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Anal. Chem. 1992, 6 4 , 134-137
Electrochemical Actinometry Using the Assembled Monolayer Film of an Azo Compound ZhongFan Liu,’ Kenichi Morigaki, Kazuhito Hashimoto, a n d Akira Fujishima* Department of Synthetic Chemistry, Faculty of Engineering, University of Tokyo, Hongo, Tokyo 113, J a p a n
A simple, convenient chemical actlnometer based on an assembled monolayer film of an amphiphiiic azo compound is proposed. I n contrast to spectrophotomefr/c quantitlcation in conventional chemical actinometries, an elecfrochemlcal approach was employed. This novel actinometry does not require any sophisticated mathematical treatment, and the light intensity is directly obtained from its linear relationship to the faradaic charge in the electrochemical reduction process, allowing simple and nearly instantaneous measurement.
INTRODUCTION Azo compounds have been recommended for chemical actinometry in the UV-visible light range for years because of their simple and well-defined photochemistry (1-5). In general, actinometry requires both complicated experimental operations and sophisticated mathematics for evaluation of light intensity, and hence is time-consuming work. Such problems are often encountered in chemical actinometry with spectrophotometric quantification techniques. In this paper, we present an electrochemical approach for actinometric measurements by employing the assembled monolayer film of 4-octyl-4’-(5-carboxylpentamethyleneoxy)azobenzene (ABD). This novel approach is established with a photochemical trans-cis isomerization and an electrochemical reduction-oxidation based on the following experimental observations (6, 7): the photocreated cis-ABD isomer is electrochemically reduced to its hydrazobenzene derivative (-NH-NH-) at substantially more anodic potential than is the trans-ABD isomer, and the hydrazobenzene compound thus produced is exclusively oxidized to the original trans-ABD isomer. Because of the distinct difference in electrochemistry of the trans- and cis-ABD isomers, the formation of the cisABD isomer due to photoisomerization of trans-ABD induced by actinometric irradiation could be followed by its selective electrochemical reduction. As compared to spectrophotometry, this method has advantages of simple operation, nearly instantaneous evaluation, and reusability. ACTINOMETRIC PRINCIPLE The photochemistry of ABD employed in the present actinometric study has been confirmed to be the same as that of azobenzene (7-10). Figure 1 shows its spectral changes in chloroform solution in response to UV and visible irradiations. The spectral change was reversible, and the absorption spectra have two excellent isosbestic points a t 309 and 415 nm, indicating that the photoisomerization consists of two linearly-dependent reaction steps, trans to cis and cis to trans, respectively. In addition to the photochemical isomerization, ABD molecules also undergo a thermal cis-trans isomerization (11). Under the assumption that the irradiation light is monochromatic and the quantum yields in both directions are independent of irradiation intensity, the differential rate *To whom correspondence should be addressed. Present address: Institute for Molecular Science, Myodaiji, Okazaki 444,Japan.
equation for the actinometric irradiation process is (4,12,13) dN,/dt = @,“S - 4J;S - kthNc
(1)
where N , = quantity of cis-ABD in monolayer film (moles), 4 = quantum yield for photoisomerization (subscripts t and c refer to trans-cis and cis-trans, respectively), I, = amount of absorbed light by trans (superscript t) and cis (superscript c) (Einsteins cm-2 s-l), S = irradiated area on monolayer film (cm2),kth = rate constant for thermal cis-trans isomerization (s-l), t = irradiation time (9). The first two terms on the right are the photoisomerization rates in both directions, and the third term is the thermal reaction rate. Since only molecules of one monolayer absorb the incident light, the amounts of photons absorbed by transand cis-ABD molecules can be expressed as a modification form of the Lambert-Beer law (13, 14): Iet= IottC, and IaC = loccCc,where I , is the incident light intensity (Einsteins cm-* s-l); Ct and C, are the respective lateral densities of trans- and cis-ABD in the monolayer film (mol cm-2), with C, = C, C, being a constant depending slightly on the fabrication pressure and temperature conditions of monolayer f i i (10);and ct and cc refer to the proportionality factors, or two-dimensional absorptivities of trans- and cis-ABD isomers (mol-’ cm2), Substituting into (1) gives
+
cW,/dt + (&ctIo + @ c d n +
- &etInCoS =
0
(2)
Clearly, the time-dependent photokinetic factor relating to sample absorption (A), F(A) = (l-lO-A)/A, which always complicates the mathematical treatment (4,13) disappears from eq 2. Hence eq 2 can be integrated in a closed form
N, =
(3) using the initial condition, N,(t=O) = 0. The exponential term in eq 3 can be further expanded as the power series, exp(-pt) = 1 - p t + 1/2(pt)2- ..., where p = dtetIo+ dccJo + k t h . For
p t < 0.2, it is possible to write exp(-pt) = 1 - p t with an error within &2% (vide infra), leading to
I o = Nc/4Jt&oSt
(4)
In the present actinometry, the cis-ABD isomer in the monolayer film (N,) is electrochemically quantified on the basis of its selective reduction cis-ABD
+ 2e- + 2H+ = -NH-NH-
(5)
Assuming a 100% reduction to the corresponding hydrazobenzene species (-NH-NH-) (7,8), the amount of cis-ABD ( N J produced during actinometric irradiation can be evaluated from the faradaic charge (Qc, in Coulombs) in the reduction process by N , = Qc/2F. As a result, a linear relationship between the irradiation light intensity and the faradaic charge is obtained K
In = -Q
0003-2700/92/0364-0134$03.00/0 0 1992 American Chemical Society
St
ANALYTICAL CHEMISTRY, VOL. 64, NO. 2, JANUARY 15, 1992
2.0
I
135
Light Off
J.
1.0
n " 250
3 50 450 Wavelengthlnm
550
4
Light On
Flgure 1. Spectral changes of ABD in chloroform solution in response to UV and visible irradiations: (a) trans-ABD (b) cis-ABD; (---) trans-
and
cis-ABD
mixtures.
where K = l/(2F&etC0), the system constant depending only on irradiation wavelength, and F = 96484.6C (mol-l) (Faraday constant).
EXPERIMENTAL SECTION Materials. The azobenzene derivative, 4-octyl-4'-(5carboxylpentamethy1eneoxy)azobenzene(ABD) is commercially available from Dojindo Laboratory (Kumamoto, Japan). All chemicals were of reagent grade and used without further purification. Sample Preparation. The ABD monolayer film was deposited onto a transparent SnOz glass substrate in the trans form with the Langmuir-Blodgett method using a commercial instrument (Kyowa, HBM-AP). A 0.2 mM CdC12aqueous 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 1.5-2.5 mM in the solution. The SnOpglass with a lateral resistance of 10 D (Asahi Glass, Tokyo) was hydrophilidy treated by immersing it into hot sulfuric 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 SnOz glass surface was hydrophilic, only one monolayer of ABD with the hydrophobic alkyl group exposed to the air was formed on the SnOz glass surface during the dipping and raising process. All samples were prepared at a constant surface pressure, 25 mN m-l, and the subphase temperature was controlled at 20 "C by a thermostat (Tokyo Rikakikai, UC-55). Actinometric Measurements. A xenon lamp and a highpressure mercury lamp (Ushio Electric, 500 W) were used for testing the validity and determining the system constant of the actinometer. The monochromatic light of the xenon lamp and the specific mercury lines were isolated by a monochromator with a grating of 1200 grooves mm-' (Shimadzu). The various light intensities at each wavelength were obtained by using suitable neutral fiiters. A ferrioxalate actinometer (15) was employed for calibration of the light intensity. The actinometric measurements were conducted using the same three-electrode cell as mentioned in the previous paper (II),where the ABD monolayer film-modified Sn02 glass was used as the working electrode (WE), and a Pt wire as the counter electrode (CE). The potential of the working electrode was controlled versus a Ag/AgCl (saturated KC1) reference electrode (RE) by a POtentiostat (Toho, 2040). A 0.1 M aqueous sodium perchlorate solution, buffered to pH 7.0 with Britton-Robinson buffer (16), was employed as the electrolyte. Before each experiment, the electrolyte was deaerated with high-purity Ar for 10 min. RESULTS AND DISCUSSION Establishment of Actinometric Procedure. The selective electrochemical reduction of the cis-ABD isomer can be carried out either directly after or simultaneously with the actinometric light irradiation. In the former case, the re-
lOsec
H
Time
Flgwo 2. Cathodic current-time behavior in the simultaneous cis-ABD reduction process, In which the ABD monolayer film was biased at - 0 . N vs AgIAGCI. The shadowed part corresponds to the faradaic
charge for
cis-ABD
reduction.
duction is best performed immediately after irradiation to minimize the influence of thermal cis-trans back-reaction. The thermal isomerization was investigated in a separate experiment and the rate constant at room temperature (20 O C ) found to be 1.38 X lo4 s-l (11). This indicates that a few minutes of reduction operation may not cause an appreciable error in light-intensity determination. On the other hand, in the case of simultaneous reduction, the cis-trans thermal isomerization and photoisomerization terms in eq 1 can be neglected completely owing to the fast transformation of cis-ABD molecules to the -NH-NH- species. Since the trans-cis isomerization rate depends only on the irradiation intensity and is not affected by the reduction kinetics of the cis-ABD isomer, eq 6 is still valid in this case. In Figure 2, a representative cathodic current-time curve is shown, obtained at a constant-potential bias, -0.4 V (vs Ag/AgCl), where the shadowed part corresponds to the faradaic charge for cis-ABD reduction under irradiation. T o assure a 100% reduction of cis-ABD molecules, the electrode potential should be controlled at a suitable bias condition. For this reason, the cis-AJ3D isomer was created by a constant 20-8 irradiation a t 340 nm and the potential dependence of the cis-ABD amount reduced electrochemically was investigated. It is found that an adequate potential bias may have a value within -0.4 to -0.6 V (vs Ag/AgCl). I t should be emphasized that since the non-faradaic charging current may not be involved in the constant-bias reduction process, the faradaic charge can be directly read from a commonly-used coulombmeter, leading to a nearly instantaneous determination of irradiation intensity. Hence the simultaneous reduction method is particularly recommended. T o meet the initial condition N,(t=O) = 0 required for derivation of eq 6, it is necessary to initialize the monolayer film to the trans-ABD state. This can be easily done with one cycle of reduction and reoxidation before actinometric irradiation, because the hydrazobenzene derivative produced from cis-ABD reduction is exclusively reoxidized to trans-ABD (7),the original state of the actinometer. This also implies that no special care is required for protecting the ABD monolayer film from normal laboratory lighting. One needs only to initialize the ABD monolayer filmactinometer before use. Validity Test. Equation 6 predicts a linear relationship between incident light intensity (I,) and faradaic charge (8,). In order to confirm its validity and to determine the system
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 2, JANUARY 15, 1992
3.0 2.0 1.0
0 0
2
4
6
8
1 0 1 2 1 4
Irradiation Intensity(xlO1O Einsteins sT1)
Flgure 3. Linear dependences of faradaic charge on the irradiation and 405-nm (0)mercury lines intensity at 334-nm (A), 365-nm (0). and at 340-nm light of Xenon lamp ( 0 ) .
Table I. System Constants ( K ) for Various Mercury Lines A, nm
10iK,Einsteins C-'
A, nm
103K,Einsteins C-i
334
4.39 f 0.03 4.72 f 0.02
365
5.66 f 0.01 51.36 f 0.12
340"
405
For Xenon lamp. Table 11. Comparisons of Irradiation Intensities Differently Evaluated Using Ferrioxalate a n d ABD Monolayer Film Actinometers (A = 340 nm)
irradiation intensity, 10'OEinsteinss-i entry no.
irradiation time, s
1 2 3 4
20 20 20 20
ABD
ferrioxalate
monolayer film
5.93 f 0.02
4.85 f 0.02 2.62 f 0.01
5.94 f 0.025 4.83 f 0.02 2.60 f 0.015
0.77 f 0.01
0.78 f 0.018
constant (K), the well-established ferrioxalate actinometer (15) was employed for calibration of the irradiation intensity. Figure 3 illustrates experimental results for some UV mercury lines (unfilled symbols) and for 340-nm light of a xenon lamp (filled circles), obtained from the simultaneous reduction method. The faradaic charge for cis-ABD reduction in all cases increases linearly with the increase of irradiation intensity. This indicates that the mathematical treatment performed in deriving eq 6 is reasonable for the experimental conditions employed. From the least-squares slope of the obtained linear plot, the system constant ( K ) at each irradiation wavelength was calculated; these are summarized in Table I. To demonstrate further the reliability of the proposed actinometry, a number of unknown intensity irradiations with a xenon lamp were respectively evaluated by the ABD monolayer film actinometer using the obtained K value and the ferrioxalate actinometer. In Table 11, the results are summarized and clearly exhibit the excellent coincidence between the two different evaluation methods. Range of Irradiation Intensity. For the present ABD monolayer film actinometer, the measurable light intensity is determined by the following two factors: The first is the mathematical approximation, exp(-pt) = 1- pt, in deriving the Z, - Q, relation. As mentioned previously, the error of this approximation is f 2 % when p t < 0.2 or (dtctZo + &cJ, + kth)t < 0.2. By neglect of the contribution from cis-trans thermal isomerization and photoisomerization reasonable for the recommended experimental condition, the measurable range of light intensity is estimated to be I , < 1.21 x 10-;/t (Ein-
300
425
550
Wavelength (nm) Flgure 4. Absorption spectra of ABD molecules: (a) trans monolayer film; (b) UV irradiating for 30 s;(c) trans isomer in chloroform solution.
steins cm-2 s-l). For stronger irradiation, deviation from the linear dependence of irradiation intensity on faradaic charge may occur and a precise evaluation of light intensity will need more complicated mathematics. The second is the saturation effect, which may occur in even stronger irradiation because of monolayer absorption. The maximum faradaic charge for cis-ABD reduction at the saturation state was found to be ca. 25 WCcm-2. This is equivalent to 1.18 X 10-'/t (Einsteins cm-* s-l). Obviously, both factors result in similar limiting values in irradiation intensity. Regarding the time of actinometric irradiation (t),a few seconds or less are enough for an accurate integration of current to obtain the faradaic charge. Taking into account these limitations, the ABD monolayer film actinometer is suitable for evaluating light intensity Einsteins cm-2 s-l. Such a range sufficiently covers most irradiation intensities used in photochemical and photoelectrochemical studies. In addition, we note that the quantum yield for trans-cis photoisomerization may be influenced by a large change of light intensity. In the absence of two-dimensional absorptivity data, the quantum yield could not be determined separately at the present. However, from the K-dt relationship ( K = 1/(2F4,etC,)) and the excellent linearities of Qc-Zo plots, it is reasonable to predict small dependence of the quantum yield on the irradiation intensity provided that the two-dimensional absorptivity is a constant. On the other hand, according to Zimmerman et al. (12), the quantum yield for trans-cis photoconversion of azobenzene in the liquid phase only depends on the excited transition. In correspondence to r - ~ * and n l r * excitation, the quantum yields were found to be 0.12 and 0.24, respectively, which were independent of both irradiation wavelength within the same transition band and irradiation intensity over a wide range. A detailed investigation of the present system is now in progress. Wavelength Response. The applicable wavelength range is determined by the absorption of the trans-ABD monolayer film.Figure 4 shows its absorption spectrum (curve a, actually two monolayers considering the double sides of glass substrate). A few percent of cis-ABD may exist in the film because of the omission of initialization operation before spectral measurement. For the sake of comparison, the absorption spectra of the trans-ABD isomer in chloroform solution and the cis-trans mixed monolayer film produced by UV irradiation are exhibited in the same figure. The small wavelength variation of the absorption base line (not the high-frequency noise) is attributed to the incomplete subtraction of the interference wave caused by the thin SnO, layer of glass sub-
ANALYTICAL CHEMISTRY, VOL. 64, NO. 2, JANUARY 15, 1992
strate, which actually enhanced the absorption peak of the ABD monolayer film around 445 nm. The intense absorption band around 300-400 nm observed in the trans-ABD monolayer film is ascribed to the T-T* transition of the ABD molecules (7,9),which show a large blue shift as compared to that of ABD in chloroform solution, indicating that a strong coupling between the -N=N- chromophores occurs owing to the highly-ordered film structure (7,9). This transition band is decreased when photoisomerization toward cis-ABD takes place, as exhibited in Figure 4b. Since the quantification is based upon the difference in electrochemistry rather than in absorption of trans- and cis-ABD molecules, the wavelength range 300-400 nm in which the trans-ABD molecules have effectively large absorptions would be suitable for the present actinometer though a decrease in absorption (followed by an increase of system constant K ) may enhance the evaluation error of irradiation intensity. For response to a wide wavelength range, a series of similar actinometers could be constructed in principle by molecular design. Advantages. A specific chemical actinometer usually has its own advantages and disadvantages. Parker’s actinometer (15) is simple to use, whereas the calculations after irradiation require some effort. The azobenzene actinometer (1,2,4) is simple and can be read on-line, but the mathematics for evaluation are complex. In the case of the present electrochemical actinometer, two distinct advantages are available. First, the evaluation process is very simple and fast. Especially with the recommended simultaneous reduction method, the faradaic charge can be directly read from the coulombmeter, leading to a nearly instantaneous determination of irradiation intensity. Secondly, this actinometer is reusable due to the following two experimental observations (6, 7, 11): (1) the original trans-ABD state can be reproduced by a simple reduction-oxidation treatment; (2) no appreciable change in film structure is observed for several tens of such treatments on the ABD monolayer film. The disadvantage of the present actinometer arises from the use of transparent SnOz glass substrates. Obviously, it is difficult to obtain SnOz glass substrates having exactly identical physical properties. This may cause an evaluation error of irradiation intensity owing to the reflectivity difference of the SnOz glass substrates, a
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problem also arising in physical actinometers. Such deviations are estimated to be
