Langmuir 1995,11, 705-707
705
Dependence of Molecular Aggregation and Orientation on the Surface Pressure and Number of Monolayers in Langmuir-Blodgett Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane Studied by Ultraviolet-Visible Spectroscopy Yan Wang and Yukihiro Ozaki* Department of Chemistry, 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 July 11, 1994. In Final Form: December 14, 1994@ Ultraviolet-visible (W-vis) absorption spectroscopyhas been employed to investigate surface pressure and number of monolayer dependences of molecular aggregation and orientation in Langmuir-Blodgett (LB) films of the title compound (abbreviated as octadecyl-TCNQ). It has been found that the LB films consist mainly of species with stacked structure of TCNQ chromophore and that the percentage of the species changes little with the surface pressure and number of monolayers. Tilt angles of the TCNQ chromophore in the LB films have been calculated from the measurements of their polarized W-vis spectra. The results show that the orientation of the TCNQ chromophore depends on the number of monolayers and surface pressure.
Introduction In recent years Langmuir-Blodgett (LB) films have been explored extensively because the use of LB films enables one to control the construction offunctional organic materials at the molecular A variety of molecular devices have been proposed that are based on LB films, such as films with nonlinear optical properties, photovoltaic cells, piezoelectric and pyroelectric devices, resistance and conducting materials, and chemical and biological sensors. We have been studying the molecular orientation and structure in LB films with a tetracyanoquinodimethane (TCNQ) chromophore by using infrared and Raman spectroscopy to provide basic knowledge for understanding the structure-function relationship of conducting LB films based on TCNQ or its Thus far, we have investigated (1)the molecular orientation and structure of TCNQ LB films and their dependences upon the length of the hydrocarbon chain s u b ~ t i t u t e d ,(2) ~ , ~the orderdisorder transitions in the TCNQ LB films,5 and (3) comparison of the molecular orientation and structure between the LB and evaporated films.6 The present study has two purposes. One purpose is to investigate molecular aggregation in the LB films of octadecyl-TCNQ and its dependency on the surface pressure and number of monolayers. Another purpose is to examine quantitatively the molecular orientation of
* To whom correspondenceshould be addressed. *Abstract published in Advance ACS Abstracts, February 1, 1995. (1)Roberts, G. G.Langmuir-Blodgett Films; Plenum Press: New York and London, 1990. (2)Ulman, A. An Introduction to Ultrathin Organic Films, From Langmuir to Selfhsembly;Academic Press, Inc.: San Diego, CA, 1991. (3)Kubota, M.; Ozaki,Y.; Araki,T.; Ohki, S.;Iriyama, K. Langmuir 1991,7,774. (4)Terashita, S.;Nakatsu, K.; Ozaki, Y.; Mochida, T.; Araki, T.; Iriyama, K. Langmuir 1992,8,3051. (5)Terashita, S.; Ozaki, Y.; Iriyama, K. J . Phys. Chem. 1993,97, 10445. (6)Nakagoshi, A.;Terashita, S.; Ozaki,Y.; Iriyama, K.Langmuir 1994,10,779.
0743-7463/95/2411-0705$09.00/0
the TCNQ chromophoric part in the films. Knowledge about the molecular aggregation and the orientation of the chromophore is essential to understand the physical properties of the conducting LB films with TCNQ or its derivatives. There have been no reports dealing with the molecular aggregation and the quantitative determination of the molecular orientation of the TCNQ chromophore in the LB films. Quantitative determination of the molecular orientation of the chromophoric part has been performed for only a few LB film~.~-lO To the best of our knowledge, this is the first time that tilt angle of a chromophoric part has ever been calculated for a one-monolayer LB film. We have employed W - v i s spectroscopyto investigate the number of monolayer and surface pressure dependences of the aggregation and orientation of the TCNQ groups in the LB films of octadecyl-TCNQ.
Experimental Section Octadecyl-TCNQwas obtained from the Japanese Research Institute for Photosensitizing Dyes Co., Ltd., and used without further purification. Thin-layer chromatographicexamination revealed that it did not contain any other colored components. The Z-typeLB films of octadecyl-TCNQwere fabricated by using a Kyowa Kaimen Kagaku Model HBM-APLangmuir trough with a Wilhelmy balance. Several drops of octadecyl-TCNQ in chloroform(1.18x M)were placed onto an aqueoussubphase ofwater which was doubly distilled from deionized water. After evaporation of the solvent, the monolayers were compressed at a constant rate of 30 cm2/minup to the surface pressure of 5 or 10 mN m-l (293 K). The n-A isotherm showed that the monolayers were liquid and solid condensedfilms at the surface pressures of 5 and 10 mN m-l, re~pectively.~They were transferred by the vertical dipping method onto quartz plates which had been subjected to ultrasonification in a hot 50% aqueous solution of DCN 90 of Decon Laboratories, Ltd. (the (7)Wang, Y.; Zhou, Y.; Wang, X.; Chen, W.; Xi, S. J. Chem. SOC., Chem. Commun. 1992,12, 873. (8) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1990,6, 672. (9)Yoneyama, M.; Sugi, M. Jpn. J . Appl. Phys. 1986,25,961. (10)Kamata, T.; Umemura, J.; Takenaka, T.; Koizumi, N. J. Phys. Chem. 1991,95,4092
0 1995 American Chemical Society
Letters
706 Langmuir, Vol. 11, No. 3, 1995
I
300
400
500
600
Wavelength Inm
Figure 2. Temperature dependence of UV-vis absorption spectrum of a one-monolayer LB film of octadecyl-TCNQon a
fused silica.
300
1
I
I
I
350
400
450
500
550
Wavelength Inm
Figure 1. (a)UV-vis absorption spectrum of octadecyl-TCNQ in a chloroform solution. (b) UV-vis absorption spectrum of a one-monolayer LB film of octadecyl-TCNQdeposited under the surface pressure of 10 mN m-l. The result of curve-fitting
is also shown. alkaline surfactant), and then in distilled water. The transfer ratio was found t o be 0.90 f 0.03 throughout the experiments. W-visible spectra of the LB films were measured on a Shimadzu W-visible 3101 spectrophotometer. In order to calculate the orientation of the TCNQ chromophore in the LB films, W-visible spectra were measured by changing polarization direction of the incident light as well as the incident angle with respect to the substrate. The optical setup and detail Log procedure for the measurements were described el~ewhere.~ normal function was employed for curve-fittingby using GRAMS software.
Results and Discussion Molecular Aggregation. Figure l a shows a UV-vis absorption spectrum of octadecyl-TCNQ in a chloroform solution at room temperature. An absorption band a t 405 nm is assigned to a z-z* transition of the TCNQ chromophore. Figure l b shows a W-vis absorption spectrum of a one-monolayer LB film of octadecyl-TCNQ deposited on a quartz plate under a surface pressure of 10 mN m-l. The spectrum of the LB film is quite different from that of the solution; the former has a n absorption maximum a t 365 nm with the shoulder near 405 nm while the latter shows a peak a t 405 nm with the shoulder near 370 nm. As shown in Figure 2, the temperature-dependent changes in the spectrum of the LB film show an isosbestic point near 400 nm (we shall report more detailed results on temperature dependence of the W-vis spectra elsewhere soonll). These observations suggest that the Wvis spectrum in the 300-550 nm region of the LB film consists of two components having a peak near 365 and 410 nm, respectively. The result of curve-fitting shown in Figure l b supports this conclusion. We assign the absorption bands a t 365 and 405 nm of the LB film to the stacked structure and monomeric form of the TCNQ chromophore, respectively. The assignment for the stacked structure is based upon the fact that, in general, TCNQ groups can form various kinds of stacked (11)Wang, Y.; Terashita, S.;Ozaki,Y.; Iriyama, K. To be submitted for publication.
structures quite easily and they always give an absorption maximum near 350-380 nm.l2-l5 For example, in the unit cell of single crystal of l-methyl-3’-ethyl-2,2’-quinoselenacyanine-[TCNQIz, four TCNQ molecules stack along the a-axis,16 and its KBr salt gives an absorption maximum a t 350 11m.l’ Therefore, the result in Figure l b suggests that the one-monolayer LB film of octadecylTCNQ consists mainly of species with the stacked structure of TCNQ chromophore, although the monomeric species coexist to some extent. Our recent study on 2-dodecyl-7,7,8,8-TCNQ(dodecyl-TCNQ)may support this conclusion.ls According to the study, the TCNQ chromophore forms stacked structure along the a-axis in the single crystal of dodecyl-TCNQ, and the LB film of octadecyl-TCNQ gives very similar X-ray diffraction pattern to that of the single crystal. Figure 2 includes the result of annealing. The spectrum noted with “annealing)) was measured a t 30 “C after the cyclic thermal treatment to 140 “C. Note that the spectrum returns only partially to its original one. This result indicates that the stacked structure is not fully recovered. We have also investigated dependences of molecular aggregation on the surface pressure and number of monolayers. The spectrum shown in Figure l b is similar to the spectrum of the one-monolayer LB film prepared under the surface pressure of 5 mN m-l and to those of five- and nine-monolayer films fabricated under the pressure of 10 mN m-l. The ratio of the peak intensities for the stacked and monomeric species changes little with the surface pressure and number of monolayers. Therefore, it seems that the surface pressure and number of monolayers have little effect on the molecular aggregation. Molecular Orientation. In functional LB films with a chromophore it is particularly important to investigate the molecular orientation of the chromophoric part quantitatively. However, such knowledge has been very Especially, the orientation of a chromophoric part in a single monolayer film has never been reported. (12) Hanson, A. W. Acta Crystallogr. 1968, B24, 768. (13) Konno, M.; Ishii, T.; Saito, Y. Acta Crystallogr. 1977, B33,763. (14) Iida, Y. Bull. Chem. SOC.Jpn. 1989, 42, 637. (15)Tanaka, J.; Tanaka, M.; Tawai, T.; Takabe, T.; Maki, 0. Bull. Chem. SOC.Jpn. 1976,49, 2139. (16) Umeura, T.; Takagi, S.; Okuda, K . ; Date, M. J . Phys. SOC.Jpn. 1982, 51, 760. (17) Takagi, S. Ph.D. Thesis, Osaka University, 1979. (18) Nakatu, K.; Terashita, S.; Sazaki, S.; Ozaki, Y.; Iriyama, K. Submitted for publication.
Letters
.l
Langmuir, Vol.11,No.3, 1995 707 Table 1. Surface Pressure and Number of Monolayer Dependences of the Tilt Angles between the Normal Direction of the TCNQ Plane and the Substrate Normal in the LB Films of Octadecyl-TCNQ
surface al
pressure (mN m-l) 5 10 10 10
no. of monolayers 1 1 5 9
tilt angles (deg) stacked species monomer 17 36 60 60
31 32
71 76
stacked and monomeric species. However, they change little between the five- and nine-monolayer films. Thus, it seems that the behavior ofthe first layer is quite different from the rest in the LB films due to the direct interaction of the chromophore with the substrate. In our previous study we concluded from comparison of the infrared transmission and reflection-absorption spectra of LB films of octadecyl-TCNQ that the TCNQ plane is nearly perpendicular to the substrate surface a t least above the third monolayer but it is inclined appreciably in the first monolayer deposited on the Auevaporated glass slide probably due to the interaction between the TCNQ chromophore and the ~ u b s t r a t eThe .~ present results are in good agreement with the previous one, although the substrates used are different from each other. As the calculated area of the flat plane of the TCNQ part is 5.7 x cm2,3the projected area of the TCNQ part on the substrate plane can be calculated as 4.7 x cm2and 2.4 x cm2,for the monomeric species in the one-monolayer LB film and the five- and ninemonolayer films prepared under the surface pressure of 10 mN m-l, respectively. Yet, according to the n-A isotherm, the molecular area was found to be 2.5 x cm2 a t the surface pressure of 10 mN m-l. Thus, it can be concluded that the molecular orientation of the first monolayer is changed during the deposition process from the air-water interface to the substrate due to the interaction with the substrate and the successive layers deposited onto the substrate almost keep their orientation as in the air-water interface.
Acknowledgment. We thank Dr. Douglas Borchman (Kentucky Lions Eye Research Institute, School of Medicine, University of Louisville) for correcting the English. LA940549+
