- Email: [email protected]
Structure Studies on Ion Coordination in 2,4-Dihydroxy-N-Octadecybenzylideneamine LB Films by UV–Vis Absorption and FTIR Spectroscopy
Journal of Colloid and Interface Science 231, 59–65 (2000) doi:10.1006/jcis.2000.7082, available online at http://www.idealibrary.com on
Structure Studies on Ion Coordination in 2,4-Dihydroxy-N-Octadecybenzylideneamine LB Films by UV–Vis Absorption and FTIR Spectroscopy Shu-feng Pang and Ying-qiu Liang1 Institute of Mesoscopic Solid State Chemistry and State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, People’s Republic of China Received January 21, 2000; accepted July 5, 2000
The LB film prepared by this method will be referred to as a surface film. (ii) A solution of the appropriate ligand in chloroform was spread on an aqueous subphase containing selected metal ions. In this case the complex formation takes place as an interface reaction between the ligand monolayer and the metal ions in this subphase and then transfers the monolayer onto a solid substrate. Such an LB film will be called a subphase film. The aim of this work is to probe spectral changes by coordination reaction and investigate the molecular structure of ioncoordination films, another type of Schiff base metal complex LB films. In order to obtain detailed information on molecular interactions and aggregations in ion-coordination film, the orientation of the alkyl chain and the phase behavior were studied in detail by comparing them with those of the subphase and the surface film.
The coordination reaction of Cu2+ ions with the Schiff base amphiphile 2,4-dihydroxy-N-octadecylbenzylideneamine in ultrathin organic LB films was probed by Fourier transform infrared (FTIR) transmission spectroscopy and UV–Vis electronic absorption spectroscopy. FTIR spectral changes owing to LB films of 2,4-dihydroxyN-octadecylbenzylideneamine contacting aqueous Cu(OAC)2 solution suggest the formation of ion-coordination films, which is corroborated by UV–Vis absorption spectra. The coordination reaction of the annealing film at different temperatures suggests that the microstructure of LB films had an important effect on the reaction rate. Detailed analysis shows that ion-coordination films take an isotropic unaxial orientation and exhibit phase transition behavior, which is determined by the different complex structures of headgroups and of subphase and surface film. °C 2000 Academic Press Key Words: ion-coordination; LB film; FTIR spectra; UV–Vis absorption; orientation.
2. EXPERIMENTAL 1. INTRODUCTION
2.1. Material
Metal complexes are compounds with fascinating chemical, optical, electrical, thermal, and electrooptical properties (1–5). Metal complexes incorporated into LB films can be employed as semiconductors, molecular metals, biosensors, and environmental sensors (6–8). This led scientists to study more kinds of metal complexes assembled into LB films and to explore more extensive potential applications. Complexes of Schiff bases, for instance salicylideneamines, have been well characterized, and they exhibit very interesting properties. For example, the low-molecular-weight amphiphile (1,3-bis(2,5dihydroxybenzylidene)amino)-2-hexadecylpropanyl)cobalt(II) has been used as an oxygen carrier in LB multilayers (9). In the literature, the widely used ways of forming metal complex films are the following: (i) The metal complex was synthesized in solution, isolated, dissolved in chloroform, and spread on a water subphase and fabricated monolayer onto a solid substrate. 1
Preparation and purification of 2,4-dihydroxy-N -octadecylbenzylideneamine and its copper(II) complex bis(2,4-dihydroxy-N -octadecylbenzylideneamino)copper(II) were reported in a separate paper (10). 2.2. Method Preparation of ion-coordination film. All the monolayer measurements and transfers were made on a KSV5000 Langmuir trough system (KSV Instrument, Finland) at 300 K. The CaF2 substrates were refluxed in chloroform solvent for at least 24 h and quartz plates were cleaned in sulfuric acid containing potassium dichromate and rinsed with doubly distilled water before the experiments. Chloroform was used as the spreading solvent. After spreading, 30 min was allowed for solvent evaporation, and the compression was made at a rate of 20 mm/min up to 30 mN/m. Prolonging the relaxation time up to 1 h did not cause significant change in the surface pressure–area isotherm, indicating that the monolayer reached
To whom correspondence should be addressed. Fax: 86-025-3317761. 59
0021-9797/00 $35.00
C 2000 by Academic Press Copyright ° All rights of reproduction in any form reserved.
60
PANG AND LIANG
a stable state. Then the monolayer was fabricated onto solid substrates by the vertical dipping method at a rate of 2 mm/min. After the deposition of the first monolayer, the solid substrates were kept dry in air for 30 min, so that the following monolayers were successfully transferred. The transfer ratio was almost unity during the upward and downward cycles. Hence, the monolayer transfer was Y-type. After immersion of the solid substrate with LB films of 2,4dihydroxy-N -octadecylbenzylidene-amine into an aqueous solution containing Cu(OAC)2 at a concentration of 1 mM for a preset time, it was taken out and rinsed with doubly distilled water. The result is called ion-coordination film in this paper. The process was repeated and time was noted. Measurements of spectra. Fourier transform infrared transmission spectra were recorded through a Bruker IFS 66v spectrometer equipped with a DTGS detector. All spectra were collected for 500 interferrograms with a resolution of 4 cm−1 . Ultimate spectra were obtained by subtracting spectra of corresponding blank substrates from those of substrates with LB films. For the measurement of FTIR spectra at elevated temperatures, the CaF2 substrate with LB films was mounted in a heating cell and temperature control was achieved with a p/N 21.500 automatic temperature controller (Graseby Specae Inc.) through a copper-constant thermocouple with an accuracy of ±1◦ C. After the temperature was raised to the preset value, 15 min was allowed for the thermal equilibrium. A Schimadzu recording spectrometer (Model UV-3100) was employed to measure UV–Vis electronic absorption spectra. Quartz plates were used as supports for LB films for spectra investigations.
FIG. 1. π–A isotherm of 2,4-dihydroxy-N -octadecylbenzylideneamine monolayer on pure water (pH 6.09) at 300 K.
3. RESULTS AND DISCUSSION
3.1. Surface Pressure–Molecular Area Isotherm of 2,4-Dihydroxy-N-octadecylbenzylidene-amine Monolayer The π –A isotherm of 2,4-dihydroxy-N -octadecylbenzylideneamine, shown in Fig. 1, indicates that the amphiphile with Schiff base moiety as headgroup can form a fairly stable monolayer at the air/water interface and exhibits a distinct phase transition region in the surface pressure range of 30–40 mN/m during continuous compression. The mean cross-sectional area per ˚ 2 , is obtained by extrapolating amphiphilic molecule, about 27 A the linear region of a condensed film to zero surface pressure. The appearance of the phase transition region may be due to the conformation transition and/or to changes in the fashion of molecular interaction. 3.2. FTIR Transmission Spectra Infrared spectroscopy is one of the most effective tools for examining molecular configuration, conformation, group orientation, and chain packing in LB films at the molecular level. So FTIR transition spectra were used to detect coordination and changes of molecular structure. Figure 2 shows FTIR
FIG. 2. FTIR transmission spectra collected at various times: (a) 0 min, (b) 5 min, (c) 130 min, (d) 15 h.
ION COORDINATION IN LB FILMS
FIG. 3. The tautomeric structure of 2,4-dihydroxy-N -octadecylbenzylideneamine and coordination reaction process between copper(II) ion and ligand.
transmission spectra collected at various times before and after the exposure of nine-layer LB films prepared at a deposition pressure of 30 mN/m to an aqueous Cu(OAC)2 solution. The conversion of 2,4-dihydroxy-N -octadecylbenzylideneamine to its copper complex bis(2,4-dihydroxy-N -octadecylbenzylideneamino)copper(II) was measuring using the C==O stretching mode near 1650 cm−1 , which is ascribed to the keto form arising from intramolecular hydrogen bonds and intramolecular proton transfer in 2,4-dihydroxy-N -octadecylbenaylideneamine (Fig. 3) (11), and the azomethine absorption band near 1544 cm−1 . The C==O stretching vibration was strong before coordination of the metal ion to the ligand. When LB films were immersed in aqueous Cu(OAC)2 solution for 5 min, the sign decreased with a concurrent increase in intensity of the band at 1613 cm−1 , assigned to the benzene stretching mode (12), and an upward shift of the azomethine absorption band, implying that the coordination reaction had taken place. As the exposure time increased, the changes became more marked. After LB films were immersed in aqueous Cu(OAC)2 solution for 15 h, the two bands at ca. 1650 and 1544 cm−1 disappeared and a broad band with a shoulder appeared at ca. 1616 cm−1 , indicating that the coordination reaction was complete. The possible coordination reaction is given in Fig. 3. In the range of higher frequencies, the strong bands at 2919 and 2850 cm−1 (not shown here), attributed to the CH2 antisymmetric and symmetric stretching modes, indicate that alkyl chains have the highly ordered all-trans conformation (13, 14). The peak positions remained almost unchanged with exposure time, showing that the hydrocarbon regions are little permeable
61
to Cu2+ ions. The coordination reaction in the present studies suggested that there are defects or pores in LB films, which are the same as in the fatty acid LB films (15). Ion-coordination dynamics should be related to the morphological features of LB films, such as the defects, surface roughness, and film thickness, the investigation of which is now in progress. Plotting the width of the CH2 antisymmetric stretching vibration as a function of exposure time in aqueous Cu(OAC)2 solution (presented in Fig. 4a) shows that the width of the band increased with immersion time. It is well known that the width of the methylene antisymmetric stretching mode is related to the alkyl chain mobility (16). The change in Fig. 4a was indicative of the mobility increase of hydrocarbon chains with an ongoing coordination reaction of copper(II) ions with the Schiff base polar headgroup of amphiphile. In other words, the modification of the headgroups has an influence on the mobility of the hydrocarbon chains. Recent investigations (17) have also demonstrated that the conformations of alkyl chains and their vibrations were also modified by the modification of the headgroup. In the present paper, copper(II) ions penetrate into LB films of 2,4-dihydroxyN -octadecylbenzylideneamine and react with it, which induces a minor rearrangement of the molecules owing to change in the interaction between headgroups. The integrated values of the CH2 antisymmetric stretching vibration decreased in intensity with increasing immersion time of contacting LB films with 1 mM aqueous Cu(OAC)2 solution (shown in Fig. 4b). It is well known that integrated intensity is determined by packing density and orientation of alkyl chains (18). The observation in Fig. 4b may be due in part to film
FIG. 4. (a) The plotted width of the CH2 antisymmetric stretching vibration as a function of exposure time. (b) The integrated intensity of the CH2 antisymmetric stretching vibration.
62
PANG AND LIANG
FIG. 5. The integrated absorption of the band at 1650 cm−1 dependence on the time.
reorganization and crystallite formation, which has been observed for LB films of cadmium salts of fatty acids by atomic force microscopy (19). Such reorganization can cause surface roughness, which increase the thickness of the film in some region and can indeed decrease the film absorbance, as will be detailed in a future report. We will now emphasize the time dependence of the integrated absorbance of the band at ca. 1650 cm−1 (Fig. 5). The curve did not exhibit any periodicity, suggesting that the transport rate through the film is fast compared to the (Cu2+ ion coordination to Schiff base polar group) reaction rate. If the ion transport was slow compared to the reaction rate, then for a perfectly lamellar structure the reaction rate would oscillate as consecutive Schiff base layers were fully coordinated. For a uniform one-dimensional diffusion process, this means that the ratio of the reaction rate to the diffusion rate is small. However, if there is transport through the film via defects, followed by lateral transport and reaction along the polar group planes, then no periodic reaction rate would be observed either. In both cases, material in all of the polar regions would be reacting simultaneously. In the first scenario, the reaction rate would determine the overall coordination rate. In the second, either the lateral transport along the polar planes or the reaction rate, or both, could determine the overall ion-coordination rate. 3.3. FTIR Spectra of Annealing Film In order for the effect of the microstructure on the ioncoordination rate to be investigated, LB films previously treated at 45 and 80◦ C were contacted with aqueous Cu(OAC)2 solution.
In the 45◦ C treated films there was a slight slowing of the overall ion-coordination rate. However, for the 80◦ C treated films, the effect was quite dramatic. The entire coordination process needs only 1 h for annealing film at 80◦ C. Clearly, the disruption of the structure of the LB films by melting at 80◦ C and refreezing markedly altered the rate-controlling mechanism in the ion-coordination process. From the fast coordination compared to the untreated film, it is inferred that for the nomelted films there were significant ion transport limitations on the determination of the ion-transfer rate. This experiment provided a low bound for the intrinsic rate of ion coordination. The above observations suggest that structure defects may also be important in determining the ion transfer rate. The 45◦ C treatment may have healed some defects within the films and increased the effective ion transfer resistance, which slowed the coordination reaction. However, the film probably melted at 80◦ C; when it was cooled and recrystallized, a large number of defects (or grain boundaries) probably formed, resulting in an increased ion transfer rate. The above discussion shows that FTIR spectra exhibit marked change after contact of LB films of 2,4-dihydroxy-N octadecylbenzylideneamine with aqueous Cu(OAC)2 solution. In order to obtain further information on the aggregation and structure of molecules in ion-coordination films, orientation of the alkyl chain and phase behavior were studied by comparing ion-coordination film with surface and subphase film. 3.4. Comparison of FTIR Spectra among Different Complex LB Film The spectral changes have been studied as discussed above. Now we analyze molecular structure in ion-coordination films. Figures 6a, 6b, and 6c compared FTIR transmission spectra of nine-monolayer complete ion-coordination films with that of subphase films and surface films. There are significant differences among spectra of the three metal complex LB films. In spectra of surface and subphase films, two distinguished bands appear at 1606 and 1544 cm−1 , while for ion-coordination film, the corresponding band is a broad band at ca. 1616 cm−1 with a shoulder. These differences imply that the molecular orientation and aggregation of headgroups in ion-coordination films are different from those of surface and subphase films. In the region of CH scissors vibration, a single band at 1467 cm−1 appeared in spectra of ion-coordination films, while for surface and subphase films, two distinguished peaks at 1447 and 1466 cm−1 can be observed. These differences indicate that the molecules were packed less densely in ion-coordination films (20). 3.5. Orientation of Alkyl Chain in Ion-Coordination Film Polarized FTIR spectra can be used to probe the orientation of an alkyl chain. For 2,4-dihydroxy-N -octadecylbenzylideneamine LB film, the polarized dichroic ratio (D = As /Ap ) is 1.004, while for ion-coordination film, the value changes to 1.04. It is clear that the orientation of hydrocarbon
ION COORDINATION IN LB FILMS
63
β = As (i 6= 0)/As (i = 0) = [n 1 n 3 (n 1 + n 3 )/(n 1 cos r + n 3 cos i)] £¡ ¢ 00 00 ¤ /εxx + cos i cos r/n 1 n 3 , × n 21 sin2 r/n 42 εzz where i is the angle of incidence in the medium of air, r is the angle of refraction in the substrate, p and s polarization are electronic fields perpendicular and parallel to the plane on incidence, respectively, n 1 sin i = n 3 sin r , and the refractive index n 1 = 1.0 (in air) n 2 = 1.5 (LB film), n 3 = 1.415 (CaF2 substrate). In addition cos2 φa + cos2 φs + cos2 γ = 1. So we determine that the orientation angle of ion-coordination film is 34.9◦ , larger than that of ligand film (30.3◦ ). By the same method, the orientation angles of subphase film (22.1◦ ) and surface film (23.7◦ ) can be obtained. The values imply that head-coordination directs the packing of complex molecules in coordination-film tilted farther away from the surface normal than surface film and subphase film, which can be explained by a spacing of the polar groups leading to a rearrangement of the CH2 dipole moments of the methylene groups along the hydrocarbon chain (17). 3.6. Order–Disorder Transition of LB Film
FIG. 6. FTIR transmission spectra of (a) ion-coordination film, (b) subphase film, and (c) surface film.
Variable-temperature FTIR spectroscopy is an effective tool for measuring conformational change in alkyl chains and determining order–disorder phase transition temperature Tc (24). In the present paper, transition temperature Tc of ion coordination
chains in ion-coordination films is slightly changed. Comparison of those in subphase film (0.894) and surface film (0.899) indicates that ion-coordination films take an isotropic unaxial orientation, while subphase and surface films take anisotropic biaxial orientation. It is accepted that anisotropic biaxial orientation of metal complex LB films is caused by a close-packing superlattice network of complex head that is subject to the shear forces induced during film transfer (21). Ion-coordination film was obtained by penetrating metal copper(II) ions into polar head regions and reacting them with materials in LB films, so the transfer-induced orientation effect cannot be seen with ioncoordination film. The orientation of alkyl chains in LB films is a major component characterizing two-dimensional structure and also relates to many functions, for example, the efficiency of energy transfer and the rate of photoinduced electron transfer (22). To analyze the polarization data in terms of molecular orientation, it is necessary to measure the dependence of absorbance on the rotation angle with respect to the plane of polarization and also on the incident angle i with respect to the surface normal of the film plane. Polarized infrared spectra of complete ion coordination films are shown in Fig. 7. Detailed calculation of orientation was carried out according to the formulae by Vanderyver et al. (23), α = Ap (i = 0)/As (i = 0) cos 2ω = (1 − α)/(1 + α)
FIG. 7. Polarized infrared spectra of complete ion-coordination film with the different incident angles.
64
PANG AND LIANG
highest and lowest values of the antisymmetric CH2 stretching frequency, reflects the order–disorder transition temperature of the alkyl chains in LB films. For ion-coordination film, only a phase transition appeared at ca. 70◦ C, which is higher than that of 2,4-dihydroxy-N -octadecylbenzylideneamine (ca. 45◦ C). However, surface film and subphase film both exhibit two-phase transition. They are near 30 and 110◦ C for surface film and 55 and 140◦ C for subphase film, respectively. According to the literature (25), the two-phase transitions were the character of liquid crystal arising from a close-packing superlattice network. The absence of the two-phase transition behavior in ion-coordination film agrees well with the results of molecular orientation. 3.7. UV–Vis Electronic Absorption Spectra
FIG. 8. Temperature dependence of wavenumber of antisymmetric CH2 stretching bands: (a) ion-coordination film, (b) surface film, (c) subphase film.
film was measured. In order to investigate headgroup structures in ion-coordination film, Tc of surface film and subphase film are also given. Figure 8 shows frequencies of the CH2 antisymmetric stretching mode plotted as a function of temperature. The temperature, corresponding to the middle point between the
UV–Vis absorption spectra of untreated films ion-coordination films and surface films are shown in Fig. 9. Two absorption peaks near 308 and 380 nm in untreated film correspond to the π–π ∗ transition of the aromatic ring in the headgroup and the n–π ∗ transition of the aromatic Schiff base chromophore. For ion-coordination film and surface film, two absorption peaks near 298 and 349 nm appeared (within experimental errors), indicating an obvious hypsochromatic shift from that seen with the untreated film. So it is concluded that the complex has a less conjunctive effect caused by formation of complex (26). The similar absorption peaks in surface film and ion-coordination film provided further evidence that copper(II) coordinated to Schiff base polar groups by contacting LB film of 2,4-dihydroxy-N octadecylbenzylideneamine in aqueous Cu(OAC)2 solutions. CONCLUSION
The ion-coordination films were prepared by contacting builtup multilayers of 2,4-dihydroxy-N -octadecylbenzylideneamine with aqueous solutions containing Cu(OAC)2 . Spectral changes owing to coordination of Cu(II) ions to ligands have been probed by FTIR spectra and UV–Vis absorption spectra. Annealing films at different temperature shows that microstructure of LB films has important effects on the rate of coordination reactions. Studies on molecular orientation and phase behavior show that ioncoordination films take isotropic unaxial orientation and exhibit a phase transition, which is determined by the complex structure of the headgroup, which is different from that of subphase and surface films. The detailed mechanism is now under study. ACKNOWLEDGMENT This work was financially supported by a major research project grant from the State Science and Technology Commission of China.
REFERENCES
FIG. 9. UV–Vis absorption spectra of (a) untreated ligand film, (b) complete ion-coordination film, and (c) surface film.
1. Naito, K., Minra, A., and Azuma, M., Thin Solid Films 210/211, 527 (1992). 2. Hopf, F. R., M¨obius, D., and Whitten, D. G., J. Am. Chem. Soc. 98, 1584 (1976).
ION COORDINATION IN LB FILMS 3. Sun, L. Y., Gu, C., Wen, K., Chao, X., Li, T. O., Hu, G. Y., and Sun, I. Y., Thin Solid Films 210/211, 486 (1992). 4. Blunkenburg, G., Meller, P., Ringsdorf, H., and Salesse, C., Biochemistry 28, 8214 (1989). 5. Xia, W. S., Huang, C. H., and Zhou, D. J., Langmuir 13, 80 (1997). 6. Gust, D., Moore, T. A., Luttrull, D. K., Degraziano, J. M., Blodt, N. J., Van der Auweraer, M., and Schryver, F. C., Langmuir 7, 1483 (1991). 7. Ko, W. H., Fu, C. W., Wang, H. Y., Barzel, D. A., Kenny, M. E., and Lando, J. B., Sens. Mater. 2, 39 (1990). 8. Bonos, F., Lely, F., Ricciardi, G., Ricciaridi, G., Romanelli, M., and Martini, G., Langmuir 9, 268 (1993). 9. Fukuda, A., Hanabusa, K., and Shirai, H., Jpn. Kokai Tokkyo koho 6228386 (87258386). 10. Pang, S. F., Huang, J. G., and Liang, Y. Q., Spectrosc. Lett. 33, 301 (2000). 11. John, W., and Ledketter, Jr., J. Phys. Chem. 81, 54 (1977). 12. Hadjoudisw, E., Vittorakis, M., and Moustakali-Mavridis, I., Tetrahedron 43, 1345 (1987). 13. Synder, B. G., Strauss, H. L., and Elliger, C. A., J. Phys. Chem. 86, 5145 (1982). 14. Synder, R. G., Hsn, S. L., and Krimm, S., Spectrochim. Acta. Part A 34, 395 (1978).
65
15. Marshbanks, T. L., and Franses, E. I., J. Phys. Chem. 98, 2166 (1994). 16. Katuyama, N., Enomoto, S., Sato, T., Ozaki, Y., and Kuramoto, N., J. Phys. Chem. 97, 6880 (1993). 17. Girard-Egrot, A. P., Morelis, R. M., and Coulet, P. R., Langmuir 13, 6540 (1997). 18. Robe, J. P., Rabott, J. F., Brown, C. A., and Swalen, J. D., J. Chem. Phys. 84, 4096 (1986). 19. Schwartz, D. K., Viswanathan, R., and Easadzinski, T. A. N., J. Phys. Chem. 96, 10444 (1992). 20. Marshbanks, T. L., Ahn, D. J., and Franses, E. I., Langmuir 10, 276 (1994). 21. Hsu, Y., Penner, T. L., and Whitten, D. G., Langmuir 10, 2757 (1994). 22. Liese, G., Tieke, B., and Wegner, G., Thin Solid Films 68, 77 (1980). 23. Vanderyver, M., Barrand, A., and Raudel-Teixier, J. Colloid Interface Sci. 85, 571 (1982). 24. Du, X. Z., Shi, B., and Liang, Y. Q., Langmuir 14, 3631 (1998). 25. Kusumizu, S., Kato, R., Yamada, M., and Yano, S., J. Phys. Chem. B 101, 10666 (1997). 26. Noh, D. Y., and Mizuno, M., Inorg. Chim. Acta 216, 147 (1994).
Structure Studies on Ion Coordination in 2,4-Dihydroxy-N-Octadecybenzylideneamine LB Films by UV–Vis Absorption and FTIR Spectroscopy Shu-feng Pang and Ying-qiu Liang1 Institute of Mesoscopic Solid State Chemistry and State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, People’s Republic of China Received January 21, 2000; accepted July 5, 2000
The LB film prepared by this method will be referred to as a surface film. (ii) A solution of the appropriate ligand in chloroform was spread on an aqueous subphase containing selected metal ions. In this case the complex formation takes place as an interface reaction between the ligand monolayer and the metal ions in this subphase and then transfers the monolayer onto a solid substrate. Such an LB film will be called a subphase film. The aim of this work is to probe spectral changes by coordination reaction and investigate the molecular structure of ioncoordination films, another type of Schiff base metal complex LB films. In order to obtain detailed information on molecular interactions and aggregations in ion-coordination film, the orientation of the alkyl chain and the phase behavior were studied in detail by comparing them with those of the subphase and the surface film.
The coordination reaction of Cu2+ ions with the Schiff base amphiphile 2,4-dihydroxy-N-octadecylbenzylideneamine in ultrathin organic LB films was probed by Fourier transform infrared (FTIR) transmission spectroscopy and UV–Vis electronic absorption spectroscopy. FTIR spectral changes owing to LB films of 2,4-dihydroxyN-octadecylbenzylideneamine contacting aqueous Cu(OAC)2 solution suggest the formation of ion-coordination films, which is corroborated by UV–Vis absorption spectra. The coordination reaction of the annealing film at different temperatures suggests that the microstructure of LB films had an important effect on the reaction rate. Detailed analysis shows that ion-coordination films take an isotropic unaxial orientation and exhibit phase transition behavior, which is determined by the different complex structures of headgroups and of subphase and surface film. °C 2000 Academic Press Key Words: ion-coordination; LB film; FTIR spectra; UV–Vis absorption; orientation.
2. EXPERIMENTAL 1. INTRODUCTION
2.1. Material
Metal complexes are compounds with fascinating chemical, optical, electrical, thermal, and electrooptical properties (1–5). Metal complexes incorporated into LB films can be employed as semiconductors, molecular metals, biosensors, and environmental sensors (6–8). This led scientists to study more kinds of metal complexes assembled into LB films and to explore more extensive potential applications. Complexes of Schiff bases, for instance salicylideneamines, have been well characterized, and they exhibit very interesting properties. For example, the low-molecular-weight amphiphile (1,3-bis(2,5dihydroxybenzylidene)amino)-2-hexadecylpropanyl)cobalt(II) has been used as an oxygen carrier in LB multilayers (9). In the literature, the widely used ways of forming metal complex films are the following: (i) The metal complex was synthesized in solution, isolated, dissolved in chloroform, and spread on a water subphase and fabricated monolayer onto a solid substrate. 1
Preparation and purification of 2,4-dihydroxy-N -octadecylbenzylideneamine and its copper(II) complex bis(2,4-dihydroxy-N -octadecylbenzylideneamino)copper(II) were reported in a separate paper (10). 2.2. Method Preparation of ion-coordination film. All the monolayer measurements and transfers were made on a KSV5000 Langmuir trough system (KSV Instrument, Finland) at 300 K. The CaF2 substrates were refluxed in chloroform solvent for at least 24 h and quartz plates were cleaned in sulfuric acid containing potassium dichromate and rinsed with doubly distilled water before the experiments. Chloroform was used as the spreading solvent. After spreading, 30 min was allowed for solvent evaporation, and the compression was made at a rate of 20 mm/min up to 30 mN/m. Prolonging the relaxation time up to 1 h did not cause significant change in the surface pressure–area isotherm, indicating that the monolayer reached
To whom correspondence should be addressed. Fax: 86-025-3317761. 59
0021-9797/00 $35.00
C 2000 by Academic Press Copyright ° All rights of reproduction in any form reserved.
60
PANG AND LIANG
a stable state. Then the monolayer was fabricated onto solid substrates by the vertical dipping method at a rate of 2 mm/min. After the deposition of the first monolayer, the solid substrates were kept dry in air for 30 min, so that the following monolayers were successfully transferred. The transfer ratio was almost unity during the upward and downward cycles. Hence, the monolayer transfer was Y-type. After immersion of the solid substrate with LB films of 2,4dihydroxy-N -octadecylbenzylidene-amine into an aqueous solution containing Cu(OAC)2 at a concentration of 1 mM for a preset time, it was taken out and rinsed with doubly distilled water. The result is called ion-coordination film in this paper. The process was repeated and time was noted. Measurements of spectra. Fourier transform infrared transmission spectra were recorded through a Bruker IFS 66v spectrometer equipped with a DTGS detector. All spectra were collected for 500 interferrograms with a resolution of 4 cm−1 . Ultimate spectra were obtained by subtracting spectra of corresponding blank substrates from those of substrates with LB films. For the measurement of FTIR spectra at elevated temperatures, the CaF2 substrate with LB films was mounted in a heating cell and temperature control was achieved with a p/N 21.500 automatic temperature controller (Graseby Specae Inc.) through a copper-constant thermocouple with an accuracy of ±1◦ C. After the temperature was raised to the preset value, 15 min was allowed for the thermal equilibrium. A Schimadzu recording spectrometer (Model UV-3100) was employed to measure UV–Vis electronic absorption spectra. Quartz plates were used as supports for LB films for spectra investigations.
FIG. 1. π–A isotherm of 2,4-dihydroxy-N -octadecylbenzylideneamine monolayer on pure water (pH 6.09) at 300 K.
3. RESULTS AND DISCUSSION
3.1. Surface Pressure–Molecular Area Isotherm of 2,4-Dihydroxy-N-octadecylbenzylidene-amine Monolayer The π –A isotherm of 2,4-dihydroxy-N -octadecylbenzylideneamine, shown in Fig. 1, indicates that the amphiphile with Schiff base moiety as headgroup can form a fairly stable monolayer at the air/water interface and exhibits a distinct phase transition region in the surface pressure range of 30–40 mN/m during continuous compression. The mean cross-sectional area per ˚ 2 , is obtained by extrapolating amphiphilic molecule, about 27 A the linear region of a condensed film to zero surface pressure. The appearance of the phase transition region may be due to the conformation transition and/or to changes in the fashion of molecular interaction. 3.2. FTIR Transmission Spectra Infrared spectroscopy is one of the most effective tools for examining molecular configuration, conformation, group orientation, and chain packing in LB films at the molecular level. So FTIR transition spectra were used to detect coordination and changes of molecular structure. Figure 2 shows FTIR
FIG. 2. FTIR transmission spectra collected at various times: (a) 0 min, (b) 5 min, (c) 130 min, (d) 15 h.
ION COORDINATION IN LB FILMS
FIG. 3. The tautomeric structure of 2,4-dihydroxy-N -octadecylbenzylideneamine and coordination reaction process between copper(II) ion and ligand.
transmission spectra collected at various times before and after the exposure of nine-layer LB films prepared at a deposition pressure of 30 mN/m to an aqueous Cu(OAC)2 solution. The conversion of 2,4-dihydroxy-N -octadecylbenzylideneamine to its copper complex bis(2,4-dihydroxy-N -octadecylbenzylideneamino)copper(II) was measuring using the C==O stretching mode near 1650 cm−1 , which is ascribed to the keto form arising from intramolecular hydrogen bonds and intramolecular proton transfer in 2,4-dihydroxy-N -octadecylbenaylideneamine (Fig. 3) (11), and the azomethine absorption band near 1544 cm−1 . The C==O stretching vibration was strong before coordination of the metal ion to the ligand. When LB films were immersed in aqueous Cu(OAC)2 solution for 5 min, the sign decreased with a concurrent increase in intensity of the band at 1613 cm−1 , assigned to the benzene stretching mode (12), and an upward shift of the azomethine absorption band, implying that the coordination reaction had taken place. As the exposure time increased, the changes became more marked. After LB films were immersed in aqueous Cu(OAC)2 solution for 15 h, the two bands at ca. 1650 and 1544 cm−1 disappeared and a broad band with a shoulder appeared at ca. 1616 cm−1 , indicating that the coordination reaction was complete. The possible coordination reaction is given in Fig. 3. In the range of higher frequencies, the strong bands at 2919 and 2850 cm−1 (not shown here), attributed to the CH2 antisymmetric and symmetric stretching modes, indicate that alkyl chains have the highly ordered all-trans conformation (13, 14). The peak positions remained almost unchanged with exposure time, showing that the hydrocarbon regions are little permeable
61
to Cu2+ ions. The coordination reaction in the present studies suggested that there are defects or pores in LB films, which are the same as in the fatty acid LB films (15). Ion-coordination dynamics should be related to the morphological features of LB films, such as the defects, surface roughness, and film thickness, the investigation of which is now in progress. Plotting the width of the CH2 antisymmetric stretching vibration as a function of exposure time in aqueous Cu(OAC)2 solution (presented in Fig. 4a) shows that the width of the band increased with immersion time. It is well known that the width of the methylene antisymmetric stretching mode is related to the alkyl chain mobility (16). The change in Fig. 4a was indicative of the mobility increase of hydrocarbon chains with an ongoing coordination reaction of copper(II) ions with the Schiff base polar headgroup of amphiphile. In other words, the modification of the headgroups has an influence on the mobility of the hydrocarbon chains. Recent investigations (17) have also demonstrated that the conformations of alkyl chains and their vibrations were also modified by the modification of the headgroup. In the present paper, copper(II) ions penetrate into LB films of 2,4-dihydroxyN -octadecylbenzylideneamine and react with it, which induces a minor rearrangement of the molecules owing to change in the interaction between headgroups. The integrated values of the CH2 antisymmetric stretching vibration decreased in intensity with increasing immersion time of contacting LB films with 1 mM aqueous Cu(OAC)2 solution (shown in Fig. 4b). It is well known that integrated intensity is determined by packing density and orientation of alkyl chains (18). The observation in Fig. 4b may be due in part to film
FIG. 4. (a) The plotted width of the CH2 antisymmetric stretching vibration as a function of exposure time. (b) The integrated intensity of the CH2 antisymmetric stretching vibration.
62
PANG AND LIANG
FIG. 5. The integrated absorption of the band at 1650 cm−1 dependence on the time.
reorganization and crystallite formation, which has been observed for LB films of cadmium salts of fatty acids by atomic force microscopy (19). Such reorganization can cause surface roughness, which increase the thickness of the film in some region and can indeed decrease the film absorbance, as will be detailed in a future report. We will now emphasize the time dependence of the integrated absorbance of the band at ca. 1650 cm−1 (Fig. 5). The curve did not exhibit any periodicity, suggesting that the transport rate through the film is fast compared to the (Cu2+ ion coordination to Schiff base polar group) reaction rate. If the ion transport was slow compared to the reaction rate, then for a perfectly lamellar structure the reaction rate would oscillate as consecutive Schiff base layers were fully coordinated. For a uniform one-dimensional diffusion process, this means that the ratio of the reaction rate to the diffusion rate is small. However, if there is transport through the film via defects, followed by lateral transport and reaction along the polar group planes, then no periodic reaction rate would be observed either. In both cases, material in all of the polar regions would be reacting simultaneously. In the first scenario, the reaction rate would determine the overall coordination rate. In the second, either the lateral transport along the polar planes or the reaction rate, or both, could determine the overall ion-coordination rate. 3.3. FTIR Spectra of Annealing Film In order for the effect of the microstructure on the ioncoordination rate to be investigated, LB films previously treated at 45 and 80◦ C were contacted with aqueous Cu(OAC)2 solution.
In the 45◦ C treated films there was a slight slowing of the overall ion-coordination rate. However, for the 80◦ C treated films, the effect was quite dramatic. The entire coordination process needs only 1 h for annealing film at 80◦ C. Clearly, the disruption of the structure of the LB films by melting at 80◦ C and refreezing markedly altered the rate-controlling mechanism in the ion-coordination process. From the fast coordination compared to the untreated film, it is inferred that for the nomelted films there were significant ion transport limitations on the determination of the ion-transfer rate. This experiment provided a low bound for the intrinsic rate of ion coordination. The above observations suggest that structure defects may also be important in determining the ion transfer rate. The 45◦ C treatment may have healed some defects within the films and increased the effective ion transfer resistance, which slowed the coordination reaction. However, the film probably melted at 80◦ C; when it was cooled and recrystallized, a large number of defects (or grain boundaries) probably formed, resulting in an increased ion transfer rate. The above discussion shows that FTIR spectra exhibit marked change after contact of LB films of 2,4-dihydroxy-N octadecylbenzylideneamine with aqueous Cu(OAC)2 solution. In order to obtain further information on the aggregation and structure of molecules in ion-coordination films, orientation of the alkyl chain and phase behavior were studied by comparing ion-coordination film with surface and subphase film. 3.4. Comparison of FTIR Spectra among Different Complex LB Film The spectral changes have been studied as discussed above. Now we analyze molecular structure in ion-coordination films. Figures 6a, 6b, and 6c compared FTIR transmission spectra of nine-monolayer complete ion-coordination films with that of subphase films and surface films. There are significant differences among spectra of the three metal complex LB films. In spectra of surface and subphase films, two distinguished bands appear at 1606 and 1544 cm−1 , while for ion-coordination film, the corresponding band is a broad band at ca. 1616 cm−1 with a shoulder. These differences imply that the molecular orientation and aggregation of headgroups in ion-coordination films are different from those of surface and subphase films. In the region of CH scissors vibration, a single band at 1467 cm−1 appeared in spectra of ion-coordination films, while for surface and subphase films, two distinguished peaks at 1447 and 1466 cm−1 can be observed. These differences indicate that the molecules were packed less densely in ion-coordination films (20). 3.5. Orientation of Alkyl Chain in Ion-Coordination Film Polarized FTIR spectra can be used to probe the orientation of an alkyl chain. For 2,4-dihydroxy-N -octadecylbenzylideneamine LB film, the polarized dichroic ratio (D = As /Ap ) is 1.004, while for ion-coordination film, the value changes to 1.04. It is clear that the orientation of hydrocarbon
ION COORDINATION IN LB FILMS
63
β = As (i 6= 0)/As (i = 0) = [n 1 n 3 (n 1 + n 3 )/(n 1 cos r + n 3 cos i)] £¡ ¢ 00 00 ¤ /εxx + cos i cos r/n 1 n 3 , × n 21 sin2 r/n 42 εzz where i is the angle of incidence in the medium of air, r is the angle of refraction in the substrate, p and s polarization are electronic fields perpendicular and parallel to the plane on incidence, respectively, n 1 sin i = n 3 sin r , and the refractive index n 1 = 1.0 (in air) n 2 = 1.5 (LB film), n 3 = 1.415 (CaF2 substrate). In addition cos2 φa + cos2 φs + cos2 γ = 1. So we determine that the orientation angle of ion-coordination film is 34.9◦ , larger than that of ligand film (30.3◦ ). By the same method, the orientation angles of subphase film (22.1◦ ) and surface film (23.7◦ ) can be obtained. The values imply that head-coordination directs the packing of complex molecules in coordination-film tilted farther away from the surface normal than surface film and subphase film, which can be explained by a spacing of the polar groups leading to a rearrangement of the CH2 dipole moments of the methylene groups along the hydrocarbon chain (17). 3.6. Order–Disorder Transition of LB Film
FIG. 6. FTIR transmission spectra of (a) ion-coordination film, (b) subphase film, and (c) surface film.
Variable-temperature FTIR spectroscopy is an effective tool for measuring conformational change in alkyl chains and determining order–disorder phase transition temperature Tc (24). In the present paper, transition temperature Tc of ion coordination
chains in ion-coordination films is slightly changed. Comparison of those in subphase film (0.894) and surface film (0.899) indicates that ion-coordination films take an isotropic unaxial orientation, while subphase and surface films take anisotropic biaxial orientation. It is accepted that anisotropic biaxial orientation of metal complex LB films is caused by a close-packing superlattice network of complex head that is subject to the shear forces induced during film transfer (21). Ion-coordination film was obtained by penetrating metal copper(II) ions into polar head regions and reacting them with materials in LB films, so the transfer-induced orientation effect cannot be seen with ioncoordination film. The orientation of alkyl chains in LB films is a major component characterizing two-dimensional structure and also relates to many functions, for example, the efficiency of energy transfer and the rate of photoinduced electron transfer (22). To analyze the polarization data in terms of molecular orientation, it is necessary to measure the dependence of absorbance on the rotation angle with respect to the plane of polarization and also on the incident angle i with respect to the surface normal of the film plane. Polarized infrared spectra of complete ion coordination films are shown in Fig. 7. Detailed calculation of orientation was carried out according to the formulae by Vanderyver et al. (23), α = Ap (i = 0)/As (i = 0) cos 2ω = (1 − α)/(1 + α)
FIG. 7. Polarized infrared spectra of complete ion-coordination film with the different incident angles.
64
PANG AND LIANG
highest and lowest values of the antisymmetric CH2 stretching frequency, reflects the order–disorder transition temperature of the alkyl chains in LB films. For ion-coordination film, only a phase transition appeared at ca. 70◦ C, which is higher than that of 2,4-dihydroxy-N -octadecylbenzylideneamine (ca. 45◦ C). However, surface film and subphase film both exhibit two-phase transition. They are near 30 and 110◦ C for surface film and 55 and 140◦ C for subphase film, respectively. According to the literature (25), the two-phase transitions were the character of liquid crystal arising from a close-packing superlattice network. The absence of the two-phase transition behavior in ion-coordination film agrees well with the results of molecular orientation. 3.7. UV–Vis Electronic Absorption Spectra
FIG. 8. Temperature dependence of wavenumber of antisymmetric CH2 stretching bands: (a) ion-coordination film, (b) surface film, (c) subphase film.
film was measured. In order to investigate headgroup structures in ion-coordination film, Tc of surface film and subphase film are also given. Figure 8 shows frequencies of the CH2 antisymmetric stretching mode plotted as a function of temperature. The temperature, corresponding to the middle point between the
UV–Vis absorption spectra of untreated films ion-coordination films and surface films are shown in Fig. 9. Two absorption peaks near 308 and 380 nm in untreated film correspond to the π–π ∗ transition of the aromatic ring in the headgroup and the n–π ∗ transition of the aromatic Schiff base chromophore. For ion-coordination film and surface film, two absorption peaks near 298 and 349 nm appeared (within experimental errors), indicating an obvious hypsochromatic shift from that seen with the untreated film. So it is concluded that the complex has a less conjunctive effect caused by formation of complex (26). The similar absorption peaks in surface film and ion-coordination film provided further evidence that copper(II) coordinated to Schiff base polar groups by contacting LB film of 2,4-dihydroxy-N octadecylbenzylideneamine in aqueous Cu(OAC)2 solutions. CONCLUSION
The ion-coordination films were prepared by contacting builtup multilayers of 2,4-dihydroxy-N -octadecylbenzylideneamine with aqueous solutions containing Cu(OAC)2 . Spectral changes owing to coordination of Cu(II) ions to ligands have been probed by FTIR spectra and UV–Vis absorption spectra. Annealing films at different temperature shows that microstructure of LB films has important effects on the rate of coordination reactions. Studies on molecular orientation and phase behavior show that ioncoordination films take isotropic unaxial orientation and exhibit a phase transition, which is determined by the complex structure of the headgroup, which is different from that of subphase and surface films. The detailed mechanism is now under study. ACKNOWLEDGMENT This work was financially supported by a major research project grant from the State Science and Technology Commission of China.
REFERENCES
FIG. 9. UV–Vis absorption spectra of (a) untreated ligand film, (b) complete ion-coordination film, and (c) surface film.
1. Naito, K., Minra, A., and Azuma, M., Thin Solid Films 210/211, 527 (1992). 2. Hopf, F. R., M¨obius, D., and Whitten, D. G., J. Am. Chem. Soc. 98, 1584 (1976).
ION COORDINATION IN LB FILMS 3. Sun, L. Y., Gu, C., Wen, K., Chao, X., Li, T. O., Hu, G. Y., and Sun, I. Y., Thin Solid Films 210/211, 486 (1992). 4. Blunkenburg, G., Meller, P., Ringsdorf, H., and Salesse, C., Biochemistry 28, 8214 (1989). 5. Xia, W. S., Huang, C. H., and Zhou, D. J., Langmuir 13, 80 (1997). 6. Gust, D., Moore, T. A., Luttrull, D. K., Degraziano, J. M., Blodt, N. J., Van der Auweraer, M., and Schryver, F. C., Langmuir 7, 1483 (1991). 7. Ko, W. H., Fu, C. W., Wang, H. Y., Barzel, D. A., Kenny, M. E., and Lando, J. B., Sens. Mater. 2, 39 (1990). 8. Bonos, F., Lely, F., Ricciardi, G., Ricciaridi, G., Romanelli, M., and Martini, G., Langmuir 9, 268 (1993). 9. Fukuda, A., Hanabusa, K., and Shirai, H., Jpn. Kokai Tokkyo koho 6228386 (87258386). 10. Pang, S. F., Huang, J. G., and Liang, Y. Q., Spectrosc. Lett. 33, 301 (2000). 11. John, W., and Ledketter, Jr., J. Phys. Chem. 81, 54 (1977). 12. Hadjoudisw, E., Vittorakis, M., and Moustakali-Mavridis, I., Tetrahedron 43, 1345 (1987). 13. Synder, B. G., Strauss, H. L., and Elliger, C. A., J. Phys. Chem. 86, 5145 (1982). 14. Synder, R. G., Hsn, S. L., and Krimm, S., Spectrochim. Acta. Part A 34, 395 (1978).
65
15. Marshbanks, T. L., and Franses, E. I., J. Phys. Chem. 98, 2166 (1994). 16. Katuyama, N., Enomoto, S., Sato, T., Ozaki, Y., and Kuramoto, N., J. Phys. Chem. 97, 6880 (1993). 17. Girard-Egrot, A. P., Morelis, R. M., and Coulet, P. R., Langmuir 13, 6540 (1997). 18. Robe, J. P., Rabott, J. F., Brown, C. A., and Swalen, J. D., J. Chem. Phys. 84, 4096 (1986). 19. Schwartz, D. K., Viswanathan, R., and Easadzinski, T. A. N., J. Phys. Chem. 96, 10444 (1992). 20. Marshbanks, T. L., Ahn, D. J., and Franses, E. I., Langmuir 10, 276 (1994). 21. Hsu, Y., Penner, T. L., and Whitten, D. G., Langmuir 10, 2757 (1994). 22. Liese, G., Tieke, B., and Wegner, G., Thin Solid Films 68, 77 (1980). 23. Vanderyver, M., Barrand, A., and Raudel-Teixier, J. Colloid Interface Sci. 85, 571 (1982). 24. Du, X. Z., Shi, B., and Liang, Y. Q., Langmuir 14, 3631 (1998). 25. Kusumizu, S., Kato, R., Yamada, M., and Yano, S., J. Phys. Chem. B 101, 10666 (1997). 26. Noh, D. Y., and Mizuno, M., Inorg. Chim. Acta 216, 147 (1994).