Tuning the photophysical properties of metal-free room temperature organic phosphors via compositional variations in bromobenzaldehyde/dibromobenzene mixed crystals

Article abstract of DOI:10.1021/cm503678r

Spurred by several recent discoveries and a broad and largely unexplored design space, purely organic phosphorescent materials are starting to garner interest for potential applications in organic optoelectronics and sensors. One particularly promising cl

Full text of DOI:10.1021/cm503678r

Article
pubs.acs.org/cm
Tuning the Photophysical Properties of Metal-Free Room
Temperature Organic Phosphors via Compositional Variations in
Bromobenzaldehyde/Dibromobenzene Mixed Crystals
Onas Bolton,†,⊥ Dongwook Lee, Jaehun Jung, and Jinsang Kim*
‡,⊥
‡
,†,‡,§,∥
†
‡
§
Department of Materials Science and Engineering, Macromolecular Science and Engineering, Department of Chemical
∥
Engineering, and Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States
*
S Supporting Information
ABSTRACT: Spurred by several recent discoveries and a
broad and largely unexplored design space, purely organic
phosphorescent materials are starting to garner interest for
potential applications in organic optoelectronics and sensors.
One particularly promising class of purely organic phosphor is
the family consisting of bromobenzaldehyde emitters doped
into crystals of dibromobenzene hosts. These stand out by
featuring bright, robust, and color-tunable room temperature
phosphorescence. However, despite these attractive qualities,
the mixed crystal motif defining these materials puts complex
demands on crystal packing, chemical structure, and sample preparation in ways that are not yet well understood. Here we report
a detailed study on these materials to optimize emission efficiency and fine-tune color. Overall, data suggest that achieving
efficient inclusion of the emitter into the host crystal is critical to optimizing quantum efficiency. Phosphorescent emission from
the mixed crystals is polarized, indicating that bromobenzaldehyde guest is ordered in the dibromobenzene host crystal. While
host compounds tolerate a surprising variety of emitter sizes, both oversized and undersized, maximum quantum efficiency is
reached when emitters and hosts are identically sized and the former is present at 1−10 wt % of total solids. The optimum
quantum efficiency for these systems appears, empirically, to be in the range of 45−55%. To fine-tune emission color, altering the
halogen substitution of the emitter molecule affords sequential 5−30 nm changes to emission maxima within the green region.
The relatively minor impact these alterations have on the overall chemical structure affords color tuning with minimal detriment
to mixing efficiency.
INTRODUCTION
members of this family of organic phosphor by varying
compositions and chemical structures. These organic materials
consist primarily of an emitter compound that features an
aromatic aldehyde with bromine substitution. These emitters
are nonphosphorescent when in solution or liquid states, but
they produce photoluminescent phosphorescence when crystal-
lized. This, we have hypothesized, is because the bromine
atoms in their structures both promote intersystem crossing via
the heavy atom effect and prevent carbonyl vibration via a
■
Until recently phosphorescence from metal-free purely organic
materials was regarded as an intractably inefficient phenomen-
on, leading these compounds to receive little attention from
1
relevant applications, such as phosphorescent organic light
2
−6
7−9
emitting diode (PhOLED) and photovoltaic fields. More
recently, however, reports have begun to awaken interest in
purely organic phosphorescence for applications in sensing
and potential uses in optoelectronic devices.
10−13
1
4−19
Several
37−39
halogen bond.
This hypothesis is built upon the existing
families of purely organic phosphors including fluorobor-
40
2
0−22
23−25
26,27
understanding of the heavy atom effect, the spin−orbit
ons,
ketones
acenes,
thiones,
and classic aromatic
41
2
8,29
coupling activity of aromatic carbonyls, and the fact that all
phosphorescent crystal structures in this family feature such
halogen-carbonyl oxygen contact. Also, the phosphorescent
efficiency of these materials has been observed to correlate with
crystal quality, suggesting that the emission mechanism is
are emerging or re-emerging with promise for
use in optical applications. One such family of purely organic
phosphors that has emerged from the growing field of crystal
3
0−36
engineering
and has received recent attention is that of
bromobenzaldehyde/dibromobenzene-style crystal phosphors
14
14
influenced by the solid state condition of this compound.
reported recently by our group. These materials feature
robust high room temperature quantum efficiency and tunable
emission color from simple small molecules. Realizing the
attractive functional qualities of these materials, however,
depends greatly on optimizing crystal and chemical structures,
a combination that is nontrivial. Here we report phosphor-
escent quantum efficiency and fine color tuning of the first
When these aldehyde emitters are condensed into their pure
crystals, they suffer greatly from self-quenching and produce
Received: October 6, 2014
Revised: October 29, 2014
Published: October 30, 2014
©
2014 American Chemical Society
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suboptimal phosphorescence. It is to avoid this consequence
that a mixing approach is taken: a noncarbonyl, dibromo host
crystal provides the desired crystal lattice and halogen contact
while allowing the emitter to emit in functional isolation.
Compounds 2,5-dihexyloxy-4-bromobenzaldehyde (Br6A) and
perfectly into a high quality host crystal, and, unfortunately,
though the emitter is very similar to the host, it is not identical.
It is an impurity in the host crystal and, thus, not
thermodynamically favored in the host crystal. This renders
these mixed crystal phosphors susceptible to a number of
emission-reducing conditions (Figure 3). Optimizing quantum
efficiency thus requires optimizing mixing so that the maximum
population of emitters present are isolated and ordered within
the host crystal.
2
,5-dihexyloxy-1,4-dibromobenzene (Br6) are such a combina-
tion of emitter and host, respectively. These compounds
Figure 1a) represent one of the simplest and brightest emitter/
(
Figure 3. Sources of reduced quantum efficiency in mixed crystal pure
organic phosphors (upper three) and the emissive outcome of a
perfectly included and isolated emitter (bottom).
EXPERIMENTAL SECTION
■
All chemicals used were purchased from Sigma-Aldrich and used
without further purification. Deuterated solvents for NMR (nuclear
magnetic resonance) were purchased from Cambridge Isotope
Laboratories. Proton NMR was conducted on a Varian Inova 500
Figure 1. Bromobenzaldehyde purely organic phosphorescent crystals.
(
(
a) Benchmark aldehyde emitter and host, Br6A and Br6, respectively.
b) Schematic illustration of dim Br6A (emitter) crystals, nonemissive
equipped with a Varian indirect detection probe using CDCl solvent
3
with chemical shifts identified relative to 0.05 v/v% tetramethylsilane
standard (0.00 ppm). 13C NMR was conducted on a Varian MR400
equipped with a Varian 5 mm PFG AutoX Dual Broadband probe
using CDCl3 for which the solvent peak was used to calibrate.
Anhydrous tetrahydrofuran was generated by distilling over sodium
metal and benzophenone, collected only from deep purple solution.
UV−vis electronic absorption measurements were collected using a
Varian Cary 50 Bio spectrometer with solution samples held in a
quartz cuvette. Photoluminescent (PL) emission, excitation, and
quantum yield data were collected using a Photon Technologies
International (PTI) Quantamaster system equipped with an
integrating sphere. Gated photoluminescence was collected with a
delay time of 150 μs after 365 nm excitation of each sample at room
temperature. X-ray diffraction for dropcast mixed crystals was carried
by a Rigaku Rotating Anode X-ray Diffractometer (X-ray wavelength:
Br6 (host) crystals, and bright phosphorescent Br6A/Br6 (mixed
crystals).
host combinations in this young family of phosphors. Pure
Br6A crystals are only weakly emissive, but when Br6A is doped
into crystals of Br6, which are otherwise completely non-
emissive, phosphorescent emission from Br6A becomes very
bright (Figure 1b). Our hypothesis that Br6A becomes included
into the Br6 crystal lattice by substitution, and thus experiences
the same order and halogen contact, is supported by the fact
that phosphorescent emission from Br6A/Br6 mixed crystals is
polarized, indicating that Br6A is ordered in the Br6 crystal
(
Figure 2).
1
.54 Å). Microscope images were collected on an Olympus BX51 W/
Unfortunately, the emitter/host relationship that gives these
DP71 fluorescent microscope equipped with cross-polarizers.
Quantum yield (QY) was measured using an integrated sphere
whose accuracy was verified using a 10 mM Rhodamine 6G/ethanol
materials their high quantum efficiency also complicates
optimization. An ideal system features emitter substituted
4
2
solution. Samples for quantum yield measurement were prepared by
dropping 0.1 g/mL chloroform solutions of the desired compounds,
mixed in the weight ratios reported, onto an unmodified glass
substrate. Crystal “films” would form as the solvent evaporated. Each
sample had a total mass of 2 mg. Absorption and emission inside the
sphere was determined by comparison to a blank sample (glass only).
A neutral density filter was used to allow for maximization of the
emission signal without saturating the photomultiplier tube detector
with excitation light. Each sample type was run in quadruplicate (or
more) with each quantum yield measurement coming from a freshly
dropcast sample. Measurements proved highly repeatable, and errors
are given as ±1 standard deviation.
Figure 2. Microscope images of polarized phosphorescent emission
from a thin Br6A/Br6 crystal. Arrows indicate relative polarizer
direction.
Synthesis of 2,5-Dihexyloxybenzaldehyde (H6A). 2-Bromo-
1,4-dihexyloxybenzene (1 equiv) was loaded into a two-neck round
bottomed flask and vacuum purged with argon three times. Anhydrous
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tetrahydrofuran was added by syringe (ca. 25 mL solvent/g reagent),
and the vessel was placed into a bath of dry ice and 2-propanol at −78
°C. A solution of n-butyllithium in hexanes (2.5 M, 1 equiv) was added
dropwise via syringe, and the reaction was then stirred at −78 °C for 1
h. Anhydrous DMF (4 equiv) was then added, and the reaction was
allowed to warm to 23 °C for 3 h. The reaction was quenched carefully
with water and extracted with diethyl ether. The organic layer was
collected and dried over MgSO before being filtered, and the solvent
4
was removed by rotary evaporation. Products were purified by column
chromatography with ethyl acetate/hexane (1:30) eluent. Viscous oil
eventually crystallizing into a white solid was collected at a yield of
1
7
9%. H NMR (500 MHz, CDCl ): δ 10.47 (s, 1H, CHO), 7.31 (d,
3
4
4
3
J(H−H) = 3.0 Hz, 1H, Ar-H), 7.11 (q, J
Hz, 1H, Ar-H), 6.92 (d, J(
.5 Hz, 2H, OCH ), 3.94 (t, J
H, CH ), 1.55 (m, 2H, CH ), 1.42 (m, 4H, CH ), 1.32 (m, 8H,
= 3.0 Hz, J
= 4.0 Hz, 1H, Ar-H), 4.03 (t, J
= 6.5 Hz, 2H, OCH ), 1.81 (m,
(H−H) 2
= 4.0
=
(H−H)
(H−H)
(H−H)
3
3
H−H)
3
6
2
2
2
2
2
3
13
CH ), 0.91 (t, J
= 6.0 Hz, 6H, CH3). C NMR (100 MHz,
2
(H−H)
CDCl ): δ 14.0, 22.5, 25.6, 29.2, 31.5, 68.7, 69.2, 110.8, 114.4, 124.1,
3
+
1
[
25.1, 153.0, 156.3, 189.8. HRMS (ESI) m/z 307.2273 (C H O
M + H] requires 307.2268).
Synthesis of 4-Chloro-2,5-dihexyloxybenzaldehyde (Cl6A).
19 31 3
+
Cl6A was made by formylation of 4-chloro-2,5-dihexyloxy-1-
iodobenzene in a method similar to that used to produce H6A.
White crystals were collected at a yield of 59%. H NMR (400 MHz,
1
Figure 4. Emitter inclusion perturbed by size exclusion. (a) Chemical
structures of emitters Br(5-8)A and hosts Br(5-8). (b) Photograph of
dropcast crystals made of mixed aldehyde and host compounds
illuminated by 365 nm light. Quantum efficiencies measured from
samples of the same compositions and growth conditions.
CDCl ): δ 10.41 (s, 1H, CHO), 7.36 (s, 1H, Ar-H), 7.04 (s, 1H, Ar-
3
3
3
H), 4.02 (t, J
= 6.4 Hz, 2H, OCH ), 4.01 (t, J
= 6.4 Hz,
(
H−H)
2
(H−H)
2
H, OCH ), 1.82 (m, 4H, CH ), 1.48 (m, 4H, CH ), 1.34 (m, 8H,
2 2 2
3
13
CH ), 0.91 (t, J
= 6.0 Hz, 6H, CH3). C NMR (100 MHz,
(H−H)
2
CDCl ): δ 14.0, 22.5, 25.6, 29.0, 31.4, 69.4, 69.7, 111.1, 115.4, 123.6,
3
+
1
[
31.0, 148.9, 155.8, 188.8. HRMS (ESI) m/z 341.1884 (C H ClO
M + H] requires 341.1878).
Synthesis of 2,5-Dihexyloxy-4-iodobenzaldehyde (I6A). I6A
19 30 3
+
Å longer than their hosts, produce appreciable emission even
when mixed with undersized hosts. This experiment also
suggests, for these compounds and this growth method, a
possible optimum empirical quantum efficiency of ca. 45−55%,
as each ideally matched combination is in near agreement. XRD
was performed for mixed crystals grown from each emitter
Br(5-8)A as a guest in Br6 host in order to examine how the
size mismatch affects the crystal packing of host Br6. XRD data
were analyzed based on the XRD pattern of Br6 single crystal.
As shown in Figure 5, they show three characteristic peaks
appearing at 10.15, 20.5, and 23.7° corresponding to (001)
plane from Br−Br contact, (110) plane from H−Br contact,
and (020) plane from H−Br contact, respectively, which
indicates that by the inclusion of 1 wt % mismatched guest
emitter the crystal structure of Br6 is not significantly altered to
be responsible for the reduced quantum efficiency. The
was made by lithiation of 1,4-diiodo-2,5-dihexyloxybenzene in a
method similar to that used to produce H6A and Cl6A. White crystals
1
were collected at a yield of 31%. H NMR (400 MHz, CDCl ): δ 10.42
3
3
(
=
4
6
2
1
4
s, 1H, CHO), 7.46 (s, 1H, Ar-H), 7.19 (s, 1H, Ar-H), 4.02 (t, J
(
H−H)
3
6.0 Hz, 2H, OCH ), 4.00 (t, J
= 6.0 Hz, 2H, OCH ), 1.82 (m,
2
(H−H) 2
3
H, CH ), 1.49 (m, 4H, CH ), 1.35 (m, 8H, CH ), 0.91 (t, J
.8 Hz, 6H, CH3). C NMR (100 MHz, CDCl ): δ 14.0, 16.2, 22.5,
2.6, 25.6, 25.7, 29.0, 31.4, 69.5, 69.9, 108.8, 124.5, 125.1, 152.1, 155.8,
89.3. HRMS (ESI) m/z 433.1232 (C H IO [M + H] requires
=
(H−H)
2
2
2
13
3
+
+
19
30
3
33.1234).
RESULTS AND DISCUSSION
■
The limits of inclusion efficiency were probed via a size
exclusion experiment. In order to perturb mixing, a series of
analogous aldehydes and hosts were synthesized with alkoxy
chain lengths variable from five to eight carbon atoms (Figure
4
a). These were intermixed in crystals grown by dropcasting
chloroform solutions containing 1 wt % aldehyde, and their
photoluminescence quantum efficiencies were measured using
an integrating sphere. As expected, emission was brightest when
aldehyde and host were the same size. Mismatched emitter and
host combinations were seen to mix surprisingly well, though
distinctly worse than ideally matched pairs, with quantum
efficiency dropping as the size difference grows. This was true
for both undersized and oversized emitters, though when the
emitter was larger than the host the quantum efficiency was
markedly lower than in the opposite case. This can be seen in
Figure 4b, where the lower left corner of the image is
qualitatively darker and quantitatively weaker than the upper
right.
The fact that phosphorescence is generated from mis-
matched emitter-host pairs, some dramatically mismatched,
indicates that inclusion is kinetically quite favorable, even if not
thermodynamically so. This is made especially clear when
oversized emitters, in some cases six methylene carbons and 7.5
Figure 5. XRD pattern of mixed crystal with different emitters in Br6
prepared by dropcast on a glass substrate. Data were analyzed based
on the XRD pattern of Br6 single crystal. Doping concentration was 1
wt % for all experiments.
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emission lifetime of bromobenzaldehyde guests also remains in
a similar range regardless of the size mismatch as shown in
Table S1 in the Supporting Information.
sparse Br6A into the increasingly pure Br6 crystals.
Phosphorescence microscopy reveals that Br6A/Br6 crystals
are brightest near nucleation sites (Figure 8) and often feature
sharp drop-offs in emission brightness. This indicates that the
inclusion rate is not constant throughout the growth of the host
crystals. If Br6A molecules are not leaving solution soon
enough to catch Br6 crystals in early growth in the dropcast
films, as would likely be the case when Br6A concentrations are
low, inclusion will suffer. So, just as recrystallization is done in
the wet lab successively to purify crystalline materials, at these
low levels of Br6A the emitter is less likely to become included
into the relatively pure Br6 crystal formed from dropcast.
Note that because these samples were made by dropcast,
crystal quality is worse than that of crystals grown slowly from
supersaturated solution. This is why pure Br6A here (dropcast)
measured a quantum efficiency of 0.1%, while Br6A reported in
our earlier work, there measured as high quality single crystals,
exhibited a quantum efficiency of 2.9%. While that trend is true
for pure crystals, dropcasting is preferable to solution growth
for mixed crystals because the likelihood of kinetically
entrapped inclusion of the emitter into the host is much better
in the faster process, dropcasting. Based on this understanding
and our observations, it follows that inclusion, and thus
quantum efficiency, appears to prefer fast crystal growth. Also
supporting this hypothesis, attempts to thermally anneal Br6A/
Br6 mixed crystals led to reduced quantum efficiency in all
cases.
In order to determine an optimal mixing ratio of emitter and
host, several samples were made with varying concentrations of
Br6A in Br6. Both compounds were mixed in a chloroform
solution to the desired amounts before being dropcast onto
unmodified glass slides. Br6A content was varied from 0.0001
wt % to 100 wt %, as total solids. No fewer than four samples
were made from each combination, and these were measured
for quantum efficiency in an integrating sphere. Efficiencies
were charted showing a mixing ratio of 1 wt % Br6A to produce
the brightest samples, though samples of 10 wt % Br6A were in
close agreement (Figure 6). A rapid reduction in quantum
Achieving efficient inclusion also becomes an issue in color
tuning. In earlier work broad changes to emission color were
accomplished by altering the electron density of the emitter
through the addition of electron deficient or rich atoms or
Figure 6. Phosphorescent quantum efficiency versus weight
percentage of Br6A in Br6A/Br6 mixed crystal sampled from dropcast.
Error bars represent ± one standard deviation.
14
extended conjugation. As these chemical structures strayed
from the original models inclusion became more complicated
and quantum efficiencies suffered. For example, in the case of
the blue-emitting alkyl substituted compounds, inclusion
efficiency was problematically low, in large part because the
aldehyde had a freezing point below room temperature. A more
elegant, and subtler, approach to color tuning would be to
simply vary the halogen atom of the emitters. Such compounds
were synthesized to explore the effects halogen variation would
have on emission color. Alternative emitters 4-chloro-2,5-
dihexyloxybenzaldehyde (Cl6A) and 2,5-dihexyloxy-4-iodoben-
zaldehyde (I6A) have nonbromine halogens, while 2,5-
dihexyloxybenzaldehyde (H6A) was designed to be halogen-
free (Figure 9a). Even though H6A is a halogen-free molecule it
has the same core benzaldehyde unit that will form halogen
bonding with a bromine of matrix Br6, which activates the
heavy atom effect. We excluded the fluoride version because F
does not form halogen bonding. These compounds alter the
electron density of the emitter in ways that have less effect on
the size, shape, and electronic character of the molecule and are
thus less likely to alter pertinent materials properties such as the
crystal growth mechanism and freezing temperature.
efficiency was observed as Br6A concentration was either
increased or decreased. Quantum efficiency tapered from a
maximum of 55% at 1 wt % Br6A to 0.1% at 100 wt % Br6A
and 0.4% at 0.0001 wt % Br6A. Gated phosphorescence spectra
(Figure S1) of the samples used for this mixing ratio study
confirm that the green emission is phosphorescence emission
from the embedded Br6A. Regardless of the side chain length
the same trend was observed in all cases where emitter from
Br5A to Br8A was intermixed at varying concentration from
0
.01 wt % to 30 wt % with its matching host from Br5 to Br8
(Figure 7). Substituting different solvents for drop casting and
dropcast solution concentration and changing substrate
temperature and hydrophobicity did not make any meaningful
change in the maximum phosphorescence quantum yield of
5
5% (Figure S2).
While it is not unexpected that quantum efficiency would
drop as Br6A content increases, a drop as Br6A content
decreases is surprising. At high concentrations of Br6A self-
quenching is anticipated as Br6A-Br6A contact becomes more
likely and the sample trends toward pure Br6A. The same is not
true at low concentrations of Br6A where emitter isolation
becomes extremely likely. Simple statistical analysis eliminates
the possibility that this trend is evidence that a dimer or
multialdehyde interaction is facilitating phosphorescence
because, for example, at concentrations of 1 wt % Br6A the
likelihood of Br6A-Br6A contact in the crystal is far below 55%,
the quantum efficiency measured at that loading of Br6A. We
hypothesize that quantum efficiency loss at low Br6A
concentration is due to poor inclusion of the increasingly
Each alternative aldehyde was included into Br6 host crystals
and measured spectrally. Br6 was chosen as the host when it
was observed to produce brighter samples than either 1,4-
dichloro-2,5-dihexyloxybenzene (Cl6) or 1,4-diiodo-2,5-dihex-
yloxybenzene (I6). Mixed crystals of Br6 were grown by
dropcasting chloroform solutions of 1 wt % emitter/Br6 onto
unmodified glass. As shown in Figure 9b, varying the halogen
from chlorine to bromine to iodine shifts the emission
spectrum in sequential 5 nm steps (λmax, Cl6A: 510 nm, λmax, Br6A
:
515 nm, λmax, I6A: 520 nm) commensurate with the varying
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Figure 7. Phosphorescence quantum yield as a function of weight percentage of emitters Br(5-8)A in mixed crystal with hosts Br(5-8). Error bars
represent ± one standard deviation.
of the more dramatically reduced electron density this emitter
exhibits for having no halogen.
We measured phosphorescence lifetime of guest emitters
Cl6A, Br6A, and I6A) in their analogous hosts (Cl6, Br6, and
(
I6) at room temperature (Table 1). The phosphorescence
Table 1. Phosphorescence Lifetime (ms) of Mixed Crystals
Was Estimated at Room Temperature by Means of Time-
Resolved Phosphorescence Measurement of 515 nm
Emission with Single Decay Curve Fitting
Figure 8. Markedly brighter regions surrounding apparent nucleation
sites in Br6A/Br6 mixed crystals.
Cl6
Br6
I6
Cl6A
Br6A
I6A
6.62
7.95
0.65
6.19
7.60
0.56
4.76
3.33
0.05
lifetime of guest emitters decreases when the halogen of the
host becomes heavier from Cl6 through Br6 to I6 likely due to
enhanced heavy atom effect between halogen atom of the host
and oxygen of the emitter. This lifetime decrease was
maximized at I6/I6A mixed crystals because it contains the
heaviest atom, iodine, both in host and emitter. When
combined with the broader color shifts presented in earlier
work, these fine-tuning steps give this family of organic
phosphors the capability to fine-tune emission color with both
breadth and precision.
Figure 9. (a) Halogen-variable emitters for fine color tuning. (b)
Emission (each excited at 365 nm) of crystals grown from solutions
containing a 1 wt % ratio of the emitter in Br6. H6A (black), Cl6A
(
red), Br6A (blue), and I6A (green).
CONCLUSIONS
■
In summary, we have demonstrated the optimization of the
quantum efficiency and fine-tuning of emission color from
emerging metal-free purely organic phosphors. Quantum
efficiency optimization relies on attaining efficient inclusion of
electron density afforded by the halogen. A halogen-free
emitter, H6A, produces an emission spectrum with a more
dramatic 30 nm blue shift (λmax, H6A: 485 nm), which is because
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the emitter into the host crystal, which is a kinetic equilibrium
maximized when both components are similarly sized and the
emitter is present at 1−10 wt %. Replacing or removing the
halogen on the emitter facilitates sequential 5−30 nm steps in
green emission region, allowing the designer to fine-tune the
emission spectrum with minimal change to the chemical
structure. These findings elucidate the complex optical and
solid state behavior these materials exhibit and illustrate the
extreme influence that material preparation has on their
performance.
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ASSOCIATED CONTENT
Supporting Information
Table S1 and Figures S1 and S2. This material is available free
of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
E-mail: jinsang@umich.edu.
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Author Contributions
⊥
These authors contributed equally to this work.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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(31) Shen, Q. J.; Wei, H. Q.; Zou, W. S.; Sun, H. L.; Jin, W. J.
This work was partly supported by the National Science
Foundation (DMR Career 0644864) and Samsung GRO grant.
D.L. was partly supported by a fellowship from LG Chemicals.
CrystEngComm 2012, 14, 1010−1015.
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