Infrared External Reflection Spectroscopic Study on the Structures of a

Yoshie Urai, Chikaomi Ohe, and Koichi Itoh*. Department of Chemistry, School of Science and Engineering, Waseda University, Shinjuku-ku, Tokyo 169-855...
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Langmuir 1998, 14, 4873-4879

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Infrared External Reflection Spectroscopic Study on the Structures of a Conducting Langmuir-Blodgett Film of a Tetrathiafulvalene Derivative Yoshie Urai, Chikaomi Ohe, and Koichi Itoh* Department of Chemistry, School of Science and Engineering, Waseda University, Shinjuku-ku, Tokyo 169-8555, Japan Received April 16, 1998. In Final Form: June 15, 1998 Infrared (IR) external reflection absorption spectroscopy was applied to study the structure and its change associated with iodine doping of a Langmuir-Blodgett (LB) film of a tetrathiafulvalene (TTF) derivative, 1,3-dithiole-4-carboxylic acid-2-(1,3-dithiol-2-ylidene)hexadecyl ester (HDTTF, Figure 1), prepared on a silicon wafer. The incidence angle (φ) dependence of the p- and s-polarized IR external reflection spectra were measured and analyzed by a theoretical calculation of reflection absorbance (Ap and As) as a function of the tilt angle of the transition moment of each IR band with an assumed complex refractive index. The results indicated that the LB film before doping consists of cis and trans structures, which have the CdO bonds with cis and trans configurations, respectively, with respect to the neighboring CdC bond of the TTF ring. The TTF ring in both structures are almost perpendicular to the substrate surface with the central CdC bond parallel to the surface. After iodine doping the LB film consists of the cis structure and gives rise to charge-transfer-induced vibronic bands at 1352 and 1252 cm-1. The analyses of the φ dependence of Ap of these bands proved that the transition moments of both bands make a tilt angle of about 70° with respect to the surface normal of the substrate, which is consistent with a face-to-face stacking of the TTF rings nearly perpendicular to the surface and the high conduction parallel to the substrate.

Introduction Iodine doping of a Langmuir-Blodgett (LB) film of a tetrathiafulvalene (TTF) derivative such as 1,3-dithiole4-carboxylic acid-2-(1,3-dithiol-2-ylidene)hexadecyl ester (HDTTF, see Figure 1) has known to result in the formation of a thin film with a high conductivity.1-7 The conductivity has been attributed to a mixed valence state consisting of the cation radical of the TTF derivative.2-5 The structures of the conducting LB films have been studied by using various methods including infrared (IR) transmission, IR reflection absorption, and IR attenuated total reflection (ATR) spectroscopy.4,5 Cooke et al.6 reported that the iodine-doped LB film of HDTTF exhibits a high-conductivity value, 10-2 S‚cm-1. On the basis of UV-polarization measurements they proved that the TTF ring of HDTTF in the doped film is aligned at a high angle to a substrate surface. They also made ATR measurements on the film, observing IR bands at 1348 and 1254 cm-1, which are characteristic of the high-conductive state of the film. An IR band corresponding to the former band was found at 1360 cm-1 for a cation radical of TTF (TTF+) by Bozio et al.8 According to them, the appearance of the * To whom correspondence should be addressed. Fax: +81-35273-2606. Tel.: +81-3-5286-3243. E-mail: [email protected]. (1) Bryce, M. R.; Cooke, G.; Dhindsa, A. S.; Lorcy, D.; Moore, A. J.; Petty, M. C.; Hursthouse, M. B.; Karaulov, A. I. J. Chem. Soc., Chem. Commun. 1990, 816. (2) Dhindsa, A. S.; Ward, R. J.; Bryce, M. R.; Lvov, Y. M.; Munro, H. S.; Petty, M. C. Synth. Met. 1990, 35, 307. (3) Dhindsa, A. S.; Bryce, M. R.; Ancelin, H.; Petty, M. C.; Yarwood, J. Langmuir 1990, 6, 1680. (4) Vandevyver, M.; Roulliay, M.; Bourgoin, J. P.; Barraud, A.; Gionis, V.; Kakoussis, V. C.; Mousdis, G. A.; Morand, J.-P.; Noel, O. J. Phys. Chem. 1991, 95, 246. (5) Dhindsa, A. S.; Song, Y.-P.; Badyal, J. P.; Bryce, M. R.; Lvov, Y. M.; Petty, M. C.; Yarwood, J. Chem. Mater. 1992, 4, 724. (6) Cooke, G.; Dhindsa, A. S.; Song, Y. P.; Williams, G.; Batsanov, A. S.; Bryce, M. R.; Howard, J. A. K.; Petty, M. C.; Yarwood, J. Synth. Met. 1993, 55-57, 3871. (7) Yip, C. M.; Ward, M. D. Langmuir 1994, 10, 549.

Figure 1. Structure of HDTTF.

band is associated with the formation of the (TTF+)2 dimer and attributable to a vibronic intensity borrowing from a charge-transfer (CT) transition by a vibrational mode of the dimer corresponding to the out-of-phase coupling of an originally IR-inactive ag mode of the constituent radicals. The direction of the transition moment of the CT-induced IR band is perpendicular to the TTF molecular plane. According to Cooke et al.,6 the transition moments of the ATR bands are parallel to the substrate surface, which conforms to the high conduction parallel to the substrate surface. Dhindsa et al.3 also applied IR spectroscopy to study the structure and conductivity process of an iodine-doped LB film of hexadecanoyltetrathiafulvalene deposited onto a silicon wafer as well as metal substrates and reported that the degree of conductivity as a function of doping can be monitored by observing the spectral features of the vibronic band which appears around 1350 cm-1. Despite these studies, information about the structures of the LB film of HDTTF before and after iodine doping is still at a qualitative level, and more explicit and quantitative information should be obtained in order to clarify the origin of the high conductivity in terms of the structural change associated with the doping process. In the present paper, we performed IR external reflection (8) Bozio, R.; Zanon, I.; Girlando, A.; Pecile, C. J. Chem. Phys. 1979, 71, 2282.

S0743-7463(98)00439-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/25/1998

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spectroscopic measurements on the LB films of HDTTF prepared on a silicon wafer before and after iodine doping. The IR external reflection spectroscopy is a method in which s- and p-polarization IR reflection absorbances are measured for an adsorbate on nonmetallic substrates such as silicon and GaAs;9-12 the reflection absorbance, A, is defined by A ) log(R0/R), where R0 and R are reflectances without and with the adsorbate on a substrate. From the analyses of the spectral changes as a function of the incidence angle, the direction of the transition moment of each vibration band can be determined, which provide detailed information with regard to structures and orientations of constituent molecules of the adsorbate. Then, the analyses of the IR external reflection spectra are expected to clarify the structures of the HDTTF-LB film and their changes associated with the iodine doping. Experimental Section Materials. HDTTF was prepared by using a procedure reported by Bryce et al.1 and purified following the method of Green et al.13 All chemicals used in this study were either of reagent grade or of spectroscopic grade. Preparation of LB Film. An LB film was prepared by spreading a 1 × 10-3 mol/L chloroform solution of HDTTF on the water surface of a Langmuir trough with a Whilhelmy balance (Kyowa Kaimen Kagaku Co. Ltd., model HBM-AP2). After a time period of 30 min to allow the solvent to evaporate, the Langmuir film was compressed to 20 mN/m and transferred to a silicon wafer (2.5 × 7.0 cm, p-type, 7.4 Ω cm, Shinetsu Chemicals Co. Ltd) by a usual vertical dipping and withdrawing (5 mm/ min) method. For several initial downstrokes the transfer ratios were appreciably larger than one (1.2-1.6) and for several initial upstrokes the ratios were less than 1.0 (ca. 0.6). Although the ratios were improved toward unity as further layers were deposited, giving an average transfer ratio of 0.93 ( 0.05, the observation indicated that the prepared LB film was more or less unstable on the substrate. This result conforms to the fact that, as explained later, the actual thickness of the LB film (ca. 10 nm) was appreciably smaller than that expected from the number, 15, of the down- and upstrokes employed for the preparation. Water used for the preparation was purified by a Millipore water purification system (Milli-Q, 4-bowl). The substrate with the (100) crystalline plane was etched by treatment with a mixture of distilled water, aqueous ammonia, and hydrogen peroxide solution (8:1:1) for a few minutes at 80 °C and then cleaned by successive ultrasonication in distilled water and acetone for 10 min each at 20 °C. Iodine Doping of the LB Film. Iodine doping was achieved by exposing the LB film to iodine vapor in a sealed container for 1 h. After the film was kept under atmosphere for about 30 min, infrared spectral measurements were started. IR Spectral Measurements. IR transmission and external reflection spectra were measured by using a Bio-Rad FTS-45A Fourier transform IR spectrophotometer equipped with an MCT detector. A JEOL IR-RSC110 reflection attachment was used for IR external reflection spectral measurements at incident angles between 30° and 80°. Each spectrum was run for 1024 scans at a resolution of 4 cm-1. Ellipsometry. An ordinary ellipsometric apparatus (ULVAC Co. Ltd., model ESM-1) was used for the estimation of the thickness of LB films.

Results and Discussion IR Transmission Spectrum of HDTTF and Its Assignments. Figure 2 exhibits the IR transmission spectra of HDTTF in the 3100-2700- and 1800-1150(9) Dluhy, R. A. J. Phys. Chem. 1986, 90, 1373. (10) Mielczarski, J. A.; Yoon, R. H. J. Phys. Chem. 1989, 93, 2034. (11) Hasegawa, T.; Umemura, J.; Takenaka, T. J. Phys. Chem. 1993, 97, 9009. (12) Hasegawa, T.; Takeda, S.; Kawaguchi, A.; Umemura, J. Langmuir 1995, 11, 1236. (13) Green, D. C. J. Org. Chem. 1979, 44, 1476.

Figure 2. Infrared spectra in the 3100-2700 cm-1 (a) and 1800-1150 cm-1 (b) regions observed for HDTTF dispersed in a KBr disk. Table 1. Frequencies and Assignments of the Infrared Bands of HDTTF and References for the Assignments frequencies (cm-1)

assignmentsa

refs

2957 2916 2853 1721 1701 1570 1541 1474 1290 1196

νas(CH3) νas(CH2) νs(CH2) ν(CdO) ν(CdO) ν(C3dC4) or ν(C8dC9) ν(C3dC4) or ν(C8dC9) δ(CH2) νas(C3-C(dO)-O

16 16 16 6 6 17 17 2 7

a ν and ν denote asymmentric and symmetric stretching modes, as s respectively, of a group in the following parentheses. ν(CdO) and ν(CdC) denote CdO and CdC stretching modes, respectively. δ(CH2) denotes a CH2 scissoring mode.

cm-1 regions. Assignments of IR bands are summarized in Table 1 together with the references on which they are based. According to Cooke et al.,6 HDTTF exists in the solid state in two forms: a yellow solid giving a CdO stretching ν(CdO) band at 1721 cm-1 and a red one giving the ν(CdO) band at 1699 cm-1. A prominent band at 1721 cm-1 and a shoulder at 1701 cm-1 in Figure 2b are due to the yellow and red solids, respectively. The 1570 and 1541 cm-1 bands in Figure 2b are assigned mainly to C3d C4 and C8dC9 stretching vibrations. IR External Reflection Spectra in the 1800-1150 cm-1 Region of the LB Film before Iodine Doping. Figure 3a,b illustrates the incidence angle (φ) dependence of the s- and p-polarized IR external reflection spectra, respectively, observed for the LB film of HDTTF in the 1800-1150-cm-1 region. In the s-polarized spectra the ν(CdO) bands are observed as split negative ones at 1724 and 1701 cm-1. When φ increases, the absolute intensities (or amplitudes) of these bands decrease monotonically as in the case of other bands in the s-polarized spectra. The bands associated with the ν(C3dC4) and ν(C8dC9) bands are hardly observed in the s-polarized spectra. The p-polarized spectra in Figure 3b consist of negative as well as positive bands in contrast to the s-polarized spectra. The ν(CdO) band at 1719 cm-1 (This corresponds to the 1724-cm-1 band in the s-polarized spectra. Presumably, the frequency discrepancy is due to overlapping with the

IR Reflection Absorption Spectroscopy of TTF

1701-cm-1 band.) give a positive absorbance, which increases with φ in the 30-60° region, while the band gives a negative absorbance at φ ) 80°. On the other hand, the ν(CdO) band at 1701 cm-1 gives a negative band at 30° and its amplitude decreases with φ. The ν(C3dC4) and ν(C8dC9) bands are observed at 1570 and 1541 cm-1 as positive bands in the region, 30° < φ < 60°, and change to negative ones at 80°. The prominent feature at 1290 cm-1, which gives positive absorbances in the region, 30° < φ < 60°, and a negative one at 80°, is ascribable to a C3-C(CdO)-O asymmetric stretching vibration. Calculation of Incidence-Angle Dependence of the Reflection Absorbances for s- and p-Polarization Beams. To analyze the spectra in Figure 3 and to determine the direction of the transition moment of each IR band, we performed a model calculation of reflection absorbance as a function of incidence angle for s- and p-polarized IR bands of an anisotropic film on a silicon substrate. The optical constant of the film at the frequency of the band in question is expressed by a complex refractive index tensor, n* ) n + ik, where n and k are real and imaginary parts, respectively, of the tensor. Assuming that the film has a uniaxial symmetry with a unique axis parallel to the surface normal (z-axis) of the substrate, we have the following relationships among the principal values, (nx, ny, nz), of n and those, (kx, ky, kz), of the extinction coefficient, k.

nx ) ny and kx ) ky By applying the method already reported,9,11,12 the calculation was performed on a three-layered system consisting of air(n ) 1.0, k ) 0.0)/LB film/silicon (n ) 3.42, k ) 0.0). The optical constant of the silicon substrate was taken from ref 14. The substrate was assumed to be isotropic and transparent; the k value of silicon ranges from 2.5 × 10-9 at 3684 cm-1 to 2.49 × 10-5 at 1300 cm-1.15 The extinction coefficients of the LB film were estimated by using the following equations:12

kx ) ky ) 3/2kbulk sin2 θ kz ) 3kbulk cos2 θ where kbulk is the extinction coefficient of the band in question and θ the tilt angle of the transition moment with respect to the surface normal of the substrate. In the calculation kbulk was assumed to be 0.065, which corresponds to the ν(CdC) band at 1541 cm-1 in Figure 3b. The value was estimated from the relative intensity of the ν(CdC) band to the νs(CH2) band at 2853 cm-1 in Figure 2 under the assumption that the latter band has the kbulk value of 0.17, which was determined by the relative intensities of the νs(CH2) bands of HDTTF and stearic acid in KBr disks; the kbulk value of the latter sample has been reported to be 0.2.12 Since no reliable data have been reported for the refractive indices of HDTTF in infrared regions, we assumed the following values to the ordinary refractive index (no ) nx ) ny) and the extraordinary refractive index (ne ) nz) of the anisotropic LB film:

no ) 1.48 and ne ) 1.56 (14) Wong, J. S.; Yen, Y.-S. Appl. Spectrosc. 1988, 42, 598. (15) Edwards, D. F. Handbook of Optical Constants of Solids; Palik, E. D., Ed.; Academic Press: Orlando, FL, 1985; p 566.

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Figure 3. s-polarized (a) and p-polarized (b) IR external reflection spectra of the LB film of HDTTF on a silicon wafer at angles of incidence, 30°, 40°, 50°, 60°, and 80°. The vertical bar in each spectrum indicates an absorbance scale.

These values were borrowed from those derived by ellipsometry for a behenic acid monolayer.16 The thickness of the anisotropic LB film was determined to be approximately 10 nm by using ellipsometry. The thickness is much smaller than that estimated from the result of X-ray crystallographic analysis of o-hexadecylthiocarboxytetrathiafulvalene.5 This can be explained in terms of several factors. First, the ellipsometric measurement for such a thin film may accompany a serious error. Second, as explained in the Experimental Section, the effective number of the monolayers may be smaller than the expected number, 15. Third, in contrast to the case of the TTF derivative, where the long axis of the TTF moiety and the hydrocarbon chain takes a tilt angle of approximately 30° with respect to the substrate normal,5 the axis of HDTTF in the LB film, which as explained later, consists of trans and cis structures, may take a larger tilt angle on the average, reducing the thickness to be much smaller than the expected thickness. Figures 4 and 5 exhibit the results of the calculation of the reflection absorbance for the s- and p-polarized IR bands (As and Ap), respectively. From Figure 4 the following facts are clarified: (i) As shows the largest negative value at θ ) 90° (the transition moment parallel to the substrate), decreases its intensity as the tilt angle decreases, and becomes zero at θ ) 0° (the transition moment perpendicular to the substrate). (ii) The absolute value of As shows a monotonic decrease as the incidence angle (φ) increases. In contrast to As, Ap exhibits drastic changes at a Brewster angle of incidence (φB ) 74°), as can be seen from Figure 5. The trends in the θ and φ dependence of Ap are summarized as follows: (i) For θ > 70°; Ap in the region of 30° < φ < ca. 65° has a negative value and its absolute value increases with φ, while Ap has a positive value at φ ) 80°. (ii) For 50° < θ < 60°, Ap has a small negative value at a lower incidence angle (30° < φ < ca. 50°) and as φ increases, Ap decreases its amplitude, changing finally its sign from negative to positive. At φ ) 80° Ap has a negative value and its absolute value increases as θ decreases. (iii) For θ < 40°, Ap has a positive value and increases with φ in the region (16) Paudler, M.; Ruths, J.; Riegler, H. Langmuir 1992, 8, 184.

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Figure 4. Reflection absorbance curves for s-polarization (As) calculated for the ν(CdC) band at 1541 cm-1 by assuming the tilt angles of the transition moment. The solid bars indicate the observed As for the 1541-cm-1 band in Figure 3a. The length of each bar corresponds to an estimated experimental error. The inserted lines and numbers indicate the correspondence between the assumed tilt angles and the calculated curves (see text).

Figure 6. Schematic representations of the structures of HDTTF in the LB film before ((a) trans structure and (b) cis structue)) and after (c) iodine doping (see text).

Figure 5. Reflection absorbance curves for p-polarization (Ap) calculated for the ν(CdC) band at 1541 cm-1 by assuming the tilt angle of the transition moment. The solid bars indicate the observed Ap for the 1541-cm-1 band in Figure 3b. The length of each bar corresponds to an estimated experimental error. The inserted lines and numbers indicate the correspondence between the assumed tilt angles and the calculated curves (see text).

of 30° < φ < 74°. At φ ) 80° Ap has a negative value and its absolute value increases as θ decreases. These trends do not depend on an assumed kbulk in the region of 0.050.2, which is taken by ordinary IR bands. Thus, the result of the simulation indicates that the measurement of Ap and As of an IR band as a function of φ allows us to make an estimate of the direction of the transition moment. Structures and Orientation of HDTTF in the LB Film before Iodine Doping. As already explained, the IR bands due to ν(C3dC4) and ν(C8dC9) of HDTTF in the LB film are observed as positive bands at 1570 and 1541 cm-1 for 30° < φ < 60° in the p-polarized spectra and change to negative ones at φ ) 80° (Figure 3b), while the corresponding bands in the s-polarized spectra (Figure 3a) are hardly observed for 30° < φ < 80°. The As and Ap values observed for the ν(CdC) band at 1541 cm-1 are plotted by solid bars in Figures 4 and 5. The plots lie in the region 10° < θ < 20°, proving that the transition moment is nearly perpendicular to the substrate surface. In the case of TTF itself with the D2h symmetry, the ν(C3dC4) and ν(C8dC9) bands are factorized into ag (1555 cm-1) and b1u (1530 cm-1).17 The IR bands at 1570 and 1541 cm-1 for HDTTF, however, take on a more or less (17) Bozio, R.; Girlando, A.; Pecile, D. Chem. Phys. Lett. 1977, 52, 503.

localized mode of either one of the ν(C3dC4) and ν(C8dC9) vibrations because the ν(C3dC4) mode is mixed with vibrational modes of the substituent at the C3 atom (see Figure 1). Then, we can conclude from the IR external reflection spectra that the TTF ring of HDTTF is virtually perpendicular to the substrate surface with its long axis parallel to the surface, as schematically shown in Figure 6a,b. The IR bands due to ν(CdO) are observed as doublet negative bands at 1724 and 1701 cm-1 in the s-polarized spectra (Figure 3a). In the p-polarized spectra (Figure 3b) the band at 1719 cm-1, which corresponds to the 1724cm-1 band in the s-polarized spectra, gives positive absorbance at a lower angle of incidence (φ), increases its amplitude with φ in the 30-60° region, and at φ ) 80° gives a negative absorbance. On the other hand, the ν(CdO) band at 1701 cm-1 gives a negative band at φ ) 30°, its amplitude decreases as φ increases, and at 80° the amplitude becomes almost negligible. The observation for the higher frequency ν(CdO) band conforms to the simulation for the tilt angle of the transition moment θ < 40° and that for the lower frequency ν(CdO) band to the tilt angle (θ) near 60°. Thus, the CdO group giving the IR band at 1719 cm-1 takes on an orientation more or less perpendicular to the substrate surface, which corresponds to the structure shown in Figure 6a, and the CdO group giving the IR band at 1701 cm-1 takes an orientation more or less parallel to the surface, which corresponds to the structure in Figure 6b. The CdO group in Figure 6a takes a trans conformation relative to the C3dC4 bond, while the CdO group in Figure 6b takes a cis one relative to the C3dC4 bond. Thus, the analysis clearly proves that HDTTF in the LB film consists of both the trans and cis structures. The prominent band at 1290 cm-1 in Figure 3b is due to the C3-C(CdO)-O asymmetric stretching vibration,

IR Reflection Absorption Spectroscopy of TTF

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Figure 7. s-polarized (a) and p-polarized (b) IR external reflection spectra in the 1800-1150-cm-1 region of the iodinedoped LB film of HDTTF on a silicon wafer at angles of incidence, 30°, 40°, 50°, 60°, and 80°. The vertical bar in each spectrum indicates an absorbance scale.

Figure 8. s-polarized (a) and p-polarized (b) IR reflection spectra in the 3100-2700-cm-1 region of the LB film of HDTTF on a silicon wafer at angles of incidence, 30°, 40°, 50°, 60°, and 80°. The vertical bar in each spectrum indicates an absorbance scale.

the transition moment of which is approximately perpendicular to the CdO bond. The φ dependence of the band indicates that the tilt angle of the transition moment is in the region of θ < 40°. Then, the 1290-cm-1 band can be ascribed to HDTTF with the cis structure because, as can be seen from Figure 6b, only the cis strucutre makes the tilt angle in that region. IR External Reflection Spectra in the 1800-1150 cm-1 Region and Structure Change of HDTTF in the LB Film after Iodine Doping. Figure 7a,b illustrates the s- and p-polarized IR external reflection spectra, respectively, observed for the iodine-doped LB film of HDTTF. The s-polarized spectra give only one IR band due to ν(CdO) at 1707 cm-1 in contrast to the spectra observed before iodine doping, which give the doublet bands for ν(CdO). In the p-polarized spectra the 1707cm-1 band gives a negative peak for 30 < φ < 60° and exhibits a positive peak at 80°. These results correspond to the simulation for the tilt angle (θ) of the transition moment near 70°. As can be seen from Figure 7a, the s-polarized spectra measured after iodine doping give prominent negative bands at 1356 and 1252 cm-1, which are absent in the s-polarized spectra before iodine doping (Figure 3a). These bands are the counterparts of the 1360- and 1245-cm-1 bands observed for the (TTF+)2 dimer by Bozio et al.,8 which are assigned to CT-induced vibronic bands of the originally infrared-inactive CdC stretching vibrations; TTF gives these ag modes at 1555 (ν2) and 1518 (ν3) cm-1.17 Figure 7a indicates that the amplitudes of the negative bands at 1356 and 1252 cm-1 decrease on increasing φ. The corresponding bands are observed as negative peaks at 1352 and 1252 cm-1 in the p-polarized spectra (Figure 7b). The amplitudes of these bands remains almost unchanged with increasing φ in the 30°-60° region, and at 80° the bands give positive peaks. These results prove that the transition moments of these bands make the tilt angle (θ) near 70°. Since the transition moments are perpendicular to the plane of the TTF ring,8 the results indicate that the plane is a little tilted (about 20°) with respect to (or nearly parallel to) the surface normal of the substrate.

The analyses of the IR external reflection spectra of the iodine-doped LB film suggest that the CdO group of HDTTF makes a tilt angle of about 70° with respect to the surface normal of the substrate, while the molecular plane of the TTF ring is almost perpendicular to the surface. On the basis of these results we can conclude that the film takes on a structure schematically depicted in Figure 6c. Thus, the doping induces the conversion from the trans to cis structure of HDTTF. The band at 1308 cm-1 in Figure 7b is the counterpart of the band at 1290 cm-1 in Figure 3b, which has been assigned to the C3-C(CdO)-O asymmetric stretching vibration of HDTTF with the cis structure. The φ dependence of the band in Figure 7b is similar to that observed for the corresponding band in Figure 3b, which conforms to the conclusion that HDTTF in the doped LB film takes on the cis strucutre. From Figure 6 we can anticipate that the doping accompanies a large orientation change of the alkyl groups of HDTTF. The next section explains how the incidence angle dependence of the IR external reflection spectra reflect this orientation change. Orientation Change of the Alkyl Chain of HDTTF in the LB Film Induced by Iodine Doping. Figure 8a,b illustrates the φ dependence of the s- and p-polarized IR external reflection spectra, respectively, observed for the LB film of HDTTF before iodine doping in the 31002700-cm-1 region. The bands at 2920 cm-1 and those at 2853 cm-1 are due to νas(CH2) and νs(CH2) of an alkyl chain, respectively.18 The transition moments of the νas(CH2) and νs(CH2) bands are perpendicular to the axis of the alkyl chain. Then, if the alkyl group takes on a uniaxial orientation, the bands of both the νas(CH2) and νs(CH2) modes should show similar φ dependence, as actually observed for the LB films of cadmium stearate.11 (18) The νas(CH2) and νs(CH2) bands are generally observed below 2920 and 2850 cm-1, respectively, for all-trans alkyl chains, while the νas(CH2) and νs(CH2) bands of the alkyl chains containing gauche conformations are generally observed above 2920 and 2850 cm-1, respetively.19 These criteria, however, do not hold for the frequencies observed in Figure 8. Although the reason for this discrepancy has not been clarified yet, we tentatively assumed that the alkyl chain of HDTTF in the LB film takes on the all-trans conformation.

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Figure 10. Reflection absorbance curves for p-polarization (Ap) calculated for the νas(CH2) band at 2920 cm-1 by assuming the tilt angle of the transition moment. The solid bars indicate the observed Ap for the 2918-cm-1 band in Figure 9b. The length of each bar corresponds to an estimated experimental error. The inserted lines and numbers indicate the correspondence between the assumed tilt angles and the calculated curves (see text). Figure 9. s-polarized (a) and p-polarized (b) IR external reflection spectra in the 3100-2700-cm-1 region of the iodinedoped LB film of HDTTF on a silicon wafer at angles of incidence, 30°, 40°, 50°, 60°, and 80°. The vertical bar in each spectrum indicates an absorbance scale.

The 2920-(νas(CH2)) and 2853-cm-1 bands (νs(CH2)) in the p-polarized spectra (Figure 8b), however, exhibit different φ dependence, while all the bands in the s-polarized spectra (Figure 8a) give negative bands. For example, the φ value, at which Ap of the 2853-cm-1 band changes from negative to positive lies between 40° and 50°, while the corresponding angle for the 2920-cm-1 band is larger than 60°. Model calculations explained below indicate that these results can be explained by considering that the alkyl groups take on several orientations, the tilt angles of which are largely different, as in the case of the cis and trans structures shown in Figure 6a,b. Figure 9a,b illustrates the φ dependence of the s- and p-polarized IR external reflection spectra in the 31002700-cm-1 region observed for the iodine-doped LB film. The s-polarized spectra are similar to those observed before iodine doping, while the p-polarized spectra exhibit quite different φ dependence from that observed before the doping. As can be seen from Figure 9b, the νas(CH2) (2920 cm-1) and νs(CH2) (2853 cm-1) bands in the p-polarized spectra give negative peaks at φ ) 30°. The amplitudes of both bands remain almost constant in the region of 30° e φ e 60° and become positive at 80°. To interpret the results in Figures 8 and 9 more explicitly, model calculations of Ap versus φ for the νas(CH2) and νs(CH2) bands were performed, the results being shown in Figures 10 and 11. The kbulk value for the νas(CH2) band was estimated to be 0.35 from the relative intensity, νas(CH2)/νs(CH2), in Figure 2 by assuming the value for νs(CH2) to be 0.17, as already explained.20 The thickness of the iodine-doped LB film was estimated to be 19.5 nm by referring to the result of ellipsometric measurement. The thickness increased appreciably compared to that observed before the iodine doping (10 nm). Although there may exist an error inherent in the ellipsometric measurement for such a thin film, the increase may be explained as being due to the conversion (19) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316. (20) The kbulk values of the νas(CH2) and νs(CH2) bands of the iodinedoped LB film were assumed to be the same as those of the undoped LB film because it is difficult to observe the IR spectrum of the doped HDTTF in an isotropic or dispersed state.

Figure 11. Reflection absorbance curves for p-polarization (Ap) calculated for the νs(CH2) band at 2853 cm-1 by assuming the tilt angle of the transition moment. The solid bars indicate the observed Ap for the 2853-cm-1 band in Figure 9b. The length of each bar corresponds to an estimated experimental error. The inserted lines and numbers indicate the correspondence between the assumed tilt angles and the calculated curves (see text). Table 2. Frequencies (cm-1), Assignments,a and Tilt Angles (θ) of the Transition Moments of the IR Band Observed for the HDTTF-LB Film before and after Iodine Doping 1719 1701 1570 1541 1290

Before Doping ν(CdO), trans form ν(CdO), cis form ν(C3dC4) or ν(C8dC9) ν(C3dC4) or ν(C8dC9) νas(C3-C(dO)-O), cis form

θ < 40° θ ≈ 60° 10° < θ < 20° 10° < θ < 20° θ < 40°

2920 2853 1707 1352 1252 1308

After Doping νas(CH2), cis form νs(CH2), cis form ν(CdO), cis form CT-induced band CT-induced band νas(C3-C(dO)-O), cis form

60° < θ < 70° 60° < θ < 70° θ ≈ 70° θ ≈ 70° θ ≈ 70° θ ≈ 40°

a

Abbreviations are the same as those in Table 1.

from the trans to cis structure induced by the doping because as can be seen from Figure 6, the tilt angle of the alkyl chain in the cis structure is much smaller than that in the trans structure. As can be seen from Figures 10 and 11, the φ value, at which Ap changes from negative to positive, depends critically on the tilt angle (θ). In addition, comparison between the results of calculation for a common value of the tilt angle indicates that the

IR Reflection Absorption Spectroscopy of TTF

amplitude of Ap for the νas(CH2) band is much larger than that for the νs(CH2) band. These results suggest that a linear combination of the Ap versus φ curves for the θ values estimated from the trans and cis structures (see Figure 6a,b) explains the φ dependence of the νas(CH2) and νs(CH2) bands in Figure 9b. The calculation based on this idea, however, has not been performed because of difficulty in determining effective thicknesses of the trans and cis structures in the LB film. The experimental values observed for the νas(CH2) and νs(CH2) bands in Figure 9b are plotted by solid bars in the figures. The plots in both figures lie between the calculated curve for θ ) 60° and that for θ ) 70°, indicating that the tilt angle of the alkyl group in the doped LB film is between 30°and 20° ()180° - θ), which conforms to the proposed model for the cis form in Figure 6b.

Langmuir, Vol. 14, No. 17, 1998 4879

Conclusion The measurement of the φ dependence of the s- as well as p-polarized IR external reflection spectra and the analysis of the spectra based on theoretical calculations allow us to determine the directions of the transition moments of infrared bands of the HDTTF-LB film before and after iodine doping, as summarized in Table 2. The results give more explicit and quantitative information about the structure and orientation changes of HDTTF induced by the doping than the earlier works.3,6 Thus, the IR external reflection spectroscopy is one of the most powerful methods for structural investigation of functional thin films deposited on nonmetallic substrates such as a silicon wafer. LA980439T