Langmuir 1995,11, 1515-1523
1515
Reduction of TCNQ in Mixed Monolayers with Cationic Amphiphiles at the Air/Water Interface? Ramesh C. Ahuja" and Bernhard J. Dringenberg Max-Planck-Institutfur biophysikalische Chemie, Postfach 2841, 0-37018 Gottingen, Germany Received December 7, 1994. I n Final Form: February 6, 1995@ Investigations of the molecular organization characteristics of octadecyltetracyanoquinodimethane (C18TCNQ)in mixed monolayerswith cationic amphiphiles such as dioctadecyldimethylamonium bromide (DOMA) has revealed that the TCNQ moiety is spontaneously reduced at the aidwater interface. The reduced TCNQ molecules react further with oxygen resulting in the formation of decay products. No such reduction is observed either in pure C18TCNQ monolayers or in mixed monolayers with lipids having neutral or negatively charged head groups. The reduction process depends on the molecular composition (molar ratio of DOMA and Cl8TCNQ in the mixed monolayer), subphase pH (e.g. no reduction at pH 21, and the nature (e.g. no reduction in the presence of clod-) and concentration of co-counterions in the subphase. The reduction process and the molecular organization characteristics have been studied using surface pressure- and surface potential-area isotherms and surface enhanced UV-vis reflection spectroscopy. The results are discussed in terms of interfacial pH, dielectric constant, and the redox potential of electroactive species at the aidwater interface. 1.0. Introduction
Most of the organic conducting systems are based on the intermolecular interactions and spatial organization of donor and acceptor components. The conductivity of these systems has been found to depend, to a large extent, on the nature, degree of charge transfer, stoichiometry, and the organization characteristics (spatial arrangement, relative orientation, etc.) of the donor and acceptor components. A mixed valence state of the donor and acceptor species is an essential requirement concerning the molecular conducting systems based on charge transfer interactions. The Langmuir-Blodgett (LB)t e ~ h n i q u e l - ~ offers a convenient and elegant way to organize amphiphilic molecules in two and three dimensions. Therefore, several classical donor and acceptor molecules have been d e r i ~ a t i z e d ~with - ~ aliphatic chains to prepare ordered conducting thin films. Recently, Ruaudel-Teixier et a1.8have introduced the concept of homodoping for the development of organic conducting systems based on LB films. The practical realization of the homodopingconcept calls for the organization of oxidizedreduced and neutral forms of the same electroactive component (donor or acceptor) in a n organized molecular array. The conductivity of such systems is anticipated to be controlled by the molecular composition (molar ratio of the oxidized reduced and neutral forms) and the molecular arrange-
' Dedicated to Professor Dr. Hans Kuhn on the occasion of his 75th birthday. Abstract published in Advance A C S Abstracts, April 1, 1995. (1)Langmuir-Blodgett Films; Roberts, G. G, Ed.; Plenum: London, 1990. (2) Ulman, A. A n introduction to ultrathin organic films; Academic Press: San Diego, CA, 1991. (3) Kuhn, H.; Mobius, D. Inrnuestigations ofSurfaces and Interfaces, Part B , 2nd ed.; Rossiter, B. W., Baetzold, R. C., Eds.; Physical Methods of Chemistry Series, John Wiley & Sons, Inc.: New York, 1993; Vol. Em,p 375. (4) Delhaes, P.; Yartsev, V. M. In Spectroscopy of New Materials; Clark, R. J . H., Hester, R. E., Eds.; John Wiley & Sons: New York, 1993; p 199. ( 5 ) Vandevyver, M. Thin Solid Films 1992,210/211,240. (6) Nakamura, T; Yunome, G.; Azumi, R.; Tanaka, M.; Tachibana, H.; Matsumoto, M.; Horiuchi, S.;Yamochi, H.; Saito, G. J . Phys. Chem. 1994. ~ . , 98. -. - , 1882. (7) Dhindsa, A. S.; Ward, R. J.;Bryce, M. R.; Lvov, Y. M.; Munro, H. S.; Petty, M. C. Synth. Met. 1990,35,307. (8)Ruaudel-Teixier, A,; Vandevyver, M.; Roulliay, M.; Bourgoin, J . P.; Barraud, A,; Lequan, M.; Lequan, R. M. J . Phys. D: Appl. Phys. 1990,23,987. @
~
ment (lattice constant, orbital overlap, and spatial architecture) of the electroactive molecules. The preparation of LB films requires, as a first step, the formation ofmixed monolayers of the component molecules a t the aidwater interface. Surface pressure andlor surface density of the molecules is often used to control the molecular organization characteristics of the monolayers a t the aidwater interface. The monolayer preparation procedure, however, may also lead to some unexpected oxidatiodreduction of the electroactive components due to a specific combination of various interfacial parameters such as dielectric constant, pH, concentration of counterions, and the GouyChapman potential. Thus, some reactions which do not occur in the spreading solvent or in the bulk aqueous phase may become possible a t the aidwater or monolayer/ subphase interface. Therefore, in addition to the optimization of the molar composition of the mixed monolayers, it is important to optimize the subphase composition (pH, nature, and concentration of counterions) in order to have a certain degree of control on the redox state and the molecular organization of the electroactive species a t the monolayer/water interface. Tetracyanoquinodimethane (TCNQ)and its derivatives have been extensively used9as electron acceptor molecules in a large number of organic conducting systems. In conducting systems based on LB films, octadecyltetracyanoquinodimethane (C18TCNQ) has been used extensively by many workers.6,8,10-12However, C l8TCNQ does not form well-defined compressible monolayers a t the air/ water interface.13 Immediately after spreading of C l8TCNQ a t the aidwater interface, monolayer islands consisting of crystallites of Cl8TCNQ are formed which are then pushed together during the compression process. The aredmolecule a s derived from the surface pressurearea isotherms is always less than that of an aliphatic chain.13 The small value of aredC18TCNQ molecule has to be attributed to the strong tendency of TCNQ to form ~
~~~
~~~~~~~~~~~~~~
~~
(9) Simon, J.; Andre, J. J Molecular Semiconductors; Springer: New York, 1985. (10) Nakamura, T.; Takei, F.; Matsumoto, M.; Tanaka, M.; Sekiguchi, T.; Manda, E.; Kawabata, Y.; Saito, G. Synth. Met. 1987,19, 681. (11)Ruaudel-Teixier, A.; Vandevyver, M.; Barraud, A., Mol. Cryst. Liq. Cryst. 1986,120,319. (12) Rustichelli, F.; Dante, S.;Mariani, P.; Myagkov, I . V.; Troitsky, V. I. Thin Solid Films 1994,242,267. (13)Naito, K.; Miura, A. Thin Solic Films 1994,243,342.
0743-746319512411-1515$09.00/0 0 1995 American Chemical Society
Ahuja and Dringenberg
1516 Langmuir, Vol. 11, No. 5, 1995 H-aggregates (hypsochromic shift in the absorption spectrum relative to the monomer species) due to strong x-x interactions between the TCNQ moieties. Many attempts have therefore been made to control the organization and aggregation of amphiphilic TCNQ species in monolayers. For many applications, it is important that the TCNQ species remain in the monomer form. Naito et al.l3-l5 have investigated extensively the relationship between the structure of the hydrophobic part and the monomolecular film properties of several amphiphilic derivatives of TCNQ. They have established that a disteroid skeleton ofthe hydrophobic region is most suitable for the preparation of homogeneous LB films of amphiphilic TCNQ. The absorption spectrum of the multilayer LB films obtained by Naito et al.13is red-shifted relative to the solution spectrum, and this shift has been attributed to the interlayer TCNQ aggregation in the multilayer films. This chemical approach however requires the synthesis of sophisticated amphiphilic derivatives of electroactive molecules, often with low synthetic yields. Another classical approach16 to control the molecular aggregation is to prepare mixed monolayers of amphiphilic chromophores with inert matridspacer lipids. The function of the matridspacer molecules is to physically separate the chromophore bearing amphiphiles. This strategy has been used extensively but no specific criteria for the selection of spacer or matrix lipids have been established. For this approach to work, it is crucial that in the mixed monolayer, the amphiphilic derivative of the functional chromophore and the m a t d s p a c e r lipids show molecular level mixing a t all stages of compression. We have been using this strategy in our laboratory for some time,17 and during the search for a n appropriate spacer lipid, we have found that CHTCNQ, when incorporated in cationic amphiphilic matrix monolayers, shows spontaneous chemical reduction a t the airlwater interface. The phenomenon of TCNQ reduction in monolayers has been independently reported by Perez in his Ph.D. thesis workla and subsequently additional work has been published in this direction by other team^.^^,^^ In the present study, we report the results of our investigations on the molecular organization and chemical reduction of Cl8TCNQ in the mixed monolayers using surface pressure- and surface potential-area isotherms and the W-vis reflection spectroscopy techniques a t the airlwater interface.
2.0. Materials and Methods Dioctadecyldimethylammonium bromide (DOMA, cf. Figure 1 for chemical structure) was obtained from Eastman Kodak. Octadecyltetracyanoquinodimethane(C18TCNQ, cf. Figure 1for chemical structure) was kindly donated by Dr. Matsumoto (National Institute of Materials and Chemical Research, Tsukuba, Japan). Arachidic acid, methyl arachidate (AME), octadecanol, dimyristoylphosphatidylcholine(DMPC, cf. Figure l for chemical structure) were obtained from Larodan Fine Chemicals, Sweden, and were used as received. Eicosylpyridinium bromide (C22Pyr)was synthesized by Dr. Sondermann i n our laboratory. Chloroform (HPLC, stabilized with 2% ethanol) solutions of the lipids were used for spreading at the aidwater interface.
NcYcN
C18TCNQ
Y DOMA / N
+/
\
DMPC
AME / \
CH3
Br.
'CH,
e
H0 C' @ 3 3
Figure 1. Chemical structures of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (ClSTCNQ), dioctadecyldimethylammonium bromide (DOMA), dimyristoylphosphatidylcholine (DMPC),and methyl arachidate (AME) amphiphilic molecules. Milli-Q filtered water, which has a pH of ca. 5.6 when in equilibrium with the atmosphere, was used as the subphase unless otherwise stated. Monolayer compression isotherms were measured on a thermostated Fromherz type round trough enclosed in a dark plastic cabinet which was flushed with humid nitrogen during the measurements. The temperature of the subphase was kept constant at 294 Kunless otherwise specified. A Wilhelmy balance (15 m m wide filter paper) was used for the surface pressure measurements. Surface potential measurements were done using a vibrating condenser electrode setup as described i n ref 16. A fiber optics detector head for the surface-enhanced UV-vis reflection measurements was located above the water surface at a lateral distance of ca. 3 cm from the Wilhelmy balance. All reflection measurements were done with the light beam incident normal to the water surface using a n apparatus similar t o that described previously.21 The reflectance (AR) refers to the difference in the reflectivity of the monolayer covered surface and the clean water surface. The absorption spectrum of LB films on substrates was measured using a home-built apparatus described in ref 16. A chloroform solution of the lipids, C18TCNQ, and mixtures thereof in the desired composition was spread a t the aidwater interface using the continuous flow technique with the syringe tip barely touching the water surface. The isotherms were measured stepwise: The moving barrier stops when the mean area per molecule decreases by 0.02 nm2 or when the surface pressure increases by 1mN/m with the surface pressure being recorded after a relaxation time of 10 s after the barrier stop.
(15) Naito, K.; Miura, A,; Azuma, M. Langmuzr 1991, 7, 627. (16)Kuhn, H.; Mobius, D.; Biicher, H. In Physical Methods of Chemistry;Weissberger, A., Rossiter, B., Eds.; John Wiley& Sons: New York, 1972; Vol. 1,Part 3B; pp 577-702. (17) Ahuja, R. C.; Mobius, D. Thin Solid Films 1994,243, 547. (18) Perez, J. Ph.D. Thesis, Universities Pans XI Orsay, 1993. (19) Perez, J.;Bourgoin,J. P.;Barisone,C.;Vandevyver,M.; Barraud, A. Thin Solid Films 1994,244, 1043.
3.0. Experimental Results 3.1. Pure ClSTCNQ Monolayers. The chemical structure of the amphiphilic derivative of TCNQ (C18TCNQ)used in the present study is shown in Figure 1. The molecular area requirements of the TCNQ moiety (based on CPK model) in different conformations are 0.92 nm2 (1.2 x 0.76 nm, flat; long and short molecular axes parallel to the water surface), 0.43 nm2 (1.2 x 0.36 nm, side-on; long molecular axis parallel and short molecular axis normal to the water surface), and 0.26 nm2 (0.76 x
592.
3701.
(14) Naito, K.; Iwakiri, T.; Miura, A,; Azuma, M. Langmuir 1990,6,
1309.
(20) Fichet, 0.;Agricole, B.; Kassi, H.; Desbat, B.; Gionis,V.;Leblanc, R. M.; Garrigou-Lagrange, C.; Delhaes, P. Thin Solid Films 1994,243,
(21) Griiniger, H.; Mobius, D.; Meyer, H. J . Chem. Phys. 1983, 79,
Reduction of TCNQ
Langmuir, Vol. 11, No. 5, 1995 1517
,
I
CIBTCNQ
:
I
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0
0.0
0.1
0.2
0.3
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Area/Molecule [nm2] Figure 2. Surface pressure- and surface potential-area isotherms of the pure Cl8TCNQ monolayer at the aidwater interface. 0.36 nm, edge-on; long molecular axis normal and short molecular axis parallel to the water surface). The x-electron system along with four hydrophilic cyano groups makes the TCNQ moiety a hydrophilic part of the C18TCNQ. Thus when the chloroform solution of Cl8TCNQ is spread a t the aidwater interface, the TCNQ moiety due to its hydrophilicity is expected to be in contact with the water surface while the hydrophobic aliphatic chain remains out of the water subphase. The surface pressure-area (x-A)and surface potentialarea (AV-A)isotherms of Cl8TCNQ a t the aidwater interface are shown in Figure 2. The area/molecule a t 20 mN/m is 0.14 nm2 which is even less than the area value of 0.19 nm2 required for a n aliphatic chain in the tightly packed condensed state. Similar isotherm data have been reported by Naito et al.13 The isotherm data show that Cl8TCNQ alone does not form a well-definedcompressible monomolecular layer a t the aidwater interface. The same conclusion can be drawn from the surface potential-area isotherm (cf. Figure 2) and the reflection spectroscopic data. The reflection spectrum of the pure Cl8TCNQ monolayer (data not presented) shows a strong and asymmetric band with a maximum at 366 nm. In contrast, the absorption spectrum of Cl8TCNQ has a maximum a t 401 nm in chloroform and a t 395 nm in acetonitrile solvents. The strong blue shift of ca. 30-35 nm of the reflection spectrum band of the Cl8TCNQ monolayer relative to the solution spectrum is to be attributed to the formation of H-aggregates of TCNQ with transition moments parallel (strong reflection signal) to the water surface. For the optical transition moment to be parallel to the water surface, TCNQ moiety should adopt either a side-on (0.43 nm2)or a parallel (0.92 nm2)arrangement a t the aidwater interface. Thus the area requirement in both conformations is much larger than the experimentally observed molecular area value of 0.14 nm2 indicating multilayer formation. The surface potential-area (cf. Figure 2) and the reflection (366 nm)-area isotherm profiles also confirm the formation of condensed multilayer aggregated islands of Cl8TCNQ after spreading of the solution. These islands are then pushed together during the compression process resulting in a film of undefined thickness and structure. The relatively high reflection signal of the Cl8TCNQ film may also be understood in terms of the multilayer formation. 3.2. Mixed Monolayers of Cl8TCNQ with Lipids. In order to suppress the aggregate formation of TCNQ, we have investigated the mixed monolayers of Cl8TCNQ with a variety of lipids. These lipids can be conveniently divided into three classes according to charge on the lipid
o.io
0.i5
0.20
0.25
0.30
0.35
Mean Molecular Area [nm2] Figure 3. Surface pressure-area isotherms of the M E / Cl8TCNQ mixed monolayersat the aidwater interface. Molar ratio values of the two components in the mixed monolayer are given for the respective isotherms. Compression speed was 15 cm2 min-l. head group: (a) neutral or zwitterionic, (b) anionic, and (c) cationic. The purpose of these investigations was to establish selection criteria for the matridspacer lipids to achieve molecular level mixing with Cl8TCNQ and to obtain monomeric Cl8TCNQ species in the mixed monolayers. 3.2.1. Cl8TCNQRdeutral Lipids. Different amphiphilic molecules with neutral head groups such as octadecyl alcohol (C180H), methyl arachidate (AME),and dimyristoylphosphatidylcholine (DMPC, cf. Figure 1for chemical structure) were investigated for their capability to form molecularly mixed monolayers with C18TCNQ. The n-A isotherms of the mixed monolayers of C l8TCNQ and AME in different molar compositions are shown in Figure 3. The isotherms show break pointshumps between 15 and 20 mN/m which are to be attributed to a major phase separation of the two components. It is to be noted that even for highly diluted mixed monolayers such as AME/C18TCNQ, molar ratio lO:l, phase separation ofthe Cl8TCNQ in the mixed monolayer occurs. The reflection spectra of the AME/C18TCNQ mixed monolayers show TCNQ aggregation (Amm = 366 nm) and are analogous to that of pure C l8TCNQ monolayers confirming phase separation of Cl8TCNQ and AME. The mixed monolayers of Cl8TCNQ and C180H, a t different molar ratios of the two components, also show phase separation and the reflecton spectra are similar to those obtained with AME. The shortening of the hydrocarbon chain length of the aliphatic alcohol to 16 CH2 units does not result in any significant difference in the monolayer organization characteristics of C18TCNQ. The n-A isotherms of the mixed monolayers of C l8TCNQ and DMPC, a t different molar ratios of the two components, are shown in Figure 4. DMPC differs from the other neutral lipids in that it has a zwitterionic head group and it shows a liquid expanded isotherm a t 294 K. In a recent study17 on the mixed monolayers of dioctadecyloxacyanine with phospholipids of varying chain lengths, we have established that lipids with expanded isotherms are better suited as matrix molecules to prepare molecularly mixed monolayers. The results presented here lend further support to the importance of this criterion. Assuming that the two components ofthe mixed monolayers do not show any specificinteractions or special packing arrangements such as matrix molecules located on top of the flat lying TCNQ moieties, the isotherms of
Ahuja and Dringenberg
1518 Langmuir, Vol.11, No. 5, 1995
-$ E
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AredDMPC Molecule [nm2] Figure 4. Surface pressure-area isotherms of the DMPC/ Cl8TCNQmixed monolayers at the aidwater interface. Molar ratio values of the two components are given for the respective isotherms. 404 nm
-8
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DMPC : CllTCNQ (5 : 1)
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Wavelength [nm] Figure5. UV-vis reflection spectrumofthe DMPC/C18TCNQ mixed monolayer, molar ratio 51, at the aidwater interface. Surface pressure was 30 mN/m. the mixed monolayers may be analyzedzzin terms of the isotherms of the component monolayers using the following expression
A," = X l A I + X Z A Z
(1)
where A,, is the average aredmolecule in the mixed monolayer, x1 and x2 are the molar ratios, and AI and A2 are the aredmolecule values of the two components, respectively. Analysis of the DMPC/ClSTCNQ mixed monolayer isotherms, molar ratio 5:1, in terms of eq 1 yields a value of 0.89 nm2 for the aredmolecule of C18TCNQ. This area value is close to the molecular area required by TCNQ (0.92 nm2;TCNQ moiety lying flat on the water surface) in the parallel orientation, indicating that Cl8TCNQ mixes well with DMPC a t the molecular level and that no phase separation of the two components occurs. The reflection spectrum of the DMPC/C18TCNQ mixed monolayer, molar ratio 5:1, a t 30 mN/m is shown in Figure 5. This spectrum is similar to the absorption spectrum of Cl8TCNQ in chloroform solution and shows a maximum a t 404 nm. Thus, both the isotherm and the reflection data show that DMPC and CMTCNQ, molar ratio 5:1, form molecularly mixed monolayers. However, as the (22) Gaines, G. Insoluble Monolayers at the Liquid-Gas Interface, 1st ed.; Wiley Interscience: New York, 1966.
0.0
0.4
a
C18TCNQ
b
DOMA
c
DOMA:ClBTCNQ(l:l)
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Mean Molecular Area [nm2] Figure 6. Surface pressure-area isotherms of DOMA, CMTCNQ,and DOWC18TCNQ mixed monolayers at the air/ water interface: (a)C18TCNQ;(b)DOMA,(c)DOWC18TCNQ (1:l). molar fraction of Cl8TCNQ in the mixed monolayers is increased further, phase separation (bumps in the n-A isotherms, cf. Figure 4)of the two components is again observed and the reflection spectra of the mixed monolayers are similar (Ama = 366 nm) to that of the pure Cl8TCNQ monolayers. On the basis of these results it is to be concluded that DMPC can be used as a matrix molecule to prepare well-defined mixed monolayers only a t relatively lower molar fractions of C18TCNQ. 3.2.2. Cl8TCNQ/Anionic Lipids. Dimyristoylphosphatidic acid and arachidic acid, both lipids with anionic head groups, were also investigated for their usefulness in preparing molecularly mixed monolayers with Cl8TCNQ. Due to the negatively charged head groups of these lipids, concentration of H+ ions a t the interface is expected to be significantly higher than in the bulk phase. As a consequence, in the interfacial pK value of the head groups of the lipids is shifted toward higher pH values. Both the anionic lipids therefore tend to remain protonated at the aidwater interface (pH 5.6) and behave as if they were neutral. The anionic lipidC18TCNQ molar ratio was varied between 1 : l and 10:1. The n-A isotherm and the reflection spectroscopic data of the mixed monolayers show that the two components do not mix well and that phase separation occurs a t all stages of compression. The reflection spectra of the mixed monolayers show a maximum a t 366 nm and are similar to that of pure Cl8TCNQ monolayers. 3.2.3. Cl8TCNQ/Catioinic Lipids. Dioctadecyldimethylammonium bromide (DOMA) and eicosylpyridinium bromide (C22Pyr) were used as cationic amphiphiles for the investigation of mixed monolayers with C18TCNQ. The molecular organization characteristics of DOMA have been studied e x t e n s i ~ e l y . ~We ~ -have ~ ~ shown earlier that the morphology and molecular organization characteri s t i c (n-A) ~ ~ ~ and AV-A isotherms) of the DOMA monolayers depend strongly on the nature and concentration of counterions in the subphase. 3.2.3.1. DOMA/Cl8TCNQ 1:l Mixed Monolayers. The x-A isotherms of the components DOMA, CMTCNQ, and the mixed monolayers DOMA/C 18TCNQ,molar ratio 1:1, are presented in Figure 6. The value of average area! molecule as obtained from the isotherm (Figure 6; curve (23)Goddard, E. D.; Kao, 0.;Kung, H. C. J . Colloid Interface Sci. 1968,27,616. (24) Marra, J. J.Phys. Chem. 1986,90,2145. (25) Ahuja, R. C.; Caruso, P.-L.; Mobius, D. Thin Solid Films 1994, 242,195.
Langmuir, Vol. 11, No. 5, 1995 1519
Reduction of TCNQ 0.15
462 nm
i
A
DOMA: Cl8TCNQ (1 : 1)
d 0.0
1 ,
I
I
I
1 .o 1.5 2.0 Area/Molecule [nm2] Figure 7. Surface potential-area isotherms of DOMA, C 18TCNQ, and D O W C l8TCNQ mixed monolayers, molar ratio 1:l at the aidwater interface. 0.0
0.5
c) in the mixed monolayer is 0.32 nm2, which is to be attributed to the aliphatic chains of the component molecules, andindicates that the Cl8TCNQ does not form multilayers and that the TCNQ moiety does not contribute to the surface pressure or area in the tightly packed condensed state of the mixed monolayer. This is in contrast to the case of DMPC/C18TCNQ mixed monolayers, where it was found that C18TCNQ occupies a n area of 0.89 nm2. The molecular organization characteristics of the DOWC18TCNQ mixed monolayer are solely determined by the packing requirements of the aliphatic chains of DOMA and Cl8TCNQ and not by the bulky TCNQ moiety. The area required by the TCNQ moiety in the side-on arrangement with the long molecular axis parallel to the water surface is 0.43 nm2which is less than the minimum area (0.6 nm2) available under the three aliphatic chains of DOMA and Cl8TCNQ in the tightly packed state ofthe mixed monolayer. These results suggest that TCNQ moieties in the DOWC18TCNQ mixed monolayer are organized under the aliphatic chains and that the two components show mixing at the molecular level. The AV-A isotherms presented in Figure 7 show that the surface potential of the DOWC18TCNQ mixed monolayer is significantly lower (AV = 225 mV a t A = 0.54nm2)compared to that ofthe pure DOMA (AV= 1010 mV) monolayer. Under the assumption that the component molecules Cl8TCNQ and DOMA do not show any specific interactions, the surface potential of the mixed monolayer may be analyzed in terms of the linear combination26-2sof the surface potential contributions of the individual component molecules. The surface potential data of C18TCNQ alone is, however, not known exactly, as the pure Cl8TCNQ does not form a well-defined monolayer. Therefore, we have estimated its value by analyzing the AV-A isotherm data of the relatively welldefined DMPC/C18TCNQ, molar ratio 5:1, mixed monolayers. For the DMPC/ClSTCNQ mixed monolayers, we have shown above that TCNQ remains in the neutral and monomeric form (cf. Figure 5; reflection maximum a t 404 nm). This analysis shows that the measured contribution of Cl8TCNQ to the AV data of the DOWC18TCNQ mixed monolayer cannot be accounted for in terms of the additivity of surface potential values of DOMA and (26) Demchak, R. J.; Fort, T.J. J . Colloid Interface Sci. 1974,46, 191. (27) Vogel, V.; Mobius, D. J . Colloid Interface Sci. 1988,126, 408. (28) Taylor, D. M.;Oliveira,0.N., Jr.; Morgan, H. J . Colloidlnterface Sci. 1990,139,508.
\
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-
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400
450
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Wavelength [nm] Figure 8. UV-vis reflection spectrum ofthe DOWC18TCNQ mixed monolayer, molar ratio 1:l at the aidwater interface. Surface pressure was 20 mN/m. C18TCNQ and is significantly lower compared to the value expected from the additivity rule. The negative contribution of Cl8TCNQ to the surface potential of D O W Cl8TCNQ mixed monolayers indicates the change in redox state of TCNQ moieties leading to a drastic reduction in the surface potential of the mixed monolayer. Further strong evidence regarding the spontaneous change in redox state ofthe TCNQ moieties, in the D O W C18TCNQ mixed monolayers, is obtained from the Wvis reflection spectroscopy of the mixed monolayers a t the aidwater interface. The reflection spectrum ofthe D O W Cl8TCNQ mixed monolayer, molar ratio 1 : l (cf. Figure 8), shows two reflection bands with maxima a t 336 and 462 nm, respectively. The spectrum of the mixed monolayer differs strongly from that of the pure Cl8TCNQ monolayers, which show a single reflection band with a maximum at 366 nm (H-aggregates) and C18TCNQ/ DMPC mixed monolayer (Amm = 404 nm). The absorption spectrum of Cl8TCNQ and of DOWC18TCNQ mixture in chloroform shows a maximum a t 401 nm (monomer species) leading to the conclusion that the TCNQ redox state changes only after spreading of the DOMA/ C18TCNQ mixture a t the aidwater interface. What is the nature of the TCNQ species in the D O W Cl8TCNQ mixed monolayers a t the aidwater interface? The absorption spectra of TCNQ in its various redox and aggregation states such as TCNQO, TCNQ-, (TCNQ-h, (TCNQ)z*-,TCNQ2,etc. have been discussed extensively in the l i t e r a t ~ r e . ~It~ -has ~ ~ been f o ~ n d that ~ ~ the , ~ ~ TCNQ2- dianion in the absence of oxygen shows absorption bands with maxima a t 330 and 240 and 210 nm, respectively. In the presence of oxygen however, the dianion is not stable and forms a negatively charged oxygen decay product36 (a,a-dicyano-p-tolyoyl cyanide, DCTC-) with absorption maxima34,35a t 287 nm ( 6 = 8.38 x lo3 M-l cm-l), 330 nm ( 6 = 1.98 x lo3 M-l cm-l), and 480 nm ( 6 = 3.92 x 104M-lcm-'1. The reported absorption maxima of DCTC- are close to the reflection spectrum maxima of the DOWC18TCNQ mixed monolayers. Thus (29) Acker, D. S.; Harder, R. J.; Hertler, W. R.; Mahler, W.; Melby, L. R.; Benson, R. E.; Mochel, W. E. J . Am. Chem. SOC.1960,82,6408. (30)Boyd, R.H.; Philips, W. D. J . Chem. Phys. 1965,43,2927. (31)Bieber, A.; Andre, J. J. Chem. Phys. 1974,5, 166. (32) Oohashi, Y.; Sakata, T.Bull. Chem. SOC. Jpn. 1973,46,3330. (33) Malek, J.;Drchal, V.;Hejda, B.; Zalis, S.Chem.Phys. 1984,89, 361. (34) Suchanski, M. R. J.Electrochem. SOC.1976,123,181C. (35) Suchanski, M. R.; Van Duyne, R. P. J . Am. Chem. SOC.1976,98, 250. (36) Lombardo, A.;Rico, T. R.; J . Org. Chem. 1979,44,209.
Ahuja and Dringenberg
1520 Langmuir, Vol. 11, No. 5, 1995
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... p H 2
. pH 12
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I
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(I
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Wavelength [nm] Figure 9. Influence of the subphase pH on the redox state of TCNQ moieties, W-vis reflection spectra of the D O W C18TCNQ, molar ratio 1:1, mixed monolayers at the air/ subphase interface: n,20 mN/m; (- * - pH 2, 10 mM HC1; (-1 pH 5.6, water; (- - -1 pH 12, 10 mM NaOH.
Wavelength [nm] Figure 10. Influence of the presence of co-counterionsin the subphase on the redox state of TCNQ moieties, UV-vis reflection spectra of DOWC18TCNQ, molar ratio 1:1,mixed monolayers at the airhubphase interface; n, 20 mN/m; (-1 water, (- - -) 1 mM NaF, (- -1 1 mM NaC104.
we tend to conclude on the basis of spectroscopic data that in the DOWC18TCNQ mixed monolayers a t the air1 water interface, Cl8TCNQ is doubly reduced and reacts with oxygen to form the negatively charged oxygen decay product (C18DCTC-). The infrared spectroscopic investigations on LB films and monolayers of Cl8TCNQ and various cationic amphiphiles by Perez et al.19 and Fichet et a1.20have also led to the conclusion that TCNQ moieties are spontaneously reduced in the mixed monolayers a t the airlwater interface. The relative intensities of the reflection spectrum maxima of the mixed monolayers in the present case are however not in agreement with that of DCTC- solution spectrum. This may in part be due to (a)the orientational anisotropy of the C18DCTC- and (b) the presence of reduced and other TCNQ species, in the mixed monolayers. The present results should be compared with those of Perez et al.19who have shown that the reduction of LB films of C18TCNQ by NzH4 vapor leads to the appearance of absorption bands in the W - v i s spectrum, with maxima a t 310,380, and 640 nm. They have attributed these bands to the C18TCNQH2, C18TCNQ'-, and (C18TCNQ-)2 species, respectively. These results and the fact that no reduction of TCNQ takes place in mixed monolayers of Cl8TCNQ with neutral lipids even a t high pH values show that the reduction process and the follow-up reactions with oxygen take place efficiently only in the presence of both the high concentration of OH- ions and cationic lipids. The central role of OH- ions in the reduction process is confirmed by the pH dependence of the reflection spectra of the DOWC18TCNQ mixed monolayers, molar ratio 1:1, as shown in Figure 9. These spectra show that a t pH 2, no measurable reduction of TCNQ takes place. The reflection spectrum of the mixed monolayer at pH 2, with a maximum a t 366 nm, is similar to that of pure Cl8TCNQ monolayers. In addition, the reflection signal a t 366 nm is relatively strong indicating the phase separation and multilayer formation of neutral Cl8TCNQ in the DOMA/ Cl8TCNQ mixed monolayer. The surface pressure- and surface potential-area isotherms ofthe DOWC18TCNQ mixed monolayers on pH 2 subphase also clearly show that Cl8TCNQ does not get reduced and the two components are phase separated. The reduction of TCNQ moieties proceeds only when sufficiently high concentration of OH- ions is available at the interface. However, increasing the pH of the subphase above a certain value does not lead to an increase in the concentration of reduced TCNQ species. This is due to the fact that already a t the
bulk pH value of 5.6, the interfacial pH value is much higher (cf. section 4) as a result of OH- ion accumulation a t the cationic monolayerlsubphase interface. The presence of OH- ions alone is, however, not sufficient for the reduction process as the reflection spectrum (aggregated TCNQ with maximum a t 366 nm) of the pure Cl8TCNQ monolayers has been found to be independent of pH until pH values as high as 12. Thus, the presence of a cationic head group amphiphile along with a sufficient concentration of OH- ions is necessary for the efficient reduction of TCNQ. Further evidence of the importance of OH- ions in the reduction process is established through investigations of the influence of different co-counterions present in the subphase. The reflection spectra of the DOWC18TCNQ mixed monolayers a t subphases containing different cocounterions are shown in Figure 10. These spectra show that in the presence of C104- co-counterions, the reduction of TCNQ is completely suppressed (relatively strong reflection signal with maximum a t 366 nm attributed to H-aggregates of TCNQ) whereas F- ions suppress the reduction of TCNQ only partially. The n-A and AV-A isotherm data of the DOWC18TCNQ mixed monolayers on subphases with different co-counterions also clearly confirm the conclusions drawn from the reflection spectra. This strong influence of co-counterions is to be attributed to the selectivity of adsorption of different ions a t the cationic monolayerlsubphase interface. The F- ion is much smaller than the C104- ion and is therefore strongly hydrated. It has been established e l ~ e w h e r ethat ~ ~ C104,~~ counterions are adsorbed selectively over the strongly hydrated OH- and F- ions resulting in a sublayer of C104ions tightly bound to the cationic head groups of the monolayer. As a consequence the concentration of OHions a t the monolayerhbphase interface is negligible, and the C18TCNQ molecules do not get reduced. The degree of TCNQ reduction in the DOWC18TCNQ mixed monolayers depends on the competitive adsorption and interfacial concentration of the OH- and other co-countenons at the cationic head group of DOMA. 3.2.3.2. Molar Composition Dependence. Investigations of the D O W C l8TCNQ mixed monolayers containingvarying amounts ofDOMA and Cl8TCNQ have shown (cf. Figure 11) that as the molar fraction of Cl8TCNQ is increased, not all the TCNQ moieties are reduced and a phase separation of the neutral Cl8TCNQ molecules takes place. Thus, the reflection spectrum of the DOWC18TCNQ mixed monolayer, molar ratio 1:4,
-
Langmuir, Vol. 11, No. 5, 1995 1521
Reduction of TCNQ 1.o
n o
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Wavelength [nm] Figure 11. Influence of molar composition of the D O W C18TCNQ mixed monolayers on the redox state of TCNQ moieties, UV-vis reflection spectra of the DOMA/C18TCNQ, molar ratio x:y,mixed monolayers at the aidwater interface: (a)4:l; (b)2:l; (c) 1:l;(d) 1:2, (e)1:4. For clarityofpresentation, curve e has been divided by a factor of 2. n was 20 mN/m. is analogous to that of the neutral (A, x 366 nm) Cl8TCNQ monolayers. The reflection spectrum of the DOWC18TCNQ mixed monolayer, molar ratio 2:1, shows the presence of TCNQ species in various redox and aggregated states. However, for mixed monolayers with excess of DOMA over C18TCNQ, the TCNQ moieties are completely reduced and molecularly mixed monolayers are formed. The reflection signal of the mixed monolayers is proportional to the surface density of C18DCTC- and shows the characteristic maxima a t 336 and 460 nm. 3.2.3.2. C22Pyr/Cl8TCNQ Mixed Monolayers. In addition to DOMA, we have also investigated mixed monolayers of C18TCNQwith eicosylpyridinium(C224rl.1, which is also a cationic amphiphile. The results obtained on the C22PyrlC18TCNQ system are analogous to those obtainedwith the DOWC18TCNQ system and show that the TCNQ moieties are completely reduced in both cases. The reflection spectra of the C22PyrIC18TCNQ mixed monolayers show the presence of a mixture of reduced TCNQ and DCTC- species. These results are in agreement with those obtained by Perez et al.I9and Fichet et a1.20who have found that the TCNQ reduction process is independent on the nature of cationic lipid head group and depends only on the molar composition of the cationic lipidC18TCNQ mixed monolayer. 3.3. DOMA/Cl8TCNQ LB Films. Most of the applications of the molecularly organized systems involve the transfer of Langmuir and complex monolayers from the airlwater interface onto the solid substrates. Therefore it is important to establish as to whether the redox state and the molecular organization characteristics of TCNQ species are maintained after the transfer of the DOMA/ Cl8TCNQ mixed monolayers onto the substrates. Figure 12 (solid curve) shows the absorption spectrum of a single DOMA/C18TCNQ, molar ratio 1:1, mixed monolayer transferred directly onto the hydrophilic quartz substrate in the upstroke mode. In this configuration, the quaternary ammonium head group of DOMA and the TCNQ moieties are in contact with the substrate. It is remarkable that the absorption spectrum of the DOMNC18TCNQ mixed film in direct contact with the hydrophilic quartz substrate differs markedly from the reflection spectrum of the mixed monolayer a t the airlwater interface. The formation of DCTC- is still observed as indicated by the strong shoulder a t 462 nm. The spectrum has a n additional band with a maximum at 414 nm which is to be attributed to a m i x t ~ r eof~the ~ ,reduced ~ ~ TCNQ- (A,,,
Il
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Wavelength [nm] Figure 12. Absorption spectra of the DOWC18TCNQ, molar ratio 1:1, mixed monolayer transferred onto the hydrophilic quartz substrate: transfer pressure, 20 mN/m. The solid curve corresponds to the TCNQ species in direct contact with the substrate. The broken curve correspondsto the TCNQ species not in contact with the substrate. The first and the last monolayers on the substrate are eicosylamine and arachidic acid, respectively. Transfer ratio = 0.9. = 422 nm) radical anion and neutral (Ama = 400 nm) TCNQ
species. However, if the DOWC18TCNQ mixed monolayer is transferred onto a hydrophobic substrate (quartz substrate coated with a monolayer of eicosylamine) in the downstroke and then a n arachidic acid monolayer is transferred in the upstroke, the absorption spectrum (cf. Figure 12; broken curve) of the DOWC18TCNQ mixed film is similar (absorption maxima a t 345 nm and 462 nm) to the reflection spectrum of the mixed monolayer a t the airlwater interface. These results show that during the transfer process, water is more effectively drained in the first transfer step while sufficient OH- ions are available at the mixed monolayerlarachidic acid monolayer interface to maintain the redox state of the TCNQ species after the transfer. To obtain information about the molecular orientation of the TCNQ species in the LB films, we have measured the polarized light absorption spectra ofthe LB films under oblique incidence. The absorption spectra of the D O W C18TCNQ mixed monolayer on hydrophobic quartz substrate using s and p polarized light a t an incidence angle of 45" are shown in Figure 13. The absorption spectra were analyzed in terms of the polarized light absorption of thin films. From this analysis, it has been determined that the optical transition moments of the TCNQ species contributing to the absorption maximum a t 460 nm are oriented with respect to the surface normal a t a n angle of 77 f 2". The respective orientation for the transition moments with maximum at 340 nm comes out to be 72 f 3". We have also transferred the DOWC18TCNQ mixed monolayers with different molar compositions to the hydrophobic quartz substrates and measured the corresponding absorption spectra. These spectra are in agreement with the reflection spectra of the DOWC18TCNQ mixed monolayers at the airlwater interface as shown in Figure 11. 4.0. Discussion The results presented here clearly show that TCNQ moieties in the cationic amphiphilelC18TCNQ mixed monolayers are reduced a t the airlwater interface. It has (37) Omit, M.; Mobius, D.; Lehman, U.; Meyer, H.J. Chem. Phys. 1986,85,4966.
Ahuja and Dringenberg
1522 Langmuir, Vol. 11, No. 5, 1995 1.0
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/\s
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the cationic head group plays a n important role in the formatiodstabilization of DCTC- and that other interfacial properties and parameters are of critical importance in the reduction process. We now consider the effect of the cationic head group in influencing the interfacial OH- ion concentration. A monolayer with a cationic head group at the airlwater interface is expected to lead to the accumulation of negatively charged OH- ions or other anionic counterions a t the monolayerlsubphase interface. The concentration of the counterions a t the interface depends on the GouyChapman potentia141of the monolayer which is given by the following expression
600
Wavelength [nm] Figure 13. Polarized light absorption spectra of the DOMAI CHTCNQ, molar ratio 1:1,mixed monolayer transferred onto the hydrophobic quartz substrate: angle ofincidence, 45";s-light polarized normal to the plane of incidence; p-light polarized parallel t o the plane of incidence. been established that the redox state of the TCNQ moiety is controlled by the interfacial concentration of OH- ions, the charge of the head group, the molar composition of the mixed monolayer and the nature of the co-counterions in the subphase. What is the mechanism of TCNQ reduction? The relevant parameters that have to be considered in this context are redox potentials of TCNQ-I TCNQO, TCNQ2-PTCNQ- pairs with respect to the OH-/ OW pair, interfacial OH- ion concentration, local dielectric constant, reactivity of the reduced TCNQ with oxygen, and the important role of the cationic head group in the reaction. Esumi et al.38 have found that TCNQ when adsorbed from acetonitrile solution on different metal oxide surfaces exhibits different colors due to the formation of TCNQ anion radicals and its aggregates. Raman spectra of thin TCNQ films ,On Ag electrodes39show that the first few layers (ca. 20 A) of TCNQ near the underlying Ag electrode are converted into DCTC- layers. No DCTCbands were however observed in the case of TCNQIAu and TCNQlCu films. This behavior was r a t i ~ n a l i z e din~ ~ terms of the catalytic properties of Ag in the formation of DCTC-. In addition, it has been found40 that strong electron acceptor molecules such as TCNQ and TCNE (tetracyanoethylene) are reduced in aqueous solutions. The reduction of TCNQ in these cases has been rationalized in terms of electron transfer from OH- ions to the TCNQ. In addition, Lombard0 and R ~ chave o ~ found ~ that during the recrystallization of D+TCNQ- from acetonitrile solutions, DCTC- is produced. The reaction chemistry of OH- ion includes Bransted proton transfer, nucleophilic displacement, and electron transfer whereby OH- ion acts as a one-electron reducing agent. The redox potentials of TCNQ35are E:, = f0.20.3 V and E$ = -0.33 Vvs SCE. In view of these redox data, spontaneous reduction of TCNQ in water is impossible. This is in confirmation with the results presented here where the pure C18TCNQ and C18TCNQlanionic lipids mixed monolayers do not show any reduction of TCNQ even on a subphase of pH 12. The fact that the reduction of Cl8TCNQ takes place exclusively in mixed monolayers with the cationic head group lipids shows that (38) Esumi, K.;Miyata, K.; Meguro, K. Bull. Chem. SOC. Jpn. 1985, 58,3524. (39)Yoshikawa, M.; Nakashima, S.;Mitsuishi, A. J.Raman Spectrosc. 1986,17,369. (40) Blyumendeld, L. A.;Bryukhovetskaya, L. V.; Fomin, G.V.; Shein, S. M.Russ. J.Phys. Chem. 1970,44, 518.
where e is the electronic charge, a the degree of dissociation of the head group, A the aredmolecule, E the dielectric constant of water, c b the bulk counterion concentration in the subphase, and T the absolute temperature. The interfacial counterion concentration is given by the following relation
(3) where c, and c b refer to the counterion concentrations a t the interface and in the bulk, respectively. The pH a t the interface is related to the Gouy-Chapman potential through the following relation41
(4) where pHi and pHb are the interfacial and bulk pH values. According to eq 4, the interfacial pH increases by one unit per 58 mV of VO. For the DOWC18TCNQ, molar ratio 1:1,mixed monolayer, A = 0.6 nm2 (a = 1, one positive charge per three aliphatic chains), Cb = 3.3 pM (pH 5.6), and taking E = 80, one obtains V O= 420 mV from eq 2. This means that the interfacial OH- ion concentration is higher by ca. a factor of lo7 compared to the bulk concentration a t pH 5.6. The higher OH- ion concentration at the interface is expected to influence not only the kinetics of the reduction process but also the electrochemical potential ofthe involved species (OH- and TCNQ) a s predicted by the Nernst equation. Another parameter of importance is the local dielectric constant a t the monolayerlsubphase interface. The dielectric constant a t any point in the interfacial region will be intermediate between that of the hydrophobic region of the monolayer ( E 2) and the bulk water ( E = 80). The value of interfacial dielectric constant has been measured to lie between 10 and 30. The value of E will influence the redox potential of the OH-/OH couple as it is controlled by the solvation energy of the OH- anion and its value has been found42to be 1.89 V in water and 0.92 V in acetonitrile. Hydroxide ion is thus expected to be a stronger base and a better electron donor a t the interface than in bulk water, because ofits reduced solvation energy. The reduced solvation of OH- decreases its ionization energy and causes it to have a more negative redox potential and to be a strong electron donor. The electron transfer between TCNQ and OH- reaction may be thought to proceed as
-
(41) Davies, J. T.;Rideal, E. K. Interfacial Phenomena; Academic Press: New York, 1963. (42) Sawyer, D. T.Acc. Chem. Res. 1988,21, 469.
Reduction of TCNQ
H0:-
+ TCNQ - HO' + TCNQ-
Langmuir, Vol. 11, No. 5, 1995 1523
(5)
Because HO'is a strong oxidant, a single electron transfer would be greater than 1V endergonic for TCNQ. Only a synchronous coupling of the electron transfer to a chemical reaction that results in a covalent bond formation of the HO' can account for the spontaneous redox activity. Although the coupling of two HO' to form H202 is one possibility, a more likely primary step for the reactants is attack by HO' of a n unsaturated center in the TCNQ or reduced TCNQ species. The role of cationic lipids in the reduction process lies therefore in the accumulation of OH- ions a t the interface, the formation and stabilization of TCNQ- radical anions, disproportionation of TCNQ- to TCNQO and TCNQ2-, the direct precursor to DCTC-, and the catalytic oxidation of TCNQ2- to DCTCthrough electrostatic interactions.
5.0. Conclusions Investigations of the molecular organization characteristics of ClBTCNQ in pure and in mixed monolayers with a variety of lipids have shown that C18TCNQ alone or in mixed monolayers with anionic and neutral lipids does not form well-defined monomolecular layers. The reflection spectrum (A, = 366 nm) ofthe pure ClBTCNQ monolayers and of the mixed monolayers with anionic and neutral lipids is to be attributed to H-aggregates of TCNQ moieties and multilayer formation. Monomer
species of TCNQ are observed only a t lower molar fraction values of C18TCNQ in mixed monolayers with DMPC. It has been shown that in the cationic lipidK18TCNQ mixed monolayers, a n unexpected spontaneous reduction of TCNQ moieties takes place a t the aidwater interface. The reduction of TCNQ is dependent on the OH- ion concentration, charge on the head group, molar composition of the mixed monolayer, and the nature of co-counterions in the subphase. The reduction process is discussed in terms of the interfacial OH- ion concentration, dielectric constant, and the specific role played by the cationic head group in the stabilization of reduced TCNQ species and the catalytic formation of DCTC-. The reduction ofTCNQ can be effectively stopped either by lowering the subphase pH or through the introduction of co-counterions which adsorb selectively a t the cationic monolayer/subphase interface or compete effectively with OH-. The results presented here show the crucial importance of optimizing the subphase conditions in controlling the molecular organization characteristics and the redox state of electroactive species in monolayers at the aidwater interface.
Acknowledgment. We thank Professor Dietmar MObius for support of this work and a critical reading of the manuscript. We also thank Mr. Werner Zeiss and Mrs. Gisela Debuch for helping out with some of the measurements. This work was funded by the BMFT (Grant No. 03M4008D). LA941029H
