11
J . Phys. Chem. 1991, 95, 11-12
Photophysical Properties of CBo James W. Arbogast, Aleksander P. Darmanyan, Christopher S. Foote,* Yves Rubin, Franqois N. Diederich, Marcos M. Alvarez, Samir J. Anz, and R. L. Whetten Department of Chemistry and Biochemistry, Unioersity of California. Los Angeles. Los Angeles, California 90024 (Received: November 20, 1990)
A number of important photophysical properties of Cm have been determined, including its lowest triplet-state energy (near
33 kcal/mol), lifetime, and triplet-triplet absorption spectrum. The triplet state is formed in near quantitative yield and produces a very high yield of singlet oxygen by energy transfer. Cmdoes not react with singlet molecular oxygen and quenches it only slowly by an unknown mechanism. These results are discussed in terms of the unusual geometry of this molecule.
The recent isolation and purification of the intriguing spherical all-carbon molecule, c60, in high yields is a n astonishing accompli~hment.'-~This compound is produced in relatively high yield along with lesser amounts of its congeners C,o and Ca4 in the carbon arc and is readily purified by chromatography. We report that Ca possesses interesting photophysical properties, summarized in Table I , which are most likely due to the unique geometry of this compound.
Go
7
lC60
- (Q)
3%,
C60+3Q
1
hv'
hv' Fluorescence Phosphorescence
46.1 kcal/molb 37.5 f 4.5 kcal/mole (2.8 f 0.2) X 10' M-' cm-' 40 f 4 psd (1.9 f 0.2) x 109 M-1 s-I 0.76 f 0.0Y 0.96 f 0.04' ( 5 f 2) x I 05 M-1 s-1 g
-
Triplet Quencher
intersystem Crossing
TABLE I: PhotoDhvsical Prowrties of C.2
'02 Energy Transfer
The longest wavclength absorption maximum3 of c 6 0 lies at 620 nm, corresponding to a singlet energy (Es) of 46.1 kcal/mol. No fluorescence from C,, was detected a t room temperature in either hexane (UV excitation) or benzene (visible excitation). Phosphorescence has not yet been observed: however, the triplet is formed in high yield (see below). The triplet-triplet absorption spectrum for c 6 0 is shown in Figure 1. The extinction coefficient a t 480 nm (eT - tSa),2.4 X IO3 M-l cm-I , was estimated by the method of Bensasson and Land6 by comparison with the T-T absorption of acridine.' The triplet lifetime under our experimental conditions is 40 f 4 ps. T h e triplet state of C,, is efficiently quenched by 302; in airsaturated C6H6, the lifetime is 330 f 25 ns. This yields a quenching rate constant by oxygen of k , ( 0 2 ) = 2 X IO9 M-' s-l, which is typical for aromatic hydrocarbons. (1) Haufler, R. E.; Conceicao, J.; Chibante, L. P. F.; Chai, Y.; Byrne, N. E.; Flanagan. S.; Haley, M. M.; O'Brien, S. C.; Pan, C . ; Xiao, 2.;Billups, W. E.; Ciufolini, M. A.; Hauge, R. H.; Margrave, J. L.; Wilson, L. J.; Curl, R. F.: Smalley, R. E. J . Phys. Chem. 1990, 94, 8634. (2) Kratschmer, W.; Fostiropoulos, K.; Huffman, D. R. Chem. Phys. Left. 1990, 170, 167. Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. " i r e 1990, 347, 354. Taylor, R.; Hare, J. P.; Abdul-Sada, A. K.; Kroto, H. W. J . Chem. SOC.,Chem. Commun. 1990, 1423. (3) Ajie, H.; Alvarez, M. M.; Anz, S . A,; Beck, R. D.; Diederich, F.; Fostiropoulos, K.; Huffman, D. R.; Kratschmer, W.; Rubin, Y.; Schriver, K. E.; Sensharma, D.; Whetten, R. L. J . Phys. Chem. 1990, 94, 8630. (4) Schmidt, R.; Afshari, E. J . Phys. Chem. 1990, 94,4377. ( 5 ) Redmond, R. W.; Braslavsky, S . E. Chem. Phys. Leu. 1988, 148,523. (6) Bensasson, R. L.; Land, E. J. Trans. Faraday SOC.1971, 67, 1904. (7) Determined by excitation with a Nd:YAG laser pulse at 355 nm in argon-saturated C6H6. The extinction coefficient for Cm was obtained by comparing the triplet-triplet absorption of acridine and C,o under the same experimental conditions. The extinction coefficient for acridine at A, is 2.43 X IO4 M''
0022-3654/91/2095-0011$02.50/0
"All measurements in C6H6or C6D, at room temperature, with concentrations of c60 -3 X and -3 X IO4 M (OD 0.5) at A,, = 355 and 532 nm, respectively. bCalculated from the lowest energy absorbance maximum at 620 nm (see ref 3). cAverage of the triplet energy of TPP (33.0 kcal/mol) and anthracene (42 kcal/mol); however, the low quenching rate with TPP suggests the actual energy is closer to the level of TPP. argon-saturated solution. ?Average of five determinations with TPP (0,= 0.62)4 and acridine (0, = 0.84)5 standards. fAverage of three determinations with TPP (0,= 0.62)4 as standard. gTotal of physical and chemical quenching of IO2by C60 (TPP, 532 nm). The maximum concentration of Ca was limited by saturation. The energy level of the triplet (ET)was estimated by triplettriplet energy transfer. c60 quenches the triplet states of both acridine (ET = 45.3 kcal/mol)8 and anthracene (ET = 42 kcal/mol)a efficiently, with rate constants of (9.3 f 1.1) X IO9 and (6.1 f 1.0) X IO9 M-I s-I, respectively. The rate constant of quenching of the tetraphenylporphine (TPP) triplet state (ET = 33.0 k ~ a l / m o l by ) ~ c 6 0 is significantly lower, kg = (3.5 f 0.3) X IO7 M-' s-I. The reason for the slower quenching in this case is probably endothermic energy transfer. Energy transfer from the Cb0triplet to produce the rubrene triplet (ET = 26.6 kcal/ mol)Io is diffusion-controlled. Thus, we conclude that the energy level of the triplet state of Ca lies near that of TPP and well below anthracene, i.e., 33 kcal/mol 5 ET C 42 kcal/mol (Table I). The low ( 1 7 5 0 M-' cm-I) extinction coefficients for the absorption spectrum) of c 6 0 between 492 and 620 nm indicate that this band, probably So SI*,is connected with a strongly symmetry-forbidden transition. Similarly, low extinction coefficients are associated with the longest wavelength TI Tz absorption. The small S-T splitting in Cb0 (AEs,T 9 kcal/mol) is probably a result of the large diameter of the molecule and the resulting small electron-electron repulsion energy." This small splitting, the very low value of the fluorescence rate constant, and the expected large spin-orbital interaction in the spherical C60 explain why intersystem crossing (ISC) is a dominant process. C , produces singlet oxygen in large quantities as measured by IO2 luminescence at 1268 nm.I2 The quantum yield (@lo,) is 0.76
-
--
(8) Birks, J. B. Photophysics ofhomaric Molecules; Wiley-Interscience: New York, 1970. (9) McLean, A. J.; McGarvey, D. J.; Truscott, T.G.; Lambert, C. R.; Land, E. J. J . Chem. Soc., Faraday Trans. 1990.86, 3075. (10) Darmanyan, A. P. Chem. Phys. Left. 1982, 86, 405. ( I I ) McGlynn, S . P.;Azumi, T.;Kinoshita, M. Molecular Spectroscopy o f t h e Triplet State; Prentice-Hall: Englewood Cliffs, NJ, 1969.
0 199 1 American Chemical Society
J . Phys. Chem. 1991, 95, 12-19
12 4.5 4.0 10.'
3a
9
3.5 1o.2
1
3.0 10" 2.5 10.' L
2.0 1o.2 1.5 1o.2
I I I I I I I I I I I I I I I I I 350
400
450
500
550
600
Wavelength, nm
Figure 1. Triplet-triplet absorption spectrum of C6,,in benzene. f 0.05 (355 nm) and 0.96 f 0.04 (532 nm). Formation of IO2 occurs by energy transfer from the highly populated c60 triplet state to molecular oxygen. These values represent a lower limit for the quantum yield of triplet production (aT). (12) Ogilby, P. R.: Foote, C. S . J . Am. Chem. Soc. 1982, 104, 2069.
The lower singlet oxygen quantum yield a t 355 nm appears to be outside the experimental error and is currently being investigated. One possible explanation is that there is a lower intersystem crossing yield from upper singlet excited states than from SI.The higher singlet states may be connected with additional deactivation channels which compete with ISC. C60 also quenches singlet oxygen with an approximate rate constant k , ( ' 0 2 ) = (5 f 2) X IOs M-'s-' , a s shown by a small shortening of the lifetime of the singlet oxygen luminescence with a nearly saturated solution of c 6 0 in C6D6. The mechanism of quenching by c 6 0 is presently unknown; chemical reaction is unlikely because no loss of starting material or formation of new product (by UV-visible absorption spectra) occurs following hundreds of laser pulses under O2 at either excitation wavelength. Cm is a potent generator of singlet oxygen. Its very high singlet oxygen yield and inertness to photooxidative destruction suggests a strong potential for photodynamic damage to biological systems. Thus, the degree to which c 6 0 is present in the environment becomes a very important question.
Acknowledgment. This work was supported by NSF Grants CHE89-11916 and C H E 89-21133 (F.N.D. and R.L.W.) and N I H Grant GM-20080.
FEATURE ARTICLE How To Observe the Elusive Resonances in H or D Scattering
+ H,
-
H, or HD
+ H Reactive
William H. Miller* and John Z. H. Zhangt Department of Chemistry, University of California, Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, Berkeley, California 94720 (Received: August 14, 1990)
+
-
Short-lived collision complexes in H or D H2 (u = j = 0) H2or HD (v', j') + H reactive scattering give rise to broad resonance structure. Though this structure is not observable in the energy dependence of the integral cross section, it is readily seen in the energy dependence in the differential cross section u(B,E),as a peak along a line in the E 4 plane. The equation of this resonance line is E = E,(J(B)), where E,(J) is the resonance energy as a function of total angular momentum J (i.e., the rotational quantum number of the complex) and J(B) is the inverse function of e(J), the effective classical deflection function for the transition. Observation of this resonance structure requires cross sections to individual final (ut, j') states; it is quenched by summing over j'. It is even more enhanced in cross sections to specific final m'states with m' # 0. (m' is the helicity of the final state, the projection of the final diatomic molecule rotational angular momentum onto the final relative translational velocity vector.) The results reported are all from rigorous three-dimensional quantum mechanical reactive scattering calculations for these cross sections.
1. Introduction
Resonances continue to attract a great deal of attention in state-to-state reactive scattering and also other areas of chemical dynamics.' A scattering resonance is due to a short-lived complex of the two colliding species. Collision complexes exist also in classical mechanics2 but do not produce resonance behavior because in classical mechanics the energy of the collision complex is not required to be quantized. (In the early days of quantum mechanics the quantization of metastable states was referred to as "weak quantizationn3 because the resonance structure has a 'Current address: Department of Chemistry, New York University, New York, N Y 10003.
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finite width r in energy space that is related to the finite lifetime of the complex, T = h / r . Infinitely long-lived states, Le., bound states, have r = 0 and thus correspond to "strong quantization".) Quantization of the collision complex, Le., resonance structure, is thus a purely quantum mechanical phenomenon that can be understood semiclassicallqP as an interference effect between the (1) (a) Truhlar, D.G., Ed. Resonances in Electron-Molecule Scattering, van der Waals Complexes, and Reactive Chemical Dynamics Calculations; ACS Symposium Series No. 263; American Chemical Society: Washington, D.C., 1984. (b) Schatz, G. C. Ann. Reo. Phys. Chem. 1988, 39, 317. (2) Brumer, P.; Karplus, M.Discuss. Faraday SOC.1973, 55, 80. (3) Kemble, E. C. The Fundamental Principles of Quantum Mechanics with Elementary Applications; Dover: New YOrk, 1958; pp 178-19s.
0 1991 American Chemical Society
