Click -functionalization of [60]fullerene and graphene with an unsymmetrically functionalized diketopyrrolopyrrole (DPP) derivative

Article abstract of DOI:10.1021/ol301028r

A synthetic strategy is developed that allows for the facile functionalization of carbon nanostructures thus providing the possibility of comparing the striking different optical and electrochemical properties of ensembles based on the diketopyrrolopyrrole (DPP) chromophore covalently attached to either [60]fullerene or graphene.

Full text of DOI:10.1021/ol301028r

ORGANIC
LETTERS
2012
Vol. 14, No. 11
2798–2801
“Click”-Functionalization of [60]Fullerene
and Graphene with an Unsymmetrically
Functionalized Diketopyrrolopyrrole
(DPP) Derivative
†,‡
Marta Castelaın, Horacio Salavagione,* and Jose L. Segura*
,‡
,†
ꢀ
´
´
Departamento de Quımica Organica, Facultad de Quımica, Universidad Complutense de
Madrid, E-28040 Madrid, Spain, and Departamento de Fısica de Polımeros,
ꢀ
´
´
´
ꢀ
CSIC, c/Juan de la Cierva 3, 28006 Madrid, Spain
ꢀ
´ ´
Elastomeros y Aplicaciones Energeticas, Instituto de Ciencia y Tecnologıa de Polımeros,
Horacio@ictp.csic.es; Segura@quim.ucm.es
Received April 19, 2012
ABSTRACT
A synthetic strategy is developed that allows for the facile functionalization of carbon nanostructures thus providing the possibility of comparing
the striking different optical and electrochemical properties of ensembles based on the diketopyrrolopyrrole (DPP) chromophore covalently
attached to either [60]fullerene or graphene.
Recently, diketopyrrolopyrrole (DPP)-based materials
have attracted much interest due to their promising per-
formance as a subunit in OFETs,1 organic photovoltaics
(OPV),2organic light-emitting diodes (OLEDs),3chemosensors,4
two-photon absorption,5 and solid-state dye lasers6 among
others.
For these purposes, both polymers7 and small mole-
cules8 have been synthesized based on the DPP core. In
this regard, because of the interesting photovoltaic proper-
ties observed in bulk heterojunction solar cells based
on blends of fullerene and DPP derivatives,9 very recently-
dyads10 and triads11 based on covalently linked DPP
derivatives and C60 have received some attention with
† Universidad Complutense de Madrid.
‡ CSIC.
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Adv. Mater. 2010, 22, 4617. (b) Li, Y. N.; Singh, S. P.; Sonar, P. Adv.
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r
10.1021/ol301028r
Published on Web 05/14/2012
2012 American Chemical Society

the aim of investigating charge separation and photo-
physical processes as well as their performance in photo-
voltaic devices.
dialkylated (3)14 and monoalkylated (4) derivatives in a
33% and 25% yield respectively. Both compounds can be
separated chromatographically, and further reaction of 4
with 1-azido-6-bromohexane (5)15 afforded the target un-
symmetrical DPP derivative(6) with an azido functionality
in a 36% yield (Scheme 1; see Supporting Information for
further details).
Here, we describe the synthesis of a novel unsymmetric
DPP derivative bearing azide moieties only in one of the
pyrrole units of the DPP core (6, Scheme 1). Additionally,
we use an extended side chain [2-hexyldecyl (HD)] on
the other nitrogen of the DPP moiety, to obtain proces-
sable materials. We show that this novel building block
can be used for the facile functionalization of not only
[60]fullerene but also other carbon nanostructures such as
graphene by means of a click coupling through the Cu-
catalyzed azideÀalkine cycloaddition (CuAAC) reaction.12
We subsequently investigated the Cu(I)-catalyzed Huis-
gen 1,3-dipolar cycloaddition of the DPP derivative 6 with
both [60]fullerene (7)16 and graphene (G-ALK)17 deriva-
tives endowed with alkine functionalities (Scheme 2). The
reaction with the [60]fullerene derivative (7) was carried
out by using the conditions previously described by some
of us, i.e. with (CH3CN)4PF6 in THF for 3 days at rt in the
presence of 1 equiv of Cu powder.16 The resulting sample
was separated on silica gel in a relatively straightforward
fashion to obtain DPP-C60 in a 32% yield. The 1H NMR
spectrum of 6 is characterized by the typical singlet of the
1,2,3-triazole unit at 7.50 ppm as well as by the signal
corresponding to the CH2-triazole protons at 4.31 ppm.
The conversion of the azido to 1,2,3-triazole groups is also
confirmed by the disappearance of the strong azido band
at 2092 cmÀ1 in 6 and the appearance of the CÀC double
bond stretching vibration of the triazole ring at 1504 cmÀ1
in DPP-C60 (Figure S1).The structure of DPP-C60 has also
been confirmed by MALDI-TOF mass spectrometry
showing the expected molecular ion peak at m/z 1656.5.
Having succeeded in the reaction between the stable
DPP-azide building block 6 and the [60]fullerene deriva-
tive, we further investigated the reactivity of 6 with an
alkyne-modified graphene (G-ALK) by taking advantage
of our recently developed strategy to produce this type of
graphene derivative.17 The presence of the modified alkyne
moiety on the graphene surface significantly improves
solubility (e.g., in DMF) which allows the click reaction
to take place in an homogeneous phase.
Scheme 1. Synthesis of Azido-Functionalized DPP Derivative 6
Reaction of 3,6-dithiophen-2-yl-2,5-dihydropyrrolo[3,4-c]-
pyrrole-1,4-dione (1)13 with 1-bromo-2-hexyldecane
(2) under stoichiometric control afforded a mixture of
The existence of click linkage between DPP and gra-
phene was confirmed by complementary FTIR and
Raman spectroscopies. While the FTIR allows following
changes in the vibrational spectrum of the organic dye,
Raman spectroscopy is very sensitive to the presence of
graphitic species, even though some remaining fluores-
cence of DPP can worsen the quality of the spectrum.
Thus, the FTIR spectrum of DPP-G, obtained by ATR
microspectroscopy, shows the almost complete disap-
pearance of the azide band around 2100 cmÀ1 and the
presence of new bands at 1488 and 1160 cmÀ1 correspond-
ing respectively to the CdC and C;N stretching vibra-
tions of the triazole ring (Figure S3).17 In addition, the
originally symmetric band due to the CdO in the DPP
(at 1663 cmÀ1) becomes wider and asymmetric and shifts
to higher frequency (1690 cmÀ1) in DPP-G because it now
has contributions from the carbonyl group of DPP as well
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Org. Lett., Vol. 14, No. 11, 2012
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Scheme 2. “Click” Reactions for the Synthesis of DPP-C60 and DPP-G
as the remaining carboxylic groups in graphene. Further-
more, the Raman spectrum of a DPP-G thin film onto Si/
DPP-[60]fullerene ensembles this quenching is due to the
very fast decay of the S1 state of the DPP moiety, by either
energy transfer to the C60 units or electron transfer from
the DPP to the C60.18
SiO2 displays only two strong signals at 1598 and 1356 cmÀ1
.
As widely known these signals correspond to the G
and D modes of graphitic structures, related to the sp2
carbon network and structural defects on it, respectively
(Figure S3).
UV/vis absorption spectra of the symmetric DPP deri-
vative 3 and the corresponding dyad DPP-C60 are given in
Figure 1. The absorption of the dyad resembles a super-
position of the spectra of the separate DPP and N-methyl-
fulleropyrrolidine 7. Absorption peaks at 547, 511, 356,
339, and 292 nm correspond to the DPP derivative
overlapping with a broad absorption below 400 nm corre-
sponding to fullerene. No significant electronic interac-
tions (e.g., charge-transfer absorptions) can be observed in
the absorption spectrum of the dyad DPP-C60 in agree-
ment with that previously observed for other DPP-full-
erene ensembles.18 In the cyclic voltammogram (CV) of the
symmetric DPP derivative 3 two reversible oxidations and
one reversible reduction can be observed (Figure 2a).
From the onsets of oxidation and reduction, the elec-
trochemical band gap of 3 can be calculated as 2.04 eV
which is in good agreement with the optical band gap.
On the other hand, dyad DPP-C60 gives rise, in addition
to the redox processes observed for the DPP derivative
6, to three quasireversible one-electron reduction
waves that reflect the first three one-electron reduction
steps of the fullerene core. This behavior is another
indication of the lack of significant interactions be-
tween both moieties in the ground state.
Figure 1. UV/vis absorption spectra of DPP, DPP-C60 (left y-axis),
and DPP-G (right y-axis) in N-methyl-2-pyrrolidone (NMP)
solutions.
In comparison with the DPP-[60]fullerene dyad DPP-
60 it is noted that the absorptions of the DPP derivative at
C
511 and 547 nm in DPP-G are broadened, weakened, and
blue-shifted to 435 nm (Figure 1). The weakening of the
absorption is due to the strong π plasmon absorption of
graphene that partially masks the DPP transitions.19
The blue shift of the DPP absorption in DPP-G can be
rationalized in terms of a strong πÀπ stacking between the
planar DPP moiety and the flat surface of graphene that
Concerning the emission properties of the dyad DPP-
C60, a significant quenching of the characteristic fluores-
cence of the DPP derivative 1 was observed which is a
strong fluorophore with anemission quantum yield ashigh
as 0.62 (Figure S2). As in other previously reported
(19) (a) Cioffi, C.; Campidelli, S.; Sooambar, C.; Marcaccio, M.;
Marcolongo, G.; Meneghetti, M.; Paolucci, D.; Paolucci, F.; Ehli, C.;
Rahman, G. M. A.; Sgobba, V.; Guldi, D. M.; Prato, M. J. Am. Chem.
Soc. 2007, 129, 3938. (b) Garlaschelli, L.; Messina, I.; Pasini, D.; Righett,
P. P. Eur. J. Org. Chem. 2002, 20, 3385.
(18) Karsten, B. P.; Bouwer, R. K. M.; Hummelen, J. C.; Williams,
R. M.; Janssen, R. A. J. Photochem. Photobiol. Sci. 2010, 9, 1055.
2800
Org. Lett., Vol. 14, No. 11, 2012

may be attributed to the fact that πÀπ stacking may render
an effective intermolecular orbital overlap. This orbital over-
lap between the LUMO level of the DPP and the graphene
work function accounts for a splitting of the empty level22
translating into a larger LUMO bandwidth which is in
agreement with the anodic shift of the reduction potential
of the DPP moiety. Given that the orbital overlap depends on
the intermolecular distance and orientation,23 a densely
packed structure may be responsible for an efficient orbital
overlap and therefore a pronounced effect observed for the
sample with graphene.
Finally, the emission of the DPP-G system shows no
emission under visible excitation at 433 nm, as occurred
with DPP-C60 (Figure S4).
In summary, we have developed a synthetic strategy that
allows for the facile functionalization of carbon nano-
structures thus providing the possibility of comparing the
striking different optical and electrochemical properties of
ensembles based on either [60]fullerene or graphene. The
differences in dimensions and shape of the two different
carbon nanostructures account for the more efficient
intramolecular contact and effective interactive cross-
section between the planar DPP moiety and the flat
graphene. The DPP clicked to [60]-fullerene, even in its
most stable configuration, can only interact with a small
part of the curved C60 surface, whereas in DPP-G the
organic molecule is surrounded by large amounts of
carbonaceous surface with which to interact (Figure S5).
This synthetic strategy can be extended to molecular and
polymeric DPP derivatives with lower band gaps. Work is
in progress to further investigate the photophysical behav-
ior of this kind of novel ensemble and to explore their
performance in photovoltaic devices.
Figure 2. (a) CV of DPP, C60, and DPP-C60 recorded in Cl2CH2/
0.1 M tetrabutylammonium hexafluorophosphate (TBAHFP)
electrolyte. Scale bars for all curves correspond to 3 μA. (b) CV
of DPP and DPP-G in NMP/0.1 M TBAHFP electrolyte. Scale
bars corrrespond to 3.5 and 20 μA for DPP and DPP-G,
respectively.
may render an effective intermolecular orbital overlap.
This is facilitated by the long flexible alkyl chain used as a
linker between the DPP and graphene moieties.
Acknowledgment. Financial support from MICINN
ꢀ
(CTQ2010-14982, MAT2009-09335), Comunidad Autonoma
de Madrid (S2009/MAT-1467), and the UCM-BSCH joint
project (GR58/08) is gratefully acknowledged. H.J.S. thanks
MICINN for a Ramon y Cajal Fellow. M.C. is indebted to
CSIC for a JAE-predoc fellowship. Authors thank Dr. G. Ellis
(ICTP-CSIC) for help with ATR microspectroscopy measure-
ments and Elena Mena-Osteritz (Univ. Ulm) for fruitful
discusions and valuable suggestions.
Regarding the electrochemical behavior, the presence
of graphene remarkably alters the CV of DPP, further
highlighting the differences with the DPP-C60 system.
Despite the fact that bands at potentials higher than
1.1 V cannotbe observed due tothelimitedelectrochemical
windows of NMP, the reduction bands of DPP shift
dramatically to less cathodic potentials (over 900 mV)
suggesting a strong communication between graphene
and DPP (Figure 2b). This large displacement has already
been observed for other π-conjugated electroactive moi-
eties covalently attached to carbon nanotubes20 and non-
covalently assembled with graphene.21 Qualitatively, this
Supporting Information Available. Complete experi-
mental details, spectroscopic characterization, and en-
ergies of HOMOs/LUMOs and band gaps of click pro-
ducts. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Jones, T. G. J.; Compton, R. G. J. Phys. Chem. B 2005, 109, 3971. (b)
Heald, C. G. R.; Wildgoose, G. G.; Jiang, L.; Jones, T. G. J.; Compton,
R. G. ChemPhysChem 2004, 5, 1794.
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(23) Bredas, J. L.; Calbert, J. P.; da Silva, D. A.; Cornil, J. Proc. Natl.
Acad. Sci. U.S.A. 2002, 99, 5804.
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The authors declare no competing financial interest.
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