Synthesis, crystallography, phosphorescence of platinum complexes coordinated with 2-phenylpyridine and a series of β-diketones

Article abstract of DOI:10.1016/j.ica.2008.05.012

Metallocyclic platinum(II) complexes coordinating with 2-phenylpyridine (ppy) and a series of β-diketone ancillary O^∧O ligands, (ppy)Pt(acac), (ppy)Pt(ba), (ppy)Pt(dbm), and (ppy)Pt(tta) (acac = acetylacetone, ba = benzoylacetone, dbm = dibenzoylmethane, tta = thenoyltrifluoroacetone) were synthesized. The crystal structure, absorption, emission, quantum yield and phosphorescence life time were characterized. As the conjugative π system of the O^∧O ligand increases in the order acac < ba < dbm, or there is a group -CF₃to attract the electron density of the tta ligand, the quantum yield decreases in the order (ppy)Pt(acac) > (ppy)Pt(ba) > (ppy)Pt(dbm) > (ppy)Pt(tta) due to an energy back-flow from ppy to the O^∧O ligand, a trend also in contrast to the phosphorescence emission spectra and time decay (biexponentially, ～0.7-13 μs).

Full text of DOI:10.1016/j.ica.2008.05.012

Inorganica Chimica Acta 362 (2009) 575–579
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, crystallography, phosphorescence of platinum complexes coordinated
with 2-phenylpyridine and a series of b-diketones
Jian Liu, Chang-Jian Yang, Qian-Yong Cao, Min Xu, Jian Wang, Hua-Nan Peng, Wen-Fang Tan,
*
Xiao-Xia Lü, Xi-Cun Gao
Department of Chemistry, School of Science, Nanchang University, 999, Nanchang 330031, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Metallocyclic platinum(II) complexes coordinating with 2-phenylpyridine (ppy) and a series of b-dike-
tone ancillary O^O ligands, (ppy)Pt(acac), (ppy)Pt(ba), (ppy)Pt(dbm), and (ppy)Pt(tta) (acac = acetylace-
tone, ba = benzoylacetone, dbm = dibenzoylmethane, tta = thenoyltrifluoroacetone) were synthesized.
The crystal structure, absorption, emission, quantum yield and phosphorescence life time were character-
Received 25 November 2007
Received in revised form 9 May 2008
Accepted 9 May 2008
Available online 22 May 2008
ized. As the conjugative
p
system of the O^O ligand increases in the order acac < ba < dbm, or there is a
group –CF3to attract the electron density of the tta ligand, the quantum yield decreases in the order
(ppy)Pt(acac) > (ppy)Pt(ba) > (ppy)Pt(dbm) > (ppy)Pt(tta) due to an energy back-flow from ppy to the
O^O ligand, a trend also in contrast to the phosphorescence emission spectra and time decay (biexponen-
Keywords:
Platinum complexes
b-diketones
Phosphorescence
Absorption, Emission
Quantum yield
tially, ꢀ0.7–13
ls).
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
(ligand-centered) state, mixed with MLCT (metal to ligand charge
transfer) character by strong spin–orbit-coupling [25,26]. How-
ever, the role of the ancillary b-diketones in the photophysics of
the platinum complexes is still not much clear.
In this paper, we synthesized Pt(II) complexes using 2-phenyl-
pyridine and four different b-diketones: benzoylacetone (ba),
dibenzoylmethane (dbm), acetylacetone (acac) and thenoyl-
trifluoroacetone (tta), in order to examine their photophysical
properties.
Luminescent square planar platinum complexes have been
extensively used in DNA probing, photosensitizers and optoelec-
tronic devices [1–4]. Recently, researches for organic light-emitting
electrophosphorescent display (OLED) have attracted much atten-
tion because of their high internal efficiency using all of the
electrogenerated singlet and triplet excitons [5–9]. In order to
understand the electron transfer processes in the excited state
and apply this knowledge to practical applications, numerous
emitting phosphors have been synthesized. Structurally, these
emitting materials usually incorporate a heavy metal iron (Pt, Ir,
Os, Ru, Rh, Re) to induce spin flips so that both singlet and triplet
states can be used electronically. Porphines [5,9–13], phenylpyri-
dines [13–16], thienylpyridines [17], and N^N ligands (bipyridines,
isoquinolines [18–23]), are previously used as the ligands to syn-
thesize phosphorescent dyes. Besides the C^N and N^N ligands,
Pt(II) can also coordinate with an ancillary bidentate O^O ligand,
e.g. b-diketones [13,15,24], which also take part in the photophys-
ical processes and greatly affect the physical properties. These
metallocyclic complexes are known to be strongly emissive and
have long luminescent lifetimes. Reports on the structured emis-
sion pattern and long luminescent lifetime indicate that the emis-
sion arises from an excited state that is predominantly a 13LC
2. Experimental
2.1. Materials and reagents
2-Phenylpyridine (ppy), dibenzoylmethane (dbm) and thenoyl-
trifluoroacetone (tta) were purchased from Acros Chemicals Inc.
The platinum
l-dichloro-bridge dimer (ppy)Pt(l-Cl)2Pt(ppy) was
prepared according to the published procedures [24,25]. The other
reagents are of analytical grade and were used as received. All sol-
vents were purified and distilled by standard procedures before
use.
2.2. Synthesis of (ppy)Pt(O^O) complexes: general procedure
Synthetic strategy for the complexes (ppy)Pt(O^O) and their
chemical structures were shown in Scheme 1 [27,28]. One equiva-
lents of dimer (ppy)Pt(l-Cl)2Pt(ppy), 1 equiv. of the appropriate
* Corresponding author. Tel./fax: +86 791 8339575.
E-mail address: xcgao@ncu.edu.cn (X.-C. Gao).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.05.012

576
J. Liu et al. / Inorganica Chimica Acta 362 (2009) 575–579
Table 1
Crystallographic data for (ppy)Pt(dbm)
Formula
C26H19NO2Pt
572.51
Mo Ka 0.71073
monoclinic
P2(1)/c
13.949(4)
7.779(2)
19.541(5)
90.00
Formula weight
Radiation (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a
(°)
b (°)
109.869
90.00
1994.2 (9)
4
1.907
7.060
c
(°)
V (Å3)
Scheme 1. The synthetic route of the platinum complexes.
Z
Dcalc (g/cm3)
Absorption coefficient (mmꢁ1
F(000)
)
diketone and 10 equiv. of Na2CO3 in 50 ml of 2-ethoxyethanol were
reacted at 100 °C for 16 h. The solvent was removed under reduced
pressure, and the compound was purified by chromatography
using dichloromethane as the eluent. The product was recrystal-
lized in dichloromethane/methanol solution. All the complexes
are air-stable and yellow-orange crystals with moderate yield
(30–50%).
1104
Goodness-of-fit
R1, Rw
Temperature (K)
1.037
0.0309, 0.0645
294 (2)
in a distilled 2-methyltetrahydrofuran (2-MeTHF) solution. Before
measuring, the solution was degassed by several freeze-pump-
thaw cycles performed in a Mbraun glove box at <0.1 ppm O2
2.2.1. (ppy)Pt(tta)
1H NMR (CDCl3, 400 MHz) d, ppm: 8.84 (dd, 1H, J = 5.6, 5.7 Hz),
7.80 (m, 3H), 7.61 (d, 2H, J = 6.4 Hz), 7.43 (q, 1H, J = 3.8 Hz), 7.12
(m, 4H), 6.40 (d, 1H, J = 4.4 Hz). Anal. Calc. for C19H12NF3O2SPt: C,
39.93; H, 2.11; N, 2.46. Found: C, 39.23; H, 1.50; N, 2.45%.
and H2O level. Quinine in 1.0 N H2SO4
(U = 0.48 at 313 nm and
U
= 0.54 at 366 nm) [29] was used as a reference. To obtain the
quantum yield data as precise as possible, the absorption and PL
must be measured in a single machine run to compare the inte-
grated emission area. The difference between refractive index of
the solvent and that of the standard should also be accounted
[24,29]. The quantum yield data obtained in this paper was strik-
ingly consistent with data reported for (ppy)Pt(acac) in degassed
2.2.2. (ppy)Pt(dbm)
1H NMR (CDCl3, 400 MHz) d, ppm: 9.15 (s, 1H), 8.07 (t, 4H,
J = 9.6 Hz) 7.80 (m, 2H), 7.67 (d, 1H, J = 7.6 Hz ), 7.53 (m, 8H),
7.16 (m, 2H), 6.77 (s, 1H). Anal. Calc. for for C26H19NO2Pt: C,
54.45; H, 3.32; N, 2.45. Found: C, 54.43; H, 3.43; N, 2.79%.
2-MeTHF (
U = 0.15) [24]. Emission life times were obtained on a
FLS 920 combined fluorescence lifetime and steady state spectrom-
eter in CH2Cl2 solution at room temperature.
2.2.3. (ppy)Pt(ba)
1H NMR (CDCl3, 400 MHz) d, ppm: 9.06 (d, 1H, J = 27.3 Hz), 7.99
(s, 2H) 7.65 (m, 3H), 7.44 (s, 3H), 7.25 (s, 2H) 7.10 (s, 2H), 6.11 (s,
1H), 2.11 (s, 3H). Anal. Calc. for C21H17NO2Pt: C, 49.41; H, 3.33; N,
2.75. Found: C, 49.20; H, 3.00; N, 2.43%.
3. Results and discussion
3.1. Crystal structure
Single crystals of (ppy)pt(dbm) were grown from dichlorometh-
ane/ethanol solution and characterized by X-ray crystallography.
The N(1)–Pt(1)–O(2) and C(7)–Pt(1)–O(1) angles are 175.74(17)°
and 174.29(19)°, respectively (Fig. 1) indicating a very slight dis-
tortion of the square plane. The Pt1–O1 (2.082(4) Å) and Pt1–O2
(2.006(4) Å) bond lengths and the C7–Pt1–N1 (80.9(2)°) and O2–
Pt1–O1 (90.55(15)°) bond angles are typical for metallocyclic and
b-diketonate derivatives of Pt [15,24,30]. There is only one mole-
cule in a single crystal cell. The molecules pack head-to-tail from
unit to unit with a plane-to-plane separation of 3.422 Å indicative
2.2.4. (ppy)Pt(acac)
1H NMR (CDCl3, 400 MHz) d, ppm: 8.99 (d, 1H, J = 5.6 Hz), 7.78
(t, 1H, J = 7.6 Hz), 7.61 (t, 2H, J = 6.8 Hz), 7.43 (d, 1H, J = 7.5 Hz)
7.16 (m, 3H), 5.47 (s, 1H), 2.05 (s, 3H), 2.00 (s, 3H). Anal. Calc. for
C16H15NO2Pt: C, 42.86; H, 3.35; N, 3.12. Found: C, 42.52; H, 3.37;
N, 3.15%.
2.3. X-ray data collection and structure determination
Diffraction data were collected at room temperature with
graphite monochromatized Mo K
of moderate p–p interaction. Metal–metal interaction is very weak
because the closest Pt–Pt distance is too long, 4.960 Å (Fig. 2).
a radiation (k = 0.71073 Å) on a
Brucker CCD X-ray diffractometer. The data were corrected for
absorption using the SADABS program. The structures were solved
by program SHELXS 97 direct methods and refined by full-matrix
least squares on F2 using the SHELXL 97 program package. The
non-hydrogen atoms were refined anisotropically. Some details
of (ppy)Pt(dbm) single crystal data collection and refinement is
summarized in Table 1.
3.2. Absorption and emission spectroscopy
Absorption spectra of the four complexes were recorded in
CH2Cl2 solution at room temperature (Fig. 2). Table 2 summarizes
the data. Strong spin–orbit-coupling is needed to efficiently mix
the singlet and triplet excited states in order for the complexes
to be useful as phosphors in OLEDs. Clear evidence for significant
mixing of the singlet and triplet excited states is seen in both the
2.4. Measurements and instrumentation
1H NMR was collected by a Bruker 400 MHz spectrometer.
Absorption spectra were obtained by a Perkin Elmer Lambda-35
UV–Vis spectrometer. Photoluminescence spectra (PL) were car-
ried out with a Hitachi F-4500 fluorescence spectrophotometer.
Emission quantum yields were carried out at room temperature
absorption and emission spectra. The four complexes show intense
*
absorption from ligand
p–p
and MLCT transitions. The extinction
*
coefficients for these bands are in the ranges expected for p–p li-
gand-centered and MLCT transitions, 10358–50526 cmꢁ1 Mꢁ1 and
>6210 cmꢁ1 Mꢁ1, respectively. The higher energy, more intense

J. Liu et al. / Inorganica Chimica Acta 362 (2009) 575–579
577
of methyl group in acac of (ppy)Pt(acac). This may due to the elec-
tron-withdrawing character caused by ‘‘inductive effect” of the –
CF3 group. However, in the lower energy region in the range of
347–480 nm, the absorption for (ppy)Pt(tta) increases markedly.
The heavy atom of S and F may contribute to the increase of the
MLCT absorption. In addition, the spectrum feature of ppyPt(tta)
is different from those of the other three. In this region, the absorp-
tion bands are not observed in the free HC^N and O^O ligand (see
Fig. 3). The weaker absorption bands are assigned as metal-to-li-
gand charge transfer (MLCT) transitions. Note that for (ppy)Pt(a-
cac), since 347 nm, the extinction coefficient increases till the
maxima at 364 nm (6880 cmꢁ1Mꢁ1). Although the extinction coef-
ficients are sometimes greater than 6000 cmꢁ1 Mꢁ1 for (ppy)Pt(ba)
and (ppy)Pt(dbm), the absorption in the range 347–480 nm cannot
*
be attributed to p–p LC transitions. It is caused by the increased
aromaticity of the ancillary b-diketone ligand.
As the aromaticity of the free ligand decreases in the order of
dbm > ba > acac, the first absorption maxima show bathochromic
shifts, indicating a remarkable decrease in the S1 energy state of
the ligands (Fig. 3).
All of the complexes are emissive both in solution and solid
state at room temperature. The emission spectra in 2-meTHF are
shown in Fig. 4. The highly structured emission spectra for (ppy)-
Pt(acac) and (ppy)Pt(ba) display vibronic progressions of ca.
1400 cmꢁ1, which are typical for the breathing modes of the aro-
matic ppy ring. The highly structured emission may originate from
Fig. 1. Molecular structure of (ppy) Pt(dbm) (50% probability ellipsoids, H atoms
omitted).
Fig. 3. Absorption spectra of ppy ligand and the four O^O ligands in CH2Cl2 at room
Fig. 2. Absorption spectra of the four complexes in CH2Cl2 at room temperature.
temperature.
*
absorption bands are assigned to
p–p
LC transitions. In this
region, the extinction coefficient decreases regularly as the aroma-
ticity of the b-diketone ring decreases in the order of
(ppy)Pt(dbm) > (ppy)Pt(ba) > (ppy)Pt(acac)
(except
for
the
256 nm band for (ppy)Pt (ba)). ppyPt(tta) has the lowest absorp-
tion value in the 250–280 nm region, even though the aromaticity
of the thiophene ring in tta ligand is thought to be higher than that
Table 2
Absorbances and extinction coefficients of the Pt complexes
Complex
Abs, kmax/nm, (e )
/cmꢁ1 Mꢁ1
ppyPt(acac) 250 (33157), 277 (23760), 313 (11560), 327 (10358), 347 (6210),
364 (6880), 410 (1864), 430 (184), 479 (10.9)
ppyPt(ba)
256 (52526), 278 (41032), 312 (20086), 330 (17933), 347 (18633),
353 (19355), 410 (3206), 430 (494), 476 (24.4)
ppyPt(dbm) 260 (50827), 279 (42129), 312 (25057), 328 (22459), 347 (21148),
366 (21958), 410 (7735), 430 (2456), 475 (144)
ppyPt(tta)
256 (24093), 276 (22861), 328 (17786), 347 (15473), 369 (15766),
410 (6474), 430 (2227), 477 (242)
Fig. 4. Emission spectra of the four platinum complexes in 2-meTHF solution,
Absorbances at 347 nm, 410 nm, 420 nm are not maxima, but the others are.
excitation wavelength 313 nm.

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J. Liu et al. / Inorganica Chimica Acta 362 (2009) 575–579
a mixed 13LC-MLCT excited state while the flattened emission
spectra for (ppy)Pt(dbm) and (ppy)Pt(tta) may indicate a pure
MLCT excited state contribution. Evidently, the b-diketonato ligand
significantly perturb the excited state since the emission spectra
for (ppy)Pt(dbm) and (ppy)Pt(tta) significantly differ from those
of (ppy)Pt(acac) and (ppy)Pt(ba). This is in contrary to the conclu-
sion of Ref. [24], in which fewer examples of b-diketonato ligands
were observed.
Table 3
Quantum yields (%) of the platinum complexes
Complexes
Quantum yield at 313 nm
Quantum yield at 366 nm
(ppy)Pt(acac)
(ppy)Pt(ba)
(ppy)Pt(dbm)
(ppy)Pt(tta)
14.56
3.85
1.33
0.69
17.02
3.93
1.47
0.80
The solid-state emission spectra of all of the four complexes
become in some extent ‘‘fat” and show bathochromic shift and
are poorly resolved (Fig. 5). These bathochromic shifts may be
caused by the intermolecular interaction of the solid sample com-
pared to molecules in solution, a phenomenon commonly observed
for most organic dye compounds. These red-shifted flattened emis-
sions may be called as excimer emission. As indicated in the first
section of this paper, The molecules pack with a plane-to-plane
separation of 3.422 Å, this distance is short enough to bring about
excimer emissions [31].
dominated by 13LC and MLCT transitions of PPY, leading to high
efficiency. However, if the triplet state energy of the ancillary b-
diketone ligand lies lower in energy than the 13LC and MLCT of
PPY, a triplet O^O level will be the lowest energy excited state.
Thus, a switch from C^NPt to O^O-based emission can be seen in
the other complexes coordinated with different b-diketones (Table
3). The phosphorescence efficiency falls in the order of (ppy)Pt
(acac) > (ppy)Pt(ba) > (ppy)Pt(dbm) > (ppy)Pt(tta). This efficiency
increase order corresponds to the aromaticity decrease order
dbm > ba > acac very well. Anyway it is confusing that (ppy)Pt(tta)
possesses the lowest quantum yield for the F and S atom in tta li-
gand should contribute a ‘‘heavy atom” effect to increase the phos-
phorescence efficiency as it does to strengthen the MLCT
absorption of the complex. The following two luminescent quench-
ing mechanisms may come into play: (1) for (ppy)Pt(tta), the CF3–
group has a strong electron-withdrawing effect therefore greatly
influence the electron density of the tta ligand as a whole and fur-
ther decreases the triplet energy of tta ligand and therefore greatly
induces energy back-flow; (2) as tta ligand’s MLCT contribution in-
creases, a switch of phosphorescence emission from previously
mainly from ppyPt centered to ttaPt centered makes the emission
not as efficiently as the ppyPt centered. In contrast, (ppy)Pt(acac)
gives the highest quantum efficiency of >0.14. The triplet level of
the three b-diketones falls in the order acac > ba > dbm. Appar-
ently, in the case of acac, the highest triplet energy level effectively
hampers the energy back-flow from ppy to acac. In contrast,
in complexes comprising O^O ligands ba, dbm and tta, energy
back-flow or loss is inevitable, leading to weak phosphorescence.
3.3. The Quantum yield
The triplet level of acac lies well above the energies of the C^N
ligand and the MLCT exited state [6]. Thus, the luminescence is
3.4. The phosphorescence life time
Although the difference is small, the phosphorescence decay
pattern can be divided into two groups with one group for (ppy)-
Pt(acac) and (ppy)Pt(ba) faster and another slower for
(ppy)Pt(dbm) and (ppy)Pt(tta) (Fig. 6). The phosphorescence of
all of four complexes decays biexponentially with components of
Fig. 5. Emission spectra of the solid crystalline platinum complexes.
1.0
ls and 13
ls for (ppy)Pt(acac) and (ppy)Pt(ba) and components
of 0.7
ls and 9.0
l
s for (ppy)Pt(dbm) and (ppy)Pt(tta). These are
consistent with emission from a triplet excited state. The slower
decay time for (ppy)Pt(acac) and (ppy)Pt(ba) and faster decay time
for (ppy)Pt(dbm) and (ppy)Pt(tta) corresponds to the classification
of the emission spectra shape of the four complexes: the former
two’s vibronic emission structure versus the latter two’s broad
and featureless emission. The decay time pattern is also in contrast
to the quantum yields measurement: the faster decay time corre-
sponds to the lower quantum yields for (ppy)Pt(dbm) and
(ppy)Pt(tta).
4. Conclusion
In summary, we synthesized a series of platinum complex coor-
dinating with ppy and different ancillary b-diketone O^O ligands
and investigated their crystal structure, absorption, emission and
quantum yield and emission lifetime. The ancillary O^O ligands
perturb the excited state of the complex as a whole. In general, lar-
Fig. 6. Emission decay curves for the Pt complexes (measured in CH2Cl2 at room
temperature).
ger conjugating p
systems lower the triplet state of the O^O ligand

J. Liu et al. / Inorganica Chimica Acta 362 (2009) 575–579
579
and cause the energy back-flow from ppy ligand to the O^O ligand
and switch previously mainly ppyPt centered phosphorescence
emission to ttaPt centered emission, therefore cause spectral and
quantum yield differences: the quantum yield decreases and the
emission spectrum becomes featureless. The life time decay falls
in the order of several microseconds, a clear indication of a triplet
excited state for the complexes.
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Thompson, Inorg. Chem. 41 (2002) 3055.
CCDC 687199 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.ica.2008.05.012.
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