CF2-Bridged C60 Fullerene Dimers and their Optical Transitions

Article abstract of DOI:10.1002/cphc.201701182

Fullerene dyads bridged with perfluorinated linking groups have been synthesized through a modified arc-discharge procedure. The addition of Teflon inside an arc-discharge reactor leads to the formation of dyads, consisting of two C₆₀ fullerenes bridged by CF₂ groups. The incorporation of bridging groups containing electronegative atoms lead to different energy levels and to new features in the photoluminescence spectrum. A suppression of the singlet oxygen photosensitization indicated that the radiative decay from singlet-to-singlet state is favoured against the intersystem crossing singlet-to-triplet transition.

Full text of DOI:10.1002/cphc.201701182

Accepted Article
Title: CF2-bridged C60 dimers and their optical transitions
Authors: Panagiotis Dallas, Shen Zhou, Stuart Cornes, Hiroyuki Niwa,
Yusuke Nakanishi, Tim Puchtler, Robert Taylor, Andrew
Briggs, Hisanori Shinohara, Kyriakos Porfyrakis, and Yasuhiro
Kino
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10.1002/cphc.201701182
ChemPhysChem
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CF2-bridged C60 dimers and their optical transitions
Panagiotis Dallas *[a], Shen Zhou [a], Stuart Cornes [a], Hiroyuki Niwa [b], Yusuke Nakanishi [b], Yasuhiro
Kino [b], Tim Puchtler [c], Robert.A.Taylor [c], G.Andrew.D.Briggs [a], Hisanori Shinohara *[b] and Kyriakos
Porfyrakis *[a]
Fullerene dyads bridged with perfluorinated linking groups have been
synthesized through modified arc-discharge procedure. The
applications since it has been demonstrated that the presence of
the highly electronegative fluorine atoms in conjugated polymers
leads to high-performing materials for field-effect transistors with
a
addition of Teflon inside an arc-discharge reactor leads to the
formation of dyads, consisting of two C60 fullerenes bridged by –CF2-
groups. The bridging groups consisting of electronegative atoms, lead
to different energy levels and to new features in the
photoluminescence spectrum. A suppression of the singlet oxygen
photosensitization, indicated that the radiative decay from singlet to
singlet state is favoured against the intersystem crossing singlet to
triplet transition.
[9]
a bright blue photoluminescence.
Scandium
Scandium-PTFE
a)
C76
b)
(C60)2-(CF2)2
C84
(C60)2-CF2
[1]
Fullerene dimers
have been synthesized by utilizing various
C90
bridging groups that link the two buckyballs. A wide range of
methods can be found in the literature, with examples
including the dimerization of pristine C60 [2] through high pressure
-100
-80
-60
ppm
-40
1250 1500 1750 2000 2250
12 14 16 18 20 22 24 26 28
Time (min)
[3]
treatment, mechanochemical synthesis using KCN and K2CO3
m/z
[4]
or dipolar cycloaddition reactions.
Besides the [2+2] carbon
linking between the two buckyballs, typical bridging groups
[5]
include oxygen
and methylene units, as well as photo-
switchable azobenzenes. [6] A dimerization of adjacent fullerenes
inside carbon nanotube peapods occurs as well. A fullerene
dimerization was monitored and distinguished between
formations of dyads in the first stage with small nanotubes to be
formed in subsequent stages through bond fusion. [7] Fullerene
dimers provide a class of materials for applications in artificial
photosynthesis and molecular electronic devices due to their
richer energy level structure compared to the single cage fullerene
counterparts. [8] Despite the number of fullerene dimers presented
in the literature, the synthesis of -CFn- bridged dimers has not
been reported due to difficulties arising from the poor reactivity of
the C-F bond. We achieved the synthesis of this unexpected dyad
in the high temperature plasma conditions of an arc-discharge
reactor by evaporating Teflon simultaneously with graphite. We
also demonstrated their reactivity with a Bingel-Hirsch reaction.
The fluorine addition could be beneficial in terms of optoelectronic
c)
Figure 1. a) HPLC traces of fullerene extract from scandium rods,
without (black) and with (blue), Teflon and the proposed structure
of the dyads (inset). b) MALDI-TOF analysis indicating the two
systems and various other perfluorinated fullerenes with –CF3
and/or –CF2 groups. 19F NMR as inset. c) Fullerene monomers
and dyad. Red arrow indicates a decrease in the yield and a green
an increase after the addition of Teflon.
A modified arc-discharge procedure was used to synthesize the
dyads.
[10]
Our initial target was the synthesis of perfluorinated
empty cages and endohedral metallofullerenes. The
decomposition of Teflon close to the arc resulted in reactive CF2n
and C2n fragments as precursor molecules. Addition of the -CF2-
on the fullerene cage results to perfluorinated empty cages and
metallofullerenes, and surprisingly, to the formation of the unusual
-CF2- bridged dimers, isolated through HPLC (Fig.1 and S1). By
integrating the HPLC peaks, we calculated the following yields:
C60: 72.94 %, C70: 24.41 %, C76-78: 0.63 %, C84: 0.39 %, C90: 0.1 %,
(C60)2-(CF2)2: 0.29 %, (C60)2-CF2: 1.25 %. Our first analysis on
elucidating the structure of the dimers was based on detailed
mass spectrometry (Fig. S2). We observed two unique structures
with an m/z=1541 (HPLC retention time 24-26 min), and an
m/z=1491 (HPLC retention time 27-29 min), while the yield of C84
and metallofullerenes was suppressed after the addition of Teflon
(Fig.1). We assigned the two dyads as C60-(CF2)2-C60 and C60-
[a]
Dr.P.Dallas, Mr.S.Zhou, Dr.S.Cornes, Prof.G.A.D.Briggs,
Prof.K.Porfyrakis
Department of Materials, University of Oxford
Oxford OX1 3PH, United Kingdom
E-mail: panagiotis.dallas@materials.ox.ac.uk;
kyriakos.porfyrakis@materials.ox.ac.uk
Mr.H.Niwa, Dr.Y.Nakanishi, Mr.Y.Kino, Prof.Hisanori Shinohara
Department of Chemistry
Nagoya University, Japan
E-mail: noris@nagoya-u.jp
[b]
[c]
Dr.T.Puchtler, Prof.R.A.Taylor
Department of Physics
Clarendon Laboratory, University of Oxford, United Kingdom
Supporting information for this article is given via a link at the end
of the document.
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10.1002/cphc.201701182
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CF2-C60. In the mass spec of the C60-CF2-C60 we distinguished
two fragments, the C60-CF2 and C60 at m/z=771 and m/z=720,
respectively. The C60-(CF2)2-C60 dimer is not subject to
fragmentation. We suspect this is due to the rigid structure of its
bridge consisting of two linking CF2 groups. The m/z=50
difference between the dyads, hints to the presence of CF2-
groups. The presence of fluorine is further confirmed by NMR
spectroscopy with the 19F NMR spectra providing evidence for the
presence of fluorine atoms, (Fig. S3a). The (C60)2-CF2 dyads is
showing a peak located at -67.04 ppm. [11] As a comparison, the
19F spectrum of (C60)2-(CF2)2 presents two peaks at -67 ppm and
-66.55 ppm, due to different environment of the fluorine atoms
with respect to the carbon cage. The 13C NMR is shown in Fig.
S4b. The spectrum for the (C60)2-(CF2)2, demonstrates weak
signals for fullerene sp2 carbons, seen in the expected region for
a C60 dimer, 140-152 ppm. [12] A signal for sp3 carbon environment
was also seen at 29.1 ppm. The signal for the sp3 CF2 carbon was
not observed as it was obscured by residual solvent peaks. The
FTIR spectra (Fig. S3c) provide a strong indication on the
presence of perfluorinated groups from the appearance of the
very strong antisymmetric and symmetric C-F vibration fingerprint
peaks at 1092 and 1031 cm-1 in both samples. Fig. S3d-f also
shows the Raman spectra of C60 and of the two dyads revealing
the presence of all eight vibration modes. The symmetry groups
representing them are given above each peak. The Ag(1)
represents an in and out of plane vibration and while it is quite
strong in the pristine C60, it nearly disappears in both dyads. The
Hg(8) hexagon shear mode [13] appears with increased intensity in
the dimers. The pentagonal pinch Ag(2) and the pentagon shear
mode, assigned to the isolated pentagon ring, both remain sharp
and relatively strong in all samples with a blue shift in the dyads
compared to pristine fullerene, something expected for
polymerized or functionalized C60. The five-fold degenerate, Hg(1)
mode shifts from 33.4 meV (269 cm-1) for pristine C60 to 30.5 meV
(246 cm-1) for the dyads. Due to change of symmetry, a peak
appears at 15 meV (122 cm-1). These low energy features
originate from inter-ball vibrational modes. In previous studies on
C120, two peaks at 127 and 139 cm−1 were observed, in relative
agreement with the work herein. [13] The materials exhibit a limited
reactivity with Lewis acids such as TiCl4, something that is
indicating that the oxidation potentials are expected to be close to
the threshold of 0.62-0.72 V vs Fc/Fc+. [14] The mass spectra after
the Lewis acid reaction are shown in Fig. S4. Since the surface
functionalization of fullerenes is a necessary step for their
utilization in various applications, the reactivity of the dimers was
tested through a Bingel-Hirsch reaction. The MALDI-TOF analysis
revealed the presence of mono-, bis-, tris-, tetra-, and penta-
malonate adducts. All peaks were separated by m/z=158, the
value assigned to each malonate (Fig.2). A more careful analysis
of the mass spectrum reveals fragments at m/z=878 and
m/z=1036 of the mono- and bis-adducts of C60.
(C60)2-(CF2)2
(C60)2-(CF2)
Bis
1857
Mono
1699
Tris
1965
Tetra
2123
Bis
1807
Tris
2015
Tetra
2173
1541
Mono
1649
Penta
2281
1491
1600 1800 2000 2200 2400 2600
a)
m/z
Figure 2. a,b) Schematic representation of the Bingel reaction
and mass spectra of the malonate derivatives.
The cyclopropane derivatives were isolated through HPLC
(Fig.S5) and their UV-Visible spectra are presented in Fig.3f in
comparison with parent, pristine dyads. The absorbance UV-vis
spectra (Fig. 3) were recorded in both toluene and CS2 and the
dyad with one -CF2- group resembles the pristine C60 more, with
very weak absorbance peaks at 540, 605 and 688 nm. In contrast,
the dimer with two bridging -CF2- groups has a featureless
spectrum without any pronounced absorbance peaks except a
shoulder at 428 nm, a fingerprint absorbance for functionalized
C60. We then studied the fluorescence of the samples, since the
light emitting properties of fullerenes are important for biomedical
applications or even for potential readout schemes of quantum
information processing architectures. [15]
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10.1002/cphc.201701182
ChemPhysChem
COMMUNICATION
treatment and they called them graphene quantum dots. However,
Malonate (C
-CF
2
60)2
60)2
since fullerene contains a pentagon ring, these clusters cannot be
(C ) -CF
60 2
2
(C
-CF
60)2
2
[18]
considered as analogues of graphene.
The corannulene
Malonate (C
-(CF
2)2
(C ) -(CF )
60 2
2 2
(C -(CF
60)2 2)2
moiety with an isolated pentagon ring surrounded with hexagons
is the core of the fullerene and upon surface functionalization it is
Toluene
fluorescent with 57 % QY; a value 8 times higher than of the
[20]
parent corannulene.
Furthermore, fluorinated conjugated
550
600
650
700
750
a)
400
b)
systems have lower HOMO-LUMO and bright electrogenerated
chemiluminescence [9] with a series of perfluorinated compounds
behaving as n-type semiconductors with blue emissions and QY
value of 68 %. In general, the electronegative fluorinated groups
have been considered to lower the HOMO-LUMO levels and to
open the HOMO-LUMO gap compared to non-fluorinated
polymers with similar backbones. [9] The dominant red emission
after green excitation in pristine fullerenes arises from the fivefold
degenerate h1u to the three-fold degenerate t1u. In order to clarify
the presence of aggregation dependent optical properties, we
recorded the PL spectra in different concentrations (Fig. S6). No
spectral differences where observed over a broad concentration
range.
400 500 600 700 800 900
Wavelength (nm)
500
600
700
800
Wavelength (nm)
(C ) -CF in CS
2
(C ) -(CF ) CS
2
c)
d)
60 2
2 2
60 2
2
19.00
18.14
17.27
16.41
15.55
14.68
13.82
12.95
12.09
11.23
10.36
9.500
8.636
7.773
6.909
6.045
5.182
4.318
3.455
2.591
1.727
0.8636
0.000
38.50
36.75
35.00
33.25
31.50
29.75
28.00
26.25
24.50
22.75
21.00
19.25
17.50
15.75
14.00
12.25
10.50
8.750
7.000
5.250
3.500
1.750
0.000
500
490
480
470
460
450
440
500
490
480
470
460
450
440
690 700 710 720 730 740 750
Emission (nm)
690 700 710 720 730 740 750
Emission (nm)
Bingel (C ) -(CF )
60 2
Bingel-(C60)2-CF2
f)
c)
2 2
An important application of fullerenes is their use as
photosensitizers with regard to the very high yield of the reactive
singlet oxygen state, generated through an energy transfer from
their triplet state to oxygen. However, singlet oxygen can be
detrimental to other biomedical applications due to its high
reactivity and toxicity. The formation of an intermediate between
the photosensitizer, that is subsequently deactivated, results in a
phosphorescence at 1270 nm corresponding to the
14.50
13.84
13.18
12.52
11.86
11.20
10.55
9.886
9.227
8.568
7.909
7.250
6.591
5.932
5.273
4.614
3.955
3.295
2.636
1.977
1.318
0.6591
0.000
120.4
114.9
109.5
104.0
98.51
93.04
87.56
82.09
76.62
71.15
65.67
60.20
54.73
49.25
43.78
38.31
32.84
27.36
21.89
16.42
10.95
5.473
0.000
500
490
480
470
460
450
440
500
490
480
470
460
450
440
690 700 710 720 730 740 750
Emission (nm)
690 700 710 720 730 740 750
Emission (nm)
-
O2(1Δg)O2(3Σg ) decay. We recorded the NIR emission of C60
Figure 3. a) UV-Visible spectra recorded in toluene b) UV-visible
spectra for the dyads and their Bingel derivatives recorded in CS2
Excitation-dependent photoluminescence maps for c) (C60)2-CF2
d) (C60)2-(CF2)2 e) Bingel derivative of (C60)2-CF2. f) Bingel
derivative of (C60)2-(CF2)2. Concentration: 0.025 mg/ml.
and the dimers in CS2, since toluene is quenching the singlet
oxygen due to formation of toluene radicals. The Bingel (C60)2-CF2
is showing a weak photosensitization (Fig. 4), while for the Bingel
derivative of (C60)2-(CF2)2 the singlet oxygen phosphorescence is
almost completely absent (see Fig.S8) following the same trend
with the QY (Fig.4c). Notice that for the optimum singlet oxygen
generation, the λexc=400 nm for the (C60)2-(CF2)2 is significantly
blue shifted compared to pristine C60 (λexc=550 nm, see Ref.22).
Fullerenes have a notoriously weak fluorescence, due to the
intersystem crossing that leads to a long lived and weak decay
from T1 to the S0 state and a subsequent energy transfer to
oxygen. As such, there is an unambiguous correlation between
the increased fluorescence quantum yields and the singlet
oxygen generation. Since this photocycle is suppressed, the
predominant transition is radiative relaxation from S1 to the
ground S0 and results to the high quantum yield. Similar
suppression of the singlet oxygen generation has been previously
observed in fullerenes functionalized with electronegative oxygen,
for example C60O3, C60O4, C60O5 [21] and dyads with pyrene. [22]
In order to elucidate the differences between the photophysical
properties of these new dimers and conventional fullerenes, we
functionalized C60 with the same malonate unit (Fig.S9). The
photosensitization efficiency of the malonates with two CF2
bridging groups (Fig. 4) was tested and compared with the
behavior of a C60 functionalized with one malonate unit. The
singlet oxygen generation is weaker in the dyads signaling a
quenching through the predominant S0-S1 transition. The
optimum is λexc =473 nm for the Bingel (C60)-CF2 with a weaker
feature at 550 nm and at 400 nm for the (C60)2-(CF2)2. For the
pristine C60 the optimum λexc is 550 and 600 nm, significantly red
In Fig.3 and Fig. S6 we present the excitation-dependent PL maps
for the red region under 470 and the PL spectra under 365 nm
excitation. The particles as expected aggregate in solution, hence
the data presented here are for an agglomeration of fullerenes as
proven by Dynamic Light Scattering. Dynamic Light Scattering
measurements, indicated a formation of an aggregate consisting
of 8 fullerene dyads for a Bingel derivative (Fig. S7).
The dyads exhibit similar emission patterns with a quantum yield
of 2 % for the (C60)2-CF2 and 14 % for (C60)2-(CF2)2 using pyrene
as standard. These high quantum yields closely resemble
carbogenic quantum dots which are synthesized in the range of
2-20 nm and have an emission maximum in the range of 430-480
[16]
nm with an optimum excitation range: 320-380 nm.
The
fullerene S1S0 transition appears at λexc=476 nm with λem=709
nm, while for the pristine C60 it is λexc=495 nm with λem=715 nm.
This dual fluorescence is reminiscent of other ultra-small
nanoscale carbon nanomaterials. The size of our materials is very
close to the mean diameter of the dual fluorescent carbon dots
reported by Zhou et al. [17] The second emission at 700-720 nm is
coming from the fullerene core. Methods for making carbon dots
include synthesis from chemical oxidation and ring opening of C60,
pyrolysis of small molecules and surface passivation of nano-
[17-19]
graphite.
Chua et al reported the formation of highly
fluorescent carbon dots by fullerene fragmentation through acid
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10.1002/cphc.201701182
ChemPhysChem
COMMUNICATION
[22]
shifted.
Both the pristine dyad, (C60)-CF2, and the malonate
Graphite rods were vaporized in an arc-discharge reactor where
Teflon tubes have been added. The arc-discharge took place
under 80 mbar of constant helium flow and a dc current of 500
amps. The carbon soot was Soxhlet extracted with toluene. Bingel
reaction: 0.05 mg of the (C60)2-(CF2) were dissolved in 0.1 ml of
CS2. Solutions of 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) and
diethyl bromomalonate were prepared accordingly: 0.005 ml of
DBU in 4.995 ml toluene and 0.0057 ml diethyl malonate in 4.994
ml toluene. 0.005 ml of the DBU solution and 0.005 ml of the
malonate solution were added in the fullerene solution. Further
0.01 and 0.0114 ml of the DBU and malonate solutions were
added, the same after 2 hours, and 0.02+0.02 ml were finally
added after 41/2 hours. A C60 monomer functionalized with the
same malonate unit was synthesized through 1:1:1 C60: DBU:
bromomalonate molar ratio.
(C60)-(CF2)2, exhibit
a biexponential decay time resolved
fluorescence with one long and one short lifetime component (Fig.
4b). The lifetimes derived from the bi exponential decays are
0.53;2.85 ns for 420 nm, 0.73;3.24 ns for 440 nm, 0.84;3.58 ns;
for 460 nm, 1;4 ns for 480 nm.
a-I)
a-II)
700
600
500
400
0.005900
0.005287
0.004675
0.004063
0.003450
0.002837
0.002225
0.001613
1.000E-03
700
600
500
400
9.700
8.587
7.475
6.362
5.250
4.137
3.025
1.912
0.8000
1200 1225 1250 1275 1300
Emission (nm)
1200 1225 1250 1275 1300
Emission (nm)
420 nm
440 nm
460 nm
480 nm
500 nm
520 nm
420 nm
440 nm
460 nm
480 nm
Mono exp.
Bi exp
Acknowledgements We would like to acknowledge EPSRC for
grant support (EP/K030108/1).
Supporting information section: HPLC curves, Raman/FTIR
spectra, 19F/13C NMR, photosensitization studies are available
free of charge via the internet.
b-I)
b-II)
6
9
12
15
4
6
8
10
ns
12
14
ns
Keywords: Fullerene dyads • Fluorescence • Photosensitization
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COMMUNICATION
Text for Table of Contents
Panagiotis Dallas *[a], Shen Zhou
[a], Stuart Cornes [a], Hiroyuki
Niwa [b], Yusuke Nakanishi [b]
Yasuhiro Kino [b]
,
,
G.Andrew.D.Briggs [a], Hisanori
Shinohara *[b] and Kyriakos
Porfyrakis *[a]
Page No. – Page No.
–CF2- bridged C60 dimers and
their optical transitions
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