RAFIK0. LOUTFYAND RAOUF0. LOUTFY
1650 cients, since they depend exponentially on the adsorption potentials, seems a futile task indeed until such time as adsorption heats can be predicted with more
confidence. It is hoped that the extension of the sorts of comparisons made here to other zeolites and adsorbates will make this possible.
Correlations between the Electrochemical and Spectroscopic Behavior of Some Benzophenones and Thiobenzophenonesl by Rafik 0. Loutfy” and Raouf 0. Loutfy University of Western Ontario, Photochemistry Unit,London 79, Ontario, Canada (Received September 87, 1971) Publication costs assisted by the Photochemistry Unit, University of Western Ontario
The electrochemistry of a number of benzophenones and the corresponding thioketones have been investigated, using ac polarographic technique. The half-wave potentials (E112)s)were found to be linearly related to the n, T* triplet energies. This relation enabled an estimate to be made of the position of the n, T* triplet energies of those compounds where n, T * triplet states were not observed spectroscopically. A linear dependence between the half-wave potentials of the carbonyl and the corresponding thiocarbonyl has also been observed. The usual Hammett correlation was obtained for both series of compounds. The heterogeneous rate constants for several of these compounds have been determined by Randel’s method of analyzing the ac data. A reversible (Kappg > 1 x 10-2 cm/sec) one-electron reduction was found for these compounds. The heterogeneous rate constants have been compared with the hyperfine splitting constants and a fair prediction of the variation in the experimental rate constant with structure was found.
Introduction It has been well established that polarographic half-wave potentials are linearly related to the calculated energy of the lowest vacant molecular orbital, both for the unsaturated2 and aroma ti^^,^ hydrocarbons. This linear dependence has also been extended to the energy of the lowest excited statea6 The electrochemical reduction of benzophenones, in aqueous media, has been studied by Zuman, et a l e The substituent effects on E,,,’s were determined using Hammett plots. Recently the electrolytic reduction of a number of derivatives of benzophenone037 and thiobenzophenones has been studied in aprotic solvents. It has been shown that these compoundsO-sundergo a one-electron reduction to form a stable anion radical in dimethylformamide and acetonitrile. However, there have been no kinetic studies reported of the electron transfer for these compounds. I n the present paper, the results of a kinetic study of the electroreduction of benzophenone, 4,4’-dimethoxybenzophenone, and bis(dimethy1amino) benzophenone and the corresponding thio ketones are reported. A correlation between the n, n* triplet energies and the El/,’sare presented and discussed. The Journal of Physical Chemistry, Vol. Y6,No. 11, 19YB
Experimental Section All experiments were carried out in acetonitrile (AN) containing 0.05 M tetraethylammonium perchlorate (TEAP) . The solvent purification procedure is described in detail in ref 9. The accessible potential range of the solvent was +2.7 to -2.1 V on Pt and +0.6 to -2.8 V on dropping mercury electrode vs. (Ag~AgCl~O.1 M TEAP in AN/10.05 M TEAP in AN). TEAP was recrystallized from hot water and dried for (1) Publication No. 26 from the Photochemistry Unit. (2) A. Maccok, Nature (London), 163, 178 (1949). (3) (a) A. Streitwieser, Jr., “Molecular Orbital Theory for Organic Chemists,” Wiley, New York, N . Y., 1961, pp 173-185; (b) M. E. Peover in “Electroanalytic Chemistry,” Vol. 11, A. J. Bard, Ed., Marcel Dekker, New York, N. Y., 1967, Chapter 1. (4) H. B. Mark, Jr., Rec. Chem. Progr., 29, 217 (1968). (5) (a) E . 8. Pysh and N. C. Yang, J. Amer. Chem. Soc., 8 5 , 2124 (1963); (b) A. Maaaenga, D . Lomnitz, J. Villegas, and C. J. Palowczyk, Tetrahedron Lett., 21, 1665 (1969). (6) P. Zuman, 0. Exner, R. F. Rekber, and W. Nauta, Collect. Czech. Chem. Commun., 3 3 , 3213 (1968). (7) J . M. Saveant and L. Nadjo, J. Electroanal. Chem., 30, 41 (1971). (8) (a) L. Lunazai, G. Maccagnani, G. Mazzanti, and G. Plaucci, J. Chem. Soc. B , 162 (1971); (b) R . M . Elofson, F . F . Gadallah, and L. A. Gadallah, Can. J. Chem., 47,3979 (1969). (9) W. R. Fawcett, P. A. Forte, R . 0. Loutfy, and J. M. Prokipcsk, ibid., 50, 263 (1972).
1651
BEHAVIOR OF BENZOPHENONES AND THIOBENZOPHENONES 48 hr at 60" under vacuum. Benzophenones, commercially available (Aldrich), were purified by recrystallization followed by zone refining. Thiobenzophenones were used as supplied by (K & K Laboratories). Table I shows the structures of the compounds investigated. The numbers on the table will be used to designate the particular compounds throughout this paper. -
Table I: Number Assignments of Compounds Studied
NO.
X
Ri
R1
1
0 0 0 0 0 0 0 0 S S S
H CHaO N (CHI)a c1 CFa CHa Br OH
H CHaO N(CHa)a c1 H CHs Br OH H CHsO N(CHs)a
2 3 4 5 6 7 8 9 10
11
H CHaO N (CHI )a
The electrochemical cell was specially designed for use with a controlled dropping mercury electrode (dme). All experiments were conducted in a drybox containing a nitrogen atmosphere at a temperature of 25.0 f 0.5". The counter electrode was a cylinder of platinum ( 2 cmz), whereby the working electrode was positioned to be in the center to achieve a symmetrical field around the working electrode. The reference electrode (AglAgC110.1 M TEAP in ANI/), placed close Lo the working electrode, was enclosed in a glass sleeve and joined to the system under investigation by a center glass disk. Current-voltage curves were obtained using a PAR Model 170 electrochemistry system, in conjunction with PAR Model 171 controlled-drop timer. The latter was fixed to 5-sec drop time. A currentsampled ac polarographic technique was used in which a small amplitude (5 mV) alternating potential was superimposed on the dc potential, which varies linearly with time. The application of operational amplifier circuitry, built into the PAR Model 170, with a threeelectrode system, drastically reduced the sources of ohmic resistence, and the use of a phase-selective locking amplifier method for recording the ac currents made this method superior to conventional dc polarography. For more deteails on ac polarographic techniques, refer to ref 10. Six frequencies (25, 45, 75, 95, 200, and 400 cps) were employed to determine the kinetics of the reaction. Depolarizer concentrations were in the range 1 X 10-4-5 X M.
Results and Discussion Polarography. All compounds gave well-characterized ac waves with a width at half-height of 90 1 mV. This corresponds to a one-electron reduction to give radical anions, in agreement with previous The summit potentials were independent of frequency or scan rate (2-20 mV/sec), ;.e., as a first approximation the reduction processes proceed reversibly. lo The half-wave potentials for all the compounds were obtained from cathodic and anodic sweeps of the ac wave in the usual manner. The El/,'s are shown in Table 11. Table 11: Electrochemical Data and n, T * Triplet Energies of Benzophenones and Thiobenzophenones Studied
9 10 11
1.84 2.01 2.16 1.68 1.61 1.883 1.695 2.372 1.17 1.35 1.51
68.6a 69.4a (70.l)d 68.Oa 67.6a 68.905 68.07" 69.50" 40.6" 42.la 43 * 4a
77.2 78.33 (79.5)"
45.67c 47 * 4c 49.OC
5 From emission spectra in EPA a t 77°K; see ref 13 for experimental detail. a See ref 12 and references therein. From ultraviolet absorption spectra at 77°K. d Predicted n, T * triplet energy. e Reference 14.
On substitution of a group with a lone pair of electrons, such as NR2, OR, or X, into the aromatic rings of benzophenones or thiobenzophenones, resonance interaction or even the inductive effect will cause a pronounced effect on both the a and a* levels, while the n level wiIl be largely unaffected by such interaction owing to its orthogonality with respect to the a system.l' It is anticipated, therefore, that a substituent would perturb the a* level in the benzophenone series to the same extent as it would for the thiobenzophenone series. Since the El/* is a measure of the relative position of the lowest vacant molecular orbital, ;.e., a* level, and, assuming the above hypothesis is valid, a linear correlation, with a slope of 1, between the El/,'s of thiobenzophenones and benzophenones is to be expected. This relation was found (see Figure l), and a slope close to unity was obtained. The equation giving the best fit for the Ei/a(C=s)and EI/,(c,o)is (10) D.E.Smith in ref 3b, Vol. I, 1966,Chapter 1. (11) S.Nagakura, Bull. Chem. SOC.Jap., 2 5 , 164 (1952). The JOUTVA~ of Physical Chemistry, Vol. 76,No. 11, 197%
1652
RAFIK0. LOUTFY AND RAOUF 0. LOUTFY
El/,(c=s)= ~.O~EI/,(C,O) - 0.785
(1)
The above results are contradictory to those reported in ref Sa, where a slope of 2 was obtained. This discrepancy could be due to the different techniques for determining the reduction potential. Correlation between the El,21sand the n, a* Singlet and Triplet Energies. It is reasonable to predict that thc El,,'s, assuming that the n level of the carbonyl or the thione compounds will be unaffected by t,he substituents, would correlate with the n, a * transition. The lowest triplet state of the benzophenones have been thoroughly studied,12 and those of thiobenzophenones have recently been in~estigated.'~The n, a * triplet and singlet energies (ET)and ( E s ) ,respectively, for the compounds investigated are given in Table 11. It has been noted14 that the singlet-triplet splitting of benzophenones and thiobenzophenones (shown in Figure 2 ) are remarkably constant. It has also been found14 that the differences in singlet or triplet energies between benzophenones and the corresponding thio ketones, for a given substituent, are almost constant and approximately equal to the differences in the bond strength. As an implication one would expect that both ET and Es would correlate linearily with El/:s. These relations are given in Figure 3 for the thiones. The slopes of E,,, us. ET and ESwere found to be 0.357 and 0.425, respectively. These slopes are much lower than the theoretical value of 215 expected from the plot of the absorption spectra of the a + a* band against the half-wave potentials. It is worth mentioning that the slopes obtained here are also lower than 0.6, which was reported previouslysb for the n-a* us. EI/,'sof some thiones. However, these authors based the singlet energies of the thiones studied on the shortest wavelength maxima, with absorption spectra measured at room temperature. It has been shown13 that in some cases (xanthione and
,"
17
18
19
+
-E,
C C O
20
21
22
VOLTS
Figure 1. Relationship between EI/,'s of thiobenzophenones (Ec8)and analogous benzophenones (Eco). The Journal of Physical Chemistry, Val. 76, N o . 11, 1972
f i
Figure 2. Correlation diagram of n, T* singlet ( E s ) and triplet (ET)levels of benzophenones and anologous thiobenzophenones.
-Eliz
Volts
Figure 3. Half-wave potentials of thiobenzophenones us. n, r* (a) singlet and ( 0 )triplet energies.
thiocamphor) the shortest wavelength maxima under the above conditions, does not represent the S -+ SI transition, but rather the S + T transition, which makes their calculations erroneous. A linear correlation of El/,'s vs. ET'S for the benzophenones is given in Figure 4,with a slope of 0.192, which is about half of that obtained for the thiones. In polar solvents, the lowest triplet state of Michler's ketone, 3, is believed to be of a charge-transfer (CT) character, ET = 61 kcal/mol.16 This assumption was (12) D. R. Arnold, Advan. Photochem., 6, 306 (1968), and references therein. (13) D. Blackwell, C. C. Liao, R. 0 . Loutfy, P. de Mayo, and 8. Paszyc, J. Mol. Photochem., in press. (14) Rafik 0. Loutfy, Ph.D. Thesis, University of Western Ontario, London, Ontario, 1972. (15) A. T. Watson and F. A. Matsen, J . Chem. Phys., 18, 1305 (1950). (16) J. Pitts, H. Johnson, and T . Kuwana, J . Phys. Chem., 66, 245 (1962).
BEHAVIOR OF BENZOPHENONES AND THIOBENZOPHENONES
t
o
-
67t L
15
/""
4,4'-di-Cyo 4tf-di-Br-
I
16
1
17
I
I
I
1.8 1.9 2.0 -E kVOLTS
I
I
2.1
2.2
Figure 4. Half-wave potentials of benzophenones us. the n, triplet energies, ET.
R*
based on spectroscopic evidence together with the very low reactivity in the polar solvents toward photoreduction.'7 Suppan, et al.,17318reported a large change in the phosphorescence lifetime of Michler's ketone, in hydrocarbon solvents, and an increase in reactivity. These factors, were considered as important evidence that crossing of a n, a* and a CT state had taken place. As mentioned before, the energy of the lowest triplet state of Michler's ketone in a polar solvent is estimated to be =62 kcal/mol. From the measured half-wave potential, the energy of the n, a* triplet state was found to be =70 kcal/mol (see Figure 4). Even allowing for the known large shift of the charge-transfer state energy level, on changing the solvent from polar to nonpolar, we believe that inversion of states is unlikely. Nevertheless, the CT and n, a* states may almost be degenerate. The emission spectra of 4,4'-dibromo- and 4,4'-dihydroxybenzophenone were measured in a 4: 1 mixture of MCH and n-P at 77°K (for experimental details, see ref 13). Both compounds were found to have lowlying n, a* triplets, with ET = 68.07 and 68.50 kcal/mol, respectively. The latter compound showed n, a* triplet emission in EPA a t 77°K with an ET = 69.6 kcal/mol, while at p H 8, a long-lived emission was observed (ET= 70.6 kcal/mol) which probably originates from a CT state. For 4,4'-dibromobenzophenone, two ac waves have been observcd, with the second wave located at the same potential as in the unsubstituted benzophenone. This is in a,greement with the results obtained by Saveant, et where the first wave corresponds to anion radical formation of the original compounds and the second ac wave is due to the formation of benzophenone anion radical after a reductive cleavage of the carbon-bromine bonds. It can be seen from
1653
Figure 4 that the E,,, obtained from the first ac wave correlates with the other substituted benzophenones, indicating that there is no change in the reduction mechanism. The half-wave potential obtained for the 4,4'-dihydroxybenzophenone (8) is remarkably high and does not correlate with the n, a* triplet energy. A possible reason is that under our experimental conditions, the compound was in a deprotonated form, in which case the lowest excited state is CT in character. From Figure 4 the expected n, a* triplet energy of the deprotonated form of compound 8 was found to be 71 kcal/mol. The E,,, of the protonated compound can be predicted, from Figure 4, using the n, a* triplet energy obtained spectroscopically in EPA, which was found to be -2.03 V. From Figure 3 it is obvious that the lowest triplet state of Michler's thione (11) is of n, a* character rather than a CT triplet, which is in agreement with work done previously.la Therefore, a correlation of the type obtained in this work, Figures 3 and 4, can be used to determine the nature of the lowest triplet state. It could also be utilized to estimate the n, a* triplet energy of compounds of known electrochemistry. Knowing the electrochemistry of a compound in a one class, it is possible to obtain the half-wave potential of the corresponding compound in the other series (see Figure 1). From this value one can obtain the n, a* triplet state using Figure 3 or 4. For example, from the E,,, of 4,4'-dichlorobcnzophenone, E,,, of the corresponding thione was found to be -1.02 V; accordingly, the expected ET is 39.4 kcal/mol. The same argument can be applied to the E,,, - ES correlation for thiobenzophenones, giving a singlet energy of 42.2 kcal/mol. The half-wave potentials have been correlated with the Hammett constant for both series. The Hammett reaction constant for both the benzophenones and thiobenzophenones is p = 0.384, which is in agreement with the value obtained for benzophenone reduction in DMF.7 The fact that the slope of the Hammett plot is almost identical for both series indicates a remarkable parallel change of properties due to the substituents. Measurement of Heterogeneous Electron-Transfer Rates. The effects of substituents on electrochemical kinetics through stabilization of the transition state are much less understood than the substituent effects on electrochemical thermodynamics, which are well d o ~ u m e n t e d . ~ JAccordingly, ~ the heterogeneous rate constants K , of the three thiones investigated and the (17) G. Porter and P. Suppan, Trans. Faraday SOC.,61, 1664 (1965). (18) P. Suppan, Ber. Bunsenges. Phy8. Chem., 72, 321 (1968). (19) 0. Ryba, J. Pilar, and J. Petranck, Collect. Czech. Chem. Commun., 33, 26 (1968). The Journal of Physical Chemistry, Vol. 76, No. 11, 1972
1654
RAFIK0. LOUTFY AND RAOUF 0. LOUTFY
corresponding carbonyl compounds were measured and correlated to the esr spectra obtained from ref 8, in an attempt to elucidate the dependence of the electrode kinetics on the structure. Of the many techniques for measuring electrode kinetics, the ac technique, with a phase-sensitive locking amplifier to separate the in-phase and out-of-phase components, was used. This technique involved measuring the Faradaic impedance in the presence of a direct current flow. However, Randles20 showed that the classical theory of the Faradaic impedance, derived from an electrode a t equilibrium, remains valid in the presence of a direct current provided that the electrode reaction is sufficiently rapid. The advantage of this technique is, of course, the on-site generation of the reactants; also, problems with adsorption are immediately apparent from the experimental results. The present work employs the classical approach developed by Randles,21 in which the resistive (Rr) and reactive (C,) components of the Faradaic impedance are plotted against w - l / ’ , where w is the angular frequency of the alternating voltage. The K,’s were obtained from these measurements through the following relationz1
Rr - l/wCf
=
RT/n2F2ACKa
(2) where A is the area of the electrode, C is the concentration of the depolarizer, and the other terms have their usual meaning. The diffusion coefficient of the depolarizer, Do,, can be obtained from the slope, S , of impedance plots, where S at E l / , is given byzz
S
2.83RT/n2F2CDo,’/z (3) Figure 5 shows some representative plots of the components of the Faradaic impedance. The rate constants (K,’s) and the diffusion coefficients are given in Table 111. =
Table 111: The K,’s and DoU)sof Benzophenones and Thiobenzophenones
NO.
1 2
3 9
10 11 Q
KlU crn/sec
Dox1/2 X 105 crn/aec1/8
0.177 0.154 0.127 0.173 0.143 0.114
6.70 6.25 6.20 5.35 5.45 5.50
apn
5.8 15.9 8.15 17.0
Reference 8.
It can be observed from Table I11 that Ka’s are in the reversible range ( K , > 1 X 10-2 cm/sec) and that benzophenones have slightly higher values than the corresponding thiobenzophenones. The Do,% of benThe Journal of Physical Chemistrg, Vol. 76, N o . 11, 1978
WbXKJZ
Figure 5. Components of the Faradaic impedance a t E l / , of thiobenzophenones. (For w read w.)
zophenones are also higher than those of the thiobenzophenones; this is probably due to difference in the magnitude of solvation. Correlation between K , and Structure. Peover and Powellza have published the only known direct correlation between the hyperfine splitting (hfs) constants and the electrode kinetics. The logarithms of the rate constants for one-electron reduction were found to vary linearily with the hfs constants of the reaction site. This behavior is explained in terms of the influence of excess charge a t the reaction site on the solvation energy of the radical anion. The more the odd electron is localized on the reaction site, the higher the energy of the transition state and the slower the reaction. From the esr results obtained for compounds 1, 2, 9, and 10 in ref 8, the hyperfine splitting constants, up, at the reactive site (C=S or C=O) were calculated. These values are shown in the last column of Table 111. However, it was not possible to obtain the values for compounds 3 and 11 owing to the complex nature of the spectra.24 It can be seen qualitatively that the rate constants, K,, are inversely related to a,,. This will explain the result that thiobenzophenones have slightly lower Ka)s than the corresponding benzophenones because the C=S group has a higher charge density than the C=O group. To correlate this relation on a quantitative basis, one has to consider the solvation energy of the anion radical on the basis of a model consisting of three spheres, one representing the functional group of radius approximately r,, = 2.75 A, and two spheres, one repre(20) J. E.B. Rtndles in “Transaction of the Symposium on Electrode Processes, E. Yeager, Ed., Wiley, New York, N. Y., 1961, Chapter 11. (21) J. E.B. Randles, Discuss. Faraday SOC.,No. 1, 11 (1947). (22) T.Biegler and H. ,4. Laitinen, Anat. Chem., 37, 572 (1965). (23) M.E. Peover and J. S. Powell, J. Electroanat. Chem., 20, 427 (1969). (24) E.G.Janzen and C . M. Dubose, Jr., J . Phys. Chem., 70, 3372 (1966).
ELECTRIC CONDUCTION OF A~-AURAMINE--Sn02SYSTEM
1655
f(ap) = C ( a p Q p > z / r+p (1
0.1 5
0.20
025
- ap&p)’/2rring
+ 2ap&p(l - ap&p)/R
Qp is McConnell’s constant, taken as ‘ / Z T ; ~ 6 is the distance from the center of the reactant to the electrode surface; D o , and D , are the local optical and static dielectric constants, respectively; R is the distance between the spheres in the above model, assumed to be 5 k ; Z is the collision frequency, w 1 0 4 cm/sec; and other terms have their usual meanings. 6, Do,, and D , are constants; accordingly, eq 4 predicts a straight-line relation between the logarithms of the KB’s and the f(a,) term; this relation is shown in Figure 6. The slope is equivalent to the theoretical value of
I
0.30
f(ap)Xldcm-’
Figure 6. Dependence of experimental rate constant on spin density at the reaction site. senting each phenyl part ,Of the molecule with an average radius of rring = 3.5 A for compounds 1 and 6 and 5.3 d for 2 and 7. Peover and showed that the heterogeneous rate constant can be given as follows In K, = In Z
- Ne2(1/Do,- l / D , ) f ( a , ) / 8RT
+ Ne2(1/D,, - l/D,)/166RT
Ne2(1/Do,- 1/D8)/18.424RT This shows t-i fair prediction of the variation of the experimental rate constant with the structure, i.e., substituents and functional groups.
Acknowledgment. The authors wish ‘to express their sincerest thanks and appreciation for having this work completed to Professor P. de Mayo, University of Western Ontario, Chemistry Department, London, Ontario, and Department of Chemistry, University of Guelph, Guelph, Ontario.
(4)
where, using our
(25) J. M. Hale, “Reactions of Molecules a t Electrodes,” N. S. Hush, Ed., Wiley-Interscience, New York, N. Y., 1971, Chapter 4.
Electric Conduction of the Aluminum-Auramine-Stannic Oxide System by H. Shimoda, M. Sukigara, T. Sakata,* and K. Honda Institute of Industrial Science, University of Tokyo, Roppongi, Minuto-ku, Tokyo, Japan
(Received November 1 , 1971)
Publication costs borne completely by The Journal of Physical Chemistry
Electric conduction in an n-type SnOz-auramine hydrochloride-A1 sandwich cell was studied. The cell showed a rectification characteristic in the dark conduction, but a larger photocurrent density was obtained when the applied field was in such direction that the dark current was suppressed. These phenomena were interpreted as follows. Electrons are the majority carriers in the dark conduction regardless of the direction of the electric field, but the probability of electron transfer from the conduction band of SnOz to the conductive level of auramine is very small. In the photoconduction electrons are still the majority carriers in the bulk of the auramine film, but the recombination of photogenerated holes with electrons accumulated in the SnOz electrode at the interface greatly increases the photbcurrent.
Introduction In a photoelectric cell containing an organic photoconductor, carrier transport in the bulk of the organic material has been a main subject for many workers.1-a Although the mechanism of the bulk conduction of very
slow carriers in a crystal and in an amorphous state is a very interesting Problem, a h m s f e r of Photogenerated charge carriers through an interface of a semiconductor
* Address correspondence to Department of Electro-Photo-Optics, Tokai University, Kanagawa, Japan. The Journal of Physical Chemistry, Vol. 76, No. 11, 1072
