Nonplanar Ladder-Type Polycyclic Conjugated Molecules: Structures and Solid-State Properties

Article abstract of DOI:10.1021/acs.cgd.5b00429

Using an efficient intramolecular carbon-carbon cross-coupling reaction, a series of new ladder-type conjugated molecules have been prepared successfully in high yields. Such a pyran-fused polycylic structure possesses an extended π-conjugated backbone with flexible conformation, which gives these molecules interesting properties, including high solubility in common organic solvents, excellent thin film-forming abilities, blue fluorescent emission with good quantum yields, and aggregate formation in a binary solvent. The self-assembly behaviors of these molecules as well as various nanostructures can be finely tailored by varying the substituted group on the molecular periphery. The powder and single-crystal X-ray diffraction analyses revealed that the synergetic effect of π-π stacking and van der Waals interactions play a key role in controlling the morphologies of these aggregates. More importantly, self-assembled molecules exhibit good fluorescent performance, due to their twist backbone conformation. (Figure Presented).

Full text of DOI:10.1021/acs.cgd.5b00429

Article
pubs.acs.org/crystal
Nonplanar Ladder-Type Polycyclic Conjugated Molecules: Structures
and Solid-State Properties
Kuojin Liu,†,‡ Feng Qiu,†,§,‡ Chongqing Yang,§ Ruizhi Tang,§ Yubin Fu,§ Sheng Han,*,†
Xiaodong Zhuang,§ Yiyong Mai,§ Fan Zhang,*,§ and Xinliang Feng§,∥
†School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Haiquan Road 100, 201418, Shanghai, P. R.
China
§School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University,
Dongchuan Road 800, 200240, Shanghai, P. R. China
∥Technische Universitat Dresden, Helmholtzstraße 10, 01062, Dresden, Germany
̈
S
* Supporting Information
ABSTRACT: Using an efficient intramolecular carbon−
carbon cross-coupling reaction, a series of new ladder-type
conjugated molecules have been prepared successfully in high
yields. Such a pyran-fused polycylic structure possesses an
extended π-conjugated backbone with flexible conformation,
which gives these molecules interesting properties, including
high solubility in common organic solvents, excellent thin film-
forming abilities, blue fluorescent emission with good quantum
yields, and aggregate formation in a binary solvent. The self-
assembly behaviors of these molecules as well as various
nanostructures can be finely tailored by varying the substituted
group on the molecular periphery. The powder and single-
crystal X-ray diffraction analyses revealed that the synergetic effect of π−π stacking and van der Waals interactions play a key role
in controlling the morphologies of these aggregates. More importantly, self-assembled molecules exhibit good fluorescent
performance, due to their twist backbone conformation.
In comparison with the aromatic π-conjugated molecules, π-
conjugated polycyclic aromatic carbonhydrates (PAHs) with
saturated carbon atoms, featuring reduced aromaticity, has been
still rarely documented.
INTRODUCTION
■
Heterocycle-fused π-conjugated molecules have drawn much
attention as organic semiconductor materials for applications in
next-generation flexible devices, such as organic field-effect
transistors (OFETs), organic light emitting diodes (OLEDs),
and organic solar cells (OSCs), due to their outstanding
optoelectronic properties.1−5 Their rigid architectures typically
endow them with extended π-conjugated systems, strong
intermolecular interactions, and thus exhibiting versatile self-
assembly behaviors and excellent charge-carrier mobility.
However, such kinds of molecules are prone to form excimers
or aggregates through π−π interactions, leading to fluorescence
quenching or photoluminescence with additional undesired
long wavelength emission.6 Rational complementary of the
advantages in one molecule, demanding for a promising
functionality, is still a big challenge for molecular design.
Recently, supplanting of unsaturated sp2-hybridized carbon
atoms with saturated sp3-hybridized carbon atoms in a
molecular backbone is an amazing approach to narrow the
optical band gap or weaken the aggregation effect through
reducing the aromatic characters or the planar rigidity of a π-
conjugated system, respectively.7 For example, dimethylene C2-
bridged acene (Chart 1, I) containing nonplanar moieties
exhibits high fluorescence performance in the aggregate state.8,9
Incorporation of heteroatoms into the π-conjugated back-
bone of a molecule can not only tune the molecular energy
levels but also provide noncovalent intermolecular interactions
favorable for the formation of stable superstructures, compared
with the full-carbon analogues.10−15 Very recently, pyran has
emerged as a highly available moiety accessing to fused
polycyclic π-conjugated molecules (e.g., compound (II))16 or
D (donor)-A (acceptor) type conjugated macromolecules,17−19
for achieving high-performance electronic devices. For example,
organic solar cell fabricated from cyclopenta[2,1-b;3,4-b′]-
dithiophene (III)-based copolymers exhibited a very high
power conversion efficiency up to 10.6%.17 Obviously, the
pyran moiety, featuring one small-radius oxygen atom and one
saturated sp3-hybridized carbon atom, serves as the electron-
donating role as well as reducing the rigidity of the skeleton in a
conjugated system, with the consequence of the improvement
Received: March 27, 2015
Revised: May 8, 2015
© XXXX American Chemical Society
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Chart 1. Typical Examples of Nonplanar ansa-Bridged Acene Derivatives
of the solubility and the tuning of the intermolecular
interactions, etc. for the resulting materials. These intrinsic
characteristics give the pyran moiety great promise for the
construction of advanced functional materials.
different substituents was obtained by silica-gel column
chromatography as a red liquid in yields of 19% and 23%,
respectively. The target compounds (4a, 4b, and 4c) were
prepared by intramolecular carbon−carbon coupling reactions
of 3a, 3b, and 3c, respectively, using palladium diacetate as the
catalyst in DMF at 130 °C for 48 h.22,23 The pure compounds
were afforded as a white solid after purification by silica gel
column chromatography in good yields (83%−90%). Detailed
preparation information on intermediates and targeted
molecules is presented in the Supporting Information. All
In the present work, we designed and synthesized a series of
nonplanar ladder-type pyran-fused conjugated molecules (IV).
The geometric and electronic structures of these molecules
have been fully characterized by single crystal X-ray diffraction
and optical spectroscopies. Their molecular packing in solid
state is highly dependent on the side chains of the molecular
periphery. In these molecules, the pyran-fused structures
essentially generated the extended π-conjugated systems and
rich optical properties. Meanwhile, self-organization of such
types of molecules enables the formation of different stable
superstructures, and their morphologies can be tailored through
the introduction of alkyloxy chains on the molecular periphery.
1
targeted compounds were fully characterized by H and 13C
NMR spectroscopy as well as high-resolution mass spectrom-
etry. In the 1H NMR spectra, the proton signals of the aromatic
skeletons of compounds 4a−4c are shifted to low field in
comparison to those of compound 3a−3c, demonstrating that
the conjugated lengths of the molecules increased after the
formation of the pyran-fused polycyclic structures by cyclization
reaction. In addition, the proton peak at 5.44 ppm is assigned to
the methane group of pyran, indicating the nonaromatic
structure of the pyran fragment.
Thermal Behaviors. These molecules show good thermal
stability with a weight loss of 5% over 290 °C (Figure S1,
Supporting Information) as estimated by thermal gravimetry
analysis (TGA) measurements. On the basis of differential scan
calorimetry (DSC) analyses (Figure 1), all of conjugated
molecules show two endothermic peaks on the heating curve
and one exothermic peak on the cooling curve. The first small
endothermic peak corresponds to the molecular motility in the
solid state, indicating the flexible conjugated backbone of these
pyran-fused compounds. The second endothermic peaks are
attributed to their melting points. With the increase of the
length of alkyloxy chains on the molecular periphery, the
melting point of the molecule decreases, caused by the motility
of alkyloxy chains. The unique phase transitions for such type
of molecules seem to be attributable to their less-rigid
backbones arising from the pyran moieties.
RESULTS AND DISCUSSION
■
Synthesis of Ladder-type PAHs. The synthetic route of
nonplanar ladder-type pyran-fused conjugated molecules is
described in Scheme 1. Starting from 2,6-dimethylnaphthalene
Scheme 1. Synthetic Route to the Nonplanar Ladder-Type
Polycyclic Conjugated Molecules
Crystal Structure Analysis. The chemical structures of the
as-prepared compounds were further confirmed by X-ray
crystallographic analysis (Figure 2). Single crystals of 4a and
4c were obtained at ambient temperature by slowly diffusing
hexane and methanol into their chloroform solutions,
respectively. The central naphthyl ring of 4a or 4c shows
slightly twisted deformation with an angle of around 13.4°. For
compound 4a, the backbone exhibits large torsion angles of
30.3° and 34.2° between the central naphthyl ring and the
outer benzene rings, respectively. In compound 4c, these
torsion angles drop down to 25.8° and 27.7°, respectively. The
oxygen atoms in the pyran units deviate from the mean planes
of the naphthyl cores by about 24.1°, 25.0° for compound 4a
and 21.1°, 19.8° for compound 4c, respectively. Such twisted
backbone structures are attributed to the nonaromatic character
by using a two-step sequence of bromination with bromine and
N-bromobutanimide, respectively, the key intermediate 1,5-
dibromo-2,6-bis(bromomethyl)naphthalene (2) was prepared
in a good yield (74%).20 Afterward, the intermediate ether
compounds (3a−3c) were prepared by condensation coupling
of 2 with various phenolic compounds in dimethylformamide
(DMF) at 80 °C by using an improved synthetic protocol.21
Here, except for phenol, 4-hexyloxy phenol and 4-dodecyloxy
phenol (5a−5b) were synthesized by condensation reaction of
hydroquinone with 1-bromohexane and 1-bromododecane,
respectively, under the catalysis of potassium hydroxide
(Scheme S1, Supporting Information). The pure 5a−5b with
B
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Figure 1. DSC curves of compound 4a (a), 4b (b), and 4c (c).
parallel columns, the shortest distance between the neighboring
molecules is 3.49 Å between the α-terminal carbon atoms of the
benzene moieties from the neighboring molecules. No apparent
π−π stacking intermolecular interactions of the central
naphthyl rings were observed, due to the twist backbone of
the pyran-fused conjugated molecule. The solid-state packing of
compound 4c is significantly different from that of compound
4a. The molecules adopt staggered packing motif, in which each
molecule is partly overlapped with two neighboring molecules
in a face-to-face fashion with the shortest distance of 3.60 Å,
which suggests the presence of π−π stacking interactions. The
two adjacent molecules are facial isomers and in mirror
symmetry, relative to the mean plane of the backbone. In
addition, the presence of C−H···π interactions from
dodecyloxy groups to the central naphthyl rings of the
neighboring molecules also stabilize the close-packed structures
in solid state. The parallel stacking columns are interdigitated
with each other through the alky side chains, manifesting the
distinct effect of the substituted groups on the solid state of
such kinds of molecules.
Figure 2. Crystal structures of compound 4a (a, b) and 4c (c, d) from
front- and side-view representations, respectively.
of the fused pyran units. The lengths of C−C (1.50/1.49 Å)
and C−O bonds (1.45/1.43 Å and 1.39/1.38 Å) in pyran
moieties are longer than those of C−C bond lengths in the
naphthyl and benzene rings, further manifesting the less
aromatic character of the pyran parts. Moreover, two terminal
dodecyloxy groups in compound 4c are oriented away from the
attached benzene rings with a dihedral angle of around 62.5°.
In Figure 3, the packing diagram of compound 4a reveals a
slipped stacking in an edge-to-edge fashion. In each of the
Optical Properties. The optical properties of compound
4a−4c in chloroform were investigated. In UV−vis spectra of
compound 4a, a set of absorption bands appears at around 340,
Figure 3. Crystal packing of compound 4a (a, b) and 4c (c, d) from front- and side-view representations, respectively.
C
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Figure 4. Normalized absorption spectra (a−c) and PL spectra (d−f) of 4a−4c in CHCl3 solution and thin films, respectively.
Table 1. Photophysical Data of Pyran-Fused Conjugated Molecules 4a−4c in CHCl3
in CHCl3 solution
in thin film
a
b
c
d
a
b
c
d
λabs [nm]
εmax [m−1 cm−1
]
λem [nm]
SS [nm]
Φf
%
λabs [nm]
λem [nm]
SS [nm]
Φf
%
4a
4b
4c
357
369
369
16730
21770
418
453
453
61
84
84
22.8
15.8
15.4
362
372
372
426
438
436
64
66
64
9.4
17.1
15.2
20200
a
b
c
The longest absorption maxima. Emission maxima upon excitation at the longest absorption maxima. Stokes shift, evaluated from the absorption
d
maxima and emission spectra maxima. quantum yields determined by using an integrating sphere.
357, and 372 nm, and a small shoulder peak at around 321 nm
is observed (Figure 4a−c), which is typically attributed to the
π−π* transitions of the aromatic backbones in a conjugated
system. Compared with that of compound 3a prior to
cyclization, the main absorption bands of 4a are significantly
red-shifted (Figure S2), manifesting the presence of an
extended π-conjugated system after the formation of the
pyran-fused ring. Such vibronically split features of these
absorption bands for 4a arise from the rigid molecular
structure, in which the intramolecular rotation of phenyl
group is restricted by the methyleneoxy moiety. In comparison
to that of compound 4a at 357 nm, the maximum absorption of
4b is red-shifted by 12 nm, due to the electron-donating effect
of the attached hexyloxy groups. As expected, UV−vis
absorption profile of 4c is similar to that of compound 4b.
Notably, the absorption spectra of all compounds exhibit an
almost solvent-independent effect, revealing their nonpolar
characters in the ground state (Figures S3−S5). The UV−vis
absorption spectra of pyran-fused PAHs were further calculated
at the time-dependent density functional theory (TD-DFT)
level (Figures S6−S8). The profiles of UV−vis spectra are in
good agreement with their experimental results, in which the
main absorption bands at the low-energy window are
dominated by electronic transitions between HOMO and
LUMO orbitals. Regarding to their excellent solubility in some
common organic solvents, compounds 4a−4c are facile to form
thin films under spin-coating treatment. And, attaching long
chains onto the peripheries of the molecules enable essentially
improving their film-forming abilities. Remarkable red shifts
and peak broadening of the absorption spectra for compounds
4a−4c were observed in the thin films compared with those in
solution (Figure 4a−c), indicating the strong intermolecular
interactions and the tendency to aggregate in solid state for
such kinds of molecules.24
The molecules 4a−4c show strong blue fluorescence
emissions in CHCl3 (Figure 4d−f). The emission maxima of
compound 4a locate at 418 nm, while the emission bands of 4b
and 4c exhibit a significant bathochromic shift of 35 nm, with
respect to that of 4a. The large Stokes shifts (SS) with 61−84
nm for these molecules manifest their remarkable structural
deformations between the ground and excited states of the
molecules, likely due to the reduced rigidity of the pyran-fused
backbones.25 The moderate fluorescence quantum yields of
compounds 4a−4c (from 0.23 to 0.15) are comparable to those
of pyran-fused PAHs.16 Interestingly, compounds 4a−4c also
exhibited strong blue emission in the thin films formed through
spin-coating on the quartz plate (Figure 4d−f). The
fluorescence quantum yields of compounds 4a−4c in solid
state were also determined by using an integrating sphere
(Table 1). Compounds 4a−4c in the solid state still maintained
certain quantum yields, likely attributed to the twisted
backbones associated with the pyran moieties as shown in
their single crystal structures. Compared with the value of
∼23% in solution, compound 4a shows a much lower quantum
yield (∼9%) in the solid state, reasonably arising from the
stronger intermolecular interactions. The quantum yields of
17.1% for 4b and 15.2% for 4c in solid state are quite similar to
those of 15.8% and 15.4% for 4b and 4c in solution,
respectively, indicating that the alkyloxy chains on the
molecular peripheries indeed enable effectively suppressing
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the chromosphere interactions of compound 4b or compound
4c in solid state. These results are in agreement with their
crystal structure analysis discussed above. The photophysical
properties of 4a−4c were summarized in Table 1.
solvent mixtures were investigated. Upon addition of methanol
(<10% volume fraction) to a chloroform solution of 4c, the
intensity of the main absorption bands in the UV−vis only
slightly increases (Figure 6a). With the increase of methanol
fraction, the intensity of the maximum absorption reduces
gradually, and the leveling-off of long-wavelength tails is
observed at higher methanol contents (>70% volume fractions)
as a consequence of the formation of aggregates.26,27
Importantly, the hypochromatic shift of the absorption peak
demonstrates that compound 4c likely undergoes H-
aggregation.28 In the fluorescent spectra of compound 4c, the
emission maxima at ∼453 nm slightly red-shift, with a continual
decrease of the intensity under the addition of methanol into its
chloroform solution (Figure 6b), presumably due to the
formation of aggregates in the mixed solvents. Similarly, the
fluorescent intensities of compound 4a and 4b also decrease
with the addition of methanol (Figure S10, Supporting
Information). However, the fluorescence profiles of all
compounds seem not to be obviously changed as the formation
of the aggregates, probably attributable to the weak π−π
stacking interaction in the aggregate state for these molecules,
completely different from those for the typical PAHs.29,30
After the drop-casting chloroform/methanol solutions of
compound 4a−4c on silicon wafers and the removal of the
solvent by evaporation under ambient temperature,31 the
morphologies of the resulting aggregates were investigated by
scanning electron microscopy (SEM). Compound 4c could
self-assemble into ribbon-like aggregates with a uniform width
of approximately 300 nm and lengths ranging from 0.8 to 5 μm
(Figure 7a). Transmission electron microscopy (TEM) image
of the sample also confirms the formation of the ribbon-like
aggregates (Figure 7b). Furthermore, atomic force microscopy
(AFM) image reveals that the ribbons show a smooth surface
with an average height of ∼78 nm (Figure 7c−d). Interestingly,
with the decrease of the length of the side chains from dodecyl
group to hexyl group, compound 4b self-organizes into helical
fibers with the lengths of 10−20 μm and the diameters of 0.5−
5 μm. Such twisted aggregates were probably induced by the
intrinsic twisting of the molecular backbone, due to the flexible
character of the pyran rings. Notably, both left-hand and right-
hand helical superstructures coexist in the aggregates,
suggesting that the aggregates are racemism, while in the case
of compound 4a without side chain substituents, 2D nano-
sheets are formed using a similar self-assembly method.
However, it has been difficult to control the formation of the
superstructures with distinct edges and uniform sizes so far.
To explore the influence of the molecular structure on the
supramolecular arrangement and morphology, powder X-ray
diffraction (XRD) was employed to analyze the aggregation
behavior of compound 4c. As shown in Figure 8a, the intense
and sharp peaks of the diffraction pattern indicated the high
crystallinity of the assembled nanoribbons. Simulated by
Materials Studio program version 6.1, both crystal cells of the
aggregates and single crystal were monoclinic crystals, and the
XRD pattern of the aggregates is also similar to that of single
crystal (see Figure S11). Accordingly, the peaks at 2θ = 3.4°
and 6.9° can be indexed as (100) and (200) reflections,
respectively, and 2θ = 2.3°, 5.3°, 8.2° and 10.9° can be indexed
as (001), (002), (003), (004) reflections, respectively, which
are associated with a lamellar packing.27,32,33 In addition, the
reflection peak at 2θ = 20.6° is typically originated from the
ordered packing of alkyl chains, and the reflection peak at 2θ =
23.3° can be attributed to the π−π stacking of aromatic
Electrochemical Properties. The electrochemical behav-
iors of pyran-fused PAHs 4a−4c were subsequently inves-
tigated by cyclic voltammetry (CV) in CH3CN (Figure S9). In
the CV profiles of 4a−4c, one irreversible oxidation processes
are observed. The HOMO energy level of 4a is −5.63 eV,
deriving from the oxidation potential (assuming Fc/Fc+ at 4.8
eV), while the LUMO energy level is −2.48 eV, which is
calculated based on the absorption edges. Owing to the
existence of alkyloxy chains with electron-donating ability,
HOMO energy levels of 4b and 4c decrease to −5.50 eV and
−5.48 eV, respectively. The LUMO and HOMO values of
pyran-fused PAHs were summarized in Table 2.
Table 2. Electrochemical Properties of Compounds 4a−4c
a
b
b
c
c
c
HOMO
(eV)
LUMO
(eV)
Egopt
HOMO
(eV)
LUMO
(eV)
Egopt
(eV)
(eV)
4a
4b
4c
−5.63
−5.52
−5.48
−2.48
−2.50
−2.46
3.15
3.02
3.02
−5.39
−5.25
−5.25
−1.66
−1.67
−1.66
3.73
3.58
3.59
a
For all of the molecules, HOMO = −[Eonset+4.8] eV, Eox(ferrocene)
=
0.46 eV vs Ag/AgCl, Eonset is the onset values of oxidation potentials.
b
LUMO = HOMO − Egopt, Egopt: optical band gap, estimated from
c
UV−vis absorption edge. Obtained by DFT calculations.
To further insight into the geometric and electronic
structures of these compounds, the theoretical calculations of
pyran-fused PAHs have been performed based on TD-DFT
(RB3LYP/6-31G (d) level). As shown in Figure 5, the LUMOs
Figure 5. Calculated molecular orbitals of compounds 4a−4c.
for all compounds primarily reside on the core of 1,5-diphenyl
naphthalene, indicative of a similar LUMO level. The HOMOs
for all compounds are distributed over the 1,5-diphenyl
naphthalene with partly contributed by pyran, demonstrating
that pyran is not an effective π-electrons delocalized structure
on the molecular skeleton. Compared with that of compound
4a, the oxygen atom in alkyloxy chains contributes to the
HOMO level in compounds 4b and 4c, revealing its electron-
donating effect. Therefore, the HOMO levels of 4b and 4c are
lower than that of 4a. The calculated HOMO energy levels of
pyran-fused PAHs are in good agreement with the values
estimated from CVs.
Self-Assembly Behaviors. To elucidate the influence of
the solvent polarity on the aggregation behaviors of 4a−4c, the
optical properties of these molecules in a series of binary
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Figure 6. UV−vis absorption (a) and fluorescence emission (b) spectra of compound 4c (5 × 10−5 M) with varied intensities in chloroform/
methanol mixtures with different methanol fractions.
be composed of 20 layers of the repeat units. The proposed
molecular arrangement for compound 4c was evaluated on the
basis of the single crystal analysis, as depicted in Figure 8b,
clearly disclosing the nanostructure formation mechanism.
CONCLUSIONS
■
A new family of nonplanar ladder-type conjugated molecules
has been prepared successfully in good yields by using Pd-
catalyzed intramolecular carbon−carbon cross-coupling reac-
tion. The geometric and electronic structures of these
molecules have been fully characterized by single crystal X-
ray analysis and optical spectroscopies. The pyran-fused
polycyclic structures give such kinds of molecules unique
backbones featuring extended π-conjugated systems and flexible
conformation, and thus leading to interesting physical proper-
ties including large Stokes shifts, blue fluorescent emission with
good quantum yields, and a strong tendency to aggregate in
binary solvents. Their excellent solubility in common organic
solvents endows them with good film-forming properties, and
these molecules exhibit strong intermolecular interaction in the
thin films. The self-assembly behaviors of these molecules with
various superstructures can be finely tailored by varying the
Figure 7. SEM (a), TEM (b), and AFM images (c) and height profile
substituted group in the molecular periphery. The excellent
(d) of ribbon-like aggregates formed by compound 4c; SEM images of
properties of these pyran-fused conjugated molecules in
nanostructure of compound 4b (e) and 4a (f).
aggregate state may allow them to be used as promising active
materials in electronic device fabrications, such as OLEDs with
blue emitting, p-type building blocks for OSCs devices, etc.
Meanwhile, the construction of the pyran-fused polycyclic
conjugated system through intramolecular cyclization might
pave a new avenue to form organic functional molecules with
much complex structures.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental information, NMR spectra, HRMS-ESI spectra,
Figure 8. (a) X-ray diffraction pattern of nanostructure from
UV−vis spectra, fluorescence spectra, TGA data, DSC data, and
compound 4c plotted against the angle 2θ; (b) illustration of the
X-ray diffraction patterns. The Supporting Information is
ribbon-like nanostructures on the basis of the molecular packing
available free of charge on the ACS Publications website at
pattern in the single crystals of compound 4c.
DOI: 10.1021/acs.cgd.5b00429. X-ray crystallographic data for
compound 4a (CCDC 1048145) and 4c (CCDC 1048146),
can be obtained free of charge from The Cambridge
segments.34 Given that the d spacing of the (100) reflection
Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/
datarequest/cif.
value is 3.9 nm calculated from the XRD pattern, the assembled
ribbons of compound 4c with the thickness of ∼78 nm should
F
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(21) Peng, J.; Qiu, F.; Xue, M.; Zhang, F. Imaging Sci, Photochem.
2014, 32, 282−288.
AUTHOR INFORMATION
■
Corresponding Authors
*(F.Z.) E-mail: fan-zhang@sjtu.edu.cn.
*(S.H.) E-mail: hansheng654321@sina.com.
(22) Campeau, L.-C.; Parisien, M.; Jean, A.; Fagnou, K. J. Am. Chem.
Soc. 2005, 128, 581−590.
(23) Campeau, L.-C.; Parisien, M.; Leblanc, M.; Fagnou, K. J. Am.
Chem. Soc. 2004, 126, 9186−9187.
Author Contributions
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D.; Giannakopoulos, A.; Beljonne, D.; Feng, X. Org. Lett. 2014, 16,
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‡K.L. and F.Q. contributed equally to this work.
Notes
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The authors declare no competing financial interest.
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̈
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ACKNOWLEDGMENTS
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■
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2012CB933404), the Natural Science Foundation of China
(21174083 and 21102091), the Shanghai Committee of
Science and Technology (11JC1405400), Shanghai Jiao Tong
University (211 Project), China Postdoctoral Science Founda-
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DOI: 10.1021/acs.cgd.5b00429
Cryst. Growth Des. XXXX, XXX, XXX−XXX