Synthesis and photophysical properties of platinum(II) arylacetylides with donor-acceptor structures

Article abstract of DOI:10.1016/j.jorganchem.2013.11.032

The detailed relationship between the structure and the nonlinear mechanisms, as well as whether different nonlinear mechanisms of an organic molecule restrain each other are still not clear. In this paper, eight platinum(II) arylacetylides with donor-acceptor structures were synthesized and their photophysical properties were studied systematically. The properties of HOMO and LUMO were calculated using DFT method, the contribution of the metal d_π and charge transfer process are dominated by the skeleton structure of these complexes. The ligand-to-metal charge transfer (LMCT) happens when the molecule with D-π-Pt-π-D or D-π-Pt-π-Ar-π-Pt-π-D skeleton is excited, however the metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT) happen when the molecule with D-π-Pt-π-A, A-π-Pt-π-A or A-π-Pt-π-Ar-π-Pt-π-A skeleton is excited, respectively. The weak absorption in the visible area of these molecules provides a wide observation window for the application in optical limiter. The low fluorescent quantum yields and detectable room temperature phosphorescence indicate that the system-cross quantum yields of the title complexes containing Pt(II) is effective because of the spin-orbit coupling between the d orbitals of the Pt(II) and the large π-conjugated system. The first-order polarizabilities |β_XXX| was determined by the solvatochromic method between 1.02 × 10^-29 to 3.46 × 10^-28 (cm⁵/esu).

Full text of DOI:10.1016/j.jorganchem.2013.11.032

Journal of Organometallic Chemistry 752 (2014) 6e11
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and photophysical properties of platinum(II) arylacetylides
with donoreacceptor structures
a
a
b
b
a
b
Denghui Yao , Haiping Wang , Jin Guo , Tingfeng Wang , Liying Zhang , Junfeng Shao ,
Fengqi Guo
a
a
,
*
College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, PR China
State Key Laboratory of Laser Interaction with Matter, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences,
b
Changchun 130033, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 September 2013
Received in revised form
The detailed relationship between the structure and the nonlinear mechanisms, as well as whether
different nonlinear mechanisms of an organic molecule restrain each other are still not clear. In this
paper, eight platinum(II) arylacetylides with donoreacceptor structures were synthesized and their
photophysical properties were studied systematically. The properties of HOMO and LUMO were calcu-
18 November 2013
Accepted 23 November 2013
lated using DFT method, the contribution of the metal d
the skeleton structure of these complexes. The ligand-to-metal charge transfer (LMCT) happens when
the molecule with De ePte eD or De ePte eAre ePte eD skeleton is excited, however the
metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT) happen when the
molecule with De ePte eA, Ae ePte eA or Ae ePte eAre ePte eA skeleton is excited,
p
and charge transfer process are dominated by
Keywords:
p
p
p
p
p
p
Platinum(II)
Arylacetylides
Nonlinear
p
p
p
p
p
p
p
p
Photophysical
respectively. The weak absorption in the visible area of these molecules provides a wide observation
window for the application in optical limiter. The low fluorescent quantum yields and detectable room
temperature phosphorescence indicate that the system-cross quantum yields of the title complexes
containing Pt(II) is effective because of the spin-orbit coupling between the d orbitals of the Pt(II) and the
large
p
-conjugated system. The first-order polarizabilities j
b
xxxj was determined by the solvatochromic
ꢁ
29
ꢁ28
5
method between 1.02 ꢀ 10
to 3.46 ꢀ 10
(cm /esu).
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
These conclusions can provide helpful guidelines for designing
materials with large nonlinear optical properties.
Organic and organometallic materials with large nonlinear op-
Platinum acetylide oligomers and polymers exhibit high linear
transmission and strong reverse saturable absorption from visible
to infrared spectral region because of the spineorbit coupling
interaction between the d orbitals of the heavy metal Pt(II) ion and
tical properties have been attracting great interests in the last two
decades due to their potential applications in optical limiting,
photodynamic therapy, and optical data storage, in which the two-
photon absorption and excited state absorption are the two most
important mechanisms [1e6]. A significant conclusion from the
previous work could be made that the molecules with donor (D)e
the large p-conjugated system [11e13]. The two-photon absorption
phenomenon of several platinum acetylide oligomers has also been
reported recently, and the enhanced nonlinear absorption based on
the absorption from both the singlet and triplet states also have
been confirmed [14e17]. Although some papers have reported the
relationship between the structure and reverse saturable absorp-
tion properties for the ns laser [11,13,18], or the two-photon ab-
sorption properties for ps laser [14,15], or the tripletetriplet
upconversion properties [19,20] of platinum acetylide oligomers
and polymers, respectively, the detailed relationship between the
structure, the two-photon absorption and excited state absorption
of the organic molecules are still not discussed clearly. Especially,
whether the two-photon absorption and the excited state absorp-
tion of an organic molecule restrain each other is also not discussed
acceptor (A) structures, such as DeA, DeAeD, DepeA, DepeD, De
p
eAe eD and Ae eDe eA, generally exhibit excellent two-
p
p
p
photon absorption properties, and the two-photon absorption
cross sections can be increased by extending the molecular con-
jugated system [7,8]. In addition, molecules containing heavy metal
ions and rigid structure always exhibit large reverse saturable ab-
sorption due to the enhancement of the intersystem crossing from
the first singlet excited state to the first triplet excited state [9e11].
*
Corresponding author. Tel.: þ86 371 67739186.
E-mail address: fqguo@zzu.edu.cn (F. Guo).
0
022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.11.032

D. Yao et al. / Journal of Organometallic Chemistry 752 (2014) 6e11
7
PBu
Pt
3
n-C
3
H
7
n-C
3
H
7
n-C
3
H
7
3 7
n-C H
PBu
3
1
2
PBu
Pt
3
PBu
Pt
PBu
3
3
n-C
3
H
7
NO
2
O
2
N
NO
2
PBu
3
4
3
PBu
Pt
3
PBu
Pt
PBu3
Pt
PBu3
Pt
3
O
2
N
NO
2
O
2
N
NO
2
PBu
3
PBu
3
PBu
3
PBu
3
5
6
PBu
Pt
3
PBu
Pt
PBu
Pt
PBu
3
PBu
Pt
PBu
3
3
3
n-C
3
H
7
3 7
n-C H
n-C
3
H
7
3 7
n-C H
PBu
3
3
PBu
3
7
8
Fig. 1. Structures of the synthesized platinum acetylide oligomers with donoreacceptor structures.
deeply up to now. So it is necessary and important to address these
questions for design nonlinear optical materials.
In this paper, a series of symmetric and unsymmetrical platinum
acetylides with donoreacceptor structures, as shown in Fig. 1, were
designed and synthesized. The design of complexes 1 to 4 is based
All the starting materials were purchased from commercial
available sources and used without any purification. All the sol-
vents were dried and distilled by the standard methods. The initial
arylacetylenes were synthesized from the corresponding aryl bro-
mides and 2-methylbut-3-yn-2-ol via Sonagashira coupling reac-
on the well-known Ae
peA, DepeA and De
peD motifs yielding
3 2 2
tion according to the literature [23e26]. Trans-[Pt(PBu ) Cl ] was
materials with significant two-photon absorption, and the inser-
tion of the heavy metal atom into an organic molecule often yields
materials with strong excited state absorption from the triplet
state. The synthesized platinum acetylides combine the donore
acceptor structure and heavy metal in one molecule, which make it
possible to study the interaction between the two-photon ab-
sorption and excited state absorption related to the structure of the
organic molecules. The component of metal and the conjugated
length of the complexes 5 to 8 are increased in comparison with
complexes 1 to 4. The increase of the component of the metal
should increase the intersystem crossing efficient of the molecule,
this is helpful to understand the relationship between the excited
absorption from the triplet state and the two-photon absorption, as
well as how the two nonlinear optical properties restrain each
other. This paper describes the synthesis processes and the detailed
studies of the photophysical properties of the designed molecules.
synthesized according to the literature method [27].
3. Result and discussion
3.1. Quantum calculation
The geometry of the platinum acetylides 2, 3, 4, 5 and 7 were
optimized and the HOMO and LUMO were calculated using the
density functional theory (DFT) at 6-31G(d) level for C, H, N, O, P
and Lanl2DZ level for Pt atoms, respectively. All the butyl groups in
the molecules were replaced by the methyl groups in order to
simplify the calculation process. The calculations were performed
with the Gaussian 03 software [28]. The calculated electronic dis-
tribution situation is shown in Fig. 2 and the calculated energy of
HOMO and LUMO are listed in table 1.
As shown in Fig. 2, the electronic density of the HOMO of 2 with
symmetrical DepePtepeD skeleton is spread uniformly on the
2
. Materials and methods
molecule, whereas the electronic density of the LUMO orbital is
mostly located at the center of the molecule. An obvious ligand-to-
metal charge transfer (LMCT) happens when molecule is excited,
1HNMR spectra were recorded on a DRX-400 spectrometer and
the chemical shifts are reported relative to TMS. MALDI-TOF mass
spectra were recorded on Autoflex III system. Elemental analyses
were performed on a Flash EA 1112 Analyzer. Electronic absorption
spectra were obtained on a UV-5200 spectrophotometer. Lumi-
nescence spectra were collected with a CaryEclipse fluorescence
spectrometer. Fluorescence quantum yield and phosphorescence
quantum yield were determined relative to the fluorescence stan-
and the contribution of the metal d
that in HOMO. The similar result is collected for 7. The electronic
density of the HOMO of 3 with unsymmetrical De ePte eA
skeleton is located at the donor side, whereas the electronic density
of LUMO orbital is mostly located at the acceptor side. An obvious
ligand-to-ligand charge transfer (LLCT) happens when the mole-
p
in LUMO is much higher than
p
p
cule is excited, and the contribution of the metal d
lower than that in HOMO. The electronic density of the HOMO of 4
with symmetrical Ae ePte eA skeleton is dispersed uniformly
p
in LUMO is
dard compound quinine sulfate (
F
em ¼ 0.55 at 313 nm excitation)
21] and phosphorescence stand complex [Ru(bpy) ]Cl
em ¼ 0.042 at 436 nm excitation) [22]. Lifetimes were measured
[
(
3
2
p
p
F
on the whole molecule, whereas the electronic density of the LUMO
orbital is mostly located at the two acceptor sides. An obvious
on a FM-4P-TCSPC time-resolved fluorescence spectrometer.

8
D. Yao et al. / Journal of Organometallic Chemistry 752 (2014) 6e11
Fig. 2. Electronic density distribution of the HOMO and LUMO orbitals of the platinum acetylides with donoreacceptor structure.
metal-to-ligand charge transfer (MLCT) happens when the mole-
cule is excited, and almost no contribution of the metal d involves
a clear red shift in the absorption maxima when the conjugated
system of the molecule is enlarged. However, for complexes 4, 5
and 6, in which two electron acceptors were connected to the two
sides of the molecules, there is no clear red shift in the absorption
maxima. Comparison of the absorption spectra of 2, 3 and 4, 5 and
7, 6 and 8, it can be concluded that replacing the electron donor by
electron acceptor can induce the absorption maxima red shift. This
is in accord with the calculated results and the literature report
[13,18].
Fig. 4a and b presents the luminescence spectra of 1e8 in air-
free chloroform at room temperature and the emission data are
summarized in Table 2. The maxima of the luminescence spectra of
1 and 2 are all located at 374 nm, the emission intensities of them
p
in LUMO. A similar result is collected for 5. The platinum acetylides
with donoreacceptor structure keep the conjugated system after
the insertion of Pt(II) ion into the skeleton of an organic molecule
and the electron transfer process could occur by passing through
the metal ions. In addition, the charge transfer behavior of these
complexes should be dominated by the structure of the molecular
skeleton, which makes it possible to study the effect of the insertion
of heavy metal ions on the two photon-absorption of the organic
molecules with donor-accepter structures.
According to the calculated results listed in Table 1, the energies
of the HOMO and LUMO of the complexes decrease when the
electron donor is replaced by electron acceptor, this is in accord
with the literature [6]. In addition, comparing the orbital energies
of 2, 7, 4 and 5 indicates that the insertion of the central aryl groups
increases the energy levels of HOMO and LUMO, and the energy
intervals between HOMO and LUMO decrease at the same time. In
addition, the insertion of the central aryl groups makes the elec-
tronic density of HOMO more localized on the conjugated ligands
2
don’t change greatly after the N gas was bubbled and the lifetimes
are 0.44 ns and 1.47 ns, respectively, which indicate that the
emission peaks are the fluorescence. The luminescence spectra of
3e8 in air-saturated chloroform display two main peaks. The
former peak with shorter wavelength turns weak and the latter
peak turns strong in air-free solution (Fig. S17). As shown in Table 1,
the lifetimes of former peaks are between 1 and 3 ns and the life-
with less contribution of the metal d
p
. These calculation results are
times of the latter peaks are between 1 and 6 ms. So the former
consistent with the reported results by Schanze [18,20].
emission peak can be attributed to the fluorescence and the latter
emission peak can be attributed to the phosphorescence. The
stokes shift of 1 (59 nm) is much larger than that of 2 (38 nm),
which indicates that the insertion of Pt(II) ion decreases the
structural rigidity of the molecule and decreases reorganization
energy between the ground state and the first singlet excited state.
It can be seen from comparison of the Stokes shifts of 2, 3, and 4
that replacing the electron donor by electron acceptor can induce
the increase of the Stokes shift. For complexes 4, 5 and 6, the
3.2. Photophysical properties
Fig. 3 shows the electronic absorption spectra of 1e8 in chlo-
roform and the data are summarized in Table 2. All the spectra
exhibit intense absorption bands from 300 nm to 400 nm, which
can be assigned to the
absorption bands from 200 nm to 300 nm, which can be assigned
to * transition of the aryl rings. This observation is consistent
pep* transition, and moderate intense
pep
with other reports in the literature [20,29]. The weak absorption
above 450 nm provides a wide observation window for the
application in optical limiter. The absorption maxima of 2 red
shifted by 21 nm compared to 1, which shows that the insertion of
Pt(II) ion not only retains but also enlarges the conjugated system
of the molecule. For complexes 2, 7 and 8, in which two electron
donors were connected to the two sides of the molecules, there is
Table 1
The calculated energies of HOMO and LUMO of the synthesized complexes.
Complex
1
2
3
4
5
7
LUMO/a.u.
HOMO/a.u.
ꢁ0.045
ꢁ0.205
0.160
0.086
0.041
0.034
0.040
0.087
ꢁ0.265
ꢁ0.277
ꢁ0.311
ꢁ0.265
ꢁ0.250
DE/a.u.
0.351
0.318
0.345
0.305
0.337

D. Yao et al. / Journal of Organometallic Chemistry 752 (2014) 6e11
9
4
4
3
3
2
2
1
1
0
0
.5
.0
.5
.0
.5
.0
.5
.0
.5
.0
8
a
1
2
3
4
b
5
6
7
8
7
6
5
4
3
2
1
0
2
00
250
300
350
400
450
500
200
250
300
350
400
450
500
W avelength (nm )
W avelength (nm)
Fig. 3. Electronic absorption spectra of compounds 1e8 in chloroform.
insertion of two Pt(II) ions and the central aryl groups induced the
decrease of Stokes shift. It is because that the increase of the
The fluorescence quantum yields (
plexes are measured using relative method and the data is listed in
Table 2. 1 has the highest , which indicates the heavy atom effect
decrease the because of the increase of the intersystem crossing
quantum yield. The increased intersystem crossing quantum yields
make it possible for the molecules to possess great triplet excited
state absorption. The phosphorescence of the synthesized com-
plexes is seriously overlapped with the corresponding fluorescence,
so we do not calculate the quantum yields of the phosphorescence.
f
F ) of the synthesized com-
s
bands in the molecule decease the rigidity of the molecule. In
addition, the Stokes shifts increase when the benzyl group was
replaced by the naphthanyl group as the central aryl group. It is
because that the structural rigidity of the naphthanyl group is
greater that of benzyl group. The same conclusion can be drawn
from the comparison of the Stokes shifts of 2, 7, and 8. The increase
of the Stokes shift also indicates the increase of the reorganization
energy between the ground state and the first singlet excited state.
F
f
F
f
3.3. The first-order hyperpolarizabilities
Table 2
Photophysical properties of 1e8.
The first-order polarizability ( xxx) of organic molecules can be
b
Compound
lmax/nm
f
l /nm Stokes-shift/nm
p
l /nm
F
f
f
s /ns
s /m
p
s
determined by the solvatochromism method. The simple expres-
4
sion of the first-order polarizability is shown in Eq. (1) [30]:
1
2
3
4
5
6
7
8
315(2.77 ꢀ 10 ) 374 59
e
e
0.053 0.44
0.038 1.47
e
e
4
336(3.52 ꢀ 10 ) 374 38
ꢀ
ꢁ
ꢀ
ꢁꢀ
ꢁ
4
2
2
2
2
2
2
2
390(2.75 ꢀ 10 ) 438 48
601
587
588
593
516
634
0.017 1.34 1.11
0.016 1.29 5.20
0.045 1.36 3.34
0.030 1.29 1.24
0.041 2.58 1.12
0.029 1.42 1.93
xxxð2
b u
Þ ¼ 3=2-
m
m
ꢁ
m
u
=
u
ꢁ
u
u
ꢁ 4
u
eg
e
g
eg
eg
eg
4
395(3.95 ꢀ 10 ) 468 73
4
394(7.32 ꢀ 10 ) 433 39
(1)
4
396(6.76 ꢀ 10 ) 443 47
4
361(4.36 ꢀ 10 ) 380 19
where ueg is the frequency of the transition from the ground state
4
390(1.56 ꢀ 10 ) 417 27
to the first excited state,
meg is the transition dipole moment
1
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
1.0
0.8
0.6
0.4
0.2
0.0
5
a
2
3
4
b
6
7
8
3
50
400
450
500
550
600
650
700
750
350
400
450
500
550
600
650
700
W avelength (nm)
W avelength /nm
Fig. 4. Luminescence spectra of 1e8 in air-free chloroform at room temperature.

10
D. Yao et al. / Journal of Organometallic Chemistry 752 (2014) 6e11
Table 3
The data of
contribution of the metal d
in HOMO and LUMO and the charge
p
u
eg
,
m
eg, (
m
e
ꢁ
m
g
) and j xxxj of 1e8.
b
transfer process are dominated by the skeleton structure of these
complexes. The low fluorescent quantum yields and detectable
room temperature phosphorescence indicate that the complexes
containing Pt(II) have pretty high system-cross quantum yields
because of the spin-orbit coupling between the d orbitals of the
eg (Sꢁ1)
meg (esu cm)
(
m
e
ꢁ
m
g
) (esu cm)
j
b
xxxj (cm /esu)
5
Compound
u
1
1
1
1
1
1
1
1
5
ꢁ17
ꢁ18
ꢁ29
1
2
3
4
5
6
7
8
5.61 ꢀ 10
5.61 ꢀ 10
4.82 ꢀ 10
4.77 ꢀ 10
4.82 ꢀ 10
4.76 ꢀ 10
4.77 ꢀ 10
5.22 ꢀ 10
3.27 ꢀ 10
6.02 ꢀ 10
11.2 ꢀ 10
10.2 ꢀ 10
6.19 ꢀ 10
11.0 ꢀ 10
12.0 ꢀ 10
6.65 ꢀ 10
3.15 ꢀ 10
3.75 ꢀ 10
7.48 ꢀ 10
3.48 ꢀ 10
1.87 ꢀ 10
2.67 ꢀ 10
4.14 ꢀ 10
2.83 ꢀ 10
1.02 ꢀ 10
4.13 ꢀ 10
2.60 ꢀ 10
1.00 ꢀ 10
1.97 ꢀ 10
8.91 ꢀ 10
1.65 ꢀ 10
3.46 ꢀ 10
5
5
5
5
5
5
5
ꢁ17
ꢁ17
ꢁ17
ꢁ17
ꢁ17
ꢁ17
ꢁ17
ꢁ18
ꢁ18
ꢁ18
ꢁ18
ꢁ18
ꢁ18
ꢁ18
ꢁ29
ꢁ28
ꢁ28
ꢁ29
ꢁ29
ꢁ28
ꢁ28
Pt(II) and the large
abilities j xxxj was determined by the solvatochromic method be-
tween 1.02 ꢀ 10
p-conjugated system. The first-order polariz-
b
ꢁ29
ꢁ28
5
to 3.46 ꢀ 10
(cm /esu). The detailed
nonlinear optical properties and the relationship between the
structure and properties will be studied soon.
between the ground state and the excited state,
dipole moment of the ground state, is the permanent dipole
moment of the excited state, and is the laser frequency. eg can be
found from the absorption band maximum of the title molecule.
can be gotten from the area under the absorption band [31]. The
value of could be derived from Eqs. (2)e(4).
g
m is the permanent
Acknowledgments
m
e
u
u
The authors thank the Opening Research Foundation from the
State Key Laboratory of Laser Interaction with Mater (China) [Grant
No. SKILL1005] for the financial support.
m
eg
m
e
ꢁ
m
g
Appendix A. Supplementary data
z
ꢁ
z
¼ A þ BðBKÞ
(2)
(3)
a
f
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2013.11.032.
ꢀ
ꢁ
2
B ¼ 2
m
ꢁ
m
=hca3
e
g
h
ꢀ
ꢁ.ꢀ
2 n þ 1
ꢁ
References
2
2
ðBKÞ ¼ ðε ꢁ 1Þ=ð2ε þ 1Þ ꢁ n ꢁ 1
2 n þ 1
2
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eg, and
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xxxj were finally calculated by substituting the values of
u
eg
,
m
m
e
ꢁ
m
g
into Eq. (1).
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[
[
hyperpolarizabilities of the title complexes are similar to that of the
common second-order nonlinear materials p-nitroaniline
ꢁ
30
5
[
(
j
b
xxxj w 20 ꢀ 10
cm /esu) and nitro-N,N-dimethylaniline
cm /esu) [32]. For complexes 1 to 4, xxx of the com-
ePte eA skeleton is larger than that
of the complexes with symmetrical De ePte eD skeleton, how-
ever it is lower than that of the complexes with unsymmetrical De
ePte eA skeleton. It’s probably because of the existence of the
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smaller than that of the complexes with symmetrical De ePte
Are ePte eD skeleton. The first-order hyperpolarizabilities of
ꢁ30
5
(
37 ꢀ 10
b
[
plexes with symmetrical Ae
p
p
p
p
p
p
[
[
[
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p
p
p
10694e10706.
p
pe
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p
p
[
[
[
the title complexes are still impressive although it were not
designed based on the criteria for organic second harmonic (SHG)
materials.
[
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[
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optical limiter. The calculation using DFT shows that the

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