Hole-transporting spirothioxanthene derivatives as donor materials for efficient small-molecule-based organic photovoltaic devices

Article abstract of DOI:10.1021/cm5033699

A new class of hole-transporting spirothioxanthene derivatives has been synthesized and characterized. Their photophysical, electrochemical and thermal properties have been studied. These compounds exhibit high hole mobilities of up to 1 × 10^-3 cm² V^-1 s^-1, determined by using thin film transistor technique. In addition, these spirothioxanthene derivatives are promising donor materials in the construction of high performance organic photovoltaic (OPV) devices. With a very low dopant concentration of 7%, highly efficient small molecule-based OPV devices with high short-circuit current density of 10.83 mA cm^-2, open-circuit voltage of 0.94 V, and high power conversion efficiency of 5.40% (the highest PCE of 5.46%) have been realized.

Full text of DOI:10.1021/cm5033699

Article
pubs.acs.org/cm
Hole-Transporting Spirothioxanthene Derivatives as Donor Materials
for Efficient Small-Molecule-Based Organic Photovoltaic Devices
Chin-Yiu Chan,† Yi-Chun Wong,† Mei-Yee Chan,*,† Sin-Hang Cheung,‡ Shu-Kong So,‡
and Vivian Wing-Wah Yam*,†
†Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong
‡Department of Physics, Hong Kong Baptist University, Kowloon Tong, Hong Kong
ABSTRACT: A new class of hole-transporting spirothioxanthene derivatives
has been synthesized and characterized. Their photophysical, electrochemical
and thermal properties have been studied. These compounds exhibit high hole
mobilities of up to 1 × 10−3 cm2 V−1 s−1, determined by using thin film
transistor technique. In addition, these spirothioxanthene derivatives are
promising donor materials in the construction of high performance organic
photovoltaic (OPV) devices. With a very low dopant concentration of 7%,
highly efficient small molecule-based OPV devices with high short-circuit
current density of 10.83 mA cm−2, open-circuit voltage of 0.94 V, and high
power conversion efficiency of 5.40% (the highest PCE of 5.46%) have been
realized.
small molecules (1.69%).16 Such performance improvement is
believed to be due to the suppression of intermolecular
INTRODUCTION
■
Organic photovoltaic (OPV) technology has aroused enormous
interactions by the steric bulky group in the 3D donor molecules.
attention in both academia and industry in recent years, owing to
From the design strategy, the donor materials must have a low
their distinct properties including great flexibility, higher
transparency, lightweight, and lower manufacturing cost over
the inorganic counterparts. Through a rational design on the
bandgap with high absorption coefficient, in order to comple-
ment the absorption spectrum of the fullerene acceptor. On the
other hand, Tang and co-workers recently demonstrated highly
molecular structure, a wide range of photoactive materials with
efficient OPV devices with PCE of up to 5.23% by employing a
interesting photophysical properties can be synthesized.1−7
Extensive studies on the development of novel photoactive
nonabsorbing 4,4′-cyclohexylidenebis[N,N-bis(4-
methylphenyl)aniline] (TAPC) as donor in the BHJ.20 This
materials and smart device architecture have boosted up the
opens up new opportunities to make use of nonabsorbing
power conversion efficiencies (PCE) of OPV devices of up to
9.2% for a single cell and 10.6% for a tandem device.8,9 To date,
organic materials as donor to realize highly efficient small
molecule-based OPV devices.
Spirobifluorene and its derivatives are a benchmark class of p-
type semiconductors in organic field-effect transistors and
the world-record OPV devices are made of conjugated polymer
donors and solution-processed fullerene acceptors with a bulk
heterojunction (BHJ) structure.
Although small molecule-based OPV devices are of equal
importance as the polymer counterparts, the development of
small molecule-based photoactive materials has received less
organic light-emitting devices (OLEDs) because of their
excellent hole mobilities.21−23 Spirobifluorene has a 3D structure
with a sp3 carbon connecting two orthogonal π systems together.
attention owing to the relatively low PCE of such devices.10−14 In
The rigid spiro-conjugation is found to enhance the hole-
transporting property by hopping process.21−30 Taking its
general, most of the small molecule-based donor materials have
1D or planar structures, and the mutually parallel orientation
with fullerene acceptor would cause a serious charge
advantage of high luminescence quantum yield, spirobifluorene
is also an ideal building block in the construction of stable blue
light-emitting materials.24−27 The synthesis of metal-free organic
dye sensitizers based on the use of the spiro group as π-
conjugated spacers for dye-sensitized solar cells has been
reported recently.28−30 The steric bulk of the spiro-configured
central unit can effectively suppress aggregation-induced self-
quenching as well as inhibit the intermolecular interactions,
recombination to quench the photogenerated excitons.15−19
More importantly, these donor materials, which are usually π-
conjugated molecules, have excellent charge transfer character
along the plane parallel to the π-conjugation, but considerably
lower hole mobilties in the direction perpendicular to the
molecular plane.15−19 This inevitably lowers the fill factor (FF) of
the OPV devices. As reported by Tu and co-workers, the
employment of 3D donor materials can significantly improve the
photovoltaic responses.16 Particularly, the PCE of the devices
(4.82%) is three times higher than that of devices based on 1D
Received: September 12, 2014
Revised: October 16, 2014
© XXXX American Chemical Society
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a
Scheme 1. Synthetic Scheme for 1−4
a
(i) CuI, L-Proline, K2CO3, DMF, 80 °C; (ii) nBuLi, −78 °C, 2,7-dibromo-9-fluorenone; (iii) cat. triflic acid, CH2Cl2; (iv) mCPBA, DCM; (v) THF
−aq. K2CO3 (2 M), [Pd(PPh3)4], reflux.
improving the photovoltaic properties.28−30 However, this class
of materials has been rarely employed as photoactive materials in
the fabrication of OPV devices, particularly because of their
considerably low absorption coefficients within the visible
spectrum. It was only until very recently that few examples of
spirobifluorene-based conjugated polymers or macromolecules
as donor materials in OPV devices have been successfully
reported, yielding highly efficient OPV devices with PCE of up to
4.6% and 4.82%, respectively.16,31 On the other hand, there are
no literature reports on the use of small molecule-based
heterocyclic spiro compounds as photoactive materials in the
construction of high performance OPV devices.
successful demonstration for the first time of such compounds as
promising candidates as donor materials in OPV devices.
Noticeably, highly efficient small molecule-based OPV devices
with PCE of up to 5.40% (the highest PCE of 5.46%) have been
realized.
RESULTS AND DISCUSSION
■
Synthesis of Spirothioxanthene Derivatives. The
synthetic route of compounds 1−4 is shown in Scheme 1.
Conversion of tertiary alcohol intermediate to Br−S−Spiro−Br
was carried out by ring closing reaction using catalytic amount of
triflic acid. Br−S−Spiro−Br was then oxidized by 3-
chloroperbenzoic acid to give compound Br−SO2-Spiro−Br.
Spirothioxanthene compounds 1−4 were synthesized using the
Herein, we report the design and synthesis of a new class of
heterocyclic spiro derivatives, namely spirothioxanthene, and the
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Figure 1. Electronic absorption spectrum of (a) 1, (b) 2, (c) 3, and (d) 4 in CH2Cl2 solution at 298 K.
standard Suzuki coupling reaction of Br−S−Spiro−Br or Br−
SO2−Spiro−Br with the corresponding boronic acid. All the
reaction intermediates and compounds 1−4 have been
Table 1. Photophysical data of 1−4
Photophysical data for 1−4
Absorption λmax (nm) (εmax (dm3 mol−1 cm−1))
1
Compound
Medium
characterized by H NMR spectroscopy and HR-EI and FAB
mass spectrometry. Satisfactory elemental analyses have also
been obtained for compounds 1−4.
1
2
3
4
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
294 (54540), 346 (88140)
298 (58040), 353 (79470)
300 (43420), 380 (87320)
298 (42630), 375 (76390)
Electronic Absorption Properties. The electronic absorp-
tion spectra of 1−4 were recorded in solutions of dichloro-
methane at 298 K. In general, the electronic absorption spectra of
the spirothioxanthene compounds show similar absorption
behavior. The absorption bands in the UV region could be
assigned to the π−π* transitions with absorption maxima
occurring at 346, 353, 380, and 375 nm for 1−4, respectively,17
with extinction coefficients of up to ∼9 × 104 dm3 mol−1 cm−1;
while the absorption band at ca. 300 nm could be attributed to
the n−π* transition of the triarylamine groups.17 In general, both
the π−π* and n−π* transitions show a red shift in energy upon
an increase in the electron richness of the electron-donating
amine moiety on the spirothioxanthene derivatives, with
carbazole being less electron-donating than di-tert-butylcarbazole
than diphenylamine. This is consistent with the literature where
stronger electron-donating group would destabilize both the π
and the n orbitals more than the π* orbital, giving rise to a
reduction of the π−π* and n−π* energy gap and hence smaller
transition energies [1 (346 nm) > 2 (353 nm) > 3 (380
nm)].18,19 On the other hand, upon going from 3 to 4, a blue shift
in the π−π* transition energy is observed [3 (380 nm) < 4 (375
nm)].18,19 This is in line with the lack of oxidation at the sulfur
atom to sulfone in 4, which results in the larger destabilization of
the π* orbital than the π orbital. Figure 1 shows the electronic
absorption spectra of 1−4 and their electronic absorption
spectral data are listed in Table 1.
(SCE), corresponding to the one-electron oxidation of the
triarylamine unit (Figure 2). With the attachment of electron-
rich triphenylamine groups, 3 and 4 display a quasi-reversible
couple at +0.96 V vs SCE. On the other hand, the potential for
oxidation is found to shift to more positive values when less
electron-rich carbazole groups are attached. For instance, 1
displays an irreversible oxidation wave at +1.35 V whereas 2
shows a quasi-reversible oxidation couple at +1.20 V vs SCE. For
reduction, all the compounds exhibit multiple reduction waves,
in which the first reduction leads to a quasi-reversible reduction
couple and the others give rise to irreversible waves. The
reductions have been assigned as the sequential one-electron
reduction of spirothioxanthene or the spirothioxanthene-dioxide
core (Figure 3). The reduction waves are sensitive to the nature
of electron-donating groups as well as the oxidation state of the
sulfur atom. The electrochemical properties of 1−4 are
summarized in Table 2, in which the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) levels have been estimated from the potentials for
oxidation and reduction, respectively.
Thermal Properties. The thermal properties of compounds
1−4 have been studied by thermogravimetric analysis (TGA). All
the spirothioxanthene compounds show high thermal stability
with decomposition temperatures (Td) > 450 °C (i.e., Td is
defined as the temperature at which the material shows a 5%
weight loss under a nitrogen atmosphere) (Figure 4). The
thermal data of the spirothioxanthene compounds have been
tabulated in Table 3.
Electrochemical Properties. The electrochemical behav-
iors of 1−4 have been investigated by cyclic voltammetry. The
cyclic voltammograms were recorded in N,N-dimethylforma-
mide in the presence of 0.1 M nBu4NPF6. The spirothioxanthene
compounds show one quasi-reversible to irreversible oxidation
couple at +0.96 to +1.35 V versus standard calomel electrode
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Figure 2. Cyclic voltammograms showing oxidative scans of (a) 1, (b) 2, (c) 3, and (d) 4 in N,N-dimethylformamide (0.1 M nBu4NPF6). Scan rate: 100
mV s−1.
Photovoltaic Properties. To testify the applicability of the
spirothioxanthene compounds as donor materials, we used BHJ
OPV devices with the configuration of indium tin oxide (ITO)/
molybdenum oxide (MoO3) (2 nm)/x % donor:C70 (60 nm)/
bathophenanthroline (BPhen) (8 nm)/aluminum (Al) (100
nm) were fabricated, in which 1−4 as donor materials, whereas
ITO, MoO3, C70, BPhen and Al were used as the anode, anodic
buffer layer, acceptor, exciton blocking layer, and cathode,
respectively. For fair comparison, two control devices with the
structures of ITO/MoO3/C70/BPhen/Al (i.e., C70-only device)
and ITO/MoO3/7% TAPC:C70/BPhen/Al (i.e., TAPC-doped
device) were also prepared and tested under similar conditions.
Under light illumination of 1 sun, the C70-only device showed a
poor performance with short-circuit current density (JSC) of 1.25
mA cm−2, an open-circuit voltage (VOC) of 0.97 V, and a FF of
0.37. These correspond to a low PCE of 0.45%, consistent with
other reports.20,37,38 The introduction of spirothioxanthene
compounds as donor materials could dramatically improve the
photovoltaic responses of OPV device. For instance, with
increasing the doping concentration of 3 from 0 to 7%, the JSC is
found to increase from 1.25 to 10.83 mA cm−2. Meanwhile, the
FF is increased from 0.37 to 0.53. By keeping a high VOC of 0.94
V, these correspond to a high PCE of 5.40% (the highest PCE of
5.46%), an order of magnitude higher than that of the C70-only
device (Figure 5). Further increase in the dopant concentration
would cause a slight degradation of the photovoltaic responses.
Similar performance improvement could be obtained by
employing another spirothioxanthene compound as the donor
(i.e., compound 4). The optimized device (i.e., 7% compound 4)
illustrates a high JSC of 10.81 mA cm−2, a VOC of 0.94 V, a FF of
0.49, and a PCE of 4.98% (the highest PCE of 5.02%). On the
other hand, the photovoltaic responses of devices doped with
carbazole-based spirothioxanthene (i.e., 1 and 2) are relatively
lower than those of devices with triphenylamine-based
compounds (i.e., 3 and 4). The optimized devices doped with
1 and 2 give PCEs of 1.31 and 3.58%, respectively. The
discrepancies in the device performance may be attributed to the
blue-shifted absorption spectra and the lower-lying HOMO
levels of 1 and 2. To verify the accuracy of JSC, we have examined
the incident photon to current efficiencies (IPCEs) of the OPV
devices, as depicted in Figure 6. For the C70-only device, a rather
low IPCE of 20.8% has been found at 380 nm. With the
introduction of spirothioxanthene as donor, all the doped devices
exhibit broad spectral responses in the range between 380 and
700 nm, roughly resembling the superposition of the absorption
spectra of spirothioxanthene with peak maximum at 380 nm and
C70 with peak maximum at 500 nm.20 In addition, a pronounced
improvement in the IPCE has been obtained. In particular, the
IPCEs of the devices doped with 1−4 are found to be 30.5, 61.7,
79.0, and 73.7%, respectively, at 400 nm. These values are much
higher than that of the C70-only device (23.0%). It is worth
noting that the JSC values calculated from the integration of the
IPCE spectra with the Air Mass (AM) 1.5 Global solar spectrum
are in good agreement (i.e., within 10% uncertainty) with those
measured from the current−voltage curves, which might be
attributed to the degradation of the devices without
encapsulation during measurements. Attempts had also been
carried out to investigate the thermal annealing effects, in which
the devices were annealed at 90 °C for 10 min. However, it was
observed that the PCEs of all devices were dropped after thermal
treatment, which might be attributed to a lower hole mobility in
the spiro molecules arising from the change of the crystallinity.
Table 4 lists the average cell parameters for the as-prepared
devices doped with compounds 1−4, in which the deviations
were obtained from device-to-device variation of four identical
devices.
Hole Mobility. As reported by Tang and co-workers, the hole
mobility of donor materials plays a crucial role in determining the
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Figure 3. Cyclic voltammograms showing reductive scans of (a) 1, (b) 2, (c) 3, and (d) 4 in N,N-dimethylformamide (0.1 M nBu4NPF6). Scan rate: 100
mV s−1.
Table 2. Electrochemical Properties of 1−4
b
b
b
b
Oxidation E1/2 (V vs SCE) [Epa (V vs SCE)]
Reduction E1/2 (V vs SCE) [Epa (V vs SCE)]
HOMO
LUMO
e
f
Compound
(ΔEp (mV))
(ΔEp (mV))
(eV)
(eV)
c
d
1
2
3
4
[+1.35]
−1.97 (66), [−2.31]
−1.99 (63), [−2.32] , [−2.61]
−2.05 (83), [−2.37]
−2.11 (82), [−2.42] [−2.73]
−5.70
−5.55
−5.31
−5.31
−2.38
−2.36
−2.30
−2.24
d
d
+1.20 (82)
+0.96 (86)
+0.96 (80)
d
d
d
a
b
0.1 M nBu4NPF6 (TBAH) as supporting electrolyte at room temperature; scan rate 100 mV s−1. E1/2 = (Epa + Epc)/2; Epa and Epc are peak anodic
and peak cathodic potentials, respectively. Irreversible oxidation wave. The potential refers to Epa, which is the anodic peak potential. Irreversible
reduction wave. The potential refers to Epc, which is the cathodic peak potential. EHOMO = −(Eox + 4.35) eV. ELUMO= −(Ered + 4.35) eV.
c
d
e
f
performance of the OPV devices.20 To correlate with the
photovoltaic properties, we determined hole mobilities of the
spirothioxanthene compounds in a thin film transistor (TFT)
configuration at room temperature.39 Specifically, compounds 3
and 4 which exhibit a good photovoltaic performance have been
examined. The hole mobility of TAPC was also studied for a fair
comparison. Figure 7 shows the output and transfer character-
istics of the fabricated OTFTs. At a large gate voltage (VG) of
−50 V, TAPC exhibits a larger source-to-drain current (IDS)
between 2.5 and 3.0 μA, whereas both spirothioxanthene
compounds 3 and 4 show smaller IDS. TAPC also exhibits higher
mobilities of 2.5 × 10−3 and 3.1 × 10−3 cm2 V−1 s−1 in the linear
and saturation regions, respectively. With the attachment of the
more electron-rich triphenylamine groups, compound 4
demonstrates comparable hole mobilities, where the linear
mobilities (μLinear) and saturation mobilites (μSat) are found to be
1.4 × 10−3 and 8.5 × 10−4 cm2 V−1 s−1, respectively. On the other
hand, with oxidization of the sulfur atom, compound 3 shows
smaller hole mobilities (i.e., μLinear = 2.0 × 10−4 cm2 V−1 s−1; μSat
= 1.4 × 10−4 cm2 V−1 s−1). In correlating with the photovoltaic
responses, it seems that hole mobilities measured by the OFET
method are not the determining factors governing the device
performance. Although TAPC possesses a higher hole mobility
than the spirothioxanthene compounds, the PCEs of devices
doped with compounds 3 and 4 are found to be higher than that
of TAPC (4.38%), as shown in Table 4. The higher PCEs are
likely to be due to the higher VOC values for devices doped with 3
and 4, which may be ascribed to a reduced dark current. As
depicted in Figure 8, the devices doped with 3 and 4 exhibit a
reduced dark current at −1 V than that of a similar device doped
with TAPC. The reduced dark current can significantly reduce
the electron leakage current and results in an increase in the VOC
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Figure 4. TGA thermograms of (a) 1, (b) 2, (c) 3, and (d) 4. Heating rate: 10 °C min−1 under a nitrogen atmosphere.
Table 3. Thermal Properties for 1−4
a
Compound
Tdecomp (°C)
1
2
3
4
497
462
466
473
a
Determined by thermogravimetric analysis. Heating rate: 10 °C
min−1 under a nitrogen atmosphere. Tdecomp was determined at 5%
weight loss.
Figure 6. IPCE of devices doped with 7% donor material.
CONCLUSION
■
In summary, a new class of spirothioxanthene compounds has
been successfully designed and synthesized. The functionaliza-
tion of the spiro core with various triarylamine groups and the
modification of the heteroatom of the spiro core have been
shown to be capable of tuning both the photophysical and
electrochemical properties of the spirothioxanthene compounds.
These compounds exhibit high hole mobilities of up to 1 × 10−3
cm2 V−1 s−1, comparable to the prototypical hole-transporting
TAPC. More importantly, spirothioxanthene compounds are
promising donor materials for the construction of high
performance OPV devices. With a very low dopant concentration
of 7%, highly efficient small-molecule-based OPV devices with
PCE of up to 5.40% (the highest PCE of 5.46%) have been
realized.
Figure 5. J−V characteristics of devices doped with 3 at different
concentrations under light illumination of 1 sun.
of the OPV devices.40 Nevertheless, the findings clearly
demonstrate for the first time that spirothioxanthene compounds
are a promising class of donor materials that warrants further
exploration to realize high-performance small-molecule-based
OPV devices.
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Table 4. Key Photovoltaic Responses for the As-Prepared Devices without and with Donor Material
Compound
Dopant Conc. (%)
Jsc Calcd (mA cm−2
)
Jsc (mA cm−2
)
Voc (V)
FF
PCE (%)
C70-Only
TAPC
1
0
7
3
5
7
9
3
5
7
9
3
5
7
9
3
5
7
9
1.15
10.62
2.86
3.12
3.70
4.27
6.44
7.42
7.94
8.15
9.78
9.77
10.00
9.84
9.81
10.03
9.99
9.97
1.25 0.03
10.73 0.47
2.90 0.12
3.04 0.11
3.86 0.17
4.17 0.09
6.62 0.04
7.67 0.08
8.57 0.12
8.35 0.02
9.78 0.22
10.86 0.16
10.83 0.14
10.60 0.11
10.14 0.17
10.94 0.41
10.81 0.11
10.67 0.05
0.97 0.02
0.85 0.01
1.06 0.01
1.10 0.01
1.08 0.01
1.08 0.01
1.16 0.01
1.17 0.01
1.16 0.01
1.16 0.0.1
0.96 0.01
0.95 0.01
0.94 0.01
0.92 0.01
0.96 0.01
0.95 0.01
0.94 0.01
0.93 0.01
0.37 0.01
0.48 0.01
0.27 0.01
0.26 0.01
0.27 0.01
0.29 0.01
0.34 0.01
0.35 0.01
0.36 0.01
0.37 0.01
0.47 0.01
0.50 0.01
0.53 0.01
0.54 0.01
0.43 0.01
0.48 0.01
0.49 0.01
0.49 0.01
0.45 0.02
4.38 0.13
0.83 0.04
0.87 0.03
1.13 0.05
1.31 0.04
2.61 0.09
3.14 0.03
3.58 0.02
3.58 0.01
4.41 0.10
5.16 0.03
5.40 0.06
5.27 0.07
4.19 0.09
4.99 0.08
4.98 0.05
4.86 0.09
2
3
4
dried over anhydrous MgSO4 and filtered. The solvent was evaporated
to dryness under vacuum. The crude solid was washed with hexane,
filtered and used without further purification. The tertiary alcohol
intermediate was dissolved in dichloromethane solution. A few drops of
triflic acid were added. The reaction was monitored by TLC analysis
until no starting material could be detected. The reaction was washed
with deionized water and the organic layer was dried over anhydrous
MgSO4. The organic layer was filtered and evaporated to dryness under
vacuum. The crude product was purified by column chromatography
using hexane as eluent. Further purification was achieved by
recrystallization of the product from dichloromethane-methanol.
EXPERIMENTAL SECTION
■
Materials. All chemicals used for synthesis were of analytical grade
and were purchased from Sigma−Aldrich Chemical Co. 4-
(Diphenylamino)phenylboronic acid, (4-(9H-carbazol-9-yl)phenyl)-
boronic acid, 2,7-dibromo-9-fluorenone, 4-(3,6-di-tert-butyl-9H-carba-
zol-9-yl)phenyl)boronic acid and (2-bromophenyl)(phenyl)sulfane
(PhS−C6H4Br−2) were synthesized according to the literature
reported procedures.32−36
Physical Measurements and Instrumentation. 1H NMR spectra
were recorded using a Bruker DPX-300 (300 MHz) or a Bruker Avance
400 (400 MHz) Fourier-transform NMR spectrometer with chemical
shifts reported relative to tetramethylsilane, (CH3)4Si. Positive-ion EI
and FAB mass spectra were recorded using a Thermo Scientific DFS
high-resolution magnetic sector mass spectrometer. The electronic
absorption spectra were obtained using a Hewlett−Packard 8452A
diode array spectrophotometer. Cyclic voltammetric measurements
were performed by using a CH Instruments, Inc. model CHI 620A
electrochemical analyzer. Electrochemical measurements were per-
formed in dichloromethane solutions with 0.1 mol dm−3 nBu4NPF6 as
supporting electrolyte at room temperature. The reference electrode
was a Ag/AgNO3 (0.1 mol dm−3 in acetonitrile) electrode, and the
working electrode was a glassy carbon electrode (CH Instruments, Inc.)
with a platinum wire as the counter electrode. The working electrode
surface was first polished with 1 μm alumina slurry (Linde) on a
microcloth (Buehler Co.) and then with 0.3 μm alumina slurry. It was
then rinsed with ultrapure deionized water and sonicated in a beaker that
contained ultrapure water for 5 min. The polishing and sonicating steps
were repeated twice, and then the working electrode was finally rinsed
under a stream of ultrapure deionized water. The ferrocenium/ferrocene
couple (Fc+/Fc) was used as the internal reference. All solutions for
electrochemical studies were deaerated with prepurified argon gas prior
to measurements. Thermal Gravimetric Analysis was performed by
thermal gravimetric analyzer (PerkinElmer TGA 7) with a heating rate
of 10 °C min−1 under a nitrogen atmosphere.
Synthesis and Characterization. Br−S−Spiro−Br. (2-
Bromophenyl)(phenyl)sulfane (265 mg, 1 mmol) was dissolved in
dry THF and degassed for 15 min. The resulting mixture was cooled to
−78 °C under nitrogen, and nBuLi (1.6 M in hexanes, 0.7 mL, 1.1 mmol)
was added in a dropwise manner. The resulting mixture was stirred at
−78 °C for 1 h, and then 2,7-dibromo-9-fluorenone (328 mg, 1 mmol)
was added in one portion. The reaction mixture was kept at −78 °C for
another hour before warming to room temperature, and then stirred
overnight. The resulting mixture was quenched with deionized water
and extracted with dichloromethane for three times. The combined
organic layer was then washed with deionized water. The solution was
1
Yield: 379 mg (75%) H NMR (300 MHz, CDCl3, 298 K, relative to
Me4Si): δ 6.49 (d, 2H, 8.0 Hz), 6.92 (t, 2H, 8.0 Hz), 7.19 (t, 2H, 8.0 Hz),
7.42 (d, 2H, 8.0 Hz), 7.51 (d, 2H, 8.0 Hz), 7.61 (d, 2H, 8.0 Hz), 7.67 (s,
2H). HRMS (Positive EI) calcd for C25H14Br2S: m/z = 505.9334; found:
505.9166 [M]+.
Br−SO2−Spiro−Br. Br−S−Spiro−Br (506 mg, 1 mmol) was
dissolved in dichloromethane and was cooled to 0 °C. mCPBA (690
mg) was added and the resulting mixture was slowly warmed to room
temperature, and stirred overnight. The reaction was quenched with
aqueous Na2S2O5 (2 M) and then washed with aqueous Na2CO3 (2 M),
deionized water. The organic layer was dried over anhydrous MgSO4.
The organic layer was filtered and evaporated to dryness under vacuum.
The crude product was purified by column chromatography using
dichloromethane-hexane as eluent. Further purification was achieved by
recrystallization of the product from dichloromethane-methanol. Yield:
484 mg (90%) 1H NMR (300 MHz, CDCl3, 298 K, relative to Me4Si): δ
6.57 (d, 2H, 8.0 Hz), 7.32 (t, 2H, 8.0 Hz), 7.46 (s, 2H), 7.51 (d, 2H, 8.0
Hz), 7.58 (d, 2H, 8.0 Hz), 7.68 (d, 2H, 8.0 Hz), 8.26 (d, 2H, 8.0 Hz). MS
(Positive FAB) calcd for C25H14O2Br2S: m/z = 538.3; found: 538.7
[M]+.
General Procedure for Compounds 1−4. Compounds 1−4 were
synthesized according to the standard Suzuki coupling reaction. The
precursor, Br−S−Spiro−Br or Br−SO2−Spiro−Br (1 mmol), was
dissolved in a mixture of aqueous K2CO3 (2 M) solution (4 mL) and
THF (80 mL). After that, the corresponding boronic acid (3 mmol) was
added to the reaction mixture. The solvent was degassed for 15 min and
[Pd(PPh3)4] catalyst (0.1 mmol) was added. The reaction mixture was
heated to reflux overnight. The crude reaction mixture was extracted
with dichloromethane for at least three times. Then, the organic layer
was washed with deionized water several times. The organic layer was
dried over anhydrous MgSO4 and filtered. The solvent was evaporated
to dryness under vacuum. The crude product was purified by column
chromatography using dichloromethane-hexane as eluent. Further
purification was achieved by recrystallization of the product with
THF-pentane or chloroform-acetone.
G
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Figure 7. Output and transfer characteristics of the fabricated OTFTs based on (a) TAPC, (b) 3, and (c) 4.
1
Cbz−SO2−Spiro−Cbz (1). Yield: 690 mg (80%). H NMR (400
MHz, CDCl3, 298 K, relative to Me4Si): δ 6.80 (d, 2H, 8.0 Hz), 7.27−
7.30 (m, 4H), 7.37−7.43 (m, 10H), 7.52−7.59 (m, 6H), 7.75−7.79 (m,
6H), 7.84 (d, 2H, 8.0 Hz), 8.02 (d, 2H, 8.0 Hz), 8.13 (d, 4H, 8.0 Hz),
8.32 (d, 2H, 8.0 Hz). MS (Positive FAB) calcd for C61H38O2N2S: m/z =
863.0; found: 863.3 [M]+. Elemental anal. Calcd (%) for C61H38O2N2S·
0.5(CH3)2O: C 84.04, H 4.66, N 3.16. Found: C 84.33, H 4.50, N 3.13.
7.08 (m, 16H), 7.23 (t, 8H, 7.5 Hz), 7.28 (d, 2H, 8.0 Hz), 7.37 (d, 4H,
8.0 Hz), 7.46 (t, 2H, 8.0 Hz), 7.60 (s, 2H), 7.67 (d, 2H, 8.0 Hz), 7.87 (d,
2H, 8.0 Hz), 8.24 (d, 2H, 8.0 Hz). MS (Positive FAB) calcd for
C61H42O2N2S: m/z = 867.1; found: 867.3 [M]+. Elemental anal. Calcd
(%) for C61H42O2N2S·0.5C5H12: C 84.45, H 5.36, N 3.10. Found: C
84.17, H 5.08, N 3.15.
TPA−S−Spiro−TPA (4). Yield: 584 mg (70%). 1H NMR (400 MHz,
CD2Cl2, 298 K, relative to Me4Si): δ 6.63 (d, 2H, 8.0 Hz), 6.89 (t, 2H,
8.0 Hz), 6.99−7.06 (m, 16H), 7.15 (t, 2H, 8.0 Hz), 7.23 (t, 8H, 7.4 Hz),
7.40−7.43 (m, 6H), 7.64 (d, 2H, 8.0 Hz), 7.78 (s, 2H), 7.84 (d, 2H, 8.0
Hz). HRMS (Positive EI) calcd for C61H42SN2: m/z = 834.3063; found:
834.2063 [M]+. Elemental anal. Calcd (%) for C61H42SN2·(CH2)4O: C
86.06, H 5.56, N 3.09. Found: C 86.05, H 5.42, N 3.30.
1
tCbz−SO2−Spiro−tCbz (2). Yield: 814 mg (75%). H NMR (400
MHz, CDCl3, 298 K, relative to Me4Si): δ 1.46 (s, 36H) 6.79 (d, 2H, 8.0
Hz), 7.37−7.43 (m, 10H), 7.52−7.59 (m, 6H), 7.75−7.79 (m, 6H), 7.84
(d, 2H, 8.0 Hz), 8.01 (d, 2H, 8.0 Hz), 8.12 (s, 4H), 8.31 (d, 2H, 8.0 Hz).
MS (Positive FAB) calcd for C77H70O2N2S: m/z = 1087.5; found:
1086.1 [M−1]+. Elemental anal. Calcd (%) for C77H70O2N2S: C 85.04,
H 6.49, N 2.58. Found: C 84.84, H 6.74, N 2.41.
OPV Device Fabrication and Characterization. OPV devices
were grown on indium tin oxide (ITO)-coated glass substrates with
sheet resistance of 25 ohm square−1. The substrates were cleaned with
1
TPA−SO2−Spiro−TPA (3). Yield: 560 mg (68%). H NMR (400
MHz, CDCl3, 298 K, relative to Me4Si): δ 6.71 (d, 2H, 8.0 Hz), 6.99−
H
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: wwyam@hku.hk. Fax: +852 2857-1586. Tel: +852
2859-2153.
■
*E-mail: chanmym@hku.hk.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
V.W.W.Y. acknowledges support from The University of Hong
Kong under the URC Strategic Research Theme on New
Materials. This work was fully supported by a grant from the
Theme-Based Research Scheme of the Research Grants Council
of the Hong Kong Special Administrative Region, China (Project
T23-713/11). C.Y.C. and Y.C.W. acknowledge the receipt of
postgraduate studentships from The University of Hong Kong.
Figure 8. Dark current of the OPV devices doped with TAPC, 3, and 4.
REFERENCES
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