Orientation-dependent electron transfer processes in fullerene-aniline dyads

Article abstract of DOI:10.1021/jp992369k

Synthesized o- and p-substituted fullerene-aniline dyads exhibited orientation-dependent electron transfer under photoexcitation. The experimental evidence based on steady state and time-resolved spectroscopy was in agreement with the computational studie

Full text of DOI:10.1021/jp992369k

J. Phys. Chem. A 1999, 103, 10755-10763
10755
Orientation-Dependent Electron Transfer Processes in Fullerene-Aniline Dyads
K. George Thomas,*†,‡ V. Biju,† D. M. Guldi,‡ Prashant V. Kamat,*,‡ and M. V. George†,‡,§
Photochemistry Research Unit, Regional Research Laboratory (CSIR), TriVandrum 695 019, India, Radiation
Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556, and Jawaharlal Nehru Centre for
AdVanced Scientific Research, Bangalore 560 012, India
ReceiVed: July 14, 1999; In Final Form: October 5, 1999
Newly synthesized ortho- and para-substituted fullerene-aniline dyads exhibit orientation-dependent electron
transfer under photoexcitation. Minimum energy conformation based on molecular mechanics calculations
1
yield folded conformations for ortho-substituted dyads and extended ones for the para-substituted dyads. H
NMR studies also provided evidence for the folding of the anilinic group in the case of ortho-substituted
dyads. Through-space charge transfer processes in these dyads were investigated using steady state fluorescence
and lifetime measurements. The decrease in the spatial distance between the donor and acceptor moieties of
the ortho-substituted dyads facilitated an efficient electron transfer. The marked increase in the singlet excited
state deactivation rate constants and quantum yields of charge separation observed in the case of the ortho-
substituted dyads, in polar solvents, further support a topographically controlled electron transfer process.
Introduction
have studied the effect of the linkage and solvent dependence
on the photoinduced electron transfer processes in a series of
Light-induced electron transfer processes play a key role in
photosynthetic systems1 and in the design of artificial molecular
devices based on donor-acceptor pairs.2 Energetics, distance
and orientation between the donor and acceptor moieties, nature
of the spacer, solvent, and temperature are some of the important
parameters that control light-induced charge separation in these
dyads. One of the ultimate aims of studies on donor-acceptor
systems is to develop devices that can convert light energy into
electricity or fuels.
Recent studies have shown that C60 is a suitable candidate to
be employed as an electron acceptor in the design of donor-
bridge-acceptor [D-B-A] systems.3 The photoinduced charge
transfer processes in fullerene-based dyads and triads have
received considerable attention.4-8 In all of these systems, the
singlet or triplet excited state of the photoexcited C60 accepts
an electron from the linked donor group to give the charge-
separated state. In the case of fullerene-donor dyads, extending
the lifetime of the charge-separated state, by controlling the rates
of forward and back electron transfer, has been a challenging
task.
Several strategies9 have been adopted for the design of
D-B-A systems, which can generate long-lived, charge
separated states with high efficiency and slow charge recom-
bination rates. In the case of natural photosynthetic systems,
one of the prime factors responsible for the high efficiency of
electron transfer is the well-defined orientation of chromophoric
units in the protein matrices.1 Effects of orientation of donor
and acceptor moieties on electron transfer processes in porphy-
rin-quinone model systems have been reported.9a-c It was
proposed in these studies that the nature of the bridging groups
used in the D-B-A systems plays a significant role in
controlling the orientation and distance of separation of the
donor-acceptor pair. In a recent work, Imahori and co-workers
zincporphyrin-C60 dyads.5e
Solvent-dependent intermolecular photoinduced electron trans-
fer between substituted pyrrolidinofullerenes and N,N′-di-
methylaniline has been recently reported by Luo et al.10 In a
recent study, we characterized the excited-state properties of a
para-substituted fullerene-aniline dyad.8a We failed to observe
any charge transfer products upon photoexcitation of this para-
substituted dyad in neat solvents. However, upon clustering these
dyads, in mixed solvents, yielded remarkably stable charge
transfer products which lived for several hundreds of micro-
seconds.8b The remarkable stability of the charge-separated
intermediates in the clustered dyads is attributed to the hopping
of the electron between the fullerene molecules. Similarly, rigid
donor-bridge-C60 systems with extended hydrocarbon bridges
were reported to exhibit charge separated states under photo-
excitation.4 These observations prompted us to probe how the
orientation of donor-acceptor moieties influence the intramo-
lecular electron transfer in fullerene-aniline based dyads.
Examples of orientation effects that control the photoinduced
electron transfer in a series of fullerene based D-B-A systems
are reported here. The difference in orientation between donor-
acceptor moieties has been achieved by attaching an anilinic
donor to the ortho as well as para positions of the phenyl groups
of 1-methyl-2-phenylpyrrolidinofullerene, using ethylenic and
propylenic linkers (1-4 in Chart 1). With the use of steady
state and time-resolved fluorescence techniques, the forward
electron transfer properties in these dyads were examined in
polar and nonpolar solvents.
Experimental Section
All melting points are uncorrected and were determined on
a Aldrich melting point apparatus. IR spectra were recorded on
a Perkin-Elmer model 882 IR spectrometer and UV-visible
spectra on a Shimadzu 2100 or GBC 918 spectrophotometer.
1H NMR and 13C NMR spectra were recorded either on a JEOL
EX-90 MHz spectrometer or a Bruker DPX-300 MHz spec-
† Regional Research Laboratory.
‡ University of Notre Dame.
§ Jawaharlal Nehru Centre for Advanced Scientific Research.
10.1021/jp992369k CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/10/1999

10756 J. Phys. Chem. A, Vol. 103, No. 50, 1999
Thomas et al.
CHART 1
for C10H14NBr (M•+) 227.0310, found 227.0313 (FAB high-
resolution mass spectrometry).
General Method of Preparation of N-Methyl,N-{(for-
mylphenoxy)alkyl}anilines (8, 11, 12, 13). A mixture of the
appropriate bromide (5 mmol), hydroxybenzaldehyde (5 mmol)
and potassium carbonate (5 mmol) was refluxed in dry acetone
(15 mL) for 12 h. The reaction mixture was cooled, filtered,
and concentrated under reduced pressure. The crude product
was chromatographed over silica gel (100-200 mesh) in each
case.
Compound 8. Elution of the column with a mixture (1:10) of
ethyl acetate and hexane gave 750 mg (60%) of 8. IR (neat)
νmax 2934, 2762, 2794, 1692, 1604, 1500, 1460, 1368, 1292,
1243, 1039, 832 cm-1; 1H NMR (CDCl3) δ 3.05 (3H, s, NCH3),
3.7-3.9 (2H, t, NCH2), 4.1-4.3 (2H, t, OCH2), 6.6-7.9 (9H,
m, aromatic), 10.4 (1H, s, CHO); 13C NMR (CDCl3) δ 39.02,
51.55, 65.90, 112.06, 112.18, 116.68, 120.77, 124.74, 128.26,
129.18, 135.75, 148.60, 160.87, 189.45; exact mass calcd. for
C16H17NO2 (M•+) 255.1259, found 255.1266 (FAB high-
resolution mass spectrometry).
Compound 11. Elution of the column with a mixture (1:3) of
ethyl acetate and petroleum ether (bp 60-80 °C) gave 740 mg
(55%) of 11. IR (neat) νmax 2949, 2873, 1692, 1601, 1551, 1502,
1480, 1450, 1388, 1356, 1287, 1241, 1195, 1161, 1040, 841
cm-1; 1H NMR (CDCl3) δ 2.0-2.4 (2H, m, CH2), 2.95 (3 H, s,
NCH3), 3.4-3.8 (2H, t, NCH2), 4.0-4.3 (2H, t, OCH2), 6.7-
7.9 (9H, m, aromatic), 10.55 (1H, s, CHO); 13C NMR (CDCl3)
δ 26.64, 38.50, 64.92, 65.85, 112.30, 120.71, 128.53, 129.25,
135.95, 156.66, 162.96, 189.50; exact mass calcd. for C17H19-
NO2 (M•+) 269.1416, found 269.1419 (FAB high-resolution
mass spectrometry).
trometer. The emission spectra were recorded on a Spex-
Fluorolog, F112-X equipped with a 450 W, Xe lamp and a
Hamamatsu R928 photomultiplier tube. The excitation and
emission slits were 1 and 4 nm, respectively. A 570-nm long
pass filter was placed before the emission monochromator in
order to eliminate the interference from the solvent. Solvent
spectra were recorded in each case and subtracted. Quantum
yields of fluorescence were measured by a relative method using
optically dilute solutions (absorbance adjusted to 0.1 at 470 nm).
N-Methylpyrrolidinofullerene dissolved in toluene (φf ) 6.0 ×
10-4) was used as reference.4,7a All of the lifetimes and rate
constants reported in this study are within the experimental error
of ( 5%.
Synthesis of Fullerene-Aniline Dyads 2-4. The general
method adopted for the synthesis of fullerene-aniline dyads is
shown in Scheme 1. The purity of all these dyads was confirmed
by HPLC.11 Reported procedures were followed for the prepara-
tion of N-methyl,N-{(2-bromo)-1-ethyl}aniline (7),8a N-methyl,
N-{(p-formylphenoxy)-1-ethyl}aniline (9),8a fullerene-aniline
dyad (1)8a and model compound, N-methylpyrrolidinofullerene
(5).12a
Compound 13. Elution with a mixture (1:4) of ethyl acetate
and hexane gave 1.2 g (90%) of 13. IR (neat) νmax 2957, 2887,
2854, 1697, 1606, 1512, 1260, 1219, 1162, 1036, 834 cm-1
;
1H NMR (CDCl3) δ 2.0-2.4 (2H, m, CH2), 2.95 (3H, s, NCH3),
3.4-3.7 (2H, t, NCH2), 4.0-4.2 (2H, t, OCH2), 6.6-8.0 (9H,
m, aromatic), 9.90 (1H, s, CHO); 13C NMR (CDCl3) δ 26.64,
37.89, 38.45, 64.95, 103.88, 112.21, 114.71, 116.35, 129.18,
131.96, 149.14, 163.85, 190.70; exact mass calcd. for C17H19-
NO2 (M•+) 269.1416, found 269.1419 (FAB high-resolution
mass spectrometry).
General Method of Synthesis of Fullerene-Aniline Dyads
(2-4). A mixture of C60 (0.2 mmol), N-methyl,N-{(formyl-
phenoxy)alkyl}-aniline (0.2 mmol) and N-methylglycine (0.2
mmol) in toluene (145 mL) was stirred under reflux for 10 h.
The reaction mixture was cooled, and removal of the solvent
under reduced pressure gave a solid residue, which was
chromatographed over silica gel (100-200 mesh) to give the
appropriate dyads.
N-Methyl,N-{(3-bromo)-1-propyl}aniline (10).12b To an ice-
cold solution of N-methyl,N-{(3-hydroxy)-1-propyl}aniline (15
mmol) in dichloromethane (20 mL) was added PBr3 (15 mmol),
dropwise over a period of 1 h. The reaction mixture was further
stirred at room temperature for an additional period of 3 h. The
reaction was quenched with ice-cold water and the pH was
adjusted to 7-8. The organic portion was extracted with
dichloromethane and the solvent was removed under vacuum.
The crude product was chromatographed on silica gel (100-
200 mesh) using a mixture (1:20) of ethyl acetate and petroleum
ether (bp 60-80 °C) to give 3.06 g (90%) of 10; IR (neat) νmax
3037, 2954, 2827, 1605, 1508, 1445, 1369, 1257, 1222, 1109,
Dyad 2. Elution with a mixture (1:3) of toluene and petroleum
ether (bp 60-80 °C) gave 58 mg (40%) of unchanged C60
followed by 58 mg (48%) of 2, mp > 400 °C; IR (KBr) νmax
2940, 2869, 2785, 1606, 1568, 1509, 1473, 1429, 1363, 1336,
1
1301, 1245, 1176, 1035, 746 cm-1; H NMR (CDCl3) δ 2.05
(2H, t, CH2), 2.80 (3H, s, NCH3), 2.92 (3H, s, NCH3), 3.54
(2H, t, anilinic NCH2), 4.01(2H, t, OCH2), 4.25 (1H, d, CH of
pyrrolidine), 4.88 (1H, s, CH of pyrrolidine), 4.98 (1H, d, CH
of pyrrolidine), [6.58-6.75 (m), 6.96 (d), 7.13-7.29 (m) and
7.63-7.79 (s)] (9H, aromatic); 13C NMR (CDCl3) δ 26.85,
29.74, 38.43, 40.00, 49.37, 65.31, 70.01, 79.14, 83.19, 112.17,
114.55, 116.19, 129.19, 130.49, 135.79, 140.15, 142.12, 142.56,
145.24, 145.48, 145.94, 146.14, 153.66, 154.11, 158.92; exact
mass calcd. for C79H24N2O (M•+) 1017.1967, found 1017.1989
(FAB high-resolution mass spectrometry).
1
1030, 992, 751 cm-1; H NMR (CDCl3) δ 1.8-2.3 (2H, m,
CH2), 2.95 (3H, s, NCH3), 3.2-3.6 (4H, m, NCH2 and BrCH2),
6.5-7.4 (5 H, m, aromatic); 13C NMR (CDCl3) δ 29.98, 31.29,
38.60, 50.89, 112.35, 116.50, 129.15, 149.02; exact mass calcd.

Fullerene-Aniline Dyads
J. Phys. Chem. A, Vol. 103, No. 50, 1999 10757
SCHEME 1
Dyad 3. Elution of the column with a mixture (1:3) of toluene
and hexane gave 65 mg (45%) of unchanged C60 followed by
50 mg (45%) of the dyad 3; mp > 400 °C; IR (KBr) νmax 2932,
2866, 2786, 1602, 1500, 1455, 1361, 1338, 1247, 1184, 1107,
1036, 749 cm-1; 1H NMR (CDCl3) δ 2.77 (3H, s, NCH3), 3.02
(3H, s, NCH3), 3.53-3.61 (1H, m, anilinic NCH), 3.64-3.71
(1H, m, anilinic NCH), 3.79-3.85 (1H, m, OCH), 4.21-4.29
(2H, m, OCH and CH of pyrrolidine), 4.95 (1H, d, CH of
pyrrolidine ring), 5.52 (1H, s, CH of pyrrolidine ring), [6.70-
6.81 (m), 6.68 (d), 7.10 (t), 7.14-7.32 (m) and 7.98 (d)] (9H,
aromatic); 13C NMR (CDCl3) δ 38.57, 39.84, 51.70, 64.86,
69.01, 69.60, 75.11, 86.54, 111.30, 112.46, 116.91, 121.47,
129.01, 129.26, 129.82, 135.95, 136.43, 139.15, 140.02, 141.70,
141.94, 142.09, 142.43, 142.92, 144.24, 144.44, 144.80, 145.12,
145.26, 145.50, 145.68, 145.99, 147.09, 148.69, 153.62, 156.97;
exact mass calcd. for C78H22N2O(M•+) 1002.1732, found
1002.1713 (FAB high-resolution mass spectrometry).
146.07, 146.82, 147.47, 149.16, 153.82, 153.88, 155.21, 156.81,
157.37; exact mass calcd. for C79H24N2O(M•+) 1017.1967, found
1017.1987 (FAB high-resolution mass spectrometry).
Laser Flash Photolysis. Nanosecond laser flash photolysis
experiments were performed with a Laser Photonics PRA/model
UV-24 nitrogen laser system (337 nm, 2 ns pulse width, 2-4
mJ/pulse) with front face excitation geometry. A typical
experiment consisted of a series of 2-3 replicate shots per single
measurement. The average signal was processed with an LSI-
11 microprocessor interfaced with a VAX computer. Details of
the experimental set up can be found elsewhere.13 Lifetime
measurements were carried out using a PTI Laser Stobe
Fluorescence lifetime system and a nitrogen laser system (PTI
model GL-3300) was used as excitation source. The emission
decay was monitored at 715 nm.
Results and Discussion
Dyad 4. Elution of the column with a mixture (1:3) of toluene
and hexane gave 60 mg (45%) of unchanged C60 followed by
57 mg (48%) of the dyad 4; mp > 400 °C; IR (KBr) νmax 2939,
2862, 2787, 1607, 1548, 1500, 1453, 1285, 1245, 1190, 1104,
1045, 754 cm-1; 1H NMR (CDCl3) δ 1.89-1.91 (2H, m, CH2),
2.82 (3H, s, NCH3), 2.96 (3H, s, NCH3), 3.51 (2H, t, anilinic
NCH2), 3.72-3.88 (1H, m, OCH), 3.98-4.11 (1H, m, OCH),
4.31 (1H, d, CH of pyrrolidine), 5.00 (1H, d, CH of pyrrolidine),
5.57 (1H, s, CH of pyrrolidine), [6.65-6.81 (m), 6.90 (d),7.07-
7.32 (m), 7.99 (d)] (9H, aromatic); 13C NMR (CDCl3) δ 26.78,
29.71, 38.62, 40.18, 49.85, 65.78, 69.98, 75.85, 91.32, 111.48,
112.43, 116.41, 121.24, 128.14, 129.06, 134.82, 135.88, 135.92,
135.94, 138.25, 138.27, 138.30, 138.91, 141.29, 141.35, 141.63,
141.88, 142.64, 142.69, 142.74, 144.20, 144.50, 144.62, 145.29,
Computational Studies. The geometry of the different
conformers was evaluated using the Sybyl force field method.14
Computational studies suggest that the most stable conforma-
tions in the case of para-substituted dyads (1 and 2) are the
extended ones, while both the ortho-substituted dyads (3 and
4) possess a folded conformation. Representative examples of
structures (2 and 4) are displayed in Figure 1. Based on these
calculations, the edge to edge distance between the anilinic
nitrogen and C60 is found to be much larger for the para-
substituted compounds (9.48 Å for 1 and 10.01 Å for 2) as
compared to the ortho-substituted compounds (3.28 Å for 3 and
4.01 Å for 4). Over the entire spherical structure of C60, the π-
electron cloud is more or less available and this particular feature

10758 J. Phys. Chem. A, Vol. 103, No. 50, 1999
Thomas et al.
3 and 4 (Figure 2, spectrum B). For the ortho-substituted dyad
4, having three methylene groups, the NCH2 protons are
observed as a triplet whereas for the ortho-substituted dyad 3,
having two methylene groups, the NCH2 protons are observed
as two separate multiplets. Such effects are not observed in the
case of ortho-substituted derivatives of the N-methyl,N-
{(formylphenoxy)alkyl}anilines, 88a and 11 (for details, see
Experimental Section). The difference in the splitting pattern,
observed for the ortho-substituted dyads may be, originating
from the spatial interactions of the π-electrons of C60 (as well
as phenyl group) and the methylene groups, indicative of the
proximity of C60 and anilinic donor group.
The doublets at 4.25 and 4.98 ppm correspond to the exo
and endo protons of the pyrrolidine ring of the para-substituted
dyad 2, and their positions remain more or less the same for
the ortho-substituted dyad 4 (Figure 2, spectrum A and B). The
singlet observed at 4.88 ppm for the para-substituted dyad 2 is
due to the methyne proton of the pyrrolidine ring and a large
downfield shift is observed for this proton, in the case of the
ortho-substituted dyad 4. (Note that the aldehyde protons of the
ortho-substituted derivatives of the N-methyl,N-{(formylphenoxy)-
alkyl}anilines (88a and 11) are more deshielded than the para-
substituted derivative (12 and 13), and details are given in the
Experimental Section.) These effects could be due to the
anisotropic effect of the oxygen of the ortho-phenoxy group.
Figure 1. Minimized structure of para- and ortho-substituted dyads:
(a) 2, (b) 4.
Absorption and Emission Characteristics of Dyads. Func-
tionalized fullerenes such as methanofullerenes and pyrrolidi-
nofullerenes possess a weak absorption band around 700 nm,
and the exact nature of this band is still being debated. An early
study has suggested that the 700 nm band in the case of
methanofullerenes originates from one of the orbitally forbidden
transition of C60 or from the spin-forbidden transition to the
lowest triplet state.15a Guldi and Maggini have assigned the 700
nm band of N-methylpyrrolidinofullerene (model compound 5)
to the 0 f *0 absorption band.15b Another report suggests that
the lone pair of electrons on nitrogen is less available in the
case of fulleropyrrolidines.15c Through-space interactions of the
nitrogen lone pair with the fullerene π systems was experimen-
tally confirmed through acid-base studies and from the reaction
rates of the alkylation of the pyrrolidine nitrogen by methyl
iodide.15c
All of the dyads in the present investigation possess a similar
long wavelength band at ∼700 nm, which are partially
influenced by the addition of trifluoroacetic acid (TFA) (see
ref 15d, for details). This observation suggests that protonation
of the anilinic nitrogen is responsible for the partial disappear-
ance of the absorption band. Similar observations have also been
reported earlier by Williams et al.,4a for rigid donor-bridge-
C60 dyads. Studies have shown that fullerenes and functionalized
fullerenes have a strong tendency to form ground state inter-
molecular charge transfer complexes with aliphatic and aromatic
amines.16 Nakamura et al. have investigated the intermolecular
CT complex formation between a C60-o-quinodimethane and
N,N-dimethylaniline (DMA).16e It is concluded that the DMA
molecule can move freely in solvent and overlap the C60 surface,
leading to the CT complex. We have carried out the concentra-
tion dependency, to check the Beer-Lambert law, in the case
of dyad 1. The absorption band at 700 nm increased linearly
with the concentration of the dyad, thus ruling out the possibility
of the formation of any intermolecular charge transfer. More-
over, the 700 nm absorption band can be seen even in dilute
solutions (< 5 µM). Based on these observations, we can
attribute part of the 700 nm band of dyads 1-4 to a weak
intramolecular charge transfer absorption that results from the
Figure 2. 1H NMR (300 MHz) spectra (δ 3.2-6.0 ppm) of para-
substituted dyad 2 (A) and ortho-substituted dyad 4 (B).
enhances the probability of accepting an electron through space
in the folded conformation.
1H NMR Studies. Further information regarding the folding
of the anilinic group to the proximity of C60 was obtained by
comparing the 1H NMR spectrum (300 MHz) of the para- and
1
the ortho-substituted dyads. Figure 2 illustrates the H NMR
spectra, in the region 3.2-6.0 ppm, for the para-substituted dyad
2 and the ortho-substituted dyad 4. The triplets observed at 3.54
and 4.01 ppm for the para-substituted dyad 2 (Figure 2, spectrum
A) are assigned to the NCH2 (anilinic) and OCH2 protons,
respectively. Interestingly, the OCH2 protons are observed as
two separate multiplets in the case of the ortho-substituted dyads

Fullerene-Aniline Dyads
J. Phys. Chem. A, Vol. 103, No. 50, 1999 10759
TABLE 1: Fluorescence Quantum Yield (Of) of the Dyads
1-4 in Various Solvents
Φf (104)
solvent
toluene
dichloromethane
benzonitrile
1
2
3
4
6.9
4.4
3.1
6.8
3.3
3.1
6.7
1.7
0.2
6.6
1.0
0.25
ground state interaction between the lone pair of anilinic nitrogen
and fullerene π-electron systems.
The fluorescence properties of the dyads were investigated
using solvents of widely varying polarity (Table 1). The
fluorescence spectrum and the quantum yield of the dyads 1-4
in nonpolar solvents (e.g., toluene) remained the same as that
of the model compound 517 (Figure 3A). These results indicated
that the singlet excited state properties of the dyads are
unaffected in nonpolar solvents.
A decrease in fluorescence quantum yield was observed with
increasing solvent polarity for all the dyads under investigation
(Table 1). The decrease in fluorescence quantum yield indicates
that a charge transfer route (Reaction 3) competes quite
efficiently with other deactivation routes for the deactivation
of the singlet excited state (Reactions 1 and 2).18 These
observations are in line with the dependence of photoinduced
electron transfer on the polarity of the medium for other
fullerene based donor-acceptor dyad molecules.4-7
1(C60-aniline)* f (C60-aniline) + hν
1(C60-aniline)* f 3(C60-aniline)* f (C60-aniline) (2)
1(C60-aniline)* f (C60•--aniline•+)
(3)
(1)
Figure 3. (A) Emission spectra of fullerene-aniline dyads 1-4 in
toluene: (a) 1, (b) 2, (c) 4, and (d) 3. (B) Emission spectra of fullerene-
aniline dyads 1-4 in benzonitrile: (a) 1, (b) 2, (c) 4, and (d) 3
(absorbance of the solutions was adjusted to 0.1 at the excitation
wavelength, 470 nm).
One of the interesting observations in the present study is
the marked decrease in the fluorescence yield, observed in the
case of ortho-substituted compounds in polar solvents (Table 1
and Figure 3B). This effect is attributed to the folding of the
aniline group in ortho-substituted dyads, which in turn results
in an increase in the probability of electron transfer. To confirm
this effect, we have further compared the fluorescence lifetimes
of ortho- and para-substituted dyads.
The short fluorescence lifetimes, observed in polar solvents,
are again indicative of reductive quenching of the singlet excited
state. However, the difference in the rate constants and quantum
yields of charge separation (Table 2) between the ortho- and
para-substituted dyads highlight the effect of orientation of the
aniline donor moiety as an important factor in controlling the
rate of forward electron transfer. The large φcs values observed
for the ortho-substituted compounds (>75%) arise from the
decrease in the donor-acceptor distance caused by the folding
of the aniline group. As shown earlier,20 a specific orientation
between the donor and acceptor moieties in the dyad molecules
is necessary for achieving an efficient electron transfer in the
excited state. In the case of fullerene-aniline dyads 3 and 4,
the folding of the anilinic group, in close proximity to the π
cloud of C60, is sufficient to achieve an efficient charge transfer.
Intersystem Crossing vs Photoinduced Charge Separation.
We have performed nanosecond laser flash photolysis experi-
ments (using 337 nm laser pulse as the excitation source) in
two solvents of differing polarity, viz., toluene (ꢀ ) 2.38) and
benzonitrile (ꢀ ) 25.2), to investigate the different photochemi-
cal processes (viz., reactions 2 and 3) and characterize the
intermediates formed.
Fluorescence Lifetime Measurements. Fluorescence life-
times of the dyads 1-4 were measured in toluene and
benzonitrile, respectively. Representative fluorescence decay
profiles of the para-substituted dyad 1 and ortho-substituted dyad
3, in toluene and benzonitrile solutions, are presented in Figure
4. The fluorescence decay profiles of the dyads 1-4 and the
model compound 5 were analyzed by the deconvolution method
using a single-exponential decay function (see the fitted curves
in Figures 4A and B). The singlet lifetimes of the dyads 1-4
and model compound 5 were found to be nearly identical (τs )
1.3 ( 0.1 ns) in toluene. In benzonitrile solutions the para-
substituted dyads, 1 and 2, exhibited fluorescence lifetimes of
0.90 and 0.70 ns, respectively, whereas the ortho dyads 3 and
4 possess much shorter lifetimes (<0.30 ns). The decrease in
the singlet lifetimes observed for the ortho-substituted dyads is
in tune with decrease in the fluorescence quantum yeilds
(compare Figures 3B and 4B and the φf and τs values in Tables
1 and 2, respectively). The rate constants and quantum yields
for charge separation (kcs and φcs)19 in excited singlet dyad
molecules were estimated from the corresponding fluorescence
lifetimes of the dyads and the model compound. These results
are summarized in Table 2.
Transient absorption spectra (350-1100 nm region) of the
dyads (1-4) in toluene were recorded following 337 nm laser
pulse excitation. The difference absorption spectra showed two
absorption bands at 370 and 700 nm, and their spectral features
are similar to those obtained with the model compound 5. A
representative example of time-resolved transient spectra re-
corded following 337 nm laser pulse excitation of 2 in deaerated

10760 J. Phys. Chem. A, Vol. 103, No. 50, 1999
Thomas et al.
Figure 5. Difference absorption spectra, recorded after 337 nm laser
pulse excitation of the dyad 2 (∼30 µM) in deoxygenated toluene in
the visible-NIR region.
Figure 4. Fluorescence decay profiles for the para-substituted fullerene-
aniline dyad (1) and ortho-substituted fullerene-aniline dyad (3) in
(A) toluene and (B) benzonitrile.
TABLE 2: Single and Triplet Excited State Propertiesa,b of
the Fullerene-Aniline Dyads 1-4
toluene
benzonitrile
dyad τs(ns)c τΤ(µs) φT τs(ns)c τΤ(µs) kcs (109s-1
)
φcs
φT
1
2
3
4
1.27 16.2 0.92
1.20 13.2 0.95
1.25 13.4 0.95 <0.30 2.2
1.14 14.4 0.95 <0.30 6.6
0.90 8.5
0.70 7.4
0.35
0.67
>2.5
>2.5
0.32 0.67
0.47 0.49
>0.77 0.18
>0.77 0.15
a τs anb τT are the lifetimes of the singlet and triplet excited states;
φT and φcs are the quantum yield of the triplet excited state and the
charge separation; kcs is the rate constant for the charge separation (ref
19). b Photophysical properties of model compound were reported
earlier (ref 21). c Fluorescence decay profile of the dyads 1-4 and
model compound 5 were analyzed by the deconvolution method using
a single-exponential decay function (see the fitted curves in Figure 4A
and B). The ø2 values for the fitting was within the error limits (ø2
)1.0 ( 0.1).
Figure 6. The absorption-time decay profile at 700 nm following
337 nm laser pulse excitation of the fullerene-aniline dyad (∼30 µM)
in deoxygenated benzonitrile: (A) dyad 2 and (B) dyad 4. Insets show
the transient absorption spectrum of the dyads in benzonitrile.
toluene are shown in Figure 5. Based on previous reports,8 we
can assign these absorption bands to the triplet excited state of
the dyads. Absorption-time profiles of optically matched
solutions (absorbance at 337 nm ) 0.7) of the para-substituted
dyad 2 and the ortho-substituted dyad 4, in toluene and
benzonitrile, are compared in Figure 6. Lifetimes of the triplet
excited states of the dyads in toluene were estimated by fitting
the absorption-time decay curves to first-order kinetics, which
yielded values in the range of 13 to16 µs.
Quantum yields of the triplet excited states were estimated
by a relative method using model compound 5 as actinometer
(φT ) 0.95).21 In toluene solutions, all of the dyads (1-4)
exhibited efficient intersystem crossing to yield a triplet quantum
yield of nearly unity (Table 2). These results indicate that the
intersystem crossing is the dominant pathway for the deactiva-
tion of the excited singlet state in a nonpolar solvent such as

Fullerene-Aniline Dyads
J. Phys. Chem. A, Vol. 103, No. 50, 1999 10761
toluene. It may be noted that this conclusion was similar to the
one drawn from the fluorescence measurements. In the case of
polar solvents such as benzonitrile, a drastic decrease in the
triplet quantum yield was evident for all of the dyads (Table
2). The decrease in the fluorescence yields and lifetimes as well
as the lower triplet yields observed in benzonitrile, relative to
toluene, are supportive of an intramolecular electron transfer
from the singlet excited dyads. The results of the triplet excited
state properties of the dyad molecules in toluene and benzonitrile
are summarized in Table 2.
It is noteworthy that the sums of triplet quantum yield and
charge transfer efficiency from the singlet excited state for the
four dyads is around unity in each case. (Please note that ΦT
and ΦCT were independently determined using transient absorp-
tion and fluorescence lifetime measurements, respectively.)
Therefore, intersystem crossing and photoinduced electron
transfer are the only two major pathways with which the excited
singlet state deactivates (reactions 2 and 3). In toluene solutions,
the high triplet quantum yield (ΦT ) 0.95) observed for dyads
1-4 reflects the dominance of intersystem crossing over the
electron transfer pathway (reaction 2). Both steady state emission
and fluorescence lifetime measurements indicate that the forward
electron transfer is quite efficient in benzonitrile for all of the
dyads employed in the present investigation. This feature
becomes especially prominent for ortho-substituted dyads 2 and
4, and the forward electron transfer process becomes the major
pathway (ΦCT ) 0.77). The enhanced rate of forward electron
transfer and the high quantum yield of charge separation (Table
2) observed in the case of the ortho-substituted dyads can be
attributed to the folding of the aniline group. The computational
calculations and 1H NMR studies, discussed in an earlier section,
support this argument. Such a conformation decreases the
donor-acceptor distances in the ortho-substituted dyads, thus
favoring the electron transfer.
Figure 7. Difference absorption spectra, recorded after 337 nm laser
pulse excitation of the fullerene-aniline dyads (∼30 µM) in deoxy-
genated benzonitrile: (A) dyad 2 and (B) dyad 4.
Electron Transfer Products. The fate of electron transfer
products via reaction 3, (radical anion of C60 and the radical
cation of aniline) in dyads 1-4, was further probed by
monitoring the absorbance in the near-infrared (NIR) region
using nanosecond flash photolysis. Earlier studies have shown
that the radical cation of aniline exhibits a characteristic
absorption around 460 nm.4a However, in the present case, it is
not possible to obtain clear evidence for the formation of the
radical cation, due to the dominance of the triplet absorption in
this spectral region (Figure 5 and Table 2). The radical anion
of C60, the other charge-separated species formed, can be readily
detected from its characteristic NIR (∼1000 nm) band. Earlier
studies using pulse radiolysis have indicated that the radical
anion of the dyad 1 exhibits a prominent absorption band at
400 and 1005 nm.8a
failure to observe the C60 radical anion following the excitation
of 1 and 3 essentially indicates the dominance of faster charge
recombination in these dyads.
In the dyad systems 2 and 4, the anilinic donor is linked to
the para as well as ortho positions of phenyl group of 1-methyl-
2-phenylpyrrolidinofullerene through three methylene chains.
In the dyad systems 1 and 3, the donor and acceptor groups are
linked together by two methylene groups. Based on the flash
photolysis experiments we can conclude that the charge
recombination process in these dyads is dependent on the length
of the alkyl chain. The stability of the radical pairs, in the case
of dyads 2 and 4, is likely to arise from the presence of an
additional methylene group linking the donor and acceptor. Such
an increase in the number of carbon atoms in the alkyl chain is
likely to provide an additional degree of freedom for confor-
mational rearrangements of fullerene and aniline moieties.
The charge-separated intermediates formed can recombine
to yield either the excited triplet state of the dyad or the ground
state (reactions 4 and 5)
Time-resolved transient absorption spectra recorded following
337 nm laser pulse excitation of dyads 2 and 4 are shown in
Figure 7. The appearance of an absorption band at 1010 nm in
both these experiments confirmed the formation of the C60
radical anion. However, the absorption arising from the C60
radical anion was not so prominent in dyads 1 and 3 (in
benzonitrile) and could not be detected using nanosecond laser
flash photolysis. Although the rates and quantum yields of
charge separation (kcs and Φcs) for both the ortho-substituted
dyads (3 and 4) are similar (Table 2), the extent of charge
stabilization in these two cases is different. The values of kcs
and Φcs’ as determined from fluorescence measurements, reflect
only the forward electron transfer. On the other hand, the
stabilized electron transfer products observed in nanosecond
laser flash photolysis also reflect charge recombination. The
(C60•--aniline•+) f 3(C60-aniline)*
(C60•--aniline•+) f (C60-aniline)
(4)
(5)
If the charge-separated state recombines to yield the triplet
excited state (reaction 4), one would expect a growth in the
triplet state absorption at 700 nm in parallel with the decay of

10762 J. Phys. Chem. A, Vol. 103, No. 50, 1999
Thomas et al.
radical ions. Both picosecond8a and nanosecond laser flash
studies did not show any growth component corresponding to
the charge recombination. Hence we rule out reaction 4 as the
major deactivation pathway for the recombination of charge
separated pairs and conclude that the recombination mainly
proceeds via reaction 5.
References and Notes
(1) (a) Deisenhofor, J.; Michel, H.; Angew Chem. Int. Ed. Engl. 1989,
28, 829. (b) Huber, R.; Angew. Chem., Int. Ed. Engl. 1989, 28, 848.
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N.; Sanchez, B.; Illescas, B.; Perez, I. Chem. ReV. 1998, 98, 2527. (c) Sun,
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New York, 1997; pp 325-390. (d) Foote, C. S. In Topics in Current
Chemistry; Electron Transfer 1; Matty, J., Ed.; Springer-Verlag: Berlin,
1994; p 347. (e) Kamat, P. V.; Asmus, K.-D. Interface 1996, 5, 22. (f)
Zhou, F.; Jehoulelt, C.; Bard, A. J. J. Am. Chem. Soc. 1992, 114, 11004.
(g) Dubois, D. K.; Kadish, K. M.; Flanagan, S.; Haufler, R. E.; Chibante,
L. P. E. Wilsson, L. F. J. Am. Chem. Soc. 1992, 114, 3978. (h) Arbogast,
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The solvent may also modify to some extent the reported
conformations (Figure 1). Despite such reservations about the
exact validity of the structures, the experimental evidence based
on steady state and time-resolved fluorescence spectroscopy
indicate the attachment of the anilinic group on the ortho
positions of phenyl group of 1-methyl-2-phenylpyrrolidino-
fullerene promotes the forward electron transfer process via a
through-space mechanism. We also attempted to calculate the
free energy changes for dyads 1-4 within the frame of the
dielectric continuum model. The latter model accounts for the
sum of the ionic radii (radical cation and radical anion) and
subtracts there from the spatial separation of the donor/acceptor
couple. The close separation in dyads 3 (3.28 Å) and 4 (4.01
Å) indicates a van der Waals contact between the two moieties
and therefore makes the conclusions derived from these calcula-
tions rather uncertain. We can conclude only that the free energy
changes in the ortho-substituted dyads are much larger than in
the para-substituted ones, since, based on the close contact, a
through-space electron transfer should prevail over the through-
bond electron transfer.
(4) (a) Williams, R. M.; Koeberg, M.; Lawson, J. M.; An, Y.-Z. Rubin,
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A.; Boyd, P. D. W. J. Phys. Chem. 1996, 100, 15926. (d) Liddell, P. A.;
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Y. Angew. Chem., Int. Ed. Engl. 1997, 36, 2626.
Despite the high efficiency of charge separation in benzo-
nitrile solutions we were able to detect the C60 radical anion
only in the case of dyads 2 and 4, indicating that the charge
recombination process is more depend on the length of the alkyl
chain. In general, fast charge recombination22 is a major limiting
factor in achieving a long-lived charge separated pair in these
donor acceptor systems. An elegant approach to suppress such
a recombination will be to employ clusters of fullerene-based
dyads in mixed solvents.8b By clustering these fullerene-donor
dyads, it is possible to induce the hopping of electron to the
adjacent fullerene moieties which in turn enhances charge
stabilization.
(6) Baran, P. S.; Monaco, R. R.; Khan, A. U.; Schuster, D. I.; Wilson,
S. R. J. Am. Chem. Soc. 1997, 119, 8363.
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303-393. (b) Wasielewski, M. R. In Photoinduced Electron Transfer, Part
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Conclusions
The orientation-dependent electron transfer studies are sig-
nificant in fullerene-based systems because of the spherical cage
and the π-electron cloud that surrounds it. The experimental
evidence based on steady state and time-resolved spectroscopy
is in agreement with computational studies (folded conformation
for the ortho-substituted derivatives and extended conformation
for para-substituted derivatives). The marked increase in the
rate constants and quantum yields of charge separation observed
in the present case for ortho-substituted dyads in benzonitrile
is attributed to the folding of the anilinic group. The folding of
the anilinic group, in close proximity to the π cloud of C60,
enhances the probability of forward electron transfer for the
ortho-substituted fullerene-aniline dyads via a through-space
mechanism.
(10) (a) Luo C.; Fujitsuka, M.; Huang, C.-H.; Ito, O. J. Phys. Chem. A.
1998, 102, 8716.
(11) Confirmed on a Cosmosil Buckprep HLPC column.
(12) (a) For synthesis of 5, see: Maggini, G.; Scarrano, G. Prato, M. J.
Am. Chem. Soc. 1993, 115, 9798. (b) For the preparation of 10 through a
different route see, Liang, K.; Law, K.-Y.; Whitten, D. G. J. Phys. Chem.
B 1997, 101, 540.
(13) Nagarajan, V.; Fessenden, F. W. J. Phys. Chem. 1985, 89, 2330.
(14) Evaluated using the Sybyl force field method of PC SPARTAN
software obtained from Wavefunction, Inc.; 18401, Von Karman, Suite 370,
Irvine, CA 92612.
(15) (a) For discussions on the 700 nm band observed in the case of
methanofullerene derivatives see, Bensasson, R. V.; Bienvenu¨e, E.; Fabre,
C. Janot, J.-M.; Land, E. J.; Leach, S.; Leboulaire, V.; Rassat, A.; Roux,
S.; Seta, P. Chem. Eur. J. 1998, 4, 270. (b) Guldi, D. M.; Maggini M.
Gazz. Chim. Ital. 1997, 127, 779. (c) Prato, M.; Maggini, M.; Acc. Chem.
Res. 1998, 31, 519. (d) Protonation of the anilinic nitrogen by adding
trifluoroacetic acid (25 mM) to CH2Cl2 solutions of dyads 2-4 causes a
partial disappearance of the band around 700 nm. This change is completely
reversed by adding pyridine (30 mM) to the above solution. The absorption
spectrum of the model compound was found to be unaffected by the addition
of TFA (250 mM), ruling out the possibility of a ground state charge transfer
interaction.
Acknowledgment. The authors thank the Office of Basic
Energy Science of the Department of Energy, the Council of
Scientific and Industrial Research, Government of India, and
the Jawaharlal Nehru Center for Advanced Scientific Research,
Bangalore for their support of the work described herein. This
is contribution no. RRLT-PRU 95 from RRL, Trivandrum and
NDRL 4137 from Notre Dame Radiation Laboratory.
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Fullerene-Aniline Dyads
J. Phys. Chem. A, Vol. 103, No. 50, 1999 10763
96, 764. (e) Nakamura, Y.; Minowa, T.; Hayashida, Y.; Tobita, S.; Shizuka,
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(17) Since the emission spectral profile of the dyads and spectral region
of emission of model compound 5 are nearly identical, 5 was used as
reference.4,7a
(18) An increase in fluorescence was observed on addition of trifluoro-
acetic acid (25 mM) to benzonitrile solution of dyads 1-4 (∼80% of φf of
that in toluene) and is due to the protonation of the anilinic nitrogen which
inhibits the electron transfer processes.
) 1320 ps (present study); previous reports4,7a τref ) 1280 ps]; (b) φcs values
are estimated as kcs/[(1/τref) + kcs].
(20) (a) Sakata, Y.; Nakashima, S.; Goto, Y.; Misumi, S.; Asahi, T.;
Hagihara, M.; Nishikawa, S.; Okada, T.; Mataga, N. J. Am. Chem. Soc.
1989, 111, 8979. (b) Wasielewski, M. R.; Niemczyk, M. P.; Johnson, D.
G.; Svec, W. A.; Minsek, D. W. Tetrahedron 1989, 45, 4785. (c) Sakata,
Y.; Tsue, H.; Goto, Y.; Misumi, S.; Asahi, T.; Nishikawa, S.; Okada, T.;
Mataga, N. Chem. Lett. 1991, 1307.
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Huang, C.-H. J. Chem. Soc., Faraday Trans. 1998, 94, 527.
(19) (a) kcs values were estimated as 1/τ - 1/τref where τ and τref are
the lifetimes of the corresponding dyad and of the model compound 5 [τref