Heteroanalogues of PCBM: N-bridged imino-PCBMs for organic field-effect transistors

Article abstract of DOI:10.1002/anie.200805228

(Figure Presented) Crossing the bridge: Two isomeric iminofullerenes, [5,6]-open azafulleroid 1 and [6,6]-closed aziridinofullerene 2, were prepared by cycloaddition C60. These "azalogues" enable the study of the effects of the bridging atom in a fulleren

Full text of DOI:10.1002/anie.200805228

Communications
DOI: 10.1002/anie.200805228
Fullerenes
Heteroanalogues of PCBM: N-Bridged Imino-PCBMs for Organic
Field-Effect Transistors**
Changduk Yang, Shinuk Cho, Alan J. Heeger, and Fred Wudl*
The extraordinary electronic properties of the fullerene C60 as
an n-type semiconductor with relatively high carrier mobility
have evoked continued interest in the development of a wide
variety of chemically modified fullerene derivatives.[1–13] One
of the most reliable and versatile routes to functionalization
has been studied by our research group and involves the
addition of diazo compounds to C60 ,[11] which results in two
isomeric products such as [5,6]-open fulleroids and [6,6]-
closed methanofullerenes.[2,3] From an electronic-structure
point of view, the [5,6]-open isomers are appealing since they
conserve the 60 p electrons of the parent C60. However, they
readily transform into the thermodynamically more stable
[6,6]-closed isomers by thermal, electrochemical, pH, and
photochemical means,[2,3,14–16] thus making their intrinsic
properties in electronic applications difficult to assess.
Therefore, we directed our attention toward the reaction
of azides with C60 for the following reasons: 1) the possibility
of an increased degree of stabilization of the [5,6]-open versus
[6,6]-closed isomers[17–19] and 2) the potential influence of the
nitrogen heteroatom of iminofullerenes on the electronic
properties of the fullerene cage,[20] which can possibly tune the
device performance. This is important since it enables us to
study the electronic device performance whilst comparing two
isomeric configurations, that is, hexagon–pentagon (C60-like)
versus hexagon–hexagon junction (PCBM-like; PCBM =
[6,6]-phenyl-C61-butyric acid methyl ester), as a function of
the perturbation of the fullerene p system. To the best of our
knowledge, no examples of performance evaluation between
methanofullerene and iminofullerene C60 adducts in elec-
tronic applications exist.
applications such as organic photovoltaics (OPV) as well as
organic field-effect transistors (OFETs). Herein, we report
the one-pot preparation of two isomeric imino-PCBMs, that
is, [5,6]-open azafulleroid ([5,6]-open APCBM; PCBM =
[6,6]-phenyl-C61-butyric acid methyl ester, A = aza) and
[6,6]-closed aziridinofullerene ([6,6]-closed APCBM;
Scheme 1). The characteristics of OFETs with [5,6]-open
Scheme 1. Structures of PCBM, [5,6]-open APCBM, and [6,6]-closed
APCBM.
APCBM and [6,6]-closed APCBM in solution-processed
devices have been determined and compared with those of
PCBM, as part of our study to assess a range of n-type
semiconductor properties.
In our original work, it was shown that 1,3-dipolar [3+2]
cycloaddition of azides to C60 followed by thermal extrusion
of N2 from the triazoline intermediate leads to [1,6]azafulle-
roids and [1,2]aziridinofullerenes.[18,19] Depending on the
nature of the substituents, the thermal elimination of N2
affords different ratios of two isomeric imino adducts.[21–23]
Following previous work,[23] the preparation of two imino-
PCBMs ([5,6]-open azafulleroid; [5,6]-open APCBM and
[6,6]-closed aziridinofullerene; [6,6]-closed APCBM) was
performed by thermal reaction of methyl 5-azido-5-phenyl-
pentanoate[24] with C60, followed by in situ thermal extrusion
of N2 (Scheme 2). Column chromatography on silica gel
yielded [5,6]-open APCBM as a major isomer (45% yields
based on consumed C60) and [6,6]-closed APCBM as a minor
Of the many monoadducts of C60 , PCBM is one of the
most successful fullerene adducts in organic electronic
[*] Dr. C. Yang,[+] Prof. F. Wudl
Mitsubishi Chemical Center for Advanced Materials
Department of Chemistry and Biochemistry and
Center for Polymers and Organic Solids, University of California
Santa Barbara, CA 93106 (USA)
Fax: (+1)805-893-4120
E-mail: wudl@chem.ucsb.edu
1
adduct (4.8%; see the Experimental Section). The H NMR
Dr. S. Cho, Prof. A. J. Heeger
Center for Polymers and Organic Solids, University of California
Santa Barbara, CA 93106 (USA)
[+] Current address:
School of Energy Engineering
Ulsan National Institute of Science and Technology
Ulsan 689-805 (Korea)
[**] Synthesis and characterization was supported by MC-CAM.
PCBM=[6,6]-phenyl-C61-butyric acid methyl ester.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200805228.
Scheme 2. Synthesis of [5,6]-open APCBM and [6,6]-closed APCBM.
o-DCB=o-dichlorobenzene.
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spectrum of [5,6]-open APCBM shows the tertiary hydrogen
atom adjacent to the nitrogen atom as a doublet of doublets at
5.20 ppm (J = 9.33 and 3.39 Hz). The 13C NMR spectrum
reveals 42 signals between d = 148 and 132 ppm, which are
attributable to the fullerene carbon atoms. In the case of [6,6]-
closed APCBM, the 1H NMR spectrum shows that the
hydrogen atom a to the nitrogen atom not only exhibits a
high-field shift from the open isomer to d = 4.56 ppm, but also
the 13C NMR spectrum displays 12 highly symmetrical full-
erene resonances (see the Supporting Information).
by the inductive effect of the bridged atoms in the fullerenes,
that is, the electron-donating and -withdrawing nature of the
corresponding functionalizing addends.
The cyclic voltammograms of [5,6]-open APCBM and
[6,6]-closed APCBM not only show three reduction waves but
also all reductions exhibit clear quasireversibility (see the
Supporting Information). The data for all materials are
summarized in Table 2. The reduction potentials of the
functionalized C60 adducts are shifted toward more negative
values with respect to C60 because of fewer p electrons and
release of strain energy.[25]
The UV/Vis absorption characteristics of C60 , PCBM,
[5,6]-open APCBM, and [6,6]-closed APCBM in chloroform
solution are shown in Figure 1. To identify the features that
Table 2: Electrochemical data for compounds in this study.[a]
Compound
E1red
E2red
E3red
C60
PCBM
[5,6]-open APCBM
[6,6]-closed APCBM
À1.071
À1.158
À1.114
À1.110
À1.484
À1.540
À1.512
À1.504
À1.969
À2.039
À1.984
À2.000
[a] Experimental conditions: values for 0.5(Epa+Epc) in V versus Fc/Fc+
(Fc=ferrocene) in o-DCB solution (10À4 to 10À3 molLÀ1) with Bu4NClO4
(0.1m) as the supporting electrolyte and Pt wire as the counterelectrode;
scan rate=50 mVsÀ1
.
[5,6]-open APCBM and [6,6]-closed APCBM show less
negative reduction values than PCBM. A likely rationale for
this redox behavior is the higher electronegativity of the
nitrogen atom relative to the analogous carbon atom, which
results in slightly reduced electron density in the fullerene
shell. It is also interesting to note that the reduction potentials
reveal remarkable similarities between [5,6]-open APCBM
and [6,6]-closed APCBM, which is in contrast to most all-
carbon systems, in which the reduction waves of the open
form occur at almost the same potential as in C60 since they
are isoelectronic analogues of C60 (60 p electrons).[3] These
results suggest that the reduction potentials of the function-
alized C60 derivatives stem from a combination of the nature
of the bridge atoms and the binding mode in fullerene
framework.
Figure 1. UV/Vis absorption spectra of solutions of C60, PCBM, [5,6]-
open APCBM, and [6,6]-closed APCBM in CHCl3.
change through iminofunctionalization on C60 , the pristine
C60 , and PCBM data obtained in this study are also listed in
Table 1. The absorption features of PCBM and [6,6]-closed
APCBM both show a distinguishable sharp band at 430 nm
Table 1: Optical data for compounds in this study.
Compound
Absorption [nm]
The differences in the properties of n-channel OFETs that
contain the isomeric structures can provide an important
guideline for the design of C60 derivatives for electronic
applications. All n-type OFETs were fabricated on heavily
doped Si wafers with a 200 nm thick SiO2 layer with top
contact geometry as shown in Figure 2a. Figure 2b shows the
C60
PCBM
[5,6]-open APCBM
[6,6]-closed APCBM
330, 405, 540, 600, 623(sh)
328, 430, 490, 603(sh), 695
332, 435(sh), 547, 602(sh)
326, 423, 495, 607(sh), 685
1/2
and 423 nm, respectively, which reflects the partially broken
symmetry (C2v) of the fullerene core relative to pristine C60
(Ih). The UV/Vis spectra of C60 and [5,6]-open APCBM are
almost identical in shape, with a peak at 330 nm and 332 nm
and broad bands at 540 nm and 547 nm, respectively, which
are bathochromically shifted compared to the closed forms
(PCBM and [6,6]-closed APCBM). These results are clearly
suggestive that the absorption characteristics of the function-
alized C60 derivatives are governed by the p system of the
fullerene. Another noticeable difference occurs in the solu-
tion colors, which are purple, brown, dark greenish-brown,
and dark wine-red for C60, PCBM, [5,6]-open APCBM and
[6,6]-closed APCBM, respectively. This observation implies
that the optical features of the C60 analogues are also affected
transfer characteristics, Ids against Vgs and Ids against Vgs
(Vds = 60 V), of the OFETs fabricated with [5,6]-open
APCBM and [6,6]-closed APCBM as an active layer,
respectively, compared to the PCBM measured under iden-
tical conditions. Both [5,6]-open APCBM and [6,6]-closed
APCBM display typical behavior of ideal n-type OFETs as
well as excellent output characteristics with clear saturation
(Figure 2c,d). Notably, a non-negligible difference between
[5,6]-open APCBM and [6,6]-closed APCBM in the plot of Ids
1/2
against Vgs is observed (the plot of Ids against Vgs clearly
shows this difference), which allows the alternation of OFET
performance in the characteristic output curves. The linear
plot of Ids1/2 against Vgs derived from the measurements of Ids
against Vgs yields electron mobilities of m1 = 4.1 ꢀ 10À2 cm2 Vs,
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and 13C NMR spectra were recorded on a Varian Unity Inova
500 MHz spectrometer and referenced to the solvent signal.
[5,6]-open azafulleroid ([5,6]-open APCBM) and [6,6]-closed
aziridinofullerene ([6,6]-closed APCBM): A mixture of 5-azido-5-
phenylpentanoate (0.7 g, 3.0 mmol), C60 (2.16 g, 3.0 mmol) and o-
DCB (150 mL) was placed in a round-bottom flask under argon and
stirred at room temperature for 15 min. The homogeneous mixture
was stirred at 608C under argon overnight. The solution was heated to
1358C and was again stirred overnight. The resulting mixture was
concentrated in vacuo to 100 mL, adsorbed onto SiO2 (40 ꢀ 10 cm)
and pre-eluted with chlorobenzene (100 mL) and then toluene. The
first fraction, which contained the remaining C60 (1.35 g, 63%), was
collected. Subsequently, the fraction that contained [5,6]-open
APCBM (Rf = 0.57) was collected. The solution was concentrated
in vacuo, redissolved in a minimal amount of toluene, and transferred
to a centrifuge tube. The product was precipitated with MeOH,
centrifuged, and decanted. The product was treated with MeOH
several times in the same manner. The product (470 mg, 45% based
on consumed C60) was isolated as a brown solid. 1H NMR (CS2/CDCl3
1:3, 500 MHz): d = 7.68 (d, J = 7.10 Hz, 2H), 7.43 (t, J = 7.57 Hz, 2H),
7.34 (t, J = 7.57 Hz, 1H), 5.20 (dd, J = 9.33, 3.39 Hz, 1H), 3.64 (s, 3H),
2.41–2.23 (m, 4H) 1.69–1.62 ppm (m, 2H); 13C NMR (CS2/CDCl3 1:3,
125.70 MHz): d = 172.13, 147.03, 144,71, 144.57, 144.40, 144.00,
143.87, 143.75, 143.70, 143.61, 143.49, 143.21, 143.06, 143.04, 142.84,
142.77, 142.68, 142.41, 142.35, 142.30, 142.26, 134.07, 141.94, 141.02,
140.95, 140.56, 140.51, 140.31, 139.74, 139.04, 139.01, 138.96, 138.32,
138.11, 138.03, 137.22, 136.33, 136.25, 136.15, 136.03, 135.95, 133.78,
133.38, 128.50, 128.07, 127.79, 64.10, 50.80, 35.78, 33.38, 20.47 ppm;
FABMS (nitrobenzyl alcohol) m/z: 926 ([M+H]+C); elemental
analysis calcd (%) for C72H15NO2: C 93.40, H 1.63, N 1.51, O 3.46;
found: C 93.29, H 2.10, N 1.70.
Upon further elution, [6,6]-closed APCBM (Rf = 0.53) was
collected as a second product fraction and further purified by triple
preparative column chromatography (toluene/CS2 1:1). Workup was
carried out as described above (using MeOH). Isolated yield = 50 mg
(4.8% based on consumed C60) as a dark-reddish solid. 1H NMR (CS2/
CDCl3 1:3, 500 MHz): d = 7.75 (d, J = 7.02 Hz, 2H), 7.48 (t, J =
7.67 Hz, 2H), 7.40 (t, J = 7.67 Hz, 1H), 4.56 (dd, J = 7.66, 5.42 Hz,
1H), 3.66 (s, 3H), 2.51–2.41 (m, 4H) 1.97–1.89 ppm (m, 2H).
13C NMR (CS2/CDCl3 1:3, 125.70 MHz): d = 172.62, 144.76, 144.68,
144.26, 144.18, 144.07, 143.33, 142.61, 142.44, 141.85, 141.67, 140.31,
139.31, 128.48, 127.81, 127.64, 63.54, 50.97, 36.11, 33.63, 21.16 ppm.
FABMS (nitrobenzyl alcohol) m/z: 926 ([M+H]+C). Elemental
analysis calcd (%) for C72H15NO2: C 93.40, H 1.63, N 1.51, O 3.46;
found: C 93.54, H 1.86, N, 1.71.
Figure 2. a) Schematic diagram of the n-type OFET structure
(L=50 mm, W=1.5 mm). b) Transfer characteristics in the saturated
~
&
*
regime ( ; PCBM, ; [5,6]-open APCBM, ; [6,6]-closed APCBM).
Output curves at different gate voltages for c) [5,6]-open APCCM and
d) [6,6]-closed APCBM.
m2 = 2.3 ꢀ 10À2 cm2 Vs, and m3 = 2.8 ꢀ 10À2 cm2 Vs for [5,6]-
open APCBM (m1), [6,6]-closed APCBM (m2), and PCBM
(m3).[26] The higher electron mobility in the [5,6]-open
APCBM OFET can be attributed to its 60p-electron nature,
which affords a stronger electron-acceptor strength than that
of the [6,6] junctions.[27] On the other hand, the mobility value
of the [6,6]-closed APCBM reveals an obvious similarity to
the isoelectronic isomer PCBM. These results are indicative
that the nature of the bridged heteroatom may not engage
with the n-type electronic properties of C60 but a perturbation
of the p electrons of the fullerene can indeed tune the
electron-transfer characters that can be responsible for the
mobility variation in OFETs.
In summary, two isomeric stable imino-PCBMs ([5,6]-
open APCBM and [6,6]-closed APCBM) have been pre-
pared. The former isomer retains the intact p electron system
of the parent C60, whereas the fullerene core of the latter has
two p electrons less than C60; this electron loss results in a
difference in the electron-accepting properties of the isomers.
It is of particular significance that an n-channel OFET that
contains [5,6]-open APCBM exhibits a substantial electron
Received: October 25, 2008
Published online: January 22, 2009
Keywords: azides · cycloaddition · electron acceptors ·
.
field-effect transistors · iminofullerenes
mobility (m = 4.1 ꢀ 10À2 cm2 Vs) in
a solution-processed
device, which is the first example of a fullerene with mobility
that exceeds that of the optimized high performance of
PCBM (m = 2.8 ꢀ 10À2 cm2 Vs). This work augurs well for the
future preparation of fullerene derivatives that have
improved mobility.
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