1640
J. Phys. Chem. 1981, 85, 1640-1643
was somewhat larger than in the emission experiments so that no information about the fast part is available from the bleaching experiments. However, the time profile of the bleaching agreed with the above values of kl and k2. The enhanced triplet-triplet annihilation of 3CT Ru(bpy)32+resulted with a fast decay to the ground state. An attempt to make use of the combined energy from the triplets by the addition of Zn2+or Mn2+gave no effect of these ions on the emission at 600 nm, and hence neither of these ions appear to react in the system.17 These ions (16) U. Lachish, P. Infelta, and M. Griitzel, to be submitted for publication.
cannot be reduced by one 3CT Ru(bpy)32+because such a reduction requires a higher energy than is available. Our experiments indicate that that the triplet-triplet reaction is not a useful way for the accumulation of energy in our system, even when all the potential reactants are highly concentrated by the polyelectrolyte.
Acknowledgment. The authors are indebted to M. S. Matheson, M. Ottolenghi, and T. Greenvald for helpful discussions. This research was supported by the American-Israeli Binational Science Foundation. (17) S. Kelder and J. Rabani, unpublished data,
Vibrational Structures in Circular Dichroism of Monosubstituted Benzenes Included in ,8-Cyclodextrin Hiroyuki Yamaguchi" and Suemi Abe Department of Chemistry, Facuhy of Science, Kumamoto University, Kumamoto 860, Japan (Received: September 22, 1980; In Final Form: February 5, 198 1)
The circular dichroism (CD) spectra of 8-cyclodextrin (0-CyD) complexes with fluorobenzene, bromobenzene, toluene, and benzonitrile were measured. By using vibrational analysis of the absorption spectra in the gaseous state, we analyzed the vibrational structure of the CD spectra. From the signs of the CD spectra, the a1 and b2 symmetries in the vibronic state were assigned to the vibrational modes near 960 and 520 cm-l, respectively. Because of the low resolution of our spectrometer, the a2 and bl vibrations could not be resolved in the CD spectra.
Introduction It is well-known that the cyclodextrins (CyDs) include aromatic compounds, alkyl halides, etc., as guest molecules in the CyD cavity, resulting in the formation of inclusion The cavity of the CyD ring is hydrophobic in nature and binds a hydrophobic portion of the guest molecule, usually forming a 1:l complex. The inclusion phenomena of CyD have been employed as an approach to model the enzyme, since CyD exhibits steric specificity in a way similar to the formation of enzyme-substrate c~mplexes.~-~ The CyDs are composed of chiral glucose units and Cotton effects are induced in achiral guest molecules upon binding to CyD~.~-llRecently, measurement of the circular dichroism (CD) of the 0-CyD complex has been shown to be a very simple tool for the assignment of absorption spectra.12-14 In a previous paper, one of the (1) W. J. James, D. French, and R. E. Rundle, Acta Crystallogr., 12, 385 (1959). (2) A. Hybl, R. E. Rundle, and D. E. Williams, J.Am. Chem. SOC., 87, 2779 (1965). (3) F. Cramer, W. Saenger, and H-Ch. Spatz, J.Am. Chem. SOC.,89, 14 (19fi91. _. (4) W. Saenger, K. Beyer, and P. C. Manor, Acta Crystallogr., Sect. B, 32, 120 (1976). (5) R. Breslow, J. Doherty, G. Guillot, and C. Lipsy, J. Am. Chem. SOC.,100, 3227 (1978). (6) R. Breslow, M. Hammond, and M. Lauer, J.Am. Chem. SOC.,102, 421 (1980). (7) W. Saenger, Angew. Chem., 92, 343 (1980). (8) K. Sensse and F. Cramer, Chem. Ber., 102, 509 (1969). (9) K. Takeo and T. Kuge, Stdrke, 24, 281 (1972). (10) A. L. Thakkar, P. B. Kuhen, J. H. Perrin, and W. L. Wilham, J. Pharm. Sci., 61, 1841 (1972). (11) K. Harata and H. Uedaira, Bull. Chem. SOC.Jpn., 48,375 (1975). \----,-
present authors has shown that the symmetry species of the vibrational modes can be identified from the signs of the CD spectra of the @-CyD complex with chlorobenzene.15 The influence of substitution on the benzenoid transitions has been investigated by the technique of linear dichroism in stretched polyethylene films. Accurate spectroscopic moments derived from linear dichroism measurements have been employed in the CD analysis of some chiral aromatic compounds.16J7 The aim of the present paper is to analyze the vibrational structures in the CD spectra of the p-CyD complexes with fluorobenzene, bromobenzene, toluene, and benzonitrile and to demonstrate again the validity of the experimentally simple @-CyDcomplex method to determine the symmetries of vibrations in the vibronic state. Experimental Section Chemicals. Fluorobenzene, bromobenzene, toluene, benzonitrile, and @-CyD were commercial pwducts. Fluorobenzene, bromobenzene, toluene, and benzd'nitrile were distilled under reduced pressure. @-CyDwas recrystallized five times from water. (12) H. Yamaguchi, N. Ikeda, F. Hirayama, and K. Uekama, Chem. Phys. Lett., 55, 75 (1978). (13) N. Ikeda and H. Yamaguchi, Chem. Phys. Lett., 56, 167 (1978). (14) H. Yamaguchi, K. Ninomiya, and M. Ogata, Chem. Phys. Lett., 76, 593 (1980). (15) H. Yamaguchi, N. Ikeda, K. Uekama, and F. Hirayama, 2. Phys. Chem. (Frankfurt am Main), 109, 173 (1978). (16) J. Sagiv, Tetrahedron, 33, 2303 (1977). (17) J. Sagiv, Tetrahedron, 33, 2315 (1977).
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The Journal of Physical Chemistry, Vol. 85, No. 72, 1981 1641
CD of Benzenes Included in @-Cyclodextrin
TABLE I: Vibrational Analysis of the CD Spectrum of the p-Cyclodextrin Complex with Fluorobenzene symmetry of vi bronic CDrnax, CD state vibration x 1 0 3 cm" sign assignment, cm-I BZ A1 B* A1 B* A,
BZ
a1 b2 a1 b2 a1 b2 a1
37.58 38.10 38.55 39.06 39.51 40.03 40.48
-
0-0
+ + +
+ 520 0 + 520 + 970 O + 2 X 970 0 + 5 2 0 + 2 x 970 0 + 3 X 970 0
O + 970
-
TABLE 11: Vibrational Analysis of the CD Spectrum of the p-Cyclodextrin Complex with Bromobenzene symmetry of vibronic CDrnax, CD state vibration x 1 0 3 cm-' sign assignment, cm-' B2
240
250
260
270 Wavelength,nm
280
B2
Flgure 1. Circular dichroism (top) and absorption (bottom) spectra of fluorobenzene (5.73 X lo4 M) in the presence of @-cyclodextrin(1.00 X lo-' M) in water. I
A1 AI BZ A1 B2
a1 bZ a1 b2 a1 b2 a1
36.74 37.26 37.70 38.21 38.67 39.18 39.62
-
0-0
+ + +
O + 520
+ 960 t 960 0 + 2 X 960 0 + 5 2 0 4 2 x 960 0
O + 520
O + 3 X 960
-
I
TABLE 111: Vibrational Analysis of the CD Spectrum of the p-Cyclodextrin Complex with Toluene symmetry of vi bronic CD,,, CD state vibration X103 cm-' sign assignment, cm-'
B2 A1 B2
A1 B2
a1 b2 a1 b2 a1
37.31 37.84 38.28 38.81 39.24
-
+ + -
0-0 0 530 0 970 0 530 + 970 0 2 X 970
+ + + +
plots were carried 0 ~ t . lFrom ~ Job's continuous variation plots of observed ellipticities at the 0-0 peaks, it was concluded that the composition ratio of p-CyD to monosubstituted benzenes was 1:l.
250
260
2 70
2 80
Wavelength,nm
Flgure 2. Circular dichroism (top) and absorption (bottom) spectra of bromobenzene (1.01X lo3 M) in the presence of @-cyclcdextrin(1.00 X lo-' M) in water.
Measurements. The absorption spectra were obtained with a Hitachi 200-20 recording spectrophotometer. The CD spectra were recorded on a JASCO J-40A recording spectropolarimeter. In order to obtain an adequate signal-to-noise ratio, we used a computer for multiple scanning and averaging. Each memory unit in the computer stored the CD signal for a spectral band of 0.1 nm. The wavelength of both instruments was calibrated at 253.7 and 296.8 nm by using a Hg lamp. The wavelengths of sharp bands were accurate to within fO.l nm for absorption and f0.2 nm for CD. All measurements were made on deaerated samples a t room temperature. Composition Ratio of p-Cy0 Complexes. According to the method of Shimizu et a1.,18 Job's continuous variation (18) H. Shimizu, A. Kaito, and M. Hatano, Bull. Chem. SOC.Jpn., 52, 2678 (1979).
Results and Discussion The observed CD spectra of the O-CyD complexes with fluorobenzene, bromobenzene, toluene, and benzonitrile are shown in Figures 1-4, respectively, where the absorption spectra are also shown. As these substituted benzenes have Czu symmetry, the electronic transitions in this wavenumber region have been assigned to the lAl 'B2 transition.20 WollmanZ1has analyzed the absorption spectrum of fluorobenzene vapor and has shown that the band at 37 818.8 cm-l is assinged to the 0-0 transition and to this transition the totally symmetric vibration (al) of 966 cm-l and the nontotally symmetric vibration (b2)of 517 cm-l are combined. On the basis of the analysis given by Wollman,21we made a vibrational analysis of the CD spectrum. The results are summarized in Table I. According to the analysis of Asagoe and Ikemoto22for the absorption spectrum of bromobenzene vapor, the band at 36 997 cm-l is ascribed to the 0-0 transition and to this transition the totally symmetric vibration (al) of 962 cm-'
-
(19) P. Job, Ann. Chim. Phys., 9, 113 (1928). (20) R. C. Hirt and J. P. Howe, J. Chem. Phys., 16, 480 (1948). (21) S. H. Wollman, J. Chem. Phys., 14, 123 (1946). (22) K. Asagoe and Y. Ikemoto,Phys. Math. SOC.Jpn., 22,685 (1940).
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Yamaguchi and Abe
The Journal of Physical Chemistry, Vol. 85, No. 12, 1981
TABLE IV: Vibrational Analysis of the CD Spectrum of the p-Cyclodextrin Complex with Benzonitrile symmetry of vibronic CDmaX, CD state vibration x103 cm-' sign assignment, cm:' BZ a1 36.04 - 0-0 bZ 36.56 - 0 520 A, 37.00 - 0 t 960 BZ a1 + 0 t 520 + 960 A1 b2 37.52 - 0 t 2 X 960 B2 a1 37.96 f 0 e 520 + 2 x 960 A, b* 38.48 - 0 t 3 X 960 B2 a1 38.93
un
4
240
250 260 Wavelength, nm
270
3
2
3
280
Figure 3. Circular dichroism (top) and absorption (bottom) spectra of toluene (5.62 X lo4 M) in the presence of P-cycidextrin (1.00 X lo-' M) in water.
I
Y
Figure 5. Coordinate system and structure of 6-cyciodextrin. (Reprinted with permission from ref 11. Copyright 1975 Chemical Society of Japan.)
250
260
270 280 Wavelength, nm
290
Figure 4. circular dichroism (top) and absorption (bottom) spectra of benzonitriie (5.71 X lo-' M) in the presence of P-cyciodextrin (1.00 X lo-' M) in water.
and the nontotally symmetric vibration (b2) of 518 cm-l are coupled. Based on the analysis proposed by Asagoe and Ikemoto,22the vibrational structures of the CD spectra are analyzed. The results are given in Table 11. Ginsburg et aLZ3have analyzed the vibrational structures of the absorption spectrum of toluene vapor and have pointed out that there is a 0-0 transition at 37 477.4 cm-' and the totally (964 cm-l, al) and nontotally (528 cm-l, bz) symmetric vibrations are combined with the 0-0 transition. According to the vibrational analysis of Ginsburg et we made a vibrational analysis of the CD spectra. The results are listed in Table 111. On the basis of the analysis proposed by Hirt and Howe,2O the vibrational structures of the CD spectra of the ~~
~~~
~
(23) N. Ginsburg, W. W. Robertson, and F.A. Matsen, J. Chem. Phys., 14, 511 (1946).
0-CyD complex with benzonitrile are analyzed. The results are shown in Table IV. The frequencies of the totally (al) and nontotally (b,) symmetric vibrations obtained from the vibrational analysis of the CD spectra of the p-CyD complexes with these monosubstituted benzenes agree closely with the frequencies obtained from the vibrational analysis of the electronic absorption spectra of gaseous monosubstituted benzenes. It is very interesting to see that the signs of CD bands of the P-CyD complexes with these substituted benzenes are changed by the symmetries of vibronic states. Harata and Uedairall have theoretically shown that the transition of the guest molecule in the cavity of 6-CyD with transition dipole moment perpendicular to the z axis of p-CyD takes a negative CD value and the transition possessing a transition dipole moment parallel to the z axis takes a positive CD value. The geometrical structure of P-CyD7 excludes the formation of equatorial inclusion complexes for these substituted benzenes. We aSsume that the theoretical conclusions given by Harata and Uedairall can be applied to the vibronic states. As the vibronic state B2 has the transition dipole moment perpendicular to the z axis of 0-CyD, the CD spectra show a negative CD value. On the other hand, as the vibronic state Al has the transition dipole moment parallel to the z axis of P-CyD, the CD spectra indicate a positive CD value. In the case of benzonitrile, the CD curve indicates a negative peak a t 36.56 X lo3 cm-'. It can be considered that the peak a t 36.56 X lo3 cm-l inherently takes a positive CD value. A reduction of the positive CD value of the peak may occur due to the overlapping of two peaks which are superimposed on either side of the peak and possess large negative CD values, so that the peak as a whole shows a negative
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
