Monolayers of Novel Calix[4]arene Derivative and Its Palladium(II

Jan 24, 2001 - Feng-Feng Lv , Shi-Ling Yuan , Li-Qiang Zheng , Na Li , Li-Zhu Wu .... a U.S. Army doctor stationed in Afghanistan in 2003, Geoffrey Li...
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Langmuir 2001, 17, 1143-1149

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Monolayers of Novel Calix[4]arene Derivative and Its Palladium(II) Complexes Formed at the Air-Water Interface Weijiang He, Fang Liu, Zhifeng Ye, Yu Zhang, Zijian Guo, and Longgen Zhu* State Key Laboratory of Coordination Chemistry, Institute of Coordination Chemistry, Nanjing University, Nanjing 210093, People’s Republic of China

Xiuhong Zhai and Junbai Li Laboratory of Colloid and Interface Science, Center for Molecular Science, Institute of Chemistry, the Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received July 31, 2000. In Final Form: October 12, 2000 5,11,17,23-Tetra-tert-butyl-25,27-di{[(2′-amino-4′-methylthio)butyryl]aminoethoxy}-26,28-dihydroxycalix[4]arene (L) and its mono- and binuclear palladium(II) complexes (PdLCl2 and Pd2LCl4) were synthesized. Their monolayers at the air-water interface were investigated by film balance measurements (compression/ expansion experiments, area relaxation experiments, viscosity measurements) and Brewster angle microscopy. The monolayer of L is inclined to form domains and multilayers, while the monolayer of PdLCl2 demonstrates good reversibility and high collapse pressure. The monolayer formed by Pd2LCl4 exhibits extraordinary cohesiveness, stability, and robustness. Its extraordinary properties are likely due to the intermolecular cross-linkage of precursor complex of Pd2LCl4 via strong chloride bridges. This provides an example of improving mechanical properties of film through intermolecular metal complexing.

Introduction Calixarenes are a class of materials possessing excellent inclusion abilities and offering promising potentials in the fields of molecular recognition, transportation, and separation.1,2 Membranes such as monolayers, LangmuirBlodgett films (LB films) made of calixarenes, are the practical forms to realize these potentials.3-12 Monolayers of calixarenes and their derivatives are able to recognize C60 and C703 and Na+ and K+ at the air-water interface.4 The relationship between the molecular structures of calixarenes and their abilities to form monolayer at the air-water interface has been thoroughly studied.5 The LB film technique is one of the most realizable strategies to obtain devices made from calixarenes. The weak mechanical strength of normal LB films however hinders their practical applications.13,14 Therefore, several (1) Bo¨hmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713. (2) Shinkai, S. Tetrahedron 1993, 8947-8968. (3) Ishikawa, Y.; Kunitake, T.; Matsuda, T.; Otsuka, T.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1989, 736. (4) Kazantseva, Z. I.; Lavrik, N. V.; Nabok, A. V.; Dimitriev, O. P.; Nesterenko, B. A.; Kalchenko, V. I.; Vysotsky, S. V.; Markovskiy, L. N.; Marchenko, A. A. Supramol. Sci. 1997, 4, 341. (5) Merhi, G.; Munoz, M.; Coleman, A. W.; Barrat, G. Supramol. Chem. 1995, 5, 173. (6) Brake, M.; Bo¨hmer, V.; Kramer, P.; Vogt, W.; Wortmann, R. Supramol. Chem. 1993, 2, 65. (7) Conner, M.; Janout, V.; Regen, S. L. J. Am. Chem. Soc. 1993, 115, 1178. (8) Markowitz, M. A.; Janout, V.; Castner, D. G.; Regen, S. L. J. Am. Chem. Soc. 1989, 111, 8192. (9) Dedek, P.; Webber, A. S.; Janout, V.; Hendel, A.; Regen, S. L. Langmuir 1994, 10, 3943. (10) Admas, H.; Davis, F.; Stirling, C. J. M. J. Chem. Soc., Chem. Commun. 1994, 2527. (11) Nabok, A. V.; Richardson, T.; Davis, F.; Stirling, C. J. M. Langmuir 1997, 13, 3198. (12) Dutton, P. J.; Conte, L. Langmuir 1999, 15, 613. (13) Roberts, G. G. In Langmuir-Blodgett Films; Roberts, G., Eds.; Plenum Press: New York, 1990; Chapter 7.

approaches have been adopted in order to enhance the strength of calixarene LB films. Regen’s disulfide calixarenes can be cross-linked to give perforated robust monolayers when exposed to ultraviolet light (254 nm).14 When the monolayer is transferred onto polymeric supports (PTMSP), the formed composite membrane demonstrates high permselectivity toward He, N2, and SF6. In addition, boronic acid calixarene derivative is also able to form robust film, and the extraordinary cohesiveness of the monolayer is ascribed to intermolecular hydrogen bonding.15 In this report, we propose a new strategy to form robust films using metal complexes of calixarene derivatives. On the basis of the knowledge on coordination chemistry, intermolecular metal complexing may be a suitable substitute for intermolecular hydrogen bonding to enhance the mechanical properties of monolayers. Because Pd(II) atoms can be connected by a chloride-bridge in polynuclear Pd(II) complexes,16 molecules of calix[4]arene Pd(II) complex containing chloride atoms may be associated at the air-water interface. Methionine is introduced into calix[4]arene for its high affinity toward Pd(II).16-19 Our preliminary studies showed that 1,3-(distal) derivatives of p-tert-butylcalix[4]arene such as 2 (Scheme 1) could form monolayers at the air-water interface. So 5,11,17,23-tetra-tert-butyl-25,27-di{[(2′-amino-4′-methylthio)butyryl]aminoethoxy}-26,28-dihydroxycalix[4]arene (L) with two low rim amino groups acylated by two methionine (14) Conner, M. D.; Janout, V.; Kudelka, I.; Dedek, P.; Zhu, J.; Regen, S. L. Langmuir 1993, 9, 2389. (15) Hendel, R. A.; Janout, V.; Lee, W.; Regen S. L. Langmuir 1996, 12, 5745. (16) Goggin, P. L.; Goodfellow, R. J.; Haddock, S. R.; Reed, F. J. S.; Smith, J. G.; Thomas, K. M. J. Chem. Soc., Dalton Trans. 1972, 1905. (17) McAuliffe, C. A. J. Chem. Soc. A 1967, 641. (18) Warren, R. C.; McConnell, J. F.; Stephenson, N. C. Acta Crystallogr., Sect. B 1970, 26, 1402. (19) Pettit, L. D.; Bezer, M. Coord. Chem. Rev. 1985, 61, 97.

10.1021/la001090o CCC: $20.00 © 2001 American Chemical Society Published on Web 01/24/2001

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moieties is synthesized (Scheme 1). It can coordinate to Pd(II) by amino groups and sulfur atoms of methionine to form the mono- or binuclear complexes PdLCl2 and Pd2LCl4. When the complexes are compressed at the airwater interface, the intra- and intermolecular interactions between palladium(II) centers via chloride bridging are likely to strengthen the film. Experimental Section Materials Dicyano (1) and diamino (2) derivatives of p-tertbutyl-calix[4]arene (Scheme 1) and trans-[Pd(Py)2Cl2] were synthesized according to the literature procedures.20-23 N-BOCmethionine was obtained from Sigma. PdCl2 and chloroform (spectrum reagent) used as spreading solvent were obtained from Aldrich. All other chemicals are of reagent grade. The synthesized compounds were characterized by 1H NMR (Bruker AM500), electrospray mass spectrometer (LCQ, Finnigan), and elemental analysis (Perkin-Elmer 240C). The geometry optimization was carried out using MM+ of Hyperchem 3.0 (Hypercube, Inc.). Synthesis of 5,11,17,23-Tetra-tert-butyl-25,27-di{[(2′amino-4′-methylthio)butyryl]aminoethoxy}-26,28dihydroxycalix[4]arene (L). The acylation of compound 2 by N-BOC-methionine was carried out using a procedure similar to that used in peptide synthesis.22 A 200-mg (0.8 mmol) portion of N-BOC-methionine was dissolved in 5 mL of CHCl3 and cooled with an ice-water bath. A 165 mg (0.8 mmol) portion of N,N′dicyclohexylcarbodiimide (DCC) was added in 0.5 h. Then, 294 mg (0.4 mmol) of 2 in chloroform was added dropwise for 2 h at 0 °C. The reaction was finished in 3 days at room temperature. After the solid was filtered off, the filtrate was evaporated to dryness in vacuo. The residue was treated with 120 mL of CH3OH containing 1.62 g of HClO4 (72%) at 45 °C for 9 h. After the (20) Zhang, W.-C.; Huang, Z.-T. Synthesis 1997, 1073. (21) Szemes, F.; Hesek, D.; Chen, Z.; Dent, S. W.; Drew, M. G. B.; Goulden, A. J.; Graydon, A. R.; Grieve, A. Mortimer, R. J.; Wearl, T.; Weightman, J. S.; Beer, P. D. Inorg. Chem. 1996, 35, 5868. (22) Huang, W.-D.; Chen, C.-Q. Synthesis of Peptides; Science Express: Beijing, 1985; p 102. (23) Chen, X.-H.; Luo, X.-M.; Song, Y.-C.; Zhou, S.-Z.; Zhu L.-G. Polyhedron 1998, 17, 2271-2278.

He et al. solvent was removed, the gel-like residue was dissolved in water. NH3 aqueous solution was added to adjust pH of the solution to 9.0 at 0 °C. The solution was then extracted with CHCl3 several times. The collected organic phase was evaporated to dryness. Recrystallization of the residue from methanol gave 207 mg of L. Yield 52.0%. Mp (uncorrected) 200-202 °C. Rf ) 0.75 (silica gel, CHCl3/CH3OH, 8:1). 1H NMR (CDCl3, 500 MHz): 8.37 (br, 2H, CONH), 7.78 (s, 2H, OH), 6.97, 6.80 (2s, 8H, Ar-H), 4.14 (d, 4H, endo-ArCHAr, J ) 13.0 Hz), 4.02 (m, 4H, OCH2), 3.86, 3.81 (m, 4H, NCH2), 3.54 (m, 2H, COCH), 3.28 (d, 4H, exo-ArCHAr, J ) 13.0 Hz), 2.52 (m, 4H, SCH2), 2.07 (m, 2H, CHCH2S), 1.80 (m, 2H, CHCH2S), 1.77 (br, NH2), 1.92 (s, 6H, SCH3), 1.19 (s, 18H, C(CH3)3), 0.94 (s, 18H, C(CH3)3). Anal. Calcd for C58H84N4O6S2: C, 69.84; H, 8.49; N, 5.62. Found: C, 69.58; H. 8.62; N. 5.93. ESMS (m/z) found m/z values: 997.5, 998.5, 999.5, 1000.5, 1001.5 for [M + H]+; 499.4, 499.9, 500.4, 500.9, 501.4 for [M + 2H]2+. Calcd m/z values: 997.6, 998.6, 999.6, 1000.6, 1001.6 for [M + H]+; 499.3, 499.8, 500.3, 500.8, 501.3 for [M + 2H]+. Synthesis of Mononuclear-Palladium(II) Complex of L (PdLCl2). A 112.8-mg (0.336 mmol) portion of trans-[Pd(Py)2Cl2]23 was refluxed with 296.6 mg (0.297 mmol) of L in methanol for 36 h. The product was purified by column chromatography (silica gel, CHCl3/CH3OH/H2O, 60:35:9), and 250 mg of yellow solid was obtained. Yield 71.6%. Mp decomposition at 190-198 °C. Rf ) 0.5 (silica gel, CHCl3/CH3OH/H2O, 60:35:9). Anal. Calcd for C58H84N4O6S2Cl2Pd: C, 59.30; H, 7.21; N, 4.77. Found: C, 59.08; H, 7.56; N, 5.05. ESMS (m/z) found m/z values: 1099.4, 1100.4, 1101.4, 1102.4, 1103.4, 1104.4, 1105.4, 1106.4, 1107.4 for [M - 2Cl - H]+; 550.3, 550.8, 551.2, 551,7, 552.2, 552.7, 553.2, 553.7, 554.3 for [M - 2Cl]2+. Calcd m/z values: 1099.5, 1100.5, 1101.5, 1102.5, 1103.5, 1104.5,1105.5, 1106.5, 1107.5 for [M - 2Cl - H]+; 550.2, 550.7, 551.2, 551.7, 552.5, 552.7, 553.2, 553.7, 554.2 for [M - 2Cl]2+. Synthesis of Binuclear Palladium(II) Complex of L (Pd2LCl4). The synthesis of Pd2LCl4 was realized using an improved literature procedure.24 A 191.7-mg (0.192 mmol) portion of compound L was refluxed with 72 mg (0.405 mmol) of PdCl2 powder in methanol for 48 h. The product was purified by column chromatography (silica gel, CHCl3/CH3OH, 15:1), and 158 mg of yellow solid was obtained. Yield 60.8%. Mp decomposition at 230-235 °C. Rf ) 0.6 (silica gel, CHCl3/CH3OH, 8:1). Anal. Calcd for C58H84N4S2O6Cl4Pd2: C, 51.52; H, 6.26; N, 4.14. Found: C, 51.34; H, 6.03; N, 4.42. far-IR (cm-1): 328.9, 294.4 (νPd-Cl); 462.0 (νPd-S). Found m/z values: 1275.2, 1276.2, 1277.2, 1278.2, 1279.2, 1280.2, 1281.2, 1282.2, 1283.2, 1284.2, 1285.2 for [M - 2Cl H]+; 1311.2, 1312.2, 1313.2,1314.2, 1315.2, 1316.1, 1317.2, 1318.1, 1319.2, 1320.2, 1321.2 for [M - Cl] +; 1382.5, 1383.5, 1384.5, 1385.5, 1386.5, 1387.5, 1388.5, 1389.5, 1390.5, 1391.5, 1392.5 for [M + Cl]-. Calcd. m/z values: 1275.3, 1276.3, 1277.3, 1278.3, 1279.3, 1280.3, 1281.3, 1282.3, 1283.3, 1284.3, 1285.3 for [M 2Cl - H]+; 1311.3, 1312.3, 1313.3, 1314.3, 1315.3, 1316.3, 1317.3, 1318.3, 1319.3, 1320.3, 1321.3 for [M - 2Cl - H]+; 1382.2, 1383.3, 1384.3, 1385.3, 1386.3, 1387.3, 1388.3, 1389.3, 1390.3, 1391.3, 1392.3 for [M + Cl]-. Surface Pressure-Area Isotherms and Compression/ Expansion Experiments. Surface pressure-area isotherms (π-A isotherms) were determined using KSV 5000 (alternate trough). The temperature was kept at 15 ( 0.2 °C. Monolayers were formed by spreading 90 µL of 5.0 × 10-4 M solutions of surfactants in chloroform dropwise onto the surface of deionized water (purified by Milli-Q system, 18 MΩ, pH ∼ 5.6). A time of 15 min was given for the evaporation of the solvent. The π-A isotherms of each sample were measured three times at a barrier speed of 10 mm/min and were found reproducible. For the twocycle compression/expansion experiments, the monolayer was compressed by barriers to the desired surface pressure. Then the barriers went back immediately at the same speed of compression to expand the monolayer. After the barriers reached the starting position, the second compression/expansion cycles was then carried out. Surface Viscosity Measurements. A procedure similar to that reported in the literature was used.14 A Teflon block (140 mm × 2 mm × 40 mm) with a centrally located 2.0 mm slit was (24) Burgess, C.; Hartley, F. R.; Searle, G. W. J. Organomet. Chem. 1974, 76, 247.

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Figure 2. Top view of the molecular structure of low-rim 1,3disubstituted calix[4]arene obtained by MM+ molecule mechanics calculation. The low rim substitution groups were omitted for clarity. Figure 1. Surface pressure-area isotherm of L, PdLCl2, and Pd2LCl4 recorded at 15 ( 0.2 °C and pH ∼5.6 with a barrier speed of 10 mm/min. placed on the alternate trough of KSV 5000 as a stationary barrier. The position of the block divided the trough into two regions: one occupying 2/3 of the total area at one side acted as the dispersing region and the other 1/3 at the other side as the measuring region. A 40-µL portion of 5.0 ×10-4 M solution of surfactant in chloroform was spread on the water surface in the measuring region. The barrier at this end started to compress 15 min later. At the desired surface pressure, 20 min was given for equilibrium. The slit was then opened cautiously to allow the molecules of the monolayer to disperse to the dispersing region, and the surface pressure in the measuring region was recorded. Area Relaxation Experiments. The area relaxation experiments were carried out using a procedure similar to that of Hawker.25 The film was first compressed to the desired surface pressure, and then the surface pressure was kept constant for 90 min. The film area was recorded in this period. A new film was spread for each relaxation measurement. To maintain the surface pressure, the forward and backward speeds of barriers were kept as 2 mm/min. Brewster Angle Microscopy (BAM) Observation. BAM observation was carried out using an OPTKEL Brewster angle microscope (Germany). The beams reflected from the monolayers were imaged by means of a CCD camera and recorded on videotapes for further analysis. BAM images were obtained during the first compression/expansion cycle. The compression and expansion speeds were 6.0 cm2/min.

Results and Discussion Monolayers of the Three Compounds at AirWater Interface. As shown in Figure 1, all three compounds form monolayers at the air-water interface. The limiting areas obtained by drawing a tangent from the condensed portions of the π-A isotherms to 0 mN/m are 162 ( 3, 145 ( 3, and 152 ( 3 Å2 for L, PdLCl2, and Pd2LCl4, respectively. The monolayer of PdLCl2 has the highest collapse pressure (∼53 mN/m), while the monolayers of L and Pd2LCl4 have lower collapse pressures which occur at ∼38 mN/m. The molecular mechanics study suggested that the cone conformation of low rim 1,3-substituted calix[4]arenes was pinched by two parallel distal phenol units with other two units twisting outward.26 The 1H NMR result of L supports its cone conformation. On the basis of these, the pinched cone conformation of compound 2 (the two substitution groups are connected with the twisting units) was used as the platform to construct the molecular structure of L (amino groups are protonated). The conformation of L was optimized using MM+ molecular mechanics in vacuo with a gradient of 0.01 kcal/mol. The molecules of complexes (25) Kampf, J. P.; Frank, C. W.; Malmstro¨m, E. E.; Hawker, C. J. Langmuir 1999, 15, 227. (26) Yu, X.-D.; Wu C.-Y. Chem. J. Chin. Univ. 1998, 19, 1492.

were built up on the optimized conformation of L. The optimization results show that the pinched cone is still kept, and the cross-sectional areas of upper rims are 158 ( 5 Å2 (L), 150 ( 4 Å2 (PdLCl2), and 150 ( 4 Å2 (Pd2LCl4), respectively. The top view of the typical conformation illustrates a roughly rhombic framework (Figure 2). Calixarene molecules are proposed to lie perpendicularly at air-water interface in a packed array with the low rim arms anchoring into water. Therefore, the limiting areas of the three monolayers are mainly determined by the cross-sectional areas of their upper rims.2,27,28 The calculated results agree well with the limiting areas obtained from the isotherms. When the monolayers of L and PdLCl2 are compressed at the rates of 20 and 10 mm/min, the isotherms obtained are almost identical and highly reproducible for the same compound (the errors of area per molecule are no more than 3 Å2). However, the isotherms of Pd2LCl4 obtained at the rate of 20 mm/min are different from those at 10 mm/min, suggesting that the isotherm of Pd2LCl4 is compression-rate-dependent. The errors of area per molecule reach 18 Å2 at the rate of 20 mm/min, while at 10 mm/min, the errors of area per molecule are no more than 3 Å2. This suggests that the intermolecular interaction of Pd2LCl4 is strong, which results in poor mobility of molecules and slows down the self-assembly of molecules. L has a plateau at 47 mN/m, which is a coexistence region of monolayer and multilayers. It suggests there is a fold of monolayer that leads to the formation of multilayers or other three-dimensional (3D) aggregates beyond the critical point. Similar to the phenomenon observed by Hawker,25 further compression has no effect on the two-dimensional (2D) molecular packing and the surface pressure in this region. The reason for the formation of multilayers will be discussed later. As for PdLCl2 and Pd2LCl4, second compressible regions are observed at higher surface pressure. The surface pressures increase at the rates of 0.12 mN/mÅ2 for PdLCl2 and 0.21 mN/mÅ2 for Pd2LCl4. The rates are much slower than those in their condensed monolayers. Different from L, their phase transitions are slower with the surface pressure still increasing. The lower increase rate of PdLCl2 compared to Pd2LCl4 suggests that the phase transition of the former is quicker than the latter. This is confirmed by the area relaxation experiments (vide infra). The onsets of the three π-A isotherms are in the following order: Pd2LCl4 > L . PdLCl2. This order reflects the relative magnitudes of their own intermolecular associations which are relevant to the self-assembly of the molecules. (27) Nakamoto, Y.; Kallinowski, G.; Bo¨hmer, V.; Vogt, W. Langmuir 1989, 5, 1116. (28) Markowitz, M. A.; Bielski, R.; Regen, S. L. Langmuir 1989, 5, 276.

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Figure 3. First two cycles of compression/expansion isotherm of compound L at 15 ( 0.2 °C and pH ∼ 5.6: (a) maximum surface pressure is 35 mN/m; (b) maximum surface pressure is 45 mN/m.

Figure 4. First two cycles of compression/expansion isotherm of compound PdLCl2 at 15 ( 0.2 °C and pH ∼5.6: (a) maximum surface pressure is 42 mN/m; (b) maximum surface pressure is 58 mN/m.

Compression/Expansion Isotherms of the Three Compounds. To gain more information about the reversibility and the intermolecular interaction of monolayers, compression/expansion isotherms were measured. Figure 3 shows the π-A isotherms of the first two compression/expansion cycles for L at different maximum surface pressure. When the maximum surface pressure is inferior to the critical point at 38 mN/m (Figure 3a, πmax ) 35 mN/m), the two cycles almost coincide, except for the second cycle having a slightly larger onset. The small difference may be due to the change of initial states created by the reorganization of the film after the first cycle of compression and expansion.25 The reversibility of the isotherms suggests there is no strong intermolecular interaction which would have led to a serious irreversible change. The multilayers or other 3D aggregates are formed when the monolayer is compressed over 38 mN/m (Figure 3b, πmax ) 45 mN/m). The serious hysteresis demonstrates that the new phases are difficult to recover to monolayer, which is also supported by the far distance between the two compression lines of the two cycles. The loss in molecule area after first cycle may result from residual multilayer patches in the film after expansion. Further evidence will be given by BAM observation. In the case of PdLCl2, when the maximum surface pressure is inferior to the critical point at ∼53 mN/m, the two cycles of compression/expansion are almost identical (Figure 4a, πmax ) 42 mN/m), indicating the good reversibility of its monolayer. The intermolecular interaction of PdLCl2 in monolayer is weak, so the molecules can be adjusted by self-assembly with low barriers, and the hysteresis is negligible. When the monolayers are compressed over 53 mN/m, the limiting areas estimated from the compression part of the two cycles are almost equal (Figure 4b, πmax ) 58 mN/m), although the hysteresis of the surface pressure-area isotherm is notable. This

phenomenon reveals that the new phase formed over ∼53 mN/m can be readily recovered to monolayer when adequate expansion is given. The weak intermolecular association of PdLCl2 may be the origin for it. Compared with L and PdLCl2, the hysteresis for the monolayer of Pd2LCl4 is evident at low surface pressure (Figure 5a, πmax ) 15 mN/m), suggesting that the strong intermolecular association in its monolayer can make the associated molecules difficult to separate from each other. Since the molecules cannot occupy the space left by expansion timely, the surface pressure decreases much more quickly. In its condensed phase (Figure 5b, πmax ) 35 mN/m), the formed intermolecular association is slightly influenced by compression and expansion, so the surface pressure decreasing line is almost coincide with the compression line in the initial period of expansion. When the monolayer is expanded into the liquid phase, the strong intermolecular association again makes the hysteresis very evident. The strong intermolecular association may also be the reason for a significant reduction of compressibility of the monolayer in the second cycle. In addition, the two lines of the second cycle are all close to the expansion isotherm of the first cycle, again suggesting that once the intermolecular association is formed, the molecules are difficult to disperse from each other. When compressed over the critical point at 38 mN/m (Figure 5c, πmax ) 52 mN/m), the great loss of the molecular area and reduction of compressibility for the film can be found. The result implies the lower mobility of molecules in the new phase makes the reconstruction of monolayer difficult. The reconstruction is far from finishing, even the barriers have gone back to the start position. Viscosity of the Monolayers Formed by the Three Compounds. The mechanical strength of monolayer was investigated by a viscosity measurement at 15 ( 0.2 °C. A significant difference in viscosity was found for the three

Monolayers of Calix[4]arene Derivative

Figure 5. First two cycles of compression/expansion isotherm of compound Pd2LCl4 at 15 ( 0.2 °C and pH ∼5.6: (a) maximum surface pressure is 15 mN/m; (b) maximum surface pressure is 35 mN/m; (c) maximum surface pressure is 52 mN/m.

compounds. When the monolayer of Pd2LCl4 is exposed to a 2 mm slit, the surface pressure drops from 38 to 32 mN/m within 3 h, and the decreasing rate is inclined to be zero. However, for the monolayers of L and PdLCl2, the surface pressure drops from 38 to 0 mN/m within 30 s. The much slower decreasing rate of Pd2LCl4 than those of L and PdLCl2 can be attributed to the poor mobility of the Pd2LCl4 molecules. As far as L and PdLCl2 are concerned, the mutual association of molecules at the airwater interface is weak. The amphiphilic molecules tend to occupy more area at the air-water interface immediately after the removal of the exterior force. The results demonstrate that only the monolayer of Pd2LCl4 has the improved mechanical properties. Area Relaxation Experiments. The stability of the monolayer at high surface pressure was studied by area relaxation experiments. In these experiments the resulting strain (A/A0, the ratio of film area at t to the original film area) were obtained at a constant surface pressure. As can been seen from parts a and b of Figure 6, monolayers of L and PdLCl2 are inclined to form multilayers or other 3D aggregates in or near their second compressible regions

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Figure 6. Relaxation experiment plots showing the changes in relative area over time for monolayers of (a) compound L, (b) compound PdLCl2, and (c) compound Pd2LCl4 at 15 ( 0.2 °C at pH ∼5.6.

of their π-A isotherms, so the monolayer-occupied areas decrease evidently. L collapses to ca. 3/4 of its original area at 43 mN/m in 90 min and PdLCl2 loses 1/5 of its original area at 52 mN/m in 90 min. However, only 5% area loss can be found for Pd2LCl4 suggesting that the monolayer of Pd2LCl4 has better stability, even at 43 mN/m (in its second compressible region, Figure 6c). In condensed phase, the area loss of L is also evident, and the decreasing rate does not change with the time. Moreover, the area losses and decreasing rates at 33 and 38 mN/m are almost identical. The surface pressureindependent area loss in condensed phases suggests that the molecules of L have a constant ability to form multilayers or other 3D aggregates. This may be due to the intermolecular hydrogen bonding formed between L molecules. For PdLCl2, the area loss increases as the surface pressure increases from 36 to 48 mN/m. The tighter the molecules are packed, the greater the area loses. The van der Waals force between molecules may be the main intermolecular interaction.

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Figure 7. Brewster angle microscopy images of the monolayers during the first cycle of compression and expansion, at 15 ( 0.2 °C and pH ∼5.6, the range of area per molecule is from ∼250 to ∼50 Å2: (a) L at 0 mN/m, during compression; (b) L at ∼5 mN/m, during compression; (c) L at ∼40 mN/m, during compression; (d) L at 0 mN/m, during expansion; (e) PdLCl2 at 0 mN/m, during compression; (f) PdLCl2 at 0 mN/m, during expansion; (g) Pd2LCl4 at 0 mN/m, during compression; (h) Pd2LCl4 at 5 mN/m, during compression; (i) Pd2LCl4 at 0 mN/m, during expansion. The bars represent 30 µm.

As shown in Figure 6c, the area decreasing rate of Pd2LCl4 in condensed phase is inclined to be zero (at 33 mN/ m). The decreasing rates are slow and almost equal in or near the second compressible region. The lowest area loss caused by the strong intermolecular association of Pd2LCl4 demonstrates the extraordinary stability of the monolayer of Pd2LCl4 at high surface pressure. BAM Images of the Monolayers Formed by the Three Compounds. Figure 7 exhibits the BAM images of the three compounds at different surface pressures. After the molecules of L are spread on the water surface, bright 2D network of hexagons can be observed (Figure 7a, 0 mN/m), and the scale of polygons turns smaller during compression. When surface pressure increases abruptly, the polygons disappear and a few scattered bright islands appear as the domains of the monolayer (Figure 7b, ∼5 mN/m). Further compression gives rise to many wormlike islands. Beyond the critical point of the π-A isotherm, the total field of vision of the microscopy is randomly filled with the bright islands (Figure 7c, ∼40 mN/m). As the monolayer is expanded, the reverse process can be observed. When the surface pressure descends to 0 mN/ m, the 2D network appears again. But there are still some bright regions that are not found before compression (Figure 7d, 0 mN/m). The domains readily found by BAM indicate L tend to form multilayers or other 3D aggregates. Moreover, the monolayer cannot be recovered completely from the new phases through expansion. Therefore, the bigger bright regions are still observed, even completely expanded. This phenomenon is in accordance with the compressibility reduction and the loss in molecule area of its monolayer when compressed over 38 mN/m (Figure 3b).

He et al.

For PdLCl2, no evident bright regions can be observed before compression. During compression, scattered regions of weak brightness with black dots appear (Figure 7e, 0 mN/m). As the surface pressure increases abruptly, the total field of the vision is almost black. Without strong intermolecular association, the molecules are randomly oriented, so its BAM images are almost black. In addition, the film immediately turns into a 2D network when expanded to near 0 mN/m (Figure 7f, 0 mN/m). As for Pd2LCl4, after the solvent of the spread solution is evaporated in 15 min, big blocks of bright regions are found to float on the water surface (Figure 7g, 0 mN/m). This image demonstrates that the molecules may be associated or polymerized. When compressed, the blocks become close to each other and coalesce into a bright uniform film. At the same time, the surface pressure begins to rise abruptly (Figure 7h, 5 mN/m). Further compression and expansion do not lead to notable change to the uniform film, even the surface pressure descends to near ∼1.0 mN/ m. Black rifts appear gradually on the bright film when further expansion is imposed (Figure 7i, 0 mN/m). Although several black rifts are found, the relative big block remains unity. It seems that once the solvent had been evaporated, the molecules of Pd2LCl4 tend to be crosslinked at the air-water interface. The exterior compression accelerates the coalescence of these aggregates to a form uniformly bright film which is quite stable. According to the limiting area given by the isotherm, this film is two-dimensional. When surface pressure increases beyond the critical point, 3D aggregates may be formed, but it is difficult to find the difference in brightness by BAM in this case. Molecular Structures and Properties of Monolayers. The above results indicate that the properties of monolayers are closely associated with their molecular structures. If identically oriented L molecules are in the same plane, the intermolecular hydrogen bonding between their methionine moieties can stabilize the monolayer of L. But at the air-water interface, the hydrogen bonding between water molecules and methionine moieties of L cannot make the molecules of the monolayer be crosslinked, yet it may play an essential role at the air-water interface. Therefore, the stability of L monolayer at airwater interface cannot be improved evidently. For L molecules oriented reversely, their methionine moieties can form intermolecular hydrogen bonding in a “tail to tail” form (low rim to low rim). This kind of intermolecular hydrogen bonding together with the hydrophobic interaction between the upper rims may be beneficial to the formation of multilayers or other 3D aggregates. Therefore, the monolayer of L readily forms domains and multilayers. Moreover, without the participation of water, the 3D aggregates are relatively difficult to recover to the monolayer. In the PdLCl2 molecule, both low rim arms coordinate to the same Pd(II) atom, so there is no intermolecular metal complexing or hydrogen bonding in its monolayer. The weak intermolecular interaction not only affords the monolayer good reversibility but also provides a lower barrier to pack the molecules tightly, so its monolayer has the lowest limiting area.15 In addition, the complex cations (Scheme 1) have good affinity to water; the more suitable hydrophobic/hydrophilic balance therefore makes its collapse pressure higher. Moreover, the molecules find it relatively difficult to escape from the water surface to form multilayers or other 3D aggregates. Different from the molecular structures of the other two, in Pd2LCl4, Pd(II) is coordinated by two chlorides, an amino group, and the sulfur atom of methionine (Scheme

Monolayers of Calix[4]arene Derivative

Langmuir, Vol. 17, No. 4, 2001 1149

Conclusion

Scheme 2

1).16-19 Polymerization (intermolecular metal complexing) of Pd2LCl4 via chloride bridges at the air-water interface could be the origin of superior properties of its monolayers. Although the chloride-bridged Pd(II) complexes are wellknown, it appears to be the first example that a robust film is assembled through this type of association (Scheme 2). The cross-linkage also prevents molecules from forming multilayers or other 3D aggregates and causes the small area loss in area relaxation experiments.

This work demonstrates that the methionine derived calix[4]arene and its palladium complexes form stable monolayers at the air-water interface. The molecules are perpendicular to the water surface, so the limiting areas are mainly determined by the upper rims of calix[4]arenes. In the three compounds, only Pd2LCl4 forms a monolayer with extraordinary cohesiveness and remarkable mechanical strength. These properties are associated with its polymerization (intermolecular metal complexing) via chloride-bridging. Neither the intermolecular hydrogen bonding of L nor the metal complex cations of PdLCl2 can enhance the mechanical properties of monolayers. The intermolecular metal complexing at the air-water interface offers a promising new strategy to compose robust film. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 29771019, 29871017, and 29823001). LA001090O