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Ion-pair effects on the electroreduction and electrochromic properties of ortho-chloranil in dipolar aprotic solvents
203
Chem., 216 (1987) 203-212 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
J. Electroanai.
ION-PAIR EFFECI’S ON THE ELECI’ROREDUCTION AND ELX~OCHROMIC PROPERTIES OF ortko-CHLORANIL IN DIPOLAR APROTIC SOLVENTS
A. DESBENE-MONVERNAY,
P.C. LACAZE, J.-E DUBOIS and A. CHERIGUI
Instilut de Topologie et de Dynamique des SystPmes de I’Universit6 Paris VII, associk au C.N.RS., rue Guy de la Brosse, 75@3 Paris (France)
1,
(Received 30th May 1986)
ABSTRACT The use of ortho-cbloranil (o-CA) as an electrocbromic material requires three conditions to operate properly:
(1) the reversibility of the electronic transfer of the reaction &a
z&z
o-CA’-; (2) the
formation of a visible-light absorbing charge-transfer complex between o-CA’- ani ethe counterion M+, and (3) tbe use of an insoluble salt &AI& as a film adherent to the electrode surface. The behavior of tbe redox o-CA/o-CA’- couple has been investigated in different solvents and in the presence of several alkaline and alkaline earth cations (M’+ ). The reversibility of the electron transfer and tbe formation of an adhesive salt layer on the electrode are strongly related to the counter ion M’+. The ion-pair formation depends on the solvent and particularly on its weak electron donor character. The best elcctrochromic properties of o-CA were obtained in 0.1 M NaClO, a&o&rile solutions.
INTRODUCTION
Recently, 3,4,5,6-tetrachloro-3,5-cyclohexadien-1,Zdione (ortho-&loran& O-CA) was shown to display electrochromic properties [l]. Upon reduction in particular experimental conditions, o-chloranil yields blue films, adherent to the electrode surface, which are bleached by oxidation. The electrochromic process follows the scheme (l-3) o-CA-&-CA+eo-CA--+M+so-CAo-CA’- . ..M+%o-CA-M+
(I) . ..M+
ion-pair in solution precipitate on the electrode surface
(2) (3)
In the present work we wish to study the influence of the solvent and of the supporting electrolyte on the behavior of this electrochemichromic system.
204 EXPERIMENTAL
The reagents and electrochemical measurements were described previously (1). The electrochromic effects were observed directly on a horizontal Pt disc (5 mm d&m.). RESULT8
The color changes of the Pt disk surface, occurring when submitted to cyclic potential scans at a rate of 100 mV/s, were utilized to evaluate qualitatively the electrochromic performance of the o-CA/o-CA’system. The effect of various perchlorate salts on the electrochromic properties of o-CA was studied in acetone (Table 1). No color was detected during the electrochemical reduction of o-CA when quatemary ammonium salts, NBu,ClO, or NEt &104, were used. Lithium and sodium perchlorates gave the strongest and most persistent contrast between the oxidation and reduction processes. The electrochromic performance of KClO,, which is poorly soluble, was less satisfactory. In the case of alkaline-earth ions the intensity varied from a very light blue with Mg2+, to a more pronounced color for Ca2+ and to a deep blue for Ba2+, similar to that obtained with lithium or sodium perchlorate.
TABLE 1 Electrochromic effect of o-CA (1.6 X lop2 M) in acetone during redox cycles (potential scan rate 100 mV S-1)
supporting salt (lo-’
M)
Potential limits/V vs. Ag/Ag+
Observations in reduction
Reduction
Oxidation
NBu,ClO, NEt .&IO4 LiClO,
-0.6 - 0.6 -0.3
+os
NaClO.,
-0.3
+o.s
KClO., (saturated)
-0.8
+0.8
MgKW
-0.8
+1.0
- 0.6
+0.8
-0.3
+0.8
2
wao4)z
WC104)
2
+0.5 +0.5
no coloration no coloration intense blue and homogeneous cola ration intense blue and homogeneous coloration intense blue but not homogeneous coloration very light blue coloration light blue coloration intense blue and homogeneous coloration
Duration (cycles)
1os-104
104-10s
1os-10s
102-10s
205 TABLE 2 Electrochromic effect of o-CA (10-l (potential scan rate 100 mV s-l) c
Solvent
38.0 69.0 20.7 36.1
Acetonitrile Propylene carbonate Acetone Dimethylformamide
M) in various solvents using NaC104 (10-l
Potential limits/V vs. Ag/Ag+ Reduction
Oxidation
-0.3 -0.5 - 0.45 -1.2
+ 0.4 +0.8 +0.6 +1.0
contrast
M) during redox cycles Duration (cycles)
very good lwd very wd medium
>105 104-10s 104-10s 102-10’
However, it must be stressed that for most of the salts the films formed did not adhere. Only in the case of sodium perchlorate an adherent, deep-blue film was formed which remained stable when the electrode was removed from the solution. As shown in Table 1, the electrochromic properties of o-CA decrease from Na+ to NR:, according to the order Na+ > Li+ > Ba*+ > K+ > Ca*+ > Mg*+ > NR:. The solvents were chosen according to the solution resistivity, the alkaline-salt solubility (particularly NaClO,), and the stability of the o-CA/o-CA’- system in the medium considered. Although they produced an electrochromic effect, solvents with a dielectric constant value smaller than 15 (such as THF) were not utilized, even at high-voltage operating conditions, because they gave a poor contrast and a short lifetime. Other solvents, such as nitromethane, were not used, because NaClO, is poorly soluble in them. The stability of the o-CA solutions was followed by cyclic voltammetry. We observed that o-CA solutions are not stable in 0.1 M LiClO,, nitromethane or in 0.1 M NaClO,, dimethylsulfoxide solutions. This instability produced deep modifications of the current-potential curves which occurred after periods of 10 min in CH,NO, or 2 h in DMSO. As a consequence, we limited our studies to four dipolar aprotic solvents, namely, acetonitrile (AN), propylene carbonate (PC), acetone (A) and dimethylformamide (DMF). The electrochromic performance of the o-CA/o-CAsystem improves in the order DMF +z A, PC < AN (Table 2). In order to explain this sequence, we studied the reversibility of the o-CA/o-CA’system and the stability of the o-CA-M+ ion pair. Electrochemical
reduction of o-CA
The reduction of o-CA in a dipolar aprotic solvent containing a quaternary ammonium salt (NBu,ClO, or NEt,ClO,) displays two distinct monoelectronic waves corresponding to the electrochemical steps (Fig. 1): o-CA&-CA’+eo-CA’- 2 o-CA* +e-
206
Fig. 1. Cyclic voltammogram carbonate + NEt,ClO, (10-l
of o-CA (lo-’ M).
M) recorded at 50 mV s-l
with a Pt disc in propylene
We can deduce from our cyclic voltammetric results (Table 3) that the first electronic transfer process is reversible. This conclusion is strengthened by two facts: (1) the cathodic peak potential (J?,,~) does not vary with the potential scan rate u and (2) the cathodic peak current (I,,,) is proportional to uii2. Using a rotating Pt disc electrode (rotation speed w), we found that the current intensity varies linearly with wl/2 for different potential values, which also confirms the reversibility of the o-CA/o-CAsystem. In the case of alkaline perchlorates, the reversibility is conserved with lithium and sodium cations but decreases with potassium ion, whose solubility is weak, especially in acetonitrile and acetone. For alkaline-earth perchlorates the o-CA reduction process appears to be more complex. Indeed, in the case of Mg2+ cation, the reduction waves of o-CA are irreversible in AN, PC and A, while for barium perchlorate, two waves can be distinguished in DMF, but these overlap in the other solvents to give a single two-electron wave. The degree of reversibility of this wave depends on the nature of the solvent. These results confirm some observations reported previously for other orthoquinones. Thus, it has been mentioned that in the case of l,Znaphthoquinone, the reversibility of the first electronic transfer was always more pronounced with alkaline than with alkaline-earth cations [2]. Moreover, it has been proved that for different carbonyl compounds no stable radical anion could ,be obtained with alkaline-earth ions, particularly with Mg2+ [3]. From Table 3, it can also be seen that the difference in potential between the two successive electronic transfer processes varies with the nature of the cation. As stated previously [l],’ the lifetime of the electrochromic effect is shortened by the formation of o-CA’- dianions. LiClO,, Ca(ClO,), and Ba(ClO,), give rise to two reduction waves close together, and in this case it is more difficult to prevent the formation of o-CA’- than for other salts. We have to stress that, while the nature of the cation of the supporting electrolyte is determining for the electron transfer [l], the choice of a dipolar solvent seems to play only a minor role in the electrochemical step. The formation of o-CA’- . . . M + ion pairs It is well known that the associations between organic anion-radicals and bulky cations are negligible. This is true for NBua and NEt : ions which give the same
201
TABLE 3 Voltammetric data of the reduction of o-CA (lo-’ S-t) solvent
supporting salt (10-l M) (ClOL ), M’+
Acetonitrile
NBu: NEt: Li+
Na+ K+ Mgs+ Ca2+ Ba2+ Propylene carbonate
Acetone
Dimethylformamide
First wave &SC //LA 67 61 68
(F’ ,:G
T:V - 225 - 225 -25
-70 -65 -60
-
4c/2)
(qw-
/mV -70 -60 -75
72 -75 -60 -70 65 -240 -84 -140 64.5 +looB poorly reversible waves 63.5 +150 * 107 +86 -54 - 128
NBu; NEt: Li+
28.2 -247 24.4 -247 25.0 - 103
Na+ K+ Mgs+ Gas+
21.4 -139 -64 -84 22.4 -235 -72 -100 25.9 +25 a poorly reversible waves 22.0 i-57 -60 -64
Ba2+
35.6
NBu: NEt; Li+
63 64 65
Na+ K+ Mgs+ Ca2+
62.8 -135 -68 -76 62.5 -305 -100 -180 66 0 ’ poorly reversible waves 59.3 -15 -110 -180
Ba2+
98.7
NBu:
37.8 34.0 34.9 34.2 40.2 40.0
NEt :
Li+ Na+ K+ Mg2+ Ca2+ Ba2+ B r-n-...:,-.
M) in different media (potential scan rate 50 mV
0 - 260 - 260 100
-60
-12 -72 -72
-84 -80 -100
&a)
&
1, 1.0 1.1 reduction waves too close together 0.5 0.9
-100
-80 -80 -82
-1%
- 1070 - 975 -190
-490 -790 0 +95
1.0 1.0 1.0 Reduction waves too close together 1.0 1.0 reduction waves too close together
- 1060 - 865 -255
- 495 -715 -30 -15
-
poorly reversible waves -70 -70 -72
Second wave Ep ,/mV
1.1 1.1 reduction waves too close together 1.0 1.0 reduction waves too close together 1.0
1.2 -290 -70 -75 -290 -70 -70 1.2 - 273 -65 -76 1.0 -260 -70 -75 1.0 -270 -70 -95 1.0 - 200 a waves close together and sufficiently reversible 36.7 -223 -66 -80 1.0 36.5 -257 -64 -70 1.0
-1060 -1000 - 285
-515 -815 -70 -195
- 1150 - 1070 - 555 - 720 - 820 -240 -390 - 515
208
-.&I+= b--&====S==
Fig. 2. Cyclic voltammograms of o-CA (10m3 M) recorded at 50 mV s-l with a Pt disc in propylene carbonate + supporting salt (10-l M). (a) LiClO,; (b) NaC104; (c) KClO,; (d) Mg(Cl0,) 2; (e) Ca(C104) *; (f)
fW304)2.
Er,, value for the first reduction wave of o-CA (Table 3). In the case of a.lkaline and alkaline-earth cations, a positive shift of the reduction waves of o-CA was observed (Fig. 2). Kalinowski [4] described the influence of the size of the cation on the potential shift observed in polarography. He found an empirical equation AE1,* = PP, where AE,,, represents the difference between the half-wave potential in the presence and in the absence of ion pairs, and + corresponds to the ionic potential (charge/crystallographic radius) of the cation. For reversible systems, the peak potential obtained in voltammetry is related to the half-wave potential by the expression E,, = E,,, - 0.0285/n. Thus, AE,,, can be replaced by 6Ep = Er’+ EARa, where EM’+ is the reduction potential of o-CA in the presence of a metallic
&on
M’+, id
Eph 4 is the reduction
potential measured in the presence of a
209
Fig. 3. Correlation between the potential shift of the first reduction wave of o-CA and the ionic potential I& of the cations in various solvents (UT, = 0 for quatemary ammonium salts).
quaternary ammonium cation which gives no association with o-CA’-. The correlation between 8Ep and the ionic potential cpcf,[5] is given in Fig. 3. It is worthwhile to note that the K+ and Mg*+ cations deviate from the correlation, as expected from the above-mentioned chemical characteristics of these species. The concentration effect of the complexing salt was first studied qualitatively with o-CA (lop3 M) solution containing NR,ClO, (10-l M). Increasing concentrations of LiClO, and NaClO, led to a progressive positive shift of the first reduction wave and improved the reversibility of the second o-CA reduction wave, as in the case of other quinones [6]. Using Ca(ClO,), and Ba(ClO,),, we observed more complex modifications of the o-CA voltammograms, which we attributed to successive changes in the reduction mechanism. A quantitative study was performed at constant ionic strength with Li+ and Na+ ions only. The dependence of the reduction-peak potential on ion concentration due to ion-pair formation: p M++Q’-+(M+)p...Q’-
(4)
was first discussed by Peover [6]. It may be expressed by the relation [7-91: EP =
K/2
-
0.0285 + @T/F)
log(1 + KIM+lP)
(5)
By plotting 8E,, vs. log[M+], we obtained a straight line from which we calculated the association constants K and the coordination numbers p of Li+ and Na+ (Table 4). Its slope reaches the theoretical value of 58 mV with solvents which favor strong o-CA--M+ associations. The coordination number p, which is close to unity
210 TABLE 4 Electrochemical reduction of o-CA (lo-’ MClO,) 10-l M Solvent
Cation M+
Acetonitrile Acetone Propylene carbonate Dimethylformamide d
Li+ Na+ Li+ Na+ Li+ Na+ Li+ Na+
M) in various solvents at fixed ionic strength: (NEt,ClO., Correlation coefficient a
Pb
KC
/mV 50.5 57.8 55.2 55.6 58.1 61.0
0.98 0.99 0.99 0.99 0.97 0.99
0.87 1.00 0.95 0.96 1.00 1.05
2.6~10~ 1.7x103 5.5 x 10’ 1.1x103 1.6~10~ 6.3 x 10’ 3 x10’ 2 x10’
Slope B
+
a From the plot 8Ep = f(logIOIM+ I). b Coordination number of the ion-pair formation o-CA’- + p M+ + o-CA’- . JI M+. o-CA” . ..p M+l mol-l ’ Association constant K = 1 ’ [o-CA’- ][M+ 1’ d The plot SEp = f(log,,[M+ 1) is not linear. K is calculated on the basis of eqn. (5).
for LiClO, and NaClO,, is in agreement with a l/l association. The association constants K show higher values with Li+ than with Na+ ions. When the metallic ion is kept constant, the association phenomena decrease going from acetonitrile to dimethylformamide. This result confirms the well-known fact that donor solvents shift equilibrium (6) to the right. (Q.- . ..M+)
solv. + (Q’-)
solv. + (M+) solv.
Fig. 4. Correlation between c, the parameter measuring the sensitivity of o-CA’associations, and the donor number of the solvents, D.
(6)
towards ion-pair
211 TABLE 5 Slope of the regression lines SEP = f(&)
in various solvents characterized by donor numbers D
solvent
D
P
Acetonitrile Propylene carbonate Acetone Dimethylformamide
14.1 15.1 17.0 26.6
390 310 270 50
Krygowski [lo] quantified the effect of the solvent on ion-pair formation property with the relation SE,,, = xD, where D is Gutmann’s donor number [ll]. In the case of o-CA reduction, linear correlations (SE, = xD) were obtained with Li+ and Na+ ions. Defining p as the regression coefficient of the correlation lines of Fig. 3, we obtained linear plots p = XD (Fig. 4). The p values calculated for the different solvents (Table 5) give the ability of the o-CA’- radical ions to form ion-pair associations. Therefore, the formation of o-CA--metallic cation ion pairs decreases from acetonitrile to dimethylformamide, similarly to the electrochromic properties of the o-CA/o-CAsystem. Consequently, solvent effects on the electrochromic behavior of o-CA can be explained in terms of the difference in ability to form o-CA--M+ ion pairs. Gutmann’s donor numbers are related to the order established earlier. Although we can explain the solvent classification by means of ion-pair formation, this process does not predict the correct order of the various cations. Indeed this sequence NR:
< K+ < Na+ < Li+ < Ba’+ < Ca*+ < Mg*+
is different from that given above for the electrochromic performance of o-CA. This discrepancy between both classifications (based on ion-pair formation and on electrochromic effects) suggests that the cation has a greater specific action on the electron transfer [l] as well as on the adsorption step [3] than on the ion-pair formation [2]. CONCLUSION
We conclude that the best electrochromic effects can be achieved only when good conductivity, solubility and stability conditions are established. Moreover, three requirements have to be fulfilled: (1) The existence of a reversible monoelectronic transfer. (2) The formation of ion-pairs with a charge transfer inducing a visible-light-absorbing band. (3) The adhesiveness of the colored salts to the electrode surface. Our study shows that the first condition is not satisfied with alkaline-earth perchlorates. The second process does not take place with quatemary ammonium salts. The third is best realized with Na+ . . . o-Ca’- associations. We have demon-
212
strated that the electrochromic performance of o-CA is improved by using the alkaline perchlorates, particularly NaClO,, which fulfills the three conditions mentioned above. The solvent effect plays an important role in ion-pair formation, since the electrochromic behavior of o-CA increases when the donor number decreases. REFERENCES 1 A. De&&e-Monvemay, A. Cherigui, P.C. Lacaze and J.E. Dubois, J. Electroanal Chem., 169 (1984) 157. 2 T. Fujinaga, S. Okazaki and T. Nagaoka, Bull. Chem. Sot. Jpn., 53 (1980) 2241. 3 T. Nagaoka, S. Okazaki and T. Fujinaga, J. EIectroanaI. Chem., 133 (1982) 89. 4 M.K. Kahnowski, Chem. Phys. Lett., 7 (1970) 55. 5 T.M. Krygowski, M. Lipsztajn and Z. Gahrs, J. Electroanal. Chem., 42 (1973) 261. 6 M.E. Peover and J.D. Davies, J. Electroanal. Chem., 6 (1%3) 46. 7 M.D. Ryan and D.H. Evans, J. Electroanal. Chem., 67 (1976) 333. 8 E. Lesniewska-Lada and M.K. Kahnowski, Electrochim. Acta, 28 (1983) 1415. 9 R.S. Nicholson and I. Sham, Anal. Chem., 36 (1964) 706. 10 T.M. Krygowski, J. Electroanal. Chem., 35 (1972) 436. 11 V. Gutmann, Chem. Br., 7 (1971) 102.
Chem., 216 (1987) 203-212 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
J. Electroanai.
ION-PAIR EFFECI’S ON THE ELECI’ROREDUCTION AND ELX~OCHROMIC PROPERTIES OF ortko-CHLORANIL IN DIPOLAR APROTIC SOLVENTS
A. DESBENE-MONVERNAY,
P.C. LACAZE, J.-E DUBOIS and A. CHERIGUI
Instilut de Topologie et de Dynamique des SystPmes de I’Universit6 Paris VII, associk au C.N.RS., rue Guy de la Brosse, 75@3 Paris (France)
1,
(Received 30th May 1986)
ABSTRACT The use of ortho-cbloranil (o-CA) as an electrocbromic material requires three conditions to operate properly:
(1) the reversibility of the electronic transfer of the reaction &a
z&z
o-CA’-; (2) the
formation of a visible-light absorbing charge-transfer complex between o-CA’- ani ethe counterion M+, and (3) tbe use of an insoluble salt &AI& as a film adherent to the electrode surface. The behavior of tbe redox o-CA/o-CA’- couple has been investigated in different solvents and in the presence of several alkaline and alkaline earth cations (M’+ ). The reversibility of the electron transfer and tbe formation of an adhesive salt layer on the electrode are strongly related to the counter ion M’+. The ion-pair formation depends on the solvent and particularly on its weak electron donor character. The best elcctrochromic properties of o-CA were obtained in 0.1 M NaClO, a&o&rile solutions.
INTRODUCTION
Recently, 3,4,5,6-tetrachloro-3,5-cyclohexadien-1,Zdione (ortho-&loran& O-CA) was shown to display electrochromic properties [l]. Upon reduction in particular experimental conditions, o-chloranil yields blue films, adherent to the electrode surface, which are bleached by oxidation. The electrochromic process follows the scheme (l-3) o-CA-&-CA+eo-CA--+M+so-CAo-CA’- . ..M+%o-CA-M+
(I) . ..M+
ion-pair in solution precipitate on the electrode surface
(2) (3)
In the present work we wish to study the influence of the solvent and of the supporting electrolyte on the behavior of this electrochemichromic system.
204 EXPERIMENTAL
The reagents and electrochemical measurements were described previously (1). The electrochromic effects were observed directly on a horizontal Pt disc (5 mm d&m.). RESULT8
The color changes of the Pt disk surface, occurring when submitted to cyclic potential scans at a rate of 100 mV/s, were utilized to evaluate qualitatively the electrochromic performance of the o-CA/o-CA’system. The effect of various perchlorate salts on the electrochromic properties of o-CA was studied in acetone (Table 1). No color was detected during the electrochemical reduction of o-CA when quatemary ammonium salts, NBu,ClO, or NEt &104, were used. Lithium and sodium perchlorates gave the strongest and most persistent contrast between the oxidation and reduction processes. The electrochromic performance of KClO,, which is poorly soluble, was less satisfactory. In the case of alkaline-earth ions the intensity varied from a very light blue with Mg2+, to a more pronounced color for Ca2+ and to a deep blue for Ba2+, similar to that obtained with lithium or sodium perchlorate.
TABLE 1 Electrochromic effect of o-CA (1.6 X lop2 M) in acetone during redox cycles (potential scan rate 100 mV S-1)
supporting salt (lo-’
M)
Potential limits/V vs. Ag/Ag+
Observations in reduction
Reduction
Oxidation
NBu,ClO, NEt .&IO4 LiClO,
-0.6 - 0.6 -0.3
+os
NaClO.,
-0.3
+o.s
KClO., (saturated)
-0.8
+0.8
MgKW
-0.8
+1.0
- 0.6
+0.8
-0.3
+0.8
2
wao4)z
WC104)
2
+0.5 +0.5
no coloration no coloration intense blue and homogeneous cola ration intense blue and homogeneous coloration intense blue but not homogeneous coloration very light blue coloration light blue coloration intense blue and homogeneous coloration
Duration (cycles)
1os-104
104-10s
1os-10s
102-10s
205 TABLE 2 Electrochromic effect of o-CA (10-l (potential scan rate 100 mV s-l) c
Solvent
38.0 69.0 20.7 36.1
Acetonitrile Propylene carbonate Acetone Dimethylformamide
M) in various solvents using NaC104 (10-l
Potential limits/V vs. Ag/Ag+ Reduction
Oxidation
-0.3 -0.5 - 0.45 -1.2
+ 0.4 +0.8 +0.6 +1.0
contrast
M) during redox cycles Duration (cycles)
very good lwd very wd medium
>105 104-10s 104-10s 102-10’
However, it must be stressed that for most of the salts the films formed did not adhere. Only in the case of sodium perchlorate an adherent, deep-blue film was formed which remained stable when the electrode was removed from the solution. As shown in Table 1, the electrochromic properties of o-CA decrease from Na+ to NR:, according to the order Na+ > Li+ > Ba*+ > K+ > Ca*+ > Mg*+ > NR:. The solvents were chosen according to the solution resistivity, the alkaline-salt solubility (particularly NaClO,), and the stability of the o-CA/o-CA’- system in the medium considered. Although they produced an electrochromic effect, solvents with a dielectric constant value smaller than 15 (such as THF) were not utilized, even at high-voltage operating conditions, because they gave a poor contrast and a short lifetime. Other solvents, such as nitromethane, were not used, because NaClO, is poorly soluble in them. The stability of the o-CA solutions was followed by cyclic voltammetry. We observed that o-CA solutions are not stable in 0.1 M LiClO,, nitromethane or in 0.1 M NaClO,, dimethylsulfoxide solutions. This instability produced deep modifications of the current-potential curves which occurred after periods of 10 min in CH,NO, or 2 h in DMSO. As a consequence, we limited our studies to four dipolar aprotic solvents, namely, acetonitrile (AN), propylene carbonate (PC), acetone (A) and dimethylformamide (DMF). The electrochromic performance of the o-CA/o-CAsystem improves in the order DMF +z A, PC < AN (Table 2). In order to explain this sequence, we studied the reversibility of the o-CA/o-CA’system and the stability of the o-CA-M+ ion pair. Electrochemical
reduction of o-CA
The reduction of o-CA in a dipolar aprotic solvent containing a quaternary ammonium salt (NBu,ClO, or NEt,ClO,) displays two distinct monoelectronic waves corresponding to the electrochemical steps (Fig. 1): o-CA&-CA’+eo-CA’- 2 o-CA* +e-
206
Fig. 1. Cyclic voltammogram carbonate + NEt,ClO, (10-l
of o-CA (lo-’ M).
M) recorded at 50 mV s-l
with a Pt disc in propylene
We can deduce from our cyclic voltammetric results (Table 3) that the first electronic transfer process is reversible. This conclusion is strengthened by two facts: (1) the cathodic peak potential (J?,,~) does not vary with the potential scan rate u and (2) the cathodic peak current (I,,,) is proportional to uii2. Using a rotating Pt disc electrode (rotation speed w), we found that the current intensity varies linearly with wl/2 for different potential values, which also confirms the reversibility of the o-CA/o-CAsystem. In the case of alkaline perchlorates, the reversibility is conserved with lithium and sodium cations but decreases with potassium ion, whose solubility is weak, especially in acetonitrile and acetone. For alkaline-earth perchlorates the o-CA reduction process appears to be more complex. Indeed, in the case of Mg2+ cation, the reduction waves of o-CA are irreversible in AN, PC and A, while for barium perchlorate, two waves can be distinguished in DMF, but these overlap in the other solvents to give a single two-electron wave. The degree of reversibility of this wave depends on the nature of the solvent. These results confirm some observations reported previously for other orthoquinones. Thus, it has been mentioned that in the case of l,Znaphthoquinone, the reversibility of the first electronic transfer was always more pronounced with alkaline than with alkaline-earth cations [2]. Moreover, it has been proved that for different carbonyl compounds no stable radical anion could ,be obtained with alkaline-earth ions, particularly with Mg2+ [3]. From Table 3, it can also be seen that the difference in potential between the two successive electronic transfer processes varies with the nature of the cation. As stated previously [l],’ the lifetime of the electrochromic effect is shortened by the formation of o-CA’- dianions. LiClO,, Ca(ClO,), and Ba(ClO,), give rise to two reduction waves close together, and in this case it is more difficult to prevent the formation of o-CA’- than for other salts. We have to stress that, while the nature of the cation of the supporting electrolyte is determining for the electron transfer [l], the choice of a dipolar solvent seems to play only a minor role in the electrochemical step. The formation of o-CA’- . . . M + ion pairs It is well known that the associations between organic anion-radicals and bulky cations are negligible. This is true for NBua and NEt : ions which give the same
201
TABLE 3 Voltammetric data of the reduction of o-CA (lo-’ S-t) solvent
supporting salt (10-l M) (ClOL ), M’+
Acetonitrile
NBu: NEt: Li+
Na+ K+ Mgs+ Ca2+ Ba2+ Propylene carbonate
Acetone
Dimethylformamide
First wave &SC //LA 67 61 68
(F’ ,:G
T:V - 225 - 225 -25
-70 -65 -60
-
4c/2)
(qw-
/mV -70 -60 -75
72 -75 -60 -70 65 -240 -84 -140 64.5 +looB poorly reversible waves 63.5 +150 * 107 +86 -54 - 128
NBu; NEt: Li+
28.2 -247 24.4 -247 25.0 - 103
Na+ K+ Mgs+ Gas+
21.4 -139 -64 -84 22.4 -235 -72 -100 25.9 +25 a poorly reversible waves 22.0 i-57 -60 -64
Ba2+
35.6
NBu: NEt; Li+
63 64 65
Na+ K+ Mgs+ Ca2+
62.8 -135 -68 -76 62.5 -305 -100 -180 66 0 ’ poorly reversible waves 59.3 -15 -110 -180
Ba2+
98.7
NBu:
37.8 34.0 34.9 34.2 40.2 40.0
NEt :
Li+ Na+ K+ Mg2+ Ca2+ Ba2+ B r-n-...:,-.
M) in different media (potential scan rate 50 mV
0 - 260 - 260 100
-60
-12 -72 -72
-84 -80 -100
&a)
&
1, 1.0 1.1 reduction waves too close together 0.5 0.9
-100
-80 -80 -82
-1%
- 1070 - 975 -190
-490 -790 0 +95
1.0 1.0 1.0 Reduction waves too close together 1.0 1.0 reduction waves too close together
- 1060 - 865 -255
- 495 -715 -30 -15
-
poorly reversible waves -70 -70 -72
Second wave Ep ,/mV
1.1 1.1 reduction waves too close together 1.0 1.0 reduction waves too close together 1.0
1.2 -290 -70 -75 -290 -70 -70 1.2 - 273 -65 -76 1.0 -260 -70 -75 1.0 -270 -70 -95 1.0 - 200 a waves close together and sufficiently reversible 36.7 -223 -66 -80 1.0 36.5 -257 -64 -70 1.0
-1060 -1000 - 285
-515 -815 -70 -195
- 1150 - 1070 - 555 - 720 - 820 -240 -390 - 515
208
-.&I+= b--&====S==
Fig. 2. Cyclic voltammograms of o-CA (10m3 M) recorded at 50 mV s-l with a Pt disc in propylene carbonate + supporting salt (10-l M). (a) LiClO,; (b) NaC104; (c) KClO,; (d) Mg(Cl0,) 2; (e) Ca(C104) *; (f)
fW304)2.
Er,, value for the first reduction wave of o-CA (Table 3). In the case of a.lkaline and alkaline-earth cations, a positive shift of the reduction waves of o-CA was observed (Fig. 2). Kalinowski [4] described the influence of the size of the cation on the potential shift observed in polarography. He found an empirical equation AE1,* = PP, where AE,,, represents the difference between the half-wave potential in the presence and in the absence of ion pairs, and + corresponds to the ionic potential (charge/crystallographic radius) of the cation. For reversible systems, the peak potential obtained in voltammetry is related to the half-wave potential by the expression E,, = E,,, - 0.0285/n. Thus, AE,,, can be replaced by 6Ep = Er’+ EARa, where EM’+ is the reduction potential of o-CA in the presence of a metallic
&on
M’+, id
Eph 4 is the reduction
potential measured in the presence of a
209
Fig. 3. Correlation between the potential shift of the first reduction wave of o-CA and the ionic potential I& of the cations in various solvents (UT, = 0 for quatemary ammonium salts).
quaternary ammonium cation which gives no association with o-CA’-. The correlation between 8Ep and the ionic potential cpcf,[5] is given in Fig. 3. It is worthwhile to note that the K+ and Mg*+ cations deviate from the correlation, as expected from the above-mentioned chemical characteristics of these species. The concentration effect of the complexing salt was first studied qualitatively with o-CA (lop3 M) solution containing NR,ClO, (10-l M). Increasing concentrations of LiClO, and NaClO, led to a progressive positive shift of the first reduction wave and improved the reversibility of the second o-CA reduction wave, as in the case of other quinones [6]. Using Ca(ClO,), and Ba(ClO,),, we observed more complex modifications of the o-CA voltammograms, which we attributed to successive changes in the reduction mechanism. A quantitative study was performed at constant ionic strength with Li+ and Na+ ions only. The dependence of the reduction-peak potential on ion concentration due to ion-pair formation: p M++Q’-+(M+)p...Q’-
(4)
was first discussed by Peover [6]. It may be expressed by the relation [7-91: EP =
K/2
-
0.0285 + @T/F)
log(1 + KIM+lP)
(5)
By plotting 8E,, vs. log[M+], we obtained a straight line from which we calculated the association constants K and the coordination numbers p of Li+ and Na+ (Table 4). Its slope reaches the theoretical value of 58 mV with solvents which favor strong o-CA--M+ associations. The coordination number p, which is close to unity
210 TABLE 4 Electrochemical reduction of o-CA (lo-’ MClO,) 10-l M Solvent
Cation M+
Acetonitrile Acetone Propylene carbonate Dimethylformamide d
Li+ Na+ Li+ Na+ Li+ Na+ Li+ Na+
M) in various solvents at fixed ionic strength: (NEt,ClO., Correlation coefficient a
Pb
KC
/mV 50.5 57.8 55.2 55.6 58.1 61.0
0.98 0.99 0.99 0.99 0.97 0.99
0.87 1.00 0.95 0.96 1.00 1.05
2.6~10~ 1.7x103 5.5 x 10’ 1.1x103 1.6~10~ 6.3 x 10’ 3 x10’ 2 x10’
Slope B
+
a From the plot 8Ep = f(logIOIM+ I). b Coordination number of the ion-pair formation o-CA’- + p M+ + o-CA’- . JI M+. o-CA” . ..p M+l mol-l ’ Association constant K = 1 ’ [o-CA’- ][M+ 1’ d The plot SEp = f(log,,[M+ 1) is not linear. K is calculated on the basis of eqn. (5).
for LiClO, and NaClO,, is in agreement with a l/l association. The association constants K show higher values with Li+ than with Na+ ions. When the metallic ion is kept constant, the association phenomena decrease going from acetonitrile to dimethylformamide. This result confirms the well-known fact that donor solvents shift equilibrium (6) to the right. (Q.- . ..M+)
solv. + (Q’-)
solv. + (M+) solv.
Fig. 4. Correlation between c, the parameter measuring the sensitivity of o-CA’associations, and the donor number of the solvents, D.
(6)
towards ion-pair
211 TABLE 5 Slope of the regression lines SEP = f(&)
in various solvents characterized by donor numbers D
solvent
D
P
Acetonitrile Propylene carbonate Acetone Dimethylformamide
14.1 15.1 17.0 26.6
390 310 270 50
Krygowski [lo] quantified the effect of the solvent on ion-pair formation property with the relation SE,,, = xD, where D is Gutmann’s donor number [ll]. In the case of o-CA reduction, linear correlations (SE, = xD) were obtained with Li+ and Na+ ions. Defining p as the regression coefficient of the correlation lines of Fig. 3, we obtained linear plots p = XD (Fig. 4). The p values calculated for the different solvents (Table 5) give the ability of the o-CA’- radical ions to form ion-pair associations. Therefore, the formation of o-CA--metallic cation ion pairs decreases from acetonitrile to dimethylformamide, similarly to the electrochromic properties of the o-CA/o-CAsystem. Consequently, solvent effects on the electrochromic behavior of o-CA can be explained in terms of the difference in ability to form o-CA--M+ ion pairs. Gutmann’s donor numbers are related to the order established earlier. Although we can explain the solvent classification by means of ion-pair formation, this process does not predict the correct order of the various cations. Indeed this sequence NR:
< K+ < Na+ < Li+ < Ba’+ < Ca*+ < Mg*+
is different from that given above for the electrochromic performance of o-CA. This discrepancy between both classifications (based on ion-pair formation and on electrochromic effects) suggests that the cation has a greater specific action on the electron transfer [l] as well as on the adsorption step [3] than on the ion-pair formation [2]. CONCLUSION
We conclude that the best electrochromic effects can be achieved only when good conductivity, solubility and stability conditions are established. Moreover, three requirements have to be fulfilled: (1) The existence of a reversible monoelectronic transfer. (2) The formation of ion-pairs with a charge transfer inducing a visible-light-absorbing band. (3) The adhesiveness of the colored salts to the electrode surface. Our study shows that the first condition is not satisfied with alkaline-earth perchlorates. The second process does not take place with quatemary ammonium salts. The third is best realized with Na+ . . . o-Ca’- associations. We have demon-
212
strated that the electrochromic performance of o-CA is improved by using the alkaline perchlorates, particularly NaClO,, which fulfills the three conditions mentioned above. The solvent effect plays an important role in ion-pair formation, since the electrochromic behavior of o-CA increases when the donor number decreases. REFERENCES 1 A. De&&e-Monvemay, A. Cherigui, P.C. Lacaze and J.E. Dubois, J. Electroanal Chem., 169 (1984) 157. 2 T. Fujinaga, S. Okazaki and T. Nagaoka, Bull. Chem. Sot. Jpn., 53 (1980) 2241. 3 T. Nagaoka, S. Okazaki and T. Fujinaga, J. EIectroanaI. Chem., 133 (1982) 89. 4 M.K. Kahnowski, Chem. Phys. Lett., 7 (1970) 55. 5 T.M. Krygowski, M. Lipsztajn and Z. Gahrs, J. Electroanal. Chem., 42 (1973) 261. 6 M.E. Peover and J.D. Davies, J. Electroanal. Chem., 6 (1%3) 46. 7 M.D. Ryan and D.H. Evans, J. Electroanal. Chem., 67 (1976) 333. 8 E. Lesniewska-Lada and M.K. Kahnowski, Electrochim. Acta, 28 (1983) 1415. 9 R.S. Nicholson and I. Sham, Anal. Chem., 36 (1964) 706. 10 T.M. Krygowski, J. Electroanal. Chem., 35 (1972) 436. 11 V. Gutmann, Chem. Br., 7 (1971) 102.