Electrochemistry of cis-Azobenzene Chromophore in Coulombically

image recording devices,3,4 to stimulate a reversible conforma- tional transition ... electrochemical behavior of cis-form of the azobenzene chro- mop...
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J. Phys. Chem. 1996, 100, 17337-17344

17337

Electrochemistry of cis-Azobenzene Chromophore in Coulombically Linked Self-Assembled Monolayer-Langmuir-Blodgett Composite Monolayers Zhongfan Liu,* Chunxia Zhao, Ming Tang, and Shengmin Cai Center for Intelligent Materials Research (CIMR), College of Chemistry and Molecular Engineering, Peking UniVersity, Beijng 100871, China ReceiVed: December 11, 1995; In Final Form: March 6, 1996X

A SAM-LB composite film containing azobenzene chromophore was fabricated on gold by combining the self-assembled monolayer (SAM) technique and the Langmuir-Blodgett (LB) technique. Reflection absorption FTIR studies indicate that the composite film has an ionic bonding character at the SAM-LB interface, which has greatly improved the film stability and effectively prevented the destructive intermolecular aggregation, as evidenced by atomic force microscopy observations. The relatively loose packing structure provides enough free volume to undergo a reversible trans-cis photoisomerization and thus enabled us to perform the electrochemical studies of the cis-azobenzene redox group in such organized monolayer assemblies. The cis-azobenzene SAM-LB composite film shows a stable and clearcut Faradaic response within the pH range 3.0-9.0, attributable to the 2e- and 2H+ electrochemical reduction-oxidation of cis-azobenzene/hydrazobenzene. Comparing with the trans-azobenzene composite film, the LB films deposited on SnO2, and the azobenzene-functionalized thiolate SAMs on gold, its reaction kinetics is more reversible as seen from the larger electron-transfer rate constant, smaller peak-to-peak splitting, and smaller half-peak width. The logarithmic rate constant shows a complicated but unique pH dependence, characteristic of azobenzene redox kinetics, which is explained by the varied sequences of electron transfers and protonations in different pH ranges.

Introduction Highly organized organic films such as Langmuir-Blodgett (LB) films and self-assembled monolayers (SAMs) have attracted a great deal of interests in the past decade because of their numerous potential applications.1,2 We have been paying particular and continuous attention to the azobenzene-functionalized monolayer assemblies. Azobenzene chromophore is photochemically reactive, originating from its reversible transcis photoisomerization, and is also electrochemically reactive owing to its reversible reduction-oxidation. This makes the azobenzene system rich of possibilities for various technological and fundamental studies. For instance, one may use the transcis isomerization phenomenon to create optical switching or image recording devices,3,4 to stimulate a reversible conformational transition of polypeptides,5 and to control the ion permeability of polymer membranes.6 With the electrochemical reactivity, one may construct a multimode chemical signal transducer,7 or employ the azobenzene chromophore as a probe to study the long-range electron-transfer kinetics and proton migration into a densely packed film.8-10 In our previous work, we have intensively studied the photochemistry and electrochemistry of azobenzene-functionalized LB films11-18 and found quite a novel photoelectrochemical hybrid phenomenon which combines the trans-cis photoisomerization and electrochemical reduction-oxidation together in a route-specific way on the basis of the remarkable difference in electrochemical reductivities of trans and cis isomers.11,12 With this hybrid reaction cycle, we have proposed a number of unique applications such as ultrahigh-density photoelectrochemical information storage,13 in situ electrochemical actinometry,14,15 and electrochemical quantification of cis and trans isomers of azobenzene in ultrathin molecular films for kinetic studies.16,17 For such kinds of * To whom correspondence should be addressed. Telephone and FAX: 86-10-6275-7157. E-mail: [email protected]. X Abstract published in AdVance ACS Abstracts, September 15, 1996.

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studies, the LB films are often deposited on conductive substrates like SnO2 or Au-evaporated glasses. What racked our brains greatly is the instability of the LB film structures. Aggregation (or, in other words, rearrangement) of molecules on the supporting substrates, which leads to the collapse of organized film structure, and simple peeling off of the LB films into the adjacent solution frequently occur due to the weak interactions between the LB film and the substrate. To overcome this instability problem, we have attempted to introduce the azobenzene chromophore into a self-assembled monolayer where the molecules are nearly covalently bound to the substrate surface via, for example, an Au-S linkage.8 However, such azobenzene SAMs are usually too closely packed to provide enough free space to allow the trans to cis photoisomerization, which is accompanied by a volume-increasing structural change.18,19 In this paper, we demonstrate that a SAM-LB composite film in which the SAM and LB film are linked with each other by a Coulombic attractive interaction can effectively improve the film stability and also allow the photoelectrochemical hybrid reactions to occur in a way similar to that in the pure azobenzene LB films. The supporting Au substrate was first modified by a 2-aminoethanethiol SAM via Au-S bonding, and then a 4-octyl-4′-(3-carboxytrimethyleneoxy)azobenzene LB film was deposited. Grazing angle incident reflection absorption Fourier transform infrared (referred to as RA-FTIR) spectroscopic results indicate that the terminal groups, -NH2 and -COOH, of the SAM and LB film were in the electrically charged states, -NH3+ and -COO-, at the SAM-LB interface. The Coulombic interaction between the differently charged terminal groups is expected to stabilize the film structure and prevent possible molecular aggregations. Indeed, the atomic force microscopy (AFM) data have strongly supported this expectation. The electrochemical behavior of cis-form of the azobenzene chromophore in the SAM-LB composite film was studied and © 1996 American Chemical Society

17338 J. Phys. Chem., Vol. 100, No. 43, 1996 compared with other organized molecular assemblies like LB films and SAMs. Since the electron will surmount a Coulombic barrier before reaching the destination, the SAM-LB composite system also provides a way to investigiate how the Coulombic barrier affects the long-range electron-transfer kinetics. Experimental Section Materials. The amphiphilic azobenzene derivative, 4-octyl4′-(3-carboxytrimethyleneoxy)azobenzene (referred to as ABD), was purchased from Dojindo Laboratory (Kumamoto, Japan). 2-Aminoethanethiol (AET) was obtained from Tokyo Kasei (TCI, Tokyo). Both were of reagent grade and used without further purification. Other chemicals were of analytical grade. Preparation of the SAM-LB Composite Films. Gold substrates were prepared by conventional vacuum evaporation of 99.99% gold on mica for AFM studies or on glass slides for electrochemistry and FTIR studies. To improve the adhesion of Au film to the substrate, the glass slide or mica was precoated with a thin Cr layer (typically Au (150 nm) with Cr (10 nm)). Prior to use, the gold substrate was cleaned in piranha solution (a hot solution of 30% H2O2 and 70% concentrated H2SO4, volume ratio) followed by rinsing with milli-Q water and absolute ethanol. The self-assembled monolayer of AET was then fabricated by immersing the Au substrate into an ethanol solution of AET for 24 h. Upon removal from the deposition solution, the substrate was extensively rinsed with absolute ethanol and then water. The LB film of ABD in its trans form was deposited onto the AET/Au substrate by the conventional Langmuir-Blodgett technique using a commerical instrument (FACE, Japan). A milli-Q water (>17 MΩ cm) was used as the subphase, and chloroform was used as the spreading solvent with the concentration of ABD being 1.0 mM in the solution. The SAM-LB composite film was obtained by dipping the AET/Au sustrate into the aqueous subphase and raising it at a rate of 10 mm/ min. Because of the hydrophilic property of the AET/Au substrate, only 1 monolayer of ABD was formed on the substrate during the dipping and raising process, with the hydrophobic alkyl terminal chain of ABD being exposed to the air. All the LB films were made at a constant surface pressure of 20 mN/ m, and the subphase temperature was controlled at 20 °C by a thermostat (UC-55, Tokyo Rikakikai). Photoelectrochemical Measurements. Photoelectrochemical experiments were conducted in a single-compartment, threeelectrode cell, which has a small quartz window for introducing the irradiation light. The ABD-AET/Au substrate was used as a working electrode. An Ag/AgCl, saturated KCl and a Pt wire were employed as the reference and counter electrodes, respectively. Cyclic voltammograms (CVs) of the composite films were obtained with a Hokuto Denko HA-150 potentiostat paired with a HB-111 function generator. The signals were recorded on a Riken Denshi F-35A X-Y recorder. All the CVs were taken in 0.1 M aqueous sodium perchlorate solution buffered with the Britton-Robinson method, which was freed from oxygen by bubbling with nitrogen. The photochemical trans-cis isomerization of ABD molecules in the composite films was induced by using a 500 W xenon lamp (Ushio Electric, UI-501C, Japan). The excitation wavelength was isolated with a suitable glass filter (Kenko, U-360 band-pass filter for UV output and cutoff filter at 440 nm for visible output). FTIR Measurements. Infrared spectra were acquired by using a Perkin-Elmer System 2000 FTIR spectrometer equipped with a DTGS detector. A variable-angle reflection attachment was used at an incidence of 82° (near grazing angle). All the

Liu et al. spectra were obtained by referencing 1000 sample scans to 1000 bare gold background scans at 4 cm-1 resolution with strong apodization. The sample chamber was purged with dry N2 to eliminate the spectral interference from water vapor in air. AFM Observations. AFM images were taken with a Nanoscope III (Digital Instruments (DI), Santa Barbara, CA) in a contact mode. The AFM cantilever was also purchased from DI (Integral Si3N4 nanoprobes, 200 µm leg). The force between the AFM tip and the sample was kept constant during scanning, and the typical value used was ca. 2 nN. To ensure the reliability of data, the instrument was frequently calibrated on mica or HOPG before sample imaging. All the measurements were performed at ambient conditions. Contact-Angle Measurements, Contact-angle experiments were performed with a contact-angle goniometer (Model JJC2, The Fifth Optical Instrument Factory of Chang-chun) under ambient conditions (15-20 °C, 50-60% relative humidity) using yellow light to illuminate the water droplet. The advancing contact angle was obtained by expanding a water droplet from a microsyringe until it advances smoothly across the surface and then measuring the angle within 10-20 s after expansion. Each given value represents an average of at least four measurements. Results and Discussion 1. Structural Characterization of a SAM-LB Composite Film. LB Film on Bare Au. The bare gold substrate pretreated with piranha solution showed a relatively poor hydrophilic property. The advancing contact angle of the water droplet was ca. 40-60°, which sensitively depended on the pretreating condition of the substrate and its exposure to air. The ABD monolayer film was transferred from the air-water interface onto it with the hydrophilic carboxylic head group facing to the gold surface. However, the thus-prepared LB film was extremely instable on the bare gold surface. Figure 1 shows the typical AFM morphologies of an ABD monolayer film on gold, which were obtained immediately after deposition. The white spots and snowflake-like regions represent the aggregated ABD molecules, which were never observed on naked Au substrates. The possibility of contaminants was excluded by the following preliminary µ-Raman experiment:20 a ca. 5 µm laser spot (λex ) 632.8 nm) was focused on the snowflake-like region, and Raman spectra in the 1100-1610 cm-1 region were collected, which were typical of the spectra of the azobenzene moiety of ABD;10 in constrast, such a spectral feature was not observed in the blank region of Figure 1b. Actually intermediate aggregation states have also been obtained, which somehow depended on the storing time after LB deposition, the evaporating condition of gold substrate, and also the fabricating condition of the LB film. Generally with increasing the exposure time to air, the aggregation tended to grow, as indicated by the dimensional increase of the aggregates. The aggregation of ABD molecules on the Au surface has also been proven by FTIR measurements. Figure 2a presents the RA-FTIR spectrum of an ABD monolayer film on naked gold. The monolayer film exhibits a spectral feature for the carbonyl stretch that is comprised of two peaks, a major one at 1710 cm-1 and a small one at 1748 cm-1, respectively. The peak at 1710 cm-1 is characteristic of a carboxylic acid in a hydrogen-bonding environment, as noted in several published works.21-28 In contrast, the carbonyl stretching frequencies around 1748 cm-1 are typical of acid groups that are unassociated,22 which are, in fact, usually taken as evidence for the presence of non-hydrogen-bonded acid groups in monolayer assemblies. Apparently, both the unassociated free carboxylic

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Figure 1. Typical AFM images of ABD LB monolayer on naked gold taken soon after LB deposition, where a and b were obtained with a different batch of evaporated Au films. The white spots and snowflake-like regions represent the aggregated ABD molecules.

Figure 2. RA-FTIR spectra of the ABD LB monolayer (a) and SAM-LB composite monolayer (b) on gold.

group and its hydrogen-bonded dimer exist in the ABD film. At the present stage, our spectral data do not yet allow us to make quantitative statements about the relative percentage of monomeric and dimeric states because the intensities of IR bands are determined by a lot of factors such as refractive index, extinction coefficient, chemical environment, and orientation effect. The above FTIR data indicate that ABD molecules have, at least partially, spontaneously rearranged or aggregated in the surface normal direction and the monolayer film structure has broken on the gold substrate. In fact, similar aggregation phenomenon of an LB film on bare gold has also been reported by J. L. Dote et al.29 They studied the infrared reflectanceabsorption spectra (IRAS) of stearic acid monolayers on gold and found that the LB molecules tend to aggregate into a threedimensional structure with time aging, in nice accord with our present observation. In short, both AFM and FTIR data indicate that the ABD monolayer film is not stable on the naked Au surface and

intermolecular aggregation occurs spontaneuosly. From AFM images, we can clearly see that no more uniform monolayer film exists at all on the substrate surface. This structural instability is believed to result from weak interaction between Au substrate and ABD molecules. The weak van de Waals and hydrophilic interactions between them are not strong enough to prevent ABD molecules from interaggregation. SAM-LB Composite Monolayer. To stabilize the original highly organized LB film structure, it is necessary to enhance the adhesion force of the LB film to the supporting substrate. For this reason, we modified the gold substrate by the selfassembled monolayer of 2-aminoethanethiol. The pKb value of the surface amino group of AET/Au was evaluated to be 1.8 ( 0.2 from contact-angle titration, suggesting that the AET/Au surface is more easily positively charged within a wide pH range. On the other hand, the carboxylic terminal group -COOH of ABD molecules is more easily dissociated to -COO- because of its low pKa value. Therefore, a Coulombic

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Figure 3. Structural illustration of the SAM-LB composite film on gold. An ionic bonding is formed at the SAM-LB interface.

attractive interaction is expected when depositing ABD monolayer film onto the AET-modified Au surface. This additional adhesion force may then effectively prevent the destructive molecular aggregation which leads to the collapse of the organized film structure as mentioned above. In the following, we call the thus-fabricated monolayer assembly an SAM-LB composite film or SAM-LB composite monolayer. Figure 2b shows the RA-FTIR spectrum of the SAM-LB composite film. Comparing with the ABD monolayer film on naked Au, a number of new spectral bands appeared at 2962, 1552, and 1394 cm-1, which are attributed to NH3+ stretching, and antisymmetric and symmetric stretching of COO-, respectively.30 This is direct evidence for the formation of ionic bond -NH3+--OOC at the SAM-LB interface as we expected. Although there still exists some unassociated free carboxylic acid group as indicated by the carbonyl stretching band at 1736 cm-1, the -NH3+--OOC ionic bond would predominate in the SAM-LB composite film, which is strongly evidenced by the greatly improved film stability. Figure 3 presents the schematic structural illustration of the SAM-LB composite film. Figure 4 gives the AFM images of the SAM-LB composite film on gold obtained soon after preparation (Figure 4a) and after 30-day time aging (Figure 4b). Obviously the morphological feature is completely different from the pure ABD LB film case (see Figure 1). The composite film is quite uniformly distributed on the gold surface, and less aggregation has occurred during time aging. This is the strong and direct evidence that the -NH3+--OOC Coulombic interaction at the SAM-LB interface has effectively stabilized the film structure and prevented the ABD molecules from destructive interaggregations. Some granular structures are also seen in the AFM images, which may be contaminants or ABD aggregates occasionally formed. The above results give us two significant implications. First, the stability of a highly organized LB film strongly depends on the surface physicochemical properties of the supporting substrate. Second, one can effectively improve the structural stability of LB films through molecular level designing of the substrate surface.

Liu et al. 2. Trans-Cis Photochemical Isomerization in the Composite Film. PhotoresponsiVe Electrochemical BehaVior. ABD molecules in organic solution undergo reversible trans T cis isomerization under light illuminations.17,18 However such a photochromic property is often restricted in highly organized SAMs and LB films because the closely packed film structure cannot provide enough free volume to allow the trans to cis photoisomerization which is accompanied by a volume-increased structural change.18,19 This makes it difficult to conduct the electrochemistry studies of cis-azobenzene in such molecular assemblies. Figure 5 shows the cyclic voltammograms (CVs) of SAMLB composite film under different experimental conditions. Curve a is the dark case, corresponding to the voltammetric behavior of trans-ABD. The trans-ABD isomer is electrochemically inactive within the potential range investigated because it did not exhibit any Faradaic response. When exposing the SAM-LB monolayer to UV light, a clearcut Faradaic response was observed, curve b. This is attributed to the electrochemical reduction-oxidation of UV-created cis-ABD isomers. Similar phenomena have been observed for azobenzene-functionalized LB films on transparent SnO2 conductive glass in our previous work,11-16 in which the formation of cis isomers and its electrochemical redox process have been well studied by an in situ spectrophotoelectrochemical method.11 The photochromic property of ABD molecules provides an alternative approach for corroborating the above reactions. In curve c of Figure 5, the SAM-LB monolayer was exposed to visible illumination after being irradiated with UV light. Since the UV-generated cis-ABD isomers were mostly reconverted into trans-ABD by the visible illumination, the Faradaic response was greatly decreased, in nice agreement with our prediction. By repeating the UV and visible irradiations, the increase and decrease of electrochemical response was also changed reversibly, indicative of the reversible trans-cis photoisomerization of ABD molecules. The electrochemical reduction product of ABD was its hydrazobenzene derivative (or hydraABD). In the anodic potential scan, hydra-ABD was exclusively reoxidized to the thermodynamically stable trans-ABD, as evidenced by the fact that the Faradaic response was obtained only in the first cycle of potential scan after each UV irradiation.11-16 The above observations indicate that the SAM-LB composite film has enough free space to undergo the reversible trans-cis photoisomerization and then the electrochemical reductionoxidation of cis isomers. It is obvious that cis isomers are more easily reduced than the trans isomers. The electrochemical reduction of trans-ABD molecules was kinetically very sluggish since no clear reduction peak was observed in the available potential window of the employed electrode. With these experimental observations, we can also construct a route-specific hybrid reaction cycle, trans f cis f hydra f trans, which is linked by the photochemical trans-cis isomerization and electrochemical reduction-oxidation reactions, similar as that obtained with azobenzene LB films.11-16 Permeation Test. The packing degree of the SAM-LB composite film was assessed by investigating its blocking effect on the redox electrochemistry of water-soluble Fe(CN)63-/4ions. Figure 6 presents the well-known reversible voltammetric behavior of Fe(CN)63-/4- on naked gold electrode (- - -), 2-aminoethanethiol SAM-modified gold electrode (‚ ‚ ‚), and SAM-LB modified gold electrode (s). Obviously not much difference exists among the three different cases, indicating that Fe(CN)63-/4- ions can easily pentrate the SAM or SAM-LB composite film and exchange electrons with the underlying gold

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Figure 4. AFM images of the SAM-LB composite film obtained soon after preparation (a) and after 30-day time aging (b).

Figure 5. Cyclic voltammetric responses of the SAM-LB composite film at different experimental conditions: (a) in the dark; (b) after 1 min of UV irradiation; (c) exposed to 1 min of visible illumination immediately after 1 min of UV irradiation. Scan rate: 20 mV/s; pH 6.0.

Figure 6. Cyclic voltammograms of 10 mM Fe(CN)63-/4- on naked Au (- - -), on 2-aminoethanethiol SAM-modified Au (‚‚‚), and on SAMLB composite film-modified Au (s). Scan rate: 200 mV/s.

electrode. This demonstrates that the SAM-LB composite film has a different packing structure from that of azobenzenefunctionalized alkanethiol SAMs on a gold surface.8,10,31 We studied the electrochemical behavior of a series of azobenzene-

derivatized alkanethiol SAMs on a gold electrode in our previous work.8,31 We found that, even with a relatively short alkyl chain, the SAM layer effectively hindered the redox reaction of Fe(CN)63-/4- ions, leading to a voltammetric behavior characteristic of ultramicroelectrodes. After further studies, we found that azobenzene molecules in such SAMs are closely packed and form a highly organized structure.32 Similar results were also reported on the p-HS(CH2)11OC6H4NdNC6H5 SAM by C. A. Mirkin and his colleagues.10 With such closely packed SAMs, we did not obtain any photoinduced electrochemical response.19 Both the light irradiation and the ion permeation experiments indicate that the SAM-LB composite film has a relatively loose packing structure, which provides the free volume necessary for the trans-to-cis photoisomerization and the ion penetration. The possible explanation for the derived loose film structure is the existence of small defects and pinholes. After deposition of the ABD monolayer on the 2-aminoethanethiol SAM, structural reorganization may occur for creating the ionic bonding at the SAM-LB interface. It is difficult to anticipate that all ABD molecules are paired one by one with 2-aminoethanethiol molecules on the substrate surface considering the difference in their molecular cross areas. Such pairing process may be a source of forming more pinholes and defects as compared with ordinary LB films deposited directly on an appropriate substrate surface. Taking into account of the uniform AFM morphology, the pinholes and defects in the SAM-LB composite film may have sub-micrometer dimensions. The integration of the redox waves of Figure 5b yields a value of ca. 5.5 × 10-11mol/cm2, corresponding to the surface packing density of the photoelectroactive ABD molecules in the composite film. As emphasized by C. A. Mirkin et al.,10 this value does not represent the actual surface coverage of filmforming molecules, which is, in fact, substantially lower than the estimated surface coverage of trans-ABD isomers (ca. 3.7 × 10-10mol/cm2). The disparity between the two values may well reflect the structural feature of the composite film, which may arise from the structural inhibition on the volume-increased trans-to-cis photoisomerization: the composite film can only provide a limited free volume, most probably around defects and pinholes, for inducing a partial photoisomerization of ABD molecules, and most of ABD molecules remain in the original trans state. The ion transport effect may also contribute to the observed difference as pointed out in a number of published works:9,10,33 the structure of the SAM-LB composite film is

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Figure 7. Cyclic voltammetric behavior of the SAM-LB composite film at different solution pH values: (a) pH 3.6; (b) pH 5.0; (c) pH 8.6. Scan rate: 200 mV/s.

Figure 8. pH dependences of anodic peak potential Ep,a (b), cathodic peak potential Ep,c (O), and midpoint potential Ea,c (2) for the composite film on gold (Britton-Robinson buffer; scan rate: 20 mV/s).

inhibiting the incorporation of charge compensating ions into the film. Since Fe(CN)63-/4- ions are quite easily penetrating the film, such inhibition may occur at local sites where a number of ABD molecules are forming a closely packed aggregate, which would be difficult to access by the compensating ions. At the present stage, we believe that the structural inhibition effect plays a major role in the observed behavior. Further investigations are in progress. 3. pH-Dependent Electrochemical Behavior of Cis Isomers. Figure 7 shows the typical cyclic voltammograms of the cis-form of the SAM-LB composite film taken at different pH values, where the cis-ABD isomers were created by 1 min UV irradiation. Since the electrochemical reduction-oxidation of ABD molecules involves 2e- and 2H+, the CV behavior was dependent on the pH value of electrolyte solution, but the total reacted amount of cis-ABD isomers remains approximately constant as indicated by the encompassing area of cathodic or anodic waves. For pH 3.0-9.0, the voltammetric behavior was reproducible, and a linear relationship was observed between anodic peak potential Ep,a, cathodic peak potential Ep,c, midpoint potential Ea,c (Ea,c ) (Ep,a + Ep,c)/2), and the solution pH value, which is clearly seen in Figure 8. This suggests that the -NH3+-OOC- ionic bond was stable within this wide pH range. At higher pH, the CV behavior became instable and no clear-cut Faradaic response was observed. This is believed to originate from the cleavage of the ionic bonding due to the deprotonation of -NH3+ to -NH2. Similar results have been

Liu et al.

Figure 9. Variation of the cathodic peak current as a function of potential scan rate for the SAM-LB composite film (pH 6.0, BrittonRobinson buffer, 0.1 M NaClO4).

reported by Sun et al.34 They observed an adsorption wave of anthraquinone-2,6-disulfonic acid at pH 2 on a 4-aminothiophenol-modified gold electrode. However it disappeared at pH > 7. They explained their observation by the electrostatic interaction of the surface amino group with the redox group dissolved in the electrolyte solution. Comparing with the cis-ABD LB monolayer simply deposited on a transparent SnO2 glass electrode,35,36 the electrochemical reversibility becomes greatly increased in the SAM-LB composite film as evidenced by the substantially smaller peak-topeak separation. In fact, the increase of reversibility is characteristic of an ABD monolayer on a gold surface covered by relatively short thiol SAMs.35 At present, it is difficult to give a satisfactory explanation for such difference; there may be some enhancement of electronic coupling between the ABD molecules and the underlying gold electrode via the intermediate thiol SAMs. Nevertheless, the cyclic voltammetric behavior of the cis-ABD/hydra-ABD couple in the SAM-LB monolayer is still far from the ideal reversible case. For an ideal Nernstian surface waves under Langmuir isotherm conditions, the peakto-peak separation (∆Ep) is zero, and the total width at halfheight of either the cathodic or anodic wave (∆Ep,1/2) is equal to 45.3 mV (25 °C) for a two-electron process.37 The CV feature obtained in this work severely deviates from these ideal values: for example, ∆Ep ) 75 mV and ∆Ep,1/2 ) 75 mV at pH 7.0 for a scan rate of 20 mV/s, but the peak current was found to be proportional to the scan rate of the electrode potential as expected for a surface reaction (see Figure 9). Although there may be some contribution of uncompensated solution resistance, the large peak separation should be mainly attributed to the sluggish electron-transfer kinetics through the highly assembled composite film. Obviously the organized film structure will restrict the conformational change of the azobenzene chromophore which is associated with electron-transfer and protonation processes. While Fe(CN)63-/4 ions can easily penetrate the film, the composite film may also inhibit, to some extent, the incorporation of charge compensating ions into it.9,10 Both effects slow the reaction kinetics. Occasionally we also observed an inversion phenomenon on the reduction peak and oxidation peak at a very low scan rate of potential, i.e., the reduction peak appeared at more anodic potential than the oxidation peak. We have specifically discussed this phenomenon in our previous work35 and will not touch the details here. What should be emphasized is that the origin for this inversion is the involvement of three different molecular states, cis-, trans-,

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Figure 10. Dependences of the cathodic peak potential (Ep,c), anodic peak potential (Ep,a), and midpoint potential (Ea,c) on the logarithmic scan rate (pH 6.0).

and hydra-ABD in the reduction-oxidation process, and the cis-to-trans conversion has simultaneously occurred via hydraABD. From Figure 8, the values of dEp,c/dpH, dEp,a/dpH, and dEa,c/ dpH were calculated as -63, -51, and -57 mV, respectively. All these values are close to the theoretical value, -60 mV/pH unit for a 2e-/2H+ process, confirming that, in the composite film, the global two-electron exchange is accompanied by twoprotonation reactions.37,38 Interestingly these pH dependences are remarkably different from that of the cis-ABD LB monolayer simply deposited on an SnO2 glass electrode, in which dEp,c/ dpH, dEp,a/dpH, and dEa,c/dpH are -80, -27, and -54 mV, respectively.36 Somehow the latter is quite similar to the 4-ethoxy-4′-((N-(2′′-mercaptoethyl)amino)carbonyl)azobenzene SAM on gold in the trans form, which has the corresponding slopes of -62, -27, and -42 mV/pH.31 The distinctly small absolute dEp,a/dpH value may suggest that the deprotonation in an anodic process is severely hindered by kinetic reasons or there exist lower protonation states in the reaction scheme. Uosaki’s recent work on mercaptohydroquinone SAMs on gold has supported this consideration.39 From the above comparison, we can understand that the SAM-LB composite film has a quite different effect on the electron transfer and proton migration into it in contrast with pure LB film and pure SAM. The composite film provides a more favorate environment for electron and proton transferring, which results in a more reversible electrochemical behavior. 4. Electron-Transfer Kinetics. Figure 10 shows the scan rate dependences of the cathodic peak potential, anodic peak potential, and midpoint potential for the cis-form of the SAMLB composite film at pH 6.0. The plots clearly indicate that all these specific potentials are linearly changed with the logarithmic scan rate at a relatively high potential scanning rate. According to Laviron’s treatment,40 the kinetic parameters of electron-transfer such as standard heterogeneous rate constant (ks), transfer coefficient (R), and electrons involved in the ratedetermining step (n) can be evaluated by using the following equations under totally irreversible conditions (e.g., n∆Ep > 200 mV):

Ep,c ) E°′ Ep,a ) E°′ +

( )

RnFV RT ln RnF RTks

(

(1)

)

(1 - R)nFV RT ln RTks (1 - R)nF

(2)

Figure 11. Variations of logarithmic standard rate constant as a function of the pH value of solution for the SAM-LB composite film (Britton-Robinson buffer, 0.1 M NaClO4).

where the notations V and E°′ are the potential scan rate and formal potential, respectively, and R, T, and F have their usual meanings. From Figure 10, the standard rate constant ks was calculated to be 0.2 s-1 at pH 6.0 for the cis-form of the composite film. The ks value here is about 1 order larger than the cis-ABD LB monolayer (ks ) 0.022 s-1) and 3 orders larger than the transABD LB monolayer (ks ) 1.8 × 10-4 s-1) on the SnO2 glass electrode, but it is about 1 order smaller than that of unsubstituted cis-azobenzene simply adsorbed on mercury.38 The sequence of these ks values is excellently consistent with that of the packing degree of these films: a loosely packed film leads to a relatively larger rate constant, or say, to a more reversible electrochemical behavior. Figure 11 gives the graph log(ks) ) f(pH), which consists of three segments within the experimental pH range. The slope of each segment was -0.33, 0.03, and -0.12 from left to right. The surface reaction is apparently more reversible in acid media and becomes less reversible in the intermediate pH range. This complicated pH dependence feature is actually characteristic of azobenzene redox kinetics, which have been observed in the simple adsorption case,38 pure LB film,36 and azobenzene SAMs.8,31 Typically the ks value becomes increasing again in the alkaline side and shows a V-shaped pH dependence in the whole pH range. However the instability of the composite film in the alkaline region disenabled us to have a look of the whole picture. As Laviron et al. pointed out,38 the ks value here has a complex meaning, since it is a function of several actual rate constants and of parameters relative to several protonation reactions. Therefore it should be explained as an apparent rate constant. Laviron has developed a sophisticated theory for such a proton-involving multistepped electrochemical reaction, in which the reduction pathway of azobenzene is represented by a so-called "nine-member square" scheme.41 Assuming that the protonation reactions are at equilibrium (faster than electron transfer), he was able to show that such systems behave like two sequential one-electron-transfer processes coupled with protonation reactions. On that basis, he explains the V-shaped behavior as a result of varied reaction sequences in different pH regions, and the pKa, the E°′, and the rates of the individual steps determine the apparent kinetic parameters. In the case of SAM-LB composite fillm, the dEa,c/dpH value was found to be independent of the potential scan rate (-57 mV), indicating that

17344 J. Phys. Chem., Vol. 100, No. 43, 1996 the protonation reaction is practically at equilibrium. Therefore Laviron’s explanation may also hold true for the SAM-LB composite film. The reaction sequences are H+-e--H+-eat lower pH, e--H+-e--H+ at higher pH, and e--H+-H+e- or H+-e--e--H+ at the intermediate pH range, which account for the observed unique pH dependence of the logarithmic rate constant. Conclusion We have designed a unique SAM-LB composite film on gold by combining the molecular self-assembling technique and Langmuir-Blodgett technique together. A strong ionic bonding is formed at the SAM-LB interface, which is evidenced by the grazing angle incident reflection absorption FTIR results. AFM studies show that the film stability has been greatly improved as compared with the LB film simply deposited on naked gold, and no destructive molecular aggregation occurs in such a composite film. Our work indicates that the stability of highly organized LB films strongly depends on the surface properties of the supporting substrate. Thus, one can stabilize the structure of the ordered film through molecular level designing of the substrate surface. The SAM-LB composite film has a relatively loose packing structure, which enables the azobenzene chromophore to undergo a reversible trans-cis photoisomerization, and hence makes it possible to investigate the electrochemistry of the cisazobenzene chromophore in the organized films. The cis-form composite monolayer shows a clear-cut cyclic voltammetric response, which is attributed to the 2e- and 2H+ electrochemical reduction-oxidation of azobenzene/hydrazobenzene. Compared with the pure ABD LB films on SnO2 and thiol-derivatized azobenzene SAMs on gold, the composite film shows more reversible kinetics, which are reflected in the larger rate constant, smaller peak-to-peak separation, and narrower peak shape. The complicated pH dependence of the logarithmic rate constant is characteristic of the azobenzene redox kinetics, as usually observed in simple adsorption films, pure LB films, and SAMs. This suggests that the electrochemical reaction in SAM-LB composite film is multistepped and that the sequence of electron transfers and protonations is varied with the pH values of the solution. Acknowledgment. The authors gratefully acknowledge the financial supports from the State Science & Technology Committee, the State Education Committee, and the National Natural Science Foundation of CHINA (NSFC). References and Notes (1) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press, Inc.: New York, 1991. (2) Michel, B. Highlights in Condensed Matter Physics and Future Prospects; Esaki, L., Ed.; Plenum: New York, 1991; p 549-572.

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