Design of cyclometallated 5-π-delocalized donor-1,3-di(2-pyridyl)benzene platinum(II) complexes with second-order nonlinear optical properties

Article abstract of DOI:10.1016/j.poly.2017.11.051

The effect on the second-order nonlinear optical properties of the nature of the π-delocalized moiety in the position 5 of a 1,3-di(2-pyridyl)benzene cyclometallated to a platinum(II) center was investigated. The influence of the substitution of a double bond with a triple bond as bridge between a triphenylamino group and the cyclometallated phenyl ring was studied and turned out to be negligible. Remarkably, the novel easily-prepared platinum(II) complex with a cyclometallated 5-styryl-1,3-di(2-pyridyl)benzene is characterized by a good second-order NLO response, as determined in solution by the EFISH technique; it is a good candidate for application in photonics.

Full text of DOI:10.1016/j.poly.2017.11.051

Accepted Manuscript
Design of cyclometallated 5-π-delocalized donor-1,3-di(2-pyridyl)benzene
platinum(II) complexes with second-order nonlinear optical properties
Nicolò Baggi, Eleonora Garoni, Alessia Colombo, Claudia Dragonetti, Stefania
Righetto, Dominique Roberto, Julien Boixel, Véronique Guerchais, Simona
Fantacci
PII:
S0277-5387(17)30779-9
https://doi.org/10.1016/j.poly.2017.11.051
POLY 12953
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
17 October 2017
29 November 2017
Please cite this article as: N. Baggi, E. Garoni, A. Colombo, C. Dragonetti, S. Righetto, D. Roberto, J. Boixel, V.
Guerchais, S. Fantacci, Design of cyclometallated 5-π-delocalized donor-1,3-di(2-pyridyl)benzene platinum(II)
complexes with second-order nonlinear optical properties, Polyhedron (2017), doi: https://doi.org/10.1016/j.poly.
2017.11.051
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Design of cyclometallated 5--delocalized donor-1,3-di(2-pyridyl)benzene
platinum(II) complexes with second-order nonlinear optical properties.
Nicolò Baggi,a Eleonora Garoni,a Alessia Colombo,*a,b,c Claudia Dragonetti,a,b,c Stefania
Righetto,a,b,c Dominique Roberto,a,b,c Julien Boixel,d Véronique Guerchais,*d and Simona
Fantacci*e
aDip. di Chimica dell’Università degli Studi di Milano, bUdR di Milano dell’INSTM, cIstituto
d
di Scienze e Tecnologie Molecolari del CNR, via Golgi, 19, I-20133 Milano, Italy;
eComputational Laboratory for Hybrid/Organic
Photovoltaics (CLHYO), CNR-ISTM, Via Elce di Sotto 8, I-06213, Perugia, Italy
Corresponding authors: alessia.colombo@unimi.it,
simona@thch.unipg.it
Abstract
The effect on the second-order nonlinear optical properties of the nature of the -delocalized
moiety in the position 5 of a 1,3-di(2-pyridyl)benzene cyclometallated to a platinum(II) center
was investigated. The influence of the substitution of a double bond with a triple bond as
bridge between a triphenylamino group and the cyclometallated phenyl ring was studied and
turned out to be negligible. Remarkably, the novel easily-prepared platinum(II) complex with
a cyclometallated 5-styryl-1,3-di(2-pyridyl)benzene is characterized by a good
Keywords:
1

Introduction
Compounds with second-order nonlinear optical (NLO) properties are of increasing
interest as building blocks for the preparation of materials with potential applications
in optical communications, optical data processing and storage, or electro-optical
devices [1-2]. Their NLO response is closely related to intramolecular charge transfer
and a donor-acceptor push-pull system is the traditional structure of second-order
NLOphores [1-3]. Among them, coordination compounds are fascinating because they
give additional flexibility, when compared to organic compounds, due to the presence
of NLO-active charge-transfer transitions between the metal and the ligands, at
relatively low energy and of high intensity, and tunable by the nature, oxidation state
and coordination sphere of the metal center which may act as electron-donor or
electron-acceptor [4-12].
In particular, the second-order NLO activity of substituted phenylpyridines
increases upon cyclometallation, an effect that has led to cyclometallated Ru(II) [13],
Ir(III) [14-24] and Pt(II) [24-25] complexes characterized by interesting NLO
properties. Platinum(II) compounds incorporating an N^N^C-cyclometallated 6-
phenyl-2,2’-bipyridine constitutes another interesting NLO-active family [26-27].
Besides, it was reported that Pt(II) complexes with tridentate ligands based on a
cyclometallated 1,3-di(2-pyridyl)benzene, which offer the metal ion an N^C^N
coordination environment, are characterized by an enhanced NLO efficiency with
respect to the corresponding N^N^N-coordinated terpyridine complexes, as measured
by the Hyper-Rayleigh Scattering technique [28]. The good second-order nonlinear
optical properties of various platinum(II) complexes with a cyclometallated 1,3-di(2-
pyridyl)benzene have also been evidenced by the electric-field induced second
harmonic generation (EFISH) technique [29]. In particular, the cyclometallated Pt(II)
chloro complex of trans-5-(p-(N,N-diphenylamino)styryl)-1,3-di(2-pyridyl)benzene
(Figure 1, Pt-1) is characterized by a good second-order NLO response mainly
determined by a charge transfer transition from platinum- to the cyclometallated ligand
[30].
These results prompted us to investigate by the EFISH technique the known Pt(II)
complex bearing a cyclometallated 5-(p-(N,N-diphenylamino)phenylethynyl)-1,3-di(2-
pyridyl)benzene (Pt-2), previously studied for its luminescence properties and
application in OLEDs [31], in order to understand the effect on the NLO properties of
the substitution of a double bond by a triple bond as bridge between triphenylamino

moiety and the cyclometallated phenyl ring. Besides, to verify the NLO importance of
the p-(N,N-diphenylamino) donor group on the styryl moiety, we prepared the simple
novel platinum(II) complex bearing a trans-5-(styryl)-1,3-di(2-pyridyl)benzene (Pt-3).
Figure 1. Investigated cyclometallated Pt(II) complexes.
Material and methods
Synthesis
General comments
All solvents were dried by standard procedures: THF was freshly distilled from
Na/benzophenone under nitrogen atmosphere; N,N-dimethylformamide (DMF) was dried
over activated molecular sieves; triethylamine (Et3N) was freshly distilled over KOH. All
reagents were purchased from Sigma-Aldrich and were used without further purification.
Reactions requiring anhydrous or oxygen-free conditions were performed under nitrogen.
Thin layer chromatography (TLC) was carried out with pre-coated Merck F254 silica gel
plates. Flash chromatography (FC) was carried out with Macherey-Nagel silica gel 60 (230-
400 mesh).
1H and 13C spectra were recorded at 400 MHz on a Bruker AVANCE-400 instrument.
Chemical shifts (δ) are expressed in ppm relative to internal Me4Si as standard. Signals are
abbreviated as s, singlet; bs, broad singlet; d, doublet; t, triplet; q, quartet; m, multiplet. Mass
spectra were obtained with a FT-ICR Mass Spectrometer APEX II & Xmass software (Bruker
Daltonics) - 4.7 Magnet and Autospec Fission Spectrometer (FAB ionization). Elemental
analyses were performed using an Exeter Analytical E-440 analyser. UV-visible spectra were
recorded for solutions of the compounds in dichloromethane, in 1 cm pathlength quartz
cuvettes using a Biotek Instruments XS spectrometer.
3

EFISH measurements
EFISH measurements [32] were carried out at the Dipartimento di Chimica of the
Università di Milano, in DMF solutions at a concentration of 10–3 M, with a non-
resonant incident wavelength of 1.907
fundamental 1.064 m wavelength produced by a Q-switched, mode-locked
Nd3+:YAG laser manufactured by Atalaser.
The apparatus used for EFISH
m, obtained by Raman-shifting the

measurements is a prototype made by SOPRA (France). The μEFISH values reported
are the mean values of 16 measurements performed on the same sample. The sign of
EFISH is determined by comparison with the solvent.
Dipole moments calculation
All the reported calculations were performed with Gaussian09 (G09) [33] without any
symmetry constraints. The molecular geometry of Pt-1, Pt-2 and Pt-3 has been optimized in
DMF solution by a Density Functional theory (DFT) approach using the B3LYP [34]
exchange-correlation functional. For Pt a LANL2DZ basis set [35] along with the
corresponding pseudopotentials have been used, while all the other atoms have been
described with a 6-31G* basis set [36]. DMF solvation effects were included in the
calculations by means of the conductor-like polarizable continuum model (C-PCM) as
implemented in G09 [37]. Ground state dipole moments of the Pt compounds have been
computed in DMF solution and have been used in the evaluation of βEFISH from βEFISH
.
Synthesis of the platinum (II) complexes
Diethyl 3,5-dibromobenzylphosphonate was prepared according to the literature [38] Stille
couplings were performed following a reported procedure [39], using 2-(tri-n-
butylstannyl)pyridine freshly prepared according to the literature [40]. Complexes Pt-1 and
Pt-2, previously studied for their luminescence properties, were prepared as reported by some
of us [30-31]. The novel complex Pt-3 was prepared as shown in Scheme 1 by reaction of
K2PtCl4 with the new pro-ligand L-3 obtained as follows.

Scheme 1. Reagents and conditions: (i) Diethyl 3,5-dibromobenzylphosphonate, t-BuOK, THF, Ar, 0°C, then rt,
15 h, 58% yield; (ii) 2-(tri-n-butylstannyl)pyridine, LiCl, PdCl2(PPh3)2, toluene, 110°C, 48 h, 48%yield; (iii)
K2PtCl4, AcOH, 118°C, 48 h, 77% yield.
1: Diethyl 3,5-dibromobenzylphosphonate (698 mg, 1.8 mmol) and benzaldehyde (230 mg,
2.1 mmol) were dissolved in THF (20mL) under Ar atmosphere at 0°C. t-BuOK (808 mg, 72
mmol) was added in small portions to the solution over 20 minutes, keeping the mixture under
stirring. The mixture was then allowed to warm gradually at room temperature and stirred
overnight. Afterwards, H2O was added causing the formation of a precipitate and the mixture
was stirred for 30 minutes. The solvent was partially removed under reduced pressure, the
residue was extracted into CH2Cl2, the organic layer was dried over MgSO4 and the solvent
removed under reduced pressure. The crude product was purified by flash chromatography
(silica, hexane:CH2Cl2, 7:3). The pure compound was obtained as a yellow solid (412 mg).
1
Yield: 58%. H-NMR (400 MHz, CDCl3) δ (ppm): 7.56 (2H, d, J = 1.6 Hz), 7.53 (1H, t, J =
1.7 Hz), 6.49 (2H, d, J = 8.0 Hz), 7.38 (2H, t, J = 7.8 Hz), 7.30 (1H, m), 7.09 (1H, d, J = 16.2
13
Hz), 6.92 (1H, d, J = 16.2 Hz). C-NMR (101 MHz, CDCl3) δ (ppm): 141.0, 136.3, 132.6,
131.5, 128.8, 128.4, 128.8, 126.8, 125.6, 123.2.
L-3. A mixture of the bromo derivative (410 mg, 1.06 mmol), 2-(tri-n-butylstannyl)pyridine
(858 mg, 2.33 mmol), PdCl2(PPh3)2 (37 mg, 0.05 mmol) and LiCl (360 mg, 8.48 mmol) were
suspended in toluene (10 mL) and heated at reflux under argon atmosphere for 48 h. After
cooling to room temperature, an aqueous solution of NaOH 1 M was added. The resulting
solution was extracted with AcOEt and the organic layer was dried over MgSO4 and the
solvent removed under reduced pressure. The crude product was purified by flash
chromatography (silica, petroleum ether/Et2O, 1:1). The desired product was obtained as a
white solid (160 mg). Yield: 45%. 1H NMR (400 MHz, CD2Cl2) δ (ppm): 8.76 (2H, d, J = 4.2
Hz), 8.51 (1H, s), 8.26 (2H, s), 7.89 (2H, d, J = 7.8 Hz), 7.80 (2H, t, J = 7.6 Hz), 7.58 (2H, d,
13
J = 7.4 Hz), 7.40, (2H, t, J = 7.5 Hz), 7.29 (5H, m). C-NMR (101 MHz, CD2Cl2) δ (ppm):
157.1, 149.7, 140.3, 138.5, 137.3, 136.8, 129.6, 128.8, 128.4, 127.8, 126.6, 125.7, 124.7,
5

122.4, 120.8. Elem. Anal. calcd for C24H18N2: C, 86.20; H, 5.43; N, 8.38. Found: C, 86.60; H,
5.45; N, 8.42.
Pt-3. The pro-ligand L-3 (100 mg, 0.3 mmol) was dissolved in acetic acid (5 ml). The
solution was degassed through freeze-pump-thaw cycles. K2PtCl4 (149 mg, 0.36 mmol) was
added and the mixture was heated at reflux for 2 days. A yellow precipitate formed: it was
filtered, washed with MeOH, H2O, EtOH and Et2O and dried under vacuum to obtain pure Pt-
3 as a yellow powder (130 mg). Yield: 77%. 1H NMR (400 MHz, CD2Cl2) δ (ppm): 9.33 (2H,
dt, J = 16.1 Hz, 6.6 Hz), 8.05 (2H, t, J = 7.7 Hz), 7.71 (2H, s), 7.62 (2H, d, J = 7.9 Hz), 7.36
(8H, m), 7.21 (1H, d, J =16.2 Hz). Elem. Anal. calcd for C24H17ClN2Pt: C, 51.12; H, 3.04; N,
4.97. Found: C, 51.34; H, 3.05; N, 4.95. MS (FAB+), calculated: (C24H17N2Pt): m/z 528.10
[M-Cl]. Found: m/z 528 [M-Cl].
Results and Discussion
The investigated platinum(II) complexes (Figure 1) are readily prepared by reaction of
K2PtCl4 with the related 5--delocalized donor-1,3-di(2-pyridyl)benzene pro-ligand dissolved
in acetic acid (see Material and methods).
The main absorption bands in the UV-visible spectra, carried out in CH2Cl2 at 298 K,
are given in Table 1. Like other platinum(II) complexes with a cyclometallated 1,3-di(2-
pyridyl)benzene, Pt-3 shows a strong band at 263 nm, which can be assigned to intraligand
1π–π* transitions of the cyclometallated 1,3-di(2-pyridyl)benzene, and a less-intense
absorption band at 421 nm in agreement with transitions of mixed charge-transfer/ligand-
centered character [29].
Table 1. Main absorption bands in the UV-visible spectra, dipole moments and second-order
NLO response.
Absorptiona
λmax / nm
βEFISH
b
βEFISH
c
(D)
(×10-30 esu)
(×10-48 esu)
(ε / M–1 cm–1)
Pt-1d
Pt-2
Pt-3
295 (21000), 378 (15700), 430sh (4900)
-880
-764
-660
10.7
11.0
9.5
-82
-69
-69
240 (35 100), 292 (27100), 338sh
(18 200), 363 (22800), 416sh (6380)
263 (9441), 421 (2364)
(a) In CH2Cl2. (b) in DMF at 1.907 m; estimated uncertainty in EFISH measurements is ±10%. (c) computed dipole
moments in DMF solution using B3LYP//6-31g*/LANL2DZ// C-PCM level of theory. (d) from reference 30.

through eq 1:
γ EFISH = (μβλ/5kT) + γ(-2ω;ω,ω, 0)
(1)
where μβλ/5kT is the dipolar orientational contribution and γ(-2ω; ω, ω, 0) is the third-order
polarizability corresponding to the mixing of two optical fields at ω and the dc poling field at
ω=0. This latter term is often referred as the electronic cubic contribution to γEFISH, which is
usually negligible. βλ is the projection along the dipole moment (μ) axis of the vectorial
component of the tensor of the quadratic hyperpolarizability at the incident wavelength λ. All
EFISH measurements were carried out working in CH2Cl2 solutions at a concentration of 10–3
M, with a non resonant incident wavelength of 1.907 μm, obtained by Raman-shifting in a
high pressure H2 of the fundamental 1.064 μm wavelength produced by a Q-switched, mode-
locked Nd3+:YAG laser.
All complexes are characterized by a negative value of EFISH (Table 1), as
determined by the EFISH technique working in DMF solution. This negative sign is in
agreement with a decrease of the excited state dipole moment with respect to the ground state
[41], suggesting that the second-order dipolar NLO response is dominated by the charge
transfer from platinum to the cyclometallated ligand [29-30]. In order to obtain the EFISH
values, we calculated the dipole moments in DMF solution. The optimized structures were
oriented in all cases with the 1,3-di(2-pyridyl)benzene moiety in the xy plane and with the y
axis crossing the Pt center. The optimized geometries of complexes Pt-1, Pt-2 and Pt-3 are
reported in Figure 1 along with the corresponding y component of the dipole moments (y),
whereas the total dipole moments () are reported in Table 1. It appears that y and  are
basically coincident, reflecting that the dipole moment is located mainly along the y axis,
which is in agreement with a C2v symmetry of the molecular architecture.
7

Figure 2. The optimized geometry of complexes Pt-1, Pt-2 and Pt-3 and the dipole moment,
calculated in DMF and expressed in Debye, along the y axis.
Complexes Pt-1 and Pt-2 are characterized by a similar dipole moment, a bit higher than that
of Pt-3 which lacks the triphenylamino moiety. The EFISH value of Pt-1 is quite similar to
that of Pt-2, showing that the substitution of a double bond by a triple bond, as bridge
between the triphenyl amino moiety and the cyclometallated ring (Figure 2), has a negligible
NLO influence in this kind of complexes. This is an interesting result because in push-pull
organic compounds an acetylenic bridge leads to a much lower NLO response than an
ethylenic bridge [42]. Besides, remarkably, it turned out that the easily-prepared novel
complex Pt-3 is characterized by a EFISH value quite similar to Pt-1. Therefore it is not
necessary to have a donor substituent on the styryl group in order to reach a high second-order
NLO response, an important observation from a synthetic point of view. Besides, it is worth
pointing out that all the complexes investigated here are characterized by better EFISH
values, due to better EFISH values, than the platinum(II) complex bearing a cyclometallated
5-methyl-1,3-di(2-pyridyl)benzene (EFISH = -480 x10-48 esu; EFISH = -47 x 10-30 esu) [29],
showing the crucial role of the -delocalized polarizable moiety on the NLO properties.

Conclusion
In conclusion, our work gives some indications for the design of easily prepared 5--
delocalized donor-1,3-di(2-pyridyl)benzene platinum(II) complexes with interesting second-
order nonlinear optical properties. Remarkably, the simple platinum complex bearing a
cyclometallated 5-styryl-1,3-di(2-pyridyl)benzene shows a good quadratic hyperpolarizability
and has a great potential for application in photonics.
Acknowledgements
This work was supported by MIUR and CNR in Italy. We thank CNRS for a financial support
(PICS Rennes-Milan) and l’Università Italo Francese (Progetto Galileo 2015/2016; G15_50)
for mobility support.
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The novel easily-prepared platinum(II) complex with a cyclometallated 5-styryl-1,3-di(2-
pyridyl)benzene is characterized by a good
13