J. Phys. Chem. C 2007, 111, 17025-17031
17025
Novel Metal Coordinations in the Monolayers of an Amino-Acid-Derived Schiff Base at the Air-Water Interface and Langmuir-Blodgett Films Huijin Liu, Xuezhong Du,* and Yan Li Key Laboratory of Mesoscopic Chemistry (Ministry of Education), State Key Laboratory of Coordination Chemistry, and School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed: June 5, 2007; In Final Form: September 3, 2007
The metal coordinations in the monolayers of an amino-acid-derived Schiff base, 4-(4-dodecyloxy)-2hydroxybenzylideneamino)benzoic acid (DSA), at the air-water interface and Langmuir-Blodgett (LB) films have been investigated using surface pressure-area isotherms, infrared reflection absorption spectroscopy (IRRAS), FTIR (transmission and reflection absorption), and UV-vis spectroscopy, respectively. A combination of the three modes of infrared spectroscopy is employed to study interfacial metal coordinations on different substrates (water, CaF2, and gold). The DSA molecules form a condensed monolayer on pure water, in which the Schiff base segments take a nonplanar conformation and their long axes are oriented almost perpendicular to the water surface. The CdN stretching bands are assigned for the monolayers on the H2O-based subphases. In the presence of Ca2+ and Zn2+ in the subphases, the monolayers are expanded to a certain extent. Ca2+ and Zn2+ ions only coordinate to the carboxylate oxygen atoms in the fashion of chelating bidentate coordination but not to the imino nitrogen atoms or phenolate oxygen atoms of the salicylideneamino moieties. The long axes of the Schiff base segments are preferentially oriented along the monolayer normals. The twist angle for the nonplanar conformations of the Schiff base segments is considered to increase in the case of Ca2+ and decrease in the case of Zn2+ in comparison with that on pure water. In the presence of Cu2+, the monolayer is expanded and the alkyl chains are disordered. Cu2+ ions coordinate not only to the carboxylate groups in the fashion of bridging bidentate coordination but also to the salicylideneamino moieties. The Schiff base segments are significantly tilted away from the monolayer normal.
Introduction Schiff bases are one of the most important classes of organic compounds and have extensive applications in biological functional materials,1-3 polymer ultraviolet stabilizers,4,5 laser dyes,6 and molecular switches in logic and memory circuits.7 Schiff bases of salicylidene derivatives exhibit photochromism and/or thermochromism in the crystalline state and have been the focus of nonlinear optical (NLO) studies for potential applications, such as data storage, information processing, telecommunication, and optical switching.8,9 Besides, the other important property of this type of Schiff base is ready to coordinate with transition metals.10 Their metal complexes exhibit fascinating chemical, optical, electric, thermal, and catalytic properties and have been well characterized since the late 1920s; however, their applications are restricted because of their poor stability. Langmuir and Langmuir-Blodgett (LB) techniques are powerful means to control molecular orientation and packing at the molecular level necessary for molecular devices.11,12 It has been shown that their stability and properties are greatly enhanced after the introduction of the Schiff base segments into the monolayers at the air-water interface.13 Stable monolayers of the Schiff base derivatives are formed, and related hydrolysis is suppressed. So far, LB films of Schiff bases and their metal complexes have been investigated including different kinds of amphiphiles.13-20 * To whom correspondence should be addressed. E-mail: xzdu@ nju.edu.cn. Fax: 86-25-83317761.
The monolayers and LB films of fatty acids and their metal complexes have been studied extensively since the invention of Langmuir and LB techniques.21 Obviously, both Schiff bases of salicylidene derivatives and fatty acids are easy to coordinate with metal ions; however, few studies on the two moieties of Schiff base and carboxyl incorporated into one amphiphile have been reported.22,23 The “push-pull” molecules (with electrondonating and attracting groups) exhibit high quadratic hyperpolarizabilities, where properties can be largely modified by photochemical, electrochemical, or acido-basic reaction.9 Among them, several examples of efficient molecules for photoswitching have been reported.24,25 Saito and co-workers investigated the photochromism of the LB films of amino-acid-derived Schiff bases in the case of Ba2+.22 Huang and co-workers investigated the aggregation behaviors of this kind of Schiff base amphiphiles in aqueous dispersions.26,27 In this paper, novel metal coordinations of the monolayers of amino-acid-derived Schiff bases have been investigated at the molecular level. Infrared reflection absorption spectroscopy (IRRAS) has been a leading structural method for in situ characterization of the monolayers at the air-water interface28-39 since the early works by Dluhy and co-workers in the mid-1980s.40 The IRRAS technique can provide abundant information not only on headgroup structure and chain conformation but also on molecular orientation. In this paper, surface pressure-area (πA) isotherms and the IRRAS technique are used to investigate the monolayers of 4-(4-dodecyloxy)-2-hydroxybenzylideneamino)benzoic acid (DSA, see Figure 1) on aqueous subphases
10.1021/jp074341b CCC: $37.00 © 2007 American Chemical Society Published on Web 10/16/2007
17026 J. Phys. Chem. C, Vol. 111, No. 45, 2007
Figure 1. Chemical structure and synthesis route of 4-(4-dodecyloxy)2-hydroxybenzylideneamino)benzoic acid (DSA).
containing metal ions (Ca2+, Zn2+, and Cu2+). Taking advantage of the LB technique, FTIR [transmission (TR) and reflection absorption (RA)] and UV-vis spectroscopy are also employed to study the metal complexes. Novel metal coordinations and molecular structures of the monolayers and LB films have been investigated. Experimental Section Materials. 2,4-Dihydroxybenzaldehyde, 1-bromododecane, and 4-aminobenzoic acid were commercially available. The chemical reagents used were of analytical grade, and the water used was double distilled (pH 5.6) after a deionized exchange. The pH of metal ion-containing solutions (1 mmol/L) was pH 7.5 for CaCl2, pH 6.8 for ZnCl2, and pH 5.3 for CuCl2. The aqueous ZnCl2 and CuCl2 solutions were not adjusted with HCl or NaOH. 4-Dodecyloxysalicylaldehyde. 2,4-Dihydroxybenzaldehyde (2.76 g, 20 mmol) was dissolved in 30 mL of methanol containing 1.3 g (23 mmol) of potassium hydroxide and refluxed under an N2 atmosphere for 1 h. Then, a methanol solution containing 6.63 g (30 mmol) of 1-bromododecane was added dropwise to this mixture and refluxed for 20 h under an N2 atmosphere. The solution was concentrated to give a red-brown powder. The raw product was purified by silica column chromatography (light petroleum/ethyl acetate/chloroform, 30: 1:5, v/v/v) to give a white granular solid 4-dodecyloxysalicylaldehyde. 4-(4-Dodecyloxy)-2-hydroxybenzylideneamino)benzoic acid (DSA). Potassium hydroxide (0.2 g, 3.6 mmol) and 0.47 g (3.6 mmol) of 4-aminobenzoic acid were dissolved in 20 mL of ethanol, and then 1.0 g (3.6 mmol) of 4-dodecyloxysalicylaldehyde was added. The solution was then gently stirred for 6 h in the presence of a small amount of acetic acid at room temperature. The white solid was filtered off and recrystallized with ethanol/chloroform (1:1, v/v) and then dried in a vacuum for 24 h to obtain a yellow DSA. mp 203-204 °C. 1H NMR (DMSO): 11.5 (s, 1H, -COOH); 8.6 (s, 1H, -CHdN); 6.6-8.2 (m, 7H, phenyl); 5.4 (s, 1H, -OH); 4.1 (s, 2H, -OCH2-); 1.3-2.1 (m, 20H, -(CH2)10-); 0.96 (t, 3H, -CH3). Elem. Anal. Calcd for C26H35NO4: C, 73.38; H, 8.29; N, 3.29; O, 15.04. Found: C, 72.96; H, 8.34; N, 3.71; O, 14.99. Isotherms and LB Film Preparation. The monolayer spreading and transfer were performed on a Nima 611 Langmuir trough (England). A Wilhelmy plate (filter paper) was used as the surface pressure sensor and situated in the middle of the trough. Two mobile barriers compressed or expanded symmetrically at the same rate. Monolayers were obtained by spreading the chloroform/ether (1:1, v/v) solutions of DSA on pure water and ion-containing subphases. The subphase temperature was kept at 22 °C. For the preparation of LB films, the monolayers were first compressed to a desired surface pressure of 20 mN/m, and then 30 min was allowed for the
Liu et al. monolayer to reach equilibrium. Eleven-layer LB films were transferred onto CaF2 and quartz substrates and 21-layer films onto gold-coated silicon wafers by the vertical method at a dipping rate of 2.0 mm/min in the Y type. IRRAS Spectrum Measurements. IRRAS spectra of the monolayers at the air-water interface were recorded on an Equinox 55 FTIR spectrometer (Bruker, Germany) connected to an XA-511 external reflection attachment with a shuttle trough system and a liquid-nitrogen-cooled MCT detector. Sample (film-covered surface) and reference (film-free surface) troughs were fixed on a shuttle device driven by a computer-controlled stepper motor for allowing collection from the two troughs in an alternating fashion. A KRS-5 polarizer was used to generate perpendicularly polarized lights, and the efficiency of the polarizer was determined to be about 99.2%.41 These experiments were carried out at 22 °C. The film-forming molecules were spread from chloroform/ether (1:1, v/v) solutions of desired volumes, and 20 min was allowed for solvent evaporation. The measurement system was then enclosed for humidity equilibrium and monolayer relaxation for 4 h prior to compression. The external reflection absorption spectra of pure water and ioncontaining solutions were used as references, respectively. The spectra were recorded with a resolution of 8 cm-1 by coaddition of 1024 scans. A time delay of 30 s was allowed for film equilibrium between trough movement and data collection. Spectra were acquired using p-polarized radiation followed by data collection using s-polarized radiation. The IRRAS spectra were used without smoothing or baseline correction. UV-vis and FTIR Spectrum Measurements. The UVvis spectra of the LB films on quartz plates were recorded on an Lambda-35 spectrometer. FTIR transmission spectra of the LB films on CaF2 substrates were recorded on an IFS 66V spectrometer (Bruker, Germany) with a DTGS detector. Typically 1000 scans were collected to obtain a satisfactory signalto-noise ratio with the resolution 4 cm-1. The film spectra were obtained by subtracting the spectra of CaF2 blank substrates from the corresponding sample spectra. The RA spectra of the LB films on gold-coated silicon wafers were recorded on the same FTIR spectrometer for the measurements of IRRAS spectra. A gold-coated silicon wafer with LB films and a bare gold-coated wafer were placed onto the sample and reference troughs, respectively. p-Polarized light was incident on the wafer plane at 83° from the surface normal. The spectra were recorded with a resolution of 4 cm-1 by coaddition of 1024 scans. Results and Discussion Isotherms of Monolayers at the Air-Water Interface. Figure 2 shows π-A isotherms of the monolayers of DSA on pure water and the ion-containing subphases, respectively. On pure water, DSA molecules form a condensed monolayer with a collapse pressure of about 40 mN/m. The linear part of the isotherm is extrapolated to zero surface pressure, and a limiting area of 0.254 nm2 can be obtained, which is very close to the cross-sectional area of the long axis of the salicylideneaniline nucleus.9,22 This means that the long axis of the chromophore is oriented almost perpendicular to the water surface. The crystal structure of methyl 4-(2,4-dihydroxybenzylideneamino)benzoate, very similar to the chromophore in the amphiphile studied here, was determined recently by X-ray diffraction.9 The compound shows the typical distortion of anils: the mean planes of the six-membered rings make an angle of 47.69°.9 That means that the Schiff base segments take a nonplanar conformation in the crystalline state. Although no clear interaction such as π-π stacking is evidenced, the molecules are very tightly packed.9
Novel Metal Coordinations
Figure 2. π-A isotherms of DSA monolayers on pure water and ioncontaining subphases (Ca2+, Zn2+, and Cu2+), respectively: compression rate, 8 mm/min; temperature, 22 °C.
Figure 3. UV-vis absorption spectra of 11-layer LB films of DSA deposited from pure water and ion-containing subphases (Ca2+, Zn2+, and Cu2+) at a surface pressure of 20 mN/m, respectively.
As a consequence, the density of the solid is 1.446 g/cm3, which is among the three highest values known for anils without heavy atoms.9 Obviously, a close-packed DSA monolayer is formed on pure water. The presence of metal ions in the subphases gives rise to expansion of the monolayer to different extents, which implies that the metal complexes between the headgroups and metal ions are formed and vary depending on the metal ion. In the case of Ca2+, a limiting area of 0.288 nm2 is obtained, which is very close to the limiting area (0.27-0.28 nm2) of the DSA monolayer in the case of Ba2+ at 15 °C.22 The limiting area is 0.333 nm2 in the case of Zn2+, which is similar to those for the monolayers of azobenzene-containing fatty acids and their barium salts at 20 °C.42 However, a significant expansion occurs in the presence of Cu2+, which suggests that different molecular orientation and/or coordination fashion probably take place. UV-vis Spectra of LB Films. Figure 3 shows UV-vis spectra of 11-layer LB films of DSA deposited from pure water surface and the aqueous subphases containing Ca2+, Zn2+, and Cu2+ ions, respectively. In the case of pure water, two absorption maxima appear around 280 and 318 nm with comparable intensities. The maximum at 318 nm is assigned mainly to the π-π* charge-transfer transition from the salicylideneamino moieties to the benzoic acid moieties, and the maximum at 280 nm is ascribed to the π-π* transition of a local excitation
J. Phys. Chem. C, Vol. 111, No. 45, 2007 17027 in the salicylideneamino moieties.43 In general, the planarity of benzylideneaniline derivatives is attributed to the favorable π delocalization throughout the molecules by intramolecular charge-transfer interaction between the substituents.44 In the case of benzylideneanilines, the bands in the long wavelength region (band I) are weaker than those in the short wavelength region (band II).43 With an increase in the twist angle of nonplanar conformations, band I is expected to decrease in intensity, and on the contrary, band II is to increase in intensity.43 When the aniline moieties carrying electron-attracting nitro groups are at the p-substituted positions, the electron-attracting groups increase the intensity of band I so that band I is stronger than band II.43 However, the planarization in each case is accompanied by an increase in the intensity of band I.43,45,46 In the crystalline state-like LB films, the Schiff base segments obviously take a nonplanar conformation. In the presence of the Zn2+ and Ca2+ ions in the subphases, the absorption spectra are similar to those on pure water except for the relative intensity. In the case of Ca2+, band I at 325 nm is a little weaker than band II at 276 nm. The spectrum is very similar to that in the case of Ba2+.22 In the case of Zn2+, band I at 321 nm is stronger than band II at 278 nm. Compared with the spectrum in the case of pure water, the changes in relative intensity of band I to band II indicate that the twist angle of nonplanar conformations is increased in the case of Ca2+ and decreased in the case of Zn2+. In the presence of Cu2+, a different spectral behavior is presented. The absorption band at 376 nm is due to the ligand transition and a charge transfer from the ligand to Cu2+ ion, which confirms that Cu2+ ions coordinate to the salicylideneaniline portions. The metal complex shows a large shift to longer wavelength because of the greater degree of charge interactions between the salicylideneanilines and Cu2+ ions.17 It is inferred that Zn2+ and Ca2+ ions do not coordinate to the salicylideneaniline portions but to the carboxylic groups and that the Cu2+ ions coordinate not only to the salicylideneaniline portions but also to the carboxylic groups. FTIR Spectra of LB Films. Figure 4a and b shows the FTIR spectra of LB films of DSA deposited from pure water and aqueous subphases containing Ca2+, Zn2+, and Cu2+ in the TR and RA modes, respectively. The bands and their assignments for different aqueous subphases in the two modes are listed in Table 1. The wavenumbers of vibrational bands in the two modes are slightly different. Herein, most of the band frequencies in the RA mode are higher than those in the TR mode. The electric field vector is parallel to the substrate surface in the TR mode and almost perpendicular to the surface in the RA mode. The intensities of the bands for the 21-layer LB films on the gold surfaces are much stronger than those for the 11layer LB films on the CaF2 substrates (a total of 22 layers taking into account two sides of the substrates). The relative intensities of the bands are quite different between the TR and RA spectra. In the case of pure water, the antisymmetric and symmetric CH2 stretching bands [νa(CH2) and νs(CH2)] appear at 2918 and 2849 cm-1 in the TR spectrum, respectively. The frequencies of the two vibrations are sensitive to conformation order of alkyl chains.47,48 Low wavenumbers are characteristic of preferential all-trans conformations in the highly ordered alkyl chains, while the number of gauche conformers increases with frequency and width of the bands. The νa(CH2) and νs(CH2) bands are stronger in relative intensity than those in Figure 4b, which indicates that the alkyl chains are preferentially oriented perpendicular to the substrate surfaces. The CdO stretching band [ν(CdO)] of the carboxylic groups is clearly observed in the vicinity of 1680-1690 cm-1.49,50 The band due to the CdN stretching
17028 J. Phys. Chem. C, Vol. 111, No. 45, 2007
Figure 4. FTIR spectra of multilayer LB films of DSA deposited from pure water and ion-containing subphases (Ca2+, Zn2+, and Cu2+) at a surface pressure of 20 mN/m, respectively: (a) 11-layer films on CaF2 substrates in the transmission mode; (b) 21-layer films on gold surfaces in the RA mode.
vibration [ν(CdN)] is very weak in the TR spectrum (at 1620 cm-1)51-53 and very strong in the RA spectrum (at 1630 cm-1),51-53 which indicates that the long axes of the salicylideneaniline segments are nearly perpendicular to the substrate surface. In the presence of metal ions in the aqueous subphases, the ν(CdO) bands of the carboxylic groups completely disappear, and the bands related to their ν(C-O)/δ(OH) modes are not observed or decrease in intensity due to the complex of carboxylic acids with metal ions. Some new absorption bands appear in the 1600-1500 cm-1 and 1430-1380 cm-1 regions and are attributed primarily to the antisymmetric and symmetric stretching vibrations of carboxylate groups [νa(COO) and νs(COO)], respectively. These observations indicate that the metal complexes between the metal ions and carboxylate oxygen atoms are formed. The ν(CdN) band undergoes a different change depending on the metal ion. In the case of Ca2+, two sharp peaks at 1548 and 1512 cm-1 in the TR spectrum are assigned to the νa(COO) vibration, and a strong band at 1449 cm-1 in the RA spectrum is predominantly due to the νs(COO) vibration. The frequency separation (∆) between the νa(COO) and νs(COO) wavenumbers is usually used as a diagnostic tool to gain an insight into the respective coordination fashions.56 The ∆ values of 99 and 63 cm-1 indicate the formation of chelating bidentate coordination between the carboxylate groups and calcium atoms.56 The change in the ν(CdN) band is not significant, but the ν(CdC)
Liu et al. band at 1598 cm-1 shifts to 1590 cm-1 and the other ν(CdC) band at 1570 cm-1 is not observed in the TR spectrum. The δ(OH) band at 1342 cm-1 related to the phenols in the RA spectrum remains almost unchanged. In the case of Zn2+, a broad band at 1527 cm-1 in the TR spectrum is assigned to the νa(COO) vibration, and a strong band at 1435 cm-1 in the RA spectrum is primarily due to the νs(COO) vibration. A ∆ value of 92 cm-1 indicates the formation of chelating bidentate coordination between the carboxylate groups and zinc atoms.56 The other bands are similar to those in the case of Ca2+. The strong νs(COO) bands are observed in the RA spectra, which indicates that the νs(COO) transition moment is almost perpendicular to the substrate surface and that the salicylideneaniline segments are oriented preferentially vertical to the support surface. In the combination of the FTIR and UV-vis spectra, it is obvious that only the carboxylate oxygen atoms coordinate with metal ions in the cases of Ca2+ and Zn2+. This kind of metal complex is demonstrated by the crystal structures of a series of organotin (IV) complexes with the ligands 4-(2hydroxybenzylideneamino)benzoic acid,57 the same as the Schiff base portions in the amphiphiles. The two oxygen atoms of the carboxylate groups coordinate to the tin atoms and neither imino nitrogen atoms nor phenolate oxygen atoms participate in coordination to the tin atom.57 In the case of Cu2+, the νa(CH2) and νs(CH2) bands shift up to higher wavenumbers, respectively, which indicates that the alkyl chains are in disordered state with the gauche conformations. The ν(CdN) band shifts down by 8-11 cm-1. The large shift means that Cu2+ ions coordinate with the imino nitrogen atoms. The δ(OH) band at 1342 cm-1 in the RA spectrum is not observed, and the corresponding ν(dC-O) band is shifted up to about 1310 from 1290 cm-1. These changes suggest that the phenolate oxygen atoms are involved in the coordination with Cu2+ ions.19 In the TR spectrum, two νa(COO) bands appear at 1589 and 1526 cm-1 and two νs(COO) bands at 1423 and 1386 cm-1, while in the RA spectrum, the νa(COO) bands appear at 1599 and 1533 cm-1 and the νs(COO) bands at 1421 and 1375 cm-1. It is inferred that the sharp band at 1590 cm-1 is related to the band at 1420 cm-1 and the broad band around 1530 cm-1 to the band at ca. 1380 cm-1 on the basis of the νa(COO) and νs(COO) band profiles. The ∆ values of about 170 and 150 cm-1 indicate the formation of bridging bidentate coordination.56 The FTIR spectral studies of a series of anhydrous long-chain copper alkanoates were reported previously.58 For the crystalline phase, the νa(COO) and νs(COO) bands appeared at 1585 and 1425 cm-1, respectively, whereas in the columnar mesophase the two bands were present at 15401530 and 1415 cm-1.58 Herein, Cu2+ ions coordinate not only to the carboxylic groups but also to the salicylideneaniline portions. According to Irving and Williams59 among the bivalent ions of the first transition series, the most stable complexes are formed for the Cu2+ ion, irrespective of the nature of the coordination ligand or of the number of ligand molecules involved.17 In the presence of Cu2+, the νa(COO), νs(COO), and other bands are clearly observed both in the TR spectrum and in the RA spectrum compared with the other cases. These indicate that the whole Schiff base segments are obviously tilted with respect to the surface normal. The probable molecular structures of DSA and its metal complexes in the monolayers are illustrated in Figure 5. IRRAS Spectra of the Monolayers at the Air-Water Interface. The so-called “metal selection rule” does not apply to the air-water interface because water is a low absorbing substrate and the intensity of the reflected beam is much lower
Novel Metal Coordinations
J. Phys. Chem. C, Vol. 111, No. 45, 2007 17029
TABLE 1: Observed Bands and Their Assignments of LB Films of DSA Transferred from Different Aqueous Subphases in the Transmission and Reflection Absorption Spectra wavenumber (cm-1)a Ca2+
H2O TR 2954 w 2919 s 2849 s 1683 s 1620 w 1598 w 1570 w
RA 2921 w 2873 w 2852 w 1692 m 1630 m 1604 m
Zn2+
TR
RA
TR
RA
TR
RA
2954 w 2918 s
2955 w 2919 s
2957 w 2926 w
2850 s
2957 w 2920 w 2874 w 2851 w
2955 w 2923 s
2850 s
2957 w 2922 w 2874 w 2852 w
2853 s
2855 w
1613 w 1590 m
1632 s 1603 s
1615 w 1585 s
1632 s 1604 s
1609 s
1622 s
1589 s
1599 s
1526 s
1533 m
1467 w 1423 s
1469 w 1421 m
1382 s
1395 m 1375 w
1548 w 1468 w
1471 w
1402 s
1434 m 1394 w
1527 s 1520 w
1512 m 1468 w
// // ⊥ ⊥
// 1470 w
1403 m
1472 w 1435 s
1402 m
⊥ ⊥
//
1354 w 1323 m 1342 w
1195 w 1172 w
//
1518 w
1449 s
1289 s
orientationb
1574 m 1520 w
1360 w 1342 w
Cu2+
1302 m 1260 w 1248 w 1200 s 1173 w 1146 w 1124 w
1342 w
1288 s
1292 m
1289 m
1292 m
1196 w 1173 w
1242 w 1200 s 1176 w
1192 w 1174 w
1243 w 1200 s 1175 w
1121 w
1122 w
1310 m 1293 w 1186 s
1312 w 1242 w 1202 s
⊥
⊥
1126 w
assignmentc νa(CH3) νa(CH2)47,48 νs(CH3) νs(CH2)47,48 ν(CdO)49,50 ν(CdN)51-53 ν(CdC)49-54 νa(COO) + ν(CdC) ν(CdC)49-54 νa(COO) ν(CdC)49-54 νa(COO) δ(CH2) + ν(CdC) νs(COO) + ν(CdC) ν(CdC)50,52 ν(CdC)55 νs(COO) δ(CH) in CHdN51,52 δ(OH) in phenol + ν(C-O)/δ(OH) in COOH49,53,55 δ(OH) in phenol49,53,55 ν(dC-OM) ν(dC-O)49,53,55 ν(dC-C)51 ν(dC-N)51,52 β(dC-H)52 β(dC-H)52 ν(C-O)
s, strong; m, medium; w, weak. //, parallel to substrate surface; ⊥; perpendicular to substrate surface. c ν, stretching; νa, antisymmetric stretching; νs, symmetric stretching; δ, bending; β, in-plane bending. a
b
Figure 5. Schematic illustration of probable molecular structures of DSA and its metal complexes in the monolayers.
than the intensity of the incoming beam. Therefore, all three electric field components have a sufficiently high intensity at the water surface to interact with the monolayer. For p-polarized radiation (electric field vector parallel to the plane of incidence), the band with a transition moment parallel to the water surface
is initially negative and its intensity increases with increasing angle of incidence and reaches a maximum, then a minimum in the reflectivity is found at the Brewster angle.28 The exact position of the Brewster angle φ depends on the wavelength of light and the optical properties of the substrate. For the air-
17030 J. Phys. Chem. C, Vol. 111, No. 45, 2007
Figure 6. p-Polarized IRRAS spectra of DSA monolayers on the H2Obased subphases (H2O, Ca2+, Zn2+, and Cu2+) at a surface pressure of 20 mN/m at 22 °C: (a) angle of incidence, 30°; (b) angle of incidence, 60°; (c) p-Polarized IRRAS spectra of DSA monolayers on D2O subphase at the incidence angles 30° and 60°.
water interface, the Brewster angle can be estimated by calculating tan φ ) n2/n1, where n1 and n2 is the real parts of refractive indices of air and water at a given wavenumber (φ ) 54.5° at 2920 cm-1 for H2O and 51.1° for D2O), respectively. Beyond the Brewster angle, the bands become positive and their intensities decrease upon further increase of incidence angle.28 For a vibration with a transition moment perpendicular to the water surface (along the surface normal), the situation is reversed: for incident angles smaller than the Brewster angle, a positive band will be observed, whereas above the Brewster angle a negative band will be found.28,34,36,39 Figure 6a shows the p-polarized IRRAS spectra of the DSA monolayers at the air-water interface and ion-containing
Liu et al. subphases at the surface pressure 20 mN/m at an angle of incidence of 30°. Unfortunately, the spectral baselines are distorted in the region between 1800-1500 cm-1 because of the altered structure of the water adjacent to the headgroups of the film constituents, and the distortions are maximal for small angles of incidence so that the ν(CdN) band cannot be clearly identified at the incidence angle of 30°. It is known that for p-polarization that the band position has a strong dependence both on the incidence angle and on the orientation angle.34 At an angle of incidence of 60° (Figure 6b), a positive peak at 1616 cm-1 is tentatively assigned to the ν(CdN) vibration. Two dips in the vicinity of the ν(CdN) band are considered to result from the negative-oriented baseline distortion beyond the Brewster angle, in contrast with the positive-oriented one below the Brewster angle, overlapped with the positive-oriented ν(CdN) band. Seen from the monolayer on the D2O subphase (Figure 6c), the 1618 cm-1 band below and above the Brewster angle can be unambiguously ascribed to the ν(CdN) mode. At the air-water interface, the frequencies of the bands related to the salicylideneaniline and carboxylic groups are lower than those for the LB films. This is because in aqueous solutions these groups may form hydrogen bonds to the adjacent water molecules, which are known to reduce frequencies. Above the Brewster angle, a negative peak around 1434 cm-1 is due to the ν(CdC) vibration, and the other negative peak at 1201 cm-1 to the ν(dC-N) mode, which suggests that the long axes of the salicylideneaniline segments are oriented preferentially perpendicular to the water surface. On the ion-containing subphases, the νa(CH2) and νs(CH2) band intensities are reduced depending on the metal ion at the same surface pressures, which is consistent with the expansion features of the corresponding isotherms although the factor of molecular orientation should be considered. In the presence of Ca2+, the νa(COO) bands are present around 1551 and 1514 cm-1. At an angle of incidence of 60°, a sharp strong negative peak is clearly observed at 1451 cm-1 due to the νs(COO) mode, which indicates that the νs(COO) transition moment is nearly perpendicular to the water surface. This means that the salicylideneaniline segments are oriented almost vertical to the water surface. The ν(CdN) band appears at 1614 cm-1 in this case, which implies that the imino nitrogen atoms do not coordinate to Ca2+ ions. In the presence of Zn2+, the νa(COO) band appears around 1530 cm-1, and a negative peak at 1427 cm-1 above the Brewster angle is due to the νs(COO) vibration. Similarly, the ν(CdN) band at 1613 cm-1 also suggests that the imino nitrogen atoms do not coordinate to Zn2+ ions. In the presence of Cu2+, the νa(CH2) and νs(CH2) bands shift, respectively, up to 2923 and 2853 cm-1 and decrease markedly in intensity. Two νa(COO) bands appear at 1584 and 1523 cm-1, and two νs(COO) bands appear at 1423 and 1378 cm-1. In the stearic acid monolayer on Cu2+-containing subphase, the νa(COO) bands are located at 1585 and 1530 cm-1 and the νs(COO) band at 1408 cm-1.30 In contrast with the spectra in the cases of Ca2+ and Zn2+, the νs(COO) bands are negative and strong below the Brewster angle and become positive above the Brewster angle in the presence of Cu2+, which indicates that the whole Schiff base segments take a different orientation from those in the cases of Ca2+ and Zn2+. The ν(CdN) band is shifted to 1607 cm-1 and the ν(dC-O) band related to the phenolates to 1309 cm-1, which indicates that Cu2+ ions coordinate with the imino nitrogen and phenolate oxygen atoms. Conclusions The DSA molecules form a condensed monolayer on pure water, where the salicylideneaniline segments take a nonplanar
Novel Metal Coordinations conformation and their long axes are oriented almost perpendicular to the water surface. The CdN stretching bands are assigned for the monolayers on the H2O-based subphases. In the presence of Ca2+ and Zn2+ in the subphases, the monolayers are expanded to a certain extent. Ca2+ and Zn2+ ions only coordinate to the carboxylate oxygen atoms in the fashion of chelating bidentate coordination but not to the imino nitrogen atoms or phenolate oxygen atoms of the salicylideneamino moieties. The long axes of the Schiff base segments are preferentially oriented along the monolayer normals with the νs(COO) transition moments almost perpendicular to the water surface. The twist angle for the nonplanar conformations of the Schiff base segments is considered to increase in the case of Ca2+ and decrease in the case of Zn2+ in comparison with that on pure water. In the presence of Cu2+, the monolayer is expanded and the alkyl chains are disordered. Cu2+ ions coordinate not only to the carboxylate groups in the fashion of bridging bidentate coordination but also to the salicylideneamino moieties. The Schiff base segments are obviously tilted with respect to the monolayer normal. The IRRAS spectra of the monolayers at the air-water interface nearly provide the same information on molecular orientation and metal coordination as that from a combination of the FTIR transmission and RA spectra of the LB films. It is clear that the molecular orientation is kept during the film transfers from the air-water interface to the solid supports. Acknowledgment. The work was supported by the National Natural Science Foundation of China (Grant No. 20673051 and 20635020) and the Natural Science Foundation of Jiangsu Province (Grant No. BK2007519). References and Notes (1) Rousso, I.; Friedman, N.; Sheves, M.; Ottolenghi, M. Biochemistry 1995, 34, 12059. (2) Baasov, T.; Sheves, M. Biochemistry 1986, 25, 5249. (3) Lanyi, J. K. Biochim. Biophys. Acta 1993, 1183, 241. (4) Das, K.; Sarkar, N.; Ghosh, A. K.; Majumdar, D.; Nath, D. N.; Bhattacharyya, K. J. Phys. Chem. 1994, 98, 9126. (5) Fore´s, M.; Duran, M.; Sola`, M.; Orozco, M.; Luque, F. J. J. Phys. Chem. A 1999, 103, 4525. (6) Acun˜a, A. U.; Amat-Guerri, F.; Costela, A.; Douhal, A.; Figuera, J. M.; Florido, F.; Sastre, R. Chem. Phys. Lett. 1991, 187, 98. (7) Nishiya, T.; Yamauchi, S.; Hirota, N.; Baba, M.; Hanazaki, I. J. Phys. Chem. 1986, 90, 5730. (8) Hodjoudis, E.; Mavridis, I. M. Chem. Soc. ReV. 2004, 33, 579. (9) Sliwa, M.; Le´tard, S.; Malfant, I.; Nierlich, M.; Lacroix, P. G.; Asahi, T.; Masuhara, H.; Yu, P.; Nakatani, K. Chem. Mater. 2005, 17, 4727. (10) Garnovskii, A. D.; Nivorozhkin, A. L.; Minkin, V. I. Coord. Chem. ReV. 1993, 126, 1. (11) Roberts, G. G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (12) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991. (13) Nagel, J.; Oertel, U.; Friedel, P.; Komber, H.; Mo¨bius, D. Langmuir 1997, 13, 4693. (14) Chen, X.; Xue, Q.-B.; Yang, K.-Z.; Zhang, Q.-Z. Langmuir 1995, 11, 4082. (15) Sundari, S. S.; Dhathathreyan, A.; Kanthimathi, M.; Nair, B. U. Langmuir 1997, 13, 4923.
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