1820
G. Klopman and N. Doddapaneni
l ~ c t r o c h ~Behavior ~ ~ c ~ lof Substituted Polycyclic Aromatic Quinones e
Klopman" and N. Doddapaneni
Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44 106 (Received September 4, 1973;Revised Manuscript Received May 2, 1974)
In an attempt to find molecules exhibiting different electrochemical properties in the presence and absence of light, a series of phenyl- and mesityl-substituted benzoquinones were synthesized. The half-wave potentials of these and other quinones were measured in aqueous dioxane and anhydrous DMF and correlated with theoretical values calculated by the Huckel method. The photoisomers of the phenyl- and mesitylsubstituted quinones were obtained under irradiation but, due to facile hydrogen migration, rapidly rearranged into unreducible hydroxy hydrocarbons.
In an attempt to find organic systems capable of storing light energy and restoring it at a later time, we are investigating the el'ec~rochemicalbehavior of electrochemically reducible photocbromic compounds. Such systems should be capable under light irradiation to reach a metastable structure of higher energy and return only under controlled conditions to their ground state. If the structure of both forms differ sufficiently, then a shift in the reduction potential of a suitable group attached to the system1 might allow us to take advantage of the energy gained iin the process of irradiation. In this series of communications, we are specifically interested in the €allowing process ibund to occur readily with stilbene derivatives2 and report here the synthesis and polarographic study of a series of phenyl- and mesityl-substituted benzoquinones and other polyphenyl-substituted quinones. Some of these Compounds contain a stilbene fragment and hence are susceptible to cyclize reversibly This produces a change in conjugation under irradii~tion.~ and may lead to a shift in the reduction half-wave potent i a l ~The . ~ role of the quinoidic group is to provide the required revers,ible oxidation-reduction center.
The other quinones were studied in order to provide a basis for corirelation between the half-wave reduction potential and the resonance of the photoisomeric ones under our sets of conditions,
x ~ e ~ ~ ~ e Section nt,al Synthesis. ( a ) 2,5-Ghesityl-3,6-diphenylbenzoyuinone. 2,5-I)iphenylbenzoquiaone(20 g) was dissolved in 100 ml of mesitylene in a 250 .ml conical flask and cooled in an ice bath. Anhydrous aluminum chloride (40 g) was added in small portions over a period G€ P hr. During that period, the temperature of the inixture was kept a t 0-5' with occasional stirring with a glass rod. After 2 hr a t 0°, the mixture was slowly alloweld TO reach room temperature and left undisturbed for an additional 2 hr. It was then treated with 200 ml of 25% hydrochloric. acid. The solid was filtered in a BOchner funnel, washed several times with water, treated with 200 mE of 2:1 concentrated sulfuric acid, and heated to The Journal of Physical Chemistry, Val. 78. No. 18. 1974
150' until complete decomposition of the aluminum chloride complex occurred. After 30 min at 150°, the mixture was cooled by the slow addition of ice and then diluted by the addition of cold water. The black-green solid was filtered and washed several times with water and then ovendried at 110'. The dried product was taken in a thimble and extracted for 24 hr with hot ether in a Soxhlet extractor. The ether extract was distilled off leaving a yellow-green product which was recrystallized from acetone to give white needlelike crystals. The crystals were identified by nmr and elemental analysis as 2,5-dimesityl-3,6-diphenylhydroquinone. Yield of the pure compound was lo%, mp 245'. Elemental analysis was obtained from Galbraith Laboratories, Knoxville, Tenn. Anal. Calcd for C 3 ~ H ~ ~ C, 0 2 86.76; : H, 6.83; Found: C, 86.60; H, 6.72; 0,6.68.
Oxidation of Hydroquinone to Quinone. Chromium trioxide (0.3 g) in a small amount of water was added slowly to a solution of 0.5 g of hydroquinone in hot acetic acid. After 4 hr at room temperature a solid was separated and washed with cold acetic acid followed by cold alcohol. Recrystallization from hot acetic acid gave fine orange-red needles, mp 289'. (b) 5,8-Dimethyl-6,7-diphenyl-1,4-naphthoquinone. Equal amounts of 2,5-dimethyl-3,4-diphenylpentadiene5 and p-benzoquinone were refluxed in methanol for 4 hr. The condensation product precipitated and was filtered. It was then added to a 50% potassium hydroxide solution and a stream of oxygen was passed through it for 6 hr. The solid was separated by suction filtration and crystallized twice from alcohol. Fine yellow needles were obtained, melting a t 199'. From nmr and elemental analyses, it was identified as 5,8-dimethyl-6,7-diphenyl-1,4-naphthoquinone. Anal. Calcd for C24H1~02:C, 85.20; If, 5.33; 0, 9.47. Found: C, 85.00; H, 5.43; 0,9.46.
Electrochemical Behavior of Aromatic Quinones
1821
TABLE 1: Melting Points and Solvents Used for the Recrystallization of the Various Quinones Quinone
Lit. mp,
MP, "C
Solvent
OC
--.___----
1. Benzoquinone 2. 2-Phenylbemoquinone
3. 2,5-Diphen ylbenzoquinone 4. 2,6-Diphenylbenzoquinone 5 . 2,5-Dimesitylbenzoquinone
6. Tetrapheny benzoquinone 7. 2,5-Dimesit yl-3,6diphenylbenxoquinone 8. Naph thoquiirione 9. 2-Phenylnaphthoquinone 10. Z95-Diphenylnaphthoquinone 11. 5,8-Dimetbyl-6,7diphenylnaphthoquinone 12. 9,lO-Anthraquinone 13. 2,5-B is(methylamino) benzoquinone 14. 2,s-€3 is (dimethylamino) benzc q uinoiie 15. 2-IJyc9rorr!inaphthoquinone 16. 2-Methorynaphthoquinone 17. 2-Amiraonaplhthoquinone 18. 2-M e thylaniinonaph thoquinone 19. 2-Dimethylaminonaphthoquinone 20. 2-Phim ylanr~jnonaphthoquinone
Alcohol Ligroin Acetic acid Acetic acid Alcohol Acetic acid
115.5 115 .O 214.5 134 239.5 316.5
115-3 16 115 .O 214 P 34
Acetic acid Alcohol Alcohol Ligroin
289 .O 128.5 139.5
128.5 110 139 .o
Alcohol Alcohol
199 .o 286
286
Alcohol
271-272
270
Alcohol Acetic acid Acetic acid Alcohol Alcohol Water Dilute alcohol
171.5 195-196 dec 183.5 204.5 233-235 120.5 192.5
171 195-196 dec 183.5 204 . O 232
315
111.o
~
I
I1 mp 199" ( e ) General Procedure for the Preparation of other PolyPhenyl-Substituted Quinones. Phenyl-p-benzoquinone, tetraphenylberrzoquinone, and 2,3-diphenylnaphthoquinone were prepared as described by Kvalnes.6 All other quinones were made according to the literature. Table I lists the various quinones that were prepared together with their melting points and the solvents used for their recrystallization. Polarograrns. Tlhe current-voltage curves were recorded with a PAR Model 170 electrochlemistry system. The mercury dropping electrode was a PAR Model 172 mechanical drop timer. A vertical dropping mercury electrode served as cathode and a platinum wire electrode served as working electrode. An aqueous saturated calomel reference electrode was used in both protrc and api~oticsolvents. All protic solutions were mixtures of 75% dioxane and 25% water. They were buffered a t pH 7.4 by 0.2 M of sodium acetate, which also serves as supporting electrolyte, and acetic acid. A l).lD2% gelatin is added as a maximum suppressor. The aprotic medium consisted of a solution of anhydrous dimethylforrnamide and 0.1 M of tetraethylammonium perchlorate (TEAP) as supporting electrolyte. The M in all solutions. concentrations of the quinones were The reversibil ity of the electrochemical processes was checked with t'ie aid of three techniques: ac polarography,
120
191--192
cyclic voltammetry, and from the E us. log[i/(id - i)] technique. Materials. Dimethylformamide was shaken with solid potassium hydroxide and dried over anhydrous calcium oxide for 2 days prior to distillation. The first 200 ml of the distillate were discarded and the fraction boiling a t 152153' was collected. 1,4-Dioxane was distilled twice over sodium metal. Eastman grade tetraethylarnmonium perchlorate was crystallized twice from water and finally dried a t 100' under vacuum for 24 hr. Baker reagent grade sodium acetate was used without further purification. Purification of Nitrogen. The nitrogen was passed through red-hot copper turnings at 400° and then through a bottle containing Drierite. In order to remove the remaining traces of oxygen, the gas was then passed through two bottles containing a Fisher solution prepared from 10 g of benzophenone and 7.5 g of a sodium-potassiurn alloy in 200 ml of "xylene. Finally, the gas was passed through a sample of the solvent under study in order to remove residual organic vapors. Results and Discussion
Half- Wave Potentials i n Aqueous Dioxane Solution. The half-wave potentials of the 20 quinones were measured in 75% aqueous dioxane solutions. In each case, a single reversible, two-electron wave was obtained, in agreement with the postulated mechanism of the reduction in protic solvents. 7-12
The measured half-wave potentials as well as the concentration of the quinones in the electrochemical cell are listed in Table 11. It was shown, in many instances, that a good correlation can be obtained between the half-wave potentials and some theoretical indices related to the structure of the quinones. The Journal
6f Physical Chemistry, Vo/. 78, No.
18. 1974
G.Klopman and N. Doddapaneni
1822
TABLE 11: Half-Wave Potentials, EI/*, of Various Quinones in 75 % Aqueous Dioxane
TABLE 111: P a r a m e t e r s Used for t h e Calculation of the Hiickel Molecular Orbitals
_I____-
Quinone
1. Benzoquinone
2. Phenylbenzoquinone 3. 2,5- Diphenylbenzoquinone 4. 2,6- Diphenylbenzoquinone 6. 2,5-Dimesity!benzoquinone 6. Tetraphenylbenzoquinone a. 2,5-Dixnesity 1-3,6diphenylbenzcquinone 8. Naphthoquinone 9. 2-Phenylnaphthoquinone 10. 2,3-Diphenylnaphthoquinone 11. 5,8-DimetliyI-6,7-
12.
13. 14. 15.
diphenylnaphthoquinone 9,lO-Atithraquinone 2,5-I3is(methylamino)benzoquinone 2,5-His (dimethylamino)benzoquinone 2-Hydroxynaphthoquinone
16. 2-Methoxyna,phthoquinone 17. 2-Aminonaphthoquinone 18. 2-Methylarninonaphthoquinone. 19. 2-Diimethyianiinonaphthoquinone 20. 2-Pheny?amnnonaphthoquinone
Concn, mM
EIIZ, V us. ace
1.o 1.o 1.o 1.o 1.o 1.o
-0.003
$0.010 -0.010 -0.005 -0.000 -0 .OB0
0.8 1 .o 1.o 1.o
-0,100 -0.252
1.o 1.o
-0.345 -0.575
1.o
-0.645
1.o 1.o 1.o 1.o
-0.515 -0,430 -0.385 -0.408
1.o
-0.450
1.o
-0.415
1.o
-0.265
-0,230
-0.315
In the case of protic media, where the reaction is reversible, such a correlation is anticipated to occur with an index related to the change in resonance incurred by the quinone upon reduction to the hydr0q~inone.l~ Thus the resonance energy of both the quinones and their hydroquinone counterparts were estimated from simple Wixkel calculations. In these calculations, the coulomb and resonance integrals pertaining to the oxygen atom ( a , and fie,, respectively) were expressed in the usual form, i.e. [Y, = a 4Pc,
=
h,P
kcxP
where a and 0 are the values corresponding to sp2 carbon and h, and kcxassumed the standard parameter values of Table 111. In Table IV, the results of these calculations are reported for several quinones. In addition to the change in resonance energy, this table also lists the energy of the lowest unoccupied orbital of the quinone (en+l) and other changes in resonance energy relevant to our discussion. A good correlation was found (Figure 1) between the half-wave potentials of the various quinones and the calcu~ EQ)between the hydrolated energy differences ( E Q Hquinones and the corresponding quinones showing that for a reversible system the half-wave potentials linearly increase with conjugation in the molecule. The correlation which was found by various authors to be valid for the simple aromatic quinones seems to be satisfactory also for our phenyl-substitul ed derivatives. The points corresponding to the latter compounds fall somewhat below the line, reflecting probably sdme deviation from coplanarity in these molecules. Half- Wave Potentials in Anhydrous DMF. The experimental half-wave potentials obtained for the reduction of our quinones in dimethylformamide are listed in Table V. Tho Journal of Physics/ Chemistry, Vol. 78, No. 18. 1974
Heteroatom or group x
h, (no H bond)
kcx
1.o
I .O 0.8
=O -0-
2.0
TABLE IV: Theoretical Indices (Huckel) Relevant to the Reduction of Quinones in Protic and Aprotic Media
-
~
AE' =
AB =
Quinone
1. Benzoquinone 2. 2-Phenylbenzoquinone
EQH,EQ (PI
2.060 2.017 3. 2,5-Diphenylbenzoquinone 1,975 4. 2,6-Diphenylbenzoquinone 1.977 8. 1,4-Naphthoquinone 1.793 9. 2-Phenyl-l,4-naphthoquinone 1 .757 12. 9,lO-Anthraquinone 1.495
AE W+l
(8)
%+I
(PI
0.25 1.810 0.24 1.777 0.23 I .745 0 . 2 3 1.747 0.14 1.653 0.12 1.637 0.00 1.495
The polarograms that were obtained consisted in each case of two well-defined one-electron reduction waves.l4-I9The first wave is attributed to the addition of an electron to the quinone to produce a semiquinone radical anion and the second one to the subsequent addition of an electron to the radical anion producing a quinone dianion.
In the absence of proton availability, the two waves are well separated and Moreover, the first reduction half-wave potentials are independent of the concentration of the depolarizer and also of the water content in the solution of dimethylformamide. However, the half-wave potential for the second wave moves toward more positive values with the addition of water and a t sufficiently high water concentrations, the wave merges with the first one to give a single wave approximately equal to the sum of the heights of the two waves obtained in anhydrous DMF. The formation of a radical anion was shown by esr studied4 and that of the dianion by the observation that during the polarographic reduction of anthraquinone, a red color develops around the mercury drop.15 Based on the polarographic similarities of the reduction waves, the conclusion can be drawn that the reduction of all quinones in DMF follows the same pattern. The half-wave potentials of the first reduction waves of unsubstituted quinones show a linear relation with the energies of the lowest unoccupied molecular orbitals ~ , + 1 (Figure 2). However here again the Eli2 values of the monoand diphenylbenzoquinones and 2-phenylnaphthoquinone, in which the phenyl groups are conjugated with the parent quinone, do not correlate as well as the other quinones. The discrepancy might be due to structural changes such as a twisting of the phenyl groups when electrons are added to the quinone. The second one-electron half-wave potential presumably represents the addition of a second electron to the same orbital as the first. We may therefore expect this value to correlate with the change in resonance energy incurred by the
Electrochemical Behavior of Aromatic Quinones
1823
TABLE V Half-Wave Potentials, El/*,and Diffusion Currents, Dimethylformamide
.-___I_
id,
for Various Quinones in Anhydrous
Ldr
#A
E ~ j eus. sce
.___
Quinone
First wave
1. Benxoquinione
2. Phenylbenzoquinone 3. 2,5-l)iphenylbenzoquinone 2,6-lliphenylbenzoquinone 2,5-XPimesitylbenzoquinone
4. 5. 6. 7.
Tetmphenylbenzoquinone 2,6-lCPimesityl-3,6diphznylbenzoquinone 8. Naphthoquinone 9. 2-Phenylnaiphthoquinone 10. 2,3-Diphenylnaphthoquinone 11. 5,8-P)imethyl-6,7diphcnylnaphthoquinone 12. 9,lO-Anthraquinone 13. 2,5-Eiis (methylamino)benzaquinone 14. 2,5-EIis(dimethylamino)bwzoquinone 15. 2-Hydroxynaphthoquinone 16. 2-Mt~thoxynaphtho~uinone 17. 2-Aminonaphihoqu&one
-_
__
I -
I _ _ _ _
Second wave
First wave
Second wave
5.6 4.8 4.7 4.6 4 .O 4.2
-0.450 -0.435 -0.425 - 0.426 -0.480 -0.505
-1.366
3.1 4.6 3.9 2.6
-0.525 -0.630 -0.625 -0.660
- 1.425 - 1.470 - 1.452 - 1.485
- I ,330
-1,300 -1.290 - 1,415 -1.390
3.8
-0.770
5.3
-0.860
-1.550 -1.600
4.3
-1.210
-1.816
3.9
-1.090
-1.640 - 1.485
18. 2-Mt?tkylaminonaphthoquinone
19. 2-Dimethy laminonaphthoquinone 20. 2-Phenylaminonaphthoquinone
- 1.475
-0.785 -0,916 -0.958 -0.910 -0.812
4.0 4.6 4.4 4 .O 4.1
I
-1.615 -1.656 - 1.570 - 1. .510
0 4-
-0.7.
Figure 1. Correlarion between measured half-wave reduction potentials, € 1 in~ valts, in 75% aqueous dioxane solutions and calculated energy differences (EOHp - fo):0, simple aromatic quinones; e, phenyl-substituted quinones. The numbers refer to the quinones listed in Table II.
system when reduced from the semiquinone radical anion to the quinone dianion. Assuming that the latter has a comparable resoinance energy to that of the hydroquinone, the relevant indices are obtained as E Q H-~ (EQ e n + l ) . These values are tabulated in Table IV for the various quinones and, when plotted us. the second half-wave potential, yielded the satisfactory correlation of Figure 3. Leibovici2' has suggested that the value of (2en+l Jn+l++l),where J,,+L,,+l i s the electron-electron repulsion in the ( n 1) orbital, represents the ability of the quinone to be reduced directly to the quinone dianions and should correlate with the second half-wave potential. We have not found this to be the ease as the correlation of our second Ell2 with Leibovici theoretical values is very poor. In our opinion this is due to the erroneous assumption that the second half- wave potential i s determined by the energy necessary to add two electrons to the quinone rather than by that of adding one electron to the semiquinone radical anion.
+
+
+
Figure 2. Measured half-wave reduction potentials, &/2, of the first one-electron wave against the energies of the lowest unoccupied molecular orbital: 0, simple aromatic quinones; 0 , phenyl-substituted quinones. The numbers refer to the quinones listed in Table II.
Comparison between t h e Reduction of Quinones in Aqueous Dioxane and Anhydrous LIMP. Interestingly enough, when the Ell2 values of reduction of the quinones in 75% aqueous dioxane were plotted us'. those in anhydrous DMF, a linear correlation was observed that included all quinones except those substituted by amino or methoxy groups (Figure 4). The correlation was to be expected as for most quinones a linear dependency is also found between the theoretical indices characterizing the two processes, i.e.
-
E Q H z - E, €"+I The deviation from this correlation observed for the amino and methoxy derivatives might possibly be attributed to a breakdown of the theoretical correlation for these heteroatoms containing derivatives. This, unfortunately, cannot be checked within the framework of a Hiickel calculation since both indices depend on the value given to the paramThe Journal of Physical Chemistry, Vol. 78, No. 78. 1974
G. K1opma.nand N. Doddapaneni
1824
I
A
1.7
d4
1.
1.5
1
Figure 3. Correlation between the half-wave potentials of the second one-electron reduction wave of quinones and the resonance energy differertces between the quinone dianions and their semiquinone radical anions, i.e., E = 6~~(6 The numbers refer to the quinones listed in Table 11.
-
+
eters that characterize the heteroatom. However,.a second possibility exists that this discrepancy is generated by some specific properties of the amino and methoxy derivatives. This is exemplified by the fact that in aqueous dimethylformamide solutions containing 0.1 M of tetraethyl perchlorate as support electrolyte, the amino derivatives are reduced in two one-electron steps of which the firstone is apparently irreversible (Table VI). The height of the irreversible reduction wave increased with methyl substitution of the amino group, i.e., NH2 < NHCH3 < N(CH& and that of the reversible wave decreased. However, the total height of the two reduction waves remains constant. On the basis of these results one can postulate that the amino quinones exist under two different forms under our experimental conditions. One interesting possibility is that a slow equilibrium exists between the free quinone and its protonated form and that the protonated form would be reduced at a lower negative potential than the unprotonated one, i.e.
Figure 4. Correlation between the first one-electron reduction wave half-wave potentials Eln (V) of various quinones in dimethylformamide and their two-electron reduction wave 'half-wave potentials Eln (V) in aqueous dioxide. The numbers refer to the quinones listed in Table V: 0,simple aromatic quinones; 0 , phenyl-substituted quinones; 0,amino- or methoxy-substituted quinones.
TABLE VI: Reduction of N-Substituted Aminonaphthoquinones l _ l _ l _ . -
0 X
H H CHs H
Y
H CH3. CH3 Ce.H.5
-Ei/P
-EI/P
0.67 0.695 0.600 0.620
0.786 0.830 0.730 0.745
Irreversible wave. Reversible wave.
where protonation could obviously not occur. This is what was observed experimentally (Figure 4). Photochromic and Polarographic Changes of 2,5-Dimesityl-3,6-diphenylbenzoquinone.The irradiation of a degassed ethanol solution of 2,5-dimesityl-3,6,diphenylbenzoquinone for 30 min by ultraviolet light from a General Electric mercury lamp No. 400 RSP33-1 at 25 cm distance resulted in the formation of a white product which did not undergo oxidation or reduction at the dropping mercury electrode in either protic or aprotic solutions. It is known that the photocyclization of diphenylcyclopentene by uv irradiation takes place snioothly to produce
bH protonated forms
The substitution of the hydrogens of an amino group by methyl groups increased the stability of the protonated form and heme the observed increase in the irreversible wave height. If this explanation can be retained and is extrapolated to the aqueous dioxane case, one might expect that here, either complete protonation or a t least a fast equilibrium exists. As a result, the reduction of the amino quinones would occur at lower negative potentials than expected from a comparison with the values obtained in anhydrous DMF The Journal of Physical Chemistry. Vol. 78, No. 78, 7974
1 C
H,
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 irraand a,a'-dicyanostilbene d i a t i ~ nof~diphenylmaleinimide ~ 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 irraone diation of the t3,5-dinnesityl-3,6-diphenylbenzoquinone, 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
