Synthesis and electron transporting properties of methanofullerene-o- carborane dyads in organic field-effect transistors

Article abstract of DOI:10.1039/c3dt50509j

A series of methanofullerene-o-carborane dyads (PCB-Ph-CB, PCB-Cn-CB, n = 1, 3, 6, 11) were synthesized via esterification of [6,6]-phenyl-C ₆₁-butyric acid (PCBA) with 2-alcohol functionalized o-carborane derivatives, 1-(4-n-BuC₆H₄)-2-R-1,2-closo-C ₂B₁₀H₁₀ (R = p-C₆H₄OH, (CH₂)_nOH, n = 1, 3, 6, 11). All the dyads are highly soluble in chlorinated and aromatic solvents under ambient conditions. UV-vis absorption and electrochemical reduction of the dyads exhibited features almost identical to each other, as well as to their parent PCBM, suggesting that the electronic properties of the dyads would be dominated by the methanofullerene moiety. Solution-processed field-effect transistors (FETs) incorporating the methanofullerene-o-carborane dyads as active layer materials were fabricated and tested. AFM images of all the thin films showed a homogeneous morphology with RMS values of 0.184-0.212 nm. The transport data are shown to be typical of n-channel FETs. Among the devices, the PCB-C1-CB FET showed the best performance, with an electron mobility of 1.72 × 10^-2 cm ² V^-1 s^-1, which is similar to that of PCBM. While the device performances gradually decreased with increasing length of the alkyl linker, they are superior to that (6.83 × 10^-4 cm ² V^-1 s^-1) of the physical blend of o-carborane and PCBM.

Full text of DOI:10.1039/c3dt50509j

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Synthesis and electron transporting properties of
methanofullerene-o-carborane dyads in organic
field-effect transistors†
Cite this: DOI: 10.1039/c3dt50509j
Maengsun Eo,a Hye Jin Bae,a Minsaeng Hong,b Youngkyu Do,*a Shinuk Cho*c and
Min Hyung Lee*b
A series of methanofullerene-o-carborane dyads (PCB-Ph-CB, PCB-Cn-CB, n = 1, 3, 6, 11) were synthesized
via esterification of [6,6]-phenyl-C61-butyric acid (PCBA) with 2-alcohol functionalized o-carborane deriva-
tives, 1-(4-n-BuC6H4)-2-R-1,2-closo-C2B10H10 (R = p-C6H4OH, (CH2)nOH, n = 1, 3, 6, 11). All the dyads are
highly soluble in chlorinated and aromatic solvents under ambient conditions. UV–vis absorption and
electrochemical reduction of the dyads exhibited features almost identical to each other, as well as to
their parent PCBM, suggesting that the electronic properties of the dyads would be dominated by the
methanofullerene moiety. Solution-processed field-effect transistors (FETs) incorporating the methano-
fullerene-o-carborane dyads as active layer materials were fabricated and tested. AFM images of all the thin
films showed a homogeneous morphology with RMS values of 0.184–0.212 nm. The transport data are
shown to be typical of n-channel FETs. Among the devices, the PCB-C1-CB FET showed the best perform-
ance, with an electron mobility of 1.72 × 10−2 cm2 V−1 s−1, which is similar to that of PCBM. While the
device performances gradually decreased with increasing length of the alkyl linker, they are superior to
that (6.83 × 10−4 cm2 V−1 s−1) of the physical blend of o-carborane and PCBM.
Received 25th February 2013,
Accepted 19th March 2013
DOI: 10.1039/c3dt50509j
www.rsc.org/dalton
properties of PCBM and thereby improve the performance of
Introduction
OPV8 and OFET9 devices. In particular, efforts to enhance elec-
Buckminsterfullerene, C60, has attracted considerable interest tron mobility of PCBM have been the focus of research interest
as an n-type semiconductor since its first report in 1985:1 it is in the field of n-type OFET applications, but the electron mobi-
considered important for its high electron affinity,2 excellent lity of most PCBM derivatives has been found to be similar to
electron transporting property,3 and low reorganization or poorer than that of PCBM.9
energy.4 Owing to the poor solubility of the pristine fullerenes
To elaborate on the electron accepting and transporting
in common organic solvents, however, these compounds have ability of PCBM, we have been interested in the introduction of
been limitedly used in solution-processed optoelectronic a novel electron-accepting auxiliary into the PCBM, which led
device applications. To circumvent this problem, soluble us to consider the use of an icosahedral carborane (C2B10H12)
fullerene derivatives such as [6,6]-phenyl-C61-butyric acid methyl as a new substituent of PCBM. Due to the existence of three-
ester (PCBM),5 which is one of the most widely studied soluble center, two-electron bonds, the carborane cages possess three-
fullerene derivatives as n-type semiconductors for organic dimensional delocalization of skeletal electrons which results
photovoltaic cells (OPVs)6 and field-effect transistors (OFETs),7 in a highly polarizable σ-aromaticity and electron-deficiency.10
have been actively investigated. A variety of PCBM analogues Because of these interesting electronic properties and excellent
have recently been reported to modulate the electronic thermal and chemical stability, carborane cages have attracted
great attention in the fields of materials science,11 organo-
metallic chemistry,12 and medicinal chemistry.13 In particular, we
and others have recently demonstrated that the introduction
of o-carborane (1,2-dicarba-closo-dodecaborane) into organic
π-systems largely stabilizes the lowest unoccupied molecular
orbital (LUMO) level by direct contribution to LUMO delocali-
zation, as well as by a strong inductive electron-withdrawing
effect.14,15 It is also known that o-carborane can be utilized as
an electron-transfer mediator in electrocatalytic reductions
aDepartment of Chemistry, KAIST, Daejeon, 305-701, Republic of Korea.
E-mail: ykdo@kaist.ac.kr; Fax: +82 42 350 2810; Tel: +82 42 350 2869
bDepartment of Chemistry and EHSRC, University of Ulsan, Ulsan 680-749, Republic
of Korea. E-mail: lmh74@ulsan.ac.kr; Fax: +82 52 259 2348; Tel: +82 52 259 2335
cDepartment of Physics and EHSRC, University of Ulsan, Ulsan 680-749, Republic of
Korea. E-mail: sucho@ulsan.ac.kr; Fax: +82 52 259 1693; Tel: +82 52 259 1268
†Electronic supplementary information (ESI) available: NMR spectra and AFM
phase images of PCB-Ph-CB and PCB-Cn-CB. See DOI: 10.1039/c3dt50509j
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due to its high stability and electron transfer ability.16 There- were prepared to modulate the distance between the C60
fore, it may be intriguing to investigate whether the introduc- moiety and the o-carborane cage. An alkyl (n-butyl) substituted
tion of o-carborane as an electron-accepting and/or phenyl ring was introduced at the C1-position of the o-carb-
transporting auxiliary into PCBM affects the electron transport- orane to endow better solubility in the organic solvent and
ing properties of PCBM, thereby modulating the electron electrochemical stability.16 It was also previously shown that
mobility in fullerene-based n-type OFET devices. It is also aryl substitution on the carborane cage leads to LUMO stabiliz-
worth mentioning that the carborane derivatives have rarely ation of the carborane moiety,15 which could be desirable for
been utilized as electroactive materials in optoelectronic possible electronic interaction with the fullerene moiety. The
device applications such as organic light-emitting diodes reaction of decaborane (B10H14) with the acetylene precursors
(OLEDs), OPVs, or OFETs,17 while they have often been in- yielded the 1-(4-n-BuC6H4)-2-substituted-o-carborane cages of
corporated into nonlinear optical systems in the form of C60-π- 1a and 2a. In the case of longer alkylene linked derivatives, the
carborane hybrids.18 Moreover, there have been no reports so lithiation of 1-(4-n-BuC6H4)-2-H-1,2-closo-C2B10H10 followed by
far on the use of o-carborane as an electron transporting the reaction with the Br-(CH2)n-OTMS (n = 3, 6, 11) in the pres-
material in n-type OFET applications.
ence of 0.2 equiv. of LiI produced 3a–5a. Compounds 1a–5a
In this report, we prepared a series of methanofullerene-o- were further converted into the corresponding alcohols via
carborane dyads via covalent-linking of the o-carborane deriva- either the demethylation reaction of the methoxy group with
tives to the ester-functionality of a PCBM moiety and utilized BBr3 (1b and 2b) or the deprotection of the trimethylsilyl
them as the electron-transporting materials in solution-pro- group with aqueous HCl (3b–5b). The subsequent esterifica-
cessed OFET devices. The results show that the electron mobi- tion of 1b–5b with [6,6]-phenyl-C61-butyric acid (PCBA) led to
lity is closely related to the distance between the fullerene the formation of the final methanofullerene-o-carborane dyads
moiety and the covalently linked o-carborane cage, implying PCB-Ph-CB and PCB-Cn-CB (n = 1, 3, 6, 11) as a brown solid.
the impact of o-carborane on the electron transporting prop- The formation of the dyads was fully characterized by multi-
erty of PCBM. Details of the synthesis and properties of the nuclear NMR spectroscopy, elemental analysis, and MALDI-
methanofullerene-o-carborane dyads and their use in the solu- TOF mass spectrometry. While the 1H and 13C NMR spectra
tion-processed, n-type OFETs are described.
showed the typical signals of a methanofullerene moiety with
ester linkage, the two broad 11B NMR signals in the regions of
δ −2 to −12 ppm confirm the presence of o-carboranyl boron
atoms (Fig. S1–S5, ESI†). All the compounds exhibited high
solubility in chlorinated and aromatic solvents such as chloro-
form, toluene, and chlorobenzene under ambient conditions
and showed solubility better than that of PCBM, making them
suitable for application to solution-processed OFET devices.
The increased solubility of dyads was further confirmed
in CH2Cl2 and tetrahydrofuran (THF) solvents where the
Results and discussion
Synthesis and characterization
The o-carborane linked PCBM derivatives, methanofullerene-o-
carborane dyads (PCB-Ph-CB, PCB-Cn-CB, n = 1, 3, 6, 11) were
designed and synthesized according to Scheme 1. Dyads
having a different length of linker (C1, C3, C6, C11, or Ph)
Scheme 1 Conditions and reagents: (i) decaborane (B10H14), CH3CN–toluene (1 : 4, v/v), reflux. (ii) BBr3, CH2Cl2, r.t. (iii) n-BuLi, Br-(CH2)n-OTMS, 0.2 equiv. LiI, THF,
−78 °C. (iv) 10% aq. HCl, MeOH–CH2Cl2 (2 : 1, v/v), 0 °C. (v) 4-(Dimethylamino)pyridine (DMAP), N,N’-dicyclohexylcarbodiimide (DCC), PCBA, CH2Cl2–CS2 (1 : 1, v/v),
r.t.
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solubility of PCBM has been reported to be 10 mg mL−1 and
2 mg mL−1 at room temperature, respectively.19 It was shown
that PCB-C1-CB freely dissolves in CH2Cl2 and THF up to at
least 2.0 wt% (20 mg mL−1), indicating that the introduction
of o-carborane may help the processability of fullerene-based
materials in a wide range of common solvents.
Table 1 Reduction potentials and LUMO energy levels of PCB-Ph-CB and
PCB-Cn-CB
Compounds
E11/2red a (V)
E21/2red a (V)
E31/2red a (V)
LUMOb (eV)
PCB-Ph-CB
PCB-C1-CB
PCB-C3-CB
PCB-C6-CB
PCB-C11-CB
PCBM
−1.07
−1.10
−1.07
−1.08
−1.08
−1.08
−1.50
−1.52
−1.50
−1.51
−1.51
−1.50
−1.89, −2.05
−2.08
−3.73
−3.70
−3.73
−3.72
−3.72
−3.72
−2.02
−2.04
−2.05
−2.05
Optical and electrochemical properties
a Potential values (V) with reference to an Fc/Fc+ redox couple. b LUMO
The UV–vis absorption properties of PCB-Ph-CB and PCB-Cn-
CB were investigated using a dilute toluene solution (5 × 10−5
M) (Fig. 1). The absorption spectra of all the dyads display fea-
tures very similar to those of their parent PCBM in the visible
region20 because the carborane cage absorbs light in the UV
region only.14b,21 While a sharp peak at 430 nm reflects the
partially broken symmetry (C2v) of the C60 moiety when com-
pared to that of the pristine C60 (Ih), the small band at 700 nm
can be assigned to the [6,6]-addition in the C60 moiety. To
investigate the LUMO level of the dyads, cyclic voltammetry
(CV) measurements were carried out in a mixed solution of
toluene–CH3CN (4 : 1, 5 × 10−4 M) (Fig. 2). For comparison, the
cyclic voltammogram of PCBM was also obtained. The cyclic
voltammograms of PCB-Ph-CB and PCB-Cn-CB show three
= −(4.8 + E11/2red) (eV).
well-defined, reversible reductions, which are very similar to
that for PCBM (Table 1).
However, inspection of the third reduction of PCB-Ph-CB
shows an additional well-defined, quasi-reversible peak at
E1/2 = −1.89 V. This peak may be ascribable to the reduction at
the 1,2-diaryl-o-carborane moiety, which is known to undergo
two reversible one-electron reductions or one two-electron
reduction depending on a scan rate.15,16,22 The high intensity
of the cathodic peak at −1.98 V with respect to that of the two
oxidation peaks at −1.82 and −1.61 V after reduction also sup-
ports one two-electron reduction in PCB-Ph-CB at a given scan
rate of 100 mV s−1. Thus, the CV trace for PCB-Ph-CB is con-
sidered the sum of the two CV traces of the methanofullerene
moiety and the 1,2-diaryl-o-carborane moiety. From the first
reduction potentials, the LUMO energy levels of the dyads
were calculated to be ca. −3.70 to −3.73 eV, indicating an
essentially identical LUMO level for all dyads (Table 1). Fur-
thermore, the values are almost the same as that of PCBM
(−3.72 eV), indicating that the electron accepting and trans-
porting properties of the dyads are dominated by the methano-
fullerene moiety. The observed absorption and reduction
features further suggest that the direct influence of an o-carb-
orane cage on the electronic properties of the methanofuller-
ene moiety will be marginal in the present methanofullerene-
o-carborane dyads, which are covalently linked via the ester
group of methanofullerene.
Fig. 1 UV–vis absorption spectra of PCB-Ph-CB and PCB-Cn-CB (n = 1, 3, 6, 11)
in toluene.
Organic field-effect transistors based on the dyads
To investigate the electron transporting properties of the
methanofullerene-o-carborane dyads, FET devices were fabri-
cated with PCB-Ph-CB and PCB-Cn-CB as active layer materials.
Fig. 3 shows the transfer characteristics, Ids vs. Vgs, of the FET
devices fabricated with the dyads. Data obtained from similar
OFETs based on a physical blend of 1-(4-n-BuC6H4)-2-Me-1,2-
closo-C2B10H10 and PCBM are also included for comparison
(Fig. 3f). All transport data are typical of n-channel OFETs; the
devices turn on with positive gate bias. The FET parameters
and results are listed in Table 2.
The saturated electron mobilities of PCB-Ph-CB and
PCB-Cn-CB were calculated using the saturation current
equation: Ids = (WCi/2L) μ(Vgs − VT)2, where W (= 50 μm) is the
channel width, L (= 3000 μm) is the channel length of the
devices, Ci (= 15 nF cm−2) is the capacitance of the SiO2
Fig. 2 Cyclic voltammograms of PCB-Ph-CB and PCB-Cn-CB.
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Fig. 4 OFET output characteristics of PCB-Ph-CB and PCB-Cn-CB.
Fig. 3 OFET transfer characteristics of PCB-Ph-CB and PCB-Cn-CB.
electron-deficient carborane cage into the methanofullerene
moiety in close proximity may have an impact on the electron
transporting properties of PCBM, although the direct electron-
withdrawing effect of o-carborane through an alkyl linker is
not apparently observed, as shown in the electrochemical
reduction.
Table 2 FET results for PCB-Ph-CB and PCB-Cn-CB
Electron mobility
(cm2 V−1 s−1
)
On/off ratio
VT (V)
PCB-Ph-CB
PCB-C1-CB
PCB-C3-CB
PCB-C6-CB
PCB-C11-CB
PCBMa
9.81 × 10−3
1.72 × 10−2
1.08 × 10−2
1.14 × 10−3
7.15 × 10−4
3.50 × 10−2
6.83 × 10−4
5 × 105
3 × 104
6 × 105
1 × 105
4 × 104
2 × 105
2 × 104
7.3
−1.2
7.5
−4.2
3.5
2
21
Surface morphology of thin films
Physical
blendb
Since the performance of FET devices often strongly depends
on the nanomorphology of the film, we examined the surface
morphology of the thin films of the dyads. Fig. 5 shows the
height images (2 μm × 2 μm) of PCB-Ph-CB and PCB-Cn-CB
thin films recorded by tapping mode atomic force microscopy
a See ref. 9. b Spin-casting from a solution of PCBM and 1-(4-n-
BuC6H4)-2-Me-1,2-closo-C2B10H10
.
dielectric, and VT is the threshold voltage. The output charac- (AFM). All AFM images show a quite homogeneous mor-
teristics, Ids vs. Vds, of the FET devices are shown in Fig. 4. The phology consisting of fine, regular grains with RMS values of
best device performance was obtained from PCB-C1-CB FET, 0.184–0.212 nm. No significant differences were observed in
with an electron mobility of 1.72 × 10−2 cm2 V−1 s−1, which is the morphological study.
similar to that of PCBM, reported in previous studies.9
Therefore, the differences between the FET properties may
However, the device performances gradually decreased with not have originated from the morphological differences. In the
increasing length of the alkyl linker.
case of the films of a physical blend of o-carborane and PCBM
In the case of PCB-C11-CB, which has the longest alkyl (Fig. 5f), however, height images showed a somewhat aggre-
linker between PCBM and o-carborane, the device performance gated morphology with a slightly increased RMS value of
was lowest and close to that of the FET fabricated with a phys- 0.360 nm. This aggregated morphology may be attributed to
ical blend of o-carborane and PCBM. Note that the data on the the phase separation of the o-carborane and PCBM. Such an
performance of the PCB-Ph-CB FET are similar to the data undesired phase separation of the two moieties could nega-
obtained from the device fabricated with PCB-C3-CB, which tively affect the charge transport pathway, thereby suppressing
has a similar physical distance between PCBM and o-carb- the electron mobility of an FET device fabricated with a phys-
orane. These results indicate that the incorporation of an ical blend of o-carborane and PCBM.
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alcohol),24 and 1-(4-n-BuC6H4)-2-H-1,2-closo-C2B10H10
were
analogously synthesized according to the published pro-
cedures. CDCl3 from Cambridge Isotope Laboratories was used
after drying over activated molecular sieves (5 Å). NMR spectra
of compounds were recorded on a Bruker Avance 400 spectro-
meter (400.13 MHz for 1H, 100.62 MHz for 13C, 128.38 MHz
for 11B) at ambient temperature. Chemical shifts are given in
ppm, and are referenced against external Me4Si (1H, 13C) and
BF3·Et2O (11B). MALDI-TOF MS was measured with a Voyager
DE-STR 4700 proteomics analyzer at the Korea Basic Science
Center (KBSC). HR EI-MS measurement (JEOL JMS700) was
carried out at KBSC. Elemental analyses were performed using
an EA1110 (Fisons Instruments) by the Environmental Analysis
Laboratory at KAIST. The UV–vis spectrum was recorded on a
Jasco V-530 in a toluene solvent with a 1 cm quartz cuvette at
ambient temperature.
25
Synthesis of 1a
To a stirred solution of decaborane (1.16 g, 9.49 mmol) in
toluene (40 mL) was added CH3CN (10 mL) and the solution
was heated to reflux for 3 h. The solution was cooled to room
temperature
and
1-(2-(4-butylphenyl)ethynyl)-4-methoxy-
benzene (2.50 g, 9.30 mmol) was added. The reaction mixture was
heated at reflux for 16 h and cooled to room temperature. After
removal of the solvent, the residue was purified by column
chromatography on silica (eluent: n-hexane–ethyl acetate =
4/1), which afforded 1a as a colorless oil (1.53 g, 43%). 1H NMR
(CDCl3): δ ppm: 7.33–7.26 (m, 4H), 6.91 (d, J = 8.5 Hz, 2H),
6.62–6.58 (m, 2H), 3.69 (s, 3H), 2.46 (t, J = 7.6 Hz, 2H),
1.51–1.42 (m, 2H), 1.27–1.17 (m, 2H), 0.85 (t, J = 7.3 Hz, 3H).
13C NMR (CDCl3): δ ppm: 160.76, 145.19, 132.06, 130.54,
128.23, 128.17, 123.17, 113.41, 85.75 (CB-C), 85.69 (CB-C),
55.26, 34.97, 32.95, 22.16, 13.85. 11B NMR (CDCl3): δ ppm:
−2.8 (br s, 3B), −9.2 (br s), −10.8 (br s) (7B). HR EI-MS: m/z
calcd for C19H30B10O, 384.3227; found, 384.3229.
Fig. 5 Topographic AFM images of (a) PCB-Ph-CB (RMS = 0.184 nm), (b)
PCB-C1-CB (RMS = 0.193 nm), (c) PCB-C3-CB, (RMS = 0.212 nm), (d) PCB-C6-CB
(RMS = 0.198 nm), (e) PCB-C11-CB (RMS = 0.203 nm), and (f) physical blend of
PCBM and 1-(4-n-BuC6H4)-2-Me-1,2-closo-C2B10H10 (RMS = 0.360 nm).
Synthesis of 1b
To a solution of 1a (1.05 g, 2.74 mmol) in CH2Cl2 (40 mL) was
added dropwise BBr3 (6.86 mmol) at −78 °C. The solution was
allowed to warm to room temperature and stirred overnight.
Experimental
General considerations
All operations were performed under an inert nitrogen atmos- After the addition of MeOH (10 mL) in an ice bath, the solvent
phere using standard Schlenk and glove box techniques. Anhy- was removed by rotary evaporation. The crude product was pur-
drous grade solvents (Aldrich) were dried by passing them ified by column chromatography on silica (eluent: n-hexane–
through an activated alumina column and stored over acti- CH2Cl2 = 3/1, then CH2Cl2), which afforded 1b as a light yellow
1
vated molecular sieves (5 Å). Spectrophotometric-grade toluene oil (0.86 g, 85%). H NMR (CDCl3): δ ppm: 7.32 (t, J = 8.6 Hz,
was used as received from J. T. Baker. Commercial reagents 4H), 6.96 (d, J = 7.7 Hz, 2H), 6.55 (d, J = 8.2 Hz, 2H), 5.16 (s,
were used without any further purification after purchasing 1H), 2.50 (t, J = 7.6 Hz, 2H), 1.55–1.45 (m, 2H), 1.32–1.21 (m,
from Aldrich (BBr3 (1.0 M solution in heptane), lithium 2H), 0.88 (t, J = 7.3 Hz, 3H). 13C NMR (CDCl3): δ ppm: 145.29,
iodide (LiI), diethyl sulfide (Et2S), 4-(dimethylamino)pyridine 132.27, 130.47, 128.24, 127.95, 123.40, 114.95, 85.82 (CB-C),
(DMAP), N,N′-dicyclohexylcarbodiimide (DCC), n-BuLi (2.5 M 85.57 (CB-C), 34.90, 32.86, 22.13, 13.81. 11B NMR (CDCl3): δ
solution in hexane), carbon disulfide (CS2), TCI (1-butyl-4- ppm: −2.8 (br s, 3B), −9.2 (br s), −10.7 (br s) (7B). HR EI-MS:
ethynylbenzene), and KatChem (B10H14, decaborane) and nano-C m/z calcd for C18H28B10O, 370.3071; found, 370.3073.
(PCBM)). 1-(2-(4-Butylphenyl)ethynyl)-4-methoxybenzene and
Synthesis of PCB-Ph-CB
1-butyl-4-(3-methoxyprop-1-ynyl)benzene (Sonogashira reac-
tion),23 [6,6]-phenyl-C61-butyric acid (PCBA),5 and Br-(CH2)n- A solution of DMAP (64 mg, 0.52 mmol), DCC (0.49 g,
OTMS (TMS; trimethylsilyl, the protection reaction of 2.37 mmol), and 1b (0.872 g, 2.37 mmol) in CH2Cl2 (150 mL)
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was added to a solution of PCBA (0.94 g, 1.05 mmol) in dry (m, 2H), 0.91 (t, J = 7.3 Hz, 3H). 13C NMR (CDCl3): δ ppm:
carbon disulfide (150 mL) via a cannula. The reaction mixture 171.03, 148.65, 147.62, 145.78, 145.15, 145.04, 144.65, 143.74,
was stirred for 20 h at room temperature. After removal of the 142.99, 142.20, 142.16, 142.12, 142.09, 141.01, 140.76, 137.99,
solvent, the residue was purified by column chromatography 137.58, 132.03, 130.94, 128.96, 128.53, 127.19, 83.29 (CB-C),
on silica (eluent: toluene–n-hexane = 1/2). The obtained solid 79.74, 78.20 (CB-C), 62.24, 51.66, 35.13, 33.62, 33.54, 33.06,
was precipitated by MeOH and filtered off, which afforded the 22.36, 22.17, 13.92. 11B NMR (CDCl3): δ ppm: −3.5 (br s, 3B),
product as a brown solid (0.54 g, 41%). 1H NMR (CDCl3): δ −10.0 (br s, 7B). MALDI-TOF MS: m/z calcd for C84H36B10O2,
ppm: 7.92–7.88 (m, 2H), 7.55–7.49 (m, 2H), 7.48–7.42 (m, 1H), 1186.4; found, 1186.5. Anal. calcd for C84H36B10O2: C 85.12, H
7.41–7.36 (m, 2H), 7.28 (d, J = 8.5 Hz, 2H), 6.92 (d, J = 8.3 Hz, 3.06; found: C 85.21, H 3.01.
2H), 6.87–6.82 (m, 2H), 2.97–2.91 (m, 2H), 2.69 (t, J = 7.4 Hz,
2H), 2.46 (t, J = 7.6 Hz, 2H), 2.28–2.18 (m, 2H), 1.52–1.41 (m,
2H), 1.27–1.16 (m, 2H), 0.85 (t, J = 7.3 Hz, 3H). 13C NMR
Synthesis of 3a
(CDCl3): δ ppm: 170.62, 151.84, 148.63, 145.77, 145.20, 145.16,
1-(4-n-BuC6H4)-2-H-1,2-closo-C2B10H10 (3.34 g, 12.08 mmol) in
144.99, 144.66, 144.52, 143.73, 143.01, 142.95, 142.16, 142.12,
THF (40 mL) was treated with one equiv. of n-BuLi (4.8 mL) at
141.00, 140.76, 137.98, 137.58, 132.04, 131.83, 130.47, 128.49,
−78 °C. The reaction mixture was allowed to warm to room
128.35, 127.81, 121.10, 85.67 (CB-C), 84.34 (CB-C), 79.71,
temperature and stirred for 2 h. To the mixture was slowly
51.58, 34.96, 34.10, 33.88, 22.18, 13.89. 11B NMR (CDCl3): δ
added Br-(CH2)3-OTMS (2.55 g, 12.08 mmol) and LiI (0.32 g,
ppm: −2.6 (br s, 3B), −10.4 (br s, 7B). MALDI-TOF MS: m/z
2.42 mmol) in THF (40 mL) at −78 °C. The reaction mixture
calcd for C89H38B10O2, 1248.4; found, 1248.7. Anal. calcd for
was slowly allowed to warm to room temperature and further
C89H38B10O2: C 85.70, H 3.07; found: C 85.77, H 2.93.
stirred overnight. The resulting solution was treated with
30 mL of a saturated aqueous solution of NH4Cl and the
organic portion was separated. The aqueous layer was further
extracted with diethyl ether (2 × 20 mL). The combined
organic portions were dried over MgSO4, filtered, and evapor-
ated to dryness. The residue was purified by column chromato-
graphy on silica (eluent: n-hexane–CH2Cl2 = 2/1), affording 3a
Synthesis of 2a
A procedure analogous to that for 1a was employed with deca-
borane (1.96 g, 16.07 mmol) and 1-butyl-4-(3-methoxyprop-1-
ynyl)benzene (3.19 g, 15.75 mmol). Purification by column
chromatography on alumina (eluent: toluene) afforded 2a as a
light yellow oil (2.27 g, 45%). 1H NMR (CDCl3): δ ppm:
7.53–7.48 (m, 2H), 7.14 (d, J = 8.6 Hz, 2H), 3.45 (s, 2H), 3.12 (s,
3H), 2.60 (t, J = 7.6 Hz, 2H), 1.62–1.53 (m, 2H), 1.38–1.28 (m,
2H), 0.91 (t, J = 7.3 Hz, 3H). 13C NMR (CDCl3): δ ppm: 145.90,
130.91, 128.69, 127.76, 82.67 (CB-C), 80.33 (CB-C), 71.96,
59.40, 35.11, 33.13, 22.32, 13.87. 11B NMR (CDCl3): δ ppm:
−2.7 (br s), −3.4 (br s) (3B), −10.5 (br s), −12.1 (br s) (7B). HR
EI-MS: m/z calcd for C14H28B10O, 322.3071; found, 322.3067.
1
as a colorless oil (3.24 g, 66%). H NMR (CDCl3): δ ppm: 7.49
(d, J = 8.4 Hz, 2H), 7.15 (d, J = 8.4 Hz, 2H), 3.35 (t, J = 5.9 Hz,
2H), 2.59 (t, J = 7.8 Hz, 2H), 1.87–1.80 (m, 2H), 1.62–1.52 (m,
4H), 1.39–1.28 (m, 2H), 0.91 (t, J = 7.4 Hz, 3H), −0.07 (s, 9H).
13C NMR (CDCl3): δ ppm: 145.80, 131.04, 128.82, 128.04, 84.13
(CB-C), 82.31 (CB-C), 61.12, 35.10, 33.16, 32.34, 31.75, 22.35,
13.87, −0.68. 11B NMR (CDCl3): δ ppm: −3.8 (br s, 3B), −10.3
(br s, 7B). HR EI-MS: m/z calcd for C18H38B10OSi, 408.3622;
found, 408.3625.
Synthesis of 2b
A procedure analogous to that for 1b was employed with 2a
(1.19 g, 3.72 mmol) and BBr3 (9.29 mmol). Purification by
column chromatography on silica (eluent: CH2Cl2) afforded 2b
Synthesis of 3b
as a dark yellow oil (1.13 g, 99%). 1H NMR (CDCl3): δ ppm: To a solution of the compound 3a (2.36 g, 5.81 mmol) in
7.51 (d, J = 8.4 Hz, 2H), 7.16 (d, J = 8.4 Hz, 2H), 3.75–3.70 (m, MeOH (40 mL) and CH2Cl2 (20 mL) was added 10% aqueous
2H), 2.60 (t, J = 7.7 Hz, 2H), 1.86–1.81 (m, 1H), 1.62–1.52 (m, HCl (5 mL) at 0 °C. The reaction mixture was stirred for 1 h at
2H), 1.39–1.28 (m, 2H), 0.91 (t, J = 7.3 Hz, 3H). 13C NMR 0 °C. After addition of water (30 mL), the resulting solution
(CDCl3): δ ppm: 146.16, 132.27, 130.85, 128.98, 127.54, 82.99 was extracted with CH2Cl2 (30 mL) and the organic phase was
(CB-C), 82.53 (CB-C), 63.03, 35.10, 33.10, 22.34, 13.86. 11B washed with water (2 × 20 mL). The organic portions were
NMR (CDCl3): δ ppm: −2.8 (br s), −3.4 (br s) (3B), −10.3 (br s), dried over MgSO4, filtered, and evaporated to dryness. The
−12.4 (br s) (7B). HR EI-MS: m/z calcd for C13H26B10O, residue was purified by column chromatography on silica
308.2914; found, 308.2916.
(eluent: CH2Cl2), affording 3b as a white solid (1.49 g, 77%).
1H NMR (CDCl3): δ ppm: 7.49 (d, J = 8.6 Hz, 2H), 7.15 (d, J =
8.4 Hz, 2H), 3.42 (q, J = 5.8 Hz, 2H), 2.60 (t, J = 7.7 Hz, 2H),
Synthesis of PCB-C1-CB
A procedure analogous to that for PCB-Ph-CB was employed 1.91–1.85 (m, 2H), 1.67–1.53 (m, 4H), 1.39–1.28 (m, 2H), 1.12
with 2b instead of 1b. The product was obtained as a brown (t, J = 5.3 Hz, 1H), 0.91 (t, J = 7.4 Hz, 3H). 13C NMR (CDCl3): δ
solid (50%). 1H NMR (CDCl3): δ ppm: 7.93–7.89 (m, 2H), ppm: 145.89, 130.98, 128.86, 127.93, 83.95 (CB-C), 82.01 (CB-
7.58–7.52 (m, 2H), 7.50–7.44 (m, 3H), 7.14 (d, J = 8.3 Hz, 2H), C), 61.48, 35.09, 33.08, 32.34, 31.57, 22.32, 13.87. 11B NMR
4.20 (s, 2H), 2.89–2.83 (m, 2H), 2.59 (t, J = 7.7 Hz, 2H), 2.46 (t, (CDCl3): δ ppm: −3.7 (br s, 3B), −10.4 (br s, 7B). Anal. calcd for
J = 7.5 Hz, 2H), 2.18–2.09 (m, 2H), 1.62–1.53 (m, 2H), 1.39–1.28 C15H30B10O, C 53.86, H 9.04; found, C 53.85, H 9.07.
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Synthesis of PCB-C3-CB
64.33, 51.84, 35.11, 34.85, 34.07, 33.65, 33.10, 29.25, 28.60,
28.23, 25.37, 22.38, 22.35, 13.91. 11B NMR (CDCl3): δ ppm:
−3.8 (br s, 3B), −10.4 (br s, 7B). MALDI-TOF MS: m/z calcd for
C89H46B10O2, 1256.4; found, 1256.7. Anal. calcd for
C89H46B10O2: C 85.15, H 3.69; found: C 85.05, H 3.60.
A procedure analogous to that for PCB-Ph-CB was employed
with 3b. The product was obtained as a brown solid (56%). H
1
NMR (CDCl3): δ ppm: 7.92–7.87 (m, 2H), 7.56–7.51 (m, 2H),
7.49–7.44 (m, 3H), 7.15 (d, J = 8.4 Hz, 2H), 3.82 (t, J = 6.2 Hz,
2H), 2.86–2.81 (m, 2H), 2.60 (t, J = 7.7 Hz, 2H), 2.35 (t, J = 7.5
Hz, 2H), 2.13–2.03 (m, 2H), 1.84–1.79 (m, 2H), 1.73–1.65 (m,
2H), 1.63–1.54 (m, 2H), 1.39–1.29 (m, 2H), 0.91 (t, J = 7.3 Hz,
3H). 13C NMR (CDCl3): δ ppm: 172.62, 148.69, 147.72, 145.81,
145.20, 145.16, 145.05, 145.02, 144.80, 144.66, 144.52, 143.76,
143.01, 142.95, 142.16, 142.13, 141.00, 137.57, 132.05, 131.01,
128.91, 128.47, 127.85, 84.05 (CB-C), 81.36 (CB-C), 79.78,
62.89, 51.70, 35.15, 33.77, 33.57, 33.15, 31.45, 28.49, 22.40,
22.23, 13.96. 11B NMR (CDCl3): δ ppm: −3.7 (br s, 3B), −10.2
(br s, 7B). MALDI-TOF MS: m/z calcd for C86H40B10O2, 1214.4;
found, 1214.6. Anal. calcd for C86H40B10O2: C 85.13, H 3.32;
found: C 85.10, H 3.15.
Synthesis of 5a
A procedure analogous to that for 3a was employed with Br-
1
(CH2)11-OTMS, affording 5a as a colorless oil (76%). H NMR
(CDCl3): δ ppm: 7.51–7.46 (m, 2H), 7.15 (d, J = 8.5 Hz, 2H),
3.54 (t, J = 6.7 Hz, 2H), 2.60 (t, J = 7.7 Hz, 2H), 1.75–1.69 (m,
2H), 1.62–1.53 (m, 2H), 1.53–1.44 (m, 2H), 1.38–1.29 (m, 4H),
1.29–0.99 (m, 14H), 0.91 (t, J = 7.2 Hz, 3H), −0.09 (s, 9H). 13C
NMR (CDCl3): δ ppm: 145.74, 130.99, 128.78, 128.12, 83.79
(CB-C), 82.63 (CB-C), 62.71, 35.11, 34.91, 33.14, 32.73, 29.51,
29.41, 29.35, 29.25, 28.92, 28.90, 25.80, 22.33, 13.89, −0.46. 11
B
NMR (CDCl3): δ ppm: −3.8 (br s, 3B), −10.4 (br s, 7B). HR
EI-MS: m/z calcd for C26H54B10OSi, 520.4874; found, 520.4877.
Synthesis of 4a
Synthesis of 5b
A procedure analogous to that for 3a was employed with Br-
(CH2)6-OTMS, affording 4a as a colorless oil (74%). 1H NMR
(CDCl3): δ ppm: 7.49 (d, J = 8.4 Hz, 2H), 7.15 (d, J = 8.4 Hz,
2H), 3.46 (t, J = 6.6 Hz, 2H), 2.60 (t, J = 7.7 Hz, 2H), 1.76–1.70
(m, 2H), 1.63–1.54 (m, 2H), 1.41–1.30 (m, 6H), 1.19–1.01 (m,
4H), 0.91 (t, J = 7.3 Hz, 3H), −0.06 (s, 9H). 13C NMR (CDCl3): δ
ppm: 145.77, 130.98, 128.80, 128.09, 83.80 (CB-C), 82.53 (CB-
C), 62.32, 35.10, 34.90, 33.11, 32.32, 29.37, 28.76, 25.22, 22.33,
−0.52. 11B NMR (CDCl3): δ ppm: −3.8 (br s, 3B), −10.4 (br s,
7B). HR EI-MS: m/z calcd for C21H44B10OSi, 450.4092; found,
450.4090.
A procedure analogous to that for 3b was employed with 5a,
affording 5b as a colorless oil (75%). H NMR (CDCl3): δ ppm:
1
7.48 (d, J = 8.5 Hz, 2H), 7.15 (d, J = 8.4 Hz, 2H), 3.61 (s, 2H),
2.60 (t, J = 7.5 Hz, 2H), 1.75–1.69 (m, 2H), 1.62–1.48 (m, 4H),
1.38–0.97 (m, 19H), 0.91 (t, J = 7.2 Hz, 3H). 13C NMR (CDCl3): δ
ppm: 145.73, 130.96, 128.76, 128.07, 83.78 (CB-C), 82.61 (CB-
C), 62.99, 35.08, 34.87, 33.10, 32.74, 29.47, 29.36, 29.35, 29.32,
29.21, 28.87, 25.68, 22.30, 13.86. 11B NMR (CDCl3): δ ppm:
−3.8 (br s, 3B), −10.4 (br s, 7B). HR EI-MS: m/z calcd for
C23H46B10O, 448.4479; found, 448.4482.
Synthesis of PCB-C11-CB
Synthesis of 4b
A procedure analogous to that for PCB-Ph-CB was employed
with 5b, affording the product as a dark brown solid (51%). 1H
NMR (CDCl3): δ ppm: 7.93–7.88 (m, 2H), 7.55–7.42 (m, 5H),
7.15 (d, J = 8.3 Hz, 2H), 4.03 (t, J = 6.7 Hz, 2H), 2.92–2.86 (m,
2H), 2.60 (t, J = 7.7 Hz, 2H), 2.49 (t, J = 7.4 Hz, 2H), 2.21–2.11
(m, 2H), 1.75–1.69 (m, 2H), 1.63–1.53 (m, 4H), 1.39–0.98 (m,
18H), 0.91 (t, J = 7.2 Hz, 3H). 13C NMR (CDCl3): δ ppm: 173.09,
148.80, 147.80, 145.84, 145.71, 145.17, 145.12, 145.06, 145.02,
144.77, 144.67, 144.63, 144.48, 144.39, 143.98, 143.74, 143.73,
143.09, 143.01, 142.97, 142.92, 142.89, 142.21, 142.16, 142.11,
142.08, 140.95, 140.72, 138.02, 137.54, 136.73, 132.08, 130.98,
128.78, 128.41, 128.22, 128.09, 83.83 (CB-C), 82.59 (CB-C),
79.88, 64.73, 51.89, 35.11, 34.91, 34.13, 33.66, 33.12, 29.45,
A procedure analogous to that for 3b was employed with 4a,
affording 4b as a colorless oil (70%). H NMR (CDCl3): δ ppm:
1
7.48 (d, J = 8.5 Hz, 2H), 7.15 (d, J = 8.4 Hz, 2H), 3.53 (t, J = 6.7
Hz, 2H), 2.60 (t, J = 7.7 Hz, 2H), 1.77–1.70 (m, 2H), 1.63–1.53
(m, 2H), 1.45–1.28 (m, 6H), 1.20–1.03 (m, 5H), 0.91 (t, J = 7.4
Hz, 3H). 13C NMR (CDCl3): δ ppm: 145.79, 130.97, 128.79,
128.05, 83.80 (CB-C), 82.47 (CB-C), 62.65, 35.08, 34.81, 33.09,
32.30, 29.29, 28.68, 25.09, 22.31, 13.86. 11B NMR (CDCl3): δ
ppm: −3.8 (br s, 3B), −10.4 (br s, 7B). HR EI-MS: m/z calcd for
C18H36B10O, 378.3697; found, 378.3694.
Synthesis of PCB-C6-CB
A procedure analogous to that for PCB-Ph-CB was employed 29.43, 29.38, 29.27, 29.25, 28.95, 28.62, 25.92, 22.41, 22.34,
with 4b, affording the product as a brown solid (55%). 1H 13.94. 11B NMR (CDCl3): δ ppm: −3.8 (br s, 3B), −10.3 (br s,
NMR (CDCl3): δ ppm: 7.92–7.88 (m, 2H), 7.55–7.42 (m, 5H), 7B). MALDI-TOF MS: m/z calcd for C94H56B10O2, 1326.5; found,
7.15 (d, J = 8.4 Hz, 2H), 3.95 (t, J = 6.8 Hz, 2H), 2.91–2.85 (m, 1326.0. Anal. calcd for C94H56B10O2: C 85.17, H 4.26; found: C
2H), 2.60 (t, J = 7.7 Hz, 2H), 2.47 (t, J = 7.4 Hz, 2H), 2.19–2.10 85.30, H 4.22.
(m, 2H), 1.75–1.69 (m, 2H), 1.62–1.53 (m, 2H), 1.50–1.42 (m,
2H), 1.40–1.28 (m, 2H), 1.20–1.02 (m, 4H), 0.91 (t, J = 7.2 Hz,
Cyclic voltammetry
3H). 13C NMR (CDCl3): δ ppm: 173.03, 148.78, 147.80, 145.85, Cyclic voltammetry measurement was carried out with a three-
145.20, 145.05, 144.41, 144.01, 143.77, 143.00, 142.95, 142.91, electrode cell configuration consisting of platinum working
142.19, 142.14, 142.10, 140.98, 138.01, 137.57, 132.09, 130.99, and counter electrodes and an Ag/AgNO3 (0.01 M in CH3CN)
128.83, 128.42, 128.25, 83.82 (CB-C), 82.34 (CB-C), 79.87, reference electrode at room temperature. Measurement was
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conducted using a solution of PCBM, PCB-Ph-CB and PCB-Cn-
CB (n = 1, 3, 6, 11) in a mixed solvent of toluene–CH3CN (4 : 1,
Abbreviations
5 × 10−4 M) and 0.1 M tetrabutylammonium hexafluoro- PCBM
[6,6]-Phenyl-C61-butyric acid methyl ester
[6,6]-Phenyl-C61-butyric acid
phosphate (n-Bu4NPF6) was used as the supporting electrolyte. PCBA
The reduction potentials were recorded at a scan rate of FET (OFET) (Organic) Field-effect transistor
100 mV s−1 and reported with reference to the ferrocene/ferro- OLED
Organic light-emitting diode
Organic photovoltaic
Lowest unoccupied molecular orbital
Cyclic voltammetry
cenium (Fc/Fc+) redox couple.
OPV
LUMO
CV
Device fabrication
AFM
Atomic force microscopy
All FETs (with the top contact geometry) were fabricated on
heavily doped n-type silicon (Si) wafers each covered with a
thermally grown silicon dioxide (SiO2) layer with a thickness of
200 nm. The active layer was deposited by spin-coating at 2500
rpm. All solutions were prepared at a concentration of 1.0 wt%
in chloroform. The thickness of the deposited films was about
70 nm. Before deposition of source–drain electrodes, the films
were dried on a hot plate and stabilized at 80 °C for 30 min.
All fabrication processes were carried out in a glovebox filled
with N2. An Ag source and drain electrodes were deposited by
thermal evaporation using a shadow mask. The thickness of
the source and drain electrodes was 75 nm. The channel
length (L) and the channel width (W) were 3000 μm and
50 μm, respectively. Electrical characterization was performed
using a Keithley semiconductor parametric analyzer (Keithley
4200) under an N2 atmosphere.
Acknowledgements
This work was supported by the Basic Science Research
Program (No. 2012039773 for M.H. Lee, and No. 2011-0009148
for S. Cho) and Priority Research Center Program (2009-
0093818 for M.H. Lee and S. Cho) through the National
Research Foundation of Korea (NRF) funded by the Ministry of
Education, Science and Technology (MEST).
Notes and references
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Conclusions
We have demonstrated that methanofullerene-o-carborane
dyads can be prepared via esterification of [6,6]-phenyl-C61-
butyric acid (PCBA) with 2-alcohol functionalized o-carbo-
rane derivatives, 1-(4-n-BuC6H4)-2-R-1,2-closo-C2B10H10 (R =
p-C6H4OH, (CH2)nOH, n = 1, 3, 6, 11). It was shown that the
dyads can act as active layer materials in n-type FET devices
with an electron mobility comparable to that of PCBM. The
performances are quite dependent on the length of the alkyl
linker between the methanofullerene and the o-carborane moi-
eties, and are better than that of the physical blend of o-carb-
orane and PCBM. Although the direct influence of an
o-carborane cage on the electronic properties of the methanoful-
lerene moiety was not apparently observed, the incorporation
of an electron-deficient carborane cage into the methanofuller-
ene moiety in close proximity may have an impact on the elec-
tron transporting properties of PCBM, suggesting that the
methanofullerene-o-carborane dyads may constitute a new
class of electron transporting materials.
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