J. Phys. Chem. 1992,96,4811-4814 (38) Spiro, T. G.; Li, X.-Y. In Biological Applications of Roman Specrroscopy; Spiro, T. G., Ed.; Wiley: New York, 1988; Vol. 3, pp 1-38. (39) Felton, R. H.; Linschitz, H.J. Am. Chem. Soc. 1966, 88, 1113. (40) Maslov, V. G. Opr. Specrrosc. 1974, 37, 580. (41) Li, X.-Y.; Czernuszewicz, R. S.;Kincaid, J. R.; Stein, P.; Spiro, T. G . J . Phys. Chem. 1990,94,47. (42) Li, X.-Y.; Zgierski, M . J. Phys. Chem. 1991, 95,4268. (43) Johnson, B. B.;Peticolas, W. Annu. Rev. Phys. Chem. 1976,27,465. (44) Spiro, T. G.; Stein, P. Annu. Rev. Phys. Chem. 1977, 28, 501. (45) Felton, R. H.; Yu, N.-T. In The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978; Vol 111, pp 347-393.
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(46) Spiro, T. G. In Iron Porphyrins; Lever, A. P. B., Gray, H. B.,Eds.; Addison-Wesley: Reading, MA, 1983; Part 11, pp 89-159. (47) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978; Vol 111, pp 1-165. (48) Shelnutt, J. A.; Cheung, L. D.; Chang, R. C. C.; Yu, N.-T.; Felton, R. H. J. Chem. Phys. 1977,66, 3387. (49) Sekino, H.; Kobayashi, H. J. Chem. Phys. 1987,86, 5045. (50) Prendergast, K.; Spiro, T. G. J . Phys. Chem. 1991, 95, 9728. (51) Fajer, J.; Davis, M. S. In The Porphyrins; Dolphin, D., Ed.; Academic: New York; Vol. 111, pp 197-256.
Triplet Excited State Behavior of Fullerenes: Pulse Radiolysis and Laser Flash Photolysis of C60and CT0in Benzene Nada M. Dimitrijevib* and Prashant V . Kamat* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: December 9, 1991; In Final Form: March 10, 1992)
Both pulse radiolysis and laser flash photolysis techniques have been employed to characterize the triplet excited state behavior of Csoand C70 in benzene at 296 K. Apart from the previously reported absorption in the visible, we were able to characterize its absorption in the UV region and determine the extinction coefficients in the visible region. Pulse radiolysis experiments give indirect confirmation for the spectral features of the triplet excited states. Self-quenching processes such as the ground-state quenching (2 X lo8 M-' s-I for %&* and 6 X lo8 M-'s-l for 3C70*)and triplet-triplet annihilation ( 2 k = 1.8 X lo9 M-I s-l for 3C60*and 2k = 2 X lo9 M-' s-l for T70*) can lead to the rapid deactivation of the triplet excited state.
Introduction The ease of laboratory synthesis and separation of fullerenes'" has prompted a burst of activities in many research laboratories. Several interesting properties of C, and C70 have been explored in recent months (for example, see ref 7 and the references cited therein). Their strong absorption in the visible and their long-lived triplet excited states make them potentially important in developing light-harvesting systems. Initial efforts have already been made to investigate the p h o t ~ p h y s i c a l ~ -p' ~h ~ t o c h e m i c a l , ' ~and '~ ph~toelectrochemical~~J~ properties of C, and C70 clusters. We have recently reported that these clusters can easily be reduced in colloidal ZnO suspension^.^^ The photochemically reduced fullerenes have distinctly different absorption features as compared to their corresponding triplet excited states. Most of the photophysical studies carried out to date involve characterization of triplet excited states by picosecond and nanosecond laser flash photolysis t e c h n i q ~ e s . ~ ~ ~An J ' - 'ESR ~ study has also been reported mntly.1° In all of these studies, the excited states were created directly with laser excitation of the carbon cluster or a triplet sensiti~er.~-I~ For the first time, we are able to employ a pulse radiolysis technique to generate triplet excited states of Cs0 and C70 and independently confirm their photophysical properties. The energy transfer from radiolytically generated triplet sensitizer also enables the precise determination of the extinction coefficients of 3C,* and 3C70*. There seems to be some disagreement regarding the lifetimes of both 3C60* and 3C70* as measured in different laboratories. For example, the reported lifetimes of 3Cso*vary from 408 to 280 ps9 and 3C70*vary from 13013to >200 psI4in aromatic solvents at room temperature. These values are only the lower limit since the triplet lifetimes of fullerenes have been found to be higher (up to 51 ms) in the solid matrices at low temperature.1° It has been recently suggested9 that photochemical processes such as triplet-triplet annihilation and ground-state quenching could also serve as the deactivating routes for the triplet excited state. We have now evaluated the kinetic parameters of these excited-state
processes that directly influence the observed decay of the triplet excited states of 3C60*and 3C,0*in benzene solutions. The results of the pulse radiolysis and laser flash photolysis experiments are described here.
Experimental Section Materials. The fullerite was obtained from Aldrich, and its components (C, and C70) were chromatographically separated (e.g., see ref 18) on a neutral alumina (activated, grade 1) column with hexane/toluene (955, v:v) as eluant for the separation of Ca and hexane/toluene (8020, v:v) for elution of C7* The solvent was evaporated from the separated fractions, and the dried material was redissolved in benzene for further investigations. Purified samples of C, and C70 were also donated to us by Dr. Ying Wang of Du Pont, and they were employed in the laser flash photolysis experiments. Biphenyl (Eastman Kodak) was recrystallized in toluene/methanol solution. The concentrations of fullerenes employed in the pulse radiolysis experiments were in the range of 3-25 pM in benzene, and in the laser flash photolysis they were in the range of 4 & toI0.4 mM in benzene. A biphenyl concentration of 0.05 M in benzene was employed in all the pulse radiolysis measurements. All the solutions were deaerated with nitrogen, and the experiments were carried out at room temperature (296 K). Optical M e " e n t s . Absorption spectra were recorded with a Perkin-Elmer 3840 Diode Array spectrophotometer. Laser flash photolysis experiments were performed with laser pulses from a Quanta-Ray CDR- 1 ND-YAG system (-64s pulse width). The photomultiplier output was digitized with a Tektronix 7912 AD programmable digitizer. A typical experiment consisted of a series of 3-6 replicate shots per single measurement. The average signal was processed with an LSI-11 microprocessor interfaced with V AX- 370 computer. Pulse Radiolysis. Pulse radiolysis experiments were performed with the Notre Dame 7 MeV A R C 0 LP-7 linear accelerator. The opening conditions and other technical descriptions of the setup
0022-3654/92/2096-48 11!§03.00/0 0 1992 American Chemical Society
4812 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
DimitrijeviE and Kamat
0.150
B T 0
-0.I 5 0 300
400
500
700
600
gI, a
, , , , 0 ' (
; 'C
800
Wavelength, nm
Figure 1. Time-resolved difference absorption spectra of ,Cm*. The spectra were recorded at (a) 1.5 ps (v),(b) 6.4 ps ( O ) , (c) 19.2 ps (A), and (d) 60 ps (0)following the 355-nm laser pulse (2 mJ) excitation of 15 pM C6,, in benzene.
"."""
I WK
0.025
I
I
0.5
1.0
I
1.5
2.0
io5 [fullerenel, M
Figure 3. (A) The absorption time profiles of )BP* at 360 nm recorded (a) in the presence and (c) in the absence of 16 pM Cmin benzene. The growth profile in b recorded at 730 nm shows the formation of )Cas. (B) The absorption time profiles (a) 'BP* (360 nm) and (b) 3C70*(470 nm) recorded following the radiolysis of benzene containing 0.05 M BP and 10 pM C70. (C) The dependence of pseudo-first-order constant of )BP* decay on the concentration of (0) C,, and (0) CT0(dose 6 Gy/pulse).
0
c 0 0
a -0.025 1
3 00
400
500
I
I
600
I
I
700
I
I
800
Wavelength, nm Figure 2. Time-resolved difference absorption spectra of 3C70*.The spectra were recorded at (a) 1.0 ps (V), (b) 4.0 ps (0). (c) 12.8 ps (A), and (d) 30.0 ps (0)following the 355-nm laser pulse (2 mJ) excitation of 10 pM C70in benzene.
are given elsewhere.*O Absorbed doses were in the range of 4-20 Gy/pulse. The experiments were carried out with a continuous flow of the sample solution.
Results and Discussion Absorption Characteristics of Triplet Excited State As Studied by Laser FlashwOtdysip Experimeats. Several research in recent months have made efforts to characterize the spectral properties of triplet excited states of Cmand C70.However most of these studies were limited to the characterization in the visible and IR region. Picosecond laser flash photolysis studies have shown that the triplet formation is a relatively slow process and is completed in 2-3 ns. The long-lived triplet lifetime of fullerenes makes it convenient to investigate their photochemistry by laser flash photolysis. The time-resolved T-T difference absorption spectra of 3C60* and 3c70* recorded with optical excitation (355-nm laser pulse) are shown in Figures 1 and 2, respectively. 3C60*exhibits maxima at 310,360,400, and 750 nm in the difference absorption spectrum recorded in Figure 1. The bleaching maximum ObSeNed at 333 nm coincided well with the ground-state maximum in this region. The isosbestic points corresponding to the conversion of Cminto 3C60*were observed at 325 and 343 nm. 3c70*exhibits absorption maxima at 300,320,345,370,395,525,and 575 nm and a broad absorption in the visible. The bleaching maxima at 380 and 470 nm and the minima in the difference absorption spectrum were observed as a result of structured ground-state absorption in this region. The isosbestic points observed at 375, 386,440, and 500 nm confirm the conversion of C70 to its triplet excited state. These spectral features of C70 in the UV region (300-350 nm) have not
been reported earlier. Both 3C60*and 3C70*exhibit significant absorption in the 400- and 800-nm region, although 3Cm*has a much stronger absorption band a t 750 nm. However, the characteristic difference absorption and bleaching maxima in the UV region and the isosbestic points provide a convenient way to spectrally distinguish these two excited species (i.e. 3C60*and 3C,0*), especially when they are generated together in a system containing both c 6 0 and CT0such as a fullerite solution. Pulse Radiolytic Generation of Triplet Excited State. a. Quenching of Biphenyl Triplet by Ca and C7@ Pulse radiolysis is another important fast kinetic spectroscopy technique which has been used to characterize the triplet excited state of several organic compounds in nonaqueous solvents such as b e n ~ e n e . ~ I - ~ ~ Radiolysis of a benzene solution containing a high concentration of biphenyl is known to yield relatively long lived excited biphenyl triplet. The high triplet energy of biphenyl (ET = 64.8 kcal mol-') thermodynamically favors energy transfer to the lower lying triplets such as c 6 0 (ET = 36.0 kcal mol-1)12and C70 (ET = 35.3 kcal mol-*).Io This method thus provides an alternate way of confirming the identity of triplet excited state. Since the triplet-triplet energy-transfer process (reaction 1) )BP*
+ c60 (or c70) -%BP + 3C60*(or 3C70*)
(1)
is in direct competition with the intrinsic decay (ko)of the biphenyl triplet (reaction 2)
-
3BP* BP (ko= 2.9 X lo4 s-I2O) (2) the observed rate constant (kobs)for the decay of 3BP* can be expressed as (eq 3) (3) kobs= ko + kct[C601 where k,, is the bimolecular rate constant for the energy-transfer process. The typical decay traces monitored a t 360 nm (abs max. of 3BP*) in the absence and in the presence of Cmand C70 are shown in Figure 3, parts A and B. The decay of 3BP* parallels the formation of 3C60*(730 nm) and C70* (470 nm). The pseudofirst-order rate constants for the decay of 3BP* and the growth of 3C60*and 3C70*werq measured at various concentrations of Cm and C70, and their ddpendence is illustrated in Figure 3C. The bimolecular rate constants for the triplet-triplet energy transfer obtained from the slopes of these plots were 1.7 X 1Olo M-I s-I and 2.0 X 1O'O M-' s-I for C60and C70, respectively. These high rate constant values suggest that the triplet-triplet energy transfer
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4813
Triplet Excited State Behavior of Fullerenes
b I
-5 f
J l
'5 v,
-
W
50 0
400
450
550
500
600
650
700
I
I
I
I
I
I
500
600
700
750
W a v e l e n g t h , nm
Figure 4. The transient absorption spectra of ,Casrecorded in the pulse radiolysis experiments: (a) (- -) difference absorption and (b) (-) corrected for ground-state absorption. The difference absorption spectrum was recorded 160 p s after the radiolysis of benzene containing 0.05 M BP and 10 pM Ca.
-
is a diffusion-controlled process. The absorption decay of both 3Ca* and 3C,0* exhibited clean first-order kinetics. The transient decay was independent of the dose (4-20 Gy/pulse) of the electron pulse, but was dependent on the ground-state concentration of c 6 0 and C7@The observed lifetimes of the 3Ca* and C70* at a ground-state concentration of 3 pM were 150 and 100 ps, respectively. No long-lived transients were observed following the decay of the triplet excited state. This indicated that the triplet excited state does not undergo any irreversible chemical changes. b. D e m t i o n of tbe Extinction Coefficientof Excited Triplet States. The energy-transfer method was further extended to determine the extinction coefficient of the T-T absorption of Ca and C7@The absorption changes at different wavelengths were recorded after the radiolysis of benzene solution containing biphenyl and Ca (or C70). The observed absorbance values can then be treated according to the following expression (eq 4) 1 / u a
= (Atd/A%ud)(l
+ (kO/ket[al))
-40001 400
I
Wavelength, nm
Figure 5. The transient absorption spectra of 3C70*recorded in the pulse radiolysis experiments: (a) difference absorption and (b) corrected for ground-state absorption. The difference absorption spectrum was recorded 160 ps after the radiolysis of benzene containing 0.05 M BP and 10 &M C70.
(4)
where AA, and hAd are the maximum changes in the absorbance of acceptor (a) and donor (d) triplets, with At, and hed as the difference extinction coefficients, at monitoring wavelengths LI and &, respectively. In the present measurements, the monitoring wavelength L1for 3BP* was 360 nm and L2was varied in the spectral region of e 7 4 0 nm. (Strong absorption of jBP* limited the precision of the measurements below 400 nm.) The intercept (Atd/AtaAAd)of the plot of l / M a versus l/[a] was used to determine the absolute value of the difference extinction mfficient of the acceptor triplet. For example, by substituting the value of q of 3BP* at 360 nm as 27 100 M-I cm-1,24we obtain the values of A€ of 3Cso*as 12 OOO M-l cm-I at 740 nm and of 3C70*as 2000 M-I cm-I at 435 nm. The corrected extinction coefficients for these two triplets are 12000 M-l cm-' (740nm) and 11 500 M-' cm-l (435 nm), respectively. These are similar to the values of 15000 M-I cm-I for 3C60*9and 11 000 M-I cm-I for 3C70*13 determined earlier by photolytic methods. The uncorrected and corrected spectra of extinction coefficient versus wavelength for 3C60*and 3C70*are shown in Figures 4 and 5, respectively. The uncorrected spectra resembled closely the spectra recorded in Figures 1 and 2. The transient spectra corrected for ground-state absorption have higher extinction coefficient values at wavelengths below 600 nm. This is especially so for 3C70*in which a prominent absorption maximum appears at 485 nm. These features are similar to the spectral characteristics reported earlier by photolytic methods. Thus with the pulse radiolysis experiments we were able to confirm independently the absorption characteristics of ,CW*and 3c70*. Deactivation of the Triplet Excited State by Self-Quenching. As indicated in the earlier section, the triplet lifetimes of 3C60*
00' 00
01
02
03
[ Fullerene],
04
I
05
mM
Figure 6. The dependence of the observed decay rate constant of (a) T,* ( 0 )and (b) ,C7,,* (W) on the concentration of C, and C70, respectively. Insert shows the absorption-time profiles (400 nm) of 3C,0* a t C70 concentrations of (a) 0.035 mM and (b) 0.15 mM. The triplet concentration was kept to a minimum (-2 pM) by employing low-energy (
