- Email: [email protected]
New applications of electrochemical techniques
J. Photo&em.
PhotobioZ.
A:
Chem.,
65 (1992)
285-292
285
New applications of electrochemical techniques Z. F. Liut, K. Morigaki, Depaltment of Synthetic Tokyo 113 (Japan)
K. Hashimoto
Chemistry,
Fatuity
and A. FujishimaW of Engineering
Zhe
University
of Tokyo,
Hongo,
Abstract Two novel applications of electrochemical techniques are discussed based on the photochemical/electrochemical hybridized reaction of azobenzene species. The first is photoelectrochemical information storage. This new recording technique has advantages of ultrahigh storage density (up to 10” bits cm-‘), non-destructive optical readout and rewritability. The second is electrochemical actinometry, in which the trans-cis photoisomerization of 4-octyl-4’-(5-carboxy-pentamethylene-oxy)-azobenzene (ABD) in an assembled monolayer film is used to detect incident light, and the quantification of the cis-ABD thus produced is performed through its selective electrochemical reduction, leading to an instantaneous evaluation of the irradiation intensity.
1. introduction
Organic molecules having both photochemical and electrochemical reactivity are potentially important in functional device studies. The azo compound is a typical example, which undergoes two types of reversible process, i.e. photochemical cis-trans isomerization [l] and electrochemical reduction*xidation [2]. In previous studies [3, 41, we have observed a route-specific phenomenon concerning both reactions of an amphiphilic azobenzene derivative in an assembled monolayer film: the photo-created cis isomer is electrochemically reduced to a hydrazobenzene species (-NH-NH-) at a substantially more anodic potential than the trans isomer, and the hydrazo species thus produced is exclusively oxidized to the original trans isomer. In this paper, we present two typical applications of this hybrid phenomenon: ultrahigh density photoelectrochemical information storage [S, 63 and electrochemical actinometry f73_
2, Experimental
details
2.1. Reagents The amphiphilic -benzene derivative, 4-octyl-4’-(S-carboxy-pentamethylene-oxy)azobenzene (ABD) was purchased from Dojindo Laboratory (Kumamoto, Japan). All chemicals were of reagent grade and were used without further purification.
+Present address: Institute for Molecular Science, Myodaiji, Okazaki 444, Japan. ++Author to whom correspondence should be addressed.
0 1992 - Elsevier Sequoia. All rights reserved
286 2.2. Fabrication of monolayer fibns The ABD monolayer film was deposited
onto a transparent SnOz glass substrate using the Langmuir-Blodgett method (Kyowa, HBM-AR). A 0.2 mM CdClz aqueous solution was used as the subphase, and no special pH adjustment was made. Chloroform was used as the spreading solvent with an ABD concentration of 1.5-2.5 mM in the solution. The SnOz glass with a lateral resistance of 10 Sz was purchased from Asahi Glass Company (Tokyo, Japan). The surface of the SnOz glass was hydrophilically treated by immersing it into hot sulphuric acid (50% by volume) for 10 min before use. The monolayer film was fabricated by dipping the substrate into the aqueous subphase and raising it at a rate of 10 mm min - ‘. Because the SnOa glass surface was hydrophilic, only one monolayer of ABD, with the hydrophobic alkyl group exposed to the air, was formed on the SnOz glass during the dipping and raising process. All of the monolayer films were prepared at a constant surface pressure of 25 mN m-r, and the subphase temperature was controlled at 20 “C by a thermostat (Tokyo Rikakikai, UC-55). 2.3. Photoelectrochemical measurements The photoelectrochemical measurements were carried out using the same cell setup as described previously [3]. The ABD monolayer film deposited onto the SnOz glass substrate was used as working electrode (WE) and a platinum wire as counterelectrode (CE). The potential of the WE was controlled vs. an Ag/AgCl (saturated KCl) reference electrode (RE) by a potentiostat (Toho, 2040). A 0.1 M aqueous sodium perchlorate solution, buffered to pH 7.0 with B&ton-Robinson buffer [S], was employed as the electrolyte. Before each experiment, the electrolyte was deaerated with high purity argon for 20 min. The trans-cis photoisomerization of ABD molecules was induced using a xenon lamp, mercury lamp (Ushio Electric, 500 W) or a 14 mV He-Cd laser (Kimmon, CD3041R, 325 nm). The monochromatic light in the first two cases was isolated by a monochromator (Shimadzu, 1200 grooves mm-‘).
3. Results
and discussion
3.1. PhotoeZectrochemical infonnatiora storage 15, 61 It is our aim in this study to organize the photochemical trans-cis isomerization and electrochemical reduction-xidation of ABD molecules using the route-specific hybrid effect mentioned above to achieve the photoelectrochemical storage of information. This is a novel strategy because both photon and electron processes are involved in the storage of information, which is therefore expected to make best use of the advantages of photochemical and electrochemical processes. 3.1.1.
Recording
scheme
thermally stable hydra-ABD (-NH-NH-) molecule, the electrochemical reduction product of the cis-ABD isomer, is used as the storage state for information instead of the thermally unstable cir-ABD. The stability of the storage state molecules has been investigated under several conditions. Although hydra-ABD molecules are slowly oxidized to trans-ABD in an oxygen atmosphere, no change is observed in these storage state molecules in deaerated or negatively-biased (-0.3 V) conditions. The photoelectrochemical hybrid feature of the ABD compound gives rise to two possible The
287
means of high-density storage: high-resolution optical writing or high-resolution electmchemical writing. For optical writing, high-density storage is obtained using a fine-beam laser to localize the trans to cis isomerization, followed by switching the potential of the entire film to a region where CL-ABD is reduced but not trans-ABD. Figure 1 shows the experimental results obtained when an He-Cd laser (beam diameter, approximately 1 mm) was used to irradiate six different areas. The number of electrochemically reacted ABD molecules was calculated by integrating the anodic current-potential curves, which showed a linear increase proportional to the increase in the number of irradiated areas (see Fig. 2). The small change in the average number of reacted molecules per area is attributed to the thermal cis to trans isomerization which occurred in the previously irradiated areas. A separate experiment was performed to determine the stability of the cis-ABD film in the dark (Fig. 2); it is obvious that only the areas subjected to illumination show an electrochemica1 response. Hence the storage density in this case clearly depends only on the irradiation area, and if such an operation was carried out to the diffraction limit of the laser light, we would be able to store 10’ bits cm-’ in a two-dimensional configuration [9]. Moreover, if several azobenzene species with similar hybrid behaviour, but substantially different spectral responses, are located in the storage film as shown in Fig. 3, i.e. using the frequency domain of light, the storage density can be further magnified by use of a tunable laser beam [9, IO]. High-density electrochemical writing is obtained by controlling the reduction area of the cis-ABD film after uniform W irradiation of the entire film. This can be achieved using a sharp glass-coated probe as the counterelectrode, with control of the distance between the probe and the storage-film-modified electrode. The scanning tunnelling microscopic technique [ll] can be used to position this probe, and a storage
24
l 3 0
total number average number per area corrected average number per area
, 0.4 -0.2 0 0.2 E (V vs Ag/AgCl)
0
1
He-Cd
3
2
laser
4 irradiated
6 5 areas
7
Fig. 1. Anodic peaks of the cyclic volt ammograms for difFerent numbers of laser-irradiated areas; 0, unit area irradiated by one beam spot for 10 s. Fig. 2. Linear relationship between the reacted AF3D molecules and the number of laser-irradiated areas.
288
tunable
laser
wavelength
glass-coated
tip electrode
potential
Fig. 3. Schematic illustrations of the use of the frequency domain of light, where A and B represent azo compounds having different spectral and electrochemical properties respectiveIy.
density of 10” bits cm-* is principally possible in a two-dimensional without using the frequency domain of light [12].
configuration
3.2.2. Readout of information The present photoelectrochemical hybrid system also offers a realistic approach for non-destructive readout of the information stored. There is a large difference between the absorption spectra of trans- and hydra-ABD films owing to the conversion of -N-Nto -NH-NHin the ABD chromophore. Thus the information stored in either writing mode can be read out by monitoring the optical changes. Since two reaction steps, i.e. the photochemical isomerization and the electrochemical reduction, are necessary for the formation of the hydra-ABD state (storage state), this optical readout method will not destroy the stored information. The intentional introduction of a hybrid process to achieve non-destructive readout of the stored information has important implications considering the intrinsic readout problem for the current photonmode techniques based on a one-photon process [9, lo]. Because no threshold value of illumination is available, in contrast with heat-mode recording, optical readout based on spectral changes is impossible for the current systems [9, 101. In addition, other readout approaches using light scattering or electric charge may also be possible in the present hybrid system, although readout based on electric charge will destroy the information stored. 3.1.3. Electrochemical erasure The information stored can be wiped by anodizing the entire film, whereupon the hydra-ABD molecules are exclusively oxidized to the original zrans-ABD state, indicating that the present storage system is rewritable. The reversible durability has been studied by repeating the isomerization + reduction ---, oxidation operation. No discernible change in the nature of the reaction was observed during several hundred cycles of repetition, although a small decrease in the reduction+rxidation peak currents
289
occurred due to dissolution of the film molecules into the aqueous electrolyte. The dissolution problem is expected to be overcome by introducing a solid electrolyte instead of using the present wet system. The idea proposed for information storage may not necessarily be limited to the present specific azo molecule, or even to the azo system. The -C=Nand -C=Csystems are also attractive in view of the similar photochemical and electrochemical behaviour. As a general rule, we can carry out a deliberate structural design to obtain a suitable molecular system having the expected hybrid effect, i.e. substantially different electrochemical behaviour between the stearic isomers of a double-bonded organic molecule_ Obviously the molecular or matrix design will play an important role in satisfying the desired requirements such as the long-wavelength response of a realistic storage system [lo]. 3.2. Electrochemical actinometry [7] In this actinometric system, the trans to cis photoisomerization of ABD molecules in the assembled monolayer film was employed to detect or memorize the photon number of the incident light, and the quantification of cis-AESD produced in the actinometric irradiation process was performed through its selective electrochemical reduction (due to the distinct difference in the reduction of the frans- and cis-ABD isomers). As compared with the spectrophotometric technique used in conventional chemical actinometry, the method developed in this work has the advantages of simple operation, nearly instantaneous evaluation of the light intensity and reusability. 3.2.1. Establishment of actinomehic procedure The selective electrochemical reduction of the cis-ABD isomer can be carried out either directly after or simultaneously with actinometric light irradiation. In the former case, the reduction is best performed immediately after irradiation to minimize the influence of the thermal cis-trans back reaction. The thermal isomerization has been investigated in a separate experiment and the rate constant at room temperature (20 “C) was found to be 1.38 X 10e4 s- ‘. This indicates that a few minutes of the reduction operation will not cause an appreciable error in the light intensity determination. In the case of simultaneous reduction, the thermal cis-trans isomer&&ion can be neglected completely owing to the fast transformation of cis-ABD molecules to the -NH-NHspecies. In Fig. 4, a representative cathodic current-time curve is shown, which was obtained at a constant potential bias of -0.4 V (vs. Ag/AgCl). To ensure 100% reduction of &-ABD molecules, the electrode potential should be controlled at a suitable bias condition. For this reason, the cis-ABD isomer was created by a constant 20 s irradiation at 340 nm and the potential dependence of the amount of cLs-ABD reduced electrochemically was investigated. It was found that an adequate potential bias has a value in the range - 0.4 to - 0.6 V (vs. Ag/AgC!l). It should be emphasized that since the non-faradaic charge current is not involved in the constant-bias reduction process, the faradaic charge can be directly read out from a commonly used coulomb meter, leading to a nearly instantaneous determination of irradiation intensity. Hence the simultaneous reduction method is particularly recommended. 3.2.2. Validity test With a careful theoretical analysis of the present actinometric system, the following linear relationship between the irradiation light intensity (lo) and the faradaic charge quantity (Qc) flowing in the selective reduction process of the cis-ABD isomer was obtained [7j
Light Off +
Light On
10s m
Time Fig. 4. Cathodic current-time behaviour in the simultaneous cir-ABD the ABD monolayer fdm was biased at -0.4 V vs. Ag/AgCi.
reduction process, in which
(1) where K=1/(2F&,C,,) is the system constant depending only on the irradiation wavelength, & is the quantum yield for the trans-cis photoisomerization, lt and CO are the two-dimensional absorptivity and the molecular density of the truns-ABD isomer in the monolayer film respectively, S and t are the area and time of irradiation respectively and F=96484_6 C mol-’ is the Faraday constant. In order to confirm its validity and to determine the system constant (K), the well-established ferrioxalate actinometer [13] was employed for calibration of the irradiation intensity. Figure 5 illustrates the experimental results for some W mercury lines and for the 340 nm light of a xenon lamp, obtained from the simultaneous reduction method. The faradaic charge for cis-ABD reduction in all cases is increased linearly with an increase in irradiation intensity. This indicates that the mathematical treatment performed in deriving eqn. (1) is reasonable for the experimental conditions employed. From the least-squares slope of the linear plot obtained, the system constant (IL) at each irradiation wavelength was calculated, and they are summarized in Table 1. To demonstrate further the reliability of the proposed actinometric system, a number of unknown irradiation intensities from a xenon lamp were evaluated by the ABD monolayer film actinometer using the Kvalue obtained and the ferrioxalate actinometer. The results are summarized in Table 2, which clearly shows the excellent agreement between the two different evaluation methods. 3.23. Advantages The present electrochemical actinometric system has two distinct advantages compared with chemical actinometry based on spectrophotometric quantification. Firstly,
291
0
2
6
4
Irradiation
8
Intensity(xlO1°
10
12
14
Einsteins s-1)
Fig. 5. Linear dependence of faradaic charge on the irradiation intensity at 334 nm (A), nm (0), 405 nm (0) (all mercury lines) and 340 nm (a) (xenon lamp).
TABLE
365
1
System constants (K) for various mercury lines .4 (nm)
ld
K (einsteins C-l)
334 340” 365 405
4.39f0.03 4.72jzO.02 5.66*0.01 51.36-+0.12
‘For xenon lamp. TABLE
2
Comparison of irradiation intensities evaluated using ferrioxalate and ABD actinometers (A = 340 nm) No.
1 2 3 4
Irradiation time (s)
20 20 20 20
monolayer
film
Irradiation intensity (X 1O’O einsteins s-3 Ferrioxalate
ABD
5.93 f 4.85 f 2.62f 0.77f
5.94f 0.025 4.83* 0.02 2.60f0.015 0.78f 0.018
0.02 0.02 0.01 0.01
monolayer f&n
the evaluation process is very simple and rapid. Especially iu the case of the recommended simultaneous reduction method, the faradaic charge quantity can be directly read out from the coulomb meter, leading to an instantaneous determination of the irradiation intensity. Secondly, the present ABD monolayer film actiuometer is reusable due to the following two experimental observations [3-51: (1) the original iruns-ABD state
292
can be completely reproduced by a suitable reduction-oxidation treatment; (2) no appreciable change in film structure is observed for several tens of treatments on the ABD monolayer film. The present actinometric system is particularly suitable for the evaluation of irradiation intensity in photoelectrochemical studies. In such cases, the actinometric measurement can be carried out using the same experimental set-up as for the photoelectrochemical measurements.
References 1 J. Griffiths, Chern. Sot. Rev., 1 (1972) 481. 2 E. Laviron and Y. Mugnier, J. EZectmanaI. Chem., 111 (1980) 337. 3 Z. F. Liu, B. H. Loo, K. Hashimoto and A. Fujishima, J. Electrounal. Chem., 297 (1991) 133. 4 Z. F. Liu, K, Hashimoto and A. Fujishima, J. EIectmanaL Chem, in the press. 5 Z. F. Liu, K. Hashimoto and A. Fujishima, Nature, 347 (1990) 658. 6 Z. F. Liu, K. Hashimoto and A. Fujishima, in K. Honda (ed.), Photochemical Processes in Organized Mokculur Systems, Elsevier, Amsterdam, 1991. 7 Z. F. Liu, K. Morigaki, K. Hashimoto and A. Fujishima, Anal. Chem., 64 (1992) 134. 8 D. D. Perrin and B. Dempsey, Buffers forpH and Metal Ion Contra, Chapman and Hall, London, 1974, p. 154. 9 A. R. Gutierrez, J. Friedrich, D. Haarer and H_ Wolfrum, IBM JY Res. Dew, 26 (2) (1982) 198.
10 11 12 13
E. Ando, J. Miyazaki and K. Morimoto, ??airzSolid Filmr, 133 (1985) 21. G. Binnig and H. Rohrer, Rev. Mod. Phys., 59 (1987) 615. J. Kwak and A. J. Bard, AnaL Chem, 61 (1989) 1794. C. G. Hatchard and C. A. Parker, Proc. R. Sot. London, Ser. A, 235 (1956) 518.
PhotobioZ.
A:
Chem.,
65 (1992)
285-292
285
New applications of electrochemical techniques Z. F. Liut, K. Morigaki, Depaltment of Synthetic Tokyo 113 (Japan)
K. Hashimoto
Chemistry,
Fatuity
and A. FujishimaW of Engineering
Zhe
University
of Tokyo,
Hongo,
Abstract Two novel applications of electrochemical techniques are discussed based on the photochemical/electrochemical hybridized reaction of azobenzene species. The first is photoelectrochemical information storage. This new recording technique has advantages of ultrahigh storage density (up to 10” bits cm-‘), non-destructive optical readout and rewritability. The second is electrochemical actinometry, in which the trans-cis photoisomerization of 4-octyl-4’-(5-carboxy-pentamethylene-oxy)-azobenzene (ABD) in an assembled monolayer film is used to detect incident light, and the quantification of the cis-ABD thus produced is performed through its selective electrochemical reduction, leading to an instantaneous evaluation of the irradiation intensity.
1. introduction
Organic molecules having both photochemical and electrochemical reactivity are potentially important in functional device studies. The azo compound is a typical example, which undergoes two types of reversible process, i.e. photochemical cis-trans isomerization [l] and electrochemical reduction*xidation [2]. In previous studies [3, 41, we have observed a route-specific phenomenon concerning both reactions of an amphiphilic azobenzene derivative in an assembled monolayer film: the photo-created cis isomer is electrochemically reduced to a hydrazobenzene species (-NH-NH-) at a substantially more anodic potential than the trans isomer, and the hydrazo species thus produced is exclusively oxidized to the original trans isomer. In this paper, we present two typical applications of this hybrid phenomenon: ultrahigh density photoelectrochemical information storage [S, 63 and electrochemical actinometry f73_
2, Experimental
details
2.1. Reagents The amphiphilic -benzene derivative, 4-octyl-4’-(S-carboxy-pentamethylene-oxy)azobenzene (ABD) was purchased from Dojindo Laboratory (Kumamoto, Japan). All chemicals were of reagent grade and were used without further purification.
+Present address: Institute for Molecular Science, Myodaiji, Okazaki 444, Japan. ++Author to whom correspondence should be addressed.
0 1992 - Elsevier Sequoia. All rights reserved
286 2.2. Fabrication of monolayer fibns The ABD monolayer film was deposited
onto a transparent SnOz glass substrate using the Langmuir-Blodgett method (Kyowa, HBM-AR). A 0.2 mM CdClz aqueous solution was used as the subphase, and no special pH adjustment was made. Chloroform was used as the spreading solvent with an ABD concentration of 1.5-2.5 mM in the solution. The SnOz glass with a lateral resistance of 10 Sz was purchased from Asahi Glass Company (Tokyo, Japan). The surface of the SnOz glass was hydrophilically treated by immersing it into hot sulphuric acid (50% by volume) for 10 min before use. The monolayer film was fabricated by dipping the substrate into the aqueous subphase and raising it at a rate of 10 mm min - ‘. Because the SnOa glass surface was hydrophilic, only one monolayer of ABD, with the hydrophobic alkyl group exposed to the air, was formed on the SnOz glass during the dipping and raising process. All of the monolayer films were prepared at a constant surface pressure of 25 mN m-r, and the subphase temperature was controlled at 20 “C by a thermostat (Tokyo Rikakikai, UC-55). 2.3. Photoelectrochemical measurements The photoelectrochemical measurements were carried out using the same cell setup as described previously [3]. The ABD monolayer film deposited onto the SnOz glass substrate was used as working electrode (WE) and a platinum wire as counterelectrode (CE). The potential of the WE was controlled vs. an Ag/AgCl (saturated KCl) reference electrode (RE) by a potentiostat (Toho, 2040). A 0.1 M aqueous sodium perchlorate solution, buffered to pH 7.0 with B&ton-Robinson buffer [S], was employed as the electrolyte. Before each experiment, the electrolyte was deaerated with high purity argon for 20 min. The trans-cis photoisomerization of ABD molecules was induced using a xenon lamp, mercury lamp (Ushio Electric, 500 W) or a 14 mV He-Cd laser (Kimmon, CD3041R, 325 nm). The monochromatic light in the first two cases was isolated by a monochromator (Shimadzu, 1200 grooves mm-‘).
3. Results
and discussion
3.1. PhotoeZectrochemical infonnatiora storage 15, 61 It is our aim in this study to organize the photochemical trans-cis isomerization and electrochemical reduction-xidation of ABD molecules using the route-specific hybrid effect mentioned above to achieve the photoelectrochemical storage of information. This is a novel strategy because both photon and electron processes are involved in the storage of information, which is therefore expected to make best use of the advantages of photochemical and electrochemical processes. 3.1.1.
Recording
scheme
thermally stable hydra-ABD (-NH-NH-) molecule, the electrochemical reduction product of the cis-ABD isomer, is used as the storage state for information instead of the thermally unstable cir-ABD. The stability of the storage state molecules has been investigated under several conditions. Although hydra-ABD molecules are slowly oxidized to trans-ABD in an oxygen atmosphere, no change is observed in these storage state molecules in deaerated or negatively-biased (-0.3 V) conditions. The photoelectrochemical hybrid feature of the ABD compound gives rise to two possible The
287
means of high-density storage: high-resolution optical writing or high-resolution electmchemical writing. For optical writing, high-density storage is obtained using a fine-beam laser to localize the trans to cis isomerization, followed by switching the potential of the entire film to a region where CL-ABD is reduced but not trans-ABD. Figure 1 shows the experimental results obtained when an He-Cd laser (beam diameter, approximately 1 mm) was used to irradiate six different areas. The number of electrochemically reacted ABD molecules was calculated by integrating the anodic current-potential curves, which showed a linear increase proportional to the increase in the number of irradiated areas (see Fig. 2). The small change in the average number of reacted molecules per area is attributed to the thermal cis to trans isomerization which occurred in the previously irradiated areas. A separate experiment was performed to determine the stability of the cis-ABD film in the dark (Fig. 2); it is obvious that only the areas subjected to illumination show an electrochemica1 response. Hence the storage density in this case clearly depends only on the irradiation area, and if such an operation was carried out to the diffraction limit of the laser light, we would be able to store 10’ bits cm-’ in a two-dimensional configuration [9]. Moreover, if several azobenzene species with similar hybrid behaviour, but substantially different spectral responses, are located in the storage film as shown in Fig. 3, i.e. using the frequency domain of light, the storage density can be further magnified by use of a tunable laser beam [9, IO]. High-density electrochemical writing is obtained by controlling the reduction area of the cis-ABD film after uniform W irradiation of the entire film. This can be achieved using a sharp glass-coated probe as the counterelectrode, with control of the distance between the probe and the storage-film-modified electrode. The scanning tunnelling microscopic technique [ll] can be used to position this probe, and a storage
24
l 3 0
total number average number per area corrected average number per area
, 0.4 -0.2 0 0.2 E (V vs Ag/AgCl)
0
1
He-Cd
3
2
laser
4 irradiated
6 5 areas
7
Fig. 1. Anodic peaks of the cyclic volt ammograms for difFerent numbers of laser-irradiated areas; 0, unit area irradiated by one beam spot for 10 s. Fig. 2. Linear relationship between the reacted AF3D molecules and the number of laser-irradiated areas.
288
tunable
laser
wavelength
glass-coated
tip electrode
potential
Fig. 3. Schematic illustrations of the use of the frequency domain of light, where A and B represent azo compounds having different spectral and electrochemical properties respectiveIy.
density of 10” bits cm-* is principally possible in a two-dimensional without using the frequency domain of light [12].
configuration
3.2.2. Readout of information The present photoelectrochemical hybrid system also offers a realistic approach for non-destructive readout of the information stored. There is a large difference between the absorption spectra of trans- and hydra-ABD films owing to the conversion of -N-Nto -NH-NHin the ABD chromophore. Thus the information stored in either writing mode can be read out by monitoring the optical changes. Since two reaction steps, i.e. the photochemical isomerization and the electrochemical reduction, are necessary for the formation of the hydra-ABD state (storage state), this optical readout method will not destroy the stored information. The intentional introduction of a hybrid process to achieve non-destructive readout of the stored information has important implications considering the intrinsic readout problem for the current photonmode techniques based on a one-photon process [9, lo]. Because no threshold value of illumination is available, in contrast with heat-mode recording, optical readout based on spectral changes is impossible for the current systems [9, 101. In addition, other readout approaches using light scattering or electric charge may also be possible in the present hybrid system, although readout based on electric charge will destroy the information stored. 3.1.3. Electrochemical erasure The information stored can be wiped by anodizing the entire film, whereupon the hydra-ABD molecules are exclusively oxidized to the original zrans-ABD state, indicating that the present storage system is rewritable. The reversible durability has been studied by repeating the isomerization + reduction ---, oxidation operation. No discernible change in the nature of the reaction was observed during several hundred cycles of repetition, although a small decrease in the reduction+rxidation peak currents
289
occurred due to dissolution of the film molecules into the aqueous electrolyte. The dissolution problem is expected to be overcome by introducing a solid electrolyte instead of using the present wet system. The idea proposed for information storage may not necessarily be limited to the present specific azo molecule, or even to the azo system. The -C=Nand -C=Csystems are also attractive in view of the similar photochemical and electrochemical behaviour. As a general rule, we can carry out a deliberate structural design to obtain a suitable molecular system having the expected hybrid effect, i.e. substantially different electrochemical behaviour between the stearic isomers of a double-bonded organic molecule_ Obviously the molecular or matrix design will play an important role in satisfying the desired requirements such as the long-wavelength response of a realistic storage system [lo]. 3.2. Electrochemical actinometry [7] In this actinometric system, the trans to cis photoisomerization of ABD molecules in the assembled monolayer film was employed to detect or memorize the photon number of the incident light, and the quantification of cis-AESD produced in the actinometric irradiation process was performed through its selective electrochemical reduction (due to the distinct difference in the reduction of the frans- and cis-ABD isomers). As compared with the spectrophotometric technique used in conventional chemical actinometry, the method developed in this work has the advantages of simple operation, nearly instantaneous evaluation of the light intensity and reusability. 3.2.1. Establishment of actinomehic procedure The selective electrochemical reduction of the cis-ABD isomer can be carried out either directly after or simultaneously with actinometric light irradiation. In the former case, the reduction is best performed immediately after irradiation to minimize the influence of the thermal cis-trans back reaction. The thermal isomerization has been investigated in a separate experiment and the rate constant at room temperature (20 “C) was found to be 1.38 X 10e4 s- ‘. This indicates that a few minutes of the reduction operation will not cause an appreciable error in the light intensity determination. In the case of simultaneous reduction, the thermal cis-trans isomer&&ion can be neglected completely owing to the fast transformation of cis-ABD molecules to the -NH-NHspecies. In Fig. 4, a representative cathodic current-time curve is shown, which was obtained at a constant potential bias of -0.4 V (vs. Ag/AgCl). To ensure 100% reduction of &-ABD molecules, the electrode potential should be controlled at a suitable bias condition. For this reason, the cis-ABD isomer was created by a constant 20 s irradiation at 340 nm and the potential dependence of the amount of cLs-ABD reduced electrochemically was investigated. It was found that an adequate potential bias has a value in the range - 0.4 to - 0.6 V (vs. Ag/AgC!l). It should be emphasized that since the non-faradaic charge current is not involved in the constant-bias reduction process, the faradaic charge can be directly read out from a commonly used coulomb meter, leading to a nearly instantaneous determination of irradiation intensity. Hence the simultaneous reduction method is particularly recommended. 3.2.2. Validity test With a careful theoretical analysis of the present actinometric system, the following linear relationship between the irradiation light intensity (lo) and the faradaic charge quantity (Qc) flowing in the selective reduction process of the cis-ABD isomer was obtained [7j
Light Off +
Light On
10s m
Time Fig. 4. Cathodic current-time behaviour in the simultaneous cir-ABD the ABD monolayer fdm was biased at -0.4 V vs. Ag/AgCi.
reduction process, in which
(1) where K=1/(2F&,C,,) is the system constant depending only on the irradiation wavelength, & is the quantum yield for the trans-cis photoisomerization, lt and CO are the two-dimensional absorptivity and the molecular density of the truns-ABD isomer in the monolayer film respectively, S and t are the area and time of irradiation respectively and F=96484_6 C mol-’ is the Faraday constant. In order to confirm its validity and to determine the system constant (K), the well-established ferrioxalate actinometer [13] was employed for calibration of the irradiation intensity. Figure 5 illustrates the experimental results for some W mercury lines and for the 340 nm light of a xenon lamp, obtained from the simultaneous reduction method. The faradaic charge for cis-ABD reduction in all cases is increased linearly with an increase in irradiation intensity. This indicates that the mathematical treatment performed in deriving eqn. (1) is reasonable for the experimental conditions employed. From the least-squares slope of the linear plot obtained, the system constant (IL) at each irradiation wavelength was calculated, and they are summarized in Table 1. To demonstrate further the reliability of the proposed actinometric system, a number of unknown irradiation intensities from a xenon lamp were evaluated by the ABD monolayer film actinometer using the Kvalue obtained and the ferrioxalate actinometer. The results are summarized in Table 2, which clearly shows the excellent agreement between the two different evaluation methods. 3.23. Advantages The present electrochemical actinometric system has two distinct advantages compared with chemical actinometry based on spectrophotometric quantification. Firstly,
291
0
2
6
4
Irradiation
8
Intensity(xlO1°
10
12
14
Einsteins s-1)
Fig. 5. Linear dependence of faradaic charge on the irradiation intensity at 334 nm (A), nm (0), 405 nm (0) (all mercury lines) and 340 nm (a) (xenon lamp).
TABLE
365
1
System constants (K) for various mercury lines .4 (nm)
ld
K (einsteins C-l)
334 340” 365 405
4.39f0.03 4.72jzO.02 5.66*0.01 51.36-+0.12
‘For xenon lamp. TABLE
2
Comparison of irradiation intensities evaluated using ferrioxalate and ABD actinometers (A = 340 nm) No.
1 2 3 4
Irradiation time (s)
20 20 20 20
monolayer
film
Irradiation intensity (X 1O’O einsteins s-3 Ferrioxalate
ABD
5.93 f 4.85 f 2.62f 0.77f
5.94f 0.025 4.83* 0.02 2.60f0.015 0.78f 0.018
0.02 0.02 0.01 0.01
monolayer f&n
the evaluation process is very simple and rapid. Especially iu the case of the recommended simultaneous reduction method, the faradaic charge quantity can be directly read out from the coulomb meter, leading to an instantaneous determination of the irradiation intensity. Secondly, the present ABD monolayer film actiuometer is reusable due to the following two experimental observations [3-51: (1) the original iruns-ABD state
292
can be completely reproduced by a suitable reduction-oxidation treatment; (2) no appreciable change in film structure is observed for several tens of treatments on the ABD monolayer film. The present actinometric system is particularly suitable for the evaluation of irradiation intensity in photoelectrochemical studies. In such cases, the actinometric measurement can be carried out using the same experimental set-up as for the photoelectrochemical measurements.
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E. Ando, J. Miyazaki and K. Morimoto, ??airzSolid Filmr, 133 (1985) 21. G. Binnig and H. Rohrer, Rev. Mod. Phys., 59 (1987) 615. J. Kwak and A. J. Bard, AnaL Chem, 61 (1989) 1794. C. G. Hatchard and C. A. Parker, Proc. R. Sot. London, Ser. A, 235 (1956) 518.