Suite of Organoplatinum(II) Triangular Metallaprism: Aggregation-Induced Emission and Coordination Sequence Induced Emission Tuning

Article abstract of DOI:10.1021/jacs.9b01283

A series of triangular metallaprisms with a kinetically inert Pt-N bond have been synthesized from the stepwise assembly of a Pt-corner, linear linker 4,4′-bipy (4,4′-bipy = 4,4′-bipyridine) and triangular ligand [tpb or tpt, tpb = tris(4-pyridyl)benzene, tpt = tris(4-pyridyl)triazine]. The use of an unsymmetrical [Pt(HL)]-corner (H₂L = 2,6-diphenylpyridine) leads to novel isostructural products. Phenyl rotation at the metal-corners endows these complexes with good aggregation-induced emission (AIE) function, with varied activities across the isostructural complexes. The coordination sequence of electron-deficient ligand tpt also imparts significant influence on the complex emission. These organoplatinum triangular metallaprisms thus provide a good model to study the influence of building blocks and coordination sequence on the luminescence of supramolecules.

Full text of DOI:10.1021/jacs.9b01283

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Communication
A Suite of Organoplatinum (II) Triangular Metalloprism: Ag-gregation-
Induced Emission and Coordination Sequence In-duced Emission Tuning
Naifang Liu, Tingting Lin, MINGDA WU, He-kuan Luo, Sheng-Li Huang, and T. S. Andy Hor
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b01283 • Publication Date (Web): 31 May 2019
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A Suite of Organoplatinum (II) Triangular Metalloprism:
Aggregation-Induced Emission and Coordination Sequence
Induced Emission Tuning
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‡
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,†
,‖
Naifang Liu, Tingting Lin, Mingda Wu, He-Kuan Luo, Sheng-Li Huang,* T. S. Andy Hor*
†
MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of
Technology, Beijing, 100081, China
‡ Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
Institute of Materials Research and Engineering, A*STAR, 2 Fusionopolis Way, Innovis, Singapore, 138634,
Singapore
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China
§
‖
Supporting Information Placeholder
organoplatinum(II) complexes harnessing Pt···Pt and π···π
interactions. Stang et al. adopted AIEgens ligands, i.e. TPE-
ABSTRACT: A series of triangular metalloprisms with
kinetically inert Pt-N bond have been synthesized from the
stepwise assembly of a Pt-corner, linear linker 4,4'-bipy (4,4'-
bipy = 4,4'-bipyridine) and triangular ligand [tpb or tpt, tpb =
tris(4-pyridyl)benzene, tpt = tris(4-pyridyl)triazine]. The use
1
5
based pyridyl unit, to construct emissive Pt(II)
6
metallasupramolecules. Compared with an abundance of
organic AIE molecules, AIE-active metal complexes, in
particular, AIE metallasupramolecules are relatively rare due
to the limited AIE generating modes of metal complex and
critical combination of AIEgens with metallasupramolecular
architectures. We herein report a series of Pt(II) trigonal
prismatic assemblies that use [Pt(HL)] as metal-corners which
carry phenyl ring whose rotations can be restricted to induce
emission in an aggregated state. To the best of our knowledge,
no AIE metallosupramolecule derived from AIEgens metal-
corner has been reported. With the aim to advance
luminescent metallosupramolecule with sophisticated AIE
effects, we have designed a set of new Pt(II)-based AIE
metallosupramolecules (Figure 1a). Upon molecular
aggregation, the trigonal metalloprisms exhibit typical AIE
effects with markedly increased quantum yields (- ).
of an unsymmetrical [Pt(HL)]-corner (H
2
L
=
2,6-
diphenylpyridine) leads to novel isostructural products.
Phenyl rotation at the metal-corners endows these complexes
with good aggregation-induced emission (AIE) function, with
varied activities across the isostructural complexes. The
coordination sequence of electron-deficient ligand tpt also
imparts significant influence on the complex emission. These
organoplatinum triangular metalloprisms thus provide a good
model to study the influence of building blocks and
coordination
sequence
on
the
luminescence
of
supramolecules.
Since the concept of AIE was coined by Tang et al. in 2001,1
numerous organic molecules with AIE effects have been
2
3
designed for OLEDs devices, cell imaging, photodynamic
4
5
therapy, liquid crystal display, etc. Some AIE luminogens
(AIEgens) like tetraphenylethene (TPE) were often
incorporated
into
the
building
blocks
of
6
7
metallosupramolecules, metal-organic frameworks and
covalent-organic frameworks8 to construct luminescent
materials based on the restriction of intramolecular rotation.
In the burgeoning field of metallosupramolecular
chemistry, coordination-driven self-assembly provides
a
powerful tool to construct metallasupramolecules with
9
,10
different levels of structural complexity.
Luminescent
supramolecular architectures have attracted considerable
attentions due to their rich excited states, structures with
precision control, building block designs and emission tuning
1
1
especially notable in host-guest adducts. These resulted in a
range of photoluminescence applications in photo-responsive
1
2
13
guest uptake/release, photodynamic therapy, biological
imaging
1
4
etc. Che et al. reported some AIE-active
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agreement with their theoretical distributions. Similarly,
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samples of the presumed isomers trans-n also showed peaks
that match the simulated isotope pattern for the intact 2+
charged cages (2117.28 for trans-1 and 2120.79 for trans-2,
respectively. Figure S19-S20).
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Figure 1. (a) Synthetic method and sequence-specific control
over the construction of triangular metalloprism isomers (tpb
for cis-1 and trans-1, tpt for cis-2 and trans-2). (b) The
photographs of metallacage cis-2 in CH Cl /hexane mixtures
2 2
with different fractions of hexane on excitation at 365 nm
using an UV lamp at 298 K.
Figure 2. Experimental (red) and calculated (blue) ESI-MS
spectra (2+) of cis-1 (a) and cis-2 (b).
Both cis-1 and trans-1 show two absorption bands at ca.
45 nm and 275 nm due to the presence of donors and
2
acceptors with - -conjugated systems. Incorporating Pt-pyridyl
moieties into the system generated metal-to-ligand charge
It is often difficult to selectively synthesize stereoisomeric
metallosupramolecules with multi-components due to the
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transfer (MLCT) absorption band at over 330 nm (Figure
S25-S26). Notably, upon increasing the hexane content in
1
6a
kinetic lability of many coordination bonds. The Pt-ligand
bond is more kinetically inert, and the ligand-exchange
labilities can be tuned by careful selection of Pt-corner
protecting groups. The cyclometalated (C^N)-group facilitates
the formation of Pt-N bond but the de-coordination is still
irreversible under ambient conditions, whereas the (C^N)-
group can also make the Pt-N bonds trans to the carbon donor
more labile. Therefore one can design a strategy in selective
construction of Pt-isomers by adjusting the sequence of
addition of N-donor linkers. Herein, the controlled synthesis
CH
varying extent and accompanied by a dramatic growth at ca.
45 nm, which is attributed to stronger - - - stacking
2 2
Cl solutions, all absorption bands exhibited red-shift with
3
interactions of the complexes when in closer proximity
state. Compared to the absorption spectrum of cis-1, cis-2
18
shows only one absorption band below 300 nm due to
incorporation of - -deficient ligand tpt. The MLCT absorption
band shows little change upon hexane addition.
The emission spectra of cis-1, cis-2 and trans-1 were
determined in pure CH Cl and CH Cl /hexane mixture
2 2 2 2
(Figure 3a, S27-S28) and trans-2 was non-emissive. The tpb-
based cis-1 and trans-1 showed slight enhancement of pure
MLCT emission at ca. 480 nm and 520 nm with addition of <
16b
of stereoisomeric metalloprism was achieved (Figure 1a).
The metal-corner [(Pt-L)(dmso)] was treated with a bridging
ligand 4,4'-bipy to yield binuclear [(Pt-L) (4,4'-bipy)]
quantitatively. After quantitative treatment with HCSA (HCSA
camphorsulfonic acid) (Figure S11-S14), a tri-pyridyl ligand
tpb or tpt) was added to the mixture to give a trigonal prism
salt cis-n·6CSA. The HCSA can protonate one carbon of
(C^N^C)-Pt]-corner thus cleave one Pt-C bond. CSA ions
2
3
0% hexane. Upon further addition of hexane, AIE appeared
=
(
and the intensity of this emission was drastically enhanced to
an approximately 10-fold accompanied by slight increase of
MLCT emission. This is
a
rare example of
a
[
metallasupramolecules that shows two set of emission bands
from different luminescence mechanism in a single spectrum.
Consequently, the tpb-based cis-1 and trans-1 exhibit weak
green emission at the outset due to pure MLCT emission. With
the addition of hexane, the two triangular metalloprisms
showed emission colour change from orange to yellow
because of the dual emission mechanisms taking effect (Figure
temporarily occupy the released coordination site at Pt-corner,
effectively stabilizing the intermediate. Metathesis with
Tetrabutylammonium Triflate gives the hexa-OTf salt in high
-
yield. Reversal of the addition sequence of 4,4'-bipy and tri-
pyridyl ligand resulted in pure trigonal prism salt trans-n. The
1
well-defined signals in H NMR spectra evidently indicated the
formation of a discrete assembly with highly symmetric
structure (Figure S15-S18).
3
c, S31). Meanwhile, their emission maximum undergoes a ca.
70 nm red shift (Figure 3b, S29) and AIE effects gradually
become a predominant emission mechanism.
Both NMR and ESI-MS analyses are consistent with the
-
proposed molecular structure. The latter analysis of the OTf
Although cis-2 has a similar structure to cis-1, the
incorporation of - -deficient ligand tpt resulted in the absence
of MLCT emission. Thus, cis-2 is non-emissive when hexane is
less than 30%. A luminescence enhancement has been
observed at ca. 560 nm and 620 nm for cis-2 upon further
salts show the molecular peaks on the parent complexes cis-1
2
+
and cis-2 of [(cis-1) − 2OTf] (m/z = 2117.29, Figure 2a) and
2
+
[(cis-2) − 2OTf] (m/z = 2120.31, Figure 2b), suggesting that
the triangular prism structures are intact in solution. The
experimental peaks were all isotopically resolved and in good
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2 2
addition of hexane to over 40% hexane-CH Cl (v/v). The
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combined emission spectra of cis-2 in different hexane
content (Figure S28, S30), clearly indicated a pure AIE effects.
In contrast, trans-2 is non-emissive both in solution and in
solid state (Figure S32), which is ascribed to the coordination
sequence of 4,4'-bipy and tpt, as well as the electron deficient
characteristic of tpt.
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Figure 4. TEM images prepared from a solution of cis-1 (a)
and cis-2 (c) in 80% hexane-CH Cl mixture. Size distributions
of cis-1 (b) and cis-2 (d) in CH Cl /hexane mixtures were
characterized by DLS. The percentages represent the hexane
content.
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2
2
2
Figure 3. Emission spectrum and plot of maximum emission
intensity and wavelength of trigonal metallaprism cis-1 (a-b).
The photographs show the CH Cl solution of cis-1 with
2 2
increasing hexane content from 0% to 90% under 365 nm UV
light (c).
These triangular metalloprisms provide the ideal model to
investigate
the
structure-property
relationship
of
isostructural metallosupramolecules. Although the building
blocks are the same, the luminescent properties of cis-n and
trans-n are completely different in some cases. In tpb-based
complexes cis-1 and trans-1, cis-trans structural changes
induced little luminescence change. However, the luminescent
properties of tpt-based cis-2 and trans-2 are completely
different. The cis-2 showed obvious AIE phenomenon, while
trans-2 is non-emissive. The combination of emissive [(Pt-
To demonstrate the formation of nanoaggregates in
CH
metalloprism cis-1 and cis-2 was investigated by transmission
electron microscopy (TEM). In 80% hexane-CH Cl mixture,
2 2
Cl /hexane solution, the morphology of the trigonal
2
2
cis-1 self-assemble into cross-linking netlike aggregates with
size over 100 nm (Figure 4a), while cis-2 exhibit globular
structures upon high hexane content (Figure 4c). Dynamic
Light Scatting (DLS) experiments were conducted to measure
the nanoaggregate sizes in different hexane content. Under
higher hexane concentration, the size of nanoaggregates
increases (Figure 4b and 4d).
L)
However, the prior combination of tpt and Pt would produce
an emission quenching intermediate product [(Pt-L) tpt],
2
(4,4'-bipy)] arm with tpt generate the luminous cis-2.
3
whose further combination with 4,4'-bipy still yield the non-
emissive trans-2. The question here is how does the
coordination sequence of tpt impose such significant influence
on emissivity of the metallosupramolecular isomers? The
geometric positioning (trans or cis) of the Pt-N bond with
respect to the carbon donor governs its lability and length,
thus the complex emission. DFT calculations were performed
to estimate Pt-N bonds length. The trans-to-carbon Pt-N bonds
invariably show bonds of extended lengths (~ 2.3Å ), whereas,
the other three bond distance are less than 2.1 Å (Figure 5,
S39). This phenomenon could be further confirmed in the
The rotation of pendant phenyl ring of Pt-corner leads to
non-emission of herein reported triangular metalloprisms. To
understand the dynamic feature of phenyl ring, we
constructed cis-2 model containing only cationic skeleton for
evaluating the dynamic feature of phenyl ring. Figure 5
illustrates the molecular structure and two optimized
conformations of cis-2. Two minimum torsion energies were
observed at 53° and -127°, demonstrating the existence of
stable conformations. The rotation of phenyl rings are not
completely free in dilute solution, but as a pendulum motion
within a certain range centered at 53° or -127°, further
confirming the availability of [Pt(HL)]-AIEgens.
16b
19
crystal structure analysis. As shown in S1
and S2 , the
trans-to-carbon Pt-N bonds are significantly longer than their
cis counterparts (Figure S40). Both the strong “trans influence”
from carbon donor and steric hindrance of pendent phenyl
ring are deemed to be responsible for the elongated Pt-N bond.
The next question is which factor plays a key role? The bond
comparison of tridentate ligand-based system was also
carried out (Figure S41). Although there is no steric group in
S3, the Pt-N bond trans to carbon is still longer than the cis
20
partner (2.096 vs. 2.020 Å with 0.076 Å difference). In S4,
the presence of a steric group significantly raises the bond
2
1
difference from 0.076 Å to 0.189 Å. This suggest that the Pt-
N bond elongation is mainly governed by the trans carbon
donor with additional contribution from the steric groups.
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The kinetically inert character of the Pt-(C,N) corner units
Experimental details on the synthesis and characterization of
cis-1, cis-2, trans-1 and trans-2, absorption and emission
spectra and DFT calculations.
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fosters the controlled synthesis of a pair of isomers whereas
the steric group helps to widen the gap of the Pt-N lengths.
The use of electron-deficient building blocks eventually
completes the strategy in using this iso-structural system as
model to exemplify the structure-property relationship of
supramolecules.
AUTHOR INFORMATION
Corresponding Author
huangsl@bit.edu.cn
andyhor@hku.hk
There are two main mechanisms of aggregation induced-
emission in metal complexes: (i) aggregation restricted
movement of flexible organic moieties on metal and (ii)
aggregation facilitated metallophilic interactions. In the
complexes reported here, the planes of coordinating pyridyl
groups from both linear and trigonal ligands are
perpendicular to the equatorial Pt plane. This precludes close
proximity or strong Pt···Pt interactions. The AIE mechanism of
cis-1, trans-1 and cis-2 are originated from aggregation
induced rigidification of phenyl rings at the Pt-corners.
ORCID
Sheng-li Huang: 0000-0003-2222-987X
T. S. Andy Hor: 0000-0001-7533-1590
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Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by “the Fundamental Research
Funds for the Central Universities”. We acknowledge IMRE
and A*STAR Computational Resource Center for
infrastructure support.
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