Coordination-enhanced luminescence on tetra-phenylethylene-based supramolecular assemblies

Article abstract of DOI:10.3390/molecules23020363

Materials with aggregation-induced emission (AIE) properties have received increased attention recently due to their potential applications in light-emitting devices, chemo/biosensors and biomedical diagnostics. In general, AIE requires the forced aggrega

Full text of DOI:10.3390/molecules23020363

molecules
Article
Coordination-Enhanced Luminescence on
Tetra-Phenylethylene-Based
Supramolecular Assemblies
Qian-Qian Yan 1,2, Shao-Jun Hu 1,2, Guang-Lu Zhang 2, Ting Zhang 2, Li-Peng Zhou 2 and
Qing-Fu Sun 2,
*
ID
1
College of Chemistry, Fuzhou University, Fuzhou 350108, China; yanqianqian@fjirsm.ac.cn (Q.-Q.Y.);
sjhu@fjirsm.ac.cn (S.-J.H.)
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter,
Chinese Academy of Sciences, Fuzhou 350002, China; glzhang@sdnu.edu.cn (G.-L.Z.);
zhangting@fjirsm.ac.cn (T.Z.); zhoulp@fjirsm.ac.cn (L.-P.Z.)
2
*
Correspondence: qfsun@fjirsm.ac.cn; Tel.: +86-591-6317-3527
Received: 14 January 2018; Accepted: 5 February 2018; Published: 9 February 2018
Abstract: Materials with aggregation-induced emission (AIE) properties have received increased
attention recently due to their potential applications in light-emitting devices, chemo/biosensors and
biomedical diagnostics. In general, AIE requires the forced aggregation of the AIEgens induced by the
poor solvent or close arrangement of AIEgens covalently attached to polymer chains. Here, we report
two coordination-enhanced fluorescent supramolecular complexes featuring hierarchically restricted
intramolecular motions via the self-assembly of tetraphenylethylene (TPE)-based tetra-dentate (La
and bidentate (Lb) ligands and the cis-Pd(en)(NO3)2 (en = ethylenediamine) unit. While the free
ligands are non-emissive in dilute solution and show typical AIE properties in both mixed solvent
system and the solid state, the self-assembled complexes maintain their fluorescent nature in the
solution state. In particular, the Pd4(La)2 complex shows remarkable 6-fold fluorescent enhancement
over La in dilute solution. We anticipate that these kinds of coordination-enhanced emissive
supramolecules will find applications in biomedical sensing or labeling.
)
Keywords: tetraphenylethylene; coordination self-assembly; fluorescence enhancement; AIE
1. Introduction
Aggregation-caused quenching (ACQ) phenomena of luminescent materials are well known,
and hinder many of their applications, such as OLED, chemo- or bio-labeling and sensing. Contrary
to ACQ, aggregation-induced emission (AIE) materials, which were first introduced by Tang and
coworkers in 2001 [
aggregated state. Molecules with AIE properties are called AIEgens, from which a variety of functional
materials, such as metal-organic frameworks (MOFs) [ ], covalent-organic frameworks (COFs) [ ],
and co-polymers [ ] have been designed. Luminescent properties of these materials have shown rich
1,2], are non-emissive in dilute solutions, but become highly luminescent in their
3
,4
5
6,7
stimuli-responsiveness towards light, aggregation, chemicals, heat, mechanical forces, etc. However,
most of the known AIE materials rely on the high local concentration of the AIEgens. It is thus
necessary to search for efficient strategies to “turn on” the AIE at low concentrations. To achieve this,
tight covalent bonding of AIE chromophores into one molecule has been shown to be an effective
strategy, but this usually requires multi-step and/or low-yield covalent synthesis.
Tetraphenylethylene (TPE) is a popular AIE-active fluorophore, whose fluorescence can be
induced by restricting C=C bond twist and phenyl-ring rotation [8,9]. Many strategies, including
covalent modifications, host-guest chemistry, anion coordination,
π
-π
stacking, electrostatic interactions
Molecules 2018, 23, 363; doi:10.3390/molecules23020363
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and embedding into MOFs, have been developed to “turn on” the emission [10
,
11]. In particular,
construction of MOFs by anchoring TPE chromophores to metal ions within a rigid matrix has been
shown to be an effective alternative strategy to get strongly luminescent nanoparticles [12
,13]. However,
the insolubility of MOFs poses inherent problems for their fabrication.
Recently, coordination-directed self-assembly has been proven to be a powerful supramolecular
approach to organize multiple AIE chromophores onto the discrete supramolecular coordination
complexes (SCCs). For example, Stang and coworkers have reported a series of light-emitting
tetraphenylethylene (TPE)-based supramolecular platforms and their multifaceted applications
in sensor devices and advanced optoelectronic materials [14]. The Su group has reported
a tetraphenylethylene (TPE)-based tetragonal Ag4L2 cage with luminescent behavior that is
significantly different from that of both AIE and ACQ mechanisms [15]. Luminescent SCCs
materials have many advantages compared to MOFs, such as the circumvention of synthetic steps,
inherent error-correction and defect-free assembly, facile and fast formation, increased solubility
and so on. In addition, the finite nature of SCC structures facilitates convenient solution-based
characterization/fabrication methods.
Although several light-emitting TPE-based discrete organoplatinum metallacages and
metallacycles have been reported [16
–20], AIE-active organopalladium SCCs have rarely been
explored [21
,
22], even though palladium has a lower cost than platinum. Pd2+ is a well-known
fluorescence quencher due to heavy atom effects and its 4d8 electronic configuration, the photo
induced electron transfer and photo induced energy transfer may both exist through the interactions
between fluorophore and Pd2+ 23]. Lots of fluorescence probes for detecting Pd2+ based on Pd2+
[
coordination-induced fluorescence quenching have been designed [24
,25]. Herein, we report the
coordination-driven self-assembly and the luminescent studies of two SCCs (1, 2) from the TPE-based
polypyridine ligands (La and Lb) and the (en)Pd(NO3)2 (en = ethylenediamine) clipping units
(Scheme 1). While the free ligands are non-emissive in dilute solution and show typical AIE properties
in the aggregated states, the self-assembled complexes maintain their fluorescent nature in dilute
solution. Such a coordination-enhanced emission behavior has been clearly confirmed by comparison
of the two SCCS featuring hierarchically restricted intramolecular motions. In particular, complex
shows a remarkable 6-fold fluorescent enhancement over La in dilute solution.
1
Scheme 1. Coordination-driven self-assembly of supramolecular complexes
ligands La,b and the cis-blocked (en)Pd(NO3)2 clips.
1 and 2 from two TPE-based

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2. Results and Discussion
For comparison purposes, ligands La and Lb, with four and two pyridine coordination
sites, respectively, were synthesized in two steps starting from the TPE precursor by following
the reported procedures [26–28]. Both were fully characterized by multi-dimensional NMR and
high-resolution ESI-TOF-MS spectroscopy (See SI for details). Coordination complex 1·NO3 was
obtained quantitatively by reacting ligand La with two equiv. of cis-(en)Pd(NO3)2 in a H2O and
◦
acetone (v:v = 1:1) mixture at 50 C for 3 h with continuous stirring. With the progression of the
1
self-assembly, the suspension of the ligand gradually turned into a clear yellow solution. H-NMR
first confirmed the quantitative formation of a highly symmetrical complex with the observation of
a single set of ligand signals (Figure 1B and Figure S8), which could be fully assigned based on a 1H-1H
COSY experiment (Figures S6 and S9). The diffusion-ordered spectroscopy (DOSY) NMR spectrum
(Figure 1C) [29] suggests the formation of a single product with a single diffusion coefficient of 1.77
10−10 m2·s−1, with an estimated diameter of 2.1 nm for the complex in solution.
×
Figure 1. 1H-NMR spectra of (
D2O:Acetone-d6 = 1:1, 298 K); (
showing the observed (Obs.) and simulated (Sim.) isotopic patterns of the 2+ peak.
A
C
) ligand La (400 MHz, DMSO-d6, 298 K), (
B
) complex
1
(400 MHz,
) 1H DOSY spectrum and (
D) ESI-TOF-MS of complex 1 with insets
In order to facilitate mass spectra measurement, 1·NO3 was transformed into its tetrafluoroborate
salt (1·BF4) by precipitation with excess amount of NaBF4 (See SI for details). The composition
of complex 1·BF4 as (en)4Pd4La2(BF4)8 was then clearly provided by electro-spray-ionization
time-of-flight mass-spectroscopy (ESI-TOF-MS) measurements. The prominent peaks observed at
m/z = 1234.2355, 793.8219, 573.6167, correspond to [M
−
(BF4−)n]n+ (n = 2–4). Moreover, the isotopic
patterns of each peak were in good agreement with the calculated values, which further confirmed
the molecular formula. Similarly, reacting ligand Lb with an equimolar amount of cis-(en)Pd(NO3)2
followed by counter-ion exchange with NaBF4 lead to the quantitative formation of the Pd2Lb2 complex
(2·BF4), which was fully characterized by multi-dimensional NMR and ESI-TOF-MS (see SI for details).

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Unfortunately, efforts to grow high-quality single crystals for both compounds were failed.
Therefore, molecular-mechanical simulations were performed to get an overview of the structures
for complexes 1 and 2 (Figure 2). In complex 1, as all the four pyridine arms are clipped together by
the (en)Pd units, the two PTE fluorophores are forced to stack to each other, with an approximate
6.06 Å separation between the ethene groups (defined by the closest C-C distance). In contrast,
the two ethene groups on the two ligands of Lb in the optimized structure of complex
2 are more
separated (9.51 Å apart). Such a gradual coordination-driven proximity of the TPE groups on these
complexes persuaded us to investigate their differences in photoluminescent properties.
Figure 2. Simulated molecular structure models for complexes 1 (A,B) and 2 (C,D). Color scheme: C,
slate sticks; H, white sticks; N, blue sticks; Pd, yellow spheres.
UV-Vis absorption spectra for both La and Lb (DMSO solution, 50.00
µ
M) were first measured,
and showed broad intense band for the π−1
-
π
* transitions from 270 to 400 nm (
λ
max
of La at 286 nm,
−1
with molar absorptivity up to 4.55
×
104 M
·
cm
;
λ
max
of Lb at 301 nm, with molar absorptivity up
to 3.09
noticed, as a result of coordination to the (en)Pd units (Figure S15).
Photoluminescence study on the ligands (Figure 3) confirmed their AIE nature. In both cases,
negligible emissions have been measured in dilute DMSO solution (50.00 M). However, when water
×
104 M−1
·
cm−1). Significant red-shifting in the absorption spectra of complexes
1 and 2 was
µ
was introduced, the emission intensity for both ligands showed great enhancement. Meanwhile,
the quantum yield of La exhibited a low value (ΦF = 0.74%) and increased remarkably to 84.2% in the
solid state. Similarly, the quantum yield of Lb was too low to be measured under dilute conditions,
but increased to 74.1% in the solid state.
Coordination-enhanced emission was confirmed after assembling La with the (en)Pd(NO3)2 clips
(Figure 4). Compared to La, the four arms on the TPE units of
1 are fixed by the (en)Pd(NO3)2 units,
which forces the two ligands to stack together, thus fully restricting the intramolecular rotations.
Though palladium is a well-known fluorescence quencher, such a forced aggregation state induced
a remarkable 6-fold fluorescence enhancement over La for complex
1
in dilute solution. The ligands
on complex
1 are more distorted than its free extended state and the perpendicular conformation
decreased the conjugation, resulting that the emission maximum undergoing a blue shift from 528
to 488 nm. These results imply that a coordination-driven self-assembly approach is effective for

Molecules 2018, 23, 363
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restricting the intermolecular rotations of the TPE units, thus attenuating the non-radiative decay
pathway and turning on the fluorescence. However, ligand Lb has two dangling phenyl rings that
remain unrestricted even after assembling with (en)Pd(NO3)2. The assembled complex
2 shows lower
emission intensity than Lb in dilute solution, probably due to the intramolecular motion [30] and the
emission quenching effect of the palladium centers.
Figure 3. Fluorescence emission spectra of (
A)
La
(λ
ex
= 365 nm); (
B
)
Lb
(
λ
ex
= 343 nm) with inserts
showing the plots of maximum emission intensity of ligands versus H2O fraction in H2O/DMSO
mixtures and photographs of assemblies in pure DMSO and 90% H2O fractions mixture solution upon
excitation at 365 nm using an UV lamp. cL = 50.00 µM.
Figure 4. Fluorescence emission spectra of ligand La
(λ
ex = 365 nm), Lb
(λex = 343 nm) and assemblies
1
(λ
ex = 372 nm) and ex = 358 nm) in H2O/acetone (v:v = 1:1) at the same ligand concentration of 50
2
(
λ
µM.
To examine whether the complexes
assemblies were measured in H2O/acetone/1,4-dioxane mixed solvent systems (Figure 5), in which
H2O/acetone(v:v = 1:1) is good solvent and 1,4-dioxane is poor solvent. For complex , the emission
1 and 2 are still AIE active, the emission spectra of the
2
intensity remains low in H2O/acetone solution with no quantum yield detectable, which indicates
that free rotation on the TPE units is still possible on the assembly in solution state. With the addition

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of 1,4-dioxane, the emission intensity gradually increased. Upon increasing the 1,4-dioxane fraction to
99%, remarkably enhanced fluorescence was observed, with a quantum yield determined to be 4.13%.
In clear contrast, complex
the fraction of 1,4-dioxane. Quantum yield (
1
remains strongly emissive in dilute solution, with low responsiveness to
) for was determined to be 4.22% in dilute solution
Φ
1
(Figure S25) and increased to 6.57% in solid state. These results clearly confirmed the forced AIE
properties by coordination self-assembly.
Figure 5. Fluorescence emission spectra of ( ) the plots of
A
)
1,
λ
ex
= 372 nm, (
B
)
2,
λ
ex
= 358 nm, and (
C
maximum emission intensity of assemblies 1, 2 versus 1,4-dioxane fraction in H2O/acetone/1,4-dioxane
mixtures. Insert: photographs of assemblies in H2O/acetone (v:v = 1:1) and 90% 1,4-dioxane fractions
mixture solution upon excitation at 365 nm using an UV lamp. cL = 50.00 µM.
3. Materials and Methods
3.1. General
Unless otherwise noted, all chemicals and solvents were purchased from commercial corporations
and used without further purification. Deuterated solvents were purchased from Admas
(Shanghai, China) and J&K scientific Co. (Beijing, China). 1D- and 2D-NMR spectra were measured
on a Bruker Biospin Avance III (400 MHz) spectrometer (Bruker Biospin, Rheinstetten, Germany).
1H-NMR chemical shifts were determined with respect to residual solvent signals of the deuterated
solvents used. ESI-TOF mass spectra were recorded on an Impact II UHR-TOF mass spectrometry
from Bruker (Bruker Biospin, Rheinstetten, Germany). Data analyses and simulation of ESI-TOF
mass spectra were processed on a Bruker Data Analysis software (Version 4.3). Molecular modelings
were performed on Marerials Studio 6.0 (Accelrys Co., San Diego, CA, USA). UV-Visible absorption
spectra were measured on a UV-2700 spectrophotometer (SHIMADZU, Kyoto, Japan). Fluorescent
spectra were measured on a FS5 spectrofluorometer (Edinburgh Instruments Ltd., Livingston, UK).
The emission quantum yields in solution and solid state were measured by FS5 Spectra Fluorometer
(Edinburgh Instruments Ltd., Livingston, UK) with integrating sphere.
3.2. Synthesis Procedure
3.2.1. Synthesis of Ligand La
4Br-TPE was prepared using reported literature procedures (Scheme 2) [26]. Tetraphenylethene
(5 g, 10 mmol) was treated with bromine (7.5 mL, 0.15 mol) and the mixture was kept for 16 h at
room temperature. The resulting solid was dissolved in hot toluene (120 mL), then concentrated to
20 mL. The precipitate was filtered and washed with hexane. The product was further purified by
chromatography on SiO2 (Hexanes/CH2Cl2, v/v = 20/1). 5.94 g (9.17 mmol, Yield: 61%) of a colorless
1
solid was obtained. H-NMR (400 MHz, DMSO-d6):
δ 7.39 (d, J = 8.3 Hz, 8H), 6.92 (d, J = 8.3 Hz, 8H).
13C-NMR (100 MHz, DMSO-d6) δ 141.50 (s), 139.38 (s), 132.79 (s), 131.19 (s), 120.48 (s).

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La was prepared using reported literature procedures [27
,28]. 4Br-TPE (161 mg, 0.25 mmol),
3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyridine (246 mg, 1.2 mmol), Pd(PPh3)4 (40 mg, 0.035 mmol)
and K3PO4 (1.1 g, 5.18 mmol) were added into a 100 mL three◦-necked flask. Under N2 atmosphere,
1,4-dioxane (50 mL) was added. The suspension was stirred at 100 C for 3 d. The mixture was evaporated
and extracted with chloroform and water. The organic phase was collected and dried over MgSO4 and
evaporated. The crude product was purified by silica gel column chromatography (CH2Cl2:CH3OH =
100:1). 64 mg (0.1 mmol, Yield: 40%) of a yellow green powder was obtained. 1H-NMR (400 MHz, CDCl3):
δ
8.83 (d, J = 1.5 Hz, 4H), 8.56 (d, J = 3.4 Hz, 4H), 7.85 (d, J = 7.9 Hz, 4H), 7.42 (d, J = 8.2 Hz, 8H), 7.33 (dd,
J = 7.8, 4.8 Hz, 4H), 7.23 (d, J = 8.2 Hz, 8H). 13C-NMR (100 MHz, CDCl3)
δ 148.52 (s), 148.18 (s), 143.26 (s),
140.60 (s), 136.11 (s), 135.95 (s), 134.10 (s), 132.16 (s), 126.57 (s), 123.54 (s).
Scheme 2. Synthetic procedures of ligand La and assembly 1.
3.2.2. Synthesis of Assembly 1
Ligand La (4.72 mg, 7.36
µ
mol) was treated with (en)Pd(NO3)2 (4.7 mg,16.2
µ
mol) in a mixture
solution of H2O and acetone (v:v = 1:1) and stirred at 50 ◦C for 3 h. 1·NO3 was obtained. H-NMR
1
(400 MHz, D2O:acetone-d6 = 1:1):
δ 9.65 (s, 4H), 9.03 (s, 4H), 8.41 (s, 4H), 7.84 (s, 4H), 7.72 (s, 8H),
7.42 (s, 8H), 3.12 (s, 8H). 13C-NMR (100 MHz, D2O:acetone-d6 = 1:1):
δ 213.88 (s), 151.20 (s), 149.24 (s),
144.23 (s), 141.47 (s), 139.20 (s), 138.65 (s), 134.48 (s), 132.34 (s), 127.80 (s), 127.14 (s), 47.30 (s).
To the solution of 1·NO3 was added excess NaBF4 aqueous solution and stirred for ten minutes.
The precipitate was filtered and washed with water. 1·BF4 was obtained and characterized
by ESI-TOF-MS: m/z 573.6163 {[(en)4Pd4La2(BF4)4]4+}, 793.8219 {[(en)4Pd4La2(BF4)5]3+}, 1234.2355
{[(en)4Pd4La (BF4)6]2+}.
2
3.2.3. Synthesis of Ligand Lb and Assembly 2
The preparation of Lb followed the literature procedure (Scheme 3) [21] and the assembly of
1
Lb with (en)Pd(NO3)2 under the similar procedure above gave complex 2·BF4. H-NMR (400 MHz,
CD3CN):
δ 9.12 (s, 2H), 8.75 (d, J = 5.0 Hz, 2H), 8.12 (d, J = 8.0 Hz, 2H), 7.77–7.55 (m, 2H), 7.48 (d,
J = 8.0 Hz, 4H), 7.22 (d, J = 8.0 Hz, 4H), 7.15 (s, 6H), 7.09 (d, J = 6.8 Hz, 4H), 4.14 (s, 4H), 2.81 (s,
4H). 13C-NMR (100 MHz, CD3CN)
δ
150.785 (s), 150.741(s), 145.69 (s), 144.27 (s), 144.13 (s), 140.08 (s),
139.61 (s), 138.98 (s), 133.99 (s), 133.09 (s), 131.81 (s), 128.95 (s), 127.98 (s), 127.71 (s), 127.46 (s),
47.62 (s). ESI-TOF-MS: m/z 326.5877 {[(en)2Pd2Lb2]4+}, 464.4514 {[(en)2Pd2Lb2(BF4)1]3+}, 740.1784
{[(en)2Pd2Lb2(BF4)2]2+}.

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Scheme 3. Synthetic procedures of ligand Lb and assembly 2.
4. Conclusions
In summary, fluorescent supramolecular coordination complexes have been self-assembled from
TPE-based polypyridine ligands and cis-blocked Pd2+ corners. Distinctively, when the four arms of
the TPE units are fixed, the assembly remains strongly emissive not only in dilute solution but also in
the aggregated state. We envisage that this coordination-enhanced emissive supramolecule may find
applications in palladium sensors, biomedical sensing, or labeling.
Supplementary Materials: The following supplementary materials are available online: synthesis, NMR spectra,
ESI-TOF-MS, UV-Vis spectra and fluorescence spectra of ligands and assemblies.
Acknowledgments: This work was supported by the National Natural Science Foundation of China (Grant
Nos. 21471150, 21521061, 21601183), Natural Science Foundation of Fujian Province (Grant Nos. 2016J06005,
2016J05051), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No.
XDB20000000). Qing-Fu Sun is grateful for the award from ‘The Recruitment Program of Global Youth Experts’.
Author Contributions: All the experiments were conducted by Qian-Qian Yan, Guang-Lu Zhang, Shao-Jun Hu
and Ting Zhang under the supervision of Qing-Fu Sun; Li-Peng Zhou contributed to the analysis of the data;
The manuscript was written jointly by Qian-Qian Yan and Qing-Fu Sun.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are available from the authors.
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