Impact of a picolinate ancillary ligand on phosphorescence and fluoride sensing properties of BMes2-functionalized platinum(II) compounds

Article abstract of DOI:10.1021/om301112u

Two new dimesitylboron (BMes₂)-functionalized 2-phenylpyridine cyclometalated Pt(II) complexes (2 and 3) with a picolinate or a methyl-picolinate as the ancillary ligand have been synthesized via a one-pot procedure at ambient conditions with high yields (>70%). The crystal structures of these Pt(II) complexes were determined by single-crystal X-ray diffraction analysis, which revealed the presence of extended π-stacking interactions in the crystal lattice of 2 and discrete dimer formation in the lattice of 3. Both complexes exhibit dual phosphorescence emission in solution at room temperature under N₂ atmosphere. These unusual photophysical properties have been systematically investigated by spectroscopic and computational studies, which established that the phosphorescent dual emission originates from admixture of ³LLCT and ³ILCT/ ³MLCT transitions. Fluoride titration experiments were conducted to further confirm the origin of phosphorescence in these compounds. The phosphorescent properties and the phosphorescent response toward fluoride ions by compounds 2 and 3 are distinctly different from the previously known BMes₂-functionalized N^aC-chelate Pt(II) compounds, which are attributed to the introduction of the low-lying π* orbital by the picolinate ancillary ligand in the Pt(II) compounds.

Full text of DOI:10.1021/om301112u

Article
pubs.acs.org/Organometallics
Impact of a Picolinate Ancillary Ligand on Phosphorescence and
Fluoride Sensing Properties of BMes2‑Functionalized Platinum(II)
Compounds
Soo-Byung Ko,† Jia-Sheng Lu,† Youngjin Kang,‡ and Suning Wang*,†
†Department of Chemistry, Queen’s University, Kingston, Ontario, K7M 3N6, Canada
‡Division of Science Education, Kangwon National University, Chuncheon 200-701, Republic of Korea
S
* Supporting Information
ABSTRACT: Two new dimesitylboron (BMes2)-function-
alized 2-phenylpyridine cyclometalated Pt(II) complexes (2
and 3) with a picolinate or a methyl-picolinate as the ancillary
ligand have been synthesized via a one-pot procedure at
ambient conditions with high yields (>70%). The crystal
structures of these Pt(II) complexes were determined by
single-crystal X-ray diffraction analysis, which revealed the
presence of extended π-stacking interactions in the crystal
lattice of 2 and discrete dimer formation in the lattice of 3.
Both complexes exhibit dual phosphorescence emission in
solution at room temperature under N2 atmosphere. These
unusual photophysical properties have been systematically
investigated by spectroscopic and computational studies, which established that the phosphorescent dual emission originates
from admixture of 3LLCT and 3ILCT/3MLCT transitions. Fluoride titration experiments were conducted to further confirm the
origin of phosphorescence in these compounds. The phosphorescent properties and the phosphorescent response toward
fluoride ions by compounds 2 and 3 are distinctly different from the previously known BMes2-functionalized N∧C-chelate Pt(II)
compounds, which are attributed to the introduction of the low-lying π* orbital by the picolinate ancillary ligand in the Pt(II)
compounds.
Ru(II),9c,d and Pt(II)6b,c,10 and high efficiency OLEDs based on
INTRODUCTION
■
Pt(II) compounds.6f−j
Triarylboron compounds with sterically bulky aryl groups such
We have recently reported the impact of the BMes2 (Mes =
as mesityl have recently attracted much research interest
mesityl) location on the phosphorescence of Pt(Bppy)(acac)
because of their broad applications in nonlinear optical
compounds (ppy = 2-phenylpyridine).11 We have found that
materials,1 charge transfer materials and emitters in organic
the meta-BMes2-functionalized ppy ligand m-Bppy can lead to
light emitting devices (OLEDs),2 and highly selective small
about 60 nm phosphorescence blue shift of its Pt(II)
anion sensors such as fluoride and cyanide.3 When triarylboron
groups are connected by proper electron-donating groups
compound, (m-Bppy)Pt(acac) (1, Chart 1), relative to the
para-substituted compound Pt(p-Bppy)(acac). This provides an
effective way to tune the emission color of N∧C-chelate Pt(II)
through π conjugation, bright luminescent compounds can be
achieved.2,4 Such compounds have been often applied as
compounds. Among phosphorescent compounds, blue phos-
switch-off sensors for fluoride anions because the binding of a
fluoride to the boron center quenches the charge transfer
(CT)-based luminescence. Furthermore, we have shown that
the binding of fluoride anions to a triarylboron center in
spatially separated donor−acceptor molecules can suppress
charge transfer and activate alternative emission pathways such
as π−π* transitions, leading to switch-on sensors for fluoride
ions.5 The attachment of a triarylboron moiety to a transition
metal center has been shown to greatly enhance metal-to-ligand
charge transfer (MLCT)6 and phosphorescence efficiency.6
This has led to the development of phosphorescence-based
sensing systems for anions using triarylborane-functionalized
transition metal complexes such as Hg(II),7 Ir(III),8 Re(I),9a,b
phorescent molecules are most in demand because stable blue
phosphorescent compounds in OLED devices remain challeng-
ing and rare. Although compound 1 has bright phosphor-
escence and impressive performance in OLEDs,11 it is a blue-
green emitter with λem = 481 nm. To examine whether
changing the acetylacetonato ancillary ligand to a picolinate
ligand can shift the emission color of the Pt(II) compound
further toward blue and to develop new dual-emissive Pt(II)
compounds, we have designed and synthesized new (m-
Bppy)Pt(picolinate) complexes. The choice of the picolinate
Received: November 19, 2012
© XXXX American Chemical Society
A
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cyclometalating ligand with K2PtCl4 in solution to produce a
dichloro-bridged Pt(II) dimer, which is then heated over 100
°C with 2-picolinic acid in the presence of excess sodium
carbonate to obtain the final product with moderate yields.12
These harsh basic reaction conditions, however, are not suitable
for the synthesis of organoboron-containing Pt(II) compounds
due to the Mes2B−C bond cleavage at high reaction
temperature.15 To overcome this problem, a one-pot synthetic
method16 for cyclometalated Pt(II) β-diketonate compounds
developed recently by our group is modified and used in the
synthesis of compounds 2 and 3. The (m-Bppy)PtMe(SMe2)
intermediate is generated by the reaction of m-Bppy and
[PtMe2(SMe2)]2 in THF at room temperature for 2 h. The
addition of 1.5 equiv of picolinic acid leads to the formation of
the corresponding (m-Bppy)Pt(II) picolinate products (2 and
3) as yellow solids under mild conditions, which are isolated by
column chromatography on silica gel with high yields (70−
72%). These two Pt(II) complexes are air stable in solution and
the solid state. These complexes have been fully characterized
by NMR spectroscopy, elemental analyses, and single-crystal X-
ray diffraction analyses.
Crystal Structures. The crystal structures of 2 and 3
determined by X-ray diffraction are shown in Figure 1 and 2,
respectively, along with important bond lengths and angles.
One important feature revealed by the crystal structures is that
the nitrogen donor atoms in both compounds have a trans-
geometry. NMR spectra indicate that the trans-isomer is the
only product for both 2 and 3. This may be caused by the
strong trans-directing effect of the phenyl group that favors the
weaker carboxylate donor over the pyridyl donor. In addition,
the preferential H bond formation between the carboxylate
oxygen atom O(1) and the ortho-H atom of the pyridyl ring
(N−H···O = 2.41 Å in 2 and 2.39 Å in 3) may also play a role
in the exclusive formation of the trans-isomer. Similar trans-
geometry in related (ppy)Pt(N∧O) compounds has been
reported previously.17 Despite the substitution by a methyl
group in the picolinate ancillary ligand in compound 3, the
bond lengths and the chelating bond angles around the Pt(II)
are similar in both compounds.
Chart 1
ligand as the ancillary ligand is based on the fact that several
nonborylated N∧C-chelate Pt(II) compounds with the
picolinate ancillary ligand have been demonstrated for
applications in OLEDs.12 In addition, it is well known that
the picolinate ligand has a higher ligand field strength than that
of acetylacetonate (acac) and its derivatives,13 thus may be very
useful in achieving highly efficient blue or blue-green
phosphorescent Pt(II) compounds by stabilizing the occupied
d orbitals and increasing d−π* transition energy. In fact, highly
efficient Ir(III) phosphorescent compounds using picolinate
ligands are well known in the literature, with an emissive triplet
state that is much higher in energy than that of acac
derivatives.13,14 We have found that the picolinate ancillary
ligand has a dramatic impact on phosphorescence and fluoride
sensing properties of the m-Bppy-chelate Pt(II) compounds.
The details of these results for our investigation and discussion
are reported herein.
RESULTS AND DISCUSSION
■
Syntheses. The synthetic procedures and conditions for the
new Pt(II) complexes (2 and 3) are shown in Scheme 1. The
m-Bppy ligand was synthesized using a modified procedure
reported by Park and co-workers.8a The traditional method for
synthesizing cyclometalated Pt(II) picolinate complexes in-
volves two steps by first heating 2.0 to 2.5 equiv of the
Scheme 1. Synthetic Procedure for the Pt Complexes 2 and 3
B
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Figure 2. Diagrams showing intermolecular stacking of complex 2, the
Pt···Pt separation distance, and the shortest π-stacking distances. Top:
Side view. Bottom: Top view of four color-coded molecules.
picolinate N atom from a neighboring molecule (3.76(1) Å)
than to the other Pt atom (as indicated by the red dashed line
in Figure 3). The π-stacking interaction within the dimer of 3 is
much less than that observed in 2, as shown by the top view of
the stacking diagram of 3, and resembles that of the acac
analogue,11 compound 1. Most noteworthy is that no
interdimer π-stacking is present in the crystal lattice of 3.
There are some hydrogen bond interactions between the
pyridyl of the ppy ligand and the oxygen atom of the picolinate
ligand between the dimers as shown in Figure 3. Overall, the
crystal structural data indicate that intermolecular π-stacking
interactions and Pt···Pt interactions in 3 are much weaker than
those in 2, which can be attributed to the methyl substituent in
the picolinate ligand in 3, which considerably reduces the π-
stacking involving the picolinate pyridyl ring and the ppy
pyridyl ring. Compared to the acac analogue, compound 1
(Pt···Pt distance = 4.64−4.67 Å),11 the Pt···Pt separation
distances of 2 and 3 are shorter, indicating that the picolinate
ligand has a greater tendency to promote intermolecular π-
stacking, which may be attributed to the aromatic pyridyl ring.
Absorption Spectra. The absorption spectra of the free
ligand m-Bppy and their Pt(II) complexes 2 and 3 were
recorded in CH2Cl2 solution and are shown in Figure 4 and
Table 1. The π to π* absorption band of m-Bppy at 313 nm is
blue-shifted approximately by 16 nm compared to that of
earlier reported para-substituted p-Bppy,10b which can be
attributed to the greater stabilization of the ground state by
meta substitution.11 Compared to the free ligand m-Bppy, the π
to π* absorption bands of Pt(II) complexes 2 and 3 are red-
shifted by ∼13 nm with increased absorbance, which can be
Figure 1. Structure diagrams of 2 (top) and 3 (bottom) with 50%
thermal ellipsoids and labeling schemes for the key atoms. Important
bond lengths (Å) and angles (deg) for 2: Pt(1)−N(1) 2.000(4),
Pt(1)−C(1) 2.000(4), Pt(1)−N(2) 2.038(4), Pt(1)−O(1) 2.093(3),
B(1)−C(4) 1.562(7), B(1)−C(12) 1.581(7), B(1)−C(21) 1.576(7)
Å, N(1)−Pt(1)−N(2) 173.23(14), C(1)−Pt(1)−O(1) 173.93(15),
C(1)−Pt(1)−N(1) 80.72(16), N(2)−Pt(1)−O(1) 79.75(13)°; for 3:
Pt(1)−N(1) 1.977(8), Pt(1)−C(1) 2.011(12), Pt(1)−N(2) 2.024(9),
Pt(1)−O(1) 2.094(9), B(1)−C(4) 1.555(17), B(1)−C(12)
1.570(16), B(1)−C(21) 1.591(15), N(1)−Pt(1)−N(2) 171.88(3),
C(1)−Pt(1)−O(1) 173.5(3), C(1)−Pt(1)−N(1) 80.8(4), N(2)−
Pt(1)−O(1) 79.1(3).
The key difference between compounds 2 and 3 is their
distinct intermolecular interaction patterns in the crystal
lattices. As shown in Figure 2, molecules of 2 form Pt(II)
dimers with a Pt···Pt separation distance of 3.710 Å. The
pyridyl ring of the ppy chelate and the pyridyl ring of the
picolinate are directly on top of each other, with the π-stacking
distances ranging from 3.53 to 3.80 Å. Most significantly, the
Pt(II) dimers of 2 further stack through the pyridyl rings with
π-stacking distances of 3.70−3.76 Å, which leads to the
extended π-stacking in the crystal lattice. A Pt(II) dimer is also
present in the crystal lattice of 3, as shown in Figure 3.
However, the Pt···Pt separation distance (4.125 Å) is much
longer than that in 2. In fact the Pt atom is much closer to the
C
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Table 1. Absorption and Luminescence Data
emission, 77
a
a
c
absorption
λmax (nm), ε (104
emission, 298 K
K
λmax (nm)/τp
λmax (nm)/τp
(μs)
b
compound
cm−1 M−1
)
(μs)
Φp
m-Bppy
251 (2.70), 287
397
0.081 364
(1.53), 313 (1.36)
d
1
271 (1.85), 327
(3.01), 344 (2.56)
481/8.7 (2)
0.430 478/9.4 (1)
2
3
261 (2.40), 326
(3.69), 390 (0.42),
415 (0.35)
450/14.1 (9), 0.013 442/29.6 (1),
493/8.0 (5) 482/16.2 (2)
263 (2.38), 326
(3.79), 388 (0.27),
412 (0.23)
450/18.8 (0), 0.032 440/34.6(5),
491/8.2 (9) 481/15.6 (8)
a
The spectra were measured in degassed CH2Cl2 solution: [M] = 1.0
b
× 10−5. The quantum efficiency for the free ligand was measured
under air using 9,10-diphenylanthracene as the standard (Φ = 0.95).18
For the Pt(II) complexes, the quantum efficiency was obtained under
N2 atmosphere using fac-Ir(ppy)3 as the standard (Φ = 0.97).19
c
d
Measured in 2-methyltetrahydrofuran solution. From ref 11.
M−1 cm−1) can be assigned to an intraligand charge transfer
(ILCT)/MLCT mixed ligand-to-ligand charge transfer (LLCT)
transition. The other lower lying absorption band at 415 nm
(2700 M−1 cm−1) for 2 and 412 nm (2300 M−1 cm−1) for 3 can
be assigned to mixed transitions involving LLCT mixed with
ILCT/MLCT transitions. These assignments are also sup-
ported by Time-dependent density functional theory (TD-
DFT) calculation results (see the TD-DFT section).
Luminescence Spectra. The luminescent spectra of the
free ligand and their Pt(II) complexes 2 and 3 in CH2Cl2 at
room temperature are shown in Figure 5 and Table 1. The free
Figure 3. Diagrams showing intermolecular stacking of complex 3, the
Pt···Pt separation distance, H-bond distances, and the shortest π-
stacking distances. Top: Side view. Bottom: Top view of four color-
coded molecules.
Figure 5. Luminescent spectra of m-Bppy and its Pt(II) complexes 2
and 3 in CH2Cl2 (1.0 × 10−5 M) under N2 at 298 K.
m-Bppy ligand displays blue luminescence at 397 nm (Φ =
0.081), which is hypsochromic shifted by 12 nm compared to
that of the para-substituted Bppy.10c The Pt(II) complexes 2
and 3 show green phosphorescence when excited in their
lowest energy absorption bands. Both compounds show two
distinct emission peaks at 450 nm and 491 or 493 nm,
respectively, that are sensitive to oxygen. The relative intensity
ratio of these two peaks is different for 2 and 3. The emission
lifetime of the peak at 450 nm (τp = 14.1−18.0 μs) is longer
than that at 491 nm (or 493 nm) (τp = 8.0−8.2 μs). From the
position and the structureless shape, the emission peak at 450
Figure 4. UV−vis absorption spectra of m-Bppy and its Pt(II)
complexes 2 and 3 in CH2Cl2 (1.0 × 10−5 M).
explained by the π-conjugation enhancement by metal
chelation. In addition, these Pt(II) complexes show two well-
resolved absorption bands in the 380−450 nm region with
moderate extinction coefficients. The absorption band at
approximately 390 nm for 2 (4222 M−1cm−1) and 3 (3798
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nm is assigned to the ILCT/MLCT mixed 3LLCT transition.20
The emission peak at 491−493 nm shows vibronic progressions
of 1168 cm−1, which become more apparent in the 77 K spectra
recorded in 2-methyltetrahydrofuran frozen glass (Figure 6).
stacking phenomenon of 2 revealed by the crystal data. No
strong concentration dependence of 3 was observed.
Electrochemical Properties. The oxidation and reduction
potentials of the Pt(II) compounds were recorded using cyclic
voltammetry (CV) in N,N-dimethylformamide (DMF). Both 2
and 3 show a quasi-reversible reduction wave that is
characteristic of the picolinate ligand and at much more
positive potentials than that of compound 1, which displays a
reduction wave typical of BMes2.10,11 A quasi-reversible
oxidation wave was also observed for both compounds. Using
the oxidation and the reduction potentials, the HOMO and
LUMO energy levels of complexes 2 and 3 were estimated and
are shown in Table 2. Compared to compound 1, the LUMO
a
Table 2. Electrochemical Data
ox
red
E1/2
E1/2
(V)
optical energy
gap (eV)
LUMO
c
b
e
complex (V)
HOMO (eV)
(eV)
d
1
−2.34
−2.18
−2.25
2.92
2.80
2.83
−5.49
−5.42 (−5.45 )
−2.46
−2.62
−2.55
b
2
3
0.62
0.58
b
e
−5.38 (−5.37 )
a
The CV was recorded in DMF solution containing 0.10 M (n-
b
Bu)4PF6 as the electrolyte, relative to Eo(Fc/Fc+) = 0.55 V. Estimated
Figure 6. Time-resolved (delay time = 180 μs) phosphorescence
spectra of 2 and 3 in 2-methyltetrahydrofuran at 77 K.
from the oxidation potentials.22 Estimated from the reduction
c
d
e
potentials.22 From ref 11. Estimated from the optical energy gap
and the LUMO energy.
3
3
This emission peak is most likely from ILCT and LLCT
transitions mixed with MLCT characters, as indicated by the
TD-DFT calculation results. The emission spectra of both
complexes display a small regiochromic blue shift with the peak
at ∼490 nm being shifted to ∼480 nm and a substantial
increase of the decay lifetime upon cooling to 77 K. Compared
to the acac compound 1 and related substituted m-Bppy Pt(II)
acac compounds, the phosphorescent quantum efficiencies of 2
and 3 are very low (Φ = 0.013 for 2 and 0.032 for 3), which
may be attributed to intermolecular interactions and the 3LLCT
contribution to the phosphorescence. In fact, compound 2 was
found to display a strongly concentration dependent
phosphorescence, as shown in Figure 7. At concentrations
above 1.0 × 10−4 M, a broad emission band at ∼625 nm,
characteristic of Pt(II) excimer emission,21 was observed,
accompanied with an emission color change from green to
orange. This is in agreement with the extensive intermolecular
level of both 2 and 3 is significantly lower, while the HOMO
level is approximately the same. This could be attributed to the
greater electron affinity of the picolinate ligand, compared to
the acac ligand in compound 1. As a consequence, the LUMO
level is no longer dominated by the boron moiety in 2 and 3, in
contrast to that observed in compound 1 and other BMes2-
functionalized N∧C-chelate Pt(II) compounds with acac or its
derivatives as the ancillary ligand.11
DFT Computational Results. The surfaces and energies of
the frontier orbitals for 2 and 3 in the ground state are depicted
in Figure 8. Complete diagrams for both complexes are
provided in the Supporting Information. For both 2 and 3, the
HOMO involves contributions mainly from the d orbital of the
Pt(II) center (33−34%), the π orbitals of the phenyl ring of m-
Bppy, and the carboxylate of the picolinate ligand. In contrast,
the LUMO is localized on the pyridyl rings of m-Bppy and
picolinate with no contributions from the boron center at all, in
agreement with the electrochemical data. This is in sharp
contrast to all previously reported N∧C-chelate Pt(II)
compounds based on BMes2-functionalized ppy and related
ligands, in which the LUMO is consistently dominated by the
boron center.6f,10,11 In 2 and 3, the main contribution of the
BMes2 group is found in the LUMO+2. The general trend of
the calculated HOMO and LUMO energies and the HOMO−
LUMO gap for compounds 2 and 3, relative to those of 1, at
the ground state is shown in Figure 9, which is in good
agreement with the experimental data shown in Table 2.
To understand the photophysical properties of 2 and 3, TD-
DFT calculations were performed to evaluate the vertical
excitation energies of each complex for the lowest six singlet
and three triplet states. The important low-lying singlet and
triplet states are shown in Table 3 (also see Supporting
Information for details). The calculation results show that the
S1 state is mainly from the HOMO → LUMO transition (95%)
for both compounds. The T1 state is also dominated by the
HOMO → LUMO transition (67−68%). Thus, the S1 and T1
Figure 7. Concentration effects for the emission spectra of 2 in
CH2Cl2 at ambient temperature.
E
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Figure 8. Isodensity surface plots and energies for frontier orbitals of 2 and 3 in the ground state (isodensity contour = 0.02 au).
a
Table 3. TD-DFT Calculation Results for 2 and 3
transition
energy
transition
configuration
oscillator
strength (f)
complex state
2
S1
S2
S6
418.9 nm
HOMO → LUMO
0.0382
0.0073
0.2014
(2.96 eV)
(95%)
366.0 nm
(3.39 eV)
HOMO → LUMO+1
(86%)
342.2 nm
(3.62 eV)
HOMO → LUMO+2
(43%)
T1 478.0 nm
(2.59 eV)
HOMO → LUMO
(67%)
HOMO → LUMO+1
(12%)
T2 412.2 nm
(3.01 eV)
HOMO → LUMO
(22%)
HOMO → LUMO+1
(31%)
T3 391.7 nm
(3.17 eV)
HOMO → LUMO+2
(46%)
3
S1
S2
S6
413.0 nm
(3.00 eV)
HOMO → LUMO
0.0409
0.0107
0.2570
(95%)
Figure 9. Calculated frontier orbital surface and energies for Pt(II)
complexes 1−3.
360.3 nm
(3.44 eV)
HOMO → LUMO+1
(77%)
341.5 nm
(3.63 eV)
HOMO → LUMO+2
(63%)
states can be assigned to LLCT mixed with ILCT/MLCT
transitions. The phosphorescent emission peak at ∼490 nm for
both compounds can be attributed to the T1 state. In
comparison, the T1 state of compound 1 involves mainly
transitions localized on the Bppy ligand with a large
contribution from the BMes2 unit. Thus, the low phosphor-
escent quantum efficiencies of 2 and 3, in comparison to that of
1, may be attributed to the large contribution of the interligand
or ligand-to-ligand CT transition to the T1 state.
The S2 state has the HOMO → LUMO+1 as the dominant
transition for 2 and 3. The T2 state has mixed contributions
from HOMO → LUMO and HOMO → LUMO+1 transitions
and is 0.42 and 0.46 eV higher in energy than T1, for 2 and 3,
respectively. The S2 and T2 can also be designated as ILCT/
MLCT mixed LLCT transitions. The HOMO → LUMO+2
transition, which is mainly localized on the m-Bppy ligand with
a large BMes2 contribution, dominates the S6 state (43% and
63% for 2 and 3, respectively) and the T3 state (46% and 57%
for 2 and 3, respectively). The intense absorption peak at 326
T1 475.9 nm
(2.61 eV)
HOMO → LUMO
(68%)
HOMO → LUMO+1
(9%)
T2 404.1 nm
(3.07 eV)
HOMO → LUMO
(20%)
HOMO → LUMO+1
(21%)
T3 391.1 nm
(3.17 eV)
HOMO → LUMO+2
(57%)
a
TD-DFT was calculated at the B3LYP level using the LANL2DZ
basis set for Pt and the 6-31G(d) basis set for all other atoms.
nm in 2 and 3 can be assigned to the S6 state. Because the T2
and T3 states are close in energy (0.10−0.14 eV difference), the
phosphorescence emission at 450 nm by 2 and 3 may have
contributions from both states.
On the basis of the computational data, we suggest that the
low phosphorescent quantum efficiencies of 2 and 3 are mainly
F
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Figure 10. UV−vis spectral change of 2 and 3 (1.0 × 10−5 M) with the addition of (n-Bu)4NF in CH2Cl2.
Figure 11. Phosphorescent spectral change of 2 and 3 (1.0 × 10−5 M) with the addition of (n-Bu)4NF in CH2Cl2 under a N2 atmosphere.
caused by the T1 state being dominated by charge transfer
between ppy and the picolinate ligand (or LLCT transitions).
chelate backbone.6b,c,10a,b For more details, please refer to the
Supporting Information.
3
As shown in Figure 11, the phosphorescence spectra of
compounds 2 and 3 do not experience drastic emission energy
change in the range 450−625 nm upon the addition of fluoride
ions, in contrast to previously reported BMes2-functionalized
transition metal compounds that usually display a distinct
emission energy and phosphorescent color change.6−10 The
moderate emission intensity decrease of 2 and 3 with fluoride
ions may be attributed partially to the dynamic association and
dissociation process of the fluoride ions with the boron atom
that increases the rate of nonradiative decay through thermal
vibrational pathways. In addition, a new fluorescent band
appears at λmax = ∼370 nm with the addition of fluoride. TD-
DFT calculation results for the fluoride adducts of [NH4][2F]
and [NH4][3F] at the ground state indicate that the frontier
orbitals are similar to those of 2 and 3, but the Pt d orbital
contributions are significantly decreased approximately 10−
12%, which may also account for the decreased phosphorescent
efficiency. In addition, the computational data confirmed that
the T1 state of the fluoride adducts has similar orbital
contributions as that of 2 and 3, although the energy is red-
shifted by ∼0.2 eV, providing a plausible explanation for why
the phosphorescent spectra 2 and 3 experience little change on
the addition of fluoride. On the basis of the computational data,
the new fluorescence band at 370 nm in the fluoride adducts
may be assigned to the S10 transition. Complete calculation
results for the fluoride adducts are provided in the Supporting
Information.
Thus, in order to achieve highly efficient phosphorescence in
the BMes2-functionalized N∧C-chelate Pt(II) compounds, it is
critical that the N∧C-chelate chromophore centered transitions
(3LC or 3ILCT) are the main contributions in the lowest triplet
state with a significant involvement of the BMes2 group.
Fluoride Titration. To further probe the origin of the
phosphorescence displayed by 2 and 3, we conducted fluoride
titration experiments, which were monitored by UV−vis,
emission, and 1H and 19F NMR spectra. The absorption
spectral change of compounds 2 and 3 with the addition of n-
NBu4F is shown in Figure 10. The low-energy absorption bands
for both compounds experience a slight red shift by ∼20−25
nm. The peak at 326 nm shows a significant intensity gain,
while the high-energy band at 270−285 nm experiences a linear
increase in intensity with the addition of F−. TD-DFT
computational data for the fluoride adducts [NH4][2F] and
[NH4][3F] at the ground state indicate that the intense peak at
∼326 nm is from the S10 state with contributions mainly from a
π (ppy)→π* (ppy-picolinate) transition, which happens to be
at a similar energy and with a similar oscillator strength to those
of the S6 state of 2 and 3, thus explaining the small change of
the low-energy absorption band with fluoride. All Pt(II)
compounds were found to have a stronger binding with
fluoride ions, compared to the free ligand, as evidenced by the
binding constants of 1.00 × 107, 3.30 × 107, and 1.24 × 106
M−1, for 2, 3, and the free m-Bppy ligand, respectively, obtained
using the Benesi−Hildebrand analysis method.23 This can be
attributed to the chelation effect of the Pt(II) center, which is
known to enhance the Lewis acidity of the boron center on the
The fluoride adducts 2F− and 3F− were observed and
confirmed by 1H and 19F NMR spectra (see Supporting
Information). In the 19F NMR spectra, one characteristic
G
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chemical shift at −174.7 ppm for 2F− and −174.5 ppm for 2F−
was observed, which is similar to the previously reported
chemical shifts of fluoride adducts with a BMes2 group
(typically in the range −175 to −180 ppm).6b,c,10 No typical
Pt−F signals24 were observed in the 19F NMR spectra, which
further supports that the fluoride anion is bound to the boron
center in the Pt(II) compounds. The results of the fluoride
titration experiments further support that the lowest triplet
states of compounds 2 and 3 do not involve the BMes2 group.
The computational calculations were performed using the
Gaussian09, revision B.01,26 software package and the High Perform-
ance Computing Virtual Laboratory (HPCVL) at Queen’s University.
The ground-state geometries were fully optimized at the B3LYP27
level using the LANL2DZ basis set for platinum metal and the 6-
31G(d) basis set for all other atoms.28 The initial geometric
parameters in the calculations were employed from crystal structure
data for geometry optimization. TD-DFT calculations were performed
to obtain the vertical singlet and triplet excitation energies.
Synthesis of Pt(II) Complexes. The starting material
[PtMe2(SMe2)]2 (100 mg, 0.174 mmol) and the m-Bppy ligand
(140 mg, 0.348 mmol) were dissolved in THF (15 mL), and this
mixture was stirred for 2 h at room temperature. 2-Picolinic acid (64
mg, 0.522 mmol) [or 4-methyl-2-picolinic acid] dissolved in THF (10
mL) solution was added to this mixture. The reaction mixture was
further stirred for 48 h at the same condition. After removal of the
solvent, the resulting mixture was extracted with dichloromethane,
washed with water, dried over magnesium sulfate, and filtered. The
filtrate was concentrated under reduced pressure and purified by
column chromatography on silica gel, eluting with ethyl acetate,
obtaining the Pt(II) compounds in 70−72% yields as pale yellow
solids.
CONCLUSIONS
■
We have shown that the replacement of an acac ancillary ligand
by a picolinate ligand has a dramatic impact on the
phosphorescent properties of the m-Bppy-based Pt(II)
compounds, leading to a red shift of the emission energy and
a substantial reduction of the emission quantum efficiency. A
TD-DFT computational study and electrochemical and fluoride
titration experiments consistently show that the low-lying
LUMO level introduced by the picolinate ligand is the primary
cause of this phenomenon, which results in no or little
contributions from the BMes2 group to the emissive states. The
phosphorescence of the (m-Bppy)Pt(picolinate) compounds is
mainly from charge transfer transitions from the phenyl ring of
the ppy to the picolinate. In addition, the picolinate Pt(II)
compounds display a greater tendency of intermolecular π-
stacking, compared to the acac Pt(II) analogues. Thus, the
picolinate ligand is not a suitable ancillary ligand in order to
achieve efficient blue phosphorescent Pt(II) compounds based
on BMes2-functionalized ppy and the related chelate ligand.
However, it may be used in achieving efficient orange or red
phosphorescent BMes-N∧C-chelate Pt(II) systems where the
N∧C-chelate ligand has a low-lying π* level that is below that of
the picolinate. The introduction of an R group on the pyridyl
ring of the picolinate ligand is an effective strategy to minimize
intermolecular stacking interactions of the Pt(II) compounds.
(m-Bppy)Pt(pic) (2). Compound 2 was obtained in 72% yield as a
yellow solid (180 mg, 0.250 mmol) from 2-picolic acid. 1H NMR (400
MHz, CDCl3, 25 °C, TMS): δ 9.14 (d, 3J = 5.6 Hz, 1H), 9.10 (d, 3J =
6.0 Hz, 1H), 8.13 (td, 3J = 6.8 Hz, 4J = 0.8 Hz, 1H), 8.07 (td, 3J = 7.2
4
3
4
Hz, J = 0.8 Hz, 1H), 7.73 (td, J = 7.6 Hz, J = 0.8 Hz, 1H), 7.56−
7.53 (m, 2H), 7.50 (d, 3J = 8.0 Hz, 1H), 7.43 (d, 3J = 8.0 Hz, 1H), 7.28
3
3
4
(d, J = 8.0 Hz, 1H), 7.08 (td, J = 6.8 Hz, J = 0.8 Hz, 1H), 6.77 (s,
4H), 2.25 (s, 6H), 1.98 (s, 12H). 13C NMR (100 MHz, CDCl3, 25 °C,
TMS): δ 171.9, 166.5, 154.3, 150.0, 148.8, 146.3, 144.2, 141.6, 141.2,
140.8, 139.4, 138.5, 138.4, 131.8, 131.7, 128.2, 127.7, 122.0, 118.8,
23.6, 21.3. Anal. Calcd (%) for C35H33BN2O2Pt: C 58.42, H 4.62, N
3.89. Found: C 58.18, H, 4.71, N 3.72.
(m-Bppy)Pt(Me-pic) (3). Complex 3 was prepared in 70% yield as a
yellow solid (179 mg, 0.243 mmol) from 4-methyl-2-picolinic acid. 1H
3
NMR (400 MHz, CDCl3, 25 °C, TMS): δ 9.12 (d, J = 5.6 Hz, 1H),
8.96 (d, 3J = 5.6 Hz, 1H), 7.97 (d, 4J = 1.2 Hz, 1H), 7.74 (td, 3J = 7.6
Hz, 4J = 1.2 Hz, 1H), 7.58 (s, 1H), 7.50 (d, 3J = 8.0 Hz, 1H), 7.44 (d,
3J = 8.0 Hz, 1H), 7.34 (dd, 3J = 5.6 Hz, 4J = 1.6 Hz, 1H), 7.28 (dd, 3J =
7.6 Hz, 4J = 1.2 Hz, 1H), 7.06 (td, 3J = 6.0 Hz, 4J = 0.8 Hz, 1H), 6.77
(s, 4H), 2.42 (s, 3H), 2.26 (s, 6H), 2.02 (s, 12H). 13C NMR (100
MHz, CDCl3, 25 °C, TMS): δ 178.2, 166.5, 153.6, 152.1, 149.9, 148.1,
146.3, 144.3, 141.6, 140.8, 139.3, 138.5, 138.4, 131.8, 129.1, 128.3,
128.2, 122.0, 118.7, 23.6, 21.6, 21.3. Anal. Calcd (%) for
C36H35BN2O2Pt: C 58.94, H 4.81, N 3.82. Found: C 58.60, H 4.85,
N 3.65.
EXPERIMENTAL SECTION
■
General Information. All reactions were carried out under a dry
nitrogen atmosphere using standard Schlenk techniques. All starting
materials were purchased from either Aldrich Chemical Co. or Strem
Chemicals, Inc. and used without further purification. 1H and 13C
NMR spectra were recorded on a Bruker Avance 400 MHz
spectrometer. UV−vis spectra were obtained on a Varian Cary 50
UV−visible spectrophotometer with all sample concentrations in the
range of 10 μM. Excitation and emission spectra were recorded on a
Photon Technologies International QuantaMaster model 2 spectrom-
eter. Photoluminescent lifetimes were measured on a Photon
Technology International Phosphorescent lifetime spectrometer,
Timemaster C-631F equipped with a xenon flash lamp, and digital
emission photon multiplier tube for both excitation and emission. All
solutions for photophysical experiments were degassed under a
nitrogen atmosphere. Solution quantum yields were calculated using
optically dilute solutions (A ≈ 0.1), using 9,10-diphenylanthracene (Φ
X-ray Crystallographic Analysis. Single crystals of 2 and 3 were
mounted on glass fibers and were collected on a Bruker Apex II single-
crystal X-ray diffractometer with graphite-monochromated Mo Kα
radiation, operating at 50 kV and 30 mA and at 180 K. Data were
processed on a PC with the aid of the Bruker SHELXTL software
package (version 6.10)29 and corrected for absorption effects.
Compounds 2 and 3 belong to the monoclinic crystal space group
P2 /c and the triclinic space group P1, respectively. The crystal of 2
̅
1
contains disordered hexane solvent molecules (0.5 hexane per
molecule of 2), which could not be modeled successfully. As a result,
the contributions from the disordered solvent molecules were
removed by the Platon Squeeze routine.30 All non-hydrogen atoms
were refined anisotropically. Complete crystal structure data can be
found in the Supporting Information. The crystal data of 2 and 3 have
been deposited at the Cambridge Crystallographic Data Center
(CCDC 910735 and 910736).
19
= 0.950)18 as the standard for m-Bppy and fac-Ir(ppy)3 for the
platinum complexes in degassed CH2Cl2 at 298 K. Cyclic voltammetry
was performed using a BAS CV-50W voltammetric analyzer with scan
rates of either 50 or 100 mV/s. The electrolytic cell used was a
conventional three-compartment cell, with a Pt working electrode, Pt
wire auxiliary electrode, and Ag/AgCl reference electrode. The CV
measurements were performed at room temperature using 0.10 M
NBu4PF6 in DMF as the supporting electrolyte. The ferrocenium/
ferrocene couple (Eo = 0.55 V) was used as the internal standard. The
starting materials (3-bromophenyl)dimesitylborane,8a m-Bppy,8a bis-
[dimethyl(μ-dimethylsulfide)platinum(II)],25 and (m-Bppy)Pt(acac)
(1)11 were synthesized according to literature procedures.
ASSOCIATED CONTENT
■
S
* Supporting Information
NMR spectra, electrochemical data, fluoride titration data,
crystal structural data, and TD-DFT computational data. This
H
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Organometallics
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material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: wangs@chem.queensu.ca.
■
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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We thank the Natural Sciences and Engineering Research
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