A molecular Ce2@ih-C80 switch-unprecedented oxidative pathway in photoinduced charge transfer reactivity

Article abstract of DOI:10.1021/ja101856j

We report for the first time the versatile Ce₂@I _h-C₈₀ building block toward synthesizing a novel electron donor-acceptor conjugate, Ce₂@I_h-C₈₀-ZnP (1). A systematic investigation of the charge transfer chemistry documents a reductive charge transfer (i.e., formation of (Ce₂@I_h-C ₈₀)^?--(ZnP)^?+) in nonpolar media (i.e., toluene/THF), while an oxidative charge transfer (i.e., formation of (Ce₂@I_h-C₈₀)^?+-(ZnP) ^?-) dominates in polar media (i.e., benzonitrile/DMF). Reduction of the [Ce₂]⁶⁺ cluster, which is highly localized and collinearly arranged with respect to the quaternary bridge carbon, is sufficiently exothermic in all solvents. Notably weak is the electronic coupling between the [Ce₂]⁶⁺ cluster and the electron-donating ZnP. The oxidation of C₈₀^6- and the simultaneous reduction of ZnP, on the other hand, necessitate solvent stabilization. In such a case, the strongly exothermic (Ce₂@I_h-C₈₀) ^?--(ZnP)^?+ radical ion pair state formation is compensated within the framework of a nonadiabatic charge transfer by a C ₈₀^6-/ZnP electronic matrix element, as the sum of good overlap and short distance, that exceeds that for [Ce₂] ⁶⁺/ZnP.

Full text of DOI:10.1021/ja101856j

Published on Web 06/10/2010
A Molecular Ce2@Ih-C80 SwitchsUnprecedented Oxidative
Pathway in Photoinduced Charge Transfer Reactivity
Dirk M. Guldi,*,† Lai Feng,‡,§ Shankara Gayathri Radhakrishnan,† Hidefumi Nikawa,‡
Michio Yamada,‡ Naomi Mizorogi,‡ Takahiro Tsuchiya,‡ Takeshi Akasaka,‡
⊥
|
|
´
Shigeru Nagase, M. Angeles Herranz, and Nazario Mart´ın
Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials,
Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, 91058 Erlangen, Germany, Center for
Tsukuba AdVanced Research Alliance, UniVersity of Tsukuba, Tsukuba 305-8577, Japan,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, Department of
Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki
444-8585, Japan, and Departamento de Qu´ımica Orga´nica, Facultad de Qu´ımica, UniVersidad
Complutense, E-28040 Madrid, Spain
Received March 13, 2010; E-mail: guldi@chemie.uni-erlangen.de
Abstract: We report for the first time the versatile Ce2@Ih-C80 building block toward synthesizing a novel
electron donor-acceptor conjugate, Ce2@Ih-C80-ZnP (1). A systematic investigation of the charge transfer
chemistry documents a reductive charge transfer (i.e., formation of (Ce2@Ih-C80)•--(ZnP)•+) in nonpolar
media (i.e., toluene/THF), while an oxidative charge transfer (i.e., formation of (Ce2@Ih-C80)•+-(ZnP)•-)
dominates in polar media (i.e., benzonitrile/DMF). Reduction of the [Ce2]6+ cluster, which is highly localized
and collinearly arranged with respect to the quaternary bridge carbon, is sufficiently exothermic in all solvents.
Notably weak is the electronic coupling between the [Ce2]6+ cluster and the electron-donating ZnP. The
oxidation of C806- and the simultaneous reduction of ZnP, on the other hand, necessitate solvent stabilization.
In such a case, the strongly exothermic (Ce2@Ih-C80)•--(ZnP)•+ radical ion pair state formation is
compensated within the framework of a nonadiabatic charge transfer by a C806-/ZnP electronic matrix
element, as the sum of good overlap and short distance, that exceeds that for [Ce2]6+/ZnP.
Introduction
radiolytical oxidants. Still, direct oxidative charge transfer within
C60 electron donor-acceptor systems has not been observed.4
In recent years, the great versatility of empty fullerenes,
especially as integrative building blocks in electron donor-acceptor
conjugates/hybrids, has been established.1 Empty fullerenes
exhibit all of the hallmarks that are found in nature’s most
sophisticated and important solar energy conversion/storage
systems, namely, photosynthetic organisms in plants, algae, and
a variety of types of bacteria.2 Owing to its rigid structure, C60
possesses small reorganization energies in charge transfer
reactions, which is of particular interest and which, as a matter
of fact, has exerted noteworthy impact on the improvement of
charge separation processes triggered by light.3 The oxidation
of C60 requires strong chemical, photochemical, or pulse
Endohedral metallofullerenes (i.e., Mn@C2m; n ) 1-2 and
m ) 30-50) bear, in contrast to empty fullerenes, additional
atoms, ions, or clusters within their inner spheres. What renders
endohedral metallofullerenes particularly appealing is that a
significant control over the chemical and physical properties
has been realized by just changing the nature and composition
of the encapsulated species.5 For instance, M2@Ih-C80 (M )
Ce or La)6 reveal, in stark contrast to empty fullerenes, strong
electron-accepting and electron-donating features.7 To this end,
M2@C80 is expected to emerge as an adjustable building block
(4) Ohkubo, K.; Ortiz, J.; Mart´ın-Gomis, L.; Ferna´ndez-La´zaro, F.; Sastre-
´
Santos, A.; Fukuzumi, S. Chem. Commun. 2007, 589–591.
(5) Endofullerenes: A New Family of Carbon Clusters; Akasaka, T.,
Nagase, S., Eds.; Kluwer: Dordrecht, The Netherlands, 2002.
(6) (a) Suzuki, T.; Maruyama, Y.; Kato, T.; Kikuchi, K.; Nakao, Y.;
Achiba, Y.; Kobayashi, K.; Nagase, S. Angew. Chem. 1995, 107, 1228;
Angew. Chem., Int. Ed. Engl. 1995, 34, 1094-1096. (b) Ding, J.; Yang,
S. Angew. Chem. 1996, 108, 2369; Angew. Chem., Int. Ed. Engl. 1996,
35, 2234-2235. (c) Akasaka, T.; Nagase, S.; Kobayashi, K.; Waelchli,
M.; Yamamoto, K.; Funasaka, H.; Kako, M.; Hoshino, T.; Erata, T.
Angew. Chem. 1997, 109, 1716; Angew. Chem., Int. Ed. Engl. 1997,
36, 1643-1645. (d) Nishibori, E.; Takata, M.; Sakata, M.; Taninaka,
A.; Shinohara, H. Angew. Chem. 2001, 113, 3086; Angew. Chem.,
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Taninaka, A.; Shinohara, H.; Nishibori, E.; Takata, M.; Sakata, M.
J. Am. Chem. Soc. 2004, 126, 364–369.
† Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg.
‡ University of Tsukuba.
§ Chinese Academy of Sciences.
⊥ Institute for Molecular Science.
| Universidad Complutense.
(1) (a) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science
1995, 270, 1789–1791. (b) Rand, B. P.; Xue, J.; Uchida, S.; Forrest,
S. R. J. Appl. Phys. 2005, 98, 124902. (c) Peet, J.; Heeger, A. J.;
Bazan, G. C. Acc. Chem. Res. 2009, 42, 1700–1708.
(2) Collings, A. F.; Critchley, C. Artificial Photosynthesis: From Basic
Biology to Industrial Application; Wiley-VCH: Weinheim, Germany,
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9
9078 J. AM. CHEM. SOC. 2010, 132, 9078–9086
10.1021/ja101856j  2010 American Chemical Society

A Molecular Ce2@Ih-C80 Switch
A R T I C L E S
Figure 2. ORTEP view of one of the enantiomers of 2; the solvent
molecules (i.e., CS2) are omitted for clarity.
the charge transfer activity, that is, from a reductive to an
oxidative mechanism, which are controlled by solvent polarity.
The syntheses of references 2 and 1 were carried out via a [2
+ 1]-cycloaddition reaction9 of diazo precursors10 with Ce2@Ih-
C80 affording 2 and 1 as major products. MALDI-TOF mass
spectrometry confirmed the integrity of 2 and 1 with molecular
ion peaks of m/z 1429.52 and 2136.18, respectively. Importantly,
single-crystal X-ray analyses, as shown in Figure 2, shed light
onto the structure of 2, namely, a [6,6]-fulleroid.11 Additional
evidence for the [6,6]-fulleroid structure came from 13C NMR
studies, which illustrate that in 2 the original π-system of
Ce2@Ih-C80 is sustained. Notable, this feature contrasts the [6,6]-
methanofullerene structure of C60, where the number of π-elec-
trons is reduced to 58.9a The [Ce2]6+ cluster is highly localized
and collinearly arranged relative to the quaternary bridge carbon
of C806-, much like that in previously reported carbene adducts
of La2@Ih-C80.12
Figure 1. Structures of Ce2@Ih-C80-ZnP (1) and Ce2@Ih-PCBM (2).
that takes on the role of either an electron acceptor (i.e.,
applicable toward the reduction of CO2) or an electron donor
(i.e., applicable toward the oxidation of water). Here, we report
for the first time using M2@Ih-C80 toward synthesizing a unique
M2@Ih-C80 containing an electron donor-acceptor conjugate,
Ce2@Ih-C80-ZnP (1) (Figure 1). A systematic investigation of
the charge transfer chemistry documents the malleable features
of Ce2@Ih-C80, that is, giving rise to reductiVe charge transfer
in nonpolar media, while oxidatiVe charge transfer dominates
in polar media. Key to the charge transfer outcome is a fine
balance in the thermodynamic driving force and in electronic
coupling. Thus, the use of endohedral metallofullerenes as an
electron donor in electron donor-acceptor conjugates opens
new avenues for carbon-based materials in the context of charge
transfer processes and photovoltaic applications, especially
considering that, so far, fullerenes act solely as electron acceptors
in photo- and redox-active electron donor-acceptor conjugates.
The structures of 2 and 1 were further corroborated by NMR
1
studies, including H NMR, 13C NMR, COSY, and HMQC
experiments. Notable are the following magnetic features of 2
and 1. First, 2 and 1 are paramagnetic owing to the two
f-electrons of the [Ce2]6+ cluster.8 Second, 2 and 1 are
magnetically anisotropic due to magnetic dipole interactions
operative between the [Ce2]6+ cluster, on one hand, and the
Results and Discussion
Endohedral metallofullerenes possess reduction potentials
comparable to that of C60, C70, C84, etc. Additionally, unlike
the latter, their oxidation potentials are close to those of widely
used electron donors such as metalloporphyrins, tetrathiaful-
valenes, etc.5,7 As a consequence of encapsulating a dimetallic
cluster (i.e., [M2]6+ cluster), M2@Ih-C80 has 86 π-electronss80
(9) (a) Hummelen, J. C.; Knight, B. W.; LePeq, F.; Wudl, F.; Yao, J.;
Wilkins, C. L. J. Org. Chem. 1995, 60, 532–538. (b) Ross, R. B.;
Cardona, C. M.; Guldi, D. M.; Gayathri, S. S.; Reese, M. O.;
Kopidakis, N.; Peet, J.; Walker, B.; Bazan, G. C.; Van Keuren, E.;
Holloway, B. C.; Drees, M. Nat. Mater. 2009, 8, 208–212. (c) Shu,
C.; Xu, W.; Slebodnick, C.; Champion, H.; Fu, W.; Reid, J. E.;
Azurmendi, H.; Wang, C.; Harick, K.; Dorn, H. C.; Gibson, H. W.
Org. Lett. 2009, 11, 1753–1756.
8
plus 6 π-electrons from C80 and [M2]6+, respectively. Such
a feature is important for stabilizing unusual redox states under
both reductive and oxidative conditions. In this light, we
designed a strategy to link Ce2@Ih-C80 to a zinc tetraphenylpor-
phyrin (i.e., ZnP) via a flexible 2-oxyethyl butyrate spacer. ZnP
with its broad absorption cross section in the visible range of
the solar spectrum generates compelling excited states that are
either strong oxidants or strong reductants. In short, Ce2@Ih-
C80-ZnP (1) should satisfy all the criteria necessary to switch
6-
(10) See Supporting Information.
(11) Crystal data for 2·2CS2: C94H14Ce2O2S4, Mr ) 1583.53, black plate,
0.41 × 0.30 × 0.04 mm, monoclinic, space group P21/c, a )
14.5938(15) Å, b ) 11.0075(9) Å, c ) 32.940(3) Å, ꢀ ) 99.129(4)°,
V ) 5224.5(9) Å3, Z ) 4; Fcalcd ) 2.018 g cm-3, µ(Mo KR) ) 1.951
mm-1, θmax ) 32.030, T ) 120 K, 83 764 total collected reflections,
18 107 unique reflections, 1112 refined parameters, GOF ) 1.118, R1
) 0.0994 and wR2 ) 0.2653 for all data; R1 ) 0.0901 for 15 772
independent reflections (I > 2.0σ(I)), min/max electron density 7.840/
-5.396 eÅ-3
.
(8) (a) Kobayashi, K.; Nagase, S. Chem. Phys. Lett. 1996, 262, 227–232.
(b) Yamada, M.; Mizorogi, N.; Tsuchiya, T.; Akasaka, T.; Nagase, S.
Chem.sEur. J. 2009, 15, 9486–9493.
(12) Yamada, M.; Someya, C.; Wakahara, T.; Tsuchiya, T.; Maeda, Y.;
Akasaka, T.; Yoza, K.; Horn, E.; Liu, M. T. H.; Mizorogi, N.; Nagase,
S. J. Am. Soc. Chem. 2008, 130, 1171–1176.
9
J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 9079

A R T I C L E S
Guldi et al.
1
Table 1. Partial H NMR Chemical Shifts (ppm) of 2 and 1 (300 MHz, in C2D2Cl4, 293 K)
a
Arpara-H (P4)
Armeta-H′ (P5)
Armeta-H (P3)
Arortho-H′ (P6)
Arortho-H (P2)
CH2CO2 (S3)
CCH2CH2 (S2a/b)
ArCCH2 (S1a/b)
2
1
4.04
4.10
2.98
3.13
2.41
2.57
-5.12
-4.87
-6.34
-6.09
-3.28
-3.43
-10.17/-9.79
-10.09/-9.74
-18.10/-17.60
-17.96/-17.43
a Ar refers to the phenyl group that is next to the bridgehead carbon in 2 and 1.
protons and carbons outside the C806- cage, on the other hand.13
Finally, the chemical shifts of all protons (2 and 1) depend, in
part, on their geometrical positions relative to the [Ce2]6+ cluster,
which is described by eq 1.14
lecular interactions. Figure 3b and Figure S7c (Supporting
Information) document that within the 6-12 ppm window a
total of 26 1H signals emerged. With the help of 2D-DQFCOSY,
NOESY, and HSQC experiments (i.e., 293 and/or 283 K), all
these 1H signals were unambiguously assigned to the porphyrin
protons (27H).10 The diagnostic NOE correlations between the
methylene protons (S6) and the phenyl protons (5b/5d) of ZnP
were taken as spectroscopic evidence for the linking of the
spacer to ZnP.10 It should be noted that the chemical shifts of
(i) the ꢀ-pyrrole protons, (ii) the ortho-, and (iii) the meta-phenyl
protons of ZnP appear separately. We conclude from the
aforementioned findings the hindered rotations of the ZnP phenyl
groups in the presence of Ce2@C80 and nonequivalent ꢀ-pyrrole
and phenyl protons. Only a close spatial Ce2@Ih-C80/ZnP
proximity, as it is known for several C60/ZnP conjugates,16
would support such a scenario. In this respect, the 2-oxyethyl
butyrate spacer is key to furnish close Ce2@Ih-C80/ZnP contacts.
Notably, C60/ZnP conjugates bearing long and flexible spacers
C(3cos2 θi - 1)
1
T2
δ ) δdia
+
(1)
∑
γi3
i)Ce(1),Ce(2)
Therefore, it is not unexpected that the presence of unpaired
f-electrons (i.e., [Ce2]6+ cluster) evokes distinguished upfield
shifts of the phenyl as well as the methylene protons (2 and 1).
Overall, significant similarities in the chemical shifts (see Table
1) reflect the structural similarity between 2 and 1. Notable is
the separation of the ortho- and meta-phenyl protons at 293 K:
∆δo-ArH ) 1.22 ppm and ∆δm-ArH ) 0.56 ppm (Figure S4 in the
Supporting Information and Figure 3a). The latter implies
rotations of the phenyl groups that are much slower than the
1H NMR time scale. The geminal methylene protons (S1a/b and
S2a/b) adjacent to the quaternary bridge carbon are also affected.
Their chemical shifts differ by ca. 0.5 and 0.4 ppm, respectively,
suggesting different magnetic environments. The overall dif-
ference vanishes, however, as the distance between the meth-
1
give rise to broad and degenerated H NMR signals of ZnP
protons due to their intramolecular flexibility.17 In spite of this
1
flexibility, the well-resolved H NMR spectra of 1 prompt a
rather rigid Ce2@Ih-C80/ZnP conformation. Considering the
previous precedents involving C60, it is safe to assume that the
giant π-system of C806- with highly delocalized negative charges
further augment such structural arrangements.
A molecular model of 1 (see Figure 3c) reveals (i) a folded
conformation, (ii) close C806-/ZnP proximity, and (iii) asym-
metric position of ZnP relative to the Ce-Ce axis.18 Such an
arrangement leads, indeed, to a magnetically and unequivalently
affected ZnP.
6-
ylene/methyl protons and C80 increases. Implicit is that both
the anisotropism of magnetic field imparted by the [Ce2]6+
cluster and the shielding from either the five- or six-membered
rings15 fall off sharply with distance.
Variable-temperature NMR experiments from 283 to 308 K
were performed to dissect possible Ce2@Ih-C80/ZnP intramo-
(13) (a) Wakahara, T.; Kobayashi, J.; Yamada, M.; Maeda, Y.; Tsuchiya,
T.; Okamura, M.; Akasaka, T.; Waelchi, M.; Kobayashi, K.; Nagase,
S.; Kato, T.; Kako, M.; Yamamoto, K.; Kadish, K. M. J. Am. Chem.
Soc. 2004, 126, 4883–4887. (b) Yamada, M.; Nakahodo, T.; Wakahara,
T.; Tsuchiya, T.; Maeda, Y.; Akasaka, T.; Kako, M.; Yoza, K.; Horn,
E.; Mizorogi, N.; Kobayashi, K.; Nagase, S. J. Am. Chem. Soc. 2005,
127, 14570–14571. (c) Yamada, M.; Wakahara, T.; Lian, Y.; Tsuchiya,
T.; Akasaka, T.; Waelchli, M.; Mizorogi, N.; Nagase, S.; Kadish, K. M.
J. Am. Chem. Soc. 2006, 128, 1400–1401. (d) Yamada, M.; Wakahara,
T.; Tsuchiya, T.; Maeda, Y.; Kako, M.; Akasaka, T.; Yoza, K.; Horn,
E.; Mizorogi, N.; Nagase, S. Chem. Commun. 2008, 558–560.
(14) (a) The chemical shift (δ) of a paramagnetic molecule in solution is
generally expressed as a sum of three contributions, namely, the
diamagnetic (δdia), Fermi contact (δfc), and pseudocontact (δpc) shifts.
Note that δfc makes much less contribution than δpc in the case of
metallofullerene derivatives. The chemical shift (δ) is briefly expressed
as eq 1, where r is the distance between Ce and the NMR-active
nucleus, θ the angle between the r vector and the vertical axis on
which the two Ce atoms are located, and C is a common constant. (b)
Bleaney, B. J. Magn. Reson. 1972, 8, 91–100. (c) Yamada, M.;
Okamura, M.; Sato, S.; Someya, C.; Mizorogi, N.; Tsuchiya, T.;
Akasaka, T.; Kato, T.; Nagase, S. Chem.sEur. J. 2009, 15, 10533–
10542.
Electronic Ce2@Ih-C80/ZnP interactions were also inferred
from the electronic absorption spectra. Although the Soret band
absorption in 1 (i.e., 426 nm) is quite close to that seen in the
corresponding C60-ZnP electron donor-acceptor system 3 (i.e.,
428 nm),10 remarkable changes evolve in range of the Q-band
absorptions of 1 (i.e., 555 and 594 nm) and 3 (i.e., 551 and 588
nm). Such shifts are particularly helpful because they emerge
as sensitive markers for determining the C806-/ZnP electronic
19
coupling of 400 ( 30 cm-1
.
Less pronounced are the differences in the electrochemical
assays. Overall, the redox potentials of Ce2@Ih-C80 1 and 2 are
quite similar (see Table 2). According to DFT calculations, the
LUMO (splitting at -4.50 and -4.01 eV) and HOMO (-5.38
eV) in Ce2@Ih-C80 involve [Ce2]6+ and C806-, respectively.8b
This places the first/second reduction potential at -0.36/-1.67
V (Ce2@Ih-C80) and -0.42/-1.75 V (2) on [Ce2]6+. Oxidation
(15) (a) Haddon, R. C. Nature 1995, 378, 249–255. (b) Saunder, S M.;
Cross, R. J.; Jime´nez-Va´zquez, H. A.; Shimshi, R.; Khong, A. Science
1996, 271, 1693–1697.
(17) (a) Dietel, E.; Hirsch, A.; Zhou, J.; Rieker, A. J. Chem. Soc., Perkin
Trans. 2 1998, 1357–1364. (b) MacMahon, S.; Wilson, S. R.; Schuster,
D. I. Proc. Elec. Chem. Soc. 2000, 8, 155–160. (c) MacMahon, S.;
Fong, R.; Baran, P. S.; Safonov, I.; Wilson, S. R.; Schuster, D. I. J.
Org. Chem. 2001, 66, 5449–5455.
(16) (a) Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.; Taniguchi, S.;
Okada, T.; Shirakawa, M.; Sakata, Y. J. Am. Chem. Soc. 1996, 118,
11771–11782. (b) Boyd, P. D. W.; Hodgson, M. C.; Rickard, C. E. F.;
Oliver, A. G.; Chaker, L.; Brothers, P. J.; Bolskar, R. D.; Tham, F. S.;
Reed, C. A. J. Am. Chem. Soc. 1999, 121, 10487–10495. (c) Schuster,
D. I. Carbon 2000, 38, 1607–1614. (d) Schuster, D. I.; Jarowski, P. D.;
Kirschner, A. N.; Wilson, S. R. J. Mater. Chem. 2002, 12, 2041–
2047. (e) Pe´rez, E. M.; Mart´ın, N. Chem. Soc. ReV. 2008, 37, 1512–
1519. (f) Lembo, A.; Tagliatesta, P.; Cicero, D.; Leoni, A.; Salvatori,
A. Org. Biomol. Chem. 2009, 7, 1093–1096.
(18) Molecular model of 1 shows an energy minimized structure obtained
with simulated annealing plus geometry optimization using the Forcite
of MS modeling. The Ce2@Ih-C80 atomic positions are imported from
the X-ray structure of 2 and kept fixed during the modeling.
(19) Guldi, D. M.; Hirsch, A.; Scheloske, M.; Dietel, E.; Troisi, A.; Zerbetto,
F.; Prato, M. Chem.sEur. J. 2003, 9, 4968–4979.
9
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A Molecular Ce2@Ih-C80 Switch
A R T I C L E S
1
Figure 3. (a,b) H NMR spectra (300 MHz) of 1 in C2D2Cl4 at 293 K with the marked signals (*) originating from impurities. (c) Two side views of a
molecular model of 1.18 The assignments are based on the 2D-DQFCOSY and NOESY experiments.
Table 2. Redox Potentials of 1 and Reference Compoundsa
nearly solvent-independently quenched. Quantum yields of
∼0.004 suggest a “through-space” mechanism that is operative
Eo4x
Eo3x
Eo2x
Eo1x
Er1ed
Er2ed
Er3ed
Er4ed
in deactivating the ZnP singlet excited state, either charge or
energy transfer. The view of through-space interactions is in
sound agreement with the conclusion of the spectroscopic
characterization (vide supra). A closer analysis (see Figure 4)
brings nevertheless some subtle changes to light. In toluene
(0.002) and THF (0.004), the quenching appears to be stronger
than in benzonitrile (0.006) and DMF (0.006). We assign this
preliminary data to a change in electronic coupling between
electron donor and electron acceptor.
Ce2@C80
2
1
ZnP
0.95 0.55 -0.36 -1.67 -2.21
0.91 0.48 -0.42 -1.75 -2.23
0.98/0.85b 0.63 0.55 0.35 -0.43 -1.76 -1.91 -2.25c
0.65
0.39
-1.94 -2.27
a All of the potentials, in volts, were measured relative to the Fc0/+
.
b Splitted one-electron oxidation process. c Two-electron reduction
process.
6-
at around +0.5 V occurs, on the other hand, exclusively at C80
.
To further exploit interactions between photoexcited Ce2@Ih-
C80 and ZnP, we turned to complementary transient absorption
measurements. Up front, the excited state properties of pristine
ZnP (Figure 5) and 2 (Figure 6) shall be discussed because they
serve as important reference points for the interpretation of 1.
The differential absorption changes recorded upon excitation
of the ZnP reference are characterized by bleaching of the ZnP
Q-band absorption at 550 nm and broad absorption between
Subtle differences were also evidenced for the reduction of ZnP
in 1 (-1.91/-2.25 V) and its reference (-1.94/-2.27 V) as
well as the oxidation of ZnP in 1 (0.35/0.63 V) and its reference
(0.39/0.65 V).
Insights into excited state interactions between Ce2@Ih-C80
and ZnP came from steady-state fluorescence measurements.
In this context, pristine ZnP evolved as a particular convenient
marker owing to its high and nearly solvent-independent
fluorescence quantum yield of 0.04.20 At first glance, the ZnP-
centered fluorescence with maxima at 605 and 650 nm in 1 is
(20) D’Souza, F.; Ito, O. Chem. Commun. 2009, 4913–4928.
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J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 9081

A R T I C L E S
Guldi et al.
Figure 4. Steady-state fluorescence spectra of 1 in solvents of different
polarity with matching absorption at the excitation wavelength (i.e., OD426nm
) 0.1).
Figure 6. Top: Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (387 nm) of 2 (∼10-5 M) in
argon-saturated toluene with several time delays between 0 and 250 ps at
room temperature (see legend for details about time progression). Bottom:
Time-absorption profiles of the spectra shown above at 550 and 1000 nm
monitoring the intersystem crossing.
In the femtosecond transient absorption measurements of 1,
immediately after the laser excitation, the strong singlet-singlet
absorptions of ZnP are discernible. This confirms, despite the
presence of Ce2@Ih-C80, the successful formation of the ZnP
singlet excited states. Instead of seeing, however, the slow
growth of the ZnP triplet features, the singlet-singlet absorp-
tions decay with accelerated dynamics in toluene (1.2 × 1012),
THF (1.3 × 1012), benzonitrile (1.4 × 1012), and DMF (2.3 ×
1012 s-1). Spectroscopically, the transient absorption changes
taken after the completion of the decay bear no resemblance to
the ZnP triplet excited state. Instead, in THF, the new transient,
as shown in Figure 7, is ascribed to that of the (Ce2@Ih-C80)•--
(ZnP)•+ radical ion pair state. In the visible region, the broad
absorption in the 600-700 nm region corresponds to the one-
electron oxidized ZnP. On the other hand, in the near-infrared
region, the broad maximum between 900 and 1200 nm
resembles the spectroelectrochemical signature of the one-
electron reduced Ce2@Ih-C80 (see Figure S1 in the Supporting
Information).
Figure 5. Top: Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (387 nm) of pristine ZnP (∼10-5
M) in argon-saturated toluene with several time delays between 0 and 3000
ps at room temperature (see legend for details about time progression).
Bottom: Time-absorption profiles of the spectra shown above at 700 and
850 nm monitoring the intersystem crossing.
570 and 750 nm.20 These spectral attributes reflect the ZnP
singlet excited state and decay slowly (4.0 × 108 s-1) to the
energetically lower-lying triplet excited state with a characteristic
maximum at 840 nm predominantly via intersystem crossing.
On the contrary, the singlet excited state features of 2 are
dominated by a rather broad singlet-singlet transition that spans
from 400 to 800 nm. The latter undergoes a rapid intersystem
crossing (3 × 1010 s-1) to yield the triplet manifold. Spectro-
scopically, a 550 nm maximum is noted for the triplet.
Interestingly, a closer inspection of the transient absorption
changes seen in benzonitrile and DMF reveal characteristics that
differ from those seen in toluene and THF. In particular, Figure
8 displays that in the visible region new minima and maxima
develop at 555/595 nm and 465/530/580 nm, respectively.
Notable is the sharp discrepancy relative to the features known
for the one-electron oxidized ZnP.
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9082 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010

A Molecular Ce2@Ih-C80 Switch
A R T I C L E S
Figure 8. Top: Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (387 nm) of 1 (∼10-5 M) in
argon-saturated DMF with several time delays between 0 and 1000 ps at
room temperature (see legend for details about time progression). Bottom:
Time-absorption profiles of the spectra shown above at 460, 530, and 808
nm, monitoring the charge separation and charge recombination.
Figure 7. Top: Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (387 nm) of 1 (∼10-5 M) in
argon-saturated THF several time delays between 0 and 50 ps at room
temperature (see legend for details about time progression). Bottom: Time-
absorption profiles of the spectra shown above at 555 and 580 nm,
monitoring the charge separation.
-∆G°). This is unmistakably secured in toluene (-1.18 eV),
THF (-1.47 eV), benzonitrile (-1.56 eV), and DMF (-1.57
eV). In contrast, the ZnP singlet excited state (2.1 eV) renders
in ortho-dichlorobenzene as energetically insufficient for driving
the oxidation of C80 and the simultaneous reduction of ZnP
(2.3 eV).
The energy diagram in Figure 10 summarizes the aforemen-
tioned. In particular, to bring the (Ce2@Ih-C80)•+-(ZnP)•- radical
ion pair state energy in 1, however, below that of the singlet
excited state of ZnP and, in turn, to activate this charge transfer
pathway necessitates solvent stabilization (i.e., ∆Gsolvent), ben-
zonitrile (0.28 eV) and DMF (0.31 eV). The strongly exothermic
(Ce2@Ih-C80)•--(ZnP)•+ radical ion pair state formation is
compensated within the framework of a nonadiabatic charge
transfer by an C806-/ZnP electronic matrix element as the sum
of good overlap and short distance that exceeds that for [Ce2]6+/
ZnP. Please note that the Ce2@Ih-C80 singlet excited state energy
(1.0 eV) fails to play any significant function in any charge
transfer chemistry.
Extending our spectroelectrochemical investigations into the
cathodic region shed light onto the nature of this transient (see
Figure 9). When applying a potential of -1.5 V (sufficient to
reduce ZnP), the corresponding spectral changes are an exact
match of what was observed during the femtosecond transient
absorption measurements. Likewise, in the near-infrared, an
apparent mismatch with the one-electron reduced Ce2@Ih-C80
is seen. Only anodic conditions in the spectroelectrochemistry
led to the expected agreement, that is, a maximum at 860 nm
(see Figure 9). In other words, the close spectral resemblance
of spectroelectrochemistry and photochemistry prompts the
unprecedented formation of the (Ce2@Ih-C80)•+-(ZnP)•- radical
ion pair state. The time absorption profiles indicate that this
highly exergonic state is metastable. In fact, multiwavelength
analyses provide lifetimes of 55 and 115 ps in benzonitrile and
DMF, respectively. It is, however, the triplet excited state of
Ce2@Ih-C80 that is formed as the stable product of charge
recombination. Support for this assumption came from comple-
mentary nanosecond experiments, where a long-lived transient
is seen, in agreement with the reference experiments performed
with 2.
6-
Conclusions
A likely rationale for the change in reactivity, that is, toluene/
THF versus benzonitrile/DMF, is associated with the energies
of the (Ce2@Ih-C80)•--(ZnP)•+ (0.78 eV) and (Ce2@Ih-C80)•+-
(ZnP)•- (2.3 eV) radical ion pair states. Reduction of the [Ce2]6+
cluster, which is, on one hand, highly localized and collinearly
arranged relative to the quaternary bridge carbon but, on the
other hand, weakly coupled electronically to the electron-
donating ZnP, requires a sufficient free energy change (i.e.,
To summarize, our investigations have shed light on the
nature of Ce2@Ih-C80 in charge transfer chemistry. In particular,
a remarkable electron donor-acceptor conjugate, involving
Ce2@Ih-C80 and ZnP, has been synthesized and characterized.
Detailed spectroscopic analyses revealed unusually strong
Ce2@Ih-C80/ZnP interactions, despite employing a flexible
2-oxyethyl butyrate linkage. These interactions are driven and
enforced by a giant π-system with highly delocalized negative
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A R T I C L E S
Guldi et al.
Time-Resolved Absorption: Femtosecond transient absorption
studies were performed with 387 nm laser pulses (1 kHz, 150 fs
pulse width) from an amplified Ti:sapphire laser system (Clark-
MXR, Inc.), and the laser energy was 200 nJ.
Electrochemistry: Differential pulse voltammetry (DPV) and
cyclic voltammetry (CV) were carried out in o-DCB using a BAS
CW-50 instrument. A conventional three-electrode cell consisting
of a platinum working electrode, a platinum counter-electrode, and
a saturated calomel reference electrode (SCE) was used for both
measurements. (n-Bu)4NPF6 was used as the supporting electrolyte.
All potentials were recorded against a SCE reference electrode and
corrected against Fc/Fc+. DPV and CV were measured at a scan
rate of 20 and 50 mV s-1, respectively. The spectroelectrochemical
measurements were done on a Varian Cary 5000 UV-vis-NIR
spectrophotometer connected to a Princeton PGstat 263A using a
home-made cell with three-electrode configuration. A light transpar-
ent platinum gauze, a platinum plate, and a silver wire were
employed as the working, counter, and reference electrodes,
respectively, in an analyte solution of o-dichlorobenzene containing
0.2 M tetrabutylammonium perchlorate supporting electrolyte. The
absorbances of the analyzed compounds were fixed between 0.1
and 0.15. The path length of the cell was determined to be 2.3
mm. The desired potential was applied, and the system output was
measured for around 20 cycles. The difference between the spectrum
with and without an applied potential was plotted as ∆OD.
Radiolysis: Pulse radiolysis experiments were performed using
50 ns pulses of 15 MeV electrons from a linear electron accelerator
(LINAC). Dosimetry was based on the oxidation of SCN- to
(SCN)2•- which in aqueous, N2O-saturated solution takes place with
G ≈ 6 (G denotes the number of species per 100 eV, or the
approximate µM concentration per 10 J absorbed energy). The
radical concentration generated per pulse was varied between 1 ×
10-6 and 3 × 10-6 M.
Figure 9. Top: Differential absorption spectrum (visible and near-infrared)
obtained upon electrochemical reduction of ZnP (∼10-5 M) at an applied
bias of -1.6 V in argon-saturated o-dichlorobenzene at room temperature.
Bottom: Differential absorption spectrum (visible and near-infrared) obtained
upon electrochemical oxidation of 2 (∼10-5 M) at an applied bias of +0.6
V in argon-saturated o-dichlorobenzene at room temperature.
Materials: All chemicals were of reagent grade and purchased
from Wako. Ce2@Ih-C80 was produced and separated according to
a previously reported method.5 Preparative and analysis HPLC was
performed on Buckyprep column (ø25 × 100 mm, Cosmosil) and
Buckyprep column, 5PYE column, and Buckyclutcher column (ø4.6
× 100 mm, Cosmosil), respectively. Toluene was used as eluent.
Synthesis: The synthesis of the diazo precursor (6•2H) and dyad
3 are described in the Supporting Information.10
charges. Most impressive is the unprecedented change in charge
transfer chemistry in toluene/THF versus benzonitrile/DMF. In
this context, the different strength in electronic coupling, [Ce2]6+/
ZnP versus C806-/ZnP, affects the charge transfer kinetics and
the charge transfer mechanism, that is, formation of (Ce2@Ih-
C80)•--(ZnP)•+ or (Ce2@Ih-C80)•+-(ZnP)•-. Especially the latter
reactivity pattern renders Ce2@Ih-C80 a promising and unprec-
edented p-type fullerene material for a solar energy conversion
system. Likewise, it creates new possibilities for fundamental
process of water oxidation triggered by light.21
1-(3-(Methoxycarbonyl)propyl)-1-phenyl[6,6]Ce2@C81 (2).Meth-
yl 4-benzoylbutyrate p-tosylhydrazone (1.4 mg, 0.0037 mmol) and
a small amount of NaOMe (ca. 0.5 mg, 0.0092 mmol) were
dissolved in 60 µL of dry pyridine under N2 for 10 min. A solution
of 1.5 mg (0.0012 mmol) of Ce2@C80 in 2.5 mL of o-DCB was
added, and the mixture was stirred at 75 °C for 2 h. After cooling
to room temperature, pyridine was removed under vacuum. The
reaction mixture was filtered, and o-DCB was removed under
vacuum. A solution of CS2/acetone (v/v ) 1/1) with 3-4 µL of
CHCl2COOH was added to dissolve the solid. The mixture was
stirred for 15 min, and 10 mL of toluene was added. After removing
CS2/acetone by vacuum, the resulting toluene solution was subjected
to HPLC analysis and separation. 2 was separated as a predominant
product: yield ca. 62% based on consumed Ce2@C80; 1H NMR (300
MHz, C2D2Cl4, 293 K) (paramagnetic) δ ) 4.10 (t, 3J ) 7.04 Hz,
1H, p-ArH), 3.13 (t, 3J ) 7.71 Hz, 1H, m-ArH), 2.57 (t, 3J ) 7.61
Hz, 1H, m-ArH), 2.23 (s, 3H, OCH3), -3.43 (t, 3J ) 7.61 Hz, 2H,
CH2CH2CO), -4.87 (d, 2J ) 8.11 Hz, 1H, o-ArH), -6.09 (d, 2J )
8.42 Hz, 1H, o-ArH), -9.74 (br m, 1Ha, PhCCH2CH2), -10.09
(br m, Hb, PhCCH2CH2), -17.43 (br t, Ha, PhCCH2CH2), -17.96
(br t, Hb, PhCCH2CH2); 13C NMR (125 MHz, C2D2Cl4, 288 K)
(paramagnetic) δ ) 198.50, 198.29, 194.12, 192.80, 189.66, 186.84,
186.73, 183.84, 183.42, 174.74, 174.48, 173.15, 172.73, 172.20,
171.71, 171.65, 171.17, 170.46, 169.43 (2C), 169.32, 169.16
(CdO), 165.90, 164.77, 161.99, 161.80, 160.77, 160.35, 159.92,
159.74, 159.53, 159.48, 158.25, 157.81, 157.15, 156.80 (2C),
156.12, 154.84, 154.69, 150.48, 150.35, 149.14, 148.89, 148.60,
148.36, 147.35, 147.24, 146.43, 146.24, 146.11, 144.57, 144.53,
Experimental Section
Spectroscopy: All NMR spectra were recorded on a Bruker AC
300 spectrometer or Bruker AV 500 spectrometer with a CryoProbe
system, locked on deuterated solvents and referenced to the solvent
peak. The 1D and 2D experiments (COSY, DQF-COSY, NOESY,
HMQC, and HSQC) were performed by means of standard
experimental procedures of the Bruker library. Absorption spectra
of all samples were recorded in toluene with a Shimadzu UV-3150
spectrometer using a quartz cell and 1 nm resolution. Matrix-assisted
laser desorption-ionization time-of-flight (MALDI-TOF) mass
spectra were recorded with a Bruker BIFLEX-III mass spectrometer
using 1,1,4,4-tetraphenyl-1,3-butadiene as the matrix. The measure-
ments were performed in both positive and negative ion modes.
Steady-State Emission: The spectra were recorded on a Fluo-
roMax 3 fluorometer (vis detection) and on a Fluorolog spectrometer
(NIR detection). Both spectrometers were built by HORIBA
JobinYvon. The measurements were carried out at room temperature.
(21) Messinger, J. ChemSusChem. 2009, 2, 47–48.
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9084 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010

A Molecular Ce2@Ih-C80 Switch
A R T I C L E S
Figure 10. Energy diagram for 1 illustrating the different excited state deactivation pathways that lead in toluene and THF (left-hand side) and benzonitrile
and DMF (right-hand side) to different charge transfer products.
142.98, 141.99, 138.28, 137.95, 137.59, 137.44, 135.86, 133.75,
133.51, 129.28, 129.07, 127.49 (Ph), 125.04, 124.62, 123.28 (Ph),
122.80 (Ph), 122.38 (Ph), 118.94, 118.81, 114.70 (Ph), 114.02 (Ph),
103.24, 99.95, 93.27, 87.34, 82.89, 82.20, 81.99, 77.76, 50.61
(OCH3), 35.70, 26.78 (CH2CO2), 23.55 (br, CH2CH2CO2), 9.21,
7.85, 4.81 (br, PhCCH2), -3.58 (spiro carbon); MS (MALDI-TOF)
m/z ) 1429.52 [M]+, calcd for Ce2C92H14O2 m/z ) 1429.91.
Ce2@Ih-C80-ZnP (1). 6•2H (4.5 mg, 4.1 × 10-3 mmol) and a
small amount of NaOMe (ca. 0.5 mg, 0.0092 mmol) were dissolved
in pyridine (150 µL) and stirred for 15 min under N2. Then,
Ce2@C80 (1.13 mg, 9.1 × 10-4 mmol) in 1.5 mL of o-DCB was
added. The mixture was stirred at 80 °C for 2 h under N2. After
cooling the reaction mixture, 500 µL of saturated Zn(OAc)2/MeOH
was added and the mixture was stirred for 30 min. After removing
the solvent under vacuum, the solid was dissolved in toluene and
filtered. The final reaction mixture underwent HPLC analysis. 1
was facilely separated and purified by two-step HPLC process: yield
181.96, 179.93, 173.50, 172.41, 171.07, 170.96, 170.70, 169.57,
169.01, 168.91, 168.76, 168.49 (CdO), 168.37, 167.29, 166.97,
164.59, 162.81, 161.38, 159.47, 159.40, 158.46, 158.38, 158.09,
157.16, 156.79, 156.72, 156.57, 156.41, 155.54, 155.36, 155.19,
154.71, 153.92, 153.21, 153.05, 153.00, 152.92, 152.60, 152.51,
152.48, 152.22, 152.09, 148.92, 148.37, 148.18, 147.52, 147.14,
146.29, 145.87, 145.44, 145.21, 145.00, 144.62, 144.32, 143.92,
143.61, 143.01, 142.01, 141.92, 141.14, 137.02, 136.89, 136.46,
136.29, 136.17, 135.96, 135.38, 134.85, 134.60, 134.11, 133.96,
133.91, 133.73, 133.63, 133.58, 133.37, 133.09, 132.88, 132.39,
129.32, 129.24, 128.70, 128.60, 128.55, 128.40, 127.84, 127.71,
127.58, 127.29 (Ph), 126.94, 126.81, 126.25, 124.62, 124.28,
123.81, 123.69, 123.27, 123.18, 123.10, 122.81 (Ph), 122.69 (Ph),
122.38, 119.09, 117.31, 114.72 (Ph), 114.08 (Ph), 112.64, 104.65,
101.76, 99.32, 92.94, 87.04, 83.86, 83.26, 81.61, 65.91 (OCH2),
62.05 (CH2OCO), 37.79, 27.29 (CH2CO2), 23.05 (br, CH2CH2CO2),
10.05, 8.57, 4.83 (br, PhCCH2), -0.96 (spiro carbon); MS (MALDI-
TOF) m/z ) 2136.18 [M + 2H]+, calcd for Ce2C137H42N4O3Zn m/z
) 2134.06.
Crystallography: Single crystals of 2•2CS2 suitable for X-ray
diffraction were obtained from slow diffusion between 2/CS2 and
hexane. Data were collected on a Rigaku Raxis-Rapid IP diffrac-
tometer using monochromatized Mo KR radiation (λ ) 0.71075
Å) at 120 K. The structure was solved by direct method and refined
by the full-matrix least-squares method. All non-hydrogen atoms
were refined with anisotropic thermal parameters, whereas all
hydrogen atoms were calculated using a riding model and were
included in the structure factor calculation.
1
ca. 40-50% based on consumed Ce2@C80; H NMR (300 MHz,
C2D2Cl4, 293 K) (paramagnetic) δ ) 10.98 (d, 2J ) 7.30 Hz, 1H),
10.53 (d, 2J ) 7.63 Hz, 1H), 10.32 (d, 2J ) 4.76 Hz, 1H-ꢀ), 10.16
2
2
(d, J ) 4.77 Hz, 1H-ꢀ), 10.12 (d, J ) 4.75 Hz, 1H-ꢀ), 9.84 (d,
2J ) 4.74 Hz, 1H-ꢀ), 9.61 (d, J ) 4.76 Hz, 1H-ꢀ), 9.53 (d, J )
2
2
2
2
4.74 Hz, 1H-ꢀ), 9.49 (d, J ) 4.77 Hz, 1H-ꢀ), 9.41 (d, J ) 6.98
3
Hz, 1H), 9.32 (m, 2H), 9.11 (t, J ) 7.01 Hz, 1H), 8.90 (m, 2H),
2
3
3
8.58 (d, J ) 5.91 Hz, 1H), 8.48 (t, J ) 7.95 Hz, 1H), 8.42 (t, J
) 7.94 Hz, 1H), 8.35 (m, 2H), 7.95 (s, 1H), 7.73 (t, 3J ) 7.79 Hz,
3
2
1H), 7.65 (t, J ) 8.11 Hz, 1H), 7.46 (d, J ) 7.46 Hz, 1H), 7.20
3
2
3
(t, J ) 7.01 Hz, 1H), 6.75 (d, J ) 8.25 Hz, 1H), 6.38 (t, J )
7.46 Hz, 1H), 4.17 (br, 1H), 4.04 (m, 2H), 3.53 (br, 2H), 2.98 (t,
3J ) 7.70 Hz, 1H, m-ArH), 2.41 (t, J ) 7.00 Hz, 1H, m-ArH),
3
Acknowledgment. This work was supported in part by the
Deutsche Forschungsgemeinschaft, Cluster of Excellence “Engi-
neering of Advanced Materials”, FCI, the Office of Basic Energy
Sciences of the U.S., a Grant-in-Aid for Scientific Research on
Innovative Areas (No. 20108001, “pi-Space”), a Grant-in-Aid for
Scientific Research (A) (No. 20245006), the 21st Century COE
-3.20 (m, 1Ha, CH2CH2CO2), -3.28 (m, 1Hb, CH2CH2CO2), -5.12
(d, 2J ) 8.45 Hz, 1H, o-ArH), -6.34 (d, 2J ) 7.83 Hz, 1H, o-ArH),
-9.79 (br m, 1Ha, CH2CH2CO), -10.17 (br m, 1Hb, CH2CH2CO),
-17.60 (br t, 1Ha, PhCCH2CH2), -18.10 (br t, 1Hb, PhCCH2CH2);
13C NMR (125 MHz, C2D2Cl4, 288 K) (paramagnetic) δ ) 196.20,
194.79, 192.79, 191.60, 190.89, 187.21 (2C), 183.59, 183.10,
9
J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 9085

A R T I C L E S
Guldi et al.
Program, The Next Generation Super Computing Project (Nano-
science Project), Nanotechnology Support Project, and a Grant-in
Aid for Scientific Research on Priority Area (Nos. 20036008,
20038007) from the Ministry of Education, Culture, Sports, Science,
and Technology of Japan. H.N. and M.Y. thank the Japan Society
for the Promotion of Science (JSPS) for the Research Fellowship
for Young Scientists. L.F. also thanks the National Natural Science
Foundation of China (20702053). We also thank the MICINN of
Spain (projects CTQ2008-00795/BQU and Consolider-Ingenio
2010C-07-25200), and the CAM (project P-PPQ-000225-0505).
Supporting Information Available: Differential absorption
spectra of one-electron reduced Ce2@Ih-C80. Synthesis of diazo
precursor 6•2H and 3. HPLC profiles and MALDI-TOF spectra
of 2 and 1. Various NMR spectra of 2 and 1. Differential pulse
voltammetry (DPV) and cyclic voltammograms (CV) of 2 and
1. This material is available free of charge via the Internet at
http://pubs.acs.org.
JA101856J
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9086 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010