3610
Langmuir 1995,11, 3610-3616
An Infrared Study on Order-Disorder Transitions in Evaporated Films of 2=Dodecyl-,2-Pentadecyl-,and 2-0ctadecyl-7,7,8,8-tetracyanoquinodimethane. A Comparison of the Transitions with Those in the Corresponding Langmuir-Blodgett Films Akihiro Nakagoshi, Yan Wang, and Yukihiro Ozaki* Department of Chemistry, School of Science, Kwansei-Gakuin University, Uegahara, Nishinomiya 662, Japan
Keiji Iriyama Division of Biochemistry, Institute of Medical Science, The Jikei University School of Medicine, Nishi-Shinbashi, Minato-ku, Tokyo 105, Japan Received February 8, 1995. I n Final Form: July 31, 1995@ Order-disorder transitions of evaporated films of 2-dodecyl-, 2-pentadecyl-, and 2-octadecyl-7,7,8,8tetracyanoquinodimethane (dodecyl-TCNQ,pentadecyl-TCNQ,and octadecyl-TCNQ)have been investigated
by using infrared spectroscopy. Four kinds of evaporated films have been prepared for octadecyl-TCNQ; two of them have been deposited on CaF2 plates at 5 and 15 "C and the other two on Au-evaporated glass slides at the same temperatures. For all the evaporated films of odadecyl-TCNQ,a CH2 symmetric stretching band of the hydrocarbon chain shows an abrupt upward shift near 125 "C, revealing that the orderdisorder transition occurs near 125 "C irrespective of the substrates used and temperatures at which the sample has been deposited. However, the molecular orientation of the hydrocarbon chain in the evaporated films depends upon the substrates and temperatures. The molecular axis of the hydrocarbon chain is nearly parallel with the substrate surface with its molecular plane nearly perpendicular to the surface in the evaporated films of octadecyl-TCNQdeposited on the CaFz plate and Au-evaporated glass slide at 5 "C. On the other hand, it is tilted with respect to the normal in the films prepared on the Au-evaporated glass slide and CaF2 plate at 15"C, respectively. There is no or little pretransitional change in the frequency of the CHz stretching symmetric band, but its bandwidth undergoes pretransitional alterations for all the octadecyl-TCNQ evaporated films. These observations lead us to the conclusion that the alkyl chain mobility begins to increase before their conformational disorder takes place. As in the case of corresponding Langmuir-Blodgett (LB) films, the order-disorder transition of the octadecyl-TCNQ evaporated film deposited on the CaF2 plate at 15 "C occurs at a higher temperature (125 "C) than that (121 "C) of the dodecyl-TCNQevaporated film prepared under the same conditions, but the evaporated film of pentadecylTCNQ with the odd-numbered hydrocarbon chain exhibits a transition temperature (121 "C) similar to that of the dodecyl-TCNQ evaporated film with the shorter even-numbered hydrocarbon chain. Cyclic temperature treatment up to 120 "C performed for the octadecyl-TCNQevaporated film deposited on the CaFz plate at 5 "C changes the molecular orientation largely; the hydrocarbon chain becomes nearly perpendicular to the substrate surface after the cycle to 120 "C.
Introduction The purpose of the present paper is to provide new insight into molecular orientation and structure and order-disorder transitions in evaporated films of 2-dodecyl-, 2-pentadecyl-, and 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (here after, we refer to them as dodecylTCNQ, pentadecyl-TCNQ, and octadecyl-TCNQ; Figure 1).For the last decade organic thin films containing TCNQ derivative as the strong accepter of an organic anionradical salt have attracted many investigators because some of them have shown high cond~ctivity.l-'~ I n order to give basic knowledge for unraveling the structure-
* Author to whom correspondenceshould be directed. Fax: +81798-51-0914. E-mail:
[email protected]. Abstract published in Advance A C S Abstracts, September 1, 1995. (1)Delhaes, P.; Yartsev, V. M. Spectroscopy of New Materials; Advances in Spectroscopy; Clark, H. J. R., Eds.; John Wiley & Sons: Chichester, 1993;Vol. 22, p 199. (2)Ruaudel-Teixier, A.;Vandevyer, M.; Barraund, A. Mol. Cryst. Liq. Cryst. 1986,120, 319. (3)Vandevyer, A.; Richard, J.; Barraund, A,; Ruaudel-Teixier, A.; Lequan, M.; Lequan, M. J . Chem. Phys. 1987,87,6754. (4) Nakamura, T.; Takei, F.; Tanaka, M.; Sekiguchi, T.; Maeda, E.; Kawabata, T. Chem. Lett. 1986,709. (5) Kawabata, Y.; Nakamura, T.; Matsumoto, M.; Sekiguchi, T.; Komizu, H.; Maeda, E.; Saito, G. Synth. Met. 1987,19, 663. @
Figure 1. Structure of 2-dodecyl- (R = C12H25),2-pentadecyl(R = C 1 a l ) ,and 2-octadecyl-7,7,8,8-tetracyanoquinodimethanes (R = C18H37)(They are referred to as dodecyl-,pentadecyl-,and octadecyl-TCNQ,respectively, in this paper).
function relationship of the conducting organic thin films based on TCNQ or its derivatives, we have been investigating the structure of Langmuir-Blodgett (LB) and evaporated films having a TCNQ chromophore by using infrared and Raman We, first, studied molecular orientation and structure in the LB films of (6)Matsumoto, M.; Nakamura,T.; Maeda, E.; Kawabata,Y.;Ikegami, K.; Kuroda, S.; Sugi,M.; Saito, G . Thin Solid Films 1988,160,61. (7)Richard, J.;Vandevyer, M.; Barraund, A,; Morand, J. P.; Lapouyade, R.; Delhaes, P.; Jacquinot, J.F.; Roulliary, M. J . Chem. SOC.,Chem. Commun. 1988,754. (8) Tachibana, H.;Komizu, H.; Nakamura, T.; Matsumoto, M.; Kawabata, Y.; Kato, T.Chem. Lett. 1989,841. (9)Richard, J.; Vandevyer, P.; Leiseur, P.; Ruaudel-Teixier, A.; Barraund, A,; Bozio, R.; Pecile, C. J . Chem. Phys. 1987,86, 2428.
0743-746319512411-3610$09.00/0 0 1995 American Chemical Society
Letters octadecyl-TCNQ,14and then compared them with those in the LB films ofdodecyl-and pentadecyl-TCNQ.16Next, order-disorder transitions in the LB films of the three kinds of alkyl-TCNQ were explored by using infrared spectroscopy.16 We also studied LB films of chargetransfer complex of octadecyl-TCNQand malachite green prepared from adsorption Langmuir monolayers.l7 Our first paper concerning evaporated films of alkyl-TCNQ dealt with molecular orientation and structure; comparison of the molecular orientation and structure between the LB and evaporated films was also discussed.ls In our last paper in this series we presented dependence of molecular aggregation and orientation on the surface pressure and number of monolayers in LB films of octadecyl-TCNQstudied by ultraviolet -visible (W-vis) spectros~opy.~~ Through the above papers, we were able to delineate the structure of the LB and evaporated films of the three kinds of alkyl-TCNQ in some detail; not only the structure but also the interactions between alkyl-TCNQ and subs t r a t e ~ ,the ~ ~degree , ~ ~ of the charge transfer,17 thermal stability,16and so on were investigated. The present study aims a t exploring further the molecular orientation and structure in the evaporated films of dodecyl-,pentadecyl-, and octadecyl-TCNQ and investigating their orderdisorder transitions. To do that, we have prepared the evaporated films of dodecyl- and pentadecyl-TCNQ on CaF2 plates at 15 "C and those ofoctadecyl-TCNQon CaFz or Au-evaporated glass slide a t 5 and 15 "C. Infrared spectra of those six kinds of evaporated films have been measured as a function of temperature. Interestingly, the four kinds of the evaporated films of octadecyl-TCNQ have shown significantly different spectra even a t room temperature, indicating that the molecular orientation and structure in the evaporated films depend upon the substrates and temperatures for the film deposition. The order-disorder transitions studied here are much more detailed than those for the corresponding LB films in terms of the number of measuring points near the transition. The cyclic temperature experiments have also been performed for the films of octadecyl-TCNQ on the CaFz plates. To our best knowledge, the order-disorder transitions in evaporated films have been investigated very little, and comparative studies ofthe order-disorder transitions between the LB and evaporated films have been virtually nonexistent, so far.
Experimental Section SamplePreparation. Dodecyl-,pentadecyl-, and octadecylTCNQwere purchased from the Japanese Research Institute for Photosensitizing Dyes Co., Ltd., and used without further purification. The thin-layer chromatographic examinations revealed that the dyes did not contain any other colored (10)Ruaudel-Teixier, A,;Vandevyer, M.; Roulliary, M.; Bourgion, P. J.; Barraund, A.; M. R. J . Phys. D: Appl. Phys. 1990,23,987. (11)Stonikov, S. T.; Troitsky, I. V.; Valter, E. R.; Karlivan, A. G.; Neiland, 0. Ya. Thin Solid Films 1989,179,267. (12)Richard, J.;Delhaes, P.; Vandevyer, M.New J.Chem. 1991,15, 137. (13)Vandevyer, M.; Ruaudel-Teixier, A.; Palacin, S.;Bourgion,P. J.; Barraund, A,; Bozio, R.; Meneghetti, M.; Pecile, C. Mol. Cryst. Liq. Cryst. 1990, 187,327. (14)Kubota, M.; Ozaki,Y.; Araki, T.; Ohki, S.;Iriyama, K. Langmuir 1991 7 . , 774 . . -. ----I
(15)Terashita, S.; Nakatsu, K.; Ozaki, Y.; Mochida, T.; Araki, T.; Iriyama, K. Langmuir 1992,8,3051. (16)Terashita, S.; Ozaki, Y.; Iriyama, K. J.Phys. Chem. 1993.97, 10454. (17)Terashita, S.; Ozaki, Y.; Yageta, H.; Kudo, K.; Iriyama, K. Langmuir 1994,10,1807. (18)Nakagoshi, A,;Terashita, S.; Ozaki, Y. Langmuir 1994,10,779. (19)Wang, Y.; Ozaki, Y.; Iriyama, K. Langmuir 1996,11, 705.
Langmuir, Vol. 11,No.10,1995 3611
a)
I
I
m 14OOC
3-
I -
3000
2600
2200
I600
WAVENUMBERlcm-I
1400
3000
13OC
2600 2200 1800 WAVENUMBERlcm.1
1400
Figure 2. Temperature-dependent changes in infrared transmission spectra of the evaporated films of octadecyl-TCNQ deposited on CaFz plates at 5 (a) and 15 "C (b).
components. The vacuum evaporated films of dodecyl-, pentadecyl-,and octadecyl-TCNQwere prepared at 5 or 15"Con CaF2 plates or Au-evaporated glass slides by using a JEOL JEE-4X vacuum evaporator. The film thickness was estimated to be about 30 nm by a Nippon Shinku ULVAC deposition monitor control,Model CRTM. The CaFzplates and Au-evaporatedglass slides had been subjected to ultrasonifications in 50% aqueous solution of DCN 90 of Decon Laboratories, Ltd. (the alkaline surfactant), and then in distilled water. Spectroscopy. Infrared transmission and reflection-absorption (RA) spectra of the evaporated films were measured on JEOL JIR-100 and JIR-6500 FT-IR spectrometers both equipped with a MCT detector, respectively. The spectra were taken at a 4 cm-I resolution and, typically, 300 interferograms were coadded to yield the spectra of a high signal-to-noise ratio. For the reflection-absorption (RA)measurements, a JEOL IR-RSC 110 reflection attachment was employed at the incidence angle of 80", together with a JEOL IR-OPT02 polarizer. In order to measure the infrared spectra at elevated temperatures, a substrate on which the evaporated film had been deposited was inserted into a sample holder in the copper block which had a ceramic heater within. Temperature control was achieved by using an Omron E5T temperature controller which gives a temperature stability of better than f0.3 "C. The temperature was monitoredwith a thermo-coupleconnectedwith the sample holder and was raised at about 0.5 "C mix1. The bandwidths of CH2 symmetric stretching and CIN stretching modes of the evaporated films were determined by computing the widths at one-half of the peak heights. The band areas were calculated by a computer system for infrared measurements and analyses on a JEOL JIR-6500FT-IR spectrometer. The precision and reproducibility of measuring the temperature-induced changesin infrared frequencies and bandwidth are between ~t0.2 cm-1.
Results Parts a and b of Figure 2 show temperature-dependent changes in infrared transmission spectra of evaporated films of octadecyl-TCNQ deposited on CaF2 plates at 5 and 15 "C, respectively. A band due to a CH2 antisymmetric stretching mode of the hydrocarbon chain is identified near 2922 cm-l as a doublet (see, Figure 3). In our previous study on LB films of octadecyl-TCNQ the doublet was thought to arise from the crystal field ~p1itting.l~ A peak a t 2850 cm-I is assigned to its CH2 symmetric stretching mode. It should be noted that the two spectra measured at 30 "C show large differences in band intensities. Particularly striking is the difference
Letters
3612 Langmuir, Vol. 11, No. 10, 1995
II
I 3050
2850 2850 WAVENUMBERlcm.1
2750
I
3050
I
I K I
I
I
2950 2850 WAVENUMBERtcm.1
I
I
2750
Figure 3. 3100-2700 cm-l regions of infrared transmission spectra in the temperature range from 110 to 130 "C of the evaporated films of octadecyl-TCNQ deposited on the CaFz plates at 5 (a) and 15 "C (b).
in the relative intensity of the CH2 antisymmetric and symmetric bands. This observation suggests that molecular orientation and structure of the hydrocarbon chain in the evaporated films change with the temperature a t which they are prepared. The 3100-2700 cm-l regions are almost unchanged in the temperature range from 30 to 90 "C. Above 90 "C, the bands due to the CH2 antisymmetric and symmetric stretching modes become broad significantly. Remarkable changes in their frequencies, bandwidths, and intensities occur upon going from 120 to 130 "C. It is also notable that the CH2 antisymmetric stretching band becomes a singlet a t 130 "C. A band at 2222 cm-l is assignable to a CSN stretching mode of the TCNQ chromophore and those a t 1547 and 1529 cm-I are attributed to its C=C stretching modes.14 Frequencies and bandwidths of the bands a t 2222,1547, and 1529 cm-l do not show large changes in the temperature range from 30 to 120 "C, but they do show marked changes between 120 and 130 "C. A band due to a CH2 scissoring mode of the hydrocarbon chain appears near 1465 cm-l as a doublet a t 30 "C. This band becomes a singlet a t 130 "C. Enlargements of the 3100-2700 cm-l region of the infrared spectra in parts a and b of Figure 2 are shown in parts a and b of Figure 3, respectively. This figure clearly demonstrates that the bands due to the CH2 antisymmetric and symmetric stretching modes show a significant upward shift between 124 and 126 "C with considerable band broadening. Parts a and b of Figure 4 compare temperaturedependent changes in infrared RA spectra of evaporated films of octadecyl-TCNQdeposited on Au-evaporated glass slides a t 5 and 15 "C, respectively. It is noted that again intensity patterns in the two spectra a t 30 "C are remarkably different from each other. The relative intensity of a band a t 2848 cm-l due to a CH2 symmetric stretching mode in Figure 4a increases with temperature up to 90 "C but decreases above 90 "C. Infrared transmission spectra in the 3 100-2700 cm-l region of an evaporated film of dodecyl-TCNQ on a CaFz plate are shown as a function of temperature in Figure 5. An evaporated film of pentadecyl-TCNQ on a CaFz plate gave similar spectra to those in Figure 5. Tem-
3000
2600
2200
1800
1400
3000
WAVENUMBERlcm-1
2600 2200 1600 WAVENUMBER/cm.l
1400
Figure 4. Temperature-dependent changes in infrared RA spectra of the evaporated films of octadecyl-TCNQ deposited on Au-evaporated glass slides at 5 (a) and 15 "C (b).
3050
2950 2850 WAVENUMBER/cml
2750
Figure 5. 3100-2700 cm-I region of infrared transmission spectra in the temperature range from 110 to 130 "C of the evaporated film of dodecyl-TCNQdeposited on the CaFz plate at 15 "C.
perature-dependent spectral changes observed for the dodecyl- and pentadecyl-TCNQ evaporated films are, in general, similar to those observed for the evaporated film of octadecyl-TCNQdeposited on the CaF2 plate a t 15 "C (Figure 3b). However, of particular note is that the temperature a t which the frequencies and bandwidths of the two CH2 stretching bands show marked changes is dependent upon the length of the hydrocarbon chain; for example, the band due to the CH2 symmetric stretching mode shifts a t 122 "C for the evaporated film of dodecylTCNQ but the corresponding band does a t 126 "C for the evaporated film of octadecyl-TCNQ. Parts a-c of Figure 6 illustrate temperature dependence of the frequency, bandwidth, and band area of the CH2 symmetric stretching band, respectively, for the evaporated films of octadecyl-TCNQdeposited on the CaF2plates
Langmuir, Vol. 11, No.10, 1995 3613
Letters
2853
-
-
A A
"
99
A
A
E2852 -
AA
$52851 -
'AA~
m
A
t-
U
f
*..'A.-*bbbbbbb.
22850 -
r2.
b A
2849 AAA
2848
15 14
..
'
f
m
:,-
Q 12
E
A
A
A I
A
A
A..
I 50
70
A
A
9
30
A
A
A
.. ..
I
I
'0
m 5'
11 10
A
I
7E 13
90
Temperalurelk
110
130
1 30
50
70
90
Temperalure/oc
110
130
Figure 6. Temperaturedependences of the frequency (a),bandwidth (b), and band area (c) of the CHz symmetric stretching band of the octadecyl-TCNQ evaporated films deposited on the CaFz plates at (A)and 15 "C (A).
a t 5 and 15 "C. The two films show very similar tendency for the changes in the frequency and bandwidth. The frequency changes abruptly a t 124 "C, and there are no or little pretransitional changes below 124 "C. In contrast, the bandwidth shows clear pretransitional changes between 90 and 124 "C. There are remarkable differences in the changes of the band area between the two films. The band area of the evaporated film prepared a t 15 "C shows a gradual pretransitional change with a sudden change a t 124 "C while that of the film deposited at 5 "C decreases up to 90 "C, increases between 90 and 120 "C, and finally shows a n abrupt decrease. Figure 7 compares temperature dependences of the frequency and bandwidth ofthe CH2 symmetric stretching mode, respectively, among the evaporated films of dodecyl-, pentadecyl-, and octadecyl-TCNQ deposited on the CaFz plates at 15 "C. Three important results can be obtained from Figure 7a. One is that the frequency of the CHz stretching band decreases with the length of the hydrocarbon chain a t room temperature. Another is that the dodecyl-, pentadecyl-, and octadecyl-TCNQ evaporated films have order-disorder transition near 121,121, and 125 "C, respectively. Yet another is that the transitions occur rather suddenly not gradually. Figure 8b also gives three noted results. First, the bandwidth of the three kinds of evaporated films show a large change a t similar temperatures to those for the frequency. Second, in contrast to the frequency, there are pretransitional changes for the bandwidth in the temperature range from 90 to 120 "C. Third, the bandwidth of pentadecyl-TCNQ with the odd-numbered hydrocarbon chain is wider than those of dodecyl- and octadecyl-TCNQ with the evennumbered chain. Parts a and b of Figure 8 show infrared spectral changes in the 3100-2700 cm-l region for the evaporated films of octadecyl-TCNQprepared on the CaFz plates a t 5 and 15
"C, respectively, subjected to cyclic thermal treatments. First, the spectra were measured a t 30 "C, and then the temperature of the samples was raised to 120 "C and the spectra were obtained. Next, the temperature was lowered to 30 "C and the subsequent spectra recorded. The last spectra are similar to the original ones, respectively, but it should be noted that the relative intensity of the CH2 symmetric stretching band a t 2848 cm-l increases significantly in Figure 8a after the thermal treatment up to 120 "C. Next, the temperature was raised to 130 "C, and the spectra were obtained.
Discussion Molecular Orientation and Structure in the Evaporated Films of Octadecyl-TCNQ. Four kinds of evaporated films have been prepared for octadecylTCNQ; two of them have been deposited on CaF2 plates at 5 and 15 "C and another two on Au-evaporated glass slides a t the same temperatures. Since molecular orientation and structure in an evaporated film change with substrates used and temperature a t which the film is deposited,20v21 it is very important to prepare the films of octadecyl-TCNQ under the various conditions. Molecular orientation of the hydrocarbon chain in an evaporated film can be investigated from intensities of its CH2 antisymmetric and symmetric stretching bands. 16,182-24If the alkyl chain is nearly perpendicular (20) Matsuzaki, F.; Inaoka, K.; Okada, M.; Sato, K. J . Cryst. Growth 1984, 69, 231.
(21) Inoue, T.; Yase, K.; Inaoka, K.; Okada, M. J.Cryst. Growth 1987, 83, 306.
(22) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (23) Chollet, P. A,; Messier, J.;Rosilio, C. J . Chem. Phys. 1976, 64, 1042. (24)Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J . Phys. Chem. 1990,94, 62.
Letters
3614 Langmuir, Vol. 11, No. 10, 1995 2855 DDDD
2854 2853
1-
2848 -
0
A
k
i
r igure J . neiauonsnip DeLween molecular orientation or cne
hydrocarbon chain and spectral pattern in the 3100-2700 cm-' region of infrared transmission and FL4 spectra.
10 1
30
50
70
90 Temperaturek
110
130
I
Figure 7. Comparison of temperature dependences of the frequency (a)and bandwidth (b) ofthe CH2 symmetric stretching band among the evaporated films of dodecyl- (01, pentadecyl(O), and octadecyl-TCNQ(A) deposited on CaF2 plates at 15 "C.
(a)
3050
I
2950 2850 WAVENUMEERlcm.1
2750
b) I
3050
2950 2850 WAVENUMBERlcm-1
2750
Figure 8. Infrared spectral changes in the 3100-2700 cm-I regions for the evaporated films of octadecyl-TCNQdeposited on the CaF2 plates at 5 (a)and 15 "C (b),subjected to the cyclic thermal treatments.
to the substrate surface, both bands should appear strongly and weakly in the transmission and RA spectra, respectively. In contrast, if both molecular axis and plane of the alkyl chain are nearly parallel with the substrate surface, the band due to the CH2 symmetric stretching mode is observed strongly and weakly in the transmission and RA spectra, respectively, while the band due to its asymmetric counterpart is seen inversely. In addition, if the molecular axis of the alkyl chain is nearly parallel with the substrate surface with its molecular plane nearly perpendicular it, completelyreverse spectral patterns from
those right above are observed. Figure 9 summarizes the relationship between the molecular orientation of the alkyl chain and spectral patterns in the 3000-2800 cm-l region. In order to discuss the molecular orientation of the alkyl chain, a reference spectrum is essential. Therefore, we used a n infrared transmission spectrum of octadecylTCNQ in a solid state as the reference spectrum (see Figure 2 in ref 14). The intensity ratio of the two CH2 stretching bands a t 2918 and 2848 cm-' (1284$12918) in the reference spectrum is 0.59. This ratio can be used as a standard indicator. The values of this indicator are 0.67 and 0.28 in the spectra of the evaporated films of octadecyl-TCNQ prepared on the CaFz plates a t 15 and 5 "C, respectively (Figure 9). Therefore, it may be concluded that the alkyl chain is nearly perpendicular to the substrate surface or tilted a little in the evaporated film deposited on the CaFz plate a t 15 "C while it is nearly parallel with the surface with its molecular plane nearly perpendicular to it in the evaporated film on the CaF2 plate deposited a t 5 "C. It is rather difficult to find a good reference for RA spectra, for which thin films must be prepared in the metal surface. However, we can still employ the above ratio as the standard. The ratios are 0.75 and 1.06 in the RA spectra of the evaporated films deposited on the Au-evaporated glass slides a t 15 and 5 "C, respectively (Figure 9). This means that the CHZ symmetric stretching bands are enhanced largely. Therefore, the observations indicate that the alkyl chain is nearly parallel with the surface with its molecular plane nearly perpendicular to it and tilted considerably in the evaporated films on the Auevaporated glass slide deposited at 5 and 15 "C, respectively. It is known from electron microscope observations that the alkyl chain is parallel with a substrate surface in evaporated films of long-chain fatty acids deposited a t temperatures below about 5 "C while it is perpendicular to the surface in those deposited a t temperatures above about 20 oC.20,21 Therefore,the present results are in good agreement with those results. It has become clear from the present study that the molecular orientation of the hydrocarbon chain depends upon both substrate and temperature for the film deposition but that the temperature effect is more critical for the orientation. Order-Disorder Transitions in Evaporated Films of Octadecyl-TCNQ Deposited on CaFz Plates. Order-disorder transition in a n evaporated film of alkylTCNQ can be investigated by the frequency, bandwidth, and band area of a CHz symmetric stretching band of the hydrocarbon chain.16 Usually, these properties of a CHz
Letters
Langmuir, Vol. 11, No. 10, 1995 3615
antisymmetric stretching band are also useful to explore the order-disorder transition, but it splits into two bands here (Figures 3 and 5). Therefore, we mainly use the symmetric stretching band for investigating the orderdisorder transition. The frequency of the CH2 symmetric stretching band is sensitive to the degree of conformational order of the hydrocarbon When the chain is highly ordered (trans-zigzag conformation), the band appears a t 2848 cm-l, while if conformational disorder is included in the chain, it shifts upward up to 2856 cm-l depending upon the content of gauche conformers. The results in Figures 5 and 6a clearly reveal that the order-disorder transition occurs near 125 "C for both evaporated films of octadecyl-TCNQdeposited on the CaF2 plates a t 5 and 15 "C. The frequency of the CH2symmetric stretching band is unchanged below the order-disorder transition except for a 1 cm-' upward shift near 60 "C (Figure 6a), indicating that there is no or little pretransitional disorder in the molecular conformation. In contrast to the frequency, the bandwidth shows pretransitional changes above about 90 "C (Figure 6b). Since the bandwidth of the CH2 symmetric stretching band reflects the results in the mobility of the hydrocarbon chain,l Figure 6b suggest that the mobility of the chain begins to increase before its conformational disorder occurs. As for the band area, the two evaporated films show largely different temperature dependences (Figure 6c). The band area ofthe CHZstretchingband ofthe evaporated film prepared at 15 "C decreases gradually up to 124 "C and then shows a n abrupt decrease. This temperaturedependent behavior bears a close resemblance to that of the bandwidth of the same band. On the other hand, the band area of the CH2 stretching band of the evaporated film deposited a t 5 "C exhibits a rather curious temperature dependence. An alteration in the band area of CH2 symmetric stretching mode results from a change in the chain packing density andor that in the orientation of a hydrocarbon chain.27It is rather unlikely that the packing density changes significantly in the pretransitional stage where the mobility of the alkyl chain is still weak. Therefore, the marked increase between 90 and 115 "C in the band area of the CH2 stretching band of the evaporated film prepared at 5 "C may be due to the change in molecular orientation of the hydrocarbon chain. It seems that the chain is nearly parallel with the substrate surface below 90 "C, but it takes random orientation above 90 "C. Probably, the alterations in the band area observed above 124 "C for both films are probably partly due to the decrease in the packing density and partly due to the molecular orientational changes. Cyclic Temperature Treatment of Evaporated Films of Octadecyl-TCNQ Deposited upon CaFz Plates. The experiments of cyclic thermal treatments provide important knowledge about transitional and pretransitional behaviors of a n evaporated film. As seen in Figure 8, after the first cycle to 120 "C both spectra of the evaporated films prepared a t 5 and 15 "C are very close to the original spectrum of the film deposited a t 15 "C in terms ofthe frequencies, bandwidths, and band areas ofthe CHz antisymmetric and symmetric stretchingbands. This result suggests that the alkyl chain becomes nearly perpendicular to the surface even in the film prepared a t 5 "C upon the cyclic temperature treatment. Probably the molecular orientation is changed by the annealing 6327
(25) Sapper, H.; Cameron, D. G.; Mantsch, H. H. Can. J.Chem. 1981,
59 - - , -2642 - -- .
(26) Cameron, D. G.; Martin, A.; Moffatt, D. J.; Mantsch, H. H. Biochemistry 1985, 24, 4355. (27) Tian, Y. J. Phys. Chem. 1991, 95, 9985.
effect. The CH2 stretching band frequencies recover even after raising the temperature to 130 "C, which is above the transition point, although the bandwidths and band areas of both CH2 stretching bands undergo significant changes. It seems therefore that the hydrocarbon chain becomes highly ordered again after the cyclic temperature treatment up to 130 "C. Comparison of Order-Disorder Transitionsin the Evaporated Films of Dodecyl-, Pentadecyl-, and Octadecyl-TCNQ on CaFz Plates. In our previous study of order-disorder transitions in LB films ofdodecyl-, pentadecyl-, and octadecyl-TCNQ, it was found that the order-disorder transition in the LB film of octadecylTCNQ with the longer even-numbered hydrocarbon chain occurs a t higher temperature (-125 "C) than that (-115 "C) in the LB film of dodecyl-TCNQwith the shorter evennumbered hydrocarbon chain, but that the LB film of pentadecyl-TCNQ with the odd-numbered hydrocarbon chain shows a transitional temperature (-1 15 "C) similar to that of the LB film of dodecyl-TCNQ.16The present results in Figures 3,5, and 6 reveal that the order-disorder transitions of the evaporated films of dodecyl-,pentadecyl-, and octadecyl-TCNQtake place a t 121,121, and 125 "C, respectively. These transition temperatures are close to those of the corresponding LB films.I6 In the present study the infrared spectrum was measured a t every 2 "C near the transition points but in the previous study it was obtained at every 20 "C. Accordingly,it is rather difficult to compare the transition temperature strictly between the evaporated and LB films. However, it may be concluded that the transition temperatures for the evaporated films of dodecyl-, pentadecyl-, and octadecyl-TCNQ (121, 121, 125 "C) are closer to those (122, 122, 127 "C) for their powders determined by differential scanning calorimetry16than those (-115, -115, and -125 "C) for the LB films. The order-disorder transitions of the evaporated films of alkyl-TCNQ may be similar to those of their bulk states. In any case it seems very reasonable that the evaporated films of octadecyl-TCNQ with the longer hydrocarbon chain show a higher transition temperature than the evaporated films with shorter hydrocarbon chains. However, it should be noted that the evaporated film of pentadecyl-TCNQshows a transition temperature similar to that of the evaporated film of dodecyl-TCNQ, although the former has a longer hydrocarbon chain than the latter. The evaporated film of pentadecyl-TCNQ shows another specific feature. The bandwidth of the CH2 symmetric stretching mode of the evaporated film of pentadecylTCNQ is wider than those of the evaporated films of dodecyl- and octadecyl-TCNQ at room temperature, indicating that the mobility of the hydrocarbon chain in the pentadecyl-TCNQ film is larger than those in the rest. These specific features of the film of pentadecyl-TCNQ were also observed for its LB films.16 They might be concerned with the fact that it has the hydrocarbon chain with the fact that pentadecyl-TCNQ has the hydrocarbon chain with a n odd number of carbon atoms while the rest have hydrocarbon chains with a n even number of carbon atoms.16
Conclusion The present study has provided new insight into the molecular orientation and order-disorder transitions in the evaporated films of dodecyl-, pentadecyl-, and octadecyl-TCNQ. For the molecular orientation of the hydrocarbon chain in the evaporated films of octadecyl-TCNQ deposited on the CaF2 plates and Au-evaporated glass
Letters
3616 Langmuir, Vol. 11, No. 10, 1995
slides a t 5 and 15 "C, the following conclusions can be reached. (i) The molecular orientation of the hydrocarbon chain changes with the temperature and substrate for the film deposition. The temperature seems to be a particularly important factor for determining the molecular orientation. The hydrocarbon chain is nearly parallel with the substrate surface with its molecular plane nearly perpendicular to the surface in the evaporated films of octadecyl-TCNQ deposited a t 5 "C irrespective of the substrates used. On the other hand, it is tilted with respect to the surface normal in the evaporated films prepared on the CaFz plate and Au-evaporated glass slide at 15 "C, and the tilt is larger for the latter. Comparison of the order-disorder transitions among the evaporated films of dodecyl-, pentadecyl-, and octadecyl-TCNQ deposited on the CaFz plates a t 15 "C has given the following conclusions.
(i) The evaporated films of dodecyl-, pentadecyl-, and octadecyl-TCNQ show the order-disorder transition a t 121,121, and 125 "C, respectively. These temperatures are identical within experimental errors to those observed for their powder samples. It seems that the orderdisorder transitions in the evaporated films of the three kinds of alkyl-TCNQ are similar to those of their bulk states. (ii) As in the case of the LB films,16pentadecyl-TCNQ with the odd number of carbon atoms shows two specific features in the order-disorder transition. One is that it shows a similar transition temperature to the dodecylTCNQ evaporated film, although the former has a longer hydrocarbon chain than the latter. Another is that the mobility of the hydrocarbon chain in the pentadecyl-TCNQ evaporated film is larger than those in the rest. LA950097G
