Conjugation-Induced Rigidity in Twisting Molecules: Filling the Gap between Aggregation-Caused Quenching and Aggregation-Induced Emission

Article abstract of DOI:10.1002/adma.201501981

Conjugation-induced rigidity in twisting molecules provides a new facile but effective avenue toward solution and solid dual-state efficient luminogens. While conjugation rigidifies the molecular conformations in solution, the twisting structure prevents or alleviates detrimental close molecular packing in the solid states, thus synergistically yielding high efficiencies in both solution and solid states.

Full text of DOI:10.1002/adma.201501981

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Conjugation-Induced Rigidity in Twisting Molecules: Filling
the Gap Between Aggregation-Caused Quenching and
Aggregation-Induced Emission
Gan Chen, Wenbo Li, Tianru Zhou, Qian Peng, Di Zhai, Hongxiang Li,
Wang Zhang Yuan,* Yongming Zhang,* and Ben Zhong Tang*
Traditional organic luminophores are mainly composed of
planar aromatic rings. They emit efficiently in dilute solutions
while being weakened or even totally quenched due to the
formation of detrimental aggregates, which facilitate exciton
interactions and nonradiative pathways.[1] This phenomenon is
referred to as aggregation-caused quenching (ACQ),[2,3] which
limits the luminogens for such practical applications as opto-
electronic devices, imaging agents, and biosensors. To realize
efficient solid-state emission, numerous endeavors have been
made to prevent or alleviate the ACQ problem, including the
introduction of bulky substituents,[4] enhanced intramolecular
charge transfer (ICT) transition,[5] cross-dipole packing,[6] and
J-aggregated formation.[7] Specifically, exactly opposite to ACQ,
Tang et al. found that a series of silole derivatives was nonemis-
sive in dilute solutions but became highly luminescent when
aggregated as solid films.[1a] They termed this phenomenon as
aggregation-induced emission (AIE), whose mechanism was
mainly ascribed to the restriction of intramolecular motions
in the solid state. The AIE effect provides a unique and pow-
erful approach to construct efficient solid emitters for diverse
applications.[1c,d]
occurring process, to achieve efficient solid-state emission;
on the other hand, for twisting AIE luminogens, despite their
emissions are highly efficient upon aggregation, they are prac-
tically nonluminescent in solution. These facts mean a large
gap between ACQ and AIE compounds. We are thus won-
dering whether we can take advantages of both ACQ and AIE
dyes to create highly efficient luminogens in both solution and
solid states, which are definitely going to enjoy much broader
applications.[8] It is assumed that solution and solid dual-
state efficient luminogens ought to simultaneously fulfill the
requirements of substantial rigidity with limited intramolecular
motions in solution[1a–1d] and considerable twisting conforma-
tions in solid to prevent detrimental exciton interactions.[9]
Substantial rigidity in solution together with twisted con-
formations in the solid within a single molecule seems con-
tradictive and virtually impossible. However, it is found that
triphenylamine (TPA) fluoresces in tetrahydrofuran (THF) and
solid state with photoluminescence (PL) efficiencies of 13.0%
and 10.2%, respectively.[1d] Considering its twisting structure,
despite these values are relatively low, they are still indicative of
somehow impeded motions of the NC single bonds in solution,
rendering TPA with partially rigidified conformations. Previous
experimental[10] as well as theoretical[11] investigations reveal
a three-bladed propeller-like structure for TPA with a (nearly)
planar NCCC center. In crystals, four non-equivalent TPA mol-
ecules are present in the unit cell.[10b] The average (over these
four molecules) angle of the CNC bonds is 119.6°, whereas the
mean dihedral angle between the NCCC center and the phenyl
rings is 44°.[11a] It is well known that ammonia has a trigonal
pyramidal shape with a bond angle of 106.7°.[12] Such dispa-
rate conformations of TPA and ammonia are suggestive of the
conjugation effect between nitrogen and nearby phenyl rings,
which endows TPA with partially rigidified conformations in
solution.
Clearly, on one hand, for traditional rigid ACQ dyes,
researchers try to frustrate aggregation, which is a naturally
G. Chen, W. Li, T. Zhou, D. Zhai,
Prof. W. Z. Yuan, Prof. Y. Zhang
Shanghai Key Lab of Electrical Insulation
and Thermal Aging
School of Chemistry and Chemical Engineering
Shanghai Jiao Tong University
No. 800 Dongchuan Rd., Minhang District,
Shanghai 200240, China
E-mail: wzhyuan@sjtu.edu.cn; ymzsjtu@gmail.com
Prof. Q. Peng
Institute of Chemistry
To gain further insights into the conjugation-induced rigidity
(CIR) effect, three TPA derivatives, namely TPAB, TPAHTPE,
and TPA3HTPE (Figure 1) were designed and synthesized.
Despite their twisting structures, they all emit intensely in
both solution and solid states (Figure 1) accompanying high
efficiencies up to 79.4% and 87.1%, respectively. Moreover, the
emission of TPAHTPE is enhanced upon aggregation rather
than quenching normally observed in conventional efficient
solution emitters. These results validate the crucial role of
CIR in twisting molecules, which affords considerable solu-
tion efficiencies. Additionally, the twisting structures avoid
or alleviate close contacts and subsequent appreciable H-type
Chinese Academy of Sciences
Zhongguancun North First Street 2
Haidian District, Beijing 100190, China
Prof. H. Li
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
No. 345 Lingling Road, Shanghai 200032, China
Prof. B. Z. Tang
Department of Chemistry
The Hong Kong University of Science and Technology (HKUST)
Clear Water Bay, Kowloon, Hong Kong, China
E-mail: tangbenz@ust.hk
DOI: 10.1002/adma.201501981
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crystal-like central-planar conformations. This result further
testifies the CIR effect.
With much longer conjugation, TPAB exhibits nearly iden-
tical absorptions with a predominant peak around 320 nm in
all solvents (Figure S10B, Supporting Information). Their emis-
sions, however, demonstrate a slight solvatochromism with
emission maxima at 383, 398, 407, and 413 nm in toluene, THF,
dichloromethane (DCM), and N,N-dimethylformamide (DMF)
(Figure 2B),[14] respectively. When aggregated, TPA generates a
common red-shifted emission (maxima at 357 nm in THF and
380 nm in crystals), whereas TPAB exhibits almost identical
maxima in both THF and crystalline solids (Figure 2A, B).
For the crystals of TPAB,[15] as depicted in Figure 2C, two
different conformers, i.e., M1 and M2 are present. The average
angle (over the two conformers) of the CNC bonds is 119.9°,
and the mean dihedral angle between the NCCC center and
P11-P13/P21-P23 planes is 39.3° (Figure S12, Table S1, Sup-
porting Information), which is even smaller than that in TPA.
These data are indicative of more planarized conformations
of the TPA moiety in TPAB compared to those of TPA. Mean-
while, the dihedral angles between the central NCCC moiety
and additional P14 and P24 planes are determined to be 14.7°
and 40.6°, respectively, implying TPAB exhibits exceedingly
planarized conformations in crystals on the whole. It is con-
jectured that the effective conjugation length of TPAB in THF
is between those of M1 and M2. It is beyond expectation that
TPAB can maintain such planarized conformations in solution,
since NC and CC single bonds are ready to freely rotate in the
common sense. These results further confirm the CIR effect.
Figure 1. Structures of TPA, TPAB, TPAHTPE, and TPA3HTPE and their
photographs in solution (10 × 10−6 M, from left to right: toluene, THF,
DCM and DMF) and crystal states taken under 365-nm UV illumination,
except for TPA in solution under 300-nm UV illumination.
exciton interactions in the neat solid, thus yielding high solid-
state efficiencies.[9,13] Apparently, suitable twisting molecules
with CIR effect take advantages of both ACQ and AIE lumi-
nogens and thus exactly fill the gap between
them, providing a new avenue to dual-state
efficient luminogens suitable for promising
applications.
TPAB, TPAHTPE, and TPA3HTPE
were prepared by Suzuki coupling reaction
between corresponding boric acids and 4-bro-
motriphenylamine or tris(4-bromophenyl)-
amine, catalyzed by palladium complex in
basic media (Scheme S1, Supporting Infor-
mation). These target compounds were fully
characterized by spectroscopic methods,
1
such as H NMR, 13C NMR, and mass spec-
troscopy, with satisfactory results obtained
(Figure S1–S9, Supporting Information).
Absorption and emission spectra of
TPA in varying solvents show subtle dif-
ferences (Figure S10A, Supporting Infor-
mation, Figure 2A). Its emission profile,
however, drew our attention: besides a peak
at ≈360 nm, a shoulder around 375 nm is
observed, which might be associated to two
primary, stable yet different conformers.
Further time-resolved fluorescence measure-
ment reveals two lifetimes for the peak and
the shoulder in varying solvents (Figure S11,
Supporting Information), thus verifying the
above speculation. Notably, the shoulder is
Figure 2. Normalized emission spectra of A) TPA and B) TPAB in different solvents (10 × 10−6 M)
approaching to the crystal emission peak
(380 nm), indicative of corresponding and in crystals. C,D) Single-crystal structure and fragmental molecular packing of TPAB.
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Table 1. Photophysical properties of the luminogens.
cryt
em
Solution emission: λem [nm]/Φf [%]a)
λ
[nm]/Φf [%]
c)
Compound
Δλ
[nm]
Toluene
357/−
THF
DCM
DMF
TPA
357/13.0b)
398/35.2
482/46.7
484/73.1
380/10.2b)
397/17.9
448/87.1
455/54.8
23
360/−
362/−
TPAB
383/29.9
450/62.6
445/73.9
407/39.7
489/45.2
494/79.1
413/49.1
505/56.5
520/79.4
−1
TPAHTPE
TPA3HTPE
−34
−29
a)Absolute PL efficiencies were measured using Absolute PLQY C11347 (Hamamatsu) at a concentration of 5 × 10−6 M; b)Data obtained from reference [1d];
cryt
THF
c)
Δλ = λem − λem
.
It is also noted that one more phenyl greatly enhances the PL
efficiency of both solutions (ranging from 29.9% to 49.1%) and
crystals (17.9%) (Table 1), which should be mainly ascribed
to the enhanced conjugation and subsequently conformation
planarization and rigidification.
conformations in the solid state and more rigid conformations
in solution due to extended conjugation. Based on these consid-
erations, TPAHTPE and TPA3HTPE were therefore designed.
Solvent exerts little influence on the absorption of TPAHTPE
and TPA3HTPE. Both compounds show a minor and a major
peak at 308/363 and 314/382 nm (Figure S14, Supporting
Information), respectively. Redder absorption of TPA3HTPE
indicates its much longer effective conjugation length. Their
emissions are slightly red-shifted as the solvent polarity
increases from toluene to DMF, with moderate to high efficien-
cies ranging from 45.2%/73.1% to 62.6%/79.4% (Figure 3A, D,
Table 1). Normally, for ICT compounds, the efficiency will be
decreased with increasing solvent polarity. Remarkable solution
efficiency and even emission enhancement in highly polar sol-
vents suggest their rigid conformations and weak ICT effect.
Similar to TPA, their solution emissions demonstrate two
peaks. Notably, in THF, the intensity of the redder peak (Ir) is
much stronger than that of the bluer one (Ib), indicating more
contribution from the molecules with somewhat more rigid
conformations. For TPA, TPAHTPE and TPA3HTPE, except
To further our understanding on the photophysical proper-
ties of TPAB, its emission behaviors in THF and THF/water
mixtures with varying water fractions (fws) were monitored
(Figure S13, Supporting Information). The emission intensity
increases with enhanced concentration and reaches its max-
imum around 20 × 10−6 M. Afterwards it swiftly decreases with
increasing concentration, demonstrating a typical concentra-
tion quenching effect. Meanwhile, the aggregation experiment
indicates TPAB is an ACQ luminogen (Figure S13, Supporting
Information), which is consistent with the efficiency results. To
realize high efficiencies in the aggregates or neat solids, more
twisted conformations in the solid state are indispensable.
Triphenylethene (HTPE) is a typical AIE unit[16] and an “ana-
logue” of TPA with a C C bond replacing the “nitrogen” core.
Incorporation of propeller-like HTPE may lead to highly twisted
Figure 3. Emission spectra of A,B) TPAHTPE and D,E) TPA3HTPE in A,D) different solvents, crystals and B,E) THF and THF/water aqueous mixtures
with varying water fractions (fw). Emission peak intensity (Ipeak) of C) TPAHTPE and F) TPA3HTPE in THF and THF/water at varying fws. Concentration =
10 × 10−6 M.
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in toluene, the Ir/Ib value successively increases with extended
conjugation length in solvents (Table S2, Supporting Informa-
tion), indicating longer conjugation offers more rigid confor-
mations in solution. The exception in toluene, however, may
result from excellent solvation effect between aromatic mole-
cules and toluene. It is arduous to keep a bulky molecule in the
extended rigid conformation when it interacts frequently with
the solvent.
Further experiments in THF and THF/water mixtures
were conducted to decipher the aggregation effect on emis-
sion. PL intensity of TPAHTPE keeps increasing and slowly
red-shifting with growing fw, owing to the increased solvent
polarity. When fw ≥ 70%, it exhibits aggregation-enhanced emis-
sion (AEE) characteristics (Figure 3B, C). TPA3HTPE shows
slight emission enhancement and red-shift before aggregation,
and reaches the maximum when aggregation first appears at
fw = 50% (Figure 3E, F, S15, Supporting Information). Further
addition of water, however, continuously attenuates the emis-
sion, which might be attributed to the aggregation-caused
close contact between the molecules.[9,17] The AEE nature of
TPAHTPE and partial ACQ property of TPA3HTPE are also
confirmed by their solution (i.e., THF) and crystal efficiencies
(Table 1), from which an enhancement of 87% and a decrease
of 33% are derived. Nonetheless, the high crystal efficiencies
of 87.1% (TPAHTPE) and 54.8% (TPA3HTPE) also clearly indi-
cate exceedingly efficient solid emissions from both lumino-
gens (Figure S16, Supporting Information).[18]
Figure 4. A) Molecular geometry and B,C) intermolecular interactions of
TPAHTPE in single crystals.
From TPA to TPAB, TPAHTPE and TPA3HTPE, longer con-
jugation leads to higher solution efficiency (Table 1), testifying
molecular rigidity in solution is highly associated with effective
conjugation length in this system. CIR is proved to be effec-
tive to obtain efficient luminogens in solution. However, we
should not neglect previous works on TPA-based derivatives.
When TPA are decorated with tetraphenylethene (TPE) or tri-
phenylacrylonitrile (TPAN), the resulting TPATPE, TPA3TPE,
TPATPAN, and TPA3TPAN (Chart S1, Supporting Informa-
tion) are all AIE-active.[1d,19] Subtle changes like an additional
phenyl or cyano group exert enormous impacts, making these
compounds virtually nonemissive in solution. It is reasonable
that more substituents result in more steric hindrance, which
subsequently decrease rather than increase the whole conju-
gation. It is verified by the absorption peaks of TPA3TPE and
TPA3HTPE, which locate at 370[1d] and 382 nm in THF, respec-
tively. Namely, three more phenyls in TPA3TPE lead to a much
shorter instead of longer effective conjugation length, which
should be ascribed to the excessive steric hindrance. Simulta-
neously, these substituents serve as additional rotating units
in solution, which further consume the exciton energies. For
cyano-containing compounds, besides steric effect, intrinsic
ICT process is also unfavorable to the light emission.
a shorter conjugation length in the solid state than that in
solution. To decipher the detailed mechanism of aggregation-
shortened conjugation, single-crystal structure of TPAHTPE[15]
was studied. The average angle of the CNC bonds (118.4°) and
the torsion angle between phenyl rings and the central NCCC
moiety (43°) are very close to those of TPA (Figure 4, Table S1,
Supporting Information). Moreover, the torsion angle between
P4 and the NCCC center is merely 12° (Table S1, Supporting
Information), implying TPA moiety and P4 depict nearly the
same planarized conformation as TPAB. Such conformation of
P1-P4 almost reaches its maximum conjugation, which would
not change much in solution. Then the distinct dissolution
induced red shift effect might mainly result from planarization
of the C C double bond, P5 and P6, which are fairly distorted
in the solid (Figure 4B).
Here, CIR has more meaning than merely rigidifying the
molecular conformations in solution. Actually, conjugation
also induces molecular planarization in solution, which further
enhances conjugation length and thus rigidity of the molecules
in return. The virtuous cycle would end when the decrease of
free energy caused by the increase of conjugation equals to
the energy required to overcome the steric hindrance. How-
ever, why on earth would a molecule present such different
conformations in solution and solid states? The secret is the
abundant intermolecular interactions formed in the solid state
(Figure 4B,C). TPAHTPE is composed of electron-donating
TPA and electron-accepting HTPE, which is beneficial for the
formation of short contacts. These contacts distort the mol-
ecules into crystal lattice with highly twisted conformations.
However, in solutions, these short contacts are completely
destructed by the solvation effect and CIR effect takes over.
Intermolecular short contacts induced distortion in the solid
It is also noted that aggregation induces blue-shifted rather
than normally red-shifted emissions[20] for TPAHTPE and
TPA3HTPE (Figure 3). Similarly, their crystal emissions
are also bluer than those in THF (Figure 3A, D). From TPA
to TPAHTPE, the difference between crystal and solution (in
crst
em
THF
em
THF) emissions (Δλ = λ − λ
) dramatically decreases
(Table 1). While TPA exhibits classical red shift upon aggre-
gation with Δλ of 23 nm, incorporation of HTPE turns it into
blue shift with Δλ of −34 nm. Somehow HTPE helps to build
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Figure 5. A) Transfer characteristics of TPA3HTPE-based OFET device at VD = −100 V. B) EL spectra and C) external quantum efficiency-current density
plots of devices I-IV.
state and CIR and planarization in solution cooperatively lead
to the aggregation-induced blue-shift.
(1 nm)/Al (100 nm) (indium tin oxide (ITO), 1,3,5-tris(N-phe-
nylbenzimidazol-2-yl)benzene (TPBi)), where TPBi is used
as electron-transport layer. For devices I (X = NPB 60 nm/
TPAHTPE 20 nm) and II (X = NPB 60 nm/TPA3HTPE 20 nm),
NPB and TPA(3)HTPE are used as hole-transport and light-
emitting layers, respectively (1,4-bis[(1-naphthylphenyl)-
amino]biphenyl (NPB)). Both devices give sky-blue electrolu-
minescence (EL) at 472 and 484 nm, respectively, which are
far from the crystal emissions of TPAHTPE and TPA3HTPE
(448 and 455 nm), indicating the EL mainly come from the amor-
phous films. The devices turn on at ≈4.4[23] and 3.7 V (Von), with
maximum luminance, current, power and external quantum
efficiencies (Lmax, CEmax, PEmax, EQEmax) of 7901/10085 cd m−2,
4.03/7.99 cd A−1, 2.26/6.97 lm W−1 and 2.1%/3.8% for I and
II (Figure 5B, C,S19, Supporting Information, Table 2), respec-
tively. Because of the considerable hole-transport ability of
the luminogens, devices III (X = TPAHTPE 80 nm) and IV
(X = TPA3HTPE 80 nm) without NPB were also constructed.
These devices perform comparably to or even better than their
counterparts with NPB, giving Von, Lmax, CEmax, PEmax, and
EQEmax of 4.5/3.6 V, 6088/10954 cd m−2, 4.80/6.82 cd A−1,
2.83/5.64 lm W−1 and 2.6/3.3% for III and IV, respectively.
These results suggest both luminogens can act as excellent
hole-transport and outstanding light-emitting materials. Supe-
rior device performance of TPA3HTPE may arise from its
higher hole mobility and better charge balance in the devices.
Highly twisted conformations in the solid state avoid close
contact leading to appreciable H-type exciton interaction, thus
offering high solid PL efficiency. Meanwhile, TPA3HTPE gives
a surprisingly smaller Δλ of −29 nm than that of TPAHTPE
(−34 nm). Three HTPE groups were believed to afford
TPA3HTPE with more distorted conformations in the solid
state, which is actually opposite. It is reasonable to conjecture
that TPA3HTPE with higher solution efficiency is more rigid
with longer conjugation compared to TPAHTPE in solution,
which makes it more difficult to distort when aggregated.
Meanwhile, besides its bulky structure, donating TPA moiety
is surrounded by three accepting HTPE groups, which are
unfavorable for the formation of short contacts. Compared to
TPAHTPE, TPA3HTPE adopts more rigid conformations in
solution and enjoys less short contacts in the solid, which coop-
eratively lead to relatively more planarized conformations in
the solid state, thus giving much smaller Δλ and lower solid
quantum efficiency (possibly owing to the relative low crystal-
linity and/or formation of H-type aggregates).
Charge transport ability, however, might be enhanced with
more planarized conformations. The highest occupied molec-
ular orbital/lowest unoccupied molecular orbital (HOMO/
LUMO) energy levels of TPAHTPE and TPA3HTPE, which
were determined by cyclic voltammetry (Figure S17, Sup-
porting Information) and absorption experiments, are respec-
tively −5.42/−2.60 and −5.13/−2.41 eV, suggesting their p-type
inherent.[21] OFET devices were then fabricated to measure their
mobility. TPA3HTPE-based device exhibits field-effect mobility
Table 2. Performance of EL device of TPAHTPE and TPA3HTPE.
(µ), threshold voltage (VT) and on/off current ratio (Ion/Ioff
)
Devicea)
Lmax
PEmax
CEmax
EQEmax
[%]
λmax
[nm]
of 7.96 × 10−4 cm2 V−1 s−1, −22.7 V and 104–105 (Figure 5A),
whereas those for TPAHTPE are 1.54 × 10−4 cm2 V−1 s−1,
−32.5 V and 102–103 (Figure S18, Supporting Information),
respectively. Better performance of TPA3HTPE might be
resulted from its more planarized conformations, which facili-
tate charge transport. It is worth noting that TPA3HTPE is a
dual-state highly efficient luminophore with considerable hole
mobility, which has rarely been reported.[22]
[cd m−2
]
[lm W−1
]
[cd A−1
]
I
472
484
472
484
7901
10 085
6088
2.26
6.97
2.83
5.64
4.03
7.99
4.80
6.82
2.1
3.8
2.6
3.3
II
III
IV
10954
a)Device configuration: ITO/X/TPBi (40 nm)/LiF (1 nm)/Al (100 nm); X = NPB
(60 nm) TPAHTPE (20 nm) (I), NPB (60 nm)/TPA3HTPE (20 nm) (II), TPAHTPE
(80 nm) (III), TPA3HTPE (80 nm) (IV). Abbreviations: λmax = EL peak, Lmax = max-
imum luminance, PEmax = maximum power efficiency, CEmax = maximum current
efficiency, EQEmax = maximum external quantum efficiency.
Owing to their high solid-state efficiencies, nondoped OLED
devices based on TPAHTPE and TPA3HTPE were fabricated
with a general configuration of ITO/X/TPBi (40 nm)/LiF
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In conclusion, this communication provides a new avenue:
CIR in twisting molecules, towards efficient luminogens in
both solution and solid states. Delicate balance between conju-
gation and twisting conformation takes advantages of both ACQ
and AIE luminogens. Despite their twisting structures, confor-
mations of TPAHTPE and TPA3HTPE are significantly rigidi-
fied by the conjugation effect in solution, resulting in strictly
restrained intramolecular motions and thus high solution effi-
ciencies (up to 79.4%). Meanwhile, effective intermolecular
interactions distort TPAHTPE and TPA3HTPE molecules into
the crystal lattice with highly twisted conformations, endowing
them with remarkable solid efficiencies of 87.1% and 54.8%,
respectively. Emission comparison reveals the conjugation
effect would further induce more planarized conformations
in solution than in crystals. Intermolecular short contacts
induced distortion in the solid state and conjugation-induced
planarization in solution cooperatively lead to the uncommon
blue-shifted emission of TPAHTPE and TPA3HTPE upon
aggregation. Such strategy of CIR in twisting molecules seems
simple but effective for the construction of solution and solid
dual-state highly luminescent materials, which are suitable for
promising optoelectronic24 and biological applications.
Supporting Information
[14] THF:
tetrahydrofuran;
DCM:
dichloromethane;
DMF:
Supporting Information is available from the Wiley Online Library or
from the author.
N,N-dimethylformamide.
[15] CCDC 1061633 (TPAB) and 1061634 (TPAHTPE) contain the sup-
plementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (51473092 and 21104044.) and the Shanghai
Rising-Star Program (15QA1402500). W.Z.Y. thanks the SMC-Chenxing
Young Scholar Program of Shanghai Jiao Tong University.
Received: April 24, 2015
Revised: June 1, 2015
Published online: July 1, 2015
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