3063
J . Phys. Chem. 1992, 96, 3063-3067
Photoracemization of Optically Active 1,l'-Binaphthyl Derivatives: Light- Initiated Conversion of Cholesteric to Compensated Nematic Liquid Crystals Mingbao Zhang and Gary B. Schuster* Department of Chemistry, Roger Adams Laboratory, University of Illinois, Urbana, Illinois 61 801 (Received: October 7, 1991; In Final Form: December 3, 1991)
The photochemistry and photophysics of a series of optically active 1,l'-binaphthyl derivatives was examined in fluid solution and in liquid crystalline media. The mechanism for the photoracemization of these compounds was examined by time-resolved laser spectroscopy. It was found that photoracemization occurs in the triplet state by atropisomerism. The efficiency of photoracemization is controlled primarily by the efficiency of intersystem crossing and the magnitude of the activation barrier for atropisomerism. The latter value is perturbable by the specific nature of the binaphthyl derivative. Attempts to partially photoresolve these binaphthyl derivatives in fluid solution and in liquid crystals were unsuccessful. However, their photoracemization induces a cholesteric to compensated nematic transition in doped liquid crystalline K- 15.
Introduction More than twenty years have passed since publication of the first descriptions1,2of photochemically induced modifications to liquid crystal phases. In their thorough review of this field in 1990, Kreysig and Stumpe recounted how this research has blossomed and taken on noteworthy technological ~ignificance.~Of special relevance to the present report are the recent discoveries that photochemical reactions initiated by irradiation with linearly polarized light are able to reorient nematic liquid crystal phases and affect their bulk proper tie^.^" The pitch of twisted nematic (cholesteric) liquid crystals is very sensitive to physical and chemical perturbation. It has been known for a long time that the addition of very small quantities of optically active compounds to a nematic liquid crystal will induce a macroscopic helical pitch in the phase.',* This phenomenon was exploited by Solladie and co-workers, who induced a cholesteric to nematic transition in liquid crystals by the photodestruction of an optically active a d d i t i ~ e . ~ It is predicted from theory1° and has been demonstrated experimentally" that irradiation of selected chiral compounds with circularly polarized light will induce an enantiomeric excess and concomitant optical activity. Similarly, irradiation of certain optically active compounds with unpolarized light will convert them to the racemic mixture.l2*I3 The possibility of switching a liquid crystal phase between a twisted and a nontwisted form by alternative irradiation of an additive with unpolarized and circularly polarized light has been analyzed the ore tic all^'^ but never explored experimentally. (1) Haas, W.; Adams, J.; Wysocki, J. Mol.Cryst. Liq. Cryst. 1%9,7, 371. (2) Sackmann, E. J . Am. Chem. SOC.1971, 93, 7088. (3) Kreysig, D.; Stumpe, J. In Selected Topics in Liquid Crystal Research; Koswig, H. D., Ed.; Verlag: Berlin, 1990; p 69. (4) Eich, M.; Wendorff, J. H. In International Symposium on Polymers
and Advanced Technology; Wendorff, J. H.; Leuin, J., Eds.;VCH Publishers: New York, 1988; p 5 0 1 (5) Vinogradov, V.; Khizhnyak, A.; Kutulya, L.; Reznikov, Yu.; Reshetnvak. V. Mol.Crvst. Lip. Crvst. 1990. 192. 273. (6) Gibbons, W. M.;'Shannon, P. J.; Sun, S.-T.; Swetlin, B. J. Nature 1991. 351. 49. (7) Buchingham, A. D.; Ceaser, G.P.;Dunn, M. B. Chem. Phys. Lett. 1969, 3, 540. (8) Solladie, G.; Zimmerman, R. G. Angew. Chem., Znt Ed. Engl. 1984, ~
-7 3~7 A.-. ,A
(9) Mioskowski, C.; Bourguignon, J.; Candu, S.; Solladie, G. Chem. Phys. Lett. 1976, 38, 456. (10) Kuhn, W.; Braun, E. Natunvissenschaften 1929, 17, 227. ( 1 1) Stevenson, K. L.; Verdieck, J. F. J. Am. Chem. SOC.1968, 90, 2974. Stevenson, K. L. J. Am. Chem. SOC.1972, 94,6652. Nelander, B.; Norden, B. Chem. Phys. Let?. 1974, 28, 384. (12) Rau, H. Chem. Rev. 1983.83, 535. (13) Mislow, K.; Gordon, A. J. J . Am. Chem. SOC.1963,85, 3521. Zimmerman, H. E.; Crumrine, D. S. J . Am. Chem. SOC.1972, 94, 498. (14) Shiyanovskii, S. V.;Reznikov, Yu. A. Sou. Phys.-Dokl. (Engl. Transl.) 1985, 30, 1053.
CHART I
1
2n-1 3n-2
4rn.3 5:n=4
We describe herein attempts to use irradiation with unpolarized and circularly polarized light to modify the pitch of twisted nematic liquid crystal phases. As part of this investigation, the photochemistry of a series of chiral 1,l'-binaphthyl derivatives (see Chart I) in isotropic solution and in liquid crystals was explored. The mechanism for photoracemization of these compounds is revealed and an efficient, light-induced conversion of a cholesteric to a nematic liquid crystal is reported.
Results 1. Photochemistry and Photophysics of Binaphthyl Derivatives 1-5. Previous studiesI5 reveal that ethano-bridged binaphthyl 1 is a highly fluorescent compound with a singlet lifetime of 3.7 ns. We find that it is relatively stable photochemically: irradiation (300 nm, Rayonet reactor) of a 4.6 X M, N2-saturated cyclohexane solution of 1 for 2 h results in no more than a 10% decrease in its absorbance at 346 nm. Irradiation of optically active 1 (5% enantiomeric excess) leads to its very inefficient photoracemization. This is not an unexpected result since it was shown previously that the fluorescence of optically active 1 is circularly polarized.15 Thus, its excited singlet state must be at least partially conformationally stable. Also, studies of 1,l'-binaphthyl itself by Irie and co-workersI6 reveal that photoracemization occurs in this case exclusively through the triplet state. Inasmuch as 1 is strongly fluorescent, intersystem crossing is inefficient and the quantum yield for photoracemization (@& will suffer accordingly. Tetreau and co-workers1' examined the photoracemization of bridged binaphthyl derivatives 2-5. They reported that acetal 2 is photoracemized rapidly by irradiation with UV light in oxygen-free solution but that ethers 3-5 racemize much less efficiently (or not at all) under these conditions. Their investigation of the mechanism for photoracemization of 2 by means of sensitization and quenching experiments showed that the reaction (15) Schippers, P. H.; Dekkers, H. P. J. M. Tetrahedron 1982, 38, 2089. (16) Irie, M.; Yoshida, K.; Hayashi, K. J . Phys. Chem. 1977, 81, 969. Yorozu, T.; Yoshida, K.; Hayashi, K.; Irie, M. J . Phys. Chem. 1981,85,459. (17) Tttreau, C.; Lavalette, D.; Cabaret, D.; Geraghty, N.; Welvart, Z. N o w . J . Chim. 1982, 6 , 461.
0022-365419212096-3063%03.00/0 0 1992 American Chemical Society
3064 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992
Zhang and Schuster
TABLE I: Spectroscopic Data binaphthyl derivative 1 2 4
5
(log absorp, nm 333 (4.1) 346 (4.1) 305 (4.2) 325 (4.1) 288 (3.9) 327 (3.7) 289 (4.0) 329 (3.7)
Amax
gA, ?h 2.5 (254 nm)
0.2 (334 nm)
ns 3.7
T,,”
triplet A,, 646
nm
iT
(room temp)! ps 8.3
iT
(77 K),s 3.8
2.5
673
9.1
11.1
5.9
693
8.3
10.0
6.8
700
8‘
14.3
Singlet excited-state lifetime at room temperature determined in N,-purged solution. Typical errors are &IO%. Calculated from the decay of the second half-life to avoid significant error introduced by triplet-triplet annihilation. Typical errors are &IO%. CThissignal is weak. The error in this determination is ca. &30%
proceeds through the triplet state of 2. Since the quantum yields for fluorescence of 2-5 vary only slightly, the difference in photoracemization behavior between the acetal and ethers was attributed to the unique availability of a bond cleavage pathway in 2. We examined the excited states of these compounds spectroscopically in order to gain additional insight into the mechanism for photoracemization. The absorption spectra of 2, 4,and 5 (we did not reexamine 3) show similar features characteristic of strong electronic interaction between the naphthyl chromophores. In T H F solution, their lowest energy band is at 330 f 5 nm with e = lo4 cm-’ M-‘. We measured the lifetime of their excited singlet states by time-correlated luminescence. The results listed in Table I are consistent with those obtained indirectly by oxygen quenching.” The triplet-triplet absorption spectra of 1, 2, 4, and 5 were measured by laser flash photolysis at room temperature in nitrogen-purged THF solution. Under these conditions, Irie and co-workersI6 reported that triplet 1,l’-binaphthyl absorbs at 610 nm, a value of significantly lower energy than that of naphthalene itself (412 nm).I8 This shift in absorption spectrum is attributed to interaction between the naphthyl rings, which should vary as the inter-ring dihedral angle changes.I9 We find that the spectra of the bridged binaphthyls vary systematically with the number of atoms in the bridging group. For 1, containing a two-atom bridge, there is a strong absorption with a maximum a t 646 nm. However, 5, linked by a six-atom bridge, shows a weaker triplet-triplet absorption spectrum with a maximum at 700 nm. The results of these experiments are summarized in Table I. If the difference between the efficiency of photoracemization of acetal 2 and ethers 3 5 is due to a bond cleavage path available uniquely to the triplet of 2,” then the triplet lifetime of 2 should be considerably shorter than that of the ethers. As is frequently the case, the decay of the triplet-triplet absorption spectra show non-first-order kinetic behavior. At early times triplet-triplet annihilation plays an important role; later unimolecular and pseudo-first-order diffusional quenching dominate the decay kinetics. Nevertheless it is clear from the data (see Table I) that the lifetime of triplet 2 under these conditions is not significantly shorter than that of 1 or of the ether-bridged binaphthyls. We also measured the triplet lifetimes in glassy methylcyclohexane at 77 K. If the difference in photoracemization behavior is due to a dihedral angle-dependent change in electronic configuration of the lowest triplet state, then there might be a meaningful difference in the radiative rate constants. No such change was found; the data in Table I show that the low-temperature triplet lifetimes vary only slightly with structure. 2. Calculation of Thermal Racemization Transition States. Molecular mechanics calculations were carried out on the bridged binaphthyls in order to gain additional insight into the factors that control their photochemistry and spectroscopy. Table I1 shows the results of PCMODEL~O calculations for the ground-state structures of 1-5. The inter-ring dihedral angles (C2C1C’lC’2; (18) Bebelaar, D. Chem. Phys. 1974, 3, 205. (19) Baraldi, I.; Ponterini, G.; Momicchioli, F. J . Chem. SOC.,Faraday Trans. 2 1987, 83, 2139. (20) PCMODEL (4.01) was purchased from Serena Software, Bloomington,
IN, and executed on a Silicon Graphics Iris 4D work station.
TABLE II: Calculated Ground-State Structures and Energies M O l (PCMODEL),
binaphthvl 1 2 3 4 5
kcal/mol 78.3 -0.9 -2.9 -2.4 -8.4
dihedral C2CICl’Ci angle, dea 37 52 60 66 63
MfO (Benson), kcal/mol 70.7 2.9 3.6 -1.4 -6.3
TABLE III: Calculations of Model Transition States for Thermal Racemization dihedral calcd Mr,” C2C,C,’C2’ AH’! biphenyl model for kcal/mol angle, deg kcal/mol 1 39.4 20 1.8 2 -34.6 40 9.5 3 -3 1.4 52 14.5 4 -37.6 55 16.5 5 -42.1 51 16.4 “Ground state. Experimental values for some compounds are available. SolladiB, G.; Zimmerman, R. G. Angew. Chem., Inr. Ed. Engl. 1984, 23, 348. Hall, D. M. J. Chem. SOC.1956, 3674.
syn planar L = 0”) vary as expected with structure. Ethanobridged binaphthyl l has the smallest dihedral angle. This value increases for the three-atom bridged acetal 2 and then is nearly constant for the four-, five-, and six-atom bridged ethers 3 5 . The heats of formation (AHf) calculated by PCMODEL are meaningfully compared with the same values calculated by means of Benson’s group equivalent method2’ (given also in Table 11). The group equivalent method does not include next-nearestneighbor interactions and therefore contains no contribution from extended conjugation of the binaphthyl r-electron system or conformational strain due to naphthalenenaphthalene interaction. These two factors operate in opposite directions on the calculated values of AHHf. Consequently, for 1, strain contributions overwhelm conjugative stabilization. But for 2, where there is less strain because of a larger dihedral angle, the opposite is true. The discrepancy is small between the molecular mechanics and group-equivalent estimates of A H p In principle, the thermal barriers to racemization for 2-5 may be estimated with molecular mechanics methods by finding the minimum energy structure with higher than C2symmetry for each compound. In practice, this approach required simplification in order to reach meaningful conclusions. Rather than calculate the energy of symmetrical bridged binaphthyls, it was necessary to reduce the number of variables by substituting the biphenyl ring system for the binaphthyls. The results of these calculations are shown in Table 111. The inter-ring bond angles and AHf for the ground-state structures calculated for the biphenyl models closely parallel the values obtained for 1-5. The dihedral angles for the model bi(21) Benson, S. D.; Cruickshank, F. R.; Golden, D. M.; Haugen, G. R.; ONeal, H. E.; Rodgers, A. S.; Shaw, R.; Walsh, R. Chem. Reu. 1969, 69,
219.
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 3065
Photoracemization of 1,l'-Binaphthyl Derivatives phenyls are smaller than for the binaphthyls, but in each case it is clear that these values are determined primarily by the nature of the bridging group. Of primary significance to this work is the trend observed in the enthalpies of activation (AH') for formation of symmetrical transition states. These structures were created by forcing the inter-ring dihedral angle to Oo and by placing the bridging chain in a conformation with local C, symmetry. This process reveals that the activation barrier for 1 is very low, that 2 has an intermediate value, and that ethers 3-5 have relatively high values that are independent of specific structure. These findings suggest that the change in activation bamer for thermal racemization of these compounds is determined by the ground-state dihedral angle. 3. Attempt To Photoresolve 1 and 2 by Irradiation with Circularly Polarized Light. Photoresolution is the converse of photoracemization. In principle, all compounds that photoracemize by a unimolecular reaction mechanism may be partially photoresolved by irradiation with circularly polarized light.I2 The achievable enantiomeric enrichment at the photostationary state [(R - S)/(R+ S)]= is directly related to the value of the Kuhn anisotropy factor (gh) as shown to good approximation in eqs 1 and 2, where eRA and tSXare the extinction coefficients of the (R)-
-I
1.20t
\
0.40tv
o~201-&==4 3.001
/\
-
-3.00
-4.00-
I
I
I
I
7
210.0 240.0 270.0 300.0 330.0 360.0
and (Qenantiomers for absorption of circularly polarized light at wavelength A, and cA is the extinction coefficient for the absorption of unpolarized light. Further, the rate at which the photostationary state is attained by irradiation of a racemic mixture with circularly polarized light also depends on the Kuhn anisotropy factor according to eq 3, where ZA is the light intensity c
+
at wavelength X and C i s the total concentration (R S). The magnitude of the enantiomeric enrichment and the efficiency of the photoresolution will increase as the anisotropy increases. The values of gA are readily determined from the circular dichroism (CD) and absorption spectra of chiral compounds having known enantiomeric purity. The CD and absorption spectra of 1 have been reported,15 the spectra of 2 in acetonitrile solution are shown in Figure 1, and g A values are presented in Table 1. Clearly, irradiation of 1 with circularly polarized iight at 254 nm could give a measurable enrichment at the photostationary state. But the rate of photoresolution will be very slow due to the small value of Experimentally, we cannot detect enantiomeric enrichment of 1 after irradiation with circularly polarized light for 20 h. The CD and UV spectra of 2 show that gAis very small. Thus, despite a large value for Que, the enantiomeric enrichment and the rate of approach to the photostationary state in this case will both be small. Experimentally, no enantiomeric enrichment was found when 2 was irradiated with circularly polarized light at 334 nm. 4. Irradiation of Optically Active and Racemic 2 in Liquid Crystal Phases. Gottarelli and co-workers22showed that addition of optically active 2, and other chiral binaphthyl derivatives, to nematic liquid crystal phases induces cholesteric behavior. They reported that the twisting power (&) of 2 in 4-cyano-4'-npentylbiphenyl (K-15)23is 55 (pm M)-I. This is a relatively large value indicating that a modest enantiomeric enrichment of 2 will induce a measurable twist when 2 is dissolved in a nematic phase. Our objectives necessitate a liquid crystal phase that is transparent to UV light in regions where 2 absorbs. K-15 satisfies this requirement, and we examined the photochemical properties of the ~~~
~~
(22) Gottarelli, G.;Hibert, M.; Samori, B.; Solladie, G.; Spada, G. P.; Zimmermann, R. J . Am. Chem. SOC.1983, 105, 7318. (23) The K-15 phase is available commercially from BDH Inc.
Wavelength (nm) Figure 1. Absorption and circular dichroism spectra of bridged binaphthalene 2 recorded in acetonitrile solution at room temperature.
K- 15 nematic phase containing small quantities of dissolved 2. Figure 2A shows K-15 containing no additive as viewed microscopically through crossed polarizers at 26 OC. The typical marbled texture of a nematic liquid crystal is readily apparent.24 Addition of a small quantity of optically active 2 to this phase converts it to a cholesteric, as is shown in Figure 2B. Irradiation of an air-saturated thin film of K- 15 containing 5 X M (-)-2 for 10 min with UV light (334 nm) causes its conversion from a cholesteric to the nematic phase that is reproduced in Figure 2c. We are unable to reverse the conversion of the cholesteric to nematic texture of K-15 containing 2 with circularly polarized light. Irradiation of a mixture formed from racemic 2 under conditions where the phase is isotropic, to avoid loss of circular polarization due to birefringen~e,~~ does not cause formation of a cholesteric liquid crystal phase when the mixture is cooled. This is evidently a consequence of the low g,, value for 2 and the concomitant slight enantiomeric enrichment expected at the photostationary state.
Discussion One objective of this work was to explore the prospect of developing an optical switch based on the reversible interconversion of twisted and nontwisted nematic liquid crystal phases. To be functional, it must be possible to both control and sense the state of such a device with light. In this work we demonstrated experimentally two of the three components required for development of the switch: a light-induced transition from a cholesteric to a nematic liquid crystal phase and the ability to sense this change optically. The third factor-arguably the most difficult-inducing a twisted nematic phase by irradiation with circularly polarized light, was not accomplished in the present system. The reason for this failing is traced to inefficient photoracemization of the studied chiral binaphthyl derivatives with large gA values. We examined the mechanism for photoracemization of these compounds in an attempt to overcome this problem. Mechanism for Photoracemization of 2. It is well-known from experimental26and theoretical19 studies of 1.1'-binaphthyl that (24) Hartshore, N. H . The Microscopy ofLi9uid Crystals; Microscope Publications Ltd.: London, 1974. (25) Sackmann, E.;Voss, J. Chem. Phys. Lett. 1972, 14, 528. Muller, W. U.; Stegemeyer, H. Ber. Bunsen-Ges. Phys. Chem. 1973, 77, 20.
3066 The Journal of Physical Chemistry, Vol. 96, No. 7 , 1992
--
Zhang and Schuster
I
Figure 2. (A) Neat K-15 at 26 OC with a glass cover slip. The typical marbled texture is observed. (€3) A cholesteric cell-like texture observed for K-15doped with (-)-2. (C) Nematic texture with a high number of surface inversions walls obtained after U V irradiation of a cholesteric thin film of K-15doped with (-)-2. All viewed at IOOX magnification through crossed polarizers. (This figure was reproduced at 55% of its original size.)
electronic excitation results in a change in the equilibrium value for the inter-ring dihedral angle. In earlier work on this parent compound, IrieI6 showed that this tendency leads to photoracemization of its triplet state. Tetreau and co-workers' expanded investigation of the bridged binaphthyls presents alternative mechanisms to the single-bond rotation (atropisomerism) thought to be operating in the parent compound.17 For 2 they favored a bond cleavage mechanism primarily because it appears to accommodate the photostability of optically active 3 5 . Our experiments seem to rule out unique chemical reactivity for 2 and support atropisomerism as the mechanism for its photoracemization. In particular, the triplet lifetime of 2 is no shorter than that of those compounds that do not photoracemize. The rate of racemization of the 1,l'-bridged binaphthyls in their excited triplet states by atropisomerization will be determined by the two primary factors that contribute to AH*.Extended electron delocalization will favor a planar triplet state as exemplified by the valence bond structures in eq 4. But unfavorable steric
2 and large for 3 5 . The former will racemize faster than the latter. These strain-energydependenttrends calculated for ground states are expected to apply equally to racemization of these compounds in their triplet states. Thus, the difference in photoracemization behavior between acetal 2 and ethers 3 5 may be explained without appeal to special reactivity.
interaction between the naphthalene rings in this geometry will inhibit its formation. If the activation barrier to achieve a planar structure is high, the rate of this reaction will be slow and it will not occur within the lifetime of the triplet state. Since there is typically little change in structure between the lower electronic states of an aromatic hydrocarbon (small Stokes shift of fluorescence, for example), we presume that the same steric factors operate in the ground and excited states of binaphthyls 1-5. With these considerations in mind, the molecular mechanics calculations provide a ready explanation for the different photochemical behavior of 2 and 3 5 . The primary structural variation between these compounds is the magnitude of the inter-ring dihedral angle in their ground states. Smaller values for this angle increase the strain energy of the ground state, but the strain of the transition state is largely independent of the nature of the bridging group. Thus, the magnitude of AHs is small for 1 and
Experimental Section Ceneral Procedure. IH NMR spectra were recorded on either a Varian XL 200 or a GE QE-300 spectrometer in CDC13solution. Chemical shifts are referenced to TMS. UV spectra were recorded on a Perkin-Elmer 552 spectrometer in either acetonitrile or cyclohexane solution. Florescence spectra at room temperature and phosphorescence spectra (77 K) were recorded on a Farrand fluorometer. The phosphorescence lifetimes at 77 K and the triplet-triplet absorption spectra at room temperature were obtained on an excimer laser apparatus with excitation at 308 nm. CD spectra were recorded on a SPEX CD VI (Jobin-Yvon, France) spectrometer in acetonitrile solution. Polarimetric measurements were performed with a Jasco polarimeter at room temperature. High-performance liquid chromatography (HPLC) was run on an IBM LC/9560 ternary gradient liquid chromatograph coupled with a Perkin-Elmer LC-75 spectrophotometric detector and a Hewlett-Packard 3390A integrator. Melting points were determined with a Nalge hot plate apparatus and are uncorrected. Materials. Nematic liquid crystal K-15 was purchased from BDH Limited (Poole, England) and used as received. The corresponding cholesteric phase was obtained by doping with (R)(-)-2 and (R)-(-)-5. Racemic mixtures of 2, 4, and 5 were prepared according to the procedure of Ernest and co-worker~.~~ The optically active forms (R)-(-)-2and (R)-(-)-5 were obtained in the same manner starting from (R)-(+)-l,l'-bis-(2-naphthol) (9996, Aldrich). 2: mp 182-184 "C; 'H NMR 6 5.7 (s, 2 H), 7.2-7.6 (m, 8 H), 8.0 (dd, 4 H). 4: mp 270-271 "C; IH NMR 6 2.0 (q, 2 H), 4.4 (m, 4 H), 7.2-7.5 (m, 8 H), 7.9 (dd, 4 H). (-)-5: mp 254-255 "C; lH NMR 6 1.8 (m, 4 H), 4.1 (m, 2 H), 4.55 (m, 2 H), 7.0-7.6 (m, 8 H), 7.9 (dd, 4 H). The optical purity of (R)-(-)-2 was determined to be ca. 1m0 by means of HPLC on a Pirkle L-leucine column (25 cm X 4.6 mm i.d., Regis) at 0 "C with hexane as the mobile phase. No attempt was made to determine the optical purity of (R)-(-)-5. Racemic 1 was prepared according to the procedure of Couture and co-workers.*' A baseline resolution of 1 was achieved on a commercial MCT column CA-1 (Regis). A small amount of
(26) Post, M. F. M.; Langrlaar, J.; Van Voorst, J. D. W. Chem. Phys. feu. 1975, 32, 59. Shank, C. V.; Ippen, E. P.; Teschke, 0.; Eisenthal, K. B. J . Chem. Phys. 1977,67,5547. Miller, D. P.; Eisenthal, K. B. J . Chem. Phys. 1985. 5076.
(27) Simpson, J. E.; Daub, G. H.; Hayes, F. N. J . Org. Chem. 1973.38, 1771. (28) Lapouyade, R.; Veyres, A.; Hanafi, N.; Couture, A.; Lablache-Combier, A. J . Org. Chem. 1982, 47, 1361.
.n
L
-I
X = Linking chain
J. Phys. Chem. 1992, 96, 3067-3072 (R)-(-)-l was obtained with a preparative MCT column prepared by following the reported procedurez9 under a pressure of 160 psi. A mixture of methanol and water (9633.5, v/v) was used as the mobile phase for both analytical and preparative purposes. Photolyses. All irradiations were performed with a Oriel high-pressure 1-kW Hg(Xe) lamp at room temperature unless otherwise indicated. Irradiations of (-)-2 and (-)-5 in solutions were carried out in dry THF at wavelengths above 305 nm under both aerated and deoxygenated conditions in a square quartz cell equipped with a latex septum. Solutions were magnetically stirred during irradiation to maintain homogeneity. Deoxygenation was accomplished by bubbling Nzthrough the solutions for 20 min before irradiation. Irradiations in liquid crystal phases were performed with a very thin cell at 334 nm isolated by means of a narrow band-pass filter. The phase transitions induced by irradiation were analyzed microscopically using a Micromaster polarizing microscope equipped with a Mettler FP82 hot stage. WOt~~&tion Of (R)-(-)-Ethano-BriageaBinapbthyl 1 and Attempted Photoresolutions. A 2-mL N,-saturated cyclohexane solution of enantiomerically enriched 1 was irradiated at 350 nm in a Rayonet reactor. The progress of the reaction was monitored
3067
by UV and CD spectroscopy. The A A z 5 4 value of the solution at the start of the irradiation was 1.1 X lo4. After 1 h of irradiation, AAl5, decreased to 6.6 X This decrease in the circularly dichroic absorption corresponds to 40% racemization of 1. Attempted photoresolutions both in solutions and in liquid crystal phase were carried out with a standard circularly polarized light setup.30 Control experiments" showed the quality of the circular polarization. The photoresolution in solutions was monitored by CD and polarimetric measurement. Attempted photoresolutions in liquid crystal phases were carried out in the isotropic temperature range and analyzed at a lower temperature liquid crystal phase.
Acknowledgment. This work was supported by a grants from the National Science Foundation and the Army Research Office for which we are grateful. We thank Professor W. H. Pirkle of this department, who provided valuable assistance and advice on the chromatographic separation of chiral compounds, and Professor F. Momicchioli of the University of Modena, Italy, who provided results of theoretical calculations on triplet binaphthyls. Registry No. 1, 139163-39-6;2, 129647-55-8;3, 86334-03-4;4, 86289-52-3;5, 86289-53-4;K-15,40817-08-1.
(29) Kooler, H.;Rimbock, K.; Mannschreck, A. J. Chromarogr. 1983,282, 89.
(30)Stevenson, K. L.; Verdieck, J. F. Mol. Phofochem. 1969, I , 271.
Effects of Ce4+/Suifato Complex Formation in the Belousov-Zhabotlnskli Reaction: ESR Studies of Malonyl Radical Formation Horst-Dieter Forsterling* Fachbereich Physikalische Chemie, Philipps- Universitat Marburg, 3550 Marburg, Germany
and Linda Stuk Center for Nonlinear Dynamics, Department of Physics, The University of Texas, Austin, Texas 78712 (Received: October 8, 1991; I n Final Form: December 10, 1991)
We investigated the effects of the formation of sulfato complexes from Ce4+ and H2SO4 on the oxidation of malonic acid (MA) by Ce4+in the Belousov-Zhabotinskii (BZ) reaction. We found from measuring malonyl radical concentrations in an ESR flow experiment that the rate of formation of sulfato complexes is very fast compared to the rate of the MA/Ce4+ reaction. This result is important in the theory of the BZ reaction; there is no difference in the reaction rate whether Ce4+ is freshly produced in the autocatalytic cycle or it is introduced in the sulfato-complexed form. Moreover, we did absolute calibrations of the malonyl radical concentrations by using Mn2+and 3-carbamoyl-PROXYL as calibration standards. From these concentrations we calculated malonyl radical self-decay rates of 4.2 X IO8 M-l s-' in 1 M H2S04and 1.5 X lo9 M-I s d in 2 M HCIO,.
1. Introduction Several recent experiments'+ have demonstrated the crucial role of malonyl radicals in the Belousov-Zhabotinskii (BZ) reaction. In a detailed modeling study, Gyorgyi et al.' raised some questions about the primary reactions determining the concentration of malonyl radicals in the oscillating system. These reactions are Ce4+ MA Ce3+ + MA' + H+ (1) (2) 2MA' + HzO products The abbreviations are as follows: MA = malonic acid, CH2(COOH),;MA' = malonyl radical, 'CH(COOH),. The products of reaction 2 are assumed' to be malonic acid and tartronic acid, but the identity of these products is not important for our present
+
--
To whom correspondence should be addressed.
0022-3654/92/2096-3067$03.00/0
considerations since it does not affect the concentration of malonyl radicals. The problem to be resolved for reaction 1 is whether freshly oxidized Ce4+, formed in the BZ autocatalytic reaction, reacts with MA significantly faster than sulfato-complexed Ce4+ which was used6*8,9to measure the rate constant, k l , for reaction 1 in (1) Forsterling, H. D.;Murlnyi, S. Z . Nafurforsch. 1990, 450, 1259. (2) Forsterling, H.D.;Murlnyi, S.;Noszticzius, Z . Reacf. Kinet. Cam/. Lett. 1990, 42, 217. (3) Forsterling, H.D.;Murlnyi, S.;Noszticzius, Z . J . Phys. Chem. 1990, 94, 2915. (4) Murlnyi, S.; Forsterling, H. D. Z . Nafurforsch. 1989, 45a, 135. (5) Forsterling, H.D.;Noszticzius, 2.J . Phys. Chem. 1989, 93, 2740. (6) Forsterling, H.D.;Pachl, R.; Schreiber, H. Z . Nafurforsch. 1987,420, 963. (7)Gyorgyi, L.;Turlnyi, T.; Field, R. J. J . Phys. Chem. 1990, 94, 7162.
0 1992 American Chemical Society
