Cyclometalated Pt(ii) complexes with a bidentate Schiff-base ligand displaying unexpected: Cis / trans isomerism: Synthesis, structures and electronic properties

Article abstract of DOI:10.1039/c7dt02323e

Square planar platinum complexes are an important class of compounds used in (nano)technology, optoelectronics, medicinal chemistry and catalysis. The major research interests in cyclometalated Pt(ii) complexes focus on the selective modulation of their electronic properties and the control of the (cis/trans) geometry. For the first time, we unveil and demonstrate that cis-trans isomers of Pt(ii) complexes can be obtained in a derivative carrying the 1-phenyl-pirazolate (Hppz) and 2-hydroxy-1-naphtyl-(N-phenyl)imine ligands. The two isomers display significant differences in both optical and electronic properties. While luminescence is quenched in solution, they are brightly emissive in the PMMA matrix at room temperature and in the 2MeTHF rigid matrix at 77 K. The phosphorescent emission of the cis isomer, blue-shifted compared to that of the trans one, results from the significantly different trans influence of the ppz ligand. Theoretical investigation highlights the almost isoenergetic potential energy of the two isomers therefore explaining their formation and evidences a large geometry distortion of their triplet state, which should be responsible for the observed luminescence efficiency.

Full text of DOI:10.1039/c7dt02323e

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DOI: 10.1039/C7DT02323E
Journal Name
ARTICLE
Cyclometalated Pt(II) complex with bidentate Schiff-base ligand
displaying unexpected cis/trans isomery: synthesis, structures and
electronic properties.
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
b
a
†b
DOI: 10.1039/x0xx00000x
A. Poma, A. Forni, C. Baldoli, P. R. Mussini, and A. Bossi
www.rsc.org/
Square planar platinum complexes are an important class of compounds used in (nano)technology, optoelectronics,
medicinal chemistry and catalysis. The major research interests in cyclometalated Pt(II) complexes focus on the selective
modulation of their electronic properties and the control of the (cis/trans) geometry. For the first time, we unveil and
demonstrate that cis-trans isomers of Pt(II) complex can be obtained in a derivative carring the 1-phenyl-pirazolate (Hppz)
and 2-hydroxy-1-naphtyl-(N-phenyl)imine ligands. The two isomers display significant differences in both optical and
electronic properties. While luminescence is quenched in solution, they are brightly emissive in PMMA matrix at room
temperature and in 2MeTHF rigid matrix at 77 K. The phosphorescent emission of the cis isomer, blue-shifted compared to
that of the trans one, results from the significantly different trans influence of the ppz ligand. Theoretical investigation
highlights the almost isoenergetic potential energy of the two isomers therefore explaining their formation and evidences
a large geometry distortion of their triplet state, which should be responsible of the observed luminescence efficiency.
6
,7
chemotherapeutic activity only in the cis form.
While significant amount of literature reports on
cyclometalated complexes with C^N donor atoms (e.g. 2-
Introduction
Luminescent d and d metal complexes, containing
conjugated ligands with N and/or C donor atoms, have
attracted widespread research interest. Their excellent
properties are particularly suited for the applications in fields
such as optoelectronics, chemo-/biosensors, bioimaging and
a
6
8
-
8,
phenylpyridine, 1-phenylpirazole or N-heterocyclic carbenes)
1a,2a
complexes with bidentate ligands having O^N donor
9
atoms, such as salicylidenimine, are less developed.
Compared to the use of salicylidenimine as ancillary ligand to
saturate the coordination sphere around metal centers, few
papers have been reported on their application as active
luminescent moieties; most of them referring to closed-shell
1
medicinal chemistry. In all the mentioned applications,
platinum(II) derivatives have been demonstrated to be
particularly advantageous. Their square planar geometry
allows for the possible decoration of the metal center with
1
0,11
Zn or Cu complexes and only very few on Pt complexes.
An
2
interesting case of bis-salicylidenimine Pt derivative was
reported by Komiya et al. displaying strong solid state
emission; feature that although quite rare is found with other
mono, di or polidentated ligands in order to control the
chemical, optical and electronic properties of the complexes as
well as to modulate their emission and radiative lifetimes in
desired way. In this contest, whereas ligand engineering plays
12
ligands or in multinuclear systems. Salicylidenimine-like
ligands would offer several advantages compared to typical
C^N ligands, including ease of synthesis, readily available
precursors, high atom economy and broad structural
variability. Such type of ligands have been applied and widely
investigated in the development of efficient (metal) catalysts
2,3
the most important role, a noticeable and niche approach to
fine-tune both the optical properties of complexes as well as
their reactivity / bio-activity is represented by geometry
4,5
engineering. For example, the well-known [Pt(NH ) (Cl) ]
3
2
2
complex, and other platinum complexes having monodentate
ligands, or mono-cyclometalated ones, display powerful
13
for enantioselective organic reactions.
Herein we report the design and synthesis of prototype neutral

heteroleptic Pt(II) complexes carrying a C N ancillary ligand
a.
Department of Chemistry, University of Milan, and SmartMatLab Center via Golgi
and an O^N bidentate one. We selected 1-phenyl-pyrazole
b.19, 20133 Milano (ITALY).

(Hppz) and 2-phenyl-pyridine (Hppy) as the C N ligand, since
Institute of Molecular Science and Technology of the CNR (ISTM-CNR), via Golgi 19
and PST via Fantoli 16/15, 20138 Milan and SmartMatLab Centre, via Golgi 19,
their corresponding acetylacetonate Pt(II) complexes emit in
2
0133 Milan (ITALY)
14,3
the deep blue and blue-green regions.
The 2-hydroxy-1-
†
Corresponding author: alberto.bossi@istm.cnr.it.
naphtyl-(N-phenyl)-imine (HNpOPh) O^N ligand was chosen to
have an emissive triplet energy level lower than the
organometallic counterpart, thus driving the whole optical and
electronic properties of the final complexes.
Electronic Supplementary Information (ESI) available: materials and methods;
synthetic procedures and characterizations; extended electrochemistry and
photophysics characterizations; theoretical calculations. Crystallographic
information can be obtained via CCDC.com website (deposition numbers 1538320-
1
538321). See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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The unexpected and worth standing result, representing an
Journal Name
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DOI: 10.1039/C7DT02323E
15
unique case so far, is that the new ppz Pt(II) complex (Pt-1
)
can be isolated in both its cis and trans forms, as defined in Fig.
16
1
. For comparative purposes, complex Pt-2
,
bearing 2-
phenylpyridine (ppy) as C^N ligand, was also synthesized, in
4
3
this case only the trans-isomer was formed.
5
N
N
N
Pt
2'
O
3
'
5'
4'
4
3
5
N
O
N
Pt
2'
N
3
'
5'
4'
1
Figure 1. a) H NMR spectra of the HNpOPh ligand, b) cis-Pt-1; c) trans-Pt-1
.
Significative protons of the ppz ligand (H
structures are reported as inset.
3 4 5
, H , H , H5’) are numbered; and the
Scheme 1. Synthetic route to complexes Pt-1 and Pt-2, isolated yields are
2
indicated. i) Hppz or Hppy, 2-ethoxyethanol/H O, 80°C, argon; ii) 2-
Ethoxyethanol, Na
2
CO
3
, T

argon.
1
NMR analysis. The H NMR spectra of cis and trans-Pt-1 and of
the HNpOPh ligand are reported in Fig. 1. The well-resolved
spectra easily allow recognizing, in both isomers, the
3
Results and discussion
Synthetic procedures. The synthesis of the Pt(II) complexes Pt-
diagnostic protons H (pyrazoyl ring, characterized by small J
3
H-
14
about 2.5 – 3.0 Hz) and H5’ (phenyl ring). These proton
H
resonances undergo the largest shift on going from one isomer
to the other, indicating a strong relationship with the
geometry of the molecule. H3 resonance shifts by almost 3
ppm, i.e. from 5.45 ppm in cis-Pt-1 to 8.15 ppm in trans-Pt-1
1
and Pt-2 is depicted in Scheme 1. The reaction of the -
17
dichloro-bridged dimer
in 2-ethoxyethanol and in the presence of Na CO , led to the
formation of a mixture of two Pt compounds having the same
molecular weight (HPLC-MS analysis Fig. S14-15). The two
products, hypothesized to be the cis/trans isomers of Pt-1
1
,
with the HNpOPh ligand, at 80 °C
2
3
(Fig. 1b and c). Such shift originates from the shielding exerted
on H by the ring-current of the phenyl ring of the O^N ligand
,
3
and this is possible only in the cis isomer (see Fig. 2). An
analogous effect shields and shifts proton H5’ to 5.8 ppm in
trans-Pt-1 compared to 7.91 ppm in cis-Pt-1. Therefore, the
cis/trans configuration of Pt-1 complex (drawings in Fig. 1) can
be safely inferred.
were separated by column chromatography and completely
characterized. The cis/trans configuration of complexes Pt-1
1
was ascertained by H-NMR studies and lately confirmed by
single crystal X-ray diffraction investigations (see below). The
reaction temperature was found to play a crucial role on the
isomeric ratio of Pt-1 (Scheme 1). At 100 °C a cis/trans ratio of
NMR analysis also provides important information relative to
the Pt - O^N ligand interactions. The coordination of the imine
N atom, results in a shift towards lower frequencies of the
imine proton from 9.36 ppm in the HNpOPh (Fig. 1a) to 9.11
ppm in cis-Pt-1 and 8.88 ppm in trans-Pt-1. We attribute this
7
5:25 (determined via HPLC) was obtained whereas at 140°C
the ratio inverts to 35:65.
For comparative purposes, the ppy derivative Pt-2 was
synthesised by the reaction of
according to the report of Liu et al. In this case, the complex
Pt-2, was isolated as trans isomer only in 43% yield,
2 with HNpOPh at 120°C,
2
1
6
progressive shielding to a partial loss of sp character reflecting
the strength of back-donation from the Pt centre to the
1
9
coordinated group. The different chemical shift between the
two isomers is ascribed to the different trans-influence of the
two coordinated atoms of the ppz ligand.
comparable to that obtained in the synthesis of Pt-1
.
A preliminary study on the thermal stability of the kinetic
isomer cis-Pt1 was conducted heating at 130-140°C, under
In trans-Pt-1 the imine nitrogen atom has the pyrazole
nitrogen in trans arrangement; the latter atom has a smaller
trans-influence with respect to the coordinated carbon atom
Argon,
a
1-1.5 mg/mL solution of the isomer in
ethoxyethanole. HPLC analysis evidenced that in the first hours
trans-Pt1 forms while, on prolonged heating (over five hours) a
black precipitate forms. Contrarily the trans isomer results in
no isomerization but eventually formation of analogue black
precipitate. Further studies are ongoing to elucidate
(
formally negatively charged), hence the trans-bond is less
1
9,20
weakened (Fig. 1c).
In the cis isomer instead, the trans
correlation of carbon-Pt-imine group weakens and lengthens
the Pt-N(imine) bond resulting in an intermediate H-shielding
18
mechanisms and degradations.
(9.1 ppm, Fig. 1b).
Similarly, the stronger Pt-ligand interaction in trans-Pt-1 is also
supported observing the more intense and resolved Pt-H long-
range coupling on the 8.88 ppm peak compared to the weaker,
Pt-H coupling, shoulder in cis-Pt-1 (i.e. peak at 9.1ppm).
2
| J. Name., 2012, 00, 1-3
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X-Ray analysis. Single crystals of the two Pt isomers were dimer, a relatively short Pt∙∙∙Pt separation distancVieew, 4Ar.t1ic0le9O6n(li5ne)
grown from slow diffusion of hexane into dichloromethane Å, was observed. The crystal structuDrOeI: 1o0f.103t9ra/Cn7sD-PT0t-21323iEs
solutions. The X-ray analysis (see SI) confirms the assignment governed as well by
- stacking interactions; however,
of cis/trans configuration to complex Pt-1. Their molecular different from the previous case, the Pt ions are far away from
structures are reported in Fig. 2 (left and right, respectively), each other, thus excluding any metal-metal interaction in
where for the trans isomer, which contains two molecules (A trans-Pt-1
and B) in the asymmetric unit, only molecule A is reported. The
two independent molecules of trans-Pt-1 differ essentially by
the inclinations of the phenyl ring with respect to the chelated
N^O ligand. The complexes are characterized by an almost
perfectly square-planar coordinated Pt ion with five-
membered and six-membered metallacycles, due to the C^N
and the O^N ligands, respectively. They evident differences in
bond lengths (see Table 1) are imputable to the stronger trans-
influence of C18 with respect to N2. In particular, the Pt–O1
bond length was found significantly longer in the trans isomer,
.
1
6
as also in the Pt-2 complex, while the Pt–N1 one was found Figure 2 – Ortep plots of complexes cis-Pt-1 (left) and trans-Pt-1 (molecule A,
right) with ellipsoids at 50% probability level.
longer in the cis isomer. Correspondingly, the N1–C11 imine
bond was shorter in the latter isomer, owing to the reduced
back-donation from the metal centre to the O^N ligand (vide
supra).
The skeleton of the cis isomer significantly deviates from
planarity, unlike that of the trans isomer, as can be clearly
seen in Fig. 3. This was due to a distortion of the six-
membered ring, with the Pt ion significantly out of the least
square plane through the non-metal atoms of this ring
(0.647(1) Å), while in trans-Pt-1 this distance was as low as
0
.243(1) and 0.188(1) Å for molecules A and B, respectively.
Such different conformation should be related to crystal
packing effects because: (i) a distortion comparable with that
16
of cis-Pt-1 was observed also in Pt-2
geometry is quite similar to that of trans-Pt-1 (Table 1); and
ii), DFT optimized geometries provide more comparable
whose molecular
Figure 3 – Partial molecular structures of cis-Pt-1 (left) and trans-Pt-1 (right) with
short Pt∙∙∙Pt (long dashes lines), C∙∙∙C (dashes lines) and C–H∙∙∙C(N) (dots lines)
intermolecular contacts.
(
distortion of the six-membered ring (see later).
The structures are characterized by the presence of
Electrochemical and optical properties
.
intramolecular C–H∙∙∙
 hydrogen bond between the phenyl
The redox properties of the three platinum complexes have
been investigated by cyclic voltammetry on glassy carbon, in
dichloromethane solution with 0.1 M TBAPF6 supporting
electrolyte. (Fig. 4, S14-21 and Table 2).
ring and the adjacent C–H bond of the C^N ligand as confirmed
by a QTAIM topological analysis of the electron density.
21
Table 1
– Selected bond lengths and intermolecular distances (Å) from X-ray
crystal structures (first line) and B3LYP/TZVP calculations when available (second
line, italic).
The first oxidation peak potentials, EpIa, are nearly the same for
+
the three complexes (0.65-0.69 V vs Fc |Fc irrespectively from
a
cis-Pt-1
trans-Pt-1
Pt-2
the C^N ligand and the configuration) and significantly more
+
14
Molecule A Molecule B
positive than in Pt(ppz)acac (0.35 V vs Fc |Fc). On the other
Pt-N1
Pt-N2
2.084 (2)
2.008 (2)
1.983 (2)
2.048 (2)
2.003 (3)
1.313 (4)
2.000 (2)
1.983 (2)
2.048 (2)
2.008 (3)
1.319 (3)
6.0323 (3)
2.005 (4)
2.061
2.009 (4)
2.046
2.065 (4)
2.119
1.980 (5)
2.016
hand, the EpIa for the HNpOPh ligand (Fig. S14-15) is more
2
.135
2.000 (2)
.028
1.998 (2)
.007
1.988 (2)
.008
1.301 (2)
.304
4.1096 (5)
2.039
2.010
2.092
2.026
1.312
+
positive (about 0.76 V vs Fc |Fc) than our complex ones. These
observations are consistent with an electron poorer Pt redox
centre by interaction via π-back donation with the electron-
enriched O^N ligand. The oxidation peaks appear chemically
irreversible at low scan rates, pointing to a chemical follow-up
of the oxidation product on the experiment timescale.
However, a portion of return peak gradually appears with
increasing scan rate (Fig. S16, 18 and 20) in the sequence
2
Pt-O1
2
Pt-C18
N1-C11
Pt∙∙∙Pt
2
1.303 (6)
1.318
7.026
1
6.0323 (3)
trans-Pt-1 > Pt-2 > cis-Pt-1, accounting for a decreasing order
of stability of the electron transfer product.
a) Data taken from ref. 16
Trans-Pt-1 results in the most stable oxidation product and it
has EpIa practically constant with scan rate implying an
The crystal structure of cis-Pt-1 also shows a dense network of
stacking interactions and for the closest centrosymmetric
-
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ARTICLE
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electrochemically reversible electron transfer. This also allows absorptions are attributable to * transitVioienwsArticolef Ontlhinee
to assess, considering the half-peak width, that the first cyclometalated ligands (i.e. ppy/ppz andDNOpI:O10P.1h0)3.9T/Ch7eDiTn0t2e3n2s3eE
oxidation process is a monoelectronic one. Instead, cis-Pt-1 absorption at 330-340 nm is a distinctive feature of the ppz
features the most reactive electron transfer product and, derivatives cis/trans-Pt
unlike the two trans complexes, has a neat second oxidation Lastly, the broad and only partially resolved bands between
peak before the background. 400-480 nm observed in Pt-2 and cis/trans-Pt-1 (Fig. 5)
-1 compared to Pt-2.
The reduction peak potentials EpIc are nearly the same for the originate from the presence of the O^N ligand, since the
+
three complexes, at -2.21 -2.25 V vs Fc |Fc, about 0.15 V less parent complexes Pt(ppy)dpm and Pt(ppz)acac have no
1
4,22
negative than in the case of Pt(ppy)acac or dpm (about -2.4 V significant absorption in this spectral region (Fig. 6 left).
+
22
vs Fc |Fc in DMF) and surely less negative also respect to We would attribute the 400-480 nm bands, in the three Pt
1
4,23
Pt(ppz)acac (see SI section 4).
This should point to a complexes, predominantly to metal-to-ligand charge transfer
significant involvement of the O^N ligand, providing a lower transitions ( MLCT) involving the Pt center and the O^N
energy LUMO, as pointed out by the theoretical computations. ligand (see DFT and section 5 SI).
1,3
However, it must be noticed that the reduction peak potential Above 420 nm the UV spectra closely match in the complexes
+
of the free O^N ligand (about -2.13 V vs Fc |Fc) undergoes a adopting the same geometry, (trans-)Pt-2 and trans-Pt-1 (Fig.
slight, but significant, negative shift upon Pt coordination. This 3b) while, albeit maintaining similar shape, an hypsochromic
shift could account for a combination of the back donation shift of about 20 nm is observed in the cis-Pt-1 complex.
from Pt (resulting in a negative shift) with increased effective
0.30
1
.0
Pt(ppy)dpm
Pt-2
DCM,RT
1
.0
Pt(ppy)dpm
Pt-2
2MeTHF, 77K
conjugation in the LUMO (resulting in a positive shift) as
accounted by DFT calculations.
0.25
0
.20
.15
0.8
0.8
0.6
0.4
0.2
0.0
0
0
.6
0.10
0
.05
.00
Pt-2
trans Pt-1
cis Pt-1
0.4
0
4
00
450
500
550

(nm)
0
0
.2
.0
250
300
350
400
450
500
550
450 500 550 600 650 700 750 800 850
(nm)


(nm)
Figure 6 – Normalised absorption (left panel DCM, RT) and emission (right panel,
7K) spectra of Pt(ppy)dpm and Pt-2
7
The three complexes are non-luminescent in degassed diluted
solutions, but become brightly emissive both at 77K in glassy
2
MeTHF (Fig. 7) as well as in doped PMMA matrix (Fig. S24).
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
+
E / V (Fc |Fc)
1
.0
cis-Pt-1, Em
cis-Pt-1, Exc
Pt-2, Em
Figure 4 – CV of the complexes in CH
2
Cl
2
with 0.1 M of TBAPF
6
at room
0.8
+
Pt-2, Exc
temperature. Potentials referred to the Fc |Fc couple.
trans-Pt-1, Em
trans-Pt-1, Exc
77K, 2MeTHF
0.6
4
4
3
3
2
2
1
1
5000
0000
5000
0000
5000
0000
5000
0000
0.4
Pt-2
trans-Pt-1
cis-Pt-1
0
.2
.0
DCM solution, rt
0
3
00 350 400 450 500 550 600 650 700 750 800 850
 (nm)
Figure 7. Excitation and emission spectra of all the Pt(II) complexes at 77K in rigid
MeTHF matrix.
2
5
000
0
At 77K, the emission maxima for the three complexes range
from 575 to 620 nm; luminescence are characterized by a
broad and featureless structure, a large Stokes-shift and their
excitation spectra match the absorption ones while gaining
2
50 300 350 400 450 500 550 600

(nm)
Figure 5 – Molar absorptivity spectra of the Pt(II) complexes in DCM solution
-
1
structured vibronic progression (spaced by 1500-1600cm ) at
low energy, Fig. 7. The identical emission profiles in Pt-2 and
trans-Pt-1 appear as the smear of the first two vibronic modes,
one as a high energy shoulder at ca. 605 nm and one with
em,max = 618-620 nm.
The electronic absorption spectra of the complexes, recorded
in dichloromethane, are reported in Fig. 5 and Table 2 (the UV
24,25
features and solvatochromism of HNpOPh
are reported in
.
4
-1
-1
Fig. S22). Below 280 nm (

> 2 10
M
cm ) the strong
4
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C7DT02323E
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Table 2. Summary of the electrochemical and optical properties of the studied complexes.Reported emission data refers either to the 2MeTHF frozem matrix at 77K
upright numbers) or to the PMMA doped thin film at room temperature (italicized numbers)
(
Electrochemistry
Absorption
Emission
s], (%)

em,max [nm]
EC
OPT
E
p,Ia
E
p,Ic
E
HOMO
E
LUMO
E
g

abs [nm]
E
g

L
-
1
(
FWHM [cm ])
4
-1
-1
[
V]
[V]
[eV]
[eV]
[eV]
( [x 10 M cm ])
[eV]
77K / PMMA
PMMA
a)
b)
77K / PMMA
5
5
75 (3200)
7.08
cis-Pt-1
trans-Pt-1
Pt-2
0.65
0.69
0.66
-2.25
-2.21
-2.22
-5.45
-5.49
-5.46
-2.55
-2.59
-2.58
2.90 333 (2.10), 438 (0.47)
2.90 340 (2.77), 456 (0.72)
2.84
2.74
2.71
92 (3470)
0.63 (25.1), 3.09 (74.9)
2.13 (38.7), 6.04 (62.4)
0.32 (31.3), 2.05 (68.7)
1.21 (30.7), 4.42 (69.3)
0.030
0.005
c
6
17, 605 (4160)
6
620, 605 (4015)
615 (4330)
05 (3880)
c
3
22 (1.17), 369 (1.18),
00 (0.61), 457 (0.47)
2.88
4
0.15 (26.5), 1.14 (73.5)
0.005
+
+
+
+
HOMO and LUMO values are calculated from CV data: EHOMO (eV) = -1e x [Ep,Ia /V (Fc |Fc) + 4.8 V (Fc |Fc vs zero)]; ELUMO (eV) = -1e x [Ep,Ic /V (Fc |Fc) + 4.8 V (Fc |Fc vs
zero)]. c) high energy shoulder, a)77K data are taken from frozen 2MeTHF diluted solution; b) PMMA data are taken from a 4% w/w dopped PMMA film on quartz
recorded at room temperature under N
2
The Huang-Rhys parameter (SHR, i.e. the ratio of the height of enhanced luminescence in rigid environment) are consistent
15
the first two vibronic peaks) greater than 1 points to a large with a strongly distorted excited emitting state.
structural distortion between the ground and excited state
geometries. The emission of the cis-Pt-1 is located, in Theoretical studies
.
agreement with the hypsochromic shift of the UV spectra, at DFT and TDDFT calculations were employed to shed light on
-1
em,max = 575 nm with FWHM of 3200 cm (Fig. 7).
The photoluminescence of Pt-2 max = 620 nm) differs formation of cis/trans-Pt-1, as well as to elucidate the
completely from that of the parent Pt(ppy)dpm ( max = 475 molecular electronic properties.
the stereochemical outcome of the reaction, leading to the
(

nm, Fig. 6 right) and, given the decay lifetime in the The geometry optimizations of the two isomers, led to almost
microsecond regime, we attribute the phosphorescence to a isoenergetic systems, with the trans form of Pt-1 more stable
triplet state localized onto the O^N ligand. Likewise, analogues than the cis one only by 1.0 kcal/mol. Thermal analysis
origin has the luminescence of cis and trans-Pt-1, since the provided even closer free energies for the formation of the
14
reference complex Pt(ppz)acac shows no emission and trans- two isomers at both the experimentally studied reaction
Pt-1 emission perfectly matches that of Pt-2 temperatures (100 and 140°C). Thus, a lower activation barrier
Excited state lifetimes (Table 2) evidence a mono-exponential for the formation of the cis complex can explain the
decay in the case of cis-Pt-1 (7.08 s) whereas in Pt-2 and prevalence of the cis form at lower temperature. Worth
trans-Pt-1 a bi-exponential decay is found with a faster noting, calculations performed on Pt-2 complex for both the
component around 1-2 s (contributing to the decay for ca. trans and the hypothetical cis isomer provided a significantly
0-38%) and a slower one at 4.5-6 s. The microsecond decay greater stability for the trans one (3.9 kcal/mol) which, indeed,
regime is in agreement with a highly perturbed LC/MLCT is the only one formed in the reaction.
.


3

3
emissive state.
Fig. 8 collects the HOMO and LUMO plots and the triplet spin
In 4% doped PMMA thin film on quartz (at room temperature density of trans-Pt-1 complex. Frontier orbital plots fully
and under N ) a quantum efficiency (L) of 0.5% is measured support the above discussions. In fact, in all cases (see Fig. 8
2
for Pt-2 and trans-Pt-1 whereas. Interestingly, cis-Pt-1 displays and S25) the HOMOs mostly involve the Pt dx2-y2 orbital
higher L = 3%. All complexes show a bi-exponential decay together with the naphtyl moiety of the O^N ligand and
overall shorter compared to 77K data (Table 2) suggesting that partially extend also on the phenyl ring of the C^N ligand.
non-radiative processes, including the coupling of the excited LUMOs are almost exclusively located onto the O^N ligand
state to the ground state vibrational modes are enhanced at with no or minor contribution from the pyrazole or pyridine
higher temperature. Interestingly, both at 77K and in PMMA at ring respectively.
room temperature the lifetime of the cis- isomer is constantly TDDFT spectrum nicely correlates to the UV-vis one (Fig. S27);
longer than that of the two trans ones. Altogether, the the lowest energy excitation, in all the complexes, are ascribed
photophysical behavior of complexes (i.e. weak spectral to a HOMO to LUMO transition, hence confirming their MLCT
overlap, featureless and broad emission, large SHR parameter, character. Higher energy excitations are essentially of HOMO
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Ple aDs ea l dt oo nn To rt a and sj ua sc tt imo na sr gins
Page 6 of 7
ARTICLE
Journal Name
to LUMO+1 character, which appears to be more of interligand influenced by the C^N ligand. On the other hand, the cis
View Article Online
DOI: 10.1039/C7DT02323E
type (see Fig. S25 and Table S1).
isomer absorption and emission are overall blue shifted and
the complex has a higher luminescence efficiency.
This research is in progress with more structural variability on
the O^N ligand (preliminary results are consistent with the
findings herein described) and further expanding its scopes,
addressing the issues of isomerization reaction mechanism
and, more interesting, on the solid state photoluminescent
emission observed in this compound class.
a)
b)
Acknowledgements
We thank for the financial support Regione Lombardia (decreto
c)
d)
Figure 8. DFT calculations of trans-Pt-1: a) HOMO, b) LUMO and c) triplet spin 3667/2013) project TIMES “Tecnologie e materiali per l′utilizzo
density plots; d) superimposed ground state geometry (yellow structure) and the
excited triplet one (pink) evidencing the significant distortion between the two efficiente dell’energia solare” and Progetto Integrato Regione
structures (see also Fig. S26).
Lombardia and Fondazione CARIPLO (decreti 12689/13, 7959/13),
Azione 1 e 2, "SmartMatLab centre" and Cariplo Foundation grant
Significant distortion is evident comparing the relaxed singlet
and triplet structures (Fig. 8d and S26). This observation justify
the absence of luminescence in fluid matrix at room
temperature. To be noted that almost negligible distortion
2013-1766. AB thanks Dr. Ivan Andreosso for performing some
preliminar optical characterizations.
occurs at the square planar platinum center, while the naphtyl Notes and references
moiety bends over 20 degrees in the excited triplet state.
1
(a) W. C. H. Choy, W. K. Chan, Y. Yuan Adv. Mater. 2014, 26
,
,
Analogues behaviour is seen in Pt-2, while an even larger bent
can be observed for cis-Pt-1 (Fig. S26). Instead, the increased
luminescence efficiency observed for cis-Pt-1 with respect to
trans-Pt-1 can be justified in terms of the smaller ground state
dihedral angle (17° vs 21° respectively; Fig. S26), between the
least-squares plane through the Pt-ppz system and that of the
NpOPh one (defined through its N-C-C-C-O chelated atoms). In
fact, coordination environment might alter Pt d-splitting and
5368; (b) Q. Zhao, F. Li, C. Huang Chem. Soc. Rev. 2010, 39
3007; (c) K. K. W. Lo, S. P. Y. Li RSC Adv. 2014, 4, 10560; (d)
Q. Zhao, C. Huang, F. Li Chem. Soc. Rev. 2011, 40, 2508; (e) Y.
You, S. Cho, W. Nam Inorg. Chem. 2014, 53, 1804; (f) A.
Aliprandi, M. Mauro, L. De Cola, Nature Chem. 2016,
5.
(a) K. Li, G. S. Ming Tong, Q. Wan, G. Cheng, W.-Y. Tong, W.-
H. Ang , W-L. Kwong, C.-M. Che, Chem. Sci. 2016, , 1653; (b)
8, 10–
1
2
7
J. A. Gareth Williams, Photochemistry and Photophysics of
Coordination Compounds: Platinum. In Top. Curr. Chem., Vol
2
non-radiative deactivation rates.
2
81, (Eds.: V. Balzani, S. Campagna) Springer Berlin
Heidelberg, 2007, pp. 205-268.
Conclusions
3
4
(a) M. E. Thompson, P. I. Djurovich, S. Barlow, S. R. Marder in
Comprehensive Organometallic Chemistry, Vol. 12 (Ed.: D.
O’Hare), Elsevier, Oxford, 2007, pp. 101; (b) J. Brooks, Y.
Babayan, S. Lamansky, P. I. Djurovich, I. Tsyba, R. Bau, M. E.
Thompson, Inorg. Chem. 2002, 41, 3055.
A. B. Tamayo, B. D. Alleyne, P. Djurovich, S. Lamansky,
I.Tsyba, N. N. Ho, R. Bau, M. E. Thompson, J. Am. Chem. Soc.
2003, 125, 7377.
In conclusion, we prepared and fully characterized the cis and
trans isomers of Pt-1, bearing an ancillary ppz cyclometalated
ligand and the NpOPh (O^N) one. We showed that the
stereoselectivity on the cis/trans ratio can be obtained
controlling the reaction conditions. To the best of our
knowledge this is the first report on the formation of cis/trans
isomers in neutral bis-cyclometaled Pt(II) complexes; the cis
isomer is made thermodynamically accessible due to the trans
influence exerted by the ppz ligand.
5
6
G. McGowan, S. Parsons, P. J. Sadler, Inorg. Chem. 2005, 44
459.
,
7
(a) B. Rosenberg, L. Van Camp, J. E. Trosko, V. H. Mansour
Nature, 1969, 222, 385; (b) B. Rosenberg Metal Ions Bwl.
Syst., 1980, 11, 127.
NMR provides important inside about trans influence and Pt-
ligand interactions. Likewise the results from the combined
electrochemical, photophysical and DFT/TDDFT studies
demonstrated that most of the electronic properties of the
complexes are determined by the O^N ligand. In fact the first
monoelectronic oxidation involves the Pt center plus O^N
ligand, while the first reduction, partially reversible, involves
the O^N ligand. Large excited state distortion quenches the
emission of the complexes but they become highly
luminescent in rigid matrix and the emissions are assigned to
7
8
9
N. Farrell, T. G. Appleton, Y. Qu, J. D. Roberts, A. P. Soares
Fontes, K. A. Skov, P. Wu, Y. Zou, Biochemistry, 1995, 34
,
1
5480; C. Shijua, D. Arisha, N. Bhuvaneshb, S. Kumaresan,
Spectrochim Acta A: Mol Biomol Spectrosc. 2015, 145, 213; J.
J. Wilson, S. J. Lippard, Chem. Rev. 2014, 114, 4470.
H. F. Xiang, S. W. Lai, P. T. Lai, C.-M. Che, in “Highly efficient
OLEDs with phosphorescent Materials”, H. Yersin Ed., Wiley-
VCH, 2008; Z. M. Hudson, C. Sun, M. G. Helander, Y.-L.
Chang, Z.-H. Lu, S. Wang, J. Am. Chem. Soc. 2012, 134
3930.
,
1
(a) J. Zhang, F. Zhao, X. Zhu, W. K. Wong, D. Ma, W. Y. Wong,
J. Mater. Chem. 2012, 22, 16448; (b) C. M. Che, C. C. Kwok, S.
W. Lai, A. F. Rausch, W. J. Finkenzeller, N. Zhu, H. Yersin,
Chem. Eur. J. 2010, 16, 233.
3
the perturbed LC states centered on the NpOPh ligand.
Complexes adopting trans configuration show similar
absorption, emission and excited state features scarcely
6
| J. Name., 2012, 00, 1-3
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Page 7 of 7
Ple aDs ea l dt oo nn To rt a and sj ua sc tt imo na sr gins
Journal Name
ARTICLE
1
0 (a) F. Dumur, L. Beouch, M.-A. Tehfe, E. Contal, M. Lepeltier,
View Article Online
DOI: 10.1039/C7DT02323E
G. Wantz, B. Graff, F. Goubard, C. R. Mayer, J. Lalevée, D.
Gigmes, Thin Solid Films, 2014, 564, 351; (b) R. Vafazadeh, V.
Hayeri, A. C. Willis, Polyhedron, 2010, 29, 1810; (c) J. Zhang,
G. Dai, F. Wu, D. Li, D. Gao, H. Jin, S. Chen, X. Zhu, C. Huang,
D. Han, J. Photochem Photobiol A: Chemistry, 2016, 316, 12.
1 (a) M. Ghedini, A. Golemme, I. Aiello, N. Godbert, R.
Termine, A. Crispini, M. La Deda, F. Lelj, M. Amati, S. Belviso,
1
1
J
. Mater. Chem. 2011, 21, 13434; (b) A. J. Howarth, R. Patia,
D. L. Davies, F. Lelj, M. O. Wolf, K. Singh, Eur. J. Inorg. Chem.
014, 3657 and ref. 7 and 11 therein.
2
2 N. Komiya, M. Okada, K. Fukumoto, D. Jomori, T. Naota, J.
Am. Chem. Soc, 2011, 133, 6493; Y. Yamaguchi, N. Nakata, A.
Ishii, Eur. J. Inorg. Chem, 2013, 30, 5233 and ref. therin; K.
Umakoshi, T. Kojima, K. Saito, S. Akatsu, M. Onishi, O.
Ishizaka, N. Kitamura, Y. Nakao, S. Sakaki, Y. Ozawa, Inorg.
Chem. 2008, 47, 5033
1
1
1
1
1
3 P. G. Cozzi Chem. Soc. Rev. 2004, 33, 410; K.C. Gupta, A. K.
Sutar, Coord. Chem. Rev. 2008, 252, 1420.
4 W. Wu, H. Guo, W. Wu, S. Ji, J. Zhao Inorg. Chem. 2011, 50
1446.
,
1
5 A. Tronnier, A. Poethig, E. Herdtweck, T. Strassner,
Organometallics, 2014, 33, 898.
6 S. Liu, H. Sun, Y. Ma, S. Ye, X. Liu, X. Zhou, X. Mou, L. Wang,
Q. Zhao, W. Huang, J. Mater. Chem., 2012, 22, 22167.
7 (a) J. Y. Cho, K. Y. Suponitsky, J. Li, T. V. Timofeeva, S. Barlow,
S. R. Marder, J. Org. Chem. 2005, 690, 4090; (b) M. M.
Mdleleni, J. S. Bridgewater, R. J. Watts, P. C. Ford, Inorg.
Chem. 1995, 34, 2334.
1
1
8 M. A. Ortuño, S. Conejero, A. Lledós, Beilstein J. Org. Chem.,
2
013,
9
, 1362; (b) F. Zheng, A. T. Hutton, C. G. E. E. van
Sittert, J. R. Moss, S. F. Mapolie, Dalton. Trans., 2013, 42
,
1
1163
9 (a) I. I. Chernyaev Ann. Inst. Platine (URSS), 1926,
F. Basolo, R. G. Pearson Prog. Inorg. Chem. 1962,
4
, 243; (b)
, 381; (c)
4
R.H. Crabtree, The Organometallic Chemistry of the
Transition Metals, John Wiley & Sons, New York, 2001; (d) J.
E. Huheey, E. A. Keiter, R. L. Keiter Inorganic Chemisty:
Principles of Structure and Reactivity, Harper Collins College
Publishers, New York, 4th edn, 1993; (e) Z. Lin, M. B. Hall
Inorg. Chem. 1991, 30, 646.
0 P. K. Sajith, C. H. Suresh Dalton Trans. 2010, 39, 815.
1 R. F. W. Bader (1990). Atoms in molecules: A quantum
theory. Oxford: Clarendon Press.
2
2
2
2 A. Bossi, A. F. Rausch, M. Leitl, R. Czerwieniec, M. T. Whited,
P. I. Djurovich, H. Yersin, M. E. Thompson, Inorg. Chem.
2
013, 52, 12403 and reference therein.
2
3 Data taken from ref.14 (cmpd. Pt-1, Pt(ppz)acac) and looking
to its CV pattern, showing no first reduction peaks before -
2
.2V vs Ag|AgNO3, that is before about -2.4 V vs Fc+|Fc and
discarding the apparently wrong corresponding table entry
4 (a) A. Ohshima, A. Momotake, T. Arai, J. Photochem.
Photobiol. A: Chemistry, 2004, 162, 473; (b) M. Sliwa, N.
Mouton, C. Ruckebusch, S. Aloıse, O. Poizat, G. Buntinx, R.
Métivier, K. Nakatani, H. Masuhara, T. Asahi, J. Phys. Chem. C
2
2
2
009, 113, 11959; (c) Y. Zhanga, Z. H. Lu, Materials Chemistry
and Physics, 2000, 63, 188.
5 (a) E. Bogdan, A. Plaquet, L. Antonov, V. Rodriguez, L.
Ducasse, B. Champagne, F. Castet, J. Phys. Chem. C 2010,
1
14, 12760; (b) A. Plaquet, M. Guillaume, B. Champagne, J.
Phys. Chem. C 2008, 112, 5638.
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