J. Phys. Chem. 1994,98, 4756-4759
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Inclusion Complex of y-Cyclodextrin-Ca: Formation, Characterization, and Photophysical Properties in Aqueous Solutions K. I. Priyadarsini, H. Mohan, A. K. Tyagi, and J. P. Mittal' Chemistry Group, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India Received: November 10, 1993; In Final Form: February 1 1 , 1994'
A simple method of preparing water-soluble c 6 0 at room temperature has been developed by complexing it with y-cyclodextrin. By this method concentrations as high as 1.O X lo4 mol dm-3 of can be solubilized in water. The complex is characterized by thermogravimetric, optical absorption, and powder X-ray diffraction analysis. The optical absorption spectrum, extinction coefficient, and equilibrium constant for a 2: 1 (CD:Cao) complex Of C60 with y-cyclodextrin are reported. With the help of picosecond and nanosecond laser photolysis techniques, the triplet characteristics of the C D - C ~ Ocomplex are determined.
Introduction Since the discovery of carbon clusters and their successful laboratory synthesis, tremendous interest has been evoked in evaluating their structure and photophysical properties.14 The triplet and singlet characteristics of Cm were studied to a great extent in many hydrocarbon and nonpolar solvents. Radiation chemical, electrochemical, and photoinduced electron-transfer studies have shown the formation of radical cations, absorbing with A, = 980 nm and having a redox potential of +1.76 V vs SCE.4-9 The high electron affinity (2.6-2.8 eV) of C6o and the presence of double bonds in its structure have made it possible to accommodate as many as 6 electrons and various free radi~a1s.IO-l~The kinetics of electron-transfer reactions, redox properties, and the addition of free radicals can conveniently be studied by the use of fast reaction kinetics using laser flash photolysis and pulse radiolysis techniques. Such studies are mainly carried out in hydrocarbon/alcohol solvents and not in aqueous medium due to the insoluble nature of c60. Recently a watersoluble complex of c 6 0 with y-cyclodextrin (CD) has been prepared and reported by Anderson et al.I5 Some pulse radiolysis studies on electron-transfer reactions from radicals to c60-7CD complexes have recently been r e p ~ r t e d . ~ In this complex, the C6o molecule is suggested to be enclosed by two C D molecules and concentrations of 8.0 X mol dm-3 could be introduced into the aqueous medium. However, the formation of the water-soluble complex is based on a tedious procedure of refluxing CD with c 6 0 in water for 18-48 h. We have now developed a simple and novel method of making water-soluble complexes of c 6 0 with CD at room temperature. By this method, concentrations as high as 1.0 X 10-4 mol dm-3 could be obtained in water. The above complex has been characterized by thermogravimetric, optical absorption, and X-ray diffraction investigations. The photophysical properties of the CD-C60 complex have been investigated using picosecond and nanosecond laser photolysis techniques, and the results are reported in this paper. Experimental Section A methanolic solution (25 cm3) of y-cyclodextrin (CD) (5.4 10-4 mol dm-3) was treated with 5.0 X 10-7 mol of solid c 6 0 . After stirring for 1 h, the insoluble powder was separated from methanol. It was then washed with excess methanol to remove any free CD. The solid dissolved readily in water. It was then centrifuged to remove any suspended particles. The optical absorption spectrum (Hitachi-330) oftheaqueous solution showed X
*Abstract published in Advance ACS Abstracts, April 1, 1994.
0022-3654/94/2098-4756$04.50/0
characteristic absorption bandsof C60 (Figure la,b). The solution containing only y-cyclodextrin in methanol showed neither precipitation on stirring for 1 h nor absorption bands similar to those observed for the CD-Cao complex (Figure IC). It was not possible to form this complex in water at room temperature. Thermogravimetric analyses of CD-Cm, CD, and Cm were carried out on a Shimadzu Thermobalance in which the sample was heated a t a rate of 5 OC/min in flowing 02 (15 mL/min). The 02 atmosphere was chosen to free the Cm of residual carbon left after the TGA run of the complex. It is essential for TGA that either the reactant or the final product be unequivocally identified to fit the weight loss data. The powder X-ray diffraction patterns were recorded on a Philips X-ray diffractometer (PW 1710) in the 28 range from 5O to 65O using Nickel-filtered CuKar radiation. Picosecond laser photolysis experiments were carried out using a pulsed mode-locked Nd-YAG laser (Continuum USA, Model 501-C-10) generating pulses of 35 ps. It is frequency doubled, tripled, and quadrupled to give wavelengths of 532,355, and 266 nm, respectively. For these studies, only 355-nm pulses were employed. The transient species were probed by an analyzing white light continuum (400-900-nm range) produced by focusing the residual fundamental laser at 1064 nm onto a cell of H20: D20 (5050). However, due to the small signal to noise ratio, the detection below 500 nm was not reliable. The arrival of probe pulses was delayed from 0 to 3800 ps using optical fibers. All other details of this setup have been given previously.16 Nanosecond laser flash photolysis experiments were carried out on a computerized laser kinetic spectrometer (Applied Photophysics, Ltd., U.K.) using a KrF excimer laser (248 nm, fwhm 12 ns, 100 mJ) whose details are given elsewhere.'' In these studies, the optical density (OD) of the solution a t 248 nm was kept at 0.5 and each signal was stored after averaging over four laser shots. Results and Discussion
Preparation and Characterization of the CD:Ca Complex. Figure 1 parts a and b show the optical absorption spectrum of the aqueous solution obtained on dissolving the complex (CDC ~ O )It. shows the characteristic bands of Cm at 213, 260, and 330 nm, a shoulder at 410 nm, and a small broad band in the region 450-550 nm. This absorption spectrum matched with that of the CD-Cso complex reported by Anderson et Therefore, the solid complex obtained on treating the methanolic solution of CD with c 6 0 must be the inclusion complex of c 6 0 with CD. The optical absorption spectrum (Figure 1 parts a and b) is, accordingly, also assigned to this inclusion complex, as the aqueous solution of CD (1 .O X 10-4 mol dm-3) alone does not 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 17. 1994 4751
Inclusion Complex of y-Cyclodextrin-Cm
W
u z 4 Lo
K 0
m
m 4
WAVELENGTH
(nm)
Figure 1. (a) Optical absorption spectrum of an aqueous solution of CD-Cm (1.02 X l e mol dm-3); (b) Spectra (a) with 12 times amplification; (c) Optical absorption spectrum of an aqueous solution of 7-CD(1 .O X l e mol dm-3).
show any absorption at these wavelengths (Figure IC). By this method, we could solubilize as much as 1.0 X 10-4 mol dm-3 of this Cm complex in water. The solid complex was prepared with different concentrations of Cm (1.0- 7.0)X mol dm-3 and a constant concentration of CD (5.4X 1V mol dm-3). The absorbance (330 nm) of the aqueous solution obtainedon dissolving the solid complex increased linearly at initial stages reaching a saturation value at 1 .O X 10-4 mol dm-3 of Cm. The extinction coefficientof the complex at 330 nm, in the initial linear region, was determined to be 1 1 000 dm3 mol-' cm-l. Higher concentrations of CD could not be taken due to solubility limitation. For the low concentrationof Cm ( 1.2 X 10-5 mol dm-3) employed for the preparation of the CD-Cm complex, the flask after transferring the contents was treated with toluene. The toluene solution did not show the characteristic optical absorption bands of Cm indicating the lack of free uncomplexed Cm. The aqueous solution obtained on dissolving the solid complex was filtered. Addition of toluene through the filter paper alsodid not show any color nor any characteristic absorption bands of Cm. Therefore, the low concentrations of Ca (1.2 X 10-5 mol dm-3) taken for the preparation of the complex have apparently been completely incorporated into the y-CD and this explains the linear increase in the absorbance with an increase in Cm concentration. For higher concentrations of Cm (>2.0 X le5 mol dm-3), some brown particles were left in the flask, which on dissolving in toluene then showed the typical absorption bands of Cm. This suggests that low concentrations of Cm should be employed in order to achieve its complete conversion into the inclusion complex. The X-ray diffractionpattern of thecomplex showed prominent peaks at 26 = 5.6O,7.7', 10.6', 14.8', 16.0°,16.7', and 21.4O (Figure 2a). The pattern of CD alone, recorded at a sensitivity of 1.5 times higher than the previous run, revealed that the crystallinity of CD is not very good. The prominent peaks observed
40
35
30
25 20 2 8 (DEGREES)
15
10
5
Figure 2 X-ray diffraction pattern of (a) CD-Cm complex and (b) CD. Arrow shows the position of Cm peaks (ref 18).
are at 26 = 5.2', 10.4'. 12.5'. 14.1°,15.6'. 16S0,18.9', and 20.5' (Figure 2b). Both CD and the complex did not show any peak above 40'. The X-ray diffraction pattern of C60 is reported to have peaks at 28 = l l . O o , 179,and 21.7O.18 By comparison it becomes evident that the complex is entirely different in structure from CD and Cm alone. If the complex was a simple mixture of CD and Cm, then the X-ray diffraction pattern would have shown peaks corresponding to the individual components, CD and Ca, exactly at same positions. It also shows that in the new
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The Journal of Physical Chemistry, Vol. 98, No. 17, 1994
Priyadarsini et al.
20
?E v 0
E
-
-
2 0.04
0,s
X
6
-/ 0.6
*s
10
$-
9
0
0.02
0 00
&
0.3
-
0
2
3
4
TIME ( n r )
0 0
5 1/
15
10
kd
-6
XlO
20
25
6 -2 dm mol
WAVELENGTH ( n m )
Figure 3. Linear plot showing variation of [Ca]/Awith l/[CD]* at 330 nm.
crystalline product no unreacted original substrate has been left. These studies support the optical absorption investigations. In order to estimate the stoichiometric ratio of CD and C a for this complex, the Benesi-Hildebrand19equation was applied where the concentration of CD was varied from 1 X lo-" to 6 X 1 V mol dm-3 at a fixed c60 concentration (1.25 X mol dm-3). The analysis of data, based on the following equations, showed better linearity for a 2: 1 complex than for any other stoichiometry.
-[C6,I --1 A
1 +L Ke[CDI2 e
(2)
In eq 2, A is the absorbance of the complex at 330 nm. A plot of [C60]/A versus 1/[CD]2 gave a straight line (Figure 3), from which the extinction coefficient and the equilibrium constant were determined to be 12 200 dm3 mol-' cm-' and 2.6 X lo7dm6 mol-2, respectively. This value for the extinction coefficient is close to that determined by the direct method. The large equilibrium constant indicates that practically an irreversible complex is formede20 The molecular diameter of c60 (6-7 A) and the cavity size (diameter) of CD are comparable, and hence it is possible that C60 may be sandwiched by two CD molecules from both sides. However, it was not possible to solubilize C a in other forms of cyclodextrins, viz., CY or 8, which may be due to their smaller cavity size (5-6 A). An attempt to solubilizeC ~ O in y-CD was also not successful, probably due to the larger size of c,o. In order to further ascertain the stoichiometry of the CD-Ca complex, thermogravimetric analysis (TGA) of pure Ca, CD, and the complex was performed. The TGA curve of Cw, recorded from 25 to 500 OC, shows weight loss only after 350 OC. Thus 325 "Cwas chosen as a suitable temperature to perform isothermal TGA experiments on the CD-C60 complex. The respective TGA curve, recorded from 25 to 325 OC followed by isothermal heating at 325 OC showed a weight loss of 77.3%. In order to avoid overlapping of CD loss with that of Ca, a very slow heating rate (2 OC/min) was used. Under identical conditions, pure CD showed 100%weight loss whereas C ~ did O not show any weight loss. Hence, the observed weight loss of 77.3% in the case of the C B c 6 0 complex can be attributed to the loss of CD alone. If CD and c60 are present in the complex in the ratio of 1:1,1:2, or 2: 1, the weight loss is expected to be 64.395, 47.496, and 78.3%, respectively. The 77.3% weight loss, observed for the complex, thus clearly supports a ratio of CD to C a of 2:l. The X-ray diffraction pattern of the residue, left on heating the complex, incidentally matched well with the X-ray diffraction pattern of c 6 0 alone, which further supports the conclusion drawn from
Figure 4. Transient optical absorption spectra obtained on picosecond laser flash photolysis of an aqueous solution of CD-Ca (1.0 X lo-' mol dm-3) (a) 0 ps and (b) 3.8 ns after the pulse and (c) time dependence variation in optical density at 740 nm as a function of delay time.
TGA studies. These results are in conformity with the optical absorption studies. All these experiments were repeated several times with different sets of samples, and the results were selfconsistent. The average value of the extinction coefficient of the complex at 330 nm, determined by two independent experiments, was 11 600 f 600 dm3 mol-' cm-I. This value is much lower than that of C a in benzene at 330 nm (51 000 dm3 mol-' cm-I). This differencecan be either due to solvent or structural effects. Strong variations in extinction coefficient have also been observed upon inclusion of Cainto a variety of vesiclestructures.~3The aqueous solution of CD-Ca was extracted with benzene, and C a came into benzene solution slowly. It showed a 4.4 times higher absorption than that in aqueous solution. On this basis, the extinction coefficient of C a in benzene was determined to be 52 950 f 2000 dm3 mol-' cm-l at 330 nm, which is close to the published value. Therefore, it may be concluded that benzene is able to remove C a from the aqueous solution of the complex, and the low value of the extinction coefficient in aqueous solution would be due to encapsulation of CWby two CD molecules. Laser PhotolysisStudies. Figure 4a shows the transient optical absorption spectrum (singlet-singlet) obtained immediately after a 35-ps laser flash photolysis (excitation at 355 nm) of an aqueous solution of CD-Ca (1.1 X 1 W mol dm-3). This spectrum changes with time, and after 3.8 ns shows maximum absorption at 740 nm (Figure 4b). The spectrum formed after 3.8 ns matched well with the reported T-T spectrum of C a in benzene and aqueous solution.3J0J4J~confirming formation of the triplets of Ca. The inset of Figure 4 shows the change in OD at 740 nm as a function of time, which is assigned to the intersystem crossing. The linear plot of ln(OD,-OD,) as a function of time gave the intersystem crossing rate (k& as (6.1 f 0.5)X 108 s-1. We have also carried out nanosecond laser flash photolysis of N2-saturated aqueous solutions of CD-Cm (2.2 X 10-S mol dm-3) and observed a transient absorption spectrum (Amx = 740 nm) similar to that observed on picosecond laser flash photolysis (Figure 4b), and reported by Anderson et al.15 Using this technique, the other triplet characteristics of Ca were determined in water. Thus, the triplet was observed to decay by first-order kinetics with a lifetime of 64 f 5 I.CS and remained the same in the concentration region of (2.0-6.0) X 10-5 mol dm-3 CD-Ca indicatingno reaction with theground state. This may be expected due to restricted diffusion of C a in the CD cavity. The extinction coefficient of the triplet at 740 nm was determined to be 4250 f 500 dm3 mol-' cm-1, by energy transfer from benzophenone triplets using benzophenone ketyl radical in water at 540 nm ( € 5 4 = 3220 dm3 mol-' cm-1) as standard.21 Using the same standard,
Inclusion Complex of y-Cyclodextrin-C60
The Journal of Physical Chemistry, Vol. 98, No. 17, 1994 4159
TABLE 1: Triplet Characteristics of CD-CQ in Aqueous Solutions 740 nm 4250 500 dm3 mol-’ cm-l 0.53 i 0.07 64 f 5 /.is (6.1 f 0.5) X lo8 s-l 8 X lo* dm3 mol-’ s-1 -1 ns
results so far indicate that c 6 0 forms a 2:1 (CD:C60) inclusion complex with y-cyclodextrin, the hydrophobic interactions being mainly responsible. The triplet lifetime of c 6 0 increased, and its reactivity with 02 decreased when it is enclosed in the y-cyclodextrin cavity.
Acknowledgment. The authors are grateful to Professor K.-D. Asmus, Hahn-Meitner-Institut, Berlin, Germany, for many useful discussions. Thanks are also due to Dr. A. V. Sapre and Mr. H. N. Ghosh, Chemistry Division, for their help in the picosecond laser experiments. References and Notes
TIME ( p s )
Figure 5. Oscilloscope traces showing the decay of the 740-nm band in (a) N+saturated, (b) aerated, and (c) 02-saturated systems and (d) bleaching of the Cw absorption at 330 nm.
the quantum yield for the Cm triplet in water was found to be 0.53 f 0.07. The extinction coefficient of the triplet at 740 nm was also determined by using biphenyl as a standard (e = 40 000 dm3 mol-1 cm-1 a t 360 nm in water), and an extinction coefficient value of 5200 f 600 dm3 mol-1 cm-1 was found for the CD-Ca complex, close to that determined with benzophenone. These values are low as compared to those of uncomplexed Cmin benzene solution. The decay of the Cm triplet was monitored at different oxygen concentrations (Figure 5, deaerated, aerated, and oxygen saturated system), from which the rate constant for quenching of the c 6 0 triplet by oxygen was determined to be 8.0 X 108 dm3 mol-’ s-l. This is slightly less than the normal diffusion-controlled rate constants in water. Figure 5d shows the bleaching of the ground state absorption of c 6 0 on laser flash photolysis of a Nz-saturated aqueous solution of CD-Cso (2.2 X 10-5 mol dm-3) at 330 nm. It matched well with the decay of c 6 0 triplets a t 740 nm (Figure Sa). Using I$T = 0.53 and k ~ s c= 6.1 X 108 s-1, the lifetime of the C a singlet excited state (SI) in aqueous solution was estimated to be 1 ns. All these properties of the triplets are listed in Table 1.
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Conclusions This paper describes a relatively simple procedure for the preparation of the c W 6 0 complex in aqueous solutions. The
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