Synthesis, tunable photophysics and nonlinear absorption of terpyridyl Pt(II) complexes bearing different acetylide ligands

Article abstract of DOI:10.1016/j.dyepig.2015.11.021

A series of 4-phenyl-2,2′; 6′,2″-terpyridyl Pt(II) complexes bearing different σ-alkynyl ancillary ligands (1a-1f) were synthesized and characterized. All complexes exhibit strong ¹π,πabsorption bands in the UV region; and broad, structureless

Full text of DOI:10.1016/j.dyepig.2015.11.021

Dyes and Pigments 126 (2016) 165e172
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, tunable photophysics and nonlinear absorption of
terpyridyl Pt(II) complexes bearing different acetylide ligands
,
Tianchu Zhao a, Rui Liu a *, Hong Shi b, Mingliang Shu a, Jinyang Hu a, Huan Li a,
,
**
Hongjun Zhu a
a Department of Applied Chemistry, College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211816, China
b Jiangsu Vocational College of Information Technology, Wuxi 214153, China
a r t i c l e i n f o
a b s t r a c t
A series of 4-phenyl-2,20; 60,200-terpyridyl Pt(II) complexes bearing different
s
-alkynyl ancillary ligands
* absorption bands in the
Article history:
(1ae1f) were synthesized and characterized. All complexes exhibit strong 1
p
,
p
Received 25 September 2015
Received in revised form
21 November 2015
Accepted 23 November 2015
Available online 2 December 2015
UV region; and broad, structureless metal-to-ligand charge transfer (1MLCT)/ligand-to-ligand charge
transfer (1LLCT) absorption bands in the visible region. When excited at the charge-transfer absorption
band, the complexes exhibit yellow to red luminescence (lmax ¼ 553e608 nm) in CH3CN at room
temperature, which is attributed to the 3MLCT/3LLCT state, except 1c. The emitting state of 1c exhibits a
significant intraligand 3p
,p* character, which is originated from the naphthalimide acetylide ligand.
Keywords:
Synthesis
Photophysics
Terpyridyl Pt(II) complex
Nonlinear absorption
Electrochemical property
Optical limiting
Complexes 1ae1c exhibit moderate triplet transient absorptions from visible to NIR region, where
reverse saturable absorption (RSA) occurs. The photophysical, electrochemical and nonlinear absorption
properties of these Pt(II) complexes can be tuned drastically by the different acetylide ligand, which
would be useful for rational design of broadband nonlinear absorption materials.
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
Among these cyclometalated square-planar platinum com-
plexes, 2,20; 60,200-terpyridyl (N∧N∧N) Pt(II) acetylide complexes are
Cyclometalated Pt(II) complexes have been attached huge
attention in the past decade owing to their unique photophysical
properties [1e3] and potential applications in probes [4e6], organic
light-emitting devices (OLED) [7e11] and nonlinear optics [12e14].
Their rich properties and versatile applications are intrinsically
based on the square-planar configuration of the cyclometalated
Pt(II) complexes and the presence of multiple charge transfer
excited states, such as metal-to-ligand charge transfer (MLCT),
ligand-to-ligand charge transfer (LLCT), intraligand charge transfer
particularly intriguing by virtue of their relatively intense emission,
broadband nonlinear absorption and the ease of structural modi-
fication on both of the terpyridine ligand and acetylide ligand
[12e18,20]. Previous studies on terpyridyl Pt(II) acetylide com-
plexes demonstrated the 1MLCT/1LLCT absorption band energy and
the emission energy of the complexes were significantly affected by
the electron-donating or electron-withdrawing ability of the sub-
stituent(s) on the acetylide ligand. However, no monotonic trend
was observed on the emission energy, lifetime and quantum yield
when varying the substituents on the acetylide ligands, which
might be caused by the admixture of multiple excited states in
proximity. By modification of the acetylide ligands, the lowest
triplet excited state of the terpyridyl Pt(II) complexes could be
(ILCT), in addition to the ligand localized
p,p* transitions. These
features lead to long-lived triplet excited states, broadband excited-
state absorption (ESA) in the visible to the near-IR region, and room
temperature phosphorescence. In addition, their photophysical
properties can be readily tuned by structural modifications of the
employed ligands to meet the specific requirements for each
unique application [15e20].
tuned to a 3MLCT state, a 3
p
,p* state localized on acetylide ligand, or
a mixture of the 3MLCT and 3p
,p
* states [12e14,16,17]. Moreover,
the strong electron-donating substituent (such as a diphenylamino
group) and strong electron-withdrawing substituent (such as the
nitro group) on the acetylide ligands could change the nature of the
lowest triplet excited state from a charge transfer triplet excited
* Corresponding author. Tel./fax: þ86 25 58139539.
state to predominantly acetylide ligand-localized 3p
,p* state
** Corresponding author. Tel.: þ86 25 83172358; fax: þ86 25 83587428.
E-mail addresses: rui.liu@njtech.edu.cn (R. Liu), zhuhj@njtech.edu.cn (H. Zhu).
[19,20].
http://dx.doi.org/10.1016/j.dyepig.2015.11.021
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

166
T. Zhao et al. / Dyes and Pigments 126 (2016) 165e172
Intrigued by this preliminary study, we are interested in further
elemental analyzer. Mass analyses were performed on an Agilent
1100 mass spectrometer.
understanding the effect of introducing aromatic substituents on
the acetylide ligand of terpyridyl Pt(II) complexes. To the best of our
knowledge, studies on the nonlinear optical properties and
structure-property correlation of the terpyridyl Pt(II) complexes
Photophysical Measurements. Optical absorption spectra were
obtained by using a Cary-5000 UV/vis/near-IR Spectrophotometer.
Photoluminescence spectra were carried out on a Fluoro-Max 4
fluorometer/phosphorometer. The nanosecond transient difference
absorption spectra, triplet excited-state lifetimes and triplet
excited-state quantum yields were measured on an Edinburgh
LP920 laser flash photolysis spectrometer. The excitation source
was the third harmonic output (355 nm) of a Nd:YAG laser (Quantel
Brilliant, pulsewidth ~4.1 ns, 1 Hz). Before each measurement, the
sample solutions were degassed with Ar for 30 min.
with extended p-conjugated substituents acetylide ligands are still
limited. Understanding how the nature of these electron-donating
or -withdrawing substituents influence the photophysics of Pt(II)
complexes is essential for rational design of Pt(II) complexes as
efficient photonic materials, such as electroluminescent materials
and nonlinear absorbing materials.
In this work, a series of novel 4-phenyl-2,20; 60,200-terpyridyl
Pt(II) complexes bearing different acetylide ligands (Scheme 1)
were designed and synthesized. The nitro- and naphthalimide-
groups were chosen as electron-withdrawing substituents, while
the carbazolyl- and diphenylamino-groups were selected as
electron-donating substituent. Their photophysical properties and
excited-state absorption were systematically investigated in order
to understand the structure-property correlations for
developing efficient nonlinear absorbing materials and organic
light-emitting materials.
Electrochemical Measurements. The electrochemical experi-
ments were carried out using a CHI 660C electrochemistry work-
station (CHI-USA). A standard one-compartment three-electrode
cell was used with a Pt electrode as the working electrode, a Pt wire
as the counter electrode and a Ag/Agþ electrode (Ag in 0.1 M AgNO3
solution) as the reference electrode. TBAP (0.1 M) was used as the
supporting electrolyte. The films of these complexes were coated
on the Pt working electrode and the scan rate was 100 mVSꢀ1
.
Nonlinear Transmission Measurement. A plano-convex lens
(f ¼ 40 cm) was used to focus the beam to the sample cuvette. The
radius of the beam waist at the focal point was approximately
2. Experimental section
96 mm. A Quantel Brilliant nanosecond laser (4.1 ns pulses) with a
repetition rate of 10 Hz was used as the light source.
2.1. Materials
2.3. Synthesis
K2PtCl4 was purchased from Jiulin Shanghai Company, and all
the other reagents were purchased from Aladdin. Tetrahydrofuran
(THF) and N,N-diisopropylethylamine (DIEA) were distilled under
N2 over sodium benzophenone ketyl. Acetonitrile were purified by
redistillation. Tetra-n-butylammonium perchlorate (TBAP) and
ferrocene were purified by recrystallization twice from ethanol. All
other reagents were used as received. 4-phenyl-2,20; 60,200-terpyr-
idyl [21,22] and arylacetylide ligands [23e26] were synthesized
according to literature methods.
2.3.1. Synthesis of complex [Pt(tpy)Cl]Cl (2)
4-Phenyl terpyridine (0.0619 g, 0.2 mmol) was dissolved in
30 mL hot CH3CN and K2PtCl4 (0.0830 g, 0.2 mmol) was dissolved in
10 mL H2O. The solutions were then mixed, and heated to reflux for
24 h. After cooling to room temperature, the mixture was filtered,
washed with water (10 mL), ether (5 mL) and CH2Cl2 (20 mL) to
afford a brown solid (0.086 g, 79.5%). 1H NMR (500 MHz, DMSO-d6):
d
ppm 8.93 (s, 2H), 8.83 (d, J ¼ 7.95, 2H), 8.70 (d, J ¼ 5.1 Hz, 2H),
8.45e8.42 (m, 2H), 8.20e8.18 (m, 2H), 7.84 (t, J ¼ 6.32, 2H),
7.68e7.65 (m, 3H).
2.2. Measurements
Synthesis and Characterization. All complexes were character-
ized by 1H NMR, elemental analysis, and MS. 1H NMR spectra were
recorded on a Bruker 300 MHz or 400 MHz spectrometer using
DMSO-d6 as the solvent, with tetramethylsilane as internal stan-
dard. The elemental analyses were performed with a Vario El III
2.3.2. General procedure for synthesis of complexes 1ae1f
Arylacetylide compound (0.1 mmol) and KOH (11.2 mg,
0.2 mmol, 2.0 equiv.) were dissolved in MeOH (20 mL) under stir-
ring at room temperature for 30 min. Then complex 2 (57.4 mg,
0.1 mmol, 1.0 equiv.) and CuI (1.2 mg, 0.06 mmol, 0.6 equiv.) were
Scheme 1. Synthetic routes for Pt(II) complexes 1ae1f.

T. Zhao et al. / Dyes and Pigments 126 (2016) 165e172
167
added. The mixture was purged under N2 for 30 min, and then
stirred at room temperature for 18 h. After the reaction, the reaction
mixture was concentrated, and ether was added. The precipitants
were separated by filtration. The solid was dissolved in 20 mL
MeOH, then 20 mL saturated aqueous solution of NH4PF6 was
added at room temperature. The mixture was stirred at room
temperature for 3 h, then the solid was filtered, washed with ether
and CH2Cl2 to give the desired product.
(100%). Anal. calcd. (%) for C41H29F6N4PPt: C, 53.66; H, 3.19; N, 6.10;
Found C, 53.61; H, 3.22; N, 6.16.
3. Results and discussion
3.1. Synthesis and characterization
Scheme 1 outlines the synthesis of the Pt(II) 4-phenyl-2,20;
60,200-terpyridyl acetylide complexes. Pt(II) acetylide complexes
derived from these precursors were prepared by employing Sono-
gashira's conditions (terminal alkynes, CuI/KOH/CH3OH) and were
obtained in 70e81% yield. All complexes are air-stable and soluble
in acetone, acetonitrile, dimethyl formamide and dimethyl sulf-
oxide. The complexes were characterized by 1H NMR spectra, MS,
and elemental analyses to confirm their structures. All of the
complexes have distinct, well-resolved patterns in their aromatic
proton resonances, which are attributable to the protons of the
pyridine rings and phenyl substituents. 1H NMR spectra showed
2.3.2.1. Complex [Pt(tpy)(C^CeC6H5)]PF6 (1a). Yield was 77.3% as
red solid. 1H NMR (DMSO-d6, 300 MHz):
d
ppm 9.20 (d, J ¼ 3.1 Hz,
2H), 9.05 (s, 2H), 8.88 (d, J ¼ 4.7 Hz, 2H), 8.55 (t, J ¼ 4.8 Hz, 2H), 8.18
(d, J ¼ 3.8 Hz, 2H), 7.95 (d, J ¼ 4.0 Hz, 2H), 7.69 (d, J ¼ 4.0 Hz, 3H),
7.51 (d, J ¼ 4.4 Hz, 2H), 7.35 (t, J ¼ 4.5 Hz, 2H), 7.28 (t, J ¼ 4.5 Hz, 1H).
ESI-MS: m/z calcd for [C29H20N3Pt]þ, 605.1; Found 605.1. Anal.
Calcd. (%) for C29H20F6N3PPt: C, 46.41; H, 2.69; N, 5.60. Found: C,
46.32; H, 2.73; N, 5.52.
obvious changes of chemical shifts for pyridine protons
(d
2.3.2.2. Complex [Pt(tpy)(C^CeC6H4eNO2)]PF6 (1b). Yield was
81.6% as orange solid. 1H NMR (DMSO-d6, 500 MHz):
d ppm 8.89 (s,
7.8e9.9 ppm) after coordination with the Pt(II) metal ion and upon
exchange of chloride for acetylide, particularly the protons nearest
to the acetylide moiety.
2H), 8.82 (d, J ¼ 5.3 Hz, 2H), 8.76 (d, J ¼ 8.0 Hz, 2H), 8.42 (t,
J ¼ 7.4 Hz, 2H), 8.04 (d, J ¼ 8.3 Hz, 4H), 7.72 (t, J ¼ 6.4 Hz, 2H), 7.59 (d,
J ¼ 6.7 Hz, 3H), 7.53 (d, J ¼ 8.5 Hz, 2H). ESI-MS: m/z calcd. for
[C29H19N4O2Pt195]þ, 650.12; Found 650 (100%). Anal. Calcd. (%) for
3.2. Optical properties
C
29H19F6N4O2PPt: C, 43.78; H, 2.41; N, 7.04; found C, 43.73; H, 2.64;
N, 7.12.
3.2.1. Electronic absorption spectra
The UVevis absorption spectra of complexes 1ae1f were
measured in CH3CN at different concentrations. No ground-state
aggregation was observed in the concentration range investigated
(1 ꢁ 10ꢀ6 to 1 ꢁ 10ꢀ4 mol Lꢀ1), which is reflected by the compliance
of the absorbance with the Beer's law. The UVeVis absorption
spectra of complexes 1ae1f in CH3CN solution are presented in
Fig. 1, and the optical characteristics are summarized in Table 1. All
of the spectra exhibit intense absorption bands below 380 nm,
2.3.2.3. Complex [Pt(tpy)(C^CeC6H4eNI)]PF6 (1c). Yield was 78.1%
as orange solid. 1H NMR (DMSO-d6, 500 MHz):
d ppm 9.09 (s, 1H),
8.98 (s, 2H), 8.88 (s, 3H), 8.74 (d, J ¼ 8.1 Hz, 1H), 8.50e8.59 (m, 3H),
8.37 (d, J ¼ 8.1 Hz, 1H), 8.20 (s, 1H), 8.04 (s, 2H), 7.91e8.01 (m, 3H),
7.68 (d, J ¼ 3.2 Hz, 1H), 7.61 (s, 2H), 4.05 (d, J ¼ 8.5 Hz, 2H), 1.65 (t,
J ¼ 6.0 Hz, 2H),1.37 (t, J ¼ 7.6 Hz, 2H), 0.96 (t, J ¼ 7.6 Hz, 3H). ESI-MS:
m/z calcd. for [C39H29N4O2Pt]þ, 780.2; Found 780.2. Anal. calcd. (%)
for C39H29F6N4O2PPt: C, 50.60; H, 3.16; N, 6.05; Found C, 50.47; H,
3.22; N, 6.07.
which can be assigned to the pep* transition within the acetylide
and terpyridyl ligands, while the broad, moderately intense ab-
sorption bands at 380e600 nm are assigned to the mixed
1MLCT/1LLCT/1ILCT transitions. These are also in line with the ter-
pyridyl Pt(II) acetylide complexes reported previously
[12e14,16,17,20]. The charge transfer nature of the low-energy ab-
sorption bands in 1ae1f is also supported by the solvent-
dependency studies. As exemplified in Fig. 2 for 1a, these low-
energy absorption bands bathochromically shift to longer wave-
lengths in solvents with lower polarity (i.e., toluene and THF)
2.3.2.4. Complex [Pt(tpy)(C^CeC6H4-Cz)]PF6 (1d). Yield was 71.3%
as brown solid. 1H NMR (DMSO-d6, 500 MHz):
d ppm 9.15 (d,
J ¼ 4.5 Hz, 1H), 8.79 (d, J ¼ 8.1 Hz, 1H), 8.62 (s, 1H), 8.42 (t, J ¼ 7.1 Hz,
1H), 8.37 (s, 1H), 8.25 (d, J ¼ 7.8 Hz, 2H), 8.12 (d, J ¼ 6.8 Hz, 2H),
7.93e7.89 (m, 2H), 7.84 (d, J ¼ 7.3 Hz,1H), 7.67e7.60 (m, 5H), 7.53 (d,
J ¼ 8.6 Hz, 2H), 7.48e7.42 (m, 4H), 7.30 (t, J ¼ 7.3 Hz, 2H), 7.18 (t,
J ¼ 7.3 Hz, 1H), 7.13 (t, J ¼ 7.3 Hz, 1H). ESI-MS: m/z calcd. for
[C41H27N4Pt]þ, 770.2; Found 770.2. Anal. calcd. (%) for
C
6.22.
41H27F6N4PPt: C, 53.78; H, 2.97; N, 6.12; Found C, 53.74; H, 2.92; N,
7x104
6x104
1a
1b
1c
1d
1e
1f
2.3.2.5. Complex [Pt(tpy)(C^CeC6H4-t-BuCz)]PF6 (1e). Yield was
70.6% as brown solid. 1H NMR (DMSO-d6, 500 MHz):
d ppm 8.89 (s,
5x104
2H), 8.86 (d, J ¼ 6.5 Hz, 2H), 8.79e8.82 (m, 4H), 8.69 (d, J ¼ 8.0 Hz,
2H), 8.64e8.66 (m, 2H), 8.31 (t, J ¼ 3.6 Hz, 1H), 8.09e8.11 (m, 2H),
7.79 (t, J ¼ 6.0 Hz, 2H), 7.63e7.66 (m, 4H), 7.54 (t, J ¼ 2.0 Hz, 2H), 7.14
4x104
(d,
J
¼
8.2 Hz, 2H), 2.50 (s, 18H). ESI-MS: m/z calcd. for
3x104
[C49H43N4Pt]þ, 882.3; Found 887.3. Anal. calcd. (%) for
49H43F6N4PPt: C, 57.25; H, 4.22; N, 5.45; found C, 57.29; H, 4.31; N,
C
2x104
ε
5.43.
1x104
2.3.2.6. Complex [Pt(tpy)(C^CeC6H4-NPh2)]PF6 (1f). Yield was
79.3% as black solid. 1H NMR (DMSO-d6, 500 MHz):
d ppm 9.07 (d,
0
J ¼ 4.6 Hz, 2H), 9.02 (s, 2H), 8.84 (d, J ¼ 7.6 Hz, 2H), 8.51 (t, J ¼ 7.4 Hz,
2H), 8.17 (d, J ¼ 3.1 Hz, 2H), 7.89 (t, J ¼ 6.5 Hz, 2H), 7.66 (d, J ¼ 2.1 Hz,
3H), 7.34 (t, J ¼ 7.7 Hz, 6H), 7.03e7.11 (m, 6H), 6.92 (d, J ¼ 8.0 Hz,
2H). ESI-MS: m/z calcd. for [C41H29N4Pt]þ, 772.2; Found 772.2
250 300 350 400 450 500 550 600 650
Wavelength (nm)
Fig. 1. UVevis absorption spectra of 1ae1f in CH3CN solution.

168
T. Zhao et al. / Dyes and Pigments 126 (2016) 165e172
Table 1
Photophysical parameters of complexes 1ae1f.
a
d
Complex
labs/nm (ε/104 L molꢀ1 cmꢀ1
)
lem/nm (tem/ns; Fem) R.T.b
lT1ꢀTn/nm (tTA/ns; εT1ꢀTn/104 L molꢀ1 cmꢀ1
; FT)
1a
1b
1c
1d
1e
1f
264 (5.54), 309 (2.91), 341 (1.55), 454 (0.79)
288 (3.31), 337 (3.10), 421 (1.21)
284 (5.43), 331 (2.37), 387 (2.08), 450 (1.02)
290 (5.03), 307 (3.74), 404 (0.64), 468 (0.42)
283 (4.74), 331 (2.82), 404 (0.87), 482 (0.18)
290 (4.95), 312 (4.72), 409 (0.57), 512 (0.56)
604 (820; 0.003)
553 (30, 0.001)
621 (10, 0.011), 674 (sh)
608 (c)
705 (815, 4.20, 0.92)
650 (40, 2.73, 0.51)
560 (8370, 1.94, 0.85)
c
c
c
608 (c)
c
a
Electronic absorption band maxima and molar extinction coefficients in CH3CN at room temperature.
b
Room temperature emission band maxima and decay lifetimes measured in CH3CN at a concentration of 1 ꢁ 10ꢀ5 mol/L. A degassed aqueous solution of [Ru(bpy)3]Cl2
(
Fem ¼ 0.042, excited at 436 nm) was used as the reference.
c
Too weak to be measured.
d
Nanosecond TA band maxima, triplet extinction coefficients, triplet excited-state lifetimes and quantum yields in CH3CN/DMF mixture (9:1, v/v). SiNc in C6H6 was used as
the reference. (ε590 ¼ 70,000 L molꢀ1.cmꢀ1
,
FT ¼ 0.20).
Because of the inductive and resonant effects from the para sub-
stituent, the energy of the acetylide-based HOMO is affected, which
in turn causes the change of the lowest excited-state energy. An
electron-donating substituent raises the acetylide-based HOMO,
while an electron-withdrawing substituent stabilizes the acetylide-
base HOMO. Consequently, the energy difference between the
acetylide-based HOMO and the terpyridine-based LUMO will be
varied and the bathochromic shift or hypsochromic shift occurs
[18,19]. These results are also in line with the bi/terpyridine Pt(II)
acetylide complexes reported previously [12e14,16e20].
1.0
0.8
0.6
0.4
0.2
0.0
Toluene
THF
CH2Cl2
CH3CN
For complex 1b, it is noted that it has an intense structureless
absorption band at lmax ¼ 421 nm (ε z 1.21 ꢁ 104 L molꢀ1 cmꢀ1),
which is significantly different from those of the other complexes.
This band is attributed to the 1MLCT/1LLCT transitions mixed with a
significant 1p
,p* transition component. This phenomenon could
also observed in the absorption spectrum of complex 1c, which
exhibits an additional intense absorption band at 387 nm
(ε z 2.08 ꢁ 104 L molꢀ1 cmꢀ1). In addition, considering the
300
350
400
450
500
550
600
extended
p-conjugation of the acetylide ligands and the strong
Wavelength (nm)
electron-withdrawing ability of the NO2 substituent in 1b, 1ILCT
within the 4-nitrophenylacetylide ligand is also likely to contribute
to this band [19,27,28].
Fig. 2. Normalized UVevis absorption spectra of complex 1a in different solvents. The
concentration of each solution was adjusted in order to obtain A ¼ 0.08 at 436 nm in a
1 cm cuvette.
3.2.2. Photoluminescence
The emission characteristics of Pt(II) complexes 1ae1f were
investigated in different solvents at room temperature. The
normalized emission spectra of Pt complexes in CH3CN at the
compared to those in more polar solvents (i.e., CH3CN and CH2Cl2).
This negative solvatochromic effect is indicative of a charge-
transfer transition, in which the dipole moment of the ground
state is larger than that of the excited state. The same solvent effect
is observed for all of the other complexes studied in this work, and
the results are provided in Fig. S1eS6 of the Supporting
Information.
For most of the complexes, except 1b, the low-energy absorp-
tion band appears to be two distinct bands in this region. The band
at about 380e430 nm appears to be insensitive to the ancillary
1.2
1a
1b
1.0
1c
1d
1e
0.8
0.6
0.4
0.2
0.0
substituents of the acetylide ligand, which emanate from the d
p
(Pt) /
p
*(N∧N∧N) 1MLCT excited state. However, in the region of
about 430e600 nm, the lower energies transitions are influenced
significantly by the nature of the ancillary substituents. Electron-
donating substituents, such as carbazole, 3,6-di-tert-butyl-carba-
zole and NPh2, cause a pronounced red-shift of this transition,
while electron-withdrawing substituents such as NO2 and naph-
thalimide induce a blue-shift compared to complex 1a. Considering
the sensitivity of the lower-energy transitions to the substituents at
the acetylide ligands, we assigned these transitions to 1LLCT tran-
sition. The calculation results indicate that the LUMO is almost
500
550
600
650
700
750
800
exclusively p*(terpyridine) based for the terpyridyl Pt(II) acetylide
complexes; while the compositions of the HOMO are delocalized on
both of the Pt metal and acetylide ligand (Fig. S7 and Table S1).
Wavelength (nm)
Fig. 3. Normalized emission spectra of Pt(II) complexes (lex ¼ 436 nm) in degassed
CH3CN solution (c ¼ 1 ꢁ 10ꢀ5 mol Lꢀ1).

T. Zhao et al. / Dyes and Pigments 126 (2016) 165e172
169
concentration of 1 ꢁ 10ꢀ5 mol Lꢀ1 are illustrated in Fig. 3, and the
emission lifetimes and the emission quantum yields are provided in
Table 1. Except for 1f, excitation of these Pt(II) complexes at their
respective charge-transfer band at room temperature results in
yellow to red luminescence. The emissions exhibit significant
Stokes shifts, and the emission lifetimes in degassed CH3CN solu-
tions vary from tens to hundreds of ns. In addition, the emission is
quite sensitive to oxygen quenching. Taking all these facts into
account, we believe that their emissions emanate from a triplet
excited state (T1 excited state).
Since 1ae1f all contain the same terpyridyl ligand, their pho-
toluminescent properties are influenced significantly by the na-
tures of the acetylide ligands, especially the electron-donating or
electron-withdrawing ability of the substituents on the ligands.
Compared to 1a, complex 1b with electron-withdrawing substitu-
ent exhibits pronounced blue-shifted emission with structureless
feature. Meanwhile, when electron-donating substituents are
introduced, complex 1d and 1e show slight red-shifted and struc-
tureless emission bands with relative weak emission character. This
trend is consistent with that observed for the 1MLCT band in the
UVevis absorption spectra, which provides another piece of
evident that the emission has 3MLCT character. For complex 1c, the
As shown in Fig. 3, 1a, 1b, 1d and 1e exhibit structureless
emission in CH3CN solution, suggesting a charge transfer character
of their emitting state. Based on the literature precedents for the
reported bi/terpyridine Pt(II) acetylide complexes, such emission
could be attributed to the 3MLCT/3LLCT states [12e14,16,17,19,20].
The charge transfer nature of the emitting state could be supported
by the solvent-dependency study (Fig. S8e12). For complex 1a, the
nature of emitting state could be 3MLCT/3LLCT mixed with some
nature of its emitting state is altered to a 3p
,p* excited states by
replacing the phenylacetylide ligand with the naphthalimide ace-
tylide ligand.
3.2.3. Electrochemical properties
The electrochemical behaviors of 1ae1f were investigated by
cyclic voltammetry (CV). The electrochemical properties of these
complexes were summarized in Table 3, and their CV curves were
provided in Fig. S13. The reduction process occurring at ꢀ1.27
to ꢀ1.62 V presumably corresponds to the one-electron reduction
of the terpyridyl ligand. This process must be affected by the
electron-donating ability of the para-substituent on the phenyl-
acetylide ligand. The reduction waves are shifted to more negative
direction by the electron-donating groups on the phenylacetylide
ligand. For example, carbazole shift the reduction peak to ꢀ1.41 V,
and diphenylamino shift the reduction peak to ꢀ1.62 V for 1d and
1f. Meanwhile, compared to 1a, the complexes bearing electron-
withdrawing groups on the acetylide ligands show more positive
reduction waves (ꢀ1.31 for 1b and ꢀ1.27 for 1c). All complexes
show an irreversible oxidation wave at 0.72e0.76 V, which can be
tentatively assigned to a PtII/III process [30].
3p
,p* character, which is supported by the minor solvatochromic
effect (Fig. 4a). Complex 1b exhibit a negative solvatochromic effect
(Fig. S9), i.e., when the dielectric constant (ε) or ET(30) value of the
solvent increases, the emission energy of the complex decreases.
This negative solvatochromic effect indicates a charge-transfer
transition of the complex, in which the dipole moment of the
ground state is larger than that of the excited state, that is also a
characteristic of the 3MLCT excited state [13,14,19,20,27,28]. In
contrast, the emission spectrum of 1c is not only red-shifted and
more structured than the other complexes (with a vibronic spacing
of approximately 1264 cmꢀ1 in CH3CN), but also exhibits minor
solvatochromic effect (Fig. 4b). This is indicative of an emitting state
with significant intraligand 3p
,p* character originated from the
naphthalimide acetylide ligand. It is also noted that complex 1d and
1e only exhibit very weak emission bands in CH3CN, which was
ascribed to the enhanced ICT (intramolecular charge transfer) state,
particularly the 3LLCT state caused by the electron-donating car-
bazoyl group and 3,6-di-tert-butyl-carbazole groups [19,27,28].
Similarly, no emission was observed for complex 1f by virtue of the
strong electron-donating NPh2 group which quenches the radiative
pathway [29]. In addition, as shown in Table 2, all of the complexes
exhibit a longer emission lifetime and a higher quantum yield in
noncoordinating solvents, such as CH2Cl2 and toluene in compari-
son to those in coordinating solvents (the solvents which are able to
coordinate with ions, like CH3CN). The quenching of the emission in
coordinating solvent like CH3CN, suggests that the emitting state
has Pt metal involved. This solvent induced quenching by coordi-
nating solvents is commonly seen in Pt(II) complexes with a 3MLCT
emitting state, which provides another solid evidence for the
involvement of the 3MLCT character in the emitting state of these
complexes.
3.2.4. Transient absorption
The nanosecond (ns) transient absorption (TA) spectra of the
complexes were studied to understand the excited-state behaviors
of the triplet excited states and to predict the spectral region where
reverse saturable absorption (RSA, i.e. transmission decreases with
increased incident energy) occurs. Fig. 5 displays the triplet TA
spectra of 1ae1c at the zero-time delay and the time-resolved TA
spectra of 1ae1c in degassed CH3CN solution. The triplet excited-
state lifetimes deduced from the TA decay profile are listed in
Table 1. The TA spectra of 1de1f were unable to be measured due to
the shorter excited-state lifetime and lower triplet excited-state
quantum yields as indicated by the emission lifetime and emis-
sion quantum yields (Table 1).
As presented in Fig. 5, all complexes exhibit bleaching bands
below 480 nm. However, the spectral features of 1ae1c are
1.2
1.2
Toluene
THF
CH Cl
(a)
(b)
Toluene
1.0
1.0
THF
CH Cl
0.8
0.6
0.4
0.2
0.0
CH CN
0.8
CH CN
0.6
0.4
0.2
0.0
500
550
600
650
700
750
800
600
650
700
750
800
Wavelength (nm)
Wavelength (nm)
Fig. 4. Normalized emission spectra of (a) complex 1a and (b) complex 1c in different solvents at room temperature, lex ¼ 436 nm.

170
T. Zhao et al. / Dyes and Pigments 126 (2016) 165e172
Table 2
Emission energy, lifetime and quantum yield of 1ae1f in different solvents at room temperature.
a
lem/nm (tem/ns; Fem
)
CH3CN
CH2Cl2
THF
Toluene
1a
1b
1c
1d
1e
1f
604 (820; 0.003)
553 (30, 0.001)
621 (10, 0.011)
608 (b)
607 (2000; 0.009)
564 (360, 0.020)
623 (200, 0.018)
608 (b, 0.003)
608 (200, 0.003)
612 (1500; 0.012)
570 (80, 0.005)
b
622 (15,0.012)
626 (1030, 0.006)
b
b
b
b
b
b
608 (b)
600 (b, 0.001)
b
b
a
A degassed aqueous solution of [Ru(bpy)3]Cl2
Too weak to be measured.
(
Fem ¼ 0.042, excited at 436 nm) was used as the reference.
b
Table 3
of the emission, implying that the TA of 1a and 1b likely emanates
from the same excited state that emits or a state in equilibrium with
the emitting state. Thus, the triplet excited state giving rise to the
observed TA for 1a and 1b could be assigned to the
Electrochemical propertiesa of complexes 1ae1f.
peak
peak
Complex
Oxidation EOX
(V)
Reduction Ere
(V)
1a
1b
1c
1d
1e
1f
0.73
0.75
0.72
0.76
0.73
0.73
ꢀ1.35, ꢀ1.62b
ꢀ1.31
3MLCT/3LLCT/3 * excited state for 1a and 3MLCT/3LLCT/3ILCT
p,p
excited states for 1b, respectively. For complex 1c, the lifetime that
obtained from the decay of the TA in CH3CN is ~8.4 s, which is
ꢀ1.27
ꢀ1.41
m
ꢀ1.46
much longer than that for a charge-transfer band and is quite
distinct from that obtained from the decay of the emission for this
ꢀ1.62
a
complex. Thus, the TA for 1c arises from the 3p
,p* state of the
Cyclic voltammetry of complex 1ae1f in CH3CN at 298 K with 0.1 M Bu4NClO4 as
supporting electrolyte; scanning rate: 100 mV/s; Value versus
E
1/2(Cp2Feþ/0
)
naphthalimide acetylide ligand, possibly mixed with some
[0.08e0.10 V versus Ag/AgNO3 (0.1 M in CH3CN) reference electrode].
3MLCT/3LLCT characters.
b
Irreversible under experimental conditions.
0.10
0.08
0.06
0.04
0.02
0.00
-0.02
-0.04
0.10
1a
0.08
0.06
0.04
0.02
0.00
-0.02
-0.04
0 ns
100 ns
200 ns
300 ns
400 ns
1a
1b
1c
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
0.04
0.02
0.04
0.03
0.02
0.01
0.00
-0.01
-0.02
1c
1b
0.00
0
μs
1.2 μs
2.4 μs
3.6 μs
4.8 μs
0
ns
15 ns
30 ns
45 ns
60 ns
-0.02
-0.04
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 5. Triplet transient difference absorption spectra of 1ae1c in CH3CN at 0 time delay and time-resolved transient difference absorption spectra of 1ae1c. (A355 ¼ 0.4 in CH3CN in
a 1 cm cuvette, excited with a 355 nm pulsed-laser).
different. Complex 1a exhibit a broad and structured absorption
band in the visible to the NIR region, while complex 1b shows a
broad and structureless absorption band in this region.
The lifetimes of 1a and 1b obtained from the decay of the TA in
CH3CN (Table 1) are consistent with those measured from the decay
3.2.5. Nonlinear absorption
The positive TA bands of 1ae1c indicate stronger triplet excited-
state absorptions with respect to the ground-state absorptions at
these wavelengths. Therefore, RSA is anticipated to occur in the
visible or near-IR spectral region for these complexes. It is well

T. Zhao et al. / Dyes and Pigments 126 (2016) 165e172
171
known that reverse saturable absorption can lead to optical
limiting, which reduces the transmittance when the laser intensity
is higher than the limiting threshold. In addition, optical limiting of
nanosecond laser pulses is generally dominated by the triplet
excited-state absorption when the intersystem crossing time is
shorter than the laser pulse width [31,32]. Thus, the optical limiting
of nanosecond laser pulses by terpyridyl Pt(II) complexes should be
dominated by the triplet excited-state absorption. In this case,
complexes 1ae1c would be expected to enhance the optical
limiting of nanosecond laser pulses.
novel photofunctional materials. The complexes exhibit ligand-
centered 1p
,p* transitions in the UV spectral region, and broad,
structureless 1MLCT/1LLCT absorption bands in the visible spectral
region. All complexes show yellow to red phosphorescence in so-
lution at room temperature, which is mainly attributed to 3p
,p*
state for 1c and 3MLCT/3LLCT state for other complexes. The
nanosecond TA spectroscopic study indicates that these complexes
exhibit moderate excited-state absorption from the visible to the
near-IR region. Due to the stronger triplet excited-state absorption
as compared to the ground-state absorption at 532 nm, moderate to
strong RSA was observed at 532 nm for ns laser pulses from some of
these complexes with a trend of 1a > 1c > 1b. It is found that the
photophysical and electrochemical properties of these complexes
were profoundly influenced by the electron-donating ability of the
substituents on the acetylide ligands. We hope these complexes
To demonstrate this, a nonlinear transmission experiment in
CH3CN solution in a 2 mm cuvette was carried out using 4.1 ns laser
pulses at 532 nm. The concentration of each complex solution was
adjusted in order to obtain a linear transmission of 90% for easy
comparison. Fig. 6 depicts the optical limiting curves for complexes
1ae1c. With increased incident fluence, the output fluence of
1ae1c decreases drastically, which clearly indicates the occurrence
of RSA at 532 nm. The strength of the RSA at 532 nm for these
complexes follows the trend of 1a > 1c > 1b. Among these, 1a ex-
hibits the strongest RSA, with an RSA threshold (defined as the
incident fluence at which point the transmittance drops to 70% of
the linear transmittance) of 0.30 J/cm2, and the output fluence
decreases to 0.97 J/cm2 when the incident energy reaches ~2.5 J/
would serve as
a model system for investigating structur-
eeproperty relationships with respect to the nonlinear optical
properties of cyclometalated square-planar Pt(II) complexes.
Acknowledgment
We greatly acknowledge the financial support in part by Natural
Science Foundation of the Jiangsu Higher Education Institutions of
China (13KJB150018), Natural Science Foundation of Jiangsu Prov-
ince (BK20130926, BK20150123), Jiangsu Planned Projects for
Postdoctoral Research Funds (1302089C) and SRF for ROCS, SEM. (R.
Liu).
cm2
mJ. The optical limiting performances of these complexes are
influenced by the different acetylide ligand, and the electron-
withdrawing groups may advance the ability of the reverse satu-
rable absorption. The strong optical limiting of 1a could be related
to its higher triplet excited-state absorption. The relatively high
emission quantum yield, broad excited-state absorption, and strong
nonlinear transmittance performance at 532 nm for 1ae1c suggest
that these complexes could be promising candidates as nonlinear
absorbing materials.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2015.11.021.
4. Conclusions
References
In summary, a series of 4-phenyl-2,20; 60,200-terpyridyl Pt(II)
complexes bearing different aryl-acetylide ligands were synthe-
sized and characterized. Their photophysical and electrochemical
properties were investigated systematically with the aim of un-
derstanding the structure-property correlations and developing
[1] Lai SW, Michael CW, Cheung KK, Che CM. Spectroscopic properties of lumi-
nescent Pt(II) complexes containing 4,40,40-tri-tert-butyl-2,20:60,20-terpyridine
(t-Bu3tpy). Crystal structures of [Pt(t-Bu3tpy)Cl]ClO4 and [Pt(t-Bu3tpy)
{CH2C(O)Me}]ClO4. Inorg Chem 1999;38:4262e7.
[2] Yang QZ, Wu LZ, Wu ZX, Zhang LP, Tung CH. Long-lived emission from Pt(II)
terpyridyl acetylide complexes. Inorg Chem 2002;41:5653e5.
[3] Crites Denise K, Cunningham Corey T, McMillin David R. Remarkable sub-
stituent effects on the photophysics of Pt(40-X-trpy)Clþ systems (trpy¼2,20;
60,20-terpyridine). Inorg Chim Acta 1998;273:346e53.
[4] Arena G, Scolaro LM, Pasternack RF, Romeo R. Synthesis, characterization and
interaction with DNA of the novel metallointercalator cationic complex (2,20:
60,20-terpyridine)methyl Pt(II). Inorg Chem 1995;34:2994e3002.
[5] Lippard SJ. Pt complexes: probes of polynucleotide structure and antitumor
drugs. Acc Chem Res 1978;11:211e7.
[6] Yam V, Tang WW, Rowena PL, Keith MC, Cheung KK. Synthesis, luminescence,
electrochemistry and ion-binding studies of Pt(II) terpyridyl acetylide com-
plexes. Organometallics 2001;20:4476e82.
[7] Zhou GJ, Wong WY, Yu XM, Kwok HS, Lin Z. New Pt(II) complexes as triplet
emitters for high-efficiency monochromatic pure orange electroluminescent
devices. J Organomet Chem 2007;692:3461e73.
[8] Ionkin AS, Wang Y, Marshall WJ, Petrov VA. Synthesis, structural character-
ization, and initial electroluminescent properties of bis-cycloiridiated com-
plexes of 2-(3,5-bis(trifluoromethyl)phenyl)-4-methylpyridine. J Organomet
Chem 2007;692:4809e27.
[9] Lyu YY, Kwak J, Jeon WS, Byun Y, Lee C, Char K, et al. Highly efficient red
phosphorescent OLEDs based on non-conjugated silicon-cored spirobi-
fluorene derivative doped with Ir complexes. Adv Funct Mater 2009;19:
420e7.
[10] Yao JH, Zhen C, Loh KP, Chen ZK. Novel iridium complexes as high-efficiency
yellow and red phosphorescent light emitters for organic light-emitting di-
odes. Tetrahedron 2008;64:10814e20.
[11] Lu W, Mi BX, Chan MW, Hui Z, Che CM, Ni Zhu, et al. Light-emitting tridentate
cyclometalated Pt(II) complexes containing alkynyl auxiliaries: tuning of
photo- and electrophosphorescence. J Am Chem Soc 2004;126:4958e71.
[12] Sun W, Wu ZX, Yang QZ, Wu LZ, Tung CH. Reverse saturable absorption of Pt
ter/bipyridyl polyphenylacetylide complexes. Appl Phys Lett 2003;82:850e2.
[13] Ji ZQ, Li YJ, Pritchett TM, Makarov NS, Haley JE, Li ZJ, et al. One-photon pho-
tophysics and two-photon absorption of 4-[9,9-Di-(2-ethylhexyl)-7-diphenyl
2.5
Tlin = 90.0%
1a
1b
1c
2.0
1.5
1.0
0.5
0.0
0.0
0.5
1.0
1.5
2.0
2.5
Incident Fluence (J/cm2)
Fig. 6. Optical limiting curves for complexes 1ae1c in CH3CN for 4.1 ns laser pulses at
532 nm.

172
T. Zhao et al. / Dyes and Pigments 126 (2016) 165e172
aminofluoren-2-yl]-2,20;60,200-terpyridine and their Pt chloride complexes.
Chem Eur J 2011;17:2479e91.
[23] Castanon SL, Beristain MF, Ortega A, Gustavo GS, Munoz E, Walser A, et al. The
synthesis, characterization and third-order nonlinear optical character of
poly(2,5-dipropargyloxybenzoate) containing a polar aromatic diacetylene.
Dyes Pigments 2011;88:129e34.
[24] Jose V, Juan GR, Zhou G, Bolink HJ, Joaquin AP. Synthesis and luminescence of
poly(phenylacetylene)s with pendant iridium complexes and carbazole
groups. J Polym Sci Part A Polym Chem 2010;48:3744e57.
[25] Serwinski PR, Lahti PM. Limits of delocalization in through-conjugated dini-
trenes: aromatization or bond formation? Org Lett 2003;5:2099e102.
[26] Hwang JJ, Tour JM. Combinatorial synthesis of oligo(phenylene ethynylene)s.
Tetrahedron 2002;58:10387e405.
[14] Zhang BG, Li YJ, Liu R, Pritchett TM, Haley JE, Sun WF. Extending the band-
width of reverse saturable absorption in Pt complexes using two-photon-
initiated excited-state absorption. ACS Appl Mater Interfaces 2013;5:565e72.
[15] Kui SF, Law YC, Tong JM, Lu W, Yuen MY, Che CM. Spectacular luminescent
behaviour of tandem terpyridyl Pt(II) acetylide complexes attributed to sol-
vent effect on ordering of excited states, “ion-pair” formation and molecular
conformations. Chem Sci 2011;2:221e8.
[16] Liu XJ, Feng JK, Meng J, Pan QJ, Ren AM, Zhou X, et al. A theoretical investi-
gation of substituent effects on the absorption and emission properties of a
series of terpyridyl Pt(II) acetylide complexes. Eur
1856e66.
J
Inorg Chem 2005:
[27] Liu R, Azenkeng A, Zhou DP, Li YH, Glusac KD, Sun WF. Tuning photophysical
properties and improving nonlinear absorption of Pt(II) diimine complexes
[17] Prusakova V, McCusker CE, Castellano FN. Ligand-localized triplet-state pho-
tophysics in Pt(II) terpyridyl perylene diimide acetylide. Inorg Chem
with extended
2013;117:1907e17.
p-conjugation in the acetylide ligands. J Phys Chem A
a
2012;51:8589e98.
[28] Liu R, Zhou DP, Azenkeng A, Li ZJ, Li YH, Glusac KD, et al. Nonlinear absorbing
platinum(II) diimine complexes: synthesis, photophysics and reverse satu-
rable absorption. Chem Eur J 2012;18:11440e8.
[18] Wang X, Goeb S, Ji Z, Castellano FN. Excited state absorption properties of
Pt(II) terpyridyl complexes bearing p-conjugated arylacetylides. J Phys Chem
B 2010;114:14440e9.
[29] Qiu D, Wu J, Xie Z, Cheng Y, Wang L. Synthesis, photophysical and electro-
phosphorescent properties of mononuclear Pt(II) complexes with arylamine
functionalized cyclometalating ligands. J Organomet Chem 2009;694:737e46.
[30] Schneider J, Du P, Jarosz P, Lazarides T, Wang X, Brennessel WW, et al.
Cyclometalated 6-phenyl-2,20-bipyridyl (CNN) Pt(II) acetylide complexes:
structure, electrochemistry, photophysics and oxidative- and reductive-
quenching studies. Inorg Chem 2009;48:4306e16.
[19] Liu R, Li Y, Li Y, Zhu H, Sun W. Photophysics and nonlinear absorption of
cyclometalated 4,6-diphenyl-2,20-bipyridyl Pt(II) complexes with different
acetylide ligands. J Phys Chem A 2010;114:12639e45.
[20] Guo F, Sun W, Liu Y, Schanze K. Synthesis, photophysics and optical limiting of
Pt(II) 40-tolylterpyridyl arylacetylide complexes. Inorg Chem 2005;44:
4055e65.
[21] Han FS, Higuchi M, Kurth DG. Diverse synthesis of novel bisterpyridines via
suzuki-type cross-coupling. Org Lett 2007;9:559e62.
[22] Das S, Brudvig GW, Crabtree RH. High turnover remote catalytic oxygenation
of alkyl groups: how steric exclusion of unbound substrate contributes to high
molecular recognition selectivity. J Am Chem Soc 2008;130:1628e37.
[31] McKay TJ, Bolger JA, Staromlynska J. Linear and nonlinear optical properties of
Pt-ethynyl. J Chem Phys 1998;108:5537e41.
[32] Guo F, Sun W. Photophysics and optical limiting of Pt(II) 40-arylterpyridyl
phenylacetylide complexes. J Phys Chem B 2006;110:15029e36.