Fullerene acting as an electron donor in a donor-acceptor dyad to attain the long-lived charge-separated state by complexation with scandium ion

Article abstract of DOI:10.1039/b612613h

A long-lived charge-separated (CS) state of fullerene-trinitrofluorenone linked dyad in which fullerene acts as an electron donor is formed by photoinduced electron transfer from C₆₀ to TNF in the presence of Sc(OTf)₃; the CS lifetime is determined as 23 ms in PhCN at 298 K. The Royal Society of Chemistry.

Full text of DOI:10.1039/b612613h

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Fullerene acting as an electron donor in a donor–acceptor dyad to attain
the long-lived charge-separated state by complexation with scandium
ion{
a
b
b
b
b
´
Kei Ohkubo, Javier Ortiz, Luis Mart´ın-Gomis, Fernando Ferna´ndez-La´zaro, Angela Sastre-Santos* and
Shunichi Fukuzumi*a
Received (in Cambridge, UK) 1st September 2006, Accepted 27th October 2006
First published as an Advance Article on the web 10th November 2006
DOI: 10.1039/b612613h
A long-lived charge-separated (CS) state of fullerene–trinitro-
fluorenone linked dyad in which fullerene acts as an electron
donor is formed by photoinduced electron transfer from C60 to
TNF in the presence of Sc(OTf)3; the CS lifetime is determined
as 23 ms in PhCN at 298 K.
Fullerene, which has a highly delocalized three-dimensional
p-system, is suitable for efficient electron transfer (ET) because
the uptake or release of electrons results in minimal structural and
solvation changes upon ET.1 Consequently, a formation of the
Fig. 1 Chemical structure of C60–TNF dyad.
long-lived CS state has been examined by using fullerene-based
D–A linked dyad systems in which fullerene acts as an electron
acceptor.2–5 However, fullerene has so far been used only as an
with those of the unlinked compounds, the first and second one-
electron reduction processes occur at the TNF moiety. The second
reduction peak due to TNF 2/TNF22 is overlapped with the peak
electron acceptor in D–A systems, because the ET oxidation of
fullerene is much more difficult than the ET reduction.6–8
?
due to C60/C60 2. The third peak is assigned to C60 2/C6022 14 On
.
?
?
We have recently reported that photoinduced intermolecular ET
oxidation of fullerene with p-benzoquinone is made possible by the
addition of Sc(OTf)3 (OTf2 = triflate) which can bind with the
product of ET.9 Such a strong binding of Sc(OTf)3 with the radical
anions of electron acceptors results in ET from electron donors to
acceptors even though photoinduced intramolecular ET is
energetically impossible in the absence of metal ion.10,11 The
elongation of the CS state lifetime of D–A linked dyads has been
achieved by the addition of metal ions.12,13 However, there has so
far been no report on formation of the CS state of D–A dyads
using fullerene as an electron donor in the presence of metal ions.
We report herein the photodynamics of a fullerene–trinitro-
fluorenone dyad (C60–TNF as shown in Fig. 1)14 in the absence
and presence of Sc(OTf)3. A long-lived CS state is formed by
photoinduced ET from C60 to TNF in the presence of Sc(OTf)3
upon photoexcitation of C60–TNF, whereas only the triplet excited
state of C60 (3C60*) is formed in absence of Sc(OTf)3.
the other hand, the one-electron oxidation occurs at the C60 moiety
of C60–TNF. When Sc(OTf)3 (30 mmol dm23) is added to a
deaerated PhCN solution of C60–TNF, a large positive shift is
observed at the first one-electron reduction due to TNF/TNF 2 as
?
shown in Fig. 2(b).16,17 The Ered value of TNF is changed from
20.40 V vs. SCE to +0.01 V in the presence of Sc(OTf)3. Thus,
2
?
TNF forms a strong complex with Sc(OTf)3, which induces a
decrease of the CS state energy.
Nanosecond laser excitation (l = 430 nm) of C60–TNF in
deaerated PhCN results in formation of the triplet excited state of
C
60 (3C60*; lmax = 750 nm)18 as shown in Fig. 3(a). The transient
absorption disappeared completely 180 ms after laser excitation.
3
The decay rate constant of C60* is determined as 3.2 6 104 s21
,
which agrees with the reported value for 3C60*. The CS energy is
The synthesis of C60–TNF has been reported previously.14 The
differential pulse voltammograms (DPV) of C60–TNF in deaerated
benzonitrile (PhCN)15 are shown in Fig. 2. The ratio of the current
of DPV of the three reduction peaks is 1 : 2 : 1. By comparison
aDepartment of Material and Life Science, Graduate School of
Engineering, Osaka University, SORST, Japan Science and Technology
Agency (JST), Suita, Osaka, 565-0871, Japan.
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp; Fax: +81-6-6879-7370;
Tel: +81-6-6879-7368
bDivisio´n de Qu´ımica Orga´nica, Instituto de Bioingenier´ıa, Universidad
Miguel Herna´ndez, Elche, 03202, Spain. E-mail: asastre@umh.es;
Fax: +34-966658351; Tel: +34-966658408
{ Electronic supplementary information (ESI) available: Spectral data for
the transient absorption measurements. See DOI: 10.1039/b612613h
Fig. 2 (a) Differential pulse voltammograms of C60–TNF dyad in
deaerated PhCN containing Bu4NClO4 (0.1 mol dm23) in the absence and
(b) in the presence of Sc(OTf)3 (30 mmol dm23) at 298 K.
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An external electron donor (trans-stilbene) was added to a
PhCN solution of C60–TNF in order to confirm the oxidizing
ability of the CS state of C60–TNF. The laser photoirradiation of a
PhCN solution containing C60–TNF, Sc(OTf)3 (30 mmol dm23
)
and trans-stilbene (10 mmol dm23) results in formation of new
transient absorption bands due to trans-stilbene radical cation
(lmax = 480 and 760 nm)22 instead of the absorption band due to
the radical cation of the C60 moiety, whereas the absorption band
due to the TNF 2/Sc3+ moiety is overlapped with that of trans-
?
stilbene radical cation as shown in Fig. 4(a) (closed circles). No
formation of trans-stilbene radical cation was observed in the
absence of Sc(OTf)3 as shown in Fig. 4(a) (open circles). This
+
?
indicated that intermolecular ET from trans-stilbene to the C60
moiety in C60 –TNF 2/Sc3+ occurs to give trans-stilbene radical
cation, because the one-electron oxidation potential of trans-
stilbene (Eox = 1.47 V vs. SCE)23 is less positive than the one-
+
?
?
electron reduction potential of the C60 + moiety (Ered = 1.49 V vs.
?
SCE). When trans-stilbene is replaced by 1,2-dimethoxybenzene
+
(Eox = 1.45 V vs. SCE), the ET oxidation by C60 –TNF 2/Sc3+
Fig. 3 Transient absorption spectra of C60–TNF (0.1 mmol dm23
)
?
?
obtained by nanosecond laser flash photolysis in deaerated PhCN (a) in
the absence and (b) in the presence of Sc(OTf)3 (30 mmol dm23) taken at
30 and 180 ms after laser excitation (430 nm) at 298 K. Decay profiles of
transient absorption at 450 nm in the absence (black) and the presence of
Sc(OTf)3 (30 mmol dm23, gray) in the (c) 100 ms and (d) 10 ms ranges.
Inset: First-order plots for decay in the presence of Sc(OTf)3.
also occurs to give 1,2-dimethoxybenzene radical cation (lmax
=
410 and 1100 nm)22c as shown in Fig. 4(b). On the other hand, no
ET oxidation was observed in the case of naphthalene (1.60 V) and
pentamethylbenzene (1.58 V),24 because the ET oxidation by the
CS state is energetically impossible. Thus, it has clearly been shown
+
that the CS state C60 –TNF 2/Sc3+ can act as a strong oxidant.
?
?
The CS lifetime of C60 –TNF 2/Sc3+ was determined by the
decay profile of the transient absorption. The transient absorption
at 450 nm due to 3C60* decays to zero in the absence of Sc3+ (black
line in Fig. 3(c)), whereas the absorption at 450 nm in the presence
+
?
?
determined from the one-electron redox potentials of C60–TNF as
1.89 eV, which is larger than the values of the singlet and the triplet
3
excited energy of C60 (1C60*: 1.75 eV; C60*: 1.56 eV).18 Thus, no
photoinduced ET occurs from the C60 moiety to the TNF moiety
as shown in the energy diagram in Scheme 1.
The addition of Sc(OTf)3 (30 mmol dm23 17
) to a PhCN solution
of C60–TNF, however, results in drastic change in the photo-
3
dynamics from the formation of C60* to ET to produce the CS
2
state in which TNF forms a complex with Sc3+ (TNF 2/Sc3+;
lmax = 450 nm) at 180 ms after laser excitation, as shown in
Fig. 3(b). The assignment of the absorption band at 450 nm due to
?
?
the TNF 2/Sc3+ complex was confirmed by that observed in the
?
photoinduced ET reduction of the unlinked TNF with dimeric
1-benzyl-1,4-dihydronicotinamide19 in the presence of Sc3+ (see
ESI,{ S1). The absorption spectrum of TNF 2 is significantly blue-
?
shifted from 548 and 970 nm to 450 nm by the complexation with
Sc3+ (see ESI,{ S1). The appearance of the absorption band at
960 nm is clear indication of formation of C60 +.9 The quantum
?
yield of the CS state was determined from the C60 + absorbance as
?
35%.20,21
Fig. 4 Transient absorption spectra of C60–TNF (0.1 mmol dm23
)
obtained by laser flash photolysis in a deaerated PhCN containing (a)
trans-stilbene (ST) and (b) 1,2-dimethoxybenzene (DMB) in the absence
(#) and presence of Sc(OTf)3 (30 mmol dm23, $) taken at 180 ms after
laser excitation (430 nm) at 298 K.
Scheme 1
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of Sc3+ does not decay completely at 180 ms after the laser
excitation (gray line in Fig. 3(c)). The residual absorption
corresponds to that due to the CS state, decaying at prolonged
reaction time as shown in Fig. 3(d). The first-order decay rate
constant of the fast component in the presence of Sc3+ agrees with
the value in the absence of Sc3+ (3.2 6 104 s21) as shown in the
first-order plots (the inset of Fig. 3(c)). This indicates that no ET
Notes and references
1 L. Echegoyen, F. Diederich and L. E. Echegoyen, Fullerenes, Chemistry,
Physics, and Technology, ed. K. M. Kadish and R. S. Ruoff, Wiley-
Interscience, New York, 2000, pp. 1–51.
2 (a) K. Ohkubo, J. Shao, Z. Ou, K. M. Kadish, G. Li, R. K. Pandey,
M. Fujitsuka, O. Ito, H. Imahori and S. Fukuzumi, Angew. Chem., Int.
Ed., 2004, 43, 853; (b) S. Fukuzumi, K. Ohkubo, H. Imahori, J. Shao,
Z. Ou, G. Zheng, Y. Chen, R. K. Pandey, M. Fujitsuka, O. Ito and
K. M. Kadish, J. Am. Chem. Soc., 2001, 123, 10676.
3 H. Imahori, D. M. Guldi, K. Tamaki, Y. Yoshida, C. Luo, Y. Sakata
and S. Fukuzumi, J. Am. Chem. Soc., 2001, 123, 6617.
4 (a) D. Gust, T. A. Moore and A. L. Moore, Acc. Chem. Res., 2001, 34,
40; (b) G. Kodis, Y. Terazono, P. A. Liddell, J. Andreasson, V. Garg,
M. Hambourger, T. A. Moore, A. L. Moore and D. Gust, J. Am.
Chem. Soc., 2006, 128, 1818.
5 S. Fukuzumi and D. M. Guldi, Electron Transfer in Chemistry, ed.
V. Balzani, Wiley-VCH, Weinheim, 2001, vol. 2, pp. 270–337.
6 Q. Xie, F. Arias and L. Echegoyen, J. Am. Chem. Soc., 1993, 115, 9818.
7 (a) C. A. Reed and R. D. Bolskar, Chem. Rev., 2000, 100, 1075; (b)
C. A. Reed, K.-C. Kim, R. D. Bolskar and L. J. Mueller, Science, 2000,
289, 101.
8 For intermolecular photoinduced ET oxidation of C60, see: (a) G. Lem,
D. I. Schuster, S. H. Courtney, Q. Lu and S. R. Wilson, J. Am. Chem.
Soc., 1995, 117, 554; (b) I. G. Safonov, S. H. Courtney and D. I. Schuster,
Res. Chem. Intermed., 1997, 23, 541; (c) C. Siedschlag, H. Luftmann,
C. Wolff and J. Mattay, Tetrahedron, 1999, 55, 7805.
9 S. Fukuzumi, H. Mori, H. Imahori, T. Suenobu, Y. Araki, O. Ito and
K. M. Kadish, J. Am. Chem. Soc., 2001, 123, 12458.
10 S. Fukuzumi and S. Itoh, Adv. Photochem., 1998, 25, 107.
11 S. Fukuzumi, Electron Transfer in Chemistry, ed. V. Balzani, Wiley-
VCH, Weinheim, 2001, vol. 4, pp. 3–67.
3
from C60* to the TNF moiety occurs in both the absence and
3
presence of Sc3+. In the absence of Sc3+, ET from C60* to the
TNF is highly endergonic and thereby energetically impossible as
3
mentioned above (Scheme 1). The ET from C60* to TNF in the
presence of Sc3+ becomes exergonic (20.08 eV in Scheme 1), but
the ET rate, which requires intermolecular ET activation by Sc3+
,
may be much slower than the decay of 3C60* because of the small
ET driving force.
On the other hand, ET from 1C60* to TNF is still energetically
impossible, but the ET becomes highly exergonic (20.27 eV) by
the addition of 30 mmol dm23 Sc(OTf)3. Femtosecond laser
excitation (l = 430 nm) of C60–TNF in deaerated PhCN results in
formation of the 1C60* at 3 ps after laser excitation. The transient
absorption of 1C60* is completely changed to 3C60* at 3000 ps (see
3
ESI,{ S3). The formation rate constant of C60* at 700 nm is
determined to be 9.0 6 108 s21. The addition of Sc(OTf)3
(30 mmol dm23) to a PhCN solution of C60–TNF also results in
formation of C60 + overlapped with the shoulder of absorption of
?
3C60*, since the absorption change at 700 nm in the presence of
Sc3+ is larger than that in the absence of Sc3+ (see ESI,{ S2). The
formation rate in the presence of Sc3+ is also determined as 9.0 6
108 s21, which is same as in the absence of Sc3+. Thus, the CS state
12 (a) K. Okamoto, Y. Araki, O. Ito and S. Fukuzumi, J. Am. Chem. Soc.,
2004, 126, 56; (b) K. Okamoto, Y. Mori, H. Yamada, H. Imahori and
S. Fukuzumi, Chem.–Eur. J., 2004, 10, 474.
13 S. Fukuzumi, K. Ohkubo, J. Ortiz, A. M. Gutie´rrez, F. Ferna´ndez-
´
La´zaro and A. Sastre-Santos, Chem. Commun., 2005, 3814.
+
1
(C60 –TNF 2/Sc3+) may be formed via ET from C60* to TNF
?
?
´
14 J. Ortiz, F. Ferna´ndez-La´zaro, A. Sastre-Santos, J. A. Quintana,
and the subsequent strong binding of TNF with Sc3+, which
2
?
J. M. Villalvilla, P. Boj, M. D´ıaz-Garc´ıa, J. A. Rivera, S. E. Stepleton,
C. T. Cox, Jr. and L. Echegoyen, Chem. Mater., 2004, 16, 5021.
15 PhCN was carefully distilled over P2O5 to remove the impurity.
16 The interaction between the neutral TNF and Sc(OTf)3 (30 mmol dm23
makes the CS process possible (Scheme 1).
The slow decay component (Fig. 3(d)) results from the charge-
recombination process. The lifetime of CS state is determined as
23 ¡ 4 ms in PhCN at 298 K from the first-order plot in the inset
of Fig. 3(d). The CS lifetime remains the same irrespective of
difference in concentrations of Sc3+ (1–30 mmol dm23).25
The activation enthalpy for the intramolecular back ET (BET)
was determined from the slope of the Eyring plot as 24 kJ mol21
(see ESI,{ S3). Such a large temperature dependence of the BET
rate indicates that the intramolecular BET process with the driving
force of 1.48 eV is deeply in the Marcus inverted region, since the
reorganization energy of BET is determined as 0.67 eV from the
activation enthalpy using the Marcus theory.26 No intermolecular
BET of the CS state was observed because the intermolecular BET
process is also slowed down in the Marcus inverted region as
reported previously for the intermolecular ET oxidation of C60.27
In summary, C60 has successfully been used as an electron donor
that is linked with an electron acceptor in the presence of Sc3+ to
attain the longest CS lifetime at 298 K (23 ¡ 4 ms) ever reported
for electron donor–acceptor linked systems.
)
is negligible, which is confirmed by UV-vis titration. See: S. Fukuzumi,
N. Satoh, T. Okamoto, K. Yasui, T. Suenobu, Y. Seko, M. Fujitsuka
and O. Ito, J. Am. Chem. Soc., 2000, 123, 7756.
17 The maximum solubility of Sc(OTf)3 to PhCN is 30 mmol dm23
.
18 C. S. Foote, Top. Curr. Chem., 1994, 169, 347.
19 S. Fukuzumi, T. Suenobu, M. Patz, T. Hirasaka, S. Itoh, M. Fujitsuka
and O. Ito, J. Am. Chem. Soc., 1998, 120, 8060.
20 The quantum yield of the CS state was determined using the
comparative method. In particular, the triplet–triplet absorption of
C60 (e740 = 18 800 mol21 dm3 cm21; Wtriplet = 0.98) served as a probe
to obtain the quantum yield for the CS state, especially for C60 (e =
11 000 mol21 dm3 cm21)9.
21 R. D. Webster and G. A. Heath, Phys. Chem. Chem. Phys., 2001, 3,
2588.
22 (a) F. D. Lewis, A. M. Bedell, R. E. Dykstra, J. E. Elbert, I. R. Gould
and S. Farid, J. Am. Chem. Soc., 1990, 112, 8055; (b) M. Hara,
S. Samori, C. Xichen, M. Fujitsuka and T. Majima, J. Org. Chem.,
2005, 70, 4370; (c) T. Shida, Electronic Absorption Spectra of Radical
Ions, Elsevier, New York, 1988.
23 The one-electron oxidation potential was determined by second-
harmonic ac voltammetry in deaerated PhCN containing 0.1 mol dm23
Bu4NClO4.
+
?
24 S. Fukuzumi, J. Yuasa, N. Satoh and T. Suenobu, J. Am. Chem. Soc.,
2004, 126, 7585.
25 Stable fullerene radical cation has been reported by means of a decrease
This work was partially supported by a Grant-in-Aid (Nos.
16205020, 17750039) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan and by Grants BQU2002-
04513-C02-01 and MAT2005-07369-C03-02 from the Spanish
Government CICYT. Prof. Osamu Ito (Tohoku University,
Japan) is gratefully acknowledged for fruitful discussions of the
results.
+
of the interaction between C60 and counter anion.7 There is little
?
+
interaction between C60 and TNF 2 in the CS state. This may be the
?
?
reason why C60 + is long-lived in the present case.
?
26 R. A. Marcus and N. Sutin, Biochim. Biophys. Acta, 1985, 811, 265.
27 S. Fukuzumi, K. Ohkubo, H. Imahori and D. M. Guldi, Chem.–Eur. J.,
2003, 9, 1585.
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