Photophysical and nonlinear optical properties of [60]fullerene derivatives

Article abstract of DOI:10.1021/jp9801121

A systematic spectroscopic study of a series of C₆₀ derivatives with different cage functionalizations is reported. The absorption spectra and absorptivities of the derivatives in solution were measured and compared. By recording the fluorescence spectra using a near-infrared-sensitive emission spectrometer (extending to 1200 nm), fluorescence quantum yields of the derivatives were determined quantitatively. Fluorescence lifetimes of the derivatives were obtained using the time-correlated single photon counting technique. The results show that both fluorescence quantum yields and lifetimes are rather similar for the different classes of C₆₀ derivatives. The nonlinear absorptive properties of the derivatives were evaluated by optical limiting measurements in solution and in polymer film using the second harmonic of a Q-switched Nd:Yag laser at 532 nm. Effects of different fullerene cage functionalizations on the photophysical properties and optical limiting responses of the C₆₀ derivatives are discussed.

Full text of DOI:10.1021/jp9801121

5520
J. Phys. Chem. A 1998, 102, 5520-5528
Photophysical and Nonlinear Optical Properties of [60]Fullerene Derivatives†
Ya-Ping Sun,* Glenn E. Lawson, Jason E. Riggs, Bin Ma, Naixing Wang, and
Dwella K. Moton
Department of Chemistry, Howard L. Hunter Chemistry Laboratory, Clemson UniVersity,
Clemson, South Carolina 29634-1905
ReceiVed: NoVember 6, 1997; In Final Form: February 3, 1998
A systematic spectroscopic study of a series of C60 derivatives with different cage functionalizations is reported.
The absorption spectra and absorptivities of the derivatives in solution were measured and compared. By
recording the fluorescence spectra using a near-infrared-sensitive emission spectrometer (extending to 1200
nm), fluorescence quantum yields of the derivatives were determined quantitatively. Fluorescence lifetimes
of the derivatives were obtained using the time-correlated single photon counting technique. The results
show that both fluorescence quantum yields and lifetimes are rather similar for the different classes of C60
derivatives. The nonlinear absorptive properties of the derivatives were evaluated by optical limiting
measurements in solution and in polymer film using the second harmonic of a Q-switched Nd:Yag laser at
532 nm. Effects of different fullerene cage functionalizations on the photophysical properties and optical
limiting responses of the C60 derivatives are discussed.
Introduction
quantum yields are larger than those in the fluorescence life-
times. Particularly interesting is the fact that the methano-C60
derivatives with very different functional groups on the methano
bridge exhibit essentially the same photophysical behavior.
In addition, the nonlinear absorptive properties of the C60
derivatives as measured by their optical limiting responses
The electronic transitions and excited state properties of
fullerenes, especially [60]fullerene (C60), have attracted much
30
1
,2
attention.
It is now well established that the low-lying
electronic transitions in C60 are only weakly allowed because
of the high degree of symmetry in the closed-shell electronic
configuration. The absorption band of C60 in the visible is very
weak, with the molar absorptivity at the band maximum of only
toward a nanosecond pulsed laser at 532 nm are similar to that
24
of C60.
However, C60 derivatives have more favorable
solubility characteristics, which are advantageous in the fabrica-
tion of the nonlinear absorptive materials into optical devices.
In this paper, we report a systematic spectroscopic study of
a series of C60 derivatives with different cage functionalizations.
The absorption spectra and absorptivities of the derivatives in
solution were measured and compared. By recording the
fluorescence spectra using a near-infrared (IR)-sensitive emis-
sion spectrometer (extending to 1200 nm), fluorescence quantum
yields of the derivatives were determined quantitatively. Fluo-
rescence lifetimes of the derivatives were obtained by the time-
correlated single photon counting technique. The results show
that both fluorescence quantum yields and lifetimes are rather
similar for the different classes of C60 derivatives. The nonlinear
absorptive properties of the derivatives were evaluated by optical
limiting measurements in solution and in polymer film using
the second harmonic of a Q-switched Nd:Yag laser at 532 nm.
Effects of different fullerene cage functionalizations on the
photophysical properties and optical limiting responses of the
C60 derivatives are discussed.
-
1
-1 3-5
∼
950 M cm .
In fact, only a small fraction of the broad
visible absorption band in the 430 ∼ 670 nm region is due to
the contribution of the transition to the lowest excited singlet
6
state. Upon photoexcitation, the excited singlet state decays
are dominated by intersystem crossing to the excited triplet state.
As a result, C60 is only weakly fluorescent, with a very low
-4
fluorescence quantum yield of 3.2 × 10 in room-temperature
toluene.6 The extremely efficient intersystem crossing produces
a high population of the excited triplet state. Because of higher
triplet-triplet transient absorption cross sections than the
ground-state absorption cross sections,7,8 C60 is an excellent
broad-band nonlinear absorber for potential optical limiting and
optical switching applications.9
-24
With a large number of methods developed for functionalizing
the fullerene cage,25,26 various C60 derivatives have recently been
studied for an understanding of the effects of derivatization on
the photophysical properties of C60.2
7-30
Among the compounds
under active investigations are methano-C60 and pyrrolidino-
C60 derivatives, which represent two important classes of
25,26,31,32
30
functionalized fullerene molecules.
It has been shown
Experimental Section
that the photophysical properties of methano-C60 and pyrroli-
dino-C60 derivatives are somewhat different from those of C60.
The derivatives have higher ground-state absorptivities than C60
due largely to a reduction in molecular symmetry. The increases
in transition probability are also reflected in the larger fluores-
cence radiative rate constants of the derivatives. As a result,
from C60 to the derivatives, the increases in the fluorescence
Materials. C60 (purity >99.5%) was obtained from Bucky-
USA and was used without further purification. All solvents
are of spectrophotometry grade. Because no interference of
possible impurities in the wavelength range of interest was found
in absorption and emission spectroscopic measurements, the
solvents were used as received.
Methano-C60 Derivatives I-III. The methano-C60 deriva-
tive I was prepared by the reaction of C60 with the stabilized
†
This paper is dedicated to Professor Jack Saltiel on the occasion of his
0th birthday.
3
0,33
6
sulfonium ylide.
The one-pot preparation was carried out
S1089-5639(98)00112-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/26/1998

Optical Properties of [60]Fullerenes
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5521
under phase transfer condition. A solution of 500 mg of C60
the reaction vessel. The solution mixture was purged with dry
nitrogen gas for ∼1 h before photoirradiation, and the loss of
solvent during the nitrogen purging was prevented by attaching
a condenser to the outlet of the reaction vessel. An aqueous
solution of potassium chromate (0.1 g/mL) was used as a liquid
cutoff filter (505 nm). The photoirradiation was for 70 min
under nitrogen atmosphere. The solution was then put on a
rota-vap to remove the solvent toluene. The solid reaction
mixture was extracted repeatedly using CS2. The CS2 solution
was then concentrated and separated on a silica gel column using
hexane-50% (v/v) toluene, methylene chloride, and then
methylene chloride -0.8% (v/v) methanol as eluents, yielding
150 mg of the amino-C60 derivative VII (23% yield). The
compound was positively identified in matrix-assisted laser
(0.69 mmol) in 500 mL of anhydrous toluene, 220 mg of
sulfonium bromide (1.2 mmol), 1 g of anhydrous K2CO3 (7.1
mmol), and 50 mg of the phase transfer catalyst tetrabutylam-
monium bromide (TBAB, 0.155 mmol) were mixed in a 1-L
round-bottomed flask. After purging with dry nitrogen gas, the
flask was sealed and the reaction mixture was stirred at room
temperature for 24 h. The stabilized sulfonium ylide generated
in situ due to the deprotonation of the sulfonium salt by K2CO3
under the catalysis of TBAB undergoes nucleophilic addition
to C60, followed by intramolecular substitution to form the
methano-C60 derivative I with the simultaneous elimination of
dimethyl sulfide. The reaction mixture was filtered to remove
any solids and then concentrated by evaporation under reduced
pressure. The matrix-assisted laser desorption ionization time-
of-flight MS analysis shows the presence of mono-, bis-, and
tris-adducts. The monoadduct was obtained after separation
from higher order adducts through silica gel column chroma-
tography using toluene as an eluent. The yield for the methano-
13
desorption ionization time-of-flight MS and proton and
C
3
8
NMR characterizations.
13
C60 derivative I is ∼50%. Proton and C NMR results clearly
show that the methylene bridge is at the [6,6]-ring junction.
The methano-C60 derivative II was obtained from the
hydrolysis of I.31 In the preparation, 350 mg of I (0.42 mmol)
and 151 mg of 4-toluenesulfonic acid (0.795 mmol) in 275 mL
toluene were refluxed for ∼8 h. Then, 275 mL water was
added, and the mixture was stirred for 30 min. The water layer
was decanted, and the toluene layer was filtered to yield a brown
solid. The solid sample was washed with water and ethanol
and then dried for ∼12 h at 60 °C, yielding 185 mg of the
derivative II (57% yield).
The methano-C60 derivative III was prepared in the coupling
reaction of II with diethylamine (DEA) using 1-ethyl-3-
(
dimethylaminopropyl)carbodimide (EDAC) as a coupling
34
agent. In the preparation, a mixture of 20 mg of II, 6 µL of
DEA, 6 µL of triethylamine, and 9.2 mg of EDAC in 6 mL of
bromobenzene was stirred for ∼20 h. The reaction mixture was
separated on a silica gel column using chloroform as an eluent,
yielding 10 mg of the derivative III (47% yield).
Measurements. Absorption spectra were obtained using a
computer-controlled Shimadzu UV2101-PC spectrophotometer.
Emission spectra were recorded on a Spex Fluorolog-2 photon-
counting emission spectrometer equipped with a 450-W xenon
source, a Spex 340S dual-grating and dual-exit emission
monochromator, and two detectors. The two gratings are blazed
at 500 nm (1200 grooves/mm) and 1000 nm (600 grooves/mm).
The room-temperature detector consists of a Hamamatsu R928P
photomultiplier tube operated at -950 V, and the thermoelec-
trically cooled detector consists of a near-IR-sensitive Hamamat-
su R5108 photomultiplier tube operated at -1500 V. In
fluorescence measurements, a Schott 540 nm (GG-540) or 610
nm (RG-610) color glass sharp-cut filter was placed before the
emission monochromator to eliminate the excitation scattering.
Minor distortion at the blue onset of the observed fluorescence
spectra due to the filter was corrected by use of the transmittance
profile of the filter. The slit of the excitation monochromator
was 5 mm (19 nm resolution). For the emission monochro-
mator, a wide slit of 5 mm (19 nm resolution) was used in
fluorescence quantum yield measurements to reduce experi-
mental uncertainties, and a narrow slit of 0.5 mm (2 nm
resolution) was used in fluorescence spectral measurements to
retain structures of the spectra. Unless otherwise specified,
fluorescence spectra were corrected for nonlinear instrumental
response with predetermined correction factors. The correction
factors for the emission spectrometer were carefully determined
with a calibrated radiation standard from Optronic Laboratories.
Fluorescence decays were measured by the time-correlated
single photon counting (TCSPC) method. The TCSPC setup
consists of a Hamamatsu Stabilized Picosecond Light Pulser
Methano-C60 Derivatives IV and V. The methano-C60
derivative IV was prepared following a procedure reported in
35
the literature, except that a lower molar ratio of bromomalonate
to C60 (1.2:1) was used to promote the formation of monoadduct.
In the reaction, nucleophilic addition of bromomalonate car-
banion from the deprotonation of bromomalonate with sodium
3
5
hydride was followed by intramolecular substitution. The
reaction mixture consists of mono-, bis-, and trisadducts. The
monoadduct was separated from the mixture in 55% yield by
silica gel column chromatography using toluene as an eluent.
The derivative V was obtained from the hydrogenolysis of IV
by a procedure reported in the literature.36
C60 Derivative VI. The C60 derivative VI was obtained as
a byproduct in the photochemical reaction of C60 with trieth-
ylamine in an air-saturated toluene solution.37 The purification
and structural characterization of the compound will be reported
separately.
Amino-C60 Derivative VII. The amino-C60 derivative VII
was prepared in the photochemical reaction of C60 with N,N′-
dimethyl-1,2-ethylenediamine. The photoirradiation was carried
out by use of an ACE Glass ACE-7861 type immersion-well
photochemical reaction assembly equipped with a 450-W
Hanovia medium-pressure mercury lamp. A solution of 518
mg (5.9 mmol) of N,N′-dimethyl-1,2-ethylenediamine in 40 mL
of toluene was added in a dropwise manner with stirring to a
solution of 580 mg (0.81 mmol) of C60 in 260 mL of toluene in

5522 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Sun et al.
Figure 1. Experimental setup for optical limiting measurements.
(
PLP-02) as the excitation source, which produces ∼33 ps
(fwhm) light pulses at 632-nm with a repetition rate of 1 MHz.
Fluorescence decays were monitored through a 695-nm color
glass sharp-cut filter. The detector consists of a Hamamatsu
R928P photomultiplier tube operated at -1 kV using an EG&G
Ortec 556 high-voltage power supply. The detector electronics
from EG&G Ortec include two 9307 discriminators, a 457
biased time-to-amplitude converter, and a 916A multichannel
analyzer. The instrument response function of the setup has a
fwhm of ∼2.5 ns. Fluorescence lifetimes were determined from
observed decay curves and instrument response functions
through deconvolution by the Marquardt nonlinear least-squares
method.
Figure 2. Absorption spectra of the methano-C60 derivatives II
(
-‚-‚-), III (- - -), and V (-‚‚-‚‚-), the C60 derivative VI (s), and
the amino-C60 derivative VII (‚‚‚) in room-temperature solutions
(Table 1).
Optical limiting measurements were performed with the setup
illustrated in Figure 1. This setup consists of a Continuum
Surelite-I Q-switched Nd:YAG laser as the light source, which
can be operated from a single shot to 10 Hz. The IR
fundamental was frequency doubled to generate the second
harmonic at 532 nm which was isolated with a Surelite harmonic
separation package. The maximum energy at 532 nm is 160
mJ/pulse, with a 5-ns pulse width (fwhm). The laser output
was varied in a range 10-160 mJ/pulse with a waveplate-
polarizer combination. The laser beam has a diameter of 6 mm,
2
corresponding to energy densities 0.035-0.57 J/cm . A Galilean
style telescope consisting of a plano-concave lens and a plano-
convex lens was used to reduce the laser beam waist to 3 mm
2
in diameter for higher energy densities up to 2.2 J/cm . A
Scientech Mentor MC2501 calorimeter and a MD10 m were
used as the detector. Solution samples were measured in a
cuvette with 2 mm optical path length.
Figure 3. A comparison of the absorption and fluorescence (inset)
spectra of different classes of C60 derivatives: the methano-C60
derivative II (-‚-‚-), the pyrrolidino-C60 derivative N-ethyl-trans-2′,5′-
dimethylpyrrolidino[3′,4′:1,2]C60 (-‚‚-‚‚-),30 the C60 derivative VI (s),
and the amino-C60 derivative VII (- - -).
Results
UV-Vis Absorption. UV-vis absorption spectra of the C60
derivatives were measured in room-temperature (22 °C) solu-
tions. The spectra of the methano-C60 derivatives shown in
in Figure 3. An interesting observation is that although the
absorption spectral profiles of different classes of C60 derivatives
are somewhat different, their absorptivities in the visible region
are rather similar.
Figure 2 are similar to those of other methano-C60 derivatives
reported previously.2
5-31
For the C60 derivative VI, the absorp-
Fluorescence Spectra. Fluorescence spectra of the C60
derivatives in room-temperature solutions were measured with
an emission spectrometer that is sensitive in the near-IR region
(up to 1200 nm). As shown in Figure 4, the spectra of the
different derivatives have similar features, with a peak at 690-
695 nm and two shoulders at longer wavelengths. Shown in
the inset of Figure 3 is a more direct comparison of the different
classes of C60 derivatives. The relative intensities of the peak
and shoulders in the fluorescence spectra of the derivative VI
and the amino-C60 derivative VII are quite similar.
tion spectrum is more similar to those of pyrrolidino-C60
30,32
derivatives.
However, the absorption spectrum of the amino-
C60 derivative VII is noticeably different. Although it has a
shoulder at ∼695 nm, as in other monofunctionalized C60
derivatives, the sharp peak at 400-450 nm as a common feature
in the spectra of methano- and pyrrolidino-C60 derivatives and
the derivative VI is absent. The similarities and differences
between the absorption spectra of different classes of C60
derivatives are easily recognized in a direct comparison shown

Optical Properties of [60]Fullerenes
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5523
Figure 5. The fluorescence decay of the C60 derivative VI in room-
temperature toluene. The solid line is from least-squares fit.
Figure 4. Fluorescence spectra of the methano-C60 derivatives II
(
-‚-‚-), III (- - -), and V (-‚‚-‚‚-), the C60 derivative VI (s), and the
amino-C60 derivative VII (‚‚‚) in room-temperature solutions (Table
).
1
TABLE 1: Photophysical Properties of the C60 Derivatives
ABS0-0
ꢀ
0-0
Φ
F
τ
F
compound solvent (nm) (M- cm-1
1
)
FLSC0-0 (10-3) (ns)
C
I
II
III
IV
V
VI
VII
60
toluene
toluene
THF
0.32 1.2
692
694
692
688
690
694
694
190
185
176
195
713
716
713
715
715
712
713
1.0
0.8
1.49
1.46
CHCl
CHCl
THF
3
0.99 1.45
1.0 1.48
3
CHCl
3
283
150
0.93 1.6
0.91 1.3
toluene
Fluorescence Quantum Yields. Fluorescence quantum
yields of the C60 derivatives in room-temperature solutions were
determined quantitatively in reference to the yield of C60 (3.2
-
4
×
10 ), which was obtained using rhodamine 101 in ethanol
6
as a fluorescence standard (ΦF ) 1.0). Because different
solvents were used because of solubility considerations, the
results of experimental measurements were corrected for effects
due to changes in solvent refractive index n.39
-2
Φ ) Φ ′(n/n )
(1)
F
F
SD
where SD represents the solvent for the standard from which
the uncorrected fluorescence yield value ΦF′ is obtained. The
C60 derivatives are apparently more fluorescent than the parent
C60, with the yields of the derivatives being ∼3 times higher
than that of C60 (Table 1). However, despite different molecular
structures of the C60 derivatives, their fluorescence yields are
-
4
rather similar, varying in a narrow range of 8 × 10 to 1 ×
-
3
1
0
(Table 1). The observed fluorescence quantum yields of
Figure 6. Optical limiting results of the methano-C60 derivatives I in
chloroform (O), II in chloroform - 10% v/v DMSO (3), III in
chloroform (0), IV in chloroform (4), and V in THF (]) solutions
with 55 and 70% linear transmittances at 532 nm.
the C60 derivatives are excitation wavelength independent.
Fluorescence Lifetimes. Fluorescence decays of the C60
derivatives in room-temperature solutions were measured by
the TCSPC method. The result for the derivative VI is shown
in Figure 5 as an example. The decays can be deconvoluted
well from corresponding instrumental response functions using
a monoexponential equation. The fluorescence lifetimes of the
C60 derivatives are somewhat longer than that of C60, varying
in a range of 1.3 to 1.6 ns (Table 1).
the setup shown in Figure 1. For the methano-C60 derivatives
I-V, optical limiting responses were compared by using
solutions with linear transmittances of 55 and 70% at 532 nm.
The solutions show significant optical limiting, with a nonlinear
relationship between the output (IOUT) and input (IIN) light
fluences (Figure 6). At high input fluences, IOUT reaches a
plateau. For the C60 derivative III, as an example, the saturated
Optical Limiting. The optical limiting properties of the C60
derivatives in room-temperature solutions were measured with

5524 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Sun et al.
Figure 8. Optical limiting responses of the C60 derivative VI in toluene
solution of 72% linear transmittance at 532 nm.
Figure 7. Plots of the output light fluences for the methano-C60
derivatives I (O), II (3), III (0), IV (4), and V (]) versus those for
C
60 at the same input light fluences and the same linear transmittance
of 70% at 532 nm.
TABLE 2: Optical Limiting Properties of the C60
Derivatives
I
OUT at saturation (J/cm2)
sample
solvent
T ) 55% T ) 70% T ) 72%
C
I
II
III
IV
V
60
toluene
0.05
0.06
0.06
0.06
0.06
0.07
5
0.1
0.12
CHCl
CHCl
CHCl
CHCl
THF
3
3
3
3
0.11
0.11
0.11
0.1
+ 10%DMSO
0.12
VI
VII
toluene
toluene
0.12
0.13
5
0.06
2
IOUT values at the plateau are ∼0.06 and ∼0.11 J/cm for the
solutions of 55 and 70% linear transmittances, respectively. A
comparison of the saturated IOUT values for the different
methano-C60 derivatives in room-temperature solutions is shown
in Table 2. The optical limiting responses of the methano-C60
derivatives are quite similar to those of C60 in toluene. For a
more direct comparison, shown in Figure 7 are plots for the
IOUT values of the methano-C60 derivatives vs the IOUT values
of C60 at the same input fluences. The plots are close to linear,
with slopes near unity, indicating that the nonlinear absorptions
in the methano-C60 derivatives and in C60 are essentially the
same.
Figure 9. Plots of the output light fluences for the C60 derivatives VI
(0) and VII (O) versus those for C60 at the same input light fluences
and the same linear transmittance of 72% at 532 nm.
tances, respectively. Shown in Figure 9 is again a more direct
comparison of the optical limiting responses between C60 and
the amino-C60 derivative VII at the linear transmittance of 72%,
where the IOUT values of VII are plotted against the IOUT values
of C60 at the same input fluences. This plot is also close to the
The optical limiting responses of the C60 derivative VI in
room-temperature toluene solution with a linear transmittance
of 72% were determined. As shown in Figure 8, the IOUT is
also strongly nonlinear with respect to IIN, reaching a plateau
4
5° line.
A systematic comparison for the optical limiting responses
2
at the input fluence of ∼0.3 J/cm . The saturated IOUT value at
of C60 and the C60 derivatives in room-temperature solutions
with similar linear transmittances at 532 nm is shown in
Table 2.
2
the plateau is ∼0.125 J/cm , which is essentially the same as
that of C60 in toluene solution. A direct comparison of IOUT
values of the derivative VI and C60 at the same IIN values is
shown in Figure 9. The plot is close to the 45° line, which
shows again that the optical limiting responses of the derivative
VI are similar to those of C60.
Optical limiting measurements were also carried out for the
methano-C60 derivative I incorporated in polymethyl methacry-
late (PMMA) films. The films of ∼0.4 mm thickness were
prepared by solution casting. In the preparation, a viscous
toluene solution of the C60 derivative I and PMMA polymer
(MW ∼ 315 000) was added to a glass mold to allow a slow
evaporation of the solvent toluene. The film samples were then
allowed to cure for several days to ensure a complete removal
of the solvent. The linear transmittance of the films was varied
by adjusting the concentration of the C60 derivative I in the
The amino-C60 derivative VII exhibits similar optical limiting
responses (Figure 10). For VII in room-temperature toluene
with linear transmittances of 55 and 72% at 532 nm, the output
fluences reach saturation at input fluences of ∼0.2 and ∼0.3
2
J/cm , respectively, with the saturated IOUT values of ∼0.06 and
2
∼
0.13 J/cm for the solutions of 55 and 72% linear transmit-

Optical Properties of [60]Fullerenes
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5525
For the films, the output fluences show no plateau even at input
2
fluences of 1 J/cm and higher (Figure 11).
Discussion
The functionalized fullerene molecules studied here represent
different classes of C60 derivatives. They have different
molecular structures, with significantly different functional
groups on the fullerene cage. However, the photophysical
properties of the derivatives, though noticeably different from
those of the parent C60, are rather similar. Qualitatively, the
overall increases in molar absorptivities from C60 to the C60
derivatives are nearly the same. Because the absorptivity
increases may be attributed to a reduction in molecular sym-
metry due to the functionalization of the C60 cage, the results
indicate that the electronic transitions in the C60 derivatives
reported here and those already reported in the literature are
likely dictated by the electronic structures of the functionalized
fullerene moiety. In addition, a common feature in the
absorption spectra of the different C60 derivatives is the weak
Figure 10. Optical limiting results of the amino-C60 derivative VII in
toluene solutions with 55% (O) and 72% (0) linear transmittances at
2
7-32
0
-0 transition band near 695 nm.
It may be concluded
qualitatively that the lowest electronic transitions in different
classes of C60 derivatives have similar energies and transition
probabilities.
5
32 nm.
In a more quantitative comparison, the absorption spectrum
of the C60 derivative VI is somewhat different from those of
methano-C60 derivatives, but rather similar to those of pyrro-
lidino-C60 derivatives (Figure 3). The similarity may have
something to do with the fact that in the C60 derivative VI and
pyrrolidino-C60 derivatives the fullerene cage is functionalized
through the [6,6]-junction with a five-member ring, though the
derivative VI has a cage-oxygen linkage. On the other hand,
the cage functionalization in the amino-C60 derivative VII is
also 1,2-addition through the closed [6,6]-ring junction, but its
absorption spectrum is noticeably different from the spectra of
other classes of C60 derivatives. The most obvious difference
in the spectrum of the amino-C60 derivative VII is the absence
of a well-resolved peak at 400 - 450 nm, which is well-known
as a characteristic feature in the spectra of methano-C60 and
2
7-32
pyrrolidino-C60 derivatives.
Although the amino-C60 de-
rivative VII consists of a six-membered ring instead of the five-
membered ring in the C60 derivative VI and pyrrolidino-C60
derivatives and the three-membered ring in methano-C60 deriva-
tives, its different absorption spectrum is probably due primarily
to the cage-nitrogen linkages because the spectra of the C60
derivatives that consist of a cyclohexane ring are similar to those
27a
of methano-C60 and pyrrolidino-C60 derivatives.
In a separate
note, because the absorption spectrum of the amino-C60 deriva-
tive has only limited structures, it seems no surprise that upon
attaching C60 cages to polyethylenimine, the absorption spec-
trum of the C60-aminopolymer becomes a structureless curve
due to spectral broadening effects associated with polymeric
4
0
structures.
The fluorescence properties of the C60 derivatives are
apparently insensitive to the molecular structural differences in
the derivatives. The fluorescence spectra of the different classes
of C60 derivatives are rather similar, with only minor changes
in the relative intensities of the vibrational bands (Figures 3
and 4). For all the derivatives, the 0-0 bands in the fluores-
cence spectra correspond well to the 0-0 bands in the absorption
spectra (Figure 3). The mirror-image relationship between the
0-0 absorption and fluorescence bands suggests that the lowest-
energy absorption and emission are associated with the same
excited singlet state for the C60 derivatives in room-temperature
solution. It may be argued that the lowest-energy electronic
Figure 11. Optical limiting results of the methano-C60 derivative I in
PMMA films (∼0.4 mm thickness) with linear transmittances of 79
and 44% at 532 nm (O). The results of the compound in toluene
solutions with the same linear transmittances (4) are also shown for
comparison.
films. Shown in Figure 11 are the optical limiting responses
of the films with linear transmittances of 79% and 44% at 532
nm, which are compared with the results of the C60 derivative
I in toluene solutions of the same linear transmittances.
Although both films show optical limiting, the responses are
much weaker in the films than in the corresponding solutions.

5526 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Sun et al.
transitions in C60 derivatives are dictated by the functionalized
C60 cage and are little affected by the functional groups in
different classes of C60 derivatives.
SCHEME 1
The observed absorption spectra of the different classes of
C60 derivatives suggest that the different cage functionalizations
have more significant effects on the higher-energy electronic
transitions. The results may be used as evidence for the
argument that the observed absorptions in the visible region
(440-650 nm) are due in large part to contributions of electronic
transitions other than the transition to the lowest excited singlet
state. The absorption contributions in the region should
therefore not be included in the calculation of fluorescence
41
transition probabilities in terms of the Strickler-Berg equation.
With the assumption that the absorption due to the lowest
electronic transition may be estimated by the mirror image of
the observed fluorescence spectrum, the calculated fluorescence
radiative rate constants kF,c of the C60 derivatives are somewhat
smaller than the experimental kF,e values obtained from the
By assuming that the populations in the upper excited states
are negligible,
30
fluorescence quantum yield and lifetime results. For C60, it
5
,6,42,43
dN /dt ) σ N i - (k + kISC)NS
(3)
(4)
has been shown
that a similar estimate of the lowest-
S
G
G
SG
lying electronic transition probability by using the assumed
fluorescence-absorption mirror image relationship results in a
kF,c value somewhat larger than the kF.e value. Nevertheless,
because the calculation of kF,c values is sensitive to the rough
assumption of the mirror-image relationship between fluores-
cence and absorption and also to other approximations associated
with the Strickler-Berg equation,41 some discrepancies between
kF,c and kF,e values might be expected. Thus, the mirror-image
assumption, though a rough approximation, still provides a
qualitative measurement for the portion in the observed absorp-
tion spectrum that is due to the transition to the lowest excited
singlet state in C60 and C60 derivatives.
The fluorescence quantum yields and lifetimes of the different
C60 derivatives are all very similar. The results further support
the conclusion that the electronic transitions in C60 derivatives
are dictated by the electronic structures of the functionalized
fullerene moiety and are little affected by the functional groups
in different classes of C60 derivatives. From C60 to the C60
derivatives, both fluorescence quantum yields and lifetimes
increase. However, the increases in quantum yields are larger
than those in lifetimes, corresponding to larger fluorescence
radiative rate constants of the C60 derivatives than that of
C60. The results are consistent with higher molar absorptivities
for the 0-0 transitions in the C60 derivatives. From the com-
parison of the C60 derivatives, it is obvious that the difference
in the fluorescence properties is much less significant among
different classes of C60 derivatives than between the derivatives
and C60.
dN /dt ) k N - k N
TG T
T
ISC
S
An expression for NG is not necessary because of the relationship
NG ) N - (NS + NT), where N is the total molecular population.
Reverse saturable absorption occurs when the excited-state cross
sections are larger than the ground-state cross section, σEFF/σG
> 1, where σEFF includes a weighted average of σS and σT.44
This situation is certainly the case for C60, with both σS and σT
7
,8,10
significantly larger than σG at 532 nm.
For modeling the
optical limiting properties of C60 in toluene solution, McLean
10
et al. have used numerical integrations to solve the differential
eqs 2-4 of the five-level model for reverse saturable absorption
(Scheme 1). The results show that the optical limiting of C60
follows the five-level model with incident light fluences at 532
2
nm up to ∼1 J/cm , and that for the 8-ns pulsed laser, the optical
limiting response is due predominantly to the strong absorption
of the excited triplet state of C60.
The optical limiting properties of the C60 derivatives may
similarly be explained in terms of the reverse saturable absorp-
tion mechanism (Scheme 1). Because photoexcited methano-
and pyrrolidino-C60 derivatives generate singlet molecular
4
5
oxygen as efficiently as C60 in solution, their intersystem
crossing yields should be close to unity like C60. For other C60
derivatives, available experimental results also show large
2
7a,46
intersystem crossing yields.
Thus, the optical limiting
responses of the C60 derivatives under consideration may again
be attributed predominantly to the stronger excited triplet-state
absorption than the ground-state absorption, σT/σG > 1.
However, what is interesting is that the optical limiting responses
of different classes of C60 derivatives are not only nearly
identical among themselves but also essentially the same as those
of C60 in room-temperature solutions (Figures 7 and 9). With
the great similarities among the excited singlet-state properties
of different classes of C60 derivatives, their excited triplet-state
properties may also be expected to be very similar. Such
similarities may explain the same optical limiting responses of
different classes of C60 derivatives.
Similar to the parent C60, the C60 derivatives exhibit signifi-
cant optical limiting responses toward a nanosecond pulsed laser
at 532 nm. The optical limiting properties of C60 have been
explained in terms of a reverse saturable absorption mecha-
1
0
nism.
A five-level model (Scheme 1) has been used to
describe the reverse saturable absorption in fullerenes and
organic dyes. As the incident light transmits through the sample
-
2
-1
with a pathlength of L, the photon flux i (photons cm s )
changes with the distance x in a nonlinear fashion due to
significant excited-state absorptions.
The transient absorption properties of some C60 derivatives
27a,d,g,28
di/dx ) -σ N i - σ N i - σ N i
(2)
have been reported.
In particular, it has been shown that
G
G
S
S
T T
the triplet-triplet molar absorptivities of the C60 derivative IV
2
28
where σ denotes absorption cross sections (cm ) and the
subscripts (G, S, and T) indicate the corresponding electronic
states as shown in Scheme 1, and N represents the molecular
populations in the different states, which are time dependent.
are smaller than those of C60. Such a decrease in the excited-
state absorptivities appears to be a general phenomenon associ-
ated with the mono-functionalization of the C60 cage.28 Even
with the characteristic blue-shift in the triplet-triplet absorption

Optical Properties of [60]Fullerenes
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5527
spectrum from C60 to the derivatives such as IV,28,47 the triplet-
state absorption cross section at 532 nm is still smaller in the
derivative than in C60. On the other hand, the ground-state
absorption cross section of the C60 derivative is somewhat higher
than that of C60. Thus, (σT/σG)Derivative < (σT/σG)C60. The fact
that the optical limiting responses of C60 derivatives and C60
are essentially the same suggests other contributions beyond
the nonlinear absorption associated with the (σT/σG) factor. It
is a possibility that the contribution due to the excited singlet-
state absorption is more significant in the C60 derivatives than
in C60. The fluorescence lifetimes of the derivatives are longer
than that of C60 (Table 1). As a result, the longer-lived singlet-
singlet transient absorption in the C60 derivatives may contribute
more to the limiting of the nanosecond laser pulse. However,
in view of the significantly different optical limiting responses
for the C60 derivative I in solution versus in PMMA film (Figure
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(
8
1
1), optical limiting contributions from mechanisms other than
the reverse saturable absorption may not be ruled out completely.
Similarly, different optical limiting responses for C60 in solution
versus in PMMA matrix have been reported.21 It seems difficult
to explain the results within the context of the reverse saturable
absorption mechanism because, according to laser flash pho-
tolysis measurements, the triplet transient absorption spectra of
C60 in solution and in PMMA film are quite similar.48 For C60,
there is a possibility that the fullerene molecules form aggregates
in the polymer matrix due to solubility characteristics. The
interactions between excited singlet-state and ground-state
fullerene molecules in the aggregates may compete with
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population and thus weaker optical limiting responses. How-
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C60 derivative because of its much improved solubilities in
common organic solvents, from which the polymer films can
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the optical limiting responses of the C60 derivative in solution
and in PMMA film may suggest more complicated mechanisms,
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Acknowledgment. We thank Bing Liu, Xian-Fu Zhang, and
Christopher Bunker for experimental assistance. Financial
support from the National Science Foundation (CHE-9320558
and CHE-9727506) is gratefully acknowledged. The research
assistantship for Bin Ma was provided in part by the Department
of Energy through DOE/EPSCoR cooperative agreement DE-
FG02-91ER75666. Dwella Moton was a participant in the
Summer Undergraduate Research Program sponsored jointly by
the National Science Foundation (CHE-9100387 and CHE-
9
5-10, 84. (c) Signorini, R.; Zerbetto, M.; Meneghetti, M.; Bozio, R.;
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9
619573) and by Clemson University.
1
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