Experimental and DFT studies of disubstituted 2-(2-hydroxyphenyl)benzothiazole-based fluorophores synthesized by Suzuki coupling

Article abstract of DOI:10.1039/c7nj01525a

Among the various fascinating fluorescent compounds, excited-state intramolecular proton transfer (ESIPT) dyes have been drawing much attention due to their anomalously large Stokes shifts. Furthermore, ESIPT fluorophores are attractive as their fluoresce

Full text of DOI:10.1039/c7nj01525a

NJC
View Article Online
View Journal
PAPER
Experimental and DFT studies of disubstituted
2-(2-hydroxyphenyl)benzothiazole-based
Cite this:DOI: 10.1039/c7nj01525a
fluorophores synthesized by Suzuki coupling†
a
a
b
a
a
Yahui Niu, Qin Wang, Haoran Wu, Yuxiu Wang and Yanrong Zhang
*
Among the various fascinating fluorescent compounds, excited-state intramolecular proton transfer (ESIPT)
dyes have been drawing much attention due to their anomalously large Stokes shifts. Furthermore, ESIPT
fluorophores are attractive as their fluorescence properties may be fine-tuned through facile chemical
modifications of existing dyes. Varying the strength of the electron-donating and electron-withdrawing
substituents anchored at different sites of the parent molecules, especially when incorporating these
substituents at several different sites simultaneously, is an effective way to achieve targeted fluorescence
properties. In the present study, a new family of 2-(2-hydroxyphenyl)benzothiazole (HBT)-based
0
0
disubstituted derivatives, with substitutions at the 4 - and 5 -positions, were readily synthesized by the
0
Suzuki coupling reaction. It was found that the electronic effects of the 5 -position substituents on the
0
molecular orbital levels were opposite to that of the 4 -position substituents. Theoretical calculations
0
0
demonstrate that the substituents at the 4 - and 5 -positions dramatically affect the HOMOs and the
LUMOs, respectively, which is in conformity with experimental results and can be applied to predict
shifts in fluorescence emission. Additionally, these derivatives show highly efficient keto-emission in the
aggregated state. The X-ray crystallographic analysis shows that weak intramolecular interactions, which
restrict intramolecular rotation, and strong intramolecular hydrogen bonding, which facilitates intramolecular
proton transfer, are responsible for the intense keto-emission observed. Our study may provide experimentally
and theoretically valuable instructions for designing other high-performance ESIPT fluorophores to meet the
demands of specific applications.
Received 6th May 2017,
Accepted 30th July 2017
DOI: 10.1039/c7nj01525a
rsc.li/njc
Stokes shifts without self-absorption, which are desirable features
for many important applications including white light-emitting
Fluorescence is one of the fundamental properties that brings materials and fluorescent probes.
Introduction
4
5
about interesting molecular functions of organic compounds.
2-(2-Hydroxyphenyl)benzothiazole (HBT) is one of the most
Among the various fascinating fluorescent compounds, excited- extensively studied ESIPT-active dyes because of its simple
6
state intramolecular proton transfer (ESIPT)-active dyes have structure and facile synthesis. Diverse chemical modifications,
1
,2
gained significant interest. ESIPT is a typical photophysical via anchoring of different substituents at its various positions,
process, whereby an excited-state proton is transferred from are simple and effective strategies for regulating ESIPT to meet
a H-bond donor to an acceptor through a pre-existing intra- specific applications. For example, Zhao et al. conjugated
0
molecular H-bond, which produces a tautomer with a molecular naphthalimide and boron-dipyrromethene at the 5 -position
structure and electronic distribution completely different from of HBT, resulting in two derivatives with long-lived triplet
3
7
the originally excited species. Consequently, compared with excited states. Wang et al. anchored different electron-donating
0
0
other ordinary dyes, ESIPT-active dyes display anomalously large amino groups at the 4 - or 5 -position of HBT, which exhibited
8
emission colors across the entire visible spectrum. These studies
a
show that the fluorescence of HBT derivatives can be controlled by
modifying the substituents. Despite this, most previous studies
focused on the structural modification of HBT involving
substituents at a single position. Few studies have explored
simultaneous disubstitution of HBT, possibly due to a lack of
practical synthetic methods. Han et al. introduced an electron-
College of Chemistry & Pharmacy, Northwest A&F University, Yangling,
Shaanxi 712100, P. R. China. E-mail: zhangyr@nwsuaf.edu.cn;
Tel: +86-29-87092226
b
Innovation Experimental College, Northwest A&F University, Yangling,
Shaanxi 712100, P. R. China
†
Electronic supplementary information (ESI) available. CCDC 1546814. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c7nj01525a
0
withdrawing trifluoromethyl group at the 4 -position and an
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017
New J. Chem.

View Article Online
Paper
NJC
0
0
Scheme 1 Synthesis route of disubstituted HBT derivatives at its 4 and 5 positions. Reaction conditions: (a) (HCHO)n, Et
3
N, MgCl
2 3
, CH CN, reflux,
2
4 h; (b) AcOH, Br
, RT, 18 h; (c) 2-aminothiophenol, 37% HCl, 30% H O /ethanol; (d) PdCl (PPh ) , K CO , H O/dioxane, 16 h, reflux.
2 2 2 2 3 2 2 3 2
0
electron-donating triphenylamine group at the 5 -position two important factors responsible for the intense keto-emission
in a single HBT derivative, which generated pure white light observed in the crystalline state.
9
emission in mixed solvents. In this system, the trifluoromethyl
group facilitated the deprotonation of the phenolic hydroxy
group, while the triphenylamine group allowed for bathochromic
emission due to intramolecular charge transfer and the ‘‘red-edge
effect’’. Therefore, exploiting the electronic effect of multiple
Results and discussion
Synthesis of disubstituted HBT derivatives
substituents at different sites is a promising tactic in designing To functionalize HBT with various moieties bearing different
1
2
0 0
novel ESIPT dyes for targeted applications.
Hammett substituent constants at the 4 -and 5 -positions
In this work, with the aim of fine-tuning the fluorescence simultaneously, we selected the Pd-catalyzed Suzuki coupling
emission properties of HBT derivatives, we designed and reaction between organoboron compounds and organic halides.
synthesized a new family of disubstituted HBT-based compounds The difference between the reaction rates of iodides and bromides
0
0
by functionalizing the 4 - and 5 -positions with different allows for selective coupling with the iodides even in the presence
substituents using Suzuki cross-coupling (molecular structures of bromides. Additionally, commercial availability of many boronic
depicted in Scheme 1). Theoretical calculations, which agreed acids and the ease of obtaining various halogenated aromatic
with the experimental observations, could be applied to predict compounds make further chemical modification feasible. The
the fluorescence properties of other new ESIPT dyes. On the synthetic strategies employed to prepare four disubstituted HBT
other hand, fluorophores with aggregation-induced emission derivatives are depicted in Scheme 1. From commercially available
enhancement (AIEE) have shown outstanding advantages com- 3-iodophenol, 2-hydroxy-4-iodobenzaldehyde was first synthesized
13
pared with those with aggregation-caused quenching in many according to a previously reported method, before bromination
1
0
11
0
practical applications such as chemo/biosensors and OLEDs.
These new HBT derivatives show AIEE characteristics. The X-ray The reaction between 5-bromo-2-hydroxy-4-iodobenzaldehyde with
crystallographic analysis shows that weak intramolecular inter- 2-aminothiophenol in the presence of HCl/H in methanol
at the 5 -position to obtain 5-bromo-2-hydroxy-4-iodobenzaldehyde.
2 2
O
actions, which restrict the intramolecular rotation, and strong yielded 2-(benzo[d]thiazol-2-yl)-4-bromo-5-iodophenol, a very
intramolecular H-bonding, which facilitates proton transfer, are important intermediate, which could be readily monitored
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017

View Article Online
NJC
Paper
Fig. 1 (A) Absorption spectra and (C) fluorescence spectra of HBT-H-H in various solvents; comparison of experimentally obtained and theoretically
calculated (B) absorption spectra and (D) fluorescence spectra in toluene solution.
through thin layer chromatography owing to its strong yellow In toluene, there are three main absorption bands at 357 nm,
fluorescence under UV irradiation. Under Suzuki reaction con- 313 nm, and 300 nm, which is in good agreement with the
ditions, the reaction of this intermediate with two equivalents calculated results (Fig. 1B). The vertical excitation calculations
of either benzeneboronic acid or 4-cyanophenylboronic acid suggested that the three absorption peaks in toluene from the
afforded a biphenyl-modified HBT derivative (HBT-H-H) and lowest to highest energy bands correspond to transitions from
a bi-4-cyanophenyl-modified HBT derivative (HBT-CN-CN), the HOMO, HOMOꢀ1, and HOMOꢀ2 to the LUMO of the
respectively. The reaction of this intermediate with one equiv- p-systems, respectively (Table S1 and Fig. S1, ESI†).
alent of 4-cyanophenylboronic acid easily afforded another
In contrast to the absorbance, HBT-H-H shows remarkable
important intermediate, compound 4, which was further solvent-dependent fluorescence (Fig. 1C), which is a character-
1
6
reacted with phenylboronic acid or 4-methoxyphenylboronic istic feature for ESIPT dyes. In the nonpolar solvent PhMe and
acid to obtain HBT-CN-H and HBT-CN-OMe, respectively. These the medium polarity solvent DCM, one dominant band with an
1
new compounds were unambiguously characterized by H and exceedingly large Stokes shift (B169 nm) was observed. In MeCN
13
C nuclear magnetic resonance spectroscopy and high-resolution and THF, which can act as H-bond acceptors, well-separated
mass spectrometry (Fig. S11–S30, ESI†). Additionally, single- dual bands were detected, one with a normal Stokes shift
crystal X-ray diffraction analysis was also performed on (B36 nm), and the other with a larger Stokes shift (B172 nm).
HBT-H-H (CCDC 1546814).† It should be noted that this This phenomenon has also been observed for other ESIPT
1
5
synthesis method could be readily applied in synthesizing other systems. We ascribe the band with the normal Stokes shift to
disubstituted or multisubstituted HBT derivatives at different the emission from a locally excited state (N*), and the band with
sites of the parent HBT molecule.
the large Stokes shift to the emission from an ESIPT state (K*).
To verify this, we conducted several measurements. First, the
excitation spectra of HBT-H-H in different solvents resembled
their respective absorption spectrum (Fig. S2, ESI†), which
Photophysical properties of HBT-H-H in solution and
theoretical calculations
We first measured the absorption and fluorescence emission reveals that the two emission bands originate from the identical
1
7
spectra of HBT-H-H in several solvents with different polarities ground state of the enol-tautomer. Second, as shown in Fig. 2,
and H-bonding abilities. Meanwhile, to gain a deeper insight from the HOMO to the LUMO of the enol-tautomer, the electron
into the electronic structures and the excited states of the density of oxygen in the phenolic hydroxyl decreases, while that of
enol- and keto-tautomers of HBT-H-H, density functional nitrogen in the benzothiazole ring increases. This change in
1
4
theory (DFT) calculations were also conducted. As shown in electron density from the ground state to the excited state
Fig. 1A, the absorption spectra have similar profiles, indicating simultaneously improves the acidity of the phenolic hydroxyl
that the solvents have little effect on the absorption spectra, group and the basicity of the nitrogen, allowing for proton
1
5
which is consistent with the previously reported result.
transfer from oxygen to nitrogen to complete the tautomerization
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017
New J. Chem.

View Article Online
Paper
NJC
As shown in Fig. 2, for the keto-tautomer, the HOMO is
mainly located on the phenol ring and the phenyl in the
0
0
5
-position, and has no distribution on the 4 -position phenyl.
In contrast, the LUMO is on the phenol ring, the benzothiazole
0
moiety and the 4 -position phenyl, and has no distribution
0
on the 5 -position phenyl. Therefore, the substituents at the
0
5
-position have more of an effect on the HOMO, whereas
0
the substituents at the 4 -position have more of an effect
on the LUMO. It is well known that regardless of the position
of the substituent, the energy levels of both the HOMO and
LUMO decrease with increasing electron-withdrawing ability of
the substituent group. Accordingly, anchoring an electron-
withdrawing substituent at the 4 -position would lead to a
2
2
0
smaller energy gap between the HOMO and LUMO, because
the decreasing magnitude of the LUMO energy level would be
bigger than that of the HOMO; and the opposite effect would
0
be observed for the substitutions at the 5 -position. The above
analysis indicates that the trend of the shift of the keto-
emission could be predicted by theoretical calculations.
Photophysical properties of other disubstituted HBT
derivatives and theoretical calculations
Fig. 2 Schematic representation of the four-level photophysical process
for HBT-H-H.
In order to further study the electric effects on the photo-
physical properties of disubstituted HBT derivatives and to
verify that the trend of the shift of the keto-emissions could
be predicted by theoretical calculations, we further synthesized
three disubstituted HBT derivatives, HBT-CN-H, HBT-CN-OMe,
and HBT-CN-CN (molecular structures shown in Scheme 1 and
photophysical data summarized in Table 1).
18,19
from the enol- to the keto-tautomer.
Meanwhile, the more
acidity of the hydroxy group and the more basicity of the nitrogen
atom in the excited state are also reflected in the elongated O–H
bond and the shortened NꢁꢁꢁH distance (Fig. S3, ESI†). Actually,
the keto tautomer in the excited state has been detected through
First, the fluorescence spectrum of HBT-CN-H, which bears
0
an electron-withdrawing CN group at the 4 -position, was
15,20
femtosecond transient absorption spectra.
Third, the energy
compared with that of HBT-H-H in different solvents. In PhMe
and DCM, both the molecules solely showed keto-emission
levels of the HOMOs and LUMOs of the enol-tautomer and
keto-tautomer were compared (Fig. 2). The LUMO of the
keto-tautomer is more stable by 0.1946 eV than that of the
enol-tautomer, and the HOMO of the keto-tautomer is less stable
by 0.4792 eV compared to the HOMO of the enol-tautomer.
Therefore, the much smaller HOMO–LUMO energy gap of the
keto-tautomer than that of the enol-tautomer underpins the
(
(
Fig. 3A and B), and HBT-CN-H shows red-shifted fluorescence
from 537 to 539 nm) with increasing F (from 1.4% to 3.7%).
f
In THF and MeCN, the intensity ratio of K* versus N* (IK*/IN*),
an important characteristic of ESIPT, increased accordingly
(Fig. 3C and D). As shown in Fig. 4, in comparison with HBT-H-H,
the energy levels of the HOMO and LUMO of its keto-tautomer were
lower by 0.1589 eV and 0.2169 eV, respectively, which results in a
smaller energy gap, and is in accordance with our above conjecture.
The difference of IK*/IN* indicates that the intramolecular H-bond in
HBT-CN-H is reinforced and less likely to be undermined by solvent
2
1
large Stokes shift due to keto-emission. Finally, the theoreti-
cally calculated emission wavelength (532.23 nm) in PhMe based
1
on the optimized S state geometry of the keto-tautomer is very
close to the experimentally observed value (537 nm) (Fig. 1D and
Fig. S4, ESI†). In nonpolar or medium polarity solvents, which
could not compete with the nitrogen for the phenolic hydroxyl
proton, the dominant form of HBT-H-H is the closed enol
bearing an intramolecular H-bond, with excitation resulting in
19
molecules, and that the ESIPT is promoted. In the optimized
ground state of the enol-tautomer of HBT-CN-H (Fig. 5), the NꢁꢁꢁO
sole keto-emission. In solvents that can act as an H-bond donor, Table 1 Photophysical data of the disubstituted HBT derivatives
intermolecular H-bonds with solvent molecules may partially or
completely destroy intramolecular H-bonds. Recently, Mcgrier
HBT-H-H HBT-CN-H HBT-CN-OMe HBT-CN-CN
s
0.00/0.00 0.66/0.00 0.66/ꢀ0.17
0.66/0.66
533
and coworkers have also shown that trifluoroacetic acid makes
the keto emission hypsochromically shift to the enol emission
in the ESIPT system. This is attributed to the protonation
of the imine nitrogen atoms, which completely muffle the ESIPT
l
l
l
em in PhMe (Exp.) 537
539
548
ex in PhMe (Cal.) 532.23
ex in PhMe (Exp.) 368
536.72
370
169
570.62
372
176
516.23
366
167
Dl in PhMe
in PhMe (%)
Chemical shift
169
F
f
1.4
12.591
lex in DCM (Exp.) 528
3.7
12.690
533
2.4
12.637
547
8.5
12.834
527
1
5
process. Therefore, excitation of HBT-H-H in H-bond donating
solvents results in both enol- and keto-emission.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017

View Article Online
NJC
Paper
Fig. 3 Comparing fluorescence spectra of the disubstituted HBT derivatives in different solvents. (A) PhMe, (B) DCM, (C) THF, and (D) MeCN.
Fig. 4 DFT-calculated HOMOs and LUMOs of the keto-tautomers of the disubstituted HBT derivatives.
and NꢁꢁꢁH distances were shortened from 2.6511 to 2.6450 Å and stronger H-bond in HBT-CN-H. The strength of the intramolecular
from 1.7697 to 1.7626 Å, respectively, and the O–H bond length was H-bond could also be directly reflected through the chemical shift of
1
also increased from 0.9938 to 0.9944 Å, all of which verified the the phenolic hydroxyl proton in the HNMR spectra (Fig. 6), which
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017
New J. Chem.

View Article Online
Paper
NJC
(from 3.7% to 8.5%). Additionally, HBT-CN-OMe demonstrated
a lower IK*/IN*, and HBT-CN-CN demonstrated a higher ratio,
which indicates that the electron-withdrawing group in
HBT-CN-CN reinforced the H-bond, while the electron-donating
group in HBT-CN-OMe weakened the H-bond. The calculated
HOMO/LUMO energy levels of HBT-CN-CN were ꢀ5.8510/
ꢀ
ꢀ
2.6542 eV, which are lower than those of HBT-CN-H (ꢀ5.6488/
2.5103 eV) (Fig. 4). In contrast, the HOMO/LUMO energy levels
(
ꢀ5.5231/ꢀ2.4811 eV) of HBT-CN-OMe are higher than those of
HBT-CN-H (Fig. 4). These results can be explained by the fact that
for their keto-tautomers, the HOMOs are spread on the 5 -position
phenyls, and have no distributions on the 4 -position phenyls,
0
0
while the distribution of the LUMOs is opposite to that of the
0
HOMOs. Therefore, the substituents at the 5 -position drama-
tically affect the HOMOs. The electron-withdrawing group at
0
the 5 -position simultaneously lowers the energy levels of the
LUMO and HOMO; the decreasing magnitude of the LUMO is
smaller than that of the HOMO, which results in a bigger energy
Fig. 5 The energy-optimized geometric structures of the enol-tautomer
of the disubstituted HBT derivatives in the ground-state and the relevant gap, and consequently, blue-shifted fluorescence emission. While
0
H-bond parameters.
the electron-donating group at the 5 -position simultaneously
elevates the energy levels of the LUMO and the HOMO, the
increasing magnitude of the LUMO energy level is smaller than
that of the HOMO, resulting in a smaller energy gap and
consequent red-shifted fluorescence emission. As expected,
the trend of the HOMO–LUMO energy gap is HBT-CN-CN 4
HBT-CN-H 4 HBT-CN-OMe, which is in line with the experi-
mental data. In the optimized ground state of enol-tautomers
for HBT-CN-OMe (Fig. 5), the distances of Nꢁ ꢁ ꢁO and Nꢁ ꢁ ꢁH
were elongated from 2.6450 to 2.6499 Å and from 1.7626 to
1.7689 Å, respectively, while the bond length of OH was
shortened from 0.9944 to 0.9941 Å. For HBT-CN-CN, although
the distances of Nꢁ ꢁ ꢁO and Nꢁ ꢁ ꢁH did not change, the OH bond
length was increased by 0.008 Å from 0.9944 to 0.9952 Å.
The change in strength of the H-bond could also be deduced
from the chemical shift of the phenolic hydroxyl proton in
1
the respective H NMR spectrum (Fig. 6). For HBT-CN-CN the
proton downshifts by 0.144 ppm, while for HBT-CN-OMe the
proton upshifts by 0.053 ppm. Therefore, the trend of H-bond
strength is HBT-CN-CN 4 HBT-CN-H 4 HBT-CN-OMe, which
Fig. 6 The chemical shift of the phenolic hydroxyl proton of the
1
3
disubstituted HBT derivatives in H NMR (CDCl ).
f
is in line with the changing trend of F . It should be noted that
shifted downfield by 0.094 ppm from 12.596 to 12.690 ppm. all the resonance signals of OH appeared significantly downfield
19
The similar correlation was also reflected in other ESIPT dyes.
at d 4 12 ppm for these HBT derivatives, clearly indicating
The improving strength of the intramolecular H-bond in HBT-CN-H strong H-bond formation in these compounds and guaranteeing
2
4
also explains its higher F , because for ESIPT fluorophores, H-bond the occurrence of keto-emission. In general, the theoretical
f
23
strength is often positively correlated with F
f
.
Therefore, introdu- calculations agree well with the experimentally observed trends.
cing electron-withdrawing groups at the 4 -position of HBT not only
red-shifts the fluorescence emission, but also improves the F
0
Photophysical properties of HBT-H-H in the crystalline state
f
.
Fixing an electron-withdrawing group like CN at the Yellow single crystals of HBT-H-H suitable for single-crystal
0
4
-position of HBT, we continued to study the substituent effect X-ray diffraction analysis were obtained by slow evaporation of
0
at the 5 -position, and introduced an electron-donating group DCM from the solution at room temperature. Suitable crystals
(OMe) and an electron-withdrawing group (CN), respectively, to of other compounds could not be obtained. Crystals of
obtain HBT-CN-OMe and HBT-CN-CN. As shown in Fig. 3A and B, HBT-H-H emitted strong yellow fluorescence under UV light
in comparison with HBT-CN-H, HBT-CN-OMe showed a red- irradiation, which prompted us to study the solid-state photo-
shifted keto-emission (from 539 to 548 nm) and a reduced F
f
physical properties of HBT-H-H. Solid-state fluorescence spectro-
(from 3.7% to 2.4%), whereas HBT-CN-CN showed a blue-shifted scopy showed that the maximum emission wavelength shifted
keto-emission (from 539 to 533 nm) and an enhanced F
f
f
bathochromically by 8 nm (Fig. S8, ESI†), and the F and t were
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017

View Article Online
NJC
Paper
Additionally, the fluorescence spectra of HBT-H-H in the THF/
H O systems were also measured, as HBT-H-H is soluble in THF
2
but not in water. The results demonstrated that the fluorescence
intensity gradually increased with increasing volume fraction of
water in the mixtures from 70% to 90% and suddenly improved at
25
9
5% (Fig. 7). Consequently, this HBT derivative is an AIE-active
dye and the nonradiative deactivation pathway was efficiently
suppressed in the aggregated state.
It is well known that the molecular spatial configuration
plays a vital role in determining the stacking mode of a dye
molecule and thus affects the photophysical properties of the
2
6
molecule in the resultant aggregated state. The crystal data
are presented in Table S2 (ESI†), and the crystal structure of
HBT-H-H is shown in Fig. 8. HBT-H-H crystallizes in the
triclinic space group P1%. HBT-H-H adopts a propeller-shaped
Fig. 7 Fluorescence spectra changes of HBT-H-H depending on the
water fractions in THF.
conformation and is not a planar molecule. In the four planar
p-units, the phenol and benzothiazole rings are coplanar with a
mere 5.531 dihedral angle because of the existence of relatively
strong intramolecular six-membered-ring H-bonds. The dihedral
angles between the phenol and the adjacent phenyl rings at the
0
0
4
- and 5 -positions are 50.21 and 50.91, respectively, which were
2
7
caused by the steric effect of biphenyl in the ortho-positions.
It should be noted that although the O–H bond length is
shortened by 0.1738 Å and the Hꢁ ꢁ ꢁN distance is increased by
0.1473 Å when moving from the solution state to the crystalline
state, the overall intramolecular H-bond distance between Oꢁ ꢁ ꢁN
is shortened by 0.004 Å from 2.651 to 2.647 Å. Therefore, the
strength of the intramolecular H-bond was reinforced in the
crystalline state, which facilitated the proton transfer from
oxygen to nitrogen via intramolecular H-bonding.
The restricted intramolecular motion is another central
2
8
f
factor in the improvement of F in the crystalline state. The
packing diagrams of HBT-H-H are shown in Fig. 9. Though the
two phenyls are not coplanar with the phenol ring, the coplanar
HBT moieties of the neighboring molecules are still parallel,
which have two kinds of intermolecular p–p stacking, one
distance is 3.510 Å, the other is 3.660 Å, both of which are
relatively larger than the characteristic distance in the crystal
stacking of the planar p-conjugated systems. Additionally, two
Fig. 8 (A) View of the intramolecular H-bonding interaction of the
HBT-H-H crystal; the distorted angle between the phenol and the phenyl
ring (B) at the 4 - position and (C) at the 5 - position.
0
0
4
9.63% and 4.8181 ns, 35.45-fold and 8.6-fold higher than those kinds of C–Hꢁ ꢁ ꢁp hydrogen bonds are also formed. One bond
measured in toluene solution, respectively (Fig. S9 and S10, ESI†). with a distance of 2.891 Å is formed between the hydrogen atom
Fig. 9 (A) Two kinds of intermolecular p–p stacking in the HBT-H-H crystal; (B) two kinds of C–Hꢁ ꢁ ꢁp hydrogen bonds in the HBT-H-H crystal.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017
New J. Chem.

View Article Online
Paper
of the 5 -position phenyl ring and the p-cloud of the phenol determined by using quinine sulfate (F
NJC
0
f
2 4
= 0.55 in 0.1 M H SO )
ring. The other bond, with a distance of 2.710 Å, is formed as a standard.
0
between the hydrogen atoms of the 4 -position phenyl ring and
the p-cloud of the phenyl ring in the benzothiazole moiety.
Computational methods
These weak forces help rigidify its conformation and hamper the All the calculations were performed using the Gaussian 09
free rotation of the C–C s bond between the phenyl ring and the package. To evaluate the solvent effect, toluene was employed
2
9
phenol ring. As a result, the excited state energy loss via as the solvent in a self-consistent reaction field calculation
the nonradiative relaxation channel is dramatically reduced, which using a polarized continuum model. The ground state (S ) and
) geometries of the enol and
0
makes the molecules emit intensively in the crystalline phase.
the first singlet excited state (S
1
keto tautomers of these HBT-based disubstituted derivatives
were optimized using density functional theory (DFT) and time-
dependent density functional theory (TDDFT) at the B3LYP/
Conclusion
6
(
-31+G(d) level, respectively. The UV-vis absorption of HBT-H-H
vertical excitation) was calculated with the TDDFT method
based on the optimized S state geometry of the enol tautomer.
For the keto emission, the emission wavelengths were calculated
based on the S optimized geometries of the keto tautomers.
We have designed and synthesized four disubstituted HBT
derivatives through Suzuki coupling. TD-DFT calculations reveal
that the maximum absorption band of HBT-H-H is attributed to
the HOMO - LUMO transition of its enol-tautomer, and its
maximum emission band is attributed to the LUMO - HOMO
relaxation of its keto-tautomer. The HOMOs distribute more on
0
1
Crystal structural determination
0
0
the 5 -position phenyl, and the LUMOs more on the 4 -position
phenyl. Thus, the electronic effect of substituents at the
The single crystals of compound HBT-H-H were obtained by the
slow diffusion of CH Cl from solution for several days at room
2
2
0
0
5
-position is opposite to that at the 4 -position. The experimentally
temperature. A single crystal of HBT-H-H with dimensions of
.40 ꢂ 0.30 ꢂ 0.20 mm was selected. The lattice constants and
observed results are in agreement with the theoretical calculations.
Therefore, the shifting trends of keto-emission could be predicted
by theoretical calculations, which would provide valuable informa-
tion for designing other high-performance ESIPT dyes by modifying
the molecular structures. Additionally, these derivatives show
highly efficient keto-emission in the crystalline state. X-ray analysis
indicates that the easier proton transfer through stronger intra-
molecular H-bonds and the restriction of the free rotation of
biphenyl rings via aggregate formation are responsible for this AIE.
0
diffraction intensities were measured at room temperature on a
Bruker SMART APEX-II CCD area detector using graphite-
monochromated Mo Ka radiation (l = 0.71073 Å). Data
reduction and integration were done by the INTEGRATE program
of the APEX2 software. The semi-empirical absorption correction
was applied using the SCALE program. The structure was solved
by direct methods and refined by the full matrix least-squares
2
method on F using SHELX.
Synthesis and characterization
Experimental
Materials
2-Hydroxy-4-iodobenzaldehyde (Compound 1). Compound 1
was synthesized according to the previously reported method.
1
3
1
H NMR (500 MHz, CDCl ) d 11.06 (s, 1H), 9.89 (s, 1H), 7.48
3
3-Iodophenol, phenylboronic acid, 4-cyanophenylboronic acid,
(
s, 1H), 7.44 (dd, J = 8.1, 1.4 Hz, 1H), 7.28 (d, J = 8.1 Hz, 1H).
4-methoxyphenylboronic acid, 2-aminothiophenol and PdCl (PPh
2
3 2
)
5-Bromo-2-hydroxy-4-iodobenzaldehyde (Compound 2). To a
were purchased from Energy Chemical (Shanghai, China). All other
materials were purchased from local commercial suppliers and were
of analytical reagent grade, unless otherwise stated. Solvents were
purified by standard procedures. Ultrapurified water was supplied
by a Milli-Q system (Millipore). All the reactions were monitored by
thin-layer chromatography (TLC) with detection by UV.
solution of Compound 1 (20 mmol, 4.96 g) in 50 mL of acetic
acid was added bromine (20 mmol, 1.04 mL). The reaction
mixture was stirred at room temperature overnight. After that,
more Br
2
(0.2 equiv., 0.2 mL) was added and allowed to react for
another 12 h. The saturated solution of Na S O was added
2
2 3
until the color of Br2 disappeared, and the mixture was
extracted with CH Cl (100 mL ꢂ 2). The combined organic
2
2
Instruments
4
layers were washed with brine and water, dried over MgSO and
1
13
The H and C NMR spectra of the products were recorded on a concentrated. The residue was purified by flash chromato-
Bruker 500 MHz NMR spectrometer in CDCl solution using graphy on silica gel to afford the targeted product as a pale-
tetramethylsilane (TMS) as the internal standard, and chemical yellow powder (3.99 g, 64%). H NMR (500 MHz, CDCl
3
1
3
) d 10.85
1
3
shifts were given in ppm and coupling constants (J) in Hz. High (s, 1H), 9.85 (s, 1H), 7.78 (s, 1H), 7.66 (s, 1H). C NMR (126 MHz,
resolution mass spectra (HRMS) were recorded on a Thermo CDCl ) d 195.09, 159.38, 135.50, 130.13, 121.65, 119.37, 112.28.
3
Scientific LTQ Orbitrap XL using standard conditions (electron
spray ionization, ESI). The UV absorption spectra were obtained Compound 2 (600 mg, 1.84 mmol) was added to DMSO (1.5 mL).
on a UV-vis spectrophotometer (U-3310). The fluorescence Under N atmosphere, 2-aminothiophenol (236 mL, 2.2 mmol) was
spectra were recorded on a fluorospectrophotometer (F-7000). added and the resulting mixture was refluxed for 2 h. The
2-(Benzo[d]thiazol-2-yl)-4-bromo-5-iodophenol (Compound 3).
2
The fluorescence quantum yield (F
f
) in the solution was reaction was cooled to RT. The crude solution was extracted
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017

View Article Online
NJC
Paper
1
with dichloromethane, and the combined organic layer was
H NMR (500 MHz, CDCl
3
) d 12.69 (s, 1H), 8.09 (d, J = 8.0 Hz,
washed with water and dried over MgSO
4
and the solvent was 1H), 7.98 (d, J = 7.5 Hz, 1H), 7.79 (s, 1H), 7.60 (t, J = 6.5 Hz, 1H),
evaporated in vacuo. The residue was further purified by 7.57 (d, J = 8.0 Hz, 2H), 7.50 (t, J = 8.0 Hz, 1H), 7.34–7.32
1
3
column chromatography (PE/DCM, 5/1) to give the targeted (m, 5H), 7.22 (s, 1H), 7.18–7.16 (m, 2H). C NMR (126 MHz,
1
product as a yellow solid. Yield: 594 mg, 75%. H NMR CDCl ) d 168.53, 157.22, 151.86, 145.30, 142.89, 139.70, 132.74,
3
(500 MHz, CDCl
3
) d 12.60 (s, 1H), 8.05 (d, J = 8.0 Hz, 1H), 132.44, 131.84, 130.57, 130.35, 129.89, 128.39, 127.02, 125.89,
7
7
1
1
.98 (d, J = 8.0 Hz, 1H), 7.91 (s, 1H), 7.72 (s, 1H), 7.59 (m, 1H), 122.37, 121.68, 119.46, 118.82, 116.89, 110.96.
1
3
.51 (m, 1H). C NMR (126 MHz, CDCl
51.55, 132.47, 130.44, 129.75, 127.12, 126.21, 122.47, 121.70, 0.24 mmol), 4-methoxybenzeneboronic acid (36.5 mg, 0.24 mmol),
18.67, 118.39, 105.24. CO (76 mg, 0.56 mmol), and PdCl (PPh ) (5/100 equiv., 8.5 mg)
HBT-H-H. Under N2 atmosphere, Compound 3 (200 mg, were added to a solution of dioxane/H O (8 mL/2 mL). The
3
) d 167.15, 156.55,
2
HBT-CN-OMe. Under N atmosphere, Compound 4 (103 mg,
K
2
3
2
3
2
0
.46 mmol), phenylboronic acid (226 mg, 1.8 mmol), K CO
resulting solution was refluxed for 14 h. The reaction mixture
2
3
(
160 mg, 1.15 mmol), and PdCl (PPh ) (5/100 equiv., 16 mg) was then partitioned between CH Cl and water, and the aqueous
2
3
2
2
were added to a solution of dioxane/H
2
O (8 mL/2 mL). The phase was extracted with CH
Cl
2 2
twice. The combined organic
resulting solution was refluxed for 14 h. The reaction mixture layer was evaporated, and the residue was purified by flash
was then partitioned between CH
2
Cl
2
and water, and the column chromatography (PE/DCM, 4/5) to give the targeted
aqueous phase was extracted with CH
organic layer was evaporated, and the residue was purified for C27H N O S (M + H) 435.11617, found 435.11618. H NMR
19 2 2
2
Cl
2
twice. The combined product as a yellow solid (78 mg, 75%). HRMS (ESI) m/z calcd
+
+
1
by flash column chromatography (PE/DCM, 3/1) to give the (500 MHz, CDCl ) d 12.64 (s, 1H), 8.08 (d, J = 8.0 Hz, 1H), 7.97
3
targeted product as a yellow solid (148 mg, 85%). HRMS (ESI) (d, J = 8.0 Hz, 1H), 7.76 (s, 1H), 7.60–7.56 (m, 3H), 7.50 (m, 1H),
+
+
m/z calcd for C H NOS (M + H) 380.11036, found 380.11078. 7.34 (d, J = 8.5 Hz, 2H), 7.19 (s, 1H), 7.07 (d, J = 9.0 Hz, 2H), 6.86
2
5
18
1
13
H NMR (500 MHz, CDCl
H), 7.97 (d, J = 8.0 Hz, 1H), 7.77 (s, 1H), 7.58 (m, 1H), 7.47 168.58, 158.74, 156.97, 151.87, 145.49, 142.84, 132.74, 132.16,
m, 1H), 7.30–7.27 (m, 6H), 7.25 (s, 1H), 7.24–7.21 (m, 4H). 131.78, 130.94, 130.40, 126.94, 125.86, 122.36, 121.67, 119.40,
3 3
) d 12.60 (s, 1H), 8.07 (d, J = 8.0 Hz, (d, J = 9.0 Hz, 2H), 3.86 (s, 3H). C NMR (126 MHz, CDCl ) d
1
(
1
3
3
C NMR (126 MHz, CDCl ) d 168.98, 157.09, 151.97, 145.26, 116.87, 113.83, 55.31.
1
1
40.50, 132.65, 130.38, 129.95, 129.66, 128.03, 127.18, 126.80,
26.57, 125.62, 122.25, 121.63, 119.63, 116.02.
HBT-CN-CN. Under N
0.48 mmol), 4-cyanobenzeneboronic acid (279 mg, 1.9 mmol),
2
atmosphere, Compound 4 (210 mg,
Compound 4. Under N atmosphere, Compound 3 (600 mg, K CO (224 mg, 2.2 mmol), and PdCl (PPh ) (5/100 equiv.,
2
2
3
2
3 2
1
.4 mmol), 4-cyanobenzeneboronic acid (205 mg, 1.4 mmol), 17 mg) were added to a solution of dioxane/H O (8 mL/2 mL).
2
K
4
2
CO
9 mg) were added to a solution of dioxane/H
The resulting solution was refluxed for 14 h. The resulting the aqueous phase was extracted with CH
solution was refluxed for 14 h. The reaction mixture was then combined organic layer was evaporated, and the residue was
partitioned between CH Cl and water, and the aqueous phase purified by flash column chromatography (PE/DCM, 3/5) to give
was extracted with CH Cl twice. The combined organic layer the targeted product (154 mg, 75%) as a yellow solid. HRMS
3
(425 mg, 3.1 mmol), and PdCl
2
(PPh
3
) (5/100 equiv., The resulting solution was refluxed for 14 h. The reaction
O (20 mL/5 mL). mixture was then partitioned between CH Cl and water, and
Cl twice. The
2
2
2
2
2
2
2
2
2
+
+
was evaporated, and the residue was purified by flash column (ESI) m/z calcd for C H N OS (M + H) 430.10086, found
chromatography (PE/DCM, 5/4) to give the targeted product 430.10101. H NMR (500 MHz, CDCl ) d 12.83 (s, 1H), 8.10
2
7
16 3
1
3
(
C
342 mg, 60%) as a yellow solid. MS (ESI) m/z calcd for (d, J = 8.0 Hz, 1H), 8.00 (d, J = 8.0 Hz, 1H), 7.76 (s, 1H), 7.65–7.59
ꢀ
ꢀ
1
20
H10BrN
2
OS (M ꢀ H) 406.98, found 406.95. H NMR (m, 5H), 7.52 (t, J = 7.7 Hz, 1H), 7.32–7.28 (m, 4H), 7.24 (s, 1H).
13
(500 MHz, CDCl
3
) d 12.68 (s, 1H), 8.08 (d, J = 8.0 Hz, 1H),
3
C NMR (126 MHz, CDCl ) d 167.99, 158.03, 151.76, 144.52,
8
7
.02 (s, 1H), 8.00 (d, J = 8.0 Hz, 1H), 7.80 (d, J = 8.5 Hz, 2H), 142.77, 132.65, 132.16, 130.62, 130.19, 127.14, 126.15, 122.48,
.67–7.55 (m, 4H), 7.52 (m, 1H), 7.13 (s, 1H). C NMR (126 MHz, 121.73, 120.01, 118.54, 117.18, 111.60, 110.94.
1
3
CDCl ) d 167.10, 157.13, 151.66, 144.62, 144.28, 132.71, 132.37,
3
1
1
31.98, 130.06, 127.14, 126.22, 122.52, 121.74, 120.36, 118.65,
18.30, 112.09, 110.74.
Acknowledgements
HBT-CN-H. Under N
.49 mmol), phenylboronic acid (72 mg, 0.59 mmol), K
(PPh ) (5/100 equiv., 17 mg)
were added to a solution of dioxane/H O (8 mL/4 mL). The
2
atmosphere, Compound 4 (200 mg,
This study was supported by the National Natural Science Founda-
tion of China (21405122), and the Natural Science Foundation of
Shaanxi (2016JM2009).
0
2
CO
3
(149 mg, 1.08 mmol), and PdCl
2
3
2
resulting solution was refluxed for 14 h. The reaction mixture
was then partitioned between CH Cl and water, and the
2
2
Notes and references
aqueous phase was extracted with CH Cl twice. The combined
2
2
organic layer was evaporated, and the residue was purified by
flash column chromatography (PE/DCM, 2/5) to give the targeted
product as a yellow solid (158 mg, 80%). HRMS (ESI) m/z
1 (a) A. P. Demchenko, K. C. Tang and P. T. Chou, Chem. Soc.
Rev., 2013, 42, 1379; (b) J. Zhao, S. Ji, Y. Chen, H. Guo and
P. Yang, Phys. Chem. Chem. Phys., 2012, 14, 8803;
(c) J. E. Kwon and S. Y. Park, Adv. Mater., 2011, 23, 3615.
+
+
17 2
calcd for C26H N OS (M + H) 405.10561, found 405.10568.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017
New J. Chem.

View Article Online
Paper
NJC
2
(a) Q. Wang, L. Xu, Y. Niu, Y. Wang, M. S. Yuan and 17 X. Zhang and J.-Y. Liu, Dyes Pigm., 2016, 125, 80–88.
Y. Zhang, Chem. – Asian J., 2016, 11, 3454; (b) Q. Wang, 18 (a) G. Granucci, J. T. Hynes and T. H. Tranthi, J. Am. Chem.
L. Xu, Y. Niu, Y. Wang, M.-S. Yuan and Y. Zhang, Dyes Pigm.,
Soc., 2000, 122, 12243; (b) W. Bartok, P. J. Lucchesi and
N. S. Snider, J. Am. Chem. Soc., 2002, 84, 1842.
19 G. Ding, Y. Lu, Y. Gong, L. Ma, Z. Luo, S. Zhang, F. Gao and
H. Li, Tetrahedron, 2016, 72, 3040.
2
2
017, 142, 365; (c) L. Xu, Q. Wang and Y. Zhang, Dyes Pigm.,
017, 136, 732.
3
(a) P. F. Barbara, L. E. Brus and P. M. Rentzepis, J. Am. Chem.
Soc., 1980, 11, 5631; (b) M. Barbatti, A. J. Aquino, H. Lischka, 20 C. L. Chen, H. W. Tseng, Y. A. Chen, J. Q. Liu, C. M. Chao,
C. Schriever, S. Lochbrunner and E. Riedle, Phys. Chem.
Chem. Phys., 2009, 11, 1406.
K. M. Liu, T. C. Lin, C. H. Hung and Y. L. Chou, J. Phys.
Chem. A, 2016, 120, 1020.
4
5
J. E. Kwon, S. Park and S. Y. Park, J. Am. Chem. Soc., 2013, 21 L. Xie, Y. Chen, W. Wu, H. Guo, J. Zhao and X. Yu, Dyes
35, 11239. Pigm., 2012, 92, 1361.
(a) H. Wang, W. Lin, K. P. Fritz, G. D. Scholes, M. A. Winnik 22 (a) S. Dey, A. Efimov, C. Giri, K. Rissanen and
1
and I. Manners, J. Am. Chem. Soc., 2007, 129, 12924;
b) L. Xiong, J. Feng, R. Hu, S. Wang, S. Li, Y. Li and
G. Yang, Anal. Chem., 2013, 85, 4113; (c) K. Y. Chen and
J. W. Hu, J. Lumin., 2015, 159, 171.
H. Lemmetyinen, Eur. J. Org. Chem., 2011, 6226; (b) Y. Qin,
I. Kiburu and A. Shimul Shah, Org. Lett., 2006, 8, 5227;
(c) F. Qi, J. Lin, X. Wang, P. Cui, H. Yan, S. Gong, C. Ma,
Z. Liu and W. Huang, Dalton Trans., 2016, 45, 7278.
(
6
7
(a) Z. Wang, F. H. Yang and R. T. Yang, J. Phys. Chem. C, 23 (a) K. Sakai, S. Takahashi, A. Kobayashi, T. Akutagawa,
2
010, 114, 1601; (b) M. Ikegami and T. Arai, J. Chem. Soc.,
T. Nakamura, M. Dosen, M. Kato and U. Nagashima, Dalton
Trans., 2010, 39, 1989; (b) K.-i. Sakai, H. Kawamura,
N. Kobayashi, T. Ishikawa, C. Ikeda, T. Kikuchi and
T. Akutagawa, CrystEngComm, 2014, 16, 3180.
Perkin Trans., 2002, 2, 1296.
(a) J. Ma, J. Zhao, P. Yang, D. Huang, C. Zhang and Q. Li,
Chem. Commun., 2012, 48, 9720; (b) P. Yang, J. Zhao, W. Wu,
X. Yu and Y. Liu, J. Org. Chem., 2012, 77, 6166.
D. Yao, S. Zhao, J. Guo, Z. Zhang, H. Zhang, Y. Liu and
Y. Wang, J. Mater. Chem., 2011, 21, 3568.
24 (a) T.-C. Fang, H.-Y. Tsai, M.-H. Luo, C.-W. Chang and
K.-Y. Chen, Chin. Chem. Lett., 2013, 24, 145; (b) G. P. Moloney,
R. W. Gable, M. N. Iskander, D. J. Craik and M. F. Mackay,
Aust. J. Chem., 1990, 43, 99.
8
9
J. Cheng, D. Liu, L. Bao, K. Xu, Y. Yang and K. Han, Chem. –
Asian J., 2014, 9, 3215.
25 (a) H. G. Lu, Y. D. Zheng, X. W. Zhao, L. J. Wang, S. Q. Ma,
X. Q. Han, B. Xu, W. J. Tian and H. Gao, Angew. Chem., Int.
Ed., 2016, 55, 155; (b) Y. Qian, S. Y. Li, G. Q. Zhang, Q. Wang,
S. Q. Wang, H. J. Xu, C. Z. Li, Y. Li and G. Q. Yang, J. Phys.
Chem. B, 2007, 111, 5861.
1
0 T. I. Kim, H. J. Kang, G. Han, S. J. Chung and Y. Kim, Chem.
Commun., 2009, 5895.
1 (a) Z. Ning, Z. Chen, Q. Zhang, Y. Yan, S. Qian, Y. Cao and
H. Tian, Adv. Funct. Mater., 2007, 17, 3799; (b) Z. Zhao, S. Chen,
1
C. Deng, J. W. Y. Lam, C. Y. K. Chan, P. Lu, Z. Wang, B. Hu and 26 (a) M.-S. Yuan, D.-E. Wang, P. Xue, W. Wang, J.-C. Wang,
X. Chen, J. Mater. Chem., 2011, 21, 10949.
2 E. L. Eliel, S. H. Wilen and L. N. Mander, Chem. Rev., 1991,
Q. Tu, Z. Liu, Y. Liu, Y. Zhang and J. Wang, Chem. Mater.,
2014, 26, 2467; (b) Z. Zhang, B. Xu, J. Su, L. Shen, Y. Xie and
H. Tian, Angew. Chem., 2011, 50, 11654.
1
1
1
1
1
91, 165.
3 H. Yin, B. Zhang, H. Yu, L. Zhu, Y. Feng, M. Zhu, Q. Guo and 27 F. Qi, J. Lin, X. Wang, P. Cui, H. Yan, S. Gong, C. Ma, Z. Liu
X. Meng, J. Org. Chem., 2015, 80, 4306. and W. Huang, Dalton Trans., 2016, 45, 7278.
4 G. J. Zhao, J. Y. Liu, L. C. Zhou and K. L. Han, J. Phys. Chem. 28 (a) J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu,
B, 2007, 111, 8940.
H. S. Kwok, X. Zhan, Y. Liu, D. Zhu and B. Z. Tang, Chem.
Commun., 2001, 1740; (b) Y. Hong, J. W. Lam and B. Z. Tang,
Chem. Commun., 2009, 4332; (c) J. Liu, J. W. Lam and
B. Z. Tang, Chem. Rev., 2009, 109, 5799.
5 P. Jagadesean, T. Whittemore, T. Beirl, C. Turro and
P. Mcgrier, Chem. – Eur. J., 2017, 23, 917.
6 (a) J. Seo, S. Kim and S. Y. Park, J. Chem. Soc., 2004, 126, 11154;
(b) L. Xie, Y. Chen, W. Wu, H. Guo, J. Zhao and X. Yu, Dyes Pigm., 29 (a) X. Q. Wang, Q. S. Liu, H. Yan, Z. P. Liu, M. G. Yao,
2
012, 92, 1361; (c) K. Benelhadj, W. Muzuzu, J. Massue,
Q. F. Zhang, S. W. Gong and W. J. He, Chem. Commun.,
2015, 46, 7497; (b) J. Mei, Y. Hong, J. W. Y. Lam, A. Qin,
Y. Tang and B. Z. Tang, Adv. Mater., 2014, 26, 5429.
P. Retailleau, A. Charaf-Eddin, A. D. Laurent, D. Jacquemin,
G. Ulrich and R. Ziessel, Chem. – Eur. J., 2014, 20, 12843.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017