1I40
J . Phys. Chem. 1988, 92, 1140-1 142
The kinetics of the oxidation of Mn2+ ions by Br03- ions is affected by silver ions only in that the induction period is prolonged (Table 111). We believe that our results can be rationalized within the framework of the FKN mechanism with silver bromide precipitation. The increase in frequency of silver-ion-perturbed BZ systems is now well-documented and according to RuofrS6 can be simulated with an Oregonator model using an additional bromide-removing step, the silver bromide precipitation term. The increase in the induction period length in the presence of silver ions can also be rationalized by this additional bromide-ion-removing term, because Br- is necessary to form H B r 0 2 which triggers the autocatalytic oxidation of the metal ion by Br03-. The results of Table I1 can be explained according to the Ruofrs6 excitability model because the system gets more and more “exhausted” as the oscillations proceed and finally reaches an excitable but nonoscillatory steady state where the system is in a reduced state. The oscillations can then be induced as this
quiescent excitable state is treated with excess silver ions (Table 11). The observation that the oscillating domain decreases with added silver ions is also consistent with the effect of silver ions as a bromide-ion-removing reagent. We get a forced oscillator with an increased frequency. If the number of oscillations is approximately constant, the oscillating domain decreases while the frequency increases correspondingly. We have shown that silver-ion-induced oscillations of the BZ type are homogeneous in nature. The presence of AgBr precipitate in the solution makes better regularity and reproducibility of oscillations, since the different volumes inside the cuvette cannot as easily differ in phase than in its absence. Acknowledgment. We are grateful to Prof. Peter Ruoff for his valuable comment to this work. Registry No. Ag*, 14701-21-4;Ce, 7440-45-1; Mn, 7439-96-5;BrOp, 15541-45-4; ethyl 3-oxobutanoate, 141-97-9.
Blmanes. 27. Rates of Intramolecular Singlet-Singlet and Trlplet-Triplet Energy Transfer wlthln One Molecule, an anfi,syn -Bisblmane Edward M. Kosower* and Rina Giniger Biophysical Organic Chemistry Unit, School of Chemistry, Sackler Faculty of Exact Sciences, Tel- Aviv University, Ramat-Aviv, Tel- Aviv 69778, Israel (Received: April 28, 1987)
-
-
Intramolecular singlet-singlet (anti syn) and triplet-triplet (also anti syn) energy transfer has been observed in the bisbimane 3, a syn-anti-bisbimane, in which a dimethylene bridge connects the photophysically active moieties. The rate constants for both types of energy transfer have been estimated on the basis of radiative rate constants and quantum yields of fluorescenceand phosphorescence of syn-1 and anti-2 bimanes and rate constants of intersystem crossing and photochemical reaction in an anti-bimane. The efficient anti syn energy transfer may involve exchange augmented by dipole-dipole transfer.
-
The photophysical and photochemical properties of syn- and anti-bimanes have been extensively studied.’-* We have now found that both singletsinglet and triplet-triplet energy-transfer rate constants can be determined in a molecule containing synand anti-bimane moieties connected by a short chemical link. Singlet-Singlet Transfer. The fluorescence spectra of the separate molecular fragments are compared to that of the composite molecule in Figure 1. The syn-(methy1,methyl)bimane [syn-(CH3,CH3)B](1) in ethanol emits at 440 nm (& = 0.53), excitation peaking at 378 nm and absorption at 369 nm. The anti-(methy1,methyl)bimane [anti-(CH3,CH3)B](2) emits at 450 nm with a very low quantum yield ($F = 0.003), excitation peaking at 341 nm and absorption a t 320 nm. With the two moieties
connected by a dimethylene chain, in the compound anti(CH3,CH3)(CH3,CH2)B-syn-(CH3,CH2)(CH3,CH3)B (“anti,syn-bisbimane”) (3), the fluorescence coincides with that of the syn-bimane (A, = 440 nm). The excitation spectrum is quite different, with a strong contribution from the anti-bimane absorption.
(1) Huppert, D.; Dodiuk, H.; Kanety, H.; Kosower, E. M. Chem. Phys. Letr. 1979, 65, 164-168. (2) Kosower, E. M.; Kanety, H.; Dodiuk, H. J . Photochem. 1983, 21, 171-1 . . 82.
(3) Kosower, E. M.; Kanety, H.; Dodiuk, H.; Hermolin, J. J . Phys. Chem. 1982, 86, 1270.
(4) Baba, M.; Hirota, N.; Kosower, E. M. J . Phys. Chem. 1981, 85, 1469-74. ( 5 ) Kanety, H.; Dodiuk, H.; Kosower, E. M. J . Org. Chem. 1982, 47, 207-13. (6) Huppert, D.; Kanety, H.; Pines, E.; Kosower, E. M. J . Phys. Chem. 1981,85, 3387-91. (7) Giniger, R.; Huppert, D.; Kosower, E. M. Chem. Phys. Lett. 1985, 118, 240-45. (8) Kosower, E. M.; Giniger, R.; Radkowsky, A. E.; Hebel, D.; Shusterman,A. J . Phys. Chem. 1986, 90, 5552-7.
A--S
The existence of bent and quasi-planar conformers of bimanes in ground and excited states has been established on the basis of
0022-3654/88/2092- 1 140$01.50/0 0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 5, 1988 1141
Energy Transfer within a Bimane Molecule
I
L
C
3
EXCITATION
L r,
e
.-n
-* L
a
t ln z
W
I-
300 350
f
400
Xlnml
290
330
370
410
450
490
530
Xlnml
Figure 1. Absorption, fluorescence, and excitation spectra of anti-
(CH3,CH3)(CH3,CH2)B-syn-(CH3,CH2)(CH3,CH3)B (antipyn-bisbimane) (3), syn-(CH3,CH3)B(syn-bimane) (l),and anti-(CH,,CH,)B (anti-bimane) (2). The difference between the excitation and absorption maxima is due to the competition of internal conversion with other processes leading to emission. photophysicals and crystal log rap hi^^^'^ results. The initially formed bent8 SI state of the anti-bimane moiety transfers energy to syn-bimane. The known' rate constant (10" s-l) for intersystem crossing in an anti-bimane is used as a clock to obtain the singlet-singlet energy-transfer rate constant (kET). Relaxation of the bent to the quasi-planar conformer (krel 10" s-l)' competes with energy transfer and intersystem crossing. On the basis of the similarities in absorption and emission between 3 and the reference bimanes, 1 and 2, we infer that all nonradiative processes in 3 excluding energy transfer (symbolized by Ck,,) are the same as those measured for 2. The energy-transfer rate constant, kET(,), can now be evaluated from the phosphorescence quantum yields (eq 1-3). In pure anti molecules, the 4pin a glass2 is 0.3, whereas in the A-S molecule 3, $p is 0.1. (The nature of the A-S phosphorescence is treated below).
k,(ss)
= 6.7ki, = 6.7 X 10"
S-'
(3)
+
Given that Ck,, kET(SS)is ~ 1 0 s-l, ' ~the fluorescence quantum yield of the bent conformer of the anti portion of 3 must be lower than the emission has not been observed. The emission maximum should be between 360 and 400 nm on the basis of a "mirror" relationship with the absorption of the donor. The spectral overlap of this putative emission with the absorption of the syn moiety is high enough to make one consider a dipole-dipole mechanism for energy tranfer. According to Forster theory," an energy-transfer rate constant of 6.7 X 10" s-l corresponds to an average distance (R) of 2.3 A between donor and acceptor species. To consider the additional contribution of exchange, we infer, on steric grounds, an s-trans arrangement for bimanes attached to the CH2CH2. The donoracceptor separation would then be 7-8 A, a distance which suggests only a small direct overlap between the two ring systems. This small overlap may not be sufficient to explain the high energy-transfer rate by an exchange mechanism.'* (The energy-transfer rate is so high that little rotational equilibration in the excited state is expected.) However, the a*-orbital of the CH2CH2bond of the connecting link is well disposed for overlap with the SI states of both rings, in either bent or planar forms; SI-SItransfer may take place via (9) Goldberg, I.; Bernstein, J.; Kosower, E. M.; Goldstein, E.; Pazhenchevsky, B. J. Heterocycl. Chem. 1983, 20, 903-12. (10) Goldberg, I.; Bernstein, J.; Kosower, E. M. Acta. Crystallogr.Sect.
E. 1982,838, 1990-2000. (1 1) FBrster, T. Discuss. Faraday SOC.1959, 27, 7-17. (12) Dexter, D. L. J . Chem. Phys. 1953, 21, 836-850.
Figure 2. Phosphorescence and excitation spectra of anri-(CH,,CH,)(CH3,CH2)B-syn-(CH3,CH2)(CH,,CH,)B (anti,syn-bimane) (3), syn(CH,,CH,)B (syn-bimane) (l),and anti-(CH,,CH,)B (anti-bimane) (2) in ethanol glass at 77 K.
a mechanism involving the a*-bond (superexchange mechanism). The mechanism of energy transfer does not involve electron transfer since varying the solvent polarity has only modest effects on the shape of the excitation spectrum, and therefore on the efficiency of the SI-SI transfer. The pure radiative lifetime for the syn,anti-bisbimane is about what would be expected for a syn-bimane, showing that there is no particular electronic effect arising from the incorporation of both moieties into a single molecule. Triplet-Triplet (Tl-Tl) Transfer. The phosphorescence spectra of the separate molecular fragments are compared to that of the composite molecule in Figure 2. The phosphorescence spectrum of syn-(methy1,methyl)bimane [syn-(CH3,CH3)B](1) in ethanol glass has a maximum at 530 nm (+p = 0.004) with an excitation maximum at 370 nm. The phosphorescence of anti-(methyl,methy1)bimane [anti-(CH3,CH3)B](2) peaks at 500 nm ($p = 0.30) with an excitation maximum at 322 nm. In the composite molecule, anti-(CH3,CH3)(CH3,CH2)B-syn-(CH3,CH2)(CH3,CH3)B ("syn,anti-bisbimane") (3), the phosphorescence is quite similar to that of the syn-bimane, with a maximum at 520 nm, the excitation spectrum peaking at 322 nm like the antibimane absorption (& = 320 nm). The $p (=0.1) for absorption by the anti-bimane portion of the 3 is corrected for the absorption of the syn portion. The results demonstrate that energy transfer from 3 ~ ~ * anti-bimane to 3 ~ ~ * - s y n - b i m a nhas e occurred. The phosphorescence maxima for syn- and anti-bimanes show that the syn triplet level lies below the anti triplet level.2 The anti triplet rearranges to a lactone5 with an estimated rate constant of 3 X lo5 s-l. The rate of formation of the lactone syn-bimane (L-S, 4) from 3 is about one-half of the corresponding rate for the anti-bimane 2. The yield of triplet in the molecule 3 is also cH3d0
CH3
kH3
4 -
loclone
- s y n - I CH3 , C H 3 )(CH j , C H 2 )B L
--s
roughly one-half of that found for the anti-bimane 2, based on the phosphorescence quantum yields under conditions in which lactone formation does not occur. Since triplet-triplet energy transfer does not complete with lactone formation, we conclude that is 1104s-I. The quantity kET(m) must be considerably greater than the phosphorescence radiative rate for the anti-bimane, since the phosphorescence of 3 appears to be purely from the syn moiety, i.e., ker(n) L lo2 s-l. Triplet-triplet energy
1142
J . Phys. Chem. 1988, 92, 1142-1 151
6-35 kLs=3 1 0 ~ s e c - I
I I
I
A- S
onti-syn
b'mone 13)
Figure 3. Simplified kinetic scheme for the photophysical behavior of the anti-syn-bis-bimane(3). The symbols are defined as follows: k,, radiative rate constant: k,,, intersystem crossing rate constant; x,,, energytransfer rate constant [ S S , SI SI,TT, T, TI];A-S, anii-syn-bisbimane (3); k,, rate constant for lactone formation of L-S, syn-bimane lactone (4). The radiative rate constant for A-'Sb* was measured in dioxane (ref 8) for 1.
-
-
transfer is almost certainly by exchange over the estimated separation of 7-8 A. The fact that the phosphorescence excitation maximum for 3 is at shorter wavelengths (322 nm) that the fluorescence excitation maximum for 3 (341 nm) may indicate that higher vibrational levels of "anti-excited" 3 ('A*-S) in the glass contribute proportionally more to the phosphorescence. The phosphorescence lifetimes for 1 and 2 have been reported. The phosphorescence lifetime for 3 (syn phosphorescence) is ca. 1 s, consistent with emission from the 3aa*-syn moiety. A kinetic scheme (Figure 3) describes the photophysical and photochemical behavior of 3, the syn,anti-bisbimane. The assumption is made that the incorporation of the syn- and anti-
bimane moieties into 3 has no effect on the photophysical rate constants, an idea supported by the fact that the sum of the absorption spectra of 1 and 2 is similar to that of 3. Conclusions. It is unusual for both SI-SI and TI-TI intramolecular energy transfer to be measurable in the same molecule and to involve the same donoracceptor couples. The only previous report to our knowledge involves benzene donors and 1,2-diketone acceptors.13 syn- and anti-Bimanes, with well-separated absorption, fluorescence, and phosphorescence emission bands, are quite convenient for quantitative photophysical studies.
Experimental Section The instruments used were a Cary Model 17 spectrophotometer (UV, visible absorption) and a Perkin Elmer Hitachi MP-4 spectrofluorimeter with a corrected spectra unit (fluorescence and excitation). Quantum yields corrected for refractive index are based on the ratio of the areas under emission curves to that of quinine sulfate in 0.1 N H2S04,the latter taken as 0.5 1. Emission lifetimes were obtained by the single photon counting technique as described elsewhere.* Clear glasses are prepared by cooling an ethanol or EPA solution of bimane in round tubes to 77 K with liquid nitrogen in a Dewar having an optically clear finger. The solvents used (with no further purification) were ethanol (Spectro, Fluka) and EPA (diethyl ether (Spectro, Merck):pentane (Spectro, Merck):ethanol, 5:5:2). The synthesis of the anti,syn-bisbimane (3) has been reported.14 Solutions were prepared by adding very small amounts of the poorly soluble compound to a volume of ethanol or another solvent and allowing the solution stand for a day. All measurements were made at very low concentrations (10-5-10-7 M) to minimize intermolecular energy transfer. Registry No. 1, 68654-22-8: 2, 68654-23-9; 3, 74235-70-4. (13)
Speiser, S.;Hassoon, S.; Rubin, M. B. J . Phys. Chem.
1986, 90,
5085-9. (14) Kosower, E. M.: Pazhenchevsky, B. J . Am. Chem. SOC.1980, 102, 4983-4993.
Kinetics and Mechanism of the Solvent Extraction of Copper W. John Albery* and Riaz A. Choudheryt Department of Chemistry, Imperial College, London SW7 2AY. England (Received: April 17, 1987)
The kinetics and mechanism of the solvent extraction of copper, using the oxime ligand Acorga P50, have been studied with the rotating diffusion cell in both the extraction and stripping directions. The reaction is shown to take place on the liquid/liquid interface between the aqueous and heptane phases. A kinetic scheme involving the sequential addition of two oxime ligands is proposed, and all the rate constants for the scheme have been measured. The data cannot be explained by models in which the reaction takes place in the aqueous phase. Sequences of free energy profiles for the extraction and stripping reactions under industrial conditions are presented.
Introduction Solvent extraction is a common industrial process for recovering and producing metals such as copper from aqueous leach liquors. The current production of copper by solvent extraction is about 0.3 million tons a year. Most common processes involve an oxime ligand dissolved in an organic phase which then is mixed with an aqueous solution of Cu2+leached from the copptkcontaining ore. The ligand extracts Cu2+into the organic phase, releasing H+ into the aqueous phase. In a subsequent process the reaction is reversed; the organic phase is treated with strong acid and the Cu2+ is stripped into the aqueous acid: 'Present address: IC1 Paints Division, Wexham Rd, Slough, Berkshire, England.
0022-3654/88/2092-1142$01.50/0
In our case the ligand HL is Acorga P50 and the organic solvent is n-heptane:
The process separates copper from other contaminating ions and can also be used to produce a higher concentration of Cu2+prior to electrolysis. 0 1988 American Chemical Society
