Synthesis and photophysical properties of a pyrazolino[60]fullerene with dimethylaniline connected by an acetylene linkage

Article abstract of DOI:10.1002/ejoc.200500903

A new triad based on pyrazolino[60]fullerene and a conjugated dimethylaniline group has been synthesized by a copper-free Sonogashira cross-coupling reaction using microwave irradiation as the source of energy. The electrochemical and photophysical proper

Full text of DOI:10.1002/ejoc.200500903

FULL PAPER
DOI: 10.1002/ejoc.200500903
Synthesis and Photophysical Properties of a Pyrazolino[60]fullerene with
Dimethylaniline Connected by an Acetylene Linkage
[a]
[a]
[b,c]
Fernando Langa,*[a]
Andreas Gouloumis, Frédéric Oswald, Mohamed E. El-Khouly,
[b]
[b]
Yasuyuki Araki, and Osamu Ito*
Keywords: Fullerenes / Electron transfer / Microwave irradiation / Photophysical properties
A new triad based on pyrazolino[60]fullerene and a conju-
gated dimethylaniline group has been synthesized by a cop-
firmed in polar and nonpolar solvents and competes with
triplet formation of the C60 moiety. The charge-separated
per-free Sonogashira cross-coupling reaction using micro- state persisted for 91 ns. Such long lifetimes are characteris-
wave irradiation as the source of energy. The electrochemical
and photophysical properties of the triad were systematically
investigated by techniques such as time-resolved fluores-
cence and transient absorption spectroscopy. Charge separa-
tion via the excited singlet state of the C60 moiety was con-
tic of long distances between the radical anion of the pyrazol-
ino[60]fullerene derivative and the radical cation of the di-
methylaniline moiety.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
Introduction
charges are well-separated and this slows the back-electron-
transfer process and increases the lifetime of the CS state.
In recent years we have focused our attention on studying
the synthesis and properties of pyrazolino[60]fullerene de-
Fullerenes are characterized by remarkably strong elec-
tron-accepting properties, similar to those of quinones.
These characteristics, together with their electronic absorp-
tion properties, make them promising chromophores in
photodriven redox processes and, as a consequence, they
have potential applications in molecular and supramolec-
[
1]
[
8]
rivatives, which can be prepared by 1,3-dipolar cycload-
[
9]
[
2]
dition of nitrile imines to C60. Nitrile imines can be pre-
pared in situ from hydrazones, meaning that the overall re-
action can be performed in one pot with good yields.[
Photophysical studies of pyrazolino[60]fullerene derivatives
have revealed efficient electron transfer from the electron
10]
[
3]
ular electronics, light harvesting and photocatalysis. The
synthesis of C -based systems that incorporate an electron-
6
0
3
[11]
pair of the pyrazoline sp nitrogen atom to the C cage.
donor fragment continues to attract considerable interest in
an effort to obtain efficient intramolecular electron transfer
and to generate long-lived charge-separated states in these
60
In an attempt to enhance intramolecular electron trans-
3
fer by utilizing a reduction gradient (through the sp nitro-
donor–acceptor (D–A) molecular assemblies.[4] In this re-
spect, various donor–spacer–fullerene systems, in which the
gen atom of the pyrazoline ring) in order to increase the
separation between the donor and the acceptor we report
here a novel donor–C60 triad 1 that incorporates a pyrazol-
donor is an amino group, have been prepared[ and some
5]
ino[60]fullerene (C Pz) as the acceptor and a dimethylami-
of them show efficient charge separation.
60
nophenylethynylphenyl moiety connected to the N1 atom
of the pyrazoline ring as the donor (Scheme 1). Theoretical
calculations have been performed along with electrochemi-
cal and photochemical studies.
In an effort to increase the lifetime of the charge-sepa-
rated (CS) state in D–A arrays, two strategies have been
used: (i) The separation between the donor (D) and the ac-
ceptor (A) has been increased and (ii) a reduction gradient
[
6]
between D and A has been introduced by insertion of
additional electroactive moieties.[ In the final CS state, the
7]
Results and Discussion
Synthesis
[
[
[
a] Facultad de Ciencias del Medio Ambiente, Universidad de
Castilla-La Mancha,
Pyrazolino[60]fullerene 1 was synthesized by two meth-
ods (Scheme 1) starting from hydrazone 2, which was ob-
tained in 75% yield by reaction of the corresponding benz-
aldehyde and 4-iodophenylhydrazine according to a stan-
dard procedure. By route (a), a solution of hydrazone 2
in benzene was treated with NCS at 0 °C under argon. A
45071 Toledo, Spain
E-mail: Fernando.LPuente@uclm.es
b] Institute of Multidisciplinary Research for Advanced Materials,
Tohoku University,
Katahira, Sendai, 980-8577, Japan
E-mail: ito@tagen.tohoku.ac.jp
[8]
c] Department of Chemistry, Graduate School of Education, Kafr
El-Sheikh, Tanta University,
Tanta, Egypt
solution of C60 in toluene and Et N were then added and
3
2344
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Synthesis and Photophysical Properties of a Pyrazolino[60]fullerene
FULL PAPER
Scheme 1. Reagents and conditions: (i) AcOH, EtOH, 30 min, reflux; (ii) 4-(dimethylamino)phenylacetylene, CuI, PPh
(PPh PdCl ], reflux, 4 h; (iii) 1) NCS, pyridine, 20 min then 2) C60, toluene, Et N, microwaves, 50 min ; (iv) 4-(dimethylamino)phenyl-
acetylene, piperidine, [Pd(Ph ], chlorobenzene, microwaves, 40 min.
3
, piperidine,
[
3
)
2
2
3
3 4
)
the mixture was irradiated for 50 min (210 W power) in a showed the expected molecular ions at m/z = 1101 and
focused microwave reactor to afford 3 in 26% yield after 1084, respectively.
purification by column chromatography (silica gel, toluene).
All attempts to perform the classical Pd/Cu-mediated
cross-coupling (Sonogashira coupling)[ reaction between Molecular Orbital Calculations
12]
3
and 4-(dimethylamino)phenylacetylene using various sol-
The most stable geometry of the targeted cycloadduct 1
was obtained by theoretical calculations at the AM1 level
and the structure is shown in Figure 1. In a similar way to
vents and bases failed in our hands. The failure of this pro-
cedure led us to turn to a second methodology based on a
copper-free Sonogashira cross-coupling reaction.
[
13]
Fol-
[
16]
other pyrazolino[60]fullerene derivatives,
the C-substitu-
lowing this procedure, 3 and 4-(dimethylamino)phenylacet-
ylene were allowed to react in chlorobenzene under micro-
wave irradiation (300 W) in a single-mode apparatus (see
Expt. Sect.) for 40 min in the presence of an excess of piper-
idine and 10 mol-% [Pd(Ph ) ]. This reaction led to the for-
mation of 1 in moderate yield after flash column
chromatography. Note that only one example of the Pd/Cu
cross-coupling reaction with compounds containing fuller-
enes has been reported previously.[
ent (the p-nitrophenyl group) is almost coplanar with re-
spect to the pyrazoline ring (dihedral angle = 3.6°) and the
N-substituent {the 4-[4-(dimethylamino)phenylethynyl]-
phenyl moiety} is twisted by 44°, indicating a lack of conju-
gation between the two moieties. The distance between the
dimethylaniline (DMA) group and the edge of the C60
3
4
14]
In order to increase the yield of 1, route (b) was at-
tempted (Scheme 1), which is based on the Sonogashira
cross-coupling of hydrazone 2 and 4-(dimethylamino)phen-
ylacetylene and subsequent cycloaddition to C . Hydra-
6
0
zone 4 was prepared in 73% yield by the reaction of 2 with
-(dimethylamino)phenylacetylene under palladium-cata-
4
[
15]
lyzed conditions (see Scheme 1) for 4 h under reflux. The
synthesis of cycloadduct 1 from hydrazone 4 was achieved
by 1,3-dipolar cycloaddition of the intermediate nitrile im-
ine to C , as described above for the synthesis of 3. Subse-
6
0
quent purification by column chromatography (silica gel,
toluene) gave 1 in 30% yield.
The structures of all compounds were fully supported by
1
13
UV/Vis, FTIR and H and C NMR spectroscopy. The
structures of both fullerene derivatives 1 and 3 were also
confirmed by their MALDI-TOF mass spectra, which and LUMO (lower).
Figure 1. AM1-optimized structure of 1 (upper) and the HOMO
Eur. J. Org. Chem. 2006, 2344–2351
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A. Gouloumis, F. Oswald, M. E. El-Khouly, F. Langa, Y. Araki, O. Ito
FULL PAPER
(RD–A) is 13.32 Å (22.11 Å with respect to the center), a model compounds 3 and 4. In model compound 4 this po-
distance that should allow a charge-separated state with a tential is observed at E = 0.18 V, meaning that an anodic
p
long life. Finally, the HOMO (E = –8.97 eV) and the displacement of about 0.17 V for this process can be ob-
LUMO (E = –3.05 eV) are centered on the 4-[4-(dimeth- served in triad 1.
ylamino)phenylethynyl]phenyl moiety and the C60 cage,
In the cathodic scan, triad 1 exhibits five reduction waves
respectively (see Figure 1). Note that an appreciable (see Figure 2) which can be easily assigned by comparison
amount of the electron density of the HOMO is present on with scans of the model compounds: The first and second
the pyrazole moiety, which acts as an electron donor with waves are assigned to the C60 fullerene cage, the third to the
respect to the C60 cage.
p-nitrophenylpyrazoline moiety and the fourth process is
due to the overlap of the third C60 fullerene potential and
the second of the pyrazoline unit. The first three waves are
Electrochemical Measurements
–1
electrochemically reversible (∆E Ϸ 60 mV at 100 mVs )
p
and the fourth is quasi-reversible (∆E Ϸ 100 mV at
p
The electrochemical behavior of C , triad 1 and model
6
0
–1
100 mVs ).
compounds 3 and 4 in benzonitrile was studied by cyclic
voltammetry (CV) and Osteryoung square-wave voltamme-
try (OSWV). The results are summarized in Figure 2 and
Table 1.
The Ered and the first Eox values enabled the free energies
(
∆GCR) of the radical ion pair to be calculated from the
[
17]
Weller relation [Equation (1)],
where ∆G refers to the
S
static energy which is calculated using Equation (2) in ben-
[
18]
zonitrile and Equation (3) in toluene.
Table 1. Electrochemical data [V vs. Ag/AgNO
cesses of compounds 1, 3, 4 and C60 fullerene detected by OSWV
in benzonitrile solution (0.1  n-Bu NClO ) at room temperature
under identical experimental conditions.
3
] for the redox pro-
–
∆GCR = Eox – Ered – ∆G
S
(1)
(2)
4
4
[a]
1[b]
2[b]
3[c]
4
5[b]
1[c]
2
E
red
E
red
E
red
E
red
E
red
E
ox
∆G
S
= e /(4πε
0
ε
R
R
D–A
)
d]
d]
[e]
1
–0.88
–0.88
–
–1.31
–1.29
–
–1.56
–1.52
–1.50[
–
–1.83[
–2.35
–2.31
0.35
–
0.18
–
3
–1.79[
2
e]
[e]
∆G = e /4πε {[1/(2R ) + 1/(2R ) – 1/R
](1/ε ) –
4
C
–1.78
–1.80
S
o
+
–
D–A
s
[1/(2R
+
) + 1/(2R
–
)](1/ε
R
)} (3)
60
–0.88
–1.31
–2.28
[
a] OSWVs were obtained using a sweep width of 25 mV, a fre-
The terms e, ε and εR refer to elementary charge, vac-
quency of 15 Hz, a step potential of 4 mV and a quiet time of
2
0
®
s on a Windows -driven Autolab PGSTAT 30 electrochemical uum permittivity and the static dielectric constant of the
analyzer. [b] C60-based reduction. [c] p-Nitrobenzene-based redox
process. [d] The third C60-based reduction wave was overlapped by
the second p-nitrobenzene-based reduction wave. [e] Hydrazone-
based reduction or oxidation.
solvent used for rate and redox-potential measurements,
respectively. The ∆GCR and excited energy (E ) values al-
00
lowed the free-energy changes for the charge-separation
process (∆GCS) to be calculated from Equation (4) (see
Table 2).
The CV shows that triad 1 is electrochemically active in
both anodic and cathodic sweep directions between +1.8 –∆GCS = E00 – (–∆GCR
)
(4)
and –2.5 V. In the anodic scan, the redox process observed
is chemically irreversible under the experimental conditions
The ∆GCS values for 1 show that the charge-separation
1
and is assigned to the dimethylaniline moiety (E = 0.35 V process via the excited singlet state of C60 ( C *) is exother-
p
60
–
1
at 100 mVs ) by direct comparison with the processes for mic in both toluene and benzonitrile.
–1
Figure 2. Cyclic voltammograms (sweep rate = 0.1 Vs ) for 1, 3, 4 and C60 in benzonitrile containing n-Bu
4
NClO
4
at room temperature.
2
346 Eur. J. Org. Chem. 2006, 2344–2351
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Synthesis and Photophysical Properties of a Pyrazolino[60]fullerene
FULL PAPER
1
f
Table 2. Free-energy changes (∆GCS and ∆GCR), fluorescence lifetimes (τ ) of C60*, rate constants (kCS) and quantum yield (ΦCS) of
charge separation, rate constants of charge recombination (kCR) and radical-ion-pair lifetime of 1 in benzonitrile (BN) and toluene.
9
–1
7 –1
Solvent
BN
Toluene –0.15
∆GCS [eV]
τ
–[b]
130 (70%)
1
f
[ps]
k
CS [10 s ]
Φ
–[a]
0.90
(0.63)
CS
∆GCR [eV]
k
CR [10 s ]
τRIP [ns]
[
a]
[a]
–0.52
–
–1.23
1.34
1.10
75
91
[
c]
[d]
[d]
[c]
6.9
–1.60
(5.0)[
e]
[e]
400 (30%)
[
a] From Equations (1), (2) and (4). In Equation (2), RD–A = 13 Å. [b] The fluorescence decay of C60 was overlapped by a broad fluores-
cence in the visible region. [c] From Equations (1), (3) and (4). In Equation (3), R = 6.0 Å and R = 4.2 Å. [d] From Equations (5) and
6) employing the shorter fluorescence lifetime as τ for 1; τf0 = 1.3 ns for 3. [e] On including the longer fluorescence lifetime as τ
+
–
(
f
f
.
Steady-State Absorption Spectra
could be due to the emission of the donor moiety, although
with a considerable red-shift from the fluorescence peak of
The absorption spectra of 1, 3 and 4 were measured in
benzonitrile and toluene in the range of 300–800 nm. Fig-
ure 3 (upper panel) shows the absorption spectrum of 3
which is characterized by an absorption band at 324 nm
with a shoulder at 414 nm. The absorption spectrum of 4 is
characterized by absorption peaks at 344 and 448 nm. The
absorption spectra of 1 (Figure 3 lower panel) are a super-
imposition of the components, indicating that there is no
significant ground-state electronic interaction. The strong
absorption of the C60 moiety appears mainly in the UV
region.
4
. Considerable quenching of the 450 nm fluorescence peak
intensity was also observed, suggesting communication be-
tween the donor and C60 moieties.
Figure 4. Steady-state fluorescence spectra of 1, 3, and 4 in toluene
(
concentrations are kept at 0.02 m; λex = 400 nm).
For 1 in benzonitrile, a broad fluorescence band was ob-
served in addition to the 700 nm fluorescence peak of the
C60 moiety.
Fluorescence Lifetime Measurements
The fluorescence time profiles of 1 in toluene are shown
in Figure 5; the 700 nm fluorescence peak is observed to
rise and the 450 nm fluorescence peak to decay, suggesting
energy transfer from the excited singlet state of the donor
moiety to the C60 moiety. From the initial decay rate of the
4
50 nm peak and the rise of the 700 nm peak, the rate of
10 –1
energy transfer can be evaluated to be 1.2×10 s .
Figure 3. Steady-state absorption spectra of: 3 and 4 in toluene
(
upper panel) and triad 1 in toluene and benzonitrile (lower panel).
The concentrations were kept at 0.02 m.
Fluorescence Spectral Studies
The fluorescence spectrum of 1 in toluene upon applying
4
00 nm light as the excitation wavelength is shown in Fig-
ure 4. Two fluorescence peaks were observed at 450 and
00 nm; the latter peak can be attributed to the C60 moiety
Figure 5. Fluorescence time profiles of 1 in toluene: (a) 450–600 nm
and (b) 670–760 nm. (λex = 400 nm; the shadow is the laser pulse
profile).
7
by comparison with the fluorescence of compound 3. The
intensity of the emission band of 1 monitored at 715 nm
The fluorescence time profile of C60 exhibited a single
was significantly quenched compared to that of the C60 ref- exponential decay with a lifetime (τ ) of 1.3 ns. The fluores-
f0
erence compound 3. It was quite difficult to record the fluo- cence decay time profile of 1 at 700 nm in toluene can be
1
rescence of C * in benzonitrile due to the weak emission fitted with two exponential components: 130 ps (70%) and
6
0
at around 700 nm. These observations suggest efficient 1400 ps (30%). The shortening of the lifetime indicates that
1
quenching of the singlet excited state of the C60 ( C *) attaching donor moieties to C60 introduces a new quench-
60
moiety by the appended donor entities. The 450 nm peak ing pathway for C . As a quenching process, it is most
60
Eur. J. Org. Chem. 2006, 2344–2351
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A. Gouloumis, F. Oswald, M. E. El-Khouly, F. Langa, Y. Araki, O. Ito
FULL PAPER
probable that charge separation from the donor moieties to zonitrile (Figure 7, upper panel) and toluene (Figure 7,
the C60 moiety occurs, yielding a charge-separated (CS) lower panel), suggesting that similar photoinduced pro-
state. The rate constants for charge separation (kCS) were cesses occur in polar and nonpolar solvents. Indeed, from
calculated from Equation (5),[ where τf0 and τ are the the calculated ∆GCS values, charge separation is possible in
19]
f
lifetimes of the reference and 1, respectively. The quantum both benzonitrile and toluene.
1
yields (ΦCS) for the generation of the CS state via C *
60
were calculated from Equation (6).[
19]
k
CS = (1/τ
f
) – (1/τf0
)
(5)
(6)
Φ
CS = kCS/(1/τ )
f
The charge-separation rate constant (kCS) and quantum
9
–1
yield (ΦCS) were evaluated to be 6.9×10 s and 0.90,
respectively, in toluene from the shorter fluorescence life-
time, as listed in Table 2. By including the shorter fluores-
cence lifetime, they are reduced to 70%. In benzonitrile, the
rate constant for the charge separation was not evaluated
due to overlapping of the fluorescence band in the visible
region.
Nanosecond Transient Spectra
The nanosecond transient absorption spectra of 1 and 3
were recorded in argon-saturated toluene and benzonitrile
by applying 355 nm laser light excitation, which excites
mainly the C60 moiety. In argon-saturated toluene, the tran-
sient spectra of 3 (Figure 6) exhibited mainly the character-
3
istic band of the triplet state of the C60 moiety ( C *) at
60
5
–1 [20]
700 nm which has a decay rate of 4.0×10 s .
The ab-
sence of radical ion pairs suggests no charge separation pro-
cess. Similar observations were made in benzonitrile.
Figure 7. Transient absorption spectra obtained by 355 nm laser
photolysis of 1 (0.1 m) in argon-saturated benzonitrile (upper
panel) and toluene (lower panel).
The broad band of the radical cation of the donor moiety
was confirmed by measuring the nanosecond transient
spectra of a mixture of C60 and 4 in benzonitrile by 355 nm
laser light excitation (Figure 8). The transient absorption
spectra exhibited growth of the absorption bands of C60·
at 1080 nm accompanied by a concurrent decay of the ab-
–
3
sorption band of C * at 760 nm. Furthermore, a slow rise
60
in absorption was clearly observed in the range 1100–
600 nm, which was attributed to the radical cation of 4
1
·
+
·+
(4 ). The assignment of the broad absorption band to 4
may be reasonable because the radical cation is widely delo-
calized from the dimethylaniline to the pyrazole ring via the
phenylacetylene bridge, as shown in the representation of
Figure 6. Transient absorption spectra obtained by 355 nm laser
photolysis of 3 (0.1 m) in argon-saturated toluene.
the HOMO of 1 in Figure 1. The rate constant for the bi-
3
On the other hand, the transient absorption spectra of 1 molecular quenching of C * (k ) was evaluated to be
60
q
9
–1 –1
(
Figure 7) show a broad transient absorption band in the 4.1×10  s . The efficiency of the intermolecular elec-
3
near-IR region with a rapid decay in addition to the C * tron transfer (Φ ) was evaluated to be 0.60 from the ratio
60
et
·
–
moiety at 700 nm with a slow decay. Although the charac- of the maximal concentration of the generated C60 to the
·
–
3
teristic peak of the radical anion of the C60 moiety (C60
)
initial concentration of C *, with the maximal absorbance
60
·
–
is expected to appear at 1000 nm, a broad band, which of C60 being evaluated after subtracting the overlapping
·
+
·+
spreads widely across the range of 1000–1600 nm, may hide absorbance of 4 . The molar extinction coefficient of 4
·
–
–1
–1
the C60 peak at 1000 nm. We tentatively attributed this was evaluated to be 18000  cm at 1250 nm employing
broad band to the radical cation of the donor moiety. Quite the molar extinction coefficient of C
·
–
–1
–1
(14000  cm at
60
[
20]
similar transient absorption spectra were observed in ben- 1080 nm).
2348
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Synthesis and Photophysical Properties of a Pyrazolino[60]fullerene
FULL PAPER
electric constants determined from electrochemistry and
photophysical measurements are reliable, although the en-
·
–
·+
ergy level of R~C60 ~CϵC–DMA in toluene may be as-
sociated with considerable error. Our observations, how-
ever, revealed that the charge-separation process via the
1
C * moiety is possible in polar and nonpolar solvents,
60
3
whereas charge separation via the C * moiety could be
60
impossible. Since the 355 nm light employed in the transient
absorption measurements directly excites the C60 moiety,
1
charge separation takes place via the C *moiety. In tolu-
60
ene, even if the donor moiety of 1 was excited, the excited
energy transfers to the C60 moiety, from which the charge
separation takes place. From the transient absorption spec-
tra of 1 in Figure 7, the relative concentration of the radical
Figure 8. Transient absorption spectra obtained by 355 nm laser
photolysis of C60 (0.1 m) in the presence of 4 (1 m) in argon-
saturated benzonitrile.
3
cation of the donor moiety to the C * moiety can be
60
From the time profiles measured at 1400 nm, the rate
evaluated to be 0.1, which implies that most of the radical
ion pair disappeared immediately after the nanosecond la-
ser pulse (6 ns) irradiation because the quantum yield of the
charge separation was larger than 0.6.
constants for the charge-recombination process (kCR) of 1
7
were evaluated to be 1.34×10 (benzonitrile) and
1
.10×107 s–1 (toluene). From kCR, the lifetimes of the radi-
cal ion pairs (τRIP) were evaluated to be around 75 (benzo-
nitrile) and 91 ns (toluene), which are quite long compared
with the lifetimes of the charge-separated state reported for
the dyads of C with dimethylaniline connected to a pyr-
6
0
[
5c,5d]
rolidine ring.
The slightly shorter τRIP value in benzo-
Conclusion
nitrile compared with toluene suggests that the charge-
recombination process belongs to the inverted region of the
A pyrazolino[60]fullerene with a dimethylaniline con-
nected by an acetylene linkage has been synthesized for the
first time and its electrochemical and photophysical proper-
ties have been systematically investigated by techniques in-
cluding time-resolved fluorescence and transient absorption
[
21]
Marcus parabola since the reorganization energy may be
less than the absolute value of the free-energy change of the
charge-recombination process (∆GCR) even in benzo-
[
22]
nitrile.
1
spectroscopy. The C * decays by intersystem crossing to
60
3
populate the C * state and by charge separation to yield
60
Energy Diagram
the radical ion pair in polar and nonpolar solvents. The
The energy diagram for the photophysical events after former process seems to be dominant compared with the
photoexcitation is based on the lowest excited-state data latter. The charge-separated state persisted for 75–91 ns de-
and calculated energy levels of R~C60· ~CϵC–DMA (Fig- pending on the solvent. The role of the rigid phenylacety-
ure 9). The excited singlet state of the C60 moiety was evalu- lene bridge may be to slow down the charge-recombination
ated from the fluorescence peak, whereas the excited triplet process. The pyrazolino[60]fullerene, which is connected to
–
·+
[
1,2,19c]
state was cited from the literature.
The energy levels dimethylaniline through an acetylene linkage, also plays a
·
–
·+
of R~C60 ~CϵC–DMA in polar solvents with similar di- role giving rise to a radical ion pair with a longer lifetime.
Figure 9. Energy diagram.
Eur. J. Org. Chem. 2006, 2344–2351
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A. Gouloumis, F. Oswald, M. E. El-Khouly, F. Langa, Y. Araki, O. Ito
FULL PAPER
4
-Nitrobenzaldehyde 4-[4-(Dimethylamino)phenylethynyl]phenylhyd-
Experimental Section
razone (4): [(PPh PdCl ] (1 mg, 1.5 µmol) in piperidine (0.5 mL)
3
)
2
2
Molecular Orbital Calculations: The computational calculations
was added to a stirred solution of 2 (184 mg, 0.5 mmol), 4-(dimeth-
ylamino)phenylacetylene (73 mg, 0.5 mmol), copper iodide (1 mg,
5.5 µmol) and triphenylphosphane (2.6 mg, 10 µmol) in freshly dis-
tilled piperidine (5 mL) under argon. The mixture was refluxed for
4 h. The reaction mixture was cooled and the solvent evaporated
under vacuum. The crude product was purified by flash column
chromatography (silica gel, hexane/dichloromethane, 1:2) to give 4
as a dark-red solid (140 mg, 73%); m.p. 169–171 °C. FTIR (KBr):
were performed by ab initio B3LYP/3-21G(*) methods using the
[
23]
GAUSSIAN03 software package on high speed computers. The
images of the frontier orbitals were generated from Gauss View-03
software.
Instruments: The UV/Vis spectral measurements were carried out
with a Jasco model V570 DS spectrophotometer. Steady-state fluo-
rescence spectra were measured with a Shimadzu RF-5300 PC
spectrofluorophotometer equipped with a photomultiplier tube
having a high sensitivity in the 700–800 nm region. MALDI-TOF
mass spectra were obtained with a VGAutoSpec spectrometer using
ditranol as the matrix. Cyclic voltammetry measurements were ob-
tained using a Versastat PAR EG & G potentiostat with analytical
electrochemical software (Mod. 250). The measurements were
taken in a double-walled cell (Metrohm EA 876–20). A glassy-car-
bon working electrode (Metrohm 6.0804.010) was used after being
polished with alumina (0.3 µ) for 1 min and platinum wire was used
–
1
ν˜ = 3313, 2932, 2853, 2186, 1610, 1523, 1335, 1247, 1106, 816 cm .
1
H NMR (200 MHz, CDCl
s, 1 H), 7.78 (d, J = 8.4 Hz, 2 H), 7.71 (s, 1 H), 7.46 (d, J = 8.4 Hz,
H), 7.40 (d, J = 8.4 Hz, 2 H), 7.15 (d, J = 8.4 Hz, 2 H), 6.67 (d,
3
): δ = 8.23 (d, J = 8.4 Hz, 2 H), 8.03
(
2
1
3
J = 8.4 Hz, 2 H), 3.02 (s, 6 H) ppm. C NMR (50 MHz, CDCl
δ = 149.87, 148.29, 146.84, 143.85, 142.06, 140.06, 136.18, 132.53,
27.76, 127.15, 123.93, 123.13, 114.51, 112.78, 111.86, 40.24 ppm.
3
):
1
General Procedure for the Cycloaddition Reactions: Pyridine (15 µL)
was added to a solution of the appropriate hydrazone (0.14 mmol)
in dry benzene (15 mL) under argon and the mixture was cooled
to 0 °C. NCS (56 mg, 0.42 mmol) was added and the mixture was
stirred for 10 min at this temperature and then warmed to room
temperature (20 min).
0.07 mmol) in dry toluene (50 mL) and triethylamine (1 mL) were
added to the mixture and single-mode microwave irradiation
3
as the counter electrode. An Ag/AgNO electrode, used as a refer-
ence, was separated from the solution using a solution of tetrabu-
tylammonium perchlorate, which was used as the supporting elec-
trolyte, in benzonitrile. The samples were purged with argon prior
to measurement.
A solution of [60]fullerene (50 mg,
The redox values were measured by differential pulse voltammetry
(
210 W) was applied for 50 min.
Ј-(4-Iodophenyl)-3Ј-(4-nitrophenyl)pyrazolino[4Ј,5Ј:1,2][60]fullerene
3): Fullerene 3 was prepared following the general procedure de-
(DPV) using a BAS CV-50W Voltammetric Analyzer (Japan). A
1
(
platinum disk electrode was used as the working electrode, while a
platinum wire served as a counterelectrode. An Ag/AgCl electrode
was used as a reference. All measurements were carried out in dif-
ferent solvents containing 0.1  tetrabutylammonium perchlorate
scribed above. After flash column chromatography (silica gel, tolu-
ene) a yield of 26% (20 mg) was obtained. FTIR (KBr): ν˜ = 2923,
–
1 1
1
590, 1515, 1334, 1258, 1105, 945, 814 cm . H NMR (200 MHz,
CDCl ): δ = 8.56 (d, J = 8.4 Hz, 2 H), 8.36 (d, J = 8.4 Hz, 2 H),
7.78 (d, J = 2.8 Hz, 4 H) ppm. C NMR (50 MHz, CDCl
47.61, 147.4, 147.16, 146.33, 146.24, 145.98, 145.88, 145.56,
[
0
(n-Bu)
4
NClO
4
] as the supporting electrolyte. A scan rate of
–1
3
.1 Vs was used. Microwave irradiation was performed in a Dis-
1
3
®
3
): δ =
cover CEM instrument.
1
The picosecond time-resolved fluorescence spectra were measured
by a single-photon counting method using the second harmonic
generation (SHG, 400 nm) of a Ti:sapphire laser (Spectra-Physica,
Tsunami 3950-L2S, 1.5 ps fwhm) and a streakscope (Hamamatsu
Photonics) equipped with a polychromator (Action Research, Spec-
traPro 150) as an excitation source and detector, respectively. Life-
times were evaluated using software attached to the equipment.
145.43, 145.23, 145.13, 145.08, 144.60, 144.15, 143.81, 143.52,
143.13, 142.89, 142.84, 142.33, 142.09, 142.00, 141.82, 141.63,
140.32, 139.77, 138.52, 138.31, 136.51, 136.35, 128.92, 125.22,
123.99, 89.86, 80.72 ppm. UV/Vis (ODCB): λmax (logε) = 429
(4.13), 330 (4.91), 298 (4.80) nm. MS (MALDI-TOF): m/z = 1084–
+
1087 [M] .
1Ј-{4-[4-(Dimethylamino)phenylethynyl]phenyl}-3Ј-(4-nitrophenyl)py-
The nanosecond transient absorption spectra in the near-IR region
were measured by laser flash photolysis; 355 nm light from a
Nd:YAG laser (Spectra-Physics and Quanta-Ray GCR-130, 6 ns
fwhm) was used as the excitation source. For transient absorption
spectra in the visible (400–600 nm) and near-IR regions (600–
razolino[4Ј,5Ј:1,2][60]fullerene (1): Compound 1 was prepared in
two different ways.
Route (a): Copper-free Sonogashira cross-coupling. Fullerene 3
(
3 4
75 mg, 0.07 mmol), [Pd(Ph ) ] (10% mol) and a large excess of
piperidine (5 mL) were added to a solution of 4-(dimethylamino)-
phenylacetylene (10 mg, 0.07 mmol) in chlorobenzene (30 mL). The
mixture was heated by single-mode microwave irradiation (300 W)
in a sealed tube for 40 min. After solvent removal, the crude prod-
uct was purified by flash column chromatography (silica gel, tolu-
ene) to yield 20% (15 mg) of fullerene 1.
1600 nm), monitoring lights from a pulsed Xe lamp were detected
with Si- and Ge-avalanche photodiode modules, respectively. The
samples were held in a quartz cell (1×1 cm) and were deaerated by
bubbling argon gas through the solution for 15 min.
4
-Nitrobenzaldehyde 4-Iodophenylhydrazone (2): A solution of 4-ni-
trobenzaldehyde (0.453 g, 3.0 mmol), 4-iodophenylhydrazine
1.05 g, 4.5 mmol) and two drops of acetic acid in ethanol (15 mL)
Route (b): Triad 1 was prepared following the general procedure
described above and after flash column chromatography (silica gel,
toluene) a yield of 30% (23 mg) was obtained. FTIR (KBr): ν˜ =
(
was refluxed for 30 min. The solvent was evaporated in vacuo and
the residual solid was purified by column chromatography (silica
gel, hexane/ethyl acetate, 4:1) to give 2 as a red solid (0.825 g, 75%);
–
1 1
2921, 1607, 1519, 1426, 1334, 1259, 1105, 943, 813, 753 cm . H
NMR (200 MHz, CDCl ): δ = 8.58 (d, J = 8.4 Hz, 2 H), 8.35 (d,
3
m.p. 154–155 °C. FTIR (KBr): ν˜ = 3283, 2926, 1591, 1561, 1514, J = 8.4 Hz, 2 H), 7.93 (d, J = 8.4 Hz, 2 H), 7.59 (d, J = 8.4 Hz, 2
–
1 1
1
479, 1334, 1262, 1157, 1104 cm . H NMR (200 MHz, CDCl
3
):
H), 7.37 (d, J = 8.4 Hz, 2 H), 6.65 (d, J = 8.4 Hz, 2 H), 3.02 (s, 6
H) ppm. C NMR (50 MHz, CDCl ): δ = 147.31, 146.16, 146.08,
3
145.75, 145.44, 145.10, 145.02, 144.66, 144.02, 143.88, 142.98,
142.73, 142.18, 141.95, 141.67, 141.08, 140.64, 140.11, 139.61,
138.44, 137.27, 136.40, 136.15, 132.53, 132.05, 128.62, 123.74,
1
3
δ = 8.22 (d, J = 8.4 Hz, 2 H), 7.96 (s, 1 H), 7.76 (d, J = 8.4 Hz, 2
H), 7.69 (s, 1 H), 7.57 (d, J = 8.4 Hz, 2 H), 6.93 (d, J = 8.4 Hz, 2
H) ppm. 13C NMR (50 MHz, CDCl
): δ = 147.23, 143.32, 141.31,
38.13, 134.68, 126.39, 124.11, 115.18, 82.86 ppm.
3
1
2350
www.eurjoc.org
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2006, 2344–2351

Synthesis and Photophysical Properties of a Pyrazolino[60]fullerene
22.99, 121.31, 111.64, 110.02, 91.95, 87.26, 40.11 ppm. UV/Vis
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Educación y Ciencia of Spain and FEDER funds (Project
CTQ2004-00364/BQU) and the Junta de Comunidades de Castilla-
La Mancha (Project PAI-05-068). This work was also supported
by a Grant-in-Aid for Scientific Research on Priority Area (417)
from the Ministry of Education, Culture, Sports, Science, and Tech-
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Published Online: March 17, 2006
Eur. J. Org. Chem. 2006, 2344–2351
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2351