Photophysical properties and optical power limiting ability of Pt(II) polyynes bearing fluorene-type ligands with ethynyl units at different positions

Article abstract of DOI:10.1016/j.jorganchem.2019.05.022

Two series of Pt(II) polyynes bearing fluorene-type ligands with ethynyl units at different positions have been synthesized. In the absorption spectra, the Pt(II) polyynes bearing fluorene-type ligands with ethynyl units at 3,6-position have blue-shift with respect to the corresponding analogs bearing fluorene-type ligands with ethynyl units at 2,7-position, showing better transparency in the visible light region. Moreover, the Pt(II) polyynes bearing fluorene-type ligands with ethynyl units at 3,6-position show stronger triplet emission than corresponding analogs bearing fluorene-type ligands with ethynyl units at 2,7-position in the photoluminescent (PL) spectra. Furthermore, these Pt(II) polyynes were applied to optical power limiting (OPL) field. The Pt(II) polyynes bearing fluorene-type ligands with ethynyl units at 2,7-position show better OPL performance than the corresponding analogs with fluorene-type ligands of ethynyl units at 3,6-position. Therefore, changing the position of the ethynyl units in fluorene-type ligands can not only effectively control the photophysical properties of the Pt(II) polyynes, but also has an important effect on their OPL ability.

Full text of DOI:10.1016/j.jorganchem.2019.05.022

Journal of Organometallic Chemistry 895 (2019) 28e36
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Photophysical properties and optical power limiting ability of Pt(II)
polyynes bearing fluorene-type ligands with ethynyl units at different
positions
*
Zhuanzhuan Tian, Xiaolong Yang, Boao Liu, Daokun Zhong, Guijiang Zhou
MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials,
Department of Chemistry, School of Science, Xi'an Jiaotong University, Xi'an, 710049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 April 2019
Received in revised form
Two series of Pt(II) polyynes bearing fluorene-type ligands with ethynyl units at different positions have
been synthesized. In the absorption spectra, the Pt(II) polyynes bearing fluorene-type ligands with
ethynyl units at 3,6-position have blue-shift with respect to the corresponding analogs bearing fluorene-
type ligands with ethynyl units at 2,7-position, showing better transparency in the visible light region.
Moreover, the Pt(II) polyynes bearing fluorene-type ligands with ethynyl units at 3,6-position show
stronger triplet emission than corresponding analogs bearing fluorene-type ligands with ethynyl units at
15 May 2019
Accepted 21 May 2019
Available online 22 May 2019
2
,7-position in the photoluminescent (PL) spectra. Furthermore, these Pt(II) polyynes were applied to
optical power limiting (OPL) field. The Pt(II) polyynes bearing fluorene-type ligands with ethynyl units at
,7-position show better OPL performance than the corresponding analogs with fluorene-type ligands of
Keywords:
Pt(II) polyynes
Triplet state
2
Transparency
Reverse saturable absorption
Optical power limiting
ethynyl units at 3,6-position. Therefore, changing the position of the ethynyl units in fluorene-type li-
gands can not only effectively control the photophysical properties of the Pt(II) polyynes, but also has an
important effect on their OPL ability.
©
2019 Published by Elsevier B.V.
1
. Introduction
acetylide ligands have attracted much attention due to their ease of
chemical modification and tuning both electronic and geometric
properties [20e23]. Moreover, the Pt(II) polyynes with fluorene-
based ethynyl ligands can show high transparency in the visible-
light region and good optical power limiting (OPL) ability. Based
on their advantages, many reported fluorene acetylide ligands were
designed by varying the substituents at their 9-position [22,24].
The photophysical properties and OPL ability of these Pt(II) poly-
ynes can be effectively tuned by the designed ligands. Clearly, the
ethynyl units at 2,7-position of the fluorene-type ligands were
shifted to 3,6-positon, which can also provide an effective tuning
the photophysical properties of the Pt(II) polyynes. However, the
Pt(II) polyynes bearing the fluorene-type ligands with ethynyl units
at 3,6-positon are relatively rare. Wong's group reported that the
Pt(II) polyynes bearing 2, 8- diethynyl- 5, 5- dioxide-
dibenzothiophene ligands show blue-shift effect in its absorption
spectrum and stronger triplet emission with respect to the Pt(II)
polyynes with 3, 7- diethynyl- 5, 5- dioxide-dibenzothiophene li-
gands [25]. The photophysical properties of the Pt(II) polyyne
bearing 2, 8- diethynyl-dibenzothiophene ligands has also been
reported by Wong's group [25], but its OPL behavior has not been
studied yet. Hence, more analogs should be synthesized and
Recently, transition metal polyynes have attracted great atten-
tion due to their various applications, such as organic light-
emitting diodes (OLEDs) [1,2], organic solar cells [3,4], ion sensors
[
5], nanomaterials [6,7] and so on. The extensive applications were
based on their unique properties through the interaction of organic
acetylide ligands and transition metal ions, such as Pt(II), Pd(II),
Au(I), Hg(II) etc. Among them, Pt(II) polyynes are the most widely
studied due to the advantages of stronger triplet emission, weak or
no absorption in visible light region, solubility and so on [8,9].
Importantly, the properties of the Pt(II) polyynes, such as, triplet
emission, absorption band, energy gap, can be fine-tuned by con-
trolling chemical structures of the organic acetylide ligands [10,11].
This provides great convenience for furnish unique properties and
applications to the Pt(II) polyynes.
So far, many organic acetylide ligands coordinating Pt(II) ions
have been reported [8e19]. It is worth noting that fluorene
*
Corresponding author.
E-mail address: zhougj@mail.xjtu.edu.cn (G. Zhou).
https://doi.org/10.1016/j.jorganchem.2019.05.022
022-328X/© 2019 Published by Elsevier B.V.
0

Z. Tian et al. / Journal of Organometallic Chemistry 895 (2019) 28e36
29
characterized to further explore their effect on the photophysical
properties and OPL behavior of the Pt(II) polyynes.
performance of each solution sample was measured in a 1 mm
quartz cell filled with CH Cl solution of the sample.
2
2
On this basis, we designed two series of Pt(II) polyynes bearing
fluorene-type ligands with ethynyl units at 2,7- and 3,6-position as
well as different structures at 9-position. In the absorption spectra,
the Pt(II) polyynes bearing fluorene-type ligands with ethynyl units
at 3,6-positon can show blue-shift effect compared with the ana-
logs bearing fluorene-type ligands with ethynyl units at 2,7-
positon, indicating their better transparency in the visible-light
region. Moreover, the Pt(II) polyynes bearing fluorene-type li-
gands with ethynyl units at 3,6-positon exhibit stronger triplet
emission in their PL spectra. In addition, these Pt(II) polyynes can
show obvious OPL response. Hence, the corresponding results will
offer valuable information about controlling the photophysical
properties and OPL performance of the Pt(II) polyynes by varying
substitution position of the organic acetylide ligands.
2.4. Computational details
Geometrical optimizations were performed using the popular
B3LYP density functional theory (DFT). The 6-311G(d, p) basis set
were used for C, H, O, S and P atoms, whereas effective core po-
tentials with a LanL2DZ basis set were employed for Pt atom
[27,28]. The energies of the excited states of the complexes were
computed by TD-DFT based on all the ground-state geometries. All
calculations were carried out by using the Gaussian 09 program
[29].
2.5. Synthesis
The ethynyl aromatic ligands were prepared by the published
Sonogashira coupling reaction [2,25,30]. All the synthetic proced-
ures are described in the Electronic Supplementary Information
(ESI).
2
. Experimental
2.1. General information
Reagents were used as obtained from commercial suppliers
2.6. General synthetic procedure for the Pt(II) polyynes
except where indicated otherwise. All the solvents were purified by
standard techniques. All reactions were preceded under a N at-
2
Under a N
and trans-[PtCl
2
atmosphere, the ethynyl aromatic ligand (1.0 equiv)
(PBu ] (1.0 equiv) were dissolved in Et N. Then,
mosphere. The reactions were monitored by thin-layer chroma-
tography (TLC) with Merck pre-coated aluminum plates. Flash
column chromatography and preparative TLC were carried out on
silica gel. All Sonogashira reactions were carried out with standard
Schlenk techniques.
2
3
)
2
3
CuI (0.01 equiv) was added and the mixture reaction was stirred at
room temperature for 12 h. After reaction, CuI was removed by
centrifugation and the solvent was removed under reduced pres-
sure. The obtained residue was dissolved in a small amount of
2 2
CH Cl and filtered by a syringe filter (PTFE, 0.45 mm) to remove
2
.2. Physical measurements
the insoluble part. Then, it was precipitated in cold methanol and
the collected precipitate was further purified by repeated pre-
cipitations to obtain the Pt(II) polyynes as a off-white solid in high
1H, 13C and 31P NMR spectra were recorded using a Bruker AXS
4
00 MHz NMR spectrometer in CDCl
3
solvent. Fast atom
yield.
1
bombardment (FAB) mass spectra were acquired using a Finnigan
MAT SSQ710 system. The molecular weights were determined by
Waters 2695 GPC and estimated by using a calibration curve of
C1-Pt: (Yield: 91%) H NMR (400 MHz, CDCl
3
):
d
(ppm) 7.50 (d,
2H, Ar), 7.23e7.16 (m, 14H, Ar), 2.06 (br, 12H, PBu
12H, PBu ), 1.35e1.33 (m, 12H, PBu ), 0.82 (t, 18H, PBu
(161.9 MHz, CDCl ): (ppm) 2.83; GPC: M
¼ 2.0 ꢀ 10 g mol
PDI ¼ 1.8 (against polystyrene standards).
O1-Pt: (Yield: 86%) H NMR (400 MHz, CDCl ): (ppm)
7.72e7.60 (m, 4H, Ar), 7.40 (d, 2H, Ar), 2.17 (br, 12H, PBu ), 1.63 (br,
12H, PBu ), 1.49e1.43 (m, 12H, PBu ), 0.94 (t, 18H, PBu ); P NMR
(161.9 MHz, CDCl ): (ppm) 3.24; GPC: M
¼ 1.8 ꢀ 10 g mol
PDI ¼ 2.2 (against polystyrene standards).
S1-Pt: (Yield: 89%) H NMR (400 MHz, CDCl ): (ppm) 7.90 (d,
2H, Ar), 7.69 (s, 2H, Ar) 7.33 (d, 2H, Ar), 2.17 (br, 12H, PBu ), 1.64 (m,
3
), 1.56e1.53 (m,
3
1
3
3
3
); P NMR
4
ꢁ1
polystyrene standards in CHCl
3
. UVevis spectra were recorded on a
3
d
n
,
PerkinElmer Lambda 950 spectrophotometer. The photo-
luminescent (PL) spectra were obtained with an Edinburgh In-
struments FLS920 fluorescence spectrophotometer. The lifetime
measurements at 298 K for the excited states were carried out using
by a single photon counting spectrometer from Edinburgh In-
struments FLS920 with a 360 nm ps LED lamp as the excitation
source, while those at 77 K were recorded with the excitation from
a Xenon flash lamp. Low-temperature PL spectra and lifetimes at
1
3
d
3
31
3
3
3
4
ꢁ1
,
3
d
n
1
3
d
3
3
1
7
7 K were measured by dipping the degassed CH
thin quartz tube into liquid nitrogen Dewar and recording data after
standing 3 min. Fluorescent quantum yields ( ) were determined
in CH Cl solution at 298 K, using the reference of quinine sulfate in
SO ca. 0.56 at 334 nm and ca. 0.55 at 365 nm) [26].
2
Cl
2
solution in a
12H, PBu
(161.9 MHz, CDCl
PDI ¼ 1.9 (against polystyrene standards).
3
), 1.52e1.42 (m, 12H, PBu
3
), 0.94 (t, 18H, PBu
3
); P NMR
4
ꢁ1
3
): (ppm) 3.27; GPC: M
d
n
¼ 1.7 ꢀ 10 g mol
,
F
F
1
2
2
PO1-Pt: (Yield: 85%) H NMR (400 MHz, CDCl ): (ppm)
3
d
1.0 M H
2
4
(F
F
7.65e7.46 (m, 7H, Ar), 7.37e7.35 (m, 4H, Ar), 2.05 (br, 12H, PBu ),
), 0.83 (t, 18H,
(ppm) 33.67, 3.12; GPC:
3
1.57e1.53 (m, 12H, PBu
3
), 1.38e1.32 (m, 12H, PBu
3
3
1
2.3. Optical power limiting measurements
PBu
3
); P NMR (161.9 MHz, CDCl ):
3
d
4
ꢁ1
M
n
¼ 1.6 ꢀ 10 g mol , PDI ¼ 1.8 (against polystyrene standards).
1
Optical power limiting (OPL) properties were recorded at
32 nm for Gaussian mode laser beam from a Q-switched Quantel
C2-Pt: (Yield: 91%) H NMR (400 MHz, CDCl
3
):
d
(ppm)
5
7.60e7.58 (m, 2H, Ar), 7.18 (m, 14H, Ar), 2.16 (br, 12H, PBu
3
), 1.63 (br,
31
Q-Smart 450 Nd:YAG laser with a repetition rate of 20 Hz. The laser
beam was split into two beams by a beam splitter. One was used as
the reference beam, which was directly received by a power de-
tector (D1). The other was focused with a lens (f ¼ 20 cm) for the
sample measurement and then received by another power detector
12H, PBu
(161.9 MHz, CDCl
PDI ¼ 2.0 (against polystyrene standards).
3
), 1.50e1.43 (m, 12H, PBu
3
), 0.93 (t, 18H, PBu
3
); P NMR
4
ꢁ1
,
3
): (ppm) 3.23; GPC: M
d
n
¼ 1.9 ꢀ 10 g mol
1
O2-Pt: (Yield: 86%) H NMR (400 MHz, CDCl ): (ppm) 7.77 (s,
3
d
2H, Ar), 7.36 (s, 4H, Ar), 2.20 (br, 12H, PBu ), 1.67 (m, 12H, PBu ),
1.51e1.46 (m, 12H, PBu ), 0.95 (t, 18H, PBu ); P NMR (161.9 MHz,
CDCl ): (ppm) 3.20; GPC:
¼ 1.8 ꢀ 10 g mol
(against polystyrene standards).
S2-Pt: (Yield: 89%) H NMR (400 MHz, CDCl ): (ppm) 7.98 (s,
3
3
31
(
D2) after transmitting through the sample. The sample to be
3
3
4
ꢁ1
measured was moved automatically along a rail to change the
incident irradiance on it. The incident and transmitted powers were
detected simultaneously by D1 and D2, individually. The OPL
3
d
M
n
,
PDI ¼ 1.6
1
3
d

30
Z. Tian et al. / Journal of Organometallic Chemistry 895 (2019) 28e36
Scheme 1. Synthesis of the Pt(II) polyynes bearing fluorene-type ligands with ethynyl units at different positions.

Z. Tian et al. / Journal of Organometallic Chemistry 895 (2019) 28e36
31
Fig. 1. UVevis absorption spectra for the Pt(II) polyynes in CH
2
Cl
2
at 298 K. (a) Series 1: C1-Pt, O1-Pt, S1-Pt and PO1-Pt (b) Series 2: C2-Pt, O2-Pt, S2-Pt and PO2-Pt.
Table 1
Photophysical data for the Pt(II) polyynes.
Compound Absorption
abs (nm)a 298 K
l
Emission
l
em (nm)a 298 K/77 K
F
b
(%) Lifetime of excited statesc
0.46 ns (418 nm)/103.6 s (559 nm)
S
1
state (ns)/T
state (
ms)
lcut-off (nm)
F
1
C1-Pt
O1-Pt
S1-Pt
PO1-Pt
C2-Pt
O2-Pt
S2-Pt
267, 310, 376sh, 400
250, 257, 283, 383
418,434,552,600/559,589,611
0.26
1.08
0.13
0.69
m
417
411
407
443
367
384
380
396
412, 438, 450 , 525/526, 550sh, 567
sh
0.54 ns (412 nm)/135
0.44 ns (406 nm)/89.3 m
0.55 ns (446 nm)/134.9
0.69 ns (366 nm)/64.3
m
s (526 nm)
s (534 nm)
s (602 nm)
ms (463 nm)
sh
245, 265 , 274, 305, 387
406, 432, 530, 577/534, 567, 584
267, 315, 348 , 371, 406sh, 423 446,463 ,586/602,656
sh
sh
m
273, 288 , 329sh, 350
sh
366, 466, 509, 546/463, 497, 514, 538, 552 0.02
sh
255, 264, 288, 344, 365
396,494, 531, 569/493,525,549
397, 494, 531, 578/493,527,549
0.06
0.04
0.03
0.45 ns (396 nm)/123.5
0.66 ns (397 nm)/129.8
2.30 ns (409 nm)/293.8
m
m
s (493 nm)
s (493 nm)
261, 269 , 296, 329sh, 341
sh
sh
sh
PO2-Pt
248, 293, 341 , 358
394 , 409, 436, 507, 542/503, 541, 561
m
s (503 nm)
a
ꢁ5
Measured in CH
2
Cl
Measured using quinine sulfate in 1.0 M H
65 nm while the of the standard is 56% and 55%.
The numbers in parentheses are the emission wavelengths of the S
2
at a concentration of ca. 10 M. sh: Shoulder.
b
2
4
SO as the standard. According to the UVevis absorption of the compounds, the excitation wavelength was set at 334 nm and
3
F
F
c
1
1 1 2 2
and T states. The lifetime of the S state was measured at 298 K in degassed CH Cl with the excitation
at 360 nm and that for the T
1
states was measured at 77 K in the same solvent with the same excitation wavelength.
Table 2
0
TD-DFT results for the repeating units of the Pt(II) polyynes based on their optimized S geometries.
Compound Contribution of d
to HOMOa (Pt)
p
orbitals Contribution of d
p
orbitals Largest coefficient in the CI
expansion of the S
stateb
Percentage contribution of the
Oscillator strength (f) of
the S /S
a
stateb
to LUMO (Pt)
1
transition to the S
1
0
1
C1-Pt
O1-Pt
S1-Pt
PO1-Pt
C2-Pt
O2-Pt
S2-Pt
6.58%
7.00%
7.20%
11.30%
9.14%
9.52%
8.34%
10.10%
0.42%
0.42%
0.41%
1.66%
0.16%
0.00%
0.00%
0.22%
H/L (0.66026) 372 nm
H/L (0.67379) 363 nm
H/L (0.67030) 363 nm
H/L (0.66028) 405 nm
H/L (0.65121) 332 nm
H/L (0.63183) 335 nm
H/L (0.59508) 339 nm
H/L (0.66872) 363 nm
87.2%
90.8%
89.8%
87.2%
84.8%
79.8%
70.8%
89.4%
1.6005
1.5427
1.6060
1.4465
0.0971
0.0337
0.0179
0.3923
PO2-Pt
a
The data were obtained by exporting DFT results with the software AOMix.
H/ L represents the HOMO to LUMO transition. CI stands for configuration interaction.
b
2
H, Ar), 7.64 (d, 2H, Ar), 7.37 (d, 2H, Ar), 2.20 (br,12H, PBu
3
), 1.68 (br,
pathways of the Pt(II) polyynes. Schemes S1 and S2 in ESI provide
the preparation of the ethynyl aromatic ligands. The designed Pt(II)
polyynes can be easily prepared using the Sonogashira cross-
coupling procedure between the different ethynyl aromatic li-
31
1
2H, PBu
161.9 MHz, CDCl
PDI ¼ 2.2 (against polystyrene standards).
3
), 1.48e1.47 (m, 12H, PBu
3
3
), 0.95 (t, 18H, PBu ); P NMR
4
ꢁ1
,
(
3
): (ppm) 3.30; GPC: M
d
n
¼ 1.7 ꢀ 10 g mol
1
PO2-Pt: (Yield: 85%) H NMR (400 MHz, CDCl ): (ppm)
3
d
2 3 2 3
gands and trans-[PtCl (PBu ) ] in Et N with CuI as catalyst (Scheme
7
(
(
.66e7.61 (m, 4H, Ar), 7.57e7.46 (m, 4H, Ar), 7.38 (m, 3H, Ar), 2.12
br, 12H, PBu ), 1.63 (br, 12H, PBu ), 1.49e1.40 (m, 12H, PBu ), 0.93
m, 18H, PBu ); P NMR (161.9 MHz, CDCl ): (ppm) 33.06, 3.70;
¼ 1.9 ꢀ 10 g mol PDI ¼ 1.6 (against polystyrene
standards).
1). The chemical structures of these Pt(II) polyynes have been fully
1
31
1
3
3
3
characterized by H and P NMR spectra. In the H NMR spectra of
these Pt(II) polyynes, the absence of the singlet peak at ca. 3.2 ppm
assigned to the proton in the alkyne groups indicates the successful
coupling between the alkyne groups in the aromatic ligands and
31
3
3
d
4
ꢁ1
,
GPC:
M
n
31
trans-[PtCl
polyynes, the strong single resonance peaks in the range of
3 2
.12e3.83 ppm indicates the presence of the trans-[PtCl (PBu ) ]
2 3 2
(PBu ) ] units. In the P NMR spectra of these Pt(II)
3
. Results and discussion
3
2
units. For PO1-Pt and PO2-Pt, the resonance peak at ca. 33.0 ppm in
their P NMR spectra can be assign to the POPh units in their
3.1. Synthesis and structural characterization
31
organic ligands.
Scheme 1 shows the chemical structures and the synthetic

32
Z. Tian et al. / Journal of Organometallic Chemistry 895 (2019) 28e36
3.2. Photophysical properties
The absorption properties of the Pt(II) polyynes were measured
in CH Cl at 298 K (Fig. 1) and the corresponding data are sum-
marized in Table 1. Obviously, the maximum absorption wave-
lengths ( max) of the Pt(II) polyynes bearing fluorene-type ligands
2
2
l
with ethynyl units at 3,6-positon (Series 2) show blue-shift effect
compared with those of corresponding Pt(II) polyynes with ethynyl
units at 2,7-positon (Series 1) (Table 1). The cut-off absorption
wavelengths (
blue-shift effect compared with the Pt(II) polyynes in Series 1.
Moreover, the cut-off of the fluorene-type ligands with
lcut-off) of the Pt(II) polyynes in Series 2 also show
lmax and l
ethynyl units at 3,6-position also show blue-shift effect compared
with those bearing ethynyl units at 2,7-position (Fig. S1 and
Table S1 in ESI). These results indicate that ethynyl units at 2,7-
position of the fluorene-type ligands are more conducive to extend
the p-conjugation in both the Pt(II) polyynes and the ethynyl aro-
matic ligands. In addition, the absorption bands of the Pt(II) poly-
ynes in Series 2 are nearly located in the UV region (Fig. 1b and
Table 1), indicating their excellent transparency in the visible-light
region (ca. 400e700 nm). However, the Pt(II) polyynes in Series 1
show inferior transparency in the visible-light region than those in
Series 2 due to their obvious absorption after 400 nm. In addition,
the major absorption bands of PO1-Pt and PO2-Pt can show red-
shift effect compared with other Pt(II) polyynes in the same se-
ries, due to the electron-withdrawing property of the P¼O unit.
It has been shown that the transition metal polyynes can show
similar transition properties to their repeating units [31]. So, the
molecular orbital (MO) patterns of the repeating unit for the cor-
responding the Pt(II) polyynes have been obtained theoretically by
time-dependent density functional theory (TD-DFT) calculations to
understand their absorption behavior (Fig. 1). Based on the TD-DFT
results in Table 2, the transitions between the key molecular or-
bitals can show the feature of the first singlet state (S
repeating units of these Pt(II) polyynes due to their large contri-
butions to the transition of the S state. From the MO distributions
1
) of the
1
of both HOMO and LUMO for the repeating units of these Pt(II)
polyynes, it can be clearly seen that their HOMO / LUMO (H / L)
transition show obvious ligand-centered
Therefore, the UV absorption of these Pt(II) polyynes can be
assigned to the metal disturbed * transitions from the ethynyl
pep* character (Fig. 2).
pep
aromatic ligands (Fig. 1). In addition, due to their higher extent of
electron delocalization in their HOMO orbitals, the repeating units
of the Pt(II) polyynes in Series 1 show longer conjugation than
those of the Pt(II) polyynes in Series 2. This adequately explains the
red-shift effect in absorption maxima of the Pt(II) polyynes in Series
1
compared with corresponding Pt(II) polyynes in Series 2. Owing
Fig. 2. Molecular orbital (MO) patterns for the repeating units of the Pt(II) polyynes
based on their optimized S geometries. (a) repeating unit for C1-Pt, (b) repeating unit
for O1-Pt, (c) repeating unit for S1-Pt, (d) repeating unit for PO1-Pt, (e) repeating unit
for C2-Pt, (f) repeating unit for O2-Pt, (g) repeating unit for S2-Pt, (h) repeating unit
for PO2-Pt.
to the strong electron-accepting ability associated with the five-
membered ring of phosphole oxide indicated by its substantial
contribution to the LUMO (Fig. 2), it should promote H / L electron
transition in PO1-Pt and PO2-Pt and lower absorption energy
associated with the electron transition process. Hence, the red-shift
effect has been observed in the absorption spectra of PO1-Pt and
PO2-Pt compared other with other Pt(II) polyynes of the same
series (Fig. 1).
0
solution at low temperature of 77 K, indicating their T
as well. Obviously, the Pt(II) polyynes in Series 2 show stronger
triplet emission and lower compared with their corresponding
Pt(II) polyynes in Series 1, which means the Pt(II) polyynes in Series
have higher triplet quantum yield. Due to the longer conjugation
length of the fluorene-type ligands of ethynyl units at 2,7-position,
the S /S transition of the repeating units of the Pt(II) polyynes in
1
characters
F
F
2 2
In CH Cl solution at 298 K, all the Pt(II) polyynes exhibit two
2
emission bands in the PL spectra (Fig. 3 and Table 1). Because of
their short lifetimes in the order of nanosecond (ns) (Table 1), the
high-energy emission bands should be induced by the decay of the
singlet states (S
ligand * transitions. Conversely, the long-wavelength emission
bands should come from the decay of the triplet states (T ) due to
their much longer lifetimes in the order of microsecond ( s) and
0
1
Series 1 is spatially further away from the Pt(II) center than those of
the repeating units of the Pt(II) polyynes in Series 2, reducing their
SOC effect and hence triplet quantum yield [10]. Meanwhile,
compared with those of the repeating units of the Pt(II) polyynes in
1
) associated with the metal disturbed organic
pep
1
m
p
Series 1, d orbitals of the Pt(II) centers can give more contribution
large Stokes shift (>150 nm). The long-wavelength emission signals
of all the Pt(II) polyynes have been substantially enhanced in
to the HOMOs of the repeating units of the Pt(II) polyynes in Series

Z. Tian et al. / Journal of Organometallic Chemistry 895 (2019) 28e36
33
2 2
Fig. 3. Photoluminescent (PL) spectra for the Pt(II) polyynes in CH Cl solution at both 298 K and 77 K.
2
(Table 2). It means that higher mixing extent between MOs from
polyynes in Series 2 can furnish higher triplet quantum yield
indicated by the much stronger triplet emission even at 298 K
(Fig. 3).
the Pt(II) centers and organic ligands to guarantee stronger SOC
effect in the Pt(II) polyynes of Series 2. As a result, the Pt(II)

34
Z. Tian et al. / Journal of Organometallic Chemistry 895 (2019) 28e36
The natural transition orbital (NTO) results for the repeating
units of these Pt(II) polyynes have been obtained based on the
optimized T geometries for the S /T excitation [32], which can
indicate the features of their triplet emission. From the NTO pat-
terns for the repeating units of the Pt(II) polyynes in Series 1
these Pt(II) polyynes should mainly show ligand-centered 3
p-p*
feature, which has been indicated by the structured line-shape of
their PL spectra at 77 K (Fig. 3). The results show good consistency
between the experimental results and the theoretical results,
which reflects the validity of the theoretical calculation.
1
0
1
(
Fig. 4), both their hole (H) and particle (P) are mainly located on
the ethynyl aromatic ligands on one side of the Pt(II) centers
Table 3). Hence, the H/P transition should indicate the ligand-
3.3. Optical power limiting behaviors
(
3
centered
pep* character for the triplet emission in the Pt(II) pol-
From the UVevis spectra of the Pt(II) polyynes (Fig. 1), it can be
yynes of Series 1. For the repeating units of the Pt(II) polyynes in
Series 2, two pairs of main H/P transitions account for their
S /T excitation (Fig. 4). Clearly, both of the two pairs of H/P
0 1
transitions are mainly located on the two ethynyl aromatic ligands.
From the NTO results in Table 3, the Pt(II) centers in polyynes of
Series 1 and 2 make very slight contributions to their H/P tran-
clearly seen that they exhibit high transparency at 532 nm, indi-
cating their low ground-state absorption at 532 nm. Hence, their
absorption behaviors are suitable to characterize their OPL prop-
erties against 532 nm laser beam. The OPL performances of these
Pt(II) polyynes have been characterized in CH
transmittance of T ~94% through Z-scan method with an open-
aperture mode. From the Z-scan curves (Fig. 5), it can be seen
2 2
Cl with a high linear
o
1 1
sitions responsible for the T emission. Hence, the T emission of
Fig. 4. Natural transition orbital (NTO) patterns for S
C1-Pt, (b) repeating unit for O1-Pt, (c) repeating unit for S1-Pt, (d) repeating unit for PO1-Pt, (e) repeating unit for C2-Pt, (f) repeating unit for O2-Pt, (g) repeating unit for S2-Pt,
h) repeating unit for PO2-Pt.
0 1 1
/ T excitation of the repeating units from the Pt(II) polyynes based on their optimized T geometries. (a) repeating unit for
(

Z. Tian et al. / Journal of Organometallic Chemistry 895 (2019) 28e36
35
Table 3
NTO results for the repeating units from the Pt(II) polyynes based on the T
1
geometries.
Compound
NTOa
Contribution percentages of metal d
p
Compound
NTOa
Contribution percentages of metal dp
orbitals and
NTOs (%)
p
orbitals of ligands to
orbitals and
p orbitals of ligands to
b
NTOsb (%)
Pt
L1
L2
Pt
L1
L2
C1-Pt
O1-Pt
S1-Pt
H
P
2.06
95.90
1.08
0.95
1.40
1.29
1.36
1.24
9.67
10.39
C2-Pt
O2-Pt
S2-Pt
H1
P1
H2
P2
H1
P1
H2
P2
H1
P1
H2
P2
H1
P1
H2
P2
2.88
0.37
0.10
0.58
3.00
0.83
1.10
0.33
3.82
0.91
0.48
1.01
3.67
0.32
0.23
0.78
53.43
59.21
59.55
60.00
85.42
87.48
49.07
48.80
72.53
74.59
68.21
69.25
65.57
67.99
66.75
67.84
37.94
38.99
37.97
38.99
10.61
10.88
49.07
48.80
22.62
23.43
29.69
29.18
29.98
31.04
31.37
31.04
0.65
2.36
0.67
2.35
0.67
3.96
1.19
97.97
95.18
97.59
95.24
97.63
85.66
87.81
H
P
H
P
PO1-Pt
H
P
PO2-Pt
a
H and P represent NTO hole and particle orbital, respectively.
Pt represent the Pt(II) centers, while L1 and L2 indicate the ethynyl aromatic ligands.
b
o
Fig. 5. Open-aperture Z-scan results for the Pt(II) polyynes (T ~94%). (a) Series 1: C1-Pt, O1-Pt, S1-Pt and PO1-Pt (b) Series 2: C2-Pt, O2-Pt, S2-Pt and PO2-Pt.
that the incident irradiance upon the Pt(II) polyynes solution
sample was changed with their Z-position (against focal point
Z ¼ 0). When the sample stays far from the focal point with the
weak incident laser irradiance, the T of the Pt(II) polyyne solution
can remains constant, indicting linear optical property (i.e. obeying
Beer's law). However, when the sample is gradually moved towards
the focus with the incident laser irradiance increase, the T of the
sample decreases to show OPL effect (Fig. 5).
(Fig. S2 in ESI).
The OPL performances of these Pt(II) polyynes based on the RSA
mechanism can be compared by using the figure of merit factor
¼ lnTsat/lnT [33], where and ex represent the ground-state
absorption cross-section and the effective excited-state absorp-
tion cross-section, respectively. T and Tsat are the linear trans-
mittance and the transmittance at the saturation fluence,
respectively. From the Z-scan results, the values of C1-Pt, O1-
Pt, S1-Pt and PO1-Pt are 16.0, 14.5, 12.9 and 14.7, respectively. In
addition, the values of C2-Pt, O2-Pt, S2-Pt and PO2-Pt are
ex
s /
s
o
o
s
o
s
o
ex o
s /s
Based on the aforementioned photophysical properties, the OPL
performance of these Pt(II) polyynes can be explained by the
ex o
s /s
reverse saturable absorption (RSA) mechanism of the T
Fig. S2 in ESI) [10]. The molecular in the ground state (S ) can be
excited to the first singlet states (S ) by initially absorbing laser
energy of laser irradiance. The S state can go to the first triplet state
) through intersystem crossing (ISC) process due to the strong
SOC effect induced by the Pt(II) centers. The T states with long
1
states
4.9, 6.0, 4.8 and 4.9, respectively. Owing to their longer conjugation
indicated by their UV absorption spectra, the Pt(II) polyynes in
Series 1 show stronger OPL responses than those in Series 2.
However, based on the RSA mechanism and their stronger triplet
emission at 298 K, the Pt(II) polyynes in Series 2 should show better
OPL ability for 532 nm ns laser than those in Series 1. This result can
be ascribed to their too low ground-state absorption at 532 nm to
induce strong enough OPL response. Critically, the excellent
transparency in visible light region still render the Pt(II) polyynes in
Series 2 good candidates as OPL materials. Hence, the position of
the ethynyl units at 2,7- and 3,6-position of fluorene-type ligands
have a great influence on the OPL behavior of the Pt(II) polyynes.
(
0
1
1
(T
1
1
lifetime in micro-second range can easily accumulate a certain
population through fast ISC process and then absorb more laser
energy to reach the higher triplet states (T
tion of nano-second order. Due to the short lifetime of the S
the contribution of the OPL effect from optical absorption accom-
panied with the S /S transition are generally neglected. Hence,
the T /T transition leads to the OPL effect of these Pt(II) polyynes
n
) in a laser pulse dura-
1
state,
1
n
1
n

3
6
Z. Tian et al. / Journal of Organometallic Chemistry 895 (2019) 28e36
Chem. Soc. Rev. 47 (2018) 4934e4953.
4
. Conclusions
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Two series of Pt(II) polyynes bearing fluorene-type ligands with
[8] G. Zhou, W.-Y. Wong, Chem. Soc. Rev. 40 (2011) 2541e2566.
two ethynyl units at different position were successfully synthe-
sized. Compared with the Pt(II) polyynes (Series 1) bearing ligands
with ethynyl units at 2,7-position, the Pt(II) polyynes (Series 2)
bearing ligands with ethynyl units at 3,6-position show blue-shift
effect in their absorption maxima and much stronger triplet
emission. Moreover, the OPL performances of the Pt(II) polyynes
can be tuned by the substitution position of the ethynyl units as
well. The Pt(II) polyynes in Series 1 can show enhanced OPL
response, while those in Series 2 exhibit much better transparency
in visible light region. The above results can provide very helpful
information to controlling the photophysical properties and OPL
ability of the Pt(II) polyynes by designing organic acetylide ligands.
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Acknowledgement
[
[
This research was financially supported by the National Natural
Science Foundation of China (no. 21572176, 21602170), the Funda-
mental Research Funds for the Central Universities (cxtd2015003,
xjj2016061), the Key Creative Scientific Research Team in Yulin City,
Shaanxi Province, the China Postdoctoral Science Foundation
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2015M580831) and the Program for New Century Excellent Talents
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The financial supporting from State Key Laboratory for Mechanical
Behavior of Materials is also acknowledged. The characterizations
of materials are supported by Instrument Analysis Center of Xi'an
Jiaotong University.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jorganchem.2019.05.022.
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