Synthesis, characterization, luminescence and non-linear optical properties of diimine platinum(II) complexes with arylacetylene ligands

Article abstract of DOI:10.1016/j.jorganchem.2014.04.002

A series of platinum(II) complexes (diimine)Pt(CCR)₂, where diimine is 2,2′-bipyridine (bpy) or 4,4′-bis(tert-butyl)-2,2′- bipyridine (dtbpy) and alkynyl ligand is biphenylacetylide or terphenylacetylide, were synthesized and their photophysical and non-linear optical properties were investigated. All compounds (1-3) were characterized using NMR spectroscopy, ESI mass-spectrometry and elemental analysis. X-Ray crystal structure of the complex containing 4,4′-bis(tert-butyl)-2, 2′-bipyridine and terphenylacetylide ligands is reported. The electronic absorption and emission spectra of the complexes have been studied. Room temperature phosphorescence was observed for the complexes in solution with the luminescence quantum yield in the 7.5-11% range and excited state lifetimes in microsecond time domain. All complexes under study exhibit two-photon luminescence and their double quantum absorption cross-section was found to be in the 9-22 GM range.

Full text of DOI:10.1016/j.jorganchem.2014.04.002

Journal of Organometallic Chemistry 763-764 (2014) 1e5
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, characterization, luminescence and non-linear optical
properties of diimine platinum(II) complexes with arylacetylene
ligands
,
c
Anna A. Melekhova a, Dmitrii V. Krupenya a, Vladislav V. Gurzhiya, Alexey S. Melnikov b
,
,
Pavel Yu. Serdobintsev b, Stanislav I. Selivanova, Sergey P. Tunik a
*
a St. Petersburg State University, Department of Chemistry, Universitesky Pr. 26, 198504 St. Petersburg, Russia
b St. Petersburg State University, Department of Physics, Ulianovskaya St. 3, 198504 St. Petersburg, Russia
c St. Petersburg State Polytechnical University, Polytechnicheskaya St. 29, 195251 St. Petersburg, Russia
a r t i c l e i n f o
a b s t r a c t
A series of platinum(II) complexes (diimine)Pt(C^CR)2, where diimine is 2,20-bipyridine (bpy) or 4,40-
bis(tert-butyl)-2,20-bipyridine (dtbpy) and alkynyl ligand is biphenylacetylide or terphenylacetylide,
were synthesized and their photophysical and non-linear optical properties were investigated. All
compounds (1e3) were characterized using NMR spectroscopy, ESI mass-spectrometry and elemental
analysis. X-Ray crystal structure of the complex containing 4,40-bis(tert-butyl)-2,20-bipyridine and ter-
phenylacetylide ligands is reported. The electronic absorption and emission spectra of the complexes
have been studied. Room temperature phosphorescence was observed for the complexes in solution with
the luminescence quantum yield in the 7.5e11% range and excited state lifetimes in microsecond time
domain. All complexes under study exhibit two-photon luminescence and their double quantum ab-
sorption cross-section was found to be in the 9e22 GM range.
Article history:
Received 31 January 2014
Received in revised form
29 March 2014
Accepted 2 April 2014
Keywords:
Pt organometallic complexes
Luminescence
Nonlinear properties
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
excited state with prolonged lifetime that makes them suitable for
upconversion applications [13]. These recent finding prompted us
The diiminebisacetylide and diphosphinebisacetylide Pt(II)
compounds attract considerable attention due to their efficient
luminescence [1] and promising nonlinear optical properties [2e
5] that open a wide area of potential applications, such solar en-
ergy conversion [6,7] nonlinear optical devices [5,8,9] and lumi-
nescence materials [10]. The photophysical and physico-chemical
characteristics of the square planar Pt(II) complexes can be readily
tuned by the variation of the diimine/phosphine and acetylide
ligand properties. Introduction of donor or acceptor substituents
into the backbone of these ligands makes possible to modify the
contribution of metal and ligand centered molecular orbitals into
emissive triplet states [2,11,12] and consequently to change the
energy of emission in a predictable way. In a series of recent
publications [13e18] it was shown that the Pt(II) complexes con-
to synthesize the Pt(II) 2,20-bipyridine complexes containing
biphenylacetylide or terphenylacetylide ligands with extended
conjugated aromatic substituents. In this communication we
report on characterization of these compounds and study of their
photophysical properties including one- and two-photon absorp-
tion characteristics.
Experimental
Materials and methods
2,20-Bipyridine (bpy) and 4,40-bis(tert-butyl)-2,20-bipyridine
(dtbpy) were purchased in Aldrich. 40-Ethynylbiphenyl [19], 40-
ethynylterphenyl [19], [bpyPtCl2] [20], [dtbpyPtCl2] [20] were
synthesized according to the published procedures. The solution
1H NMR spectra were recorded using a Bruker DPX 300 spec-
trometer. Mass spectra were determined with a Bruker APEX-Qe
ESI FT-ICR instrument in the ESIþ mode. Microanalyses were car-
ried out in the analytical laboratory of St. Petersburg State
University.
taining the acetylide ligands with extended conjugated
p-system
display phosphorescence strongly related to pep* intraligand
* Corresponding author. Tel./fax: þ7 (812)3241258.
E-mail address: stunik@inbox.ru (S.P. Tunik).
http://dx.doi.org/10.1016/j.jorganchem.2014.04.002
0022-328X/Ó 2014 Elsevier B.V. All rights reserved.

2
A.A. Melekhova et al. / Journal of Organometallic Chemistry 763-764 (2014) 1e5
Synthesis of [bpyPt(C^CC6H4Ph)2] (1)
Two-photon induced emission (TPIE) measurements
A suspension of [bpyPtCl2] (70 mg, 166 mmol) in CH2Cl2
(10 mL), and diisopropylamine (3 mL) was degassed and CuI
(10 mg, 0.060 mmol) and 40-ethynylbiphenyl (73.8 mg,
414 mmol) were added to the reaction mixture, which was then
stirred overnight at room temperature. The solvent was removed
in vacuo, and the residue was purified by column chromatog-
raphy packed with alumina using CH2Cl2 as eluent. The product
was recrystallized by slow concentration of a CH2Cl2/hexane
solution (0.055 g, 47%) to give an orange crystalline solid. 1H
The setup for two-photon induced emission measurements is
shown in Fig. S6 (Supporting information). In brief, a femtosecond-
mode-locked Ti:Sapphire laser Pulsar (Amplitude Technologies)
generates 20 fs pulses at a repetition rate of 80 MHz with an
average pulse energy 2 nJ (central wavelength of laser radiation is
790 nm). The laser beam was focused on a sample cell (square
quartz cuvette, inner size 1 cm) by a lens with 7 cm focal length. The
diameter of the caustic (full width at half maximum) was approx-
imately 0.02 mm, which corresponds to the radiation density up to
20 GW/cm2. The photodiode was used to monitor laser power. To
eliminate thermal effects the solution in the cell was stirred during
the experiment. To minimize the effects of reabsorption, the exci-
tation beam was focused as close as possible to the edge of the
quartz cell. The two photons induced emission was detected in a
direction perpendicular to the pump beam. Ocean SD2000 spec-
trometer was used to record fluorescence spectra.
NMR (CDCl3;
d
ppm): 9.77 (d, J ¼ 5.4 Hz, 1H), 8.14 (d, J ¼ 4.1 Hz,
2H, H3,4), 7.62 (d, J ¼ 8.0 Hz, 2H), 7.60 (d, J ¼ 8.0 Hz, 2H), 7.56
(d, J ¼ 7.5, J ¼ 7.4 Hz, 1H), 7.53 (d, J ¼ 8.0 Hz, 2H), 7.44 (dd, J ¼ 7.2,
J ¼ 7.3 Hz, 2H), 7.33 (t, J ¼ 7.0 Hz, 1H). ESIþ MS (m/z): 706
[M þ H]þ, 728 [M þ Na]þ, 744 [M þ K]þ; Elemental analysis (%):
Calc: C38H26N2Pt: C, 64.67; H, 3.71; N, 3.97. Found: C, 64.36; H,
3.87; N 4.06.
Comparison with the reference compound was used to deter-
mine the two-photon induced emission cross section. In this case
the two-photon induced emission (sTPE) and two-photon absorp-
tion (sTPA) cross-sections can be calculated using the equations (1)
and (2):
Synthesis of [dtbpyPt(C^CC6H4Ph)2] (2)
The complex 2 was prepared as an orange crystalline material
(yield 72%) analogously to 1 starting from [dtbpyPtCl2] and
40-ethynylbiphenyl. The 1H NMR (CDCl3;
d
ppm): 9.77 (d, J ¼ 6.1 Hz,
Fn3r Cr
1H), 7.96 (s, 1H), 7.63 (d, J ¼ 8.7 Hz, 2H), 7.63 (d, J ¼ 8.0 Hz, 2H), 7.61
(d, J ¼ 6.0 Hz, 1H), 7.53 (d, J ¼ 8.0 Hz, 2H), 7.44 (dd, J ¼ 7.7, J ¼ 7.8 Hz,
2H), 7.32 (t, J ¼ 7.1 Hz, 1H), 1.47 (s, 9H, t-Bu). ESIþ MS (m/z): 818
[M þ H]þ, 840 [M þ Na]þ, 856 [M þ K]þ. Elemental analysis (%) Calc:
sTPE
¼
¼
sTPE;r
;
(1)
(2)
Frn3C
sTPE
Q
sTPA
;
C
46H42N2Pt: C, 67.55; H, 5.18; N, 3.42. Found: C, 67.28; H, 5.12; N
3.70.
where F is the integrated emission intensity, C is the concentration
of the molecules in solution, n is the refractive index of the solvent,
Synthesis of (dtbpyPt(C^C(C6H4)2Ph)2 (3)
The complex 3 was prepared as an orange crystalline material
(yield 44%) analogously to 1 starting from [dtbpyPtCl2] and
Table 1
Crystallographic data for 3.
40-ethynylterphenyl. The 1H NMR (CDCl3;
d
ppm): 9.78
(d, J ¼ 6.0 Hz, 1H), 7.98 (s, 1H), 7.70 (ABq, DdAB ¼ 12 Hz, J ¼ 8.4 Hz,
4H), 7.67 (d, J ¼ 6.0 Hz, 2H), 7.62 (ABq, DdAB ¼ 24 Hz, J ¼ 8.4 Hz, 4H),
7.62 (d, J ¼ 6.0 Hz, 1H), 7.48 (dd, J ¼ 7.5, J ¼ 7.4 Hz, 2H), 7.37
(t, J ¼ 7.3 Hz, 1H), 1.48 (s, 9H, t-Bu). Elemental analysis (%): Calc
Compound
3
Formula
PtC61H56N2Cl6
Triclinic
13.137(2)
13.137(2)
17.565(3)
87.008(2)
87.008(2)
67.100(2)
2787.1(9)
1224.87
P-1
Crystal system
ꢀ
a (A)
ꢀ
b (A)
C
58H50N2Pt: C, 71.81; H, 5.19; N, 2.89. Found: C, 71.25; H, 5.21; N
ꢀ
c (A)
3.21.
a
b
g
(ꢁ)
(ꢁ)
(ꢁ)
Photophysical experiment
3
ꢀ
V (A )
Molecular weight
Space group
All photophysical measurements were carried out in CH2Cl2
solution, which was distilled immediately prior to use. All solu-
tions were carefully degassed before life-time and quantum yield
measurements. A light-emitting diode (LED; maximum emission
at 385 nm) was used to pump luminescence. The LED was used in
m
(mmꢂ1
)
2.845
210(2)
2
Temperature (K)
Z
Dcalc (g/cm3)
Crystal size (mm3)
Radiation
1.460
0.15 ꢃ 0.10 ꢃ 0.02
the continue and pulse mode (pulse width, 1e20
ms; duty of
Mo K
a
edge, w90 ns; repetition rate, 100 Hze10 kHz). A digital oscil-
loscope Tektronix TDS3014B (Tektronix, bandwidth 100 MHz),
monochromator MUM (LOMO, interval of wavelengths 10 nm)
and photomultiplier tube Hamamatsu were used for life-time
measurements. Emission spectra were recorded using an
HR2000 spectrometer (Ocean Optics). A halogen lamp, LS-1-CAL
(Ocean Optics), and deuterium lamp, DH2000 (Ocean Optics),
were used to calibrate the absolute spectral response of the
spectral system in the 200e1100 nm range. The absolute emis-
sion quantum yield was determined by the comparative method
using LED pumping and rhodamine 6G in ethanol
Total reflections
Unique reflections
21,561
8262
2.32e47.00
7407
0.0344
0.0501
0.1000
0.2156
0.1081
0.2185
1.278
Angle range 2
q
(ꢁ)
Reflections with jFoj ꢄ 4sF
Rint
Rs
R1 (jFoj ꢄ 4sF
)
wR2 (jFoj ꢄ 4sF
R1 (all data)
wR2 (all data)
S
)
3
ꢀ
rmin
,
rmax, e/A
ꢂ5.417, 3.298
R1
s
¼
SjjFoj
ꢂ
jFcjj/SjFoj; wR2
¼
{S[w(F2o
ꢂ
F2c)2]/S[w(F2o)2]}1/2
;
w
¼
1/
(
Fem ¼ 0.95 ꢀ 0.03) as standard with the refraction coefficients
[
2(F2o) þ (aP)2 þ bP], where P ¼ (Fo2 þ 2Fc2)/3; s ¼ {S[w(Fo2 ꢂ F2c)]/(n ꢂ p)}1/2
of dichloromethane and ethanol equal to 1.42 and 1.36,
respectively.
where n is the number of reflections and p is the number of refinement
parameters.

A.A. Melekhova et al. / Journal of Organometallic Chemistry 763-764 (2014) 1e5
3
the subscript r was used to denote the reference compound. A
Coumarin 307 solution (C13H12NO2F3, MW ¼ 271.24, LambdaChrom
Laser dye) in methanol was used as the reference compound. The
two-photon induced emission cross-section of Coumarin 307 is
19 GM [21]. Refractive index of methanol and dichloromethane are
1.32 and 1.42 respectively.
The ESIþ-MS measurements and 1H NMR spectroscopic data
indicate that in solution the complexes 1e3 retain the structures
found for 3 in solid state. The ESIþ-MS spectra of 1 and 2 show the
signals of molecular ions, ionized by the conjugation with the Hþ,
Naþ, Kþ cations. The isotopic patterns calculated for the ionized
complexes 1 and 2 fit completely the expected stoichiometry
(Figs. S1eS2, Supporting information). In turn, the 1H and 1He1H
COSY NMR spectra (see Experimental and Figs. S3eS5 in Support-
ing information) display a well resolved set of the signals corre-
sponding to the protons of the diimine ligands in the 9.8e7.6 ppm
range together with the resonances of aromatic rings of the aryla-
cetylenes at ca. 7.9e7.3 ppm. The number of the signals, their
multiplicity and relative intensities are in agreement with the
stoichiometry and symmetry of the molecules shown in Scheme 1.
X-ray diffraction measurements
For single crystal X-ray diffraction experiment, crystal of X was
fixed on a micro mount, placed on a Bruker Smart APEX II diffrac-
tometer and measured at 210 K using monochromated Mo Ka ra-
diation. Absorbance correction was applied using SADABS program
[22]. The structures were solved by direct methods using SHELX-97
program [23]. Positions of H atoms were modeled using ‘riding’
model. Crystallographic data for 3 are given in Table 1.
Absorption spectra
Fig. 2 presents the room temperature absorption and emission
spectra of the complexes 1e3 in dichloromethane solution. The
details of photophysical properties of compounds are listed in
Table 2. The absorption spectra of 1e3 exhibit a broad band near
Results and discussion
The synthetic strategy for preparation of three novel [Pt(diimi-
ne)(C^CAr)2] complexes shown below involves the CuI-catalyzed
chloride-to-alkyne metathesis that has been already used to
obtain a series of the other Pt(II) alkynyl compounds [24]. The
[Pt(diimine)(C^CAr)2] complexes, where Ar ¼ biphenyl, terphenyl
and diimine is bpy or dtbpy, were obtained in good yield by stoi-
chiometric reaction of [Pt(diimine)Cl2] with the corresponding
terminal alkyne HC^CAr. The CuI catalyst (10 mol %) was used with
dichloromethane as solvent and diisopropylamine as a base.
The complexes obtained were characterized by the 1H NMR
spectroscopy, mass spectrometry and elemental analyses. The
structural assignment for the Pt(diimine)(C^CAr)2 complexes was
confirmed by XRD analysis of the complex 3 containing 4,40-di-tert-
butyl-2,20-bipyridine and terphenylacetylene ligands (3).
400 nm, which has been assigned to the d(Pt)ep*(diimine) MLCT
transitions analogously to the assignment made earlier for similar
platinum compounds [6,25e27]. This band slightly shifts to higher
energy in 2 and 3, where bipyridine ligand contains electron-
donating tert-butyl groups [6,26].
w310 nm) corresponds to diimine- and acetylide-based intraligand
* transitions [6,10,25]. The absorption maximum shifts from
304 nm for 2 to 319 nm for 3 as the acetylide substituent is varied.
This change could be ascribed to the decreasing of * energy gap
A higher energy band (l
pep
pep
in aromatic compounds with increasing of a conjugated system
length.
Emission spectra
All complexes obtained feature moderately intense lumines-
cence in dichloromethane solution at room temperature. The
emission spectra are presented in Fig. 1, Table 2 lists emission
wavelength maxima, excited state lifetimes and quantum yields in
aerated and degassed solutions. The large values of the Stocks shift
together with excited state lifetimes in the microsecond time
domain are indicative of triplet origin of the emission observed,
similar to the other Pt(II) diiminebisacetylide complexes [10,25,26].
According to assignments for the structurally analogous platinum
compounds [10,26], the emissive triplet excited state in 1e3 may be
ascribed to the LMCT transitions. All compounds show emission
quantum yield in the range 7.5e11% in degassed solution. It should
be mentioned that the luminescence intensity of this complexes
decreases ca. three times in aerated solutions that can be consid-
ered as oxygen quenching of triplet exited state [28].
Structural characterization
The molecular structure of 3 obtained by X-ray crystallography
is shown in Fig. 1 together with the atom numbering scheme.
Important bond lengths and angles are given in the figure caption.
The structure is slightly distorted and to display a lower symmetry
pattern due to crystal packing effects. The PteN distances are
ꢀ
1.959(14) and 2.071(13) A, the PteC bond distance are 1.962(19)
ꢀ
and 2.019(15) A. These distances fit well those found for the other
acetylenideediimine platinum complexes [1,24,25]. Terphenyl
rings are non-coplanar and significantly twisted around single CeC
bonds.
Two-photon induced emission
Fig. 3 shows the TPIE spectra of 1e3 in CH2Cl2. Clearly, the TPIE,
in terms of spectral features and peak wavelengths, are identical to
that of the one-photon excited emission. For all complexes the in-
tensity of TPIE is linearly dependent on the square of the incident
laser power (Fig. 4). It is worth noting that none of the complexes
studied display any indication of decomposition under the condi-
tion of experiments. In degassed dichloromethane solution the
two-photon absorption cross sections (lex 790 nm) were measured
to be of the order of 10e20 GM for all compounds studied. The
obtained values of two-photon absorption cross-section are in a
good agreement with those obtained earlier for the diphosphine-
bisacetylide Pt(II) complexes, which also contain extended
ꢀ
Fig. 1. Molecular view of 3. Selected interatomic distances (A): Pt(1)eN(1) 1.959(14),
Pt(1)eN(2) 2.071(13), Pt(1)eC(39) 1.962(19), Pt(1)eC(19) 2.019(15), C(19)eC(20)
1.07(3), C(39)eC(40) 1.16(3).

4
A.A. Melekhova et al. / Journal of Organometallic Chemistry 763-764 (2014) 1e5
Scheme 1.
1,0
100
80
60
40
20
0
1,0
1
1
2
2
3
3
0,8
0,6
0,4
0,2
0,0
0,5
0,0
0,00
300
400
500
600
700
800
0,01
0,02
0,03
Wavelength, nm
P2, W2
Fig. 2. Absorption and emission (lexit ¼ 385 nm) spectra of 1e3 in CH2Cl2, 25 ꢁC.
Fig. 4. Power dependence of the upconverted luminescence intensity of 1e3 in CH2Cl2.
The uncertainty of experimental data and linear regression fit are also given.
conjugated aromatic system [29]. It has to be noted that the alkynyl
ligands used in this study are easy to prepare that open a pro-
spective to application of these complexes as TPA molecular sen-
sors, biolabels and NLO materials.
Table 2
Summary of one- and two-photon photophysical properties of 1e3, CH2Cl2, 25 ꢁC.
labs, nm
s,
m
s
lem
,
Q.Y., %
sTPA,
(10ꢂ3 ε, Mꢂ1 cmꢂ1
)
nm
GM
1
2
3
Degassed 306 (77), 409 (13)
Aerated
Degassed 304 (85), 401 (17)
Aerated
0.30 ꢀ 0.03 602 7.5 ꢀ 0.7
9 ꢀ 2
Conclusion
0.22 ꢀ 0.03
2.8 ꢀ 0.3
0.38 ꢀ 0.04 584
0.23 ꢀ 0.03
10 ꢀ 1.0 13 ꢀ 3
3.2 ꢀ 0.3
Three novel Pt(II) diiminebisacetylide complexes were synthe-
sized and structurally characterized using NMR spectroscopy, ESIþ
mass spectrometry and XRD. Their photophysical properties were
also studied including nonlinear optical characteristics. The com-
plexes obtained display appreciable phosphorescence as well as
two-photon absorption under pumping at 790 nm. This excitation
wavelength fits well into transparency window of biological tissues
that makes possible to use this compounds as photoactive com-
ponents of nonlinear confocal luminescent dyes.
Degassed 319 (100), 401 (19) 0.37 ꢀ 0.03 584
11 ꢀ 1
22 ꢀ 5
Aerated
0.26 ꢀ 0.03
4.6 ꢀ 0.5
1,0
0,8
0,6
0,4
0,2
0,0
1
2
3
Acknowledgments
Financial support from Saint-Petersburg State University
Research grant 0.37.169.2014 and RFBR grant 13-04-40342 is
gratefully acknowledged. The studies were performed using sci-
entific equipment of the Center of Shared Usage “The analytical
center of nano- and biotechnologies of SPbSPU” with financial
support of the Ministry of Education and Science of the Russian
Federation; Center for Magnetic Resonance, Center for X-ray
Diffraction Studies and Center for Chemical Analysis and Materials
Research of Saint Petersburg State University.
450
500
550
600
650
700
Appendix A. Supplementary material
Wavelength, nm
Fig. 3. Normalized two-photon induced emission spectra of 1e3 in CH2Cl2, 25 ꢁC,
lexit ¼ 790 nm.
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.jorganchem.2014.04.002. CCDC 883156

A.A. Melekhova et al. / Journal of Organometallic Chemistry 763-764 (2014) 1e5
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