Langmuir 1992,8, 257-261
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Fourier Transform Infrared Studies of Molecular Ordering and Interactions in Langmuir-Blodgett Films Containing Nitrostilbene and Stearic Acid Y. P. Song,tJ M. C. Petty,t J. Yarwood,*pt W. J. Feast,? J. Tsibouklis,tJ and S. MukherjeetJ Department of Chemistry and Molecular Electronic Research Group, School of Engineering and Applied Science, University of Durham, South Road, Durham, DHl 3LE, U.K. Received October 4,1990. In Final Form: August 2, 1991
Ultrathin Langmuir-Blodgett multilayer films containing a 1:l mixture of an amidonitrostilbene dye and stearic acid have been characterized by Fourier transform infrared spectroscopy. Linear dichroism measurements on semiconductor and metal surfacesshowthat the molecules are packed (withthe exception of the first monolayer) in well-orderedand strongly hydrogen-bonded arrays. We have developed a new method of calculating the average angle of a given transition dipole with respect to the surface normal, These data show that the alkyl chains are orientated at about 75 f 4 O to the substrate plane. Interactions between the two types of molecule appear to be of minor importance. The molecule-surface interactions, leading to disordering of the first monolayer, may easily be monitored through shifts of the v(NOz) modes of the dye head group. The organic matrix "engineered" in this way is an ideal starting point for the construction of novel nonlinear optical device structures.
Introduction As part of a program of research aimed at the synthesis and characterization of novel multilayer structures with nonlinear optical properties, we have recently published1 a report of the interactions between nitrostilbene (4HANS) (Figure la) and hemicyanine (HC) dyes. In that paper we were able to correlate the enhancement of second harmonic generation (SHG) coefficient (over and above the sum of SHG coefficients of the two components) with the degree of alignment and partial separation of the hemicyanine molecules. This ties in very nicely with what is expected from the optical spectra of hemicyanine dyes.2 The separation was envisaged to occur via a degree of interdigitation (see Figure 8 of ref 1)but there was a question as to whether "free" amide v(C=O) bands would be present a t >1700 cm-l (Figures 2, 3, and 6 of ref 1). After publication it was discovered3that the 4HANS samples were composed of -50% stearic acid (SA). This immediately explains the bands at 1705cm-l (cyclicacid dimers) and at 1750 cm-' (free acid) in benzene solution^^^^ and also explains the multiplicity of bands6 in the 17201740-cm-l region for 4HANS and 4HANS/HC LangmuirBlodgett (LB) multilayers which are obviously caused by stearic acid molecules in differing environments. This does not alter the principle conclusions of ref 1but makes the spectra much more easily understood. We have also recently discovered that it is very difficult to produce welldefined LB multilayers of pure 4HANS. Therefore, in order to understand other nonlinear optical materials of Chemistry. t School of Engineering and Applied Science. (1)Ancelin, H.; Briody, G.; Yarwood, J.; Lloyd, J. P.; Petty, M. C.; Ahmad, M. M.; Feast, W. J. Langmuir 1990,6, 172-177. ( 2 ) Schildkraut, J. S.; Penner, T. L.; Willand, C. S.; Ullman, A. Opt. Lett, 1988, 13, 134. (3) Tsibouklis, J.; Mukherjee, S.; Cresswell, J.; Peffy, M. C.; Feast, W. J. J. Mol. Electron. 1990, 6, 221-223. (4) Tsibouklis, J.; Cresswell,J.; Kalita, N.; Pearson, C.; Maddaford, P. J.; Ancelin, H.; Yarwood, J.; Goodwin, M. J.; Carr, N.; Feast, W. J.; Petty, M. C. J. Phys. D. 1990,22, 1608-1612. ( 5 ) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 3rd Ed.; Chapman and Hall: London, 1986; p 239. (6)Davies, G. H.; Yarwood, J. Spectrochim. Acta 1987, 43A, 1619. (7) Petty, M. C.; Barlow, W. In Langmuir-Blodgett Films; Roberts, G. G., Ed.; Plenum Press: London, 1990. + Department
0743-7463l9212408-0257$03.00/0
i i h
b
(4
b (b)
(C)
Figure 1. Chemical structuresof materials used in this study:
(a)the stilbenedye 4HANS,(b)hydrogen-bondeddye molecules, (c) cyclic stearic acid dimer.
containing nitrostilbene (and, by necessity, stearic acid), we clearly need to reassess the data obtained. This is the principal objective of the current paper. The following paper in this issue reports our structural characterization of multilayers of the nitrostilbene/stearic acid mixture in alternate LB films with a diarylalkyne derivative (JTl1)one of a series of newly synthesized materials4 for potential nonlinear optical devices.
Experimental Section Cast films of the 4HANS were made by depositing a small amount of the solution (1mg/mL) in chloroform onto the ZnSe attenuated total reflection (ATR) crystal surface and allowing the solvent to evaporate. LB film samples were prepared on standard (ZnSe) or micro (silicon) ATR crystals or on silvered glass slides by Y-type LB deposition, at a dipping pressure of 30 mN m-l and a dipping speed of 1.0 cm min-l. The micro ATR crystals were refluxed in propan-2-01to produce a hydrophilic surface (which picks up the floating monolayer on the first 'upstroke"). Both the ZnSe ATR cystal and the silvered slides were cleaned with solvent but found to pick up on the first 'downstroke". The surfaces were therefore hydrophobic to the dye material. All spectra were collected using an unpurged Mattson Sirius 100 Fourier transform (FT) infrared spectrometer operating at 0 1992 American Chemical Society
Song et al.
258 Langmuir, Vol. 8, No. 1, 1992
Table I. Summary of Band Position (cm-I) and AssignmentsKfor the four HANS/SA Cast Films ~maJ Au1,2aIcm-1 assignments 719 13.5 r(CHd rocking mode 849 13.5 out-of-plane aromatic CH deformation 968 19.3 out-of-plane trans ethylenic CH deformation 1346 17.4 vdNOz) 1406 7.7 v(C-N) amide I11 1470 11.6 NCHd 1520 28.9 v,(NOz), amide I1 appears as a shoulder 1589 17.4 aromatic v(C-C) 1663 15.4 v(C=O) amide I (influenced by hydrogen bonding) 2849 9.6 dCHd 2918 15.4 vw(CH2) 2955 15.4 vw(CH3) 3179 36.7 v(N-H) cis 3295 79.1 v(N-H) trans The full width at half-height. ~~
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I
I
3400
3200
3000
2800
2400
2600
Wavenumber 0.5
0.4 0.10
0.3
0.08 0.2
0.06 0.1
0.04
0.0
0.02
I800
14b0
16bO
12b0
iOb0
BdO
Wavenumber Figure 2. FTIR cast film spectrum of the pure nitrostilbene dye on ZnSe: (A) the v(CH) region, 3500-2400 em-'; (B) the finger print region, 1800-700 cm-'.
H \
H /
/N-c \\
0 ___
(a) trans
3298
cm-I
0 ---
\
4
/N-c
\
(b) cis
3181
cm-I
Figure 3. Trans and cis configurations of a hydrogen-bonded amide group with their corresponding v(NH) stretching frequencies (em-'). 4 em-' resolution with a liquid nitrogen cooled, broad-band MCT detector. Data collection times were about 20 min.
Results and Discussion Figure 2 shows the FTIR spectrum of a cast film of the pure nitrostilbene (4HANS). There is a strong band at 3295 cm-l (not observed previously) which may be assigned5 to the secondary amide v(NH) band of 4HANS hydrogen-bonded in the trans configuration (Figure 3a). The much weaker band at 3179 cm-' represents a small (approximately 1 in 25) proportion of the cis configuration (Figure 3b). There is also a small shoulder at 3383 cm-1, which could be attributed to a "non-hydrogenbonded" v(N-H) band. Among the peaks in the region between 3026 and 3105 cm-l, most of which are due to the
0.00 I
1800
1700
1600
1500
1400
1300
Wavenumber Figure 4. FTIR-ATR spectra of (a) one, (b) three, and (c) five LB layers of a 1:l mixture of 4HANSISA on a hydrophilic silicon substrate.
aromatic C-H stretching modes, the 3026-cm-l band may be due to the C-H stretch of the ethylene group (Figure la). Most of the rest of the spectrum is what would be expected for this nitrostilbene molecule. Assignment of the principal bands is given in Table I. The amide group is characterized51aby three bands: amide I at 1640cm-l, amide I1 at 1550 cm-', and amide I11 at 1400 cm-l. All three bands are observed (Table I), although the amide I band is to higher wavenumbers than normal because of the electrophilic substituent (the phenyl group) at the nitrogen atom5 and the amide I1 band is partly obscured by the strong va(N02) band at 1522 cm-l (whose width reflects the multiple origins of this absorption). Table I provides a useful basis with which to assign the spectra of LB films containing 4HANS. Figure 4 shows a comparison of the spectra of LB multilayers of a 1:l molar mixture of 4HANS/SA dipped onto a silicon ATR crystal. Different numbers of monolayers were laid down in order to examine differences in molecular ordering for different multilayer "structures". These spectra confirm the previous findings (a) that the "disordering'' of molecules within the first monolayer (spectrum 4a) is apparent from the extreme width and mul-
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(8) Swalen, J. D.; Rabolt, J. F. In Fourier Transform Infrared Spectroscopy; Ferraro, J. R., Basile, L. J., Eds.; Academic Press: New
York, 1985; Vol. 4, Chapter 7 and references therein.
LB Films Containing Nitrostilbene and Stearic Acid
Langmuir, Vol. 8, No. 1, 1992 259
I
I
I
Substrate
Figure 5. Schematic representation of the mixed multilayer system whose spectrum is shown in Figure 4.
tiplicity of v(C=O) bands in the 1650-1750-cm-’ region and (b) that both cyclic dimers (1701cm-l) and free stearic acid (1744 cm-l) are present in the first layer presumably as a result of “isolation” in the disordered 1st layer mixture, due to multiple interaction types at the Si/Si02/interface.lp6 As the number of layers increases the free v(C=O) band decreases rapidly in relative intensity (Figure 4b,c). This shows that, for the multilayers of the 1:l mixture, both acid and stilbene dye molecules are strongly hydrogenbonded. The continued presence of a strong “cycIicdimer” band at 1701 cm-l (Figure 4c) shows that the molecules are not isolated from their own type, and this is supported by the observation of only one nitro group environment in the multilayer spectra (Figure 4b,c). Since neither free v ( C 4 ) nor sideways dimer v(C=O) appear to be present, the molecular arrangement is expected to be as shown in Figure 5 with hydrogen-bonded 4HANS molecules as shown in Figure l b and cyclic stearic acid dimers as shown in Figure IC.Direct interactions between SA and 4HANS in the multilayers seems to be minimal, and the material seems to be two phase. Further assessment of the ordering of the molecules in such multilayer structures may be obtained by a comparison of ATR and reflection-absorption (RAIRS) spectraa8p9 While the ATR experiment couples transition dipoles regardless of their orientations, grazing angle reflection only couples transition dipoles perpendicular to the substrate.13J4 Changes in intensity between the two spectra (known as dichroism) leads to information about the degree of ordering (in principle of both chains and head groups). Comparisonof ATR and RAIRS spectra for Y-type multilayers of the 4HANS/SA mixture (dipped on hydrophobic substrates) are shown in Figures 6 and 7. Since the aliphatic chain spectra of both components strongly overlap, only one band characteristic of the stearic acid moleculesis observed unambiguously;this is the cyclic dimer v(C=O) band at 1701cm-’. There are nevertheless other weaker bands due to SA: at 1433 cm-1 (C-OH inplane bending), 1298 cm-’ (v(C-0), and 946 cm-’ (O-H out-of-plane bending). There are several Ymarkersnin these spectra pointing to the well-ordered nature of these molecular multilayers. The most obvious ones of these are the bands arising from modes of the amide group, and of the (CHz), group and from the out-of-plane C-H bending, which all show strong (9) Knoll, W.; Philpott, M. R.; Golden, W. G. J . Chem. Phys. 1982,77, 219.
(10) Hanick, N.J.;ZnternationalRefZectionSpectroscopy; John Wiley & Sons: New York, 1967; 30. (11)Debe, M. K. Prog. Surf. Sci. 1987, 24 (1-4), 1. (12) Chollet, P.A.;Messier, J. Chem. Phys. 1982, 78, 235. (13) Chollet, P. A.; Messier, J.; Rosilio, C. J. Chem. Phys. 1976, 64, 1042. (14) Mirabella, F. M., Jr. Appl. Spectrosc. Reu. 1985,21 (1, 2), 45.
I ,
3500
‘
RAIRS
3000
2500
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1000
Wavenumber Figure 6. RAIRS and ATR spectra of the mixed multilayer system showing different band intensities (i.e., dichroism) between the two techniques.
RAIRS lab0
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e00
Wavenumber Figure 7. Expanded spectra from Figure 6.
dichroism. The v(NH) band at 3287 cm-l and the v(C=O) band at 1663 cm-l have high intensity in ATR and very low intensity in RAIRS (Figures 6 and 7). This would indicate that the amide group is arranged (as shown in Figure lb) with both the NH and CO groups orientated roughlyparallel to the substrate. A very interesting feature of these spectra (shownin Figure 6) is the underlying broad absorption in the 2500-3200-cm-’ region. This is almost certainly due to the v(0H-0) band of the hydrogenbonded cyclic dimer (Figure IC)observed only in RAIRS. This confirms independently the (very nearly) perpendicular alignment of the stearic acid molecules. Both 6(CH2) and CH2 rocking bands (at 1470 and 719 cm-l, respectively; see Figure 7) show parallel dichroism and confirm that the multilayers contain well-orderedaliphatic chains. The strong parallel dichroism of the out-of-plane bending modes due to the trans C-H groups in the ethylene group (972 m-l) and those due to the C-H groups (849 cm-l) in the disubstituted benzene groups indicates that the stilbene molecular plane is perpendicular to the substrate surface. (Note that there is a peak at 947 cm-l which also shows strong dichroism; the origin of this band is not fully understood as yet). In order to quantify the way in which these molecules are orientated relative to the surface, we have calculated the “tilt” angle of each of the transition dipoles of interest. The basis of such a calculation is as follows.
260 Langmuir, Vol. 8, No. 1, 1992
Song et al.
tions in the film.1° This gives
Although the experimental conditions in ATR are different from those in RAIRS, the constant C in eqs 4 and 3 is the same, as we noted above. Substituting eq 4 into 3, we obtain
To make eq 5 useful, we again take the average ( cos2 8 i ) to be equal to cos2 8iavand write Figure 8. Definitions of the angles81 and 82 of the two transition dipoles ~1 and ~2 with respect to the substrate normal. According to the fundamental theory" of optical spectroscopy, the measured infrared absorption coefficient (a) is obtained as
where E is the electric field vector and the dipole moment association with the normal coordinate Qi. (ImlQilk) is an integral determined by the initial state k, final state m, and the normal coordinate Qi. If the electric field direction is uniquely specified by the optical arrangement-as in transmission and reflection absorption (RAIRS) measurements-then the_absorptionsare related to the average angle between dp/dOj (E pj) and the surface normal (Figure 8) by12J3 av atrms(pj) 0: E2/lj21(mlQjIk)l2 sin ej 2 av aRAIRS(pj) a E2p~1(mlQjIk)l2 COS oj
According to eq 6, if one of the angles is known, the other one can be determined. The approach we have used to determine the tilt angles of the dipole transition moments in the 4HANS/SA LB film can now be explained in two stages. Firstly, since we know that the aromatic rings are perpendicular to the substrate as discussed above, we assume that the plane of the nitro group also lies vertically aligned to the substrate. Thus, if p1 is due to v,(NO2) and p2 due to vs(N02).then 81 and 82 are interrelated by O1 = 8 2 - 90". Hence aR(111) --
aR(p(2)
aATR(p1) cos2elaV aATR(p1) -----aATR(p2)
sin28y
"ATR(p2)
1 tan281''
i.e.
2
(2)
Thus, tan2 e? is proportional to the ratio atrans(pj)/ a"(pj), and therefore can be determined. However, the proportionality factor depends on experimental conditions, and its value is difficult to determine. As an alternative approach we have employed a simple method to estimate the tilt angle of dipole transition moments in organic thin films by means of RAIRS and ATR measurements. Assume that there are two dipole transition moments in a molecule which have two tilt angles 81 and 82, respectively, as shown in Figure 8. According to eq 2, the ratio between the absorbance due to p1 and p2 in a RAIRS experiments is given by (3)
Since the two absorbances are obtained in one experiment, the experimental conditions are exactly the same for them. Therefore, the constant C in eq 3 is not governed by experimental conditions but is equal to the ratio I(m(Q11k)(2/ (m1Q21k)I2. The term C(p1)~/(p2)~ in eq 3 can be determined by ATR measurements since dipoles will absorb energy in ATR measurements regardless of their orientations.lOJ1 For this experiment, although the dipoles PI and p2 may be well ordered in LB films, their absorptions should not depend on their orientations, because the electric field has approximately equal components in all spatial direc-
(7)
Using experimental values of 2.1 for ffATR(kl)/aATR(P2) and 1.33 for aR(pl)/ffR(P2),we obtain elav= 52" and eZaV= 142". Note that eq 7 may only be used in this way provided the two transition dipoles are at 90" to each other and in a plane vertical to the substrate. Secondly,having obtained the BaVvaluefor one transition dipole moment (v,(NOp) or vs(N03 in this case), we can use it to obtain all the other tilt angles of the transition moments from eq 6 by calculating cos2 8zaVfor any other transition dipole. The results we have obtained using this technique are listed in Table 11, but some comments on the validity and accuracy of the technique are appropriate before we consider the interpretation of the figures. The procedure depends on certain assumptions about the optical properties of the system, notably that the three electric field components in ATR are approximately equal. Although this is approximately true14for the clean crystal, it is not strictly the case for a deposited organic film, especially if the material is birefringent (asit will be in a well-orientated multilayer LB film). Estimates show that these components change by no more than 20-30s on film deposition so the optical distortion produced should not be large (and the absorption coefficients are very low, so distortion of the band shape should also be small). Furthermore, as is shown in eq 3-6, all spectral ratios used are obtained using the same technique. Thus, any optical distortion caused by the sample being subjected to differing optical fields in the two techniques should be largely canceled out.
Langmuir, Vol. 8, No. 1, 1992 261
LB Films Containing Nitrostilbene and Stearic Acid Table 11. Tilt Angle of Dipole Transition Moments in the 4HANS/SA LB Films**
ad
I
!A
~ATR(C)/
a ~ ' ( u d N 0 2 ) ) a~m(u.dN02)) 8" (h4')
u(N-H) ~as(CHd va(CHd ~as(CH3) va(CH3) u(C=O) acid v(C=O) amide u(C-0 aromatic ~(CHZ) v(C-N) b(C-H) ethylenic I(C-H) aromatic r(CHd
2.1 4.2
-0 0.83 0.16 0.089 0.025 0.32
1.1
0.047 0.023 1.0 0.23 0.22 0.29 0.13 0.10 0.16 0.13
0 0.53
0.045 0.072 -0 -0 -0
*
-
90 106 or 74 103 or 77 150 or 30 48 or 132 111or 69 90 13 or 167 104 or 76 62 or 118 90 90 90
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--
a Integrated intensities used. Most calculated angles have two possible values because of the square cosine nature of the function in eq 7.
1700
1600
1500
1400
1300
Wavenumber Figure 9. Comparison of ATR and RAIRS spectra of multilayers of the 1:l 4HANS/SA mixture: (a) cast film of 4HANS on ZnSe(ATR), (b) 20-layer LB film on ZnSe(ATR), (c) 5-layer L B film on Si (ATR), (d) 10-layer LB film on silvered glass (RAIRS).
Taking into account all these potential difficulties, we estimate that our integrated intensities (for use in eqs 3-7) are accurate to &lo%. This results in an error of f4O on the angles calculated as shown in Table 11. Such errors substrate the NO2 group is adjacent to the surface (Figure take account also of the difficulty in measuring integrated 5), but for the hydrophobic substrate the first layer is intensities for overlapping bands. deposited on the first downstroke11J2so the aliphatic chain As may be seen from the table, the v(C=O) amide is adjacent to the substrate. This fundamental structural transition dipole (TD) is virtually parallel to the substrate difference is reflected in different v,(NOz) band frequencies (Figure lb) whereas the stearic acid v(C=O) TD is tilted (1342 and 1347 cm-') since the NO2 group environment only -20" away from the substrate direction. The CH2 is significantly different in the two cases. Indeed, it is alkyl chains are also tilted at about 15' to the substrate quite clear from comparison of parts b and c of Figure 9 normal. However, as mentioned above, the aromatic rings that these two v,(NOz) bands correspond to surface-adand ethylenic group are virtually perpendicular to the sorbed and non-surface-adsorbedNO2 groups,respectively. surface. From these calculated angles it is possible to build This effect is also manifested in the v(C=O) band a t 1640 up a model of the multilayer structures as shown in Figure cm-1 and in the va(NO2) band at -1520 cm-l (although l b and in Figure 5. the latter is complicated by the perpendicular dichroism Considerable interest has been generated r e ~ e n t l y l ~ * ~ of ~ the amide I1 band near 1540 cm-'1. in the importance of the substrate-monolayer interactions as a controlling feature of the structure and order of Summary and Conclusions subsequent multilayers. In order to address this question We have shown chemicallythat well-orientated and wellfor 4HANS/SA multilayers we have compared (see Figure LB multilayers containing a 1:l mixture of the aligned 9) the spectra of LB films of this mixture on different nitrostilbene dye (4HANS) and stearic acid may be built substrates under different conditions. In particular, up on a variety (semiconductor or metal) of substrates. comparison of an ATR spectrum on hydrophilic silicon Such multilayers, which are important in potential non(Figure 9c) with the ATR spectrum on hydrophobic zinc linear optical devices, may be constructed up to 300 layers selenide (Figure 9b) and with the RAIRS spectrum on a ( 7500 A) thick without any degradation in the deposition hydrophobic substrate (Figure 9d) shows that there are ratio. Such an organic matrix is an ideal one with which distinct shifts in frequency and band shape of the features to attempt the design of new types of engineered structure near 1525 and 1350 cm-' (shown by the vertical lines). with enhanced electronic properties. These shifts of the v,(NO2) and va(NO2) bands (forexample between parts c and d of Figure 9) may be associated with Acknowledgment. We thank the Molecular Elecband shape changes on going from ATR to RAIRS. tronics Committee of the SERC for continued support of However, comparison of parts b and c of Figure 9-both this work. obtained in ATR but on different substrates (one hydrophilic and the other hydrophobic)-shows that there are Registry No. H&(CH&COzH, 57-11-4; 4HANS, 13263236-1. small but real surface-induced shifts. For a hydrophilic
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