Self-assembly and phase separation of amphiphilic dyads based on 4,7-bis(2-thienyl)benzothiodiazole and perylene diimide

Article abstract of DOI:10.1039/c3ra47633b

To get highly ordered organic structures at the nanoscale, a series of new electronic donor-acceptor (D-A) dyads were synthesized. The dyads bearing different side chains (lipophilic or amphiphilic) or linkers (Long or Short) showed variable self-assembly behaviour. Long-linker dyads can fold in dilute solution, but the folding of short-linker dyads was not observed. Intermolecular D-A interactions and acceptor-acceptor (A-A) aggregation were proved to co-occur in chloroform for all four dyads. In the bulk state, amphiphilic dyads could overcome the intrinsic D-A interactions and achieve better D-A phase separation than lipophilic ones due to the incompatibility of the side chains. The dyad with a short-linker and amphiphilic side chains, S_amphi, had the ability to form a gel, and both of the amphiphilic dyads could form nanofibres. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3ra47633b

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Self-assembly and phase separation of amphiphilic
dyads based on 4,7-bis(2-thienyl)benzothiodiazole
and perylene diimide†
Cite this: RSC Adv., 2014, 4, 13078
*
Jiang Peng, Feng Zhai, Xinyan Guo, Xinpeng Jiang and Yuguo Ma
To get highly ordered organic structures at the nanoscale, a series of new electronic donor–acceptor (D–A)
dyads were synthesized. The dyads bearing different side chains (lipophilic or amphiphilic) or linkers (Long
or Short) showed variable self-assembly behaviour. Long-linker dyads can fold in dilute solution, but the
folding of short-linker dyads was not observed. Intermolecular D–A interactions and acceptor–acceptor
(A–A) aggregation were proved to co-occur in chloroform for all four dyads. In the bulk state,
amphiphilic dyads could overcome the intrinsic D–A interactions and achieve better D–A phase
separation than lipophilic ones due to the incompatibility of the side chains. The dyad with a short-linker
and amphiphilic side chains, Samphi, had the ability to form a gel, and both of the amphiphilic dyads could
form nanofibres.
Received 14th December 2013
Accepted 20th January 2014
DOI: 10.1039/c3ra47633b
www.rsc.org/advances
equipped at each end of the amphiphilic dyads.4,5 The
cooperativity of p–p stacking and incompatibility of the side
Introduction
chains enables the phase separation of both electron donors
and acceptors.
In bulk heterojunction organic photovoltaic devices, bicon-
tinuous arrays of electron donors and acceptors are a
prerequisite.1 As an inherent property of organic semi-
conductors, the exciton diffusion length is ca. 5–10 nm.2 In
traditional bulk heterojunction devices, donor and acceptor
materials are blended with insufficient microphase separa-
tion, which results in exciton loss.3 To maximize the het-
erojunction interface, researchers paid much attention to
small molecule dyads consisting of electron donor (i.e.
p-type semiconductors) and electron acceptor (i.e. n-type
semiconductors) units,4,5 which are promising for opti-
mizing charge separation and charge transportation
processes.
In the present contribution, a series of new D–A dyads
containing 4,7-bis(2-thienyl)-2,1,3-benzothiodiazole (T2BTZ)
and perylene diimide (PDI) were synthesized (Fig. 1). A ben-
zothiodiazole unit was incorporated to lower the LUMO level
and red-shi the absorption onset,8 compared with the short-
wavelength absorption (ca. 500 nm onset position) of
commonly used oligothiophene.5a,b Incompatible triethylene
glycol and dodecyl chains were utilized to improve phase
separation (Samphi and Lamphi). The corresponding dyads with
dodecyl chains on both sides (Slipo and Llipo) were synthesized
as reference compounds. The independent donor (T2BTZ)
and acceptor (PDI) were also prepared as reference
compounds. Moreover, two amino acids were used to tune the
distance between the donor and acceptor. The aliphatic
linkers would avoid conjugated electron coupling between
both parts.
The inuence of side chains and linker length on assembly
behaviour, phase separation and nano-morphology was
systematically investigated. Intramolecular D–A interaction of
long-linker dyads was rst observed in dilute solution. Inter-
molecular D–A interactions and A–A aggregation were both
observed in concentrated solution. The dyads could achieve D–
A phase separation assisted by the amphiphilic side chains, and
the assembly morphologies differed with side chains and linker
length. To the best of our knowledge, the linker length inuence
on dyad assembly and intramolecular D–A interactions has
rarely been studied.9
To link electron donors and acceptors together, various
linkages including so alkyl chains,5a rigid phenylene ace-
tylene,5b coordination bonding and hydrogen-bonding6 were
used. Linkers are of key importance in dyads, since they
affect the donor–acceptor (D–A) distance and D–A interface.
However, the donors tend to stack with acceptors, forming
charge transfer complexes that trap excitons.7 To induce D–A
separation at the nanoscale and drive the formation of well-
dened nano-morphology, incompatible wedges (e.g.
dodecyl and triethylene glycol (TEG) chains5a,b
) were
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Lab of Polymer
Chemistry Physics of Ministry of Education, College of Chemistry, Peking
&
University, Beijing, 100871, China. E-mail: ygma@pku.edu.cn; Fax: +86 10-6275-
1708; Tel: +86 10-6275-6660
† Electronic supplementary information (ESI) available: Experimental procedures,
details of spectroscopic and analytical data. See DOI: 10.1039/c3ra47633b
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aggregation, concentration-dependent UV-vis spectra of Llipo and
Lamphi were recorded (Fig. S1†). The result conrmed that the
spectral change indeed originated from intramolecular folding
instead of intermolecular aggregation. Furthermore, the 0–1
transition intensity of Llipo is slightly higher than that of Lamphi
(Fig. 2b, red arrow), and this happens at several selected
concentrations from 10ꢀ6 M–10ꢀ5 M (Fig. S1†). We tentatively
explain it as follows: when folding happens, the side chains on
both sides approach each other intramolecularly. For Llipo, the
affinity of the dodecyl chains induced folding of the D–A unit,
while for Lamphi, the incompatibility of the dodecyl and tri-
ethylene glycol chains slightly inhibited folding. As far as we
know, this is the rst report on PDI–T2BTZ interactions and
intramolecular side chain incompatibility, characterized by
absorption spectroscopy in dilute solution.
Fig. 1 Chemical structures of four dyads and reference compounds.
The donor and acceptor are 4,7-bis(2-thienyl)benzothiodiazole
(T2BTZ) and perylene diimide (PDI), respectively.
To understand the assembly behaviour of the donor and
acceptor segments in solution, all four dyads were studied by
concentration-dependent 1H NMR in CDCl3 (Fig. 3). Taking
Samphi as an example and diluting its CDCl3 solution, we found
that the peaks of the PDI and T2BTZ protons moved downeld
and the ne structure became clear (Fig. 3, black arrows), indi-
cating the assembly of Samphi. Moreover, detailed evidence for
assembly was found from the upeld chemical shis (Fig. 3, red
arrow) of protons a and b attached to PDI. Through chemical
structural analysis, we propose that proton a forms a weak
hydrogen bond with the nearby carbonyl oxygen in the aggre-
gates, which leads to a lower chemical shi of proton a. As the
solution was diluted, the aggregates slowly dissociated, enabling
free rotation of the phenyl ring. Therefore, the hydrogen bond
was weakened and the chemical shi of proton a moved upeld.
Meanwhile, proton b had a slight upeld shi due to the elec-
tronic effect from a. These phenomena demonstrate that self-
assembly of Samphi exists in chloroform at this concentration
range (ca. 1 mM to 10 mM). The other dyads showed similar
chemical shi changes when diluting their CDCl3 solutions
(Fig. S7–S10†), indicating the occurrence of self-assembly.
Fig. 2 (a) UV-visible absorption spectra of short-linker dyads and
reference compounds; (b) normalized absorption spectra of short-
linker and long-linker dyads (chloroform, 5 ꢁ 10ꢀ6 M); (c) schematic
diagram of intramolecular folding of short-linker and long-linker
dyads.
1
To further investigate the assembly details, a H NMR dilu-
tion experiment of the reference compounds PDI and T2BTZ
and a mixture of both compounds was carried out in CDCl3. The
chemical shis of the most downeld PDI proton (c) and T2BTZ
proton (d) at four different concentrations (30 mM to 9 mM) are
summarized in Fig. 4 (see the spectra in Fig. S7–S13†). The d of
Results and discussion
Self-assembly in solution
UV-visible spectroscopy was carried out to study the ground state proton c changed by a maximum value (Dd) of 0.09 indicating
electronic properties of the dyads in solution. Slipo and Samphi that PDI can self-assemble. Proton d only had a Dd value of
showed identical absorption, suggesting that the electronic 0.013, which indicates that the self-assembly tendency of T2BTZ
properties of the donor and the acceptor are not inuenced by is weak. However, the Dd of proton d increased signicantly
the side chains. The spectra of Slipo and Samphi totally overlapped (Dd ¼ 0.09) when PDI and T2BTZ were mixed together. It means
with the spectra summation of the reference compounds PDI that T2BTZ has a tendency to co-assemble with PDI. In the four
and T2BTZ (Fig. 2a), demonstrating no electronic interaction dyads, the Dd of proton d was apparently larger than 0.013
between donor and acceptor (Fig. 2c). However, aer the 0–0 (Table S1†), so the D–A co-assembly existed. Therefore, it is
transition peaks belonging to the acceptor component were necessary to introduce different side chains to overcome the
normalized, Llipo and Lamphi had relatively higher 0–1 transition D–A interactions in order to achieve phase separation.
intensities than Slipo and Samphi, along with red-shied onset
wavelengths10 (Fig. 2b, black arrows). Based on this result, we
proposed that in Llipo and Lamphi, T2BTZ may fold back to
Morphology control and phase separation
interact with PDI intramolecularly due to the exibility of the The active layer morphology is of paramount importance for
long linker (Fig. 2c). To rule out the possibility of intermolecular organic semiconductor devices (OSCs, OFET, etc.).11 Herein, the
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morphology and phase separation, which were inuenced by
side chains, linkers and aromatic p–p interactions, were
investigated.
Since all four dyads had assembly abilities as identied by 1H
NMR, we were interested in their bulk self-assembly behaviour.
The differential scanning calorimetry (DSC) of Llipo and Lamphi
showed only one peak for the rst-order phase transition, and
polarized optical microscopy (POM) showed that no liquid
crystal state existed. Lamphi had a slightly higher Tm (179 ꢂC)
ꢂ
than Llipo (166 C) (Fig. S4†). Aer thermal annealing by DSC,
Llipo and Lamphi were used to conduct small angle X-ray scat-
tering (SAXS) experiments. For Llipo, a strong scatter peak was
observed at q1 ¼ 1.36 nmꢀ1, and weak signals of 2q1 and 3q1
could also be observed, which correspond to a repeating length
of d ¼ 4.63 nm (Fig. 5a). On the other hand, diffraction peaks
were observed at a smaller angle (q1 ¼ 0.59 nmꢀ1, 2q1
¼
1.18 nmꢀ1) for Lamphi (Fig. 5b), indicating longer repeating units
of d ¼ 10.6 nm. In MM2 force eld optimization, the molecular
length of Llipo and Lamphi were both 6.4 nm. We can deduce that
a layered periodic structure exists in both compounds. The d
spacing of Llipo was close to the molecular length indicating a
repeating unit of one single molecule (Fig. 6, le), while the d
spacing of Lamphi was almost twice as long as the molecular
length corresponding to the repeating unit of two molecules,
which indicates that phase separation of TEG and alkyl chains
occur in Lamphi (Fig. 6, right). The observed length was smaller
than the real value because of the overlapping of the alkyl
chains. These results demonstrate that incompatible side
chains assist D–A dyads to achieve donor–acceptor phase
separation. For Slipo and Samphi, the DSC showed no phase
transition before they were decomposed, so thermal treatment
is not ideal for the annealing process (see DSC curves in
Fig. S5†). Thus, solvent annealing was adopted. The dyad Samphi
formed a gel during the slow solvent evaporation of its solution
in a dichloromethane (DCM)–hexane (v/v ¼ 1/1) mixture, and
the Slipo solution remained clear under the same conditions
(Fig. S6†). Aer evaporating the residual solvent under vacuum,
the solid (Slipo) and dried gel (Samphi) were used to conduct a
SAXS experiment. One scattering peak was obtained for each
dyad (q ¼ 1.40 nmꢀ1 for Slipo, q ¼ 0.83 nmꢀ1 for Samphi). The
smaller q value for Samphi indicated similar phase separation
behaviour with long-linker dyads. However, the absence of high-
order scattering demonstrated poor phase separation results or
improper assembly conditions (Fig. 5c and d).
Fig. 3 Molecular structure of Samphi and the concentration-depen-
dent 1H NMR spectra of Samphi in CDCl3.
Fig. 4 Selected proton chemical shift changes of PDI (blue triangle),
T2BTZ (black square) and their equimolar mixture (purple inverted
1
triangle for PDI and red circle for T2BTZ) studied by H NMR experi-
ments. (CDCl3, 30 mM to 9 mM, 25 ꢂC).
According to scanning electron microscopy, Slipo, Samphi and
Lamphi form bres while Llipo formed nanoparticles aer a
heating–cooling cycle in chloroform–MeOH (v/v ¼ 1/1) at a
concentration of 6 ꢁ 10ꢀ4 M (Fig. S14†).
To explore the assembly in detail, the concentration was
lowered to 6 ꢁ 10ꢀ5 M. Aer heating and then cooling, the
assemblies were observed by transmission electron microscopy
(Fig. 7). In an alcohol-containing solvent, amphiphilic dyads
can form a more regular nanobre structure. Samphi formed thin
bres with a width of around 10 nm, which is about twice the
length of the rigid core (3.5 nm). It's in agreement with the
model proposed in the SAXS experiment. In contrast, without
the regulation of side chains, Slipo could only form thick bres.
Fig. 5 Small angle X-ray scattering of Llipo (a), Lamphi (b), Slipo (c), Samphi
(d) samples. Llipo and Lamphi were treated by DSC, and Slipo and Samphi
were treated by solvent annealing.
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Lamphi may be attributed to the exible long linker. Due to the
lipophilic affinity of side chains and relative rigid molecular
structure, Slipo formed thick bres with a diameter larger than
50 nm. For Llipo, the exible long linker allows the D–A mole-
cules to fold, and the side-by-side intermolecular lipophilic
interactions were disrupted, resulting in discrete nanoparticles.
Fig. 6 Schematic phase separation diagram of long-linker dyads; Llipo
has a multilamellar structure (left) and Lamphi has a bilayer structure
(right).
Cyclic voltammetry
To verify whether the energy levels of the donor and acceptor
match, cyclic voltammetry was carried out. The data are
summarized in Table 1 (see the redox curve in Fig. S3†). The
HOMO of T2BTZ and LUMO of PDI were determined by the
onset of the CV curve relative to ferrocene. Their corresponding
LUMO or HOMO was calculated from the optical bandgap
obtained from the UV-vis absorption spectra. The short-linker
dyads had HOMO and LUMO values nearly identical to T2BTZ
and PDI, respectively, which also demonstrated the indepen-
dent D–A electronic properties in the dyads. The LUMO differ-
ence between the donor and acceptor was 0.62 eV (Table 1),
which was sufficient to provide a driving force for charge
separation. The intramolecular quenching of T2BTZ uores-
cence by PDI also indicates the matched energy levels (Fig. S2†).
Therefore, reasonable energy levels enable the dyads to be
promising materials in organic photovoltaics.
Conclusions
Lipophilic and amphiphilic dyads were synthesized and
unambiguously characterized. Their self-assembly behaviour
was studied from dilute solutions to the bulk state.
In chloroform, no obvious difference in assembly behaviour
between amphiphilic and lipophilic dyads was observed, prob-
ably due to the good solubility of these dyads. In dilute solutions
(10ꢀ6 to 10ꢀ5 M), only long-linker dyads showed intramolecular
Fig. 7 Transmission electron microscopy images of four dyads.
However, Llipo, which differed only in linker length from the D–A interactions. However, in concentrated solutions (10ꢀ3 to
Slipo dyad, formed nanoparticles with a diameter around 50 nm. 10ꢀ2 M), all four dyads showed D–A interactions and A–A
For Lamphi with a longer molecular length than Samphi, nano- aggregation. Among them, short-linker dyads showed stronger
bres narrower than 10 nm were observed. We speculate the intermolecular interactions, evidenced by 1H NMR, maybe
reasons for different morphology formation are as follows. At because the short-linker enhanced the intermolecular cooper-
least two factors contribute to the morphology differences: (1) ativity of the D–A interaction and A–A aggregation.
the properties of the side chains (amphiphilic or lipophilic); (2)
Different from its assembly behaviour in solution, in the
the linker length of the dyads (long-linker ones can fold as bulk state, with the assistance of amphiphilic chains, Lamphi can
evidenced by UV-vis absorption spectra, thus are more exible overcome D–A interactions and achieve better nanoscale D–A
than short-linker ones). The incompatible side chains enable phase separation compared with Llipo. In contrast, Samphi has
amphiphilic dyads to form thin bres. The thinner bres for poor phase separation but good crystallinity. Therefore, Lamphi
Table 1 Redox potentials and energy levels of short-linker dyads and reference compounds in DCM at 1 ꢁ 10ꢀ5 M
Compound
4ox/V
4red/V
Optical bandgapa/eV
HOMO/eV
LUMO/eV
Slipo
Samphi
PDI
T2BTZ
FeCp2
1.14
1.08
1.53
1.04
0.43
ꢀ0.49
ꢀ0.50
ꢀ0.51
ꢀ0.80
—
ꢀ5.51
ꢀ5.45
ꢀ6.08
ꢀ5.41
ꢀ3.38
ꢀ3.87
ꢀ3.86
ꢀ3.24
2.22
2.17
a
Calculated from the onset of absorption spectra (lonset).12
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Instrumentation
1H and 13C NMR spectra were collected on a Mercury plus 300
(300 MHz) or Bruker Avance 400 (400 MHz) spectrometer, using
CDCl3 or d6-DMSO as solvents. Chemical shis are reported in
parts per million (ppm) and coupling constants are reported in
Hertz (Hz). 1H NMR chemical shis were referenced to the
residual solvent peak (7.26 ppm in CDCl3 or 2.50 ppm in d6-
DMSO). 13C NMR chemical shis were referenced to CDCl3
(77.0 ppm). Electron ionization (EI) mass spectra were collected
on a VG ZAB-HS mass spectrometer. Elemental analyses were
performed using a German Vario EL III elemental analyzer.
Electro-spray ionization (ESI) mass spectrometry was performed
using a Bruker Apex IV FTMS instrument. Matrix-Assisted Laser
Desorption/Ionization Time of Flight Mass Spectrometry
(MALDI-TOF-MS) was performed using an ABI 4800 Plus MALDI
TOF/TOF™ Analyzer. The UV-vis absorption spectra were
recorded using a Hitachi U-4100 spectrophotometer with a 1 cm
quartz cell. The photoluminescence spectra were recorded on a
Horiba Jobin Yvon FluoroMax-4P spectrouorometer with right-
angle geometry, using a 1 cm quartz cuvette. Cyclic voltammetry
was carried out by using BASI Epsilon workstation. The
compounds were detected in DCM (freshly distilled over CaH2
prior to use) containing 0.1 M n-Bu4NPF6 as an electrolyte. The
working electrode was a glassy carbon electrode and the counter
electrode was a platinum sheet electrode. The redox potentials
were recorded versus a Ag/AgCl electrode (reference electrode).
Differential scanning calorimetry was carried out on a MET-
TLER TOLEDO DSC 822 with a programmed heating procedure
under nitrogen. Small-angle X-ray scattering experiments were
carried out on a SAXSess high-ux small-angle X-ray scattering
instrument (Anton Paar), equipped with a Kratky block-colli-
mation system, at room temperature. The X-ray was generated
by a sealed-tube X-ray generator (Philips PW3830) with a Cu
target (l: 0.1542 nm). The power of the generator used for
measurement was 40 kV and 40 mA. The X-ray intensities were
recorded on an imaging-plate detection system with a pixel size
of 42.3 ꢁ 42.3 mm2. The distance from the sample to the
detector was 264.5 mm and the exposure time was 600 s. The
Scanning Electron Microscopy (SEM) experiment was carried
out on a Hitachi S4800 Cold Field Emission Scanning Micro-
scope. Transmission electron microscopy was carried out on a
JEOL JEM-2100F eld-emission high resolution transmission
electron microscope.
Scheme 1 Reagents and conditions for the synthesis of the p-type
segment: (a) n-C12H25Br, K2CO3, KI, acetone, reflux, 83%; (b) bis(pi-
nacolato)diboron, PdCl2(dppf), KOAc, DMF, 60 ꢂC, 74%; (c) N-bro-
mosuccinimide, CHCl3–AcOH, 73%; (d) Pd(PPh3)4, K2CO3 (2 M), THF,
80 ꢂC, 76%; (e) LiAlH4, THF, 89%.
Scheme 2 Reagents and conditions for the synthesis of the n-type
segments: R ¼ R1, (a) n-C12H25Br, K2CO3, DMF, 80 ꢂC, 91%; (b)
N2H4$H2O, EtOH–THF, reflux, 82%; (c) n-4, 4-dimethylaminopyridine,
imidazole, 130 ꢂC, 79%; (d) p-toluenesulfonic acid monohydrate,
95 ꢂC, toluene, 88%. R ¼ R2, (a) p-toluenesulfonyl triethylene glycol
monomethyl ether, K2CO3, DMF, 90 ꢂC, 47%; (b) N2H4$H2O, EtOH,
reflux, 91%; (c) n-4, 4-dimethylaminopyridine, imidazole, 130 ꢂC, 75%;
(d) p-toluenesulfonic acid monohydrate, 95 ꢂC, toluene, 88%.
is more suitable for lm formation processes. Under the same
aggregate growth conditions, the assembly morphology of the
four dyads is as follows: both of the amphiphilic dyads could
form thin nanobres (F # 10 nm), while the lipophilic dyads
form thick bres or nanoparticles (F z 50 nm). The phase
separation and morphology results provide guidance for
organic device fabrication. Photovoltaic device performance of
the dyads is under investigation in our lab.
Synthesis
The p-type (Scheme 1) and n-type (Scheme 2) segments were
synthesized separately and then connected with amino acid
linkers (Scheme 3). Synthetic details of all compounds were
Experimental
Materials
All the chemicals were purchased commercially. The oxygen summarized in the ESI.†
and moisture sensitive reactions were performed under a
p-7 was synthesized from p-1 and p-2 in 5 steps. The
nitrogen atmosphere using standard Schlenk techniques. diphenol p-1 and aldehyde p-2 were prepared according to the
Tetrahydrofuran (THF) was freshly distilled with sodium under literature,13 and then p-1 was treated through alkylation and
a nitrogen atmosphere prior to use. N,N-Dimethylformamide Miyaura boration to afford p-4. p-6 was obtained via a Suzuki
(DMF) was stirred overnight with CaH2, distilled under vacuum coupling of p-4 and p-5. The reduction of p-6 by LiAlH4 gave the
and then dried over molecular sieves.
nal p-7(Scheme 1).
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p-7, diisopropyl azodicarboxylate, PPh3, THF, rt, 30 min, 54%; (c)
imidazole, DMF, 95 ꢂC, glycine, 92%; (d) (COCl)2, DMF, DCM, rt, 2 h;
4-nitrophenol, Et3N, DCM, overnight, 91%; (e) p-7, 4-dimethylami-
nopyridine, DMF, 72 h, 53%. R ¼ R2, (a) imidazole, DMF, 95 ꢂC,
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2011, 7, 6135; (e) T. N.
Y
n-7 and n-8 were afforded by the unsymmetrical function-
alization of perylene diimide. Starting materials n-1 and n-2
were synthesized according to the literature.14 Alkylation of n-1
followed by Gabriel synthesis afforded the aniline derivative n-
4, which can be condensed with anhydride n-2. The perylene
monoimide diester was treated with p-toluenesulfonic acid to
afford another anhydride, n-6. n-6 was condensed with 6-ami-
nocaproic acid or glycine to obtain n-7 or n-8, respectively.
Various condensation methods were tested in order to link
both segments. The Mitsunobu reaction and Mukaiyama
condensation were nally applied for Llipo and Lamphi, respectively
(Scheme 3). Carboxylic acid n-8 has lower condensation reactivity
than n-7. The possible reasons were proposed as follows: (1) n-8
bearing a glycine linker has extremely poor solubility in common
solvents (THF, CHCl3, DCM, toluene, etc.); (2) the hydroxyl group
of n-8 may be deactivated by forming intramolecular hydrogen
bonds with one of the carbonyl oxygens of a diimide unit. To
overcome these disadvantages, we utilised transesterication.15
The active ester n-9, prepared from acid chloride, has an
enhanced solubility, which allows a mild conversion to obtain the
target compounds Slipo and Samphi (Scheme 3).
¨
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Science Foundation (no. 21074004, 91227202), the Ministry of
Science and Technology (no. 2013CB933501), and the Ministry
of Education (NCET-08-0005) of China.
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