N@C60-porphyrin: A dyad of two radical centers

Article abstract of DOI:10.1021/ja209763u

Dyads of endohedral nitrogen fullerene and porphyrin have been synthesized. In the two-radical-center dyad, the copper(II) tetraphenylporphyrin suppressed the electron spin resonance (ESR) signal of N@C₆₀ through intramolecular dipolar coupling

Full text of DOI:10.1021/ja209763u

Communication
pubs.acs.org/JACS
N@C60−Porphyrin: A Dyad of Two Radical Centers
Guoquan Liu,† Andrei N. Khlobystov,‡ Georgios Charalambidis,§ Athanassios G. Coutsolelos,§
G. Andrew D. Briggs,† and Kyriakos Porfyrakis*,†
†Department of Materials, Oxford University, Oxford OX1 3PH, United Kingdom
‡School of Chemistry, University of Nottingham, Nottingham NG7 2RD, United Kingdom
§Department of Chemistry, University of Crete, Heraklion 71003, Greece
S
* Supporting Information
Chart 1. C60−H2TPP (1), C60−CuTPP (2), N@C60−H2TPP
ABSTRACT: Dyads of endohedral nitrogen fullerene and
(1N), and N@C60−CuTPP (2N) (TPP =
porphyrin have been synthesized. In the two-radical-center
dyad, the copper(II) tetraphenylporphyrin suppressed the
electron spin resonance (ESR) signal of N@C60 through
intramolecular dipolar coupling with a strength of 27.0
MHz. Demetalation of the metalloporphyrin moiety of the
dyad, which effectively turned the two-radical-center dyad
into a single-radical-center dyad, recovered 82% of the
ESR signal of N@C60. Such mechanism of switching a spin
state on and off could find use in molecular spintronics
applications.
Tetraphenylporphyrin)
n the search for molecular materials for quantum
1,2
Itechnologies, endohedral fullerenes are leading candidates.
N@C60, with a nitrogen atom trapped inside the C60 cage,3−6
exhibits the longest electron spin coherence time of any
molecular radical.7 Single-qubit operations and coherent
quantum state transfer between the electron spin and nuclear
spin of 15N@C60 have been demonstrated.8,9 To fulfill the
scalability requirement, much effort has been dedicated to the
synthesis of dimers of endohedral fullerenes. Initial progress
was made in the synthesis of fullerene dimers of the type N@
C60−C60, in which one fullerene moiety is spin-active and the
other is spin-silent,10−12 but recently we succeeded in
synthesizing N@C60−N@C60.13 At the same time, dyads of
Figure 1. HPLC traces of dyads 1N (black) and 2N (red) (5PBB
column, toluene as eluent, 18 mL/min).
C
60 and porphyrin have been widely explored in the interests of
photovoltaic energy conversion.14,15 Some of the reaction
schemes have been successfully transferred to endohedral
fullerenes such as M3N@C80 (M = Y, Sc) and La@C82.16,17 If
both the porphyrin and endohedral fullerene in the dyad are
paramagnetic, a two-radical-center system can be constructed
using this approach. A recent investigation reported ferromag-
netic spin coupling between copper porphyrin and the
metallofullerene La@C82 in a porphyrin-based inclusion
complex.18 Herein we have synthesized two covalently linked
N@C60−porphyrin dyads and demonstrated intramolecular
spin−spin coupling between the copper porphyrin and the
endohedral fullerene. The two N@C60−porphryin dyads 1N
and 2N (Chart 1), as well as their C60−porphyrin counterparts
1 and 2, were synthesized following the Prato reaction
procedure.19 Complete separation of dyad 1N (or 1) from
dyad 2N (or 2) was achieved using recycling-mode HPLC
(Figure 1). The removal of residual dyad 1N was essential
when the electron spin resonance (ESR) spectrum of dyad 2N
was measured because of the high sensitivity of ESR
spectroscopy.
The UV−vis absorption spectra of these dyads are
combinations of the spectra of the constituent fullerenes and
porphyrins, consistent with previous reports on other full-
erene−porphyrin dyads.20−22 A sample containing dyad 1N (a
mixture of 1N and 1) exhibited ESR spectra similar to those of
other pyrrolidine derivatives of N@C60.23,24 As shown in Figure
2, zero-field splitting (ZFS) features (D = 16.3 MHz and E =
0.4 MHz) were observed in the frozen-solution ESR spectrum
of dyad 1N. This finding confirms a negligible effect of the free-
base porphyrin on the electron spins in the N@C60 moiety.
Copper(II) tetraphenylporphyrin (CuTPP) and dyad 2
exhibit similar g factors and hyperfine interaction patterns,
indicating a negligible effect of the empty fullerene cage on the
Received: October 27, 2011
Published: January 11, 2012
© 2012 American Chemical Society
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Scheme 1. Demetalation of Dyad 2N
Figure 2. ESR spectra of dyad 1N in toluene at (a) 295 and (b) 77 K.
electron spin located on the copper center (Figure 3). The
splitting of nine lines in the highest-field component indicates
Figure 4. (a) UV−vis absorption and (b) ESR spectra of a dyad 2N
sample before and after demetalation. The UV−vis spectra were
recorded in a CH2Cl2 solution, and ESR spectra were recorded in a
CS2 solution.
acidification, it is reasonable to expect that the fraction of
endohedral species in the dyad 2N sample is similar to that in
the dyad 1N sample (approximately 0.014%).
Because every N@C60 is bound to CuTPP in the dyad, the
complete suppression of the ESR signal of N@C60 indicates
strong spin−spin interactions between the unpaired electrons
of the nitrogen and the copper. In the weak-coupling regime,
there are two kinds of spin−spin interactions between the two
separated radical centers, namely, dipolar coupling and
exchange coupling. Exchange coupling normally requires
overlap of the electron density distributions, but such overlap
could be excluded in dyad 2N on the basis of our DFT
calculations.25 First, both the theoretical calculations and
previous experimental results confirmed that approximately
98% of the nitrogen spin is localized on the endohedral
nitrogen atom in N@C60.6,15 Second, the copper spin is
exclusively distributed on the porphyrin moiety, as also
indicated by the similarity between the ESR spectra of dyad
2 and CuTPP (Figure 3). Third, the interaction between the
porphyrin moiety and the fullerene moiety is negligible in the
electronic ground states of the fullerene−porphyrin dyads on
the basis of their UV−vis absorption spectra. Therefore, dipolar
coupling must be the dominant mechanism of spin−spin
interaction in dyad 2N.
In solution samples of dyad 2N, both intermolecular and
intramolecular dipolar coupling are present. Whereas the
intermolecular coupling is sensitive to the sample concen-
tration, the intramolecular coupling is independent of the
concentration. Therefore, the effects of the two types of dipolar
coupling can be distinguished by the concentration-dependence
studies of the ESR spectra. In a simple mixture of N@C60 with
CuTPP, where intermolecular dipolar coupling exists exclu-
sively, an increase in sample concentration led to both line
width broadening25 and a decrease in the signal amplitude
(Figure 5a). The ESR signal of N@C60 disappeared only in
samples with very high concentrations (e.g., 4.1 × 10−3 M).
Figure 3. ESR spectra of (a) CuTPP, (b) dyad 2, and (c) dyad 2N in
CS2.
that the four nitrogen atoms in the porphyrin macrocycle are
magnetically equivalent. The differences in the line shapes of
the two spectra are caused by the different molecular
correlation times (79 ps for CuTPP and 355 ps for dyad
2).25 The incorporation of a fullerene cage in dyad 2 increases
the molecular size and slows the tumbling rate of the molecule
significantly.
Unexpectedly, the dyad 2N sample (a mixture of 2N and 2)
showed ESR spectra similar to those of dyad 2, containing only
features of the CuTPP moiety (Figure 3). The characteristic
ESR signal of N@C60 was detected in neither the room-
temperature solution nor the frozen solution of the dyad 2N
sample. The disappearance of the N@C60 signal could be
caused either by an interaction with the copper spin or
decomposition of N@C60 during the reaction, leaving only
dyad 2 in the sample.26 Therefore, it was indispensible to be
able to demonstrate the presence of dyad 2N in the sample.
Demetalation27 was employed to convert dyad 2N into dyad
1N (Scheme 1), from which the ESR signal of N@C60 could be
determined. The acidification process was monitored by UV−
vis spectroscopy. As shown in Figure 4a, after the copper ion
was removed, the absorption peaks ascribed to CuTPP
disappeared and peaks ascribed to free-base porphyrin emerged.
Accordingly, the ESR signal of N@C60 was recovered (Figure
4b). The recovered N@C60 signal was 82 4% of that of the
previous dyad 1N sample with the same concentration. Given
the possibility of decomposition of N@C60 during the
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ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis and characterization of dyads 1 and 2, simulation of
ESR spectra, demetalation process of dyads 2 and 2N, and DFT
calculation results for dyads 2 and 2N. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
Figure 5. ESR spectra of (a) an N@C60/CuTPP mixture (molar ratio
1:1) and (b) a dyad 2N sample at (I) 4.1 × 10−3 M, (II) 1.6 × 10−3 M,
and (III) 8.0 × 10−4 M. Measurements were taken at room
temperature in CS2, and the parameters were set to best demonstrate
the signal of N@C60.
kyriakos.porfyrakis@materials.ox.ac.uk
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
However, for dyad 2N, no ESR signal of N@C60 was observed
at any of the experimental concentrations (Figure 5b). Since
the intermolecular dipolar coupling is negligible in samples with
low concentrations (e.g., 8.0 × 10−4 M), we deduce that the
intramolecular dipolar coupling plays the dominant role in the
suppression of N@C60 signal.
We acknowledge funding from EPSRC (EP/F028806/01), the
European Commission (FP7-REGPOT-2008-1, Project BIO-
SOLENUTI 229927), and the Royal Society. G.L. was
supported by the China Oxford Scholarship Fund, the William
Louey Educational Fund, and a Graduate Scholarship from St
Anne’s College, Oxford. We thank Dr. Nick Rees at University
of Oxford for the acquisition of NMR spectra, Dr. Maria del
Carmen Gimenez-Lopez at Nottingham University for the
acquisition of mass spectra, and the Oxford Supercomputing
Centre for the provision of computing services. We also thank
Dr. Steven D. Karlen for help with the synthesis of the
porphyrin−aldehyde.
To calculate the intramolecular dipolar coupling strength, we
used a distance of 1.26 nm between the two radical centers,
which was determined on the basis of the optimized geometry
of dyad 2, assuming that the nitrogen atom occupies the center
of the fullerene cage.25 The coupling strength (Ddip) was
therefore found to be 2.7 × 107 Hz following the classical point-
dipole approximation. Because the spin−lattice relaxation rate
of the copper ion in solution (1 × 109 to 3.3 × 109 Hz)28 is
higher than Ddip, the main consequence of dipolar coupling is
expected to be line width broadening rather than any AB
splitting pattern.29,30 Furthermore, the molecular tumbling rate
of 2.8 × 109 Hz for dyad 2N in CS2, which falls in the
intermediate molecular motion regime, cannot effectively
counteract the line width broadening effect. In view of the
extremely narrow line width of N@C60 (<9 kHz),2 the
broadening effect should dramatically reduce the signal
amplitude of N@C60. Such a decrease in signal amplitude, as
well as the low endohedral nitrogen percentage in the dyad,
could explain the disappearance of the N@C60 signal in the
ESR spectrum of the dyad 2N sample.
In summary, two dyads of N@C60 and porphyrin have been
synthesized. The free-base porphyrin imposes a negligible effect
on N@C60 in dyad 1N. In the two-radical-center dyad 2N, the
ESR signal of N@C60 disappears. The concentration depend-
ence demonstrated strong intramolecular dipolar coupling
between the two radicals, and the dipolar coupling strength
was calculated to be 27.0 MHz. The line width broadening and
amplitude decrease resulting from dipolar coupling explain the
suppression of the ESR signal of N@C60 in dyad 2N. Removal
of the copper ion from the metalloporphyrin moiety, which led
to recovery of approximately 82% of the N@C60 signal, proved
the existence of dyad 2N in the sample. The demetalation,
which changed the two-radical-center system (dyad 2N) into a
single-radical system (dyad 1N), provides a strategy for
switching spin states on and off. This may have applications
in molecular spintronics devices. Additionally, these dyads may
provide a system in which the electron spins of N@C60 interact
with the electron pair resulting from photon-induced charge
separation in photoactive adducts.
REFERENCES
■
(1) Harneit, W. Phys. Rev. A 2002, 65, No. 032322.
(2) Benjamin, S. C.; Ardavan, A.; Briggs, G. A. D.; Britz, D. A.;
Gunlycke, D.; Jefferson, J.; Jones, M. A. G.; Leigh, D. F.; Lovett, B. W.;
Khlobystov, A. N.; Lyon, S. A.; Morton, J. J. L.; Porfyrakis, K.;
Sambrook, M. R.; Tyryshkin, A. M. J. Phys.: Condens. Matter 2006, 18,
S867.
(3) Murphy, T. A.; Pawlik, T.; Weidinger, A.; Hohne, M.; Alcala, R.;
Spaeth, J. M. Phys. Rev. Lett. 1996, 77, 1075.
(4) Jakes, P.; Dinse, K. P.; Meyer, C.; Harneit, W.; Weidinger, A.
Phys. Chem. Chem. Phys. 2003, 5, 4080.
(5) Kanai, M.; Porfyrakis, K.; Briggs, G. A. D.; Dennis, T. J. S. Chem.
Commun. 2004, 210.
(6) Nikawa, H.; Araki, Y.; Slanina, Z.; Tsuchiya, T.; Akasaka, T.;
Wada, T.; Ito, O.; Dinse, K. P.; Ata, M.; Kato, T.; Nagase, S. Chem.
Commun. 2010, 46, 631.
(7) Morton, J. J. L.; Tyryshkin, A. M.; Ardavan, A.; Porfyrakis, K.;
Lyon, S. A.; Briggs, G. A. D. J. Chem. Phys. 2006, 124, No. 014508.
(8) Morton, J. J. L.; Tyryshkin, A. M.; Ardavan, A.; Porfyrakis, K.;
Lyon, S. A.; Briggs, G. A. D. Phys. Rev. Lett. 2005, 95, No. 200501.
(9) Brown, R. M.; Tyryshkin, A. M.; Porfyrakis, K.; Gauger, E. M.;
Lovett, B. W.; Ardavan, A.; Lyon, S. A.; Briggs, G. A. D.; Morton, J. J.
L. Phys. Rev. Lett. 2011, 106, No. 110504.
(10) Goedde, B.; Waiblinger, M.; Jakes, P.; Weiden, N.; Dinse, K. P.;
Weidinger, A. Chem. Phys. Lett. 2001, 334, 12.
(11) Hormann, F.; Hirsch, A.; Porfyrakis, K.; Briggs, G. A. D. Eur. J.
Org. Chem. 2011, 117.
(12) Zhang, J.; Porfyrakis, K.; Morton, J. J. L.; Sambrook, M. R.;
Harmer, J.; Xiao, L.; Ardavan, A.; Briggs, G. A. D. J. Phys. Chem. C
2008, 112, 2802.
(13) Farrington, B. J.; Jevric, M.; Rance, G. A.; Ardavan, A.;
Khlobystov, A. N.; Briggs, G. A. D.; Porfyrakis, K. Angew. Chem., Int.
Ed. 2012, DOI: 10.1002/anie.201107490.
(14) Ohkubo, K.; Kotani, H.; Shao, J. G.; Ou, Z. P.; Kadish, K. M.; Li,
G. L.; Pandey, R. K.; Fujitsuka, M.; Ito, O.; Imahori, H.; Fukuzumi, S.
Angew. Chem., Int. Ed. 2004, 43, 853.
1940
dx.doi.org/10.1021/ja209763u | J. Am. Chem.Soc. 2012, 134, 1938−1941

Journal of the American Chemical Society
Communication
(15) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T.
K.; Kim, S. K.; Kim, D.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125,
9129.
(16) Pinzon, J. R.; Cardona, C. M.; Herranz, M. A.; Plonska-
Brzezinska, M. E.; Palkar, A.; Athans, A. J.; Martin, N.; Rodriguez-
Fortea, A.; Poblet, J. M.; Bottari, G.; Torres, T.; Gayathri, S. S.; Guldi,
D. M.; Echegoyen, L. Chem.Eur. J. 2009, 15, 864.
(17) Feng, L.; Slanina, Z.; Sato, S.; Yoza, K.; Tsuchiya, T.; Mizorogi,
N.; Akasaka, T.; Nagase, S.; Martin, N.; Guldi, D. M. Angew. Chem., Int.
Ed. 2011, 50, 5909.
(18) Hajjaj, F.; Tashiro, K.; Nikawa, H.; Mizorogi, N.; Akasaka, T.;
Nagase, S.; Furukawa, K.; Kato, T.; Aida, T. J. Am. Chem. Soc. 2011,
133, 9290.
(19) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993,
115, 9798.
(20) Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.; Taniguchi, S.;
Okada, T.; Shirakawa, M.; Sakata, Y. J. Am. Chem. Soc. 1996, 118,
11771.
(21) Kuciauskas, D.; Lin, S.; Seely, G. R.; Moore, A. L.; Moore, T. A.;
Gust, D.; Drovetskaya, T.; Reed, C. A.; Boyd, P. D. W. J. Phys. Chem.
1996, 100, 15926.
(22) Schuster, D. I.; Li, K.; Guldi, D. M.; Palkar, A.; Echegoyen, L.;
Stanisky, C.; Cross, R. J.; Niemi, M.; Tkachenko, N. V.; Lemmetyinen,
H. J. Am. Chem. Soc. 2007, 129, 15973.
(23) Franco, L.; Ceola, S.; Corvaja, C.; Bolzonella, S.; Harneit, W.;
Maggini, M. Chem. Phys. Lett. 2006, 422, 100.
(24) Zhang, J. Y.; Morton, J. J. L.; Sambrook, M. R.; Porfyrakis, K.;
Ardavan, A.; Briggs, G. A. D. Chem. Phys. Lett. 2006, 432, 523.
(25) See the Supporting Information.
(26) Liu, G. Q.; Khlobystov, A. N.; Ardavan, A.; Briggs, G. A. D.;
Porfyrakis, K. Chem. Phys. Lett. 2011, 508, 187.
(27) Tan, K.; Jaquinod, L.; Paolesse, R.; Nardis, S.; Di Natale, C.; Di
Carlo, A.; Prodi, L.; Montalti, M.; Zaccheroni, N.; Smith, K. M.
Tetrahedron 2004, 60, 1099.
(28) Drago, R. S. Physical Methods for Chemists, 2nd ed.; Saunders
College Publishing: Orlando, FL, 1992.
(29) Rakowsky, M. H.; More, K. M.; Kulikov, A. V.; Eaton, G. R.;
Eaton, S. S. J. Am. Chem. Soc. 1995, 117, 2049.
(30) Burchfield, J. M.; Du, J. L.; More, K. M.; Eaton, S. S.; Eaton, G.
R. Inorg. Chim. Acta 1997, 263, 23.
1941
dx.doi.org/10.1021/ja209763u | J. Am. Chem.Soc. 2012, 134, 1938−1941