Photodimerization of a
m-Phenylenebis(arylmethanofullerene): The First
Rigorous Proof for Photochemical Inter-Fullerene
[2 + 2] Cycloaddition
Joop Knol and Jan C. Hummelen*
Stratingh Institute and Materials Science Center
UniVersity of Groningen, Department of Chemistry
Nijenborgh 4, 9747 AG Groningen, The Netherlands
ReceiVed NoVember 8, 1999
ReVised Manuscript ReceiVed February 2, 2000
The functionalization of C60 offers a valuable tool for obtaining
new materials with special physical and electronic features. The
electron-accepting and -conducting properties of the fullerene cage
can be utilized in different types of molecular devices.1 An
example is the use of processable fullerene derivatives in bulk
heterojunction photovoltaic cells in which the fullerene functions
as a continuous n-type semiconducting network.2 For optimum
electron transport, the fullerene cages have to be assembled in
some sort of (linear) array. Therefore, we initiated a study of
functionalized molecules containing two or more C60 units that
can adopt conformations in which the fullerene cages are in close
proximity. This type of smallest “oligomeric” C60 may serve as
a model for more extended fullerene arrays and clarify certain
aspects of the reported behavior of C60 in the solid state when
exposed to light,3 high pressure,4 and reducing species.5
To date, several compounds incorporating two remote C60 units
including compounds with general structure “C60-spacer-C60”
have been reported.6,7 We envisaged that an appropriate choice
of spacer could lead to well-defined interactions between the two
independent fullerene units. On the basis of molecular models
we selected a m-phenylenebis(methanofullerene) with general
structure R-C61-(1,3-Ph)-C61-R as a synthetic target, since
conformational freedom allows the two fullerene cages to be in
close contact.
To avoid solubility problems we prepared m-phenylenebis
(arylmethanofullerene) 2b carrying four solubilizing chains via
a double Bamford-Stevens protocol8 from bis-tosylhydrazone 19
(Scheme 1). Reaction of 1 with sodium methoxide (1.96 equiv)
in the presence of excess C60 (3.6 equiv) at 70-75 °C in 1,2-
dichlorobenzene (ODCB)/pyridine afforded a complex isomeric
mixture (36%) containing the expected bisfulleroid 2a. Upon
heating (ODCB, reflux) the isomeric mixture was cleanly
converted into the highly soluble m-phenylenebis(arylmethano-
fullerene) 2b in 94% yield.
In contrast to the thermal behavior of 2a, prolonged irradiation
of a deoxygenated solution of 2a (150 W sodium flood lamp) in
ODCB afforded a clean steady-state mixture consisting of 2b
(�40%) and a new compound (�60%) according to HPLC and
TLC analysis. Irradiation of 2b under identical conditions gave
the same result, indicating that the new compound is formed from
2b, not 2a. The UV-vis characteristics of the new compound,
obtained from HPLC diode array detection (Figure 1), lacked any
resemblance with a (mono)1,2-substituted fullerene of the type
present in 2b. Especially the weak absorption bands in the 400-
800 nm region were diagnostic for a fullerene bis-addition product.
On the basis of detailed studies on the regiochemistry of 2-fold
additions to C60 by Hirsch and co-workers,10 we were able to
unambiguously assign the UV-vis data to a cis-3 regioisomer,
which is consistent with the formation of photodimer 2c from
2b. Structure 2c is in full agreement with the predicted unique
dimerization site for two specific (enantiotopic) cis-3 CdC bonds
in each fullerene moiety of 2b.11 Since 2c can be formed only as
a “meso” dimer (with CS symmetry), the asymmetry of the
dimerization site does not lead to two isomeric products.
Photodimer 2c was obtained in pure form (84% based on 60%
conversion) after separation from 2b using flash chromatography
with a silica gel column and CS2/toluene (90/10) as the eluent.
The 500 MHz 1H NMR spectrum of 2c (Figure 2) recorded in
CS2 at 25 °C showed resonances for the bridging m-phenylene
unit (8.50 (H2), 7.74 (H4, H6), and 7.29 (H5) ppm) shifted upfield
with respect to 2b (8.75 (H2), 8.12 (H4, H6), and 7.65 (H5) ppm).
These shifts are probably caused by the effect of two rigidly
interlocked fullerene cages in dimer 2c.
In sharp contrast to the unsubstituted C60 photodimer (C120)7d
and structural analogues,7a,c,e 2c displayed reasonable solubility
in common fullerene solvents (toluene, ODCB, CS2). Hence, a
13C NMR spectrum in CS2 was obtained without much difficulty
(Figure 3). As compared to 2b, photodimer 2c showed four signals
for fullerene-sp3 carbon atoms (ä 79.65, 74.71 (cyclopropyl) and
ä 76.00, 73.14 (cyclobutyl)) and a total of 53 fullerene-sp2 carbons
(of a possible 56) which is in full agreement with its CS symmetry.
* Author for correspondence. E-mail: j.c.hummelen@chem.rug.nl.
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(9) For the synthesis of 1 see Supporting Information.
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(11) The cis-3 position as unique dimerization site in 2b was predicted
from Darling molecular models and computer-generated molecular models
using Hyperchem, version 5.1. A preliminary modeling study suggests that it
is hard to design other spacers (apart from m-phenylene) in which two parallel
CdC bonds (from different C60 fragments) can cyclodimerize without the
introduction of a substantial amount of strain in the product.
3226 J. Am. Chem. Soc. 2000, 122, 3226-3227
10.1021/ja993931n CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/16/2000
The MALDI-TOF-MS data for 2c showed the expected
molecular ion with most intense peak at m/z ) 2097.0 (M + 1)
(calculated for 12C163(13C)H64O4 (M + 1): m/z ) 2097.5).
Control experiments showed that the photodimerization process
2bf2c is significantly retarded by molecular oxygen, implying
a mechanism involving the (methano)fullerene triplet excited state.
A similar effect was reported for the photopolymerization process
in thin solid films of C60.3a Furthermore we observed complete
cycloreversion of 2c to 2b within 15 min in refluxing ODCB
which is quite similar to the behavior reported for C1207d and
higher C60 oligomers,12 which revert to C60 upon heating.
The three different fullerenes 2a, 2b, and 2c displayed identical
behavior when solutions in ODCB were irradiated for 44 h at 20
°C. In all cases mixtures of 2c/2b of fairly identical composition
(58/42 from 2a, 61/39 from 2b, and 61/39 from 2c) were obtained.
This confirms the reversibility of the photodimerization process
2bf2c.
The synthesis of 2c represents the first example of a controlled
[2 + 2] cycloaddition process of fullerenes. It adds clear proof
to long-standing proposals on structures and mechanisms involved
in solid-state photo- and pressure-polymerized C60. Since [2 +
2] cycloaddition (e.g., the interconversion 2bf2c) will ultimately
influence the performance of devices in which the fullerene phase
plays a role as electron-accepting/transporting medium, the
presence of parallel oriented CdC bonds in the contact area
between adjacent C60 moieties has to be taken into account in
the design of fullerene arrays.
Acknowledgment. Mr. Albert Kiewiet is acknowledged for MALDI-
TOF measurements. This work was financially supported by The
Netherlands’ Agency for Energy and the Environment (Novem No.
146-120-008-3).
Supporting Information Available: Synthetic procedures and char-
acterization data for compounds 1, 2a, 2b, and 2c (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
JA993931N
(12) Wang, Y.; Holden, J. M.; Bi, X.-X.; Ecklund, P. C. Chem. Phys. Lett.
1994, 217, 413-417.
Scheme 1. Synthesis of m-Phenylenebis(arylmethanofullerene)
2b and Photodimerization Process 2bf2ca
a Reagents and conditions: (a) NaOMe, pyridine/ODCB, C60, 70-75
°C. (b) ODCB, ¢. (c) ODCB, hî, 17 °C.
Figure 1. UV-vis spectrum of photodimer 2c in toluene.
Figure 2. 1H NMR spectra of 2b (top) and 2c (bottom) recorded in CS2
at 25 °C.
Figure 3. 13C NMR spectra of 2b (top) and 2c (bottom) recorded in
CS2 at 25 °C.
Communications to the Editor J. Am. Chem. Soc., Vol. 122, No. 13, 2000 3227