3252 Synthetic Metals, 55-57 (1993) 3252-3257

CLUSTERS OF C,o-FULLERENE IN A WATER SOLUTION CONTAINING

y..CYCLODEXTRIN; A PHOTOPHYSICAL STUDY

MIKAEL SUNDAHL, THOMAS ANDERSSON, KAROLINA NILSSON, OLOF

WENNERSTROM, GUNNAR WESTMAN

Department of Organic Chemistry, Chalmers University of Technology, S-412 96 GOteborg,

Sweden

ABSTRACT

y-CD has been used to dissolve C60 in water. Depending on the concentration of cyclodextrin

two forms of solutions can exist. At high y-CD:C60 ratio a complex that is monomeric in C60 is

stable whereas at lower y-CD:C60 ratio a cluster of several C60 molecules m o u n d e d by y-CD

can exist. We propose that clusters of different sizes can be formed. The y-CD:C6o complex have

an electronic absorption spectrum similar to that of an organic solution of C60 and the cluster have

an electronic absorption spectrum similar to that of a thin film of C60.

For the ~FCD:C60 complex the photophysical properties are similar to those of an organic

solution with the exceptions for quenching of the triplet state by molecular oxygen and

annihilation of the triplet state. The rate constant for quenching of the triplet is reduced by a factor

two and the rate constant for annihilation of triplets is reduced by a factor four when compared to

those expected for a "free" C60 , respectively.

The properties of the clusters are different from those of an organic solution of C6o. For water

solutions of small clusters, decay of the excited state of the cluster is clearly dependent on the

intensity of the excitation laser pulse; at high laser intensity there is both a fast and a slow

component in the decay process whereas at low laser intensity only the slow process is observed.

For excitation of larger clusters we could only detect a fast decay process.

Keywords: C60-fullerene, "Fcyclodextrin, complex, cluster, photophysics, triplet state.

INTRODUCTION

The fuUerenes and in particularly C60 have gained more interest than any other molecule

recently[l]. This highly symmetric molecule have unique electronic properties which is reflected

in its photophysics. The photophysical properties of organic solutions of C60 can be summarized

as follows [21; The S0-S 1 transition is strongly forbidden and only very weak fluorescence can be

detected. The faith of almost all singlet excited C~0 molecules is to undergo intersystem crossing

0379-6779/93/$6.00 © 1993- Elsevier Sequoia. All rights reserved

to the triplet state. This triplet can (1) decay to its ground state by intersystemcrossing (2) be

quenched with oxygen, yielding singlet oxygen (3) be quenched with other organic molecules

having low enough triplet energy (4) undergo triplet-triplet annihilation.

In this paper we will present results on the photophysical properties of a water solution of a

y-CD:C60 complex [3], and preliminary results on the photophysics of cluster of C6O surrounded

by T-CD.

RESULTS AND DISCUSSION

Preparation of water solutions of complex [3] and cluster The y-CD:C6o water solution was prepared by boiling an aqueous solution of T-CD (0.02-0.1

M) containing solid C6o (>99%). The dissolution of C6o is slow and to maximize the

concentration it can be necessary to reflux the solution for more than 50 hours. The electronic

absorption spectrum of a solution prepared in this way is shown in Fig. 1.

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0.8

0.6

~0.4 <

0.2

i J i i f l i i i

x40

3OO

' t . . . . t . . . .

0 200 400 500 600 700 rtm

Fig. 1. Electronic absorption spectrum of a water solution of T-CD:C60 complex.

Interestingly, the solution has the characteristic magenta color of C60. There is also a magenta

colored precipitate formed from concentrated solutions. On addition of cold water to this

precipitate excess cyclodextrin can be washed away. The remaining precipitate was dissolved in

hot water and the color changed to yellow (similar to that of a thin film of C60). We propose that a

cluster of several C6o molecules surrounded by T-CD is formed. The electromc absorption

spectrum of the cluster depend on how long time the solution has been heated (Fig. 2). We suggest

that larger clusters are formed on prolonged heating. A solution of "large" clusters was extracted

with toluene. It was possible to quantitatively recover C6o from the cluster solution to the toluene

solution, assuming that the molar absorbtivities for the maxima at 343 rim and 331 nm for cluster

solution and toluene solution, respectively are the same.

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0.6

0.5

0.4

~ 0.3

< 0.2

0.1

0 200 400 500 600 700 nm

. . . . I . . . . I . . . . I . . . . I . . . .

, i

300

Fig. 2. Electronic absorption spectra of clusters in water solution after 1 and 5 minutes heating at

80"C respectively.

Molecular modelling of ?CD :C 60 complex

To check the size of C60 in relation to that of ~-CD we have made molecular modelling using

the program BIOGRAPH[41 (BioDesighn Inc.) Dreiding H force field. Docking of C60 and y-CD

gave a stable I: 1 complex (Fig. 3). Further docking with another y-CD unit gave a structure in

which the C60 fits nicely into the cavity of two y-CD units bond together via the longer rim

(Fig. 3). However, the effect of water exclusion from the interior of the y-CD has not been

considered in the modelling experiments.

Fig. 3. Models of T-CD:C6o complexes derived from molecular modelling.

P hotophysics of the ~CD :C 60 complex

In Fig. 4 a transient difference absorption spectrum obtained on pulsed laser excitation (7 ns, 355

nm) is shown. At longer wavelengths the spectrum is due to TI-T n absorption and at shorter

3255

wavelengths there is a combination of ground state bleaching and TI-T n absorption. The transient

is quenched by molecular oxygen and the experimental quenching rate constant is 6.4 x 108

M-Is "1. This rate constant is only 1/10 of the diffusion controlled, whereas in a toluene solution the

triplet is quenched with a rate 1/5 of the diffusion controlled. This suggests that half of the surface

of 3C60" in the complex is available for collision with molecular oxygen.

0 .04

0.03

0.02

o . o l !o -0.01

-0.02

-0.03

-0.04 250 350 450 550 650 750 85( n m

Fig. 4. Difference absorption spectrum obtained on pulsed laser excitation. The spectrum was

obtained by slicing traces at different wavelengths.

i n I

0

-1

-2

-3

-4

-5

-6

Aabsorbance) i I i L I i I t i

I , 0 1.00 10-4 2.00 10 -4 s

Fig. 5. Decay traces obtained at 750 nm, at,different powers of the excitation pulse. The traces are

shifted vertically for clarity, A absorbance values at 0 s were from top 0.40, 0.22 and 0.15,

respectively. The straight lines corresponds to a half life of 83 Its.

3256

Furthermore, the shape of the decay curve is dependent on the intensity of the excitation laser

pulse (Fig. 5). We conclude that this is due to T-T annihilation. Assuming first plus second order

decay kinetics and then fitting to the decay curves, we get a triplet state lifetime of 83 ItS and a

second order decay rate constant 1.2 x 109 M-Is "1 for a nitrogen flushed solution of the complex.

For a toluene solution the second order rate constant was determined to 7.1 x 109 M l s -1 . The

molar absorbtivity for the T-T absorption at 750 nm was taken to 2 x 104 M'tcm -l [21. If we

correct for the different diffusion rates in water and toluen respectively the annihilation rate in

water is only 1/4 of that for a toluene solution, again suggesting that half of the surface of the

3C60" is available for collision. Thus, oxygen quenching of the excited triplet and T-T annihilation

is clearly in favor of a 1:1 T-CD:C60 complex.

P hotophysics of the ~CD :C60 cluster In Fig. 6 transient difference absorption spectra of a solution of cluster of medium size at

different limes after the excitation laser pulse (7 ns, 355 nm) are shown. Decay traces were

collected at different wavelengths and where then sliced as indicated in Figure 6.

0 .16 .

0 .12

0.08 x5

0.04

0 500 550 600 650 700 750 800 850 n m

Fig. 6. Difference absorption spectra of a cluster solution at 40 ns and 400 ns after the laser pulse.

The spectrum clearly changes with time. This might be due to the presence of several excitons,

singlet or triplet, within a single cluster shortly after the laser pulse. These excitons would then be

expected to annihilate fast which we clearly see. At longer times after the excitation pulse we

conclude that there is at most one h'iplet exciton in each cluster. Furthermore, the shape of the

decay curve is clearly dependent on the intensity of the excitation laser pulse (Fig.7). At higher

intensities we get more of the fast decay process and relatively less of the slower. Another way of

changing the shape of the decay curve is to vary the size of the cluster; for solutions of large

clusters we can no longer detect the slow decay process but only the fast one. We assume that the

number of excitations in a cluster, the size of the cluster, and the excitation degree should follow

normal statistics. Then we can conclude that the smaller clusters should contain 5 to 25 C60

molecules.

3257

0.007 ~ I , I , I , - ~

0.07

0.06 ~ 0.003

o ~ o.ool

i o , o 4

0.03 " -0.001 ~ I ' t ' I ~ - q -

0.020.010 ~ . 5 1 0 "4 3.5 l 0 -4 s

-0.01 I ' ' ' ' I ' ' ' I

0 5.0 10 .8 1.0 10 -7 1.5 10 -7 s

Fig. 7. Decay curves at 720 nm, energy of excitation laser 29, 41, 57, and 74 mJ per pulse. Inset,

decay on a long time scale (29 mJ pulse). Excitation wavelength 355 nm, pulse length 7 ns.

CONCLUSIONS AND OUTLOOK

It has been possible to dissolve C60 in water with the help of y-CD. Two different solutions can

be formed, one which is monomeric in C6o and one where several C6o molecules stick together.

The photophysical properties of the T-CD:C60 complex are not consistent with a photostable 2:1

T-CD:C60 complex, but further experiments are needed to clarify the structure of the complex. If

the "/-CD:C6o ratio is low several C6o molecules prefer to stick together forming species which are

polymeric in C60 rather than forming the "/-CD:C60 complex. Different sizes of these cluster can

be formed. The shape of these clusters might be either spherical or alternatively linear. Dynamic

light scattering and other experiments will be made to elucidate the shape and size of these

clusters. Further experiments on the photophysical properties are also under way.

REFERENCES

[1] For reviews; see H. W. Kroto, A.W. Allaf and S.P. Balm, Chem. Rev., 91 (1991) 1213; R.F.

Curl and R.E. SmaUey, Sci. Am., October (1991) 32.

[2] J.W. Arbogast, A.p. Darmanyan, C.S. Foote, Y. Rubin, F.N. Diedrich, M.M. Alvarez, S.J.

Anz, R.L. Whetter,, J. Phys. Chem., 95 (1991) 11; Y. Kajii, T. Nakagawa, S. Suzuki, Y.

Achiba, K. Obl, K. Shibuya, Chem. Phys. Lett., 181 (1991) 100; T.W. Ebbesen, K. Tamgaki.

S. Kuroshima, Chem. Phys. Lett., 181 (1991) 501; R.R. Hung, J.J. Gabrowski, J. Phys. Chem.,

95 (1991) 6073.

[3] T. Andersson, K. Nilsson, M. Sundahl, G. Westman, O. Wennerstr6m, J. Chem. Soc. Chem.

Com. (1992) 604; also presented as a poster at the 14th 1UPAC symposium on

photochemistry, K.U. Leuven, Belgium, 19-25 july, 1992.

[4] S.L. Mayo, B.D. Olafson, W. A. Goodard, HI J. Phys. Chem. 94 (1990) 8897.