Luminescence properties of cyclometalated platinum(II) complexes in a dichloromethane/n-hexane system

Article abstract of DOI:10.1016/j.tetlet.2020.152802

The luminescence properties of three neutral cyclometalated Pt(II) complexes were investigated in a CH₂Cl₂/n-hexane system. The diphenylamino and trifluoromethyl modified Pt2 exhibits aggregation-induced phosphorescent emission (AIPE

Full text of DOI:10.1016/j.tetlet.2020.152802

Tetrahedron Letters 66 (2021) 152802
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Luminescence properties of cyclometalated platinum(II) complexes
in a dichloromethane/n-hexane system
1
1
⇑
Weitong Tao , Yan Chen , Lu Lu, Chun Liu
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Linggong Road 2, Dalian 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The luminescence properties of three neutral cyclometalated Pt(II) complexes were investigated in a
CH2Cl2/n-hexane system. The diphenylamino and trifluoromethyl modified Pt2 exhibits aggregation-
induced phosphorescent emission (AIPE) feature. The crystal stacking of Pt2 shows that the intermolec-
ular hydrogen bonding restricts both intramolecular rotation (RIR) and vibration (RIV), which weakens
the non-radiative decay ways and leads to an AIPE performance. TEM images reveal that Pt2 aggregates
into nanosheets.
Received 15 November 2020
Revised 18 December 2020
Accepted 21 December 2020
Available online 12 January 2021
Keywords:
Ó 2021 Elsevier Ltd. All rights reserved.
Platinum(II) complexes
Aggregation-induced phosphorescent
emission
Hydrogen bonding
Nanoaggregates
Introduction
binding strength and offer the molecules high stability [32]. Honda
et al. [33] discovered that a thioamide-based cationic pincer Pt(II)
Platinum(II) complexes are sensitive upon external stimuli on
account of their square planar geometry and conjugated ligand
structure, which are generally influenced by intra- and intermolec-
complex was AIE-active induced by hydrogen bonding (NAHÁÁÁClÀ)
in the absence of traditional PtÁÁÁPt interaction. Kuwabara et al. [34]
reported that a cationic Pt(II) complex exhibited three emission
modes: blue (monomer emission), yellow (hydrogen-bonded
dimer) and orange (aggregate emission), which was modulated
by a combination of hydrogen bonding (NAHÁÁÁClÀ) and a solvo-
phobic effect. Yang et al. [13] synthesized a neutral fluorine-substi-
tuted Pt(II) complex, whose photophysical properties, molecular
packing orientation and electron transport ability were tuned
through CAHÁÁÁF hydrogen bonding. The previous discoveries show
that the number and the type of hydrogen bonding play an impor-
tant role in adjusting the performances of luminescent materials.
We have a long-term interest in disclosing the relationship
between the molecular structures of the cyclometalated metal
complexes and their properties [35–39]. Herein, three neutral Pt
(II) complexes (Scheme 1) were synthesized using acetylacetone
as the ancillary ligand and their luminescence properties were
investigated in a CH2Cl2/n-hexane system in detail to study the
effects of the intermolecular hydrogen bonding on the properties.
ular PtÁÁÁPt and/or
p-p stacking interactions [1–6]. Pt(II) complexes
exhibit rich spectroscopic and luminescence properties such as
high luminescence quantum yields, long emission lifetimes, large
Stoke shifts and color tunability [7–11]. Therefore, Pt(II) complexes
have been used as promising candidates for applications in organic
light-emitting devices (OLEDs) [12–15], bio-imaging [16–18], drug
discovery [19–23], and so on. In the past years, most of the tradi-
tional organic luminescent materials are well known to suffer from
aggregation-caused quenching (ACQ) in the solid state [24,25],
which has greatly restricted the applications of organic lumines-
cent materials in many fields. In 2001, Tang and co-workers [26]
reported that a silole derivative was almost non-emissive in dilute
solution but strongly emissive in aggregated state. Aggregation-
induced emission (AIE) has settled a great deal of problems with
ACQ fundamentally, and has attracted attention as a new approach
for the development of novel optoelectronic materials [27–31].
It is well known that the hydrogen bonds, including CAHÁÁÁN,
CAHÁÁÁO and CAHÁÁÁF, contribute to the photophysical behaviors
of the dyes. Although individual hydrogen bonding is relatively
weak, multiple hydrogen bonding array may show much stronger
Results and discussion
Molecular design
⇑
Corresponding author.
E-mail address: cliu@dlut.edu.cn (C. Liu).
These authors contributed equally to this work.
The chemical structures of platinum(II) complexes are shown in
Scheme 1. Using Pt(ppy)(acac) as a reference, a trifluoromethyl
1
https://doi.org/10.1016/j.tetlet.2020.152802
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.

W. Tao, Y. Chen, L. Lu et al.
Tetrahedron Letters 66 (2021) 152802
of Pt1 has a slight enhancement with the addition of n-hexane,
but decreases apparently when the n-hexane fraction increases
to 90%, which shows an inconspicuous AIPE property. However,
the Pt2 demonstrates a typical AIPE feature in the CH2Cl2/n-hexane
system. As shown in Fig. 1C, the phosphorescence emission inten-
sity of Pt2 increases steadily when the fn-Hexane is less than 60%,
whereas the intensity is enhanced sharply when increasing the
n-hexane fraction from 60% to 90% and then reaches the maximum
at 90% n-hexane fraction. Fig. 1D shows a stable emission intensity
of Pt2 over time at the 90% n-hexane content in the CH2Cl2/n-hex-
ane system, which means that the aggregation process goes to the
equilibrium quickly.
Scheme 1. Chemical structures of Pt(II) complexes for this study.
Molecular packing mode
(TFM) group was introduced onto Pt1 to construct intermolecular
hydrogen bonding. Both a TFM group and a rotatable dipheny-
lamino (DPA) moiety were introduced into the cyclometalating
ligand of Pt2 to reach different luminescence properties compared
to Pt(ppy)(acac) and Pt1.
In order to clarify the relationship between the intermolecular
interaction of Pt1 and Pt2 and their luminescence properties, the
multiple intermolecular interactions of Pt1 and Pt2 were studied
in detail by analyzing molecular packing of their crystal structures,
respectively. The single crystals were cultivated by slowly evapo-
rating the solvents from a CHCl3/cyclohexane system and fully
investigated by single-crystal X-ray diffraction analysis. The crystal
analysis figures and corresponding data for Pt1 and Pt2 are illus-
trated in Figs. 2–4 and Tables S2-S4.
As shown in Fig. 2, Pt1 molecules pack as antiparallel dimers in
crystals and possess PtÁÁÁPt interactions with a 3.90 Å mean dis-
tance (Fig. 2, Table S3). Because of this special packing arrange-
ment, abundant intermolecular hydrogen bonds could be
observed in the crystal structures. As shown in Fig. 2B, CAHÁÁÁN,
CAHÁÁÁO and CAHÁÁÁF hydrogen bonds were found in the molecular
Luminescence properties
To study the luminescence properties of all complexes, the
emission spectra were investigated in a CH2Cl2/n-hexane system
with different volume ratios. As depicted in Fig. 1, Pt(ppy)(acac),
Pt1 and Pt2 exhibit different emission profiles with the increase
of n-hexane in the mixed solvent. The phosphorescence emission
intensity of Pt(ppy)(acac) decreases dramatically as the n-hexane
fraction increases, which shows an obvious ACQ phenomenon
(Fig. 1A). Nevertheless, the phosphorescence emission intensity
Fig. 1. Emission spectra of Pt(ppy)(acac) (A), Pt1 (B) and Pt2 (C) (5.0 Â 10À5 mol LÀ1) in CH2Cl2/n-hexane with different n-hexane fraction (0–90%) at room temperature.
Emission spectra (D) of Pt2 (5.0 Â 10À5 mol LÀ1) over time with a 90% n-hexane fraction at room temperature.
2

W. Tao, Y. Chen, L. Lu et al.
Tetrahedron Letters 66 (2021) 152802
Fig. 2. Molecular packing of Pt1 in crystal state. The orange dashed lines (A) represent the PtÁÁÁPt interactions. The green, red and blue dashed lines (B) represent the CAHÁÁÁF,
CAHÁÁÁO and CAHÁÁÁN hydrogen bonding, respectively.
Fig. 3. Molecular packing of Pt2 in crystal state. The red (A) and green (B, C) represent the CAHÁÁÁO and CAHÁÁÁF hydrogen bonding, respectively.
packing of Pt1. The CAHÁÁÁO hydrogen bonding exists between the
acetylacetone (acac) ligand of one molecule and the 2-phenylpyri-
dine moiety of the neighboring molecule (Fig. 2B). The presence of
CAHÁÁÁF hydrogen bonding could be found among the F atoms of
one molecule’s TFM group and the H atoms of two other molecules
in different dimers, which plays an important role in restricting the
intramolecular vibration (RIV) effectively (Fig. 2B) [40–42].
Therefore, these intermolecular interactions cause a more rigid
molecular environment to suppress the non-radiative pathways,
leading to an AIPE performance.
As shown in Figs. 3 and 4, the molecular packing of Pt2 also
shows a plane stacking with antiparallel dimers. This arrangement
allows the formation of CAHÁÁÁO hydrogen bonding between the H
atoms of 2-phenylpyridine and the O atoms at acetylacetone of the
neighboring molecule (Fig. 3A). Meanwhile, the intermolecular
CAHÁÁÁF hydrogen bonding (2.77 Å) is found in dimers (Fig. 3B,
3

W. Tao, Y. Chen, L. Lu et al.
Tetrahedron Letters 66 (2021) 152802
Fig. 4. Molecular packing of Pt2 in crystal state. The orange dashed lines (A) represent the PtÁÁÁPt interactions. The yellow dashed lines (B) represent the CAHÁÁÁ
p
intermolecular interactions.
Fig. 5. TEM images of Pt2 in CH2Cl2/n-hexane (5.0 Â 10À5 mol LÀ1) with 80% (A) and 90% (B) n-hexane fraction at room temperature (after deoxygenated). The distribution
charts of the length of nanosheets with 80% (C) and 90% (D) n-hexane fraction for Pt2 aggregates.
Table S4) between the F atoms of TFM and the H atoms of phenyl
group at the DPA moiety. It is worth noting that this molecular
packing mode results in the netlike formation of CAHÁÁÁF hydrogen
bonding between the obliquely molecule in adjacent dimers of Pt2,
which restricts the rotation of the phenyl groups at the DPA moiety
greatly (Fig. 3C).
[43–45], leading to a more remarkable AIPE property. On the other
hand, the multiple intermolecular interactions could explain the
fast aggregation of Pt2 at the 90% n-hexane content (Fig. 1D).
Aggregate morphology
Compared to Pt1, Pt2 molecules pack much denser due to the
stronger intermolecular PtÁÁÁPt interaction with a mean distance
of 3.83 Å (Fig. 4A, Table S3). In addition, the adjacent Pt2 molecules
To further investigate the formation of aggregates, transmission
electron micrograph (TEM) images of the aggregate morphology of
Pt2 in CH2Cl2/n-hexane (5.0 Â 10À5 mol LÀ1) with 80% and 90%
n-hexane fractions are shown in Fig. 5. The aggregates of Pt2 are
regular square nanosheets with different sizes (Fig. 5A, B). The
aggregates (100–120 nm, Fig. 5C) formed at an 80% n-hexane
fraction are little smaller than those (120–140 nm, Fig. 5D) at a
90% n-hexane fraction.
in opposite direction are interlocked by symmetric CAHÁÁÁ
p inter-
molecular interactions (2.77 Å) between the phenyl groups of
DPA (Fig. 4B). In the aggregated state, the synergistic effect of these
multiple intermolecular interactions contributes to a rigid molecu-
lar environment to restrict non-radiative process [40]. Thus, Pt2
demonstrates an obvious AIPE property in the CH2Cl2/n-hexane
system.
Thus, the intramolecular vibration of Pt1 is limited by a combi-
nation of PtÁÁÁPt interaction and intermolecular hydrogen bonding.
In case of Pt2, the much more abundant intermolecular interac-
tions caused by the DPA moiety result in the restriction of
intramolecular vibration (RIV) and intramolecular rotation (RIR)
Conclusion
In conclusion, three neutral cyclometalated Pt(II) complexes
with different luminescence properties have been reported. Pt2
performs a typical AIPE feature in the CH2Cl2/n-hexane system,
4

W. Tao, Y. Chen, L. Lu et al.
Tetrahedron Letters 66 (2021) 152802
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while Pt(ppy)(acac) exhibits an obvious ACQ phenomenon. The
TFM-modified Pt1 displays feeble AIPE property. The crystal stack-
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Declaration of Competing Interest
The authors declare that they have no known competing finan-
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to influence the work reported in this paper.
Acknowledgements
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