J. Phys. Chem. 1995,99, 14771-14777
14771
Azobenzene-Derivative Langmuir-Blodgett Films Deposited on Various Thiol Monolayers Kenichi Morigaki? Zhong-Fan Liu," Kazuhito Hashimoto, and Akira Fujishima* Department of Applied Chemistry, Faculty of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan Received: January 18, 1995; In Final Form: June 19, 1995@
Monolayers or bilayers of the azobenzene derivative 4-octyl-4'-((carboxyltrimethylene)oxy)azobenzene (ABD) have been deposited by the Langmuir-Blodgett (LB) method on gold substrates modified with thiol selfassembled monolayers. Here the thiol monolayer serves both as a substrate for the LB film and as a barrier for the electron transfer between the azobenzene and the gold substrate. The electrochemical properties of the ABD films depend on the length of the thiol molecule and the orientation of the ABD molecule. They become more irreversible when the thiol molecule is longer and when the ABD molecule is adsorbed on the thiol with its hydrophobic terminal. Although energetically unstable cis-ABD is reduced to the hydrazobenzene derivative (HBD) at a more anodic potential than trans-ABD, the reduction of cis-ABD was not observed in the cyclic voltammetry of ABD films on 2-mercaptoethanol. Moreover, UV light irradiation of the trans film in a certain cathodic potential range causes cathodic and anodic spikelike currents when the light intensity is increased and decreased, respectively, at the cathodic bias potential. These phenomena are explained by using the following mechanism; i.e., the reduction of cis-ABD can occur at a more anodic potential than the oxidation of HBD when the separation of the redox potentials is relatively small for trans-ABD. If the above two opposite reactions take place much faster than the voltammetric potential scan, the electron flows cancel each other out and no net faradaic current can be observed. However, the spiked current responses are generated when the amounts of reduction and oxidation currents become out of balance.
Introduction Azobenzene has been studied extensively as a photochromic material.' It has a wide range of potential applications, including information storage devices? artificial vision? chemical sensors$ and biological sensors.5 Most of the applications are based on the photochemical trans-cis isomerization of azobenzene. The isomerization was used to change chemical properties such as capacitance,6structure of supermolecular system^,^ and binding abilities of enzymes to other compound^.^ The electrochemistry of azobenzene is one of the properties which can be changed by the photochemical reaction.* We reported previously an observation of a distinct difference in the electrochemical reduction potential between trans and cis isomers in the assembled monolayer film of the azobenzene derivative (ABD) made by the LB te~hnique.~ We proposed potential applications such as high-density information storage2and photon countingl o using this phenomenon. However, the difference in electrochemistry between the two isomers has not been observed or was small in homogeneous protic solutions." In protic media (water, methanol, ethanol, etc.) the electrochemical reactions of azobenzene occur by single-step twoelectron reduction and oxidation. Considering the importance of its photochemistry, studies on the relative electrochemical behavior of trans- and cis-azobenzene are important. In spite of a considerable number of studies in the past several decades,8.11-'7the electrochemistry of the two isomers in protic media is not thoroughly understood. While there seems to be a general agreement that the cis isomer is reduced at more anodic potentials than the trans form in ethanol, controversy still
* To whom correspondence should be addressed.
+ Present address: Institut f i r Polymere, ETH-antrum, CH 8092 Zurich, Switzerland. Present address: Department of Chemistry, Peking University, Beijing 100871. China. @Abstractpublished in Advance ACS Absrracrs, September 1, 1995. ~
*
remains on the relative behavior of the two isomers in aqueous media.8,'2,14,'6The electrochemistryof azobenzene in aqueous solution is complicated because it depends on the pH of the solution, the type and concentration of the buffer used, and the effect of ad~orption.'~9'~3'~ The trans and cis forms showed little or no difference in reduction potential in purely aqueous solution when adsorption of azobenzene to the electrode occurred. Laviron et al. attributed the observability of differences in protic media to the protonation process in the reduction.'' They also pointed out the possibility of the cis-trans isomerization in the first stage of the reduction. On the other hand, the electrochemistry of ABD molecules in monolayers is affected by factors which are inherent in assembled films. In particular, the long-range electron transfer between the electrode and azobenzene is thought to have a great influence. Hence, it is very important to study the electrochemistry of ABD monolayers by controlling the film structures. On this point, thiol monolayers on gold are attractive as substrates, because they can control both the hydrophilicity of the surface and the length of the electron-transfer pathway by changing the terminal groups and the length of the thiol molecule. In the past several years, an increasing number of studies have been devoted to thiol molecules adsorbed on solid surfaces like gold, because they can realize stable and ordered monolayers with controlled thickness and wettability of the surface. Properties such as ~ t r u c t u r e , 'stability, ~ ~ ' ~ and surface wettability20-22have been extensively studied. They have provided a useful tool for basic electrochemical studies such as the experimental examination of electrical double-layer strucand of electron-transfer t h e ~ r i e s . ~ ~ - * ~ Here we report on a novel observation which can explain the discrepancy in the relative electrochemicalbehavior of cisand trans-azobenzene between aqueous solution and the monolayer. We deposited monolayers or bilayers of ABD by the LB method on thiol monolayers. The orientation of ABD films
0022-365419512099-14771$09.00/0 0 1995 American Chemical Society
Morigaki et al.
14772 J. Phys. Chem., Vol. 99, No. 40, 1995
transferred to the thiol monolayers depended largely on the surface wettability of the thiol monolayer. The distance between the electrode and the azo functional group could be varied by changing the length of the thiol molecules and the orientation of the ABD. For thiols longer than three carbon units, we could observe a difference in reduction potential between the cis and trans isomers in voltammetric measurements. However, when we used shorter thiols (ethanethiol or 2-mercaptoethanol), the reduction of cis-ABD was not observed. We propose a mechanism to explain these phenomena.
-4- I
Experimental Section
-4011
Reagents. The azobenzene derivative, 4-octyl-4’-((carboxy1trimethylene)oxy)azobenzene(ABD) was commercially available from Dojindo Laboratory (Kumamoto, Japan). Thiols (ethanethiol, n-butanethiol, n-hexanethiol, n-octanethiol, noctadecanethiol, 2-mercaptoethanol, 3-mercaptopropionic acid) were purchased from Tokyo Kasei (Tokyo, Japan). All chemicals were of reagent grade and used without further purification. Substrate Preparation. Sn02 glass substrates (Asahi Glass, Tokyo; 10 QD)were hydrophilically treated by immersing them in hot sulfuric acid (50% by volume) for 10 min before the LB film preparation. Gold substrates were prepared by vacuum evaporation of about 500 8, of gold (Tanaka Kikinzoku, Tokyo; 99.999%) onto an optical slide glass (Matsunami, Tokyo) that had been precoated with 50 of chromium (Nilaco Corp., Tokyo; 99.99%) to improve the adhesion of gold. Thiol Monolayer Formation. Gold-coated substrates were used immediately for the thiol self-assembly after being taken out of the chamber. In cases where they were stored for a few hours in room air, they were cleaned by immersing in hot sulfuric acid (50% by volume) for 15 min and were rinsed first with deionized water (Millipore Products, Bedford, MA; > 18 MQ) and then with ethanol before being placed in the thiol solutions. Adsorptions were carried out in 60 IL of ethanol at a thiol concentration of 1-3 mM. All adsorptions were performed at room temperature for 1 day. Fabrication of ABD Films, The ABD monolayer films was deposited onto self-assembled thiol-modified gold substrates (thioYAu) or SnO2 conductive glass substrates in the dark room by the Langmuir-Blodgett method using a commercial instrument (Kyowa, HBM-AP; Tokyo). A 0.2 mM CdC12 aqueous solution was used as the subphase, and no special pH adjustment was made. Chloroform was used as the spreading solvent, with the ABD concentration being 1.8 mM in the solution. The thiol/ Au substrates were removed from the thiol solution just before use and rinsed with ethanol several times. The monolayer films were fabricated by vertical dipping of the substrate into the water or by horizontal attachment of the substrate onto the water surface. The transfer ratio was close to unity for all of the experiments. All samples were prepared at a constant surface pressure, 25 mN m-’, and the subphase pH was 7.0; its temperature was controlled to 20 “C by a thermostat (Tokyo Rikakikai, UC-55; Tokyo). Electrochemical Measurements. The electrochemical measurements were conducted using a three-electrode cell setup, where the ABD monolayer film was used as the working electrode (WE), and a Pt wire, as the counter electrode (CE). The WE areas were ca. 0.1 and ca. 3 cm2 for the thioYAu and SnO2 substrates, respectively. The potential of the working electrode was controlled versus a Ag/AgCl (saturated KC1) reference electrode (RE)by a potentiostat (Toho, PS-07; Tokyo). A 0.1 M aqueous sodium perchlorate solution whose pH was controlled with citratemazHP04 buffer was employed as the
I i
v
I
I
I
I
I
-0.4 0 PotentiaNV VS. Ag/AgCI)
-0.8
i
14 0.4
Figure 1. Cyclic voltammograms of trans- (solid line) and cis-ABD (dashed line) in a monolayer deposited on conductive Sn02 glass. The surface area of the WE was ca. 3 cm2. The sweep rate was 20 mV/s. The cis isomer was created by UV light irradiation at 0.0 V. The maximum amount of cis-ABD in the monolayer was ca. 20% because of steric hindrance.
electrolyte. Before each experiment, the electrolyte was deaerated with high-purity Ar or nitrogen for 15 min. A 200 W mercury-xenon lamp (Hayashi, LA-200UV; Tokyo) was used for inducing the trans to cis isomerization of ABD in the monolayer film. The UV wavelengths were isolated by a glass filter centered at 362 nm with a band pass of 60 nm. The UV light intensity was ca. 150 mW/cm2. For the voltammetric experiments, the film was irradiated for 20 s with a potential of 0.0 V. For the photocurrent response measurements, the UV irradiation was performed under a cathodic potential bias.
Results and Discussion Electrochemical Behavior of the ABD Monolayer on an SnOz Glass Electrode. The ABD film deposited on SnO2 conductive glass is composed of almost 100% trans-ABD in its initial state. This is because trans-ABD has a smaller volume per molecule than cis-ABD and the equilibrium is shifted to the side of the.trans isomer in a densely packed monolayer. cis-ABD is generated only by the irradiation of the film with UV light. The amount of cis-ABD is limited to ca. 20% of the total number of ABD molecules by steric limitations in the monolayer. Hereafter, the ABD film containing 20% cis form is referred to as a “cis-ABD film”. Figure 1 shows the cyclic voltammograms for the trans- and cis-ABD films deposited on the Sn02 glass. The reduction and oxidation peak potentials are greatly separated, indicating electrochemical irre~ersibility.~~ The separation was about 0.72 V between the reduction of transABD and the oxidation of HBD and 0.26 V between the reduction of cis-ABD and the oxidation of HBD, respectively. cis-ABD is reduced at a considerably more anodic potential compared with the trans isomer because it is energetically unstable. When the cis-ABD film was scanned to more negative potentials, a second peak due to the reduction of trans-ABD was observed at the same potential as that for the trans-ABD film, and its peak area was ca. 80% of the solid line. Moreover, the second potential scan of the cis-ABD film returned to the shape of the original solid line, indicating that all of the HBD molecules formed were oxidized to trans-ABD. The electrochemical reactions are thought to proceed through a single-step two-electron reduction or oxidation, as in the case of azobenzene in protic media. This assumption should be reasonable considering the permeability of water and electrolyte molecules into the It was also supported by the pH dependence of the reduction potential of ABD monolayer films.29 Electrochemical Behavior of the ABD Film on the Thiol Monolayer. The electrochemical reaction of the ABD film
J. Phys. Chem., Vol. 99, No. 40, I995 14773
Azobenzene-Derivative Langmuir-Blodgett Films
Au
>AuKK Figure 2. Schematic illustration of the long range electron transfer between the gold electrode and the azo group: (A) ABD molecules are adsorbed on a thiol monolayer with the hydrophilic part; (B) ABD molecules are adsorbed with the hydrophobic part. (A) and (B) have different electron-transfer pathway lengths inside the ABD molecule. involves a long-range electron transfer between the substrate and the azo group. The electron-transfer kinetics dominates the electrochemicalbehavior of ABD monolayers. We utilized thiol monolayers as a barrier to control the rate of electron transfer. Figure 2 illustrates schematically the long-range electron transfer between the gold substrate and the azo group through the thiol monolayer. The direct through-space tunneling is thought to be suppressed, because the distance between the gold substrate and the azo group is more than 10 A. Hence, the electron transfer occurs mainly by the a-tunneling through the thiol and ABD molecules. The length of the electrontransfer pathway can be changed not only by the length of the thiol molecule but also by the orientation of the ABD on the thiol surface. As illustrated schematically in Figure 2A,B, ABD molecules can be adsorbed on the thiol monolayers in opposite orientations. In Figure 2A, the electron tunneling pathway inside the ABD molecule has three C-0 bonds, three C-C bonds, and one benzene ring. On the other hand, there are eight C-C bonds and one benzene ring in the case of Figure 2B. The pathway in Figure 2B is thought to be longer than that in Figure 2A. In Figure 3 are shown the cyclic voltammograms of the transand cis-ABD films on thioVAu substrates with different thiol molecules. Ethanethiol (C~HSSH), n-octanethiol (n-C&7SH), and n-octadecanethiol (C18H37SH) were used for curves a-c, respectively, in the figure. In all of the films, the hydrophobic part of the ABD molecule was bound to the thiol surface. In curves a and b, redox peaks appeared only with the cis-ABD film due to the reduction of cis-ABD to HBD and the oxidation of HBD to trans-ABD, whereas no redox peaks were observed for the trans-ABD film within the applied potential range. The redox peaks of curve b are more greatly separated than those of curve a, indicating the irreversibleelectrochemistry. For the longer thiol shown in curve c, the electron transfer was completely suppressed by the large barrier of the thiol spacer. Therefore, the electrochemical reaction of neither trans- nor cisABD was observed. The results in Figure 3 show that the electrochemistry of the ABD films becomes more reversible as the electron-transfer pathway becomes shorter. When the ABD monolayer was deposited on 2-mercaptoethanol, HOCz&SH, we obtained an unexpected result. The hydrophilic side of the ABD is bound to the thiol monolayer, and the pathway between the gold
Io.2pA
CIS, trans
Thiol
4.6
4.4
4.2
’
0.4
0.2
0
0.6
Potentlal ( V vs. Ag/AgCl )
Figure 3. Cyclic voltammograms of cis- and trans-ABD on thioVAu: (a) ABD monolayer deposited on an ethanethiol monolayer by the horizontal transfer method; (b) ABD bilayer deposited on an noctanethiol monolayer by the vertical dip; (c) ABD bilayer deposited on an n-octadecanethiol monolayer by the vertical dip. In all films the first ABD layer is adsorbed on the thiol monolayer surface with the hydrophobic alkyl terminal (the orientation in Figure 2B). In the case of bilayers, only the first layers are electrochemically active, because the second layers are too far From the substrates for the electron tunneling. The surface area of WE was ca. 0.1 cm2. The sweep rate was 20 mV/s. The cis isomer was generated by W light irradiation at 0.0 V.
4.4
-0.2
0
0.2
Potentlal ( V vs. Ag/AgCI )
Figure 4. Cyclic voltammogram of trans- and cis-ABD in the monolayer deposited on the 2-mercaptoethanol monolayer. The sweep rate was 20 mV/s.
substrate and the azo group is shorter than that for curve a in Figure 3. However, the cis isomer, which was generated by W light irradiation at the potential of 0.1 V (vs Ag/AgCl), did not show any faradaic peak current during the potential scan, as is shown in Figure 4.30 To explain this observation, we will propose the following “redox-potential inversion” mechanism considering the electrochemical reversibility of the ABD-HBD redox reactions in the monolayer. Inversion of Redox Potentials. According to Figure 1, trans-ABD on SnO2 glass starts being reduced at -0.4 V and is almost completelyconverted into HBD at about -0.9 V when the potential is scanned in the cathodic direction. This process can be shown schematically as curve a in Figure 5A. The HBD produced is oxidized to trans-ABD between -0.1 and +0.3 V in the potential sweep in the anodic direction, as seen in curve b. There is a large potential separation (ca. 0.8 V) between the redox reactions, indicating electrochemical irreversibility. On the other hand, the photochemically generated cis isomer is
Morigaki et al.
14774 J. Phys. Chem., Vol. 99, No. 40, I995
(4
i
HBD-trans
(B)
B
fi
" trans-HBD HBD-trans
Cis-ABD
HBD
Trans-ABD
Figure 6. Schematic illustration of the reduction of cis-ABD and the oxidation of HBD. cis-ABD is converted rapidly to trans-ABD via HBD.
Figure 5. Schematic current-voltage curves of the ABD-HBD redox reactions (A) an ABD monolayer which has irreversible eletrochemsitry; (B) an ABD monolayer which has reversible electrochemistry. Lines a-d represent the reduction of trans-ABD, the oxidation of HBD formed from trans-ABD or cis-ABD, and the oxidation of HBD formed from cis-ABD, respectively.
completely reduced to HBD in the potential region between 0.0 and -0.3 V (curve c), because cis-ABD is energetically unstable. The reoxidation of HBD (curve d) occurs at the same potential as in curve b. The shift of the reduction potential (AEin Figure 5) occurs depending on the energetic differences. These processes are summarized as follows. If the separation of the reduction potential of trans-ABD and the oxidation potential of HBD is large enough, the reduction potential of cis-ABD is situated between them. HBD generated from the reduction of cis-ABD stays stable in the monolayer unless the potential is scanned in the anodic direction. On the other hand, Figure 5B represents a system whose electrochemistry is more reversible. The reduction potential of trans-ABD (a) and the oxidation potential of HBD (b) are close to each other. If the extent of the potential shift (hE) between the two isomers is similar to that in Figure 5A, the reduction potential of cis-ABD shifts to a more anodic potential than the oxidation potential of HBD. In this case, the redox potentials of cis-ABD and HBD are inverted. Although such an inverted redox potential seems to be unusual, it is not impossible. Note that this apparent inversion arises because HBD is oxidized to energetically more stable trans-ABD (not to cis-ABD). In this case, the cyclic voltammogram which is expected for cis-ABD is as follows. First, cis-ABD is generated by W light irradiation in a potential region where no electrochemical reaaction occurs. When the potential is scanned in the cathodic direction, cis-ABD starts being reduced at the potential of the dashed line (c) in the diagram. However, electrochemically generated HBD returns immediately to trans-ABD at this potential, because the potential is more anodic than the oxidation potential of HBD (curve d). Therefore, HBD is simultaneously consumed by the re-oxidation and the concentration of HBD in the monolayer does not increase, as suggested by the dashed line (c). Hence, most of the cis-ABD molecules are converted into trans-ABD via HBD during the cathodic potential scan3' Figure 6 illustrates this process schematically. Both the electrochemical reduction of cis-ABD and the oxidation of HBD takes place simultaneously.
Two electrons are involved in each reaction, but the directions of electron transfer are opposite for reduction and oxidation. This situation is similar to that for an exchange current at the electrode. If the electron exchange is much faster than the potential scan rate, no net faradaic current is observed in the voltammetric measurement, because the electron transfers in both directions cancel out the total electron flow. This corresponds to the situation in Figure 4. This mechanism explains the absence of faradaic peaks of cis-ABD in Figure 4. The apparent inversion of redox potentials between cis-ABD and HBD prevents the observation of faradaic peaks, because the reduction of cis-ABD and the oxidation of HBD are canceling out each other. Next we show two experimental results which could be also reasonably interpreted by this mechanism. Reorganization of the ABD Film on the Thiol Monolayer. The orientation and stability of the ABD monolayer on thiol depend on the type and strength of the interaction at the thiolABD interface. When thiol molecules are terminated with hydrophobic alkyl chains, a hydrophobic interaction binds the hydrophobic terminal of the ABD to the thiol surface. However, a substrate which is covered by an ethanethiol monolayer (C2S/ Au) has a relatively,hydrophilic surface compared with longer alkanethiol~.~~ This is due to defects of packing in the ethanethiol monolayer and the effect of the gold substrate under it. Hence, in this case, it is considered that the hydrophilic side of the ABD is adsorbed on the thiol monolayer by vertical dipping for the substrate. Figure 7 shows cyclic voltammograms for the trans- and cis-ABD monolayers deposited on C2S/Au. Curves a-c in Figure 7 correspond to the same film prepared by vertical dipping, but curve d corresponds to a film prepared by horizontal transfer. Curve a was observed just after the electrolyte was introduced into the cell. The redox peaks of cis-ABD were not observed, similar to the case of 2-mercaptoethanol (ABD/HOC2S/Au). However, the redox peaks of cisABD became observable when the film was soaked in the electrolyte solution. Curves b and c are cyclic voltammograms of the same film as in curve a after being kept in the electrolyte for 30 and 60 min, respectively. Small redox peaks for cisABD appeared after 30 min, and the peaks were slightly inverted (i.e., the cathodic peak was located at a more anodic potential than the anodic peak). The peaks became less inverted and larger after 60 min. The voltammetric feature remained the same as (c) after that. Comparing the cyclic volammogams in Figure 7, it is easily noticeable that the electrochemistry of ABD/C2S/Au freshly prepared by vertical dipping (a), which resembles that of ABD/HOC2S/Au (Figure 4), became closer to the electrochemistry of Figure 7d for ABD/C2S/Au prepared by horizontal dipping in the course of time. The similarity between curves c and d indicates that the ABD molecules
Azobenzene-Derivative Langmuir-Blodgett Films I (a)
I
I
F
+-+ I (b)
I
J. Phys. Chem., Vol. 99, No. 40, 1995 14775
trans,cis
cis
trans
V
-0.4
-0.2
0
0.2
0
Potential ( V vs. Ag/AgCI )
20
40
60
80
Time (sec)
Figure 7. Cyclic voltammograms of cis- and truns-ABD on the ethanethiol monolayer: (a) freshly prepared film by the vertical dipping method; (b) and (c) the film kept in solution for 30 and 60 min, respectively; (d) freshly prepared film by the horizontal transfer method. (a)-(c) are the same film. The sweep rate was 20 mV/s. The cis isomer was generated by UV light irradiation at 0.0 V.
Electrolyte solution
CH3
I
8 4-
(CH2)n
Figure 8. Schematic illustration of the reorganization of ABD film structure at the electrolyte solution interface: (A) just after being dipped in electrolyte solution; (B) 60 min after (A).
changed their orientation so that the hydophilic part faced the electrolyte solution. This process is illustrated in Figure 8. The assumption is reasonable considering that the surface of C2S/ Au is not completely hydrophilic and interaction with the hydrophobic carbon chain is still favorable in aqueous solution. These observations can be interpreted as follows. The redox potentials of cis-ABD and HBD were inverted in Figure 7% and the reduction of cis-ABD and oxidation of HBD were canceling out the faradaic currents for each other. As the inversion became smaller, the cancellation of the faradaic current also became smaller and the redox peaks started to be observed (Figure 7b,c). Differential Response of the Photocurrent at Cathodic Potentials. UV light irradiation of the inverted redox system at cathodic potentials shows a different response depending on the applied potential. There are three potential regions in the diagram of Figure 5B at which we expect different responses to the UV light inadiation. The first is potential region I, where photochemically generated cis-ABD from trans-ABD is not reduced and stays stable in the film. The second is potential region n, where both the reduction of cis-ABD and oxidation
Figure 9. Photocurrent response of the truns-ABD monolayer film deposited on the ethanethiol monolayer by UV light irradiation at cathodic potential bias: (A) -0.1 V; (B) -0.2 V; (C) -0.35 V.
of HBD occur. Therefore, at this potential, photochemically generated cis-ABD is reduced to HBD and HBD is oxidized to trans-ABD simultaneously. The third is potential region Ill, where cis-ABD is reduced to HBD and generated HBD does not go back to trans-ABD and stays stable in the film. Figure 9 shows the observed electrochemical response of the trans-ABD monolayer to UV light irradiation at different potentials corresponding to the above three regions. Figure 9A is the case for W light irradiation at -0.1 V (vs Ag/AgC1) (potential region I in Figure 5B). No current response was observed, indicating that the cis isomer is electrochemically inactive at this potential bias. Figure 9C is the one for UV light irradiation at -0.35 V (potential region III). It caused a cathodic current response, indicating that only the reduction of cis-ABD took place and HBD was not oxidized to truns-ABD. The gradual decay of the cathodic current during the light irradiation is due to the decreasing amount of unreduced ABD molecules in the film due to electrochemical consumption. On the other hand, W light irradiation at -0.2 V (potential region 11) caused a pair of spiked electrical signals, as is shown in Figure 9B. When the irradiation started, a sharp cathodic current spike was observed, and when the UV light was extinguished, an anodic current spike was observed. Both spikes decayed quickly to the original current levels. This behavior indicates that both the reduction of cis-ABD and the oxidation of HBD occurred simultaneously at this potential. It is reasonable to speculate that .the current spikes were observed only when the amount of reduction and oxidation were out of balance. The reduction of photochemically generated cis-ABD exceeds the oxidation of HBD at the beginning of W inadiation, because not enough HBD exists. On the other hand, the oxidation of HBD becomes dominant when the light is extinguished, because the remaining HBD is consumed by oxidation. During the W light irradiation, the reduction of cis-ABD and the oxidation of HBD are in dynamic equilibrium, canceling out the faradaic currents for each other, and only small or zero cathodic current is observed. Transitions h m one region to another exist between the three potential regions, and intermediate responses were obtained at intermediate potentials. The transitions should coincide with the potentials where the reduction of cis-ABD and oxidation of HBD occur ((b) and (c) in Figure 5B). Although it was difficult to determine these potentials exactly, they were estimated approximately to be -0.15 and -0.3 V for the system in Figure
Morigaki et al.
14776 J. Phys. Chem., Vol. 99, No. 40, 1995 Anodic
Cathodic
-
0
40
80 120 160 Time (sec)
200
Figure 10. Photocurrent response of the trans-ABD monolayer deposited on the 2-mercaptoethanol monolayer by changing the UV light intensity.
9. Hence, the reduction potential of cis-ABD is located at about 0.15 V and the oxidation potential of HBD is located at about -0.3 V. In this case, the reduction potential of cis-ABD is more anodic than the oxidation potential of HBD by ca. 0.15 V. The above results support the hypothesis that the reduction potential of the cis-ABD becomes more anodic than the oxidation potential of HBD. The inversion of the redox potentials was caused because of the difference in the energetic stability between cis- and trans-ABD. Note again that HBD generated by the reduction of cis-ABD is oxidized to transABD. Neither faradaic current for the reduction of cis-ABD nor the oxidation of HBD was observed in the voltammetric measurements (Figure 4). On the other hand, sharp spikes of faradaic current were observed when the UV light irradiation started and stopped in potential region 11. These spikes arise only when the reduction of cis-ABD and the oxidation of HBD are out of balance. Figure 10 shows the electrical response of the ABD monolayer to the change of incident light intensity. Cathodic and anodic spike responses were observed when the light intensity was increased and decreased, respectively. This result clearly shows that the change of the light intensity disturbs the dynamic equilibrium of redox reactions and causes spikelike faradaic currents. Hence, the ABD film is this potential range has a differential response to the light intensity. On the Electrochemistry of cis- and trans-Azobenzene in Aqueous Media. The proposed mechanism can explain the relationship between the electrochemical reversibility of ABD monolayers and whether the reduction potential of cis isomer is observable or not in voltammetric measurements. If the ABD monolayer has irreversible electrochemistry, the reduction of cis-ABD is observed at a more anodic potential than trans-ABD. On the other hand, if the ABD monolayer has relatively reversible electrochemistry, the reduction of cis-ABD is not observable because of the fast cis to trans conversion via HBD. The change of reversibility of the ABD/HBD redox reactions in LB form may be ascribed to the difference in orientation on different substrates. It is expected that a packing condition of LB film depends on its orientation. On the other hand the redox reactions of ABD/HBD are accompanied by a volume change. Therefore the difference in packing conditions (orientation) may cause a redox potential shift. At the present stage, however, we cannot explain more quantitatively the potential shift caused by the variation of film orientation. The present experimental results and explanation can make clear partially the controversy on the relative electrochemical
behavior of cis and trans isomers in protic media. Although the factors which are responsible for the reversibility of the azobenzene-hydrazobenzene redox reactions still remain unclear, our hypothesis can explain qualitatively the experimental results of earlier work. The reduction of cis-azobenzene is observable when the electrochemical reactions are irreversible, as in the case of ethanol solutions. On the other hand, it is not observable in pure aqueous media because the adsorption of azobenzene on the electrode makes its electrochemistry reversible. The cis to trans isomerization of azobenzene is thought to occur in this case by the same mechanism shown above. Although the possibility of cis-trans isomerization in the first step in the reduction of cis-azobenzene in aqueous media was first pointed out by Laviron et al. more than a decade ago,” we believe that our findings offer a clearer scheme for the reaction mechanism and help further understanding of the photoelectrochernistxy of azobenzene.
Conclusion We have proposed the possibility that the reduction potential of cis-azobenzene becomes more anodic than the oxidation of hydrazobenzene to the trans isomer when the azobenzenehydrazobenzene system has reversible electrochemistry. This is because the cis isomer is energetically unstable compared with the trans isomer and hydrazobenzene is reoxidized exclusively to trans-azobenzene. Experimental results using ABD monolayers on short-chain thiols supported this hypothesis. ABD monolayers which had “inverted redox reactions” showed no redox peaks for cis-ABD in voltammetric measurements because of the fast conversion of cis-ABD to trans-ABD via HBD. The reduction of cis-ABD and oxidation of HBD cancel each other out in this case. On the other hand, UV light irradiation of the trans-ABD film at certain controlled cathodic potentials caused spikelike current responses when the light intensity was changed. This indicates that both the reduction of cis-ABD and the oxidation of HBD are occurring simultaneously at this potential. These spikes occur only when the two reactions become out of balance. These experimental results and explanations can give new insight into the mechanism that determines whether the difference of the reduction potentials of cis and trans isomers is observable or not for azobenzene in protic media.
Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan. References and Notes (1) Dum,H., Bouas-Laurent, H., Eds Photochromism; Elsevier: Amsterdam, 1990; pp 165-192. (2) Liu, 2. F.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 658. (3) Fujiwara, H.; Yonezawa, Y. Nature 1991, 351, 724. (4) Ueno, A.; Yoshimura, H.; Osa, T. J . Am. Chem. SOC. 1979, 101, 2779. ( 5 ) Willner, I.; Rubin, S.; Riklin, A. J . Am. Chem. Soc. 1991, 113, 3321. (6) Iwamoto, M.; Majima, Y.; Naruse, H.; Noguchi, T.; Fuwa, H. Nature 1991, 353, 645. (7) Ichimura, K.; Suzuki, Y.; Seki, T.; Hosoki, A,; Aoki, K. Langmuir 1988, 4, 1214. (8) Klopman, G . ; Doddapaneni, N. J . Phys. Chem. 1974, 78, 1825. (9) Liu, Z. F.; Loo, B. H.; Hashimoto, K.; Fujishima, A. J . Electroanal. Chem. Interfacial Electrochem. 1991, 297, 133. (10) Liu, Z. F.; Morigaki, K.; Hashimoto, K.; Fujishima, A. Anal. Chem. 1992, 64, 134. (11) Laviron, E.; Mugnier, Y. J . Elecrroanal. Chem. Interfacial Electrochem. 1980, 111, 337. (12) Hillson, P. J.; Bimbaum, P. Trans. Faraday SOC. 1952, 48, 478. (13) Castor, C. R.; Saylor, J. H. J . Am. Chem. Soc. 1953, 75, 1427.
J. Phys. Chem., Vol. 99, No. 40, 1995 14777
Azobenzene-Derivative Langmuir-Blodgett Films (14) Wawzonek, S.; Fredrickson, J. D. J . Am. Chem. SOC.1955, 77, 3985. (15) Wawzonek, S.; Fredrickson, J. D. J . Am. Chem. SOC. 1955, 77, 3988. (16) Chuang, L.; Fried, I.; Elving, P. J. Anal. Chem. 1965, 37, 1528. (17) Pezzatini, G.; Guidelli, R. J . Chem. SOC. Faraday Trans. 1 1973, 69, 794. (18) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J . Am. Chem. SOC.1987, 109, 2358. (19) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J . Am. Chem. SOC. 1987, 109, 3559. (20) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J . Am. Chem. SOC.1989, 111, 321. (21) Laibinis, P. E.; Whitesides, G. M. J . Am. Chem. SOC.1992, 114, 1990. (22) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370. (23) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1992, 8, 2560. (24) Chidsey, C. E. D. Science 1991, 251, 919. (25) Finklea, H. 0.;Hanshew, D. D. J . Am. Chem. SOC. 1992,114,3173. (26) Finklea, H. 0.;Ravenscroft, M. S.; Sinder, D. A. Langmuir 1993, 9, 223. (27) We would like to give a brief description of the term “reversibility”, which is frequently used for electrochemical reactions, because it will play a central role in our discussion. Although “reversibility” in chemistry usually means the possibility of chemical reactions to go back and forth between two chemical states, “reversibility” in electrochemistry implies the speed of a reaction to reach the chemical equilibrium where the Nernst equation applies. If an electrochemical reaction is fast enough compared
with the experimental time scale, it appears to be always following the Nernst equation and is said to be “reversible”. In cyclic voltammetry, the separation of redox peak potentials is associated with the reversibility of the electrochemical reactions. An electrochemical reaction is said to be reversible when there is no separation between the redox potentials and to be “irreversible” when there is a large separation between the redox potentials. The peak separation indicates an activation barrier for the electrochemical process. (28) Although the LB film is transferred to the substrate at its solid state packing (x = 25 mN m-l), it still contains considerable free space for the electrolyte to penetrate. This was made clear from the fact that the ABD monolayer could not block the electrochemical reaction of Fe(CN)63-/ Fe(CN)64- in solution. (29) Liu, Z. F.; Hashimoto, K.; Fujishima, A. J . Electroanal. Chem. lnte$acial Electrochem. 1992,324, 259. (30) The formation of the cis isomer was confirmed by measuring a UV-absorption spectrum change of the film. (31) Although we assumed HBD as the intermediate of cis to trans conversion in the above hypothesis, it is not clear at present whether there is enough time for the protonation and deprotonation processes. It may be also possible to consider that some other transient state like-NH-N-exists instead of the HBD.* (32) Contact angle measurements of thiol surfaces were performed to check their wettability for H20. Advancing contact angles were determined at room temperature and 100% relative humidity using a homemade setup which comprises a small chamber and an optical microscope. We obtained 78’ for ethanethiol and 110’ for octanethiol, respectively.
JP9501761
