Synthesis and photophysical properties of ruthenocene-[60]fullerene dyads

Article abstract of DOI:10.1039/b509221c

Two novel ruthenocene-C₆₀ dyads, with a 2-pyrazoline ring or a pyrrolidine ring as a linker, have been synthesized with the aim of providing a simple model of natural photosynthesis. The photophysical properties of the two ruthenocene-C₆₀ dyads have been investigated by steady-state absorption and fluorescence, time-resolved fluorescence and nanosecond transient measurements in polar and non-polar solvents. The charge separation takes place in the ruthenocene-pyrazolino[60]fullerene more efficiently than in the ruthenocene-pyrrolidino[60]fullerene dyad. The lifetimes of the charge-separated states of the ruthenocene-pyrazolino[60]fullerene and the ruthenocene- pyrrolidino[60]fullerene dyads are 100 ns in PhCN. It was found that the ruthenocene-[60]fullerenes have an ability to prolong the charge-separated states compared with those for ferrocene-[60]fullerenes. the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006.

Full text of DOI:10.1039/b509221c

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Synthesis and photophysical properties of ruthenocene-[60]fullerene
dyadsw
a
cd
a
a
´
´
Juan Jose Oviedo, Mohamed E. El-Khouly, Pilar de la Cruz, Laura Perez,
Javier Garı
n,b Jesus Orduna,b Yasuyuki Araki,c Fernando Langa*a and
Osamu Ito*c
´
´
Received (in Montpellier, France) 6th July 2005, Accepted 24th October 2005
First published as an Advance Article on the web 11th November 2005
DOI: 10.1039/b509221c
Two novel ruthenocene-C60 dyads, with a 2-pyrazoline ring or a pyrrolidine ring as a linker, have
been synthesized with the aim of providing a simple model of natural photosynthesis. The
photophysical properties of the two ruthenocene-C60 dyads have been investigated by steady-state
absorption and fluorescence, time-resolved fluorescence and nanosecond transient measurements
in polar and non-polar solvents. The charge separation takes place in the ruthenocene-
pyrazolino[60]fullerene more efficiently than in the ruthenocene-pyrrolidino[60]fullerene dyad. The
lifetimes of the charge-separated states of the ruthenocene-pyrazolino[60]fullerene and the
ruthenocene-pyrrolidino[60]fullerene dyads are 100 ns in PhCN. It was found that the
ruthenocene-[60]fullerenes have an ability to prolong the charge-separated states compared with
those for ferrocene-[60]fullerenes.
scribed the synthesis and preliminary studies on ruthenocene-
Introduction
C60 dyads.9 The first ionization potentials (IPs) for ferrocene
and ruthenocene are 6.86 and 7.45 eV, respectively.10,11 Thus,
ferrocene is a stronger donor in an electron-transfer sense. In
many electron-donor (D) and electron-acceptor (A) systems,
however, there is a little evidence to distinguish between the
donor strengths of ferrocene and ruthenocene in systems with
strong electron-acceptors. Indeed, there is electrochemical
evidence that ruthenocene is the stronger donor, despite the
fact that it has a lower energy HOMO than ferrocene.12 The
superior donating properties of ruthenocene in some D–A
dyads can be attributed to the more extensive d-orbitals of
ruthenium,13 which lead to increase direct or indirect spatial
overlap with the acceptor.14 It seems that this difference in
donor strength between ferrocene and ruthenocene is most
evident when Z6-fulvene is a significant resonance form. These
findings stimulated us to investigate in detail the electroche-
mical and optical properties of ruthenocene-fullerene dyads.
In the work described here, two new ruthenocene-C60
(RcBC60) dyads 3 and 4 were synthesized with different
linking bridges to the fullerene cage. In case of dyad 3, a
pyrazoline ring was used whereas a pyrrolidine ring was
incorporated in dyad 4. The photophysical behavior of both
dyads was investigated using different photochemical techni-
ques in polar and non-polar solvents.
The importance and complexity of natural photosynthesis1
make the study of suitable simpler models of great interest. An
important approach to build these models is to link covalently
an electron acceptor and an electron donor. The special
electrochemical2 and photophysical3 properties of fullerenes,
which include absorption in the visible region and a low
reorganization energy that can accelerate the photoinduced
charge separation and decelerate charge recombination,4 have
placed C60 and its derivatives amongst the most important
organic materials for photovoltaic applications, particularly
plastic solar cells.5 Numerous examples of molecular systems
covalently linked between electron donors and C60 have been
synthesized to date in the search for efficient photoinduced
charge separation and slow charge recombination.6 A variety
of donors have been attached to fullerenes7 and, among them,
ferrocene has been exploited intensively7e,8 due to its reversible
low oxidation potential.
Nevertheless, combinations of other metallocenes, such as
ruthenocene, with C60 remain unexplored. Recently, we de-
a Facultad de Ciencias del Medio Ambiente, Universidad de Castilla-
La Mancha, 45071 Toledo, Spain. E-mail:
Fernando.LPuente@uclm.es; Fax: +34 902 204 130;
Tel: 34 925 268 843
Departamento de Quımica Organica, ICMA, Universidad de
b
´
Zaragoza-CSIC, E-50009 Zaragoza, Spain. E-mail:
´
jorduna@unizar.es; Fax: +34 976761 194; Tel: 34 976 761 194
c Institute of Multidisciplinary Research for Advanced Materials,
Tohoku University, Katahira, Aoba-ku, Sendai, 980-8577, Japan.
E-mail: ito@tagen.tohoku.ac.jp; Fax: +81-22-217-5608
d Department of Chemistry, Faculty of Education, Kafr El-Sheikh,
Tanta University, Egypt. E-mail: elkhouly@tagen.tohoku.ac.jp
w Electronic supplementary information (ESI) available: copies of H
NMR, 13C NMR and MS spectra for compounds 2, 3 and 4 and
transient absorption spectra of 5 in toluene and benzonitrile. See DOI:
10.1039/b509221c
Results and discussion
Syntheses and characterization
The synthetic approach to prepare the 2-pyrazolino[60]fuller-
ene derivative 3 is based on the 1,3-dipolar cycloaddition
reaction of the nitrile imine generated in situ from the
corresponding hydrazone 2. This hydrazone was prepared
in 90% yield from ruthenocenecarboxaldehyde (1)15 and
1
ꢀc
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Scheme 1 (i) 4-Nitrophenylhydrazine and (ii) NBS, C60 and Et3N.
Chart 1 Reference compounds 5 and 6.
lyses for 3 and 4 revealed the molecular ion peaks to be in
agreement with the calculated ones. Finally, in the UV-Vis
spectra, measured in CH2Cl2, the characteristic absorption of
C60 monoadducts was observed at around 430 nm.
4-nitrophenylhydrazine in EtOH under reflux for 20 minutes
(Scheme 1). It was necessary to modify our previously de-
scribed general procedure to obtain compound 3 in a good
yield.16,17 The reaction of 2 with NBS in CHCl3 at room
temperature was followed by the addition of C60 and Et3N in
dry toluene at 40 1C. The mixture was stirred for 45 minutes to
give compound 3 in 16% isolated yield after flash column
chromatography (silica gel, toluene) followed by centrifuga-
tion in hexane, methanol and diethyl ether.
Optimized structures and frontier MO’s.
Optimized geometries (B3PW91/LanL2DZ) for 3 and 4 are
shown in Fig. 1. The most important structural difference
arises from the fact that conjugation of the para-nitrophenyl
group with the ruthenocene moiety across the pyrazoline ring
in compound 3 gives rise to an almost planar arrangement,
with the pyrazoline ring forming an angle of 81 with the
cyclopentadiene ring linked to it.
On the other hand, pyrrolidino[60]fullerene system 4 was
prepared in 26% yield by a 1,3-dipolar cycloaddition reac-
tion18 between the ruthenocenecarboxaldehyde (1), a-amino-
phenyl- acetic acid and C60 in chlorobenzene under microwave
irradiation19 at 210 W for 45 minutes in a focused microwave
oven (Scheme 2). 10-(4-Nitrophenyl)-30-(ferrocenyl)pyrazolino
[60]fullerene (5) (Chart 1) was prepared as previously de-
scribed.23
On the other hand, the lack of conjugation between the
ruthenocene group and the pyrrolidine ring in compound 4
allows free rotation around the bond linking these units and
the ruthenocene group is arranged away from the C60 spher-
oid. The distances (RCC) from the Rc center to the center of
the C60 moiety were estimated to be 7.55 and 8.40 A, respec-
tively, for compounds 3 and 4.
The structures of dyads 3 and 4 were confirmed by spectro-
scopic analysis (NMR, FT-IR and MS). The 1H NMR spectra
of C60 derivatives exhibited all expected signals for the organic
addends. In the case of compound 3, the 4-nitrophenyl group
signals were observed between 8.00 and 8.50 ppm and the
ruthenocene protons appeared as a singlet around 4.50 ppm
for the unsubstituted cyclopentadienyl ring and as two multi-
plets, between 4.50 and 5.50 ppm, for the C5H4 moiety. Similar
characteristic signals for the ruthenocene moiety were also
The HOMO and LUMO of compounds 3 and 4 are depicted
in Fig. 2. It can be seen that the LUMO is localized on the C60
moiety whereas the main contribution to the HOMO is from
ruthenium d orbitals in 4 and the HOMO in dyad 3 spreads
from ruthenocene to the para-nitrophenyl group. These orbital
topologies indicate that the charge-separated state has a
1
observed in the H NMR spectrum of compound 4, together
structure like Rcꢁ+BC60 ꢂ, but the positive charge in 3 is
ꢁ
with signals for the pyrrolidine ring, which appear as singlets
at 5.91 and 5.58 ppm, and the signals for the protons of the
phenyl group between 7.26 and 8.00 ppm. The 13C NMR
spectra are also consistent with the proposed structures, with
the signals from the ruthenocene fragment between 70.0 and
75.0 ppm and those due to the corresponding sp2 carbons of
the sphere between 160.0 and 134.0 ppm. In addition, the two
sp3 carbon signals for the functionalized C60 cage were ob-
served at around 89.0 and 76.0 ppm. The MALDI-MS ana-
also partially supported by the pyrazoline ring and the para-
nitrophenyl group attached to it. It should also be noted that
there is a significant difference in the calculated HOMO–
LUMO gaps, which are 1.86 eV and 2.30 eV for 3 and 4,
respectively.
Fig. 1 Optimized geometries (B3PW91/LanL2DZ) of compounds 3
(left) and 4 (right).
Scheme 2
ꢀc
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Table 1 Reduction potentials (V)a determined by CV for compounds
1–4, C60 and ruthenocene, measured in ortho-dichlorobenzene/aceto-
nitrile (4 : 1)
b
b
b
E11/2(red) E21/2(red) E13/2(red)
E41/2(red) Eo1x Eo2x
HOMO–
LUMO
1
2
3
4
0.68 1.17
0.61
0.58 1.42 1.59
ꢂ1.78c
ꢂ1.01
ꢂ1.11
ꢂ1.41
ꢂ1.52
ꢂ1.40
ꢂ1.82c
ꢂ1.96
ꢂ2.12
ꢂ1.92
0.71
1.82
C60 ꢂ0.99
Rc
0.73
a
V vs. Ag/AgNO3; GCE as working electrode; 0.1 M (n-C4H9)4
b
NClO4; scan rate was 100 mV sꢂ1
para-nitrophenyl group.
.
c
Irreversible. Assigned to the
potential is observed at 0.71 V vs. Ag/Ag+ and this corre-
sponds to the oxidation potential (Eox) of the Rc moiety. A
similar cyclic voltammogram was obtained for 3, but a new
non-reversible reduction wave at ꢂ1.82 V, assigned to the
para-nitrophenyl moiety, also appeared. It can be seen that the
Eox value for 3 is less positive by 0.13 V than that of 4,
indicating the better electron-donor capability of the Rc
connected to the pyrazoline ring. On the other hand, the Ered
value of 3 is less negative than that of 4 by 0.10 V, indicating
that the C60 moiety with the pyrazoline ring (3) is more
electron-negative than that with pyrrolidine ring (4) as re-
ported previously.17 The data obtained are summarized in
Table 1 together with comparison data for C60, ruthenocene,
formylruthenocene (1) and hydrazone 2.
Fig. 2 The HOMO and LUMO of dyads 3 (top) and 4 (bottom).
Electrochemistry
Ruthenocene (Rc) based D–A systems have been much less
widely studied than systems derived from ferrocene, presum-
ably partly due to the irreversible redox chemistry of ruthe-
nocene itself. Recently, however, it has been found that a
number of substituted Rc derivatives undergo chemically
(though not electrochemically) reversible two-electron oxida-
tions.20 With the aim of estimating the energies of charge-
separated states formed by photoinduced electron transfer, the
reduction and oxidation potentials of dyads 3 and 4 were
determined by cyclic voltammetry (CV) in ortho-dichlorobenze-
ne(DCB)/acetonitrile (4 : 1) containing 0.1 M tetra-n-butyl-
ammonium perchlorate [(n-C4H9)4NClO4] as a supporting
electrolyte.
The experimental HOMO(Rc)–LUMO(C60) gap, determined
as the difference between Ered and Eox, is significantly lower for
2-pyrazoline-based dyad 3 (1.59 eV) than that for pyrrolidine
derivative 4 (1.82 eV) according to theoretical calculations.
This difference suggests an easier photoinduced electron trans-
fer in 3 than in 4.
From the first Eox and Ered values, the free-energies (DGRIP
)
of the radical ion-pairs of Rcꢁ+BC60 ꢂ were calculated using
ꢁ
the Rehm–Weller equation (eqn (1)).21
DGRIP = Eox(Rc) ꢂ Ered(C60) ꢂ DGS
(1)
The cyclic voltammogram (CV) of compound 4 in DCB/
acetonitrile (4 : 1) is shown in Fig. 3. Three reversible peaks
can be seen in the negative potentials down to ꢂ2.5 V vs. Ag/
Here, DGS refers to the static energy calculated according to
eqn (2):21
Ag+ and these are attributed to the reduction potentials (Ered
)
DGS = e2/(4pe0eRRD–A
)
(2)
of the C60 core. In the positive potential, an irreversible
Here, the terms e, e0,and eR refer to elementary charge,
vacuum permittivity and static dielectric constant of the
solvent used for rate measurements and redox potential
Table 2 Free-energy changes for the charge-separation process
and charge-recombination (DGRIP) in benzonitrile (PhCN)
and toluene
a
(DGCS
)
Compound
Solvent
DGRIP
eV
/
DGCS/eV
DGCS/eV
3
via C60*
1
via C60
*
3
4
a
PhCN
Toluene
PhCN
Toluene
1
ꢂ1.38
ꢂ1.97
ꢂ1.61
ꢂ2.20
ꢂ0.37
0.22
ꢂ0.14
0.45
3
ꢂ0.15
0.44
0.08
0.67
Fig. 3 Cyclic voltammogram of compound 4 in ortho-dichloroben-
zene/acetonitrile (4 : 1) (V vs. Ag/AgNO3; scan rate was 100 mV sꢂ1).
E00 = 1.75 eV for C60*, and E00 = 1.50 eV for C60*.
ꢀc
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measurements, respectively. Based on DGRIP and excited en-
ergy (E00) values, the free-energy changes of the charge-
separation process (DGCS) were calculated (eqn (3)) as listed
in Table 2.
the pyrazolino group. The absorption band at around 400 nm
for dyad 3 changes with solvent polarity, as depicted in Fig.
4b. It can be seen that the broad band at around 400 nm
decreases in toluene, suggesting a polar electronic interaction
between ruthenocene and C60 in the ground state.8a,22 Similar
behaviour has been observed in the analogous ferrocene-2-
pyrazolino[60]fullerene dyad 5.23
From these DGCS values, the charge-separation process of 3
and 4 via the excited singlet state of C60 (1C60*) is sufficiently
exothermic in benzonitrile. In toluene, the CS process seems to
be slightly endothermic, taking into consideration that the
Weller model in toluene is likely an oversimplification, and the
exact values quoted above are probably not completely reli-
able. The charge-separation process via the excited triplet state
of C60 (3C60*) is exothermic for 3 in PhCN, but endothermic in
toluene.
Steady-state fluorescence spectra
The photochemical behaviour of dyads 3 and 4 was investi-
gated using steady-state fluorescence measurements. The stea-
dy-state fluorescence spectra of compounds 3 and 4 are shown
in Fig. 5. The spectra were obtained using a detector consisting
of a special photomultiplier tube with relatively high sensitiv-
ity in the wavelength region above 600 nm. The measurements
were carried out in different solvents (PhCN, DCB and
toluene) by applying an excitation wavelength of 400 nm,
which exclusively excites the C60 moiety. The emission ob-
served in the range 700–760 nm for 4 in toluene is a typical
fluorescence of the pyrrolidino-C60 moiety. In the case of 3 in
toluene, a new fluorescence was observed at 653 nm in addi-
tion to the peak at 720 nm.
ꢂDGCS = DGCR ꢂ E00
(3)
3
For dyad 4, the DGCS values of C60* are endothermic in
PhCN and toluene.
Steady-state absorption spectra
The absorption spectra of dyads 3 and 4 measured in PhCN
are shown in Fig. 4. The absorption bands above 400 nm are
attributed to the C60 moiety. The absorption band of the Rc
moiety is expected to appear at wavelengths below 400 nm, but
the intensity of this band is weak compared with the huge
absorption band of C60 in this region. The absorption inten-
sities of compound 3 are slightly stronger than those of 4 in the
380–480 nm region. The absorption spectrum of pyrrolidi-
no[60]fullerene dyad 4 is essentially a linear combination of the
absorption spectra of the reference compounds, ruthenocene
and N-methylpyrrolidino[60]fullerene, suggesting that there is
no significant electronic interaction between the ruthenocene
and fullerene in the ground state. On the other hand, the
absorption band of compound 3 in the 380–480 nm region
suggests interaction between the C60 moiety and Rc through
The emission peak at 653 nm is typical of a pyrazolino-C60
moiety attached to the Rc moiety. In polar solvents, these
emission intensities were almost completely quenched com-
pared with those in toluene. These observations suggest effi-
cient quenching of the singlet excited state of the C60 moiety by
the appended Rc moieties in polar solvents. The close spatial
separation between the fullerene and the Rc group is expected
to lead to efficient deactivation of the fullerene singlet excited
state. In toluene the 720 nm emission of 3 was weak compared
with that for 4, suggesting a weak quenching process for 3 even
in toluene.
Fig. 4 (top) Steady-state absorption spectra of compounds 3 (a, --)
and 4 (b, —) in PhCN. (bottom) UV-vis spectra of 3 in dichloro-
methane (—) and toluene (--). The concentrations were kept at
2 ꢃ 10ꢂ5 M.
Fig. 5 Steady-state fluorescence spectra of (top) dyad 3 and (bottom)
dyad 4 in different organic solvents; lex = 400 nm.
ꢀc
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Table 3 Fluorescence lifetimes (tf) of 1C60*, rate constants (kCS) and
1
quantum yield (FCS) for charge-separation via C60* for 3 and 4 in
toluene, DCB, and PhCN
Compound Solvent
t
FS/ps
kCS/sꢂ1
FCS
3
4
PhCN
DCB
Toluene 750(70%) 2000(40%)
Very fast
200(65%) 1550(35%)
>1.0 ꢃ 1010 >0.90
5.6 ꢃ 108
4.1 ꢃ 109
0.83
0.42
PhCN
DCB
150
400
5.8 ꢃ 109
1.6 ꢃ 109
5.9 ꢃ 108
0.87
0.66
0.41
Toluene 700
The fluorescence decay-time profiles at 700 nm for dyads 3
and 4 are shown in Fig. 7. The fluorescence time profile of the
reference C60 exhibited a single exponential decay with a
lifetime (tf0) of 1.3 ns. The attachment of donor moieties
introduces a new quenching pathway for the lowest excited
singlet state of the C60 moieties in dyads 3 and 4. In polar and
non-polar solvents, the recorded fluorescence lifetimes mon-
itored at 720 nm for both dyads are shorter than the (tf0)
value, suggesting that the fluorescence quenching occurs from
Fig. 6 Time resolved spectra of (top) dyad 3 and (bottom) dyad 4 in
different solvents; lex = 400 nm.
1
the C60* moiety. The fluorescence decay time profile can be
fitted with two exponential components for 3, from which two
lifetimes can be evaluated (see Table 3).
Time-resolved fluorescence spectra and fluorescence lifetimes
The time-resolved fluorescence spectral features of dyads 3 and
4 in polar solvents (DCB and PhCN) and a non-polar solvent
(toluene) are shown in Fig. 6; the emission spectra are depicted
in the time range of 0.1–0.6 ns.
The short lifetimes (tf) are in the 200 and 420 ps range in
DCB and toluene, respectively. In PhCN, the fast part of the
process was too fast to record. For compound 4, the lifetimes
were evaluated by curve-fitting with a single exponential and
gave values in the range 150–620 ps. The fluorescence lifetimes
became shorter on increasing the solvent polarity, suggesting
that the quenching process is due to the charge separation
from the donor moieties to the C60 moiety to yield the charge-
separated (CS) state. The rate constants for charge separation
(kCS) were calculated using the eqn (4):
In these experiments a streak-scope was used as the detector.
The observed emission spectra in toluene are similar to those
of the steady-state measurements, supporting the reliability of
our measurements for these weak emissions. For compound 3
in DCB, the emission band at 653 nm disappeared but the
remaining 700–720 nm emission is consistent with the steady-
state measurement thus providing evidence of efficient quench-
ing in polar solvents.
kCS = 1/tf ꢂ 1/tf0
(4)
where tf0 and tf are the lifetimes of the reference and investigated
compound, respectively. The quantum yields (FCS) for the gen-
eration of the CS state via 1C60* were calculated using the eqn (5):
FCS = (1/tf ꢂ 1/tf0)/1/tf
(5)
The charge-separation rate constant (kCS) and quantum yield
(FCS) were evaluated as shown in Table 3. The higher values of
both kCS and FCS for 3 in comparison to those of 4 suggest
better charge-separation ability in 3 than 4. In each com-
pound, the kCS and FCS values increase with solvent polarity,
indicating that the charge-separated process belongs to the
normal region of the Marcus parabola.24 It is worth noting
that in toluene the charge separation takes place for both 3
and 4. This observation is consistent with the fact that nitro-
gen atom near the C60 moiety acts as a strong electron-donor
in the non-polar solvent.25 Compared with the ferrocene (Fc)
derivatives with similar structures 5 and 6, the kCS values for
the Rc derivatives are smaller than those of the Fc derivatives
(5: kCS = 9.9 ꢃ 109 sꢂ1 in toluene (supporting information;
Fig. S1w) and 6: 2.2 ꢃ 1011 sꢂ1 in PhCN)8 by one order and
two order for 3 and 4, respectively, reflecting weaker donor
ability of the Rc moiety than the Fc moiety, as revealed by the
higher Eox of the Rc moiety than that of the Fc moiety.8
Fig. 7 Fluorescence decay profiles monitored at 720 nm of (top) 3
and (bottom) 4; lex = 400 nm.
ꢀc
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Fig. 9 Transient absorption spectra of compound 2 (0.1 mM)
obtained by 532 laser light photolysis in Ar-saturated PhCN.
rapid decay at 720 nm can be attributed mainly to rapid decay
3
of the C60* moiety due to the heavy atom effect.
The transient absorption spectra of 2 observed by exciting
the Rc moiety with 532 nm light in PhCN are shown in Fig. 9,
which exhibited the broad absorption spectra with maxima
at 640, 950 and 1200 nm in the 50 ns spectrum. These
bands might be attributed to the radical ion-pairs
(Rcꢁ+BPhNO2 ꢂ). The lifetime of the radical-pairs was eval-
ꢁ
uated as 20 ns which is shorter than that of 3.
Fig. 8 Transient absorption spectra of compound 3 (0.1 mM)
obtained by 355 laser light photolysis in Ar-saturated (top) PhCN
and (bottom) toluene.
In the case of 5, the transient absorption spectra (supporting
information; Fig. S2w) exhibited only the weak absorption
band of triplet C60 at 720 nm without evidence of the forming
of radical ion-pairs. This suggests that the charge recombina-
tion after efficient charge separation is fast, because of stronger
donor ability of the Fc moiety. On the contrary, the charge-
recombination of the Rc derivatives is slow, because the
charge-recombination process may belong to the Marcus
inverted region24 due to sufficiently negative DGRIP values in
Table 2.
Transient absorption measurements
The nanosecond transient spectra of 3 in PhCN by 355 nm
laser light photolysis (Fig. 8, top), where C60 can be predomi-
nantly excited, were measured. Immediately after laser excita-
tion (spectrum at 50 ns), the transient absorption peak
appeared at 720 nm with a broad absorption in the 1000–
1600 nm region. At 500 ns, the broad absorption in the near-
IR region disappeared and the absorption at 720 nm re-
mained. This remaining absorption band was assigned to the
triplet state of C60 (3C60*). The rapidly decaying absorption
The transient absorption spectra of compound 4 in PhCN
are shown in Fig. 10 (top). In the spectrum at 50 ns, the strong
absorption appearing at 720 nm is due to the 3C60* moiety. In
comparison to dyad 3, the absorbance of 4 at 720 nm is
stronger by a factor of 10, suggesting the predominant gen-
can be ascribed ꢂto the radical ion pair Rcꢁ+BC60 ꢂ. In
ꢁ
ꢁ
3
general, the C60 moiety shows a clear absorption peak at
eration of the C60* moiety. In the near-IR region, the broad
1000 nm; however, only a broad absorption was observed in
absorption tail seems to be extended, although the relative
intensity of the near-IR band of compound 4 is weak com-
pared with that at 720 nm. The rapid-decay in the near-IR
the case of compound 3. One possible explanation for this is
+
overlap of the absorption due to Rcꢁ
.
band can be attributed to Rcꢁ+BC60 ꢂ. On the other hand,
ꢁ
The time profiles in the inset in Fig. 8 (top) show that the
decay processes in the near-IR region give a decay rate
constant of k(1020 nm) = 9.6 ꢃ 106 sꢂ1, which is attributed to
3
the band at 720 nm can be attributed to the C60* moiety,
although the decay was very fast, probably due to the heavy
atom effect of Rc.
the charge-recombination rate constant (kCR
)
of
Rcꢁ+BC60 ꢂ. The inverse of the kCR value gave the lifetime
ꢁ
In the case of 4 in toluene, similar transient absorption
spectra were observed and these are shown in Fig. 10 (bot-
tom). Although the absorption shape is quite similar to that in
PhCN, the time-profiles were quite different. The decay rates
at 720 nm and the near-IR region were quite slow, which
suggests that these bands are due to the 3C60* moiety. Indeed,
the decay was accelerated on addition of O2 and this provides
further evidence for this assignment. From the fluorescence
(tRIP) of Rcꢁ+BC60
temperature. The decay-time profile at 720 nm shows dual
to be 100 ns in PhCN at room
ꢂ
ꢁ
decays; the initial decay rate is in good agreementꢂwith the kCR
value, suggesting the absorption of Rcꢁ+BC60 is overlap-
ꢁ
ping in this region. The difference transient absorption spec-
trum obtained subtracting the transient spectruꢁm at 500 ns
from that at 50 ns is all attributed to Rcꢁ+BC60 ꢂ. The slow
decay at 720 nm can be attributed to the lifetime of RcB3C60*.
In toluene (Fig. 8 (bottom)) the predominant absorption peak
1
quenching, the charge-separation takes place via the C60*
moiety; however, the fast charge-recombination produces the
3C60* moiety.
3
appeared at 720 nm and this is due to the C60* moiety. The
ꢀc
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98 | New J. Chem., 2006, 30, 93–101

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Fig. 11 Energy diagram of 3 and 4 in benzonitrile.
that in PhCN, although the values in toluene may be asso-
ciated with considerable error.
In the case of compound 3 in PhCN, the charge-separation
1
process takꢂes place mainly via C60* moiety generating
ꢂ
ꢁ
Rcꢁ+BC60 . Minor contribution of forming Rcꢁ+BC60
ꢁ
Fig. 10 Transient absorption spectra of compound 4 (0.1 mM)
obtained by 355 laser light photolysis in Ar-saturated (top) PhCN
and (bottom) toluene.
may arise viaꢂ the electron migration (transfer) fꢂrom
Rcꢁ+BPhNO2 to C60 to yield the final Rcꢁ+BC60 ꢂ in
ꢁ
ꢁ
PhCN. As in Table 1, the energy level of Rcꢁ+BPhNO2 is
ꢁ
higher than that of Rcꢁ+BC60 ꢂ, because the reduction
ꢁ
In Table 4, the charge-recombination rate constants (kCR
)
potential of PhNO2 group was more negative than that of
of Rcꢁ+BC60 evaluated from the decay at 1020 nm are
summarized. The kCR value in PhCN is larger than that in
toluene for 3. Lifetimes of the radical ion-pairs are as long
ꢂ
ꢁ
C60 moiety. The charge-recombination of ion-pair,
ꢂ
Rcꢁ+BC60 may produce the ground states. In toluene, the
charge-separation process of 3 is minor, because of the en-
dothermic process; therefore, intersystem crossing takes place
ꢁ
as 100 ns in PhCN for both 3 and 4. Compared with 6 (kCR
=
4.5 ꢃ 109 sꢂ1 in PhCN),8 the kCR values for the Rc derivatives
are quite smaller, probably because of the high hole trapping
ability of the Rc moiety.
competitively with the charge-separation ꢂprocess. The charge-
recombination of ion-pair, Rcꢁ+BC60
may produce the
ꢁ
3C60* in addition to the direct production of the ground states.
In the case of dyad 4 in PhCN, the charge-separation
process via the 1C60* moiety is a minor process. Similar results
Energy diagram
3
were observed for 6.8 The considerable C60* moiety is pro-
The energy diagram after photo-excitation is depicted in Fig.
11, in which the lowest excited state data were evaluated from
the 0–0 band of the longest ꢂwavelength of the transition. The
duced via intersystem crossing and via the charge-recombina-
tion process since the charge-separated state is higher than
3
3C60*. For the generation of C60* moiety in toluene, the
energy level of Rcꢁ+BC60 in PhCN is slightly lower than
ꢁ
contribution of the intersystem crossing increases compared
with that of the charge-recombination process, if any.
those in DCB/acetonitrile in Table 2. The negative DGCS
values in polar solvents render charge separation from both
1
1Rc* and C60*, suggesting possiblꢁe charge-separation. How-
ever, the energy level of Rcꢁ+BC60 ꢂ in toluene is higher than
Conclusions
In summary, we have synthesized two new donor–acceptor
dyads based on ruthenocene as the electron donor and 2-
pyrazolino[60]fullerene and pyrrolidino[60]fullerene as elec-
tron acceptors. The electrochemical and photophysical prop-
erties of the two dyads have been studied. Cyclic voltammetry
measurements show a lower HOMORc–LUMOC60 gap in the
2-pyrazolino[60]fullerene dyad (3) than in the pyrrolidino[60]-
fullerene (4) system and this order is supported by theoretical
calculations at the B3PW91/LanL2DZ level. The photophysi-
cal properties of the ruthenocene-C60 dyads were investigated
by measuring the time-resolved fluorescence and nanosecond
Table 4 Charge recombination rate constant (kCR) and lifetimes
(tRIP) of Rcꢁ+BC60
ꢁꢂ
Compound
Solvent
kCR/sꢂ1
tRIP/ns
3
PhCN
Toluene
9.6 ꢃ 106
100
—
b
b
—
4
PhCN
Toluene
(9.1 ꢃ 106)a
(100)a
—
b
b
—
a
b
Charge-separation is a minor process. Charge-separation was not
observed.
ꢀc
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New J. Chem., 2006, 30, 93–101 | 99

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transient spectra in polar and non-polar solvents. A different
type of behaviour was observed in each of these two dyads.
The photoinduced charge separation takes place efficiently in
2-pyrazolino[60]fullerene-ruthenocene, whereas the charge-se-
paration efficiency becomes quite low in pyrrolidino[60]fuller-
ene-ruthenocene. This finding reveals that the pyrazoline ring
mediates charge-separation due to the higher electron-donat-
ing ability of the pyrazoline ring compared with that of that
pyrrolidine ring. Compared with the corresponding ferrocene-
C60 derivatives, the slower charge-separation and slower
charge-recombination were confirmed for ruthenocene-C60
derivatives.
Photonics, B2834). For spectra in the visible region (400–800
nm), a Si-PIN photodiode (Hamamatsu Photonics, S1722-02)
was used as a detector. All the samples were placed in a quartz
cell (1 ꢃ 1 cm) and were deaerated by bubbling Ar through the
solution for 15 min.
The computational calculations were performed by ab initio
B3LYP/3-21G(*) methods with the GAUSSIAN 03 software
package on high-speed computers. The images of the frontier
orbitals were generated from Gauss View-03 software.
Synthesis
Ruthenocenecarboxaldehyde 4-nitrophenylhydrazone (2). A
solution of 115 (63 g, 0.24 mmol), 4-nitrophenylhydrazine (37
g, 0.24 mmol) and two drops of acetic acid in 15 mL of ethanol
was heated under reflux for 20 min. The mixture was cooled to
room temperature, filtered and the solid was recrystallized
from ethanol. The product was obtained as a red solid in 90%
yield (85 mg) (mp 207–208 1C). IR(KBr) n/cmꢂ1 3271, 1543,
1474, 1286, 1167, 808. 1H NMR (200 MHz, CDCl3) d/ppm
8.14 (d, J = 9.5 Hz, 2H), 7.69 (bs, 1H), 7.51 (s, 1H), 6.98 (d,
Experimental
General remarks
All cycloaddition reactions were performed under argon. C60
was purchased from MER Corporation (Tucson, AZ). TLC
using Merck silica gel 60-F254 was used to monitor cycloaddi-
tion reactions. 1H NMR and 13C NMR spectra were recorded
on Varian Mercury 200 and Varian Inova 500 spectrometers.
FT-IR spectra were recorded on a Nicolet Impact 410 spectro-
photometer using KBr disks. MALDI-TOF mass spectra were
obtained on a Bruker ReflexIII spectrometer. Elemental ana-
lyses were performed on a Perkin-Elmer 2400 CHNS/O Series
II analyzer.
J = 9.5 Hz, 2H), 4.99 (m, 2H), 4.70 (m, 2H), 4.59 (s, 5H). 13
C
NMR (50 MHz, DMSO) d/ppm 150.9, 142.1, 138.1, 126.8,
111.3, 84.8, 72.3, 71.9, 70.0. Anal. calcd. for C17H15N3O2Ru:
C, 51.77; H, 3.83; N, 10.65%. Found: C, 51.15; H, 3.64; N,
10.65%; UV-Vis lmax (CH2Cl2)/nm (log e) 394 (4.0), 235 (5.3).
10-(4-Nitrophenyl)-30-(ruthenocenyl)pyrazolino[40,50:1,2] [60]
fullerene (3). An Ar-blanketed solution of NBS (53 mg, 0.03
mmol) and hydrazone 2 (24 mg, 0.07 mmol) in dry CHCl3 (20
mL) was stirred at room temperature for 30 min under Ar. The
solvent was removed in vacuo, and a solution of C60 (50 mg,
0.07 mmol) and NEt3 (7 mg, 0.07 mmol) in dry toluene (50
mL) was added to the solid. The solution was stirred for 45
min at 40 1C. The solvent was removed under reduced
pressure. The resulting solid was purified by silica gel flash
chromatography using toluene as the eluent. Centrifugation
with methanol and diethyl ether was used to further purify the
solid. Compound 3 was obtained in 16% yield (12 mg).
IR(KBr) n/cmꢂ1 1585, 1493, 1320, 1274, 1105, 999, 837, 526.
1H NMR (500 MHz, CDCl3) d/ppm 8.30 (d, J = 9.8 Hz, 2H),
8.20 (d, J = 9.8 Hz, 2H), 5.52 (m, 2H), 4.83 (m, 2H), 4.47 (s,
5H). 13C NMR (125 MHz, CDCl3) d/ppm 149.2, 147.5, 147.0,
146.2, 146.1, 145.9, 145.8, 145.7, 145.5, 145.3, 145.1, 145.0,
144.9, 144.4, 144.1, 143.9, 143.0, 142.9, 142.8, 142.7, 142.3,
142.1, 142.0, 141.8, 141.78, 141.70, 139.7, 139.2, 137.7, 136.8,
134.6, 131.6, 131.3, 128.1, 125.0, 118.6, 114.6, 114.0, 112.7,
91.4, 81.3, 72.9, 71.9, 71.4. MS m/z: 1113.0 (M + 1), 720 (C60);
UV-Vis lmax (CH2Cl2)/nm (log e) 430 (4.5), 392 (4.5), 320
(4.8), 255 (5.3).
The UV/Vis spectra were obtained using a Jasco model
V570 DS spectrophotometer or a Shimatzu spectrophot-
ometer. Steady-state fluorescence spectra were measured on
a Shimadzu RF-5300 PC spectrofluorophotometer equipped
with a photomultiplier tube with high sensitivity in the
700–800 nm region.
Cyclic voltammetry measurements were carried out on an
Autolab PGSTAT 30 potentiostat using a BAS MF-2062 Ag/
0.01 M AgNO3, 0.1 M (n-C4H9)4ClO4 in acetonitrile reference
electrode, an auxiliary electrode consisting of a Pt wire, and a
Metrohm 6.1247.000 conventional glassy carbon electrode (3
mm o.d.) as a working electrode directly immersed in the
solution. A 10 mL electrochemical cell from BAS, Model
VC-2, was also used. The reference potential was shifted by
290 mV towards a more negative potential compared with the
Ag/AgCl scale. E1/2 values were taken as the average of the
anodic and cathodic peak potentials. Scan rate: 100 mV sꢂ1
.
The picosecond time-resolved fluorescence spectra were
measured by a single-photon counting method using a second
harmonic generation (SHG, 410 nm) Ti:sapphire laser (Spec-
tra-Physica, Tsunami 3950-L2S, 1.5 ps fwhm) and a streak-
scope (Hamamatsu Photonics, C43334-01) equipped with a
polychromator (Action Research, SpectraPro 150) as an ex-
citation source and a detector, respectively. Lifetimes were
evaluated with software attached to the equipment.
50-(Phenyl)-20-ruthenocenylpyrrolidino[30,40:1,2][60]fullerene
(4). A solution of 1 (57 mg, 0.22 mmol), a-aminophenylacetic
acid (83 mg, 0.55 mmol) and C60 (80 mg, 0.11 mmol) in
chlorobenzene (50 mL) was irradiated at 210 W for 45 min
in a focused microwave oven. The solvent was removed under
reduced pressure. The resulting solid was purified by silica gel
flash chromatography using toluene as the eluent. Centrifuga-
tion with methanol and pentane was used to further purify the
The nanosecond transient absorption measurements in the
near-IR region were measured by means of laser-flash photo-
lysis; 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 transient absorption spectra in the near-IR region
(600–1200 nm), monitoring light from a pulsed Xe-lamp was
detected with Ge-avalanche photodiode module (Hamamatsu
ꢀc
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100 | New J. Chem., 2006, 30, 93–101

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solid. Compound 4 was obtained in 26% yield (31 mg).
1
38, 1599–1605; (f) D. I. Schuster, Carbon, 2000, 38, 1607–1614; (g)
J. L. Segura and N. Martın, Chem. Soc. Rev., 2000, 29, 13–25; (h)
´
D. M. Guldi, Chem. Soc. Rev., 2002, 31, 22–36; (i) J.-F. Nieren-
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IR(KBr) n/cmꢂ1 3417, 2916, 2328, 1643, 1427, 1103, 702. H
NMR (500 MHz, CDCl3) d/ppm 7.95 (d, J = 7.02 Hz, 2H),
7.42 (m, 3H), 5.91 (s, 1H), 5.58 (s, 1H), 5.35 (s, 1H), 5.00 (s,
1H), 4.71 (s, 5H), 4.62 (s, 1H); 13C NMR (125 MHz, CDCl3/
CS2) d/ppm 154.7, 154.6, 153.4, 147.6, 147.5, 147.4, 147.3,
147.1, 146.8, 146.5, 146.4, 146.3, 146.27, 146.1, 146.0, 145.7,
145.5, 145.4, 145.0, 144.6, 143.4, 142.9, 142.8, 142.5, 142.4,
142.3, 142.2, 140.17, 139.9, 139.6, 138.3, 129.0, 128.9, 128.8,
89.6, 74.7, 71.7, 71.295, 71.2, 71.18, 70.5. MS m/z: 1066.1
(M ꢂ 1); UV-Vis (CH2Cl2) UV-Vis lmax (CH2Cl2)/nm (log e)
430 (4.0), 326 (5.0), 310 (5.0), 257 (5.3).
8 (a) D. M. Guldi, M. Maggini, G. Scorrano and M. Prato, J. Am.
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10 -(4-Nitrophenyl)-30-(ferrocenyl)pyrazolino[40,50:1,2] [60]
fullerene (5). This compound was prepared as previously
described.23
9 J. J. Oviedo, P. de la Cruz, J. Garın, J. Orduna and F. Langa,
´
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Acknowledgements
Financial support for this work was provided by a grant from
the Ministerio de Educacion y Ciencia of Spain, FEDER
´
funds (Project CTQ2004-00364/BQU) and the Junta de Co-
munidades de Castilla-La Mancha (Project PAI-02-023). This
research was partially supported by a Grant-in-Aid for the
COE project, Giant Molecules and Complex Systems, 2002.
This work was also supported by a Grant-in-Aid for Scientific
Research on Priority Area (417) from the Ministry of Educa-
tion, Culture, Sports, Science, and Technology of Japanese
Government.
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