 845 689 1023 Cherqaoui, D., 97 Chesta, C. A., 69 Chevalier, S., 667, 675 Cho,T., 103 Christensen, P., 459 Climent, M. A., 609 Coates, J. H., 739 Cordischi, D., 207 Corma, A., 213 Cormier, G., 755 Corradini, F., 859 Corrales, T., 83 Cosa, J. J., 69 Cottier, D., 1003 Coudurier, G., 193 Courcot, D., 895 Crawford, M. J., 817 Cullis, P. M., 727 Curtis, J. M., 239 Dang, N-T., 875 Danil de Namor, A. F., Das, T. N., 963 Davey, R. J., 1003 Davidson, K., 879 Demeter, A., 41 1 Dempsey, P., 1003 Demri, D., 501 Derrick, P.J., 239 Dewing, J., 1047 Diagne, C., 501 Dickinson, E., 173 Doblhofer, K., 745 Domen, K., 911 Doughty, A., 541 Douglas, C. B., 471 Dwyer, J., 383, 1047 Dyke, J. M., 17 Eastoe, J., 487 Easton, C. J., 739 Ebitani, K., 377 Egsgaard, H., 941 El-Atawy, S., 879 Elisei, F., 279 Elliot, A. J., 831, 837 Engberts, J. B. F. N., Eustaquio-Rincon, R., 113 Ewins, C., 969 Fantola Lazzarini, A. L., Fausto, R., 689 Favaro, G., 279,333 Feliu, J. M., 609 Filimonov, I. N., 219,227 Fogden, A., 263 Fornes, V., 213 Franck, R., 667,675 Freeman, N. J., 751 845 727 423 Hallbrucker, A., 293 Halpern, A., 721 Hamnett, A., 459 Hancock, G., 523 Handa, H., 187 Hann,K., 733 Hao, L., 133 Harada, S., 869 Haraoka, T., 911 Harland, P. W., 935 Harper, R. J., 659 Harriman, A., 697,953 Harrison, N.J., 55 Haruta, M., 1011 Hashino, T., 899 Hattori, H., 803 Heal, M. R., 523 Heenan, R. K., 487 Helmer, M., 31, 395 Herein, D., 403 Herzog, B., 403 Higgins, S., 459 Hindermann, J-P., 501 Hirst, D. M., 517 Hiyane, I., 973 Hoekstra, D., 727 Holmberg, B., 559 Holz, M., 849 Hoshino, H., 479 Hosoi, K., 349 Hutchings, G. J., 203 Hutton, R. S., 345 Ikawa, S-i., 103 Ikonnikov, I. A., 219 Indovina, V., 207 Inoue, Y., 797,815 Ishiga, F., 979 Ishigure, K., 93,591 Isoda, T., 869 Ito, O., 571 Iwasaki, K., 121 Jakubov, T., 783 Jameel, A. T., 625 Janchen, J., 1033 Jayakumar, R., 161 Jenneskens, L. W., 327 Jennings, B. J., 55 Jiang, P-Y., 591 Jiang, P. Y., 93 Johansson, L. B.-dj., 305 Johari, G. P., 883 Joseph, E. M., 387 Joshi, P. N., 387 Kagawa, S., 349 Kaler, E. W., 471 Kalugin, 0.N., 297 Kato, R., 763 Leaist, D.G., 133 Lei,G-D., 233 Lerner, B. A., 233 Leslie, M., 641 Li, J., 39 Li, P., 605 Li, Y., 947 Lin, J., 355 Lincoln, S. F., 739 Lindblom, G., 305 Liu,C-W., 39 Liu,X., 249 Loginov, A. Yu., 219,227 Lohse, U., 1033 Longdon, P. J., 315 Lunelli, B., 137 Mabuchi, M., 899 Machado, V. G., 865 Mackie, J. C., 541 Maeda, T., 899 Maestre, A., 575 Maginn, S. J., 1003 Mahy, J. W. G., 327 Maity, D. K., 703 Makarova, M. A., 383, Maksymiuk, K., 745 Malatesta, V., 333 Malcolm, B. R., 493 Mallon, D., 83 Mandal, A. B., 161 Marcheselli, L., 859 Marchetti, A., 859 Mariani, M., 423 Martins, A., 143 Maruya, K-i., 91 1 Masetti, F., 333 Massucci, M., 445 MatijeviC, E., 167 Matsuda, J., 321 May, I. P., 751 Mazzucato, U., 333 Mchedlov-Petrossyan, N.O., Merga, G., 597 Meunier, F., 369 Mezyk, S. P., 831 Mittal, J. P., Miyake, Y., 979 Mohan, H., 597,703 Moriguichi, I., 349 Morikawa, A., 377 Morokuma, M., 377 Morrison, C. A,, 755 Muir, A. V. G., 459 Lu, J-X., 39 1047 629 597, 703, 711, 825 Owari, T., 979 Ozutsumi, K., 127 Padley, M. B., 203 Pal, H., 711 Palleschi, A., 435 Paradisi, C., 137 Pardo, A., 23 Parsons, B. J., 83 Pavanaja, U. B., 825 Pedulli, G. F., 137 Peeters, M. P. J., 1033 Peng, W., 605 Pepe,F., 905 Pereira, C. M., 143 Perez, J. M., 609 Perrichon, V., 773 Peter, L. M., 149 Petrov, N. Kh., 109 Pispisa, B., 435 Pivnenko, N. S., 297 Plane, J. M. C., Plowman, R., 1003 Porcar, I., 339 Potter, C. A. S., 59 Poyato, J. M. L., 23 Prenosil, J. E., 587 Previtali, C. M., 69 Pringle, T. J., 1015 Priyadarsini, K.I., 963 Pryamitsyn, V. A., 889 Rabold, A., 843 Rama Rao, K. V. S., Ramsden, J. J., 587 Rao, B. S. M., 597 Rehani, S. K., 583 Rettig, W., 59 Rey, F., 213 Rezende, M. C., 865 Rhodes, N. P., 809 Richter, R., 17 Rocha, M., 143 Rochester, C. H., 203 Rodes, A., 609 Rofia, S., 137 Rosenholm, J. B., 733 Rosmus, P., 517 Rossi, P. F., 363 Rout, J. E., 1003 Rudham, R., 809 Ryde,N., 167 Sacco,A., 849 Sachtler, W. M. H., 233 Saitoh, T., 479 Salmon, G. A., 75 Sano,T.. 869 Sapre, A. V., 825 Sarre, P. J., 51 7 31, 395 825 Breysse, M., 193 Briggs,B., 727 Brocklehurst, B., 271 Brown, R. G., 59 Brown, S. E., 739 Bruna, P. J., 683 Frety, R., 773 Frey, J. G., 17, 817 Frostemark, F., 559 Gans, P., 315 Gao,Y., 803 Garcia, R., 339 Katsumura, Y., 93, 591 Kaur, T., 579 Kawashima, T., 127 Keil, M., 403 Kemball, C., 659 Kida, I., 103 Mukherjee, T., 711 Nagaishi, R., 93, 591 Nagaoka, H., 349 Naito, S., 899 Naito, T., 763 Navaratnam, S., 83 Sato, K., 797 Saur, O., 1029 Sbriziolo, C., 311 Schedel-Niedrig, Th., Schlogl, R., 403 Schnabel, W., 287 403 Brzezinski, B., 843 Garcia Baonza, V., 553 Kiennemann, A., 501 Neoh, K. G., 355 Scremin, M., 865 Buckley, A.M., 1003 Butt, M. D., 727 Byatt-Smith, J. G., 493 Garcia-Paiieda, E., Gautam, P., 697 Geantet, C., 193 575 Kim, J-H., 377 King, F., 203 Kirschner, J., 403 Nerukh, D. A., 297 Nicholson, D., 181 Nickel, U., 617 Seddon, B. J., 605 Shahid, G., 507, 513 Sharma,A., 625 Cabaleiro, M. C., 845 Caceres Alonso, M., 553 Gengembre, L., Gil, F. P. S. C., 895 689 Kita,H., 803 Klein, M.L., 253 Ninomiya, J., Nishihara, H., 103 321 Shaw, N., 17,817 Sheil, M. M., 239 Calado, J. C. G., 649 Caldararu, H., 213 Calvente, J. J., 575 Calvo, E. J., 987 Camacho, J. J., 23 Cameron, B. R., 935 Gill, D. S., 579, 583 Gill, J. B., 315 Goede, S. J., 327 Gomez, C. M., 339 GonGalves da Silva, A. M., 649 Kleshchevnikova, V. N., Kobayashi, A., 763 Kobayashi, H., 763 Kobayashi, T., 1011 Kondo, Y., 121 629 Nogami, T., 763 Nonaka, O., 121 Nuiiez Delgado, J., 553 Nyholm, L., 149 Occhiuzzi, M., 207,905 Ohtsu, K., 127 Sheppard, N., 507,513 Shiao, J-C., 429 Shihara, Y., 549 Shiralkar, V. P., 387 Shishido, T., 803 Shizuka, H., 533 Campa, M. C., 207 Campos, A., 339 Capobianco, J. A., 755 Caragheorgheopol, A., 213 Carlsen, L., 941 Carvill, B.T., 233 Catalina, F., 83 Cavasino, F.P., 3 11 Chen, J-S., 429, 717 Gray, P. G., 369 Green, W. A., 83 Grein, F., 683 Grimshaw, J., 75 Grzybowska, B., 895 Guelton, M., 895 Gulliya, K. S., 953 Hachey, M., 683 Haeberlein, M., 263 Kossanyi, J., 41 1 Kurrat, R., 587 Kuwamoto, T., 121 Laachir, A., 773 Lambert, J-F., 667,675 Langan, J. R., 75 Lavalley, J-C., 1023, Lamotte, J., 1029 1029 Okamura, A., 803 Oliveri, G., 363 Onishi, T., 91 1 Ono,Y., 187 Oradd, G., 305 Ortica, F., 279 Ota, K-i., 155 Otlejkina, E. G., 297 Otsuka, K., 451 Siders, P., 973 Silva, C. J., 143 Silva, F., 143 Simkiss, K., 641 Singh, J., 579, 583 Singh, R., 583 Smith, T. D., 919,931 Soares, V. A. M., 649 Soria, V., 339 Chen, Y-H., 61 7 Cheng, A., 253 Hall, D. I., 517 Hall, G., 1 Lavanchy, A., Lazzarini, E., 783 423 Ottavi, G., 333 Ouellette, D.C., 837 Spiro, M., 617 Stanley, D. R., 1003 i Stewart, B., 969 Stoeckli, F., 783 Sun, L. M., 369 Teo, W. K., 355 Teramoto, M., 979 Teraoka, Y., 349 Upadhyaya, H. P., 825 Valat, P., 41 1 Valls, M. J., 609 Weingartner, H., 849 Weir, D. J., 751 Werner, H., 403 Yamasaki, M., 869 Yanes, C., 575 Yang, Z-Q., 941 Suquet, H., 667,675 Surov, Y. N., 297 Suzuki, T., 549 Tabrizchi, M., 17 Taghazucchi, M., 859 Takagi, T., 121 Takahashi, K., 155 Takasawa, A., 91 1 Tamura, K-i., 533 Tanaka, I., 349 Thompson, N. E., 1047 Timms, A. W., 83 Timney, J. A., 459 Tosi, G., 859 Touret, O., 773 Tournayan, L., 773 Trejo, A., 113 Tsuji, H., 803 Tsunashima, S., 549 Tun& C-H., 947 van Hooff,J. H. C., van Wolput, J. H.M. C., Vedrine, J. C., 193 Venanzi, M., 435 Villamagna, F., 47 Villemin, D., 97 Vlietstra, E. J., 327 Vollarovh, O., 855 Vollmer, F., 59 1033 1033 Whitaker, B. J., 1 Whitehead, M. A., 47 Wikander, G., 305 Williams, D. E., 345 Wilpert, A., 287 Wintgens, V., 41 1 Woermann, D., 875 Wohlers, M., 403 Wolthuizen, J. P., 1033 Wormald, C. J., 445 Yano,H., 869 Yoshitake, H., 155 Yotsuyanagi, T., 93,479 Young, R. N., 271 Zanotto, S.P., 865 Zhang, X., 605 Zholobenko, V. L., 233, Zhong, G. M., 369 Ziolek, M., 1029 1047 Tassi, L., 859 Turco Liveri, M. L., 3 11 Vyunnik, I. N., 297 Xin, Q., 973 Zubarev, V. E., 721 Tateno, A., 763 Turco Liveri, V., 3 11 Wang, C. F., 605 Yagci, Y., 287 Zundel, G., 843 Taylor, A., 1003 Taylor, M. G., 641 Teixeira-Dias,J. J. C., 689 Udagawa, T., 763 Umemoto, H., 549 Unayama, S-i., 549 Watanabe, H., 571 Waters, M., 727 Weckstrom, K., 733 Yamaji, M., 533 Yamamoto, M., 899 Yamanaka, I., 451 11 ~~ FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division Annual Congress: The Reactive Interface in Electrochemistry and Catalysis To be held at the University of Liverpool on 12-15 April 1994 Further information from Dr J.F. Gibson, The Royal Society of Chemistry, Burlington House, Piccadilly, London W1V OBN Neutron Scattering Group Neutron Scattering Data Analysis To be held at the Rutherford Appleton Laboratory on 13-15 April 1994 Further information from Mrs S. Humphreys, The Rutherford Appleton Laboratory, Chilton, Didcot 0x11 ORA Colloid and Inte~ace Science Group Theoretical Modelling and Simulation in Colloid and Interface Science To be held at the University of Bristol on 18-20 April 1994 Further information from Dr R.Buscall, ICI Corporate Science Group, PO Box 11, The Heath, Runcorn WA7 4QE ~~ ~~ Division Autumn Meeting: Reactions and Mechanisms for Fine Chemicals To be held at the University of Glasgow on 6-9 September 1994 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN Gas Kinetics Group 13th International Symposium on Gas Kinetics To be held at University College, Dublin on 11-15 September 1994 Further information from Dr H. Sidebottom, Department of Chemistry, University College, Dublin Electrochemistry Group with the SCI ELECTROCHEM 94 To be held in Edinburgh on 12-16 September 1994 Further information from Professor D.E. Williams, Department of Chemistry, University College London, 20 Gordon Street, London WClH OAJ iii THE ROYAL SOCIETY OF CHEMISTRY, FAFUDAY DIVISION, GENERAL DISCUSSION 98 Polymers at Surfaces and Interfaces University of Bristol, 12-14 September 1994 Organising Committee: Professor Sir Sam Edwards (Chairman) Dr R. Buscall Professor R. H. Ottewill Dr T. Cosgrove Professor J. S. Higgins Dr R. W. Richards Dr R. A. L. Jones New experimental methods and new theoretical and computational techniques have recently led to great progress in understanding the difficult but technologically important problems associated with the conformation of polymer molecules at surfaces and interfaces.The purpose of this Discussion is to bring together experimentalists and theoreticians working towards a molecular understanding of polymers at surfaces and interactions to survey the progress in the area to date and to indicate future directions of research. The meeting will attempt to bring a unified approach to the problem, encompassing problems of the structure of surfaces and interfaces in polymer melts, the conformation of polymers at solid/liquid and liquidliquid interfaces, and extensions towards more complicated biological systems. The preliminary programme may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, Piccadilly, London W 1V OBN.THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 99 Vibrational Optical Activity: from Fundamentals to Biological Applications University of Glasgow, 19-21 December 1994 Organising Committee Professor L. D. Barron (Chairman) Dr A. F. Drake Dr D. L. Andrews Professor R. E. Hester Professor A. D. Buckingham Traditional optical activity measurements such as CD are confined to the visible and near-ultraviolet spectral regions where they provide stereochemical information on chiral molecules via polarized electronic transitions. Thanks to prompting from theory and new developments in instrumentation, optical measurements are now being made in the vibrational spectrum using both infrared and Raman methods. Studies over the past decade on a large range of chiral molecules, from small organics to biological macromolecules, have demonstrated that vibrational optical activity opens up a whole new world of fundamental studies and practical applications undreamt of in the realm of conventional electronic optical activity. The meeting seeks to bring together experimentalists and theoreticians to discuss the current and future experimental possibilities and the development of theories, including ab initio computational methods, which can relate the observations to stereochemical details. The increasing importance now being attached to molecular chirality and solution conformation in the life sciences should also encourage the partipation of biomolecular scientists. The preliminary programme may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, London W1V OBH. iv

ISSN:0956-5000    

DOI:10.1039/FT99490BP069

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(7), 935-939 Appearance Energies of Small Cluster Ions and their Fragments Brett R. Cameron, Craig G. Aitken and Peter W. Harland" Chemistry Department , University of Canterb ury , Chris tch urch ,Ne w Zealand Appearance energies of the cluster ions (CO,); (2 d n d 4), (N2,0); (2 < n < 4) and (NH,),,NH: (0 <n < 7), and the cluster ion fragments (N20 * 0)' and (N20. NO)' have been determined by electron impact ionization of neutral clusters formed in a supersonic molecular beam. Results obtained for (CO,); (2 <n d 4), (N20),+ (2 < n < 4), (N,O NO)+ and (NH,),NH,+ (0 < n d 2) are in general agreement with previously reported appear- a ance energies for these species, while the appearance energies of (N20* 0)' and (NH,),NH; (3 < n < 7) have been measured for the first time.Binding energies deduced from appearance energy measurements for the (CO,);, (CO,); and (N,O); cluster ions are observed to be in accord with results obtained using ion-molecule equilibrium methods. Possible mechanisms for the formation of the cluster fragment ions (N20-O)+ and (N20-NO)+ are discussed. Atomic and molecular clusters have been the subject of extensive experimental and theoretical research activity for more than 30 years. The fundamental aim of this research effort has been to gain an improved understanding of the evolution from the atomic or molecular properties of a system to its bulk phase properties with increasing cluster size. For example, atomic and molecular clusters represent small, isolated systems which may be used to test and further our understanding of amorphous solids, catalysis, liquid structure and solvation effects.Despite the intensive research activity involved, however, the characterization of cluster species is still in its early stages. Considerably more particle- specific information will be required in order to develop a complete understanding of the mechanisms of cluster forma- tion and elucidate the factors governing the structure and sta- bility of such species. The use of supersonic molecular beams to generate clusters which are rotationally and vibrationally cooled provides a suitable environment for obtaining this information. The essentially collision-free environment of the molecular beam provides the opportunity to investigate the clusters formed in the supersonic expansion in the absence of any further aggregation and problems relating to particle specificity may be largely resolved through the application of ionization and mass-filtering techniques. In this paper we report an investigation of the appearance potentials for the cluster ions (CO,); (2 < n G 4), (N,O),f (2 < n < 4) and (NH3),NH,' (0 < n < 7), and the cluster ion fragments (N20 * 0)' and (N,O. NO)'.The chemistry of CO, cluster ions is of considerable interest in ionospheric studies of the predominantly CO, atmospheres of Mars and Venus,' while ammonia clusters are of interest with regard to the energetics of gas-phase proton solvation.Accurate appearance energies are required for the determination of binding energies and enthalpy changes associated with various steps in cluster formation. A knowledge of cluster-ion appearance energies may also be used to obtain information on rearrangement processes and internal cluster ion-molecule reactions which may follow from electron impact or photo- ionization of neutral clusters. Appearance energies for (CO,); (2 < n < 4), (N,O)J (2 < n G 8), (N,O.NO)+ and (NH3),NH,' (0 < n < 2) have been previously while those determined for (N,O -0)' and (NH3),NH,' (3 < n < 7) represent new results. Experimental The gas mixture under study was expanded from a high-pressure stagnation reservoir through a commercial electro- magnetic pulsed valve (General Valve Corporation, model 9-181) into the first of two differentially pumped vaccum chambers.The valve was modified by the inclusion of a small stagnation volume between the 0.8 mm orifice in the valve and a 50 pm shaped orifice in the exit plate. This gave higher cluster densities than obtained from a valve fitted with a 50 pm orifice. The central core of the pulsed supersonic expan- sion was sampled by a 1.0 mm skimmer (Beam Dynamics) located cu. 300 nozzle diameters (15 mm) from the nozzle exit. The skimmed supersonic beam was allowed to enter the ion source of a Vacuum Generators SXP300 quadrupole mass filter located 10 cm downstream from the skimmer assembly in the second differentially pumped chamber.The electron energy distribution was estimated to be cu. 0.85 eV full width at half maximum (FWHM). Output pulses from the channel- tron electron multiplier were passed through a high-Q 2 MHz notch filter to eliminate rf pick-up from the quadrupole driver circuitry and amplified with a fast preamplifier fol- lowed by an amplifier and pulse amplitude discriminator combination. The signal-to-noise ratio of the beam signal was optimised using a simple gating arrangement. The TTL output pulses from the pulse-counting preamplifier were split and fed into two and-gates. Using a pulse generator and a pulse delay unit, two 5 V gates of identical width were independently delayed with respect to the nozzle trigger pulse in order to correspond with different regions of the signal pulse envelope.The first window was positioned over the ion arrival time distribution resulting from the pulsed supersonic beam in order to sample signal plus background, while the second was positioned somewhat later in time, sampling only back- ground signal. Output pulses from the two and-gates were counted through a counter-timer. Computer control of elec-tron energy and mass selection was implemented using custom-built 12-bit digit al-to-analogue converters incorpor- ated into the mass spectrometer control unit. The pulsed nozzle was generally operated at a frequency of 10 Hz, with an open time of not more than 2 ms. Background pressures of and Torr were maintained in the expansion chamber and the mass spectrometer chamber during normal operation of the nozzle.The temperature of the nozzle was monitored using a thermocouple attached to the body of the valve. The potential difference across the thermocouple was calibrated and amplified using a simple fixed-gain circuit, the output of which was supplied to one of 16 14-bit analogue-to-digital conversion channels monitored. The reservoir pressure was monitored using an MKS Baratron (loo00 Torr) connected to an MKS type 286 con-troller. Experiments were carried out automatically by scan- ning from low to high and from high to low electron energy for increments of 0.04 or 0.08 eV. Reproducibility from week to week was excellent and the ionization efficiency curves were automatically analysed using a linear least-squares pro- cedure to locate and tabulate the threshold and any breaks in the curves.All of the ionization efficiency curves reported were caljbrated against argon and the molecular ion recorded simultaneously. Results Neutral CO,, N20 and NH, clusters were produced by expanding gas mixtures containing 100 Torr of argon and 500 Torr of CO,, N,O or NH, made up to a total pressure of ca. 4OOO Torr with helium at a reservoir temperature of 295 K. These mixtures were found to produce supersonic beams of sufficiently high cluster content for reliable determi- nation of appearance energies for the cluster ions (CO,); (2 < n < 4), N20i (2 < n d 4) and (NH,),NHZ (0 d n d 7).For the N,O mixture it was also possible to determine the appearance potentials for the cluster ion fragments (N,O. 0)' and (N,O. NO)'. While monomer and cluster speed distributions were not measured, we would expect the parallel translational temperature of the cluster beams to be close to the value of CQ. 6 K measured previously for a pure helium beam under the same source condition^.^ The extent of rotational cooling which occurs during the supersonic expansion of these gas mixtures is unclear and will depend on the efficiency of rotational to translational energy transfer in collisions between the molecules and clusters and the rare-gas atoms. We tentatively suggest that the terminal rotational temperatures of the molecules and clusters will be less than 40 K in all cases. No clusters containing helium or argon atoms were observed for any of the gas mixtures examined.Ion counts were measured at up to 100 points with a typical counting period of 5 s at each electron energy repeat- ed some 15 to 20 times to obtain an average count with an acceptable standard error. Average ion counts were also recorded at an electron energy of 70 eV at the beginning and end of each run. Depending upon the number of ions exam- ined, a run could take up to 3 or 4 h to complete and it was therefore necessary to consider any potential sources of long- term experimental instability that might adversely affect the accuracy of the measurements. In particular, it is known that the cluster content of supersonic molecular beams is highly sensitive to variations in source pressure and .temperature," and these variables were carefully monitored throughout each run.Owing to the small flow of gas through the 50 pm nozzle, the reservoir pressure was observed to drop by not more than 2 or 3% over a 4 h period of continuous operation and no change in nozzle temperature was detected. The semi-log plot has been used in this study to determine the cluster-ion appearance potentials with an estimated accuracy of kO.1 eV in all cases. This error limit includes both statistical and systematic errors. Although nominally less accurate than ideal photoionization measure- ments, reproducibility is excellent and certainly good enough to allow critical comparisons to be made between our results and those of previous experimental and theoretical investiga- tions.The appearance potential of Ar' used as the primary electron energy scale calibrant for all of the appearance potential measurements was taken to be 15.76 +_ 0.01 eV.13 C02 Clusters Illustrative examples of ionization efficiency curves measured for (CO,);, (CO,); and (CO,): are shown in Fig. 1, where every second point has been omitted for clarity. Calibration of the electron energy scale was achieved through a concur- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 160 140 7v) l2OI100 v)c.5 80-8-$ 60-.-0,-v) 40 -t I *O t L 1 I 1 ,I 11.0 11.5 12.0 12.5 13.0 13.5 14.0 14.5 15. uncorrected electron energy/eV Fig.1 Measured ionization efficiency curves for (0)(CO,);, (0) (CO,): and (A) (CO,):. Every second experimental point has been omitted for clarity. rent measurement of the ionization energy for Ar'. The cor- rected values of the (CO,);, (CO,); and (CO,); appearance energies and the calculated binding energies are shown in Table 1 with literature values for comparison. N20Clusters Ionization eficiency curves for (N20)i (2 < n < 4) and the cluster ion fragments (N200)' and (N,O * NO)' are illus- * trated in Fig. 2 and 3. The appearance energies determined for these species are summarized in Table 2. The value of 12.3 eV determined for the appearance energy of (N20);is in Table 1 Appearance energies (Eapp)and binding energies (E,,) for (CO,): (2 G n G 4) EdeV for (C0,):-CO, ion E.ppleV this work literature (CO,): 13.1 f0.1 0.73 0.675a*b 0.564"*'(W,= 12.8 k 0.1 0.36 0.32b (CO,), 12.6 k 0.1 0.26 0.22d These values have been corrected to 0 K by Linn and Ng4 and are therefore lower than the values stated by the authors.Ref. 14. Ref. 1. Ref. 15. Table 2 Appearance energies (Eapp) and binding energies (EJ for N,O clusters EdeV ion E,JeV this work literature (N,O * O)+ 14.6 f 0.1 (N,O * NO)+ 14.3 f 0.1 17.0 f0.2 (N2O): 12.3 & 0.1 0.6 1 0.56" 0.57'*' w201: 12.1 f 0.1 0.22 (N,O),f 12.0 f0.1 0.12 a Ref. 4. Ref. 18. Corresponds to a temperature of 481 K and may not be directly comparable. J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 C 0300, 0L c 250 3 C I s s s-_--_-0 -_ _-44f -10 11 12 13 14 15 16 17 18 uncorrected electron energy/eV Fig. 2 Measured ionization efficiency curves for (0)(N,O):, (0)(N,P); and (A) (N20)f.Every second experimental point has been omitted for clarity. excellent agreement with that of 12.35 f0.02 eV reported by Linn and Ng4 and the value of 12.394 & 0.015 eV reported by Kamke et d6It can be seen from Fig. 4 that the shape of the ionization eficiency curve measured for the (N,O * NO)+ cluster ion fragment is significantly different to the shape of the curves obtained for any of the other ions illustrated in Fig. 1-5. Apart from the ionization threshold at 14.3 eV, there is a sharp change of slope at ca.17.0 eV, suggesting the presence of a second threshold for the formation of this ion. The shape of this curve was found to be totally reproducible. The (N,O*NO)+ion was also observed by Linn and Ng,4 who estimated an appearance energy of 14.01 eV, in reason- able accord with the lowest-energy threshold of 14.3 eV determined in the present study. A second threshold at ca. 17.2 eV is also apparent on the (N,O *NO)+photoionization efficiency curve recorded by Linn and Ng. Although they attempted no interpretation of this, it lends support to our observation of a reproducible, higher-energy threshold. The appearance energy for the cluster fragmentation product 50 -. 3. . 0 -z--O. 0 c--.C C0 3 0. 1 , -12 13 14 15 16 17 18 19 20 uncorrected electron energy/eV Fig. 3 Measured ionization efficiency curves for (0)(N,O.O)+ and (0)(N,O.NO)+, with Ar' (0)as reference. Every second experimental point has been omitted for clarity. 4500 'I , I,, I 3°1 3 OJ 0-4000 0: 0 3500 31 0 0 -3 15001 10001 L 5001 c , /,I 1 7 8 9 10 11 12 13 14 15 16 17 uncorrected electron energy/eV Fig. 4 Measured ionization efficiency curves for (0)NHf, (0) (NH,)NH:, (A) (NH,),NHf and (0)(NH,),NHf. Every second experimental point has been omitted for clarity. (N,O -0)' has not been previously reported. In view of the agreement observed between the appearance energies of (N,O)l and (N,O.NO)+ determined in the present study and those obtained by Linn and Ng4 using photoionization, we might expect the appearance energies of 12.1, 12.0 and 14.6 eV obtained for (N,O)l, (N,O)Z and (N,O.O)+, respectively, to be equally reliable.The appearance energies for (N,O): and (N,O),' reported in the photoionization study of Kamke et aL6 are 12.29 & 0.02 and 12.26 f0.04 eV, respectively, or ca. 0.2 eV higher than the electron impact threshold reported here and listed in Table 2. Although, in principle, photoionization should yield more accurate thresholds with lower uncer-tainty, experimental photoionization data do not often measure up to these expectations. The appearance energies o01 12400 01 0-3-2200 0 2000c 0 0 1800tc C 11 7* 16001 S 140Ob 0 12001 _-..-0 --.g.1000~ CI, .ii 8001 t 600 C 7 8 9 10 11 12 13 14 15 16 uncorrected electron energy/eV Fig. 5 Measured ionization efficiency curves for (0)(NH,),NHf, (0)(NH,),NHf and (A) (NH,),NHf. Every second experimental point has been omitted for clarity. The ionization efficiency curve for (NH,),NHf would be superimposed on that for (NH,),NHf and has been omitted from the figure. The higher signal level for (NH,),NHf over its neighbours, (NH,),NHf (Fig. 5) and (NH,),NHf, reflects the higher stability of this cluster. for the (N,O); (1 < n < 8) cluster ions are reported by Kamke et d6with experimental uncertainties from f0.015 eV for n = 2 to kO.04 eV for n = 8.However, inspection of the experimental data shown in Fig. 1 of ref. 6 for the (N,O)T ions shows little correspondence between the reported values and the thresholds anticipated from the data. The threshold regions are smeared, noisy and the shape of the curves varies considerably from n = 1 to n = 8. The reported thresholds and uncertainties are the result of an empirical multi- parameter fitting procedure, which cannot gurantee a unique solution. So, despite photoionization thresholds quoted to two or three decimal places with uncertainties in the meV range, some consideration must be given to the data treat- ment. Electron impact ionization thresholds do return reli- able values within the stated uncertainty, although it must be acknowledged that recoil energy and internal excitation in the ionization process are folded into the absolute values measured by either technique.Using the appearance energy value of 12.3 eV measured for (N,O)l with the ionization energy of 12.886 f0.002 eV for N2016 and the estimated intermolecular binding energy of 0.02 eV for the neutral dimer,17 the bond dissociation energy of (N20); has been calculated to be 0.61 eV, in good agree- ment with the value of 0.56 eV reported by Linn and Ng4 and the value of 0.57 eV determined by Illies" using ion- molecule methods. Note, however, that this latter value relates to a measurement of AH5 for the association reaction of N20 and N20' at 481 K and therefore may not be directly comparable. The (N20)2+ binding energy calculated using the photoionization data reported by Kamke et d6 would be 0.512 eV, which seems a little low.Assuming the same binding energy of 0.02 eV for (N,O), and (N20)4, we calculate bond dissociation energies of 0.22 and 0.12 eV for (N,O); N,O and (N20)3+N,O, respectively, compared with values of 0.124 and 0.05 eV calculated using the photo- ionization thresholds reported by Kamke et aL6 The appearance energy of 14.6 eV determined for the (N20.0)' cluster ion fragment is observed to be 2.3 eV greater than that of (N,O)l, close to the difference of 2.4 eV between the appearance energy of N20+ from N,O (12.886 ev) and the appearance energy of 0' from N20 (15.29 eV).I6 This observation may be rationalized by writing the structure of (N,O 0)' as 0' * N20 and to postulate that the forma- tion of this species involves the ionization and fragmentation of one of the N20 monomer units in (N,O), without any significant perturbation of the accompanying cluster mol- ecule.Note that the same argument would also apply to the species (CO* CO,)' and (NH, * NH,)+ observed by Stephan et An alternative mechanism proposed by them for the formation of these cluster fragment ions involved an internal ion-molecule reaction. If the ion within the (N,O), cluster is initially formed in some electronically excited state N20+*, then the (N,O.O)+ ion may be produced by the following sequence of reactions: (N,O), + e-+N,O.N,O+* + 2e-+N,O; + N, (1) The N,O; species produced in this manner would be expected to have an appearance energy greater than that of (N20); by at least the additional energy required for the electronic excitation of the parent ion.Irrespective of which- ever mechanism applies, we can use the measured appearance potential of 14.6 eV for (N,O-O)', the thermochemical threshold of 15.29 eV for the formation of 0' from N,016 and the binding energy of 0.02 eV for (N2O)i7 to estimate a lower bound of 0.71 eV for the bond dissociation energy of (N,O * 0)'. The lowest energy threshold observed for the formation of (N20* N0)'corresponds to an appearance energy of 14.3 eV, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 while the break at higher electron energy equates to an ion- ization threshold of ca.17.0 eV. Note that the differences of 2.0 and 4.7 eV observed between these thresholds and the appearance energy of 12.3 eV obtained for (N20)2f corre- spond very well to the differences of 2.124 and 4.854 eV between the ionization energy of 12.886 eV for N,O' and the thermochemical thresholds of 14.19 and 17.76 eV for the fragmentation processes N,O + e--+NO+(X'E') + N(4S)+ 2e- (2) and N,O + e-+NO+(X 'E') + N(,P) + 2e- (3) respectively.' Applying the same argument used for (N,O.O)', it seems reasonable to write the structure of (N,O NO)' as NO+ -N,O and to describe the formation of this species as involving the ionization and fragmentation of one of the N,O molecules in the neutral dimer without any significant perturbation of the other.Realistically, the mol- ecules making up a cluster must exert an influence on one another. This might well be expected to include a lowering of the ionization energy with increase in cluster size, as observed in this and other studies. This adds support to the mechanism of cluster ionization described above. Linn and Ng4 observed that the photoionization efficiency curve they measured for (N20* NO)' had essentially the same profile as that of NO' produced from the fragmentation of N20+. Such an observa- tion indicates that the fragmentations of N20+ and (N,O)l to form NO' and (N,O -NO)', respectively, follow similar reaction pathways, further supporting the notion that ioniza- tion of the neutral N,O dimer occurs on a single monomer unit to form N,O'.N,O.The neutral monomer in N20' -N,O then acts simply as a spectator in the fragmen- tation process leading to the formation of (N,O. NO)+. Note that N20' may undergo another fragmentation process leading to the formation of NO'(XIZ') and N(,D). The thermochemical threshold for the formation of NO+ by this process would be 16.57 eV,16 suggesting that another break in the ionization efficiency curve of (N,O.NO)' may be expected between 14.3 and 17.0 eV. We were unable to detect this break, although there was some evidence for such a feature on the photoionization efficiency curve measured by Linn and Ng.4 Our failure to observe this feature may be attributed to the low-energy resolution of the instrument employed for the present study.The fragmentation of N20+-N20 may then be viewed as a set of energy-dependent unimolecular cluster ion dissociation reactions as shown in eqn. (4), analogous to eqn. (2) and (3). -14.6 eV + (N,O)O' + N, + 2e-(N,O)N,O + e--14.3 eV -+ (N,O)NO+ + N(4S) + 2e--16.6 eV + (N,O)NO' + N(,D) + 2e--17.0 eV + (N,O)NO' + N(,P) + 2e-(4) NH,Clusters Ionization efficiency curves for the ammonia clusters are shown in Fig. 4 and 5 and appearance energies are listed in Table 3 for (NH,),NH: (0 < n < 7). Discrepancies between binding energies deduced from ion-molecule equilibria and from cluster-ion appearance energies suggest that electron impact and photoionization fail to yield the true adiabatic ionization energies of these weakly bound species.2- Disso- ciation energies of the (NH,),NHd ions deduced from appearance energy measurements were found to be in poor J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Appearance energies (Eapp)for (NH,),NHf (0 < n < 7) ion electron impacta photoionizationb 9.7 9.59 i-0.02 9.2 9.15 f 0.04 9.0 9.03 k 0.04 8.9 - 8.85 - 8.81 - 8.78 - 8.72 - ’’ Reproducibility fO.l eV or better (this work). Ref. 8. accord with those obtained by ion-molecule methods, indica- tive of this failure of electron impact ionization measurements to sample the true adiabatic ionization thresholds for the ammonia cluster ions. Conclusion Klots and Compton’ suggested that the equilibrium geometry of van der Waals cluster ions produced by electron impact or photoionization may be considerably different from the equilibrium geometry of the neutral precursor. In such situations the Franck-Condon factors near the true adiabatic ionization threshold may be so small that the observation of the adiabatic threshold is precluded.It has been suggested that small Franck-Condon factors near threshold are not a particularly serious problem in the ion- ization of rare-gas cluster species owing to the close spacing of many Rydberg levels throughout the region between the adiabatic and the direct ionization thresholds which may decay via autoioni~ation.~~’~Rydberg states with lifetimes greater than 50 ps and principal quantum numbers 55 < n < 75 have been reported for CO, clusters by Camp- bell and Tittes.” Such long-lived high Rydberg states can be observed only if there are some states for which non-radiative mechanisms of decay, such as autoionization and electronic predissociation, are significantly slower than radiative decay.It has been shown2’ that predissociation rates of molecular Rydberg states are considerably greater for states of low prin- cipal quantum number and while autoionization is the most probable non-radiative decay mechanism, the apparent absence of states with principal quantum number less than 55 observed in the experiment performed by Campbell and Tittes” indicates that predissociation may also be an impor- tant mechanism.It is therefore possible that lower Rydberg states of molecular clusters may, in fact, predissociate instead of decaying to levels of lower energy uia the autoionization process. For this reason, unfavourable Franck-Condon factors may not be completely compensated for in the ioniza- tion of molecular cluster species, making the observation of their true adiabatic ionization potentials unlikely. Some knowledge of Franck-Condon factors for van der Waals clus- ters may therefore be required for the reliable interpretation of cluster-ion appearance potentials. The most probable mechanism for the formation of the cluster fragment ions (N,O * 0)’ and (N20 NO)’ would appear to involve the ionization and fragmentation of one of the N,O molecules in the neutral N,O dimer without any significant perturbation of the second molecule, although alternative mechanisms cannot be discounted.Despite these recognised deficiencies, electron impact ionization efficiency curves can provide sig- nificant mechanistic information, especially where breaks are found and where comparisons with monomer measurements and data collected using other techniques are available. References 1 M. Mautner and F. H. Field, J. Chem. Phys., 1977,66,4527. 2 C. E. Klots and R. N. Compton, J. Chem. Phys., 1978,69,1636. 3 G. G. Jones and J. W. Taylor, J. Chem. Phys., 1978,68,1768. 4 S. H. Linn and C. Y. Ng, J. Chem. Phys., 1981,75,4921. 5 K. Stephan, J. H. Futrell, K. I. Peterson, A. W. Castleman Jr. and T.D. Mark, J. Chem. Phys., 1982,77,2408. 6 B. Kamke, W. Kamke, R. Herrmann and I. V. Hertel, 2. Phys. D, 1989,11,153. 7 K. Stephan, J. H. Futrell, K. I. Peterson, A. W. Castleman Jr., H. E. Wagner, N. Djuric and T. D. Mark, J. Mass Spectrom., 1982, 44,167. 8 S. T. Ceyer, P. W. Tiedemann, B. H. Mahan and Y. T. Lee, J. Chem. Phys., 1979,70,14. 9 B. R. Cameron and P. W. Harland, J. Chem. Soc., Faraday Trans., 1991,87, 1069. 10 Atomic and Molecular Beam Methods, ed. G. Scoles, Oxford University Press, London, 1988. 11 C. A. McDowell, The ionization and Dissociation of Molecules, McGraw-Hill, New York, 1963. 12 R. W. Kiser, Introduction to Mass Spectrometry and its Applica- tions, Prentice-Hall, Englewood Cliffs, NY, 1965. 13 V. H. Dibeler and R. M. Reese, Adv. Mass Spectrom., 1966, 3, 471. 14 R. G. Keese and A. W. Castleman, J. Phys. Chem. Ref: Data, 1986,15,1011. 15 K. Hiraoka, G. Nakajima and S. Shoda, Chem. Phys. Lett., 1988, 146,535. 16 H. M. Rosenstock, K. Draxl, B. W. Steiner and J. T. Herron, J. Phys. Chem. Ref: Data 6, Suppl. 1, 1977,70. 17 H. L. Johnston and K. E. McCloskey, J. Phys. Chem., 1940,44, 1038. 18 A. J. Illies, J. Phys. Chem., 1988,92,2889. 19 C. Y. Ng, D. J. Trevor, P. W. Tiedemann, S. T. Ceyer, P. L. Kronebusch, B. H. Mahan and Y. T. Lee,J. Chem. Phys., 1977, 62,4235. 20 E. E. B. Campbell and A. Tittes, Chem. Phys. Lett., 1990, 165, 289. 21 S. M. Tarr, J. A. Schiavone and R. S. Freund, J. Chem. Phys., 1981,74,2869. Paper 3/05770D; Received 24th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000935

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(7), 941-945 941 Protonated Carbamic Acid Collisional Activation and Unimolecular Dissociation of CH,NOl t Helge Egsgaard* Department of Combustion Research, Risq National Laboratory, DK-4000 Roskilde, Denmark Lars Carlsen National Environmental Research institute, Department of Environmental Chemistry, DK-4QOQ Roskilde, Denmark Protonated carbamic acid is generated by elimination of a vinyl radical from the radical cation of ethyl car- bamate. Double hydrogen transfer leads to an excited state of the stable Y-configuration in addition to other isomers. The properties of gaseous CH,NOl have been disclosed on the basis of collisional activation mass spectrometry as well as unimolecular dissociation (MIKE) in combination with an extensive use of 15N and deuterium labelling.Thermochemical aspects of the protonated carbamic acid are discussed based on semi- empirical MNDO calculations, supplementary to appearance energy measurements. A number of common and important acids, e.g. HzCO, and H2S03, are generally regarded as elusive. Attempts to gener- ate the species in their free state, i.e. in the condensed phase, lead to decomposition products only.'g2 In the gas phase, however, high energy barriers prevent their spontaneous dis- sociation. Thus, successful experiments based on neutral-ization-reionization mass spectrometry have been reported for both H2C03 and H,S0,.394 The protonated counterparts of these elusive acids are, on the other hand, remarkably stable and two classes appear to be of general importance.The highly symmetrical H,XO; system, e.g. protonated carbonic acid exhibits the so-called Y-stabilization, which has been interpreted as 'acylic aromaticity'.s*6 Thus, this class of protonated acids is charac- terised by a high stability towards unimolecular The second class of protonated acids, of the general formula H20X+,exhibit non-planar structures with significant charge localization within the m~lecules.~-~' This results in rela- tively weak H,O-.X+bonds and, hence, in an enhanced electrophilic rea~tivity.~-' ' Thus, the hypohalous acid ions H,OX+ (X = C1, Br, I) apparently play, under certain condi- tions, an important role in the aqueous halogenation of organic molecules.' Protonated nitrous acid, H,NO,f, is analogously assumed to be the carrier of the NO+ electro- phile in nitrozation reactions under similar The parallel to the well studied H2N0,f species has in the latter context to be Protonated carbamic acid represents an interesting ana- logue to the previously studied H3XOi acids.A priori four individual structures should be considered, exhibiting the possibility of elimination of HzO and NH, , respectively. The Y-configuration (1) has been subjected to a number of theo- retical studiess*6p'6 and is the species considered in the super- acid studies by Olah et ~l."*~' /OJ+ Ol+ 01. /OJ + HZN-C H,N-C; H3N--C4 HN=C 'OH OH2 'OH 'OH 1 2 3 4 The CH,NO; species is a dominant ion in the electron- impact-induced mass spectra of aliphatic carbamic esters.t Part 11 in the series Gas Phase Ion Chemistry. For Part 10 see H. Egsgaard and L. Carlsen; J. Anal. Appl. Pyrol., 1993,25,361. Consequently, mass spectrometry may provide details of the unimolecular chemistry of the protonated carbamic acid. It appears, however, that the protonated carbamic acid has not previously been the subject of detailed experimental investi- gations using more advanced mass-spectrometic techniques. The present paper describes the generation and properties of protonated carbamic acid and its isomers, rationalized on the basis of 15N and 'H labelling experiments in combination with various MS/MS techniques. The thermochemistry is discussed on the basis of semi-empirical MNDO calculations supplementary to appearance energy measurements. Experimental The mass-spectrometric investigations were carried out using a Varian MAT CH 5Ddouble-focussing mass spectrometer equipped with a combined EI/FI/FD ion source.The present study was carried out using electron impact (EI) ionization at 70 eV. The ion source temperature and pressure were 200°C and 10-Torr, respectively. Collisional activation (CA) was carried out by introducing He as the target gas in the second field-free region. The CA spectra were recorded with a reduction of the main beam to ca. 30% and are uncorrected for unimolecular processes. The resolution for the first mass analysis was ca. 400. The E2/U linked scan was established using two indepen- dent computer driven digital-to-analogue converters for the electrostatic sector in combination with two power supplies (Caneberra Industries, Model 3002).The output being based on the actual value of the pseudo-linear U-ramp. The target gas (He) was admitted into a collision cell, which consisted of a modified total-ion monitor located in the first field-free region. The main beam was attenuated to 30%. All data were acquired uia an HP3497A data acquisition/ control unit and an HP9836S computer. Signal averaging, combined with digital filtering was applied in order to study weak signals satisfactorily. Chemicals The ethyl carbamates were synthesized from ammonia (NH, and I5NH3) and the appropriate ethyl chloroformate.The labelled chloroformates were obtained uia the reaction of (CCl,O),CO and the C,D50H and used in situ.lg Direct exchange utilizing CH,OD was used for the labelling of the NH, hydrogens with deuterium. Thermochemistry and MO Calculations Appearance energies (E,) were derived from plots of ion cur- rents us. nominal electron energy. The energy scale was cali- brated by measurement of a series of ionic species with well defined appearance/ionization energies within the actual energy domain, the onsets being determined on the basis of 'vanishing current I. The MNDO calculations were carried out applying the MOPAC package implemented for Macintosh I1 computers (Serene Software, Bloomington, IN). Results Generation of Gaseous CH,NOZ The electron-impact-induced fragmentation of ethyl car-bamate discloses an abundant m/z 62 which may be assigned to protonated carbamic acid.This ion is a result of a double hydrogen transfer involving the loss of a vinyl radical which apparently is a general feature of aliphatic carboxylic esters.,' A comprehensive study based on, e.g. ethyl car- bamate as precursor for the protonated carbamic acid depends obviously on the possibility of isotope labelling. Previous investigation of the radical cations of ethyl car- bamate revealed a very complex ion chemistry without, however, isomerization of hydrogens in the amide and ethoxy groups.2' Thus, the primary reactions of the molecular ion, e.g. loss of C,H;, directly reflects the position of the label in the neutral molecule.21 This result is in accordance with pre- vious studies of methyl arba am ate,,-,^ as well as lower ali- phatic carbonates.9 Ethyl carbamate can on this background be considered as a likely precursor in studies of protonated carbamic acid and its isomers. The compounds given below were synthesized, enabling MS/MS investigation of the isotopic labelled car- bamic acids and related isomers. J+* X,N-C' \OC,Y, compound N X Y 5a 14N H H 5b 14N D H 5c 14N H D 5d 14N D D 5e 15N H H 5f 15N D H 5g5h 15N 15N H D D D The radical cations of carbamates exhibit a very complex ion chemistry with four interconverting structures 6-9 to be con- ~idered.~~-~~In addition, the distonic ion 7 rearranges irre- versibly into a series of proton-bound complexes.23 It appears, however, that the iminol structure 9 does not directly contribute to the ion chemistry of carbamates, poss- ibly due to the high energy requirement for its f~rmation.,~ This assumption gains strong support from the virtual absence of H/D exchange between the amide and alkyl hydrogens.24 The distonic ion 8 with the charge predomi- nantly localized on the nitrogen, reveals energetics compara- ble to 923and is, on this account, not considered further as a significant species in the general features of the radical cations of ethyl carbamate.Thus, the ion chemistry of the radical cations of ethyl carbamate is apparently covered by J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the ion 6 exhibiting the original geometry and the distonic ion 7, the latter being thermodynamically favoured by 0.52 eV.23,24 Hence, the elimination of C2Hj is assumed to take place from these two species only. The structure 6 can be considered as precursor only for the isomers 1 and 3, whereas the distonic ion 7 in principle can give rise to ions 1-3. It has, however, to be emphasized that neither 6 nor 7 is able of generating the isomer 4 directly. +o H,N-C' 8 '0-C-HCH, 0 I+* +,OH I3,N-C' +TH,N-C '0-CH~CHJ \O-C'HCH,, In this study the formation of 1, from the radical cation of ethyl carbamate is thermodynamically favoured by 1.28 eV, which, however, is not supported by determination of the heat of formation of the m/z 62 species. The appearance energy was determined to be 10.7 eV.In combination the heats of formation of the neutral ethyl carbamate (Af H = -4.73 eV) derived by the group-additivity principle25 and the expelled vinyl radical (Af H =0.63 eV),26 the heat of for- mation of the m/~62 species is calculated to be 5.3 eV. In combination with the theoretically predicted value for 1 [Af H(1) 3.15 eV, vide infra] this implies the generation of 1 in an excited state. In contrast to this the direct generation of 3 from 6 is apparently slightly exothermic, cf: Fig. 2. The gener- ation of protonated carbamic acid in an excited state gains support from the observation of a weak, but significant signal, in the CA spectra (vide infra) due to charge stripping, i.e. forming the corresponding doubly charged protonated carbamic acid.This is unsual for low-energy beam experi- ments and is in general associated with energetically high- lying species, e.g. Rydberg excited species of low-molecular- weight molecules or atoms. Structural and Thermochemical Considerations Based on semi-empirical MO calculations, applying MNDO, structural as well as thermochemical features of the possible isomers of protonated carbamic acid can be elucidated. The MNDO method has previously been used to study proto- nated carbonic The method predicts the experimen- tally obtained heats of formation ~atisfactorily~~ and mimics high-level MO calculations.28 However, since MNDO appar-ently tends to overestimate energy barriers associated with the 1,3-shifts ~ignificantly,~' this type of calculation was not included in the present study.A priori the four fundamentally different structures of protonated carbamic acid 1-4 have to be considered. It appears that the thermodynamically favoured structures of the isomers 1 and 3 exhibit virtually planar configurations, whereas in the cases of 2 and 4 the -OH, moieties are twisted out of the plane, the dihedral angles being CQ. 75" (to 0)and ca. 110" (to N), respectively, in agreement with the previously reported results for protonated carbonic acid.8 J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 Obviously, the single isomers can exist in varying rotameric forms.The cis isomers of carbamic acid and several deriv- atives have previous been investigated applying ab initio cal-culations, the cis isomer being the thermodynamically more favoured.16v30The subsequent protonation of this isomer is suggested to leave the protonated species in a cis configu-ration as we11.I6 However, it should be noted that this isomer emerged as the more stable by as little as 0.09 eV.16 These results are mimicked by the MNDO calculations, the equi- librium structure of 1 being visualized in Fig. 1. As in the case of protonated carbonic acid7** the so-called Y-configuration of 1 (cf. Fig. 1) (ArH = 3.15 eV), was found to be thermodynamically favoured relative to 3 (Af H = 4.53 eV), 2 (AfH = 4.72 eV) and 4 (A,H = 5.04 eV).The relative stabilities of the last three isomers are further reflected in rather weak bonds between the central carbon atom and 'stable inorganic moieties', i.e. -OH, and -NH,. Thus, the bond order for the C-NH, bond in isomer 3 was calculated to be 0.77, whereas the bond orders of the C-OH, bonds in isomers 2 and 4 were found to be 0.58 and 0.71, respectively. The calculated bond lengths of the C-0 and C-N bonds in isomer 1 are all somewhat shorter than pure C-O/C-N bonds, the corresponding bond orders being 1.17 and 1.41, respectively, reflecting the charge distribution in the isomer, exhibiting a positively charged atom (pc = +0.54e) and nega- tively charged oxygens (po = -0.21e) and nitrogen (pN= -0.23e).The possible fragmentations of the isomers 2-4 have been studied theoretically, applying the MNDO concept. It was unambiguously disclosed that the simple fragmentations, i.e. 2 -P H2N=C=O+ + H,O, 3 +O=C=OH+ + H,N and 4 -P HN=C=OH+ + H20, are all endothermic, by 0.27, 1.40 and 0.49 eV, respectively, and proceed with a reverse activation energy virtually equal to zero. In addition the frag- mentation 3 --,NHZ + CO, and 3 -,NH;' + 'C0,H have been studied, the corresponding changes in heat of formation being AAf H = -0.65 and AAf H = +2.66 eV, respectively. The fragmentation 4 -+ HN=C=O + H,Of is endothermic by 0.31 eV. In Fig. 2 the MNDO derived potential-energy diagram for the CH,NOi system is visualized. MS/MS Analyses of CH,NOi The experimental study of the CH,NOl system was based on the analyses of unimolecular dissociations (MIKE) and collision activation mass spectrometry (CAMS) in com-bination with E2/U linked scan.The resulting spectra of protonated carbamic acid as derived from the isotopomers 5a and 5b are given in Fig. 3 and 4, respectively. The elimination of NH, and H20 isotopomers from protonated carbamic acids derived from 5a-5h are given in Table 1 as relative abundances based on E2/U scans. 0.959H O-H k.009 1.321 /J119.20 1.005 H/ 119.4" (,o\ 1.955 H A, H = 72.53 kcal mol-' = 3.15 eV Fig. 1 MNDOderived geometry for the protonated carbamic acid in the thermodynamically favoured Y-configuration (1) I HNCOH' + H20 + H205. > ?5d -C02+ NH4' 3.0-I Fig. 2 MNDOderived potential-energy diagram for the CH,NOi system.The dashed lines correspond to reactions channels in which transition states were not localized 20 40 60 m/z Fig. 3 Unimolecular dissociations (a) and collision-induced mass spectra (b)of m/z 62 of H,NCO,C,H, (5a). Inserts:(a) partial MIKE and (b)CA (E2/U)scans. The signals due to scattering of the main beam and charge stripping are marked* and @@, respectively. Table 1 Collision-induced loss of neutrals from protonated car- bamic acid isotopomers based on precursors 5a-5h given as relative abundances as determined by an E2/Ulinked scan neutral loss/u 16 17 18 19 20 21 22 mfz 62 (5a) 4 17 71 7 m/z 64 (5b) 2 56 39 3 m/z 64 (9) 3 -23 24 46 4 m/z 66 (Jd) 9-82 5 4 m/z 63 (5) 8 81 74 m/z 65 (Sf) 2 56 21 19 3 m/z 65 (5g) 4 5 42 46 3 m/z 67 (5h) 4 5 66 22 2 1 'iLT/I, ..'I J" I I I I I 20 40 60 m/z Fig. 4 Unimolecular dissociations (a) and collision-induced mass spectra (b)of m/z 64 of D,NCO,C,H, (5b).Inserts:(a)partial MIKE and (b)CA (E2/V)scans.The signals due to scattering of the main beam and charge stripping are marked* and @ 6, respectively. It is immediately evident that significant differences between the MIKE and CA mass spectra can be noted. The MIKE spectra [Fig. 3(a) and 4(a)] are dominated by ions corresponding to the elimination of H,O. It is a characteristic feature that the hydrogens involved predominantly 'originate from the ethyl group' in accordance with fragmentation of 2. The predominant loss of H20 from 2 relative to that from 4 is in perfect agreement with the predicted energy barriers for the two reaction channels being 0.27 and 0.49 eV, respec- tively.A corresponding elimination of NH, is virtually absent in these spectra, reflecting the high energy requirement, 1.40 eV, for this reaction. In addition, NHf/H30+ ions are observed, however, at somewhat lower intensities. The ions are most probably a result of fragmentation of proton-bound complexe~.~ 1-33 The generation of NHf by necessity involves all four hydrogens in the molecule. The labelling experiments show, on the other hand, that the H,O+ retains both 'hydrogens from the ethyl group', i.e.isomerization prior to fragmentation apparently does not operate [Fig. 3(a)and 4(a)]. Consequently, 2 and 3 become obvious candidates as precursors for the proton- bound complexes. 2 3 Increasing the internal energy of the ions prior to fragmen- tation by collisional activation gives rise to the opening of additional reaction paths [Fig. 3(b) and 4(b)].The multiply labelled protonated carbamic acid derived from 5h is in this context a key species, since it enables unambiguous assign- ment of all the major fragments. It appears from Table 1 that J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 D20 and I5ND, are eliminated in a ratio of approximately 3 : 1 as disclosed by the ions at m/z 47 and m/z 46, respec-tively. Furthermore, it should be noted that the formal loss of 'OD giving rise to m/z 49 is virtually absent.Thus, the elimi- nation of NH, can be assigned for a series of the iso- topomers; the relative abundances of those ions are given in Table 1. The elimination of NH, as well as the formation of NH;' obviously have to be associated with the structure 3. The elimination of H20 may, as already elucidated by the MIKE spectra, take two routes, i.e. uia 2 or 4.The reaction via 4 appears to be somewhat enhanced in the CA spectra (Fig. 3 and 4). It is emphasized that a typical collision-induced fragmenta- tion of an ion with 3 keV translational energy takes place as a result of a vertical excitation with 1-1.5 eV as the most probable excitation Thus, the observation of the elimination of NH, as well as NHj+ from 3 in the CA experi-ments is completely in line with the predicted thermochemis- try for the CH4N0; system.Consequently, it appears that the observed fragments can be rationalized on the basis of the following reactions. Ol+ I4,N-C' -NH, w o=a+ \OH 3 ,OH2 I+ -H,OHN=C rn HN=C=OH+ \OH 4 Based on the results given in Table 1 the relative contribu- tions from structures 2-4 as well as the apparent isotope effects can be determined. However, note that the apparent isotope effects cannot be associated with a single reaction, e.g. a hydrogen transfer from one heteroatom to another. Thus, the apparent isotope effect is a result of a series of reactions with possible individual isotope effects. Owing to the large number of isotopomers investigated, the set of equations, even including isotope effects, is mathematically over-determined. Thus, least-squares analyses gave the following mutual ratio for the fragmentation pathways of 24: 2, 1.00 (kdkD : 1.22); 3, 0.34 (kdkD : 0.85) and 4, 0.40 (k&D : 1.08), the values being scaled arbitrarily.The apparent isotope effects for the elimination of H20 from 2 and 4 are slightly lower than that previously reported for protonated carbonic acid.* This may well be due to the possible concurrent forma- tion of the intermediate structures 2/3from 6 and 7. In addi- tion, differences in geometry, H,COi exhibiting close to ideal symmetry, are expected to increase possible isotope effects.The apparent isotope effect for elimination of NH, from 3 (0.85) is unexpected since it indicates the presence of a reverse isotope effect. This is clearly associated with the intermediate structure 3,in which a reverse secondary isotope effect appar- ently operates, i.e. the deuterium isotopomers of ammonia are preferably eliminated. The isotope effect is a consequence of the relatively high energy barrier for the NH, elimination which enables the intermediate 3 to accumulate. Conclusion Protonated carbamic acid is generated by the elimination of a vinyl radical from the radical cation of ethylcarbamate. The J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 945 ion chemistry of protonated carbamic acid involves all four conceivable isomers 1-4. The Y-configuration was found by MNDO calculations to be the most stable isomer by 1.28 eV.Analyses of the unimolecular dissociations reveal elimination of water through concurrent formation of 2 and 4. Collisional activation increases the relative contribution of 4 in addition 14 15 16 17 18 F. Cacace, M. Attina, G. de Petris and M. Speranza, J. Am. Chem. SOC., 1989,111,5481. F. Cacace, M. Attina, G. de Petris and M. Speranza, J. Am. Chem. SOC., 1990,112, 1014. M. Remko, Collect Czech. Chem. Commun., 1988,53,1141. G. A. Olah and M. Calin, J. Am. Chem. SOC.,1968,90,401. G. A. Olah, A. M. White and D. OBrien, Chem. Rev., 1970, 70, to opening a channel whereby ammonia is eliminated via 3. This reaction is virtually absent in the unimolecular pro-cesses.The relative contributions of structures 2-4 to the eventual products were determined to be 2, 1.00; 3, 0.34 and 4, 0.40. The observation of charge stripping and the interme- diacy of 4, of necessity involving 1, is taken as evidence for 19 20 21 22 561. H. Eckert and B. Forster, Angew. Chem., 1987,99,922. H. Budzikiewics, C. Djerassi and D. H. Williams, Mass Spec- trometry of Organic Compounds, Holden-Day, San Francisco, 1967. H. Egsgaard and L. Carlsen, Org. Mass Spectrom., 1992,27,535. P. C. Burgers, C. Lifshitz, P. J. A. Ruttink, G. Schaftenaar and J. the primary formation of 1 in the excited state. References 23 K. Terlouw, Org. Mass Spectrom., 1989,24,579. G. Schaftenaar, R.Postma, P. J. A. Ruttink, P. C. Burgers, G. A. McGibbon and J.K. Terlouw, Znt. J. Mass Spectrom. Zon Pro-cesses, 1990, 100, 521. 1 2 3 4 5 6 7 8 9 10 11 Comprehensive Inorganic Chemistry, ed. J. C. Bailar, H. J. Emelius, R. Nyholm and A. F. Trotman-Dickenson, Pergamon Press, Oxford, 1973. Gmelin, Handbuch der Anorganischen Chemie, Kohlenstoff, Teil G3, Verlag Chemie, Weinheim, 1973. J. K. Terlouw, C. B. Lebrilla and H. Schwarz, Angew. Chem., 1987,99,352. D. Sulzle, M. Verhoeven, J. K. Terlouw and H. Schwarz., Angew. Chem., 1988,100,352. P. Gund, J. Chem. Educ., 1972,49, 100. M. L. Williams and J. E. Gready, J. Comput. Chem., 1989,10,35. H. Egsgaard and L. Carlsen, Advances in Mass Spectrometry, ed. P. Longevialle, Heydon, London, 1989, vol. 11 A/B, p. 886. H. Egsgaard and L. Carlsen, J. Chem. SOC., Faraday Trans. 1, 1989,85,3403.H. Egsgaard and L. Carlsen, Int. J. Mass Spectrum. Zon Pro-cesses, 1992,113,233.M-T.Nguyen and A. F. Hegarty, J. Chem. SOC.,Perkin Trans. 2, 1984,2037. G. de Petris, A. di Marzio and F. Grandinetti, J. Phys. Chem., 1991,95,9782. 24 25 26 27 28 29 30 31 32 33 34 35 G. A. McGibbon, C. A. Kingsmill, J. K. Trelouw and P. C. Burgers, Org. Mass Spectrom., 1992,27, 126. S. W. Benson, Thermochemical Kinetics, Wiley, New York, 2nd edn., 1970. H. M. Rosenstock, K. Draxl, B. W. Steiner and J. T.Herron, J. Phys. Chem. Ref. Data, 1977,6, suppl. 1. J. L. Holmes and F. P. Lossing, Can. J. Chem., 1982,60,2365. G. Rasul, V.K. Reddy, L. Z. Zdunek, G. K. S. Prakash and G. A. Olah, J. Am. Chem. SOC.,1993,115,2236. K. J. van den Berg, C. B. Lebrilla, J. K. Terlouw and H. Schwarz, Chimia, 1987,41,122. M. Remko and S. Scheiner, J. Mol. Struct. (Theochem), 1988, 180,175. S. Hammerum, Fundamentals of GasPhase lon Chemistry, ed. K. R. Jennings, Kluwer, Dordrecht, 1990,379. T. H. Morton, Org. Mass Spectrom., 1992,27,353. D. J. McAdoo and T. H. Morton, Ace. Chem. Res., 1993,26,295. M. S. Kim and F. W.McLafferty, J. Am. Chem. Soc., 1978, 100, 3279. I. W. Griffiths, E. S. Mukhtar, R.E. March, F. M. Hams and J. H. Beynon, Znt. J. Mass Spectrom. lon Processes, 1981,39, 125. 12 H. M. Gilow and J. H. Ridd, J. Chem. Soc., Perkin Trans. 2, 13 1973,1321. T. A. Turney and G. A. Wright, Chem. Rev., 1959,59,497. Paper 3/06199J; Received 18th October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000941

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(7), 947-951 Intramolecular Photodimerization of 2-Naphthoates:Successful Application of Hydrophobic Forces in the Preparation of Large-ring Compoundst Chen-Ho Tung,* Yi Li and Zhi-Qiang Yang The Laboratory of Photochemistry, Institute of Photographic Chemistry, Academia Sinica, Beijing 100101,China ~ ~~ ~~ ~~ ~~ ~ ~ The fluorescence spectra and photodimerization of polymethylene bis(2-naphthoate) (N -M, -N) in aqueous organic binary mixed solvents have been studied. Strong intramolecular excimer fluorescence was observed, suggesting that hydrophobic interactions force polymethylene chains to self-coil. Photoirradiation of these solu- tions resulted in intramolecular dimerization of 2-naphthoate groups to give ring-closure products.The quantum yields for the photodimeriration are significantly greater than those in organic solvents. This work provides an example of the application of hydrophobic interactions in expediting the formation of macrocyclic entities. The construction of macrocyclic compounds is a long-standing problem in synthetic organic A bifunctional molecule may undergo either intramolecular or intermolecular reactions. The intramolecular reaction gives macrocyclic ring-closure products, while the intermolecular reaction results in polymers. The rates of the latter type of reaction are dependent on the concentration of the substrate, while those of the former are not, since the effective concen- tration for the reaction is kept constant by the function of a molecular chain linking the two functional groups.Therefore, the cyclization can occur without competition only at low concentrations, as first described by Ziegler.' Reactivity in cyclization reactions may be interpreted on the basis of the activation energy and the probability of end-to-end encounters. According to transition-state theory, the activation energy for a cyclization reaction depends on the structure of the initial open-chain state and on that of the transition state, whose conformation can reasonably be expected to resemble that of the cyclic product. In general, the activation energy reflects the strain energy of the ring to be formed, which is markedly dependent on ring size.6 The probability of end-to-end encounters of a molecular chain will decrease as the chain becomes longer.' Thus, the forma- tion of a large ring from a flexible chain demands a for-midable price in terms of entropy.398 Traditionally, chemists use energetically highly favoured reactions between the ter- minal groups to synthesize large-ring compound^.^ The new approach is to use forces to reduce the entropy expense."*" It is well established that in aqueous organic binary mixed solvents hydrophobic interactions force molecules with long hydrocarbon chains to ~elf-coil.'~*'~ Self-coiling of a chain linking two terminal groups would increase the intramolecu- lar ring-closure probability and expedite the formation of macrocyclic entities.In the present work, we report a suc-cessful example of the utilization of hydrophobic forces to promote large-ring formation.We use 2-naphthoate as the terminal groups of polymethylene chains, since this group may undergo photodimerization to give a 'cubane-like' dimer as the unique product with a reasonable quantum yield.14 The molecules we studied have the following struc- tures and are abbreviated as N -M, -N. NPCO~(CH~),CO,NP N -M, -N (n = 2, 3, 5, 8, 10) where Np = 2-naphthoate. t Hydrophobic Effects on Photochemical and Photophysical Pro- cesses. Part 15. For Part 14*see ref. 1. Through the examination of excimer formation and photo- dimerization, we obtained evidence for the self-coiling of N -M, -N in aqueous organic mixed solvents and demon- strated that the intramolecular photodimerization of the ter- minal groups of N -M, -N is enhanced by self-coiling of the chains.Results and Discussion Intramolecular Excimer Formation of N -M, -N To establish that the self-coiling of polymethylene chains is driven by hydrophobic interactions we studied the emission spectra of N -M, -N in aqueous organic binary mixed sol- vents. Fig. 1 shows the fluorescence spectra of N -MI, -N (1 x lo-' mol dm-3) in ethylene glycol-water (EG-H,O) mixed solvents at ambient temperature, which is typical of , 300 340 380 420 460 500 540 A/nm Fig. 1 Fluorescence spectra of N -M,,-N and BN in EG-H,O. -M,, -N] = 1/2[BN] = 1 x lo-' mol dm-3. 1, BN in the mixed solvents with various ratios of water to EG; 2, N -MI, -N in EG; 3, N -M,, -N in EG-H,O with H,O :EG = 30 : 70.the other polymethylene 2-naphthoates in aqueous organic mixed solvents. The fluorescence spectrum of the model com- pound, butyl 2-naphthoate (BN) is also shown. In the mixed solvents with various EG :H,O ratios, BN exhibits struc- tured fluorescence characteristic of the naphthoate monomer with maxima at 340, 350 and 370 nm. The behaviour of N -M,, -N is quite different from that of BN. In ethylene glycol, N -M,, -N also shows only monomer emission. However, in the mixed solvents, an excimer fluorescence centred at ca. 400 nm is observed. For solvents with water : EG ratios >30 : 70, the excimer emission dominates the fluorescence spectrum of N -N,, -N.At concentra- tions <5 x mol dm-3, the ratio of the fluorescence intensities of excimer to monomer, lD/IM,is independent of concentration, suggesting that the excimer is intramolecular. Thus the excimer formation for N -M, -N in EG-H,O is attributed to the self-coiling of the polymethylene chain, which makes the two terminal chromophores approach each other. The excitation spectra for the excimer and monomer emission are identical, and the maxima correspond to that in the UV absorption spectrum, suggesting the absence of strong interaction between the naphthoate chromophores in the ground state. Thus, N -M, -N in EG-H,O tends to assume the self-coiling conformation and the two end groups are in proximity but do not associate with each other in the ground state.In other aqueous organic solvents, such as dimethyl sulfoxide-water (DMSO-H20) and 1,Cdioxane-water (DX-H,O), the behaviour of N -M, -N is analogous to that in EG-H,O. Effectsof Amylose on Excimer Formation of N -M, -N In order to ascertain the self-coiling of the polymethylene chain in aqueous organic solvents we studied the effect of amylose on the excimer formation of N -M, -N in EG-H,O. Amylose is a water-soluble polymer. The confor- mation of this polymer in solution and its interaction with polar and non-polar compounds have been extensively inves- tigated in connection with the biological significance of the amylose structure. In aqueous organic solvents, amylose molecules exist in the helical form and can incorporate long- chain substrates." In the absence of amylose, the excimer emission dominates the fluorescence spectrum of N -M,, -N in EG-H20 with H20 :EG = 30 :70 (Fig.2). Addition of amylose results in the enhancement of monomer emission and a reduction in the excimer fluorescence. In the presence of sufficient amylose the 'fluorescence spectrum is dominated by monomer emission. Obviously this is due to the fact that amylose forms an inclusion complex with N -M,, -N and prevents the two terminal naphthoate groups from approach- ing each other. Enhancement of Intramolecular Photodimerization of the 2-Naphthoate End Group of N -M, -N via Hydrophobic Interaction Photodimers of naphthalene derivatives have been known since Bradshaw and Hammond reported the intermolecular (4n+ 4n) photodimerization of 2-methoxynaphthalene.' Photoirradiation of alkyl 2-naphthoates results in a 'cubane- like' photodimer as the unique product (Scheme 1)in spite of the fact that six isomeric dimers are formally possible.'4 This selectivity originates from two restrictions.First, the photo- dimerization occurs only between the substituted rings. Sec- ondly, in the dimer the substituents are in the head-to-tail orientation. The photoirradiation of a bichromophoric com- pound, like N -M, -N, can lead either to intra- or inter- molecular reactions. The intramolecular reaction gives J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2 300 340 380 420 460 500 540 A/nm Fig. 2 Effect of amylose on the fluorescence spectra of N -MI, -N in EG-H,O (H,O :EG = 40:60)[N -MI, -N) = 1 x mol drn-j.[amylose]/mol dm-3: 1,O; 2, 5 x macrocyclic ring-closure products, while the intermolecular reaction results in polymers. In order to protect the cubane- like photodimer from decomposition we used II > 280 nm light as the light source. Irradiation of 5 x lo-' mol dm-3 N -M, -N in DX-H,O with H20 : DX = 60 :40 at ambient temperature led to the formation of intramolecular ring-closure photodimers only, as shown in Scheme 1. No 0-(CH2) 0 I o=c c=o 6/> 6/> 8 Scheme 1 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 polymerization products were detected. The yield of the intramolecular products was 100% based on the consump- tion of the starting materials.The assignment of the intramol- ecular reaction relies on the observation that the m/z values of the molecular ion in the mass spectra of the products are identical with those of the corresponding starting materials N -M,-N. Furthermore, in the ‘HNMR spectra of the products no protons assignable to naphthyl group were detected. The structure proposed for the cubane-like photo- Hd-0.-/ Hd’ “\3q;Hd” C/CH2 -0’ \\ \ a’ Hd dimer rests mainly on its ‘H NMR spectrum, which is in close agreement with that reported in the 1iterat~re.l~ The ‘H NMR spectra for the intramolecular photodimers of N -M,, -N, N -M, -N and N -M, -N are shown in Fig. 3 and the spectral details and assignments for the pro- ducts are given in Table 1.Note that the product molecules belong to the C, point group and the geminate protons for the methylene groups in the neighbourhood of the carboxyl groups are not magnetically equivalent. The difference Hd-0. ’ .c Hd” \ Hd’ (CH2)6‘d-0’ \Hd I z /’ I I J I I f I ’ //7 5 4 7.0 4.5 4.0 3.5 6 Hd-0. / ,c c& Hd” \GJkg; II I L II \ \ a’ -0’ \ Hd w 0 I7.0f, 4.5 4.0 3.5 6 Fig. 3 400MHz ‘HNMR spectra of the intramolecular photodimers of N -M, -N, N -M, -N and N -MI, -N in CDCl, 950 Table 1 'HNMR data of intramolecular photodimers of N -M, -N,N -M, -NandN-MI, -N 6 split JJHZ intensity assignment intramolecular photodimer of N -M, -N 6.78-7.17 m 8H Ar 4.90 m 2H Hd 4.61 d Jc,,a= 11.2 2H Ha 3 Ha, 4.34 dd Ja, = Jas 2H H,, Hc, = 11.2 Jb, c = 'b*, c = 9.9 4.20 m 2H Hd' 3.78 d Jc,b= 9.9 2H Hb, Hb' intramolecular photodimer of N -M, -N 7.00 m 8H Ar 4.55 d Jc,,a = 11.0 2H Ha 7 Ha, 4.41 dd Jar,c= Ja,cI = 11.0 2H Hc, H,y 'b,c = 7.1 = JW, c' 4.31 m 2H Hd 4.16 m 2H Hd' 3.86 d Jc?,b = 7.1 2H Hb, Hb, intramolecular photodimer of N -M -N 6.96-7.04 m 8H Ar 4.54 d Jcr.a= 11.9 2H Ha, Ha, 4.41 dd Jar,, = Ja,cI 2H H,, H,.= 11.9 Jb. c = 'b', c' = 7.9 4.25-4.19 m 4H Hd, Hd* 3.84 d Jc,b = 7.9 2H Hb, Hb' between the chemical shifts of the geminate protons decreases with increasing ring size. We have demonstrated experimentally that the yield of product in the photodimerization of N -M, -N is depen- dent on the square of the light intensity. This suggests that the formation of the cubane-like photodimer for N -M, -N is a two-photon process.A plausible proposal for this photodimerization is shown in Scheme 2. N -M, -N absorbs the first photon to give a [4 +4] cycloaddition product (1) which then rearranges thermally to 2, as described by Sasse and co-workers." Absorption of the second photon by 2 results in the cubane-like photodimer. Another possible pathway to form the cubane-like dimer from 1 is that the naphthyl group of the unreacted starting material N -M, -N plays the role of triplet sensitizer to induce conversion of 1 into the final product, since Yang has demonstrated that in the presence of a triplet sensitizer pho- toirradiation can bring about the conversion of the adduct of naphthalene and cyclohexa- 1,3-diene into a cubane-like dimer., The photodimerization of N -M, -N in organic sol- vents, both polar and non-polar, was also studied.Photoirra- diation of these solutions gave the same products as those in aqueous organic mixed solvents, but the quantum yields were much lower. Table 2 gives the conversions of N -M, -N in J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0 0 endo (1) cubane-like end4ope rearranged (2) Scheme 2 DX-H,O with H,O : DX = 60 : 40, cyclohexane and meth- anol after irradiation for 15 min with a 450 W Hanovia high- pressure mercury lamp in a merry-go-round apparatus. The conversions of all N -M, -N in cyclohexane and methanol are smaller than those in DX-H,O.For example, the conver- sion of N -M, -N in DX-H,O is ca. 10 times greater than that in cyclohexane. Obviously the high yields of the photo- dimerization of N -M, -N in aqueous organic mixed sol- vents are attributed to the self-coiling of the polymethylene chain, This demonstrates the potential application of hydro-phobic interactions to the synthesis of macrocyclic entities. Experimenta1 'H NMR spectra were recorded either at 100 MHz with a Varian FX-100 or at 400 MHz with a Varian XL-400 spec- trometer. MS spectra were run either on a Finigan 4021C spectrometer or on a VG ZAB spectrometer. UV spectra were measured with a Hitachi UV-340 spectrometer.Fluores- cence spectra were run either on a Hitachi EM 850 or a Hitachi MPF-4 spectrofluorimeter. Photoirradiation pro- ducts were separated by using a Varian VISTA 5500 liquid chromatograph with a Lichrosorb RP 18 column. Poly- methylene bis(2-naphthoates) (N -M, -N) were synthesized by esterification of 2-naphthoyl chloride with corresponding diols, as previously reported. 10~22Spectral-grade cyclohexane, methanol and 1,4-dioxane were used without further purifi- cation for fluorescence measurements. Amylose was a gift from the Shanghai Institute of Organic Chemistry, Academia Sinica. The average degree of polymerization was 350; the purity of straight chain species was > 95%. Fluorescence Measurements The samples were purged with nitrogen for at least 30 min before measurement. The excitation wavelength was 280 nm.Table 2 Conversions of N -M, -N in various solvents at ambient temperature after irradiation for 15 min with a 450 W Hanovia lamp ([N -M, -N] x 5 x lop5mol dm-') DX-H,O 65 40 37 33 cyclo hexane 7 12 17 9 methanol 1 2 3 2 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 951 The spectra were fully corrected for instrumental response. Of particular interest was the excimer-to-monomer intensity ratio, ZD/ZM, calculated from the peak heights at 400 and 355 nm for the excimer (ID)and the monomer (IM), respectively. 5 6 7 8 K. Ziegler, in Methoden der Organischen Chemic (Houbenweyl), ed. E. Muller, Georg Thieme Verlag, Stuttgart, 1955, vol. 4/2. J. F. Liebman and A.Greenberg, Chem. Rev., 1976,76,311. L. Ruzickal, Chem. Znd. (London), 1935,54,2. L. Mandolin, in Advanced Physical Organic Chemistry, ed. V. Gold and D. Bethell, Academic Press, London, 1986, vol. 22, pp. Photoirradiation and Product Analysis 9 1-1 12, and references therein. E. G. Corey, K. C. Nicolaon and L. S. Melvin, J. Am. Chem. Photoirradiation was carried out in a quartz reactor, and the samples were purged with nitrogen. A450 W Hanovia high- pressure mercury lamp was used as the excitation source. Neat toluene was used as filter. For the photodimerization carried out in 1,4-dioxane-water7 after irradiation the solu- tion of the reaction mixture was extracted with ethyl ether and washed with water, Crude products were obtained after evaporation of the ether.For the photodimerization carried out in cyclohexane and methanol, after photoirradiation the solvents were evaporated. The crude products were separated by using HPLC, with methanol-water (95/5) as the eluting 10 11 12 13 14 SOC., 1975, 97, 653, and references therein; N. W. Porter, J. Am. Chem. SOC.,1986,108,2787. C. H. Tung and Y. M. Wang, J. Am. Chem. SOC., 1990,112,6322. X. K. Jiang, Y. Z. Hui and Z. X. Fei, J. Chem. SOC., Chem. Commun., 1988,689. X. K. Jiang, Acc. Chem. Res., 1988,21, 362. C. H. Tung and C. B. Xu, in Photochemistry and Photophysics, CRC Press, Boca Raton, 1991, vol. 4, ch. 3, pp. 177-220. P. J. Collin, D. B. Roberts, G. Sugowdz, D. Wells and W. H. F. Sasse, Tetrahedron Lett., 1972, 321; C. Kowala, G. Sugowdz, W. H.F. Sasse and J. A. Wunderlich, Tetrahedron Lett., 1972, 4721; T. Teitei, D. Wells and W. H. F. Sasse, Aust. J. Chem., 1976, 29, 1783. solvent. The products were identified by 'H NMR and mass spectrometry. 15 16 M. Yamamoto, T. Sano and T. Yasunaga, Bull. Chem. SOC.Jpn., 1983,55, 698. Y. Hui, J. C. Russell and D. G. Whitten, J. Am. Chem. SOC.,1983, 105,1374. This work was supported by the National Science Founda- tion of China. 17 18 P. V. Bulpin, A. N. Cutler and A. Lips, Macromolecules, 1987, 20,44. M. L. Bender and M.Komiyama, in Cyclodextrin Chemistry, 19 Springer Verlag, Berlin, 1978. J. S. Bradshaw and G. S. Hammond, J. Am. Chem. SOC., 1963, References 20 85, 3955. T. Teitei, D. Wells, T. H.Spurling and W. H. F. Sasses, Aust. J. 1 Z. Zhen and C. H. Tung, J. Photochem. Photobiol. A, Chem., Chem., 1978,31,85. 1992,68,247. 2 P. A. Evans and A. B. Holmers, Tetrahedron, 1991,47,9131. 21 22 N. C. Yang and J. Libman, J. Am. Chem. SOC., 1972,94,9228. C. H. Tung, G. Z. Ma,S. Y. Guo, S. K. Wu and H. J. Xu, Acta 3 G. Illuminati and L. Mandolini, Acc. Chem. Res., 1981, 14,95. Chim. Sinica, 1985,43, 1092. 4 S. Masamune, G. S. Bates and J. W. Corcoran, Angew, Chem., Int. Ed. Engl., 1977, 16, 585. Paper 3/06749A; Received 1lth November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000947

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(7), 953-961 Photophysical Properties of Merocyanine 540 Derivatives Andrew C. Benniston and Anthony Harriman" Center for Fast Kinetics Research, The University of Texas at Austin, Austin, Texas 78712,USA Kirpal S. Gulliya Baylor Research Institute, Baylor Medical Center, 3812 Elm St., Dallas, Texas 75226,USA Photophysical properties have been measured for five merocyanine dyes having (i) different alkyl substituents on the thiobarbiturate subunit [i.e. ethyl (butyl Merocyanine UO), or hexyl], (ii) the sulfonate group replaced with a methyl group, or (iii) the benzoxazole residue replaced with 1,2-naphthoxazole, and for one of the correspond- ing oxonol derivatives. Neither the nature of the alkyl group nor the absence of the sulfonate group exert any significant effect on the photophysical properties of the dyes in ethanol solution at 22 "C.However, extending the size of the benzoxazole residue increases fluorescence and triplet yields and decreases the yield for photoiso-merization. A detailed two-dimensional NMR analysis of one of the dyes has shown that the ground state exists exclusively in an all-trans conformation while FTlR indicates hydrogen bonding between the thiobarbiturate subunit and the polymethine bridge. Using this structural information, the energetics and mechanics of the isomerization processes are discussed. It is further shown that these structural modifications affect the efficacy with which the dyes kill leukemic cells under illumination due to pronounced changes in lipophilicity.Merocyanine 540 shows considerable potential as a photo- sensitizer for selective eradication of leukemia and related viral contaminants from blood products'.' and as a radiation sensitizer for treatment of solid turn our^.^ Consequently, several research groups have reported on the photochemical properties of this dye in fluid solution and in micro-heterogeneous Fluorescence is moderately intense but, in common with most cyanine dyes, intersystem crossing to the triplet manifold is extremely inefficient, and the domi- nant photoprocess involves isomerization of the first excited singlet state to form a long-lived (unstable) isomer. The photophysical properties, and in particular the rate of isom-erization, of such merocyanine dyes have been found to depend on both viscosity and polarity of the so1vent.",l2 Isomerization is assumed to occur via trans-cis intercon-version and, as such, might be expected to involve large-scale torsional motion since the terminal subunits are quite b~1ky.l~Also, the ionized sulfonic acid residue, which pro- vides modest solubility in water and which anchors the dye close to an aqueous surface when dissolved in a lipid mem- brane,I4 might be involved in specific interactions with protic solvents. On close scrutiny, however, it appears that little is known about the intimate details of the isomerization process and, in fact, the conformation of the ground-state molecule remains unresolved.This is unfortunate because the compound might be converted into a much more potent photosensitizer if isomerization could be inhibited.With this in mind, we sought to explore deeper into the mechanism of the isomer- ization process by the study of derivatives of Merocyanine 540 having different alkyl chains attached to the thiobarbitu- rate subunit, having an extended benzoxazole residue, and having the water-solubilizing sulfonic acid residue removed. It is shown that such structural modifications have little effect on the rates of isomerization and it is concluded that the isomerization process involves only a modest structural per- turbation. Furthermore, the sulfonic acid group plays no obvious role in controlling the dynamics of isomerization in homogeneous solution, but replacing the benzoxazole moiety with a second thiobarbiturate subunit, forming an oxonol, results in greatly enhanced rates of isomerization. Since the ultimate objective of this work is to design improved photo- sensitizers for removal of the leukemia virus from blood and bone marrow, some attention has been given to their in situ concentration and photochemical behaviour.Experimental Merocyanine 540 (2) was obtained from Eastman-Kodak and purified by column chromatography on silica using acetone- methanol (98:2) as eluent. For the purified material in ethanol solution, the absorption maximum (Amax) was at 560 nm and the molar absorption coefficient at the peak maximum (cmax) was 167000 dm3 mol-' cm-'.The general synthetic route used to prepare merocyanine dyes has been reported previously' ',16 and was used for the preparation of the new compounds studied here. Interestingly, in the prep- aration of 4, a minor component was separated by column chromatography and identified as 5 on the basis of 'H NMR, 13C NMR and mass spectrometry; a sample of 5 was subse- quently synthesized by direct self-condenation of the thio- barbiturate residue. Starting materials were purchased from Aldrich Chemicals or Eastman-Kodak and were used as received. Water was double-distilled and deionized with a Millipore purification system. Ethanol (Aaper, Absolute grade) was used as received. All other solvents were spectro- scopic grade, used as received, or were redistilled under vacuum. Analytical data for the new compounds are provid- ed here; all compounds were purified by extensive chroma- tography on silica and it should be noted that the reported yields were not optimised.Compound 1 Yield: 0.3 g (33%). 'H NMR (C2H6]-dimethyl sulfoxide): S = 1.14-1.19 (6 H, m); 2.11 (2 H, m);2.58 (2 H, m); 4.40- 4.43 (6 H, m); 6.49-6.54 (1 H, d, J = 13.7 Hz); 7.49 (2 H, m); 7.74-7.84 (4 H,m); 7.98 (1 H, m). FAB MS (nitrobenzyl alcohol matrix): 491 (M+). UV-VIS (C,H,OH): A,,Jnm = 560 (&,,Jdm3 mol-' cm-' = 168000). FTIR (KBr disc): 1630; 1670 cm- ' (CO stretch). Compound 3 Yield 1.4 g (74%). 'H NMR (CDCl,): 6 = 0.86 (6 H, t, J = 7.2 Hz); 1.13-1.19 (9 H, t, J = 7.2 Hz); 1.28 (12 H, s); 1.60 (4 H, br); 2.11 (2H, m); 2.51-2.57 (2 H, t, J = 6.7 Hz); 3.04-3.13 (6 H, t, J = 7.2 Hz); 4.33-4.47 (4 H, m); 4.56 (2 H, m); 6.49-6.54 (1 H, d, J = 13.5 Hz); 7.17 (1 H, d, J = 8 Hz); 7.46-7.52 (2 H, q, J = 6.4 Hz); 7.73-7.84 (3 H, m); 7.91-7.99 (1 H, m).FAB MS (nitrobenzyl alcohol matrix): 602 [M-(C,H,),NH+]. UV-VIS (C,H,OH): A,,Jnm = 560 (&,Jdm3 mol-' cm-' = 178000). FTIR (KBr disc): 1630; 1670 cm-' (CO stretch). Compounds 4 and 5 Purification by chromatography on silica of the crude mixture, eluting first with ethyl acetate-hexane (1:1) gave 4 and 5 as impure products due to incomplete separation. Further purification of 4 on a second column, eluting with ethyl acetate-hexane (3 : 7), gave a deep blue solid, which appeared as a single spot on TLC (R, = 0.38).Yield: 300 mg (19%). 'H NMR (CDCl,): 6 = 0.88-0.91 (6 H, dt, J = 7.3 Hz, J' = 2 Hz); 0.93-0.96 (3 H, t, J = 7.3 Hz); 1.31-1.41 (6 H,m); 1.62-1.68 (4 H, m); 1.72-1.78 (2 H, m); 3.86-3.89 (2 H, t, J = 7.3 Hz); 4.39-4.42 (4 H, t, J = 7.8 Hz); 5.54-5.64 (1 H, d, J = 12.5 Hz); 7.09-7.11 (1 H, dd, J = 7.7 Hz. J' = 1 Hz); 7.23-7.26 (1 H, dt, J = 7.8 Hz, J' = 1.2 Hz); 7.28-7.31 (1 H, dt, J = 7.7 Hz, J' = 1 Hz); 7.39-7.41 (1 H, d, J = 8 Hz); 7.77- 7.82 (1 H, t, J = 13 Hz); 7.84-7.89 (1 H, t, J = 12.9 Hz); 7.95- 7.98 (1 H, d, J = 13 Hz). FAB MS (nitrobenzyl alcohol matrix): 482 (M+). UV-VIS (C,H,OH): A,,Jnm = 560 (&,,Jdrn3 mol-' cm-'= 165000). FTIR (KBr disc): 1630; 1670 cm-' (CO stretch). Similarly, purification of 5 on a second column, eluting with acetone-methanol (9 :l), gave a red solid, which appeared as a single spot on TLC (R, = 0.10).Yield: 30 mg (2%). 'H NMR (CDCl,): 6 = 0.9 (12 H, m); 1.25-1.31 (8 H, m); 1.61 (8H,m); 4.32 (8H, m); 7.89 (2 H, d, J = 13 Hz); 8.30 ( 1 H, t, J = 13 Hz). 13C NMR (CD,CN): J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 14.01; 20.92; 21.09; 30.10; 47.82; 48.26; 103.37; 120.30; 160.72; 161.27; 162.87; 179.72. FAB MS (nitrobenzyl alcohol matrix): 547 (M'). UV-VIS (C,H,OH): A,,,/nm = 540 (&,,Jdm3 mol-' cm-' = 142000). FTIR (KBr disc): 1630; 1670 cm-' (CO stretch). A further sample of 5 was synthe- sized by direct condensation of an equimolar mixture of NYN'-dibutyl-2-thiobarbituricacid and 5-(3-methoxypropen- 1,3-diene)-N,N'-dibutyl-2-thiobarbituricacid in methanol containing excess triethylamine.Identical analytical data were found for the two samples. Compound 6 Yield 1.5 g (35%). 'H NMR (CDCl,): 6 = 0.86-0.93 (6 H,t, J = 7.2 Hz); 1.12-1.18 (9 H, t, J = 7.2 Hz); 1.24-1.32 (4 H, m); 1.56 (4 H,m); 2.17-2.19 (2 H, m); 2.62-2.65 (2 H, m); 3.03-3.12 (6 H, q, J = 7.2 Hz); 4.29 (4 H, m); 4.58 (2 H, m); 6.56-6.62 (1 H, d, J = 13.8 Hz); 7.61-7.81 (4 H, m); 7.96-8.14 (4 H, m); 8.32-8.36 (1 H, d; J = 8.2 Hz). FAB MS (nitrobenzyl alcohol matrix): 699 (MH'); 597 [M(C,H,),NH+]. UV-VIS (C,H,OH): A,,Jnm = 572 (&,,Jdm3 mol-' cm-' = 164000). FTIR (KBr disc): 1630; 1670 cm- '(CO stretch). 'H NMR spectra were recorded with Bruker AC250 or General Electric GN-500 FT-NMR instruments with TMS as internal standard.Absorption spectra were recorded with a Hitachi U3210 spectrophotometer and fluorescence spectra were recorded with a fully corrected Perkin-Elmer LS5 spec- trofluorimeter. Solutions for fluorescence studies were adjust- ed to possess an absorbance of <0.05 at the excitation wavelength. Singlet excited-state lifetimes were measured by time-correlated, single-photon-counting techniques using a mode-locked, synchronously pumped, cavity-dumped Rho-damine 6G dye laser. The excitation wavelength was 565 nm and fluorescence was isolated from scattered laser light with a high-radiance monochromator. The instrumental response function was ca. 60 ps and was deconvoluted from the experi- mental decay profile prior to data analysis.The fluorescence lifetime for oxonol 5 was measured with a synchronous streak camera following excitation at 532 nm with a 30 ps laser pulse. ca. 500 individual laser shots were averaged and analysed by computer iteration after deconvolution of the instrument response. Flash photolysis studies were made with a frequency-doubled Quantel YG481 Nd : YAG laser (pulse width 10 ns; pulse energy 70 mJ). Solutions were adjusted to possess an absorbance of ca. 0.2 at 532 nm and were purged with N,, 0, or air. Transient differential absorption spectra were recorded point-by-point with five individual laser shots being averaged at each wavelength. Kinetic studies were made at fixed wavelength with 50 individual laser shots being aver- aged and analysed by computer non-linear, least-squares iter- ative procedures.Where appropriate, the laser intensity was attenuated with crossed-polarizers. In several cases it was necessary to restrict the intensity of the monitoring beam to a low level in order to avoid photolysis of the photoi~orner.~ This attenuation was achieved by placing appropriate neutral density filters before the sample cell. Differential absorption coefficients for the excited triplet state were measured by the energy-transfer method using anthracene as donor. Anthra- cene was excited at 355 nm and the triplet state was moni- tored at 422 nm assuming a differential absorption coefficient of 52000 dm3 mol-' cm-'." The derived differential absorption coefficients for the excited triplet states at the respective maxima were as follows: 1, 45000; 2, 42000; 3, 44OOO; 4,45000; 5,28000 and 6,41000 dm3 mol-' cm-'.Differential absorption coefficients for the unstable isomers were measured by the complete bleaching method in 0,- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 saturated ethanol, with due allowance for triplet population. The values derived for the absorption peak maxima were as follows: 1, 15000; 2,17000; 3,16000; 4, 18000; 5,15000 and 6, 17000 dm3 mol-' cm-I. In all cases, the laser intensity was calibrated using zinc meso-tetraphenylporphyrin in benzene as standard.' '3'' The temperature was controlled using a heated metal block and was measured with a thermo-couple in direct contact with the solution.Time-resolved thermal lensing studies were made with a conventional setup2' using a Melles Griot 40 mW laser diode emitting at 835 nm as the probing beam. Output from the diode laser was separated from the excitation pulse with a narrow bandpass filter and directed through a 0.3 mm pinhole into the entrance slits of a New Focus low-noise silicon photocell. For each measurement, the intensity of the excitation laser pulse was varied using crossed-polarizers and the intensity of the probing beam was attenuated with glass neutral density filters immediately before entering the detec- tor. The instrument was calibrated using tris(2,2'-bipyridyl) ruthenium(I1) dichloride in deoxygenated ethanol, for which the triplet energy was taken as 196 kJ mol-', as measured by luminescence spectroscopy.Solutions were adjusted to possess an absorbance of 0.40at 532 nm and 40 individual traces were averaged before computer analysis. Partition coefficients were determined by dissolution of 1 mg of dye in a mixture of chloroform (10cm3) and a 2% (w/w) aqueous solution of human serum albumin (10 cm3). After sonication and standing for 24 h, a further 10 cm3 of chloroform were added and the layers separated. The concen- tration of dye in each layer was measured by absorption spectroscopy and the partition coefficient was defined as the molar ratio of dye in water relative to chloroform. Diffusion coefficients were measured in 50% (w/w) aqueous glycerol using the fluorescence photofading method developed by Axelrod et aL2' The solution was examined with a Zeiss Axiovert 35 inverted microscope and excitation was provided by a 10 ns laser pulse at 532 nm (70 mJ) focussed through the optics of the microscope.Recovery of the initial fluorescence signal, due to Brownian motion, was monitored at 610 nm. A suspension of Daudi cells (1 x lo6 cells ~m-~) in growth medium was mixed with a merocyanine dye (20 pg cm-3). After 30 min of incubation at 37"C, the suspension was placed in Falcon petri dishes (35 x 10 mm) and exposed to 514 nm light delivered with an argon ion laser. After irradia- tion for varying times, the cells were washed twice with RMPI- 1640 culture medium, resuspended in growth medium, and incubated overnight at 37°C.After 22 h of incubation, cell viability was determined by the trypan blue exclusion method.22 Comparative experiments were made with dye only and light only controls. The cell-killing efficacy is report- ed in terms of the log (reduction), which is defined as the logarithm of the number of surviving cells divided by the number of cells exposed to irradiation. For fluorescence microscopy studies, Daudi cells were stained with dye, washed and aliquots were placed on microsope slides with a cover slip on top. This wet mount was examined by polarized fluorescence spectroscopy under a Zeiss Axiovert 35 inverted microscope with 514 nm excitation and 600 nm emission wavelengths.The observed fluorescence intensity, in arbitary units, was compared to that for Merocyanine 540under iden- tical conditions. Results Ground-state Conformation of 4 A detailed understanding of the isomerization processes can be made only if the conformation of the ground state is known with certainty. Previous molecular mechanics MM2 calculations and resonance Raman results3 suggested that the ground state of MC540 possessed an all-trans conformation, but convincing structural data are lacking. A full structural analysis was undertaken, therefore, for merocyanine 4 in CDCl, solution using high-resolution and two-dimensional NMR techniques. The similarity of absorption and fluores- cence spectra and the close comparability of the photo- physical properties of merocyanines 1-4 and 6 suggest to us that all of these dyes possess identical conformations. High-field (500 MHz) 'H NMR COSY spectra were recorded for 4 in CDCl, and are shown in Fig.1 and 2. Com- plete assignment of all the protons in the molecule was pos- sible and the derived chemical shifts and coupling constants are collected in Table 1. In particular, the alkenic protons could be clearly identified and assigned. Thus, H(9) was unambiguously identified from its characteristic chemical shift (6 = 5.59) and doublet pattern. For this proton the mea- sured coupling constant, J, was found to be 12.5 Hz. Three other resonances were observed to possess very similar J values; 6 = 7.80 (J = 13.0 Hz), 6 = 7.87 (J = 12.9 Hz) and 6 = 7.96 (J = 13.0 Hz) (Fig.1). These latter three resonances are assignable, therefore, to the remaining three alkenic protons; the observed patterns of two triplets and a doublet being consistent with this assertion. Individual peak assign- ment could be made from the two-dimensional spectra which indicated mutual coupling between adjacent protons (Fig. 2). The second doublet (6 = 7.96) was clearly due to H(12) while the two triplets were assignable to H(10) (6 = 7.87) and H(11) (6 = 7.80). Since the coupling constants found for the alkenic protons were essentially the same, we can conclude that a single type of double bond prevails. On this basis, the ground-state conformation must be either all-trans or all-cis.Furthermore, since an all-cis arrangement cannot be accom- modated for merocyanine 4, we conclude that the only acceptable ground-state conformation is all-trans. Detailed analysis of all the resonances indicated that only this single isomer was present to the observable limit (~95%). The average magnitude of the coupling constant for the alkenic protons (J = 12.9 Hz) is significantly lower than the expected value for an isolated trans double bond (J NN 17 Hz); the corresponding isolated cis double bond has an expected coupling constant of about 10 Hz.*~ This finding is consistent with each of the carbon atoms in the polymethine bridge pos- sessing partial double-bond character, as expected for a merocyanine dye which can exist in zwitterionic resonance forms.Indeed, the coupling constants for individual protons Table 1 'H NMR spectral properties recorded for merocyanine 4 in CDCl, solution atom label proton shift," 6 coupling constant,b J/Hz 16, 16' 0.884.91 7.3, 2.0 8 0.93-0.96 7.3 -15, 15' 1.3 1-1.35' 7 1.36-1.41' 14, 14 1.62-1.68' -6 1.72-1.78' 5 3.86-3.89 7.3 13, 13' 4.39-4.42 7.8 9 5.54-5.64 12.5 1 7.09-7.1 1 7.7 3 7.23-7.26 7.8, 1.0 2 7.28-7.31 7.7, 1.2 4 7.39-7.41 8.0 11 7.77-7.82 13.0 10 7.84-7.89 12.9 12 7.95-7.98 13.0 ~ ~~ a kO.1 ppm referenced to TMS. kO.1 Hz. 'Too much resonance overlap to evaluate coupling constant. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ;3y 15’ 16 7\ 8 ’* . rL 0 b-,LL1 0t ‘1 0 6 5 4 3 26 High-resolution(500 MHz) twodimensional COSY NMR spectrum recorded for 4 in CDCl, showing the atom labelling Fig.1 are seen to increase as that proton nears the thiobarbiturate subunit, owing to an increased electron density. This effect can be interpreted in terms of zwitterionic structures formed by electron donation from the N atom in the benzoxazole subunit to one of the carbonyl groups in the thiobarbiturate subunit.24 -o R ao&rfS Y :+ R In the all-trans conformation, the carbonyl groups on the thiobarbiturate subunit are coplanar with the polymethine bridge and are well positioned for intramolecular hydrogen bonding. Indeed, FTIR spectra indicate the existence of such hydrogen bonding in the solid state, as evidenced by a broad absorption centred at 3400 cm-’.The carbonyl groups appear as a broad peak centred at 1630 cm-’and a some-what less intense and sharper peak centred at 1670 an-’.For a vinylogous amide carbonyl group in the absence of hydro- gen bonding, we would expect2’ to observe the CO stretching band at 1670 cm-’.Hydrogen bonding, of medium strength, is expectedZS to lower this frequency by about 30 cm-’. On s Fig. 2 The (500 MHz) twodimensional COSY NMR spectrum recorded for 4 in CDCl, showing the expanded aromatic and alkenic regions J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 this basis, the carbonyl band centred at 1670 cm-' is assign- ed to a non-hydrogen-bonded group and the corresponding peak centred at 1630 cm-' is attributed to the hydrogen- bonded counterpart.Photophysical Properties of 3 in Ethanol The photophysical behaviour of the various merocyanine dyes 1-4 followed a common pattern, as illustrated below for the hexyl derivative 3. All measurements were made in ethanol solution at 22°C under an atmosphere of air, nitrogen or oxygen. Some of the photophysical properties of the butyl derivative 2 (i.e. Merocyanine 540) have been reported previo~sly~-'~and the major goal of this study is to evaluate the effect of structural changes on the rates of isomerization. Ultimately, these results should be of value for the design of improved photosensitizers for use in photodynamic therapy. Note that previous work has established that minor structur- al changes can be accompanied by significant differences in the efficacy for light-induced cytotoxicity.26 Absorption and fluorescence spectral profiles recorded for 3 in dilute ethanol solution are shown in Fig.3. The absorp- tion spectrum shows a sharp peak centred at 560 nm [Emm = (178000 2 12000) dm3 mol-' cm-'1 while the fluorescence excitation spectrum gave a good match to the absorption spectrum over the entire visible spectral range. The Stokes' shift, measured in dilute ethanol solution, was (660 f40) cm-', indicating the absence of a substantial geometric change upon promotion to the first excited singlet state. The fluoresence quantum yield (@J, measured relative to Rho- damine 101 in ethanol,,' was found to be (0.16 f0.02) and the fluoresence lifetime (zs) was measured to be (415 20) ps.The radiative lifetime calculated from the Strickler-Berg expression2' (zo = 2.7 ns) was in good agreement with that determined experimentally (zo = = 2.6 ns). Laser flash photolysis studies carried out in N,-saturated ethanol solution indicated the presence of two transient species after excitation with a 10 ns laser pulse at 532 nm (Fig. 4). The shorter-lived transient, which aborbs predomi- nantly around 660 nm, was assigned to the triplet excited state of the dye on account of its reactivity towards molecular oxygen. The longer-lived transient, which absorbs principally at 595 nm, did not react with oxygen and, on the basis of wavelengthlnm Fig.3 Absorption and fluoresence spectra recorded for merocya- nine 3 in dilute ethanol solution. The excitation wavelength used for the fluorescence spectrum was 540 nm. previous ' was assigned to a geometric isomer formed by rotation around one of the polymethine double bonds. The differential absorption spectrum of this species (Fig. 4) shows that the isomer absorbs somewhat to the red of the ground state. The isomer was observed to be extremely photolabile and absorbed both the exciting and analysing pulses. Under low-intensity illumination, the lifetime of the unstable isomer was found to be (6.1 f0.5) ms. The differen- tial molar absorption coefficient for the isomer at 595 nm was estimated to be (1.6 f0.2) x lo4 dm3 mol-' cm-', by com- plete conversion in 0,-saturated solution, and this allowed determination of the quantum yield for formation of the isomer (0as (0.45 & 0.06).Isomerization occurs exclusively from the first excited singlet state. The triplet state decayed with a lifetime (z,) of (700 _+ 50) ps in thoroughly deaerated ethanol solution and was quenched by 0, with a bimolecular rate constant of (1.3 f0.3) x lo9 dm3 mol-' s-'. The triplet state was also populated via energy transfer from triplet anthracene in ethanol solution following laser excitation at 355 nm. From these latter studies, the bimolecular rate constant for triplet energy trans- fer was found to be (4 f1) x lo9 dm3 mol-' s-' and the differential molar absorption coefficient for the triplet stateof 3 at 680 nm was found to be (4.4 _+ 0.5) x lo4 dm3 mol-' cm-'.The triplet state, as formed by energy transfer, exhibited the same differential absorption spectrum asthat generated by direct excitation of 3 and decayed cleanly to the prepulse baseline. On the basis of actinometric laser flash photolysis ~tudies,''~~~ the quantum yield for population of the triplet excited state (0,)was determined to be (0.0030 f0.0006). wavelengthlnm t \II I 1 I I I I 1 I I I-aw! 4505#)5506006Hl7#]7sl8#) wavelengthlnm Fig. 4 Differential absorption spectra recorded 1 ps after excitation of merocyanine 3 in (a) deoxygenated and (b) oxygenated ethanol solution with a 10 ns laser pulse at 532 nm. (a) Spectra for both triplet state and unstable isomer; (b) spectrum of the isomer alone. Table 2 Photophysical properties measured for the various merocyanine dyes in dilute ethanol solution 1 560 0.163 430 0.0035 775 0.42 6.9 2 560 0.160 410 0.0030 820 0.40 6.7 3 560 0.170 415 0.0031 495 0.45 6.1 4 560 0.150 380 0.0030 210 0.47 3.7 5 540 0.092 125 0.00084 635 0.65 0.012 6 572 0.180 530 0.0038 780 0.24 42.0 Absorption maximum, +2.+_ 12%.' &20ps. +50 ps. f10%. Comparison of the Photophysical Properties of Merocyanines Comparable photophysical properties were observed for the other merocyanine dyes in ethanol solution and there were no major differences in the magnitude of any of the measured parameters.Thus, the absorption maximum of the ground- state dye remained at 560 nm and the Stokes' shift was invariant at (660 & 60)cm-'. The fluorescence quantum yield and excited-singlet-state lifetime remained insensitive to the length of the alkyl substituent on the thiobarbiturate subunit and to the presence of the sulfonato group (Table 2). Simi-larly, the quantum yield for formation of the triplet excited state and of the unstable isomer did not change, within experimental error. Lifetimes measured for the triplet and unstable isomer were similar, if not identical, throughout the series (Table 2). It is clear, therefore, that these structural changes are not manifest in a significant variation in the photophysical properties, at least in ethanol at 22 "C.Photophysical Properties of Compound 5 Compound 5 is an oxonol that is somewhat soluble in water. The absorption and fluorescence spectra recorded for 5 in ethanol are shown in Fig. 5. The absorption maximum, which is centred at 540 nm, is situated some 20 nm to the blue of that found for merocyanines 1-4. The fluoresence spectrum shows reasonable mirror symmetry with the absorption spec- trum, while the Stokes' shift of (620 _+ 30) cm-' indicates little structural difference between ground and excited states. Both the quantum yield of fluorescence and the fluorescence lifetime are significantly lower than those of the merocyanine dyes (Table 2). In oxygenated ethanol, the unstable isomer was formed in high quantum yield (Table 2).The differential absorption spectrum recorded for the unstable isomer is shown in Fig. 6 and is similar to those observed for the merocyanine dyes, although the maximum absorption now , lDL,\11 1 I I wavelength/nm Fig. 5 Absorption and fluorescence spectra recorded for oxonol5 in dilute ethanol solution. The excitation wavelength used for the fluo- rescence spectrum was 520 nm. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1 I D I 1 D h Q, C me -m P W I I I D I I I I I I I I 4505005506m69m'150800 wavelength/nm Fig. 6 Differential absorption spectra recorded (0)1 ps and (A)60 ps after excitation of oxonol 5 in deoxygenated ethanol solution with a 10 ns laser pulse at 532 nm. The shorter-lived species is attrib- uted to the unstable isomer and the longer-lived species is believed to be the triplet excited state.appears at 570 nm and the spectral features are considerably broader. The isomer was found to possess a relatively short lifetime compared with those of the merocyanine dyes (Table 2). In contrast, the triplet excited state, which was formed in extremely low quantum yield in deoxygenated ethanol (Table 2), retained a long lifetime. The differential absorption spec- trum recorded for the triplet (Fig. 6) differs from those found for the merocyanine dyes in that the absorption peak lies further into the near-infrared region and the differential molar absorption coefficient measured at the absorption maximum = (2.8 & 0.6) x lo4 dm3 mol-' cm-'1 is sig-nificantly lower.Photophysical Properties of Merocyanine 6 The general properties determined for 6 correspond very closely to those recorded for merocyanines 1-4. Extension of the benzoxazole subunit causes a modest red shift in the absorption and fluorescence maxima and exerts a small influ- ence on the various photophysical properties (Table 2). It appears that the increased size of the naphthoxazole subunit results in a decreased rate of isomerization and this effect is manifest in increased fluorescence and triplet yields and a sig- nificant decrease in the quantum yield for formation of the unstable cis isomer. The cis isomer possesses a relatively long lifetime, indicative of the increased size of the rotor.Activation Energies and Energetics for Merocyanines 4 and 5 The enthalpy difference between the ground-state trans isomer and the unstable cis isomer was measured for merocy- anine 4 and oxonol 5 by time-resolved thermal lensing tech- niques.20 Using the <Di values given above, the enthalpy changes accompanying conversion of the cis isomer to the trans isomer were found to be (75 f5) and (120 f10) kJ mol-', respectively, for 4 and 5 in ethanol solution. Clearly, the cis isomer for 5 is considerably less stable than that of 4. Activation energies for conversion of the cis isomers to trans isomers (EL) were measured by recording the relevant rate constants at different temperatures in ethanol solution and expressing the results in terms of conventional Arrhenius plots.The derived values for these thermal processes were (120 f10) and (60 f6) kJ mol-', respectively, for 4 and 5. The corresponding activation energies for photoinduced trans to cis isomerization (El) were measured by recording the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 fluorescence lifetime as a function of temperature and were found to be (24 & 3) and (19 f3) kJ mol-', respectively, for 4 and 5. It is seen that the excited-state barriers are very much less than those for the thermal reactions. The activation energies can be considered as linear com- binations of several terms. For merocyanine 4, where zwitter- ions persist, the measured activation energy will contain activation energy terms associated with solvent viscosity (E,), solvent polarity as expressed by the EA30) parameter, and twisting of the molecule (EJ.E, = E,, -p[Ed30) -301 + E, Here, expresses the fraction of the E-,-(30) parameter that is involved in a polarity-dependent activation energy.29 For oxonol 5, where zwitterions are not expected, the activation energy is assumed to be independent of solvent polarity. For ethan01,~' E, = 9.2 kJ mol-' and [Ed30) -30) = 91.5 kJ mol-'. Previously, the /lterm was measured for a structur-ally related merocyanine dye" and found to be 0.11 and -0.09, respectively, for the photoinduced and thermal isom- erization processes. On this basis, the activation energy associated with rotation around the central double bond (EJ has values of 25 and 10 kJ mol-', respectively, for light- induced trans to cis isomerization of 4 and 5. The corre- sponding values for thermal cis to trans isomerization of 4 and 5, respectively, are 103 and 51 kJ mol-'.It is clear from these values that both isomerization processes are much easier in 5 than in 4. Light-induced Cytotoxicity Studies The general photophysical behaviour of merocyanines 14 is quite similar, although the lipophilicity changes among the various compounds. This latter feature was measured experi- mentally by determining the partition coefficient (P) for dye distributed between aqueous human serum albumin and CHC1,; 1 (log P x 4.0), 2 (log P = 1.18), 3 (log P = 0.36), 4 (log P = -2.28), 5 (log P = 1.41) and 6 (log P = -1.88).The dyes demonstrate a clear preference for the aqueous protein, except for 4 and 6 which show markedly increased lipo- philicity relative to Merocyanine 540 (2). The diffusion coeffi- cients (D) were measured in aqueous glycerol, using the fluorescence photofading method, and the values are com- piled in Table 3. It can be seen that, under these conditions, the values remain very similar. The cellular concentration of merocyanine dye after incu- bation with Daudi cells was determined by polarized fluores- cence microscopy and the values are collected in Table 3. The more lipophilic derivatives are seen to exhibit the highest preference for localization within the cellular system while the most hydrophilic derivative shows little tendency to assimi- late within the cell.The percentage of cells surviving after Table 3 Partition coefficients, diffusion coefficients, relative in situ concentrations and cell-killing efficacies measured for the various dyes compound log P D/107an's-' [dye]" 10g(red[40])~ log(redC901)' 1 4.0 2.2 14 0.08 0.10 2 1.18 2.0 100 0.37 1.30 3 0.36 1.9 115 0.40 1.60 4 -2.28 2.1 290 1.22 5.40 5 1.41 2.0 - - - 6 -1.88 1.7 260 1 .oo 4.30 Relative in situ dye concentration measured by polarized fluorescence. Light-induced cell-killing efficacy in terms of log (reduction) as measured after exposure to 40 J cn-*. Light-induced cell-killing efficacy in terms of log (reduction) as measured after exposure to 90 J exposure to 40 and 90 J cm-' illumination at 514 nm was also measured for each of the dyes and the results are col- lected in Table 3.It is seen that, within experimental limi- tations, there is a direct correlation between the in situ dye concentration and the efficacy for light-induced cytotoxicity. Discussion It is apparent from the results collected in Table 2 that the photophysical properties of merocyanine 540 show little dependence on the nature of the alkyl substituents appended to the thiobarbiturate group. Similarly, the sulfonate group does not influence the photophysical properties of the dye in ethanol solution. In particular, lifetimes measured for the excited singlet state and for the unstable isomer remain com- parable, within experimental limitations, for merocyanines 14.This finding means that the rates of both light-induced (forward) and thermal (reverse) isomerization steps are essen- tially independent of the nature of these substituents. Some- what similar results have also been observed16 for thermal isomerization of merocyanine dyes based on benzimidazole, where the unstable isomer is much shorter lived than those found for merocyanines 1-4. We must conclude, therefore, that isomerization does not involve large-scale torsional motion of the thiobarbiturate subunit and that the sulfonate group does not associate closely with a solvent cluster. These findings are consistent with a model in which the thiobarbiturate subunit is held in place by hydrogen bonding between a carbonyl group and a proton in the polymethine bridge.This has the effect of inhibiting rotation around the C(5)-C(6) bond in the transition state and effectively pre- vents isomerization at this bond. It is expected that rotation around the C(l)-C(2) bond will be partially inhibited by steric interaction between the substituent attached to the ben- zoxazole N atom and H(10)and, in any case, isomerization at this bond seems unlikely to account for the observed optical absorption changes. Instead, rotation of the benzoxazole subunit around the C(3)-C(4) bond, leading to formation of the corresponding cis isomer (Scheme I), appears to give a Y IIhv li S H H--0 R Scheme 1 satisfactory account of all our experimental data.In the absence of specific solvation at the sulfonate group, the molar volumes of the rotors in merocyanines 1-4 are not too dis-similar; the molar volumes of the rotating groups31 in 1 and 4, respectively, being ca. 135 and ca. 120 cm3 mol-'. The slightly faster rates of isomerization found for 4 relative to 1-3 are consistent with this modest change in volume of the rotor. The photophysical properties observed for merocya-nine 6 also appear to be consistent with this scheme. Thus, the molar volume3' of the rotor in 6 is increased to ca. 180 cm3 mol-' and this has the effect of decreasing the rates of isomerization relative to merocyanines 1-3. For the oxonol 5 the negative charge should be considered as being delocalized over all four oxygen atoms, giving rise to the two extreme resonance forms shown in Scheme 2.Again, the non-ionized thiobarbiturate subunit is locked in place by hydrogen bonding, as evidenced by FTIR spectroscopy, and this has the effect of stabilizing the trans conformation. However, because of extensive electron delocalization, each of the C-C bonds in the polymethine bridge can be considered as being of 1.5 bond order. Photoisomerization occurs at the C(3)-C(4) bond, forming the corresponding cis isomer (Scheme 2). The rates of both forward and reverse isomer-izations are significantly faster than found for merocyanines 1-3 despite the fact that the molar volume of the rotor in 5 (V z 140 cm3 mol-') is comparable to that of 1-3 (V z 135 em3 mol-1).31 The enhanced rates of isomerization found for the oxonol most probably reflect the decreased bond order since, despite the presence of zwitterion~,~~the bond order of the isomerizing bond in merocyanines 1-3 must be closer to 2.0 than 1.5.The activation energies for isomerization, after correction for changes in viscosity and polarity, are signifi-cantly less for 5 than for 4. Again, such differences can be attributed to changes in the bond order of the isomerizing bond. The lower activation energies observed for the pho-toinduced processes relative to the corresponding thermal processes arise simply because of the higher potential energy available for the former reactions. s R R 11hv S 1I Scbeme 2 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I 1 1 1 I I I 3J 20 -to oa Lo 20 311 4.0 log P Fig. 7 Correlation between the measured partition coefficient and the cell-killingefficacy measured after exposure to 40J (m) and 90 J cm-' (A) illumination at 514 nm for the various merocyanine dyes The photophysical properties and diffusion coefficients recorded for compounds 1-6 remain comparable, but the effi-cacy for light-induced cytotoxicity towards Daudi cells varies significantly among the compounds. For the merocyanine dyes, where direct comparison is more meaningful, there is a clear correlation between the quantity of dye assimilated into the cell and the cell-killing effect (Table 3). There is a further correlation between the efficacy for light-induced cytotoxicity and the partition coefficient (Fig. 7, Table 3) and it is clear that, for the limited range of compounds studied, increased lipophilicity leads to both enhanced localization within the cell and more effective cell killing.This is an important quan-titative structure-reactivity criterion since it allows more rational design of improved photosensitizers for leukemia therapy. This work was supported by the National Institutes of Health (CA 53619). The CFKR is supported by the Uni-versity of Texas at Austin. References 1 C. J. Gomer, Photochem. Photobiol., 1991,54, 1093. 2 F. Sieber, Photochem. Photobiol., 1987,46, 1035. 3 A. Harriman, L. C. T. Shoute and P. Neta, J.Phys. Chem., 1991, 95,2415. 4 B. Kalyanaraman, J. B. Feix, F. Sieber, J. P. Thomas and A. W. Girotti, Proc. Natl. Acad. Sci. USA, 1987,84,2999. 5 J. Davila, A. Harriman and K. S. Gulliya, Photochem. Photobiol., 1991,53, 1. 6 M. Hoebeke, J. Piette and A. Van der Vorst, J. Photochem. Pho-tobiol. B: Biol., 1991,9,281. 7 J. Davila, A. Harriman and K. S. Gulliya, J. Chem. SOC., Chem. Commun., 1989,1215. 8 P. H. Aramendia, M. Krieg, C. Nitsch, E. Bittersman and S. E. Braslavsky, Photochem. Photobiol., 1988,48,187. 9 N. S. Dixit and R. A. Mackay, J. Am. Chem. SOC.,1983, 105, 2928. 10 R. J. Singh, J. B. Feix, T. J. Pintar, A. W. Girotti and B. Kalya-naraman, Photochem. Photobiol., 1991,53, 345. 11 A. Harriman, J. Photochem. Photobiol.A: Chem., 1992,6579. 12 Y. Onsager, M. Yin, D. R. Bessire and E. L. Quitevis, J. Phys. Chem., 1993,97,2344. 13 S. P. Velsko, D. H. Waldeck and G. R. Fleming, J. Chem. Phys., 1983,78,249. 14 P. R. Dragsten and W. W. Webb, Biochemistry, 1978,17,5228. 15 A. C. Benniston, K. S. Gulliya and A. Harriman, Photochem. Photobiol., submitted. 16 A. C. Benniston, K. S. Gulliya and A. Harriman, Photochem. Photobiol., submitted. 17 I. Carmicheal and G. L. Hug, J. Phys. Chem. Re$ Data, 1986, 15, 1. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 961 18 L. Pekkarinen and H. Linschitz, J. Am. Chem. Soc., 1960, 82, 25 A. J. Gordon and R. A. Ford, The Chemist’s Companion, Wiley-2407. Interscience, Chichester, 1972, p. 184. 19 J. K. Hurley, N. Sinai and H. Linschitz, Photochem. Photobiol., 26 W. H. H. Gunther, R. Searle and F. Sieber, Phosphorus, Sulfur, 1983,38,9. and Silicon, 1992,67, 417. 20 G. Rossbroich, N. A. Garcia and S. E. Braslavsky, J. Photo-27 S. R. Meech and D. Phillips, J. Photochem., 1983,23, 193. chem., 1985,31,37. 28 S. J. Strickler and R.A. Berg, J. Chem. Phys., 1962,37,814. 21 D. Axelrod, D. E. Koppel, J. Schlessinger, E. Elson and W. W. 29 C. Reichardt, Solvent Effects in Organic Chemistry, Verlag-Webb, Biophys. J., 1976,16, 1055. Chemie, Weinheim, 1979. 22 K. S. Gulliya, J. L. Mathews, J. W. Fay and R. M. Dowben, 30 J. M. Hicks, M. T. Vandersall, E. V. Sitzmann and K. B. Cancer Chemother. Pharmacol., 1988,22,211. Eisenthal, Chem. Phys. Lett., 1987, 135,413. 23 A. E. Derome, Modern NMR Techniques for Chemistry 31 A. Bondi, J. Phys. Chem., 1964,68,441. Research, Pergamon, Oxford, 1987. 24 L. G. S. Brooker, A. C. Craig, D. W. Heseltine, P. W. Jemkins Paper 3/06287B; Received 21st October, 1993 and L. L. Lincoln, J. Am. Chem. SOC.,1965,87,2443.

ISSN:0956-5000    

DOI:10.1039/FT9949000953

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(7), 963-968 Triplet of Cyclooctatetraene:Reactivity and Properties Tomi Nath Das* and K. lndira Priyadarsini Chemistry Division, Bhabha Atomic Research Centre, Bombay-400085, India Using 7 MeV electron pulses from a linear electron accelerator, the triplet excited state of cyclooctatetraene (COT), a commonly used triplet scavenger in dye lasers, has been characterised in hydrocarbon solvents. In deoxygenated cyclohexane and benzene, 3COT shows an absorption due to a triplet-triplet (T-T) transition between 300 and 400 nm with a peak at 350 nm in cyclohexane and at 360 nm in benzene, and with a lifetime of 100 ps in cyclohexane. Unlike many other solute triplets, 3COT exhibits comparatively low reactivity towards molecular oxygen and the measured rate constant was 3 x lo7 dm3 mol-' s-'.The triplet nature of the transient was confirmed by T-T energy-transfer processes from triplet donors like biphenyl, p-terphenyl and pyrene in a deoxygenated cyclohexane matrix and the molar absorptivity of 3COT at 350 nm was estimated to be 7000 k300 dm3 mol-' cm-'. In benzene, the T-T energy transfer rate constants using various 7-aminocoumarin laser dyes as donors were estimated. These values (of the order of lo9 dm3 mol-' s-') are 6.5 for Coumarin 47 (C47), 5 for C102, 6 for C120, 2.8 for C152 and 1.5 for C153. Similarly, rate constants were also measured using aromatic triplet donors. These values in the same order are 3.8for p-terphenyl, 13.3 for biphenyl, 3.3for both naphthalene and benzil, 0.74 for pyrene, 0.3 for anthracene and 0.7 for acridine.Based on the results of equilibrium studies in cyclohexane with 3anthracene as the donor, the energy of the first excited 3COT (ET)was estimated to be 41 L-1 kcal mol-'. Conjugated non-aromatic cyclic polyenes (CP) show unique properties in their excited states.lT2 These molecules, in both their singlet and triplet excited states, absorb in the UV region. The respective energy gaps between these states are large, the triplet absorption cross-sections are low and the corresponding energy levels are not yet precisely known, although they are assumed to lie in the region of 50 kcal mol-' (ET).3-5These properties allow these polyenes to be used as quenchers for the triplet excited states of various dyes used in laser systems.During the operation of a dye laser, the triplet excited levels of the dyes are populated along with the singlet states, which has a detrimental effect on their This happens because the fraction of the excited singlet state population responsible for the laser action becomes deacti- vated by the intersystem-crossing mechanism. These triplet dye molecules exhibit broad optical absorptions due to T-T transitions with high molar absorptivities. This results in sig- nificant loss of laser efficiency which becomes prominent when it operates in the CW or long-pulse mode.' Molecular oxygen is considered to be an efficient triplet scavenger, but the resulting singlet molecular oxygen produced by energy transfer from dye triplets is frequently responsible for the fast degradation of the dye molecules and the formation of non- fluorescent product^.^.'^ Hence, it is essential to use a suit- able triplet scavenger, which may preferentially remove the triplet dye molecules without interfering with the laser effi- ciency.Cyclopolyenes such as cyclooctatetraene, cyclohexadiene and cycloheptatriene fulfil these unique triplet properties and have been employed for efficient dye laser operation in the past.' '-I4 Many laser dyes have shown improved per-formance in the presence of these additives. Pappalardo et al.11*12could generate long laser pulses from Rhodamine-6G solutions using mmol dm -quantities of cyclooctatetraene (COT) as triplet scavenger. They observed that COT was as effective as oxygen for this purpose.On the other hand, Marling et a1.13 observed that dye laser output from RhodaminedG in the presence of COT was comparatively more than its output in the presence of oxygen. Thus, COT has been well accepted as an efficient triplet scavenger in dye- laser systems.'' However, the detailed mechanism for the scavenging process of dye triplets by COT is still not well understood. In addition, other parameters like the T-T absorption spectrum of 3COT and its molar absorptivity and lifetime in different solvents are not yet available in the liter- ature. In this study, triplet excited states of COT were produced and detected in cyclohexane and benzene solvents using nanosecond pulse radiolysis.Its T-T absorption spectra, life- time, reactivity with oxygen, energy-transfer reactions from a variety of donor triplets and the corresponding rate con-stants, as well as the energy level of its first excited triplet state were quantified. Experimental Laser-grade dyes Coumarin 47 (C47), Coumarin 102 (C102), Coumarin 120 (C120), Coumarin 152 (C152) and Coumarin 153 (C153) were obtained from M/s Lambda Physik and were used without any further purification. Scintillation-grade p-terphenyl, made at BARC was used for the studies. Biphenyl and benzil obtained locally from M/s SISCO were recrystallised from methanol. Pyrene, naphthalene, azulene, and anthracene, obtained from M/s Fluka, and acridine, obtained from M/s Sigma, were used as received.Spectro- grade benzene and cyclohexane obtained from M/s Spectro- chem India were purified by passing through an activated alumina column followed by distillation. Iolar grade N, , N,O and 0,used for various studies were obtained from M/s Indian Oxygen Limited. Stabiliser-free working solutions of COT, obtained from M/s Aldrich, were prepared as follows. A fixed volume of the solvent benzene (or cyclo- hexane, 10 cm3) was first deoxygenated in a vacuum line fol- lowing a repeated freeze-pumpthaw technique. Then the required amount (1 x lo-' cm3) of COT was distilled on-line under vacuum onto the frozen solvent matrix. On warming it produced the required stock solution of 1 x lo-' mol dm-3 COT in the solvent.This oxygen-free stock solution was stored inside a refrigerator and was stable for one month. Pulse radiolysis studies were carried out using 7 MeV elec- tron pulses from a linear electron accelerator providing pulses of 50 ns and 2 ps duration. The triplet excited states of various molecules produced were detected using a kinetic J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 spectrophotometric setup. The details regarding this setup have already been p~blished.'~~'~ The absorbed doses were determined with a thiocyanate dosimeter (1 x lo-, mol dm-3 KCNS aerated solution in water (using the value of GE= 2.23 x m2 J-' at 500 nm from the literature.'* The errors encountered in the measurement of various parameters varied between f10 and 15% and are not re- ported separately.Any pulse to pulse variation of the absorbed dose was minimised by averaging of the oscillo- scope traces. Results and Discussion Pulse radiolysis in hydrocarbon solution ultimately results in the formation of solvent excited states. because the primary species produced initially from ionization eventually undergo ion rec~mbination.'~ This happens because of the low rela- tive permittivity of the matrix which does not help the ions to escape from their mutual Coulombic attraction. The excited states of the solvents undergo a number of photophysical processes and sometimes return to the ground state.20 In addition to these reactions, if a solute is present in the solu- tion, these excited species transfer their excitation energy and generate the excited states of the solute.Thus, pulse radiolysis of hydrocarbon solutions provides an excellent and indirect method to produce solute excited states, especially triplets, which cannot be produced by other conventional techniques like flash photolysis. Solvents like benzene and cyclohexane are generally employed for such purposes, and the former is sometimes preferred more over the latter owing to the higher yields of triplets available., ' The typical steps involved in the formation of a solute triplet (3S) in a hydrocarbon (HC) such as cyclohexane or benzene are as follows : HC -*+ HC' + e-HC+ + e-'HC*, 3HC* --+ HC' + S-HC + S+ e-+S-S-* 3 *s++s-,I s, s 3HC* + S -HC + 3S* While in cyclohexane, ion recombination is an important process, in benzene, energy transfer is considered to be mainly responsible for triplet f~rmation.~~.~~ In this paper, while the T-T absorption spectra of 3COT were measured in both solvents, its molar absorptivity was estimated in cyclohexane.The energy-transfer studies with laser dyes and other donors were carried out in benzene with an improved signal to noise (S/N) ratio, owing to the higher triplet yields present. The absorption spectra of the transient, generated by the radiolysis of deoxygenated solutions of 1 x lov2mol dmW3 COT in cyclohexane and 2 x lo-* mol dm-3 COT in benzene, are presented in Fig. 1. These spectra show similar broad absorption profiles between 300 and 400 nm, with peaks at 350 nm in cyclohexane and 360 nm in benzene.The inset in Fig. 2 shows the kinetic trace for the first-order decay of the transient in cyclohexane at a low dose of 7 Gy, and its lifetime in cyclohexane was measured as 100 ps (k, = 1 x lo4 s-'). Although, the spectrum obtained in benzene strongly suggests the transient to be 3COT, from these measured parameters alone in cyclohexane, the possibility of formation of a different transient other than the 3COT, such as COT' or COT-, could not be ruled out. In order to confirm the nature of the transient, a cyclohexane solution of COT was 0.040 -lo.012 0.036 3 0.031 I u-0.027 t0.022 290 320 350 380 410 wavelength/nm Fig.1 Absorption spectra of 'COT generated by pulse radiolysis of an N,-saturated solution of 1 x mol dm-' COT in cyclo- hexane (0)at 60 Gy dose and 2 x lo-' mol dm-' COT in benzene (*) at 100 Gy dose saturated with N20, a frequently used electron scavenger, and the transient behaviour compared with cases where COT solutions were either deoxygenated or oxygen saturated. In Fig. 2, the transient decays under these conditions are pre- sented. The transient was observed to decay at a faster rate (k, = 2 x lo5 s-') in the presence of dissolved molecular oxygen (under saturation), while in the presence of dissolved N20,although the yield of the transient at 350 nm decreased by ca. 40%, the decay characteristics (k, = 2 x lo4 s-') did not differ appreciably from the transient decay obtained in the case of deoxygenation (nitrogen saturation).Under similar experimental conditions in the presence of dissolved N,O in cyclohexane, studies with a standard system such as anthracene revealed that the triplet anthracene yield decreased by a similar value. In both of these cases, since the decay kinetics of the transient remained unaffected and remained as under N,-saturated conditions, radical anion nature of the transient was ruled out. The absorption spectra of COT anion and dianion are reported in the literat~re~~" and these are distinctly different from the spectra presented in Fig. 1. Similarly, the possibility of formation of the radical cation of COT under these experimental conditions was neg- ligible, because in the presence of an electron scavenger like N,O, the yield of cation should have increased to a large extent and its decay slowed down. The decrease in the yield in the presence of 0, (electron scavenger) and the formation of a similar transient in benzene (cation scavenger) thus confirm its triplet nature.0.01 5 I _1 200a 0.005 0.ooop -0.005l I I I I I -0 20 40 60 80 100 120 time/ps Fig. 2 Oscilloscope traces of 'COT decay at 350 nm under (a)N, saturation, (b) N,O saturation and (c) 0, saturation at low dose. Inset: First-order decay of 'COT in cyclohexane under N, saturation and 7 Gy dose. J. CHEM. SOC.FARADAY TRANS., 1994, VOL. 90 The reactivity of 3COT with oxygen was determined by mixing different volumes of aerated cyclohexane in deaerated COT solutions at a fixed concentration, using the value of oxygen concentration in cyclohexane under air-saturated conditions of 2.3 x lop3mol dmP3 from the literat~re.~~ The rate constant for the reaction of 3COT with oxygen was esti- mated to be 3 x lo7 dm3 mol-' s-'.In the absence of dis- solved oxygen, at a higher absorbed dose (60 Gy or higher), the decay of 3COT followed second-order kinetics [2k/ ~l=5.qk2.5)x lo6 s-'1 which changed to pseudo-first-order kinetics (k, = 1 x lo4 s-') at low doses (7 Gy). At higher doses, the larger concentration of 3COT produced probably decayed by self-quenching or disproportionation processes, which became insignificant at low doses with matrix de-excitation processes taking over.Therefore, a change in the decay kinetics from second order to pseudo- first order, as observed above, is expected. Energy-transfer Studies In order to confirm further the triplet nature of the transient absorbing at 350 nm and estimate its E,, triplet energy, the following energy-transfer studies were carried out. The triplet-triplet energy-transfer process is known to proceed by an exchange mechanism.25 The donor (D) and acceptor (A) molecules collide with each other and form a collision complex. The overlap of their electron clouds leads to an exchange of electrons which finally results in excitation energy transfer. The necessary condition for an efficient energy transfer is that the energy level of 3Dmust be higher than that of the 3A by ca.3 kcal mol-'. The decay mecha- nisms of the 3Dmay be represented as follows: 3D*klD (1) 3D+ALD+ 3A* (11) where k, is the rate constant for the decay of donor triplet in the absence of acceptor and k, is the energy-transfer rate constant. The observed pseudo-first-order decay constant (k,) of 3Don account of energy transfer is given by: where [A] is the concentration of the acceptor. Therefore, a plot of observed decay rate constant against acceptor concen- tration is linear and the resulting slope gives the value of k, for the donor-acceptor combination. In order to keep the energy-transfer process predominant over the other decay modes, the reactivity of this process (k,[A]) was always kept sufficiently higher than that of all the other processes.Addi- tionally, sufficient care was taken to minimise any direct for- mation of 3A. Some representative plots of measured k, against [A] are presented in Fig. 3 for selected triplet donors and COT as the acceptor in the benzene matrix. The T-T energy-transfer rate constants obtained from similar plots with different donors are listed in Table 1. The T-T energy- transfer rate constants from the standard donors 3biphenyl and 3pyrene to COT were measured as 3.3 x lo9 and 7.4 x lo8 dm3 mol-' s-l, respectively, and were used to confirm that the transient produced from COT was indeed its triplet. The triplet energy levels, E,, of these donors are biphenyl (65.6 kcal mol- ') and pyrene (48.7 kcal mol- ').The concentration of each of the donors was kept constant at 1 x lo-' mol dm-3 in deoxygenated cyclohexane and the concentration of COT was varied from 5 x lod5to 6 x mol dm-3. Under these conditions, the concentration of donor was sufficiently high to prevent any direct formation of 3COT. In other words, under these experimental conditions c I v) v1 0 5 -Y [COT]/10-5 mol dm-3 Fig. 3 Linear plots of T-T energy transfer from various donors to COT in deoxygenated benzene solution at 7-10 Gy dose. Donors: p-terphenyl (0); C153 (A); C47 (*); C152 (+) and acri- benzil (0);dine ( x ). donor triplets were produced first, which subsequently trans- ferred the energy to the acceptor molecules.Fig. 4(a) shows oscilloscope traces for the decay of 3biphenyl at 360 nm in the absence and presence of COT. From this figure it can be seen that in the presence of COT, the initial decay due to 3biphenyl became faster than when COT was absent. However, in the presence of COT, with the expected formation of 3COT with its A,,, at 350 nm, the later part of the trace clearly shows such a behaviour and sub- sequent very slow decay (due to 3COT) in this timescale. This mixed behaviour was due to the overlap of the absorption spectra of the two triplets, viz. 3biphenyl and 3COT (with different molar absorptivities). The results in the case of pyrene (triplet absorbing at 415 nm) in Fig. 4(b)similarly show an increase in the decay rate of 3pyrene in the presence of COT.At 350 nm, where 3COT has absorption maxima, the initial part of the oscilloscope trace showed an overall mixed behaviour (due to 3pyrene decay and 3COT formation with different respective molar absorptivities) and remained virtually invariant in the time- scale of 3pyrene decay. The initial mixed behaviour was a result of the non-zero molar absorptivity of 3pyrene at 350 nm. In order to confirm the T-T spectrum of COT, time- resolved energy-transfer spectra were measured using p-terphenyl as donor in cyclohexane. p-Terphenyl was chosen because the Amax due to its triplet absorption lies at 460 nm and its absorption at 350 nm, where Lmax3COT lies, is very low. Fig. 5 shows the time-resolved spectra of T-T energy Table 1 Rate constants for T-T energy transfer between acceptor COT and various donors (D) in benzene rate constant 3D(L/nm) E,/kcal mol-' /lo8 dm3 mol-' s-' biphenyl (360) 65.6 133 naphthalene (4 14) 61 33 p-terphenyl (460) 58.3 38 benzil (480) 53.7 33 C47 (565) 57.5 65 C102 (570) 57.5 50 C120 (505) 58 60 C152 (600) 52 28 C153 (590) 50 15 pyrene (415) 49 7.4 acridine (440) 45.3 7 anthracene (422) 42 3 966 0.07 I H i 0.05 - 3 0.03 - 0.01 - ' (b) -0.01 0 I 10 I 20 I 30 I 40 I 50 ti me/p CJB 0.01 to-Ba#@0'0°1 I I I I J -0.01; 2 4 6 8 10 time/ps Fig.4 A, Oscilloscope traces showing the decay of biphenyl triplet in cyclohexane at 360 nm: (a) in the absence of COT and (b) in the presence of 4 x mol dm-3 COT (A x 2.5).B, Oscilloscope traces showing the decay of pyrene triplet at 415 nm: (a) in the absence of COT, (b) in the presence of 2 x mol dm-3 COT and (c) oscilloscope trace at 350 nm in the presence of 2 x mol dm-3 COT. transfer from p-terphenyl to COT. It is seen that with the passage of time, as absorbance due to triplet p-terphenyl decreased, the simultaneous formation of a new peak match- ing that in Fig. 1 appeared. These results once again con- ) : B wavelength/nm Fig. 5 Time resolved transient absorption spectra in deoxygenated cyclohexane by the energy-transfer mechanism from p-terphenyl donor to COT acceptor at a dose of 10 Gy: after pulse (0)and after 8 PS (0) J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 350 7-1 290 ,/' c 2F I I I J 1500 3000 4500 6000 (1/[COT])/dm3 mot-' Fig. 6 Linear plots of reciprocal 3COT absorbance at 350 nm against reciprocal [COT] in cyclohexane solution. Donors : biphenyl(0)and pyrene (*). firmed the earlier assumption that the transient spectrum shown in Fig. 1 was due to ,COT. Molar Absorptivity of 3COT It is necessary to know the molar absorptivity of the triplet accurately for laser gain measurements. This value can be determined by techniques such as the singlet depletion method or energy-transfer method.26 In this study, the latter method has been followed, the details of which are given below. From eqn. (I) and (11), the fraction of 3D transferring energy to A to give ,A can be written in terms of their respec- tive absorbances (A) and molar absorptivities (8) to give the following relation16 €3AA3D where A3D is the absorbance due to the donor triplet at its known A,,, (Table l), and A,, is the absorbance of acceptor triplet at its A,,, where its molar absorption coefficient is to be determined (350 nm for 3COT).&3D and &3A are the respec- tive molar absorptivities of the triplet donor and acceptor. From eqn.(2), the intercept of the linear plot of l/A3,, us. l/[A] gives the value of &3A by substitution of known values of A,, and &3A. For estimation of &(,COT), both biphenyl and pyrene were used as donors. Fig. 6 shows linear plots of l/A3A against l[A] with these two donors in the presence of COT in cyclohexane.From the respective measured inter- cepts and using values of E for biphenyl at 360 nm of 42800 dm3 mol-' cm-' and for pyrene at 415 nm of 30400 dm3 mol-' cm-' from the literature,26 the molar absorption coef- ficient, &(,COT),at 350 nm in cyclohexane was estimated to be 7000 f300 dm3 mol-' cm-'. Reactivity and Energy Levels of 3COT These studies were carried out in benzene where the solvent triplet yield is fairly large (Gtriplet= 4.2)27and the high energy level of ,benzene (84 kcal mol-') ensures formation of solute triplets in the ns timescale. As a result, sufficiently high con- centrations of triplets of many molecules can be produced by radiolysis of benzene solutions, especially in those cases where the intersystem-crossing efficiency is low.However, as the benzene triplet lifetime is short (few ns), high concentra- tions of solutes are required to generate measurable yields of solute triplets. It was observed that although the yield of J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3COT in benzene was higher than that in cyclohexane, its molar absorptivity (at A,,, = 350 nm) was lower [%COT(~~~~)/&~COT(~~H12)x 0.51. We have reported the triplet characteristics of many laser dyes of the 7-aminocoumarin lass.^*,^' The triplet energy levels of these dyes (viz. C47, C102, C120, C152 and C153) and their absorption maxima are listed in Table 1. Energy-transfer studies were carried out in benzene using these dyes as donors and COT as the acceptor.In these studies the ratio of donor to the maximum acceptor concentration was always kept >25. The triplet decay of the donor triplet was moni- tored at its absorption maximum at various concentrations of COT (5x 10'-8 x low4 mol dm-3). Additional energy-transfer studies were carried out with naphthalene, p-terphenyl, benzil, acridine, biphenyl and pyrene as donors. The quenching rate constants thus determined are listed in Table 1. In all these cases, T-T energy transfer was moni- tored by following the decay of donor triplet at suitable wavelengths. To determine the energy-transfer rate constants, slopes of the best-fit linear plots of eqn. (1) are used. These values were observed to approach the diffusion-controlled limit when the energy of the donor triplet was higher than ca.50 kcal mol-', as in the case with laser dyes and with p-terphenyl, naphthalene and benzil. For donors like pyrene, acridine and anthracene, with triplet energy below this value, the rates gradually dropped below the diffusion-controlled limit by almost two orders of magnitude, indicating a concur- rent decrease in the energy-transfer efficiency. These observa- tions suggest that the energy level of 3COT lies in the range 40-42 kcal mol-'. In the case of anthracene, the rates are almost 50 times lower than the diffusion limits. This suggests that the energy level of 3COT is close to the anthracene triplet energy level, with the difference between these being less than 2 kcal mol-'.This observation was further con- firmed when azulene, with its triplet energy level at 39-31 kcal mol-', was used as the donor.' In this case no energy transfer could be observed even at a very high concentration of COT. It confirmed that the triplet energy level value of COT was higher than that of azulene. In a separate study, when 3COT was used as the donor, an energy-transfer mechanism to azulene could not be confirmed quantitatively, mainly owing to the short lifetime of 3azulene and its low molar absorptivity. In Fig. 7, a plot of log (keJ against 3D energy level is presented for the various donors used in this study. As discussed above, from the order of the k,, value, the energy level of COT triplet was observed to lie in the range 40-42 kcal mol-l.Since the triplet energy level of COT has been estimated to be close to that of triplet anthracene, there is a possibility of a reverse energy-transfer mechanism from COT to anthracene LA-I 1J-. -0840 46 52 58 64 70 ET(3D)/kcal mol-' Fig. 7 Plot showing log (ke,)(T-T energy-transfer rate constant) as a function of E, (energy level of donor lowest triplet state) in benzene l8 I-----1 / /' 0' 0.0 I 0.1 I 0.2 I 0.3 I 0.4 0.5 [COTI/[ANl Fig. 8 Plot of k+/[AN] against [COT]/[AN] under equilibrium conditions for 3AN -P COT energy transfer in benzene at a dose of 7 GY in the form of an equilibrium established between these two species. It was indeed observed that with the increase of anthracene (AN) concentration the yield of 3COT decreased proportionately, suggesting the existence of such an equi-librium.kf 3AN + COT 3COT + AN (111) kb At a low absorbed dose, as the individual concentrations of the triplets of AN or COT were comparatively less (~500 times) than these solute concentrations, the forward and the backward reaction kinetics in eqn. (111)above could be rep- resented in terms of a pseudo-first-order process as given below. -dC3AN] = kf[COT][3AN] = k4,C3AN] (3a)dt The observed decay rate of the transient signal due to 3AN then could be expressed in terms of these forward and back- ward pseudo-first-order processes or the respective rate constants3' as (4) where k, and kb are the rate constants for the forward and backward reactions, respectively, for equilibrium (111).For the above relationship to be valid, in addition to the low total triplet yield maintained during the experiment, any other decay modes for these triplets were assumed to be negligible. By rearranging eqn. (4) above, a linear relation was obtained for the variation of k,/[AN] with [COT]/[AN], with its slope k, and intercept kb. The linear plot presented in Fig. 8 indicates the validity of this assumption and gives the equi- librium constant value, K x 30. From this estimate of the back energy-transfer rate constant, k, x 1 x lo7 dm3 mol-' s-', the E, of COT was confirmed to be 41 & 1kcal mol-'. Conclusions Although cyclooctatetraene has been in use as a quencher of dye triplets during laser operation, the mechanism responsible for the quenching process and the fate of the resulting 3COT is still not well understood.To study the 968 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 properties of 3COT, it was not possible to generate it by con- ventional methods such as flash photolysis and low-temperature ma trix-isolation techniques. The reasons may be due to the low intersystem-crossing yield of %OT and its small molar absorption coeficient, so that its characterisation 4 5 6 G. S. Hammond, N. J. Turro and R. S. H. Liu, J. Org. Chem., 1963,28,3297. A. A. Lamola, in Techniques of Organic Chemistry, ed. A. A. Lamola and N. J. Turro, International Science Publishers, New York, 1969, vol.14, p. 17. F. P. Schafer, in Dye-lasers, Topics in Applied Physics, ed. F. P. by optical absorption techniques was difficult. From the results obtained from these pulse radiolysis studies, it is amply clear that this indirect method of generation of solute triplets is a helpful tool for such investigations. The T-T absorption spectrum of COT shows that it has negligible absorption in the wavelength region above 400 nm, which is the most commonly used tunable range for various dye lasers. Even in the region where the 3COT has maximum absorption, its molar absorption coefficient is not very high. Therefore, any loss in laser gain due to 3COT absorption will be negligible. A low value of the energy level of %OT should allow its use as an efficient quencher of triplets of polyphenyls 7 8 9 10 11 12 13 14 Schafer, Springer-Verlag, Berlin, 1973, Vol.1, p. 1. D. P. Sorokin, J. R. Lankard, V. L. Moruzzi and E. C. Hammond, J. Chem. Phys., 1968,48,4726. W. Schmidt and F. P. Schafer, 2. Naturforsch., 1967,22,1563. W. H. Winters, H. I. Mandelburg and W. B. Mohr, Appl. Phys. Lett., 1974, 25, 723. A. N. Fletcher, Appl. Phys., 1983,32,9. R. Pappalardo, H. Samelson and A. Lempicki, IEEE J. Quant. EIectr., 1970,6, 716. R. Pappalardo, H. Samelson and A. Lempicki, Appl. Phys. Lett., 1970,16,267. J. B. Marling, D. W. Dregg and L. Wood, Appl. Phys. Lett., 1970, 17, 527. G. Jones 11, S. F. Griffin, C. Choi and W. R. Bergmark, J. Org. and coumarin dyes commonly used in lasers. In this context it needs to be mentioned that initially the probable triplet energy level of COT was assumed to lie in the range 48-50 kcal mol-'.Therefore, it was believed to be inefficient for deactivation of triplets of rhodamine dyes,31 whose triplet energy levels were in the range 40-42 kcal mol-'. This remained a paradox, as on the contrary, COT was used suc- cessfully in dye laser systems employing Rhodamine-6G. Therefore, the observations were considered to be more trivial than simple T-T energy transfer. Our studies clearly demonstrate the capabilities of COT as a quencher of rho- damine triplets by an energy-transfer process, although the actual transfer process is less efficient than that of coumarin dyes. In our studies, the ultimate fate of 3COT in these sol- vents could not be resolved.Its longer lifetime, as measured, may pose some problems during laser operation. One of the possible paths of its deactivation is via the formation of pro- ducts such as benzene and acetylene, which is an eventual degradation process for the polyene-like COT. The compa- ratively slow interaction rate of 3COT with molecular oxygen suggests chemical reaction rather than energy transfer. From the decay trace presented in Fig. 2, it is observed that in the presence of dissolved oxygen, unlike in its absence (N2 saturated), the absorption of 3COT does not decrease to the baseline value. Qualitatively, this indicates the formation of a permanent product absorbing at this wavelength. 15 16 17 18 19 20 21 22 23 24 25 26 27 Chem., 1984,49,2705. G. Jones 11, in Dye Laser Principles and Applications, ed.F. J. Duarte and L. W. Hillman, Academic Press, New York, 1990. K. I. Priyadarsini, D. B. Naik, P. N. Moorthy and J. P. Mittal, in Proceedings of the 7th Tihany Conference on Radiation Chem- istry, ed. J. Dobo, L. Nyikos and R. Schilter, Hungarian Chemi- cal Society, Budapest, 1991, p. 105. S. N. Guha, P. N. Moorthy, K. Kishore, D. B. Naik and K. N. Rao, Proc. Indian Acad. Sci., 1987,99,261. E. M. Fielden, in The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis, ed. J. H. Baxendale and F. Busi, Riedel, Dordrecht, 1982, pp. 58, 59. A. Singh, Radiat. Res. Rev., 1972,4, 1. J. K. Thomas, Annu. Rev. Phys. Chem., 1970,21,17. F. Busi, in The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis, ed. J. H. Baxendale and F. Busi, Riedel, Dordrecht, 1982, p. 417. E. J. Land and A. J. Swallow, Trans. Faraday SOC., 1968, 64, 1247. (a)Y. Ito, T. Azuma, Y.Katsumura, Y. Aoki, Y. Tabata and K. Kimura, Radiat. Phys. Chem., 1987,29,31; (b)G. Beck and J. K. Thomas, J. Phys. Chem., 1972, 76, 3856; (c) P. I. Kimmel and H. L. Strauss, J. Phys. Chem., 1968,72,2813. S. L. Murov, Handbook of Photochemistry, Dekker, New York, 1973. N. J. Turro, Modern Molecular Photochemistry, Benjamin, New York, 1978. I. Carmichael and G. L. Hug, J. Phys. Chem. Ref. Data, 1986,15, 1, and references therein. J. H. Baxendale and M. Fiti, J. Chem. SOC., Faraday Trans. 2, 1972,68,218. The authors thank Dr. J. P. Mittal for his encouragement and support during the course of this study. 28 29 K. I. Priyadarsini, D. €3. Naik and P. N. Moorthy, J. Photochem. Photobiol., A: Chem., 1990,54,251; 1989,46,239. K. I. Priyadarsini, D. B. Naik and P. N. Moorthy, Chem. Phys. Lett., 1989,157, 525. References 30 31 P. Wardman, J. Phys. Chem. Ref. Data, 1989,18, 1637. A. K. Chibisov and V. E. Korobov, Russ. Chem. Rev., 1983, 52, 1 D. P. Evans, J. Chem. SOC., 1957,1351. 27. 2 R. E. Kellogg and W. T. Simpson, J. Am. Chem. SOC., 1965, 87, 4230. 3 R. S. H. Liu, N. J. Turro and G. S. Hammond, J. Am. Chem. SOC.,1965,87, 3406. Paper 3/04645A; Received 3rd August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000963

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(7), 969-972 Formation and Structure of Langmuir-Blodgett Films of C,, and Arachidic Acid Ciaran Ewins and Brian Stewart Department of Chemistry and Chemical Engineering, University of Paisley, High Street, Paisley, UK PA I 2BE ~~ ~~ ~~ Langmuir films have been prepared using pure c60 and arachidic acid mixed with c60 in the ratio of 1 : 1 and 4.2 :1. Langmuir-Blodgett (LB) films were then deposited on a variety of substrates with Y-type deposition being observed. Films containing bilayers of arachidic acid alternating with both single and double monolayers of C,, were prepared. The structure of the arachidic acid component in the mixed films was investigated by grazing- incidence FTIR spectroscopy and was shown to be similar to that in an LB film of pure arachidic acid.The absence of structural disruption in the presence of C,, provides strong evidence for an ordered alternant arrangement of fullerenes and arachidic acid in the films. LB films incorporating c60 have been reported by several groups. c60 is one of an increasing number of new com- pounds that form LB films but are not amphiphiles with hydrophobic and hydrophilic parts to the molecule. It has been found that C,, forms stable monolayerl and multi-layer films' at an air/water interface but that these films do not transfer well to solid substrates. Transfer promoters such as arachidic acid' (CH3C18H36C02H) and stearyl alcohol3 (C,,H,,OH) have been used to aid deposition of good quality films. Transfer promoters have in the past been suc- cessfully applied to the deposition of other materials such as phthal~cyanines.~ Different values for the area per molecule (A,) have been reported for LB films of c60 ranging from 32 A2 to 96 A2.The theoretical A, calculated' for an ideal close-packed monolayer is 92 A'. The lower observed values have been attributed to the formation of floating multilayer films. The chosen solvent and the concentration of the spreading solu- tion seem to have an effect on this value.2 The transfer of these films to glass has given variable results, with no transfer' and 60% Z-type deposition2 being reported. The theoretical ratio of A, values for arachidic acid : c60 is 4.2 : 1. The use of a solution containing arachidic acid and c60 in this mole ratio is expected to produce Langmuir films in which both components occupy the same total area.The transfer ratio for such a mixture is improved to almost The interesting point about these films is that the value for A, corresponds to that expected for the arachidic acid alone. It has been suggested that this is due to the c60 being 'squeezed out' of the arachidic acid monolayer during compression to sit on top of the alkyl chains. Langmuir films of this type have been deposited in a Y-type manner, suggest- ing an alternant bilayer structure of arachidic acid and c60 [Fig. l(a)]. Some evidence for such a film structure has come from film thickness measurements. In the present work we have investigated the structure of the alternate bilayer films using grazing incidence (GI) FTIR spectroscopy.In addition we have prepared an LB film con- taining monolayers of c60 sandwiched between bilayers of arachidic acid [Fig. l(b)]. This was achieved using an alter- nate layer Langmuir trough, with separate films of arachidic acid and 4.2 :1 arachidic acid : c6,. The proposed structure is supported by thickness measurements, UV-VIS absorb-ance and GI FTIR spectroscopy. GI FTIR Spectroscopy GI FTIR has been used extensively to study LB films and can provide information about the orientation of molecules rela- tive to a metal surface.6 The success of this technique relies on the fact that the component of the IR radiation that is polarised perpendicular (p-polarised) to the metal substrate gives an enhanced absorption, while the component polarised parallel (s-polarisation) to the surface gives no measurable absorption, due to a 180" phase change upon reflection causing destructive interference.The polarisation is con-trolled by a polariser and the angle of incidence is usually 80" (also giving greater pathlength and thus sensitivity). For a fatty acid molecular chain perpendicular to a surface, the transition dipole of the methyl symmetric stretch v, (CH,) is nearly perpendicular to the surface and thus absorbs p- polarised light whereas the symmetric and asymmetric stretches of the methylenes v, (CH,) and v, (CH,) are parallel to the surface and so will not absorb p-polarised light.This allows the intensity of the CH, absorption to be measured in two different polarisations (transmission for parallel and GI for perpendicular) and hence the orientation of the alkyl chains can be calculated., Arachidic acid has been studied by this technique and found to be tilted at an angle, 0 = 25" to the surface normal.8 GI FTIR can also be used alone in a more qualitative manner to assess the order and orientation of an LB film by comparison with films of known structure. Experimental The LB trough used was a NIMA alternate layer troughg located in a class lo00 clean room. The trough is made of PTFE and the surface pressure was measured by a Wilhelmy balance. The toluene used was scintillation grade, the arachi- dic acid was purchased from Aldrich (99%)and was used as 000000 000000 I I I alternant bilayer film CG0monolayer film Fig.1 (a) Proposed structure of 4.2 :1 arachidic acid-C,, LB film. (b)Proposed structure of arachidic acid<,, monolayer film. received. The c60 was prepared in this laboratory and puri- fied by column chromatography over silica/graphite" fol-lowed by recrystallisation from cyclohexane. Hydrophobic quartz slides were prepared by treating Spectrosil B (Thermal Syndicates) with hexamethyldisilazene. Aluminium-coated glass slides were prepared in an RF sputter coater. The water was purified on a deionisation-reverse osmosis-UV sterilisa- tion system (Elgastat/UHP) and had a resistivity of 18 MR cm-'.The subphase water pH was 5.8 and the temperature was 22°C.Typical concentrations of the solutions used were: c60, 0.35 mg cm-3 (4.86 x mol dm-3) and arachidic acid, 1.1 mg cm-3 (3.52 x mol dm-3) in toluene. To prepare the LB films, the solutions were mixed and spread on the water surface uia micro-syringe. The solvent was allowed to evaporate for 20 min and the floating films were then com- pressed at a rate of between 40 and 70 A2 molecule-'min-'. Langmuir films were prepared from pure c60 and from the mixed solutions of molar ratios, 1 :1 c60 : arachidic acid and 4.2 : 1 arachidic acid : c60. The floating films were transferred at a surface pressure, n = 20 mN m-', for the pure c60 and n: = 30 mN m-' for the mixed films. To prepare the monolayer c60 LB films a two-compartment alternate layer trough was used.Solutions of arachidic acid and of 4.2 :1 arachidic acid :c60 were spread in compartments A and B, respectively. Films were deposited by alternately passing a cleaned slide down through A and then up through B until the required number of c60 mono-layers were deposited. The films produced were studied by UV-VIS spectroscopy and GI FTIR. The spectrometers used were a Perkin-Elmer Lamda 9 spectrophotometer for transmission UV-VIS spec-troscopy and a BIO-RAD FTS 40 at 2 cm-' resolution for GI FTIR. Film thickness measurements were carried out with a Watson-Barnett interferometer fitted to a Nikon Optiphot Microscope. This method assesses film thickness by measur- ing the shift in interference fringes across a step in film height.To aid this the LB films were coated with a thin film of vacuum-evaporated silver. Results and Discussion Cs0 Film C,, appears to form a compressed monolayer at the air/ water interface under the conditions stated. The isotherm [Fig. 2(a)] gives A, = 90 A2. This is obtained by extrapo- lating the linear part of the plot back to zero pressure and compares well with the calculated value of 92 A*. The float- ing film is brittle and does not transfer well, giving low trans- fer ratios and an uneven film on glass. 1 :1 Molar Ratio C,,-Arachidic Acid Film The 1 :1 C60-araChidiC acid film gave the isotherm shown in Fig. 2(b). Extrapolating back to zero pressure gives A, x 60 A', which is close to the calculated value.Since the isotherm is not that of two immiscible liquids'' this suggests that the C60 and arachidic acid are mixed in a monolayer film with the arachidic acid uniformly dispersed throughout the film. The presence of the transfer-promoter arachidic acid allowed the deposition of good quality films onto aluminium-coated slides. The transfer ratios were approximately 100% but the deposition was of Y-type, i.e. deposition on the upstroke and downstroke. GI FTIR spectroscopy of 20 layers on an aluminium-coated glass slide [Fig. 3(a)] confirms the presence of the c60 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 8ol70 7 60-E z E 50-1s? 40-2?;30-z;20-10 -n, 10 30 50 70 90 110 130 A, fA2 Fig.2 (a) Isotherm of C,, on ultrapure water. (b) Isotherm of 1 : 1 arachidic acid-C,, .(c) Isotherm of 4.2 : 1 arachidic acid<,, . in the film with all fullerene bands occurring at the same wavenumbers as in a pure sample.12 Fig. 4(a) shows the C-H stretch region. The intensities of the v, and v,(CH,) bands (transition moments perpendicular to the chain axis) are greater than would be expected in a pure arachidic acid film of the same number of layers (where 0 = 25"), suggesting greater tilt and/or disorder. The v,(CH,) bands are weak and the v,(CH,) band is very weak. This indicates disorder of the 0.08 Q, 0.06 I t I I I I I I 1 1800 1600 1400 1200 1000 800 600 400 wavenumber/cm-' Fig. 3 GI FTIR absorption: (a) 1 :1 arachidic acid-C,,; (b) arachi-dic acid-€,, monolayer LB film; (c) pure arachidic acid; (d) 4.2 : 1 arachidic acid-c,, all on aluminium 0.07O.O81 0.03--3100 3d50 3000 2$50 2900 2850 2800 27150 2700 wavenumber/cm-' Fig.4 GI FTIR absorption in the C-H stretch region for: (a) 1 : 1 arachidic acid<,,; (b) arachidic acid<,, monolayer LB film; (c) arachidic acid LB film; (d) 4.2: 1 arachidic acid-C,, all on alu-minium J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 chain ends which is expected to result from methyl-fullerene packing. The C=O stretch is only observed at 1707 cm-', indica-tive of 'facing' dimers in the carboxylic acid groups. Facing dimers are the norm for a pure arachidic acid LB film.There is a very weak band at 3020 cm-' due to aromatic C-H stretching. This is most likely to be toluene solvent trapped in the film. 4.2 :1 Molar Ratio Film The 4.2 :1 arachidic acid-C,, mixed film gives the isotherm shown in Fig. 2(c). It is identical to that previously reported' and confirms that the total area occupied is the same as that for arachidic acid alone. The film is very stable, the total area only falling by 1.5% over 2 h at 30 mN m-'. Aluminium, silicon and quartz substrates gave Y-type deposition. The presence of the c60 was confirmed by transmission UV-VIS spectroscopy and GI FTIR. The GI FTIR spectrum [Fig. 3(d)] shows the four c60 bands in the expected positions. The c60 absorbance at 1428 cm-' is somewhat obscured by the v,(CO,) at 1429 cm-'.Only one type of carbonyl is visible. This is the 'facing dimer' stretch at 1706 cm-'. Fig. 4(d) shows the C-H region for the mixed film and Fig. 4(c) the same region for a film of pure arachidic acid prepared under identical conditions. The overall similarity in the shape and relative intensities of the two spectra in the C-H region shows that the arrangement of the arachidic acid is little different from that in a pure arachidic acid film. This is only possible if the film is indeed composed of alternating double layers of c60 and arachidic acid with carboxylic acid groups facing each other. The two main differences are that the v,(CH,)ip (in-plane) stretch is weaker relative to the out-of-plane stretch [v,(CH,)op] com-pared with the mixed film and that the intensities of all C-H stretching bands are weaker overall in the mixed film.The differences in the v(CH,) in-plane and out-of-plane intensities at 2963 and 2954 cm-' are most likely due to the presence of the c60 on top of the methyl groups. Observed wavenumbers and assignments of vibrational bands are given in Table 1. C,, Monolayer Films LB films containing monomolecular layers of c60 between bilayers of arachidic acid [Fig. l(a)] were prepared with good transfer ratios using the alternate trough method. The first dip was down through a monolayer of arachidic acid and the second dip was then up through the arachidic acid-C,, film. Films were prepared on hydrophobic quartz with 5, 10 and 20 monolayers of C,, by dipping a total of 10, 20 and 40 times, respectively.A plot of absorbance (at 343 nm) us. Table 1 GI FTIR main absorbances of 4.2 :1 arachidic acid :(20-layer LB film on aluminium) _____~ wavenumber/cm - assignment notes 2963 2955 2917 2872 2850 1707 1461-1450 1429 1354-1175 1183 577 528 ~~ ~ Identical band positions (to within +2 cm-') are observed for a 1 : 1 film. 971 0.3 ~ 0.2 0 (0-e2 -", 0.1 0 0 U I I , I I 0 10 20 30 40 number of dips Fig. 5 Absorbance at 341 nm us. the number of dips for an arachi- dic acid-C,, monolayer LB film number of dips (Fig. 5) is approximately linear (gradient = 0.0048 absorbance units per dip) and is evidence of good film quality in the monomolecular layer c60 film.A similar plot for a 4.2:l arachidic acid :C60 LB film (c60 bilayer film) by Williams et aL2 has approximately twice the gradient (0.012),corresponding to the same absorbance per c60 layer. The absorbance of a multilayer system can be used to determine the layer coverage (0)if the absorption cross- section and the layer number density are known: @ = A/ (2.303aN),where A is the decadic absorbance [10g~~(~~/Z)], a is the cross-section in cm2 molecule-' and N is the mono- layer number density in molecules cm-2. The absorption cross-section may be obtained from the molar absorptivity in solution provided that there is no change in the nature of the transition in going to the solid state.For c60, the bands at 343, 268 and 220 nm show no detectable differences in wave- length or bandwidth between solution in toluene or cyclo- hexane and the solid-state film. It therefore seems justified to obtain a value of the solid-state absorption cross-section from a solution molar absorption coefficient (E) measurement. A small solvent effect on E is observed between toluene and cyclohexane. However, toluene solvation is probably a better model for the fullerene close-packing environment. The molar absorption coeficient at 343 nm is found to be (56 316 f3%) dm3 mol-' cm-' for c60 [>99% by high-performance liquid chromatography (HPLC)] in toluene [average of four determinations in the concentration range (1.69-4.15) x 10-mol dm-,].This value is considerably larger, and therefore more reliable, than that previously reported. The absorp- tion cross-section is calculated from the relation : lOOOE c=-2.3O3NA Using our solution molar absorption coefficient, = 9.352 x cm2 molecule-'. Using this, together with the molecular density in the crystalline (1 11) planes;14 Nccp= 1.21 x lo-'' molecule cm-', the calculated monolayer coverages for the three films are 5.02, 10.33 and 23.53 L. This compares well with the ideal values: 5, 10 and 20, perhaps indicating that some multilayering is occurring during the time required for higher numbers of dips. The UV-VIS absorption spectrum of the monolayer c60 film (Fig. 6) is very similar to that of an alternant bilayer film J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Oa5 t 0.4i r! f 0.3I’M 0tI I I 1 I I ,‘@I 200 300 400 500 600 700 800 /lf/nm Fig. 6 UV-VIS spectrum of arachidic acid-C,, (b)bilayer LB films containing 20 layers of C,, (a) m onolayer and with no significant shifts in peak positions being observed. The absorption background at 200 nm is approximately twice that observed in the bilayer films since double the number of arachidic acid layers are present. The thickness of a film produced from a total of 41 dips containing 20 monolayers of c60 is expected to be 126.6 nm, taking the thicknesses of the C60 and arachidic acid mono- layers to be 1.0 and 2.6 nm, respectively. The actual film thickness was measured to be 124 nm by interferometry, offering further good evidence for the proposed structure.GI FTIR was carried out on a film containing 10 mono-layers of C,o and 20 layers of arachidic acid deposited on aluminium [Fig. 3(b)]. The c60 bands are present in their usual positions with the expected intensities. In the C-H stretch region [Fig. qb)]the spectrum is very close to that of a pure arachidic acid LB film [Fig. 4(c)] and is indicative of a well ordered film. Conclusions We have shown that LB films of different c6, :arachidic acid ratios can be deposited. GI FTIR spectroscopy of films deposited on aluminium shows that the arachidic acid chains are oriented in a manner similar to that in an LB film of pure arachidic acid.This result, together with previous film thick- ness measurements, confirms that in the 4.2:1 mixed film the deposited film consists of alternating double layers of arachi-dic acid and c60. The arachidic acid/C6, monolayer films also contain well ordered bilayers of arachidic acid but only single monolayers of c60 which is confirmed by our film thickness and absorbance measurements. We have demonstrated an ability to control the formation of well ordered layers of fullerenes of varying thickness which are insulated from each other by arachidic acid. This offers a valuable tool for probing the effects of dimensionality on the electrical, optical and spectroscopic properties of fullerene films. We would like to thank Denny Wernham for preparing and purifying the 0.References 1 Y. S. Obeng and A. J. Bard, J. Am. Chem. SOC.,1991,113,6279. 2 G. Williams, C. Pearson, M. R. Bryce and M. C. Petty, Thin Solid Films, 1992,209, 150. 3 J. Milliken, D. D. Dominquez, H. H. Nelson and W. R. Barger, Chem. Muter., 1992,4,252. 4 Yansong Fu and A. B. P. Lever, J. Phys. Chem., 1991,956979. 5 J. M. Hawkins, A. Meyer, T. A. Lewis, S. Loren and F. Hollander, Science, 1991,252, 312. 6 An Introduction to Ultrathin Organic Films, ed. A. Ullman, Aca- demic Press, New York, 1991. 7 P. A. Chollet, Thin Solid Films, 1980,68, 13. 8 D. L. Allara and R. G. Nuzzo, Langmuir, 1985,1,45. 9 F. Grunfeld, Rev. Sci. Instrum., 1993,64, 548. 10 W. A. Scrivens, P. V. Bedworth and J. M. Tour, J. Am. Chem. SOC., 1992, 114, 7917. 11 Langmuir Blodgett Films, ed. G. Roberts, Plenum, New York, 1990. 12 P. Bhyrappa, A. Penicaud, M. Kawamoto and C. Reed, J. Chem. SOC.,Chem. Commun., 1992,936. 13 D. R. Haynes, A. Tokmakoff and S. M. George, Chem. Phys. Lett., 1993,214, 50. 14 W. Kratschmer, L. D. Lamb, K. Fostiropoulos and D. R. Huffman, Nature (London), 1990,347,354. Paper 3/07018B; Received 25th November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000969

出版商:RSC    

年代:1994

数据来源: RSC