Quantitative spectroscopic studies of the photoexcited state properties of methano- and pyrrolidino-[60]fullerene derivatives

Article abstract of DOI:10.1021/jp971067k

UV/vis absorption and fluorescence spectra and fluorescence quantum yields and lifetimes of a series of methano- and pyrrolidino-[60]fullerene derivatives in different solvents are studied systematically. The absorption and fluorescence properties of the

Full text of DOI:10.1021/jp971067k

5626
J. Phys. Chem. A 1997, 101, 5626-5632
Quantitative Spectroscopic Studies of the Photoexcited State Properties of Methano- and
Pyrrolidino-[60]fullerene Derivatives
Bin Ma, Christopher E. Bunker, Radhakishan Guduru, Xian-Fu Zhang, and Ya-Ping Sun*
Department of Chemistry, Howard L. Hunter Chemistry Laboratory, Clemson UniVersity,
Clemson, South Carolina 29634-1905
ReceiVed: March 25, 1997; In Final Form: May 30, 1997X
UV/vis absorption and fluorescence spectra and fluorescence quantum yields and lifetimes of a series of
methano- and pyrrolidino-[60]fullerene derivatives in different solvents are studied systematically. The
absorption and fluorescence properties of the derivatives with different substituents are somewhat different
from those of [60]fullerene, but very similar among themselves, indicating that the low-lying transitions and
the photoexcited state processes are dictated by the electronic structures of functionalized [60]fullerene cages.
The results also allow an examination of the issue concerning discrepancies between experimentally determined
transition probabilities and those calculated in terms of the Strickler-Berg equation for fullerene molecules.
In addition, quenchings of the excited singlet states of the [60]fullerene derivatives by electron donor N,N-
diethylaniline (DEA) and the formation of emissive fullerene-DEA exciplexes in solvents of different polarities
are investigated.
Introduction
electronic structures and excited state properties of fullerenes.
In this paper, we report a systematic spectroscopic study of seven
C60 derivatives in room-temperature solution. Absorption
properties of the derivatives were examined in detail in different
solvents. Fluorescence spectra and quantum yields were
obtained on a near-infrared-sensitive emission spectrometer (up
to 1200 nm), and fluorescence lifetimes were determined using
the time-correlated single-photon-counting technique. The
absorption and emission properties of the different C60 deriva-
tives are quite similar among themselves, but somewhat different
from those of C60. With the photophysical results of the C60
derivatives, the issue concerning discrepancies between experi-
mental fluorescence radiative rate constants and those calculated
in terms of the Strickler-Berg equation20 for fullerene mol-
ecules4,6,21,22 becomes clearer. In addition, quenchings of the
excited singlet states of C60 and the C60 derivatives by electron
donor N,N-diethylaniline and the formation and solvent depen-
dence of exciplexes are compared and discussed.
The electronic absorption and emission properties of fullerenes,
especially [60]fullerene (C60) and [70]fullerene (C70), have
attracted much attention.1 Because of the high degree of
symmetry in the closed-shell electronic configuration, the low-
lying electronic transitions in C60 are only weakly allowed. The
observed C60 absorption band in the visible is very weak, with
the molar absorptivity at the band maximum of only ∼950 M-1
cm-1 2-4
For C70, the symmetry-related forbidden selection is
.
somewhat relaxed, and transition probabilities of low-lying
excited states become much higher (∼21 000 M-1 cm-1 at ∼470
nm, the maximum of a broad absorption band in the visible).5,6
The fullerenes are only weakly fluorescent, regardless of
experimental conditions.3-8 The low fluorescence yields (3.2
× 10-4 for C60 and 5.7 × 10-4 for C70 in room-temperature
toluene)8 are not only due to rapid intersystem crossing to the
formation of excited triplet states but also attributable to small
fluorescence radiative rate constants as a result of weak
electronic transitions.
Experimental Section
Methano-C60 and pyrrolidino-C60 derivatives represent two
important classes of functionalized fullerene compounds.9-12
They are investigated for potential technological applications.
For example, methano-C60 derivatives are being considered as
inhibitors for the protease of the HIV virus13,14 and as efficient
photosensitizers in the generation of singlet molecular oxygen
for photodynamic activities.15 The fullerene derivatives are also
being examined for their nonlinear optical properties.16,17
Similar to the parent C60, the methano-C60 and pyrrolidino-C60
derivatives are excellent nonlinear absorbers, and their more
favorable solubility properties are advantageous in the fabrica-
tion of optical limiting devices. Apparently, many of the
potential technological applications under consideration for the
C60 derivatives are related to their photoexcited states, for which
systematic investigations are still in demand.18,19 In addition,
since methano-C60 and pyrrolidino-C60 derivatives are structur-
ally perturbed fullerene molecules, the similarities and differ-
ences in their photophysical behavior in comparison with the
parent C60 may provide a more general understanding of the
Materials. C60 (purity >99.5%) was obtained from Southern
Chemical Group. The sample purity was checked by UV/vis
absorption, 13C NMR, and matrix-assisted laser desorption
ionization time-of-flight MS methods, and the sample was used
without further purification. N,N-Diethylaniline (99%) was
obtained from Aldrich and purified through reduced-pressure
distillation before use. Hexane, toluene, o-xylene, dichlo-
romethane, chloroform, acetonitrile, acetone, carbon disulfide,
and o-dichlorobenzene (all spectrophotometric grade) were
obtained from Burdick & Jackson, and 1,2,3,5-tetramethylben-
zene (85% and the remaining fractions being other isomers)
and 1,2,4-trimethylbenzene (98%) were obtained from Aldrich.
The solvents were used as received because there was no
interference of possible impurities in the wavelength range of
interest according to absorption and emission spectroscopic
measurements of the solvents. Rhodamine 101 (99%) was
obtained from Fisher Scientific and used as a fluorescence
standard.
Methano-C60 Derivatives I-V. Methano-C60 derivatives
I-V were prepared by use of the reactions of C60 with stabilized
X Abstract published in AdVance ACS Abstracts, July 1, 1997.
S1089-5639(97)01067-0 CCC: $14.00 © 1997 American Chemical Society

Methano- and Pyrrolidino-[60]fullerene Derivatives
J. Phys. Chem. A, Vol. 101, No. 31, 1997 5627
SCHEME 1
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-
tronically cooled detector consists of a near-infrared-sensitive
Hamamatsu 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 scat-
tering. 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
monochromator, a wide slit of 5 mm (19 nm resolution) was
used in fluorescence quantum yield measurements to reduce
experimental 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 specified otherwise,
fluorescence spectra were corrected for nonlinear instrumental
response by use of predetermined correction factors. The
correction factors for the emission spectrometer were carefully
determined by using a calibrated radiation standard from
Optronic Laboratories.
Fluorescence decays were measured by using the time-
correlated single-photon-counting (TCSPC) method. One TC-
SPC setup consists of a nitrogen flash lamp obtained from
Edinburgh Instruments as excitation source. The flash lamp
was operated at 50 kHz, and the wavelength of 317 nm was
isolated using a band-pass filter with a 10 nm fwhm. Fluores-
cence decays were monitored through a 545 nm color glass
sharp-cut filter. The detector consists of a Philips X2020
photomultiplier tube in a thermoelectronically cooled housing
from the Products for Research, Inc. The photomultiplier tube
was operated at -2 kV using an EG&G 556 high-voltage power
supply. The detector electronics from EG&G include two 9307
discriminators, a 457 biased time-to-amplitude converter, and
a 916A multichannel analyzer. The instrument response func-
tion of the setup has a fwhm of ∼2.0 ns. The second TCSPC
setup with an instrument response function of ∼60 ps (fwhm)
has been described in detail elsewhere.26 Fluorescence lifetimes
were determined from observed decay curves and instrument
response functions by use of the Marquardt nonlinear least-
squares method.
sulfonium ylides with cyano, carboxy, and carbonyl groups.23
The one-pot preparation was carried out under phase transfer
conditions. A toluene solution of C60, sulfonium salt, K2CO3,
and the phase transfer catalyst tetrabutylammonium bromide
(TBAB) was mixed and reacted at room temperature. The
stabilized sulfonium ylide generated in situ due to the depro-
tonation of the sulfonium salt by K2CO3 under the catalysis of
TBAB undergoes nucleophilic addition to C60, followed by
intramolecular substitution to form a methano-C60 derivative
with the simultaneous elimination of dimethyl sulfide (Scheme
1).
In a typical one-pot reaction, 100 mg (0.14 mmol) of C60 in
60 mL of anhydrous toluene, 0.16 mmol sulfonium bromide,
200 mg (1.45 mmol) of anhydrous K2CO3, and 20 mg (0.076
mmol) of the catalyst TBAB were added to a 100 mL round
bottom flask. After purging with dry nitrogen gas, the flask
was sealed and the reaction mixture was stirred at room
temperature for 24 h. The mixture was then filtered to remove
any solids and concentrated by evaporation under vacuum.
Results from the matrix-assisted laser desorption ionization time-
of-flight MS analysis show the presence of mono-, bis-, and
tris-adducts. The mono-adduct was obtained after separation
from higher order adducts through silica gel column chroma-
tography using CS2 as an eluent. The methano-C60 derivatives
are brown solids and more soluble than C60 in common organic
solvents such as chloroform. The yields for preparations of the
methano-C60 derivatives I-V are in the range 45-60%. Proton
and 13C NMR results clearly demonstrate that the methylene
bridge is at the [6,6]-ring junction. In contrast to the preparation
of methano-C60 derivatives based on addition reactions of diazo
compounds,9,11,12,27 no isomers having the “open” [6,5]-ring
bridged form were found in the reaction mixtures of the
sulfonium ylide addition.
Methano-C60 Derivative VI. The methano-C60 derivative
VI was prepared following a procedure reported in the
literature,24 except that a lower molar ratio of bromomalonate
to C60 (1.2:1) was used to promote the formation of mono-
adduct. In the reaction, nucleophilic addition of a bromoma-
lonate carbanion from the deprotonation of bromomalonate with
sodium hydride was followed by an intramolecular substitution.
The reaction mixture consists of mono-, bis-, and tris-adducts.
The mono-adduct was separated from the mixture in 55% yield
through silica gel column chromatography using toluene as
eluent and was positively identified in matrix-assisted laser
desorption ionization time-of-flight MS, proton and 13C NMR,
and FT-IR characterizations.
Results
Pyrrolidino-C60 Derivative VII. The pyrrolidino-C60 de-
rivative VII was obtained from photochemical reactions of C60
and triethylamine in room-temperature toluene. The purification
and structural characterization of the compound have been
reported elsewhere.25
UV/Vis Absorption. UV/vis absorption spectra of the
methano-C60 derivatives I-VI in room-temperature (22 °C)
toluene are shown in Figure 1. The spectra are rather similar
among themselves, but different from that of C60.9,18,19,23
Characteristic features in the spectra of the methano-C60
derivatives include a small peak at ∼700 nm with molar
absorptivities ranging from 200 to 300 M-1 cm-1 and a sharp
Measurements. Absorption spectra were obtained using a
computer-controlled Shimadzu UV2101-PC spectrophotometer.

5628 J. Phys. Chem. A, Vol. 101, No. 31, 1997
Ma et al.
to 1.2 × 10-3, larger than that of C60. Observed fluorescence
quantum yields of the derivatives are also independent of
excitation wavelengths. For the pyrrolidino-C60 derivative VII,
fluorescence quantum yields were also determined in several
solvents of different polarities. The results were corrected for
effects due to changes in solvent refractive index n.
ΦF ) ΦF′(n/nSD)-2
(1)
where SD represents the solvent for the standard from which
the uncorrected fluorescence yield value ΦF′ is obtained. The
results show that the yields are only marginally solvent
dependent (Table 2).
Fluorescence Lifetimes. Fluorescence decays of C60 and the
C60 derivatives in room-temperature toluene or chloroform were
measured using the time-correlated single-photon-counting
method. For example, the fluorescence decay of the derivative
I is shown in Figure 4. The decay curves can be deconvoluted
well from corresponding instrumental response functions using
a monoexponential equation.30 The C60 fluorescence lifetime
of 1.2 ns thus obtained in room-temperature toluene is in
excellent agreement with the literature values.22,31 Fluorescence
lifetimes of the C60 derivatives are somewhat longer, ∼1.5 ns,
and the results of different derivatives are again very similar
(Table 2).
Figure 1. Absorption spectra of methano-C60 derivatives I (- - -), II
(-‚‚-), III (- - -), IV (- - -), V (-‚-), and VI (s) and pyrrolidino-C60
derivative VII (‚‚‚) in room-temperature toluene.
band in the 420-430 nm region.27 The absorption spectrum
of the pyrrolidino-C60 derivative VII has the same features, but
still somewhat different from the spectra of the methano-C60
derivatives such that the latter have a maximum at ∼500 nm
(Figure 1). Overall, the derivatives are stronger absorbers than
C60. The absorption spectral parameters in toluene are sum-
marized in Table 1.
Transition Probabilities and Radiative Rate Constants.
Fluorescence radiative rate constants of the C60 derivatives can
be calculated from experimentally determined fluorescence
quantum yields ΦF and lifetimes τF
kF,e ) ΦF/τF
(2)
Absorption spectra of the C60 derivatives remain essentially
the same in solvents of different polarities, for example, toluene
vs methylene chloride or o-dichlorobenzene. In saturated
hydrocarbon solvents such as hexane and methylcyclohexane,
observed UV/vis absorption spectra of the C60 derivatives are
slightly better resolved, with more detailed vibronic structures
in the band at ∼700 nm (Figure 2). However, similar to C60,
the C60 derivatives have significantly different UV/vis absorption
spectra in the solvent series of benzenes with different numbers
of methyl substituents.8,28 For example, observed UV/vis
absorption spectra of II in toluene, o-xylene, 1,2,4-trimethyl-
benzene, and 1,2,3,5-tetramethylbenzene change in a charac-
teristic fashion. As shown in Figure 3, the band at ∼430 nm
progressively shifts to the red with solvents from toluene to
1,2,3,5-tetramethylbenzene, while there are essentially no
changes in the longer wavelength portion of the spectra at all.
Fluorescence Spectra. Fluorescence spectra of the methano-
and pyrrolidino-C60 derivatives in room-temperature toluene are
shown in Figure 2. The spectra are also very similar among
themselves, but different from the fluorescence spectrum of C60.
The vibronic structures in the spectra are broad and are not
affected by increasing the resolution of the spectrometer with
narrower emission slits. Fluorescence spectra of the C60
derivatives are excitation wavelength independent and are only
marginally affected by solvent changes, even in the solvent
series of methyl-substituted benzenes (Figure 3). In saturated
hydrocarbon solvents such as hexane and methylcyclohexane,
observed fluorescence spectra are slightly better resolved (Figure
2). The fluorescence spectral parameters are also summarized
in Table 1.
where the subscript e identifies experimentally determined
fluorescence radiative rate constants. Results of C60 and the
C60 derivatives are summarized in Table 2. For comparison,
the radiative rate constants may be calculated, independent of
the fluorescence quantum yield and lifetime results, in terms
of electronic transition probabilities by use of the Strickler-
Berg equation.20
-3
kF,c ) 2.880 × 10-9n2 νF
-1(g /g ) (ꢀ/ν) dν (3)
∫
AV
0
e
where the subscript c identifies calculated fluorescence radiative
rate constants, g denotes state degeneracies, ꢀ represents molar
absorptivities, and n is the refractive index of the solvent.
-3
νF
-1 ) [ I (ν) dν]/[ ν-3I (ν) dν]
(4)
∫
∫
AV
F
F
where IF represents fluorescence intensities. While the calcula-
-3
tion of the νF
-1 term using observed fluorescence spectra
AV
is straightforward, the determination of the integration ∫(ꢀ/ν)
dν from observed absorption spectra is somewhat more difficult.
For C60, it is known8,21,22 that the inclusion of the entire long-
wavelength absorption band (440-700 nm) significantly over-
estimates the electronic transition probabilities because the band
is likely contributed by several low-lying electronic transitions.
Similarly, the electronic transition probabilities of the C60
derivatives cannot be calculated by using the entire absorption
bands in the visible. Instead, the ∫(ꢀ/ν) dν term due to the
lowest excited singlet state was calculated by assuming a mirror
image relationship between the absorption and fluorescence
spectra. For example, the assumed relationship for the deriva-
tive IV is illustrated in Figure 5. The calculated fluorescence
radiative rate constants of the C60 derivatives obtained with the
assumption of a mirror image relationship between fluorescence
and absorption bands and g0 ) ge ) 1 are also summarized in
Table 2.
Fluorescence Quantum Yields. Fluorescence quantum
yields of the C60 derivatives in room-temperature toluene were
determined carefully in reference to the yield of C60 (3.2 ×
10-4), which was obtained using rhodamine 101 in ethanol as
a fluorescence standard (ΦF ) 1.0).8,29 As summarized in Table
2, fluorescence yields of the derivatives range from 9.3 × 10-4

Methano- and Pyrrolidino-[60]fullerene Derivatives
J. Phys. Chem. A, Vol. 101, No. 31, 1997 5629
TABLE 1: Absorption and Fluorescence Spectral Parameters of the C60 Derivativesa
derivative
absorption bands (nm)
fluorescence bands (nm)
I
II
III
IV
V
VI
VII
687, 615(sh), 555(sh), 491, 430
694, 617(sh), 560(sh), 498, 430
695, 620(sh), 554(sh), 499, 431
696, 620(sh), 555(sh), 496, 431
695, 615(sh), 555(sh), 489, 430
688, 615(sh), 551(sh), 491, 427
707, 642(sh), 615(sh), 552, 480(sh), 434
711, 777, 880(sh)
718, 785, 880(sh)
720, 789, 883(sh)
719, 789, 892(sh)
716, 786, 880(sh)
715, 778, 914(sh)
726, 798, 912(sh)
a In room-temperature toluene, except for VI in chloroform.
Figure 3. Absorption and fluorescence (inset) spectra of the methano-
C60 derivative II in toluene (-‚‚-), o-xylene (- - -), 1,2,4-trimethylbenzene
(-‚-), and 1,2,3,5-tetramethylbenzene (s).
Figure 2. Fluorescence spectra of the methano-C60 derivatives IV (‚‚‚,
very similar for II and III, not shown), V (- - -), and VI (-‚‚-, very
similar for I, not shown) and the pyrrolidino-C60 derivative VII (s) in
room-temperature toluene (6 × 10-5 M). Shown in the inset is a
comparison of the absorption and fluorescence spectra of methano-C60
derivative VII in hexane (s) and toluene (-‚-).
TABLE 2: Fluorescence Properties of the C60 Derivatives
ΦF
τF
kF,c
kF,c
Electron Transfer Quenching and Exciplex Formation.
Similar to C60,1b,7a,32,33 the methano-C60 derivatives undergo
electron transfer interactions with electron donor N,N-diethyl-
aniline (DEA) in both ground and excited singlet states. The
formation of ground state charge transfer complexes requires
high DEA concentrations. At low DEA concentrations (<0.3
M), the electron transfer quenching of excited singlet states and
the formation of fullerene-DEA exciplexes may be studied
without the interference of ground state charge transfer com-
plexes. As shown in Figure 6 for the methano-C60 derivative
V in room-temperature hexane, intensities at the first fluores-
cence band (716 nm) are quenched with increasing DEA
concentrations, and the quenching is accompanied by increases
in intensities at longer fluorescence wavelengths. The increases
in fluorescence intensities at longer wavelengths with increasing
DEA concentrations, which become more evident when the
observed fluorescence spectra are normalized at the first
fluorescence peak (Figure 6, inset), are due to the formation of
an emissive fullerene-DEA exciplex. In room-temperature
toluene, quenchings of the excited singlet states of the methano-
C60 derivatives by DEA also result in the formation of emissive
fullerene-DEA exciplexes. Shown in Figure 7 are results for
VI, as an example. However, in a more polar solvent environ-
ment of chloroform or hexane-acetone mixture, there are no
exciplex emissions, while fluorescence intensities of the methano-
C60 derivatives are still quenched efficiently by DEA. Observed
fluorescence spectral profiles become independent of DEA
concentrations. Thus, the quenching of fluorescence intensities
compound
solvent
toluene
(10-3
)
(ns)
(105 s-1
)
(105 s-1
)
C60
I
II
III
IV
V
VI
VII
0.32 a
1.07
1.14
1.19
1.18
1.2
2.7
7.1
7.65
8.1
8.1
4.8a
4.8
4.9
5.1
4.4
toluene
toluene
toluene
toluene
toluene
chloroform
toluene
CH2Cl2
o-DCB
1.51
1.49
1.47
1.46
1.49
1.48
1.5
1.05
1.05
1.08
0.93
7.1
7.0
4.2
a From ref 8.
in a more polar solvent environment may be evaluated in terms
of the Stern-Volmer relationship.
ΦF°/ΦF ) 1 + kqτF°[DEA]
(5)
where kq is the quenching rate constant and τF° is the
fluorescence lifetime in the absence of quencher. For the
methano-C60 derivative V in hexane with 10% (v/v) acetone,
the Stern-Volmer plot (Figure 8) is not linear, exhibiting
significant upward deviations at high DEA concentrations. The
nonlinear Stern-Volmer behavior is most likely due to the
contribution of static fluorescence quenching, which is more
significant at high DEA concentrations.33 In order to determine
the quenching rate constant kq, fluorescence quenchings were
measured carefully at a series of low DEA concentrations
(<0.05 M). The Stern-Volmer plot of the results shown in

5630 J. Phys. Chem. A, Vol. 101, No. 31, 1997
Ma et al.
Figure 4. Fluorescence decay of the methano-C60 derivative I in room-
temperature toluene. The solid line is from a least-squares fit.
Figure 6. Observed fluorescence spectra of the methano-C60 derivative
V (∼2 × 10-5 M) in room-temperature hexane at different DEA
concentrations. The DEA concentrations are (in the direction of the
arrow) 0, 0.036, 0.09, 0.15, 0.21, and 0.3 M. Shown in the inset are
the spectra normalized at the first peak.
Figure 5. The assumed mirror image relationship for the calculation
of the fluorescence radiative rate constant kF,c of the methano-C60
derivative IV.
Figure 8 (inset) is essentially linear. A least-squares regression
yields a Stern-Volmer constant kqτF° of 28 M-1. With the
known fluorescence lifetime τF° of 1.49 ns, the quenching rate
constant kq is 1.9 × 1010 M-1 s-1
.
The results shown in Figure 8 may be treated by use of an
equation that takes into consideration static quenching contribu-
tions.33,34
Figure 7. Observed fluorescence spectra of the methano-C60 derivative
VI (7 × 10-6 M) in room-temperature toluene at different DEA
concentrations. The DEA concentrations are (in the direction of the
arrow) 0, 0.036, 0.09, 0.15, 0.21, and 0.3 M. Shown in the inset are
the spectra normalized at the first peak.
ΦF°/ΦF ) (1 + kqτF°[DEA]) exp(NV[DEA])
(6)
where N is Avogadro’s number and V is the static quenching
volume. With the known kq and τF° values, a least-squares
regression of the plot shown in Figure 8 yields the static
quenching volume V of 3022 Å3. If the quenching volume is
assumed to be spherical, the V value corresponds to a static
fluorescence quenching radius of ∼9 Å for the methano-C60
derivative V in hexane with 10% (v/v) acetone, which is larger
than the van der Waals radius of the C60 cage of ∼5 Å.
The behavior of the pyrrolidino-C60 derivative VII is appar-
ently somewhat different. In saturated hydrocarbon solvents
such as hexane and methylcyclohexane at room temperature,
fluorescence intensities are not quenched by DEA at concentra-
tions up to 0.3 M. At even higher DEA concentrations, changes
in observed absorption spectra of VII due to the formation of
ground state charge transfer complexes become evident. How-
ever, the quenching of fluorescence intensities of VII is
significant in toluene or hexane with 10% (v/v) acetone at DEA
concentrations similar to those for the methano-C60 derivatives
discussed above (<0.3 M). In toluene, there is apparently the
formation of an emissive VII-DEA exciplex (Figure 9).
Discussion
The photophysical properties of the methano- and pyrrolidino-
C60 derivatives are in many ways similar to those of C60. The
overall increases in molar absorptivities of the derivatives may
be attributed to a reduction in molecular symmetry due to the

Methano- and Pyrrolidino-[60]fullerene Derivatives
J. Phys. Chem. A, Vol. 101, No. 31, 1997 5631
classes of C60 derivatives again suggest that the 0-0 transitions
are dictated by the electronic structures of the 1,2-functionalized
C60 cages.
The pyrrolidino-C60 derivative VII is a linked fullerene-
tertiary amine system, for which the possibility of excited state
intramolecular electron transfer should be considered. However,
the fluorescence quantum yield and lifetime of VII in toluene
are very similar to those of the methano-C60 derivatives (Table
2), suggesting no electron transfer quenching. The fluorescence
quantum yields of VII in solvents of different polarities are
similar, which is also against the possibility of intramolecular
electron transfer quenching of the fullerene excited state. In
fact, it is known35 that the intermolecular electron transfer
quenching of C60 fluorescence by triethylamine is strongly
dependent on solvent polarity.
The methano-C60 derivatives I-VI and the pyrrolidino-C60
derivative VII all have a weak absorption band at ∼700 nm
and rather similar fluorescence spectra. The results may be used
as evidence for the argument that absorptions in the visible
region (440-650 nm) are due primarily to contributions of
electronic transitions other than the 0-0 transition. The
absorption contributions in the region should therefore not be
included in the calculation of fluorescence radiative rate
constants by use 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 smaller than the
experimental kF,e values obtained from fluorescence quantum
yield and lifetime results (Table 2). For C60, it has been shown8
that a similar estimate of the lowest lying electronic transition
probability with the assumed absorption-fluorescence mirror
image relationship results in a kF,c value larger than the kF,e value.
Nevertheless, because the calculation of kF,c values is sensitive
to the rough assumption of a mirror image relationship between
fluorescence and absorption and also to other approximations
associated with the Strickler-Berg equation,20 some discrep-
Figure 8. Fluorescence quenching ratios ΦF°/ΦF for the methano-C60
derivative V at different DEA concentrations in room-temperature
hexane with 10% (v/v) acetone. The solid line is from eq 6, with the
Stern-Volmer constant kqτF° ) 28 M-1 obtained from the results at
low DEA concentrations shown in the inset and NV ) 1.8 obtained
from a nonlinear least-squares regression.
ancies between kF,c and kF,e values might be expected.
A
qualitative conclusion is that for C60 and the C60 derivatives in
room-temperature solution the lowest energy absorption and the
fluorescence are associated with the same excited singlet state.
Characteristic changes in observed absorption spectra of C60
and the C60 derivatives in the solvent series of methyl-substituted
benzenes (Figure 3) are interesting. The absorption in the visible
region (480-750 nm) is insensitive to solvent changes, while
the intense second absorption band undergoes significant
solvatochromic shifts. A simple explanation might be that the
electronic transitions responsible for the visible absorption are
only weakly allowed, essentially inert to solvent effects. Other
systems exhibiting such properties include long-chain polyenes
and diphenylpolyenes, which have been investigated exten-
sively.36 In the polyenes, the absorption spectra are due to
strongly allowed electronic transitions (1Bu* f 1Ag) and undergo
significant solvatochromic shifts, whereas the fluorescence
Figure 9. Observed fluorescence spectra of the pyrrolidino-C60
derivative VII (6 × 10-6 M) in room-temperature toluene (-‚‚-) and
with 0.3 M of DEA (s).
functionalization of the C60 cage. Interestingly, however, the
absorption spectra and molar absorptivities of the methano-C60
derivatives with different substituents are very similar, indicating
that the transitions are dictated by the electronic structures of
the functionalized fullerene moiety. For the pyrrolidino-C60
derivative VII, the overall absorption spectrum is somewhat
different from those of the methano-C60 derivatives I-VI, but
it also consists of a weak absorption band at ∼700 nm that is
assigned to the 0-0 transition. It may thus be concluded on
the basis of the absorption results that the lowest electronic
transitions in the C60 derivatives have similar energies and
transition probabilities. The conclusion is further supported by
the fluorescence results. The fluorescence spectra, quantum
yields, and lifetimes of the different C60 derivatives are all very
similar. A common molecular structural feature in the methano-
and pyrrolidino-C60 derivatives under consideration is the 1,2-
addition (6,6-closed) to the C60 cage. The similarities in the
absorption and fluorescence properties of the two different
1
spectra are due to weakly allowed transitions (1Ag r Ag*)
and thus are hardly solvent dependent.36 However, a special
feature for the solvent dependence of the absorption spectra of
C60 and the C60 derivatives is that it is very solvent specific.
The nature of apparently specific interactions between the
fullerene species and methyl-substituted benzenes remains an
interesting question for further experimental and theoretical
investigations.
Similar to C60, the C60 derivatives are electron acceptors in
both ground and excited states. Quenchings of the fluorescence
intensities of C60 and the C60 derivatives by DEA apparently

5632 J. Phys. Chem. A, Vol. 101, No. 31, 1997
Ma et al.
share some common features.1b,32,33 The electron transfer
process between the excited singlet fullerene and DEA is
inefficient in nonpolar saturated hydrocarbon solvents such as
hexane and methylcyclohexane, so inefficient for the pyrroli-
dino-C60 derivative VII that the fluorescence quenching is
absent. A similar absence of fluorescence quenching in
nonpolar saturated hydrocarbon solvents has been reported for
the C60-triethylamine system.35 In a more polar solvent
environment with the presence of a polar cosolvent, quenchings
of the excited singlet states of the derivatives by DEA are not
only close to diffusion-controlled but also consist of substantial
contributions from static interactions (Figure 8).33
(8) Ma, B.; Sun, Y.-P. J. Chem. Soc., Perkin Trans. 2 1996, 2157.
(9) Isaacs, L.; Diederich, F. HelV. Chim. Acta 1993, 76, 2454.
(10) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993, 115,
9798.
(11) Hirsch, A. The Chemistry of Fullerenes; Thieme: Stuttgart, 1994.
(12) Diederich, F.; Thilgen, C. Science 1996, 271, 317.
(13) (a) Friedman, S. H.; DeCamp, D. L.; Sijbesma, R. P.; Srdanov,
G.; Wudl, F.; Kenyon, G. L. J. Am. Chem. Soc. 1993, 115, 6506. (b)
Sijbesma, R.; Srdanov, G.; Wudl. F.; Castoro, J. A.; Wilkins, C.; Friedman,
S. H.; De Camp, D. L.; Kenyon, G. L. J. Am. Chem. Soc. 1993, 115, 6510.
(14) Toniolo, C.; Bianco, A.; Maggini, M.; Scorrano, G.; Prato, M.;
Marastoni, M.; Tomatis, R.; Spisani, S.; Palu´, G.; Blair, E. D. J. Med. Chem.
1994, 37, 4558.
(15) (a) Foote, C. S. ACS Symp. Ser. 616 (Light ActiVated Pest Control)
1995, 17. (b) Jensen, A. W.; Wilson, S. R.; Schuster, D. I. Bioorg. Med.
Chem. 1996, 4, 767.
(16) (a) Signorini, R.; Zerbetto, M.; Meneghetti, M.; Bozio, R.; Maggini,
M.; De Faveri, C.; Prato, M.; Scorrano, G. J. Chem. Soc., Chem. Commun.
1996, 1891. (b) Maggini, M.; Scorrano, G.; Prato, M.; Brusatin, G.;
Innocenzi, P.; Guglielmi, M.; Renier, A.; Signorini, R.; Meneghetti, M.;
Bozio, R. AdV. Mater. 1995, 7, 404. (c) Cha, M.; Sariciftci, N. S.; Heeger,
A. J.; Hummelen, J. C.; Wudl, F. Appl. Phys. Lett. 1995, 67, 3850.
(17) Sun, Y.-P.; Riggs, E. J.; Liu, B. Chem. Mat. 1997, 9, 1268.
(18) (a) Anderson, J. L.; An, Y.-Z.; Rubin, Y.; Foote; C. S. J. Am. Chem.
Soc. 1994, 116, 9763. (b) Lin, S.-K.; Shiu, L.-L.; Chien, K. M.; Luh, T.-
Y.; Lin, T.-I. J. Phys. Chem. 1995, 99, 105. (c) Guldi, D. M.; Asmus, K.-
D. J. Phys. Chem. 1997, 101, 1472.
(19) Sun, Y.-P.; Ma, B.; Riggs, J. E.; Bunker, C. E. In Fullerenes, Recent
AdVances in the Chemistry and Physics of Fullerenes and Related Materials,
Vol 4; Kadish, K. M., Ruoff, R. S., Eds.: The Electrochemical Society
Inc.: Pennington, NJ, in press.
(20) Strickler, S. J.; Berg, R. A. J. Chem. Phys. 1962, 37, 814.
(21) Sension, R. J.; Phillips, C. M.; Szarka, A. Z.; Romanow, W. J.;
McGhie, A. R.; McCauley, J. P., Jr.; Smith, A. B., III; Hochstrasser, R. M.
J. Phys. Chem. 1991, 69, 6075.
For the formation of exciplex, a difference between C60 and
the C60 derivatives is that the C60-DEA exciplex emission can
only be observed in nonpolar saturated hydrocarbon solvents,
while for the C60 derivatives there are also exciplex emissions
in toluene, a solvent that is nonpolar but more polarizable than
saturated hydrocarbons. However, despite the somewhat dif-
ferent solvent sensitivity for the exciplex emission, the funda-
mental excited state processes of C60 and the C60 derivatives in
the presence of DEA should be similar. The absence of exciplex
emissions in a more polar or polarizable solvent environment
is likely due to solvent polarity effects. For the exciplex state,
a competing decay process to fluorescence might be ionization
for the formation of solvated ion pairs, which is strongly solvent
polarity dependent.37 In fact, the observation of radical cation
and anion species in a transient absorption study of C60-amine
systems in room-temperature toluene has been reported.38
(22) Kim, D.; Lee, M.; Suh, Y. D.; Kim, S. K. J. Am. Chem. Soc. 1992,
114, 4429.
(23) Wang, Y.; Cao, J.; Schuster, D. I.; Wilson, S. R. Tetrahedron Lett.
1995, 36, 6843.
(24) Bingel, C. Chem. Ber. 1993, 126, 1957.
(25) Lawson, G. E.; Kitaygorodskiy, A.; Ma, B.; Bunker, C. E.; Sun,
Y.-P. J. Chem. Soc., Chem. Commun. 1995, 2225.
(26) Bunker, C. E.; Sun, Y.-P.; Gord, J. R. J. Am. Chem. Soc., submitted.
(27) Isaacs, L.; Diederich, F. HelV. Chim. Acta 1993, 76, 1231.
(28) Catala´n, J.; Saiz, J. L.; Laynez, J. L.; Jagerovic, N.; Elguero, J.
Angew. Chem., Int. Ed. Engl. 1995, 34, 105.
(29) Karstens, T.; Kobs, K. J. Phys. Chem. 1980, 84, 14.
(30) For VI, the decay consists of minor contribution from a short-lived
component, which might be attributed to scattering or trace amount of
impurity.
Acknowledgment. We thank Bing Liu for experimental
assistance and Dr. James Gord of the Wright Laboratory for
the use of TCSPC equipment for some of the fluorescence
lifetimes. Financial support from the National Science Founda-
tion (CHE-9320558) is gratefully acknowledged. The research
assistantships provided to B.M. and C.E.B. were provided in
part by the Department of Energy through DOE/EPSCoR
cooperative agreement DE-FG02-91ER75666.
(31) Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. J. Am. Chem.
Soc. 1995, 117, 4093.
(32) Palit, D. K.; Ghosh, H. N.; Pal, H.; Sapre, A. V.; Mittal, J. P.;
Seshadri, R.; Rao, C. N. R. Chem. Phys. Lett. 1992, 198, 113.
(33) Sun, Y.-P.; Bunker, C. E.; Ma, B. J. Am. Chem. Soc. 1994, 116,
9692.
References and Notes
(1) (a) Foote, C. S. In Topics in Current Chemistry: Electron Transfer
I; Mattay, J., Ed.; Springer-Verlag: Berlin, 1994; p 347. (b) Sun, Y.-P. In
Molecular and Supramolecular Photochemistry, Vol. 1; Ramamurthy, V.,
Ed., Marcel Dekker: New York, 1997, in press.
(34) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Inter-
science: London, 1970.
(35) Sun, Y.-P.; Ma, B.; Lawson, G. E. Chem. Phys. Lett. 1995, 233,
57.
(2) (a) Ajie, H.; Alvarez, M. M.; Anz, S. J.; 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. (b)
Taylor, R.; Hare, J. P.; Abdulsada, A.; Kroto, H. W. J. Chem. Soc., Chem.
Commun. 1990, 1423.
(3) Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.;
Diederich, F. N.; Alvarez, M. M.; Whetten, R. B. J. Phys. Chem. 1991, 95,
11.
(4) Sun, Y.-P.; Wang, P.; Hamilton, N. B. J. Am. Chem. Soc. 1993,
115, 6378.
(5) Arbogast, J. W.; Foote, C. S. J. Am. Chem. Soc. 1991, 113, 8886.
(6) Sun, Y.-P.; Bunker, C. E. J. Phys. Chem. 1993, 97, 6770.
(7) (a) Wang, Y. J. Phys. Chem. 1992, 96, 764. (b) Sibley, S. P.;
Argentine, S. M.; Francis, A. H. Chem. Phys. Lett. 1992, 188, 187. (c)
Catala´n, J.; Elguero, J. J. Am. Chem. Soc. 1993, 115, 9249.
(36) (a) Hudson, B. S.; Kohler, B. E.; Schulten, K. In Excited States;
Lim, E. C., Ed., Academic Press: New York, 1982; Vol. 6, p 1. (b) Saltiel,
J.; Sun, Y.-P. In Photochromism, Molecules and Systems; Du¨rr, H., Bouas-
Laurent, H., Eds.; Elsevier: Amsterdam, 1990; p 64. (c) Kohler, B. E. Chem.
ReV. 1993, 93, 41.
(37) Sun, Y.-P.; Ma, B. Chem. Phys. Lett. 1995, 236, 285.
(38) (a) Ghosh, H. N.; Pal, H.; Sapre, A. V.; Mittal, J. P. J. Am. Chem.
Soc. 1993, 115, 11722. (b) Park, J.; Kim, D.; Suh, Y. D.; Kim, S. K. J.
Phys. Chem. 1994, 98, 12715. (c) Ito, O.; Sasaki, Y.; Yoshikawa, Y.;
Watanabe, A. J. Phys. Chem. 1995, 99, 9838. (d) Sasaki, Y.; Yoshikawa,
Y.; Watanabe, A.; Ito, O. J. Chem. Soc., Faraday Trans. 1995, 91, 2287.