1282
J . Phys. Chem. 1994,98, 1282-1287
Bucky(basket)ball: Stabilization of Electrogenerated CM'- Radical Monoanion in Water by Means of Cyclodextrin Inclusion Chemistry+ Pierre Boulas, Wlodzimierz Kutner,**SM. Thomas Jones,. and Karl M. Kadish' Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: October 12, 1993; In Final Form: November 19, 1993"
A water-soluble magenta y-cyclodextrin inclusion complex of c 6 0 (y-CD/Cbo) was prepared by a mild method which involved solubilizing c 6 0 in carbon disulfide followed by extracting c 6 0 with a y-CD aqueous ethanolic solution. The UV-visible spectroscopy of y-CD/Cao and spectroelectrochemistry of the electrogenerated y-CD/ c60- radical monoanion in water resemble those of c 6 0 in n-hexane and c60- in dichloromethane, respectively, indicating that the electronic properties of the c 6 0 and c60- guests are practically unaffected by molecular inclusion. The FT-IR spectrum of solid y-CD/C60 is only slightly different from that of y-CD. Cyclicvoltammetry of the y-CD/Cso complex in aqueous 0.15 M LiC104 solution shows three one-electron electroreductions of which only the first two, of E1/2 values equal to -0.57 and -1.03 V vs SCE, are reversible. The ESR spectrum in a frozen 0.2 M LiC104 aqueous solution of electrogenerated y-CD/C60- is similar to that of c60- in aprotic solvents. The ESR line shape and linewidth are dependent upon the temperature with an activation energy, E, = 0.043 eV, while the g value, equal to 1.998, is temperature independent over the 77-220 K range.
Introduction Highly hydrophobic buckminsterfullerene, c60, is moderately soluble in nonpolar aprotic solvents, only sparingly soluble in polar nonaqueous solvents,'.2 and insoluble in water. Water solubilization of c 6 0 becomes important, among others, in view of the potential ability of the fullerene to inhibit certain HIV enzyme^.^^^ Until recently, the solubilization of c 6 0 in aqueous solutions was not readily accomplished without its chemical derivatization which often leads to a partial lifting of its p~eudoaromaticity.~~ A noncovalent approach to bringing e 6 0 into water involves cyclodextrin (CD) inclusion chemistry.*JO Cyclodextrins form inclusion complexes with a variety of neutral and ionic organic species in both water and some highly polar organic solvents,11-16 and they are now commonly used for the dissolution of hydrophobic molecules, and pharmaceuticals in particular,17 in polar solvents and especially in water. The hydrophobic interior of a CD torus facilitates host-guest interaction with a nonpolar guest, while the highly polar torus exterior, rich in hydroxyls, facilitates solubilization of the resulting inclusion complex. The first reported procedure for preparing an inclusion complex of y-CD and c 6 0 involved refluxing an aqueous c 6 0 slurry containing y-CD for ca. 50 h.l0 This procedure may lead to the formation of two different inclusion complexes with different (y-CD)-to-Ca ratios,'* i.e., a monomeric C60 complex and a cluster composed of several C ~molecules O surrounded by y-CD molecules at the high and low ratio, respectively. Our laboratory developed a different milder procedure for the preparation of the y-CD/C60 inclusion c0mp1ex.I~This procedure is described in detail in the present paper. We also demonstrate that not only the neutral c 6 0 compound but also its c60- radical monoanion can be stabilized in aqueous solutions by means of CD inclusion chemistry. The FT-IR spectral characteristics of the solid y-CD/C60 inclusion complex and its UV-visible spectroscopiccharacteristics in an aqueous solution are presented. Cyclic voltammetry (CV), controlled-potential bulk coulometry, and spectroelectrochemistry of the complex are also described 7 Presented in part at Fullerenes: Chemistry, Physics and New Directions, Symposium of the 181st Meeting of the Electrochemical Society, St. Louis, MO, May 17-22, 1992. t On leave from the Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland * To whom correspondence should be addressed. e Abstract published i n Advance ACS Abstracts, January 1, 1994.
0022-3654/94/2098- 1282$04.50/0
along with the ESR characteristics of the y-CD/Cso- radical monoanion in frozen aqueous solution.
Experimental Section Chemicals. c 6 0 was obtained from SES Research, Houston, TX, and its purity verified by gel permeation HPLC.20 CY-, 8-, and y-cyclodextrin (Amaizo, American Maize-Products Co., Hammond, IN) were used as received. Absolute ethanol (Midwest Graint Products Co., Pekin, IL), carbon disulfide(Fluka Chemie, AG, Buchs, Switzerland), N,N-dimethylformamide (DMF), and dichloromethane (Aldrich Chemical Co., Milwaukee, WI) were of the highest available purity and were used without further purification. Tetra-n-butylammonium perchlorate, (TBA)c104 (Eastman Kodak Co., Rochester, NY), was recrystallized from absolute ethanol and dried at 40 OC under reduced pressure prior to use. Lithium perchlorate (Fluka) was used as received. Water for preparation of solutions was purified (18 Mil cm) with a Milli-Q Water system (Millipore Corp., Bedford, MA). Instrumentation. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) as well as controlled-potential bulk coulometry experiments were carried out with a Model EC 225 2Avoltammetric analyzer (IBM Instruments Inc., Danbury, CT) or a BAS- 100 Electrochemical Analyzer (Bioanalytical Systems, West Lafayette, IN) in a conventional three-electrode cell. A large-area platinum gauze was used as the working electrode for controlled-potential coulometry, while a 0.07 1 cmz area Tokai GC-30 glassy carbon disk (Tokai Carbon Inc., Japan) was used for CV and DPV. A platinum wire and saturated calomel electrode (SCE) served as the auxiliary and reference electrodes, respectively. All potentials are referred to the SCE. Solutions were deoxygenated by nitrogen purging. UV-visible spectra were recorded with a Model 9430 UVvisible spectrophotometer (IBM Instruments Inc., Danbury, CT) at 0.2-nm resolution. In situ thin-layer spectroelectrochemical UV-visible spectra were recorded with a Model TN-6500 rapid scan spectrophotometer/multichannelanalyzer (Tracor Northern, Middleton, WI) at 0.6-nm resolution. A large-area 52-mesh platinum grid (Fisher Scientific Co., Pittsburg, PA) served as the working electrode in the thin-layer spectroelectrochemical FT-IR spectra (CsI pellets) were recorded with a 2-cm-' resolution on a Model IR/32 FT-IR spectrometer equipped with 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 1283
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Temperature, OC Figure 1. Thermogravimetric analysis (TGA) in oxygen of y-cyclodextrin/C60 (curve l), y-cyclodextrin (curve 2), and c 6 0 (curve 3). Temperature ramp 10 K min-'.
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a 9000 data processing system (IBM Instruments Inc., Danbury, CT) . ESR spectra were recorded with an IBM Model ER lOOE spectrometer. The g values were calculated with respect to diphenylpicrylhydrazide (DPPH), g = 2.0037 f 0.0002.22 Lowtemperature measurements were performed with a stream of nitrogen passing through a heat exchanger immersed in liquid nitrogen. Thermogravimetric analysis (TGA) experiments were performed on a Model Hi-Res TGA 2950 thermogravimetricanalyzer equipped with a Thermal Analyst 2100 (TA Instruments, New Castle, DE). Typically, a 10-mg sample was heated in an oxygen atmosphere with a temperature scanning rate of 10 K min-I. All experiments were performed at ambient temperature, (22 f 1) OC, unless otherwise stated. Solubilizationof Cw in Aqueous y-CD Solution. Typically, a 3-mL sample of a concentrated carbon disulfide solution of c 6 0 (3.8 mg mL-') was extracted with 15 mL of a 30/70 (v/v) water ethanolic solution nearly saturated (ca. 0.3 wt %) with y-CD. The resulting two-phase liquid system was purged for 4 h, upon sonication, with ethanol-saturated nitrogen until the carbon disulfide was evaporated. A violet solid precipitated overnight from the remaining water ethanolic phase. The solid was partially dissolved in water and the uncomplexed c 6 0 was filtered out. The magenta filtrate was evaporated to drynessunder reduced pressure at room temperature yielding a violet solid y-CD/C60.
Results and Discussion The violet solid precipitated from the aqueous-ethanolic solutions is soluble in DMF and in water; yellow and magenta solutions are obtained, respectively. The solubility of the violet solid is 1 1 mg mL-l and ~ 0 . 3 3mg mL-I in DMF and water, respectively, as determined spectrophotometrically. The c60' to-(y-CD) mole ratio for the solid was determined independently by TGA and coulometry. The violet solid contained ca. 12%of water, as calculated from the height of the TGA mass step below 100 OC (Figure 1, curve 1). TGA curves for y-CD and c 6 0 are quite different, Le., the combustion temperature of y-CD (Figure 1, curve 2) is ca. 330 OC while that of c 6 0 exceeds 400 OC (Figure 1, curve 3). Therefore, the net mass losses of the solid due to y-CDand c 6 0 could be determined as 57% and 31%, respectively, and the c 6 0 to-(y-CD) ratio is calculated as 1: 1. This ratio differs from that of 1:2 reported in the literature,IOpresumably because of different preparation methods adopted. The solubility of the violet solid in DMF is larger than in water. Therefore, the former was chosen as the solvent for the coulometry. Exhaustive electroreduction of a 0.2 M TBAP DMF solution of the solid was performed at 4 . 4 5 V, Le., at a potential which is ca. 0.15 V more negative than the cyclic
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Wavelength tnm) Figure 2. Electronic absorption spectrum of (a) 50 pg mL-l Cm in n-hexane, (b) nearly saturated magenta y-cyclodextrin/Cm in water, and (c) solution (b) after heating to 60 OC. Insertsare the same spectra at the magnifications indicated.
voltammetry peak potential for the first (y-CD/C60°)/(y-CD/ &-) electroreduction in this solution (see below). Bulk electrooxidation at 0.0 V of the electroreduction product, followed by cyclic voltammetry at a glassy carbon working electrode of the recovered neutral complex, proved that the exhaustive electroreduction was reversible. However, the C60-tO-(yCD) ratio, calculated by using Faraday's law and taking into account the water content in the solid determined by TGA, varies in the range from 1 5 to 1:lO. Thus, either the complex stoichiometry in solution is different from that in the solid or not all of the c 6 0 sites were accessible for charge transfer during electrolysis, or both. The UV-visible spectrum of the water solution of y-CD/C60 (Figure 2b) shows three strong and narrow UV bands positioned at 215,260, and 333 nm, a spike at 409 nm, and a broad band centered at ca. 500 nm. The UV part of this spectrum is identical to that reported previously.1° The spectrum in Figure 2b, contrary to spectra of c 6 0 chemical derivatives, closely resembles the c 6 0 spectrum in n - h e ~ a n e ~which ~ - ~ shows ~ characteristic UV bands at 211,256, and 328 nm, a spike at 404 nm and a broad band, rich in vibrational features, centered at ca. 540 nm (Figure 2a). Hence, any chemical derivatization of c 6 0 upon inclusion seems rather unlikely. An overall small red shift of the UV bands of
1284 The Journal of Physical Chemistry, Vol. 98, No, 4, 1994
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Wavenumber, cm.l F i p e 3 . FT-IRspectrum (CsI pellets) ofsolid (a) Cm, (b) y-cyclodextrin, and (c) y-cyclodextrin/C60.
the water solution of the Y-CD/C60 inclusion complex, as compared to that of c 6 0 in n-hexane, is probably due to a more polar environment of c 6 0 for the former solvent.27 The broad visible band, assigned to the HOMO-LUMO transition for C60,25 is 40-nm blue shifted for y-CD/Cso, indicating that the HOMOLUMO energy gap is larger for the complex in water. IR spectroscopy is rarely used for characterization of cyclodextrin inclusion complexes because the guest molecule bands are often masked by the bands of the cyclodextrin host molecule itself." This is also the case for the FT-IR spectrum of the solid Y-CD/C60 complex recorded between 500 and 1800cm-l (Figure 3). Nevertheless, the spectrum displays certain features which may be attributed to molecular inclusion. Similarly, as for other cyclodextrin inclusion complexes,' the FT-IR spectrum of the y-CD/C60 complex (Figure 3c) combines features of the y-CD host (Figure 3b) and the c 6 0 guest (Figure 3a) spectra. The spectrum of c 6 0 (Figure 3a) shows four major tl, bands 28-30 at 527,576,1183, and 1429 cm-I. These bands are largely masked by the y-CD bands 31,32 in the spectrum of y-CD/C60 and only the strongest one, i.e., that a t 527 cm-I, can unequivocally be distinguished (Figure 3c). Although this band is located close to the 534-cm-l band of the y-CD ring vibration, it seems unlikely that the host-guest interaction between y-CD and c 6 0 in the y-CD/Cso complex would cause such a shift of the y-CD ring vibration band. There are only minute shifts,
