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Ferrocene-modified oligopeptide as model compound for charge-transfer interactions with organic electron acceptors
Materials Science and Engineering C 18 Ž2001. 121–124 www.elsevier.comrlocatermsec
Ferrocene-modified oligopeptide as model compound for charge-transfer interactions with organic electron acceptors Marek Pietraszkiewicz a,) , Agnieszka Wieckowska b, Renata Bilewicz b, Aleksandra Misicka b, Lucjan Piela b, Krzysztof Bajdor c a
Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44 r 52, 01 224 Warsaw, Poland b Chemistry Department, Warsaw UniÕersity, ul. Pasteura 1, 02 093 Warsaw, Poland c Industrial Chemistry Research Institute, ul. Rydygiera 8, 01 793 Warsaw, Poland
Abstract Two newly synthesised 21 - amino acid peptides based on L-leucine and L-lysine substituted with ferrocene groups preserve the helical structure of the peptide and behave as multi-centre donor systems with six electroactive reversible redox centres per molecule. The formal potential of the hexaferrocene compound is slightly less positive than that of its single centre analogue. CT interactions of the ferrocene peptides with chloranil are not confirmed by UV-vis spectra. q 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Advanced functional materials and nanomaterials are the focus of the current quest for futuristic technologies of the XXI century. In particular, nanocomputers are a hot topic, and many groups compete in this direction. Of special interest are molecular switches working on Aon – offB mode. The moving part of the molecule can be a molecular fragment Žslow movement., proton Žmuch faster., and electron Žthe fastest.. Stoddart et al. w1–5x contributed significantly to the molecular switch concept using rotaxanes and catenanes. However, electronically bistable supramolecular systems would be of great interest because they have the fastest switching capacity. Interest in multi-centre charge transfer compounds is attributable to their potential utility as molecular conducting materials, superconductors and molecular memory w6– 8x. Donors and acceptors can be fixed in space by suitable molecular scaffolding playing mainly a structural role. Porphyrin-laden maquettes w9,10x, peptides modified with organic moieties or transition-metal complexes capable of photoinduced electron-transfer within helical domains w11x and peptide-based assemblies of organic chromophores as moieties reminiscent of the reaction centre of photosynthetic bacteria have been reported in literature. a y Helical
)
Corresponding author. E-mail addresses: [email protected] ŽM. Pietraszkiewicz., [email protected] ŽR. Bilewicz..
peptide incorporating crown ether moieties was found to produce an ion channel from aligned crown ether molecules w12x. The peptide backbone seems to be especially useful in controlling the spatial arrangement of electron donor units. Tetrathiafulvalene units have been successfully incorporated into the peptide structure w13,14x. Our aim was to design an electroactive peptide containing six independent reversible redox centres, each one located above the other and acting as a well-defined polynuclear donor for new donor–acceptor materials, and for the studies of interactions in complex donor–acceptor systems. Electronic bistability has been theoretically predicted by Stolarczyk and Piela w15x. Two newly synthesised peptides, comprising 21 amino acids substituted with electroactive ferrocene groups, preserve the helical structure of the peptide and behave as multi-centre donor systems with six independent reversible redox centres per molecule.
2. Results and discussion Formulas of the synthesized 21-oligopeptide and its substituted analogs are shown in Scheme 1. The starting 21-oligopeptide: AcŽLeu-Lys-Leu-Leu-LeuLys-Leu. 3 NH 2 was prepared by standard solid-phase methods on Rink-resin by using fluorenylmethoxycarbonyl N a-terminal protection and t-butyloxycarbonyl N ´ protection of lysine. After cleavage from the resin by 95% trifluoroacetic acid ŽTFA., the crude peptide was purified
0928-4931r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 0 1 . 0 0 3 7 9 - 4
122
M. Pietraszkiewicz et al.r Materials Science and Engineering C 18 (2001) 121–124
Scheme 1. Structure of the compounds studied.
by RP HPLC and characterized by FAB Žcalc. 2526.5, found 2526.5.. This oligopeptide Žcompound 1. in its hexatrifluoroacetate form Ž M W 3210, 100 mg. was suspended in acetonitrile Ž4 ml. and triethylamine Ž0.4 ml. under Ar and stirred with ferrocenyl aldehyde Ž120 mg, 3-fold molar excess. at 80 8C. After 30 min, the cloudy solution was cooled, filtered and washed with MeCN. The product Žcompound 2. —hexa-Schiff base was dried in vacuum Žyield: 38 mg.. LSIMS MS: 3709.7 ŽMq. , 3747.6 ŽM q Kq. , calcd. for C 194 H 290 N28 O 22 Fe 6 : 3701. Compound 1 in its hexatrifluoroacetate form Ž115.4 mg, 0.036 mmol. was dissolved in a mixture of dry dimethylformamide Ž1 ml. and acetonitrile Ž2 ml. under Ar, and 1 ml of N-methylmorpholine and 5 mg of 4-N, N-dimethylaminopyridine was added with stirring. 0.5 mmol of ferrocenylcarboxylic acid fluoride Žprepared freshly from the carboxylic acid and cyanuric fluoride. in 1 ml of acetonitrile was added at 0 8C. Stirring was maintained for 3 h at room temperature and the progress of the reaction was monitored on TLC plate ŽMerck SiO 2 .. Methanol Ž1 ml. was added to quench unreacted fluoride, and all solvents were evaporated under reduced pressure. All soluble materials were dissolved in MeCN Ž5 ml., and the product was filtered, washed with MeCN, and dried in vacuum. Yield of 3 was 127 mg. MALDI TOF MS: 3821.2 ŽM q Naq. , calcd. for C 194 H 290 N28 O 28 Fe 6 : 3797.7. The circular dichroism spectra for unsubstituted peptide 1 and the two electroactive peptides 2 and 3 are shown in Fig. 1. CD curve recorded for unsubstituted starting 21-peptide in methanol at the concentration of 2.75 = 10y5 M showed three CD bands related to Cotton Effect at 220, 208 and 192 nm, with D ´ s y147, y178.8 and 452.1, respectively. This indicated the a-helical secondary structure of the oligopeptide. The hexa-Schiff base obtained from 21-peptide and ferrocenylcarboxaldehyde Žcompound 2, Fig. 1b. turned out to be a-helical secondary structure. The CD spectrum was recorded in 2.68 = 10y5 M solution, showing three CD bands related to Cotton Effect at 208, 218, and 193 nm
Fig. 1. Circular dichroism spectra of xx mM of the peptide Žfirst from the above. and their hexaferrocene derivatives ŽSchiff base and carboxamide, respectively. recorded in methanol.
M. Pietraszkiewicz et al.r Materials Science and Engineering C 18 (2001) 121–124
with D ´ s y115.5, y155.9, and q343.4, respectively. The hexaferrocenylcarboxamide-based 21 peptide Žcompound 3, Fig. 1c. had similar features indicating a-helical secondary structure. The CD spectrum was recorded in 4.13 = 10y5 M solution, showing three CD bands related to Cotton Effect at 207, 219, and 192 nm with D ´ s y193.3, y136.2, and q408, respectively. These results
123
are consistent with the dominance of helix secondary structure for all three peptides. The UV–VIS electronic spectra for ferrocene, compounds 2 and 3 in methanol solutions with and without presence of electron acceptor Žchloranil. are shown in Fig. 2. In the spectrum of ferrocene, an absorption band occurs in the visible region at 445 nm ŽFig. 2a..
Fig. 2. UV–VIS electronic spectra for x mM Ža,d. ferrocene and Žb,e. compound 2 and Žc,f. compound 3 in methanolrchloroform solution in the absence and presence of x mM chloranil.
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M. Pietraszkiewicz et al.r Materials Science and Engineering C 18 (2001) 121–124
The methanol solution of compound 2 exhibited similar spectrum in the visible region with the absorption band for ferrocene chromophore occurring at 455 nm ŽFig. 2b.. Mixing of deoxygenated solutions of chloranil and compound 2 in methanol lead to the formation of a new broad band at ca. 680 nm ŽFig. 2e.. The final absorbance of this band Žafter adding six molar equivalents of chloranil. was about three times lower than that of the ferrocene chromophore present in compound 2. The latter was shifting with increasing chloranil concentration towards longer wavelengths and finally appeared at 470 nm. Similar experiments with quinone, instead of chloranil, lead to the formation of a broad band at ca. 600 nm. No reactivity of simple ferrocene ŽFig. 2a,d. and compound 3 with chloranil or quinone was observed ŽFig. 2c,f.. The spectral position of the new bands formed in the solution of compound 2 and acceptor was sensitive towards the type of acceptor and could be explained either as a result of ferrocene interactions with chloranil or as connected with chemical reactions due to the presence of the imine bond in the molecule. In order to check the electron-donating ability of all ferrocene systems studied, their formal potentials were evaluated using voltammetry. The voltammetric peaks observed for the ferrocene compounds correspond to the reversible electrode processes of the ferrocene ŽFcrFc q . unit, however, some differences in the peak characteristics are observed upon incorporation of the ferrocene groups into the peptide structure. Upon incorporation, a shift of the ferrocene redox potential to more positive values is observed. This change in potential is expected since it follows the general behaviour observed for single ferrocene compounds with electron withdrawing substituents attached directly to the cyclopentadienyl ring. The formal potentials, E8, calculated as the midpoint potentials of the oxidation and reduction peak potentials, Ž Epa q Epc .r2, are 0.420, 0.536 and 0.593 V for ferrocene, compound 2 and compound 3, respectively. The formal potential of the amide compound is much more positive than that of the amine one, confirming that hexaferrocene-amide 3 has lower electron density than the corresponding hexaSchiff base 2. These properties explain the difference in
reactivity of compounds 2 and 3 with the acceptors studied. It is, therefore, conceivable that longer peptide and more ferrocenylamide units are necessary to observe CT interactions with acceptors of quinone-type and the possible electronic bistability effects w15x.
Acknowledgements This work was supported by the State Committee for Scientific Research, project No. 3 T09A 098 14 ŽMNEMON..
References w1x C.P. Collier, G. Mattersteig, E.W. Wong, Y. Luo, K. Beverly, J. Sampaio, F.M. Raymo, J.F. Stoddart, J.R. Heath, Science 289 Ž5482. Ž2000. 1172. w2x M. Asakawa, M. Higuchi, G. Mattersteig, T. Nakamura, A.R. Pease, F.M. Raymo, T. Shimizu, J.F. Stoddart, Adv. Mater. 12 Ž2000. 1099. w3x V. Balzani, A. Credi, S.J. Langford, F.M. Raymo, J.F. Stoddart, M. Venturi, J. Am. Chem. Soc. 122 Ž2000. 3542. w4x V. Balzani, A. Credi, G. Mattersteig, O.A. Matthews, F.M. Raymo, J.F. Stoddart, M. Venturi, A.J.P. White, D.J. Williams, J. Org. Chem. 65 Ž2000. 1924. w5x C.L. Brown, U. Jonas, J.A. Preece, H. Ringsdorf, M. Seitz, J.F. Stoddart, Langmuir 16 Ž2000. 1924. w6x M.R. Bryce, J. Mater. Chem 5 Ž1995. 1481. w7x S. Frenzel, S. Arndt, R.M. Gregorius, K. Mullen, J. Mater. Chem. 5 ˝ Ž1995. 1529. w8x S. Kothaokota, T.L. Mason, D.A. Tirrell, M.J. Fournier, J. Am. Chem. Soc. 117 Ž1995. 536. w9x F. Rabanal, W.F. DeGrado, P.L. Dutton, J. Am. Chem. Soc. 118 Ž1996. 473. w10x D.E. Robertson, R.S. Farid, C.C. Moser, R. Pidikiti, J.D. Lear, W.F. Degrado, P.L. Dutton, Nature 368 Ž1994. 425. w11x G.V. Kozlov, M.Y. Ogawa, J. Am. Chem. Soc. 119 Ž1997. 8377. w12x J.-C. Meillon, N. Voyer, Angew. Chem., Int. Ed. Engl. 36 Ž1997. 967. w13x H. Mihara, Y. Tanaka, T. Fujimoto, N. Nishino, J. Chem. Soc., Perkin Trans. 2 Ž1995. 1915. w14x S. Booth, E.N.K. Wallace, K. Singhal, P.N. Bartlett, J.D. Kilburn, J. Chem. Soc., Perkin Trans. 1 Ž1. Ž1998. 1467. w15x L.Z. Stolarczyk, L. Piela, Chem. Phys. 85 Ž1984. 451.
Ferrocene-modified oligopeptide as model compound for charge-transfer interactions with organic electron acceptors Marek Pietraszkiewicz a,) , Agnieszka Wieckowska b, Renata Bilewicz b, Aleksandra Misicka b, Lucjan Piela b, Krzysztof Bajdor c a
Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44 r 52, 01 224 Warsaw, Poland b Chemistry Department, Warsaw UniÕersity, ul. Pasteura 1, 02 093 Warsaw, Poland c Industrial Chemistry Research Institute, ul. Rydygiera 8, 01 793 Warsaw, Poland
Abstract Two newly synthesised 21 - amino acid peptides based on L-leucine and L-lysine substituted with ferrocene groups preserve the helical structure of the peptide and behave as multi-centre donor systems with six electroactive reversible redox centres per molecule. The formal potential of the hexaferrocene compound is slightly less positive than that of its single centre analogue. CT interactions of the ferrocene peptides with chloranil are not confirmed by UV-vis spectra. q 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Advanced functional materials and nanomaterials are the focus of the current quest for futuristic technologies of the XXI century. In particular, nanocomputers are a hot topic, and many groups compete in this direction. Of special interest are molecular switches working on Aon – offB mode. The moving part of the molecule can be a molecular fragment Žslow movement., proton Žmuch faster., and electron Žthe fastest.. Stoddart et al. w1–5x contributed significantly to the molecular switch concept using rotaxanes and catenanes. However, electronically bistable supramolecular systems would be of great interest because they have the fastest switching capacity. Interest in multi-centre charge transfer compounds is attributable to their potential utility as molecular conducting materials, superconductors and molecular memory w6– 8x. Donors and acceptors can be fixed in space by suitable molecular scaffolding playing mainly a structural role. Porphyrin-laden maquettes w9,10x, peptides modified with organic moieties or transition-metal complexes capable of photoinduced electron-transfer within helical domains w11x and peptide-based assemblies of organic chromophores as moieties reminiscent of the reaction centre of photosynthetic bacteria have been reported in literature. a y Helical
)
Corresponding author. E-mail addresses: [email protected] ŽM. Pietraszkiewicz., [email protected] ŽR. Bilewicz..
peptide incorporating crown ether moieties was found to produce an ion channel from aligned crown ether molecules w12x. The peptide backbone seems to be especially useful in controlling the spatial arrangement of electron donor units. Tetrathiafulvalene units have been successfully incorporated into the peptide structure w13,14x. Our aim was to design an electroactive peptide containing six independent reversible redox centres, each one located above the other and acting as a well-defined polynuclear donor for new donor–acceptor materials, and for the studies of interactions in complex donor–acceptor systems. Electronic bistability has been theoretically predicted by Stolarczyk and Piela w15x. Two newly synthesised peptides, comprising 21 amino acids substituted with electroactive ferrocene groups, preserve the helical structure of the peptide and behave as multi-centre donor systems with six independent reversible redox centres per molecule.
2. Results and discussion Formulas of the synthesized 21-oligopeptide and its substituted analogs are shown in Scheme 1. The starting 21-oligopeptide: AcŽLeu-Lys-Leu-Leu-LeuLys-Leu. 3 NH 2 was prepared by standard solid-phase methods on Rink-resin by using fluorenylmethoxycarbonyl N a-terminal protection and t-butyloxycarbonyl N ´ protection of lysine. After cleavage from the resin by 95% trifluoroacetic acid ŽTFA., the crude peptide was purified
0928-4931r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 0 1 . 0 0 3 7 9 - 4
122
M. Pietraszkiewicz et al.r Materials Science and Engineering C 18 (2001) 121–124
Scheme 1. Structure of the compounds studied.
by RP HPLC and characterized by FAB Žcalc. 2526.5, found 2526.5.. This oligopeptide Žcompound 1. in its hexatrifluoroacetate form Ž M W 3210, 100 mg. was suspended in acetonitrile Ž4 ml. and triethylamine Ž0.4 ml. under Ar and stirred with ferrocenyl aldehyde Ž120 mg, 3-fold molar excess. at 80 8C. After 30 min, the cloudy solution was cooled, filtered and washed with MeCN. The product Žcompound 2. —hexa-Schiff base was dried in vacuum Žyield: 38 mg.. LSIMS MS: 3709.7 ŽMq. , 3747.6 ŽM q Kq. , calcd. for C 194 H 290 N28 O 22 Fe 6 : 3701. Compound 1 in its hexatrifluoroacetate form Ž115.4 mg, 0.036 mmol. was dissolved in a mixture of dry dimethylformamide Ž1 ml. and acetonitrile Ž2 ml. under Ar, and 1 ml of N-methylmorpholine and 5 mg of 4-N, N-dimethylaminopyridine was added with stirring. 0.5 mmol of ferrocenylcarboxylic acid fluoride Žprepared freshly from the carboxylic acid and cyanuric fluoride. in 1 ml of acetonitrile was added at 0 8C. Stirring was maintained for 3 h at room temperature and the progress of the reaction was monitored on TLC plate ŽMerck SiO 2 .. Methanol Ž1 ml. was added to quench unreacted fluoride, and all solvents were evaporated under reduced pressure. All soluble materials were dissolved in MeCN Ž5 ml., and the product was filtered, washed with MeCN, and dried in vacuum. Yield of 3 was 127 mg. MALDI TOF MS: 3821.2 ŽM q Naq. , calcd. for C 194 H 290 N28 O 28 Fe 6 : 3797.7. The circular dichroism spectra for unsubstituted peptide 1 and the two electroactive peptides 2 and 3 are shown in Fig. 1. CD curve recorded for unsubstituted starting 21-peptide in methanol at the concentration of 2.75 = 10y5 M showed three CD bands related to Cotton Effect at 220, 208 and 192 nm, with D ´ s y147, y178.8 and 452.1, respectively. This indicated the a-helical secondary structure of the oligopeptide. The hexa-Schiff base obtained from 21-peptide and ferrocenylcarboxaldehyde Žcompound 2, Fig. 1b. turned out to be a-helical secondary structure. The CD spectrum was recorded in 2.68 = 10y5 M solution, showing three CD bands related to Cotton Effect at 208, 218, and 193 nm
Fig. 1. Circular dichroism spectra of xx mM of the peptide Žfirst from the above. and their hexaferrocene derivatives ŽSchiff base and carboxamide, respectively. recorded in methanol.
M. Pietraszkiewicz et al.r Materials Science and Engineering C 18 (2001) 121–124
with D ´ s y115.5, y155.9, and q343.4, respectively. The hexaferrocenylcarboxamide-based 21 peptide Žcompound 3, Fig. 1c. had similar features indicating a-helical secondary structure. The CD spectrum was recorded in 4.13 = 10y5 M solution, showing three CD bands related to Cotton Effect at 207, 219, and 192 nm with D ´ s y193.3, y136.2, and q408, respectively. These results
123
are consistent with the dominance of helix secondary structure for all three peptides. The UV–VIS electronic spectra for ferrocene, compounds 2 and 3 in methanol solutions with and without presence of electron acceptor Žchloranil. are shown in Fig. 2. In the spectrum of ferrocene, an absorption band occurs in the visible region at 445 nm ŽFig. 2a..
Fig. 2. UV–VIS electronic spectra for x mM Ža,d. ferrocene and Žb,e. compound 2 and Žc,f. compound 3 in methanolrchloroform solution in the absence and presence of x mM chloranil.
124
M. Pietraszkiewicz et al.r Materials Science and Engineering C 18 (2001) 121–124
The methanol solution of compound 2 exhibited similar spectrum in the visible region with the absorption band for ferrocene chromophore occurring at 455 nm ŽFig. 2b.. Mixing of deoxygenated solutions of chloranil and compound 2 in methanol lead to the formation of a new broad band at ca. 680 nm ŽFig. 2e.. The final absorbance of this band Žafter adding six molar equivalents of chloranil. was about three times lower than that of the ferrocene chromophore present in compound 2. The latter was shifting with increasing chloranil concentration towards longer wavelengths and finally appeared at 470 nm. Similar experiments with quinone, instead of chloranil, lead to the formation of a broad band at ca. 600 nm. No reactivity of simple ferrocene ŽFig. 2a,d. and compound 3 with chloranil or quinone was observed ŽFig. 2c,f.. The spectral position of the new bands formed in the solution of compound 2 and acceptor was sensitive towards the type of acceptor and could be explained either as a result of ferrocene interactions with chloranil or as connected with chemical reactions due to the presence of the imine bond in the molecule. In order to check the electron-donating ability of all ferrocene systems studied, their formal potentials were evaluated using voltammetry. The voltammetric peaks observed for the ferrocene compounds correspond to the reversible electrode processes of the ferrocene ŽFcrFc q . unit, however, some differences in the peak characteristics are observed upon incorporation of the ferrocene groups into the peptide structure. Upon incorporation, a shift of the ferrocene redox potential to more positive values is observed. This change in potential is expected since it follows the general behaviour observed for single ferrocene compounds with electron withdrawing substituents attached directly to the cyclopentadienyl ring. The formal potentials, E8, calculated as the midpoint potentials of the oxidation and reduction peak potentials, Ž Epa q Epc .r2, are 0.420, 0.536 and 0.593 V for ferrocene, compound 2 and compound 3, respectively. The formal potential of the amide compound is much more positive than that of the amine one, confirming that hexaferrocene-amide 3 has lower electron density than the corresponding hexaSchiff base 2. These properties explain the difference in
reactivity of compounds 2 and 3 with the acceptors studied. It is, therefore, conceivable that longer peptide and more ferrocenylamide units are necessary to observe CT interactions with acceptors of quinone-type and the possible electronic bistability effects w15x.
Acknowledgements This work was supported by the State Committee for Scientific Research, project No. 3 T09A 098 14 ŽMNEMON..
References w1x C.P. Collier, G. Mattersteig, E.W. Wong, Y. Luo, K. Beverly, J. Sampaio, F.M. Raymo, J.F. Stoddart, J.R. Heath, Science 289 Ž5482. Ž2000. 1172. w2x M. Asakawa, M. Higuchi, G. Mattersteig, T. Nakamura, A.R. Pease, F.M. Raymo, T. Shimizu, J.F. Stoddart, Adv. Mater. 12 Ž2000. 1099. w3x V. Balzani, A. Credi, S.J. Langford, F.M. Raymo, J.F. Stoddart, M. Venturi, J. Am. Chem. Soc. 122 Ž2000. 3542. w4x V. Balzani, A. Credi, G. Mattersteig, O.A. Matthews, F.M. Raymo, J.F. Stoddart, M. Venturi, A.J.P. White, D.J. Williams, J. Org. Chem. 65 Ž2000. 1924. w5x C.L. Brown, U. Jonas, J.A. Preece, H. Ringsdorf, M. Seitz, J.F. Stoddart, Langmuir 16 Ž2000. 1924. w6x M.R. Bryce, J. Mater. Chem 5 Ž1995. 1481. w7x S. Frenzel, S. Arndt, R.M. Gregorius, K. Mullen, J. Mater. Chem. 5 ˝ Ž1995. 1529. w8x S. Kothaokota, T.L. Mason, D.A. Tirrell, M.J. Fournier, J. Am. Chem. Soc. 117 Ž1995. 536. w9x F. Rabanal, W.F. DeGrado, P.L. Dutton, J. Am. Chem. Soc. 118 Ž1996. 473. w10x D.E. Robertson, R.S. Farid, C.C. Moser, R. Pidikiti, J.D. Lear, W.F. Degrado, P.L. Dutton, Nature 368 Ž1994. 425. w11x G.V. Kozlov, M.Y. Ogawa, J. Am. Chem. Soc. 119 Ž1997. 8377. w12x J.-C. Meillon, N. Voyer, Angew. Chem., Int. Ed. Engl. 36 Ž1997. 967. w13x H. Mihara, Y. Tanaka, T. Fujimoto, N. Nishino, J. Chem. Soc., Perkin Trans. 2 Ž1995. 1915. w14x S. Booth, E.N.K. Wallace, K. Singhal, P.N. Bartlett, J.D. Kilburn, J. Chem. Soc., Perkin Trans. 1 Ž1. Ž1998. 1467. w15x L.Z. Stolarczyk, L. Piela, Chem. Phys. 85 Ž1984. 451.