Energy level tunable pre-click functionalization of [60]fullerene for nonlinear optics

Article abstract of DOI:10.1016/j.tet.2013.12.018

[2+2] click chemistry followed by Prato reaction is reported the first time for energy level tunable functionalization of C₆₀. The novel kind of D-A-A C₆₀ charge-transfer complexes exhibits strikingly enhanced optoelectronic properties and significant third-order nonlinear optical potentials. Meanwhile, structures of the 'click' C₆₀ products were fully characterized by NMR, IR, and MS. By introducing TCNE, TCNQ, F ₄-TCNQ click monomers, we obtained energy level tunable fullerene derivatives in high solubility, which was beneficial for processing. Furthermore, UV-vis spectra were enlarged with end-absorptions into the near infrared region, indicating excellent nonlinear optical properties. The electrochemical properties have been studied by cyclic voltammograms, which were in good correlation with the optical spectroscopic and DFT data.

Full text of DOI:10.1016/j.tet.2013.12.018

Tetrahedron 70 (2014) 573e577
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Energy level tunable pre-click functionalization of [60]fullerene
for nonlinear optics
,
a
b,
Wan-Shu Zhang a, Dong Wang a , Hui Cao , Huai Yang
*
*
a University of Science and Technology Beijing, Beijing 100083, PR China
b Beijing University, 100871, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
[2þ2] click chemistry followed by Prato reaction is reported the first time for energy level tunable
functionalization of C60. The novel kind of DeAeA C60 charge-transfer complexes exhibits strikingly
enhanced optoelectronic properties and significant third-order nonlinear optical potentials. Meanwhile,
structures of the ‘click’ C60 products were fully characterized by NMR, IR, and MS. By introducing TCNE,
TCNQ, F4-TCNQ click monomers, we obtained energy level tunable fullerene derivatives in high solubility,
which was beneficial for processing. Furthermore, UVevis spectra were enlarged with end-absorptions
into the near infrared region, indicating excellent nonlinear optical properties. The electrochemical
properties have been studied by cyclic voltammograms, which were in good correlation with the optical
spectroscopic and DFT data.
Received 9 September 2013
Received in revised form 28 November 2013
Accepted 5 December 2013
Available online 13 December 2013
Keywords:
Nonlinear optics
[60]fullerene
Click chemistry
Energy level modulation
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
click reagents and alkynes, i.e., ‘[2þ2] click reaction’7 has gained
enormous attention. Taking advantage of this reaction, we should
Materials with large third-order optical nonlinearities and fast
nonlinear optical (NLO) responses are considered to be promising
candidates for photonic applications, such as optical communica-
tion, data processing, optical switching, and so forth.1 The study of
molecular materials for third-order nonlinear optics has focused
primarily on organic compounds,2 among which materials with
high molecular hyperpolarizabilities (strongly dipolar conjugated
be able to availably modulate energy levels and optimize electronic
states via appropriate option of click reagents with different con-
jugation lengths. To the best of our knowledge, there is no report so
far on the use of [2þ2] click reaction to improve the solubility and
optoelectronic properties of fullerenes. Therefore, the combination
of the well-known Prato reaction8 and the above-mentioned high-
yielding click reaction is a powerful and novel strategy in the
functionalization of C60 (Scheme 1).
p
-systems) exhibit strong third-order nonlinearity.3 Especially,
electron-deficient C60 stands out in view of its unique three-
dimensional -electron delocalization pattern.
p
However, the poor solubility, relatively large bandgap and low
charge delocalization of pristine fullerenes severely limit their de-
rivatization and utilization.4 To circumvent this hurdle, a variety of
approaches5 have been developed. In particular, covalent func-
tionalization is attracting wide interest as it can greatly alter the
physical and chemical properties of fullerene through attachment
of strong electron-withdrawing soluble moieties. Clearly, effective
synthetic methodologies for the appending side chains are of great
significance, in which high selective and productive click chemis-
try6 comes to the forefront.
Scheme 1. General functionalization of fullerene via Prato reaction.
As a highly efficient and catalyst-free reaction, the thermal
[2þ2] cycloaddition followed by retro-electrocyclization, between
2. Results and discussion
2.1. Synthesis
The overall synthetic scheme was depicted in Fig. 1, and high-
light was the novel type of monodonorebisacceptors (DeAeA)
* Corresponding authors. Tel./fax: þ86 10 62767426; e-mail addresses: yinuben@
163.com (D. Wang), yanghuai@coe.pku.edu.cn (H. Yang).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.12.018

574
W.-S. Zhang et al. / Tetrahedron 70 (2014) 573e577
Fig. 1. General scheme for the synthesis of click products C series (C1eC3) and further functionalized DeAeA type C60 derivatives S series (S1eS3). The percentage in the brackets
was productivity.
charge-transfer C60 complexes (S1eS3). The synthesis of the
DeAeA type C60 derivatives involved a two-step procedure. First,
the series of strong electron donors C1eC3 was synthesized by the
high-yielding [2þ2] click reaction using ditetracyanoethene
(TCNE), 7,7,8,8-tetracyanoquiodimethane (TCNQ), and 2,3,5,6-
tetrafluoro-7,7,8,8-tetra cyanoquinodimethane (F4-TCNQ) as click
reagents. Then the electron donors were appended by reacting with
fullerene and sarcosine in o-dichlorobenzene (o-DCB). The [2þ2]
click reactions had significantly high yields approximately 90%. The
novel ‘click’ C60 products S1eS3 with tunable energy levels and
different conjugation lengths exhibited high stability to moisture
and air as well as enhanced solubility in common organic solvents,
as compared to C60. All the newly synthesized compounds were
fully characterized by NMR, FTIR, UVevis, mass and other spec-
troscopic methods.
2.2. UVevis spectroscopy
The normalized UVevis absorption spectra of the click com-
pound C2 and the corresponding C60 derivative S2 were shown in
Fig. 2a. The absorption peaks of C2 were located at 270, 305, 489,
and 693 nm, respectively. Upon appending fullerene, these bands
were shifted to 291, 320, 479, and 716 nm, respectively. The band at
291 nm was much higher in intensity due to the overlap of the
fullerene absorption band.9 The 23 nm bathochromic shift of the
end absorption peak was attributed to the electronic interactions
between the click donor and C60 acceptor moieties.10 As shown in
Fig. 2b, all the prepared C60 derivatives exhibited intense charge-
transfer (CT) bands with end-absorptions under 1000 nm, in-
dicating excellent nonlinear optical materials.11 Compared with S1
(477 nm), the red-shift in the maximum absorption was 241 and
365 nm for S2 and S3, respectively. The extent of bathochromic
shifts was dramatically facilitated by the elongation of conjugation
lengths of molecular backbones and enhancement of electron af-
finities via introduction of strong electron-withdrawing sub-
stitutes,12 from which an effective way to modulate the optic
properties of C60 derivatives was achieved.
Fig. 2. UVevis absorption spectra of (a) C2 and S2; (b) S1eS3 in methylbenzene at
298 K. The inset photographs were the chemical structures.
theory (DFT) calculations.13 In Fig. 3, the optimized structures and
electron densities of the lowest unoccupied molecular orbital
(LUMO) and the highest occupied molecular orbital (HOMO) of the
‘click’ C60 derivatives were shown. From S1 to S3, The LUMO energy
was greatly decreased at the same time the HOMO was slightly
2.3. Electrochemistry
The electronic structures of the ‘click’ C60 derivatives in the
ground electronic state were visualized by density functional

W.-S. Zhang et al. / Tetrahedron 70 (2014) 573e577
575
red
On the other hand, S2 and S3 exhibited easier reductions (E at
on
ꢀ0.64 and ꢀ0.54 V, respectively), probably caused by the expanded
p-conjugation of the donor moieties and introduction of strong
electron-withdrawing radicals, such as fluorine atoms in F4-
TCNQ.15 Consequently, from S1 to S3, the LUMO energy levels and
electrochemical band gaps calculated from the onset oxidation and
reduction potentials, were both on the monotone decreasing trend
(Fig. 4(2)). Furthermore, these electrochemical band gaps were in
good correlation with the optical spectroscopic and DFT data that
they were reduced upon moving from S1 to S2 and on to S3, as
summarized in Table 1.
2.4. Nonlinear optics
Z-scan experiments were utilized to verify the possibility of NLO
enhancements via functionalization of C60, with parameters sum-
marized in Table 1. Furthermore, Fig. 5 displayed the nonlinear
absorptive and refractive behavior of S2 and a good fit between the
experimental data and the theoretical curve was achieved. As
shown in Fig. 5a, the absorption decreased with increasing in-
tensity of the incident laser, with the light transmittance increasing
to 115% at the focal point. Except for S3 (Introduction of strong
electron-withdrawing atom fluorine greatly impaired NLO prop-
erties, leading to no refractive data), the third-order nonlinear
Fig. 3. Optimized structures and electron distributions of LUMO and HOMO of (1) S1;
(2) S2; (3) S3 calculated at the B3LYP/3-21G level.
increased. Consequently, the LUMOeHOMO gap was gradually
lowered due to the increase of
p-electron numbers and strain en-
ergy release of C60 carbon cage.14 The electron densities in the
LUMO were all located on the C60 moiety, suggesting that C60 act as
the acceptor. On the other hand, except for S1, the HOMOs were
found on the click entities, indicating that the click moieties behave
as the donor groups and suggesting the hole distribution in the
charge-separated state. To confirm this computational prediction,
electrochemical studies were performed to evaluate the redox
potentials of the ‘click’ C60 derivatives.
optical susceptibility (c
(3)) was in the order of C6011<S1<S2, which
could be ascribed to the structural symmetry violation17 and the
elongation of conjugation length18 via functionalization. Therefore,
this could be an efficient guidance for molecular design to improve
NLO properties of fullerene.
3. Conclusions
The cyclic voltammograms and energy levels of the ‘click’ C60
derivatives were shown in Fig. 4(1). Compared with S1, the TCNQ
and F4-TCNQ adducted click-fullerenes S2 and S3 showed anodi-
cally shifted onset oxidation potentials (Eooxn) at 0.47 and 0.53 V,
which was assigned to the more efficient interaction of the click
donor moiety and C60 acceptor, making the oxidation more difficult.
In summary, a series of novel DeAeA type C60 derivatives was
synthesized via a two-step procedure of [2þ2] click chemistry
followed by Prato reaction. Computational studies of DFT predicted
the enlargement of conjugation lengths and electron-withdrawing
abilities to decrease energy gaps, and both electrochemical and
Fig. 4. (1) Cycle voltammograms of ‘click’ C60 derivatives S1eS3 in methylbenzene/Bu4NPF6 at 293 K; (2) LUMO and HOMO energy levels of C60 (a: see Ref. 16) and S1eS3.
Table 1
Electrochemical and NLO properties of products S1eS3
Materials
Eooxna (V)
Eroenda (V)
Ebg (eV)
Ecg (eV)
Edg (eV)
NLO properties
b
(ꢁ10ꢀ12 m/W)
n2 (ꢁ10ꢀ19 m2/W)
c
(3) (ꢁ10ꢀ13 esu)
S1
S2
S3
0.42
0.47
0.53
ꢀ0.83
ꢀ0.64
ꢀ0.54
1.25
1.11
1.07
1.87
1.30
1.06
2.55
2.31
1.60
ꢀ4.65
ꢀ7.75
ꢀ6.28
3.37
4.76
d
5.24
8.83
d

576
W.-S. Zhang et al. / Tetrahedron 70 (2014) 573e577
Fig. 5. Z-scan results of S2 in tetrahydrofuran. The half-open circles represent the Z-scan experimental data, and the solid lines are the theoretical fitting curves: (a) data collected
under the open-aperture configuration; (b) data collected under the closed-aperture configuration.
UVevis studies proved that this was indeed the case. Meanwhile,
UVevis spectra were enlarged with end-absorptions into the near
infrared region, indicating excellent nonlinear optical properties.
Furthermore, Z-scan measurements to investigate the third-order
NLO properties confirmed that the ‘click’ C60 derivatives have ex-
cellent nonlinear absorptive and refractive properties. The present
study opens up a venue for utilization of energy level tunable ful-
lerene derivatives in nonlinear optics.
461.6 g molꢀ1, found: 461.2 g molꢀ1 [MH]þ; elemental analysis
calcd (%) for C29H27N5O (461.6): C 75.46, H 5.90, N 15.17; found: C
75.49, H 5.90, N 15.19.
4.1.2. 2-(1-(4-(Dibutylamino) phenyl)-2-(4-(dicyanomethylene) cy-
clohexa-2,5-dien-1-ylidene)-2-(4-formylphenyl) ethylidene) malono-
nitrile (C2). The click monomer (0.0837 g, 0.250 mmol) and click
reagent TCNQ (0.051 g, 0.250 mmol) were dissolved in di-
chlorobenzene (10 mL). The reaction mixture was then stirred at
50 ꢂC for 3 h under argon atmosphere. The mixture was concen-
trated, rediluted with CH2Cl2, and filtered through a plug of silica
gel. The solvent was removed in vacuo and the crude product was
purified by column chromatography (SiO2, ethyl acetate/petroleum
ether¼1:3) to give C2 (0.122 g, 91%) as a yellow-brown solid. 1H
4. Experimental section
4.1. General
Reagents were purchased from commercial sources (Aldrich)
and used without further purification. 1H NMR spectra were mea-
sured on a Bruker AV300 NMR spectrometer (300 MHz) at 20 ꢂC.
Chemical shifts are reported in ppm downfield from SiMe4, using
the solvent’s residual signal as an internal reference. FTIR was
recorded on a Perkin Elmer LR-64912C Fourier transform infrared
spectrometer. All MALDI-TOF-MS spectra were measured on a Shi-
madzu AXIMA-CFR mass spectrometer. The operation was per-
formed at an accelerating potential of 20 kV by a linear positive ion
mode with dithranol as a matrix. UVevis spectra were recorded in
a quartz cuvette on a JASCO V-570 spectrophotometer. Cyclic vol-
tammetric measurements were carried out in a conventional three-
electrode cell using Glassy Carbon working electrodes of 2 mm
diameter, a platinum wire counter electrode, and an Ag/AgCl ref-
erence electrode on a computer-controlled CHI 660C instrument at
room temperature. The nonlinear optical properties (NLO) response
was measured by means of Z-scan technique, employing 20 ps laser
pulses at 532 nm delivered by a mode-locked Nd:YAG laser.
NMR (CDCl3, 300 MHz):
d
¼0.96 (6H, t), 1.34 (4H, m), 1.57 (4H, m),
3.35 (4H, t), 6.64 (2H, d, J¼9.2), 7.24 (4H, m), 7.53 (2H, d, J¼8.8), 7.77
(2H, d, J¼8.4), 7.95 (2H, d, J¼8.4), 10.04 (1H, s) ppm. FTIR (KBr,
cmꢀ1):
n
¼2958, 2930, 2201, 1699, 1584, 1523, 1400, 1348, 1292,
1209, 1186, 1110, 920, 888, 825, 795, 682. MALDI-TOF-MS (dithra-
nol): m/z: calcd for C35H31N5O: 537.7 g molꢀ1, found: 537.3 g molꢀ1
[MH]þ; elemental analysis calcd (%) for C35H27F4N5O (537.7): C
78.19, H 5.81, N 13.03; found: C 78.21, H 5.82, N 13.04.
4.1.3. 2-(1-(4-(Dibutylamino)
phenyl)-2-(4-(dicyanomethylene)-
2,3,5,6-tetrafluorocyclohexa-2,5-dien-1-ylidene)-2-(4-formylphenyl)
ethylidene)malononitrile (C3). The click monomer (0.0837 g,
0.250 mmol) and click reagent F4-TCNQ (0.069 g, 0.250 mmol) were
dissolved in dichlorobenzene (10 mL). The reaction mixture was
then stirred at 80 ꢂC for 3 h under argon atmosphere. The mixture
was concentrated, rediluted with CH2Cl2, and filtered through
a plug of silica gel. The solvent was removed in vacuo and the crude
product was purified by column chromatography (SiO2, ethyl ace-
tate/petroleum ether¼1:2) to give C3 (0.137 g, 90%) as a black solid.
4.1.1. 2-(4-(Dibutylamino) phenyl)-3-(4-formylphenyl) buta-1,3-
diene-1,1,4,4-tetracarbonitrile (C1). The click monomer (0.0837 g,
0.250 mmol) and click reagent TCNE (0.032 g, 0.250 mmol) were
dissolved in dichlorobenzene (10 mL). The reaction mixture was
then stirred at 30 ꢂC for 1 h under argon atmosphere. The mixture
was concentrated, rediluted with CH2Cl2, and filtered through
a plug of silica gel. The solvent was removed in vacuo and the crude
product was purified by column chromatography (SiO2, CH2Cl2) to
give C1 (0.103 g, 90%) as a brown solid. 1H NMR (CDCl3, 300 MHz):
1H NMR (CDCl3, 300 MHz):
d
¼0.86 (6H, t), 1.23 (4H, m), 1.42 (4H,
m), 3.57 (4H, m), 6.81 (2H, d, J¼8.8), 7.24 (2H, m), 7.72 (2H, d, J¼7.6),
7.99 (2H, d, J¼6.4), 10.07 (1H, s) ppm. FTIR (KBr, cmꢀ1):
¼2959,
n
2872, 2190, 2147, 1701, 1639,1600, 1520, 1482,1393, 1290, 1196, 970,
826, 747. MALDI-TOF-MS (dithranol): m/z: calcd for C35H27F4N5O:
609.6 g molꢀ1, found: 609.2 g molꢀ1 [MH]þ; elemental analysis
calcd (%) for C35H27F4N5O (609.6): C 68.96, H 4.46, N11.49; found: C
68.99, H 4.46, N 11.47.
d
¼0.97 (6H, t), 1.38 (4H, m), 1.61 (4H, m), 3.92 (4H, t), 6.68 (2H, d,
J¼9.2), 7.76 (2H, d, J¼9.6), 7.85 (2H, d, J¼8.4), 8.01(2H, d, J¼8.8),
10.08 (1H, s) ppm. FTIR (KBr, cmꢀ1):
1698, 1597, 1523, 1466, 1402, 1369, 1284, 1207, 1165, 1134, 927, 830,
816, 518. MALDI-TOF-MS (dithranol): m/z: calcd for C29H27N5O:
4.1.4. ‘Click’ C60 derivative S1. A mixture of C60 (115 mg, 0.16 mmol)
and sarcosine (13 mg, 0.15 mmol) was dissolved in 10 mL of o-DCB
under argon atmosphere at 60 ꢂC. Afterward, a solution of C1
(69 mg, 0.15 mmol) in 5 mL of o-DCB was added. The reaction
n
¼2957, 2872, 2727, 2206,

W.-S. Zhang et al. / Tetrahedron 70 (2014) 573e577
577
mixture was heated at 110 ꢂC for an additional 8 h. After being
cooled to room temperature the solvent was evaporated. Unreacted
C60 was eluted with methylbenzene. The crude was purified by
column chromatography on silica gel. The mixture was concen-
trated, rediluted with CH2Cl2, and filtered through a plug of silica
gel. The solvent was removed in vacuo and the crude product was
purified by column chromatography (SiO2, ethyl acetate/petroleum
ether¼1:1) to give S1 (67 mg, 37%) as a black solid. 1H NMR (CDCl3,
J¼6.0), 7.79 (2H, d, J¼5.7) ppm. FTIR (KBr, cmꢀ1):
n
¼2959, 2931,
2873, 2189, 1726, 1646, 1601, 1521, 1465, 1386, 1287, 1188, 1122,
1075,1039, 801, 744, 705, 574, 527. MALDI-TOF-MS (dithranol): m/z:
calcd for C97H32F4N6: 1357.33 g molꢀ1, found: 1357.27 g molꢀ1
[MH]þ; elemental analysis calcd (%) for C97H32F4N6 (1357.33): C
85.83, H 2.38, N 6.19; found: C 85.87, H 2.39, N 6.20.
Acknowledgements
300 MHz):
d
¼0.83 (6H, m), 0.96 (4H, m), 1.41 (4H, m), 1.68 (4H, m),
2.82 (1H, s), 4.28 (5H, m), 7.506 (2H, d, J¼6.0), 7.514 (2H, d, J¼5.6),
We are grateful for financial support from the National Natural
Science Fund for Distinguished Young Scholar (Grant No.
51025313), National Natural Science Foundation (Grant No.
51103010, 51173003, 51203011).
7.69 (2H, d, J¼5.6), 7.70 (2H, d, J¼5.7) ppm. FTIR (KBr, cmꢀ1):
n
¼2959, 2212, 1720, 1602, 1535, 1485, 1451, 1415, 1342, 1262, 1209,
1183, 1098, 1019, 810, 705, 575, 527. MALDI-TOF-MS (dithranol): m/
z: calcd for C91H32N6: 1209.27 g molꢀ1, found: 1209.46 g molꢀ1
[MH]þ; elemental analysis calcd (%) for C91H32N6 (1209.27): ele-
mental analysis calcd (%) for C91H32N6 (1209.27): C 90.38, H 2.67, N
6.95; found: C 90.40, H 2.67, N 6.92.
Supplementary data
Cycle voltammograms and UVevis spectra of ‘click’ compounds
C1eC3, Measurement of third-order nonlinear optical and the
corresponding parameters calculation. Supplementary data related
to this article can be found at http://dx.doi.org/10.1016/
j.tet.2013.12.018.
4.1.5. ‘Click’ C60 derivative S2. A mixture of C60 (115 mg, 0.16 mmol)
and sarcosine (13 mg, 0.15 mmol) was dissolved in 10 mL of o-DCB
under argon atmosphere at 60 ꢂC. Afterward, a solution of C2
(81 mg, 0.15 mmol) in 5 mL of o-DCB was added. The reaction
mixture was heated at 120 ꢂC for an additional 10 h. After being
cooled to room temperature the solvent was evaporated. Unreacted
C60 was eluted with toluene. The crude was purified by column
chromatography on silica gel. The mixture was concentrated,
rediluted with CH2Cl2, and filtered through a plug of silica gel. The
solvent was removed in vacuo and the crude product was purified
by column chromatography (SiO2, ethyl acetate/petroleum
ether¼3:1) to give S2 (50 mg, 26%) as a dark brown solid. 1H NMR
References and notes
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(4H, m), 2.19 (2H, m), 2.83 (1H, s), 4.28 (3H, m), 5.28 (4H, m), 7.52
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and sarcosine (13 mg, 0.15 mmol) was dissolved in 10 mL of o-DCB
under argon atmosphere at 60 ꢂC. Afterward, a solution of C3
(91 mg, 0.15 mmol) in 5 mL of o-DCB was added. The reaction
mixture was heated at 140 ꢂC for an additional 13 h. After being
cooled to room temperature the solvent was evaporated. Unreacted
C60 was eluted with methylbenzene. The crude was purified by
column chromatography on silica gel. The mixture was concen-
trated, rediluted with CH2Cl2, and filtered through a plug of silica
gel. The solvent was removed in vacuo and the crude product was
purified by column chromatography (SiO2, ethyl acetate/petroleum
ether¼3:1) to give S3 (30 mg, 15%) as a black solid. 1H NMR (CDCl3,
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300 MHz):
d
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