Selective In-Plane Photoelectrochemical Reaction of an Azobenzene

Received: October 4, 1994; In Final Form: December 7, 1994@ ... molecules with selective orientations in the surface plane of the film. A new type of ...
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J. Phys. Chem. 1995, 99, 3352-3356

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Selective In-Plane Photoelectrochemical Reaction of an Azobenzene Derivative in an Assembled Film R. Wang,?,*T. Iyoda,s K. Hashimoto,T3* and A. Fujishima*,t$' Department of Applied Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113, Japan, and Photochemical Conversion Materials Project, Kanagawa Academy of Science and Technology Laboratory, 1583 Iiyama, Atsugi, Kanagawa 243-02, 113 Japan Received: October 4, 1994; In Final Form: December 7, 1994@

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Trans cis photoisomerization of an azobenzene derivative in a Langmuir-Blodgett (LB) film was induced by successive linearly polarized UV illuminations along different directions. Polarized UV-visible spectroscopy was used to determine that the isomerization occurred with the highest probability along the polarization direction. After UV irradiation, the application of cathodic potential reduced the cis isomer to hydrazobenzene, which is not only thermally stable but, more importantly, insensitive to the actinic light for the trans-cis photoisomerization. By combining the electrochemical method, the utilization of successive polarized UV irradiations allows for the possibility of obtaining the composition of the hydrazobenzene molecules with selective orientations in the surface plane of the film. A new type of in-plane polarized interconversion system was developed by the photoelectrochemical hybrid technique.

Introduction Azobenzene derivatives have attracted considerable interest due to their reversible trans-cis photoisomeri~ation.~-~ The photoisomerization can result in desirable physical and chemical properties. As an example of the various applications, azobenzene-containing materials are widely regarded as one possible candidate for storage mediums or switching elements in optoelectronic system^.^-^ Excitation by linearly polarized light often causes photoactive materials to have designable structures with characteristic optical properties due to changes in anisotropy. Previous reports have shown that by polarized UV irradiation, reorientation and anisotropy in liquid crystal^,^^^^-'^ linear photopolymerization18and b i r e f r i n g e n ~ eas~well ~ ~ ~as~ ~ ~ in-plane anisotropic photobleaching2' and modification2sz2can be produced. All these studies concentrated on the reversible photochemical trans-cis isomerization with the primary application to photoimaging, photoswitching, and information storage. However, unless the recorded information is read out in a wavelength region where the photochromic compound does not absorb light, loss of information is inevitable after repeated readings. The electrochemical and photoelectrochemical properties of azobenzene moieties in solution have been investigated for the past few decade^.^^-*^ Recently, research work has focused on studies at the molecular level. The Langmuir-Blodgett (LB) technique has been accepted as an effective method to organize the molecules in a precise manner. The photoelectrochemical behavior of an azobenzene derivative in LB films has been reported by our gr0up.8326927We have observed that a stable state, namely, hydrazobenzene (HAB), is formed by electrochemically reducing the cis isomers. In this paper, a new type of in-plane polarized interconversion system incorporating this photoelectrochemical hybrid method is presented. In this system, linearly polarized UV irradiations with different directions were applied to the same film successively, and the trans cis hydrazobenzene interconversion was induced inde-

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* To whom correspondence

should be addressed.

t The University of Tokyo.

Kanagawa Academy of Science and Technology Laboratory. @Abstractpublished in Advance ACS Abstracts, February 15, 1995.

pendently along the distinct polarization directions. This method is quite reproducible and simple and offers the potential for widespread applications in information storage.

Experimental Section Materials. The amphiphilic azobenzene derivative, 4-octyl4'-(5-carboxypentamethyleneoxy)azobenzene (ABD), was purchased from Dojindo Laboratory (Kumamoto, Japan). The chemical was of reagent grade and was used without further purification. Preparation of LB film. The ABD LB film was deposited onto SnOz glass or CaF2 substrate by a conventional vertical dipping method using a standard LB trough (Kyowa Kaimen, HBM-AP). A 0.2 mM CdCL aqueous solution was used as the subphase and kept at 20 OC. Chloroform was applied as the spreading solvent, with the ABD concentration being 1.8 mM in the solution. The surface pressure for film fabrication was controlled at 25 mN/m. The procedures of LB film fabrication were performed under dim red light conditions to ensure that all the azobenzene moieties were trans isomers. The films were stored in the dark for 1 week before spectra measurement. Apparatus. Polarized UV-visible spectra of a nine-layer film of ABD on a CaF2 substrate before and after polarized UV irradiation for 20 s were recorded on a Shimadzu UV3 lOlPC UV-visible spectrophotometer with dichroic sheet polarizers. The experimental setup for the photoelectrochemical measurement is shown in Figure l. The SnOz glass substrate with ABD monolayer film was attached onto the electrochemical cell and served as the working electrode (WE). A Pt wire and an Ag/AgCl (saturated KC1) were used as the counter electrode (CE) and reference electrode (RE), respectively. A 0.1 M NaC104 aqueous solution, buffered to pH 7.0 with citric acid Na2HPO4 solution, was employed as the supporting electrolyte. Before each experiment, the electrolyte was deaerated with highpurity N2 gas for 20 min. Irradiations were carried out using a 500-W xenon lamp, employing two band-pass filters to obtain light with a wavelength of 365 nm for the trans cis photoisomerization.

0022-3654/95/2099-3352$09.00/00 1995 American Chemical Society

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J. Phys. Chem.. Vol. 99, No. 10, 1995 3353

Azobenzene Derivative in an Assembled Film

Figure 1. Schematic diagram of the photoelechochemical measurement. The working electmde (WE) is S n a glass with ABD monolayer film, The reference electmde (RE) is AgIAgCI (saturated KCI). The counter electrode (CE) is Pt wire.

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absorption band at 450 nm is assigned to the n-n* transition of the chromophore of c i s - a z o b e n ~ e n e . ~ However, ~.~~ no significant increase of the 450-nm peak is observed in the spectra before and after W irradiations. This is probably because 20 s of polarized W irradiation in the present work results in low photoisomerization yield. In addition, the absorption coefficient corresponding to the n-n* transition of cis-azobenzene is very low (E % 1500) comparing with that of the n-n* transition of trans-azobenzene (6 % 17 000)?9 The 340-nm band serves as a type of photoisomerization process in this study. The optical dichroic nature is detected in the as-transferred film prior to any W light illumination as shown in Figure Za. All and Al correspond to the absorbances of the film by polarizers aligned parallel and perpendicular to the film-dipping direction, respectively. The n-n* band shows a larger absorption for the polarized probe light set parallel to the film-dipping direction (All before the first irradiation) than that in the case of irradiation at perpendicular incidence (AI before the first irradiation). This implies that the projections of the n-n* transition moments, i.e., the long axes of the azobenzene are distributed anisotropically in the surface plane of the as-transferred film and that the trans-azobenzene moieties preferentially orientate along the film-dipping d i r e c t i ~ n . ) ~Distinct , ~ ~ photoselection in the absorption spectra (Figure 2a) is observed when the film is illuminated for 20 s by linearly polarized light with the electric field perpendicular to the filmdipping direction. In the direction parallel to the irradiated polarization plane ( A l ) , a difference in the absorption intensity before and after irradiation is clearly detected. On the contrary, in the orthogonal direction (All). no significant spectral change is obtained. The observation indicates that the trans cis isomerization induced by the linearly polarized UV irradiation occurs with the highest probability along the polarization direction, while the molecules aligned perpendicular to the polarization direction are almost unvaried. Correspondingly, when the polarized W beam is altered perpendicular to its original direction (parallel to the film-dipping direction) and irradiates the same film for 20 s (Figure 2b), the second trans cis isomerization is induced selectively along the polarization direction (All), independent of the first photoisomerization process. The molecules in the orthogonal direction ( A 3 remain in the same state as that after the first irradiation. The phenomenon implies that when the different polarized W lights are applied successively, trans cis isomerization can be induced independently along the different polarization directions in this film system. It should be noted that the azobenzene moieties almost keep their in-plane orientation during consecutive polarized W irradiations. A polarized FTIR spectroscopic study also shows that the in-plane orientation of the azobenzene moiety (1604 cm-I due to v(C-C) corresonding to the vibration of benzene ring along the long axis, 1260 cm-' due to v(benzene-0). and 1152 cm-I due to v(benzene-N)) before and after polarized W irradiation varied within *3', which can be neglected in the present experiment. This lack of change in the azobenzene moiety is probably due to the low isomerization rate which will be discussed in detail in the next section. As a result, this system might be feasible for application to information storage if both trans-ABD and cis-ABD are sufficiently stable. However, the cis isomer is relatively unstable and quickly reverts thermally back to the trans isomer after several minutes under room ~ondition?~.~* The composition of trans and cis isomers in each direction would not be conserved as a result of individual photoisomerization events. Furthermore, as with an information storage system, since the recorded image is read out in a wavelength region where the photochro-

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Figure 2. Polarized W-visible s p c w of a nine-layer LB film. All and AL correspond to the absorbances of the f h with polarizers aligned parallel and perpendicular to the film-dipping direction. respectively. (a) Before (-) and after e**) the fmt polarized UV irradiation for 20 s with the polarization plane perpendicular to the filmdippingdimtion. (b) Before (-) and after e..) the second polarized W irradiation for 20 s with the polarization plane parallel to the film-dipping direction.

Results and Discussion Polarized Photnisomerization. The polarized W irradiations in the directions parallel and perpendicular to the filmdipping direction are taken as typical examples for the present study. Identical light intensities of the irradiation beams are set in the orthogonal directions. Figure 2 shows the polarized UV-visible spectra of the nine-layer film of ABD on CaF2 substrate before and after linearly polarized W irradiations. Usually, the occurrence of trans cis photoisomerization is characterized by the decrease of the 340-nm peak and the increase of the 450-nm peak in the W-visible absorption spectra. The absorption band at 340 nm is assigned to the n-n* transition of the chromophore of trans-azobenzene. while the

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3354 J. Phys. Chem., Vol. 99, No. 10, 1995

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Irradiation Time (s) Figure 3. Dependence of the photoinduced cis-ABD yield on polarized UV irradiation time in ABD monolayer film. The polarized UV beam is set with the electric field parallel to the film-dipping direction.

Figure 4. Successive cycles of the polarized photoelectrochemical procedures of ABD LB film. The successive cycles here are (1) polarized UV irradiation, ( 2 ) electrochemical reduction, (3) another polarized UV irradiation perpendicular to the fist irradiation, (4)

mic compound still absorbs light, destruction of the information is inevitable after continuus readouts along the different directions. To circumvent this problem, an electrochemical method can introduce the HAB to this system. HAB is thermally stable as well as insensitive to the actinic light for the trans-cis isomerization. Information could be recorded as HAB in a specific direction by means of linearly polarized photoelectrochemistry and read out through the composition of the trans isomers in separate directions by linearly polarized UV light without destruction of the recorded information. Polarized Photoelectrochemical Interconversion. Photoelectrochemical properties of ABD LB monolayer films have been reported in previous paper^.^,^^,^^ Since the cis isomer is less stable than the trans isomer, cis-ABD is reduced to the HAB state at less cathodic potential than t r a n ~ - A B D . ~Because ~-~~ of the substantially different redox potentials between transABD (-0.7 V) and cis-ABD (-0.3 V), the electrode potential can be set to around -0.5 V where only cis-ABD is electrochemically active. The Faraday Principle affords an efficient method to calculate the photoinduced cis isomer yield from the cyclic voltammograms ( C V S ) . ~The ~ value of the cathodic charge can be obtained by integrating the area underneath the reduction peak. This value corresponds directly to the number of molecules that are converted from cis-ABD to HAB, based on almost 100% current efficiency. In other words, the total cathodic charge can be used to estimate the amount of photoinduced cis-ABD molecules in the monolayer ABD film. According to the CV curves measured by using the electrochemical cell in Figure 1, the dependence of the photoinduced cis-ABD yield on polarized UV irradiation time is shown in Figure 3. The polarized UV beam is set with the electric field parallel to the film-dipping direction. A similar variation tendency is observed when the polarized irradiation is set in the orthogonal direction. As shown in Figure 3, when the irradiation time is less than 20 s, the photoinduced cis-ABD yield linearly increases with the irradiation time. However, when the irradiation time is more than 20 s, the cis-ABD yield increases gradually slower until the photostationary state is reached. In the present study, we choose 20 s for the UV irradiation time since the photoisomerization rate is very low (5%) compared with that of the photostationary state under prolonged irradiation (26%). A photoisomerization with such a low conversion rate would scarcely affect the orientation of trans-ABD, so the original in-plane distribution of trans-ABD should be conserved throughout the continuous photoisomerizations as well as the trans cis HAB interconversions. The linear increase of the cis-ABD yield with the irradiation time within 20 s displays strong evidence for

the above assumption. The control of the irradiation time becomes a key point in accomplishing the in-plane polarized photoelectrochemical reaction. The successive procedures described in Figure 4 are undertaken to investigate the polarized photoelectrochemicalbehavior of the ABD monolayer film. When polarized W light irradiates the trans-ABD film for the first time, the trans cis photoisomerization is induced along the polarization direction. The resulting cis-ABD is reduced to HAB at the appropriate cathodic potential. Next, a second UV irradiation with a different polarization direction is applied to the same film. Trans cis photoisomerization is induced along the second irradiation direction, and the cis-ABD is similarly reduced to HAB. At this time, when the anodic potential is applied to the film, the accumulated HAB molecules in different directions are completely oxidized to trans-ABD molecules. Figure 5a shows the cyclic voltammograms of the ABD monolayer film after polarized W illuminations for 20 s in the orthogonal directions with the same intensity according to the processes described in Figure 5b. The CV curves corresponding to procedure I show that the cathodic current ( R I ~ ) after irradiation along the film-dipping direction exhibits a slightly greater reduction value than that ( R I ~ after ) the orthogonal direction irradiation. A possible reason is inferred as the as-transferred molecules in the film distribute anisotropically and prefer to orientate along the film-dipping direction (Figure 2a). Therefore, the amounts of photoinduced cis-ABD molecules (corresponding to the cathodic currents in the CV curves) along the orthogonal directions should be different. The photoisomerization amount can be estimated from the decrease of the percentage of the trans isomer in the film after UV irradiation, corresponding to the decrease of the absorbance at 340 nm. However, as shown in Figure 2, the decrease of the absorbance at 340 nm resulting from the first polarized UV irradiation shows no clear difference from that resulting from the second polarized UV irradiation. This is probably due to the instability of cis-ABD. In order to further confirm that the polarized UV irradiations induce the trans cis HAB reaction independently along the different polarization directions, the CV curves in Figure 5a (I1 and 111) are recorded. After polarized UV irradiation for 20 s along the film-dipping direction, the obtained cis-ABD is reduced to HAB and then oxidized completely to trans-ABD (Figure 5a(II)) so that the amount of trans-ABD is kept constant before and after this “one-way” cyclic process.8 Next, the polarized UV light, perpendicular to the film-dipping direction, irradiates the same film for another 20 s, and similarly, the film is reduced and oxidized (Figure Sa(II1)). The cathodic currents

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glass electrode after polarized UV irradiation (a) and the corresponding sequence of the polarized photoelectrochemical procedures (b). WII and Wl indicate the polarized W irradiation parallel and perpendicular to the film-dipping direction, respectively.

before and after the continuous irradiations (Figure 2). This suggests that during the successive treatments, Le., polarized W irradiations and electrochemical reductions, the aggregate acts as a reaction unit for the trans cis HAB interconversion in the film. Each aggregate is relatively independent of the adjacent aggregates. Therefore, conversion of molecules in each aggregate scarcely affects the molecules in the other aggregates, especially when the rate of conversion is low. As shown in Figure 6, trans-ABD is converted to cis-ABD with the highest probability for those aggregates which have transition moments of the packed molecules along the polarization direction, whereas the molecules in the other aggregates with different orientations are hardly influenced. The resulting cisABD is consecutively reduced to hydrazobenzene, which is thermally stable and, more importantly, insensitive to the actinic lights for both trans cis and cis trans photoisomerizations.32 The electrochemical procedure can "freeze" the first photoisomerization event as thermally stable HAB in the aggregates with orientation parallel to the polarization direction. Therefore, when the polarized W beam perpendicular to the original direction irradiates the same film for the second time, transcis isomerization could be mainly induced along the second direction. The cis-ABD induced by the second irradiation is reduced to HAB separately and fixed in the film along the second polarization direction. The total HAB accumulated by the first and second photoelectrochemical processes is oxidized to trans-ABD by applying an anodic potential. The aggregation effect may play another important role in the independent trans cis HAB interconversion besides the merits of the HAB. It allows for the possibility of developing independent reactions from along the orthogonal directions to multiple directions.

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of I1 and 111 are perfectly consistent with those of I1 and 12, respectively. The identity of the reduction amounts of I2 with III should be emphasized. In the case of lTI,the cathodic current corresponds to the reduction of cis-ABD by the polarized UV irradiation perpendicular to the film-dipping direction. As for 12, after polarized UV illumination parallel to the film-dipping direction and reduction of the resulting cis-ABD, the film is irradiated for the second time along the perpendicular direction. The reduction of the relevant cis-ABD results in the cathodic current. This consistency indicates that the resulting amounts of cis-ABD and the HAB from consecutive individual procedures, Le., polarized W irradiation and electrochemical reduction, should be governed only by the intrinsic in-plane distribution of the molecules in the film. This displays strong evidence that the independent trans cis HAB interconversion is induced along the different irradiation directions in Conclusion the ABD LB film. Such an independent process is probably related to the existence of the aggregation effect in the film. We have shown that by using successive linearly polarized UV irradiations in orthogonal directions, trans cis HAB It has been reported that a maximum absorption band at 352 interconversion was induced successively and selectively along nm in ABD chloroform solution is assigned to the n-n* the distinct polarization directions at the same position in the transition of the monomer chromophore of trans-azobenzene.28 ABD LB film. In consideration of the merits of the HAB, i.e., The W-visible spectra of the ABD LB film in Figure 2 exhibit thermally stable and inactive to the actinic light, as well as the a corresponding absorption band at 340 nm. The blue shift in existence of aggregation, the polarized photoelectrochemical the spectra implies that the molecules aggregate in an H-like hybrid process is expected to be induced independently along fashion in the ultrathin film,36337indicating that the transition multiple directions. moments of the aggregation chromophores are parallel to each other with packing direction parallel to the film s ~ r f a c e . It ~ * ~ ~ ~ As an example of the application of this system, information should be noted that, although the transition moments of the could be written using the linearly polarized W beam with chromophores in the same aggregate are packed orderly in a different directions at the same position in the film and definite dire~tion:~the orientations of the projections of different subsequently frozen in the film as HAB by electrochemical aggregates in the film surface plane are dis0rdered.4~1~~ The H reduction. The stored information in a definite direction would aggregation is not destroyed by the successive W illuminations, be indestructible during the successive writings along the other directions at the same position. In addition, the information since the shape of the 340-nm absorption band is unchanged

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3356 J. Phys. Chem., Vol. 99, No. 10, 1995 could also be read out through the composition of trans-ABD by linearly polarized UV beam without destruction. This provides a prospective storage process for improving storage density. Moreover, in order to read out the recorded information, only a polarized UV beam is feasible.

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