Enhancing Organic Phosphorescence by Manipulating Heavy-Atom Interaction

Article abstract of DOI:10.1021/acs.cgd.5b01400

Achieving highly efficient phosphorescence in metal-free materials under ambient conditions remains a major challenge in organic optoelectronics. Herein, we report a concise approach to obtaining pure organic phosphorescence with high quantum efficiency of up to 21.9% and millisecond-scale lifetime by manipulating heavy-atom interaction based on a class of dibromobenzene derivatives in the solid state under ambient conditions. By comparing two pairs of the organic compounds designed, the one with two more bromine atoms on the alky terminals (PhBr₂C₆Br₂/PhBr₂C₈Br₂) showed higher luminescence efficiency than the other one (PhBr₂C₆/PhBr₂C₈). From the single-crystal analysis, it was proposed that the enhancement of phosphorescence resulted from increased intermolecular heavy-atom interaction in the organic crystals. Furthermore, a temperature sensor was demonstrated by using a model probe of this kind of organic phosphorescent crystals. This work not only provides a concise alternative to enhance phosphorescence in metal-free materials but also extends the scope of pure organic phosphorescent materials with high luminescent efficiency in a single component.

Full text of DOI:10.1021/acs.cgd.5b01400

Article
pubs.acs.org/crystal
Enhancing Organic Phosphorescence by Manipulating Heavy-Atom
Interaction
Huifang Shi,†,‡,⊥ Zhongfu An,‡,⊥ Pei-Zhou Li,† Jun Yin,§ Guichuan Xing,‡ Tingchao He,§
Hongzhong Chen,† Jingui Wang,† Handong Sun,§ Wei Huang,*,‡ and Yanli Zhao*,†,∥
†Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
21 Nanyang Link, 637371, Singapore
‡Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation
Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, People’s
Republic of China
§Division of Physics and Applied Physics, Centre for Disruptive Photonic Technologies (CDPT), School of Physical and
Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore
∥School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore
S
* Supporting Information
ABSTRACT: Achieving highly efficient phosphorescence in
metal-free materials under ambient conditions remains a major
challenge in organic optoelectronics. Herein, we report a
concise approach to obtaining pure organic phosphorescence
with high quantum efficiency of up to 21.9% and millisecond-
scale lifetime by manipulating heavy-atom interaction based on
a class of dibromobenzene derivatives in the solid state under
ambient conditions. By comparing two pairs of the organic
compounds designed, the one with two more bromine atoms
on the alky terminals (PhBr2C6Br2/PhBr2C8Br2) showed
higher luminescence efficiency than the other one (PhBr2C6/PhBr2C8). From the single-crystal analysis, it was proposed that
the enhancement of phosphorescence resulted from increased intermolecular heavy-atom interaction in the organic crystals.
Furthermore, a temperature sensor was demonstrated by using a model probe of this kind of organic phosphorescent crystals.
This work not only provides a concise alternative to enhance phosphorescence in metal-free materials but also extends the scope
of pure organic phosphorescent materials with high luminescent efficiency in a single component.
compounds into an amorphous polymer matrix.7,10,22 The
triplet-promoting heavy-atom and rigid solid-state conditions
INTRODUCTION
■
Organic phosphorescent materials have evoked considerable
could enhance the spin-forbidden singlet-to-triplet excited-state
interest due to their potential applications in organic light-
emitting diodes,1 photovoltaics,2,3 photocatalytic reactions,4
transition. Adachi and co-workers showed phosphorescence
and bioimaging.5 So far, most room temperature phosphor-
with a lifetime of above 1 s by using a doping method along
with deuteriation substitutions.8 Very recently, our group
escent materials are typically limited to heavy-metal-containing
complexes, such as Ir3+, Pt2+, and Ru3+ complexes,6 which can
proposed a rational design strategy by stabilization of triplet
excited states through H-aggregation to realize ultralong
facilitate intersystem crossing to enhance phosphorescence
generation.4 These metals are not only costly but also resource-
phosphorescence with a lifetime of 1.35 s and a lasting time
of up to 56 s.19 Nevertheless, the exploration of new strategies
limited, which have hindered the development of the organic
to enhance phosphorescence in organic metal-free materials is
phosphorescence. Phosphorescence from organic metal-free
still of great importance and challenge.
materials under ambient conditions has been widely considered
Phosphorescence is usually not seen in fluid solutions
because there are many deactivation processes that compete
to be impossible due to weak spin−obit coupling and highly
forbidden triplet−singlet transitions, despite the emerging
demand in the optoelectronics.7 To defy this imprisonment,
with the emission, such as nonradiative decay and collisional
quenching by oxygen and other impurities.30 As shown in
many efforts have been made to investigate the metal-free
organic phosphorescence recently.7−28 Crystallization-induced
Figure 1a, a certain amount of additives with heavy atoms, such
phosphorescence based on pure organic luminogens was
observed at room temperature by Tang and co-workers.29
Received: September 29, 2015
Kim et al. largely enhanced pure organic phosphorescence
Revised: December 9, 2015
efficiency by impurity doping method and introducing organic
© XXXX American Chemical Society
A
DOI: 10.1021/acs.cgd.5b01400
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Figure 1. Schematic presentation for the manipulation of heavy-atom interaction to enhance organic phosphorescence. (a) Typical approach to
enhance phosphorescence by replenishing additives with heavy atoms in solution. (b) Proposed strategy of manipulating heavy-atom interaction in
the solid state. (c) Synthetic routes of pure organic phosphorescence molecules.
a
Table 1. Structure Data of Single Crystals
name
formula
PhBr2C6
PhBr2C6Br2
PhBr2C8
PhBr2C8Br2
C18H28Br2O2
1.54184
C18H26Br4O2
1.54184
C22H36Br2O2
1.54184
C22H34Br4O2
1.54184
wavelength (Å)
space group
a (Å)
P1
̅
P1
P1
̅
P1
̅
̅
6.9534 (8)
8.2493 (14)
9.7157 (12)
107.062 (13)
106.936 (10)
99.153 (12)
491.107
8.0564 (19)
8.5752 (13)
9.5028 (13)
66.877 (14)
87.324 (16)
66.414 (19)
548.533 (2)
2
6.7829 (5)
9.6771 (6)
9.6955 (7)
74.616 (6)
85.034 (6)
73.541 (6)
588.411
8.4619 (9)
9.0000 (5)
9.5811 (7)
115.671 (6)
92.610 (6)
99.466 (5)
643.072
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
vol (Å3)
Z
2
2
2
density (g/cm3)
F(000)
1.475
1.798
1.3893
1.6786
222
287.8
253.1622
8, 11, 11
319.8326
10, 9, 11
h
max, kmax, lmax
8, 10, 8
9, 10, 11
a
The obtained crystal structures have been deposited at the Cambridge Crystallographic Data Centre and allocated the deposition number: 1404734
(PhBr2C6), 1404733 (PhBr2C6Br2), 1405036 (PhBr2C8), and 1405039 (PhBr2C8Br2).
as 1-ethyl iodide31 and propyl halide,32 as well as sodium
iodide,33 was added into the solutions to enhance the
phosphorescence quantum yield, which is acknowledged to
be one of the most efficient alternatives for increasing
phosphorescence generation.33 The enhancement of the spin-
coupling by surrounding heavy atoms, with the consequent
increase in the probability of T1 → S0 transition, can be affected
by external perturbation of heavy atoms within the absorbing or
emitting molecules. Inspired by such an effective strategy, we
thus made our efforts on tailing molecular alkyl groups with
various numbers of bromine atoms to enhance the phosphor-
escence in pure organic materials in the solid state (Figure 1b),
providing an alternative approach for enhancing the phosphor-
escence efficiency. The bromine atoms at the end of the alkyl
units play the same role as the isolated heavy atoms shown in
Figure 1a.
In this study, we designed a modification of alkyl terminal
groups with bromine atoms to enhance the phosphorescence
efficiency by manipulating heavy-atom interaction, as illustrated
in Figure 1b. Bright green phosphorescence was observed for all
of the compounds in the solid state under ambient conditions
due to the heavy-atom effect. By comparing the two pairs, the
ones with four bromine atoms showed higher luminescence
efficiency than the ones with two bromines. As observed from
the single-crystal structures, there are three Br−Br interaction
pairs for PhBr2C6Br2 and two Br−Br interaction pairs for
PhBr2C8Br2 between every two molecules, while there is only
one for PhBr2C6 and PhBr2C8. Therefore, we proposed that
more efficient phosphorescence could be obtained by
increasing the intermolecular heavy-atom interaction ratio in
the solid state. To the best of our knowledge, this is the first
system that the enhancement of phosphorescence in organic
metal-free materials was realized by manipulating molecular
heavy-atom interaction through tailoring terminal groups of
alkyl substituents with bromine atoms.
RESULTS AND DISCUSSION
■
In order to validate our hypothesis, a type of dibromobenzene
derivatives, namely, 1,4-dibromo-2,5-bis(hexyloxy)benzene
(PhBr2C6), 1,4-dibromo-2,5-bis(octyloxy)benzene (PhBr2C8),
1,4-dibromo-2,5-bis((6-bromohexyl)oxy)benzene
(PhBr2C6Br2), and 1,4-dibromo-2,5-bis((8-bromooctyl)oxy)-
benzene (PhBr2C8Br2), were synthesized in high yields of
above 70% by following a concise one-step approach shown in
1
Figure 1c. These molecules were fully characterized by H
NMR, 13C NMR (Figures S1−S8 in the Supporting
Information (SI)), elemental analysis, and single-crystal X-ray
diffraction (Table 1). The melting points of PhBr2C6 and
PhBr2C8 were around 63.5 and 66.8 °C (SI Table S1). It was
noteworthy that compounds of PhBr2C6Br2 and PhBr2C8Br2
B
DOI: 10.1021/acs.cgd.5b01400
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
with alkyl groups substituted by bromines exhibited a higher
thermal feature with melting points of around 89 °C.
In a further set of experiments, photophysical properties of
the four compounds were investigated in detail through
measuring both steady-state and time-resolved emission spectra
in solution and the solid state. For these dibromobenzene
derivatives in dilute dichloromethane solutions, the same
profiles of the absorption spectra with peaks at around 300
nm were presented in Figure 2a. No emission was observed in
Figure 2. Photophysical characteristics of pure organic phosphorescent
molecules. (a) Normalized absorption spectra in dilute dichloro-
methane solutions. (b) Normalized steady-state photoluminescence
spectra in crystal state excited at 325 nm. (c) Luminescent images of
the four compounds under hand-held UV lamp excitation at 365 nm.
Figure 3. Phosphorescent characteristics of four organic phosphor-
escent molecules in crystal state. (Left) Phosphorescence excitation
and emission spectra of the four compounds with time delay in crystal
state at room temperature excited at 350 nm (total decay time, 0.02 s;
delay time, 0.20 ms; gate time, 5.0 ms). (Right) green band
luminescence decay of the four compounds under ambient conditions.
solution, ascribing to the enhancement of the singlet-to-triplet
transition induced by the heavy atoms. In the solid state when
excited at 325 nm, the chromophores of PhBr2C6 and PhBr2C8
showed intense green emissions with major emission bands at
around 500 nm and small blue bands at about 380 nm, where
the blue emission area occupied only about 5% of the total
emission. After one of the terminal hydrogen atoms in alkyl
chains was substituted with a bromine atom, the blue emissions
disappeared completely, leaving only strong green emission
bands for both PhBr2C6Br2 and PhBr2C8Br2 (Figure 2b). With
excitation by a hand-held UV lamp at 365 nm under ambient
conditions, intense green luminescence was observed for all of
the crystals, particularly for PhBr2C6Br2 (Figure 2c).
To gain deeper insight into the nature of the green emissions,
time-resolved excitation and emission spectra were collected
under ambient conditions. As shown in Figure 3, only green
emissions were observed with the excitation at 350 nm, and the
emission band at ∼380 nm thoroughly disappeared after 0.2 ms
time delay for all of the compounds. The excitation peaks
monitoring the green emission bands at 500 nm were around
350 nm for all of the compounds. Furthermore, the
luminescence decay curves were recorded at around 500 nm
as shown in Figure 3 (right part). A lifetime at green emission
was as long as several microseconds, which further verified that
the green emission originated from the radiative decay of the
triplet excited state. The fluorescence feature of the emission at
380 nm was proven by the data of short lifetimes in
nanosecond scale from photoluminescence decay curves
shown in Figures S13 and S14 of the SI. The absolute
quantum efficiencies (AQEs) of the phosphorescence for
PhBr2C6 and PhBr2C8 were only 3.4% and 8.9%, respectively.
With an increase of the heavy atoms at the end of alkyl chain
units, the corresponding AQEs of PhBr2C6Br2 and PhBr2C8Br2
compounds were significantly enhanced up to 21.9% and
13.1%, respectively. This increase may be attributed to the
influence of the heavy atoms at the end of the alkyl groups. To
prove the effect of the bromine heavy atoms on the generation
of phosphorescence, a control experiment using two com-
pounds without any bromine atoms (Scheme S1 in the SI) was
conducted. It was found that the crystals of PhC6 and PhC8
showed fluorescence emission with short lifetimes of around
2.9 and 6.1 ns, respectively, but not phosphorescence (Figure
S15 in the SI).
To further understand the origin of enhanced pure organic
phosphorescence in this study, the single-crystal structures and
molecular packing modes of the four compounds were
examined by X-ray diffraction analysis. In crystals, all of these
organic phosphors are arranged in layers with a distance of
around 3.7 Å between neighboring layers presented in Figure
S16 of the SI. In the same layer, all luminogens are locked by
various intermolecular forces including C−Br···Br−C and C−
H···Br interactions (Figure 4). In both PhBr2C6 and PhBr2C8
single-crystal systems, there are only Br···Br short-range
interactions with the distances of 3.405 and 3.380 Å,
respectively. For PhBr2C6Br2 and PhBr2C8Br2 single crystals,
analogous bromine clusters near benzene chromophores were
observed. Within the allowable interaction distance of <4.4 Å,34
it is important to note that there are triple Br···Br interactions
for a single chromophore in PhBr2C6Br2 single crystal, double
Br···Br interactions for PhBr2C8Br2, and only one Br···Br
interaction for PhBr2C6 and PhBr2C8, which lead to different
phosphorescence efficiency among these four compounds. The
improvement of the phosphorescence quantum yields for both
PhBr2C6Br2 and PhBr2C8Br2 solids may be ascribed to the
synergistic effects from the promotion of Br−cluster interaction
C
DOI: 10.1021/acs.cgd.5b01400
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Figure 4. Perspective views of the heavy-atom interaction in single crystals of dibromobenzene derivatives.
and the suppression of triplet−triplet annihilation between
adjacent molecules. The hypothesis was further proven by
multimolecule triplet spin density distribution simulated by
density functional theory (DFT) calculation. As shown in
Figure S17 of the SI, the spin density distribution of PhBr2C6
was localized on two adjacent molecules for every four
molecules, while it was only distributed on one molecule for
PhBr2C6Br2. Thus, the lowest phosphorescence efficiency of
PhBr2C6 solid was expected because of the accessorial triplet−
triplet quenching process. By comparing the spin density
distribution of PhBr2C6 and PhBr2C8, it was found that the spin
density was only localized on one molecule for PhBr2C8 (Figure
S17 in the SI), which may lead to higher phosphorescence
efficiency by suppressing triplet−triplet annihilation.
Taking advantage of the temperature sensitivity of
phosphorescence,35 a temperature sensor was developed on
the basis of PhBr2C6Br2 crystal with the highest phosphor-
Figure 5. Phosphorescent spectra of PhBr2C6Br2 crystal at different
escence yield of 21.9%. As shown in Figure 5, upon increasing
temperature from 20 to 90 °C, the phosphorescence intensity
of PhBr2C6Br2 crystal decreased gradually along with fixed
emission profiles, which are attributed to the enhanced
nonradiative transition of the excited states at elevated
temperature. Unexpectedly, despite the relative low melting
point (88.6 °C) for PhBr2C6Br2 solid, the phosphorescence
intensity fell about 50% when the temperature reached 90 °C,
suggesting that the compound still showed strong phosphor-
escence even in the melting state. The relationship between the
temperatures.
phosphorescence intensity changing rate (I) and the temper-
ature (T) can be accurately fitted by a parabola equation of (I0
− I)/I = 3.24T2 + 3.04 × 10−3T − 0.07, which revealed the
quantification possibility of temperature sensing in the range of
20−90 °C using the pure organic phosphors.
D
DOI: 10.1021/acs.cgd.5b01400
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
General Procedure. These dibromobenzene derivatives were
synthesized by referencing the previous literature.7 To a 50 mL
round-bottomed flask with 2,5-dibromohydroquinone (1 equiv) and
potassium carbonate (3 equiv) dissolved in dimethylmamide was
added bromoalkanes (2.5 equiv). The mixture was stirred at 80 °C for
12 h. The product was purified by flash column chromatography with
hexane and dichloromethane as the eluent to give target compounds
with yields of 70−80%.
CONCLUSIONS
■
In summary, a series of pure organic phosphorescent
compounds with concise molecular structures have been
designed and prepared. Pure organic phosphorescence with
high quantum efficiency of up to 21.9% and millisecond-scale
lifetime has been obtained by manipulating heavy-atom
interaction. The relationship between molecular structures
and phosphorescence feature has been studied through single-
crystal analysis and DFT calculation, indicating that the
phosphorescence generation could be promoted by increasing
bromine−cluster interactions and decreasing triplet−triplet
annihilation chance. Furthermore, the potential application of
these kinds of materials as temperature sensors has been
demonstrated by using PhBr2C6Br2 solid as a model probe,
which revealed the possibility of the quantification of
temperature sensing in the range of 20−90 °C. This work
outlines a fundamental approach to enhance pure organic
phosphorescence under ambient conditions, providing a major
step forward expanding the scope of pure organic phosphor-
escence applications.
1
1,4-Dibromo-2,5-bis(hexyloxy)benzene (PhBr2C6). H NMR (400
MHz, CDCl3): δ 7.09 (s, 2H, ArH), 3.95 (t, 4H, CH2O), 1.80 (m, 4H,
CH2), 1.48 (m, 4H, CH2), 1.34 (m, 4H, CH2), 0.91 (t, 6H, CH3). 13
C
NMR (100 MHz, CDCl3): δ 150.1, 118.5, 111.1, 70.3, 31.5, 29.1, 25.6,
22.6, 14.0. Anal. Calcd for C18H28Br2O2: C, 49.56; H, 6.47; Br, 36.63;
O, 7.34. Found: C, 49.45; H, 6.41.
1,4-Dibromo-2,5-bis((6-bromohexyl)oxy)benzene (PhBr2C6Br2).
1H NMR (400 MHz, CDCl3): δ 7.08 (s, 2H, ArH), 3.96 (t, 4H,
CH2O), 3.43 (t, 4H, CH2Br), 1.90 (m, 4H, CH2), 1.82 (m, 4H, CH2),
1.55 (m, 8H, CH2). 13C NMR (100 MHz, CDCl3): δ 150.1, 118.5,
111.2, 70.5, 33.8, 32.7, 28.9, 27.8, 25.2. Anal. Calcd for C18H26Br4O2:
C, 36.40; H, 4.41; Br, 53.81; O, 5.39. Found: C, 36.48; H, 4.47.
1
1,4-Dibromo-2,5-bis(octyloxy)benzene (PhBr2C8). H NMR (400
MHz, CDCl3): δ 7.08 (s, 2H, ArH), 3.94 (t, 4H, CH2O), 1.80 (m, 4H,
CH2), 1.48 (m, 4H, CH2), 1.29 (m, 4H, CH2), 0.89 (t, 6H, CH3). 13
C
NMR (100 MHz, CDCl3): δ 150.1, 118.5, 111.2, 70.4, 31.8, 29.3, 29.2,
29.1, 25.9, 22.7, 14.1. Anal. Calcd for C22H36Br2O2: C, 53.67; H, 7.37;
Br, 32.46; O, 6.50. Found: C, 53.49; H, 7.26.
1,4-Dibromo-2,5-bis((8-bromooctyl)oxy)benzene (PhBr2C8Br2).
1H NMR (400 MHz, CDCl3): δ 7.08 (s, 2H, ArH), 3.95 (t, 4H,
CH2O), 3.41 (t, 4H, CH2Br), 1.86 (m, 8H, CH2), 1.50 (m, 8H, CH2),
1.37 (m, 8H, CH2). 13C NMR (100 MHz, CDCl3): δ 150.1, 118.5,
111.2, 70.2, 34.0, 32.8, 29.1, 28.7, 28.1, 25.9. Anal. Calcd for
C22H34Br4O2: C, 40.64; H, 5.27; Br, 49.16; O, 4.92. Found: C, 40.48;
H, 5.18.
Fabrication of Temperature Sensors. In this work, PhBr2C6Br2
crystal was selected as a temperature probe because of its high
phosphorescence yield of 21.9%. The organic phosphors were put into
a cuvette with a volume of 0.35 mL. The capped cuvette was then
placed in a holder of the fluorescence spectrophotometer with
temperature-control equipment. To eliminate the effect of the external
physical factors such as residual stress, the sample was first heated with
a heating ratio of 5 °C/min to 90 °C and then cooled to the room
temperature. After that, the phosphorescent spectra of the temperature
probe at different temperatures were collected. The phosphorescence
spectra were obtained by a Varian Cary Eclipse fluorescence
spectrophotometer with the following detailed settings: total decay
time, 0.02 s; delay time, 0.20 ms; gate time, 5.0 ms; excitation and
emission slit, 10 nm.
EXPERIMENTAL SECTION
■
General Methods. The steady-state photoluminescence and
excitation spectra in solution and the solid state were measured by
Shimadzu RF5300 equipment with a xenon lamp as an excitation light
source. The phosphorescence spectra at room temperature were
obtained by a Varian Cary Eclipse fluorescence spectrophotometer
with the following detailed settings: total decay time, 0.02 s; delay
time, 0.20 ms; gate time, 5.0 ms; excitation and emission slits, 10 nm.
Emission lifetime and absolute quantum efficiency were measured by
Edinburgh FL 920 fluorescence spectrometer with a μF900 μs flash
lamp and an integrated sphere at room temperature. The lifetimes (τ)
of the luminescence were obtained by fitting the decay curve with a
multiexponential decay function of
i
I(t) =
A e−t/τ
∑
i
i
where Ai and τi represent the amplitudes and lifetimes of the individual
components for multiexponential decay profiles, respectively. UV−
visible absorption spectra in solution were detected by UV-3600
Shimadzu. 1H and 13C NMR spectra were recorded on Bruker BBFO-
400 and -100 MHz spectrometers, respectively. Tetramethylsilane was
chosen as the internal standard. Elemental analysis was performed on
an Elementar Vario MICRO elemental analyzer. The crystal
luminescent photographs were taken by a Cannon EOS 700D camera
under the excitation at 365 nm with a hand-held UV lamp at room
temperature.
The X-ray crystallography was carried on an Agilent Technologies
Super Nova diffractometer using microfocus graphite-monochromated
Cu Kα radiation (λ = 1.5418 Å) at room temperature. The structure
was solved by direct methods and refined by full matrix least-squares
analyses on SHELXTL-97S3.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.cgd.5b01400.
Characterizations of the four compounds, DTF results,
and absorption spectra (PDF)
Accession Codes
Calculation Method. The DFT simulations were performed to
study the electronic properties of singlet and triplet states of PhBr2C8
and PhBr2C8Br2 crystals by using exchange-correlation functional
B3LYP and the unrestricted formalism (UB3LYP), respectively,
together with basis sets 6-31G(d,p). The spin density distribution of
triplet states was plotted in SI Figure S17 with isovalue of 0.02 e/Å3,
where green and blue areas represent the positive and negative spin
densities, respectively. All of the DFT calculations were based on the
experimental crystal structures without further crystal optimizations.
Synthesis and Characterization. Materials. Unless otherwise
noted, all reactions were carried out under a nitrogen atmosphere. All
of the solvents and chemicals were used as received from commercial
sources.
CCDC 1404733−1404734, 1405036, and 1405039 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
*(Y.Z.) E-mail: zhaoyanli@ntu.edu.sg.
E
DOI: 10.1021/acs.cgd.5b01400
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
(21) Xu, J. J.; Takai, A.; Kobayashi, Y.; Takeuchi, M. Chem. Commun.
2013, 49, 8447−8449.
*(W.H.) E-mail: iamwhuang@njtech.edu.cn.
Author Contributions
(22) Lee, D.; Bolton, O.; Kim, B. C.; Youk, J. H.; Takayama, S.; Kim,
J. J. Am. Chem. Soc. 2013, 135, 6325−6329.
⊥H.S. and Z.A. contributed equally to this work.
(23) Kuno, S.; Akeno, H.; Ohtani, H.; Yuasa, H. Phys. Chem. Chem.
Phys. 2015, 17, 15989−15995.
Notes
The authors declare no competing financial interest.
(24) Reineke, S.; Baldo, M. A. Sci. Rep. 2014, 4, 3797.
(25) Kwon, M. S.; Lee, D.; Seo, S.; Jung, J.; Kim, J. Angew. Chem., Int.
Ed. 2014, 53, 11177−11181.
ACKNOWLEDGMENTS
■
(26) Fermi, A.; Bergamini, G.; Roy, M.; Gingras, M.; Ceroni, P. J. Am.
Chem. Soc. 2014, 136, 6395−6400.
This work is supported by the National Research Foundation
(NRF), Prime Minister’s Office, Singapore under its Campus
for Research Excellence and Technological Enterprise
(CREATE) ProgrammeSingapore Peking University Re-
search Centre for a Sustainable Low-Carbon Future, as well as
the NTU-A*Star Silicon Technologies Centre of Excellence
under Program Grant No. 11235100003. It was also supported
by the National Basic Research Program of China (973
Program; Grant No. 2015CB932200), the National Natural
Science Foundation of China (Grant Nos. 21304046,
61505078, 61136003, and 51173081), a project funded by
the China Postdoctoral Science Foundation (Grant No.
51235005), and the Natural Science Fund for Colleges and
Universities in Jiangsu Province (Grant No. 14KJB430015).
(27) Maity, S. K.; Bera, S.; Paikar, A.; Pramanik, A.; Haldar, D. Chem.
Commun. 2013, 49, 9051−9053.
(28) Yong, G. P.; Zhang, Y. M.; She, W. L. CrystEngComm 2012, 14,
3923−3929.
(29) Yuan, W. Z.; Shen, X. Y.; Zhao, H.; Lam, J.; Tang, L.; Lu, P.;
Wang, C. L.; Liu, Y.; Wang, Z. M.; Zheng, Q.; Sun, J. Z.; Ma, Y. G.;
Tang, B. Z. J. Phys. Chem. C 2010, 114, 6090−6099.
(30) Kohler, A.; Bassler, H. Mater. Sci. Eng., R 2009, 66, 71−109.
(31) Graham-Bryce, I. J.; Corkill, J. M. Nature 1960, 186, 965−966.
(32) McGlynn, S. P.; Daigre, J.; Smith, F. J. J. Chem. Phys. 1963, 39,
675−679.
(33) Seybold, P. G.; White, W. Anal. Chem. 1975, 47, 1199−1200.
(34) Kelch, A.; Dix, I.; Hopf, H.; Jones, P. G. Acta Crystallogr., Sect. C:
Cryst. Struct. Commun. 2013, 69, 627−629.
(35) Wang, X. D.; Wolfbeis, O. S.; Meier, R. J. Chem. Soc. Rev. 2013,
42, 7834−7869.
REFERENCES
■
(1) Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151−154.
(2) Shao, Y.; Yang, Y. Adv. Mater. 2005, 17, 2841−2845.
(3) Wong, W.; Wang, X.; He, Z.; Djurisic, A. B.; Yip, C.; Cheung, K.;
Wang, H.; Mak, C. S. K.; Chan, W. Nat. Mater. 2007, 6, 521−527.
(4) Zhao, J. Z.; Wu, W. H.; Sun, J. F.; Guo, S. Chem. Soc. Rev. 2013,
42, 5323−5351.
(5) Zhang, G.; Palmer, G. M.; Dewhirst, M. W.; Fraser, C. L. Nat.
Mater. 2009, 8, 747−751.
(6) Zhao, Q.; Huang, C.; Li, F. Chem. Soc. Rev. 2011, 40, 2508−2524.
(7) Bolton, O.; Lee, K.; Kim, H.-J.; Lin, K. Y.; Kim, J. Nat. Chem.
2011, 3, 205−210.
(8) Hirata, S.; Totani, K.; Zhang, J.; Yamashita, T.; Kaji, H.; Marder,
S. R.; Watanabe, T.; Adachi, C. Adv. Funct. Mater. 2013, 23, 3386−
3397.
(9) Al-Attar, H. A.; Monkman, A. P. Adv. Funct. Mater. 2012, 22,
3824−3832.
(10) Kwon, M. S.; Lee, D.; Seo, S.; Jung, J.; Kim, J. Angew. Chem., Int.
Ed. 2014, 53, 11177−11181.
(11) Reineke, S.; Seidler, N.; Yost, S. R.; Prins, F.; Tisdale, W. A.;
Baldo, M. A. Appl. Phys. Lett. 2013, 103, 093302−093305.
(12) Gao, H. Y.; Zhao, X. R.; Wang, H.; Pang, X.; Jin, W. J. Cryst.
Growth Des. 2012, 12, 4377−4387.
(13) Yong, G. P.; Zhang, Y. M.; She, W. L.; Li, Y. Z. J. Mater. Chem.
2011, 21, 18520−18522.
(14) Gao, H. Y.; Shen, Q. J.; Zhao, X. R.; Yan, X. Q.; Pang, X.; Jin, W.
J. J. Mater. Chem. 2012, 22, 5336−5343.
(15) Zhang, G.; Chen, J.; Payne, S. J.; Kooi, S. E.; Demas, J. N.;
Fraser, C. L. J. Am. Chem. Soc. 2007, 129, 8942−8943.
(16) Bergamini, G.; Fermi, A.; Botta, C.; Giovanella, U.; Di Motta, S.;
Negri, F.; Peresutti, R.; Gingras, M.; Ceroni, P. J. Mater. Chem. C 2013,
1, 2717−2724.
(17) Menning, S.; Kramer, M.; Coombs, B. A.; Rominger, F.; Beeby,
̈
A.; Dreuw, A.; Bunz, U. H. F. J. Am. Chem. Soc. 2013, 135, 2160−2163.
(18) Koch, M.; Perumal, K.; Blacque, O.; Garg, J. A.; Saiganesh, R.;
Kabilan, S.; Balasubramanian, K. K.; Venkatesan, K. Angew. Chem., Int.
Ed. 2014, 53, 6378−6382.
(19) An, Z.; Zheng, C.; Tao, Y.; Chen, R.; Shi, H.; Chen, T.; Wang,
Z.; Li, H.; Deng, R.; Liu, X.; Huang, W. Nat. Mater. 2015, 14, 685−
690.
(20) Zhang, X.; Xie, T.; Cui, M.; Yang, L.; Sun, X.; Jiang, J.; Zhang,
G. ACS Appl. Mater. Interfaces 2014, 6, 2279−2284.
F
DOI: 10.1021/acs.cgd.5b01400
Cryst. Growth Des. XXXX, XXX, XXX−XXX