Luminogens for Aggregation-Induced Emission via Titanium-Mediated Double Nucleophilic Addition to 2,5-Dialkynylpyridines: Formation and Transformation of the Emitting Aggregates

Article abstract of DOI:10.1002/chem.201905611

New luminogens for aggregation-induced emission (AIE), which are characterized by a branched cross-conjugated 2,6-bis(1,2,2-triarylvinyl)pyridine motif, have been synthesized exploiting the one-pot Ti-mediated tetraarylation of 2,6-bis(arylethynyl)pyridin

Full text of DOI:10.1002/chem.201905611

A Journal of
Accepted Article
Title: Luminogens for Aggregation-Induced Emission via Titanium-
Mediated Double Nucleophilic Addition to 2,5-Dialkynylpyridines:
Formation and Transformation of the Emitting Aggregates
Authors: Lutz Gade, Francesco Foschi, Kevin Synnatschke, Sebastian
Grieger, Wen-Shan Zhang, Hubert Wadepohl, Rasmus R.
Schröder, and Claudia Backes
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To be cited as: Chem. Eur. J. 10.1002/chem.201905611
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Luminogens for Aggregation-Induced Emission via Titanium-
Mediated Double Nucleophilic Addition to 2,5-Dialkynylpyridines:
Formation and Transformation of the Emitting Aggregates
Francesco Foschi,[a] Kevin Synnatschke,[b] Sebastian Grieger,[b] Wen-Shan Zhang,[c] Hubert
Wadepohl,[a] Rasmus R. Schröder,[c] Claudia Backes*[b] and Lutz H. Gade*[a]
Dedicated to Professor Richard R. Schrock on the occasion of his 75th birthday
Abstract:
New
luminogens
for
aggregation-induced
in common polar solvents but display intense fluorescence when
an "antisolvent" is added, which induces molecular aggregation.
Intramolecular rotation and vibration, which enables non-
radiative relaxation pathways in solution, are thought to be
restricted upon aggregation, thus leading to AIE.11–14 Solid-state
and colloidal luminogens, including HPS and TPE derivatives,
have been exploited for various analytical applications, including
detection of metal ions,15–19 pH and small molecules,20–27 and
biomolecules,28–40 as well as mechanofluorochromic sensing.41,42
We recently developed a Ti(OiPr)4-mediated double aryl
Grignard addition to 2-alkynylpyridines and related alkynylated
emission(AIE), which are characterized by a branched cross-
conjugated 2,6-bis(1,2,2-triarylvinyl)pyridine motif, have been
synthesized exploiting the one-pot Ti-mediated tetraarylation of
2,6-bis(arylethynyl)pyridines. Thin layer solid-state emitters were
prepared by spin coating of the luminogens, while AIE-colloidal
dispersions were investigated in terms of optical density and
scattering behaviour. This has given insight into particle size
distributions, time evolution of the aggregation and the influence
of different functionalization patterns on the luminescence of
molecular aggregates. In particular, a combination of extinction
spectroscopy and dynamic light scattering is being proposed as
a powerful method for investigating the dynamic aggregation
process in AIE-type colloids.
N-heterocycles, which could be employed for
a one-pot
synthesis of 2-(1,2,2-triarylvinyl)-pyridines (Scheme 1).43
work:
Ar
Previous
N
1. iPrSSiPr
2. Ti(OiPr)4
Ar
Ar
Introduction
N
H
2O
3. Ar
MgBr
Ar
The development of highly efficient organic solid state emitters
for both fluorescent and electroluminescent light sources is an
important focus of contemporary materials science. While being
potent emitters in dilute solutions, for many common polycyclic
aromatic fluorophores the emission is at least partially quenched
at higher concentration. The formation of aggregates, which may
inter alia involve π-π stacking of aromatic rings, generally results
in aggregation-caused quenching (ACQ) of fluorescence and
many efforts, including functionalization of the fluorophores with
bulky residues, have been described over the years to overcome
this limitation.1–4 In addition, a different class of aromatic emitters,
that luminesce upon aggregation, was first reported by Tang and
collaborators about twenty years ago, including in particular
work:
This
Ar
Ar
Ar
Ar
Ar
Ar
N
N
Ar
Ar
Scheme 1. Ti(OiPr)4-mediated multiple aryl Grignard addition to alkynylated N-
heterocycles based on a 2-alkynylpyridine motive, giving 2-(1,2,2-triarylvinyl)-
pyridines.
propeller-shaped
tetraphenylethylene
(TPE)4–7
and
hexaphenylsilole (HPS)8–10 derivatives, which are non-emissive
The method was a conceptual extension of earlier work on the
carbo- and azaphilic Grignard addition to the nitrile group in
ortho-cyanido N-heterocycles.44 The originally reported protocol
could now be adapted to provide a straightforward synthetic
access to 2,6-bis(1,2,2-triarylvinyl)-pyridines in two steps from
2,6-dibromopyridine via Sonogashira coupling and subsequent
double aryl Grignard addition to the C≡C triple bonds.
The target compounds were found to be AIE-type
luminogens, to an extent which was found to depend on the aryl
substituents. The aim of this work has been the investigation of
the stability and optical properties of the emitting aggregates
using a combination of fluorescence spectroscopy and optical
extinction spectroscopy. Moreover, their time-dependent
evolution has been studied by analyzing the scattering
contribution to the extinction spectra combined with dynamic
[a]
[b]
[c]
Dr. F. Foschi, Prof. Dr. H. Wadepohl and Prof. Dr. L. H. Gade*
Institute of Inorganic Chemistry, Heidelberg University
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
E-mail: lutz.gade@uni-heidelberg.de
K. Synnatschke, S. Grieger, Dr. C. Backes
Applied Physical Chemistry, Heidelberg University
Im Neuenheimer Feld 253, 69120 Heidelberg, Germany
E-mail: backes@uni-heidelberg.de
Dr. W. S. Zhang and Prof. Dr. R. R. Schröder
Centre for Advanced Materials
Heidelberg University
Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
Supporting information for this article is given via a link at the end of
the document.
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light scattering (DLS). Finally, the AIE-properties of the colloidal
dispersions have been compared with the alternative,
aggregation independent, viscochromic enhancement of the
emission intensity.
(Scheme 2, C). In particular, the use of a mild and soft oxidizing
agent, such as isopropyl disulfide, within a one-pot reaction
protocol allows for the use of the Ti reagent either
stoichiometrically or substoichiometrically, the latter giving rise to
the optimized reaction protocol.43 The titanium-mediated double
arylation of each 2-(arylethynyl)pyridine branch results in the
generation of titanacyclopropanes (Ti-icyclopropane) with a strong
titanium(II) π-complex character (Ti-iπ-complex) (Scheme 3).43 The
organic ligand, which is the product of double aryl Grignard
addition, can be liberated by oxidative addition of isopropyl
disulfide (Scheme 3, A), thus allowing for the regeneration of a
titanium(IV) center that can undergo a new reactive cycle by
ligand exchange (Scheme 3, B). This type of reactive behaviour
is key to achieving the desired one-pot tetraarylation of 2,6-
bis(arylethynyl)pyridines under mild conditions.
Results and Discussion
Synthesis and structural characterization of the new 2-
(1,2,2-triarylvinyl)-pyridine based AIE luminogens
Based on our previous work on metal mediated nucleophilic
additions to alkynes and nitriles, the Ti(OiPr)4-mediated
tetraarylation of 2,6-bis(arylethynyl)pyridines was applied to the
synthesis of 2,6-bis(1,2,2-triarylvinyl)pyridines, as shown in
Scheme 2.43
R
R
R
R
R
R
Ar
Ti-i cyclopropane
Ti-i
π
-complex
2 iPrS
A)
A)
PdCl
PdCl (PPh ) (0.040 equiv)
equiv)
2(PPh3)2 (0.040
CuI
equiv)
CuI (0.080 equiv)
(0.080
PPh
PPh (0.080 equiv)
equiv)
3 (0.080
MgBr
Ar
Ar
Ar
Ar
II
TiLn
IV
TiLn
IV
Br
Br
N
N
Br
Br
Ti iPr)
(O
2 Ar
MgBr
4
N
N
+
Ar
Ar
+
N
NEt
NEt (solvent)
3 (solvent)
2-5 days
2-5 days
N
N
50-80%
50-80%
2.4
equiv
2.4 equiv
B)
B)
R
R
R
R
B
R
R
A
R
R
R
R
Ligand
Exchange
iPrSSiPr
iPrSSiPr
iPrSSiPr (3.0 equiv)
e iv)
(3.0
Ti(OiPr)
quiv)
equiv)
4 (1.0
Oxidative Addition
SiPr
IV
LmTi
N
N
N
N
R
R
Ph
Ar
Ar
Ar
Ph MgBr (5.0 eequuiv)
iv)
RMgBr (5.0
Release
Ligand
THF
THF
3 days
3 days
SiPr
N
R
R
R
R
25-50%
25-50%
F
F
F
F
Scheme 3. Proposed role of isopropyl disulfide in the titanium-mediated
tetraarylation of V-shaped 2,6-bis(arylethynyl)pyridines. (A) Liberation of the
product and regeneration of a titanium(IV) species by oxidative addition of
C)
C)
F
F
F
F
N
N
N
N
isopropyl disulfide. (B) Regeneration of
a titanacyclopropane by ligand
exchange and double aryl Grignard addition to the 2-alkynylpyridine motif.
Φs
=
29%
F
F
F
F
2
, 50%, Φs = 29%
2
,
The detailed geometries of the propeller-shaped
tetraarylethylene units in 1–5 were determined by single crystal
X-ray diffraction (Figure 1 & SI). Disorder in the crystal structures
of 3 and 4 reflects alternative orientations of 3-methyl-4-
fluorophenyl and 3-fluorophenyl groups, respectively.
Φ
=
1
47%
45%,
s
1, 45%, Φs = 47%
F
F
F
F
F
F
F
F
F
F
N
N
F
F
F
F
F
F
N
N
F
F
F
F
Φs
=
4
4, 55%, Φs = 30%
F
F
F
F
Φ
=
3
12%
30%,
s
3
, 30%, Φs = 12%
F
F
F
F
F
F
F
F
F
F
F
F
N
N
N
N
F
F
F
F
F
F
F
F
F
F
F
F
Φ
Φs
=
18%
=
6
6
5
5
4%
, 46%, Φs = 4%
25%,
s
, 25%, Φs = 18%
Scheme 2. (A) Synthesis of V-shaped 2,6-bis(arylethynyl)pyridines by double
Sonogashira coupling. (B) Synthesis of 1–6 by Ti(OiPr)4-mediated quadruple
aryl Grignard addition to 2,6-bis(arylethynyl)pyridines. (C) Compounds 1–6;
given are isolated yields and the emission quantum yields of spin coated films.
From the commercially available 2,6-dibromopyridine
a
straightforward 2-step protocol was employed for the synthesis
of target compounds 1–6: a double Sonogashira coupling with
differently substituted arylacetylenes followed by Ti(OiPr)4-
mediated quadruple nucleophilic arylation of the bis(alkynes).
This yielded the 2,6-bis(1,2,2-triarylvinyl)pyridine derivatives 1–6
Figure 1. Molecular structures of the 4-fluorophenyl substituted compound 1
(top, left), the 3-methyl-4-fluorophenyl substituted compound 3 (top, right), the
3-fluorophenyl substituted compound
difluorophenyl substituted compound
4
5
(bottom, left) and the 3,5-
(bottom, right), illustrating the
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propeller-shaped geometries of the tetraarylethylene units involved [fluorine
atoms: green; nitrogen atoms: blue].
neighboring molecule (twist angle: 79.30°; Cg-Cg’ contact
The propeller-shaped arrangements of the tetraarylethylene
units observed in the crystal structures of all five compounds are
consistent with the previously identified structural criteria for the
observation of AIE. The presence of the para-fluorinated phenyl
groups on the pyridine-cored backbone of 1 is associated with
the highest luminescence quantum yield (ΦS=47%, Scheme 1) in
the 1–6 series. In the structure of 1, ethylene C=C bond lengths
of 1.348~1.354 Å and C-F bond lengths of 1.360~1.365 Å fall
within the range previously reported for aromatic luminogens of
related structure (Table 1).45
length: 3.868 Å, see SI, Figure S2). These findings may play a
role in the reduced luminescence quantum yield of 2 (ΦS=29%)
compared to the performance of 1 (vide infra).
Table 1. Selected inter- and intramolecular interactions in single crystals of
1
X···Y
Distance
(H···Y)/Å
Distance
(C···Y)/Å
Angle
(C-H···Y)/°
Figure 2. Hirschfeld surface mapped over dnorm (top), showing the length of
intermolecular contacts as a color gradient between red (shortest contacts)
and blue (longest contact). Propeller-shaped structure of 1 (bottom), where
selected intra- and intermolecular contacts have been highlighted for clarity.
Cg1: Centroid(C20, C21, C22, C23, C24, C25); Cg2: Centroid (C34, C35, C36,
C37, C38, C39).
C(18)-H(18)···F(6)inter
C(44)-H(44)···F(2)inter
C(45)-H(45)···F(4)inter
C(3)-H(3)···F(5)inter
2.443
2.634
2.584
2.669
2.799
2.785
3.043
3.248
3.281
3.544
3.389
3.305
3.395
3.406
3.982
4.164
146.93
160.76
142.64
124.85
121.67
123.75
169.98
162.58
C(4)-H(4)···F(1)inter
Finally, intermolecular interactions in the crystal network of
the meta-substituted 3–5 may be mostly related to
intermolecular H···F contacts, with only minor contribution of
C(42)-H(42)···F(1)inter
C(33)-H(33)···Cg(1)intra
C(13)-H(13)···Cg(2)intra
As recently noted for fluorinated aromatic AIE-luminogens,
intermolecular aggregation based on weak F···H and C-H···π
interactions may give rise to ordered supramolecular units,
which in turn restricts the roto-vibrational freedom of the aryl
groups.45 This effect may counterbalance the higher tendency of
fluorinated aromatics to undergo π-π stacking-mediated ACQ,
resulting in an overall enhanced fluorescence intensity upon
aggregation.4,45,46
Two intramolecular C-H···π contacts (C(33)···Cg1 and
C(13)···Cg2 distances of 3.982 and 4.164 Å, respectively, Table
1) and several intermolecular C-H···F interactions of similar
strength may be viewed as a particularly effective source of
restriction of intramolecular motion (RIM) and consequent boost
of the luminescence quantum yield (Figure 2).
C-H···π contacts. The observed luminescence quantum yields
as low as ΦS(5) = 4% and ΦS(3) = 12% can also be interpreted
as the consequence of reduced RIM due to a more pronounced
disorder in the solid state and, at the same time, a possibly
enhanced ACQ due to multiple functionalization of the aromatic
rings.52
In contrast, in spite of a loose packing of the aromatic rings
in the solid state, the observed intra- and intermolecular contact
distances are still consistent with moderate ACQ due to π−π
interactions.47 The overall fluorescence intensity (vide infra) can
therefore be seen as the net outcome of competing RIM and
ACQ processes upon aggregation.45 In order to visualize the
intermolecular interactions in the crystal, normalized contact
distances (dnorm) were mapped onto the molecular Hirschfeld
surface (Figure 2).48–51 As expected, the local minima of dnorm are
often associated with fluorine substituents involved in weak
intermolecular H···F bonds (Figure 2). The lack of fluorine
substituents in 2 and therefore the absence of intermolecular
H···F contacts is expected to result in a less pronounced RIM.
Moreover, one of the six phenyl substituents of 2 is involved in
parallel-displaced π-π stacking with another phenyl group from a
The AIE properties of the para-fluorophenyl substituted
compound 1: Luminescence in a solvent/antisolvent system
In order to characterize the AIE-type luminescence in more
detail, we investigated aggregated colloids in liquid suspension.
Aggregation was induced by adding water as an antisolvent to a
solution of 1 in acetonitrile (Figure 3A). The photoluminescence
(PL) intensity of 1 was found to be negligible in diluted
acetonitrile solutions, as well as in water/acetonitrile mixtures
with a water content below 50%. A strong monotonic increase of
the PL intensity (up to a factor of ~300) was observed with
increasing water content in mixtures ranging from 50% to 90%,
indicating aggregation induced emission. The extent of the boost
in emission intensity can be extracted from photoluminescence
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resonant regime on a double logarithmic scale. The linear decrease in E is
characteristic of scattering.
excitation (PLE) contour plots of the mixtures with a water
content of 50% and 90% shown in Figure 3B,C, respectively (for
more data, see SI, Supplementary Figure S3). The emission
maximum is centered at ~470 nm in both cases, giving rise to
the blue color in Figure 3A. The maximum in the excitation shifts
from ~310 nm for 50% volume percent (v/v) of water to ~325 nm
for 90% H2O.
This contrasts with the spectra of the 90% v/v H2O sample
(Figure 3E) in which about ~1/3 of the measured extinction in the
resonant regime is due to contributions from scattering. The
scattering spectrum in the resonant regime is red-shifted with
respect to the pure absorbance, but similar in shape, leading to
an apparent bathochromic shift of the peak positions.53,54
In addition, scattering is manifested in the extinction
spectra in the non-resonant regime, where it decreases with
wavelength in intensity as a power law.53,54 The power law
scaling can be best discerned when plotting the spectral region
of the non-resonant regime on a double logarithmic scale (insets
in Figure 3D-E), the linear scaling being characteristic for non-
resonant scattering. The aggregation process itself may be
followed in more detail by recording time-dependent extinction
spectra.
Particle formation and AIE: Wavelength dependent optical
extinction
Wavelength-dependent optical extinction spectra can contain
rich information on the number and size of particles due to light
scattering in the non-resonant regime. This approach was taken
to analyze the evolution of the particles sizes in the aggregation
of 1 and its impact on the PL response. The overall extinction
ε(λ) of colloidal suspensions at a given wavelength λ arises as
the sum of the absorption α(λ) and scattering σ(λ):
ε(λ) = α(λ) + σ(λ)
Time dependence of aggregation and particle formation
In Figures 4A-C extinction spectra of 1 are shown, measured at
various time intervals after preparing the acetonitrile/water
mixtures (also see SI, Supplementary Figure S4).
Both components were deconvoluted with the aid of an
integrating sphere which allowed to collect scattered light and
thus to record the true absorbance spectra (for the latter, see
also SI). The scattering background σ(λ) was determined as the
difference between the measured extinction ε(λ) and
absorbance values α(λ) at a given wavelength. Examples for
50% and 90% v/v H2O mixtures are shown in Figure 3D,E. The
absorbance spectra of 1 are generally characterized by a broad
band over a spectral range of 300-350 nm with an unresolved
vibronic fine structure.
50% H2O
For the 50% v/v H2O sample, absorbance and extinction
measurements agree well (Figure 3D) which is consistent with
the absence of solid bodies in suspension and hence negligible
scattering in this homogeneous system.
70% H2O
A
60 %
H2O 0 % 10 % 20 % 30 % 40 % 50 %
B
70 % 80 % 90 %
90 % H2O
C
50 % H2O
C
F
90% H2O
D
E
Figure 4. A-C) Optical extinction spectra of 1 in different acetonitrile-water
mixtures measured at various time intervals after preparation. A) 50% v/v H2O,
B) 70% v/v H2O, C) 90% v/v H2O. D) Plot of the optical extinction at 280 nm
(resonant regime) as function of time. E) Plot of the optical extinction at 450
nm (non-resonant regime) as function of time. F) Plot of the scattering
exponent (from 400-650 nm) as function of time. The horizontal broken lines in
E and F are guides for the eye.
Figure 3. A) Photograph of solutions of 1 in different water/acetonitrile
mixtures under UV irradiation showing pronounced increase in
photoluminescence intensity at water volume percentages above 50%. B,C)
Photoluminescence excitation contour plots of 1 in 50% v/v H2O (B) and 90%
v/v H2O (C). D-E) Absorbance, extinction and scattering spectra of 1 in 50%
v/v H2O (D) and 90% v/v H2O (E). The insets show the scattering in the non-
a
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exponent indicating the formation of larger particles and vice
versa.53 To this end, the extinction in the 400–650 nm range was
plotted on a double logarithmic scale and fit by linear regression
(Supplementary Figure S5). The slope of this fit corresponds to
the respective negative scattering exponent, and the time
evolution of these exponents for the different solvent mixtures
(50, 70 and 90% v/v H2O in acetonitrile) are shown in Figure 4F.
Scattering exponents are in the range of ~2 and 4 as
would be expected from Mie theory53 (see SI for details). In the
case of the 70% v/v H2O solvent mixture, a sharp decrease in
the scattering exponent is observed within the first 8h after
mixing the solvents, implying a rapid growth of particles, which
eventually precipitate from the suspension. This accounts for the
decrease in extinction in both the resonant (Figure 4D) and non-
resonant (Figure 4E) regime noted above. In case of the 90%
v/v H2O solvent mixture, the scattering exponent drops slightly
before equilibrating at ~8h which is consistent with a slow and
minor growth of the suspended particles. Overall, these findings
suggest the formation of larger particles in solvent mixtures with
an intermediate water content of 70%, which is consistent with
previous reports by Tang and collaborators for related aromatic
AIE-luminogens.8,55
Neither the extinction nor the barely detectable PL of 1 change
over time in the water:acetonitrile = 50:50 mixture (Figure 4A)
which is consistent with negligible aggregation. This solvent
mixture may be considered as the threshold mixture triggering
aggregation (see also Figure 3A) in the water/acetonitrile system.
In contrast, a strong apparent red-shift of the main extinction
signal from 330 nm to around 360nm, a general broadening of
the spectral profile and non-negligible extinction in the non-
resonant regime (λ>400 nm, due to scattering) take place within
a few hours after the preparation of the sample for 70% v/v H2O
(Figure 4B). Finally, a moderate red-shift is observed over time
in the 90% v/v H2O mixture, together with a slight increase of the
optical extinction on the long wavelength slope of the non-
resonant regime (Figure 4C).
The extinction in the resonant regime can be used to monitor
the amount of 1 sedimenting from the suspension because the
scattering follows the absorbance in shape, and therefore
extinction (as well as absorbance) can be used to track the
concentration of 1. Figure 4D depicts the time-dependence of
the extinction at 280 nm, i.e. the resonant regime, indicating that
concentration of dissolved 1 drops roughly exponentially with
time in the 70% v/v H2O solvent mixture, while it is essentially
invariant for both the 50% v/v H2O and 90% v/v H2O solvent
mixtures.
Determination of particle size distributions by dynamic light
scattering
In addition, the measurement of optical extinction at a given
wavelength in the non-resonant regime can be used to indicate
the evolution of the scattering strength.53 Figure 4E displays a
plot of the extinction at 450 nm as a function of time. In the 50%
v/v H2O solvent mixture, the extinction remains approximately
zero due to the absence of larger aggregates. In contrast, the
extinction in the 70% v/v H2O solvent mixture increases at first,
before falling off rapidly at t>8h. This suggests the initial
formation of large aggregates (with higher scattering strength) in
the first few hours, which then precipitate. This gives rise to the
drop in extinction as well as the decreasing chromophore
concentration observed in Figure 4D in the resonant regime for
this solvent mixture. In contrast, the extinction at 450 nm
increases only slightly over time in the 90% v/v H2O mixture,
suggesting a slight increase in aggregate content (or size) with
time, mirroring the essentially constant extinction in the resonant
regime (Figure 4D). Both observations indicated the presence of
an overall stable colloid in that particular solvent mixture.
To obtain quantitative evidence for the conclusions proposed
above, dynamic light scattering (DLS) measurements were
conducted on 1 in 50%, 70% and 90% v/v H2O solvent mixtures
as a function of time. A subset of the data is shown in Figure 5
(the complete data set is provided in the SI, Supplementary
Figure S6). In good agreement with the analysis of the extinction
spectra, DLS confirms that the mean particle size of the colloids
of 1 increases within the first two hours from 215 nm to 530 nm
in the case of the 70% v/v H2O solvent mixture (Figure 5A). After
~8h, the average size of the colloids is smaller (145 nm) than the
initial size. The extinction spectra suggest that this is due to
selective precipitation of larger aggregates, leaving a minor
fraction of smaller particles in suspension. In contrast, a
moderate increase of the mean size of the colloids of 1 is
observed for the 90% v/v H2O solvent mixture (Figure 5B) from
~130 nm to ~160 nm. Importantly, even such a subtle change in
aggregate size can be well captured by changes in the
scattering coefficient as represented in Figure 4E,F. The overall
evolution of particles size over time is shown in Figure 5C.
It is possible to derive information on aggregate size by
analyzing the scattering exponent in the non-resonant regime
B
A
C
70 % H2O
90 % H2O
which changes systematically with particle size, a smaller
Overall, DLS strongly supports the conclusions drawn from the
Figure 5. Aggregate size according to dynamic light scattering as function of time of 1 in two different acetonitrile/water mixtures. A) DLS size for the 70% v/v
H2O solvent mixture. B) DLS size for the 90% v/v H2O solvent mixture. C) Evolution of the DLS size as function of time for both mixtures.
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extinction spectroscopic analysis, demonstrating the rich
information encoded in optical extinction spectroscopy and its
potential to systematically investigate aggregate formation of
discrete molecules.
Optical and scanning electron microscopy of the particles
formed in the aggregation process
Optical microscopy (OM) and scanning electron microscopy
(SEM) further confirmed the findings discussed above. In a
freshly prepared mixture of 1 in a 70% v/v H2O mixture, optical
microscopy confirmed the presence of small, uniformly
distributed, luminescent deposits along with few bigger particles
(Figure 6A). According to SEM, these initial aggregates are
amorphous (Figure 6B). During incubation of the suspension at
room temperature, the average size of the aggregates increased
and the fluorescence emission blue shifted, according to optical
microscopy (Figure 6C). Micrometer sized solid particles of 1,
including crystals of regular shape and dimensions, were
collected together with essentially bi-dimensional aggregates
(Figure 6D) from the mixture with a 70% v/v (H2O) by drop
casting under mild conditions. Notably, the fraction of crystalline
material in the sample was found to grow over time and
accompanied the observed separation and sedimentation of
solid material from the mixture.
Figure 7. Amorphous, crystalline and bi-dimensional aggregates after three
hours of incubation in a 70% v/v H2O mixture at room temperature.
Aggregation-induced emission vs viscochromicity
As discussed above, the observed AIE of the 2,6-bis(1,2,2-
triarylvinyl)pyridines may be understood in terms of frozen
intramolecular motion upon aggregation and thus suppression of
associated non-radiative deactivation pathways. An alternative
approach to slow down intramolecular mobility is based on the
increase in viscosity of the surrounding medium, i. e. the solvent
system. We therefore compared the response to solutions of
compound 1 to anti-solvent addition, on the one hand, and to
addition of a viscous co-solvent, on the other.
A) 70 % H2O fresh
B) 70 % H2O fresh
Figure 8A,B summarizes the aggregation-induced emission
of 1 in water/acetonitrile mixtures immediately after preparation
of the samples. As seen visually from the photograph in Figure
3A, the PL increases abruptly after a certain threshold of water
5 μm
100 nm
content, when aggregation occurs (Figure 8A). Above
a
C) 70 % H2O aged
D) 70 % H2O aged
threshold of ~60%, a linear increase in PL intensity is observed
(Figure 8B). The investigations described above clearly
demonstrated that aggregates are larger at intermediate water
contents, on the one hand, and that a linear scaling of PL with
increasing volume fraction of water can be observed, on the
other. This strongly suggests that the PL enhancement is
independent of the size of aggregates and rather a fingerprint of
aggregation strength, i.e. intermolecular interaction, which can
be understood in terms of RIM.
100 nm
5 μm
As indicated, such a RIM of AIE-type luminogens may also
be induced by increasing the viscosity of the solvent system. In
this case, aggregation-independent fluorescence enhancement
arises from the restriction of rotational and vibrational
intramolecular motion, resulting from increased solvent viscosity
(Figure 8C,D).8,55
Figure 6. Solid amorphous particles of 1. A-B) Freshly prepared
water:acetonitrile = 70:30 suspension imaged with optical microscopy (A,
excitation wavelength: 385 nm) and SEM image (B). C-D) Images after 48h of
incubation at room temperature. The presence of larger aggregates emitting at
lower wavelegths is observed in optical microscopy (C, excitation wavelength:
385 nm), while crystalline and thin layered particles of 1 are seen in SEM (D).
This aggregation-independent viscochromic behaviour of 1
was investigated measuring the PL intensity of its DMSO
solution upon addition of different amounts of glycerol, the
viscosity of latter being approximately 700 times greater than
that of the former. The fluorescence intensity of the solution
indeed increased upon increasing the amount of glycerol in the
mixture (Figure 8C and Supplementary Figure S7). In contrast to
Amorphous, crystalline and bi-dimensional aggregates coexist in
the system three hours after preparation of the mixture, as
clearly shown in Figure 7.
In summary, the characterization of the colloidal systems
formed from 1 in acetonitrile-water mixtures established the
aggregation-induced emission, with aggregate size scaling non-
linearly with the content of the antisolvent and a characteristic
time evolution. In each case, the PL data can be heavily
influenced by changes in concentration and particle size.
AIE, which occurs abruptly at
a given threshold in the
antisolvent/solvent ratio (Figure 8B), the PL increase displayed
exponential dependence on the glycerol content in the range
20–90%, as indicated by the exponential fit in Figure 8D.
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B
A
Influence of para- and meta-substitution patterns on the
aggregation
Having analyzed the aggregation-induced emission of
compound 1, we performed a comparative study for the 2,6-
bis(1,2,2-triarylvinyl)pyridine derivatives 1–5 assessing the
formation of the colloidal aggregates of all five compounds in
terms of the magnitude of the PL increase and the water content
threshold for aggregation (Supplementary Figure S8-14).
Figure 9A displays a plot of the relative PL increase of all
compounds under study as
a function of water content,
D
C
determined immediately after addition of the anti-solvent. As
delineated above, the data could be fitted to a linear function
above the aggregation threshold, allowing the determination of
the onset at which aggregation occurred and the enhancement
factor (extrapolated to 100% water content, Figure 9C). The
onset of AIE was found to occur at around 60% v/v H2O (Figure
9B) in all cases, except compound 3, thus suggesting reduced
solubility of the 4-fluoro-3-methylphenyl functionalized 3 even at
very low water content. However, the magnitude of PL
enhancement (Figure 9C) was found to be strongly dependent
on the structure, the largest PL enhancement factor of > 400
being found for the phenyl-based luminogen 2, followed by the
para-fluoro substituted 1 with an enhancement > 300. Meta-
functionalization of the aryl groups in 3, 4 and 5 appeared to
hamper the increase of the PL intensity upon aggregation to
some extent.
Notably, coexistence of two conformers of 4 in a C6D6
solution was shown by 19F{1H} NMR (see SI, Supplementary
Figure S1). Moreover, in the X-ray diffraction structures of 3 and
4, disorder in the meta-functionalized aryl groups indicated the
coexistence of different rotamers which differ for a 180° rotation
of the rings about their spinning axis (Figure 1). Such disorder
can obviously not result from rotation of the aryl groups in 1 and
2 for symmetry reasons.
Figure 8. A-B) Aggregation induced emission of 1 enforced by the addition of
water to 1 dissolved in acetonitrile. A) PL spectra (excitation at 330 nm)
normalized to the maximum at 90% v/v H2O. The normalized absorbance
spectrum of 1 in acetonitrile is added. B) Evolution of the PL intensity as
function of water volume fraction normalized to the PL intensity in acetonitrile.
C-D) Viscochromic luminescence enhancement of 1 enforced by addition of
glycerol to increase solvent viscosity. C) Normalized PL spectra (excitation at
330 nm). D) Evolution of the PL intensity as function of glycerol volume
fraction normalized to the PL intensity in DMSO.
The classic Grunberg-Nissan mixing rule predicts an
exponential relationship between the relative amount of solvents
in the ideal mixture and the viscosity of the system.56 Thus, the
changes in PL intensity can be easily and quantitatively linked to
changes in viscosity in the nearly ideal DMSO/glycerol mixture,
with the potential of viscometric application of the luminogen.57
In absolute terms, the photoluminescence is enhanced by a
factor of > 300 through the formation of aggregates with aid of
water as antisolvent, while the viscochromic effect is significantly
weaker resulting in a ~ 60-fold increase compared to the initial
solution.
Meta-functionalization of the aryl rings appears therefore to
be associated with a less pronounced increase of fluorescence
intensity upon aggregation and lower luminescence quantum
yields for spin coated solid films, as expected in case of reduced
RIM in the solid state. Notably, the meta-arrangement of the
fluoro-substituents on aryl substituents of 4 and 5 did not spectra
compared to the para-fluorinated aryl substituents of 1, while
A
C
B
Figure 9. A) Evolution of the PL intensity as a function of water volume fraction, normalized to the PL intensity, in acetonitrile for compounds 1–5. B) PL
threshold of AIE. (C) Enhancement factor extrapolated from the fits in (A) for the different compounds at 100% water content.
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major deviations were observed for 2 and 3.
In particular, a water content of 70% or 60% in the solvent
mixture induced a dramatic change in the time-zero spectral
profile of
2 and 3, respectively, when compared to the
corresponding spectra of 1, which consisted of an apparent red-
shift of the main extinction signal from 330 to 345 – 355nm, a
general broadening of all the signals and very strong extinction
in the non-resonant regime (Figure 10). These features are a
clear fingerprint of a significant contribution of light scattering to
the extinction spectra. Generally, the presence of phenyl (2) or
4-fluoro-3-methylphenyl (3) substituents appears to result in a
more rapid formation of highly scattering suspensions, in
comparison to 1.
F
F
F
F
Figure 11. Time evolution of PL emission of 3 in a water/acetonitrile mixture
N
N
with a 70% v/v (H2O).
2
3
F
F
Table 2. Selected emission maxima of 1–4
λ
MAX(1)/n
m
λ
MAX(2)/nm
λ
MAX(3)/nm
λ
MAX(4)/nm
FilmSPIN COATED
90% v/v H2O t0
90% v/v H2O 24h
70% v/v H2O t0
70% v/v H2O 24h
464
471
466
466
463
493
479
474
471
470
468
471
468
466
420
464
468
470
452
445
Figure 10. Time-zero extinction spectra of
2 (left) and 3 (right) in
acetonitrile/water mixtures.
In addition, a pronounced blue-shift in the fluorescence
emission spectra of 2, 3 and 4 was observed over time in
mixtures with an intermediate water content (60–80%,
Supplementary Figures S11-13). The effect is particularly
pronounced for 3, where a blue shift of around 60 nm takes
place gradually within the first 5 hours after preparation of the
sample (Figure 11). Only limited sedimentation of fluorescent
particles and no severe drop in the PL intensity were observed
48 h after preparation of the sample, as opposed to the
corresponding suspension of 1, thus suggesting the formation of
a relatively stable colloid of 3 at intermediate water contents.
This is consistent with the main features of the optical extinction
profile of the system, where a gradual shift of the main extinction
band towards longer wavelength values and an increase of the
scattering background in the non-resonant regime can be
observed in aged samples (24 h, 48 h) of 3 as a result of the
absence of substantial sedimentation of the luminogen (see SI,
Supplementary Figure S12).
Conclusion
The titanium-mediated one-pot quadruple aryl Grignard addition
to 2,6-bis(arylethynyl)pyridine triple bonds has been employed
as
a convenient method to synthesize various bis(1,2,2-
triarylvinyl)pyridine derivatives that act as luminogens for
aggregation-induced emission. The synthetic method itself is
straightforward and may be extended to other target compounds
in future studies.
The colloidal dispersions of the luminogens have been
investigated and thereby insight into particle size distributions,
time evolution of the aggregation and physical stability of the
suspensions was obtained. Spin coating of the luminogens in
solid films was used for the preparation of solid-state emitters
with low to moderate quantum yields. Finally, an exponential
aggregation-independent relationship between viscosity of the
medium and fluorescence intensity has been determined.
It is evident that the exploration of the optical properties of
AIE-type colloids by straightforward spectroscopic methods may
provide the basis for a better understanding and a finer tuning of
AIE-based analytical methods of practical relevance, thus
improving reliability and reproducibility of AIE-sensing systems.
This will be the aim of future work on these systems in our
laboratory.
Finally, the comparison between the wavelength of
maximum emission of 1–4 as colloids and spin-coated thin films
is presented in Table 2. With the exception of 2, the spin-coated
and colloidal luminogens with
a 90% water content are
characterized by emission maxima at comparable wavelengths,
thus suggesting similar molecular and packing geometries in the
two aggregated states, both resulting from rapid aggregation of
the luminogen on the one hand. On the other hand, in line with
what was observed for 1 and 3 (Figure 11), emission of colloids
of 4 with a 70% water content and of 2 with a 80% water content
(see SI, Supplementary Figures S11, S13) is blue-shifted, as
expected for a different, possibly crystalline character of slowly-
forming aggregates.4
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Experimental Section
single droplets of the mixture, deposited on the same spot of the wafer
surface, by use of absorbent filter paper at room temperature.
All reactions were carried out in oven-dried flasks (150 °C) under dry
inert gas atmosphere, according to Schlenk standard techniques. 1H and
13C{1H} NMR spectra were recorded in benzene-d6 at room temperature
on a Bruker Avance II (400MHz) or a Bruker Avance III (600MHz)
spectrometers. Chemical shifts are reported in parts per million (ppm)
and referenced internally to the benzene residual proton or carbon
signals (δ = 7.16 or δ = 128.06 ppm, respectively). 19F{1H} NMR spectra
were recorded in benzene-d6 at room temperature on a Bruker Avance II
(400MHz). Chemical shifts are reported in parts per million (ppm) and
referenced to an external standard (CFCl3). High resolution mass spectra
were acquired on Brucker ApexQe Hybrid 9.4 T FT-ICR and JEOL JMS-
700 magnetic sector (EI, LIFDI) spectrometers.
X-ray diffaction study: Full shells of intensity data of single
crystal X-ray diffraction analysis were collected at low temperature with a
Bruker AXS Smart 1000 CCD diffractometer (Mo-Kα radiation, sealed X-
ray tube, graphite monochromator) or an Agilent Technologies
Supernova-E CCD diffractometer (Mo-Kα radiation, microfocus X-ray
tube, multilayer mirror optics). Molecular structures of 1,3–5 have been
included in the main text: Nitrogen atoms: blue. Fluorine atoms: green.
Thermal ellipsoids set at the 50% probability level. H-atoms have been
omitted for clarity. For the discussion of compound 1 (Table 1, Figure 2)
bond lengths and angles were calculated with normalized hydrogen
positions.58 For further details see supporting Information.
General Procedure (GP-1) for the preparation of 2,6-bis(1,2,2-
triarylvinyl)pyridines.
To a solution of a 2,6-bis(arylethynyl)pyridine in dry THF (2.0 mL for each
mmol of pyridine) isopropyl disulfide (3.0 equiv) and Ti(OiPr)4 (1.0 equiv)
were added consecutively while stirring the solution at room temperature.
After cooling the mixture to –78 °C, a THF solution of arylmagnesium
bromide (5.0 equiv) was added dropwise, the cooling bath was then
removed and the mixture was stirred at 40 °C for 3 days, hence turning
black quickly. The reaction vessel was then cooled to rt in a water bath,
distilled water (0.5 mL for each mmol of pyridine) was added dropwise
and the mixture was stirred for 1 h. After addition of THF (5.0 mL) and
dichloromethane (30 mL) the mixture was stirred vigorously for 2 h. The
white precipitate was removed by filtration, the crude product was
adsorbed over celite and purified by column chromatography (SiO2,
eluent gradient: petroleum ether:ethylacetate = 100:1 to 25:1, 0.25 vol%
NEt3). After evaporation of the volatiles and recrystallization in ethyl
acetate/pentane, products 1–6 were obtained as white solid.
Recrystallization in benzene/pentane gave single crystals of 1–5 suitable
for X-ray diffraction analysis.
Compound 1. Colourless solid, 3.1 g, 4.5 mmol, 45%. Alkyne: 2,6-
bis((4-fluorophenyl)ethynyl)pyridine (3.2 g, 10 mmol); iPrSSiPr (4.5 g, 4.8
mL, 3.0 equiv); Ti(OiPr)4 (2.8 g, 3.0 mL, 1.0 equiv); ArMgBr: 4-
fluorophenylmagnesium bromide (1.0 M THF solution, 50 mL, 5.0 equiv).
1H NMR (600.13 MHz; C6D6; 295.2 K): δ [ppm] = 6.86 – 6.55 (m, 21 H),
6.55 – 6.50 (m, 6 H). 13C{1H} NMR (150.90 MHz; C6D6; 295.7 K): δ [ppm]
Starting materials: Synthetic procedures of the double
Sonogashira coupling for the synthesis of the precursors (2,6-
bis(arylethynyl)pyridines) have been included in Supporting Information.
2,6-bis(phenylethynyl)pyridine
and
2,6-bis((4-fluorophenyl)ethynyl)-
pyridine are known compounds and have been previously
characterized.43
= 162.3 (d, 1JCF= 247.4 Hz), 162.13 (d, 1JCF= 247.1 Hz), 162.05 (d, 1JCF
=
4
4
247.4 Hz), 161.9, 140.5, 140.3, 139.4 (d, JCF= 3.3 Hz), 138.4 (d, JCF
=
Optical extinction and absorbance measurements were carried
out with an Agilent Cary 6000i spectrometer in quartz cuvettes. The
spectrometer was equipped with an integrating sphere (external DRA-
1800) for absorbance measurements. In this case, the cuvettes were
placed in the center of the sphere. The measurements of both, extinction
and absorbance spectra allow for the calculation of scattering spectra
(Scattering = Ext − Abs). All spectra were acquired with 0.5 nm
increments and 0.1 s integration time.
Fluorescence measurements were carried out in quartz cuvettes
at 20 °C using a Fluorolog-3 Horiba Scientific Fluorescence spectrometer
equipped with a Syncerity PMT detector and a 450 W xenon light source
for excitation. Typical acquisition parameters were 0.2 s integration time
in a range of 350-645 nm with increments of 0.5 nm. Light at 330 nm was
used for excitation and the selected bandpass was 3 nm for both,
excitation and emission. The PLE maps were acquired with excitation
wavelengths between 250 and 400 nm and emission between 350 to
650 nm. A 400 nm cut off filter was used to avoid second order
harmonics for the PLE mapping.
Solid state emission spectra were recorded using an integrating
sphere (diameter 6’’, coated with Spectraflect®) on a Jasco FP-6500
spectrofluorometer. Spin coating of the luminogens from DCM solutions
(10 mg/mL; 25 °C, 1500 rpm) has been used for the production of thin
films of 1–6 on regular glass. Photoluminescence Quantum Yields (PLQY,
ΦS) were calculated with the FelixGX software (PTI). All operations have
been performed in the laboratory of Prof. Dr. Uwe H. F. Bunz, Institute of
Organic Chemistry, Heidelberg University.
Dynamic light scattering experiments were conducted with a
Zetasizer Nano ZSP from Malvern Panalytical equipped with a 633 nm
HeNe laser. Measurements were performed in a Glass cuvette at an
173° backscatter angle with general purpose analysis. The viscosity of
water was assumed to be the sample viscosity due to the generally high
fraction of water in the samples (1.0031 mPa·s at 20 °C).
4
3
3.4 Hz), 138.0 (d, JCF= 3.4 Hz), 135.7, 133.12 (d, JCF= 7.8 Hz), 133.08
(d, 3JCF= 7.7 Hz), 132.7 (d, 3JCF= 7.8 Hz), 123.8, 115.3 (d, 2JCF= 21.5 Hz),
2
2
115.2 (d, JCF= 21.4 Hz), 114.9 (d, JCF= 21.2 Hz). 19F{1H} NMR (C6D6,
376.27 MHz, 295.0 K): δ [ppm]= –113.8, –114.18, –114.20. MS (HR-
DART(+)): calcd 696.2126 (C45H28F6N, [M+H]+), found 696.2116.
Compound 2. Colourless solid, 4.1 g, 7.0 mmol, Yield: 50%.
Alkyne: 2,6-bis(phenylethynyl)pyridine (4.0 g, 14 mmol); iPrSSiPr (6.3 g,
6.7 mL, 3.0 equiv); Ti(OiPr)4 (4.0 g, 4.1 mL, 1.0 equiv); ArMgBr:
phenylmagnesium bromide (1.0 M THF solution, 70 mL, 5.0 equiv). 1H
NMR (399.89 MHz; C6D6; 295.3 K): δ [ppm] = 7.14 – 6.77 (m, 30 H), 6.71
– 6.64 (m, 3 H). 13C{1H} NMR (100.55 MHz; C6D6; 295.2 K): δ [ppm] =
162.4, 144.1, 143.3, 142.6, 142.4, 141.4, 135.3, 131.74, 131.67, 131.4,
128.1, 127.91, 127.88, 127.1, 126.79, 126.71, 123.9. MS (HR-
DART(+)):calcd 588.2691 (C45H34N, [M+H]+), found 588.2679.
Compound 3. Colourless solid, 0.42 g, 0.54 mmol, Yield: 30%.
Alkyne: 2,6-bis((4-fluorophenyl)ethynyl)pyridine (0.61 g, 1.8 mmol);
iPrSSiPr (0.81 g, 0.86 mL, 3.0 equiv); Ti(OiPr)4 (0.51 g, 0.53 mL, 1.0
equiv); ArMgBr: 4-fluoro-3-methylphenylmagnesium bromide (1.0 M THF
solution, 9.0 mL, 5.0 equiv). 1H NMR (399.89 MHz; C6D6; 295.3 K): δ
[ppm] = 6.97 – 6.56 (m, 21 H), 1.98 (br, 6 H), 1.96 (br, 6 H), 1.86 (br, 6 H).
13C{1H} NMR (100.55 MHz; C6D6; 296.1 K): δ [ppm] = 161.1, 160.8 (d,
1
1
1JCF= 246.0 Hz), 160.7 (d, JCF= 245.7 Hz), 160.6 (d, JCF= 245.8 Hz),
4
4
140.9, 140.3, 139.5 (d, JCF= 3.7 Hz), 138.7 (d, JCF= 3.8 Hz), 138.3 (d,
4JCF= 3.8 Hz), 135.5, 134.44 (d, JCF= 5.0 Hz), 134.41 (d, JCF= 5.0 Hz),
3
3
134.2 (d, 3JCF= 5.0 Hz), 130.61 (d, 3JCF= 7.8 Hz), 130.58 (d, 3JCF= 7.7 Hz),
3
2
2
130.20 (d, JCF= 7.7 Hz), 124.7 (d, JCF= 17.4 Hz), 124.5 (d, JCF= 17.4
Hz), 124.4 (d, 2JCF= 17.4 Hz), 123.8, 114.84 (d, 2JCF= 22.4 Hz), 114.77 (d,
2JCF= 22.4 Hz), 114.5 (d, JCF= 22.3 Hz), 14.5 – 14.2 (m). 19F{1H} NMR
2
(376.27 MHz, C6D6, 295.7 K): δ [ppm]= –118.6, –118.8, –118.9. MS (HR-
DART(+)): calcd 780.3065 (C51H40F6N, [M+H]+), found 780.3053.
Fluorescence micrographs were recorded with a Zeiss Axio
Imager 2 equipped with a Colibri LED light source for fluorescence
illumination. SEM micrographs were obtained with a Zeiss Delta field-
emission scanning electron microscope using a landing energy of 500 eV
and an Inlense detector. Probes were prepared on silicon wafers by
drop-casting in mild conditions, involving gentle and reiterated soaking of
Compound 4. Colourless solid, 3.8 g, 5.5 mmol, Yield: 55%.
Alkyne: 2,6-bis((3-fluorophenyl)ethynyl)pyridine (3.3 g, 10 mmol);
iPrSSiPr (4.5 g, 4.8 mL, 3.0 equiv); Ti(OiPr)4 (2.8 g, 2.9 mL, 1.0 equiv);
ArMgBr: 3-fluorophenylmagnesium bromide (1.0 M THF solution, 50 mL,
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10.1002/chem.201905611
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1
5.0 equiv). H NMR (399.89 MHz; C6D6; 295.1 K): δ [ppm] = 6.95 – 6.55
7.
M. Wang, G. Zhang, D. Zhang, D. Zhu, B. Z. Tang, J. Mater. Chem.
2010, 20, 1858–1867.
(m, 25 H), 6.55 – 6.38 (m, 2 H). 13C{1H} NMR (100.55 MHz; C6D6; 295.1
1
1
K): δ [ppm] = 162.97 (d, JCF= 245.2 Hz), 162.96 (d, JCF= 246.7 Hz),
8.
9.
Y. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Commun., 2009, 4332–4353.
J. Liu, J. W. Y. Lam, B. Z. Tang, J. Inorg. Organomet. Polym. 2009, 19,
249–285.
162.8 (d, 1JCF= 245.9 Hz), 161.1, 145.1 (d, 3JCF= 7.6 Hz), 144.3 (d, 3JCF
=
3
7.6 Hz), 143.9 (d, JCF= 7.6 Hz), 141.3 (br), 141.2 (br), 136.0 (br), 129.9
3
3
3
(d, JCF= 8.2 Hz), 129.7 (d, JCF= 8.2 Hz), 129.6 (d, JCF= 8.2 Hz), 127.0
10.
11.
12.
J. Chen, Z. Xie, J. W. Y. Lam, C. C. W. Law, B. Z. Tang,
Macromolecules 2003, 36, 1108–1117.
4
4
4
(d, JCF= 2.7 Hz), 126.92 (d, JCF= 2.7 Hz), 126.85 (d, JCF= 2.7 Hz),
124.1, 118.1 (d, 2JCF= 21.8 Hz), 118.0 (d, 2JCF= 21.8 Hz), 117.7 (d, 2JCF
=
Q. Peng, Y. Yi, Z. Shuai, J. Shao, J. Am. Chem. Soc. 2007, 129, 9333–
9339.
2
2
21.8 Hz), 114.7 (d, JCF= 21.4 Hz), 114.4 (d, JCF= 21.4 Hz), 114.4 (d,
2JCF= 21.1 Hz). 19F{1H} NMR (376.27 MHz, C6D6, 295.1 K): δ [ppm]= –
112.7 (s, 2 F), –113.10 (s, 1 F), –113.11 (s, 1 F), –113.27 (s, 1 F), –
113.28 (s, 1 F). MS (HR-DART(+)): calcd 696.2126 (C45H28F6N, [M+H]+),
found 696.2116.
Q. Wu, Q. Peng, Y. Niu, X. Gao, Z. Shuai, J. Phys. Chem. A 2012, 116,
3881–3888.
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1862–1869.
14.
15.
Q. Peng, Y. Yi, Z. Shuai, J. Shao, J. Chem. Phys. 2007, 126, 114302.
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Compound 5. Colourless solid, 1.7 g, 2.1 mmol, Yield: 46%.
Alkyne: 2,6-bis((3,5-difluorophenyl)ethynyl)pyridine (1.6 g, 4.6 mmol);
iPrSSiPr (2.1 mg, 2.2 mL, 3.0 equiv); Ti(OiPr)4 (1.3 mg, 1.4 mL, 1.0
16.
M. Shyamal, P. Mazumdar, S. Maity, S. Samanta, G. P. Sahoo, A.
Misra, ACS Sen. 2016, 1, 739−747.
equiv); ArMgBr: 3,5-difluorophenylmagnesium bromide (1.0
M THF
solution, 23 mL, 5.0 equiv). 1H NMR (399.89 MHz; C6D6; 295.3 K): δ
[ppm] = 6.69 – 6.61 (m, 1 H), 6.49 – 5.83 (m, 20 H). 13C{1H} NMR
(100.55 MHz; C6D6; 295.2 K): δ [ppm] = 164.5 – 161.6 (m), 159.6, 144.8
– 144.7 (m), 143.9 – 143.7 (m), 141.2 – 141.1 (m), 140.3 (br), 136.5,
124.2, 113.9 – 113.3 (m), 104.2 – 103.2 (m). 19F{1H} NMR (C6D6, 376.27
MHz, 295.3 K): δ [ppm]= –108.6, –109.0 (br), –109.3 (br). MS (HR-
DART(+)): calcd 804.1561 (C45H22F12N, [M+H]+), found 804.1536.
17.
18.
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20.
21.
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Y. Chen, W. Zhang, Y. Cai, R. T. K. Kwok, Y. Hu, J. W. Y. Lam, X. Gu,
Z. He, Z. Zhao, X. Zheng, B. Chen, C. Gui, B. Z. Tang, Chem. Sci.
2017, 8, 2047−2055.
S. Chen, J. Liu, Y. Liu, H. Su, Y. Hong, C. K. W. Jim, R. T. K. Kwok, N.
Zhao, W. Qin, J. W. Y. Lam, K. S. Wong, B. Z. Tang, Chem. Sci. 2012 ,
3 , 1804–1809.
Compound 6. Colourless solid, 0.60 g, 0.65 mmol, Yield: 25%.
Alkyne: 2,6-bis((4-(tert-butyl)phenyl)ethynyl)pyridine (1.0 g, 2.6 mmol);
iPrSSiPr (1.2 g, 1.3 mL, 3.0 equiv); Ti(OiPr)4 (0.74, 0.77 mL, 1.0 equiv);
ArMgBr: 4-tert-butylphenylmagnesium bromide (1.0 M THF solution, 13
Y. Liu, Y. Tang, N. N. Barashkov, I. S. Irgibaeva, J. W. Y. Lam, R. Hu,
D. Birimzhanova, Y. Yu, B. Z. Tang, J. Am. Chem. Soc. 2010, 132,
13951–13953.
1
mL, 5.0 equiv). H NMR (399.89 MHz; C6D6; 295.1 K): δ [ppm] = 7.19 –
M. Gao, S. Li, Y. Lin, Y. Geng, X. Ling, L. Wang, A. Qin, B. Z. Tang,
ACS Sen. 2016, 1, 179−184.
7.16 (m, 3 H), 7.16 – 7.13 (m, 3 H), 7.11 – 7.05 (m, 14 H), 7.00 – 6.94
(m, 4 H), 6.78 – 6.68 (m, 2 H), 6.68 – 6.62 (m, 1 H), 1.19 (s, 18 H), 1.09
(s, 18 H), 1.04 (s, 18 H). 13C{1H} NMR (100.55 MHz; C6D6; 295.1 K): δ
[ppm] = 162.8, 149.4, 149.2, 149.0, 141.9, 141.8, 141.2, 140.7, 140.3,
135.4, 131.8, 131.6, 131.4, 125.0, 124.8, 124.7, 123.8, 34.6, 34.45,
34.43, 31.6, 31.40, 31.36. MS (HR-DART(+)):calcd 924.6447 (C69H82N,
[M+H]+), found 924.6426.
23. G. Liang, F. Ren, H. Gao, Q. Wu, F. Zhu, B. Z. Tang, ACS Sen. 2016, 1,
1272−1278.
24. D. Li, J. Liu, R. T. K. Kwok, Z. Liang, B. Z. Tang, J. Yu, Chem. Commun.
2012, 48 , 7167–7169.
25.
T. Han, J. W. Y. Lam, N. Zhao, M. Gao, Z. Yang, E. Zhao, Y. Dong, B.
Z. Tang, Chem. Commun. 2013, 49, 4848–4850.
W. Zhang, J. Kang, P. Li; H. Wang, B. Tang, Anal. Chem. 2015, 87,
8964−8969.
26
Acknowledgements
27.
28.
Y. H. Seo, A. Singh, H. J. Cho, Y. Kim, J. Heo, C. K. Lim,
S. Y. Park, W. D. Jang, S. Kim, Biomaterials 2016, 84, 111−118.
Y. Li, R. T. K. Kwok, B. Z. Tang, B. Liu, RSC Adv. 2013, 3, 10135–
10138.
This work has been supported by the Deutsche
Forschungsgemeinschaft (DFG) within the collaborative
research centre SFB1249 ”N-Heteropolycycles as Functional
Materials” (Projects A02, B05, and C02).
29. H. Wang, J. Liu, A. Han, N. Xiao, Z. Xue, G. Wang, J. Long, D. Kong, B.
Liu, Z. Yang, D. Ding, ACS Nano 2014, 8, 1475–1484.
30. H. Shi, J. Liu, J. Geng, B. Z. Tang, B. Liu, J. Am. Chem. Soc. 2012, 134,
9569–9572.
Conflict of Interest
31.
32.
33.
34.
H. Shi, R. T. K. Kwok, J. Liu, B. Xing, B. Z. Tang, B. Liu, J. Am. Chem.
Soc. 2012, 134, 17972–17981.
The authors declare no conflict of interest.
D. Ding, J. Liang, H. Shi, R. T. K. Kwok, M. Gao, G. Feng, Y. Yuan, B.
Z. Tang, B. Liu, J. Mater. Chem. B 2014, 2, 231–238.
H. Shi, N. Zhao, D. Ding, J. Liang, B. Z. Tang, B. Liu, Org. Biomol.
Chem. 2013, 11, 7289–7296.
Keywords: Titanium tetraisopropoxide • One-pot tetraarylation •
Aggregation-Induced Emission • Extinction spectroscopy •
Scattering exponent
Y. Yuan, R. T. K. Kwok, B. Z. Tang, B. Liu, J. Am. Chem. Soc. 2014,
136, 2546–2554.
35. S. Chen, Y. Hong, Y. Liu, J. Liu, C. W. T. Leung, M. Li, R. T. K. Kwok, E.
Zhao, J. W. Y. Lam, Y. Yu, B. Z. Tang, J. Am. Chem. Soc. 2013, 135,
4926–4929.
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Entry for the Table of Contents
Francesco Foschi, Kevin
Synnatschke, Sebastian Grieger,
Wen-Shan Zhang, Hubert Wadepohl,
Rasmus R. Schröder, Claudia
Backes* and Lutz H. Gade*
Aggregate and shine! New luminogens
for aggregation-induced emission(AIE)
have been synthesized exploiting the
one-pot Ti-mediated tetraarylation of 2,6-
bis(arylethynyl)pyridines. Their
aggregation-related photophysics and
viscocromicity have been studied in
detail.
Page No. – Page No.
Luminogens for Aggregation-
Induced Emission via Titanium-
Mediated Double Nucleophilic
Addition to 2,5-Dialkynyl-
pyridines: Formation and
Transformation of the Emitting
Aggregates
[a]
[b]
[c]
Dr. F. Foschi, Prof. Dr. H. Wadepohl and Prof. Dr. L. H
Institute of Inorganic Chemistry, Heidelberg University
Im Neuenheimer Feld 270, 69120 Heidelberg, German
E-mail: lutz.gade@uni-heidelberg.de
K. Synnatschke, S. Grieger, Dr. C. Backes
Applied Physical Chemistry, Heidelberg University
Im Neuenheimer Feld 253, 69120 Heidelberg, German
E-mail: backes@uni-heidelberg.de
Dr. W. S. Zhang and Prof. Dr. R. R. Schröder
Centre for Advanced Materials
Heidelberg University
Im Neuenheimer Feld 225, 69120 Heidelberg, German
Supporting information for this article is given via a link
the document.
This article is protected by copyright. All rights reserved.