The Journal of
Physical Chemistry
0 Copyright. 1990, by the American Chemical Society
VOLUME 94, NUMBER 25 DECEMBER 13, 1990
LETTERS Photoinduced Organic Donor to Metal Electron Transfer across a Rigid Spacer Thomas A. Perkins, Brian T. Hauser, John R. Eyler, and Kirk S. Schanze* Department of Chemistry, University of Florida, Gainesville, Florida 3261 I (Received: August 20, 1999)
Two compounds (b)ReCH-DTF, which contain the metal complex chromophore (b)Re’(C0)3 (where b = 2,2’-bipyrazine or 5,5’-bis(N,N-diethylcarbamido)-2,2’-bipyridine)covalently linked to a 1,3-benzodithiafulvene (DTF) electron donor via a trans-l,4-cyclohexane (CH) spacer, have been prepared and studied by emission and transient absorption spectroscopy. Steady-state and time-resolved emission data indicate that the metal-to-ligand charge-transfer (MLCT) excited state of the (b)ReCH-DTF complexes is quenched compared to the model compounds (b)ReCH which contain the (b)Re1(CO)3 chromophore but not the DTF donor. The MLCT quenching is ascribed to a rapid intramolecular DTF Re electron transfer (ET) which competes with normal radiative and nonradiative MLCT decay. The Occurrence of intramolecular ET is confirmed by nanosecond transient absorption studies which reveal that the charge-transfer state, (b’-)Re1(C0)3-CH-DTF‘+, is formed rapidly following photoexcitation. The rate constants for forward ET (DTF Re) in the (b)ReCH-DTF complexes at 25 OC were determined from time-resolved emission data in CH$N and in CH2C12,and the activation parameters for forward ET in CH3CN were determined from temperature-dependent emission data. The rates for back ET were determined from the decay of the transient absorption in CH3CN and CH2CI2.
-
-
Introduction A remarkable research effort during the past 8 years has focused on measuring the rates of elec!ron transfer (ET) between redox sites that are held apart at fixed distances.’ A variety of spacers have been utilized in these studies, including rigid’-5 and semirigid6 organic spacers, peptides,’-” and proteins.’2-20 This work has ~
~
~~~~~~~
( I ) (a) Wasielewski, M. R. In Photoinduced Electron Transfer, Part A, Conceptual Basis; Fox, M. A.. Chanon, M., Eds.; Elsevier: Amsterdam, 1988; p 161. (b) Marcus. R. A.; Sutin, N . Biochim. Biophys. Acta 1985,811,265. (2) Closs, G. L.;Calcaterra, L. T.; Green, N . J.; Penfield, K. W.; Miller, J. R. J. Phys. Chem. 1986, 90, 3673. (3) Leland, B. A.; Joran, A. D.; Felker, P. M.; Hopfield, J. J.; Zewail, A . H.; Dervan, P. B. J. Phys. Chem. 1985, 89, 5571. (4) Paddon-Row, M. N.; Oliver, A. M.; Warman, J. M.; Smit, K. J.; de Haas, M. P.; Oevering, H.; Verhoeven, J. W. J. Phys. Chem. 1988, 92, 6958. ( 5 ) Hermant. R. M.; Bakker, N . A. C.; Scherer, T.; Krijnen, B.; Verhoeven, J. W. J. Am. Chem. SOC.1990, 112, 1214. (6) Finckh, P.; Heitele, H.; Volk, M.; Michel-Beyerle, M. E. J. Phys. Chem. 1988. 92, 6584.
unequivocally demonstrated that ET occurs between redox sites that are separated by significant distances on the molecular scale. (7) Isied, s. S.; Vassilian, A,; Magnuson, R. H.; Schwarz, H . A. J. Am. Chem. SOC.1985, 107, 7432. (8) Isied, S. S . ; Vassilian, A.; Wishart, J. F.; Creutz, C.; Schwarz, H. A.; Sutin, N . J. Am. Chem. SOC.1988, 110, 635. (9) Schanze, K. S . ; Sauer, K. J. Am. Chem. SOC.1988, 110, 1180. (IO) Schanze, K. S.; Cabana, L. A. J. Phys. Chem. 1990, 94, 2740. ( I I ) DeFelippis, M. R.; Faraggi, M.; Klapper, M. H. J. Am. Chem. SOC. 1990, I 12, 5640. ( 1 2 ) Kostic, N. M.; Margalit, R.; Che, C.-M.: Gray, H. B. J . Am. Chem. Soc. 1983, 105, 7765. (13) Axup, A. W.; Albin, M.; Mayo, S. L.; Crutchley, R. J.; Gray, H. B. J . Am. Chem. Soc. 1988, 110, 435. (14) McGourty, J. L.; Blough, N. V.; Hoffman, B. M. J . Am. Chem. Soc. 1983 - - --, .107 - - , 4470 . . -.
(15) Peterson-Kennedy, S. E.; McGourty, J. L.: Hoffman, B. M. J. Am. Chem. SOC.1984, 106, 5010. (16) McLendon. G. L.; Miler. J. R. J . Am. Chem. Soc. 1985, 107,i81 I . ( I 7) Bechtold, R.; Gardineer, M. B.; Kazmi, A,; van Hemelryck, B.; k e d , S . S . J . Phys. Chem. 1986, 90, 3800.
0 1990 American Chemical Society
8746
The Journal of Physical Chemistry, Vol. 94, No. 25, 1990
Letters
TABLE I: PhotoDhvsical Data" CHpCN
E,, (Eo,o),cm-l 14120 (16500)
compound (bpz)ReCH (bpz)ReCH-DTF (dab)ReCH (dab)ReCH-DTF
CH2CI,/TBAP
@ernb
4 2 3 2
14 120 (16500)
15620 (17870) 15620 (17870)
( 1 ) x 10-3 (I) x 10-3 ( 1 ) x 10-3 ( I ) x 10-3
T,,,
21.8 7.3 24.3 15.6
Eem (Eo,o),cm-'
ns (0.2) (0.2) (0.2) (0.2)
'All data a t 25 OC. Estimated errors given in oarentheses. bActinometer [Ru(bpy),CI,] in H,O rSolvent CH2C12with 0.1 M tetrabutylarkonium' perchlorate
We have developed a synthetic methodology that allows the use of transition-metal chromophores such as (b)Re'(CO), (where b is a bidentate diimine ligand such as 2,2'-bipyridine) in studies of photoinduced electron transfer (ET) across rigid organic spacers. This effort affords a method to tune the redox properties of the excited-state chromophore and thereby to study the effect of driving force on ET across a variety of rigid organic spacers.*' In a project aimed at testing the new synthetic methodology, the two novcl trans- 1,4-cyclohexane-bridged complexes (b)ReCH-DTF were prepared. The 1,3-benzodithiafulvene (DTF)
(b)ReCH
CH-DTF
T,,
ns
14880 (16900) 14880 (16900) 16100 (1840C) 16100 (18400)
36.7 (0.4) 1.2 (0.2) 36.7 (0.4) 1.9 (0.2)
(ae,,,= 0.055. ref
31); estimated error &IO%.
0 15 0 12 0 09 0 06
A A
0 03 0 00
350
400
450
500
550
600
Wavelength, nm m
Figure 1. Transient absorption spectra a t 15-11s delay following pulsed laser excitation (Nd:YAG 355 rim, 6 ns fwhm. 8 mJ): (a) (bpz)ReCH; (b) (bpz)ReCH-DTF. Both samples in CH2C12 with 0.1 M TBAP.
( b) Re CH- DTF
termination of the thermodynamics and kinetics for forward and back ET across the cyclohexane spacer in the two donor-substituted Re(1) complexes.
where b is one of the two following diimine ligands
bPZ
dab
moiety in these complexes is an electron donor and quenches the metal-to-ligand charge-transfer (MLCT) excited state of the (b)Re(CO)] chromophore by intramolecular DTF to Re electron transfer. Emission and transient absorption studies on the (b)ReCH-DTF complexes and the model compounds (b)ReCH and CH-DTF confirm that MLCT excitation of the (b)Re(CO)] chromophore leads to formation and subsequent decay of a charge-transfer state (CT) by the following sequence of intramolecular ET reactions:
Experimental Section Details concerning the synthesis and characterization of all compounds described herein are available as supplementary information. (See paragraph at end of paper regarding supplementary material.) Fluorescence and electrochemical experiments were performed as previously d e s ~ r i b e d . l ~Laser * ~ ~ flash photolysis was carried out at the Center for Fast Kinetics Research and the University of Florida by using the third harmonic of a Nd:YAG laser for excitation (Aex = 355 nm, 6 ns fwhm, 8 mJ).
1986, 322, 286.
Results and Discussion Cyclic voltammetry was carried out on the (b)ReCH-DTF complexes in CH3CN and CH2CI2 with tetrabutylammonium perchlorate (TBAP) as a supporting electrolyte. Potentials are referenced to the SSCE electrode (+0.236 V vs NHE). Cathodic scans of each complex reveal a single reversible wave with a half-wave potential of -0.65 V (bpz) and -0.90 V (dab) in CH3CN and -0.5 V (bpz) and -0.86 V (dab) in CH2C12. In each case the cathodic wave corresponds to reduction of the coordinated diimine ligand. Anodic scans reveal a single irreversible wave with a peak potential of +0.97 V (bpz) and +1.06 V (dab) in CH,CN and +1.06 V (bpz) and +1.07 V (dab) in CH2CI2. A similar irreversible wave was observed for CH-DTF. The irreversible anodic process is assigned to oxidation of the DTF moiety. By use of the electrochemical data and the energy of the d*(Re) x*(diimine) MLCT excited state, the free energy changes for forward and back ET in the (b)ReCH-DTF complexes were estimated (AGFETand AGBET, respectively; Table II).25 The data
( I 9) Elias. H.: Chou. M. H.: Winkler, J. R. J . Am. Chem. SOC.1988, / l o , 429. (20) Osvath, P.; Salmon, G.A.; Sykes, A. G.J . Am. Chem. Soc. 1988, 110, 71 14. (21) The excited-state reduction potential for a series of (diimine)Rel(CO),L complexes can be varied by nearly 0.5 V simply by variation of the diimine ligand (refs 22 and 23). (22) Chen. P.: Duesing. R.; Tapolsky. G.; Meyer. T. J. J . Am. Chem. SOC. 1989. 1 1 1 , 8305. (23) MacQueen, D. B.; Perkins, T. A.; Schmehl, R. H.; Schanze, K. S. Mol. Crysr. Liq. Crysf.. in press.
(24) Perkins, T. A,; Pourreau, D. B.; Netzel, T. L.; Schanze, K. S. J . Phys. Chem. 1989, 93, 45 1 I . (25) The thermodynamic quantities in Table 11 were estimated from the equations A G F E=~ E,(DTF/DTF+) - E , 2(diimine/diimine-) - E M L Cand .~ AGBET= E , 2(diimine/diimine-) - E,(DI(F/DTF+), where E , and E l , 1 are peak and hajf-wave electrochemical potentials, respectively, and EMLCTIs the energy of the MLCT excited state. E M L was~ estimated ~ from fits of emission spectra of the (b)ReCH complexes in the two solvents (ref 26). No Coulombic term was used because forward and back ET are charge-shift reactions.
9-
(b)Re1(C0)3-DTF
(b-)Ren(C0)3-DTF MLCT
kFm
e-
A (b-)Rei(C0)3-DTF
+
CT lkeET
where b = bpz or dab and DTF = 1,3-benzodithiafulvene I n the present Letter we report the results of electrochemical, emission, and transient absorption experiments that allow de(18) Bechtold, R.; Kuehn, C.; Lepre, C.; tsied, S. S. Nature (London)
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Letters
The Journal of Physical Chemistry, Vol. 94, No. 25, 1990 8747
TABLE 11: Electron-Transfer Data for (b)ReCH-DTF Comdexes' CHICN ligand bpz dab
~FET
AGFET
9.1 (1) X IO7 2.3 (0.2) X I O 7
-0.43 -0.26
CH2CI2/TBAPb
AH'FET U'FET AGBET BET 2.3 (0.2) -14 (2) -1.62 3.6 (0.3) X IO7 3.3 (0.2) -14 (2) -1.96 1.3 (0.2) X IO7
'Units: AGFETand A G B E T in eV, kFETand k B E T in s-I, with 0.1 M tctrabutylammonium perchlorate.
AH'FET
in kcal, and
indicate that the photoinduced forward ET process is moderately exothermic in both solvents and is slightly more exothermic in (bpz)ReCH-DTF than in (dab)ReCH-DTF. Note that the back ET reaction is strongly exothermic in both solvents and is more exothermic in (dab)ReCH-DTF than in (bpz)ReCH-DTF. The lowest energy absorption band for all of the Re complexes occurs at approximately 400 nm (c = 4 X I O 3 M-I cm-I). This band is assigned to the dn(Re) n*(diimine) MLCT absorption. Photoexcitation of each complex into this absorption results in the appearance of a broad, long-wavelength emission band which is assigned to the MLCT excited state. The interesting feature is that the lifetime ( 7 ) and quantum yield (@em) of the MLCT emission is significantly lower for each of the (b)ReCH-DTF complexes compared to the corresponding (b)ReCH complex (Table I). The emission quenching in the DTF-substituted complexes is attributed to the occurrence of forward ET (see eq
AS'FET
~GFET ~FET -0.54 8 ( I ) X IO8 -0.35 5 (0.5) X IO8
~GBET -1.56 -1.93
P BET 3.1 (0.3) X IO7 1.0 (0.1) X IO7
in eu. Estimated errors given in parentheses. *Solvent CH,Cl2
-
1 ).
The fact that ET is the predominant mechanism for excitedstate quenching in the (b)ReCH-DTF complexes is supported by the results of several transient absorption experiments. For example, pulsed laser photolysis of the model complex (bpz)ReCH (Nd:YAG, X = 355 nm, 5 ns fwhm, 8 mJ) produces a transient that absorbs weakly throughout the visible region that is assigned to the dn(Re) r*(bpz) MLCT excited state (Figure la). The transient decays with a lifetime of 36 ns in CH2C12/TBAP,27which is in excellent agreement with the lifetime determined from the MLCT emission decay (37 ns). By contrast, laser photolysis of (bpz)ReCH-DTF produces a transient that absorbs strongly in = 530 nm, Figure Ib) that decays with the visible region ,X(, a lifetime of 27 ns in CH3CN and 32 ns in CH2CI2/TBAP. The strong transient absorption observed for (bpz)ReCH-DTF suggests that the charge-transfer state, (bpz-)Re'(CO),-DTF+, is produced by photoinduced intramolecular ET; the visible transient absorption band is very likely due to the donor radical cation, DTF'+. The transient absorption spectrum of (dab)ReCH-DTF is qualitatively similar to the spectrum of (bpz)ReCH-DTF, suggesting that forward ET occurs in this complex as well. The decay lifetime of the transient absorption for (dab)ReCH-DTF is 77 ns in CHJN and 98 ns in CH2CI2/TBAP. Further evidence that the visible absorption band is due to DTF" was provided by laser flash photolysis of a mixture of the model complex (bpz)ReCH and CH-DTF in C H j C N (c = 5 X and 2 X M, respectively). In the bimolecular experiment, a visible transient absorption was observed that was identical with that observed following flash excitation of the covalently linked complex (bpz)ReCH-DTF; however, in the bimolecular system the transient absorption decayed with a half-life of approximately 25 ks. These observations are consistent with the following reaction sequence:
-
[(bpz)ReCH]+
hu
*[(bpz)ReCH]+
[(bpz)ReCH]O
+ CH-DTF
+ CH-DTF'+
-+
k,
kb
[(bpz)ReCH]+
CH-DTF (2)
Photoexcited *[(bpz)ReCH]+ is quenched by a diffusion-controlled encounter with CH-DTF ( k , ) which produces the free cation radical CH-DTF'+. This species decays via diffusion-controlled (26) Caspar, J. V.; Westmoreland, T. D.; Allen, G. H.; Bradley, D. G.; Meyer, T. J.: Woodruff, W. H. J . Am. Chem. SOC.1984, 106. 3492. (27) CH,CIz/TBAP indicates methylene chloride solvent with 0.1 M tetrabutylammonium perchlorate.
Figure 2. Projection of a three-dimensional structure of (bpz)ReCHDTF generated by using computer-based molecular modeling (SYBYL). The coordinates of the metal complex were obtained from the crystal (bpy = 2,2'-bipyridine and MQ structure of [(bpy)Re(CO)l(MQ)][PF,]l = N-methyl-4,4'-bipyridinium, ref 28).
back-reaction (kb) with the reduced metal complex [(bpz)ReCH]O. The half-life observed for the decay is consistent with the rate expected for diffusion-controlled reaction of two species present at an initial concentration of approximately M. Under the assumption that ET is the predominant mechanism for quenching of the MLCT lifetime in the (b)ReCH-DTF complexes, the emission lifetime data in Table I can be utilized to calculate rate constants for forward ET by the equation kFET = 1 / 7 - I / T , ~ ~ , , where T is the emission lifetime of the (b)ReCH-DTF complex and 7,del is the emission lifetime of the corresponding (b)ReCH c o m p l e ~ . ~ The . ~ . ~rate constants for forward ET in CH$N and CH2C12/TBAPat 25 OC determined by using this equation are listed in Table 11. In addition, by measuring the temperature dependence of the emission lifetimes of each donor-substituted complex and the corresponding model in CH3CN at five temperatures ranging from -3 to 35 OC the temperature dependence of kFET was determined and used to and AS'FET, Table calculate the activation parameters ( AH*FET 11). The rate constants for back ET (kBET) in CH3CN and CH2CI,/TBAP were calculated directly from the decay rate of the transient absorption which is assigned to the C T state (Table 11).
Figure 2 shows the energy-minimized structure of (bpz)ReCH-DTF which was generated by using the crystal structure coordinates of a structurally similar (diimine)Re(CO),L complex28 and molecular mechanics (SYBYL, Tripos force field)29to calculate the geometry of the DTF-substituted cyclohexane spacer. Force field calculations indicate that the trans- 1 ,Ccyclohexane spacer is restricted to the equatorial, equatorial conformation and that the only degrees of freedom involve rotation about the bonds between the ring and the DTF and styryl-pyridine substituents. The separation distance between the Re atom and the center of the DTF moiety is approximately 16 8, and is not significantly affected by the bond rotations mentioned above. Note that both kFETand kBET are comparatively large (ranging from IO7 to IO9 s-l), despite the large separation distance between the Re center and the DTF donor. The observation of relatively large negative A S * F E T (Table 11) for both complexes suggests that forward ET is n o n a d i a b a t i ~ this ; ~ ~ fact ~ ~ ~coupled ~~ with the comparatively small A H * F E T values indicates that kFETis limited primarily by the fact the electronic interaction between the metal and the DTF donor is relatively (28) Chen. P.; Curry, M.; Meyer, T. J. Inorg. Chem. 1989, 28, 2271. (29) Clark, M.;Cramer, R. D., 111; Van Openbosch, N. J . Comput. Chem. 1989, IO, 982. (30) Taube, H. In Tunneling in Biological Systems; Chance, B., DeVault, D.,Frauenfelder. H.. Marcus, R. A., Schreiffer. J . R., Sutin, N., Eds.; Academic Press: New York, 1979; p 173. (31) Harriman, A. J. Chem. Soc., Chem. Commun. 1917, 717.
8748
J . Phys. Chem. 1990, 94. 8748-8750
Conclusion Interestingly, k F E r is larger for (bpz)ReCH-DTF than for (dab)ReCH-DTF in both solvents. Since forward ET is only The experiments on the (b)ReCH-DTF complexes clearly moderately exothermic, the process is expected to be in the demonstrate that by using a metal complex chromophore and a "normal" free energy region. As a result, the larger kFET for rigid organic spacer rates of rapid intramolecular ET between (bpz)ReCH-DTF is expected due to the fact that AGFET is more redox sites that are held at a well-known separation distance can exothermic for this complex. be readily determined. Experiments in progress are focused on By contrast, kBET is larger for (bpz)ReCH-DTF than for using rigid organic spacers of varying lengths and on increasing (dab)ReCH-DTF, despite the fact that AGBET is more exothermic the range of driving forces for forward and back ET by varying for the dab complex. I n each complex the back-reaction involves the diimine ligands on the Re chromophore. These studies will ET from the diimine anion to the DTF cation; this is essentially allow examination of the solvent and distance dependence of the ET from an organic radical anion to an organic radical cation. reorganization energy for ET in these organic donor-metal comAs a result, there should be a relatively small inner-sphere replex chromophore-quencher systems. organization energy for back ET (e.g., 0.3-0.5eV).2 Taking into Acknowledgment. Acknowledgment is made to the donors of consideration the separation distance between the Re center and the Petroleum Research Fund, administered by the American the DTF moiety, the total reorganization energy for the back ET Chemical Society, for support of this research. Some of the reaction is probably in the range 1 .+I .4 eV.la.2-IoTherefore, back transient absorption experiments were performed at the Center ET is in the inverted free energy region in both solvents; this fact for Fast Kinetics Research, which is supported jointly by the very likely explains the observed inverted dependence of kBET on Biomedical Research Technology Program of the Division of AGBET observed for the two (b)ReCH-DTF c ~ m p l e x e s . ~ ~ ~ - * ~-~~ Research Resources of the National Institutes of Health (RR00886) and by The University of Texas at Austin. We thank (32) Based on the assumption that SoFET IS small or zero, the temperature Professor Russell Schmehl and Professor J.-M. Lehn for comments dependence data can be utilized to estimate the reorganization energy (A) and and advice and Mrs. Dorothy Freeto for travel support. the donor-acceptor electronic coupling (HAB) (refs 8 and IO). The assumption that A S ° F Eis~ small for the (b)ReCH-DTF complexes is probably valid (see ref IO) but cannot be tested due to the fact that the DTF oxidation is irreversible on the electrochemical time scale. Under this assumption the activation parameters for forward E T for ReCH-DTF in CH3CN suggest that X = 1 e V and H A 8 = 5 cm''
Supplementary Material Available: Preparation of and analytical data for the (b)ReCH-DTF and (b)ReCH complexes and CH-DTF (3 pages). Ordering information is given on any current masthead page.
Network Recrystatllzation in a Supercritical Fluid D. C. Steytler,* P.S. Moulson, S. A. Clark, and M. L. Parker AFRC Institute of Food Research, Colney Lane, Norwich NR4 7UA. U.K. (Receiued: September 25, 1990)
A new mechanism of network recrystallization has been demonstrated in supercritical C 0 2 in which a "saw tooth" pressure oscillation has been employed to induce growth of a filament microstructure in a crystalline material (aspartame). During the pressure cycles repeated deposition of material occurs onto the filaments which ultimately grow and interconnect to form small, discrete clusters of approximately 100 p n diameter.
Introduction
The benefits of supercritical fluids (SCF's) have been widely reviewed' and reports of their application in a variety of extraction processes are continually appearing2v3 Moreover, in recent years growing consumer awareness of residual solvent levels has led to increasing interest in applications of S C F C 0 2 extraction in the food indus t ry.435 Apart from the natural benefits of a nontoxic, nonflammable medium, C 0 2 , with a critical temperature of 31.05 OC, has many advantages over conventional petrochemical solvents. In particular, the high degree of compressibility in the S C F state enables large changes in density which in turn affect solvation. Under the control of pressure (and temperature) this facilitates selective separation of components. Also the high vapor pressure of C 0 2 ensures its complete removal at low temperatures so that volatile ( I ) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction. Principles and Practice; Butterworths: Boston, MA, 1986. ( 2 ) Proceedings of International Symposium on Supercritical Fluids, Nice, Or/. 17-19. 1988; Societe Francaise de Chimie: Paris, 1988; Vols. I , 2 . (3) Supercritical Fluid Science and Technology; Johnston, K. P., Penninger, J . M . L.,Eds.; ACS Symposium Series 406; American Chemical Society: Washington, DC, 1989. (4) Coenen, H.; Kriegel, E. Ger. Chem. Eng. 1984, 7, 335. ( 5 ) O'Toole. C.: Richmond. P.; Reynolds, J . Chem. Eng. 1986. June, 74.
0022-3654/90/2094-8748$02.50/0
and thermolabile substances are completely retained. This is particularly important in the recovery of more "natural" flavors and essential o i k 6 Chemical degradation is also minimized in C 0 2 extraction by the provision of a chemically inert, nonoxidative environment. Recently, interest in SCF's has broadened to include applications as a reaction and as a solvent for controlled recrystallization. The unique control of density through pressure (and temperature) in SCF's offers a new route to nucleation of solutes that is not provided by liquid solvents. One technique that has been commonly adopted, and is inherent in the separation stage of many extraction processes, is rapid expansion/decompression (6) Moyler, D. A.; Heath, H. B. "Flavours and Fragrances: A World Perspective". Proc. 10th Int. Congr. Essential Oils, Flavours and Fragrances, Washington DC, 16-20 Noo. 1984 1984, 41. ( 7 ) Nakamura. K.; Min Chi, Y.; Yano, T . Agric. Biol. Chem. 1988, 52, 1541. (8) Poliakoff, M.; Turner, J.; Upmacis, R. K. J . Am. Chem. Soc. 1986, 108, 3645. (9) Johnston, K. P.: Flarsheim. W.; Hrnjez, A. M.; Metha, A,; Fox, M.; Bard, A. Proceedings of Internationol Symposium on Supercritical Fluids, Nice, Oct 17-19 1988; Societe Francaise de Chimie: Paris, 1988; Vol. 2, p 907. (IO) Suppes, G.J.; Occhiogrosso, M. A,; McHugh, M. A. Reference 9, p 911.
0 1990 American Chemical Society
