Pushing fullerene absorption into the near-IR region by conjugately fusing oligothiophenes

Article abstract of DOI:10.1002/anie.201203981

Fusing two in one: The π-electron systems of fullerene and an oligothiophene were conjugately fused by an open-cage process. This led to novel fullerene-oligothiophene chromophores with significantly enhanced light-absorbing capability, which covers a wid

Full text of DOI:10.1002/anie.201203981

Angewandte
Chemie
DOI: 10.1002/anie.201203981
Fullerenes
Pushing Fullerene Absorption into the Near-IR Region by Conjugately
Fusing Oligothiophenes**
Zuo Xiao, Gang Ye, Ying Liu, Shan Chen, Qian Peng,* Qiqun Zuo, and Liming Ding*
Research in organic electronics and optics is rapidly advanc-
ing, and fullerene materials have received more and more
attention in this field owing to their unique electronic and
photophysical properties.[1] Fullerenes are among the best n-
type semiconductors for organic solar cells (OSC).[2] The
power conversion efficiency of conjugated polymer–fullerene
solar cells has exceeded 8%.[3] Besides, fullerenes have shown
their potential in organic thin-film transistors (OTFT) and
Scheme 1. Open-cage strategy for improving fullerene absorbance.
nonlinear optics (NLO).[4] However, the weak visible absorb-
ance and zero near-infrared (NIR) absorbance of fullerene
materials are their drawbacks for these applications. For
example, in typical polythiophene–fullerene solar cells, the
photocurrent contributed from fullerene absorption is only
13% of the total.[5] To expand the application of fullerenes in
organic electronics and optics, the synthesis of highly light-
absorbing fullerene materials is a necessary yet challenging
task.[2a] Herein, we present the incorporation of an oligothio-
phene into the fullerene p-system by an open-cage strategy.
The absorbance of the new fullerene materials was not only
significantly enhanced at the visible region, but also extended
to the NIR region. The band gap of the fullerene can be
lowered to about 1 eV.
Iwamatsu et al. reported the synthesis of 20-membered-
ring open-cage fullerenes through a novel ring-enlargement
reaction between skeleton-modified fullerenes and ortho-
substituted aromatic diamines.[7] Inspired by this work, we
designed two ortho-diamino-containing oligothiophene chro-
mophores 3T and 5T (Scheme 2). 3T and 5T were prepared
through straightforward methods including Stille coupling,
reduction, and bromination reactions with 2,5-dibromo-3,4-
dinitrothiophene as the starting material (see the Supporting
Information). The skeleton-modified fullerene 1 was pre-
The weak visible absorbance of fullerene originates from
its high molecular symmetry, which makes the lowest-energy
transitions dipole-forbidden.[6] The open-cage strategy pro-
posed here involves cutting the framework of fullerene and
fusing an external chromophore into the p-system of fullerene
(Scheme 1). Through this modification, the symmetry of
fullerene will be reduced, which relaxes the dipole selection
rules of low-energy transitions, and the p-system of fullerene
will be greatly extended. Therefore, the light-absorbing
property of fullerene can be greatly improved.
[*] Dr. Z. Xiao,[+] G. Ye,[+] Y. Liu, S. Chen, Prof. Dr. L. Ding
National Center for Nanoscience and Technology
Beijing 100190 (China)
E-mail: opv.china@yahoo.com
Dr. Q. Peng
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (China)
E-mail: qpeng@iccas.ac.cn
Q. Zuo
Jiahong Optoelectronics
Suzhou 215151 (China)
[+] These authors contributed equally to this work.
[**] This work was supported by the “100 Talents Program” of Chinese
Academy of Sciences, National Natural Science Foundation of
China (21102028), and Jiahong Optoelectronics.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203981.
Scheme 2. Synthesis of OC60-3T and OC60-5T.
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pared according to Murataꢀs open-cage process.[8] Treatment
of 1 with 3T (or 5T) and pyridine at 808C afforded the
expected orifice-enlarged product, OC60-3T, in 50% yield
(18% yield for OC60-5T). Through this reaction, the p-
systems of oligothiophene and fullerene are completely fused
to each other by a conjugated pyrazine bridge. OC60-3T and
OC60-5T can be viewed as a new type of fullerene-chromo-
phore dyads, but they are very different from traditional
fullerene dyads in which the p-systems of fullerene and the
chromophore are separated by a non-conjugated bridge.[9]
The conjugated bridge helps to realize one integrated
conjugation system and may facilitate cross-talking between
fullerene and chromophore.[10]
OC60-3T and OC60-5T were thoroughly characterized by
spectroscopic methods. A striking feature of the 1H NMR
spectra of these two compounds is that they both show
Figure 2. UV/Vis–NIR spectra of C60, C70, OC60-3T, and OC60-5T in
CHCl3 (10À5 m). Inset: enlargement of the spectra at 500–1100 nm.
a
singlet signal at high field (Figure 1). The signals,
tion than pristine C60 and C70. Taking the absorbance at
550 nm for example, the molar extinction coefficients for
OC60-3T and OC60-5T are 1.4 ꢁ 104 and 2.1 ꢁ 104 LmolÀ1 cmÀ1,
respectively, which are 15 and 22 times higher than that of C60,
and 1.4 and 2.1 times higher than that of C70. More
interestingly, OC60-3T and OC60-5T exhibit distinct and
broad absorption at the NIR region. A step-like absorption
covering the range of 700–900 nm is observed for OC60-3T. A
stronger absorption covering the range of 700–1100 nm with
the
absorption
maximum
at
856 nm
(e = 0.54 ꢁ
104 LmolÀ1 cmÀ1) is observed for OC60-5T. The absorption
edges of OC60-3T and OC60-5T are at 920 nm and 1100 nm,
respectively. The NIR absorption bands of OC60-3Tand OC60-
5Tare sensitive to solvent. The bands are slightly blue-shifted
in toluene or dioxane compared with that in CHCl3 (Support-
ing Information, Figure S12). The solvatochromism of OC60-
3Tand OC60-5T indicates that their NIR absorption probably
involves with a charge-transfer process.[14]
The long-wavelength absorption of OC60-3Tand OC60-5T
implies low band gaps of the molecules. We thus investigated
the electrochemical properties of the compounds by cyclic
voltammetry. OC60-3T and OC60-5T both show one quasi-
reversible oxidation peak and three quasi-reversible reduc-
tion peaks. The onset potentials (vs. Fc/Fc+) for oxidation and
reduction are 0.42 Vand À0.88 V for OC60-3T and 0.20 Vand
À0.85 V for OC60-5T, respectively. The HOMO and LUMO
energy levels of OC60-3T and OC60-5T were estimated from
their onset oxidation and reduction potentials through
empirical equations (Table 1).[15] The band gaps for OC60-3T
and OC60-5T derived from the difference between LUMO
and HOMO are 1.30 eVand 1.05 eV, respectively. To the best
of our knowledge, the 1.05 eV band gap for OC60-5T is among
the lowest band gaps for a fullerene-related p-system.[16]
Compared to the 1.9 eV band gap for pristine C60,[17] the
much lower band gaps of OC60-3T and OC60-5T indicate that
the open-cage strategy is very effective for tuning the band
gaps of fullerenes.
Figure 1. 1H NMR spectra of OC60-3T and OC60-5T.
À11.26 ppm for OC60-3T and À11.25 ppm for OC60-5T, can
be assigned to the protons of the H2O encapsulated inside
fullerene cage. As demonstrated in previous work, water
molecules in the solvent enter the fullerene automatically
only if the opening on fullerene is large enough for H2O to
pass through.[11] The driving force for this supramolecular
phenomenon can be attributed to the strong Lewis acidity of
the fullerene cage.[12] The large negative chemical shifts of the
encapsulated H2O are caused by the shielding of fullerene
electrons and can in turn provide the solid evidence for the
formation of large-orifice open-cage fullerenes.[13] Signals at
3.1–4.7 ppm belong to the four characteristic protons of the
methylene groups located at the rim of fullerene orifice,
which further confirm the structures of OC60-3T and OC60-
5T.[7,11b] High-resolution ESI-MS gives the expected molec-
ular ion peaks, 1481.3005 and 1645.2794, for OC60-3T and
OC60-5T, respectively.
The dark color of the dilute solution of OC60-3T (dark
red) and OC60-5T (dark green) indicates the dramatically
improved light-absorbing ability of the compounds. The UV/
Vis–NIR spectra of OC60-3T and OC60-5T as well as the
references C60 and C70 are shown in Figure 2. As expected,
OC60-3T and OC60-5T show greatly improved visible absorp-
To gain further understanding of the electronic and optical
properties of OC60-3T and OC60-5T, we performed quantum
chemical calculations for OC60-3T and OC60-5T without alkyl
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Table 1: Optical and electrochemical data.
might originate from intramolecular charge transfer (ICT)
on
on
[b]
from the oligothiophene donor to the ITPO acceptor. To
verify this hypothesis, we focus on a model compound N-5T
consisting of the same oligothiophene and ITPO moieties as
in OC60-5T. N-5T was synthesized by condensation of
ninhydrin and 5T in 63% yield (Scheme 3). As shown in
Compound
lmax
[nm]
Eox
[V]
Ered
[V]
HOMO[a]
[eV]
LUMO[a]
[eV]
Eg
[V]
OC60-3T
OC60-5T
338,
543
421,
553,
856
0.42
0.20
À0.88
À0.85
À5.52
(À5.16)[c]
À5.30
À4.22
(À3.24)
À4.25
1.30
(1.92)
1.05
(À4.86)
(À3.26)
(1.60)
[a] HOMO and LUMO energy levels were estimated using the following
equations: HOMO=À(Eoxon+5.1) eV, LUMO=À(Eredon+5.1) eV.
[b] Eg =LUMOÀHOMO. [c] Values in the parentheses are theoretical
values.
Scheme 3. Synthesis of the model compound N-5T.
chains. The geometries for OC60-3T and OC60-5T were
optimized at the B3LYP/6-31G* level. The HOMO and
LUMO for the two compounds are shown in Figure 3. It is
Figure 4, N-5T exhibits two characteristic absorption bands,
one in visible region and the other in NIR region, with
absorption maxima at 401 nm (e = 4.3 ꢁ 104 LmolÀ1 cmÀ1) and
702 nm (e = 0.83 ꢁ 104 LmolÀ1 cmÀ1), respectively. A solvato-
Figure 3. Frontier orbitals for a) OC60-3T and b) OC60-5T.
clear that in both cases, the HOMO is localized at the
oligothiophene moiety, while the LUMO is mainly localized
at the 9H-indeno[1,2-b]thieno[3,4-e]pyrazin-9-one (ITPO)
tetracyclic fragment connecting oligothiophene with fuller-
ene, and with some distribution on fullerene sphere. The
calculated HOMO and LUMO energy levels are À5.16 and
À3.24 eV for OC60-3T and À4.86 and À3.26 eV for OC60-5T,
respectively. Either experimental or theoretical values of the
HOMO and LUMO energy levels indicate that from OC60-3T
to OC60-5T, the LUMO drops slightly while the HOMO rises
significantly by about 0.3 eV (Table 1). Therefore, the reduc-
tion in band gaps of the open-cage fullerenes is mainly caused
by the rise of the HOMO, which is located at the oligothio-
phene moiety. This band-gap tuning process through the
open-cage strategy for fullerenes is quite similar to that of the
donor–acceptor strategy for developing low-band-gap con-
jugated copolymers.[18]
Figure 4. UV/Vis–NIR spectra of OC60-5T and N-5T in CHCl3 (10À5 m).
chromism study indicates that the high-energy band is not
sensitive to solvent polarity, while the NIR band is quite
sensitive to solvent and shows remarkable blue-shifts (ca.
30 nm) in polar solvents such as dioxane, DMF, and DMSO
compared with that in CHCl3 (Supporting Information,
Figure S12). Therefore, the visible absorption could be
attributed to the integration of the respective absorption of
oligothiophene and ITPO,[19] and the NIR absorption might
result from ICT.[14] Comparison between N-5T and OC60-5T
absorption spectra indicates that the fullerene p-system has
a strong coupling with the p-system of ITPO fragment, which
enhances the electron-accepting strength of ITPO and leads
to a significant red-shift (152 nm) for ICT band.
In summary, by fusing the p-electron systems of oligo-
thiophene and fullerene through the open-cage strategy, we
significantly enhanced the visible absorbance of fullerenes
and pushed their absorption into NIR region. This optical
property improvement concerns the following factors:
1) symmetry breaking of the fullerene by open-cage reactions
allows more low-energy transitions to take place; 2) addi-
TD-DFT calculations indicate that the low-energy tran-
sition from HOMO to LUMO of OC60-3T or OC60-5T
dominates their NIR absorbance (see the Supporting Infor-
mation). Considering that the HOMO is at the oligothio-
phene and the LUMO is mainly at the ITPO fragment, we
suggest that the NIR absorption of OC60-3T and OC60-5T
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Received: May 23, 2012
Revised: June 27, 2012
Published online: && &&, &&&&
Keywords: band gaps · fullerenes · molecular electronics ·
.
NIR absorption · quantum chemistry
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Angew. Chem. Int. Ed. 2012, 51, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Fullerenes
Fusing two in one: The p-electron sys-
tems of fullerene and an oligothiophene
were conjugately fused by an open-cage
process. This led to novel fullerene–
oligothiophene chromophores with sig-
nificantly enhanced light-absorbing
capability, which covers a wide spectral
range. The fullerene band gap could be
tuned to about 1 eV by a chemical
approach.
Z. Xiao, G. Ye, Y. Liu, S. Chen, Q. Peng,*
Q. Zuo, L. Ding*
&&&& — &&&&
Pushing Fullerene Absorption into the
Near-IR Region by Conjugately Fusing
Oligothiophenes
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
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