Langmuir–Blodgett Films Containing TCNQ Incorporated from the Aqueous Subphase

Journal of Colloid and Interface Science 243, 156–164 (2001) doi:10.1006/jcis.2001.7886, available online at http://www.idealibrary.com on

Langmuir–Blodgett Films Containing TCNQ Incorporated from the Aqueous Subphase Pilar Cea, Hector Artigas, Jose S. Urieta, Maria C. Lopez, and Felix M. Royo1 Departamento de Qu´ımica Org´anica–Qu´ımica F´ısica, Facultad de Ciencias, Plaza de San Francisco, Ciudad Universitaria, 50009 Zaragoza, Spain Received February 20, 2001; accepted July 26, 2001

Films containing TCNQ (tetracyanoquinodimethane) anions were prepared by incorporating it from a LiTCNQ aqueous subphase. The TCNQ moiety was transferred from the subphase as the counterion of a positively charged monolayer of DPOP+ (diphenylbis(octadecylamino)phosphonium). The isotherm shape, the area per molecule, the collapse pressure, and the monolayer stability were dependent on the LiTCNQ concentration in the subphase. The obtained Langmuir–Blodgett films were classified into three types depending on the TCNQ oxidation state. The oxidation state depends on the transference pressure as well as on the monolayer age. The degree of both order and architecture of the films was studied by means of SEM, IR, UV–vis spectroscopy, and X-ray diffraction. Conductivity studies were also performed. °C 2001 Academic Press Key Words: Langmuir–Blodgett films; deposition; tetracyanoquinodimethane.

INTRODUCTION

Following with our line of work in the field of electroactive molecules (1) arranged in thin films (2–4) as well as in the technique of incorporation of ions from the subphase attracted by a charged monolayer (5–7), we present here the results of detailed research into the properties of Langmuir and Langmuir–Blodgett (LB) films containing tetracyanoquinodimethane (TCNQ). Since the discovery of the highly conducting charge-transfer (CT) complex TTF(tetratiafulvalene)TCNQ in the early 1970s, the efforts of many scientists have been focused on these kinds of compounds, opening a new field of interdisciplinary research with far-reaching consequences for material science. Inside this field of research, TCNQ as well as its derivatives have been extensively used as electron acceptor molecules in a large number of organic conducting systems (8), MIM (metal/insulator/metal) devices (9), COS (conducting organic salt) electrodes and biosensors (10, 11), and potentiometric sensors (12, 13), and even incorporated into electrochromic devices (14). To our knowledge all routes to incorporate TCNQ in LB films have implied the spreading of a TCNQ derivative in an original 1 To whom correspondence should be addressed. E-mail: femer@posta. unizar.es.

0021-9797/01 $35.00

C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.

neutral state (15), a TCNQ salt (16–19), or even a compound containing TCNQ in a direct mixed oxidation state (20–22), namely the homodoping route. The compulsory requirement in all cases was the synthesis of a TCNQ derivative containing long alkyl chains to prevent the compound from being dissolved in the subphase. We introduce here a different way of preparing films containing TCNQ, in which no TCNQ derivative is spread onto water. This method was originally applied to achieve TCNQ-ordered films and it is based on the well-known route of incorporating an ion from the subphase when an ionized monolayer is obtained at the gas–liquid interphase. This route has been used to prepare both organic/organic (23, 24) and inorganic/organic layers (5–7, 25). Our aim in this work is to see if it is possible to incorporate the TCNQ moiety from an aqueous subphase and we will try to analyze the advantages and disadvantages of this method versus the traditional ways mentioned above. Obviously one advantage is the synthesis process. With this method we only need a water-soluble TCNQ salt easily obtainable (e.g., LiTCNQ) and an organic salt capable of forming a stable ionized monolayer at the air–water interface (there are many salts commercially available). This fact makes it possible to experimentally assay many combinations, a useful ability given the importance of both theoretical and practical uses of thin films (26) containing TCNQ. Another advantage is that with this method we can decrease the number of alkyl chains (these chains often hinder the use of films for practical applications). A good example might be the preparation of MIM devices formed by hetero-LB films containing TCNQ. These devices require the presence of several molecules containing each one of the needed functional groups (for example sensitizer/electron acceptor) and traditionally they are prepared using alternating troughs (9). The incorporation of one of the moieties from the subphase would simplify the experimental work and decrease the number of alkyl chains. The two molecules employed in this paper, DPOPBr and LiTCNQ, were chosen for the following reasons. DPOPBr is a molecule capable of forming very stable Langmuir films (27) and is easily transferable onto a solid substrate yielding to LB films to a highly ordered degree. Besides, the cation DPOP+ has been used as a counterion in TCNQ salts (28–31) with good conductivity values (although not arranged on LB films, but in a 3D salt). The lithium salt of TCNQ·− was used because it

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FIG. 1. Chemical formula of (a) DPOP+ and (b) TCNQ·− .

is reasonably soluble in water. This has allowed us to prepare solutions in which the TCNQ·− concentration let this moiety compete with other anions (OH− and Br− ) in the formation of the double ionic layer and the ulterior transference process. The chemical formulas of DPOP+ and of TCNQ·− are given in Fig. 1.

The substrates (glass, CaF2 , quartz, and Lindemann glass capillaries for the SEM, IR, UV–vis, and X-ray measurements, respectively) were cleaned thoroughly, as was previously described (2, 27). The monolayers were transferred onto the substrates by the vertical dipping method. The dipping speed was 0.2 cm min−1 for the first layer and 0.6 cm min−1 for the following ones. Scanning electron microscopy was performed by means of a JEOL JSM 6400 microscope. The transmission IR spectra were recorded with a Jasco 410 Fourier transform IR spectrometer. The X-ray diffraction experiments were carried out using a Pinhole camera (Anton-Paar) operating with a Ni-filtered CuKα beam. The sample was prepared on the surface of a Lindemann glass capillary, whose axis was perpendicular to the X-ray beam. The diffraction pattern was collected on a photographic film. The UV–vis spectra were acquired on an UVIKON 941 double-beam spectrophotometer. The solution spectra were registered with a quartz cell 1 cm in path length. The spectra of LB films were collected with a normal incident angle with respect to the film plane. Conductivity studies in the film plane were undertaken using a two-probe technique. The resistance between the two wires was measured with a Hewlett Packard 4329 A, High Resistance Meter.

EXPERIMENTAL SECTION

DPOPBr has been provided by Prof. F. Palacios and Prof. D. Aparicio from the Departamento de Qu´ımica Org´anica de la Universidad del Pa´ıs Vasco (Spain). LiTCNQ was provided by Dr. Ballester from Universidad Complutense de Madrid (Spain). Their purity (better than 99%) was checked by spectroscopic techniques. Solutions of DPOPBr were prepared using chloroform as solvent; this was HPLC grade (99.9%) purchased from Sigma. The solutions were kept in dark bottles wrapped with aluminium foil in a refrigerator. The DPOPBr concentration used in this work was always 10−4 M. Aqueous solutions of LiTCNQ were prepared employing Millipore Milli-Q water (resistivity 18.2 MÄ · cm). The solutions were stored in darkness and they were always used within 1 month. Several concentrations (from 5 × 10−5 to 10−6 M) were used as subphase. This study has been performed with a Teflon trough (460 * 210 mm2 ) designed by us and whose details have been reported before (32). The surface pressure of the monolayers was measured with a Wilhelmy paper plate pressure sensor and it was tested periodically with the π–A curve of the docosanoic acid monolayer. The uncertainty in the area per molecule obtained from the isotherms is about ±5%. In order to guarantee the reproducibility of the results each isotherm was registered at least three times. Concerning the procedure, first, the DPOPBr solution was carefully and slowly spread on the aqueous surface of the trough. After waiting about 15 min to allow the solvent to evaporate, the compression slowly began at a controlled sweep˚ 2 mol−1 min−1 ). All the experiments were caring speed (1.5 A ried out at 20◦ C. The gaseous phase was air and no degasification of the LiTCNQ solutions was performed.

RESULTS

Langmuir Films Given that the molecule DPOPBr leads to the formation of positive-charged monolayers at the air–water interface, it is expected that both the nature and the concentration of the counterions would condition the isotherm shape as well as the properties of the Langmuir films. We have previously studied (27) the influence of the subphase nature, showing how it determines the isotherm shape, the stability of the monolayer, the transfer ratio, and the LB film quality. In this work we focus our attention on the second point, the influence of the TCNQ·− concentration. Figure 2 shows isotherms of DPOP+ monolayers registered at several concentrations of LiTCNQ in the subphase. We can see that the higher the concentration, the lower the collapse pressure, and the higher the area per molecule at a certain pressure. Moreover, we would say that the kind of collapse will differ depending on the subphase concentration. When concentrations of 5 × 10−5 M of LiTCNQ are used in the subphase a collapse at 25 mN/m can be observed in the isotherm form. After the collapse long, sharp, dark-blue needles can be seen on the water surface. Such needles are perpendicular to the sweeping direction; that is, they are parallel to the barrier. When concentrations of 5 × 10−6 M are used the collapse pressure appears at 30 mN/m, and in this case bluish aggregates with no particular order are formed at the air–water interface. Finally, if concentrations of 10−6 M are employed the collapse pressure is 32 mN/m and now no needles or visible aggregates appear. All the isotherms present an overshoot and a plateau. In those concerning concentrations of 5 × 10−6 M or greater a collapse

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FIG. 2. Isotherms of DPOP+ using in the subphase (a) pure water, (b) LiTCNQ 10−6 M, (c) LiTCNQ 5 × 10−6 M, and (d) LiTCNQ 5 × 10−5 M.

in 3D is evident. Nevertheless, we believe that at 10−6 M the origin of this plateau is different. In the latter case, the pressure increases again after the plateau and the area per molecule after it is one-half the area just before it begins. This plateau also appears in the isotherms of DPOP+ when pure water is used as subphase and a detailed study of this phenomenon (27) concluded that the molecules suffered a rearrangement of the head groups (overshoot) and the formation of a bilayer (plateau). The experiments performed using LiTCNQ in the subphase (values of the area per molecule, hysteresis cycles, and the different slopes of the films absorbance vs numbers of layers transferred before and after the plateau) indicate that we are facing again the same situation as when pure water is used as subphase. The stability of the monolayers is quite good in all the cases although it decreases slightly when the subphase concentration increases. In fact, the best stability is achieved when pure water is used in the subphase (the molecule area lost is only 1% in a 12-h-old monolayer), and the worst using 5 × 10−5 M LiTCNQ solutions (7% in 12 h). To sum up, the isotherms show a great dependence on the nature of the subphase and the LiTCNQ concentration. All this seems to indicate that the nature of the double ionic layer is quite different, and this means different molecular interactions and different rearrangements (different values of the area/molecule) that lead to monolayers showing different interactions among the molecules, different packaging (see later X-ray experiments and compare with DPOP in pure water (27)), and in consequence different physical properties. We will try to take advantage of this situation to prepare several types of LB films and to maneuver thin films into the desired properties. Langmuir–Blodgett Films The concentration of the subphase and the transference pressure are also expected to determine the type of deposition, the

transference ratio, and obviously the LB film properties. Several multilayers were prepared, changing the LiTCNQ concentration in the subphase (between 5 × 10−5 and 10−6 M), and deposited at transference pressures between 20 and 40 mN/m. Lower concentrations of LiTCNQ in the subphase lead to LB films with a poor transference of TCNQ; they are especially low if many layers are transferred. The monolayer deposition was made onto substrates of glass, Lindemann glass capillaries, CaF2 , and quartz. All the experiments were performed several times in order to guarantee the reproducibility and quality of the obtained results. The first evidence, later confirmed by spectroscopic techniques, of the TCNQ incorporation onto the LB films was their blue color, given that films of DPOPBr using pure water in the subphase are white. The transferences were always Y-type in spite of changing both the concentration and the transference pressure. It is important to remark here that DPOP+ monolayers onto pure water as subphase lead to a Y-type deposition only in the first transference and to a Z-type in the following ones. This result is in agreement with previous papers reporting how the nature of the subphase can determine the type of transference and even the packaging state of the alkyl chains in the monolayer (19). On the other hand, both the transference and the concentration pressure influence the deposition ratio. At a fixed transference pressure the deposition ratio improves when the TCNQ·− concentration decreases. At a fixed concentration, it improves when the transference pressure increases. Depositions using 10−6 M LiTCNQ at 30 mN/m have transference ratios close to 1, although after the plateau the transference ratio decreases until 0.8. Experiments at different concentrations of LiTCNQ have been performed with extremely similar results. Then, given that 10−6 M LiTCNQ concentration leads to the best transference ratios all the results reported correspond to films prepared using this concentration. To assess the reproducibility of the deposition process, the variation in LB film absorption with the number of layers at a fixed wavelength has been obtained. Figure 3 depicts the absorbance at 226 nm (band due to the DPOP+ moiety) and 630 nm (band due to the TCNQ moiety) versus the number of layers of an LB film on quartz substrate. The relationship between absorbance and number of layers, which is linear up to 25 layers, demonstrates a constant transfer ratio during the deposition and a constant architecture in LB films. An X-ray diffraction study has been made with 27 layers of transferred films onto Lindemann glass capillaries at 30 mN/m. Two diffuse rings corresponding to θ = 1.3 and θ = 3.6 were obtained. Following the Bragg law (λ = 2d sin θ ; ˚ these angles correspond to 33 ± 1, A ˚ and with λ = 1.54 A) ˚ ˚ 12 ± 1 A, respectively. The value of 33 A might correspond to the distance of two polar heads separated by the alkyl chains corresponding to two DPOP+ moieties in a bilayer. On the other ˚ distance might be due to the distance between two hand, the 12-A neighbor DPOP+ moieties in the same layer. Both interpretations are in agreement with the size of the DPOP+ cation obtained

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time, changing their initial color to pink–violet. This transition is highly dependent on the temperature, revealing the presence of a kinetic effect. At 20◦ ± 2◦ C it takes several days (7–10). However, at 28◦ ± 2◦ C it takes only some hours (10–12). If the films are kept into a refrigerator (−10◦ C) they do not evolve to the pink–violet color (at least for 1 year). Figure 5 shows the SEM of a LB film whose color was pink after 4 days at 25◦ C. In some regions of the film the initial homogeneity disappears and some 3D crystals and irregular domains appear. Films of type 2. Films transferred from old monolayers (here old means more than 4 h), at lower pressures than those corresponding to the plateau in the isotherm, are type 2 films. Their original color is blue–grey, again more or less intense depending on the number of layers and the transference pressure. In this case no evolution with time has been observed. Films of type 3. Films transferred after the plateau in the isotherm are type 3 films. They have a quite peculiar appearance. Their color is green if they are looked from the side view and brown if from the front. Moreover, they have a special brightness, resembling the typical metallic luster. Again no evolution is observed and they are stable with time. FIG. 3. Absorbance vs number of layers at (j) 226 nm and (d) 630 nm in a LB film transferred at 30 mN/m using a 10−6 M LiTCNQ subphase.

with simulation programs (chem 3D). On the other hand, this architecture is in concordance with a Y deposition (polar head + alkyl chain − alkyl chain + polar head). The quality and homogeneity of the films was also confirmed by SEM experiments. Figure 4a shows a photograph of a LB film of DPOP+ TCNQ·− transferred at 20 mN/m using a subphase 5 × 10−6 M of LiTCNQ. The LB film looks very homogeneous and is reminiscent of a weave shape. Figure 4b shows a LB film of DPOP+ TCNQ·− transferred at 30 mN/m using a subphase 10−6 M of LiTCNQ. Again the film is very homogeneous, and it also is reminiscent of a weave. No significant difference was observed in the morphology shown in the pictures of LB films transferred from young monolayers or old ones (what we later call films of type 1 and 2, respectively). Finally, Fig. 4c shows a film that was transferred after the above-mentioned collapse in which sharp long needles are formed, and Fig. 4d is a magnification in which we can observe that the needles are formed by an infinite number of microcrystals in 3D, oriented in a preferential direction, that is, perpendicular to the dipping direction. Finally, the physical appearance of the LB films as well as their properties have been found to be dependent both on the transference pressure and on the age of the monolayer. We have classified the obtained films into three groups, whose characteristics and properties have been summarized in Table 1 and discussed in the following. Films of type 1. Films transferred from young monolayers whose initial color is blue, and more or less intense depending on the number of layers and the transference pressure, are type 1 films. They are not stable and show a clear evolution with

Infrared Spectra The frequencies of methylene stretching vibration modes, νs (CH2 ) and νa (CH2 ) are indicative of the ordered status of packing of monolayer molecules in the solid state (33–37). When the alkyl chain is highly ordered (trans-zigzag conformation), the bands appear near 2918 and 2850 cm−1 , respectively, while if conformational disorder is included in the chain, they shift near 2927 and 2956 cm−1 , depending on the ratio of gauche TABLE 1 Properties of LB Films as a Function of the Preparation Conditions Films Determinants

Type 1

Type 2

Type 3

Transferred from Langmuir films Transfer ratio Color of the pristine film λ max/nm

Not aged

Aged

≈1 Blue

≈1 Blue–grey

630 and 670

610 and shoulder at 665 Yes

After the plateau in the isotherm ≈0.8 Green–brown luster 400–700 weak and structureless Yes

Stability Mixed valence state in the pristine films and majoritary oxidation state CT band (IR conductivity) Macroscopic dc conductivity of the pristine film

No No

No

Yes TCNQ·− and TCNQ0 Yes

10−7 S cm−1

10−5 S cm−1

TCNQ·−

No TCNQ0 No 10−8 S cm−1

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FIG. 4. SEM photographs of (a) LB film of DPOP+ TCNQ·− transferred at 20 mN/m using a subphase 5 × 10−6 M of LiTCNQ; (b) LB film of DPOP+ TCNQ·− transferred at 30 mN/m using a subphase 10−6 M of LiTCNQ; (c) sharp, long, dark needles formed at the air–water interface after the collapse; and (d) magnification of (c).

conformations. The hydrocarbon tails in the LB films prepared on pure water as subphase present bands at 2918 and 2848 cm−1 , while the LB films prepared with LiTCNQ have the bands at 2923 and 2852 cm−1 in films of type 1, 2919 and 2850 cm−1 in films of type 2, and 2918 and 2850 cm−1 in films of type 3. These data indicate that in all cases the hydrocarbon chains present a high degree of conformational order. On the other hand, the CH2 scissoring band of the hydrocarbon chain is sensitive to the intermolecular interactions. The splitting of δ(CH2 ), a 1468 cm−1 peak, observed in all types of DPOP–TCNQ films, provides an indication of orthorhombic subcell packing with two molecules in one unit cell (37–39).

Figure 6 shows the IR spectra of the three types of films in the region 3600–1800 cm−1 . We can observe that either pristine films of type 1 or 3 do not present an initial CT band, or at least it is very weak. Meanwhile films of type 2 show a CT band, and then IR conductivity. Jacobsen et al. (40) have proposed a quantitative correlation between the position of the maximum of the CT absorption band and the dc conductivity value measured at room temperature. Following this relationship, films of type 2 with CT centered about 3000 cm−1 (more or less depending on the specific conditions of preparation) should have a conductivity at room temperature of about 0.1–1 S cm−1 .

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TABLE 2 Characteristic IR Bands of TCNQ, Assignation, and Their Presence in Several Types of Film Wavenumbers (cm−1 )

Relative intensities in films of

Mode

TCNQ0

TCNQ·−

Type 1

Type 2

Type 3

b1u ν19 b1u ν19 b1u ν20 b1u ν20

2211 — 1545 —

— 2189 — 1507

m vs m m

s m s s

vs; br sh m w

Note. vs, very strong; s, strong; m, middle; w, weak; sh, shoulder; br, broad. Assignations from Refs. (41–43).

FIG. 5. SEM photograph of an aged LB film (4 days at 25◦ C) transferred at 30 mN/m using LiTCNQ 10−6 M in the subphase.

The IR bands of TCNQ in several oxidation states have been widely studied and reported (41–43). Following the assignations made by these authors, Table 2 shows some of the most helpful bands in the determination of the redox state of the TCNQ moieties. From these data we can see that in films of type 1 the

TCNQ has been mainly transferred in the radical anion state (strong band situated at 2189 cm−1 ). Although the bands attributable to the neutral oxidation state also appear their relative intensities are lower. On the other hand, films of type 2 contain both TCNQ0 and TCNQ·− , confirmed by the presence of bands at 2211 (strong) and 2189 (weak), respectively. Chappell et al. (44) have established a relationship between the frequencies in the IR spectra and the degree of charge transfer, ρ. According to this relationship, films of type 2 present a ρ = 0.3. These facts together with the CT band confirm that a mixed valence state has been achieved and films of type 2 are expected to show a macroscopic conductor character. Finally, films of type 3 have a band at 2211 cm−1 as well as a very small shoulder about 2190 cm−1 (not even seen in Fig. 6 unless it is amplified). In this case the TCNQ is mainly in the neutral oxidation state and the films are supposed to be insulators. UV–vis Spectra

FIG. 6. IR spectra of LB film (a) type 1, (b) type 2, and (c) type 3.

The UV–vis spectrum of a 5 × 10−5 LiTCNQ solution in water has two bands at 315 and 610 nm (Fig. 7a). Such bands are attributable to local excitations of the TCNQ radical anions forming the dimeric units (42, 45). The spectrum of a LB film transferred at 30 mN/m using a 10−6 M LiTCNQ subphase is shown in Fig. 7b. Now the bands appear at 350, 630, and 670 nm; that is, all of them are redshifted (J-aggregates) with respect to the solution. To our knowledge no author has reported the existence of two clear separated bands in the region 600–700 nm although in some cases a shoulder has appeared (46). Some authors (47) have observed the presence of two bands at low temperatures (77 K). Richard et al. (46) considered that these bands are the residue of the very pronounced vibronic structure observed for the corresponding locally excited transition at 850 nm in monomeric TCNQ·− . Figure 7c corresponds to a LB film of type 1 allowed to evolve with time under air exposure; a final pink color is achieved, and now the region from 600 to 900 nm presents a weak, smooth, and structureless absorption. Besides, a new band at 470 nm appears. This band is due (48–50) to the decomposition of the dianion, TCNQ2− , to form a negatively charged oxygen decay product DCTC− (α, α-dicyano- ptolyoyl cyanide). The broad band at 530 nm also observed in this

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they have values oscillating between 10−8 and 10−6 S cm−1 , and finally they have a conductivity of 10−5 S cm−1 . Pristine films of type 2 have conductivities of 10−5 S cm−1 and films of type 3 are insulators (10−8 S cm−1 ). DISCUSSION

FIG. 7. UV–vis spectra of (a) LiTCNQ solution 10−5 M; (b) pristine film (type 1) transferred at 30 mN/m using a 10−6 M LiTCNQ subphase; (c) final state of (b) after its evolution at air; (d) film type 2; and (e) sharp long dark needles.

spectrum might be due to the presence of a large number of electronic configurations that lie in a relatively narrow energy region and can interact with each other. The spectrum of a film of type 2 can be seen in Fig. 7d. Its main band is centered at 610 nm and presents a shoulder at 665 nm that is slightly blueshifted with respect to pristine films of type 1. Films of type 3 present a very weak and structureless absorption in the region 400–700 (not shown spectrum). In this case no clear band appears. The UV–vis spectrum of the sharp, long, dark needles obtained after the collapse at the air–water interface has two bands at 510 and 650 nm, revealing the different rate of aggregation with respect to true LB films (Fig. 7e). Macroscopic Conductivity Measurements The conductivity measurements of the LB films were performed using two electrical contacts to the film. It was assumed ˚ per layer (46). No that the electroactive phase thickness is 10 A anisotropy of conductivity was found in the film plane. Films of type 1 showed an initial conductivity of 10−7 S cm−1 . The films were doped with iodine. The doping iodination process was carried out by dipping the films into an atmosphere saturated with iodine vapor for 5–10 min. Afterward, the films were withdrawn from the vessel containing the iodine and then the films were allowed to equilibrate without the I2 vapor. This process was followed by both IR and UV–vis spectroscopy, obtaining results in complete agreement with other previously reported in TCNQ films (47). Just after the oxidation process with iodine

We have seen that it is possible to prepare LB films containing the TCNQ moiety by incorporating it from an aqueous subphase. In addition, the TCNQ is incorporated in several oxidation states depending on the specific experimental conditions. In films of type 1 an electrostatic interaction between the TCNQ·− and the DPOP+ takes place, leading to the formation of films in which no mixed valence state is achieved. Films of type 2 show a nonintegral degree of ionicity. Here the question is why a nonintegral oxidation state is achieved if initially the subphase only contains TCNQ·− . Obviously we have to admit that an oxidation process takes place at any step in the LB film preparation, specifically at the air–water interface or at the air–water–substrate interface. It is well known (3, 19, 51–53) that some reactions which do not take place in the bulk aqueous phase may become possible at the air–water or monolayer–subphase interface and this is the case we are now facing. While solutions of LiTCNQ are stable for several weeks and they present a slow rate of degradation several hours are enough to induce an oxidation process at the air–water interface. To explain such a phenomenon the relevant parameters that have to be considered are not only the redox potentials 0/− of TCNQ (E 1/2 = +0.2 − 0.3 V), but also the local dielectric constant, the role of the DPOP+ in the reaction, and the other anions or cations situated in or near the double ionic layer. The presence of these ions (OH− , H+ , Li+ , TCNQ·− ) as well as their concentrations (that depends on the Gouy–Chapman potential (54)) are expected to influence both the electrochemical potential of the involved species (as predicted by the Nernst equation) and the kinetics of the reaction (19). In young monolayers the oxidation process of the TCNQ·− has had not enough time to occur or at least not in a high proportion, given that multilayers obtained from young Langmuir films present a few ratio of TCNQ0 . Nevertheless, multilayers obtained from old monolayers are formed by both TCNQ·− and TCNQ0 . This fact reveals the great importance of the kinetic effect, which we can take advantage of to control the final oxidation state of the LB films. In films of type 3 a rearrangement of the head groups and the whole monolayer to form a bilayer takes place and obviously this will have an important influence on the double ionic layer, the concentration of the ions in the subphase, and the interactions among them. We have also to take into account that in order to reach 40 mN/m more time is needed than to reach 30 mN/m (remember that there is a plateau in the isotherm), making it unavoidable that these Langmuir films are older than those before the plateau, so this also might contribute to the final situation although in a shorter proportion. Another important point in this study is the different stability of the three types of films obtained. Films of type 1 are unstable.

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FIG. 8. IR spectra of (a) pristine film (type 1) transferred at 30 mN/m using a 10−6 M LiTCNQ subphase and (b) final state at air.

Such instability has been previously reported in some films containing TCNQ (55–57) and it is due to the degradation of the TCNQ2− in the presence of oxygen to form DCTC− (49). The explanation of such degradation in films of type 1 can be attributed to the spontaneous disproportionation of TCNQ·− to TCNQ0 and TCNQ2− . The latest one is then transformed into DCTC− by means of the atmospheric oxygen. The degradation process can be avoided, maintaining the films in a free oxygen atmosphere or keeping them at low temperatures. The IR spectrum of type 1 films supports this affirmation, given that a clear evolution with time has been observed in which the band at 2189 cm−1 (TCNQ·− ) decreases while the band at 2211 cm−1 (TCNQ0 ) increases its intensity (Fig. 8). The bands due to the TCNQ2− presence (42, 58) (2164, 1576, 1498, and 1299 cm−1 ) also appear in the spectrum although their intensity is very weak, probably because of a quick evolution to the decay product, confirmed by the continuous increase of the band at 480 nm in the UV–vis spectrum. Films of type 2 do not show this evolution with time. In these films there are both TCNQ·− and TCNQ0 . Probably the equilibrium of the two species and the interaction between them to form stable aggregates prevents the formation of TCNQ2− and its ulterior transformation into DCTC− . Films of type 3 are mainly made up of TCNQ in the neutral state and again no evolution is observed because the TCNQ0 reduction is not a spontaneous process and thus the direct precursor to DCTC− is not formed.

Films containing TCNQ present many applications, as was described in the Introduction. One of them is their use as conducting or semiconducting organic materials. Meanwhile pristine films of types 1 and 3 are insulators, and films of type 2 have IR conductivity, although the experimental values of dc conductivity show differences of 4 orders of magnitude with respect to the data expected from IR measurements. The disagreement of IR and dc conductivity values has been reported before and attributed to the polycrystalline nature of the films (59). The homodoping route mentioned in the Introduction allows obtainment of LB films with macroscopic conductivities between 10−2 and 10−7 S cm−1 , depending on the stoichometry and the cation employed (55). The values obtained with the method here employed (10−5 S cm−1 ) are situated in that range although more experiments are needed to see if other cations are able to lead to homogeneous and stable films without being impenetrable barriers to the electron movement due to the steric hindrance of alkyl chains. Moreover, the cation presence probably will also determine the final degree of ionicity. To sum up, this route of TCNQ incorporation into LB films leads to homogeneous and stable multilayers in which an exhaustive control of the experimental conditions allows a determination of the final oxidation state, depending on the desired practical use of the films, which are many and varied as was said in the Introduction. It also simplifies the synthesis process as well as decreases the number of alkyl chains in mixed multilayers. Nevertheless, more experiments are needed in order to see if other cations in the Langmuir film or other TCNQ salts (using TCNQ directly in a mixed oxidation state) in the subphase can improve or modify final LB film properties. ACKNOWLEDGMENTS We are grateful for financial assistance from the project of Ministerio de Educaci´on y Ciencia (PB96-0723 and BQU2000-1165), and from Diputaci´on General de Arag´on (P57/97). Diphenyl bis(octadecylamino)-phosphonium bromide was kindly provided by Profs. F. Palacios and D. Aparicio from the Departamento de Qu´ımica Org´anica de la Facultad de Farmacia de la Universidad del Pa´ıs Vasco. The authors thank Dr. Ballester et al. from Universidad Complutense de Madrid for the synthesis of LiTCNQ. Finally, we are especially grateful to Dr. J. Barber´a from the Departamento de Qu´ımica Org´anica de la Universidad de Zaragoza for his valuable advice on the X-ray studies.

REFERENCES 1. Cea, P., Pardo, J., Urieta, J. S., L´opez, M. C., and Royo, F. M., J. Electrochim. Acta 46, 589 (2000). 2. Cea, P., Morand, J. P., Urieta, J. S., L´opez, M. C., and Royo, F. M., Langmuir 12, 1541 (1996). 3. Cea, P., Lafuente, C., Urieta, J. S., L´opez, M. C., and Royo, F. M., Langmuir 13, 4892 (1997). 4. Cea, P., Lafuente, C., Urieta, J. S., L´opez, M. C., and Royo, F. M., Langmuir 14, 7306 (1998). 5. Mingotaud, C., Lafuente, C., Amiell, J., and Delhaes, P., Langmuir 15, 289 (1999). 6. Lafuente, C., Mingotaud, C., and Delha`es, P., Chem. Phys. Lett. 302, 523 (1999). 7. Ravaine, S., Lafuente, C., and Mingotaud, C., Langmuir 14, 6347 (1998).

164

CEA ET AL.

8. Simon, J., and Andre, J. J., “Molecular Semiconductors.” Springer-Verlag, New York, 1985. 9. Choi, J.-W., Jung, G.-Y., Oh, S. Y., Lee, W. H., and Shin, D. M., Thin Solid Films 284–285, 876 (1996). 10. Centonze, D., Losito, I., Malitesta, C., Palmisano, F., and Zambonin, P. G., J. Electroanal. Chem. 435, 103 (1997). 11. Scaboo, K. M., and Chambers, J. Q., Electrochim. Acta 43, 3257 (1998). 12. Ruzicka, J., and Lamm, C. G., Anal. Chim. Acta 54, 1 (1971). 13. Sharp, M., Anal. Chim. Acta 85, 17 (1976). 14. Yasuda, A., and Seto, J., J. Electroanal. Chem. 247, 193 (1988). 15. Vandevyver, M., Ruaudel-Teixier, A., Palacin, S., Bourgoin, J. P., and Barraud, A., Mol. Cryst. Liq. Cryst. 187, 327 (1990). 16. Nakamura, T., Yunome, G., Azumi, R., Tanaka, M., Tachibana, H., Matsumoto, M., Horiuchi, S., Yamochi, H., and Saito, G., J. Phys. Chem. 98, 1882 (1994). 17. Nakamura, T., Takei, F., Matsumoto, M., Tanaka, M., Sekiguchi, T., Manda, E., Kawabata, Y., and Saito, G., Synth. Met. 19, 681 (1987). 18. Naito, K., and Miura, A., Thin Solid Films 243, 342 (1994). 19. Ahuja, R. C., and Dringenber, B. J., Langmuir 11, 1515 (1995). 20. Ruaudel-Teixier, A., Vandevyver, M., Roulliay, M., Bourgoin, J. P., Barraud, A., Lequan, M., and Lequan, R. M., J. Phys. D: Appl. Phys. 23, 987 (1990). 21. Yartsev, V. M., Bourgoin, J. P., Delha`es, P., Vandevyver, M., and Barraud, A., Synth. Met. 57, 3801 (1993). 22. Yartsev, V. M., Fichet, O., Bourgoin, J. P., and Delha`es, P., J. Phys. II 3, 647 (1993). 23. Gr¨uniger, H., M¨obius, D., and Meyer, H., J. Chem. Phys. 79, 3701 (1983). 24. Takahashi, M., Kobayashi, K., and Takaoka, K., Langmuir 16, 6613 (2000). 25. Tachibana, H., Yamanaka, Y., Sakai, H., Abe, M., and Matsumoto, M., Chem. Mater. 12, 854 (2000). 26. Petty, M. C., Thin Solid Films 210, 417 (1992). 27. Cea, P., Lafuente, C., Urieta, J. S., L´opez, M. C., and Royo, F. M., Langmuir 12, 5881 (1996). 28. Bryce, M. R., Ahmad, M. M., Friend, R. H., Obertelli, D., Fairhurst, S. A., and Winter, J. N., J. Chem. Soc., Perkin Trans. 2 1151 (1988). 29. Allen, D. W., Millar, I. T., and Tebby, J. C., Tetrahedron Lett. 6, 745 (1988). 30. Bryce, M. R., Moore, A. J., Kim, Y. H., Liu, A. X., and Nowak, M. J., Tetrahedron Lett. 28, 4465 (1987). 31. Lequan, M., Lequan, R. M., Batail, P., Halet, J. F., and Ouahab, L., Tetrahedron Lett. 24, 3107 (1983). 32. Royo, F. M., L´opez, M. C., Ruiz, B., Camacho, A., Lozano, J. M., and Urieta, J., Rev. Acad. Cienc. Exactas, Fis., Quim. Nat. Zaragoza 48, 177 (1993). 33. Tasumi, M., and Shimanouchi, T., J. Chem. Phys. 43(4), 1245 (1965). 34. McKean, D. C., Biedermann, S., and B¨urger, H., Spectrochim. Acta, Part A 33, 845 (1974).

35. Snyder, R. G., Aljibury, A. L., Strauss, H. L., Casal, H. L., Gough, K. M., and Murphy, W. F., J. Chem. Phys. 81, 5352 (1984). 36. Gericke, A., and K¨uhnerfuss, H., Ber. Bunsen-Ges. Phys. Chem. 99, 641 (1995). 37. Sapper, H., Cameron, D. G., and Mantsch, H. H., Can. J. Chem. 59, 2543 (1981). 38. Snyder, R. G., and Schachtschneider, J. H., Spectrochim. Acta 19, 85 (1963). 39. Umemura, J., Cameron, D. G., and Mantsch, H. H., Biochim. Biophys. Acta 606, 32 (1980). 40. Jacobsen, C. S., Johannsen, I., Bechgaard, K., Barraud, A., and Holezer, K., Phys. Rev. Lett. 53, 194 (1986). 41. Girlando, A., and Pecile, C., Spectrochim. Acta, Part A 29, 1859 (1973). 42. Bozio, R., Zanon, I., Girlando, A., and Pecile, C., J. Chem. Soc., Faraday. Trans. 2 74, 235 (1978). 43. Meneghetti, M., and Pecile, C., J. Chem. Phys. 84, 4149 (1986). 44. Chappell, J. S., Bloch, A. N., Bryden, W. A., Maxfield, M., Poehler, T. O., and Cowan, D. O., J. Am. Chem. Soc. 103, 2442 (1981). 45. Boyd, R. H., and Phillips, W., J. Chem. Phys. 43, 2927 (1965). 46. Richard, J., Vandevyver, M., Lesieur, P., Barraud, A., and Holczer, K., J. Phys. D: Appl. Phys. 19, 2421 (1986). 47. Richard, J., Vandevyver, M., Lesieur, P., Ruaudel-Teixier, A., Barraud, A., Bozio, R., and Pecile, C., J. Chem. Phys. 86, 2428 (1987). 48. Hertler, W. R., Hartzler, H. D., Acker, D. S., and Benson, R. E., J. Am. Chem. Soc. 84, 3387 (1962). 49. Lombardo, A., and Rico, T. R., J. Org. Chem. 44, 209 (1979). 50. Grossel, M. C., Duke, A. J., Hibbert, D. B., Lewis, I. K., Seddon, E. A., Horton, P. N., and Weston, S. C., Chem. Mater. 12, 2319 (2000). 51. Goldenberg, L. M., Khodorkowsky, V. Y., Becker, J. Y., Lukes, P. J., Bryce, M. R., Petty, M. C., and Yarwood, J., Chem. Mater. 6, 1426 (1994). 52. Ward, R. J., Dhindsa, A. S., Bryce, M. R., Munro, H. S., and Petty, M. C., Thin Solid Films 198, 363 (1991). 53. Dhindsa, A. S., Song, Y. P., Badyal, J. P., Bryce, M. R., Lvov, Y. M., Petty, M. C., and Yarwood, J., Chem. Mater. 4, 724 (1992). 54. Davies, J. T., and Rideal, E. K., “Interfacial Phenomena.” Academic Press, New York, 1963. 55. Bourgoin, J. P., Th`ese de l`Universit´e de Paris XI, Orsay, 1991. 56. Perez, J., Bourgoin, J. P., Barisone, C., Vandevyver, M., and Barraud, A., Thin Solid Films 244, 1043 (1994). 57. Fichet, O., Agricole, B., Kassi, H., Desbat, B., Gionis, V., Leblanc, R. M., Garrigou-Lagrange, C., and Delha`es, P., Thin Solid Films 243, 592 (1994). 58. Khoo, S. B., Foley, J. F., Korzaniewski, C., Pons, S., and Marcott, C., J. Electroanal. Chem. 233, 223 (1987). 59. Richard, J., Delha`es, P., and Vandevyver, M., New J . Chem. 15, 137 (1991).