Electrochemical Behavior of Cis and Trans Azobenzenes C
1825
Further work is in progress to synthesize and investigate the properties of some tetramesitylbenzoquinone and other quinones where proton migration cannot take place.
C hv
---L
References and Notes
1 C
cyclopentenedihydrophenanthrene.22 However, the uv irrad i a t i ~ nof~diphenylmaleinimide ~ and a,a'-dicyanostilbene resulted in the formation of 9,10-dicarboximido-9,lO-dihydrophenanthrene and trans-9,10-dicyano-9,lO-dihydrophenanthrene, respectively. On the basis of these observations and also on the basis of our uv and ir analysis of the product obtained upon irradiation of the t3,5-dinnesityl-3,6-diphenylbenzoquinone, one may conclude that proton migration takes place in this compound. The resulting product, a dihydroxy hydrocarbon, has no reducible group and hence is inert toward the dropping mercury electrode. Similar experiments on 5,8-dimethyl-6,7-diphenyl-1,4naphthoquinone resulted also in proton migration.
P. Zuman, "Substituents Effects in Organic Polarography," Plenum Press, New York, N. Y., 1967. F. B. Mallory, C. S. Wood, J. T. Gordon, L. 6.Lindquisl, and M. L. Savitz, J. Amer. Chem. SOC.,84, 4361 (1962). R. B. Woodward and R. Hoffmann, J. Amer. Chem. Soc., 87, 395 (1965). G. Lober, Ber. Bunsenges. Phys. Chem., 70,524 (1966). M. A. Ogliaruso, M. G. Romanelli, and E. I. Becker, Chem. Rev., 65, 261 (1965). D. E. Kvalnes, J. Amer. Chem. SOC.,56, 2478 (1934). J. B. Conant and L. F. Fieser, J. Amer. Chem. SOC.,46, 1858 (1924). J. B. Conant, J. Amer. Chem. Soc., 49, 293 (1927). L. F. Fieser, J. Amer. Chem. Soc., 51, 3101 (1929). 0. H. Muller and J. P. Baumberger, Trans. Nectrochem. Soc., 71, 181 (1937). N. H. Furman and K. G. Stone, J. Amer. Chem. Soc., 70, 3055 (1948). M. G. Evans and J. De Heer, Quart. Rev., Chem. Soc., 4,94 (1950). A. Streitwieser, "Molecular Orbital Theory for Organic Chemists," Wiley, New York, N. Y., 1961. R. L. Edsberg, D. Eichlin, and J. J. Garis, Anal. Chem,, 25, 798 (1953). S. Wawzonek, R. Berkey, E. W. Blaha, and M. E. Runner, J. Electrochem. SOC.,103, 456 (1956). S. Wawzonek, Anal. Chem., 30,661 (1958). I. M. Kolthoff and T. B. Reddy, J. Nectrochem. Soc., 1 M. E. Peover, J. Chem. Soc., 4540 (1962). E. Mueller and W. Dilger, Chem. Ber., 106, 1643 (1973). G. J. Hoijtink, J. Van Schooten, E. de Boer, and W. V. Aalbersberg, Recl. Trav. Chim. Pays-Bas, 73,355 (1954). C. Leibovici, Tetrahedron Lett.,4073 (1967). A. A. Lamola, G. S. Hammond, and F. B. Mailory, fho~ocbem,Photobiol, 4, 259 (1965). M. V. Sargent and C. J. Timmons, J. Amer. Chem. SOC., 85, 2186 ( 1963).
Electrochemical Behavior of Cis and Trans Azobenzenes 6. Klapman" and N. Doddapaneni Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44 106 (Received September 4, 1973; Revised Manuscript Received May 2, 1974)
The polarographic behavior of a series of substituted trans and cis azobenzenes was investigated in both protic and aprotic solvents. The influence of the substituents on the relative stability of the cis derivative and on the shift in half-wave potential between the trans and the cis derivatives are discussed and found to relate with Hamrnett substituent indices.
Since the discovery of the effect of light on electrochemical systems by Beequerell in 1839,much work has been devoted to attempts to find a practical way of converting solar energy into electrical current and storing it in a form suitable for later utilization. One of the possible routes for achieving such a goal consists in taking advantage of changes that occur in the chemical properties of some specific organic molecules under irradiation. This line of thought had stimulated several recent attnmpts to investigate redox properties of excited state^.^,^ So far, however, no satisfactory results were reported, whether because of the instability and low con-
centration of excited molecules in solution* or because either dark or photochemical reactions were superimposed. An alternative way of attacking this problem consists in the study of the redox potential of unsaturated photoisorneric compounds. Unlike excited molecules, the photoisomeric molecules are stable and their behavior in both forms is familiar to organic and physical chemists. For our purpose, the properties required from these molecules is, in addition to being able to exist in two stable forms (photoisomers), that they be reducible in a reversible manner and in both forms electrochemically. In the preceeding paper,5 we reported on the electrochemical behavior of The Journal of Physical Chemistry. Vol. 78. No. 18. 1974
G. Klopman and N. Doddapaneni
1826
some quinoidic der.ivatives of stilbenes. In these cases, it was found that an irreversible proton migration takes place in the photoisomers, yielding a dihydroxy hydrocarbon.
1
-04
Azobenzenes may be considered to have electronic structures that are similar to those of stilbenes since the azo and vinyl groups are isoelectronic. One may therefore anticipate that similar structural changes occur upon ultraviolet irradiation of azobenzenes. In 193'7 Hartley6 reported the formation of a metastable cis-azobenzene by iriradiation of a solution of stable transazobenzene in petroleum ether and was subsequently able to separate it by cf~romatography.~ Thus, in contrast to stilbenes, trans azobenzenes upon irradiation only reach the cis structure and do not easily cyclize.s However, since the cis structure is sufficiently stable to be isolated, it can be studied polarographically. Because the azobenzenes, upon uv irradiation, are not susceptible i;o nydrogen migration, and that the azo group itself is reducible, they are obvious choices for our examination. We report in this paper the effect of substitutents on the polarographic behavior of azobenzenes and on their photoisomerjc properties. A linear relationship was found between the half-wave potentials and Hammett substituent constantsg for both the cis and trans isomers. Experimental Section Materials. The azobenzenes were made according to the methods reported in the literature. All were recrystallized at least twice from alcohol. Tetraethylammonium perchlorate (TEAP, Eastman Grade) was recrystallized twice from water and dried at 100° under vacuum for at least 24 hr. Eastman grade tetraethylammonium chloride (TEAC) was used without further purification. Solutions. All aqueous solutions contain 5 X lo-* M of the azo compound in 70% dioxane, 0.1 M of tetraethylammonium chloride (TEAC) as supporting electrolyte and (9.005%gelatin. AH nonaqueous solutions contain M of the depolarizer in anhydrous dimethylformamide and 0.1 A4 of tetraethylammonium perchlorate (TEAP). Polarograms. The current-voltage curves were recorded with a PAR Model 170 electrochemistry system. A vertical dropping mercury electrode served as cathode and a platinum wire electrode served as working electrode. A platinum plate decitrode was used for cyclic voltammetric studies. For the study of the photochemical behavior of trans azobenzenes in anhydrous DMF, the platinum electrode consisted of a thin layer of platinum coated on a quartz plate and was obtained from Harrick scientific corporation. AH other experimental conditions remained similar to those described pre~iously,~ Results and I)is@ussisn Polarographic Stud-y in Aqueous Dioxane. The halfwave potentials of reduction of the trans azobenzenes were measured in 7O9/Cl aqueous dioxane solutions. After recordThe Journalof Physicel Chernistry, Vol. 78, No. 18, 1974
-06
-08
-10
-12
E \'*>
'4
-16
-'E
B
Figure 1. Polarograms of pmethoxyazobenzene in 70% aqueous dioxane: (a) before irradiation; (b) after irradiation with ultraviolet light for 40 min.
ing the single reduction wave potentials, the solutions were irradiated for 40 min with a General Electric mercury lamp No. 400 RSP 33-1. The irradiated solutions were then submitted again to polarographic analysis. The new polarograms consisted in each case of two waves, one at the same potential as that of the pure trans azo compound and the other at a more positive potential. The potential a t which the new wave appears was found to be the same as that of a pure sample of the cis isomer obtained independently. Depending on the duration of irradiation, the relative heights of the two waves vary but their sum remains constant and equal to that of the original single wave. The results are illustrated in Figure 1 by the polarograms of p methoxyazobenzene with and without irradiation. From these and similar polarograms for our other azo compounds, we evaluated the steady-state concentration of the cis isomer in the irradiated solutions to be approximately 30-50%. The largest percentage is found for the compounds substituted by electron-withdrawing groups para to the azo group and the lowest for those substituted by electron-donating group. The half-wave potentials from Table 1 show that the metastable cis azobenzenes undergo reduction more easily than the corresponding trans isomers. This is probably due to the reduced conjugation in the cis azobenzenes produced by steric interaction. Moreover, the reduction of the cis isomers are irreversible in contrast to those of the trans. The reason is that the reduction of both isomers results in the formation of the same hydroazobenzene which is oxidized at the same potentials as that where the reversible reduction of the trans-azobenzene took place. The following diagram illustrates what is believed to be the global mechanism of the process. trans-ezobenzene
cis.azobenzene
ae- c 2 ~ '
hydroazobenzene
Electrochemical Behavior of Cis and ‘Trans Azobenzenes
1821
TABLE I: Half-Wave Potentials of Stable Trans and Metastable Cis Azobenzenes E ~ / zV, us.
X--CBH~-N=N-C~H~-Y X
NO.
Y
1 2 3
4 5 6 7 8 9 10 11 I2 13
14 15 a Taken
8ce
Resultant substituent
Trans Eijz
Metastable E m
constantsQ2hp-
-0.986 -1.031 -1.050 -1.046 -1.05 -1.160 -1.126 - 1.085 -0.932 -0.929 -0.916 -0.944 -0.925 -1 .060 -1.145 -1.130
-0.800 -0.832 -0.853 -0.850
0 .OO -0.17 -0.27 -0.25 -0.37 -0.66
-0,881 -0.748 -0.748 -0.739 -0.767 -0.743 -0.906 -1.005 -0.986
-0.42b + O .23 +0.23 +0 .27
+Q . I 5 +0.22 -0.34
-0.54 -0.50
from thrr review by H. H. Jaffe, Chem. Reu., 63, 191 (1953). bDerivedfrom the Figure 2 of this work.
I
08
-00
‘>4
-32
oc
I 4 2
I
104
Figure 2. Half-wave potentials, Eln (V), for the reduction wave of substituted azobenzenes in 70 % aqueous dioxane solutions vs. Hammett substituent constants crp-x: (A) trans azobenzenes; (6)cis azobenzenes. ‘The numbers refer to the azobenzenes listed in Table I.
The half-wave potentials of both the cis and the trans azobenzenes (Table I) correlated well with Hammett substituent constants except for some of the disubstituted compounds (no. L3, 14, and 15 in Table I) where the additivity of Hanimett u’s might not be valid. The observed linear relationships, irhown in Figure 2, have approximately the same slope: ptrans = 0.25; pels = 0.21. The polarographic study of o-hydroxyazobenzenes1° reveals the existence of i3 tautomerism between an o-quinoidic structure itnd an azo structure. Thus the polarographic reduction of ,@-NaphtholOrange ( X = SOS-) produces two waves repre5,entb.g each a two-electron reduction. The height of the first wave increases when the polarographic solution i s irradiated with ultraviolet light and reverts to its original height in the dark. The total height of both curves remains constant. Cyclic voltammetry studies show that the first wave (€3112 = -0.90 V ) is highly reversible and the second one (El/? = -1.05 V) is irreversible. One may therefore conclude that the reversible wave is due to the reduction of the quinonic group and that the second wave is due to the reduction of the azo group. A similar behavior was observeal for benzene-@-naphthol [(X = H) €3112 = -0.956 and -1.21 V]
IT
If a methyl substituent is placed para with respect to the hydroxy group in o-hydroxyazobenzene, the polarographic wave consists only of one wave due to the reduction of the azo group (E112 = -0.98 V). No wave due to the quinoidic group reduction was detected even after exposing the solution for 3 hr to ultraviolet light. This might be due to the destabilization of the quinoid form by the electron-donating methyl group which enhances the electron density on the oxygen atom.
Half- Wave Potentials in Anhydrous DMF. In aprotic solutions the azobenzenes are reduced reversibly in two oneelectron steps according to the following scheme.lOJ1
The half-wave potentials of the two reduction waves of substituted trans azobenzenes in anhydrous dimethylformamide containing 0.1 M tetraethylammonium perchlorate (TEAP) as supporting electrolyte are summarized in Table 11. They both give a good Hammett correlations1°J2 as shown in Figure 3. The u value used for the dimethylamino substituent was derived from Figure 2 and found equal to -0.42. A linear relation (Figure 4) was also found between the first one-electron reduction half-wave potentials of the various azobenzenes in dimethylformamide and their twoelectron reduction half-wave potentials in aqueous dioxane. This shows that the steric and mesomeric effects of substitThe Journal of Physical Chemistry. Vol. 78, No. 18. 1974
G. Klopman and N. Doddapaneni
-1:
--38
-36
-02,
-04
00
*02
-12
-7 3
-1-4
-15
-1 6
I
47
*0,4
P-x
Figure 3. Relation between the measured h a h a v e Potentials, f i n 9 of the reduction wave of azobenzenes in DMF and the Hammett sub(A) first reduction wave: (B)second reduction stituent constant q,--x: wave. The numbers refer to the compounds listed in Table 11.
TABLE I 1 HTalf-Wave Potentials of Stable Trans Azobenzenes IL--CBH~---N=N-C~H~--Y
E m , V, us.
see
x
First wave
Second wave
Ea 6
w
7 8 9
€I H H H CHI QCHB OC2Hs
-1.340 -1.410 - 1.466 - 1.430 -1.315 -1.586 - 1.500 -1.235 -1.230 -1.198 - 1.415 -1,470 -1.452
-1.880 -1.928 -2.016 - 1.968 -1.890 -2.100 - 2.042 - 1.765 -1.766 - 1.766 - 1.950
No.
x
1
H
2 3 4
34 H
10 11 12
13
H
H
-2.00 - 1.990
uents in these complounds in both protic and aprotic solvents are minimal. We have then prepared the cis azobenzenes by ultraviolet irradiation of petroleum ether solutions of the trans isomers and characterized them by melting points and uv analysis. Their reduction was studied polarographically in anhydrous dimethylformamide. The polarograms were found in each case to be identical with those of the corresponding trm6 isomers. Furthermore, the uv irradiation of the solutions of the trans azobenzenes in DMF did not produce any change in their polarograms. Even the cyclic voltammograms obtained during irradiation through a transparent electrode failed to produce any significant change in the electrochemical behavior of the azobenzenes. These observations are evidently indicative of the short lifetime of the cis isomers under our polarographic conditions. The polarographic study of p-sulphobenzeneazo-@naphthol (El/:, -- -1.005, -1.74 V) and benzeneazo-pnaphthol (E112= -1.02, -1.82 V) in dimethylformamide reveals that both compounds exist completely in the azo
The Journal of Physical Chemistry, Vol. 78, No. 78, 1974
Figure 4. Dependence of the first one-electron reduction wave halfwave potential (Eln) of substituted azobenzenes in DMF on the twoelectron reduction half-wave potentials in 70 % aqueous dioxane solutions of the corresponding azo ,-oa)pounds. The numbers refer to the compounds listed in Table II.
form. No trace of quinoidic form was observed after irradiation with ultraviolet light for a long period of time. However, the addition of a little water to the DMF solutions of these compounds resulted in the formation of the quinoid structure as shown by the appearance of an additional polarographic wave. Similar results were found for the aminoquinones. Hence it is concluded that the quinoidic structure is stabilized by protic solvents. In conclusions, we thus found that both photoisomers of azobenzenes undergo reduction a t the dropping mercury electrode. Although the photochemical process is reversible, the electrochemical process is reversible only with trans azobenzenes, Furthermore, the difference AEli2 between the half.wave potentials of the reduction of the two isomers is only in the order of 0.2 V, which is marginal for practical utilization. Further investigation of the behavior of the azobenzenes must take into consideration the important fact that the electron-donating substituents increase the value of A33112 but decrease the stability of the cis isomer, whereas the e ~ e c ~ r o n - w i t h d ~ a wsubstituents in~ decrease the value of but increase the stability of the cis isomer, making it difficult to design an optimal structure. References and Notes (1) E. Becquerel, C. R. Acad. Sci., 9, 561 (1839). (2) R. Excelby and R. Grinter, Chem. Rev., 65, 247 (1965). (3) (a) E. Rabinowitch, J. Chem. Phys., 8, 560 (1940); (b) M. Eisenberg and H-P. Silverman, Electrochem. Acta, 5, 1 (1961). (4) H. Berg and F. A. Gollmick, Colltct. Czech. Chem. Co,mmun., 30, 4192 (1965). (5) G. Klopman and N. Doddapaneni, J. Phys. Chem., 78, 1820 (1974). (6) G. S. Hartley, Nature(London),140, 281 (1937) (7) G. S. Hartley, J. Chem. SOC.,633 (1938). (8) G. M. Badger, R. J. Drewer, and 6.E. Lewis, /"lust. J. Chem., 19, 643
(1966). (9) L. P. Hammelt, "Physical Organic Chemistry," IWcGraw-Hill, New York, N. Y., 1940, p 184. (IO) (a) S. Millefiori, Ric. Sci., 39,616 (1969); (b) S. Millsfioriand G.Favini, 2. Phys. Chem. (Frankfurtam Main), 75, 23 (1971). (11) (a) G. H. Aylward, J. L. Garnett, end J. W. Sharp, Anal. Chem., 39,457 (1967); (b) P. Tomasik, Rocz. Chem., 44, 1211 (1970); (c) N. M. Atherton, Trans. Faraday Soc., 67, 2510 (1991): (d) A. G, Evans, J. Chem. SOC.B, 1484 (1971). (12) K. G.Bot0 and F. 6.Thomas, Ausf. J. Chem., 2.4, 975 (1979).
