Utilization of Sc3N@C80 in long-range charge transfer reactions

Article abstract of DOI:10.1039/c0cc04159a

Electron accepting Sc₃N@C₈₀ promotes long-range charge transfer events evolving from photoexcited metalloporphyrins to afford radical ion pair states with lifetimes in the range of μs.

Full text of DOI:10.1039/c0cc04159a

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COMMUNICATION
Utilization of Sc N@C in long-range charge transfer reactionsw
3
80
Silke Wolfrum, Julio R. Pinzo
´
n, Agustin Molina-Ontoria, Andreas Gouloumis,cd
a
b
cd
cd
e
a
Nazario Martın,* Luis Echegoyen* and Dirk M. Guldi*
´
Received 30th September 2010, Accepted 24th November 2010
DOI: 10.1039/c0cc04159a
Electron accepting Sc N@C promotes long-range charge
3 80
transfer events evolving from photoexcited metalloporphyrins
to afford radical ion pair states with lifetimes in the range of ls.
Artificial photosynthetic systems for practical solar fuels
production must incorporate both molecular and supramolecular
assemblies to collect light energy, separate charge, and trans-
port charge to catalytic sites. The design and development of
light-harvesting, photo-conversion, and catalytic modules
capable of self-ordering and self-assembling into an integrated
functional unit could lead to an efficient artificial photo-
1
,2
synthetic system.
Scheme 1 Molecular structures of 1 and 2.
Empty fullerenes replicate some of the features that are found
in nature’s most sophisticated and important solar energy
3
conversion/storage system. Owing to its rigid structure, C60
In this work, we wish to report for the first time the long-
range charge transfer processes of electron donor–acceptor
conjugates possessing ZnP—as chromophore/electron donor—
possesses small reorganization energies during charge transfer
reactions, which has already led to the improvement of charge
4
separation processes triggered by light. In contrast to empty
3
and Sc N@C80—as electron acceptor—1 and 2—see Scheme 1.
fullerenes, endohedral metallofullerenes bear additional atoms,
5
ions, or clusters within their inner spheres. What renders
This investigation has documented the successful radical ion
pair state formation with lifetimes that range well into ms.
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
1 and 2 were prepared by heating Sc N@I -C80, the
3 h
corresponding aldehydes (7 or 10) and sarcosine in an
o-DCB/dimethylformamide (DMF) 4 : 1 solvent mixture between
15 minutes to 3 hours depending on the aldehyde used—see ESI.w
A preliminary purification step was made using a silica gel
6
the encapsulated species. However, it is only recently that
endohedral metallofullerenes have begun to be used in electron
donor–acceptor systems and they have shown superior ability
to stabilize charge separated states when compared to their C60
column eluting first with CS
Sc N@I -C80 followed by CS
2
2
in order to remove the unreacted
3
h
/toluene mixtures to elute the
7
analogue dyads. Sc N@I -C is the most abundant metallo-
3
desired products. Further purification was made by HPLC using
h
80
fullerene obtained as a single isomer and can be readily purified
8
by non-chromatographic methods.
a Cosmosil—pentabromobenzyl (PBB) column (4.6 Â 250 mm)
À1
eluting with toluene at 2.0 mL min . 1 and 2 have retention times
of 9.6 and 11.4 min, respectively.
1
The structure of 1 and 2 was corroborated by H NMR
a
Department of Chemistry and Pharmacy and Interdisciplinary Center
spectroscopy. The pyrrolic b protons in the porphyrin ring are
observed around 9.0 ppm, the protons in the aromatic rings
connected to the meso positions are observed as a group of
four signals with integration ratio 2 : 6 : 2 : 3 in the region
from 8.4 to 7.7 ppm and the protons in the aromatic rings and
the double bonds are observed in the region from 7.7 to 6.3 ppm.
The signals of the methylene groups directly connected to the
oxygen atoms in the n-hexyl groups in 1 and 2 are overlapped
with one of the pyrrolidine proton signals at 4.2 ppm. In 1 and
2, the methylene groups in the [2,2]paracyclophanes are
observed in the region from 4.0 to 2.8 ppm overlapped with
the other two protons of the pyrrolidine rings. The protons in
the N-methyl are observed at B2.50 ppm and the remaining
for Molecular Materials, Friedrich-Alexander-Universita¨t
Erlangen-Nu¨rnberg, 91058 Erlangen, Germany.
E-mail: silke.wolfrum@chemie.uni-erlangen.de,
guldi@chemie.uni-erlangen.de
Department of Chemistry, Clemson University, Clemson, SC, USA.
E-mail: jpinzon@clemson.edu; Fax: +1 864 656 6613
Departamento de Quı´mica Orga´nica I, Facultad de Quı´mica,
Universidad Complutense, 28040 Madrid, Spain.
E-mail: nazmar@quim.ucm.es
IMDEA-Nanociencia, Campus de Cantoblanco,
b
c
d
28049 Madrid, Spain
e
Department of Chemistry, University of Texas at El Paso, El Paso,
TX, USA. E-mail: echegoyen@utep.edu; Fax: +1 915 747-8807
w Electronic supplementary information (ESI) available: Details on
synthesis, NMR characterization and electrochemistry. See DOI:
1
0.1039/c0cc04159a
2
270 Chem. Commun., 2011, 47, 2270–2272
This journal is c The Royal Society of Chemistry 2011

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protons in the aliphatic chains are observed as complex broad
signals at: 2.0, 1.6, 1.4 and 1.0 ppm with an integration ratio of
À1.62/À1.56, À2.00/À1.98, and À2.32/À2.27
V versus
+
Fc/Fc . The reduction processes at À1.22/À1.18, À1.62/À1.56,
and À2.32/À2.27 V are based on Sc N@C , while the reduc-
2
1
: 2 : 4 : 3. The tBu groups are observed as a strong signal at
3
80
.5 ppm. All the assignments were confirmed by COSY and
tion of ZnP occurs at À2.00/À1.98 V. The reduction processes
HSQC experiments—see ESI.w
The absorptions of 1 and 2 (Fig. S11, ESIw) feature in
for the dyads are reversible, as previously reported for
1
3,14
[5,6]-Sc N@I -C fulleropyrrolidine derivatives.
80
Con-
3
h
the near-infrared region the broad onset of Sc
3
N@C80. The
sidering these data we calculate a driving force of about
0.63 eV for intramolecular charge transfer. Finally, an
‘‘endohedral heavy-atom effect’’ should also be considered to
accelerate the excited state deactivation.
visible region is dominated by the ZnP features including
the Soret-band (i.e., 428 nm) and the Q-band-transitions
9
(
i.e., 550/594 nm).
Insights into the excited state properties were derived from
Transient absorption measurements provided conclusive
evidence about the nature of the excited state deactivation,
namely energy transfer versus charge transfer versus endo-
hedral heavy-atom effect. All of the compounds were probed
by femtosecond laser excitation at 387 nm. The differential
absorption changes recorded upon excitation of ZnP are
characterized by bleaching around 530 nm and a transient
between 570 and 750 nm. Such spectral features reflect the ZnP
fluorescence measurements. Particularly useful is the strong
1
0
quantum yield of 0.04) and long-lived (2.1 ns) ZnP
11
(
fluorescence with maxima at 605 and 650 nm. To this end,
we measured the fluorescence of 1 and 2 upon excitation at
4
20 nm and noted a distinct fluorescence quenching of the ZnP
chromophore/fluorophore—Fig. S12 (ESIw). The fluorescence
quenching revealed a marked increase with solvent polarity
1
1
(
i.e., toluene o THF o benzonitrile) and a decrease with
singlet excited state—Fig. S13 (ESIw). The latter decays
8
À1
donor–acceptor distance (i.e., 2 o 1) resulting in fluorescence
quantum yields ranging from 0.033 (2 in toluene) to 0.020
slowly (4.0 Â 10 s ) to the energetically lower-lying triplet
excited state with a characteristic maximum at 840 nm,
1
1
(
1 in THF). In complementary time-resolved measurements we
predominantly via intersystem crossing.
Photoexciting
determined ZnP fluorescence lifetimes of 500 (1) and 1600 ps (2).
Sc N@C80 led to differential absorption changes that include
3
Considering the difference in excited state energy of ZnP
1
a minimum (650 nm), which corresponds nicely to the ground
state absorption, followed by strong maxima in the near-
infrared region (1025 nm)—Fig. S14 (ESIw). Unlike ZnP, the
singlet excited state features of Sc N@C decay quickly
2
(
2.1 eV) and Sc N@C (1.5 eV) a thermodynamically driven
3 80
(
0.6 eV) transduction of excited state energy emerges as a
feasible deactivation pathway. The low fluorescence quantum
yield of Sc N@C hampers, however, a conclusive and firm
3
80
9
À1 15
with a lifetime of 84 ps (1.1 Â 10 s ). An ‘‘endohedral
heavy-atom effect’’ is likely to be responsible for the ultrafast
singlet excited state decay to yield the triplet manifold.
The transient absorption spectra of 1 and 2 are—at early
times—virtually identical to those seen for ZnP, indicating the
instantaneous formation of the ZnP singlet excited state
3
80
analysis. Charge transfer might be an alternative deactivation
pathway for photoexcited ZnP. To evaluate the thermo-
dynamic feasibility of such a deactivation we turned to
electrochemistry—see Fig. 1 and S5 (ESIw). For 1 and 2, we
3
note in line with ZnP and Sc N@C80 three one-electron
oxidation processes at +0.25/+0.28, +0.44/+0.40, and
+
+0.57/+0.55 V versus Fc/Fc that correlate with the first
3
despite the presence of Sc N@C80—see Fig. 2. The ZnP
centered singlet excited states decay, however, rapidly and
transform over the course of 1500 ps. This fast decay is
associated with marked changes observed in the differential
absorption spectra—a new set of maxima, which evolve in the
visible region (i.e., 580–700 nm) as well as in the near-infrared
oxidation of ZnP, the first oxidation of Sc N@C , and the
3 80
second oxidation of ZnP, respectively. In terms of reduction,
four one-electron reduction processes are seen at À1.22/À1.18,
Fig. 2 Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (387 nm, 150 nJ) of 1 in
THF with several time delays between 0 and 1300 ps at room
temperature with color code from black and red to blue and orange.
Inset—time–absorption profiles of the spectra at 467, 533, and
1295 nm, monitoring the charge separation dynamics.
3 h
Fig. 1 CVs of (a) Sc N@I -C80, (b) 1, and (c) 2 in tetra-n-butyl-
ammonium hexafluorophosphate (0.05 M) containing o-DCB.
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2270–2272 2271

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system back into the normal Marcus region with a lifetime
of 2.6 ms.
This work was supported by the MEC of Spain (projects
CTQ2008-0795/BQU and Consolider-Ingenio 2010C-07-
25200), the CAM (MADRISOLAR-2 project S2009/PPQ-1533),
EU (FUNMOLS FP7-212942-1) and the Deutsche
Forschungsgemeinschaft (SFB and Clusters of Excellence
Engineering of Advanced Materials). A. M. O. and A. G.
thank the CAM, and the MEC of Spain for a research grant
and a Ramon y Cajal contract, respectively. We also thank the
´
National Science Foundation (Grant No. DMR 0809129
to L.E.).
Fig. 3 Differential absorption spectra (visible) obtained upon nano-
second flash photolysis (355 nm, 150 nJ) of 1 in THF with a time delay
of 80 ns at room temperature. Inset—time–absorption profile of the
spectra at 460 nm, monitoring the charge recombination dynamics.
Notes and references
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region (i.e., 800–1200 nm). Of particular importance is the close
resemblance of the visible and the near-infrared part with the
radiolytically and spectroelectrochemically generated spectrum
of the one-electron oxidized ZnP radical cation and the one-
3
12,14
electron reduced Sc N@C radical anion, respectively.
3 80
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D. Gust, T. A. Moore and A. L. Moore, Acc. Chem. Res., 2001, 34,
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À
3
N@C80) –(ZnP) governs
ꢀ
ꢀ
+
tion leading to the formation of (Sc
the fast deactivation of the ZnP singlet excited state.
ꢀ
ꢀ
+
À
The (Sc
3
N@C80
)
–(ZnP)
radical ion pair states are
6 D. M. Guldi, L. Feng, S. G. Radhakrishnan, H. Nikawa, M. Yamada,
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persistent on the femto, pico, and nanoseconds time-scales.
It is only on the microsecond time regime where they start to
decay—Fig. 3. Thus, to determine the charge-recombination
rates complementary nanosecond experiments, that is, excitation
with a 6 ns laser pulse were performed. The spectral signatures
of the one-electron oxidized ZnP radical cation and the one-
7
(a) J. R. Pinzo
A. J. Athans, S. S. Gayathri, D. M. Guldi, M. A
N. Martın, T. Torres and L. Echegoyen, Angew. Chem., Int. Ed.,
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n, M. E. Plonska-Brzezinska, C. M. Cardona,
´
. Herranz,
´
2
´
S. G. Radhakrishnan, G. Bottari, T. Torres, D. M. Guldi and
L. Echegoyen, J. Am. Chem. Soc., 2009, 131, 7727–7734.
electron reduced Sc N@C
3
radical anion—as detected
80
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J. Am. Chem. Soc., 2005, 127, 16292–16298; (b) S. Stevenson,
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immediately after the laser pulse—decay synchronously and
give rise to kinetics that obey an unimolecular rate law. From
˚
the latter analyses lifetimes of 1.0 ms for 1 (RCC = 32.74 A)
and 1.2 ms for 2 (RCC = 45.94 A) were determined in THF in a
strictly oxygen free environment. Addition of molecular oxygen
˚
2
005, 61, 5655–5662.
1
1
0 F. D’Souza and O. Ito, Chem. Commun., 2009, 4913–4928.
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to THF solutions resulted in shorter lifetimes with values of
0
.15 ms. From the latter we infer an activation-controlled
ꢀ ꢀ
+
À
80
interaction of molecular oxygen with (Sc N@C ) –(ZnP)
3
ꢀ
via charge transfer to generate O2 as the mechanism. The
12 J. R. Pinzo
´
n, C. M. Cardona, M. A. Herranz, M. E. Plonska-
guez-
À
16
Brzezinska, A. Palkar, A. J. Athans, N. Martin, A. Rodrı
´
lifetimes measured in benzonitrile—3.2 ms for 1 and 2.6 ms for
Fortea, J. M. Poblet, G. Bottari, T. Torres, S. G. Radhakrishnan,
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2
—are interesting. Owing to the short donor–acceptor separation
and the cathodically shifted reduction of Sc N@C80 relative to
charge recombination in 1 is evidently pushed into the
1
1
3
2
006, 128, 6480–6485.
7b,8c
60
C ,
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17
inverted region of the Marcus parabola. In other words,
Sc N@C80 like C60 features small reorganization energies during
(
b) J.-C. Chambron, A. Harriman, V. Heitz and J. P. Sauvage,
J. Am. Chem. Soc., 1993, 115, 6109.
3
18
charge transfer reactions. In 2 the larger donor–acceptor
˚
separation of nearly 46 A seems to dominate any of the
1
5 D. M. Guldi and M. Prato, Acc. Chem. Res., 2000, 33, 695–703.
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aforementioned effects and brings the charge recombination
process into the normal region of the Marcus parabola.
In summary, we have shown that electron transfer reactions
between ZnP, as an electron donor, and Sc N@C , as an
1
25, 12803–12809.
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(
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80
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electron acceptor, are possible over center-to-center distances
˚
of up to 45 A. Moreover, we have shown that changing the
1
solvent diminishes the distance effects and pushes the charge
recombination reaction of the long range electron transfer
2
272 Chem. Commun., 2011, 47, 2270–2272
This journal is c The Royal Society of Chemistry 2011