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Synthesis and electron transfer property of phthalocyanine derivatives
PROGRESS IN ORGANIC COATINGS
ELSEVIER
Progress
in Organic
Coatings
Original
31 (1997)
139-145
Paper
Synthesis and electron transfer property of phthalocyanine Keiichi Of Industrial
Department
Chernistv,
Sakamoto”,
of Itzdustrial
College
Received
Tee-hnolog.Y,
2 December
Eiko Nihon
Ohno
I/niversity,
1996; accepted
derivatives
12 March
I-2-1
Izurni-cho,
Nurashino.shi,
Chiba
275,
Japan
1997
Abstract Metal phthalocyanine derivatives exhibit high redox abilities. In this paper, soluble metal phthalocyanine derivatives, l:oklalt phthalocyanine tetrasulfonic acid and cobalt phthalocyanine octacarboxylic acid have been synthesized. Cyclic voltammograms and other electrochemical measurements were carried out for their cobalt phthalocyanine derivatives in order to estimate l.heir electron transfer abilities and electrochemical mechanism in an organic solvent. Electrode processes were diffusion controlled at almost one electron transfer involving some weak adsorption and having the following chemical reaction. 0 1997 Elscvicr Science S.A. Keywords: Soluble cobalt phthalocyanine Electron transfer mechanism
derivatives;
Electrochemistry;
1. Introduction Metal phthalocyanine derivatives have been used as dyes, pigments and other chromophores to date. In the last 10
years, metal phthalocyanine derivatives are attractive as functional materials [l-4]. Metal phthalocyanine derivatives are known to exhibit high electron transfer abilities. In spite of their high electron transfer abilities, metal phthalocyanines and their derivatives are utilized only in a few fields becauseof their lower solubility in common organic solvents. The functions of metal phthalocyanine derivatives are almost all based on electron have been few electrochemical
transfer reactions [S]. There studies on the electron trans-
fer properties of the solid state of metal phthalocyanine derivatives.
It is necessary
then to examine
the electron
transfer behavior of metal phthalocyanine derivatives in an organic solvent. It is felt that the application useof cobalt phthalocyanine derivatives are much more efficient than metal onesfor our previous studies[6,7]. Recently,
computer
aided
analytical
instruments
have
developed into electrochemical measurements. Organic electrochemical the properties * Corresponding
procedures are utilized to do analysis of of organic molecules and to estimate the
PII
SO300-9440(97)00029-5
Chronoamperometry;
Chronocoulometry;
function of materials. One of the apparatus,cyclic voltammetry (CV), is used to observe the relation between the current and potential of electrodes so that its milization is on the increaseto easily estimate the reversibility and the electron number on the reaction. An electrode reaction involves not only elect.ron charge transfer, but alsothe ensuingvarious processes,for instance masstransfer, the adsorption of reaction speciesand products, and the contribution Iof coupled chemical reactions. Especially, very exactly electrochemical analyses are required in the development of functional materials. In such cases,the potential step analysis has advantagesover the potential scan method, CV, therefore employment of potential step analyses, chronoamperometry and chronocoulometry, bring about qualitative measurementswith higher accuracy when compared with CV. To date, there are very few reports about detailed studies on electrochemical analysis for metal phthalocyanine derivatives using CV, chronoamperometryand chronocoulometry, in spite of the fact that important information can be obtained for the development of functional materials. The authors have synthesized cobalt phthalocyanine tetrasulfonic acid (1) and phthalocyanine octacarboxylic acid (2), and measured cyclic voltammograms
(CVs), chronoam-
perometry and chronocoulometry in order to estimatetheir electron transfer abilities and mechanism.
author
0300-9440/97/$17.00
Cyclic voltammetry;
0 1997 Elsevier
Science
S.A. All rights reserved
140
K. Sakumoto,
E. Ohno
/ Progwss
in Orgcmic
Coatings
31 (1997)
139-145 SolH
HO25
(NH@CO DBU MX 210”C/180min
(1) Scheme
2. Experimental
I.
The initial potential in each case was -1200 mV vs. Ag/ AgCl, the step width was 250 ms, and the step potential was +1600 mV vs. Ag/AgCl.
2. I. Equipment Infrared (IR) spectra were recorded on a Shimadzu IR435 spectrometer. Electron spectra were measured on a Shimadzu UV-2100, spectrometer in pyridine solution (0.025 mg cm-“). Elemental analyses were carried out using a Perkin-Elmer 2400 equipment. Melting points were measured with a Mettler FP-5 apparatus. Cyclic voltammetry of cobalt phthalocyanine derivatives were carried out with a BAS CV-SOW voltammetric analyzer at room temperature in dimethyl sulfoxide (DMSO) containing 0.10 mol drn-s solution of tetrabutylammonium perchlorate (TBAP). Cyclic voltammograms were recorded by scanning the potential at the rate of 50 mV s-‘. The working and counter electrodes were platinum (Pt) wires and the reference electrode was a silver/silver chloride (Ag/AgCl) saturated sodium chloride electrode. The area of the working electrode was 2.0 x lo-’ cm’. The chronoamperometry of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) was measured following an applied voltage pulse from -1200 to 0 mV vs. Ag/AgCl and from 0 to +1600 mV vs. Ag/AgCl, and the reversible pulse. The chronocoulometry of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) was measured under the following conditions.
2.2. Preparation
of cobalt phthalocyanine
derivatives
2.2.1. Cobalt phthalocyanine tetrasuljonic. acid (1) [8] Cobalt phthalocyanine-4,4’,4”,4”‘-tetras,ulfonic acid was synthesized from 4-sulfophthalic acid, metal halide and urea (Scheme 1). A 200-ml three-necked flask equipped with a reflux condenser, a thermometer and a motor was charged with 12.3 g (50 mmol) of 4-sulfophthalic acid, 30.0 g (0.50 mol) of urea, 30 mmol of cobalt chloride (CoClz), 1 g of 1,8-diazabicyclo[5.4,0]-undec-7-ene (DBU) as catalyst and 150 ml of 1,2,4trichlorobenzene as solvent. The reaction mixture was heated to 210°C for 180 min. The product was washed with benzene until no 1,2,4-trichlorobenzene remained and was filtered. The solid was dried in vacuum until reaching a constant weight; yield 75%. Calculated for C3?Hi6 NROI$ZLrCo: C, 42.86; H, 1.80; N. 19.45. Found: C, 42.91; H, 2.08; N, 19.67. X,,,(pyridine)/nm: 688.5. 649.0, 354.0, 213.0. v,,,lcm-‘: 3000 (~c-n), 1700 (vcc), 1450 (&-n), 1380 (usso), 1050 (6,-n), 770 (&cmn). 2.2.2. Cobalt phthalocyanine octacarboxylic acid (2) [9] Cobalt phthalocyanine-2,3,9,10,16,17,23,24-octacarboxylic acid (2) was synthesized from benzene-1.2,4,.5-tetracar-
Scheme 2.
K. Sakamoto.
E. Ohm
/Progress
in Organic
boxylic dianhydride (pyromellitic dianhydride), metal halide and urea. The synthetic route is shown in Scheme 2. A loo-ml two-necked flask equipped with a reflux condenser and a thermometer was charged with 2.50 g (11.5 mmol) of pyromellitic dianhydride, 13.0 g (0.22 mol) of urea, 23.5 mmol of CoC12, and 0.1 g of DBU. The flask was heated to 250°C until the reaction mixture was fused. The reaction product was washed with water, acetone and 6 N hydrochloric acid (HCl). After being dried, the solid obtained was hydrolyzed. Thirty grams of crude product, 30 g of potassium hydroxide (KOH) and 90 ml of water were charged into a 300 ml beaker. The beaker was heated for 480 min at 100°C. The mixture was diluted with 200 ml water and was filtered. The filtrate was acidified to pH 2 with concentrated HCl. The product precipitated as a blue solid at this point. The blue product was separated from the solution by a centrifuge. The solid was washed with water three times, and was dried; yield 30%. Calculated for CasHrzsNsHr&o: C, 70.87; H, 8.63; N, 7.51. Found: C, 70.18; H, 8.40; N, 7.43. X,,, (pyridine)/nm: 684.0. v,,,lcm ‘: 3300 (Y&, 2900 ( uCmH), 1590 (UC-C), 1150 (Z&c), 1150 @c-o), 1090 (6c-n).
3. Results and discussion The synthetic pathway and the synthetic result of cobalt phthalocyanine tetrasulfonic acid (1) is shown in Scheme 1. Metal phthalocyanine sulfonic acids are prepared by two processes, i.e. the urea synthesis method from sulfonated phthalic derivatives and the sulfonation of phthalocyanine rings. In this work, the former process was employed since the latter could not be controlled in the substituted number and position of sulfonic acid. Metal phthalocyanine tetrasulfonic acids are prepared
Coatings
31 (1997)
139-145
141
from monosulfonic phthalic derivatives. Then the metal phthalocyanine tetrasulfonic acids are water soluble. Metal phthalocyanine tetrasulfonic acids have been used in dyestuffs for natural and synthetic textile fibers. In IR, electron spectra and elemental analytical data show the position of the absorption peaks and remainder of the product is in positive agreement with the literature data [lo]. This confirmed the formation of cobalt phthalocyanine tetrasulfonic acid (1). The synthesis of cobalt phthalocyanine oct.acarboxylic acid (2) is shown in Scheme 2. In general, the synthesis of metal phthalocyanines start from 1,2-benzenedicarbonitriles or 1,2-benzenedicarboxylic anhydride. Pyrornellitic dianhydride used in this work gives monomeric to pentameric phthalocyanine rings [9]. This is because the starting material is a bifunctional reagent to bring about the polycyclotetramerization in phthalocyanine synthesis. For this reason, the product was synthesized under the reaction conditions for the monomer preparation [9]. Boston and his co-worker reported that octasubstituted phthalocyanines synthesized from pyromellitic anhydride have functional groups such as carboxylic acid, imide or amide at the peripheral points [9]. The presence of these functional groups was analyzed by IR spectroscopy. The IR spectrum of synthesized cobalt octasubstituted phthalocyanine gave a characterjstic pattern of imide groups in the region of 1600-1800 cm-‘. The synthesized cobalt octasubstituted phthalocyanine changed the functional group from imide to acid by hydrolysis with KOH. The data of IR and electron spectra of cobalt phthalocyanine octacarboxylic acid (2) are also shown in Scheme 1. The analytical data of metal phthalocyanine octacarboxylic acid (2) were in good agreement with the required values of IR, electron spectrum and elemental analysis. It is confirmed that cobalt phthalocyanine octacatboxylic acid (2) was synthesized.
(2)
I
1.0
0
I
-1.0
Potential, V vs. Ag/AgCl
I
I
1.0
0
I
-1.0
Potential, V vs. Ag/AgCI
Fig. I. Cyclic voltammograms and their first differential curves of cobalt phthalocyanine tetrasulfonic acid and cobah phthalocyanine octacarboxylic acid in DMSO with 0.1 M TBAP, scan rate: 50 mV/s. Potentials of the reversible wave are midpoint potential of anodic and cathodic peaks for each Icouple, El,>. *, irreversible peak; AE. the separation between the anodic and cathodic peaks for reversible couple. (I) Cobalt phthalocyanine tetrasulfonlc acid; (2) cobalt phthalocyanine octacarboxylic acid.
142
K. Sakamo~o,
E. Ohm
/ Progre.w
in Organic
Cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) have the strongest absorption peaks at 688 and 684 nm, respectively. The strongest absorption peak assignees are the Q band which could be attributed to the allowed P-T* transition [ 11,121. The Q band absorption of metal phthalocyanine derivatives synthesized in this work shifted by around 80 nm to a longer wavelength in comparison with unsubstituted metal phthalocyanines which appeared around 600 nm. The shift of absorption maxima depends upon the change in electron distribution in the phthalocyanine macroring by substituents. In CV, the potential is linearly varied back and forward between two potential. Therefore, if a molecular is reduced in the forward scan, it will be reoxidized on the reverse scan. The current response shows upper half, cathodic (reduction) and lower half, anodic (oxidation) peaks. Fig. 1 shows CVs and their first differential curves of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2). The reported potentials are the midpoint potential of anodic and cathodic peaks for each couple, Ellz, and the peak potential for the irreversible step which have a mark on superscript. The AE values in Fig. 1 are the anodic peak to cathodic peak separation located in the reduction (negative) potential region. The CV of cobalt phthalocyanine tetrasulfonic acid (1) showed two cathodic peaks and four anodic peaks. The peaks of cobalt phthalocyanine tetrasulfonic acid (1) are attributed to four reduction stages. The first oxidation potential of cobalt phthalocyanine tetrasulfonic acid (1) appeared at 0.67 V vs. Ag/AgCl and the first reversible reduction potential at -0.62 V vs. Ag/AgCl. The CV of cobalt phthalocyanine tetrasulfonic acid (1) was sorted into five waves which consist of two reversible reduction couple, one irreversible reduction wave and two irreversible oxidation waves. Clack [ 131, Rollmann [ 141 and Orihara [ 151 reported that cobalt phthalocyanine and cobalt phthalocyanine tetrasulfo-
Coarings
,
0’ 50
100
150
200
Scan rate /mv*s-’
250
300
139-145
nit acid possessed five redox waves. Results in this work are in agreement with previously reported data [ 13- 151. In the case of cobalt phthalocyanine octacarboxylic acid (2), three cathodic peaks and six anodic peaks appeared. The peaks were sorted into three reversible reduction couples and three irreversible oxidation waves. Orihara 1151 reported that the oxidation of the phthalocyanine ring occurs after the central metal is oxidized and reduction is followed by reduction of met,sl. The reduction and oxidation of metal phthalocyanine derivatives are due to the interaction between the phthalocyanine ring and the central metal [ 16,171. Since the change of CV was recorded in the region of reduction potential, reversible peaks were assigned to the phthalocyanine ring and irreversible peaks were attributed to the central metal. Carboxylic and sulfonic groups are electron-withdrawing groups so they are expected to reduce the electron charge in the phthalocyanine ring. The change in the redox potential is due to the kind and number of substituents. The AE value is around 100 mV of cobah phthalocyanine tetrasulfonic acid (1) and over 100 mV of cobalt phthalocyanine octacarboxylic acid (2), but the redox processes are the same for these molecules. Fig. 2 shows the change in the ratio of anodic peak current to the cathodic current of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) with a scan rate, u. This relationship between the anodic to cathodic peak current ratio of a reversible couple, i,li, and the scan rate, u, serves as a quick test for electrochemical mechanism associated with a preceding or succeeding reversible or irreversible chemical equilibrium. In the case of the ratio, i$i,, and scan rate, u, is unity, it is a simple electron transfer reaction. In the case of the i$ic decreases with u, it is assigned as a reversible electron transfer reaction followed by a reversible chemical reaction, while the i$i, increases with u, it is a chemical reaction proceeding a reversible electron transfer. The scan rate varied from 0.05 to 0.3 V s-’ in this work. 4
0
31 (1997)
0
I
’
’
’
’
’
’
50
100
150
200
250
300
Scan
’
rate /mv*s-I
Fig. 2. Change in the anodic to cathodic current ratio with scan rate for cobalt phthalocyanine tetrasulfonic acid and cobalt phthalocyanine acid. (I) Cobalt phthalocyanine tetrasulfonic acid: (2) cobalt phthalocyanine octacarboxylic acid; 0. first redox couple; 0, seconcl redox redox couple.
octacarboxylic couple: 0. third
K. Sakamoto.
E. Ohm
/Progress
in Organic
Coatings
31 (1997)
139-145
200
300
143
Reverse step Slope
0
100
1.020
?ccl
300
400
500
100
400
500
Timelmsec
Time/msec
Fig. 3. Chronoamperometry and the slope calculated from the Cottrell boxylic acid, potential step -1.2-1.6 V vs. Ag/AgCl, time interval octacarboxylic acid.
0
plot of cobalt phthalocyanine tetrasulfonic acid and cobalt phthalocy.snine octacar250 ms. (I) Cobalt phthalocyanine tetrasulfonic acid, (2) cobalt phthalocyanine
It is shown that the ratio of anodic to cathodic peak current, i$i, decreased continuously with an increasing scan rate, u, for all reversible couples of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2). The reversible reduction couples of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) are characterized as a fast reversible electron transfer followed by a reversible chemical reaction. Except for the third reduction potential of cobalt phthalocyanine octacarboxylic acid (2), the value of anodic to cathodic peak current ratio, i$i,, is extrapolated to zero of scan rate, u, which converges close to one. Extrapolated to zero of scan rate, v, the AE values approach close to 60 mV. These data suggest that the electrode processes of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) take place in almost one-electron transfer. The midpoint potential of cathodic to anodic peak, El,* is independent of scan rate, v and have constant values. Reactants move to the surface of an electrode in electrolyte for the mass transfer process. And then for the electron transfer process, the reactants receive electrons from the electrode. Electrolysis causes a flow of electron in the electron circuit. The current of electrolysis is proposed to the reduced mass and means the rate of reaction on the electrode. The mass transfer process is slower, in comparison with the electron transfer process. Since the electrochemical reaction in this system is carried out the stationary solution containing electrolyte, the mass transfer is only controlled by diffusion. Cathodic peaks, i,, are directly proportional to the square root of the scan rate, while anodic peaks, i,, are not affected with scan rate, II, for reversible and irreversible peaks of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2). Consequently, it is thought that these electrode processes are diffusion-controlled complicated electron transfer involving some weak adsorption with the oxide of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2). Chronoamperometry is the measurement of current with
time. Chronoamperometry involves the measurement of the current-time response to a potential step excitation signal. A large cathodic current flows immediately when the potential is stepped up from the initial value, after that it slowly attenuates. The reduction step exhibited the same behavior in comparison with both potential steps. The current-time curves are converted into the relation between the current and square root of time, which is called the Cottrell plot (Appendix A). Electron processes in the systems are diffusion-controlled electron transfers mentioned above. Relationships between the current and square root of time are conside:red to be a finite diffusion for cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2). Fig. 3 shows chronoamperometry and the slope calculated from Cottrell plots of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2). The current of the Cottrell plot is a measurement of the rate for electrolysis at the electrode surface. Electrolysis is controlled with a mass transfer by diffusion on the electrode so that the diffusion constant: implies the rate of electrolysis. The slope means the diffusion constant in each step, and the forward step indicates the reduction reaction and the reverse step is oxidation. The oxidation processes of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phtlnalocyanine octacarboxylic acid (2) are slower than the reduction. The oxidation of cobalt phthalocyanine tetrasulfonic acid (1) was lower than cobalt phthalocyanine octacarboxylic acid (2), while cobalt phthalocyanine octacarboxy iic acid (2) was lower reduction ability than cobalt phthalocyanine tetrasulfonic acid (1). The chronocoulometry was taken by one treatment of chronoamperometry in which the current response was integrated to give a response to the charge. The charge-time curve of the forward step for chronocoulometry is the integral of chronoamperometry (Appendix B). The extent of diffusion control increases systematically as the standard potential becomes positive fclr cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2). In the case of cobalt phthalocyanine
144
Fig. 4. Anson plots of cobalt phthalocyanine tetrasulfonic (2) cobalt phthalocyanine octacarboxylic acid.
acid and cobalt phthalocyanine
tetrasulfonic acid (l), the electron charge was reached at about 30 PC in the forward step and was decreased to 15 PC, and reverse steps were attenuated to 0 PC with 70 ms in the reduction side from -1200 to 0 mV vs. Ag/AgCl potential. In the oxidation side from the 0 to +1600 mV vs. Ag/ AgCl step, chronocoulometry had a linear forward step and a flat reverse step indicating no Faradic activity for all compounds. It is thought that reduction and oxidation take place in different pathways. Fig. 4 shows Anson plots which are converted from the charge-time curve of chronocoulometry into the relation between charge and square root of time. Chronocoulometry gives rise to a double layer charging (Qd,), and electrolysis of adsorbed species charge (Q& and solution species (Q) in the initial potential step (Appendix Cl. Only the value of Q in three term depends upon the scanning time. The intercept of the Anson plot expresses the sum of Qdl and Qads. Since double step chronocoulometry is used in this work, Qads can take away Qdl which is a value of the difference of intercepts between forward and reverse steps. When no adsorption of reactant or product, the intercept of Anson plot for both forward and reverse steps are equal (Qd,). While reactant adsorbs but product does not, the intercept of the reverse is a measure of Qd, in the presence of adsorbed reactant, and the intercept of the forward step contains both Qd, and QadFfor adsorbed reactant. The chronocoulometry of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) shows that the reactant is adsorbed but not the product. The adsorbed species charge (Q& was found from the calculation to be 7.40 and 8.92 PC for cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2), respectively. Hence, the double layer charge (Qdl) was estimated to be 0.53 & at 0 ms of the chronocoulometry for cobalt phthalocyanine tetrasulfonit acid (1) and cobalt phthalocyanine octacarboxylic acid (2). The relation between Qr/Qf and the square root of time can be estimated by the mechanism and rate of the following
octacarboxylic
acid. (1) Cobalt phthalocyanine
tetrasulfonic
acid;
chemical reaction. The value QJQf indicates the base line for the chronocoulometry of the reverse step charge Q, divided by the final value of forward step Qr. It was found that the following chemical reaction o.beyed first-order kinetics which found the calculation to be 0.20 and 0.26 s-’ for cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2). Consequently, it is considered that electron transfer mechanisms are proposed as the following. For cobalt phthalocyanine tetrasulfonic acid (l), the oxidation step is Co” - PC(SO,H),
-+ [Co’” - PC(SO,H),]
[Co”’ -PC(SOjH)J+
+
+ + e
[Co” -PC(SO:,H),]*+
+ e
The reduction steps are Co” - PC(SO,H),
+ e 5
[Co” -PC(S03H)J]-
[Co” -PC(SO.,H),]-
+ e G [Con -PC(S03H)J2-
[CO”-PC(SO~H)~]~-
+ e + [Co’-PC(SO,H)J-
For cobalt phthalocyanine dation steps are Co” - PC(COOH),
octacarboxylic
acid (2), the oxi-
-+ [Co”’ - PC(COOH)s]+
[Co”’ - PC(COOH)s]+
+
[co”1 -PC(COOH),]‘+
+ e
[Con’ - PC(COOH)s]”
-+ Co’” - PC(C00H)s]31
+ e + e
The reduction steps are Co” -PC(COOH)
8+ e 5
[Co” -PC(COOH)s][Co” - PC(COOH)s]2-
[Co” - PC(CIOOH)~] -
+ e ‘G [Con -PC(COOH)& + e 5
[Co’ - PC(COOH)s]“-
4. Conclusion The authors used electrochemical appa,ratus and obtained precise data of the electrode process of soluble cobalt phtha-
K. Sakamo~o,
E. Ohno
/ Pro,q-rss
in Organic
locyanine derivatives in an organic solvent. The results may be summarized as follows: 1. Cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) were synthesized. 2. Electrode processes were diffusion-controlled one electron transfer involving some weak absorption and having chemical reaction. 3. The adsorbed charge was found from the calculation to be 7.40 and 8.92 PC for cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2), and the double layer charges were estimated to be 0.53 PC. 4. The following chemical reactions obeyed first-order kinetics. The kinetic constants for cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) are calculated to be 0.20 and 0.26 s-‘, respectively. Appendix
A
The current-time curve for chronoamperometry expressed by the Cottrell equation (Eq. (l)), iz
nFACD’/’ Tw+/2
= Kt-(i2
is
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31 (1997)
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145
Appendix C The Anson plot is a strai.ght line with an intercept, as mentioned in Appendix B. Chronocoulometry is useful to study adsorption on an electrode surface. When adsorbed species exist on an electrode surface, it is electrolyzed immediately, whereas solmion species must diffuse the electrode in order to react. The total charge Q,,,,, is measured in a potential step experiment. 2nFACD’i’t’i’ 7$/z
Qtotal=
+
c!dl
+
Qads
(4)
Qads= nFAI’
t-3
where Qd, is the double layer charge (C), Qadsisthe adsorbed species charge (C), r is the amount adsorbed (mol/cm’). Qtotalis obtained by summing Q., Qadsand Qdi. As the expression of Q in Eq. (2), a plot of Q vs. r”’ is a straight line. The Anson plot. shouldbe linear with intercept that is equal to the secondand third terms in Eq. (4). If Q,,, is known, then the value Qadscan be calcul,ated for an electrode of the known electrode area. When double step chronocoulometry is used, the difference in the intercepts of forward and reverse stepsis Qads.
(1)
where i is the current (A), n is the number of electrons transferred per ion or molecule (mol-‘), F is Faraday’s constant (96485 C/mol), A is the electrode area (2.0 x lo-* cm’), C is the concentration (mol/cm3), D is the diffusion constant (cm/s) and t is time (s).A plot of the current i vs. square root of time, t”‘, which is called the Cottrell plot, gives a straight line. The slope means the diffusion constant in forward and reverse steps. Appendix B
References Cl1K.F.
Schoch Jr, B.R. Kundalder
101
and T.J. Meaks,
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121P.J.
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kaishi),
34 (1991)557.
[71 K. Sakamoto,
Chronocoulometry was taken by one treatment of chronoamperometry in which the current responsewas integrated to give a responseto the charge. The charge-time curve of the forward step for chronocoulometry is the integral of Eq. (1): this is called the Anson equation (Eq. (2)).
Q=
2nFACD”‘ti’2 1r1/2
= 2Kt’i2
G-9
.I. Jpn. Sot. Colour Mater., 6R (1905) 67. Weber and D.H. Busch. Inorg. Chem., 4 (1965) 469. II (1972) 1578. [91 D.R. Boston and J.C. Bai, Irwrg. Chew., [lOI L.D. Rollmann and R.T. Iwamoto. J. Am. Chern. Sot., 90 (1968)
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90 (1968)
133.
The reverse step is the following equation:
[14] L.D.
Rollmann
and R.T. Iwamoto,
J. Am.
Chem
1455.
Qr =
2nFACD’I’ {p &I?
+ (t+.)“2
- p>
(3)
A plot of the charge Q vs. squareroot of time, t”‘, is called the Anson plot. The Anson plot is a straight line with an intercept.
[ 151 Y. Orihara, Buff.
M. Nishikawa,
Ser.,
Sot.
H. Ohno,
E. Tsuchicla
and H. Matsuda,
60 (I 987) 373 1. [ 161 K. Kasuga and M. Tsutsui. Coordination Chern. R~L.. 32 (1980) 1171 A.B.P. Lever, S. Licoccis, K. Magrell and B.S. Ramanamy, Chern.
Chem.
201
Jpn.,
(1982)
237.
67. Adv.
ELSEVIER
Progress
in Organic
Coatings
Original
31 (1997)
139-145
Paper
Synthesis and electron transfer property of phthalocyanine Keiichi Of Industrial
Department
Chernistv,
Sakamoto”,
of Itzdustrial
College
Received
Tee-hnolog.Y,
2 December
Eiko Nihon
Ohno
I/niversity,
1996; accepted
derivatives
12 March
I-2-1
Izurni-cho,
Nurashino.shi,
Chiba
275,
Japan
1997
Abstract Metal phthalocyanine derivatives exhibit high redox abilities. In this paper, soluble metal phthalocyanine derivatives, l:oklalt phthalocyanine tetrasulfonic acid and cobalt phthalocyanine octacarboxylic acid have been synthesized. Cyclic voltammograms and other electrochemical measurements were carried out for their cobalt phthalocyanine derivatives in order to estimate l.heir electron transfer abilities and electrochemical mechanism in an organic solvent. Electrode processes were diffusion controlled at almost one electron transfer involving some weak adsorption and having the following chemical reaction. 0 1997 Elscvicr Science S.A. Keywords: Soluble cobalt phthalocyanine Electron transfer mechanism
derivatives;
Electrochemistry;
1. Introduction Metal phthalocyanine derivatives have been used as dyes, pigments and other chromophores to date. In the last 10
years, metal phthalocyanine derivatives are attractive as functional materials [l-4]. Metal phthalocyanine derivatives are known to exhibit high electron transfer abilities. In spite of their high electron transfer abilities, metal phthalocyanines and their derivatives are utilized only in a few fields becauseof their lower solubility in common organic solvents. The functions of metal phthalocyanine derivatives are almost all based on electron have been few electrochemical
transfer reactions [S]. There studies on the electron trans-
fer properties of the solid state of metal phthalocyanine derivatives.
It is necessary
then to examine
the electron
transfer behavior of metal phthalocyanine derivatives in an organic solvent. It is felt that the application useof cobalt phthalocyanine derivatives are much more efficient than metal onesfor our previous studies[6,7]. Recently,
computer
aided
analytical
instruments
have
developed into electrochemical measurements. Organic electrochemical the properties * Corresponding
procedures are utilized to do analysis of of organic molecules and to estimate the
PII
SO300-9440(97)00029-5
Chronoamperometry;
Chronocoulometry;
function of materials. One of the apparatus,cyclic voltammetry (CV), is used to observe the relation between the current and potential of electrodes so that its milization is on the increaseto easily estimate the reversibility and the electron number on the reaction. An electrode reaction involves not only elect.ron charge transfer, but alsothe ensuingvarious processes,for instance masstransfer, the adsorption of reaction speciesand products, and the contribution Iof coupled chemical reactions. Especially, very exactly electrochemical analyses are required in the development of functional materials. In such cases,the potential step analysis has advantagesover the potential scan method, CV, therefore employment of potential step analyses, chronoamperometry and chronocoulometry, bring about qualitative measurementswith higher accuracy when compared with CV. To date, there are very few reports about detailed studies on electrochemical analysis for metal phthalocyanine derivatives using CV, chronoamperometryand chronocoulometry, in spite of the fact that important information can be obtained for the development of functional materials. The authors have synthesized cobalt phthalocyanine tetrasulfonic acid (1) and phthalocyanine octacarboxylic acid (2), and measured cyclic voltammograms
(CVs), chronoam-
perometry and chronocoulometry in order to estimatetheir electron transfer abilities and mechanism.
author
0300-9440/97/$17.00
Cyclic voltammetry;
0 1997 Elsevier
Science
S.A. All rights reserved
140
K. Sakumoto,
E. Ohno
/ Progwss
in Orgcmic
Coatings
31 (1997)
139-145 SolH
HO25
(NH@CO DBU MX 210”C/180min
(1) Scheme
2. Experimental
I.
The initial potential in each case was -1200 mV vs. Ag/ AgCl, the step width was 250 ms, and the step potential was +1600 mV vs. Ag/AgCl.
2. I. Equipment Infrared (IR) spectra were recorded on a Shimadzu IR435 spectrometer. Electron spectra were measured on a Shimadzu UV-2100, spectrometer in pyridine solution (0.025 mg cm-“). Elemental analyses were carried out using a Perkin-Elmer 2400 equipment. Melting points were measured with a Mettler FP-5 apparatus. Cyclic voltammetry of cobalt phthalocyanine derivatives were carried out with a BAS CV-SOW voltammetric analyzer at room temperature in dimethyl sulfoxide (DMSO) containing 0.10 mol drn-s solution of tetrabutylammonium perchlorate (TBAP). Cyclic voltammograms were recorded by scanning the potential at the rate of 50 mV s-‘. The working and counter electrodes were platinum (Pt) wires and the reference electrode was a silver/silver chloride (Ag/AgCl) saturated sodium chloride electrode. The area of the working electrode was 2.0 x lo-’ cm’. The chronoamperometry of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) was measured following an applied voltage pulse from -1200 to 0 mV vs. Ag/AgCl and from 0 to +1600 mV vs. Ag/AgCl, and the reversible pulse. The chronocoulometry of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) was measured under the following conditions.
2.2. Preparation
of cobalt phthalocyanine
derivatives
2.2.1. Cobalt phthalocyanine tetrasuljonic. acid (1) [8] Cobalt phthalocyanine-4,4’,4”,4”‘-tetras,ulfonic acid was synthesized from 4-sulfophthalic acid, metal halide and urea (Scheme 1). A 200-ml three-necked flask equipped with a reflux condenser, a thermometer and a motor was charged with 12.3 g (50 mmol) of 4-sulfophthalic acid, 30.0 g (0.50 mol) of urea, 30 mmol of cobalt chloride (CoClz), 1 g of 1,8-diazabicyclo[5.4,0]-undec-7-ene (DBU) as catalyst and 150 ml of 1,2,4trichlorobenzene as solvent. The reaction mixture was heated to 210°C for 180 min. The product was washed with benzene until no 1,2,4-trichlorobenzene remained and was filtered. The solid was dried in vacuum until reaching a constant weight; yield 75%. Calculated for C3?Hi6 NROI$ZLrCo: C, 42.86; H, 1.80; N. 19.45. Found: C, 42.91; H, 2.08; N, 19.67. X,,,(pyridine)/nm: 688.5. 649.0, 354.0, 213.0. v,,,lcm-‘: 3000 (~c-n), 1700 (vcc), 1450 (&-n), 1380 (usso), 1050 (6,-n), 770 (&cmn). 2.2.2. Cobalt phthalocyanine octacarboxylic acid (2) [9] Cobalt phthalocyanine-2,3,9,10,16,17,23,24-octacarboxylic acid (2) was synthesized from benzene-1.2,4,.5-tetracar-
Scheme 2.
K. Sakamoto.
E. Ohm
/Progress
in Organic
boxylic dianhydride (pyromellitic dianhydride), metal halide and urea. The synthetic route is shown in Scheme 2. A loo-ml two-necked flask equipped with a reflux condenser and a thermometer was charged with 2.50 g (11.5 mmol) of pyromellitic dianhydride, 13.0 g (0.22 mol) of urea, 23.5 mmol of CoC12, and 0.1 g of DBU. The flask was heated to 250°C until the reaction mixture was fused. The reaction product was washed with water, acetone and 6 N hydrochloric acid (HCl). After being dried, the solid obtained was hydrolyzed. Thirty grams of crude product, 30 g of potassium hydroxide (KOH) and 90 ml of water were charged into a 300 ml beaker. The beaker was heated for 480 min at 100°C. The mixture was diluted with 200 ml water and was filtered. The filtrate was acidified to pH 2 with concentrated HCl. The product precipitated as a blue solid at this point. The blue product was separated from the solution by a centrifuge. The solid was washed with water three times, and was dried; yield 30%. Calculated for CasHrzsNsHr&o: C, 70.87; H, 8.63; N, 7.51. Found: C, 70.18; H, 8.40; N, 7.43. X,,, (pyridine)/nm: 684.0. v,,,lcm ‘: 3300 (Y&, 2900 ( uCmH), 1590 (UC-C), 1150 (Z&c), 1150 @c-o), 1090 (6c-n).
3. Results and discussion The synthetic pathway and the synthetic result of cobalt phthalocyanine tetrasulfonic acid (1) is shown in Scheme 1. Metal phthalocyanine sulfonic acids are prepared by two processes, i.e. the urea synthesis method from sulfonated phthalic derivatives and the sulfonation of phthalocyanine rings. In this work, the former process was employed since the latter could not be controlled in the substituted number and position of sulfonic acid. Metal phthalocyanine tetrasulfonic acids are prepared
Coatings
31 (1997)
139-145
141
from monosulfonic phthalic derivatives. Then the metal phthalocyanine tetrasulfonic acids are water soluble. Metal phthalocyanine tetrasulfonic acids have been used in dyestuffs for natural and synthetic textile fibers. In IR, electron spectra and elemental analytical data show the position of the absorption peaks and remainder of the product is in positive agreement with the literature data [lo]. This confirmed the formation of cobalt phthalocyanine tetrasulfonic acid (1). The synthesis of cobalt phthalocyanine oct.acarboxylic acid (2) is shown in Scheme 2. In general, the synthesis of metal phthalocyanines start from 1,2-benzenedicarbonitriles or 1,2-benzenedicarboxylic anhydride. Pyrornellitic dianhydride used in this work gives monomeric to pentameric phthalocyanine rings [9]. This is because the starting material is a bifunctional reagent to bring about the polycyclotetramerization in phthalocyanine synthesis. For this reason, the product was synthesized under the reaction conditions for the monomer preparation [9]. Boston and his co-worker reported that octasubstituted phthalocyanines synthesized from pyromellitic anhydride have functional groups such as carboxylic acid, imide or amide at the peripheral points [9]. The presence of these functional groups was analyzed by IR spectroscopy. The IR spectrum of synthesized cobalt octasubstituted phthalocyanine gave a characterjstic pattern of imide groups in the region of 1600-1800 cm-‘. The synthesized cobalt octasubstituted phthalocyanine changed the functional group from imide to acid by hydrolysis with KOH. The data of IR and electron spectra of cobalt phthalocyanine octacarboxylic acid (2) are also shown in Scheme 1. The analytical data of metal phthalocyanine octacarboxylic acid (2) were in good agreement with the required values of IR, electron spectrum and elemental analysis. It is confirmed that cobalt phthalocyanine octacatboxylic acid (2) was synthesized.
(2)
I
1.0
0
I
-1.0
Potential, V vs. Ag/AgCl
I
I
1.0
0
I
-1.0
Potential, V vs. Ag/AgCI
Fig. I. Cyclic voltammograms and their first differential curves of cobalt phthalocyanine tetrasulfonic acid and cobah phthalocyanine octacarboxylic acid in DMSO with 0.1 M TBAP, scan rate: 50 mV/s. Potentials of the reversible wave are midpoint potential of anodic and cathodic peaks for each Icouple, El,>. *, irreversible peak; AE. the separation between the anodic and cathodic peaks for reversible couple. (I) Cobalt phthalocyanine tetrasulfonlc acid; (2) cobalt phthalocyanine octacarboxylic acid.
142
K. Sakamo~o,
E. Ohm
/ Progre.w
in Organic
Cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) have the strongest absorption peaks at 688 and 684 nm, respectively. The strongest absorption peak assignees are the Q band which could be attributed to the allowed P-T* transition [ 11,121. The Q band absorption of metal phthalocyanine derivatives synthesized in this work shifted by around 80 nm to a longer wavelength in comparison with unsubstituted metal phthalocyanines which appeared around 600 nm. The shift of absorption maxima depends upon the change in electron distribution in the phthalocyanine macroring by substituents. In CV, the potential is linearly varied back and forward between two potential. Therefore, if a molecular is reduced in the forward scan, it will be reoxidized on the reverse scan. The current response shows upper half, cathodic (reduction) and lower half, anodic (oxidation) peaks. Fig. 1 shows CVs and their first differential curves of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2). The reported potentials are the midpoint potential of anodic and cathodic peaks for each couple, Ellz, and the peak potential for the irreversible step which have a mark on superscript. The AE values in Fig. 1 are the anodic peak to cathodic peak separation located in the reduction (negative) potential region. The CV of cobalt phthalocyanine tetrasulfonic acid (1) showed two cathodic peaks and four anodic peaks. The peaks of cobalt phthalocyanine tetrasulfonic acid (1) are attributed to four reduction stages. The first oxidation potential of cobalt phthalocyanine tetrasulfonic acid (1) appeared at 0.67 V vs. Ag/AgCl and the first reversible reduction potential at -0.62 V vs. Ag/AgCl. The CV of cobalt phthalocyanine tetrasulfonic acid (1) was sorted into five waves which consist of two reversible reduction couple, one irreversible reduction wave and two irreversible oxidation waves. Clack [ 131, Rollmann [ 141 and Orihara [ 151 reported that cobalt phthalocyanine and cobalt phthalocyanine tetrasulfo-
Coarings
,
0’ 50
100
150
200
Scan rate /mv*s-’
250
300
139-145
nit acid possessed five redox waves. Results in this work are in agreement with previously reported data [ 13- 151. In the case of cobalt phthalocyanine octacarboxylic acid (2), three cathodic peaks and six anodic peaks appeared. The peaks were sorted into three reversible reduction couples and three irreversible oxidation waves. Orihara 1151 reported that the oxidation of the phthalocyanine ring occurs after the central metal is oxidized and reduction is followed by reduction of met,sl. The reduction and oxidation of metal phthalocyanine derivatives are due to the interaction between the phthalocyanine ring and the central metal [ 16,171. Since the change of CV was recorded in the region of reduction potential, reversible peaks were assigned to the phthalocyanine ring and irreversible peaks were attributed to the central metal. Carboxylic and sulfonic groups are electron-withdrawing groups so they are expected to reduce the electron charge in the phthalocyanine ring. The change in the redox potential is due to the kind and number of substituents. The AE value is around 100 mV of cobah phthalocyanine tetrasulfonic acid (1) and over 100 mV of cobalt phthalocyanine octacarboxylic acid (2), but the redox processes are the same for these molecules. Fig. 2 shows the change in the ratio of anodic peak current to the cathodic current of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) with a scan rate, u. This relationship between the anodic to cathodic peak current ratio of a reversible couple, i,li, and the scan rate, u, serves as a quick test for electrochemical mechanism associated with a preceding or succeeding reversible or irreversible chemical equilibrium. In the case of the ratio, i$i,, and scan rate, u, is unity, it is a simple electron transfer reaction. In the case of the i$ic decreases with u, it is assigned as a reversible electron transfer reaction followed by a reversible chemical reaction, while the i$i, increases with u, it is a chemical reaction proceeding a reversible electron transfer. The scan rate varied from 0.05 to 0.3 V s-’ in this work. 4
0
31 (1997)
0
I
’
’
’
’
’
’
50
100
150
200
250
300
Scan
’
rate /mv*s-I
Fig. 2. Change in the anodic to cathodic current ratio with scan rate for cobalt phthalocyanine tetrasulfonic acid and cobalt phthalocyanine acid. (I) Cobalt phthalocyanine tetrasulfonic acid: (2) cobalt phthalocyanine octacarboxylic acid; 0. first redox couple; 0, seconcl redox redox couple.
octacarboxylic couple: 0. third
K. Sakamoto.
E. Ohm
/Progress
in Organic
Coatings
31 (1997)
139-145
200
300
143
Reverse step Slope
0
100
1.020
?ccl
300
400
500
100
400
500
Timelmsec
Time/msec
Fig. 3. Chronoamperometry and the slope calculated from the Cottrell boxylic acid, potential step -1.2-1.6 V vs. Ag/AgCl, time interval octacarboxylic acid.
0
plot of cobalt phthalocyanine tetrasulfonic acid and cobalt phthalocy.snine octacar250 ms. (I) Cobalt phthalocyanine tetrasulfonic acid, (2) cobalt phthalocyanine
It is shown that the ratio of anodic to cathodic peak current, i$i, decreased continuously with an increasing scan rate, u, for all reversible couples of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2). The reversible reduction couples of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) are characterized as a fast reversible electron transfer followed by a reversible chemical reaction. Except for the third reduction potential of cobalt phthalocyanine octacarboxylic acid (2), the value of anodic to cathodic peak current ratio, i$i,, is extrapolated to zero of scan rate, u, which converges close to one. Extrapolated to zero of scan rate, v, the AE values approach close to 60 mV. These data suggest that the electrode processes of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) take place in almost one-electron transfer. The midpoint potential of cathodic to anodic peak, El,* is independent of scan rate, v and have constant values. Reactants move to the surface of an electrode in electrolyte for the mass transfer process. And then for the electron transfer process, the reactants receive electrons from the electrode. Electrolysis causes a flow of electron in the electron circuit. The current of electrolysis is proposed to the reduced mass and means the rate of reaction on the electrode. The mass transfer process is slower, in comparison with the electron transfer process. Since the electrochemical reaction in this system is carried out the stationary solution containing electrolyte, the mass transfer is only controlled by diffusion. Cathodic peaks, i,, are directly proportional to the square root of the scan rate, while anodic peaks, i,, are not affected with scan rate, II, for reversible and irreversible peaks of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2). Consequently, it is thought that these electrode processes are diffusion-controlled complicated electron transfer involving some weak adsorption with the oxide of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2). Chronoamperometry is the measurement of current with
time. Chronoamperometry involves the measurement of the current-time response to a potential step excitation signal. A large cathodic current flows immediately when the potential is stepped up from the initial value, after that it slowly attenuates. The reduction step exhibited the same behavior in comparison with both potential steps. The current-time curves are converted into the relation between the current and square root of time, which is called the Cottrell plot (Appendix A). Electron processes in the systems are diffusion-controlled electron transfers mentioned above. Relationships between the current and square root of time are conside:red to be a finite diffusion for cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2). Fig. 3 shows chronoamperometry and the slope calculated from Cottrell plots of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2). The current of the Cottrell plot is a measurement of the rate for electrolysis at the electrode surface. Electrolysis is controlled with a mass transfer by diffusion on the electrode so that the diffusion constant: implies the rate of electrolysis. The slope means the diffusion constant in each step, and the forward step indicates the reduction reaction and the reverse step is oxidation. The oxidation processes of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phtlnalocyanine octacarboxylic acid (2) are slower than the reduction. The oxidation of cobalt phthalocyanine tetrasulfonic acid (1) was lower than cobalt phthalocyanine octacarboxylic acid (2), while cobalt phthalocyanine octacarboxy iic acid (2) was lower reduction ability than cobalt phthalocyanine tetrasulfonic acid (1). The chronocoulometry was taken by one treatment of chronoamperometry in which the current response was integrated to give a response to the charge. The charge-time curve of the forward step for chronocoulometry is the integral of chronoamperometry (Appendix B). The extent of diffusion control increases systematically as the standard potential becomes positive fclr cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2). In the case of cobalt phthalocyanine
144
Fig. 4. Anson plots of cobalt phthalocyanine tetrasulfonic (2) cobalt phthalocyanine octacarboxylic acid.
acid and cobalt phthalocyanine
tetrasulfonic acid (l), the electron charge was reached at about 30 PC in the forward step and was decreased to 15 PC, and reverse steps were attenuated to 0 PC with 70 ms in the reduction side from -1200 to 0 mV vs. Ag/AgCl potential. In the oxidation side from the 0 to +1600 mV vs. Ag/ AgCl step, chronocoulometry had a linear forward step and a flat reverse step indicating no Faradic activity for all compounds. It is thought that reduction and oxidation take place in different pathways. Fig. 4 shows Anson plots which are converted from the charge-time curve of chronocoulometry into the relation between charge and square root of time. Chronocoulometry gives rise to a double layer charging (Qd,), and electrolysis of adsorbed species charge (Q& and solution species (Q) in the initial potential step (Appendix Cl. Only the value of Q in three term depends upon the scanning time. The intercept of the Anson plot expresses the sum of Qdl and Qads. Since double step chronocoulometry is used in this work, Qads can take away Qdl which is a value of the difference of intercepts between forward and reverse steps. When no adsorption of reactant or product, the intercept of Anson plot for both forward and reverse steps are equal (Qd,). While reactant adsorbs but product does not, the intercept of the reverse is a measure of Qd, in the presence of adsorbed reactant, and the intercept of the forward step contains both Qd, and QadFfor adsorbed reactant. The chronocoulometry of cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) shows that the reactant is adsorbed but not the product. The adsorbed species charge (Q& was found from the calculation to be 7.40 and 8.92 PC for cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2), respectively. Hence, the double layer charge (Qdl) was estimated to be 0.53 & at 0 ms of the chronocoulometry for cobalt phthalocyanine tetrasulfonit acid (1) and cobalt phthalocyanine octacarboxylic acid (2). The relation between Qr/Qf and the square root of time can be estimated by the mechanism and rate of the following
octacarboxylic
acid. (1) Cobalt phthalocyanine
tetrasulfonic
acid;
chemical reaction. The value QJQf indicates the base line for the chronocoulometry of the reverse step charge Q, divided by the final value of forward step Qr. It was found that the following chemical reaction o.beyed first-order kinetics which found the calculation to be 0.20 and 0.26 s-’ for cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2). Consequently, it is considered that electron transfer mechanisms are proposed as the following. For cobalt phthalocyanine tetrasulfonic acid (l), the oxidation step is Co” - PC(SO,H),
-+ [Co’” - PC(SO,H),]
[Co”’ -PC(SOjH)J+
+
+ + e
[Co” -PC(SO:,H),]*+
+ e
The reduction steps are Co” - PC(SO,H),
+ e 5
[Co” -PC(S03H)J]-
[Co” -PC(SO.,H),]-
+ e G [Con -PC(S03H)J2-
[CO”-PC(SO~H)~]~-
+ e + [Co’-PC(SO,H)J-
For cobalt phthalocyanine dation steps are Co” - PC(COOH),
octacarboxylic
acid (2), the oxi-
-+ [Co”’ - PC(COOH)s]+
[Co”’ - PC(COOH)s]+
+
[co”1 -PC(COOH),]‘+
+ e
[Con’ - PC(COOH)s]”
-+ Co’” - PC(C00H)s]31
+ e + e
The reduction steps are Co” -PC(COOH)
8+ e 5
[Co” -PC(COOH)s][Co” - PC(COOH)s]2-
[Co” - PC(CIOOH)~] -
+ e ‘G [Con -PC(COOH)& + e 5
[Co’ - PC(COOH)s]“-
4. Conclusion The authors used electrochemical appa,ratus and obtained precise data of the electrode process of soluble cobalt phtha-
K. Sakamo~o,
E. Ohno
/ Pro,q-rss
in Organic
locyanine derivatives in an organic solvent. The results may be summarized as follows: 1. Cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) were synthesized. 2. Electrode processes were diffusion-controlled one electron transfer involving some weak absorption and having chemical reaction. 3. The adsorbed charge was found from the calculation to be 7.40 and 8.92 PC for cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2), and the double layer charges were estimated to be 0.53 PC. 4. The following chemical reactions obeyed first-order kinetics. The kinetic constants for cobalt phthalocyanine tetrasulfonic acid (1) and cobalt phthalocyanine octacarboxylic acid (2) are calculated to be 0.20 and 0.26 s-‘, respectively. Appendix
A
The current-time curve for chronoamperometry expressed by the Cottrell equation (Eq. (l)), iz
nFACD’/’ Tw+/2
= Kt-(i2
is
Coatings
31 (1997)
139-145
145
Appendix C The Anson plot is a strai.ght line with an intercept, as mentioned in Appendix B. Chronocoulometry is useful to study adsorption on an electrode surface. When adsorbed species exist on an electrode surface, it is electrolyzed immediately, whereas solmion species must diffuse the electrode in order to react. The total charge Q,,,,, is measured in a potential step experiment. 2nFACD’i’t’i’ 7$/z
Qtotal=
+
c!dl
+
Qads
(4)
Qads= nFAI’
t-3
where Qd, is the double layer charge (C), Qadsisthe adsorbed species charge (C), r is the amount adsorbed (mol/cm’). Qtotalis obtained by summing Q., Qadsand Qdi. As the expression of Q in Eq. (2), a plot of Q vs. r”’ is a straight line. The Anson plot. shouldbe linear with intercept that is equal to the secondand third terms in Eq. (4). If Q,,, is known, then the value Qadscan be calcul,ated for an electrode of the known electrode area. When double step chronocoulometry is used, the difference in the intercepts of forward and reverse stepsis Qads.
(1)
where i is the current (A), n is the number of electrons transferred per ion or molecule (mol-‘), F is Faraday’s constant (96485 C/mol), A is the electrode area (2.0 x lo-* cm’), C is the concentration (mol/cm3), D is the diffusion constant (cm/s) and t is time (s).A plot of the current i vs. square root of time, t”‘, which is called the Cottrell plot, gives a straight line. The slope means the diffusion constant in forward and reverse steps. Appendix B
References Cl1K.F.
Schoch Jr, B.R. Kundalder
101
and T.J. Meaks,
.I’. Am. Chem.
Sot..
(1979)7071.
121P.J.
Regensburger. Photochem. Photo&o/., 8 (1968) ,429. and S. Nitta, !Lwsnr and Actuutor.F B. 9 (1992) 85. and F. Shibamiya, J. Jpn. Sot. Colour Mater., 64 [41 K. Sakamoto (1991) 291. and A.B.P. Laver, Phthalocyunine Properties and 151 CC. Lezonoff Applications, VHC, New York, 1993. 161K. Sakamoto and F. Shibamiya, J. Jpn. Petrol. frost , (Sekiyu Gak[31 S. Kanefusa
kaishi),
34 (1991)557.
[71 K. Sakamoto,
Chronocoulometry was taken by one treatment of chronoamperometry in which the current responsewas integrated to give a responseto the charge. The charge-time curve of the forward step for chronocoulometry is the integral of Eq. (1): this is called the Anson equation (Eq. (2)).
Q=
2nFACD”‘ti’2 1r1/2
= 2Kt’i2
G-9
.I. Jpn. Sot. Colour Mater., 6R (1905) 67. Weber and D.H. Busch. Inorg. Chem., 4 (1965) 469. II (1972) 1578. [91 D.R. Boston and J.C. Bai, Irwrg. Chew., [lOI L.D. Rollmann and R.T. Iwamoto. J. Am. Chern. Sot., 90 (1968)
181J.H.
1455.
1111N.E.L. Can.
[I21 C. [I31
Khatib,
B. Boudjema,
J. Chem.,
Fierro,
(1988)
6902.
D.W.
Clack,
66 (1988)
A.B.
M. Maitrot,
H. Chermette
and L. Pote.
2313.
Anderson
and D.A.
Scherson,
N.S. Hush and I.S. Wolsey,
J’. Phys.
Inorg,
Chem..
92
C/tern.,
19 (1976)
Sot.,
90 (1968)
133.
The reverse step is the following equation:
[14] L.D.
Rollmann
and R.T. Iwamoto,
J. Am.
Chem
1455.
Qr =
2nFACD’I’ {p &I?
+ (t+.)“2
- p>
(3)
A plot of the charge Q vs. squareroot of time, t”‘, is called the Anson plot. The Anson plot is a straight line with an intercept.
[ 151 Y. Orihara, Buff.
M. Nishikawa,
Ser.,
Sot.
H. Ohno,
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