Langmuir 1996, 12, 5869-5874
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Investigation of 2-(Docosylamino)-5-nitropyridine Monolayers and Langmuir-Blodgett Films R. Ricceri,† L. Dei,† M. F. Ottaviani,† D. Grando,‡ and G. Gabrielli*,† Department of Chemistry, University of Florence, via G. Capponi, 9-50121-Firenze, Italy, and DEE, Politecnico di Bari, via Orabona, 4-70100-Bari, Italy Received April 29, 1996X The behavior of 2-(docosylamino)-5-nitropyridine (DCANP) spreading monolayers was investigated as a function of temperature, pH, and subphase composition. The interactions of DCANP monolayers with copper ions dissolved into the subphase at different concentrations were also studied. The investigations were carried out by measuring surface pressure and surface potential-area isotherms. Langmuir-Blodgett (LB) films transferred from pure water and copper aqueous subphases were also studied by Fourier transform infrared (FTIR), UV-vis, and electron paramagnetic resonance (EPR) spectroscopies, X-ray photoelectron spectroscopy (XPS), ellipsometry, and refractive index measurements. The interactions between copper ions and DCANP molecules were investigated by means of various techniques in the bulk phase, in monolayers, and in LB films. Clear evidence of interactions between DCANP and copper ions was provided only from EPR spectroscopy in the LB films, because of the very small molar ratio of bound/free DCANP in the LB films. The refractive index of Cu-doped LB films was shown to be increased by the presence of metal ions.
Introduction
Materials and Methods
The interest in the field of Langmuir-Blodgett (LB) films for second-order nonlinear optics is increasing rapidly.1 However, only few substances are known which can provide stable LB films of a thickness more than 20 layers. One of these substances is 2-(docosylamino)-5nitropyridine (DCANP), which can provide LB films of Y-type of good optical quality up to the thickness of more than 1 µm.2 This characteristic makes DCANP one of the most studied substances in the field of LB films for nonlinear optics wave guiding.3 In spite of this, characterization of DCANP monolayers has not been reported exhaustively till now in the literature. The aim of this work is to study the behavior of DCANP monolayers as a function of different parameters (temperature, pH, different concentrations of copper ions in the subphase), in order to control the characteristics of the LB films of this molecule for optical applications. In fact, the buildingup of LB films with different characteristics depends on the properties of the monolayers and thus on the suitable conditions (temperature, pH, and composition of the subphase, etc.) chosen for the transfer onto solid support. An investigation on the characteristics of LB films transferred from pure water and from copper aqueous subphases is also carried out by means of various techniques, such as UV-vis, EPR, and FTIR spectroscopies, in order to show the potential advantages in using a metal-doped LB film (higher refractive index and thus smaller cutoff thickness) in wave guiding applications (copper ions were chosen because of the high affinity of these cations for nitrogen-containing ligands).4
2-(Docosylamino)-5-nitropyridine (DCANP) was synthetized and purified according to the procedure reported in the literature.2 Stearic acid (purity >99%) was provided by Aldrich. Ethanol, KBr, Cu(ClO4)2, Cu(NO3)2, and CuCl2 (analytical grade) were purchased from Fluka. Ethanol and CdCl2‚2.5H2O (analytical purity) were provided by Carlo Erba. Chloroform (analytical grade, Fluka) was used as spreading solvent; the concentration of spreading solutions of DCANP was about 1 mg/L. NaOH and HCl (analytical purity) were provided by Fluka and used to prepare subphases at different pH values. Water was purified with a Millipore Milli-RO 6 and a Milli-Q water system (organex system), obtaining a specific resistance g18 MΩ‚cm. Surface pressure-area isotherms (π vs A) were recorded using a computer-controlled Lauda MGW Filmwaage Balance with discontinuos compressions to allow the monolayer to reach equilibrium after each variation of area (variation of area was 0.01-0.02 m2/mg, and intervals of 30 s were allowed between two successive compressions for all measurements; the accuracy was (0.01 m2/mg for areas and (0.05 mN/m for surface pressures). Surface potential as a function of molecular area (∆V vs A) was recorded using two radioactive 241Am electrodes according to the procedure reported in the literature.5 The accuracy of the measurements was (1 mV. Y-type Langmuir-Blodgett films of DCANP were prepared using a KSV3000 balance at a compression rate of 10 cm2/min (corresponding to 0.02 m2 mg-1 min-1). The accuracies in surface pressure measurements were the same reported for the Lauda instrument. LB films were deposited at π ) 19 mN/m (T ) 288 K) at a transfer rate of 10 mm/min both for the downstroke and the upstroke for all substrates. The subphases were pure water and 0.05 M CuCl2. Quartz slides (hydrophobized with trichlorooctadecylsilane) were used as substrates for samples for UV-vis absorption spectra (performed using a Perkin Lambda 5 spectrophotometer; accuracy in measuring absorbance (0.001) and for XPS measurements, which were performed with an ESCA 100 instrument of VSW. The binding energies for XPS spectra were referred to the methylmethylene components of the C 1s spectrum at 284.8 eV. LB monolayers of cadmium stearate were deposited at a transfer rate of 3 mm/min from a 5 × 10-5 M KHCO3 and 10-6 M CdCl2 subphase at π ) 40 mN/m in order to make hydrophobic the chromium slides used for ellipsometric measurements. Chromium slides and germanium crystals covered by a cadmium stearate monolayer were subsequently covered with DCANP
* To whom correspondence should be addressed. † University of Florence. ‡ DEE, Politecnico di Bari. X Abstract published in Advance ACS Abstracts, November 1, 1996. (1) Non linear optical properties of organic molecules and crystals; Chemla, D. S., Zyss, J., Eds.; Academic Press: New York, 1987; Vols. 1 and 2. (2) Decher, G.; Tieke, B.; Bosshard, C.; Gu¨nter, P. Ferroelectrics 1989, 91, 193. (3) Bosshard, C.; Ku¨pfer, M.; Flo¨rsheimer, M.; Gu¨nter, P.; Pasquier, C.; Zahir, S.; Seifert, M. Makromol. Chem., Macromol. Symp. 1991, 46, 27. Bosshard, C.; Ku¨pfer, M.; Gu¨nter, P.; Pasquier, C.; Zahir, S.; Seifert, M. Appl. Phys. Lett. 1990, 56, 1204.
S0743-7463(96)00419-2 CCC: $12.00
(4) Hathaway, B. J.; Billig, D. E. Coord. Chem. Rev. 1970, 5, 1. Hathaway, B. J.; Tomlinson, A. A. G. Coord. Chem. Rev. 1970, 5, 143. (5) Puggelli, M.; Gabrielli, G. Colloid Polym. Sci. 1985, 263, 879.
© 1996 American Chemical Society
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Figure 1. Surface pressure-area and surface potential-area isotherms of DCANP spreading monolayers at 278, 283, and 288 K on a pure water subphase.
Figure 2. Surface pressure-area and surface potential-area isotherms of DCANP spreading monolayers on aqueous subphases at pH ) 0.5, 2, 5.6, and 11 (T ) 288 K).
layers deposited according to the usual procedure previously described. Ellipsometric measurements were performed on LB films deposited on chromium slides using a Rudolph Research ellipsometer (accuracy in these measurements was about (10% on the average thickness). EPR spectra were obtained orienting the LB film with respect to the magnetic field by inserting the quartz plate in the EPR cavity. At least 50 layers (transferred on silanized quartz from a 0.05 M CuCl2 subphase) were necessary to obtain an adequate signal-to-noise ratio in the EPR spectra. The EPR spectra were recorded at room temperature (293 K) by means of a Bruker 200D spectrometer operating in the X-band, interfaced with Stelar software to a PC-IBM computer for data acquisition and handling. Magnetic parameters were measured by field calibration with the 2,2-bis(4-tert-octylphenyl)-1-picrylhydrazyl (dpph) radical (g ) 2.0036). Infrared spectra were obtained by a BIO-RAD FTS-40 FTIR spectrometer at a resolution of 2 cm-1. An ATR variable-angle ATR accessory Graseby Specac was used for FTIR-ATR spectra: the incident angle of the infrared beam was 45° from the surface normal: with this angle 14 reflections were perfomed for each interferogram. Germanium crystals were used for FTIRATR spectra: 1000 interferograms were collected for each measurement performed on twenty-layer LB films. For FTIR transmission spectra, 32 interferograms were collected for each measurement performed on KBr pellets and on 162 DCANP layers deposited on silanized quartz. Refractive index measurements were performed at the wavelength of 488 nm by dark m-lines spectroscopy (TE and TM polarization). A CW Ar-Kr Innova 70 laser and a Contek rotation stage were employed. One hundred sixty-two layers of pure DCANP and Cu-doped DCANP were transferred on a Pyrex substrate for these measurements. The sensitivity in measuring the refractive index was (0.0005, though the eperimental error was (0.002, because of film defects and irregularities. A coordinate system was introduced: Y is the dipping direction, Z is the direction perpendicular to Y and parallel to the substrate, X is the axis perpendicular to both Y and Z.
suggests that DCANP chromophores cannot lie face down on the water surface. The compressibility modulus shows the same values for all curves, indicating the existence of a liquid condensed region in the high-surface pressure region. ∆V vs A curves do not show appreciable differences as a function of temperature. The value of the vertical component of the molecule dipole moment, as calculated by the formula7
Results and Discussion DCANP Spreading Isotherms as a Function of Temperature. Figure 1 shows the π vs A and ∆V vs A isotherms of the monolayers of DCANP on a pure water subphase at various temperatures (278, 283, and 288 K). Spreading monolayers at higher temperatures than 288 K are not stable because of incipient three-dimensional nucleation.6 π vs A curves show that all the isotherms are very similar. Collapse pressure increases with decreasing temperature, and the shape of the collapse region seems to be influenced by the temperature. The limiting area per molecule of about 36 Å2 for all curves (6) Vollhardt, D. Adv. Colloid Interface Sci. 1993, 47, 1.
µvert ) µ cos θ ) A∆V/12π (A, area of the molecule in Å2/molecule corresponding to the value of the surface potential ∆V (in volts); θ, angle between the dipole of the molecule and the normal to the surface), is 0.5 D. Taking into account that the dipole moment of packed alkyl chains is about 0.4 D,8 surface potential values indicate that the aromatic ring of DCANP lies parallel to the water surface along the direction connecting the amino and nitro groups; in fact, pnitroaniline has a dipole moment of 5.35 D,9 aniline’s is around 1 D,9 nitrobenzene’s is 4.28 D,9 and pyridine’s is 2.15 D,9 so it is reasonable to assume that the dipole moment of the polar head of DCANP must be an order of magnitude higher than the value found from surface potential data, and that implies the dipole of the DCANP polar head to be nearly perpendicular to the normal to the surface. DCANP Spreading Isotherms as a Function of pH. Figure 2 shows the π vs A and ∆V vs A isotherms of DCANP monolayers spread on a water subphase at 288 K at various pH values (pH ) 0.5, 2, 5.6, and 11). The π vs A isotherms show little or no variation in a wide range of pH. Only at pH ) 0.5 is a change in the isotherm detected: the limiting area shifts to 39 Å2/molecule from the 36 Å2/molecule found at the other values of pH examined, and there is a decrease in the collapse pressure in comparison with those of the other pH cases examined. In any case, the compressibility modulus does not seem to change much with respect to those of the other curves (in the condensed region, at π ) 19 mN/m, Cs-1 is 130 mN/m instead of 140 mN/m for all the other curves examined). The ∆V vs A isotherm at pH ) 0.5 is different from all the others, showing much higher values at all the (7) Gaines, G. L. Insoluble monolayers at liquid-gas interfaces; Interscience: New York, 1966. (8) Taylor, D. M.; Oliveira, O. N., Jr.; Morgan, H. J. Colloid Interface Sci 1990, 139, 508. (9) Landort, H.; Bo¨rnstein, R. Numerical data and functional relantionships in science and technology; New series, group II; Springer Verlag: Berlin, Vol. 4 (1967) and Vol. 14 (1982).
2-(Docosylamino)-5-nitropyridine Monolayers
Langmuir, Vol. 12, No. 24, 1996 5871 Table 2. Molecular Areas and ∆V Values of DCANP Spreading Monolayers at π ) 19 mN/m and T ) 288 K as a Function of Subphase Composition
Figure 3. Surface pressure-area and surface potential-area isotherms of DCANP spreading monolayers on different subphases: 0.05 M CuCl2, 10-7 M CuCl2, 0.15 M NaCl, and pure water. Table 1. ∆V Values at π ) 0 and at Area/Molecule ) 65 Å2 for DCANP Spreading Monolayers as a Function of Subphase Composition subphase
∆V (mV)
water 10-7 M CuCl2 10-6 M CuCl2 10-5 M CuCl2 10-3 M CuCl2 0.05 M CuCl2 0.15 M NaCl
5 106 142 170 165 146 35
areas. These differences are due to protonation of the polar head of DCANP at very low pH values. The ∆V vs A isotherms at pH ) 2, 5.6, and 11 show all the same values, indicating (together with π vs A curves) that no change in monolayer structure takes place in a wide range of pH. DCANP Spreading Isotherms as a Function of Cu2+ Concentration. Figure 3 shows the π vs A and ∆V vs A isotherms of DCANP monolayers on 0.05 M CuCl2, 10-7 M CuCl2, 0.15 M NaCl, and pure water at 288 K. The π vs A isotherms present all the same behavior, even if in the presence of 0.15 M NaCl (same ionic strength of 0.05 M CuCl2) the isotherm is shifted toward larger areas (limiting area, 38 Å2/molecule) and in the precence of 0.05 M CuCl2 the opposite behavior is found (limiting area, 34 Å2/molecule, a slightly smaller value than that on pure water). On pure water and on 10-7 M CuCl2 the curves are almost identical. The π vs A isotherms in the presence of 10-3, 10-5, and 10-6 M CuCl2 as subphases (not shown) do not present differences with respect to the isotherm on pure water. The compressibility moduli for all curves show the same values. The analysis of ∆V vs A isotherms can provide more information about the interactions between copper ions and DCANP polar heads. Table 1 shows that in the expanded phase surface potential values are significantly larger in the presence of 10-7 M CuCl2 with respect to those on pure water and on 0.15 M NaCl. In the condensed phase (π ) 19 mN/m, see Table 2) ∆V values are significantly smaller as Cu2+ concentration increases. This trend is due only to the presence of copper ions, because the use of other salts with different anions but the same cation provides the same results. ∆V values can therefore allow us to suppose that interactions between DCANP and Cu2+ ions are present. These interactions do not affect the structure of the monolayers, as is shown by π vs A curves. Formation of Langmuir-Blodgett Films. DCANP LB films of Y-type were easily transferred from a pure
subphase
area/mol (Å2)
∆V (mV)
water 10-7 M CuCl2 10-6 M CuCl2 10-5 M CuCl2 10-3 M CuCl2 0.05 M CuCl2 0.15 M NaCl
32 32 32 32 33 30 34
627 640 611 539 510 536 630
water subphase, obtaining good transfer ratios (1.00 ( 0.05) up to 170 layers without showing any turbidity due to crystallization, in good accordance with the literature.2 The transfer from a 0.05 M CuCl2 subphase with the same compression and dipping conditions gave also good Y-type LB films with the same transfer ratios as the LB films transferred from a pure water subphase. Optical Absorption of LB Films. UV-vis absorption spectra of DCANP LB films (50 layers) transferred from the pure water subphase and from 0.05 M CuCl2 are shown in Figure 4a. The spectra are in good agreement with those reported in the literature.2 The spectrum of the LB film transferred from the copper chloride subphase is identical to the one for the film transferred from the pure water. Polarized UV-vis absorption spectra (not reported; they closely resemble those reported in the literature10 ) show that LB films of DCANP transferred from the pure water present the same anisotropy reported in previous works (the ratio of optical densities parallel and perpendicular to the dipping direction is 1.62,10). The DCANP LB films transferred from 0.05 M CuCl2 showed the same polarization-dependent effect as those transferred from pure water, indicating the same orientation of the molecules. The UV-vis spectrum in chloroform solution (see Figure 4b) of the powder obtained by evaporating a solution (an ethanol/chloroform 1:5 mixture) of copper chloride and DCANP (DCANP/CuCl2 in about a 1:100 molar ratio) and then washing away with water the excess copper chloride shows that a broad band appears at 768 nm (not present in the spectrum of a chloroform solution of pure DCANP), which cannot be due to the copper aquo complex but only to the DCANP-complexed Cu2+.4 Therefore, UV-vis spectroscopy shows that DCANP forms a complex with Cu2+ in bulk; however, this interaction between copper ions and DCANP in LB films was not detected by UV-vis spectroscopy in LB films. XPS Measurements. XPS measurements were performed in order to confirm the presence of copper ions in the LB films and to obtain information about the interactions between DCANP and Cu2+. XPS spectra performed on one transferred monolayer of DCANP from a 0.05 M copper chloride subphase revealed the presence of a Cu2+ 2p3/2 band (Figure 5), but the signal was too weak to make a quantitative analysis of the nitrogen/copper ion ratio and to deduce the entity of the possibly bound copper ions (the Cu2+ peak could be split in the single components due to the bound and unbound copper ions), meaning that the presence of these ions is very low. This explains why UV-vis spectroscopy did not detect the presence of copper ions in the LB films. Ellipsometric Measurements. Ellipsometric measurements were performed on different LB samples: (i) a twenty-layer Y-type LB film of DCANP deposited on the chromium slide beginning with the upstroke (polar heads in contact with the substrate); (ii) a twenty-layer Y-type (10) Decher, G.; Klinkhammer, F.; Peterson, I. R.; Steitz, R. Thin Solid Films 1989, 178, 445.
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a
b
Figure 6. EPR spectra of 50-layer LB films of DCANP transferred from a 0.05 M CuCl2 subphase at different orientations (0°, 45°, and 90°) of the film plate with respect to the magnetic field direction.
Figure 4. (a) UV-vis absorption spectra of 50-layer LB films of DCANP transferred from (i) a pure water subphase (dashed line) and (ii) a 0.05 M CuCl2 subphase (solid line). (b) UV-vis absorption spectra in chloroform solution of (i) pure DCANP (dashed line) and (ii) a mixture of pure DCANP and Cucomplexed DCANP (solid line). The inset shows the 650-850 nm region.
Figure 5. XPS spectrum for the LB film (one monolayer) of DCANP transferred from a 0.05 M CuCl2 subphase. The absorption peak of Cu2+ is shown.
LB film of DCANP, deposited beginning with the downstroke (hydrophobic chains in contact with the substrate, for this layer the rate of dipping was 2 mm/min); and (iii) a twenty-layer Y-type LB film of DCANP deposited on a chromium slide previously covered with a monolayer of cadmium stearate. In all cases we obtained an average thickness for a DCANP monolayer of 23 ( 2 Å, for the
films transferred both from a pure water subphase and from 0.05 M CuCl2. This value is in agreement with the literature2 and shows that the possible presence of copper ions in the LB films does not influence the orientation of the aliphatic chains of the multilayers. EPR Measurements. The EPR spectra recorded on a 50-layer LB film transferred from a 0.05 CuCl2 subphase clearly indicate the presence of Cu(II), as reported in Figure 6. This figure shows three EPR spectra at different orientations of the film plate with respect to the magnetic field direction. The EPR spectra contain several signals, and their complete interpretation is not straightforward. However, they clearly indicate that a large fraction of Cu(II) provides a signal that depends on the orientation of the film. This can be interpreted only in terms of Cu2+ complexation by DCANP molecules in the LB film.11 Work is in progress in order to interpret the spectra accurately and obtain information about the molecular orientation of the film. FTIR Measurements. Figure 7 shows the KBr pellet FTIR spectrum of the pure DCANP and of the powder obtained by evaporating a solution (an ethanol/chloroform 1:5 mixture) of DCANP/CuCl2 in about a 1:100 molar ratio and then washing away with water the excess copper chloride (see above). The spectrum of the bulk DCANP agrees with the literature,12 and the assignment of the bands is reported in Table 3. It is possible to notice two remarkable differences between these two spectra: first, the shift of the N-H stretching band at 3402 cm-1 toward lower frequencies (together with a considerable broadening); second, the shift of the N-H bending band at 1611 cm-1 toward higher frequencies (see shoulder at 1624 cm-1 in the spectrum of the powder). This suggests that the powder prepared in the way above described contains a certain amount of DCANP-Cu2+ adduct where the copper ions are bound to the NH-py group. In fact, the two abovementioned features of the spectrum agree with Cu2+ (11) Richards, P. M.; Salamon, M. B. Phys. Rev. B 1974, 9, 32. (12) Haerri, H. P.; Tang, Q.; Tieke, B.; Zahir, S. Thin Solid Films 1992, 210/211, 234.
2-(Docosylamino)-5-nitropyridine Monolayers
Figure 7. FTIR spectra in a KBr pellet of pure DCANP (solid line) and a mixture of pure DCANP and Cu-complexed DCANP (dashed line): top, 3500-2700 cm-1; bottom, 1650-1250 cm-1. Table 3. Assignments of FTIR Main Bands of DCANP wavenumber (cm-1)
assignment
3401 3179 and 3096 2955 2916 2850 1612 1532 1472 1332 717
N-H stretching C-H aromatic stretching CH3 antisymmetric stretching CH2 antisymmetric stretching CH2 symmetric stretching N-H deformation (bending) NO2 antisymmetric stretching CH2 scissoring NO2 symmetric stretching CH2 rocking
complexation by N-H groups.13 From the FTIR spectrum it is possible to deduce that the described powder contains both the complex and the pure DCANP ligand. In order to check if this effect occurs even in the LB films, we made FTIR spectra of DCANP transferred onto different substrates and from different subphases. It is interesting to note that the spectra of a twenty-layer LB film transferred onto germanium ATR crystals (untreated and treated with cadmium stearate) in the 4000-1000 cm-1 wavelength range (see Figure 8) closely match the one of pure DCANP in a KBr pellet. This means that the ATR technique allows us to obtain well defined and resolved spectra even without the MCT detector (we used the usual DTGS). The two transmission spectra of 162-layer LB films transferred on silanized quartz plates refer to two different subphases: pure water and aqueous 0.05 M CuCl2 (see (13) Nakamoto, K. Infrared spectra of inorganic and coordination compounds; J. Wiley & Sons: New York, 1962.
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Figure 8. FTIR-ATR spectra of DCANP: 20-layer LB film transferred from a pure water suphase on a germanium substrate (solid line); 20-layer LB film transferred from a pure water subphase on a germanium substrate made hydrophobic with a cadmium stearate monolayer (dashed line); top, 35002700 cm-1; bottom, 1650-1250 cm-1.
Figure 9. FTIR spectra of DCANP: 162-layer LB film transferred from a pure water subphase on a quartz substrate (solid line); 162-layer LB film transferred from a 0.05 M CuCl2 subphase on a quartz substrate (dashed line).
Figure 9). The two spectra are perfectly identical, and they exactly match the corresponding spectral region of the DCANP bulk spectrum. In particular, no shift and no broadening of the N-H stretching band at 3402 cm-1 are observed in passing from the water to the CuCl2 aqueous subphase. In the presence of Cu2+ complexation by the N-H group, we expect both the shift of the N-H stretching band toward lower frequencies and a broadening of the
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same band.13 In the absence of such a feature we can infer either no complexation in the LB films or a very low molar ratio of Cu-DCANP complex to pure DCANP in the LB films. The results found by means of EPR spectroscopy (see previous section) indicate rather a low molar ratio of complex to pure ligand than the absence of complexation, as EPR spectra clearly show the presence of a Cu-DCANP adduct. On the other hand, we recall that a 0.05 CuCl2 M bulk concentration implies a very low surface concentration of the copper ions, which leads to a low complex/ligand molar ratio in the LB transferred film. Work is in progress in order to obtain information about the molecular orientation in LB films by means of polarized light FTIR spectroscopy and thus show possible differences in orientation between pure DCANP and Cu-doped LB films. Refractive Index Measurements. Refractive index measurements were performed on the pure DCANP and on the Cu-doped DCANP LB films (162 layers transferred on a Pyrex substrate, as reported previously). For the Cu-doped DCANP LB films, the TE in-plane index components corresponding to the dipping direction and the normal one, that is nyy and nzz, were 1.668 and 1.628, while the TM indices were 1.609 and 1.605, respectively. For the pure DCANP LB film an index decrease (∆n) of 0.016 was found, the TE index components being nyy ) 1.652 and nzz ) 1.612 and so on. ∆n is well above the experimental error: this states that the index change is a consequence of Cu doping.
pH ) 0.5 are changes in the monolayer produced due to protonation of the polar heads. The presence of copper ions in the aqueous subphase does not induce changes in the π vs A curves even at concentrations of 0.05 M, while variations in the surface potential isotherms are present also with 10-7 M CuCl2. LB films of good optical quality have been obtained up to 170 layers. The films have been characterized by means of UV-vis, FTIR, and XPS spectroscopies and ellipsometry. The presence of copper in the films was shown by means of the XPS technique; the binding between copper and DCANP was evidenced by means of EPR spectroscopy. The effective binding between Cu2+ and DCANP could be exploited in order to build up LB films of DCANP with different characteristics with respect to the refractive index, which is increased by the presence of copper ions in the multilayer. It is likely that a control of the refractive index of DCANP LB films could be obtained by varying the concentration of copper ions in the subphase. This control could be of great importance in order to build up wave guides for nonlinear optics in which the cutoff thickness for propagation could be decreased by metal ion doping. This could mean that it would be possible to obtain LB films wave guides with less layers and thus less structural defects, i.e. more efficient wave guides with small light losses.14 Work is in progress in order to investigate if the binding of Cu2+ to DCANP modifies the nonlinear optical properties of the organic molecule, but it is very likely that a small content of copper ions cannot alter remarkably the nonlinear optical properties of DCANP LB films.
Conclusions
Acknowledgment. Financial support from M.U.R.S.T., C.N.R., and C.S.G.I. is gratefully acknowledged. R.R. wishes to thank Prof. A. Sabatini for useful discussions.
DCANP forms stable monolayers at the water-air interface at 288 K or lower temperatures. The properties of the monolayer are not affected by temperature. Changes in the pH of the subphase do not produce remarkable effects on monomolecular films, and only at
LA960419W (14) Pitt, C. W.; Walpita, L. M. Thin Solid Films 1980, 68, 101.
