J. Phys. Chem. 1981, 85,1643-1645
CD value a t 36.56 X lo3 cm-l. The molecules in the cavity of P-CyD exhibit CD due to the perturbation from the chiral P-CyD. According to S ~ h e l l m a nsuch , ~ ~ CD may arise from either an electric dipole transition that consists of the product of zero-order electric dipole moment and first-order magnetic dipole moment or a magnetic dipole transition that is composed of the product of zero-order magnetic dipole moment and first-order electric dipole moment. We apply the theoretical considerations given by S ~ h e l l m a nto~the ~ vibronic states. As it is obvious from the character table of the C2, point group that both the zero-order electric and zero-order magnetic dipole moments are nonzero, it is expected that the progression based ob the transition to the B1 vibronic state can be observed in the absorption and CD spectra. (24) J. A. Schellman, J. Chern. Phys., 44,55 (1966).
1643
A bl vibration couples with the B2 electronic state and produces the A2 vibronic state. The bl vibration can be observed in the CD spectra because the zero-order magnetic dipole moment is nonzero. According to the theoretical considerations given by Sagiv,17to produce optical rotation, small in-plane magnetic components (A2,B,) and out-of-plane electric components (B,) have to be induced, when a chiral perturbing potential is brought in the neighborhood of the chromophore. Therefore, a2 and bl vibrations should be observed in CD spectra. It is well-known that in the excited state of monosubstituted benzenes the a2 and bl vibrations appear at about 200 cm-1.23125 However, we cannot observe the a2 and bl vibrations in the CD spectra of the p-CyD complexes with these monosubstituted benzenes. ( 2 5 ) S. Takagi, H. Nomori, and M. Hatano, Chern. Lett., 611 (1974).
Anomalously Enhanced Dissociation-Field Effects on an Aqueous Copper( 11) Poly(styrenesu1fonate) Solution Aklhlko Yamagishl Deparfment of Chemistw, Faculty of Science, Hokkaldo University, Sapporo 060, Japan (Received: December 3 1, 1980; In Final Form: March 3, 198 1)
The dissociation-field effect is studied on an aqueous solution of copper poly(styrenesu1fonate)(Cu(PSS)2). At zero field, the dissociation constant, K,, for the equilibrium, Cu2+.2SS- Cu2++ 2SS-, is less than 9 X lo4 M, where SS- denotes the styrenesulfonate residue. Kz increases by a factor of 1.3 X lo3 under a electric field of E = 5-15 kV cm-'. This anomalously enhanced dissociation is beyond the scope of Onsager's theory. The results are interpreted in terms of the collapse of the chelate structure around a bound Cu2+ion due to the stretching out and orientation of a coiled PSS chain.
Many biological processes are sensitive to the voltage across a cell membrane., A specific ion permeability, for example, increases by several orders of magnitude with the millivolt jump experiments across a few-nanometer cell membrane. We show below that even a simple divalent metal-polyelectrolyte system exhibits the on-off type dissociation of a counterion at an electric field strength of 10-20 kV cm-'. The investigated system is an aqueous copper(I1) poly(styrenesulfonate) (CU(PSS)~) solution. A solution is prepared by adding 0.5 equiv of copper(I1) perchlorate salt to potassium poly(styrenesu1fonate) (KPSS). The concentration of a free Cu2+ ion, [Cu2+If,is determined spectrophotometrically by the use of the following complexation equilibrium: Cu2+ + Mx- CuMx+ K1 (1) where Mx- is a monoanionic murexide ion (shown in Figure 2).2 Since neither Mx- nor CuMx+ binds with CU(PSS)~ from dialysis experiments, reaction 1takes place in a bulk medium. An electric field pulse was generated by discharging through a coaxial cable of 157 m at the voltage of 13.5 kV.3 An electric field arises to 27 kV cm-l within 2 ps and decays (1) G . Roy, Prog. Biophys. Mol. Biol., 29, 59 (1975). (2) G. Schwarzenbach and H. Gysling, Helu. Chirn. Acta, 32, 1314
(1949). (3) A. Yamagishi, J.Phys. Chern., 80, 1271 (1976).
with a half-life time of -300 FS. By paralleling a reference cell with higher conductivity the temperature rise of the sample cell was suppressed to below 2 "C. The transof CuMx+) mittance change was followed at 475 nm (A, or 522 nm (Ama of Mx-). We utilized the fact that the complexation rate of (1)is faster than the decay rate of the electric fields4 The incident light is polarized a t an angle of 55O with respect to the electric field direction in order to avoid orientational dichrosim, if p r e ~ e n t . ~ First, an electric field is applied to a solution containing Cu2+ and Mx- only (no KPSS). A small transmittance increase of about 0.5% is observed at 475 nm with a simultaneous decrease of transmittance at 522 nm. Thus a part of CuMx+ dissociates into Cu2+and Mx- under the electric field. The ratio K,(E)/Kl (0) is obtained as 0.991 f 0.005 at E = 27 kV cm-l. The results are understandable within the framework of Onsager's theory on the field-induced dissociation of a simple weak electrolyte.6 When the same solution contains KPSS, most Cu2+ions bind with PSS-. At [KPSS] = 2[Cu(C104),] = 8.20 X 10" (4) G. Geier, Helu. Chirn. Acta, 51, 94 (1968). (5) Actually, the transient transmittance change observed in the present work is completely isotropic, or it does not depend on the polarization direction of the incident light. (6) According to Onsager's theory (L.Onsager, J. Chem. Phys., 2,599 (1934)), K,(E)/K1(0)= 1 - a E with a = 1.8 X lo* V" cm. Thus K1( E ) / K l ( 0= ) 0.95 at E = 27 kV cm-'. The measured Kz(E)/Kz(0)is larger than the theoretical value, partly because the Cu2+-Mx- bond is strengthened by the coordinating interaction.
0022-3654/81/2085-1643$01.25/00 1981 American Chemical Society
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Yamagishi
The Journal of Physical Chemistry, Vol. 85, No. 12, 1981
a
I
I
I
I
I
I
I
li
MX-
v
I
0
I
5
10 15 Electrlc Field Strength/kv 0 - 1
I
I
Figure 2. The dependence of &(.E) on E. Each plot is generated from the single decay curve observed after the initial 27 kV cm-' pulse.
I
400
I
300
I
I
I
I
200
100
Time / psec
I
I,
0
Figure 1. Transient changes of the transmittance at 475 nm (a), the electric birefringence at 600 nm (b), and the electric field strength (c), after electric dlscharge is performed on a solution of Cu(PSS), (8.20 X lo-' M) and NH,+Mx- (4.55 X M).
M and [NH4+Mx-] = 4.55 X M, for example, the M, concentration of a free copper ion is less than 6 X if [Cu2+]fis calculated from [Cu2+]f= [CuMx+l/K1[Mx-l with K1 = 3.75 X lo4 M-l a t 25 "C. Accordingly, if the dissociation constant, Kz, for the dissociation equilibrium of Cu2+from Cu2+2SS-(SS- = styrenesulfonate residue) cu2+2ss- + cu2+ 2ss(2) is defined by
+
K2 = [CU~+]~[~SS-]/[CU~+~SS-](3) K2 is estimated to be less than 9 X lo4 M. The same upper limit is obtained for Ni(PSSI2, Zn(PSS)z,and Cd(PSS)z. The values are lo2 times smaller than the dissociation constant of Cd(PMS)2,which was obtained by means of electromotive force measurements.' The discrepancy of the measurements may lie in the way of discriminating a free ion from a bound one. A negative Mx- ion experiences a repulsive interaction with the negatively charged PSSchain. Therefore, it reacts with such a Cu2+ion since it is located apart from PSS-. On the other hand, the emf method gives the free Cu2+ion concentration averaged over a wider space including the periphery region of a PSSchain. When an electric field is applied to the above solution, about a 6% decrease in transmittance is observed a t 475 nm (Figure la) with the concomitant increase of transmittance at 522 nm. The change corresponds to a 25% conversion of Mx- into CuMx+. Since K1 is little affected by E , the observed shift is caused by the field-assisted dissociation of Cu2+from Cu2+2SS-in eq 2. The relaxation rate of the initial transmittance decrease in Figure l a is 2.6 X lo4s-l, which is close to the relaxation rate of (1)in a bulk medium, 1.2 X lo4 s-l. We conclude therefore that CuMx+ is produced by the equilibration of (1)in the time range of 100 ps, a t which stage the dissociation of a bound Cu2+from Cu2+2SS-is already complete. Once reaction 1 attains equilibrium (or t > t , in Figure (7) S. Oman and D. Dolar, Z. Phys. Chem. (Frankfurt on Main), 56, 13 (1967) (PMS stands for poly(methylstyrenesu1fonate)).
la), the amplitude of the signal gives the free Cu2+concentration according to the equation [Cu2+]f = [CuMx+]/K,(E)[Mx-],where Kl(E) is nearly equal to Kl(0) as stated above. Inserting this [Cu2+Ifinto eq 3, we can determine K2(E)as a function of E. The results are shown in Figure 2. The transient electric birefringence is measured on the same solution at 600 nm, where the solution is transparent. The transient birefringence is caused by the orientation of a PSS- chain under electric field.8 As shown in Figure l b , the orientational process is complete within 10 ps. From the E dependence of the amplitude of the electric birefringence, the degree of orientation is saturated above 10 kV cm-'. Two points are noteworthy from the results in Figure 2. (i) &(E) is virtually saturated above E = 10 kV cm-l. (ii) The saturated &(E) is larger than K2(0)by more than 1.3 X lo3 times. These features are contrasted with the field-induced dissociation of a monovalent counter cation. By measuring the Wien effect on a NaPSS solution, Patterson and Wissbrun obtained that K2(E)/K2(0) for the binding of Na+ with PSS- increases in proportion to E in the range of 0-6 kV ~ m - l . Extrapolating ~ their results to E = 27 kV cm-l, K 2 ( E ) / K 2 ( 0is) estimated to be a t most 3. They concluded that a randomly coiled PSS- chain is stripped of part of the bound Na+ ions by the field, and orients with the rest of Na+ ions as a highly associated electrolyte. In the present case, it is not certain whether the dissociation of a Cu2+ion takes place ahead of or behind the orientation of a polyelectrolyte. Both processes are complete within the time resolution of the present apparatus (10 ps). It seems, however, reasonable to relate the anomalously large field effects of C U ( P S S )with ~ the orientational motion of the polyelectrolyte itself. For randomly coiled polyelectrolytes,to which the present polymer belongs, the field may affect the binding of a counterion in at least the two ways. (i) The electrostatic potential ($) for a counterion decreases as the shape of a polyion changes from random coil to rigid rod. The difference of rc/ between these two conformations is, however, estimated to be less than 1kcal mol-I under the present conditions.1o This value is too small to explain the magnitude of Kz~~~~
~
~
~
(8) K. Kikuchi and K. Yoshioka, J. Phys. Chem., 77, 2101 (1973). (9) K. F. Wissbrun and A. Patterson, Jr., J. Polymn. Sci., 33, 235
(1958). (10) The difference of $ between the coil and rigid-rod conformations is estimated to be -0.034n* kcal mol-' with n* = the effective charge per one polymer chain (F. Oosawa, "Polyelectrolytes", Marcel Dekker, NY, 1971). n* is less than 10 in the present cases.
J. Phys. Chem. 1981, 85, 1645-1653
(E)/K,(O)as observed presently. (ii) The effective concentration of the SS- residue decreases, due to the above conformational change, if a counterion is bound with more than two residues. Theoretically, K2 is expected to be proportional to wf/wb where wf and wb are the numbers of configurations of the free and counterion-bound polymer chains, respectively. At saturated binding, each copper ion is considered to occupy two SS- residues. In case of a rigid-rod configuration, only two adjacent residues are accessible to the same Cu2+ion. Thus only one binding state exists for each configuration of a rigid rod or Wf/ wb = 1. On the other hand, for a randomly coiled configuration, even two distants residues can bind with the same Cu2+ ion, as long as they are located close enough. Accordingly, Wf/ w b is less than one, leading to a decrease of K 2 Although an accurate calculation of this effect is difficult, we are able to estimate a minimum value of wf/wb, if part of a polymer chain is completely collapsed. Under that assumption, wf/wb is approximately equal to ( 2 N / N ! ) ' / 2 )where , N is the number of residues of that part.l' Since all Cu2+ions have their possibilities limited by the previously bound ions, Wf/ wb is much larger than (11) This is the number of combinations of choosing N residues.
pairs out of
1645
this value. However, this effect seems to explain a t least qualitatively the present increase of K2(E).12 The following fact supports the view that chelate formation by polymer residues around a bound counter ion is essential to cause the large field-dissociation effect. Cu(PAN)+, binds Copper(I1) 1-(2-pyridylazo)-2-naphthol, with PSS- with a binding constant larger than lo6 M-l. When an electric field is imposed on the Cu(PAN)+PSSsolution, no isotropic transmittance change is observed at the wavelength where the spectrum of a bound Cu(PAN)+ is different from that of a free Cu(PAN)+. Instead, a large dichroism is observed, implying that the entire bound Cu(PAN)+does move along with the orienting PSS chain. Since PAN- is a terdentate ligand, Cu(PAN)+ may bind with a single SS- residue in forming a square-planar Cu(PAN)*-SS- complex. For a complex of this kind, no increase in dissociation constant is expected due to the collapse of the chelate structure by polyelectrolyte residues around the metal ion.
Acknowledgment. Thanks are due to Professor Junichi Aihara of Shizuoka University for valuable discussions. (12) According t o the derived equation, N is about 15 to lead to the experimental K&?2)/K2.
Optically Detected Time-Resolved Electron Paramagnetic Resonance. Excited States and Radical Ion Kinetics in Pulse Radiolysis of Aromatics in Cyclohexane Solutions Joseph P. Smith and Alexander D. Trifunac" Chemistry Divlslon, Argonne Natlonal Laboratory, Argonne, Illinois 60439 (Received: January 6, 198 1; In Final Form: February 23, 198 1)
Excited-state production and radical ion recombination kinetics in pulse-irradiated solutions of aromatic solutes in cyclohexane were studied by a new method of optical detection of time-resolved electron paramagnetic resonance (EPR) spectra. The EPR spectra of radical ion pairs which recombine to yield excited solute singlets were detected by the resonant decrease in fluorescence intensity occurring after application of a single (30-500 ns) microwave pulse. Fluorescence detection of magnetic resonance (FDMR) was used to obtain EPR spectra of radical ion pairs with lifetimes between 0.02 and 1.4 ~s at solute concentrations of 10-5-10-' M. The decay rate of the FDMR intensity observed for M solutions of 2,5-diphenyloxazole (PPO) was independent of radiation dose over a wide range of low doses but increased at high radiation doses. Analysis of the FDMR decay kinetics showed that the FDMR effect is a resonant modulation of the singlet yield of geminate ion recombination and suggested that a dose-dependent relaxation process contributes to the decay at high radiation doses. A short-lived component with a broad EPR line width was detected at PPO concentrations below M. This finding suggested that the FDMR method could be used to distinguish between excited states produced by solute anion-solute cation and solute anion-solvent cation recombination reactions. The resolution of hyperfiie components in the FDMR spectrum of M biphenyl solutions was strongly dependent on microwave magnetic field and pulse length. The best resolution was obtained with microwave magnetic fields below 0.4 G and pulse widths in excess of 300 ns. The hyperfine structure of the EPR spectra of radical ions involved in excited-state production could be thus obtained by sacrificing some of the time resolution and sensitivity of the FDMR method.
Introduction Irradiation of solutions of aromatic molecules in hydrocarbon solvents results in the production of excited states.' The results of several studies suggest that ion recombination is a major pathway for the production of excited states.24 The following scheme, in which solvent (1) Thomas, J. K. Creat. Detect. Excited State 1971, I B , 481.
and aromatic solute molecules are indicated by S and Ar, respectively, summarizes the ion recombination pathways for excited-state Production. (2) Dainton, F.; Ledger, M. B.; May, R.; Salmon, G. A. J . Phys. Chen.
1973, 77,45.
(3)Jonah, C.D.;Sauer, M. C., Jr.; Cooper, R.; Trifunac, A. D. Chem. Phys. Lett. 1979, 63, 535. (4)Beck, G.;Thomas, J. K. J.Phys. Chem. 1972, 76, 3856.
0022-3654/81/2085-1645$01.25/00 1981 American Chemical Society
