332
J. Phys. Chem. 1985, 89, 332-335 H
0 H-0'
H Figure 7. Proposed structures for the 2:l a-CD-adamantanecarboxylic acid complex.
acids.21 Insertion of the charged carboxylate group of a ligand into a CD cavity is probably energetically prohibitive. The large ApK, of 1.7 units for the a-CD-adamantanecarboxylate complex, which seems to indicate a much higher affinity for the neutral form of the ligand, is actually misleading. As Gelb and c o - ~ o r k e r s have ' ~ shown, and as our data in Figures 1 and 5 corroborate, the large ApK, in this case appears to be due almost entirely to a preferentially binding of the neutral ligand in the 2: 1 a - C D complex, instead of preferential binding in the 1:l complex. That is, in terms of Scheme 11, the data are consistent with K1 being equal to Kl. and K2 being much greater than K2.. Thus 2:l complexes are formed with a-CD and such ternary complexes are preferentially formed with the neutral form of adamantanecarboxylic acid. The K,, K1., K2, and K2. values used to fit Figures 1 and 5 may not be unique but they do suggest a couple of interesting molecular interpretations. If K, is approximately equal to K1- there is no preferential binding of the carboxylic acid in the 1:l complex with a-CD. This suggests that the functional group of adamantanecarboxylate is directed away from the CY-CDcavity when its carboxylic acid group is both protonated and unprotonated. Also it is interesting that K2 > K1. In other systems in which 2:l a-CD inclusion complexes are formed K2 is less than K,.11,12If K2 > K, there must be positive cooperativity in binding the second a-CD to the complex. The enthalpy change for formation of a 2:l a-CD-adamantanecarboxylic acid complex has a rather large negative value of -12.0 kcal/mol. When one substracts the AHo value of -3.2 kcal/mol for forming the 1:l complex, the enthalpy change for the binding ~
of the second a-CD molecule to the 1:1 complex is calculated to be -8.8 kcal/mol. This large exothermic value suggests that the binding of the second CYCDinvolves a significant improvement in hydrogen bonding or van der Waals interactions. A possible explanation for the cooperative and exothermic binding of the second CY-CDis that hydrogen bonding occurs between the secondary hydroxyl groups of the two a-CD molecules in the ternary complex, as depicted by Figure 7. For adamantanecarboxylate complexes with 6-CD and y C D the data in Figure 6 indicate that the complexes are 1:1, at least in the concentration range studied (Breslow et a1.22have evidence for 1:2 /3-CD-adamantanecarboxylate complex formation at high concentrations of the latter species). This being the case the observed ApK,s of 1.3 and 0.8 units, for 0-CD and y C D , respectively, can be attributable to a preferential interaction with the protonated carboxylic acid form of the ligand. As mentioned above, this may be due to a deeper penetration of the neutral ligand into the cavities or to a carboxylic acid group first binding mode. It is particularly striking that neutral adamantanecarboxylic acid binds to p-CD with a very large association constant. From Scheme I, a K I .of 2.0 X lo4 M-I determined microcalorimetrically, and a ApK, of 1.3 units, one can calculate the association constant of adamantanecarboxylic acid to p-CD to be 4.0 X los M-I. The latter value is approximately equal to the value determined by a direct microcalorimetric binding study at pH 4.08 (after correcting the apparent binding constant at this pH to obtain K l ) . The stronger binding of p-CD to the neutral ligand is due entirely to a more negative enthalpy change for the association. This improved AHo could be consistent with either of the abovementioned possibilities that (a) deeper penetration and hydrogen bonding with the carboxylic acid group occurs for the neutral ligand or that (b) carboxylic acid first binding occurs.
Acknowledgment. This work was supported by research grants from the National Science Foundation (PCM 82-06073), and the donors of the Petroleum Research Fund, administered by the American Chemical Society, and by a fellowship (for K.B.) from the Swedish Natural Science Council. Registry No. a-CD, 10016-20-3; @-CD,7585-39-9; y-CD, 1746586-0; adamantanecarboxylate,6501 2-54-6; adamantanecarboxylic acid, 828-51-3.
~~~~~~~
(21) R. J. Bergeron, M. A. Channing, and K. A. McGovern, J. Am. Chem. SOC.,100, 2878-2883 (1978).
(22) R. Breslow, M. F. Czarniecki, J. Emert, and H. Hamaguchi, J . Am. Chem. Soc., 102, 762-770 (1980).
Photoinduced Electron-Transfer Reactions between Optical Isomers Youkoh Kaizu, Takahiro Mori, and Hiroshi Kobayashi* Department of Chemistry, Tokyo Institute of Technology. 0-okayama, Meguro- ku, Tokyo 152, Japan (Received: June 5, 1984)
The luminescence quenching of (-)D-[Ru(bpy)s]2+(bpy: 2,2'-bipyridine) by (+)D-, rac-, and (-)D-[Co(edta)]- (edta: ethylene diaminetetraacetate) was studied in aqueous and also aqueous methanol media. The quenching rate constant k, in mixtures of methanol and water of (-)D-[Ru(bpy)3]2+found with (-)D-[Co(edta)]- was greater than the k, found with (+)D-[Co(edta)]and their mean value was obtained with the racemic mixture of [Co(edta)]-. A difference in the quenching constants observed between the optical isomers reveals that the electron transfer to the quencher in the luminescent excited state of [R~(bpy)~]*' can occur only in a contact ion pair but not in the solvent-separated ion pair.
Excited [ R ~ ( b p y ) ~(]*~[+R ~ ( b p y ) ~ ] ~in' ) solution is quenched by three possible processes: *[Ru(bpy)3I2++ Q == [Ru(bpy)312+ + Q*
(1)
0022-3654/85/2089-0332$0l.50/0
where Q denotes the quencher. In process 1, the excitation energy 0 1985 American Chemical Society
Photoinduced Electron-Transfer Reactions
The Journal of Physical Chemistry, Vol. 89, No. 2, 1985 333
p ~ r t e d . ~ JThe ~ C D measurements confirmed that no actual racemization was induced in both [ R u ( b p ~ ) ~and ] ~ +[Co(edta)]upon irradiation as far as the intensity, wavelength, and duration used in the present work were concerned. However, irradiations of C W laser beam give rise to fast racemization of the complexes.Il The absorption spectra were measured on a Hitachi spectro*[Ru(bpy)3I2+ + e- + [Ru(bpy)31+ photometer Model 220A and the CD spectra were taken on a Jasco spectropolarimeter Model J-5OOC. The excitation and emission Ee = 0.84 V vs. NHEI spectra were observed on a Hitachi fluorescence spectrophotometer [Ru(bpy)J3+ e*[Ru(bpy)J2+ Model MPF-2A with a Hamamatsu Photonics photomultiplier ~e = -0.84 v VS. NHEZ R928. The temperature of the cell compartment was kept constant by circulating water from a Haake thermostat Model FK. The The reduction potential [Co"'(edta)]-/ [Co"(edta)12- has been solution in a rectangular cell of 1-cm path was bubbled with determined as 0.37 V vs. NHE.3 Thus, the quenching of oxygen-free nitrogen gas prior to the measurements. The emission * [ R ~ ( b p y ) ~ is ] ~ascribed + to the electron transfer from *[Ruof [ R ~ ( b p y ) ~was ] ~ +detected by using the excitation wavelength (bpy)J2+ to [Co(edta)]-. The rate of the quenching process is 440 nm, where [Co(edta)]- showed no actual absorption, and the determined by the diffusion-controlledencounters of * [ R ~ ( b p y ) ~ ] ~ + scattered light of shorter wavelength was cut by using a Toshiba and [Co(edta)]- in solution. For the dynamic quenching, a glass filter L-39. To avoid the reabsorption effect due to [CoStern-Volmer plot of luminescence yields conforms to that ob(edta)]-, the quantum efficiency of luminescence of [Ru(bpy)JZ+ tained from lifetime measurements. It may be possible that a was measured at 670-690 nm. nonluminescent ion pair is formed between [Ru(bpy)J2+ and its The luminescence decay was determined with excitation by a counteranion [Co(edta)]- in solution. In such a case, however, second-harmonic (347 nm) of the giant pulse generated from an the slope of a Stern-Volmer plot of luminescence yields is not N E C SLG ruby laser type 2008 with a kryptocyanine Q-switch. coincident with that of decay lifetimes and the lifetime does not The second-harmonic pulse was obtained by means of an ADP vary with the concentration of quencher in solution. (ammonium dihydrogen phosphate) crystal doubler. A layer of In the present work, the luminescence quenching of (-)Daqueous solution of C u S 0 4 and a Toshiba glass filter DUV 1A [Ru(bpy)J2+ by (+)D-, rac-, and (-)D-[C~(edta)]-,all of which were used to cut the principal harmonic component of excitation should have identical redox and diffusional properties, was studied light. The luminescence from the sample solution was detected in aqueous and also aqueous methanol media. A difference in by a Hamamatsu Photonics photomultiplier R928 placed in a the rates of oxidative quenching of (-)D-* [Ru(bpy)J2+ by the direction perpendicular to the propagation of the excitation pulse. optical isomers of the quencher was detected. The scattered excitation light was reduced by a Toshiba glass filter Y-50and an Oriel glass filter G722-5400. Experimental Section To improve the linearity of the response, a potential of 500-800 V was applied to the photomultiplier by a John Fluke high-voltage [Ru(bpy),]Brz was obtained from the corresponding chloride supply Model 412B. The photomultiplier output was detected which was prepared by the literature method: The chloride was by using a termination of 50 D after amplification by means of dispersed and vigorously stirred in a saturated aqueous solution an Avantek module amplifier SAG3060B. The signal was disof KBr and recrystallized 3 times from a mixture of acetone and played on a Tektronix oscilloscope Model 475A (250 MHz) and water. (-)D-[Ru(bpy)3](clo4)2was prepared as follows: Krecorded on a Polaroid camera. The luminescence decay curve [RuCl5HZ0]and sodium (+),-tartrate were reacted in aqueous thus obtained was digitized on a Hewlett-Packard computer Model media and then the product was heated with 2,2'-bipyridine. From 9825A with its graphic plotter Model 9872A. The time constant the resultant solution, (-)D-[R~(bpy)3](C104)2was precipitated of the system used in the present luminescence decay measureby addition of NaC104. (-)D-[Ru(bpy)3]Br2 (At(282 nm) = +152 ments was less than 50 ns. M-' cm-') was obtained by treating the perchlorate salt with a The rate constant of first-order decay was evaluated from the saturated solution of KBr and recrystallized 3 times from aqueous observed decay curve by means of the square-intensity-weighted acetone mediae5 least-squares method. The rate constants were determined at a K[Co(edta)] was prepared from cobalt chloride and sodium variety of temperatures. The temperature of the sample comethylenediaminetetraacetate and purified as described in the partment was controlled by circulating water from a Haake FK literature.6 The mixture obtained by rection of (-)D-[C~thermostat. (N02)z(en)z]Br(en: ethylenediamine) and AgCl was filtered. By addition of K[Co(edta)] to the filtrate, (-)D-[Co(NOz)2Results and Discussion (en)2](-)D-[Co(edta)] was precipitated. K(-)D-[Co(edta)] (At(585 Mixed solutions of (-)D-[Ru(bpy)3]2+and (+)D-[Co(edta)]-, = -1.73 M-l cm-I) was obtained from the precipitate by nm) (-)D-[Co(edta)]- or their racemic mixture display a superposed dispersing and stirring it in a concentrated ethanol solution of KBr absorption spectrum of the components. The intensity of the and recrystallized from aqueous acetone media.7 K(+)D-[Columinescence of [Ru(bpy)J2+, however, decreased and its decay (edta)] (Ae(585 nm) = +1.73 M-I cm-I) was precipitated by lifetime also decreased with increase in the concentration of addition of KBr to the filtrate obtained when the precipitate of [Co(edta)]-. The plots of ro/r and +o/+ against the concentrations (-)D- [Co(N0,) 2( en)2](-) D- [Co(edta)] was collected by filtration .7 of [Co(edta)]- give straight lines with an intercept of unity, where The compounds used in the present work were identified by r , ro are the lifetimes and 4, +o are the luminescence yields in elemental analysis. The absorption and luminescence spectra of the presence and absence of the quencher [Co(edta)]-, respectively. [ R ~ ( b p y ) ~conformed ]~+ to the spectra in the literature.* The The rate constant of luminescence quenching k, is evaluated from C D spectra of the diastereomers indicated that a higher resolution K S V / Twhere ~ the slope of the straight line is Ksv. The rate of the optical isomers was obtained than those previously reconstant determined by lifetime measurements k , ( ~ was ) in good agreement with that obtained from luminescence yield measurements kq(+). The quenching arises from diffusion-controlled (1) Sutin, N.; Creutz, C. Adu. Chem. Ser. 1978, No. 168, 1. encounters of * [ R ~ ( b p y ) ~ and ] ~ +[Co(edta)]- in aqueous solution, (2) Lin, C.-T.; Bottcher, W.; Chou, M.; Creutz, C.; Sutin, N. J . Am. Chem. SOC.1976, 98, 6536. but is not a static process attributable to the formation of a is transferred to the quencher. On the other hand, reactions 2 and 3 are bimolecular electron-transfer processes in which the excited complex * [ R ~ ( b p y ) ~ is ] ~involved. + The reduction and oxidation potentials of [Ru(bpy)JZ+ are remarkably enhanced in the excited state as follows:
+
(3) Tanaka, N.; Ogino, H. Bull. Chem. SOC.Jpn. 1965, 38, 1054. (4) Burstall, F. H. J . Chem. SOC.1936, 173. Palmer, R. A,; Piper, T. S. Inorg. Chem. 1966, 5 , 864. ( 5 ) Liu, C. F.; Liu, N. C.; Bailar, J. C., Jr. Inorg. Chem. 1964, 3, 1085. (6) Dwyer, F. P.; Gyarfas, E.; Mellor, D. J . Phys. Chem. 1955, 59, 296. (7) Dwyer, F. P.; Garvan, F. L. Inorg. Synth. 1960, 6, 192. (8) Fujita, I.; Kobayashi, H. J . Chem. Phys. 1973, 59, 2902.
~~
~~
~
~~
(9) Mason, S. F.; Peart, 9.J. J. Chem. SOC.,Dalton Trans. 1973, 949. (IO) Ogino, H.; Takahashi, M.; Tanaka, N. Bull. Chem. SOC.Jpn. 1970, 43, 424. (11) Porter, G. 9.; Sparks, R. H. J . Phorochem. 1980, 23, 123. (12) Sutin, N. Acc. Chem. Res. 1982, 15, 275.
334 The Journal of Physical Chemistry, Vol. 89, No. 2, 1985 TABLE i: Rate Constants for Quenching of (-)D-*[Ru(bpy)3p+
ionic strength,
medium H*0
quencher
rac- [Co(edta)]-
M
90% CHjOH 50% CH3OH
0.00 3.93 x 10-2 1.20 x 10-1 0.00 0.00 4.24 X 1.26 X lo-' 0.00 0.00
81% CH3OH
0.00
90% CH,OH
0.00
50% CH30H 81% CH3OH
(+)D-[C~(edta)](-)D- [Co(ed ta) ](+)D-[C~(edta)](-)D- [Co(edta)](+)D-[Co(edta)](-) D- [Co(edta)]-
Kaizu et al.
by [Co(edta)r kq(7)"/(109 M-I S-I) 6.65 3.68 3.03 3.89 8.48 1.74 1.09 12.7 3.50 4.18 8.20 9.14 11.8 14.1
kdb/(109
M-'
S-I)
13.3 7.75 6.47 9.35 17.7 5.30 3.86 24.8 9.35 9.35 17.7 17.7 24.8 24.8
kdc/(109 SKI) 1.08 1.08 1.08 0.46 0.48 0.48 0.48 0.50 0.46 0.46 0.48 0.48 0.50 0.50
Kd/M-I 12.3 7.2 6.0 20.5 36.9 11.1 8.0 49.6 20.5 20.5 36.9 36.9 49.6 49.6
k,/kd 0.50 0.47 0.47 0.42 0.48 0.33 0.28 0.5 1 0.37 0.45 0.46 0.52 0.47 0.57
(ll70') + k,F/(109 s-I) 1.08 0.96 0.96 0.33 0.44 0.24 0.19 0.52 0.27 0.38 0.41 0.52 0.44 0.66
'The values determined by lifetime measurements. bCalculated according to eq 6 and 7 assuming a = 11 A. CCalculatedaccording to eq 8 assuming a = 11 A. d K kd/kd. CEvaluatedfrom kq, kd, and kd according to eq 5. nonluminescent ion pair in solution. Only very small differences were found between the quenching rate constants k, of (-)D[ R ~ ( b p y )2+~ ]obtained with (+)D- [Co(edta)]-, (-)D- [Co(edta)]-, and their racemic mixture present in aqueous media. Since the molecular dimensions of (+)D-[Co(edta)]- and (-)D-[Co(edta)]are exactly equal, no difference is expected for the diffusioncontrolled rate. In mixtures of methanol and water, however, (-)D-[Co(edta)]- was more effective than (+)D-[Co(edta)]- in quenching (-)D-[Ru(bpy)3]2+. The k,(4) conformed again to the kq(T). In 90% C H 3 0 H at 25 OC, the k,(4) found for (-)D[Co(edta)]- ((1.38 f 0.03) X IOLoM-I s-l) was greater than the corresponding value for (+)D-[Co(edta)]- ((1.22 f 0.01) X 1O'O M-' S-I) and the mean of these values (( 1.29 f 0.03) X 1Olo M-' s-I) was obtained with the racemic mixture. The k,(4). of (-)D-*[Ru(bpy)3]2+in the presence of rac-[Co(edta)]- varies with the concentration of C H 3 0 H in aqueous methanol: k, = (0.67 f 0.05) X 1Olo M-' s-I in H20;k, = (0.39 f 0.01) X 1Olo M-I s-l in 50% C H 3 0 H ;and k, = (1.29 f 0.03) X 1Olo M-I s-I in 90% C H 3 0 H . The rate constants may be correlated with the viscosities of media: 0.893 CP(H20), 1.544 CP (50% C H 3 0 H ) , and 0.776 CP (90% C H 3 0 H ) . The rate constants were also measured for a variety of ionic strengths obtained by addition of KBr in 81% C H 3 0 H : k,(4) = (8.40 f 0.10) X IO9 M-'s-l, 1.1 = 0.0 M; k, = (1.92 f 0.12) X lo9 M-I s-l, 1.1 = 0.042 M; k, = (1.04 f 0.09) X IO9 M-l S-I, p = 0.126 M. The encounters of *[Ru(bpy),12+ and [Co(edta)]- in solution are reduced with increase in the ionic strength. The decay lifetime was dependent upon the ionic strength but not on a variety of the salts used for adjustment of the ionic strength. At high ionic strength, no appreciable difference in the k, values could be detected between (+)D-[Co(edta)]- and (-)D-[Co(edta)]-. The quenching of * [ R ~ ( b p y ) ~by ] ~ [Co(edta)]+ in solution is described by the following scheme: *A
+
kd
Q
k ' * l / r o k
'-'
/
*AIQ
+ kel(l/r,,* + k * , )
A + Q
where A and Q denote [ R ~ ( b p y ) ~ ]and ~ ' [Co(edta)]-, respectively, *AIQ is the precursor ion pair, kd and k4 are the rate constants of association and dissociation of the ion pair, respectively, k (= 1/ T ~ and ) k' (= 1/T,,') are the reciprocals of the lifetimes of * [ R ~ ( b p y ) ~in ] ~bulk + solution and in the precursor ion pair, and kelis the rate constant of the electron-transfer process which gives rise to the oxidative deactivation of * [ R ~ ( b p y ) ~ ] ~ + . The ratio of the lifetimes in the absence and presence of the quencher is given by (4) 7 0 / 7 = 1 + kdTo[QI/[l + k-d/(k'+ keJI and thus the quenching rate constant k, is rewritten as kq = + k-d/(k'+ keJI (5)
The rate constant of diffusion-controlled encounters of A and Q is given by the equation12
+
kd = kdo ~ X P [ ~ ~ Z A Z Q I . ~ @Up1")] '/~/(~
(6)
where kdo = [4TN(DA + DQ)a/'OOO1(~O(~)/kT)/[exp(~O(~)/kT) (7) zA and zQ are the charges on A and Q, 1.1 is the ionic strength, a = (e2/2ekT)@,/3 = ( 8 ~ N e ~ / 1 0 0 0 e k T ) 'N / ~is, Avogadro constant, e is the electronic charge, c is the dielectric constant of the medium, k is the Boltzmann constant, DA and DQ are the diffusion constants obtained by the Stokes-Einstein equation, a is the distance between the metal centers at the extreme approach, and U(a)= zAzQe2/ca. On the other hand, the rate constant of dissociation is described as follows:12 k-d = [3(DA + DQ)/n21[(UO(a)/kT)/(1 - exp(-UO(@)/kr)~l (8)
In Table I, the rate constants of quenching of (-)!-*[Ru(bpy)3I2+ by (+)D-, (-)D-, and rac-[Co(edta)]- determined by lifetime measurements for the variety of media are summarized. The table also contains the calculated rate constants of association and dissociation of the precursor ion pair. For aqueous solutions, the rate constant of quenching by rac-[Co(edta)]- was determined as 6.65 X lo9, 3.68 X lo9, and and 1.20 3.03 X lo9 M-l SKIat ionic strengths 0.0, 3.9 X X lo-' M. According to eq 5, the ratio k,/kd is not dependent upon the ionic strength of the medium. Thus, the ionic strength dependence of k, is attributable to that of kd. The observed dependence can be reproduced when @a= 3.5 M-ll2 and thus a = 11 8,. The value a = 11 8, is in good agreement with the sum of the radii of [ R ~ ( b p y ) ~ (6 ] ~&I3 + and [Co(edta)]- (5 This indicates that the electron transfer essential to the quenching process arises only in a contact ion pair. Assuming a = 11 A, the values of kd are evaluated as 1.33 X lolo, 1.75 X IO9, and 6.47 X lo9 M-' s-I at ionic strengths 0.0, 3.9 X and 1.2 X 10-1 M, respectively. The ratio of the observed k, and the calculated kd in a lower ionic strength is equal to 'I2,as shown in Table I. This implies that k4/((l/7{) + kel]is almost equal to unity and thus the rate (l/ro') + kel is comparable to that of dissociation of the precursor ion pair, k4 = 1.08 X lo9 s-l. If l / ~ {is not so different from = 1.7 X lo6 S-I, kel should be predominant (13) Rillema, D. P.; Jones, D.S.; Levy, H.A. J . Chem. SOC.,Chem. Commun. 1979, 849. (14) Okamoto, K.; Tsukihara, T.; Hidaka, J.; Shimura, Y . Chem. Lett. 1973, 145.
J. Phys. Chem. 1985,89, 335-338 TABLE 11: Rate Constants for Quenching of (-)n-*[Ru(bpy)3]zt by ICo(edta)l- in 90%CHIOH as a Function of Temperature
k,(r)/(109 M-'s-' )
temp/OC
(+)D-
[Co(edta)]-
(-)D- [Co(edta)l-
9.7 11.8 14.4 16.7
15 25 35 45
AG' '/(kcal mol-') AH*/(kcal mol-') T U *"/(kcal mol-')
10.3 14.1 16.5 20.5
(+)D-[Co(edta)]-
(-)D-[Co(edta)]-
3.70 0.02 2.7 -1.0
3.59 i 0.05 3.4 -0.2
*
'At 25 'C.
+
335
dependent upon the assumed values of a, DA,and DQ, However, it should be noted that the kd in high ionic strength methanol media is lower than expected for the diffusion-controlled encounters of *[Ru(bpy),12+ and [Co(edta)]- in solution. A difference in the quenching cross section between (+)D- and (-)D-[Co(edta)]- is ascribed to a difference in the rate of electron transfer within the precursor ion pair, although the electrontransfer rates are reduced in methanol media. The A-(-)D-*[ R ~ ( b p y ) ~leads ] ~ + to preference for A-(-)D-[Co(edta)]- in the oxidative deactivation. A rather slow oxidation of rac-[Co(edta)]" by ~ i - ( + ) ~ - [ R u ( b p y ) ~also ] ~ +yields a slight excess of A-(+)D[Co(edta)]- in aqueous s01ution.I~ The rate constants for quenching of (-)D-* [ R u ( b p ~ ) ~ by ]~+ (+)D-[Co(edta)]- and (-)D-[Co(edta)]- in 90% C H 3 0 H were determined at a variety of temperatures from 15 to 45 OC. From the plot of In k,(r)/ T against 1/ T, AH*and AS*were evaluated. In Table 11, the observed rate constants are summarized as a function of temperature, and the thermodynamic parameters AG*, AH*, and AS* are also tabulated. The rate of electron transfer is increased when reorientation of the counterions in the precursor ion pair can yield closer contact of the reactants or provide a more effective path of electron transfer. (-)D-[Co(edta)]- can make closer contact with (-)D-*[R~(bpy)~]*+ than (+)D-[Co(edta)]-. The preference for (-)D-[Co(edta)]- is achieved only for a higher activation enthalpy barrier which is in turn compensated by a sizable increase in activation entropy. This is ascribed to a greater solvent reorientation accompanying the effective electron-transfer process.
in 11701 k,, and as high as lo9 s-l. Typical concentrations used in the present work were M for [ R ~ ( b p y ) ~ and ] ~ +(1-5) X lo4 M for [Co(edta)]-. For a value of the equilibrium constant of ion-pair formation K = 10, which is the case in aqueous solutions of lower ionic strengths, the fraction of [ R ~ ( b p y ) ~ forming ]~+ an ion pair with [Co(edta)]- is less than 0.2%. The fraction increases up to 2%if K = 40,which is obtained for 81% C H 3 0 H of lower ionic strength. It is inferred that only when the concentration of the nonluminescent ion pair is low, do the luminescence yield measurements give an intrinsic rate constant close to the value obtained from lifetime measurements. As shown in Table I, the quenching rate is larger in the media, in which the ion-pair formation is accelerated by an enhancement in the rate of diffusion-controlled association kd. The value of kd calAcknowledgment. We are grateful to Professor H. Ogino, culated for the diffusion-controlled encounters of * [ R ~ ( b p y ) ~ ] * + Tohoku University, for informative discussions on the reduction and [Co(edta)]-in 81% CH30H of = 0.126 M is 3.86 X lo9 potential of [Co(edta)]-. M-' s-l as shown in Table I. Assuming a = 11 A, the kd can be Registry No. (-)-[R~(bpy)~]~+, 52389-25-0;(+)-[Co(edta)]-, reevaluated as 2.56 X lo9 M-'s-l for the diffusion-controlled 18661-70-6; (-)-[Co(edta)]-, 27829-16-9. encounters of ( * [ R ~ ( b p y ) ~ ] B r )and + [Co(edta)]- in the same medium. The value yields K = kdJk4 = 2.4 M-I, k,/kd = 0.43, (15)Geselowitz, D. A,; Taube, H. J . Am. G e m . SOC.1980,102, 4525. and thus 11701 k,, = 0.36 X lo9 s-I. The results are not seriously
+
Ionizatlon of the Hydroxycyclohexadienyl Radical in Concentrated KOH: A Measure of the Actlvity of OH- in Highly Basic Media' Hitoshi Taniguchi and Robert H. Schuler* Radiation Laboratory and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556, and Department of Chemistry, Yamaguchi University, Yamaguchi 753, Japan (Received: June 8, 1984)
The ESR spectrum of the hydroxycyclohexadienyl radical has been examined in aqueous solutions containing up to 10 M KOH. In strongly basic solution the radical exists, as a result of ionization of the OH proton, predominantly in an anionic form where the hyperfine constant of the c6 proton (a(H,))is -4 G greater than that of the neutral radical. The other ESR parameters are virtually unaffected by ionization. At pH values above 12, equilibration of the neutral and anionic forms is rapid with respect to the frequency of the hyperfine interaction so that the observed hyperfine constant represents the weighted average of the two forms and directly provides information on the equilibrium composition. From the dependence observed up to 1 M OH- the pKb for deprotonation of this radical by base is found to be -0.58 i 0.05. At higher base concentrations a(H6),because it can be accurately measured even in the presence of reaction products, provides a good measure of the apparent activity of OH-. While the present measurements indicate that the activity coefficient of OH- in strong base considerably exceeds one, the values obtained are substantially less than those given by the spectrophotometricmeasurements of Yagi1 using indoles as indicators. Implications of the differences noted on the significances of any given basicity scale are discussed.
@-Hydroxyradicals ionize considerably more readily than do aliphatic alcohols and, in general, undergo acid dissociation of the O H proton in the pH range of 13-16.24 Because acid-base equilibria of these radicals can be readily examined by in situ
* Address correspondence to this author at the University of Notre Dame. 0022-365418512089-0335$01SO10
radiolysis-ESR techniques these radicals are potentially very valuable indicators for determining the activity of OH- in con(1) The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2598 from the Notre Dame Radiation Laboratory.
0 1985 American Chemical Society
