Tuning the dipolar second-order nonlinear optical properties of cyclometalated platinum(II) complexes with tridentate N^C^N binding ligands

Article abstract of DOI:10.1002/chem.201301131

Appropriate functionalization of the cyclometalated ligand, L, and the choice of the ancillary ligand, X, allows the dipolar second-order nonlinear optical response of luminescent [PtLX] complexes - in which L is an N^C^N-coordinated 1,3-di(2-pyridyl)benzene ligand and X is a monodentate halide or acetylide ligand - to be controlled. The complementary use of electric-field-induced second-harmonic (EFISH) generation and harmonic light scattering (HLS) measurements demonstrates how the quadratic hyperpolarizability of this appealing family of multifunctional chromophores, characterized by a good transparency throughout much of the visible region, is dominated by an octupolar contribution. Minding our L and X: Appropriate functionalization of the cyclometalated 1,3-di(2-pyridyl)benzene ligand, L, and the choice of the ancillary X ligand allow the dipolar second-order nonlinear optical response of [PtLX] complexes to be controlled (see figure; MLCT=metal-to-ligand charge transfer, ILCT=Intraligand charge transfer). Copyright

Full text of DOI:10.1002/chem.201301131

FULL PAPER
Minding our L and X: Appropriate
functionalization of the cyclometalated
1,3-di(2-pyridyl)benzene ligand, L, and
the choice of the ancillary X ligand
allow the dipolar second-order nonlin-
ear optical response of [PtLX] com-
plexes to be controlled (see figure).
Nonlinear Optics
E. Rossi, A. Colombo, C. Dragonetti,
S. Righetto, D. Roberto,* R. Ugo,
A. Valore, J. A. G. Williams,
M. G. Lobello, F. De Angelis,
S. Fantacci,* I. Ledoux-Rak, A. Singh,
J. Zyss .......................... &&&& — &&&&
Tuning the Dipolar Second-Order
Nonlinear Optical Properties of Cyclo-
metalated Platinum(II) Complexes
with Tridentate N^C^N Binding
Ligands
Chem. Eur. J. 2013, 00, 0 – 0
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DOI: 10.1002/chem.201301131
Tuning the Dipolar Second-Order Nonlinear Optical Properties of
Cyclometalated Platinum(II) Complexes with Tridentate N^C^N Binding
Ligands
Ester Rossi,[a] Alessia Colombo,[a] Claudia Dragonetti,[a] Stefania Righetto,[a]
Dominique Roberto,*[a] Renato Ugo,[a] Adriana Valore,[a] J. A. Gareth Williams,[b]
Maria Grazia Lobello,[c] Filippo De Angelis,[c] Simona Fantacci,*[c]
Isabelle Ledoux-Rak,[d] Anu Singh,[d] and Joseph Zyss[d]
Dedicated to Dr. Rinaldo Psaro for his 60th birthday
Abstract: Appropriate functionaliza-
tion of the cyclometalated ligand, L,
and the choice of the ancillary ligand,
X, allows the dipolar second-order
nonlinear optical response of lumines-
cent [PtLX] complexes—in which L is
an N^C^N-coordinated 1,3-di(2-pyri-
dyl)benzene ligand and X is a mono-
dentate halide or acetylide ligand—to
be controlled. The complementary use
of electric-field-induced second-har-
monic (EFISH) generation and har-
monic light scattering (HLS) measure-
ments demonstrates how the quadratic
hyperpolarizability of this appealing
family of multifunctional chromo-
phores, characterized by a good trans-
parency throughout much of the visible
region, is dominated by an octupolar
contribution.
Keywords: chromophores · density
functional calculations · ligand ef-
fects · nonlinear optics · platinum
Introduction
Recently we reported that various luminescent, cationic
cyclometalated IrIII complexes that contain a substituted
1,10-phenanthroline[3] or 2,2’-bipyridine[4] ligand are charac-
terized by interesting second-order NLO properties. The use
of the electric-field-induced second-harmonic (EFISH) gen-
eration method,[5] supported by a sum-over-states time-de-
pendent density functional theory (SOS-TD-DFT) investiga-
tion, revealed that the NLO properties are determined
mainly by MLCT (metal-to-ligand charge-transfer) process-
es from the orbitals of the cyclometalated Ir-containing
moiety, which acts as a donor system to p* orbitals of the
N^N bidentate ligand, which acts as an acceptor system.[3]
Also, some luminescent b-diketonate IrIII and PtII complexes
with a cyclometalated 2-phenylpyridine ligand show a signif-
icant second-order NLO response, as determined by the
EFISH technique, attributed through a SOS-TD-DFT inves-
Organometallic and coordination complexes that show both
luminescent and second-order nonlinear optical (NLO)
properties are new molecular multifunctional materials that
might offer additional flexibility relative to organic molecu-
lar materials. Their luminescent and NLO properties can be
tuned by introducing charge-transfer transitions between the
metal and the ligands, the influence of which can be control-
led according to the nature, oxidation state, and coordina-
tion sphere of the metal center.[1,2]
[a] Dr. E. Rossi, Dr. A. Colombo, Dr. C. Dragonetti, Dr. S. Righetto,
Prof. Dr. D. Roberto, Prof. Dr. R. Ugo, Dr. A. Valore
Dipartimento di Chimica, Universitꢁ degli Studi di Milano
UdR dellꢂINSTM di Milano and ISTM-CNR
via Golgi 19, 20133 Milano (Italy)
tigation to mainly intraACTHUNTGRNENUGliACHUTNTGERNNUGgand charge-transfer transitions that
involve the cyclometalated ligand.[6] Tris-cyclometalated IrIII
complexes are also characterized by interesting second-
order NLO properties that can be tuned by the nature of
the cyclometalated 2-phenylpyridine substituents.[7]
Fax : (+39)02-50314405
E-mail: dominique.roberto@unimi.it
[b] Prof. Dr. J. A. G. Williams
Department of Chemistry, University of Durham
South Road, Durham, DH1 3LE (UK)
It was recently reported[8] that a series of unconventional
terpyridine and cyclometalated dipyridylbenzene platinu-
m(II) complexes, with the tridentate ligand carrying a strong
electron-acceptor group, display large quadratic hyperpolar-
izabilities, as measured by the hyper-Rayleigh scattering
technique (HRS). The N^C^N-coordinated cyclometalated
dipyridylbenzene platinum complexes are characterized by
an enhanced NLO efficiency with respect to the correspond-
[c] Dr. M. G. Lobello, Dr. F. De Angelis, Dr. S. Fantacci
ISTM-CNR, Via Elce di Sotto 8, 06100 Perugia (Italy)
Fax : (+39)075-585-5606
E-mail: simona@thch.unipg.it
[d] Prof. Dr. I. Ledoux-Rak, Dr. A. Singh, Prof. Dr. J. Zyss
Laboratoire de Photonique Quantique et Molꢃculaire
UMR CNRS 8531-Institut dꢂAlembert, ENS Cachan
61 avenue du Prꢃsident Wilson, 94235 Cachan (France)
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Cyclometalated Platinum(II) Complexes
FULL PAPER
ing structurally related N^N^N-coordinated terpyridine
complexes.[8] Indeed, various luminescent 4,4’-stilbenoid-sub-
stituted N^C^N platinum(II) complexes show significant
quadratic hyperpolarizabilities, as determined by the HRS
technique, which are higher than those typical of purely or-
in a transparent region of the absorption spectra of all the
PtII complexes investigated.
Experimental Section
ganic 4,4’-disubstituted stilbenes, probably due to the polari-
[9]
ꢀ
zation along the C Pt bond.
General comments: Reagents were purchased from Sigma–Aldrich or
Fluorochem. Complexes 1–5 were prepared as previously reported,[14–18]
whereas new complexes 6–10 were prepared as described below. Products
were characterized by 1H NMR (Bruker DRX-300 spectrometer) and
UV-visible (Jasco V-530 spectrophotometer) spectroscopy, and by ele-
mental analysis.
EFISH measurements: All EFISH measurements[5c] were carried out at
the Dipartimento di Chimica of the Universitꢁ degli Studi di Milano, on
solutions of samples in DMF at a concentration of 1ꢄ10ꢀ4 m, with a non-
resonant incident wavelength of 1.907 mm, which was obtained by Raman
shifting the fundamental 1.064 mm wavelength produced by a Q-switched,
mode-locked Nd3+:YAG laser manufactured by Atalaser. The apparatus
These results prompted us to carry out an in-depth inves-
tigation of the electronic origin and the tuning of the
second-order NLO properties of cyclometalated 1,3-di(2-
pyridyl)benzene platinum(II) complexes. This class of mate-
rials is of particular interest for study because of their re-
markable luminescent properties.[10] The effect of the nature
of the substituents on the cyclometalated 1,3-di(2-pyridyl)-
benzene ligand and of the monodentate ancillary ligand
(chloride or various substituted phenylacetylides) on the
quadratic hyperpolarizability has been investigated by using
complexes 1–10. The interpretation of their electronic struc-
used for EFISH and THG measurements is
a prototype made by
SOPRA (France). The mbEFISH values reported are the mean values of 16
successive measurements performed
on the same sample. The sign of mb
was determined by comparison with
the reference solvent (DMF).
HLS measurements: The HLS techni-
que[11–13] involves the detection of the
incoherently scattered second-harmon-
ic light generated by a solution of the
molecule under irradiation with a laser
of wavelength l, thus leading to the
measurement of the mean value of the
bꢄb tensor product, <bHLS >. All
HLS measurements were carried out
at the ꢅcole Normale Supꢃrieure de
Cachan in [D7]DMF at a concentration
of 1ꢄ10ꢀ3 m by working with a low-
energy nonresonant incident radiation
of 1.907 mm.
Preparation of platinum complexes:
For the numbering used to assign the
1H NMR spectroscopic signals, see
Scheme 1.
N^C^N-5-Methyl-1,3-di(2-pyridyl)-
benzene platinum(II) pentafluorophe-
nylacetylide (6): A mixture of penta-
fluorophenylacetylene
(40.3 mg,
0.210 mmol) and 0.5m sodium methox-
ide (0.4 mL, 0.210 mmol) in methanol
(1 mL) was stirred for 30 min at room
temperature. Then complex 1[11]
(101.0 mg, 0.210 mmol) dissolved in
MeOH/CH2Cl2 (4:1 v/v) was added,
and the mixture was left for one day
at room temperature. The solution
changed color by passing from yellow
to dark green. Then the solvents were
removed, and the crude product was
washed with water, methanol, and n-
tures and optical properties has been facilitated by DFT and
TD-DFT calculations. Meanwhile, the dipolar and octupolar
contributions to the quadratic hyperpolarizability[11,12] of
these platinum(II) complexes have been determined experi-
mentally by a combination of EFISH and harmonic light
scattering (HLS)[13] techniques, working with a nonresonant
incident wavelength of 1907 nm, the second (2w) and third
harmonic (3w) of which lie at 953 and 636 nm, respectively,
hexane. Further purification by precipitation with dry pentane from
CH2Cl2 gave the desired product as a green solid in almost quantitative
yield (128.0 mg, 97%). 1H NMR (300 MHz, CDCl3): d=9.46 (d, J=
5.6 Hz, JACHTUNGRNENUG(
195Pt)=48.0 Hz, 2H; H6), 7.93 (t, J=8.0 Hz, 2H; H4), 7.67 (d,
J=8.0 Hz, 2H; H5), 7.35 (s, 2H; H6’), 7.23 (t, J=6.0 Hz, 2H; H5),
2.40 ppm (3H; CH3); 19F NMR (300 MHz, CDCl3): d=ꢀ136.7 (d, 2F),
ꢀ
ꢀ157.7 (t, 1F), ꢀ159.9 ppm (t, 2F); IR (CH2Cl2): n˜ =2081 cmꢀ1 (C C);
ꢁ
elemental analysis calcd (%) for C25H13F5N2Pt: C 47.55, H 2.08, N 4.44;
found: C 47.67, H 2.10, N 4.32.
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D. Roberto, S. Fantacci et al.
desired product as a green solid in almost quantitative yield (145.3 mg,
1
98%). H NMR (400 MHz, THF): d=9.87 (s, J
(
195Pt)=48.0 Hz, 2H; H6),
8.48 (d, J=8.8 Hz, 2H; H4), 8.14 (d, J=8.8 Hz, 2H; H3), 7.43 (d, J=
7.7 Hz, 2H; H2’’), 7.27 (t, J=7.6 Hz, 2H; H3’’), 7.16 (t, J=7.6 Hz, 1H;
ACTHNUTRGNEUNG
H4’’), 6.96 pm (t, J(19F)=11.0 Hz, 2H; H4’’); IR (CH2Cl2): n˜ =2091 cmꢀ1
ꢁ
(C C); elemental analysis calcd (%) for C26H12F8N2Pt: C 44.65, H 1.73,
N 4.01; found: C 44.72, H 1.72, N 4.06.
N^C^N-1,3-Bis[5-(trifluoromethyl)pyridin-2-yl]-4,6-difluorobenzene plat-
inum(II) p-dimethylaminophenylacetylide (10): A solution of (4-ethynyl-
phenyl)dimethylamine (30.8 mg, 0.213 mmol) and 0.5m sodium methox-
ide (0.4 mL, 0.213 mmol) in methanol (1 mL) was stirred for 30 min at
room temperature. Then the mixture was added to a solution of complex
4[16] (134.7 mg, 0.212 mmol) in MeOH/CH2Cl2 (5:1) that had been previ-
ously cooled to ꢀ208C. The mixture was left stirring at ꢀ208C for 4 h
and then left at room temperature overnight; its color changed from
yellow to dark red. The solvents were removed, and the crude product
was washed with water, methanol, and n-hexane. Further purification by
precipitation with dry pentane from CH2Cl2 gave the desired product as
a red solid (63.2 mg, 40%). 1H NMR (400 MHz, THF): d=9.58 (s, J-
ACTHNGUTRNENUG(
195Pt) = 48.0 Hz, 2H; H6), 8.21 (d, J=8.5 Hz, 2H; H4), 7.86 (d, J=
8.5 Hz, 2H; H3), 7.15 (d, J=8.8 Hz, 2H; H3’’), 6.68 (d, J=8.8 Hz, 2H;
H2’’), 6.58 (d, J
G
N
ꢀ
(CH2Cl2): n˜ =2085 cmꢀ1 (C C); elemental analysis calcd (%) for
Scheme 1. 1H NMR spectra numbering scheme.
C28H17F8N3Pt: C 45.29, H 2.31, N 5.66; found: C 44.87, H 2.72, N 5.42.
Computational methods: The molecular geometry of all the PtII com-
plexes was optimized in vacuo by means of a DFT approach using the
B3LYP[19] exchange-correlation functional and a LANL2DZ basis set[20]
for all atoms along with the corresponding pseudopotentials for Pt. We
also reoptimized the geometries in DMF solution, finding a small differ-
ence of dipole moment compared to geometries in vacuo, which are
therefore used throughout the manuscript. All the calculations were per-
formed with Gaussian 03[21] without any symmetry constraints. Single-
point calculations were performed in DMF solution on the optimized ge-
ometry in vacuo using a LANL2DZ basis set. Solvation effects were in-
cluded by means of the conductor-like polarizable continuum model (C-
PCM)[22] as implemented in Gaussian 03. Since in Gaussian 03 the DMF
solvent was not parameterized, we employed the cavity parameters of
the similar acetonitrile solvent and the e and eINF values for DMF from
Gaussian 09. Ground-state dipole moments have been computed for
complexes 1–10 both in vacuo and in DMF solution. In the evaluation of
bEFISH from mbEFISH, the value of mtot calculated in DMF solution (Table 1)
was always used. Analysis of the electronic structure in terms of energy
and character of the frontier molecular orbitals has been carried out for
complexes 1–10 in DMF solution. TD-DFT calculations have been per-
formed in DMF solution for complex 4 using the B3LYP functional, the
SDD basis set for Pt, and the 6-31G** basis set for all the other atoms.
Fifty singlet–singlet transitions have been computed and interpolated by
Gaussian functions with s=0.15 eV, which roughly corresponds to a full
width at half-maximum (FWHM) value of approximately 0.36 eV. To
qualitatively understand the electronic factors that govern the second-
order NLO response of complex 4, the lowest 50 singlet–singlet excita-
tion energies, transition dipole moments, oscillator strengths, and excited-
state dipole moments of complex 4 were calculated by TD-DFT to evalu-
ate the quadratic hyperpolarizability by means of the sum-over-states
(SOS) approach. The excited-state dipole moments were calculated by
using the RhoCI density for each calculated excited state, as implement-
ed in Gaussian 03. The SOS approach requires in principle the calcula-
tion of dipole matrix elements between all possible couples of excited
states (three level terms) in addition to the ground-to-excited-state transi-
tion dipole moments (two level terms).[23] It turns out, however, that
three-level terms show approximately the same scaling with the number
of excited states as two-level terms, so that the latter can be used for a
qualitative assessment of the contributions to the quadratic hyperpolariz-
ability.[23]
N^C^N-5-Methyl-1,3-di(2-pyridyl)benzene platinum(II) p-nitrophenyl-
acetylide (7):
A
mixture of 1-ethynyl-4-nitrobenzene (22.3 mg,
0.152 mmol) and 0.5m sodium methoxide (0.3 mL, 0.152 mmol) in metha-
nol (1 mL) was stirred for 30 min at room temperature. Then complex
1[14] (75.6 mg, 0.152 mmol) dissolved in MeOH/CH2Cl2 (4:1 v/v) was
added, and the mixture was left for one day at room temperature. The
solution changed color by passing from yellow to red. Then the solvents
were removed, and the crude product was washed with water, methanol,
and n-hexane. Further purification by precipitation with dry pentane
from CH2Cl2 gave the desired product as a red solid in quantitative yield
(89.0 mg, 100%). 1H NMR (400 MHz, CDCl3): d=9.38 (d, J=5.6 Hz, J-
ACHTUNGTRENNUNG(
195Pt)=40.8 Hz, 2H; H6), 8.16 (d, J=8.9 Hz, 2H; H3’’), 7.95 (td, J=1.4,
7.8 Hz, 2H; H4), 7.67 (m, 4H; H5–2’’), 7.23 (td, J=1.4 Hz, 2H; H3),
ꢀ1
ꢀ
ꢁ
2.62 ppm (s, 3H;
G
analysis calcd (%) for C25H17N3O2Pt: C 51.20, H 2.92, N 7.16; found: C
50.97, H 2.77, N 7.03.
N^C^N-1,3-Di(2-pyridyl)-4,6-difluorobenzene platinum(II) phenylacety-
lide (8): A solution of 1-ethynylbenzene (15.8 mg, 0.155 mmol) and 0.5m
sodium methoxide (0.3 mL, 0.155 mmol) in methanol (1 mL) was stirred
for 30 min at room temperature. Then the mixture was added to a solu-
tion of complex 3[16] (77.5 mg, 0.155 mmol) in MeOH/CH2Cl2 (5:1) that
had been previously cooled to ꢀ208C. The mixture was left stirring at
ꢀ208C for 4 h and then left at room temperature overnight. The solvents
were removed, and the crude product was washed with water, methanol,
and n-hexane. Further purification by precipitation with dry pentane
from CH2Cl2 gave the desired product as a yellow solid (70.3 mg, 80%).
1H NMR (400 MHz, [D8]THF): d=9.58 (d, J=5.5 Hz, J 195Pt)=41.0 Hz,
ACHUTNGRNENUG(
2H; H6), 8.17 (d, J=7.8 Hz, 2H; H4), 8.00 (td, J=7.8 Hz, 2H; H3), 7.44
(m, 4H; H5–2’’), 7.27 (m, 2H; H3“), 7.15 (m, 1H; H4’’), 6.85 ppm (t, J-
ꢁ
A
analysis calcd (%) for C24H14F2N2Pt: C 51.16, H 2.50, N 4.97; found: C
50.95, H 2.36, N 4.88.
N^C^N-1,3-Bis[5-(trifluoromethyl)pyridin-2-yl]-4,6-difluorobenzene plat-
inum(II) phenylacetylide (9): A solution of phenylacetylene (24.0 mg,
0.234 mmol) and 0.5m sodium methoxide (0.4 mL, 0.212 mmol) in metha-
nol (1 mL) was stirred for 30 min at room temperature. Then the mixture
was added to a solution of complex 4[16] (134.7 mg, 0.212 mmol) in
MeOH/CH2Cl2 (5:1) that had been previously cooled to ꢀ208C. The mix-
ture was left stirring at ꢀ208C for 4 h and then left at room temperature
overnight; the color changed from yellow to dark red. The solvents were
removed, and the crude product was washed with water, methanol, and
n-hexane. Further purification by precipitation with dry pentane gave the
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Cyclometalated Platinum(II) Complexes
FULL PAPER
Table 1. Experimental electronic spectra and mbEFISH, calculated total m, and related bEFISH of the investigated PtII complexes.
[b]
[c]
[b]
Absorbance[a]
mbEFISH
[ꢄ10ꢀ48 esu]
mtot
bEFISH
[ꢄ10ꢀ30 esu]
lmax [nm] (e [mꢀ1 cmꢀ1])
[D]
1
2
3
4
5
6
7
8
9
334 (8000), 382 (9600), 412 (9000)
325 sh (7100), 377 (8900), 420 (9500), 489 (400)
321 (11300), 334 (14100), 366 (13000), 375 (15300), 438 (100), 467 (100)
331 (10000), 342 (10700), 387 (11300), 474 (300)
335 (8500), 388 (8400), 410 (7300)
ꢀ480
ꢀ1060
ꢀ720
10.2
6.0
5.4
13.1
7.4
12.7
17.9
2.4
ꢀ47
ꢀ177
ꢀ144
ꢀ188
ꢀ296
ꢀ180
ꢀ104
ꢀ442
ꢀ215
ꢀ470
ꢀ2470
ꢀ2190
ꢀ2290
ꢀ1860
ꢀ1060
ꢀ1980
ꢀ2020
340 (6600), 368 (4000), 384 (3200), 410 br (2300)
329 (13300), 361 (10100), 394 br (7800), 465 (700)
328 (9700), 334 (8900), 374 (8000), 410 (3200), 469
346 (7200), 399 (7200), 449 (3700)
ACHTUNGTNER(NUNG 200)
9.2
4.3
10
341 (14600), 384 (9200), 493 (4100)
[a] From 320 nm in DMF. [b] Working in DMF with an incident radiation wavelength of 1.907 mm; estimated uncertainty in EFISH measurements is
ꢂ10%. [c] Calculated at the B3LYP level in DMF; values computed under vacuum were 7.2, 3.8, 3.6, 9.3, 5.7, 10.7, 14.3, 1.7, 6.3, and 1.8 Debye for com-
plexes 1–10, respectively. The error on m is ꢂ1 D.
Results and Discussion
Synthesis and spectroscopic characterization: The known
complexes (1–5) were prepared as previously reported,[14–18]
whereas the new complexes (6–10) were readily obtained in
good to excellent yield by reaction of the cyclometalated
platinum(II) chloride precursors with the suitable phenyla-
ꢁ
cetylide anion, prepared by the deprotonation of HC CC6F5
ꢁ
or HC CC6H4-4-R (R=H, NO2, NMe2) with sodium meth-
oxide (see the Experimental Section). All platinum(II) com-
plexes were fully characterized by elemental analyses and
by IR, NMR (see the Experimental Section), and UV-visible
(see Table 1) spectroscopies. The electronic absorption spec-
tra obey Lambert-Beerꢂs law when the concentration is
lower than 1.0ꢄ10ꢀ4 m, which suggests in this range of con-
Figure 1. Absorption spectra of complex 4 in various solvents.
centrations a lack of significant aggregation such as through
Dipole moments, electronic structure, and absorption spec-
tra: The molecular geometries of complexes 1–10 were opti-
mized by DFT calculations (see the Experimental Section).
The 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 PtII center (see Figure 2). The optimized geome-
tries of the structure of complexes 1–10 are reported in
Figure 2 along with the corresponding y component of the
dipole moments (my) calculated in DMF solution, whereas
the total dipole moments (mtot) calculated both in vacuo and
in DMF solution are reported in Table 1. It appears that the
dipole moment is located mainly along the y axis, which is
in agreement with a C2v symmetry of the molecular architec-
ture. Moreover, the calculated dipole moments in DMF sol-
ution are higher than those calculated in vacuo (there is an
increase of about 1.2–1.6 times for complexes 1–9, with the
largest increase of a factor of 2.4 for complex 10). For com-
plexes 1–10, the energies of the frontier HOMO–LUMO
molecular orbitals computed in solution are schematized in
Figure 3, and the corresponding HOMO and LUMO isoden-
sity plots are reported in Figure 4.
[24]
ꢀ
p–p stacking or Pt Pt interactions. The absorption band
maxima at wavelengths greater than 330 nm and their ex-
tinction coefficients are presented in Table 1.
All complexes show intense bands in the 260–320 nm
1
region, which can be assigned to intra
li
N
tions of the cyclometalated 1,3-di(2-pyridyl)benzene[25] and
of the ancillary acetylide[26] ligands. The less-intense absorp-
tion bands at 330–450 nm were reported to correspond to
transitions of mixed charge-transfer/ligand-centered charac-
ter.[27] In accordance, they exhibit a pronounced negative
solvatochromic behavior (for example, complex
4 in
Figure 1), typical of electronic transitions with an apprecia-
ble degree of charge-transfer character; the blueshift of the
peaks with increasing solvent polarity suggests a lower
dipole moment in the excited state than in the ground
state.[25] Although the substitution of the chloride ligand by
various phenylacetylide ligands does not change the spectral
profile substantially, the individual bands in the region 330–
450 nm become rather less well resolved, with evidence of a
tail to longer wavelengths. This tail might tentatively be at-
tributed to a pCꢁC!p*N^C^N ligand-to-ligand charge-transfer
(LLCT) transition, by analogy with other platinum(II) ace-
tylide complexes.[26,28]
Comparison between the computed (TD-DFT calcula-
tions) and experimental absorption spectra of complex 4
(Figure 5) confirms that the simulated spectrum nicely re-
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D. Roberto, S. Fantacci et al.
Figure 2. The optimized geometry of complexes 1–10 and the dipole
moment, calculated in DMF and expressed in Debye, along the y axis.
Figure 4. Isodensity plot (isodensity value=0.035) of the HOMO and
LUMO levels of complexes 1–10 calculated in DMF solution.
small participation of the Pt dyz orbital (see Figure 5). The
LUMO+1 of complex 4 has an electronic distribution simi-
lar to that of the LUMO+1 of complexes 2, 7, and 10, and
to the LUMO of complexes 1, 3, 5, 6, and 8. The experimen-
tal shoulder at 365 nm originates from a HOMOꢀ2!
LUMO transition, in which HOMOꢀ2 is a nonbonding
combination of the Pt dxz orbital and the p orbital on the
pyridyl and benzene rings, whereas the LUMO is a p* orbi-
tal completely delocalized on the 1,3-bis(5-(trifluoromethyl)-
pyridin-2-yl)-4,6-difluorobenzene ligand with a small metal
contribution. Both these transitions have MLCT character
Figure 3. Schematic representation of the energy levels of complexes 1–
10 calculated in DMF.
with partial intra
sorption bands at 342 and 330 nm are also of MLCT charac-
ter with small contribution of intraligand charge-transfer
ACHUTGTNRENNUGliACTHNGUTRENNUgG and p–p* nature. Other experimental ab-
A
A
produces the main experimental features, with the calculat-
ed low-energy absorption band redshifted only by 0.05 eV
with respect to the experimental one. Moreover, in corre-
spondence with the experimental shoulder, we confirmed
that we have a transition computed at 365 nm that is charac-
terized by a very low oscillator strength.
The lowest-energy absorption band of complex 4 is char-
acterized by a HOMO!LUMO+1 transition in which the
HOMO level is delocalized along the yz plane on the metal
center, the ancillary Cl ligand, and the cyclometalated ben-
zene ring, whereas the LUMO+1 level is delocalized on the
pyridyl rings and on the central phenyl carbon bound to the
metal center of the 1,3-di(2-pyridyl)benzene ligand, with
character on the 1,3-di(2-pyridyl)benzene ligand.
Second-order NLO properties: The second-order NLO prop-
erties of the various platinum(II) complexes were first inves-
tigated by the EFISH[5] technique (see Table 1). It is known
that this technique can provide direct information on the in-
trinsic dipolar molecular second-order NLO properties
through Equation (1):
gEFISH ¼ ðmbEFISH=5kTÞ þ gðꢀ2w; w, w, 0Þ
ð1Þ
in which mbEFISH/5kT is the dipolar orientational contribu-
tion, and g(ꢀ2w; w, w, 0), a third-order term at frequency w
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Cyclometalated Platinum(II) Complexes
FULL PAPER
by a fluorine atom (complex 2) on the metalated benzene
ring causes an increase in the absolute value of mbEFISH and
a decrease in the dipole moment, therefore leading to a
large increase (by a factor of 3.8) of bEFISH, as expected for a
stronger charge-transfer process. The absolute value of
mbEFISH for complex 3, with two fluorine atoms on the 4- and
6-positions of the metalated benzene ring, is lower than that
of complex 2 in part due to a decrease of the dipole
moment, whereas the absolute value of bEFISH is only slightly
lower. A slightly higher value of bEFISH is shown by complex
4, which has a structure that shows two additional CF3
groups on the pyridine rings. Since the dipole moment is en-
hanced by a factor of 2.4, a large value of mbEFISH (ꢀ2470ꢄ
10ꢀ48 esu) is obtained.
A large mbEFISH value (ꢀ2190ꢄ10ꢀ48 esu) is also reached
upon substitution of the chloride ancillary ligand of complex
1 by a phenylacetylide ligand (complex 5) due to a strong in-
crease in the absolute value of bEFISH (by a factor of 6.3),
probably caused by an increased charge transfer from the
Pt–phenylacetylide moiety. Interestingly, the value of mbEFISH
is not strongly affected by increasing the electron-withdraw-
ing properties of the acetylide ligand, as observed upon
going from the phenylacetylide 5 to the pentafluorophenyla-
cetylide 6, due to the expected large decrease of the abso-
lute value of bEFISH buffered by an accompanying increase of
the dipole moment. Substitution of the chloride ancillary
ligand of complexes 3 and 4 by the phenylacetylide ligand to
give complexes 8 and 9, respectively, also leads, as in the
case of complexes 1 and 5, to an increase in the absolute
value of bEFISH with a parallel significant decrease in the
dipole moment.
The addition of electron-withdrawing substituents on the
1,3-di(2-pyridyl)benzene cyclometalated ligand of PtII–chlor-
ide complexes thus leads to an enhancement of its acceptor
properties, as confirmed by a decrease in the energy of the
LUMO orbitals (Figure 3), and therefore to a more facile
MLCT from the Pt atom, which corresponds to an increase
in the absolute value of bEFISH. Substitution of chloride with
phenylacetylide as ancillary ligand (complexes 1, 3, 4 and 5,
8, 9, respectively) leads to an increase in bEFISH in agreement
with the calculated decrease in the HOMO–LUMO gap (by
0.40, 0.59, and 0.68 eV, respectively) that is caused by a shift
of the HOMO orbitals, which are located mainly on the
phenylacetylide ligand, at higher energy (see Figure 3). As
shown in the isodensity plots reported in Figure 4, the
HOMO–LUMO transition corresponds, with the exception
of complex 7, to a charge transfer from the phenylacetylide
ligand to the acceptor 1,3-di(2-pyridyl)benzene cyclometa-
lated ligand. In accordance, for complex 6 (with the penta-
fluorophenylacetylide ligand) such charge transfer is less im-
portant (Figure 4). This lower charge transfer from the pen-
tafluorophenylacetylide ligand relative to that of complex 5
(with phenylacetylide) and the increase in the HOMO–
LUMO gap due to a strong stabilization of the HOMO with
respect to the LUMO (Figure 3) are the origin of the de-
crease of the absolute value of b1.907EFISH upon going from 5
to 6.
Figure 5. Top: Comparison between the TD-DFT simulated spectrum
(continuous line) and the experimental spectrum (dotted line) of complex
4 in DMF. Vertical lines correspond to calculated excitation energies and
oscillator strengths. Inset: Isodensity surface plots (isodensity counter=
0.035) of molecular orbitals involved in the lowest intense transition that
gives rise to the absorption band at 396 nm. Bottom: Calculated contri-
butions to the quadratic hyperpolarizability b for complex 4 as a function
of the excited-state wavelength.
of incident light, is a purely electronic cubic contribution
that can usually be neglected when studying the second-
order NLO properties of strongly dipolar molecules.[2]
The experimental mbEFISH values of all the investigated PtII
complexes, measured as solutions in DMF and working with
a nonresonant incident wavelength of 1.907 mm, are reported
in Table 1. The values of bEFISH, the projection along the
dipole moment axis of the vectorial component of the
tensor of the quadratic hyperpolarizability, were obtained by
using the values of the dipole moments calculated by DFT.
All complexes are characterized by a negative value of
mbEFISH owing to the negative value of Dmeg upon excita-
tion,[2] which reflects a decrease in the dipole moment upon
excitation as suggested by the negative solvatochromic be-
havior[25] of the low-energy MLCT transitions reported
above. It follows that the second-order dipolar NLO re-
sponse is probably dominated by the charge transfer from
platinum to the cyclometalated ligand. In accordance, when
taking the PtII–chloride complexes (1–4; Table 1) into con-
sideration, the substitution of a methyl group (complex 1)
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D. Roberto, S. Fantacci et al.
To qualitatively gauge the electronic factors that govern
the second-order NLO properties for the prototype complex
4, we determined the contribution of each excited state to
the static quadratic hyperpolarizability by means of the SOS
method.[23] In the bottom panel of Figure 5 the contribution
to the quadratic hyperpolarizability as a function of the ex-
cited-state wavelength for complex 4 is reported. The over-
all quadratic hyperpolarizability has a negative value, as ex-
perimentally found, because the negative contributions dom-
inate the small positive contributions. We found that three
main transitions determine the negative sign of the hyperpo-
larizability, which are associated to the S3, S7, and S36 excited
states. These excited states are characterized by (relatively)
strong transition dipole moments (absolute values of 0.30,
0.97, and 1.49 a.u., for the S3, S7, and S36 excited states, re-
spectively, along the y axis) and by a sizable decrease in the
dipole moment with respect to the ground state (Dmeg 3.6,
4.9, and 4.7 D for the S3, S7, and S36 excited states, respec-
tively). Thus, even the high-lying S36 excited state partly con-
tributes to the second-order NLO response by virtue of the
very strong transition dipole moment. S2 and S7 have a
strong MLCT character that involves as starting state the
Pt-based orbitals (HOMO and HOMOꢀ2, respectively) and
the LUMO+1 as the arriving state (see Figure 5).
In addition to the transitions responsible for the dipolar
NLO response, our SOS analysis for complex 4 has also
highlighted the presence of several excitations endowed
with high oscillator strength but that also give rise to a small
variation in the excited-state dipole moments (e.g., S9 and
S10, with f=0.18 and 0.17, and Dmeg ꢃ1 D), which could be
associated with an octupolar NLO response.
Interestingly, complex 7, with a phenylacetylide ancillary
ligand carrying a nitro group in the para position of the
phenyl ring, shows a completely different electronic struc-
ture. In fact, the HOMO level is not really localized on the
nitrophenylacetylide ligand, whereas the LUMO level is,
particularly on the strong acceptor nitro group (Figure 4),
and not on the two pyridinic rings of cyclometalated 1,3-
di(2-pyridyl)benzene, which is different to the other PtII
complexes investigated. For complex 7, the lowest unoccu-
pied orbitals delocalized on the pyridyl rings of the cyclome-
talated ligand are the LUMO+1 and LUMO+2, with the
LUMO+1 being 1.16 eV above the LUMO and the
LUMO+2 still 0.10 eV above.
Dmeg upon excitation, prevail over the overall bEFISH sign,
thus leading to an overall negative (albeit reduced) value of
bEFISH.
On the contrary, in the case of complex 10, which has a
phenylacetylide ligand with a strong donor dimethylamino
in the para position of the phenyl ring, the LUMO level is
completely centered on the cyclometalated 1,3-di(2-pyridyl)-
benzene chelated ligand and the HOMO on the phenylace-
tylide ligand. It follows a strong HOMO–LUMO electron
transfer completely opposite to that of 7, thereby producing
one of the largest values of bEFISH (ꢀ470ꢄ10ꢀ30 esu) among
the PtII complexes investigated (Table 1).
It turns out that in all cases, even if the direction of the
HOMO–LUMO charge transfer is strongly opposite (com-
plex 7 and 10), the platinum atom might act as an efficient
bridge. Clearly the nature of the substituents on the phenyl-
acetylide ligand can tune the second-order NLO response
and the value of the dipole moment. Also, the substitution
on the cyclometalated 1,3-di(2-pyridyl)benzene ligand might
tune the second-order NLO response, since in the majority
of complexes the LUMO level is delocalized on this chelat-
ed ligand, which is thus involved in the charge-transfer proc-
ess. For instance, the introduction of a CF3 group on the pyr-
idinic rings upon going from 3 to 4 causes an increase in
both bEFISH and the dipole moment (Table 1) to afford a
high value of mbEFISH. Similarly, upon going from 8 to 9,
there is an increase in the dipole moment and of mbEFISH
,
but the bEFISH value decreases, probably because the LUMO
of complex 9 is not on both pyridinic rings (Figure 4).
To have a complete understanding of the various compo-
nents of the second-order NLO properties of these platinu-
m(II) complexes, and in particular to evaluate not only the
dipolar but also the octupolar contribution to the quadratic
hyperpolarizability, we carried out an HLS investigation of
some representative complexes (1, 2, 6, and 10) by working
at an incident wavelength of 1.907 mm (Table 2). The dipolar
Table 2. HLS experimental values of hbHLSi and related dipolar and octu-
polar contributions and modulus of the quadratic hyperpolarizability of
some of the investigated PtII complexes, working in DMF with an inci-
dent radiation wavelength of 1.907 mm.[a]
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ ꢀ
J¼1
J¼3
ꢀ ꢀ
Complex
hbHLS
i
b
b
b
[ꢄ10ꢀ30 esu]
[ꢄ10ꢀ30 esu]
[ꢄ10ꢀ30 esu]
[ꢄ10ꢀ30 esu]
1
2
6
10
285
432
345
278
36
136
140
364
922
1384
1097
709
923
1391
1105
797
Calculation of the lowest SOS contributions to the hyper-
polarizability for complex 7 shows that the HOMO–LUMO
charge transfer along the y axis from the metalated benzene
part of the chelated ligand to the nitrophenylacetylide
ligand gives rise to a dominant positive contribution to
bEFISH, opposite in direction to the HOMO–LUMO charge
transfer of all the other PtII complexes investigated.
The decrease in the bEFISH value of complex 7, which
maintains its negative sign, is due to the counteracting
HOMO–LUMO positive contribution, which partly cancels
the important negative contribution of the HOMO–
LUMO+1/HOMO–LUMO+2 excitations to the NLO re-
sponse. These excited states, characterized by a negative
[a] Estimated uncertainty in HLS measurements is ꢂ10%.
(J=1) and octupolar (J=3) contributions and the modulus
ꢀ ꢀ
ꢀ ꢀ
of the quadratic hyperpolarizability ( b ) have been calcu-
lated by Equations (2), (3), and (4)[11,12,29] since all com-
pounds have a C2v symmetry:
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ ꢀ
2
2
2
J¼1
J¼3
ꢀ ꢀ
ð2Þ
b
¼
b
þ b
ꢀ
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Cyclometalated Platinum(II) Complexes
FULL PAPER
rffiffiffi
ꢀ
ꢀ
ꢀ
ꢀ
3
5
ed by MLCT or LMCT transitions with a direct involvement
of the metal; however, examples of ligand-to-ligand charge-
transfer processes mediated by the metal have been report-
ed for IrIII,[3a] and for d8-metal complexes coordinated to two
dithiolate ligands[30] and to mixed diimine and dithiolate li-
gands.[31] Finally, the presence of CF3 groups on the 5-posi-
tion of the pyridine ring of the cyclometalated chelated
ligand is a tool to increase the dipole moment and lead to
large mbEFISH values.
J¼1
ð3Þ
ð4Þ
b
¼
bEFISH
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢁ
ꢂ
ꢁ
ꢂ
ꢁ
ꢂ
2
2
2
9
2
21
J¼1
J¼3
b2HLS ¼ jbXXX
j
þ jbZXX
j
¼
b
þ
b
2
2
ꢀ
ꢀ
ꢀ
ꢀ
J¼3
ꢀ
ꢀ
The resulting values of
b
reported in Table 2 show that
the dipolar contribution is much lower than the octupolar
one, as previously observed for various b-diketonates
LnIII,[29] IrIII,[6b] and PtII[6b] complexes, [Ru(substituted 1,10-
In conclusion, the PtII complexes investigated are a new
family of organometallic second-order NLO chromophores
with a response that is easily tunable by a rational approach,
whereby in specific cases the Pt atom plays the role of a
central bridge of the transfer process from the donor to the
acceptor moieties of the molecular structure.
phenanthroline)
[4-EtPhCO2]2,[6c] and cationic bipyridyl iridium
plexes. In particular, complexes 1, 2, and 6 are characterized
ACHTUNGTRENNUNG
(PPh3)2Cl2],[6b] [Ru(substituted terpyridine)2]
AHCTUNGTRENNUNG
(III)[4] com-
by
a
remarkably high value of the quadratic hyper-
ꢀ ꢀ
ꢀ ꢀ
polarizability b owing to an important contribution of the
octupolar component, with the dipolar component being
less than 13% of the octupolar one. As expected, the dipo-
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
J¼1
lar contribution to the quadratic hyperpolarizability
b
Acknowledgements
of complex 10 is relatively high (51%) due to a particularly
strong charge transfer from the ancillary NMe2–phenylacety-
lide to the cyclometalated 1,3-di(2-pyridyl)benzene ligand.
In agreement with this suggestion, by enhancing the accept-
or properties of the cyclometalated chelated ligand, the in-
crease in the quadratic hyperpolarizability is more relevant
for the dipolar contribution than for the octupolar one (en-
hancement factor of 3.8 and 1.5, respectively, upon going
from 1 to 2; Table 2).
This work was supported by Fondazione Cariplo (grant no. 2010-0525),
by MIUR (FIRB 2003: RBNE033KMA, FIRB 2004: RBPR05JH2P and
PRIN 2008: 2008FZK5AC_002), and by the Consiglio Nazionale delle
Ricerche (INSTM-PROMO). S.F., F.D.A., and M.G.L. thank the Istituto
Italiano di Tecnologia, Platform Computation, Project SEED 2009
“HELYOS” for financial support.
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dyl)benzene platinum(II) complexes demonstrates the facile
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Received: March 25, 2013
Revised: April 20, 2013
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