Supramolecular organization of fullerenes by quadruple hydrogen bonding

Article abstract of DOI:10.1039/b008006n

Quadruple hydrogen bonded fullerene dimer 8 with a very high dimerisation constant (K_a ≥ 1.0 × 10⁶ M^-1) was synthesised and fully characterised.

Full text of DOI:10.1039/b008006n

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Supramolecular organization of fullerenes by quadruple hydrogen bonding†
Minze T. Rispens, Luis Sánchez, Joop Knol and Jan C. Hummelen*
Stratingh Institute and Materials Science Centre, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The
Netherlands. E-mail: J.C.Hummelen@chem.rug.nl; Fax: (+31) 50-3634296
Received (in Liverpool, UK) 27th September 2000, Accepted 13th November 2000
First published as an Advance Article on the web 4th January 2001
Quadruple hydrogen bonded fullerene dimer 8 with a very
high dimerisation constant (Ka ! 1.0 3 106 M21) was
synthesised and fully characterised.‡
Dimer 8 showed 29 signals for C60-sp2 carbons in 13C NMR,
in accordance with C2h symmetry. The resonances at d 172.8
(a), 156.5 (b) and 154.4 (c) are characteristic for the
2-ureidopyrimidin-4-one moiety.5a The signals at d 80.6 and
52.4 are diagnostic for the cyclopropyl moiety.8 The 1H NMR
spectrum recorded in CDCl3 showed the typical resonances5a
for the four dimer bonding hydrogen atoms of 8 at d 11.81 (Hb)
and d 10.40 (Hc), a signal at d 12.98 for the intramolecularly
bonded Ha, and one at d 5.75 for the vinylic proton Hd (Scheme
1). When the concentration of compound 8 was lowered to 1.0
3 1025 mol l21 no dissociation was observed, thus giving a
lowest estimate of the dimerisation constant of 1 3 106 M21, in
good agreement with the values obtained for other ureidopyr-
imidinone derivatives.5a To probe a possible influence of the
fullerene core on the hydrogen bonding unit, equimolar
amounts of dimer 8 and dimer 9 of N-butylaminocarbonyl-
6-methylisocytosine (9)5 were dissolved in CDCl3. A statistical
mixture of the possible dimers was obtained (Fig. 1) as was
concluded from the integration of the absorptions of all protons
Fullerenes have interesting properties that may be utilised in
applications such as organic photovoltaic (PV) devices.1 The
bulk-heterojunction version of such cells especially has re-
ceived much attention recently. In this type of device, a blend of
an electron donor (for example a p-conjugated polymer) and an
electron acceptor (for example a fullerene derivative) serves as
the active PV layer.2 Very recently, influencing the morphology
of the bulk heterojunction layer by simply changing the
spincoating solvent resulted in a substantial increase of the PV
power conversion efficiency to a record value of 2.5%.3 A more
intimate mixing of the components most likely resulted in both
a larger internal donor–acceptor interface area and increased
percolation of at least one of the components. Charge carrier
transport in the polymer phase was further improved on the
nanoscopic scale due to enhanced polymer inter-chain inter-
actions.
Another potential way to obtain desired bulk-heterojunction
architectures is through supramolecular assembly of the
constituents. Hydrogen bonding is particularly useful in the
construction of supramolecular structures.4 Strong non-cova-
lent binding can be obtained using multiple hydrogen bond
arrays, especially with Meijer’s self-complementary 2-ur-
eidopyrimidin-4-ones.5 With this unit, ultra-strong non-cova-
lent coupling can be achieved, reaching association constants of
> 108 M21 5c
.
Here we report on the preparation of a [60]fullerene
derivative bearing one 2-ureidopyrimidin-4-one moiety as the
hydrogen bonding unit. This molecule serves as a model
compound for supramolecular fullerene arrays based on
multiple hydrogen bonding interactions.
The synthesis of target molecule 8 started from easily
accessible 1,2-bis(hexyloxy)benzene.6 Friedel–Crafts acylation
yielded keto ester 1,7 which was transformed into the corre-
sponding p-tosylhydrazone 2. Subsequently, carboxylic acid 4
was prepared using the following one pot procedure:8 first,
heating the anion of 2 in the presence of [60]fullerene in o-
dichlorobenzene (ODCB) at 80–90 °C gave fulleroid 3a,
together with methanofullerene 3b, higher adducts and [60]full-
erene. This crude mixture was photoisomerised quantitatively
to a mixture of 3b, higher adducts and [60]fullerene. Hydrolysis
of this mixture yielded crude acid 4, which was purified by
column chromatography. The overall yield for the conversion of
2 into 4 was 36%. Carboxylic acid 4 was converted into the
corresponding acid chloride 5, after which reaction with sodium
azide yielded the corresponding acyl azide 6. Heating 6 in the
presence of 6-methylisocytosine at 80 °C for 2 h afforded 8 in
good yield.
† Electronic supplementary information (ESI) available: details of prepara-
tion and spectroscopic characterization of all compounds. See http://
www.rsc.org/suppdata/cc/b0/b008006n/
‡ See also the following paper in this issue: J. J. Gonza´lez, S. Gonza´lez,
E. M. Priego, C. Luo, D. M. Guldi, J. de Mendoza and N. Mart´ın, Chem.
Commun., 2001, DOI: 10.1039/b008005p.
Scheme 1 (a) p-TsNHNH2, MeOH, 6 h., D, 61%; (b) 1. NaOMe, py, 30
min., rt, C60, ODCB, 80–90 °C, 16 h; (c) ODCB, 500 W flood lamp, 100
min; (d) ODCB, HOAc, HCl, H2O, 16 h, 36% (2?4, 3 steps); (e) SOCl2,
CHCl3, 1 h; (f) NaN3, ODCB, DMAC, rt 75 min; (g + h) 2-amino-
4-hydroxy-6-methylpyrimidine, py, 70–80 °C, 2 h, 71% (4?8, 3 steps).
DOI: 10.1039/b008006n
Chem. Commun., 2001, 161–162
This journal is © The Royal Society of Chemistry 2001
161

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When filtered solutions of MDMO-PPV10 and 8 [both 1.0 (m/v)
% in chloroform] were mixed in a ratio of 1+4 and the mixture
was subsequently spincoated on a glass substrate, an optical
quality film was obtained.
Other supramolecularly interacting fullerene compounds
have been reported recently.11 The synthesis of 8, however,
represents the first example of a hydrogen bonded fullerene
dimer. With both fullerene cages connected through the very
strong quadruple hydrogen bonding motif, provided by the
ureidopyrimidinone moiety, the application in supramolecular
electronics is within reach. Non-covalent multifullerene arrays
based on bis(ureidopyrimidinone) substituted fullerenes are
currently under investigation.
These investigations were financially supported by the Dutch
Ministries of EZ, O&W and VROM through the EET program
(EETK97115) and by the Netherlands Organization for Energy
and the Environment (NOVEM) through the NOZ-PV Program
(146.120-008.3).
Fig. 1 (a) 1H NMR of 8 in CDCl3. (b) 1H NMR of equimolar solution of 8
and 9 in CDCl3.
Table 1 Redox properties
b
b
c
b
c
b
Com.a
E1
E2
E1red
E3
E2red
E4
red
red
red
red
Notes and references
3b
4
8
C60
9
20.66
20.72
20.68
20.60
—
21.04
21.09
21.01
21.09
—
—
—
21.15
—
21.55
21.60
21.56
21.63
—
—
22.00
22.02
22.02
22.07
—
21.80
21.84
—
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21.02
21.60
a Experimental conditions: V: vs. Ag wire, GCE as the working electrode,
Bu4NPF6 (0.1 M) as the supporting electrolyte, ODCB–MeCN (4+1) as the
solvent, 100 mV s21 scan rate. b Waves corresponding to the reduction
processes of the C60 cage. c Waves corresponding to the reduction processes
of the ureidopyrimidinone moiety.
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Ha–d in 1H NMR. The most pronounced shift is observed for the
resonance of Hc.
While 8 shows characteristic absorptions for a methanofuller-
ene in UV-Vis, the FTIR-spectrum showed a peak pattern at
1695, 1658, 1586 and 1513 cm21, indicative of a pyrimidin-
4(1H)-one dimer9 and the typical fullerene absorption at 526
cm21. The MALDI-TOF spectrum of 8 featured a parent peak
at m/z = 1217.8 for the monomer.
The redox behaviour of fullerene derivatives 3b, 4, 8, and that
of 95a was determined by cyclic voltammetry (Table 1). The
cyclic voltammogram of 3b showed four reversible waves
corresponding to reduction of the fullerene core, with values
typical for methanofullerenes.8 In the case of carboxylic acid 4
an additional wave was observed at 21.80 V, which was
attributed to the reduction of the acid moiety. Finally, the cyclic
voltammogram of 8 showed the similar four waves correspond-
ing to reduction of the fullerene core as well as a weak wave at
21.84 V and a shoulder at 21.15 V. The latter two waves are
related to the 2-ureidopyrimidin-4-one moiety as was confirmed
upon comparison with 95a (21.02 and 21.60 V).
In an exploratory experiment towards application in a PV
device, the processability of dimer 8 was shown as follows:
162
Chem. Commun., 2001, 161–162