Langmuir 1993,9,2185-2189
2185
Vibrational Spectra of Langmuir-Blodgett Monolayers of Magnesium and Palladium Tetra-tert-butylphthalocyanine M. I. Gobernado-Mitre and R. Aroca* Materials and Surface Science Group, Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario N9B 3P4,Canada
J. A. DeSaja Department of Condensed Matters Physics, University of Valladolid, 47011 Valladolid, Spain Received November 13, 1992. I n Final Form: May 11, 1993 Langmuir-Blodgett (LB) monolayers of magnesium and palladium tetra-tert-butylphthalocyanine complexes (MPct, M = Mg, Pd) have been prepared on various substrates. The formation of stable Langmuir layers and the surface pressure versus molecular area isotherms are reported for these nonamphiphilic molecules. A detailed spectroscopic characterization of MPct materials and LB layers is given using spontaneous Raman scattering (RS), resonance Raman scattering (RRS), surface-enhanced resonanceRaman scattering(SERB),transmissionand reflection-absorption FT-IR spectroscopy(RAIRS), and UV-visible spectroscopy. The orientation of MPct monomolecular films transferred to smooth gold films and ZnS substrate was probed by RAIRS and transmission FT-IR spectroscopy, respectively. The effect of the adsorption of N02/N204,S02, and NH3 gases on LB films of MPct materials was monitored in the visible spectrum.
Introduction The primary objectives of the present work were the fabrication and the spectroscopic characterization of Langmuir-Blodgett monolayers of magnesium and palladium tetra-tert-butylphthalocyaninecomplexes. The MPct complexes are nonamphiphilic molecules soluble in common organic solvents (unlike the insoluble unsubstituted Pc materials), a property that makes them suitable for LB work. The formation and the stability of monolayers fabricated with nonamphiphilic molecules (or "nonconventional materials"), is mainly governed by intermolecular interactions. For phthalocyanine derivatives, the u-r interactions determine the molecular packing in the solid state including LB film fabrication. LB film studies using nonamphiphilic moleculeshave been carried out with a large number of different materials.lV2 Surface pressure (T)versus molecular area (A) measurementa or T-A isotherms are taken by a direct measurement of the surface tension y (the Wilhelmy method) or by detecting the difference in y between the subphase and the subphase in the presence of a monolayer (the Langmuir balance method).g The limiting area is obtained in the F A isotherm by extrapolating the low compressibility region to zero surface pressure. Several research groups have reported LB fabrication of nonamphiphilic molecules and, in particular, mono-Pc derivatives. The present paper reports the successful fabrication of LB films of MgPct and PdPct. The molecular organization in the LB films of MgPct and PdPct was probed using transmission and reflection-absorption FT-IRspectroscopy. The effect of adsorption of NOz/N204, SO2 and NH3 gases on the LB films of MPct is also discussed.
* Author to whom correspondence should be directed.
(1)Ulman, A. Ultrathin Organic F i l m ; Academic Press, Inc.: New, York, 1991;p 111. (2) Hann, R. A. In Longmuir-Blodgett Film;Roberta, G., Ed.;New York, Plenum Press, New York, 1990; p 17. (3) Kumaki, J. Macromolecules 1988,21, 749. 0743-7463/93/2409-2185$04.00/0
Experimental Section MPct compleses were supplied and purified (usingthin-layer chromatography)by Dr. L. Tomilova at the Institute of Organic Intermediatesand Dyes in Moscow. Smooth Au for RAIRS and metal island films of Au and Cu were prepared using a Balzers evaporating system, and the thicknesswas monitored by an XTC Inficon quartz crystal oscillator. Smooth metal fiisof 110-nm Au were made by evaporatinggold in vacuum (P = 2 X lo-' Pa) at a rate of 5 Ws onto a Corning 7059 glass substrate maintained at 200 O C . The metal island f i s were prepared by metal evaporation at a rate of 1 A/sonto glass kept at 200 O C . Floating monolayers were spread onto a Lauda Langmuir fii balance equipped with an electronicallycontrolleddippingdevice, Lauda Filmlift FL-1.Monolayers of MPct were spread from 1.3 X lo-' Mtoluene (HPLCgrade from Aldrich)solutionsontoa deionized and filtered (Millipore Milli-Q Plus unit) subphase, with a resistivity of 18.2 MS2 cm. The temperature of the subphasewas controlled at 15 O C for both *-A measurementa and monolayer transfer to solid supports. For isotherm studies, the monolayer was compressed at a rate of 8.1 X lo-' nm2 molecule-' s-l. Monolayers of MgPct at a constant surfacepressure of 20 mN/m were transferred to various substrates (ZnS, Au island film,bare glass and smooth Au on glass) duringthe upstroke (%deposition). The PdPct monolayers were transferred at 25 mN/m. The substrate was lowered into the subphase prior to spreading the phthalocyaninesolution. The transfer ratio varied between 0.95 and 0.98 for each LB layer. Electronic absorption spectra were recorded on a ResponseUV-visible spectrophotometer interfaced to an IBM-PC computer. A SpectraPhysics Model 2020 Kr+ion laser was used to obtain Raman Shih which were measured with a Spes-1403double spectrometer. A Lese1 Ar+ laser was used to excite the nonresonance spectra measured with a THR-3000 spectrograph equipped with a liquid nitrogen cooled Spex CCD detector. Infrared spectra were measured on a BOMEM DA3 FTIR spectrometer. For data analysis, all files were imported to SpectraCalc software availablefrom GalacticIndustriesCorp. Results and Discussion Langmuir-Blodgett Films of MPctMaterials. The MPct complexes are nonamphiphilic surfactante, Le., hydrophobic molecules without hydrophilic groups that 0 1993 American Chemical Society
2186 Langmuir, Vol. 9, No. 8,1993
could participate in dipoledipole interaction with water. However, it has been pointed out that the central metal atom could interact with the water subphase. The P A isotherm obtained for these materials in a Langmuir balance (involving the direct measurement of the horizontal force on the float separating the film from the clean water surface) could be different from those obtained with a Wilhelmy plate (direct measurement of surface ten~ion).~ The experimental analysis of the thermodynamic mechanism of monolayer spreading and film transfer has been attempted for amphiphilic m o l e c u l e ~ . ~The ? ~ *theoretical ~ basis for monolayer spreading and film transfer of nonamphiphilic molecules is, however, not well-known. There exists an expanding data base of T-A isotherms for nonamphiphilic molecules. In particular, the data for monophthalocyanine derivatives are collected in Table I. The tetra-tert-butyl derivatives placed at the beginning of the table showed a range of values from 0.53 to 1.20 nm2 for the limiting area for these compounds. For instance, the T-A isotherm obtained by a Langmuir balance gave a limiting area of 0.53 nm2/molecule8 for CuPct. Measurements of limiting areas using the Wilhelmy plate method produce values of area/molecule from 0.96 and 0.87 nm2/molecule7to 0.60 nm2/molecules for CuPct and 0.42 nm2/molecule for (cumylphenoxy)~cCu,14 as can be seen in Table I. The data clearly suggest that the limiting area is affected by the quality of the floating monolayer (assuming a correct calibration of T and A scales) and by the fact that, for nonamphiphilic molecules, the partial formation of a multilayer on the subphase is probable. It should also be pointed out that for alkyl Pc derivatives, the limiting area values do not show a clear dependence on the size of the alkyl substituent. In fact, all reported values are scattered within the limits given for the tertbutyl Pc molecules, indicating that the limiting area is mainly determined by the Pc ring orientation on the water subphase. In summary, the observed differences seem to be due to factors such as solvent, compression rate, and temperature of the subphase, that would affect the isotherm and the limiting areas. The question of usefulness of the isotherm data has prompted a comprehensive study of these and other factors affecting the quality of the monolayer of t-BuPc molecules and is presently under way in our laboratories using both the Langmuir balance and a Wilhelmy plate based instrument. In the present study, reproducible isotherms were obtained (using a Langmuir balance) for both materials. We report a limiting area of 0.76 nm2/molecule for both MgPct and PdPct as
Gobernado-Mitre et al. Table I. Limiting Areas from r A Isotherms of Mono-Pc Derivatives. ~~~~~
molecule lim. area (nmz), solvent (tert-buty1)QcCu 0.96 (W), xylene xylene (tert-buty1)QcCu 0.60 (W), xylene (tert-buty1)QcCu 0.87 (W), (tert-butyl)&Cu 0.53 (L), toluene (tert-buty1)QcCo 0.85 (L),toluene (tert-buty1)QcNi 0.45 (L),toluene (tert-butyl)&Zn 1.20 (L),xylene (tert-butyl)&Zn 0.92 (L),CHCla/xylene (tert-buty1)QcZn 0.5 (L),toluene (tert-butyl)&VO 0.5 (L),toluene (tert-buty1)QcMn 0.86 (W), xylene/DMF (tert-butyl)QcHz ( 0 . 2 ~ DMF (tert-butyl)QcHz 0.70 (W), toluene (2,4-di-tert-amylphenoxy)QcCu 0.67 (W), chloroform (cumy1phenoxy)QcHz (0.38) (W), chloroform chloroform (cumy1phenoxy)QcHz (0.45) (W), chloroform (cumy1phenoxy)QcCu (0.42) (W), chloroform (cumylpheonxy)&Pd (0.57) (W), (phenoxy)QcHz (0.32) (W), chloroform chloroform (octadecoxy)QcHz (0.38) (W), (neopentoxy)QcHn (0.75) (W), chloroform (propoxy)ePcHz o.air0.711~ TCE TCE (propoxy)ePcCu 1.05 (W), (butoxy)ePcHz 0.71[0.79] (W), TCE TCE (butoxy)&'cCu 0.97 (W), (penty1oxy)ePcHz 0.85[0.86] (W), TCE (penty1oxy)ePcCu 1.20 (W), TCE (hexy1oxy)scHz 0.69[0.93] (W), TCE xylene/TTH (n-hexyl)*cH2 0.35[0.861 (W), xylene/TTH (n-hexyl)&Cu 0.48 (W), xylene/'M'H (n-heptyl)&Hz 0.73[0.93] (W), (n-heptyl)&'cCu 0.84 (W), xylene/TTH (n-octyl)acH2 0.90 (W), xylene/TTH
ref 5 6 7
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8 5 7 9 9 7 (w), 10 11 12 13 14 14 14 13 13 13 (w), 15 15 15 15 15 15 15 16 16 16 16 16 a (L)= Langmuir balance; (W) = Wilhelmy plate. TCE = l,l,ltrichloroethane;TTH = tetrahydrofuran; DMF = dimethylformamide. Parentheses indicate data calculated from the published isotherm. Bracketa indicate data estimated from CPK s p a c e - f i g models, assuming that the molecules are arranged with the plane of the ring system perpendicularto the water surface.
shown in Figure 1. The isotherm for PdPct contained a second phase transition with a low compressibility region giving a limiting area of about 1.00 nm2. The isotherms were repeated for a new monolayer of MgPct and PdPct. The features and the limiting areas obtained from the isotherms were reproducible within 0.05 nm2/molecule. The latter results were obtained a t a subphase temperature of 15 "C. It has become evident in our laboratory that the temperature of the subphase plays an important role in the formation of good quality floating monolayers of nonamphiphilic Pc molecules. The isotherm for PdPct (4) Eguea, S.;Gemma, N.; Azuma, M. J. Phys. Chem. 1990,94,2512. showed hysteresis during the compression and expansion (5) Hann,R. A.; Gupta, S.K.; Fryer, J. R.; Eyres, B. L. Thin Solid F i l m 1986,134, 35. cycles when carried to high surface pressure (>40 mN/m). (6) Brynda, E.;Koropecky, I.; Kalvoda, L.; Nespurek, S. Thin Solid However, the monolayer exhibited negligible hysteresis F i l m 1991,199,375. for measurements carried out up to the beginning of the (7) Robe&, G.G.;Petty, M. C.; Baker, S.; Fowler, M. T.;Thomas,N. J. Thin Solid Film 1985,123, 113. solid state transition (15 mN/m). Therefore, the isotherm (8) Souto,J.; DeSaja,J. A.; Gobernado-Mitre, M. I.; Rodriguez,M. L.; measurements suggest that in the condensed phase the Aroca R. Sene. Actuators B, in preas. molecules are closely packed and this packing is irrevers(9) Aroca, R.; Battisti, D. In Recent Deuelopments in Molecular Spectroscopy;Jordanov, B., Kirov, N., Simova, P., E&.; World Scienible. tific: Singapore, 1989; p 213. The electronic absorption spectra for LB films of MgPct (10) Baker, S.;Petty, M. C.; Roberts, G. G.; Twigg, M. V. Thin Solid F i l m 1983,99,53. and PdPct are shown in Figure 2. For MgPct,the electronic (11) Kovace, G. J.; Vmcett, P. 5.;Sharp, J. H. Con. J. Phys. 1986,63, spectrum of the LB film was similar to the toluene solution 346. spectrum with a typical solid-state &-band broadening. (12) Lu,A. D.;Pang, X. M.; Li, Y. J.; Jiang, D. P.; Hua, Y. L.; Chen, W. Q.; Zhou, E.L. Thin Solid F i l m 1991,196,323. The aging of LB films of MgPct on glass, followed for a (13) Barger, W.R.; Snow,A. W.; Wohltjen,H.; Jarvis, N. L. Thin Solid period of 6 months, showed no signs of degradation Films 1986, 133, 197. according to the electronic spectrum. However, it is well1984,106,4706. (14)Snow, A. W.;Jarvis, N. L. J. Am. Chem. SOC. (15)Cook,M.J.;Dunn,A.J.;Daniel,M.F.;Hart,R.C.O.;Richardson,known that the solutions of MgPct exposed to visible light R. M.;Roser, S. J. Thin Solid Film 1988, 159,395. changed color from blue-green to yellow. The solution (16) McKeown, N. B.; Cook, M. J.; Thomson, A. J.; Harrison, K. J.; absorption spectrum of the PdPctwas noticeably different Daniel, M. F.;Richardson,R. M.; Roser, S.J. Thin Solid F i l m 1988,159, 489. from that of the LB film, showing that strong intermo-
Monolayers of MPc'
Langmuir, Vol. 9, No.8,1993 2187
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lecular interactions in the solid monolayer have altered considerably the electronic energies of the Pc complex. Vibrational Characterization of MPct Complexes. FTIR Spectra. The vibrational assignment of the observed infrared (IR) and Raman frequenciesk a classical problem in vibrational spectroscopy and is required for further studies of molecular association, molecular interactions, and interfacial problems. The Pc macrocycle in these molecules has a 4-fold main symmetry axis. The 4-tert-butyl substitutes on the benzene ring may, however, produce different isomers and lower the symmetry. It should be pointed out that IR, RS,and RRS spectra of MgPctandPdPctwereverydifferentinsofarasthe relative intensity of vibrational frequencieswere concemed. The observed differences should be mainly related to the effect of the central metal atom on the optical parameters of the macrocyclic ligand. The transmission infrared spectrum and the off-resonance Raman spectrum of MgPct and PdPct are shown in Figures 3 and 4, respectively. The IR frequencies of PdPc and MgPc have been reported by Kobayashi et ale1' and Sidorov et aZ.,18 respectively. A direct comparison of the IR and RS spectra of the MPc molecule with MPct molecule allowed a positive identification of the tert-butyl frequencies. Similarly, strong bands in the vibrational spectra of MPc molecules such ~
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Figure 4. Infrared transmission and Raman spectra of PdPct in a KBr pellet. as the C-H wagging (Pc) at ca. 728 cm-l, and the C-H bending (Pc) ca. 1119 cm-l, were not seen in the spectra of the tert-butyl molecules.1° The C-H stretching vibrations of the tert-butyl moieties were clearly observed in the IR. A partial assignment of the vibrational fundamentals is presented in Table 11. Transmission FTIR spectra of a 60 LB monolayer film of PdPct and of 50 LB of MgPct deposited onto ZnS were also recorded. In transmission FTIR the electric field component of the incident light was parallel to the f i i s surface. A drastic
Gobernado-Mitre et al.
2188 Langmuir, Vol. 9,No. 8, 1993 Table 11. Observed Infrared Frequencies of KBr Pellets in cm-1 interpretation PdPct MgPct tert-butyl def 517 m 524 w tert-butyl def 561 w benzene def 600 w 005 w tert-butyl def 663 m 673 m 668m Pc breathing 690m 692 m Pc ring 748 8 755 v8 C-H wag(tb) 767 m 763 w 790 vw 782 vw C-H wag(tb) 827 s 829 m 862 vw 859 vw C-H wag(tb) 892 w 896 W benzene ring 913 w 921 m benzene ring 943 m 955 vw C-H bend(tb) 1022 vw 1023 vw C-H bend(tb) 1054 w 1046 m C-H bend(Pc) 1091 m 1082 s 1113 w 1124 m C-H bend(tb) 1143 w C-H bend(tb) 1150w 1149 w C-H bend(tb) 1192 m 1188 w C-H bend(tb) 1202 w 1199 w C-H bend(tb) 1258 m 1256 m C-H bend(tb) 1281 m 1281 w pyrrole str 1349 m 1329 m C-H bend(tb) 1369 s 1362 m isoindole str 1385 w isoindole str 1393 m 1392 w isoindole str 1463 m 1487 m isoindole str 1524va 1521 m benzene str 1573 m 1569 vw benzene str 1601 m 1612 m 1617 w C-H str. (tb) 2867 m 2856 vw C-H str. (tb) 2903 m 2903 vw C-H str. (tb) 2956 s 2956 w
increase in the relative intensity in the in-plane vibrations would be expected for a flat-on organization of the Pc macrocycle. In fact, very little change was observed in comparison with the spectra of the material dispersed in a KBr pellet (random distribution of microcrystals). Reflection-absorption infrared spectra (RAIRS) of ten LB monolayers of MgPct and PdPct transferred to a smooth Au film were acquired. In the RAIRS spectrum the relative intensity of the molecular vibrations with dynamic dipole component perpendicular to the metal surface would be favored.lg The patterns of relative intensities of the transmission and RAIRS spectra were very similar. The flat-on organization can therefore be rejected for both the LB films of MgPct and PdPct transferred to ZnS and Au substrates. The infrared data seem to suggest that the tilted organization of the Pc rings given by the limiting area of the isotherms was preserved during transfer to the solid substrate. Raman and Surface-EnhancedRaman Spectra of MgPct and PdPct. Both dyes absorb in the visible region of the spectrum and the 647.1-nmlaser line is in resonance with the electronic absorption of both molecules. The Ar+ ion laser line at 514.5 nm is off-resonance, and correspondingly, the absolute intensity of the Raman scattering is very weak. In fact, good quality spectra as those given in Figures 3 and 4 were only obtained using a spectrographequipped with a CCD detector. Resonance Raman scattering of MgPct at 647.1 nm was easily detectable with Raman bands arising from a weak fluo(19)Hayden, B. E. In Vibrational Spectroscopy of MOkCUk8 on Surfaces,Methods of Surface Characterization;Yatea, J. T., Madey, T. E., Eds.;Plenum Preaa: New York, 1987;Vol. 1,p 267.
687
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Figure 5. Surface-enhanced resonant Raman spectra of MgPct and PdPct on a 4-nm Au islands film measured at room temperature.
rescence background. The RRS from PdPct was overpowered by strong fluorescence in the Stokes region. SERRS of MgPct and PdPct on Au island filmspermitted observation of the enhanced RRS, while the fluorescence in the Stokes region was being quenched by the metal particles as illustrated in Figure 5. It should be pointed out that unlike the IR spectra, the RS and RRS (SERRS) spectra were dominated by the vibrational frequencies of the parent Pc molecule. This is a consequence of the fact that the Raman cross section of molecular vibrations associated with a conjugated system are much larger than those of saturated groups. Reported frequencies in the resonanceRaman spectra of PdPc and MgPPZl were used for the vibrational assignment of the tert-butyl analogs. The most prominent features in the RRS (SERRS)spectra (asin most Pc molecules) were the macrocycle vibrations at 689 and 687 cm-l for MgPct and PdPct, respectively. The observation is in agreement with a soft ?r-system in the Pc ring, giving rise to a large change of the polarizability with the normal modes of the macrocycle. The RRS effect in the MgPct also enhances the pyrrole stretching vibrations a t 1330 and 1507 cm-l. The RS spectrum (offresonance) of MgPct presents a pattern of relative intensities that is brought about by the appearance of vibrationalmodes with a large contribution from the outer section of the isoindole moieties. Therefore, C=C benzene stretching frequenciesat 1586and 1612cm-l, and isoindole frequencies at 1393 and 1439 cm-l were observed as seen in Figure 3. The external tert-butyl substituents are only seen with very weak relative intensity. The RRS or SERRS spectra of PdPct and MgPct were different due to the nature of the metal-ligand interaction, and the fact that the 647.1-nmlaser line is in full resonance with PdPct but is only on the edge of the MgPct &-band. The same SERF@ spectrum as shown in Figure 5 was repeatedly obtained for one LB monolayer at 298 and 148 K on a Au island film and at room temperature on a Cu island film of 410,and 15 nm thickness. The bandwidth of each Raman band measured at 298 K and at 148 K was the same. The RS spectrum of PdPct followed the normal pattern found in Pc rings, where the pyrrole stretching at 1324 and 1523cm-1, benzene stretch at 1613cm-l, and isoindole stretches at 1395 and 1452 cm-1 were observed. The RS (20)Huang,T.H.;Chen,W.H.;Rieckhoff,K. E.; Voigt, E.M.J.Chem.
Phys. 1984,80,4061.
(21)Jennings, C.; &oca, R.; Hor,A. M.; Loutfy, R. 0. J. Raman Spectrosc. 1984,15, 34.
Monolayers of MPct Table 111. Observed Raman Frequencies in cm-1 MgPct PdPct RRS SERRS RS SERRS RS interpretation 528vw 526w 522w 522vw tert-butyl def 549vw 532w 536vw 539w tert-butyldef 552vw 556w ringdef 600w 600m 600m 609m 611s benzenedef 620w 622w 622m tert-butyl wag 631 w 632w 633w 637m tert-butylwag 689vs 689va 689s 687w 687s Pcbreathing 717vw 716w 732 w 749s 749s 747m 755m 757w Pcring 784vw 788w 824m 827m 829m 84Ovw Pcring 843vw 844sh 863 vw 861 vw 882 vw 924vw 920w 959m 955m 956w 964vw 962w benzenering 1028vw 1021m 1030w 1097w 1098vw 1099sh C-H bend(tb) 1138w 1137w 1136m 1134vw 1143w pyrrolering 1173 m 1175 m 1199 m 1182vw 1178 w C-H bend(tb) 1217m 1217m 1224vw 1220 w C-H bend(Pc) 1288w 1286m 1287w 1284 w C-H bend(Pc) 1330s 1330s 1330m 1336vw 1324m pyrrolestr 1394w 1394m 1393vs 1395 w isoindole str 1425w 1425m isoindole str 1437w 1441sh 1439m 1452 m isoindole str 1507s 1507s 1507vs 1525w 1523vs pyrrolestr 1586 m benzene str 1612 s 1613 w benzene str
spectrum of PdPct is given in Figure 4. Observed frequencies and partial assignment are given in Table 111. Gas Adsorption on LB Films. LB films of MgPct and PdPct on glass were placed in a glass cell of 0.4 dm3volume
Langmuir, Vol. 9,No. 8,1993 2189
and treated with S02, NH3, and NO2 gas. A rotary pump was used to pump the chamber to 300 Pa and pure gas was admitted until 575 Pa. The samples were exposed to gas for 1 min and then let in open air for spectroscopic measurements. The effect of the gas adsorption was monitored in the visible spectrum. The Q absorptionband observed in the spectrum of an LB film of MgPct, shown in Figure 2, was very sensitive to the NO2 adsorption. The adsorption of the electron acceptor gas molecule leads to a complete disappearance of the red band. The gas desorption is very slow and the film does not recover its spectral properties even after 3 days. However, the &-band observed in the visible spectrum of an LB film of MgPd exposed to NH3 and SO2 was not affected by the presence of these gases. Similarly, intensity of the &-band in the electronic spectrum of a PdPct LB film was lost due to NO2 adsorption. Partial recovery of the PdPct film was evident after being in air for 2 h. The PdPct LB film was not affected by exposure to ammonia or S02.
Conclusion The fundamental vibrational frequenciesof MgPct and PdPct were recorded and assigned. Langmuir-Blodgett films of MgPct and PdPct materials have been prepared. The formation of a stable floating layer on the water surface,with a tilted molecular organization,was supported by the P A isotherm data. According to transmission and RAIRS data, the same molecular organization seems to be preserved after the transfer to solid substrates. The shortrange quality of the film would have to be studied using imaging techniques with molecular resolution.
