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Fourier Transform Infrared Spectroscopic Studies on Alternate-Layer Langmuir-Blodgett Films with Nonlinear Optical Properties Y. P. Song,??*J. Yarwood,*tt J. Tsibouklis,tl* W. J. Feast,t J. Cresswell,*and M. C. Petty$ Department of Chemistry and Molecular Electronics 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 The nature and extent of molecular “ordering” in alternate-layer Langmuir-Blodgett films of 4-nheptadecylamido-4’-nitrostilbene/stearic acid and a functionalizeddiarylalkyne(JT11)have been elucidated using infrared linear dichroism. Comparison of intensity ratios of certain infrared bands in the attenuated total reflection and the reflection-absorption spectra has been used to determine the details of the molecular geometrical arrangement from the resulting directions of the selected transition moments. This information, along with spectra of hydrogen-bonded stilbene and stearic acid C=O groups, leads to a predicted “structure”which is consistentwith the observationof a rather modest nonlinear opticalcoefficient. This should lead to an improved design strategy for the successful production of organic nonlinear optical devices. Introduction
c 22 H+6
In the preceding paper’ we reported the use of infrared linear dichroism to characterize the molecular order and interactions in Langmuil-Blodgett (LB) multilayers containing 4-n-heptadecylamido-4‘-nitrostilbene(4HANS) and stearic acid (SA). We have also previously described4 the nonlinear optical (nlo) properties of alternate layers of this material and a functionalized diarylalkyne known as JTll.213(The 4HANS and JTll molecules are shown in Figure 1.) It was anticipated that such a molecular arrangement, in which the donor group of one of the dye components was adjacent to the acceptor group in the other, would lead to enhanced second-order nlo a ~ t i v i t y . ~ Although high-quality films exhibiting waveguiding could be produced, only modest nlo coefficients were noted. In this paper we present a detailed infrared spectroscopic investigation, using attenuated total reflection (ATR) and reflection-absorption (RAIRS) techniques of the molecular ordering in (4HANS/SA)/JT11 multilayers. JTll has the very useful C=C chemical group (a vibrational ”chromophore”) which allows (at least in principle) convenient study of the structural arrangement in these LB films. The overall objective is to identify and understand the molecular interactions which are ultimately responsible for the electronic behavior of the material deposited. For example, our previous work5 shows that hydrogen bonding plays an important role in the separation and ordering of the active molecules involved in second Department of Chemistry. School of Engineering and Applied Science. (1)Song, Y. P.;Petty, M. C.; Yarwood, J.; Feast, W. J.; Tsibouklis, J.; Mukherjee, S. Preceding paper in this issue. (2) 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 1989,22, 1608. (3) Cresswell, J. P.; Tsibouklis, J.; Petty, M. C.; Feast, W. J.; Carr, N.; Goodwin, M.; Lvov, Y. M. Proceedings of the 1990 SPIE Conference on Nonlinear Properties of Organic Materials, San Diego 1990; SPIE Proceedings Series, Vol. 1337, pp 358-363. (4)Neal, D. B.; Petty, M. C.; Roberta, G. G.; Ahmad, M. M.;Feast, W. J.; Girling, I. R.; Cade, N. A.; Kolinsky, P . V.; Peterson, I. R. Electron. Lett. 1986, 22, 460. (5) Ancelin, H.; Briody, G.; Yarwood, J.; Lloyd, J. P.; Petty, M. C.; Ahmad, M. M.; Feast, W. J. Langmuir 1990,6, 172. t
t
17 H36
//c\N/ 0
H I
C
Ill C
b@ (a)
JTll
(b)
4HANS
Figure 1. Chemical structures of the molecules employed in this work: (a) JT11, (b) 4HANS.
harmonic generation. In the present situation the stearic acid also provides an important structural component, the role of which was not appreciated in the past (see ref 1). Experimental Section The experimental procedures have been outlined in our previous papers.lv4 The multilayers were deposited from an ultrapure water subphase by the standard techniques using an alternate layer trough described elsewherea6Y-type deposition with good transfer ratios (1.0 f 0.1) waa achievedon hydrophobic substrates: ZnSe (forATR studies)and silvered glass (for RAIRS studies). Spectra were obtained with a Mattaon Sirius ‘100” spectrometer (equippedwith a liquid Nz cooled MCT detector) at a resolution of 4 cm-1. Data collection times were about 20 min/spectrum.
Results and Discussion Before we can elucidate the spectra of the alternate 4HANS/JT11 layers, it is necessary to provide spectral assignments for the principal bands of JTll which has not been studied in detail before. Figure 2 shows the Fou(6) Holcroft, B.; Petty, M. C.;Roberts, G. G.; Russell, G. J. Thin Solid Films 1985, 134, 83.
0 1992 American Chemical Society
LB F i l m with Nonlinear Optical Properties
Langmuir, Vol. 8, No. I , 1992 263
I
1 3000
3500
I
3500
2500
1
(Cast Film 3000
2500
2000
1500
1000
Wavenumber
Wavenumber
0.3 0,
0
9 P
0.2
h
0 P
4
0.1
0.0
I
1600
1400
1200
lo00
800
1600
1400
1200
1000
eo0
Wavenumber Figure 2. Infrared ATR spectra of a cast film of JTll in (a) 2000-4000-~m-~ and (b) 600-1800-~m-~regions. The film was deposited on a hydrophobic ZnSe crystal.
Wavenumber Figure 3. Comparison of RAIRS spectra (10 monolayers on silvered glass) of JTll and cast film (on ZnSe). In both cases the substrate is hydrophobic.
Table I. Summary of Band Position ( c d ) and Assignments for JTll Cast Films iLJcm-1 fwhdcm-1 assignment 3412 185 HzO, 4OH) 23 aromatic v(C-H) 3107 aromatic v(C-H) 3017 52 17 2917 va. (CHd 12 2851 vs(C&) v(C=C) 17 2222 U(C=c) 17 2184 16 v(C-N) of the pyridinium ring 1636 10 aromatic v(C-C) (disubstituted 1584 benzene) 17 1468 WHz) out-of-plane 6(CH) 39 826 12 721 r(CH2)
rier transform (FT) infrared spectrum of a cast film of JTll on a ZnSe ATR crystal. The band assignments' are shown in Table I. Special comment is necessary only for two bands. The first is a very broad band at 3412 cm-' which may be due to the adsorbed water because the JTll is probably very hygroscopic. We note that, in the Aldrich Library of Infrared Spectra, many spectra of pyridinium derivatives do have this broad peak, suggesting that such materials are usually hygroscopic. The second band is the v(C=C) doublet at 2184/2222 cm-'. The
splitting of this band (which is present in all JT compounds we have studieda)may be due to a Fermi resonance of the v ( C 4 ) mode with a skeletal v(C-C) mode near 1125 cm-l.lo The relative intensity of these two components (in this case 1O:l) varies from one analogue to another and with the quality of LB multilayer deposited.8 There is no evidence so far that the structure of this band is related to different numbers of chemical species. It may, however, be associated with different chemical environments. Figure 3 shows a comparison of a RAIRS spectrum of a sample consisting of 10 layers of JTll dipped on a silvercoated glass substrate, with the cast film (ATR) spectrum of Figure 2. The dichroism displayed here is not as pronounced as that observed for the 4HANS/stearic acid spectra.' The orientation of the molecules in the JTll LB f i i is clearlydifferent from those in either the 4HANS/ SA or in the 4HANS/ JTll multilayer5 (see below). Some notable features in the JTll spectra on going from cast film to LB film include (a) the loss of a band at 754 cm-I, (b) the appearance of a new band at 1047 cm-l, and (c) the intensity change of a band at 1138 cm-l. These changes are not fully explained, but they may be related to possible reactions occurring during dipping when the JTll molecule are in contact with water.
(7) Bellamy, L. J. The Infrared Spectra of Complex Molecules 3rd ed.; Chapmen and Hall: London, 1986; Vol. 1.
(8) Maddaford, P. J.; Christopher, D. J.; Petty, M. C.; Yarwood, J. Unpublished data.
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Langmuir, Vol. 8,No. 1, 1992
'
RAIRS
3500
3000
2500
2000
1500
3500
3000
2500
2000
1500
Wavenumber Figure 5. ATR and RAIRS infrared spectra of J T l l (in the alternate-layerLB film) obtained by subtraction of 4HANS/SA spectra (demonstratesthe excellent ordering of J T l l alternate layers).
Wavenumber (C=O) acid 1720 cm-l ) amide 1664 cm-l
Table 111. Tilt Angle of the Transition Dipoles in the JTll Lavers
I 1700
1600
1500
1400
Wavenumber Figure 4. Comparison of 13-layer ATR and 18-layer RAIRS spectraof alternatelayers of J T l l and 4HANS/SA. ATR spectra were obtained on Si and RAIRS spectra on silvered glass. Table 11. Tilt Angle of the Transition Dipoles in the Alternated LB Films
4.4 0.20 18 3.9 0.18 4.3 0.40 0.32 0.38 1.1
-0 0.16 3.2 0.54 0.28 0.46 -0
0.67 0.53 0.16
-
90 58 or 122 75 or 105 78 or 102 44 or 136 79 or 101 90 33 or 147 47 or 133 78 or 102
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In order to assess the degree of ordering of these potentially electronically active (Y-type) multilayers of JTll with 4HANS/SA, we have compared ATR and RAIRS spectralsgof alternate layers. These are shown in Figure 4. The strong dichroism displayed in these spectra suggests that the molecules in the LB alternate layer film show definite orientations. Following the method described in the previous paper1 and making the same assumptions, the tilt angles of the v,(NOz) and v,(NOz) have been determined to be 55' and 145' (from the ratios of these bands; i.e. [CYR ~ , ( N O ~ ) / Cv,(NOz)l YR = 0.70 and [CYATRY,(NO~/CYATR v,(NOa)l = 0.35). From these values, (9)Davies, G . H.; Yarwood, J. Langmuir 1989, 5, 229. (10)Sheppard, N.; Simpson, D. M. Q.Reu. Chem. SOC.1952, 6, 1.
vm(CH3) v,(CHz) va(CH3) Ua(CH2) u(C-N) 6(CHz)
0.60 66 0.32 16 3.4 5.4
0.24 8.1 0.07 1.4 2.4 0.93
63 or 117 75 or 105 71 or 109 77 or 103 52 or 128 73 or 107
the tilt angle of all the other dipole transition moments may be obtained. The data are listed in Table 11. From this table one can see that the average tilt angle of the carbon chains of the LB films, including the JTll layers, is only about 15' away from the surface normal. The ordering of the JTll layers alone may be then assessed by comparing the RAIRS and ATR spectra obtained after subtraction of the QHANS/SA LB film spectra from the alternating layer LB film spectra. The subtracted spectra are shown in Figure 5 in which dichroism can be clearly seen. From these spectra, the average tilt angle of the dipole transition moments in the JTll layers can be estimated by means of the same method as we used for the complete film. To do so, we have used the known tilt angle of the v(C=C) transition moment as given in Table 11. Having obtained this angle, all the others can be obtained, and the results are listed in Table 111. From this table we can see that the tilt angle of the chains is again about 15'. This angle is very similar to that for the complete film layers. Thus, the orientation perpendicular to the substrate in the JTll layers is better in the alternated LB films than in the JTll multilayers. We conclude that the orientation in the 4HANSlSA layers helps to create order in the JTll layers during the dipping process. This is undoubtedly associated with hydrogenbonded interactions (see below). The most obvious major change between the alternate layer spectra shown in Figure 4 and the spectra of multilayers of 4HANSlSA alone (Figure 6 of the preceding paper in this issue) is the disappearance of the cyclic acid dimer band at 1700 cm-l. A replacement band arises at 1720cm-l which is immediately assignable to a sideways dimeric7s8or oligomeric hydrogen-bonded acid molecule (Figure 7a). (Spectrum d in Figure 6 shows this band very clearly.) The sideways hydrogen bonding (Figure 7 is
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Langmuir, Vol.8, No. 1, 1992 265
LB F i l m with Nonlinear Optical Properties o
Table IV. Electrooptie x(*) (-w: w,o) Coefficients at 633 nm for LB Monolayer and Bilayer Samples
0.l
material JTll monolayer 4HANS/SA monolayer bilayer
I ieoo
I La 1700
isoo
1600
1400
1300
Wavenumber Figure 6. Infrared (ATR) spectra of (a) 3, (b) 5, (c) 7, and (d) 13 monolayers of 4HANSISA in the alternate-layer LB films (the spectra of the appropriate number of layers of JTll have been subtracted). /o-*, o=C
/o-*\
/o-*, o=C
'a=$
/o-*, O=C,
'
Figure 7. (a) Molecular arrangement of "sideways"acid dimers whose v(C=O) band arises at about 1720 cm-l. (b) Proposed molecular arrangementof alternate layers of JTll and 4HANS/ SA on a hydrophobic substrate.
clearly confirmed by the strong dichroism of the broad v(0H-0) band between 2800 and 3500 cm-l (ATR curve of Figure 4A). Cyclic dimers are not, of course, expected to be present if one examines the likely structure of these alternate multilayers (Figure 7b). Figure 6 shows the spectra of 3,5,7, and 13layers of 4HANS/SA in altematedlayer LB films. Spectrum a shows that for one layer of 4HANS/SA the amide C=O group probably feels more than one environment and the acid C=O group finds many different hydrogen-bonding opportunities. As the number of layers increases from b c d the bands in this region become better defined, eventually giving a simple broad absorption band centered at -1720 cm-l. The band at
--
x(*) (-w;
w,o)/ mV-1
7.4 x 10-12 4.5 x 10-12 2.6 x 10-l2
1660 cm-' is due to the hydrogen-bonded amide C-0 groupsas we described in the previ0uspaper.l These bands are an indication of sideways acid dimers and strong hydrogen-bonded amide C=O groups. The type of interaction envisaged is shown in Figure 7 (see below). From this figure it can be seen that four possible interactions could exist: (1)acid C=O groups bonded to the hydroxyl group of another acid; (2) acid C-0 group bonded to the hydrogen of an amide group; (3)amide C-0 group bonded to amide H-N group; (4) amide C - 0 group bonded to the acid hydroxyl group. However, only two dominant v(C=O) bands are observed in the spectra. Using these spectra and the information collected in Tables I1 and 111, it is now possible to discuss the orientation and interactions to be expected in alternate layers containing JTll and the stilbene/stearic acid mixture. Figure 7b shows a scheme which is consistent with the data reported here. It is clear from Figure 6 that the acid v(C=O) band, being broad and asymmetric, provides evidence for more than one "sideways-type" hydrogen-bonding interaction. These are marked 1 and 2 in Figure 7b. There may also be small amounts of cyclic acid dimer, but there is no clear indication of significant absorption at 1705cm-l; any cyclic dimer which is present must arise, of course, from rearrangement of acid molecules. The amide v(C=O) band is rather narrower and better defined, indicative, perhaps, of a narrower range of amide group environments (or maybe a lower sensitivity to environmental variations). We still expect two amide C=O group interactions, however, as mentioned above. These are numbered 3 and 4 in Figure 7b. These are necessarily present if dye and acid aggregation occurs as we believe it does (and as we deduced in the preceding paper in this issue). The "tilt" angles determined for the v,(NO)a and vs(NO2) bands show that the stilbene dye head group is tilted significantly away from the substrate normal. This situation ties in nicely with the way in which JTll molecules in the next layer may show the alignment ( 8 c d = 43O) and probably interdigitation projected in Figure 7b. All the other angles determined from the intensity ratios are consistent with this proposed molecular arrangement. The various tilt agles for the dye chromophores and alkyl chain obtained from this work would lead to an overall bilayer spacing of 6.0 nm. This compares very favorably with the value of 6.3 f 0.05 nm from a low-angle X-ray diffraction s t ~ d y . ~ In terms of nlo properties it is important that the molecule arrangement is noncentrosymmetric in an electrical (or electronic) sense. Interdigitation, as displayed in Figure 7b, is likely to lead to a lower total dipole due to sideways (rather than head-on) D-A-D-A alignment. Indeed, this has been found to be the case from out optical experiments.3 Table IV contrasts the second-order d o susceptibilities, x(*)(-a;w,o), measured using the Pockels effect, for the 4HANSISA and JTll monolayers and for the bilayer. Clearly the anticipated improvement in the susceptibility for the alternate-layer structure hm not been a ~ h i e v e d .The ~ interdigitation suggested by our infrared experiments results in a multilayer structure with a high degree of structural order. Unfortunately, the arrangement of the chromophores is not conducive to high nlo activity.
266 Langmuir, Vol. 8, No. 1, 1992
spectroscopy (ATR and RAIRS) of the molecul& ordering in alternate-layer LB films based on two dye materials. The data show strong evidence for the interdigitation of the dye chromophores, explaining the modest nlo activity in the films. These results emphasize that knowledge of the detailed arrangement of the
Song et al.
Acknowledgment. We thank the Molecular Electronics Committee of the Science and Engineering Research for continuing support of this work. Registry No. H3C(CHd1&01H, 57-11-4; (HANS, 13263236-1; JT11, 136984-70-8.
