Photoinduced charge separation of the covalently linked fullerene-triphenylamine-fullerene triad. effect of dual fullerenes on lifetimes of charge-separated states

Article abstract of DOI:10.1246/bcsj.80.2465

Photoinduced intramolecular events of the newly synthesized fullerene-triphenylamine-fullerene (C₆₀-TPA-C₆₀) triad, in which the TPA entity was substituted with an electron-donating CH ₃O-group to increase electron-donating ability, were investigated in relation to a C₆₀-TPA dyad. The molecular orbital calculations showed that the radical cation is located on the TPA entity, whereas the radical anion is located on two C₆₀ entities in the radical ion pair. The fluorescence intensity of the singlet excited state of C₆₀ was efficiently quenched by the attached TPA moiety in polar solvents. The quenching pathway involves a charge-separation process from the TPA to the singlet excited state C₆₀. The lifetimes of the radical ion-pairs for C ₆₀-TPA-C₆₀ evaluated from nanosecond transient absorption measurements were found to be 600 and 454 ns in benzonitrile and dimethylformamide, respectively. These lifetimes of radical ion-pairs of C ₆₀-TPA-C₆₀ are longer than those of the C₆₀-TPA dyad, which reflects the effect of the second C₆₀ moiety in stabilizing the radical ion-pairs.

Full text of DOI:10.1246/bcsj.80.2465

Ó 2007 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 80, No. 12, 2465–2472 (2007) 2465
Photoinduced Charge Separation of the Covalently Linked
Fullerene–Triphenylamine–Fullerene Triad. Effect of Dual
Fullerenes on Lifetimes of Charge-Separated States
ꢀ1;2
Mohamed E. El-Khouly,
Chan Soo Choi,4 Osamu Ito,1 and Kwang-Yol Kay
Jong Hyung Kim,3 Minseok Kwak,3
ꢀ3
1Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
Katahira, Aoba-ku, Sendai 980-8577
2Department of Chemistry, Faculty of Education, Tanta University, Kafr El-Sheikh, Egypt
3Department of Molecular Science and Technology, Ajou University,
Wonchon-dong, Youngtong-gu, Suwon 443-749, Korea
4Department of Applied Chemistry, Daejeon University, Korea
Received June 26, 2007; E-mail: mohamedelkhouly@yahoo.com
Photoinduced intramolecular events of the newly synthesized fullerene–triphenylamine–fullerene (C60–TPA–C60)
triad, in which the TPA entity was substituted with an electron-donating CH3O-group to increase electron-donating abil-
ity, were investigated in relation to a C60–TPA dyad. The molecular orbital calculations showed that the radical cation is
located on the TPA entity, whereas the radical anion is located on two C60 entities in the radical ion pair. The fluores-
cence intensity of the singlet excited state of C60 was efficiently quenched by the attached TPA moiety in polar solvents.
The quenching pathway involves a charge-separation process from the TPA to the singlet excited state C60. The lifetimes
of the radical ion-pairs for C60–TPA–C60 evaluated from nanosecond transient absorption measurements were found to
be 600 and 454 ns in benzonitrile and dimethylformamide, respectively. These lifetimes of radical ion-pairs of C60–
TPA–C60 are longer than those of the C60–TPA dyad, which reflects the effect of the second C60 moiety in stabilizing
the radical ion-pairs.
Photoinduced intramolecular electron transfer of donor–
acceptor systems is the most elementary and ubiquitous of all
photochemical reactions, and it plays a crucial role in many es-
sential photophysical processes.1,2 Functionalization of fuller-
ene (C60) with electron-donating moieties has drawn consider-
able attention as building blocks to construct artificial light
energy harvesting systems and also to develop molecular elec-
tronic devices.3,4 Among the electron-donors, aromatic amines
are the most fundamental donors used in optoelectronic mate-
rials owing to their low oxidation potential and good film-
forming properties.5
To date, various C60–amine dyads with different linkages
have been synthesized, and their photophysical properties
have been investigated.6 In our continuous efforts to explore
the intramolecular events of C60–aromatic amine connected
systems, we have reported the photoinduced processes of the
covalently linked C60–triphenylamine dyads with the H- and
CH3-groups,7 in which we have shown that there is efficient
charge separation in these systems by employing the time-
resolved and nanosecond transient techniques. It is notable
that the charge recombination of such systems occurs quickly
to the ground state, from which the lifetimes of the radical
ion-pairs (RIPs) can be evaluated in the range of a few nano-
seconds.
valently linked with different electron-donor compounds have
been investigated, e.g., porphyrins, phthalocyanines, ꢀ-conju-
gated oligomers, tetrathiafulvalenes (TTFs), substituted poly-
fluorene, etc.8 The presence of two C60 units could contribute
to establish well-ordered solid phases, which is a crucial factor
in the field of optoelectronic devices research.8c In this manu-
script, we designed a new molecular C60–TPA–C60 triad 1
(Fig. 1), in which the TPA moiety is bounded by two C60 units,
aiming to use it as a constituent of the electronic devices. Here,
the CH3O-group on triphenylamine was expected to increase
the electron-donating ability compared to our previous CH3–
substituted C60–TPA dyad.7d Both picosecond fluorescence
and nanosecond transient absorption spectral studies were per-
formed in order to characterize the electron-transfer species in
polar solvents and to explore the mechanistic details of the
light-induced electron-transfer reactions. Finally, by compar-
ing to the C60–TPA dyad 2, the effect of the second C60 moiety
for prolonging the lifetimes of the charge-separated state was
clarified in this study.
Results and Discussion
Synthesis and Characterization.
The most straight-
forward preparation of C60–TPA–C60 1 and C60–TPA 2 can
be envisaged to proceed through a reaction sequence of the
following steps, i.e., the Ullmann,9 Vilsmeier,10 Prato’s 1,3-
dipolar cycloaddition11 reactions depicted in Scheme 1. Every
In order to reveal the effect of the second fullerene to the
C60–donor dyads, related triads consisting of two C60 units co-
Published on the web December 13, 2007; doi:10.1246/bcsj.80.2465

2466 Bull. Chem. Soc. Jpn. Vol. 80, No. 12 (2007)
PET of C60–Triphenylamine–C60 Triad
CS
CH
hν
3
N
CS
OCH
3
hν
N CH
3
N
N
CR
H C
3
N
CR
OCH
3
C60-TPA-C60 (1)
C60-TPA (2)
Fig. 1. Molecular structures and expected photoinduced processes: CS; charge separation, CR; charge recombination. In the present
study, TPA refers as to CH3O-substituted triphenylamine otherwise cited.
dipolar cycloaddition of the azomethine ylide afforded C60–
TPA 2 in 29.0% yield.
The structure and purity of the new compounds were
O
H
N
H3C
a)
b)
c)
O
H
confirmed by 1H NMR, 13C NMR, and IR spectroscopies,
MALDI-TOF mass spectrometry, and elemental analysis (see
experimental section). The 1H NMR spectra of 1 and 2 are
consistent with the proposed structures, and 13C NMR spectra
showed characteristic aliphatic carbon peaks: seven resonance
lines for 1 and four resonance lines for 2. The complex pattern
of 13C NMR signals between 114.82–158.68 ppm was assigned
to the aromatic carbons and also the resonance peaks of C60.
These patterns of signals are unambiguously diagnostic for
the proposed structures of 1 and 2. The MALDI-TOF-MS
provided a direct evidence for the structures of 1 and 2 by a
singly charged molecular ion peak at m=z ¼ 1826:71 and
1051.14, respectively.
N
NH
N
N
3
H3CO
4
CH3
H3CO
H3CO
N
b)
CH3
N
O
c)
N
1
N
H
5
H3CO
H3CO
Scheme 1. Synthesis of compounds 1 and 2; a) 4-iodoani-
sole, K2CO3, Cu, 18-crown-6, o-dichlorobenzene (DCB),
180, 48 h, 86.0% yield, b) POCl3, DMF, o-dichloroethane,
reflux, 36.7% yield for 4 and 73.0% yield for 5, and c) sar-
cosine, fullerene, reflux, 10.0% yield for 1, 29.0% yield
for 2.
Photophysical Studies. The absorption spectra of C60–
TPA–C60 1 and C60–TPA 2 are shown in Fig. 2. In the UV–
vis region, the absorption spectra exhibited the characteristic
peaks of a 1-methyl-3,4-fullero-2,3,4,5-tetrahydropyrrol (ab-
breviated as fulleropyrrolidine) below 350 nm in addition to
432 and 704 nm. The TPA moiety showed an absorption in
the UV region with a maximum at 298 nm. The UV–vis spec-
trum is a superimposition on the sum of the two fulleropyrro-
lidine units and the TPA moiety, indicating that there is no
significant ground-state electronic interaction between the
components. Spectral features observed in o-dichlorobenzene
(DCB) and benzonitrile (BN) were the same as in toluene.
Photoinduced intramolecular events of both compounds
were investigated by using steady-state fluorescence spectros-
copy on exciting the C60 moiety (ꢁex ¼ 400 nm). In toluene,
step of the reaction sequence proceeded smoothly and effi-
ciently to give a good or moderate yield of the product. Di-
phenylamine was coupled with 4-iodoanisole under Ullmann
condition to give 4-methoxytriphenylamine (3, abbreviated
as TPA) in 86.0% yield followed by Vilsmeier formylation
with a large excess of POCl3/DMF to produce diformylated
TPA 4. The functionalization of C60 was based on azomethine
1,3-dipolar cycloaddition. Treatment of C60 in toluene with
dialdehyde 4 and sarcosine or methylglycine afforded dyad 1
in 10.0% yield. Although C60–TPA–C60 1 should be obtained
as a stereoisomeric mixture due to the formation of two asym-
metric centers in two-fold cycloaddition reaction, high resolu-
tion 1H NMR spectrum (400 MHz) of C60–TPA–C60 1 showed
that one of the two stereoisomers is predominant.
1
ꢀ
the intensity of the C60 moiety of C60–TPA–C60 at 710 nm
was found to be almost the same as that obtained for the
fulleropyrrolidine reference.3,4,12 The spectral shape is insen-
sitive to solvent changes, whereas the fluorescence properties
are found to be sensitive to solvent polarity. Changing the
solvent from toluene to DCB gave rise to significant quenching
Analogously, monoaldehyde 5 was obtained in 73.0% yield
by the Vilsmeier reaction under normal stoichiometry of
POCl3/DMF (up to 3.0 equivalents), and subsequent 1,3-
ꢀ
of the excited singlet state of the C60 moiety (1C60 ) compared
to that in toluene. These observations suggest efficient quench-

M. E. El-Khouly et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 12 (2007) 2467
0.6
0.08
0.06
0.04
0.02
0.00
2 / DCB
1 / TN
1 / DCB
1 / BN
0.5
0.4
0.3
0.2
0.1
0.0
300 400 500 600 700 800
300 400 500 600 700 800
Wavelength / nm
Fig. 2. Steady-state absorption of C60–TPA–C60 1 and C60–TPA 2 (left) and fluorescence spectra (right) of C60–TPA–C60 in
toluene (TN), and o-dichlorobenzene (DCB); ꢁex ¼ 400 nm. Concentrations are kept at 1 ꢁ 10ꢂ6 mol dmꢂ3
.
similar decay as C60–TPA–C60, where the fluorescence lifetime
of the major component was evaluated to be 107 ps (90%).
Based on the short lifetimes of the major components, the rate
(kCS) and quantum yield (ꢂCS) for the charge-separation pro-
cess in DCB were determined to be 9:9 ꢁ 109 sꢂ1 and 0.94
for C60–TPA–C60 and 8:6 ꢁ 109 sꢂ1 and 0.93 for C60–TPA, re-
spectively. In more polar solvents, such as BN and DMF, high-
er values for kCS and ꢂCS were obtained, as listed in Table 1.
Electrochemical Studies. The charge-separation process
via 1C60 was supported from the viewpoint of thermodynam-
ꢀ
ics of electron-transfer processes. The cyclic voltammogram of
C60–TPA–C60 in DCB showed the first reduction potential
(Ered) of the C60 moiety at ꢂ930 mV vs. Ag/AgCl, and the
corresponding first oxidation potential (Eox) of the TPA moiety
was located at 610 mV vs. Ag/AgCl. The potential difference
between the first oxidation of the TPA moiety and the first
reduction of the C60 moiety was found to be 1.54 V. Compared
to the redox values of C60–TPA, the first oxidation (618 mV
vs. Ag/AgCl) and reduction (ꢂ930 mV vs. Ag/AgCl) poten-
tials of C60–TPA–C60 were slightly change (ca. 10 mV), sug-
gesting appreciable interaction between the two fullerene
moieties of C60–TPA–C60. Based on the first oxidation poten-
tial of the TPA moiety, the first reduction potential of the
C60 moiety and a solvent correction term (ꢀGS), the thermo-
dynamic driving forces for charge-recombination process
1
ꢀ
Fig. 3. Time profiles of C60 -fluorescence at 710 nm of
C60–TPA–C60 1 in toluene (TN) and DCB; ꢁex ¼ 400 nm.
1
ꢀ
ing of the C60 moiety by the appended TPA moiety in the
polar solvents. The fluorescence quenching of the 1C60 moie-
ꢀ
ty became more prominent with an increase in the solvent po-
larity, providing evidence for intramolecular electron transfer
in polar solvents.6 Similar behavior was observed for C60–
TPA. By using a streak-scope as a detector, time-resolved fluo-
rescence spectra were observed by irradiating with a 400-nm
laser for 0–2 ns, which afforded similar fluorescence spectra
to those of the steady-state fluorescence spectra, supporting
the reliability of the both fluorescence measurements.
(ꢂꢀGCR) and the intramolecular charge-separation (ꢂꢀGCS
)
1
ꢀ
via the C60 moiety calculated from the Rehm–Weller equa-
tion are listed in Table 1.13 These values suggest an exother-
The fluorescence lifetime measurements (Fig. 3) track the
above considerations in a more quantitative way, giving kinet-
ic data of the charge-separation processes in polar solvents. In
1
ꢀ
mic charge-separation process via the C60 moiety to form
the RIPs in polar solvents, whereas endothermic in toluene.
The ꢂꢀGCS values of C60–TPA–C60 were found to be 0.28,
0.43, and 0.48 eV in DCB, BN, and DMF, respectively, which
are slightly more negative than the corresponding values of
C60–TPA.
1
ꢀ
toluene, the time-profiles of the C60 of C60–TPA–C60 and
C60–TPA exhibited a single-exponential decay with a lifetime
of 1400 ps, which is almost the same as the fulleropyrrolidine
reference,12 suggesting no fluorescence quenching of the 1C60
ꢀ
unit by the TPA moiety. The time-profile of C60–TPA–C60 in
DCB could be satisfactorily fitted by a bi-exponential decay:
one has a short lifetime of 92 ps (80%) which reflects the
Molecular Orbital Calculations. Moreover, the charge-
1
ꢀ
separation process from the TPA unit to the C60 moiety
forming the RIPs was supported by studying the molecular
geometries and electronic structures of C60–TPA–C60 and
C60–TPA on the basis of molecular orbital calculation using
density functional method (DFT) at B3LYP/3-21G level.14
From the optimized structure of C60–TPA–C60, shown in
Fig. 4, the center-to-center distance (RCC) between the C60
1
ꢀ
actual intramolecular deactivation of the C60 moiety, and
the other has a longer lifetime, which resembles that of the
C60 reference. In BN and dimethylformamide (DMF), the de-
ꢀ
cays of 1C60 were too fast to be detected using our instrument
(ca. 20 ps). The fluorescence time-profiles of C60–TPA show a

2468 Bull. Chem. Soc. Jpn. Vol. 80, No. 12 (2007)
PET of C60–Triphenylamine–C60 Triad
ꢀ
Table 1. Free-Energy Changes (ꢂꢀGCR and ꢂꢀGCS) via 1C60 , Initial-Quick Fluorescence Lifetimes
1
ꢀ
1
ꢀ
of C60 (ꢃf), Rates (kCS) and Quantum Yields (ꢂCS) of Charge-Separation via C60 , Rates of
ꢃꢂ
Charge-Recombination (kCR) and Lifetimes of Radical Ion-Pairs (ꢃRIP) of C60–TPAꢃþ–C60 and
C60ꢃꢂ–TPAꢃþ
a)
a)
b)
b)
Compound Solvent ꢂꢀGCR
ꢂꢀGCS
ꢃf
/ps
kCS
/sꢂ1
ꢂCS
kCR
/sꢂ1
ꢃRIP
/ns
/eV
/eV
C60–TPA–C60 TN
DCB
BN
1.92
1.59
1.32
1.27
ꢂ0:17
0.28
0.43
1400
93
<20
<20
—
—
—
—
9:9 ꢁ 109
>5 ꢁ 1010
>5 ꢁ 1010
0.94 2:5 ꢁ 107 40
>0:99 1:6 ꢁ 106 600
>0:99 2:2 ꢁ 106 450
DMF
0.48
C60–TPA
TN
DCB
BN
2.03
1.59
1.32
1.31
ꢂ0:28
0.26
0.43
1390
107
<20
<20
—
—
—
—
8:6 ꢁ 109
>5 ꢁ 1010
>5 ꢁ 1010
0.93 2:5 ꢁ 107 40
>0:99 2:1 ꢁ 106 480
>0:99 2:7 ꢁ 106 360
DMF
0.44
a) The driving forces for ꢀGCR and ꢀGCS were calculated by equations; ꢂꢀGCR ¼ eðEox ꢂ EredÞ þ
ꢀGS and ꢂꢀGCS ¼ ꢀE00 ꢂ ðꢂꢀGCRÞ, where ꢀE00 is the energy of the 0–0 transition (1.75 eV for
ꢀ
1C60 ). ꢀGS refers to the static energy, which was calculated by ꢂꢀGS ¼ ꢂðe2=ð4ꢀ"0ÞÞ½ð1=
ð2R Þ þ 1=ð2R Þ ꢂ ð1=RCCÞ="S ꢂ ð1=ð2R Þ þ 1=ð2R ÞÞ="RÞ, where R and R are radii of the
þ
ꢂ
þ
ꢂ
þ
ꢂ
radical cation (0.37 nm) and radical anion (0.42 nm), respectively; RCC is the center–center distance
between C60 and TPA (1.2 nm), which were evaluated from optimized structure. "R and "S refer
to solvent dielectric constants for electrochemistry and photophysical measurements, respectively.
ꢀ
b) The kCS and ꢂCS via 1C60 were calculated from the relation; kCS ¼ ð1=ꢃfÞ ꢂ ð1=ꢃf0Þ and ꢂCS
¼
kCS=ð1=ꢃfÞsample, in which ꢃf0 is the lifetime of C60 reference (1400 ps).
LUMO+1
LUMO
HOMO
Fig. 5. Transient absorption spectra of C60–TPA–C60
obtained by irradiating 532 nm laser light in Ar-saturated
toluene.
1
and TPA moieties was estimated to be 1.2 nm. The electron
distribution of the highest occupied molecular orbital (HOMO)
was found to be entirely located on the TPA entity, whereas
the electron distribution of the lowest unoccupied molecular
orbital (LUMO and LUMO+1) was found to be entirely locat-
ed on the C60 spheroids. This suggests the charge-separated
state as C60ꢃꢂ–TPAꢃþ–C60 or C60–TPAꢃþ–C60
Transient Absorption Studies. Conclusive evidence
about the charge separation from the TPA unit to the C60
.
ꢃꢂ
1
ꢀ
moiety was obtained from the nanosecond transient absorption
measurements by irradiating at 532 nm with 6 ns laser pulses,
which selectively excited the C60 moiety. In toluene, the tran-
sient spectra of C60–TPA–C60 (Fig. 5) exhibited absorption
peak at 700 nm corresponding to the the excited triplet state
Fig. 4. Optimized structure and the HOMO, LUMO, and
LUMO+1 of C60–TPA–C60 calculated by ab initio molec-
ular orbital method at B3LYP/3-21G level.
ꢀ
of the C60 moiety (3C60 ), which is almost identical to that

M. E. El-Khouly et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 12 (2007) 2469
Fig. 6. Transient absorption spectra of C60–TPA–C60 obtained by 532 nm laser light in Ar-saturated BN (left) and DMF (right).
of the excited triplet state of fulleropyrrolidine reference;
1C60 *-TPA
the slow decay at 700 nm in inset of Fig. 5 also supports this
assignment. The absence of the characteristic peaks of the
TN
1C60 *-TPA-C60
RIPs indicates no electron transfer, which is in good agreement
with the no-emission decay and with the positive ꢀGCR value
kCS
isc
DCB
3
ꢀ
system crossing (isc) from the C60 moiety.
kCR
in toluene. Therefore, the C60 moiety is generated via inter-
1
3C60 *-TPA
ꢀ
In DCB, although the transient absorption spectra of C60–
TPA–C60 (see Supporting Information, Fig. S1) are almost
the same as those in toluene, the absorption intensity of the
kCS
BN
DMF
ꢀ
3C60 moiety is considerably lower than that in toluene, indi-
.-
.+
cating the presence of competitive path along with intersystem
C60 - TPA
1
ꢀ
crossing (isc) via C60 . In addition, a weak absorption band
was observed at 1000 nm. The time-profile at 1000 nm showed
the quick decay (2:5 ꢁ 107 sꢂ1), followed by a slow decay
(ꢄ105 sꢂ1). The slow decay part was ascribed to the absorption
.-
C60 - TPA .+-C60
hν
kT
kCR
in TN
in DCB
in BN and DMF
3
ꢀ
tail of C60 , whereas the quick decay part can be assigned
to the radical anion of the C60 moiety (C60ꢃꢂ),3,4,15–17 since
formation of RIPs via intramolecular charge-separation pro-
C60 -TPA
C60 -TPA-C60
1
ꢀ
cess via C60 was evidenced by the efficient quenching of
ꢀ
Fig. 7. Energy diagram of C60–TPA–C60 and C60–TPA in
the studied solvents.
1C60 -fluorescence. The quick decay of the C60
may be related to the relatively higher energy of the charge-
moiety
ꢃꢂ
ꢀ
separated state compared to the energy level of the 3C60 moi-
ety, which promotes the charge-recombination process to the
106 sꢂ1 (DMF), and the ꢃRIP values were 480 and 360 ns, re-
spectively. The ꢃRIP values of C60–TPA–C60 are longer than
those of C60–TPA. Since the energy levels of the charge-sepa-
rated states of C60–TPA–C60 and C60–TPA are almost similar,
the slightly longer ꢃRIP values of C60–TPA–C60 may reflect an
effect of the second C60 moiety on stabilizing the RIPs due to
electronic interaction between the two C60 moieties, including
electron migration between the two C60 moieties.
ꢀ
3C60 moiety, in addition to the ground state.
As shown in Fig. 6, quite different spectra and time profiles
were observed for C60–TPA–C60 in highly polar solvents, such
as BN and DMF, compared to those in toluene and DCB. The
narrow band in the near-IR region at 1000 nm served as a di-
agnostic probe for identification of the C60 moiety,3,4,15–17
ꢃꢂ
and the absorption band in the vis region at 740 nm was as-
signed to the radical cation of the TPA moiety, which overlaps
Furthermore, the ꢃRIP values of C60–TPA–C60 and C60–TPA
are much longer than those the reported for C60–tetrathiaful-
valene (TTF)–C60 and C60–substituted-polyfluorene–C60 tri-
ads, revealing the role of the TPA moiety in stabilizing the
charge-separated states, due to of geometric and electronic rea-
sons.8a,d As for the substituent effect on triphenylamine donors,
the ꢃRIP values with CH3O-group are apparently longer than
the reported amine–C60 dyads with the H- and CH3-groups;7d
this may due to the effect of the CH3O-group on increasing the
electron-donating character of the triphenylamine moiety.
Energy Diagram. Figure 7 illustrates energy level dia-
grams summarizing the observed photoinduced intramolecular
3
ꢀ
with the C60 moiety.
The characteristic band of the C60 moiety in the near-IR
region was employed as reliable probe to determine the rate
ꢃꢂ
ꢃꢂ
constant of the charge recombination, kCR, because the C60
band at 1000 nm does no overlap the bands of other species.
The time profiles (insets) were well-fitted by a single-exponen-
tial function. The kCR values of C60–TPA–C60 were evaluated
to be 1:6 ꢁ 106 sꢂ1 (BN) and 2:2 ꢁ 106 sꢂ1 (DMF), from
which the lifetimes of the radical ion-pairs (ꢃRIP) were evalu-
ated as 600 and 450 ns, respectively. For C60–TPA, the kCR
values were evaluated to be 2:1 ꢁ 106 sꢂ1 (BN) and 2:7 ꢁ
1
ꢀ
events of C60–TPA–C60 and C60–TPA. In toluene, the C60

2470 Bull. Chem. Soc. Jpn. Vol. 80, No. 12 (2007)
PET of C60–Triphenylamine–C60 Triad
rinsed with acetone before use. The counter electrode was a plat-
inum wire. The measured potentials were recorded with respect to
an Ag/AgCl (saturated KCl) reference electrode.
moiety decayed without forming the RIPs, where the charge-
1
ꢀ
separation is thermodynamically unfavorable. The C60
moiety relaxes to the 3C60 moiety by an intersystem crossing
ꢀ
3
ꢀ
Lifetime measurements were performed by using a single-pho-
ton counting method with a second harmonic generation (SHG,
400 nm) of a Ti:sapphire laser (Spectra-Physica, Tsunami 3950-
L2S, 1.5 ps fwhm) and a streak scope (Hamamatsu Photonics)
equipped with a polychromator (Action Research, SpectraPro
150) as an excitation source and a detector, respectively. Lifetimes
were evaluated with software attached to the equipment. The
nanosecond transient absorption measurements in the near-IR re-
gion were measured by means of laser-flash photolysis; 532 nm
light from a Nd:YAG laser (Spectra-Physics and Quanta-Ray
GCR-130, 6 ns fwhm) was used as an excitation source. For tran-
sient absorption spectra in the near-IR region (600–1100 nm),
monitoring light from a pulsed Xe-lamp was detected with a
Ge-avalanche photodiode module (Hamamatsu Photonics).19 All
of the samples in a quartz cell (1 ꢁ 1 cm) were deaerated by
nitrogen bubbling through the solution for 20 min.
process. Finally, the C60 decays to the ground state. In
polar solvents, fluorescence quenching takes place through
electron transfer from the TPA entity to the covalently bonded
ꢀ
1C60 moiety, which was confirmed by observing the charac-
ꢃꢂ
teristic bands of C60 and TPAꢃþ with nanosecond transient
technique. The charge-recombination of the charge-separated
states may direct to the ground state in BN and DMF, whereas
ꢀ
in DCB, charge-recombination may produce mainly the 3C60
moiety in addition to the ground state. The longer lifetimes of
the radical ion-pairs (ꢃRIP) in BN compared to that in DMF
suggest that the charge-recombination process with a highly
negative ꢀGCR (ca. ꢂ1:30 eV) belongs to the Marcus inverted
region,18 because the reorganization energy of the C60–donor
dyads has been reported to be as small as 0.6 eV.15
Conclusion
Synthetic Details. 4-Methoxytriphenylamine (3): To a so-
lution of 4-iodoanisole (4.15 g, 17.7 mmol) in o-dichlorobenzene
(20 mL) were added copper powder (1.50 g, 23.6 mmol), K2CO3
(6.55 g, 54.4 mmol), 18-crown-6 (0.31 g, 1.17 mmol), diphenyl-
amine (2.00 g, 11.8 mmol). After refluxing for 48 h, the reaction
mixture was cooled to room temperature and filtered off. The sol-
vent was evaporated from the filtrate, and the residue was washed
with ethanol and recrystallized from ethanol to give compound
3 (1.43 g, 86.0%) as a brown solid. Mp 108–109 ꢅC; 1H NMR
(400 MHz, CDCl3): ꢄ 7.18 (m, 4H), 7.05 (d, J ¼ 4:4 Hz, 2H), 7.01
(d, J ¼ 4:8 Hz, 4H), 6.92 (m, 2H), 6.82 (d, J ¼ 4:4 Hz, 2H), 3.78
(s, 3H); IR (KBr): ꢅ ¼ 1036, 1238, 1506, 1585 cmꢂ1; Anal. Calcd
for C19H17NO: C, 82.88; H, 6.22; N, 5.09%. Found: C, 82.87; H,
6.25; N, 5.12%.
The photophysical events of a C60–triphenylamine–C60 triad
were studied by using time-resolved emission and nanosecond
transient techniques. From the collected data, presence of the
second fullerene unit seems to afford a stabilizing electronic
or structural effect on the photo-generated ion radical pair.
In line with the enhanced stabilization of the charge-separated
state, higher quantum yields of charge separation were also
found in the triad system. This clearly points to cooperative
effects evolving from the two fullerene moieties. Despite the
simplicity of studied compounds, the photo-generated charge-
separated states possess a significant stability, suggesting their
potential to be photosynthetic models.
4-Methoxy-40,400-diformyltriphenylamine (4): To a solution
of DMF (1.41 mL, 18.2 mmol) in 1,2-dichloroethane (20 mL) was
added compound 3 (0.50 g, 1.81 mmol), and carefully poured
POCl3 (1.69 ml, 18.2 mmol). The mixture was refluxed for 6 h,
then cooled to room temperature, and poured into saturated aque-
ous sodium acetate solution (50 mL). The product was extracted
with dichloromethane (3 ꢁ 50 mL) and the extract was dried over
MgSO4. The solvent was then evaporated. The residue was chro-
matographed on silica gel with dichloromethane to give com-
pound 4 (0.22 g, 36.7%) in a yellow solid. Mp 92–95 ꢅC; 1H NMR
(400 MHz, CDCl3): ꢄ 9.86 (s, 2H), 7.72 (d, J ¼ 4:8 Hz, 4H), 7.15
(d, J ¼ 4:8 Hz, 4H), 7.10 (d, J ¼ 4:4 Hz, 2H), 6.92 (d, J ¼ 4:4 Hz,
2H), 3.83 (s, 3H); IR (KBr): ꢅ ¼ 1032, 1161, 1246, 1504, 1587,
1693, 2731, 2833 cmꢂ1; Anal. Calcd for C21H17NO3: C, 76.12;
H, 5.17; N, 4.23%. Found: C, 76.09; H, 5.20; N, 4.20%.
N-4-Methoxyphenyl-N,N-bis[4-(1-methyl-3,4-fullero-2,3,4,5-
tetrahydropyrrol-2-yl)phenyl]amine (1): Compound 4 (50 mg,
0.15 mmol) and sarcosine (33 mg, 0.37 mmol) were added to a so-
lution of fullerene (0.22 g, 0.30 mmol) in chlorobenzene (30 mL).
The reaction mixture was refluxed for 18 h, and then filtered. The
solvent was evaporated from the filtrated and the residue was sub-
jected to chromatography on silica gel with toluene to give com-
pound 1 (27 mg, 10.0%) in a black solid. Mp > 410 ꢅC (dec.);
1H NMR (400 MHz, CS2): ꢄ 7.56 (br, 4H), 6.95 (d, J ¼ 4:4 Hz,
4H), 6.91 (d, J ¼ 4:8 Hz, 2H), 6.74 (d, J ¼ 4:4 Hz, 2H), 4.94 (d,
J ¼ 5:6 Hz, 2H), 4.86 (s, 2H), 4.24 (d, J ¼ 5:6 Hz, 2H), 3.77 (s,
3H), 2.84 (s, 6H); 13C NMR (CS2): ꢄ 156.55, 156.37, 154.00,
153.60, 153.52, 147.98, 147.42, 146.90, 146.61, 146.49, 146.43,
146.36, 146.32, 146.25, 146.08, 145.93, 145.77, 145.65, 145.55,
Experimental
Reagents and Instruments. All chemicals were purchased as
reagent grade and used without further purification. All solvents
employed for the photophysical studies were purchased from
Aldrich and used as received. All reactions were performed using
dry glassware under nitrogen atmosphere. Analytical TLC was
carried out on Merck 60 F254 silica gel plates, and column chro-
matography was performed using Merck 60 silica gel (230–400
mesh). Melting points were determined on an Electrothermal IA
9000 series melting point apparatus and are uncorrected. NMR
spectra were recorded on a Varian Mercury-400 (400 MHz) spec-
trometer with the TMS peak as a reference. IR spectra were
recorded on a Nicolet 550 FT infrared spectrometer and measured
as KBr pellets. UV–vis spectra were recorded on a JASCO V-550
spectrometer. MALDI-TOF-MS spectra were recorded with an
Applied Biosystems Voyager-DE-STR. Elemental analyses were
performed with a Perkin-Elmer 2400 analyzer.
UV–vis spectral measurements were carried out on a JASCO
model V570 DS spectrophotometer. Steady-state fluorescence
spectra were measured on a Shimadzu RF-5300 PC spectrofluoro-
photometer equipped with a photomultiplier tube having high sen-
sitivity in the 700–800 nm region. Molecular orbital calculations
were performed by using density functional B3LYP/3-21G meth-
od with the Gaussian 98 package.14 Cyclic voltammetry was per-
formed on a BAS CV-50 W electrochemical analyzer in deaerated
DCB solution containing tetrabutylammonium perchlorate as a
supporting electrolyte at 298 K. The glassy carbon working elec-
trode was polished with BAS polishing alumina suspension and

M. E. El-Khouly et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 12 (2007) 2471
145.44, 145.42, 145.31, 145.29, 144.87, 144.83, 144.54, 143.32,
143.19, 142.86, 142.79, 142.76, 142.41, 142.34, 142.28, 142.25,
142.19, 141.97, 141.84, 141.79, 141.78, 140.40, 140.33, 140.18,
139.98, 139.45, 136.92, 136.78, 136.03, 135.96, 130.48, 127.50,
125.81, 115.17, 83.36, 70.35, 69.16, 55.52, 50.00, 30.89, 30.63.
IR (KBr): ꢅ ¼ 1032, 1178, 1240, 1504 cmꢂ1; UV–vis (toluene):
ꢁmax ¼ 323, 432, 705 nm; MS (MALDI-TOF); m=z for C145H27-
N3O Calcd 1826.79. Found 1826.71; Anal. Calcd: C, 95.33; H,
1.49; N, 2.30%. Found: C, 95.29; H, 1.50; N, 2.34%.
Storage of Solar Energy, ed. by J. S. Connolly, Academic,
New York, 1981. e) H. D. Roth, A Brief History of Photoinduced
Electron Transfer and Related Reactions in Topics in Current
Chemistry, ed. by J. Mattay, Springer-Verlag, Berlin, 1990,
Vol. 156, p. 1.
2
a) D. Gust, T. A. Moore, Top. Curr. Chem. 1991, 159, 103.
b) M. R. Wasielewski, Chem. Rev. 1992, 92, 435. c) Fundamen-
tals of Photoinduced Electron Transfer, ed. by G. J. Kavarnos,
VCH Publisher, New York, 1993. d) The Photosynthetic Reaction
Centre, ed. by J. Deisenhofer, J. R. Norris, Academic Press,
San Diego, 1993. e) Electron Transfer in Chemistry, ed. by V.
Balzani, Wiley-VCH, Weinheim, 2001, Vols. I–V.
4-Methoxy-40-formyltriphenylamine (5): To a solution of
DMF (0.60 mL, 7.63 mmol) in 1,2-dichloroethane (20 mL) was
added compound 3 (0.70 g, 2.54 mmol), and carefully poured
POCl3 (0.71 mL, 7.63 mmol), The mixture was refluxed for 4 h,
then cooled to room temperature, and poured into a saturated
aqueous sodium acetate solution (50 mL). The product was ex-
tracted with dichloromethane (3 ꢁ 50 mL), and the extract was
dried over MgSO4. The solvent was evaporated. The residue was
chromatographed on silica gel with dichloromethane/hexane (3:1)
to give compound 5 (0.56 g, 73.0%) in a yellow solid. Mp 88–90
3
a) R. Taylor, D. R. M. Walton, Nature 1993, 363, 685.
b) C. S. Foote, in Physics and Chemistry of the Fullerenes, ed.
by K. Prassides, Kluwer Academic Publishers, Amsteradm,
1994. c) D. Kuciauskas, S. Lin, G. R. Seely, A. L. Moore, T. A.
Moore, D. Gust, T. Drovetskaya, C. A. Reed, P. D. W. Boyd, J.
Phys. Chem. 1996, 100, 15926. d) O. Ito, Res. Chem. Intermed.
1997, 23, 389. e) D. M. Guldi, P. V. Kamat, in Fullerenes, Chem-
istry, Physics and Technology, ed. by K. M. Kadish, R. S. Ruoff,
Wiley-Interscience, New York, 2000. f) C. A. Reed, R. D.
Bolskar, Chem. Rev. 2000, 100, 1075.
1
ꢅC; H NMR (400 MHz, CDCl3): ꢄ 9.74 (s, 1H), 7.64 (d, J ¼ 4:8
Hz, 2H), 7.31 (m, 2H), 7.14 (d, J ¼ 4:8 Hz, 2H), 7.10 (m, 3H),
6.93 (d, J ¼ 4:8 Hz, 2H), 6.88 (d, J ¼ 4:4 Hz, 2H), 3.80 (s, 3H);
IR (KBr): ꢅ ¼ 1034, 1161, 1244, 1506, 1585, 1689, 2733, 2835
cmꢂ1; Anal. Calcd for C20H17NO2: C, 79.19; H, 5.65; N, 4.62%.
Found: C, 79.23; H, 5.64; N, 4.64%.
4
a) D. M. Guldi, Chem. Soc. Rev. 2002, 31, 22. b) T. D. M.
Bell, K. P. Ghiggino, K. A. Jolliffe, M. G. Ranasinghe, S. J.
Langford, M. J. Shephard, M. N. Paddon-Row, J. Phys. Chem.
A 2002, 106, 10079. c) Synthesis to Optoelectronic Properties,
N-4-Methoxyphenyl-N-[4-(1-methyl-3,4-fullero-2,3,4,5-tetra-
hydropyrrol-2-yl)phenyl]-N-phenylamine (2):
´
Compound 5
ed. by D. M. Guldi, N. Martın, Kluwer Academic Publishers,
(0.10 g, 0.33 mmol) and sarcosine (88 mg, 1.00 mmol) were added
to a solution of fullerene (0.26 g, 0.36 mmol) in toluene (110 mL).
The reaction mixture was refluxed for 18 h, and then filtered.
The solvent was evaporated from the filtrated, and the residue
chromatographed on silica gel with toluene/hexane (2:1) to give
compound 2 (0.10 g, 29.0%) in a black solid. Mp > 410 ꢅC
(dec.); 1H NMR (400 MHz, CDCl3): ꢄ 7.57 (br, 2H), 7.17 (m, 2H),
7.03–6.95 (m, 5H), 6.91 (d, J ¼ 4:8 Hz, 2H), 6.80 (d, J ¼ 4:4 Hz,
2H), 4.96 (d, J ¼ 5:6 Hz, 1H), 4.87 (s, 1H), 4.24 (d, J ¼ 5:6 Hz,
1H), 3.78 (s, 3H), 2.84 (s, 3H); 13C NMR (CDCl3): ꢄ 158.68,
156.24, 154.18, 153.86, 153.67, 148.34, 147.95, 147.41, 147.02,
146.57, 146.40, 146.32, 146.05, 145.67, 145.41, 144.76, 144.50,
143.10, 142.80, 142.68, 142.39, 142.27, 142.14, 141.71, 140.59,
140.27, 138.48, 136.82, 135.97, 130.10, 129.27, 127.42, 123.36,
122.49, 122.33, 114.91, 83.39, 55.71, 40.47, 30.11; IR (KBr):
Norwell, 2002. d) M. Fujitsuka, O. Ito, in Handbook of Photo-
chemistry and Photobiology, ed. by H. Nalwa, American Science
Publisher, California, 2003, Vol. 2. e) H. Imahori, Y. Mori, Y.
Matano, J. Photochem. Photobiol., C 2003, 4, 51.
5
a) W. Zhu, Y. Xu, Y. Zhang, J. Shen, H. Tian, Bull. Chem.
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1999, 32, 4849. d) T. D. M. Bell, A. Stefan, S. Masuo, T. Vosch,
M. Lor, M. Cotlet, J. Hofkens, S. Bernhardt, K. Mullen,
M. van der Auweraer, J. W. Verehoeven, F. C. De Schryver,
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6
a) R. M. Williams, J. M. Zwier, J. W. Verhoeven, J. Am.
Chem. Soc. 1995, 117, 4093. b) Y.-P. Sun, B. Ma, G. E. Lawson,
Chem. Phys. Lett. 1995, 233, 57. c) R. M. Williams, M. Koeberg,
J. M. Lawson, Y.-Z. An, Y. Rubin, M. N. Paddon-Row, J. W.
Verhoeven, J. Org. Chem. 1996, 61, 5055. d) K. G. Thomas, V.
Biju, M. V. George, D. M. Guldi, P. V. Kamat, J. Phys. Chem.
A 1998, 102, 5341. e) K. G. Thomas, V. Biju, D. M. Guldi,
P. V. Kamat, M. V. George, J. Phys. Chem. A 1999, 103,
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ꢅ ¼ 1038, 1178, 1242, 1504 cmꢂ1; UV–vis (toluene): ꢁmax
¼
311, 430 nm; MS (MALDI-TOF); m=z for C82H22N2O Calcd:
1051.09. Found 1051.14; Anal. Calcd: C, 93.70; H, 2.11; N,
2.67%. Found: C, 93.67; H, 2.14; N, 2.71%.
K.-Y. Kay acknowledges the Brain Korea 21 program of the
Ministry of Education for the financial support. M. El-Khouly
thanks JSPS fellowship.
7 a) S. Komamine, M. Fujitsuka, O. Ito, K. Moriwaki,
T. Miyata, T. Ohno, J. Phys. Chem. A 2000, 104, 11497.
Supporting Information
b) A. S. D. Sandanayaka, K. Matsukawa, T. Ishi-i, S. Mataka,
Y. Araki, O. Ito, J. Phys. Chem.
Transient absorption spectra of C60–TPA–C60 in Ar-saturated
DCB. This material is available free of charge on the Web at
http://www.csj.jp/jorunals/bsj/.
B 2004, 108, 19995.
c) A. S. D. Sandanayaka, H. Sasabe, Y. Araki, Y. Furusho,
O. Ito, T. Takata, J. Phys. Chem. A 2004, 108, 5145. d) H.-P.
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