Langmuir 1995,11, 4495-4498
4495
Orientation Control of Functional Molecules in Langmuir-Blodgett Films Caused by a Trigger Molecule: Infrared Spectroscopic Study on the Orientation of n-Alkane,Trigger Molecule Reiko Azumi" and Mutsuyoshi Matsumoto National Institute of Materials a n d Chemical Research, Higashi, Tsukuba, Ibaraki 305, J a p a n
Shin-ichi Kuroda Electrotechnical Laboratory, Umezono, Tsukuba, Ibaraki 305, J a p a n
Maxwell J. Crossley School of Chemistry, The University of Sydney, New South Wales 2006, Australia Received May 25, 1995. I n Final Form: August 7, 1995@ We previously reported that the orientation of a functional molecule meso-tetrakis(3,5-di-tert-butylphenyl)porphinatocopper(II), PM, was controlled by the addition of a small amount of the trigger molecule, n-hexatriacontane (CH3(CH2)34CH3),in the mixed Langmuir-Blodgett (LB)films with cadmium icosanate. In this paper we investigated the orientation of n-hexatriacontane, the trigger molecule, in the mixed LB film by comparing the infrared transmission and reflection-absorption spectra of the mixed LB films. Deuterated n-hexatriacontane (CD3(CD2)34CD3,HA-d)was used to separate the infrared absorption bands of the trigger molecule from those of cadmium icosanate. In the mixed LB film with a mixing ratio PM/ cadmium icosanatem-d = 0/10/0.5, the tilt angles y of the alkyl chains for HA-d and cadmium icosanate (C20) with respect to the film normal were estimated at 12" and 13", respectively. The introduction of PM with the mixingratio 1.5/10/0.5 changed y of HA-d to 37", whereas that of C20 stayed unchanged. The existence of the anisotropy of CDZstretching bands around the molecular axis indicates that HA-d was oriented with its C-C-C plane almost perpendicular to the film surface. The orientation of HA-d was further studied with various mixing ratios of 1.5/10/x to investigate the mechanism of orientation control of PM by HA-d. A crossover point was clearly seen around the mixing value x = 0.5. With the mixing value x in the region 0 < x 5 0.5, the electron spin resonance spectra can be simulated satisfactorily by adding (0.5 - xYO.5 times the spectrum with the mixing ratio 1.5/10/0 and x/0.5 times the spectrum with the mixing ratio 1.5/1/0.5, suggesting specific molecular interaction of the two components. At x = 0.1, the C-C-C plane of HA-d was distributed almost homogeneously around its own molecular axis; whereas with increasing x in the region 0 < x 5 0.5, the C-C-C plane of HA-d was oriented gradually in an anisotropic manner or the fraction of the anisotropically oriented HA-d had increased. In the region x 2 0.5, the fraction of HA-d not participating in the orientation change of PM took an orientation similar to that of HA-d in the mixed LB film with C20 alone. In contrast to the case of HA-d, the alignment of C20 did not change by the introduction of HA-d, supporting the argument that the orientation change of PM occurred through the direct interaction between HA-d and PM.
Introduction
resonance (ESR)and Uvlvis absomtion sDectroscoDiesfor mixed LB films with the molar m i k n g rako of porphyrin/ Since one of the most attracting subjects in the cadmium icosanateln-hexatriacontane = 1.5IlOlx in the Langmuir-Blodgett (LB) films research is to precisely range 0 5 x 5 5 . Without n-hexatriacontane, the major arrange the functional molecules in a planned manner, component of the porphyrin had a n orientation with the various kinds of strategies to control the orientation or most probable angle 8 0 = 58" between the plane normal arrangement of the molecules incorporated have been ofthe molecule and the film normal, while the macrocycle tried.1-5 We have reported that a small amount of alongchain n-alkane such as n-hexatriacontane ( C H ~ ( C H Z ) ~ ~ -plane of the major component was oriented almost vertically (80 = 80") with respect to the LB film surface CHd, when added in the preparation of monolayers, can with x = 0.5 for n-hexatriacontane. This mixing ratio drastically change the orientation of copper porphyrin shows that to change the orientation of the porphyrin the molecules in the LB film^.^-^ The orientation of one of number of moles of n-hexatriacontane can be as small as the porphyrins was elucidated by using electron spin one-third the number ofmoles ofporphyrin. These results indicate that by using a trigger molecule, which in this Abstract published inAduanceACSAbstracts, October 15,1995. case is a long-chain n-alkane, we can control the orienta(1)Proceedings of the Sixth International Conference on LangmuirBlodgett Films. Thin Solid Films 1994,242-244. tion of functional molecules in the LB films. @
(2) Fukuda, K.; Nakahara, H.; Kato, T. J. CoZloidInterfaceSci.1976,
54, 430.
(3)Nakamura, T.; Tanaka, M.; Sekiguchi, T.; Kawabata, Y. J. Am. Chem. SOC.1986, 108, 1302. (4) Kawabata, Y.; Sekiguchi, T.; Tanaka, M.; Nakamura, T.; Komizu, H.; Honda, K.; Manda, E.; Saito, M.; Sugi, M.; Iizima, S. J . Am. Chem. SOC.1986, 107, 5270. (5) Matsumoto,M.;Nakamura, T.;Tanaka,M.; Sekiguchi,T.; Komizu, H.; Matsuzaki, S. Y.; Manda, E.; Kawabata, Y.; Saito, M.; Iizima, S.; Sugi, M. Bull. Chem. SOC.Jpn. 1987, 60, 2737.
0 743-7463/95/24 11-4495$09,00/0
Azumi, R.; Matsumoto, M.; Kawabata, Y.; Kuroda, S.; Sugi, M.; L. G.; Crossley, M. J. J. Am. Chem. SOC.1992, 114, 10662. Azumi, R.; Matsumoto, M.; Kawabata, Y.; Kuroda, S.; Sugi, M.; L. G.; Crossley, M. J. J. Phys. Chem. 1993,97, 12862. Azumi, R.; Tanaka, M.; Matsumoto, M.; Kuroda, S.; Sugi, M.; Le, , Crossley, M. J. Thin Solid Films 1994,242, 300. (9) Azumi, R.; Matsumoto, M.; Kuroda, S.; King, L. G.; Crossley, M. J. Langmuir, in press.
0 1995 American Chemical Society
Azumi et al.
4496 Langmuir, Vol. 11, No. 11, 1995
r
I
I
Va(CH2)
h
To.001
1
Figure 1. Molecular structure of PM.
In this paper we focused on the mechanism oforientation control: how the trigger molecule controls the orientation of other molecules. The orientation of n-hexatriacontane in the mixed LB film is investigated using infrared transmission a n d reflection-absorption (RA) spectroscopies. The combination of these two spectroscopies is a powerful tool for investigating the orientation of molecules in LB films.1°-14 Deuterated n-hexatriacontane (CD3(CD2)34CD3)is used to separate the infrared absorption bands ofthe trigger molecule from those ofthe matrix molecule, cadmium icosanate. Experimental Section Preparation of the Mixed LB Films. meso-Tetrakis(3,5di-tert-butylphenyl)porphinatocopper(II),PMIS(Figure l),was synthesized by slight modification of the literature method.16J7 Icosanoic acid (Eastman Kodak) and n-hexatriacontane-d74 (CD3(CD2)34CD3,Cambridge Isotope Laboratories, D 98%, abbreviated hereafter as H A 4 were used as received. Achloroform solution containing molar ratio of PWicosanoic acidlHA-d = 0/10/0.5 and 1.5/1O/x (0 5 x 5 2) was spread onto an aqueous subphase of pH 6.0, containing 4.0 x M CdClz and 5.0 x M KHCO3. Icosanoic acid was converted to cadmium salt ((320) on this subphase. All the monolayer experiments were performed at 17 "C on a Lauda Filmwaage. The details were described el~ewhere.~ The monolayers were transferred onto a solid substrate by the vertical-dipping method at a surface pressure of 25 mN m-l. The dipping speed was 10 and 15 mm min-l for upward and downward strokes, respectively. Twentyone monolayerswere deposited onto both sides ofa CaF2 substrate for infrared transmission spectroscopy and 20 monolayers onto a glass substrate coated with gold for reflection-absorption spectroscopy. Transfer ratios around unity were obtained for all the samples. Infrared Spectroscopy. Infrared spectra of the mixed LB films were measured using a Perkin-Elmer System 2000 FTIR. The spectrometer was purged with nitrogen gas to minimize the amount of water vapor present in the sample chamber. The spectra were recorded at a 4-cm-I resolution by coadding 10100 scans in the 4000-800 cm-' region. For RA spectroscopic measurements, a wire grid polarizer was placed before the reflection attachment (Spectratech) and provided polarization selection. The p-polarized light was introduced with an incident angle of 80". In-plane anisotropy was not observed for any of the LB films examined. Estimation of the Orientation ofAlkyl Chains in the LB Films.ls The enhancement factor m for the RA measurement with respect to the transmission measurement can be obtained ~~~
~
(10)Chollet, P.-A,; Messier, J.; Rosilio, C. J. Chem. Phys. 1976,64, 1042. (11)Allara. D.L.:Swalen. J. D. J. Phvs. Chem. 1982. 86.2700. (12)Rabolt, J. F.; Burns, 'F. C.; Schloker, N. E.; Swalen; J. D. J. Chen. Phys. 1983,78,946. (13)Allara, D.L.;Nuzzo, R.G.Langmuir 1986,1, 45, 52. (14)Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990,94,62. (15)Crossley, M. J.; Burn, P. L. J. Chem. SOC.,Chem. Commun. 1987,39. Longo, F. R.; Finarelli, J. D.; Goldmacher, J.;Assour, (16)Ad1er.A. D.; J.; Korsakoff, L. J. Org. Chem. 1967,32,476. (17)Adler, A. D.;Longo, F. R.; Kampas, F.; Kim, J. J.Inorg. Nucl. Chem. 1970,32,2443. (18)Umemura et al. (ref 14)have demonstrated a more detailed estimation of the enhancementfactors and the tilt angles of the transition moments with respect to the film normal. Here we employ a simpler estimation method, since it is not the purpose ofthis paper to estimate precisely the tilt angles but rather to investigate the change of the angles with varying composition of the mixed LB film.
3000
2800
2900
"
ZOO
2100
2000
Wavenumber / cm" Figure 2. Infrared transmission and RA spectra of the mixed LB film without PM (C20IHA-d = 10/0.5). The transmission spectrum is multiplied by 20/42, considering the difference in layer number. as follows:10
m=
4nI3sin2e
n2 cos 8
(1)
where nl and n2 are the refractive indices of air and the film, and f3is the incident angle. The value m = 6.62 was obtained for the parameters nl = 1.0, n2 = 1.5, and 6 = 80". The tilt angle 4 of the uniaxially oriented transition moment from the film normal is thus roughly described as f01lows:'~J~
(2) where AT is the absorbance of the peak in the transmission spectra, ARis that in the RA spectra, and m is the enhancement factor calculated with eq 1. Using the orthogonal relationship, the tilt angle y for the alkyl chain of C20 and HA-d from the film normal can be obtained as follows:14 cos2 a
+ cos2 p + cos2 y = 1
(3)
where a and represent the tilt angles of the transition moments of CH2 (or CD2) antisymmetric and symmetric stretching, respectively.
Results and Discussion Orientation of HA-d in the Mixed LB Film with C20. We investigated the orientation of HA-d in a mixed LB film with the C20 matrix alone (without PM). Figure 2 shows the infrared transmission and RA spectra of the mixed LB film of C2OIHA-d = 10/0.5. The peaks at 2194 and 2089 cm-l are assigned to CD2 antisymmetric and symmetric stretching of HA-d, respectively. The peaks at 2917 and 2850 cm-' are assigned to the corresponding CH2 stretching of C20. These peaks are strong in the transmission spectrum and weak in the RA spectrum, indicating that the alkyl chains of both HAd and C20 are rather perpendicular to the film surface. Using eqs 2 and 3, the a, B, and y angles are estimated for HA-d and C20 as being 82,81, and 12" and 80,81, and 13", respectively. This indicates that the molecular axis of HA-d is aligned in the mixed LB film in much the same manner as that of c20. Orientation of H A 4 in the Mixed LB Film with PM and C20. The above results give the basis of the orientation analyses 0fHA-d in the case where H A 4 works efficiently as a trigger molecule. Figure 3 shows the infrared transmission and RA spectra of the mixed LB film with the mixing ratio PWC2O/HA-d = 1.5/10/0.5, where the orientation of most of the porphyrin molecules
Langmuir, Vol. 11, No. 11, 1995 4497
IR Study on the Orientation of n d l k a n e
a
f 0.001
3000
2900
2800
'I
2200
2100
I\
1
2000
Wavenumber I cm"
Figure 3. Infrared transmission and RA spectra of the mixed LB film with the mixing ratio PMICPOIHA-d = 1.5/10/0.5.The transmission spectrum is multiplied by 20/42,considering the
difference in layer number.
L--0.1 ~I
I
1f
0
0.001
11
e--.
0.1 -----L----0 _/.------~
I
I
Y
Figure 4. Schematic view of the orientation of HA-d in the mixed LB film with the mixing ratio PMIC20IHA-d = 1.5/10/ 0.5.The C-C-C plane is almost perpendicular to the film surface.
in the LB film is controlled by HA-d according to our previously reported result^.^ The ratio of absorbance of the peak a t 2195 cm-l (CD2 antisymmetric stretching) over the one a t 2092 cm-' (CD2 symmetric stretching) was ca. 4.0 in the transmission spectrum whereas it was below unity in the R4 spectrum. This indicates that the transition moment of the CDZ antisymmetric stretching is almost parallel to the film surface though the one for the CDZsymmetric stretching has a certain angle with respect to the film surface. This is shown schematically in Figure 4 where the C-C-C plane is almost perpendicular to the film surface. The estimated tilt angle y is 37" for HA-d. The tilt angle y of HA-d is apparently larger than that for the mixed LB film without PM (Figure 2), indicating that HA-d aligns in a different manner due to the interaction with PM. The ,A values of the two peaks slightly shifi to higher wavenumbers as compared to those of the LB film without PM, suggesting the existence of HA-d in the gauche conformation.lg The spectra of the C-H stretching region assigned to C20, on the other hand, are similar to those of the mixed LB film without PM. The a, p, and y angles are 81, 82, and 13",respectively, indicating that the structure of C20 is not affected so much by the existence of PM. This rules out the mechanism of orientation control in which the orientation change of PM occurs through the structure (19)Casal, H.L.; Mantsch, H.H.Biochim. Biophys. Acta 1984,779, 381.
4498 Langmuir, Vol. 11, No. 11, 1995
a (VaCDz) 0
Azumi et al.
0
O
O
In the region x L 0.5, the fraction of HA-d not participating in the orientation change of PM should increase with increasingx since the fraction of the affected PM withx = 2.0 is almost equal to that withx = 0.5.7Here we assume that the number of HA-d molecules associated with the Orientation control is independent of the mixing value x in this region and that the additional H A d molecules are irrelevant to the orientation control. It is clearly seen that the angles a and /3 increase and that the angle y decreases with increasing x. Furthermore, the difference between a and p also decreases. The angles seem to approach the values of HA-d in the mixed LB film with C20 alone: a and 3, 82" and y 12". Taking into account that the estimated angles a, p, and y are the weighted means ofthe corresponding values of each H A d component, these results suggest that the fraction ofHA-d not controlling the orientation of PM takes an orientation similar to that of H A d in the mixed LB films with C20 alone. This component may exist only in the C20 domains and have no intermolecular interaction with PM in the PM/C20/HA-d mixed LB films. In contrast to the case of HA-d, the angles a, p, and y of C20 show no significant dependence on x as seen in Figure 7. This indicates that the alignment of C20 stays unchanged by the introduction of HA-d. The results support the argument that the orientation change of PM occurs through the direct interaction between the two components and not through the structural change of the matrix molecule C20.
-
Ob. 0.5
0.0
1.0
1.5
2.0
mixing value x
Figure 6. Angles a, /3, and y of HA-d aa a function of x in the mixed LB film with PM/CSO/HA-d = 1.5/10/x.
80-8
*
p (VsCHz) 6
6
b
a (VaCHz)
.
-
Conclusions
.
y (molecular axis)
0.0
0.5
1.0
1.5
2.0
mixing value x
Figure 7. Angles a, B, and y of C20 as a function of x in the mixed LB film with PWC20/HA-d = 1.5/10/x. controlled by HA-d is proportional to the amount of HA-d, also suggesting a specific molecular interaction of the two components. The angles a and /3 depend significantly on the value of x in this region. At x = 0.1, the mean angles a and p of HA-d are substantially the same, indicating that the C-C-C plane of HA-d is distributed almost homogeneously around its own molecular axis. As x increases, a increases and p decreases while y depends to a much lesser extent on x. This suggests that the C-C-C plane of H A d is oriented gradually in a n anisotropic manner or that the fraction of the anisotropically oriented H A d increases with increasing x. In a sense, HA& molecules seem to be packed more regularly as x increases. One of the possible explanations is that the C-C-C plane ofHAd is anisotropically oriented through the interaction with neighboring HA-d molecules, considering that this type of interaction can become significant as the number of HA-d molecules increases.
In this report we investigate the orientation of a trigger molecule, n-hexatriacontane, in mixed LB films using infrared transmission and reflection-absorption spectroscopies with the molar mixing ratio PM/CBO/HA-d = 0/10/0.5 or 1.5/10/x, 0 5 x 5 2. In the mixed LB film with a mixing ratio 0/10/0.5, i.e. without PM, HA-d takes an orientation similar to that of C20: the alkyl chains of both components are almost parallel to the film normal. The introduction of PM strongly influences the orientation of HA-d but not the orientation of C20, suggesting that HA-d aligns in a different manner through the specific molecular interaction with PM. The variation of x shows two regions of orientation of HA-d below and above x = 0.5, while the orientation of C20 is almost constant irrespective of x. As the value x increases in the region 0 < x 5 0.5, HA-d molecules seem to be packed more regularly through the specific molecular interaction between HA-d and PM. In the regionx 2 0.5, the additional HA-d molecules which do not participate in the orientation change ofPM take an orientation similar to that of HA-d in the mixed LB film with C20 alone, suggesting that this component has no intermolecular interaction with PM in the PM/CBO/HA-d mixed LB films. The above results indicate the importance of specific molecular interaction between the trigger molecules and the affected molecules. The study of this interaction will give rise to a better understanding of the mechanism by which the trigger molecules control the orientation of other molecules. LA950405A
