Synthesis, structure, photophysics, and a DFT study of phosphorescent C*N∥N- and C∥N∥N-coordinated platinum complexes

Article abstract of DOI:10.1021/ic400732g

The reaction of N,N-diphenyl-2,2′-bipyridin-6-amine (L1) and N,N-diphenyl-6-(1H-pyrazol-1-yl)pyridin-2-amine (L2) with K₂PtCl ₄ produced C*N^∥N-coordinated cycloplatinated compounds with a five-six fused metallacycle 1a and

Full text of DOI:10.1021/ic400732g

Article
pubs.acs.org/IC
Synthesis, Structure, Photophysics, and a DFT Study of
Phosphorescent C*N∧N- and C∧N∧N‑Coordinated Platinum
Complexes
Caleb F. Harris,†,§ Dileep A. K. Vezzu,† Libero Bartolotti,† Paul D. Boyle,‡,∥ and Shouquan Huo*,†
†Department of Chemistry, East Carolina University, Greenville, North Carolina 27858, United States
‡Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States
S
* Supporting Information
ABSTRACT: The reaction of N,N-diphenyl-2,2′-bipyridin-6-
amine (L1) and N,N-diphenyl-6-(1H-pyrazol-1-yl)pyridin-2-
amine (L2) with K2PtCl4 produced C*N∧N-coordinated
cycloplatinated compounds with a five−six fused metallacycle
1a and 2a, respectively, which were then converted into their
phenylacetylide derivatives 1b and 2b, respectively. Similar
reactions starting from 2-phenyl-6-(1H-pyrazol-1-yl)pyridine
(L3) produced C∧N∧N-coordinated platinum complexes 3a
and 3b with a five−five-fused metallacycle. The structures of
1a, 1b, 2b, 3a, and 3b were determined by X-ray
crystallography. The C*N∧N-coordinated platinum complexes
are closer to a square geometry, whereas the C∧N∧N-coordinated complexes display a nearly perfect planar geometry. The π···π
interactions were revealed in the crystal packing for 1a, 2b, and 3a with a π···π contact of 3.450, 3.422, and 3.414 Å, respectively.
Two conformers were revealed in the crystal structure of 2b, one with the phenyl ring of the phenylacetylide being approximately
parallel with the coordination plane and the other with the phenyl ring being approximately perpendicular to the coordination
plane. Both 1a and 1b are weakly emissive in the red region. Complexes 2a and 3a are also weakly emissive, but their acetylide
derivatives 2b and 3b emitted strongly green light at room temperature with quantum yields of 43 and 62%, respectively. DFT/
TDDFT calculations were performed to elucidate the nature of their electronic transitions. The calculations suggested that lowest
singlet and triplet excited states are characteristic of a mixed state involving one or more charge-transfer transitions such as ILCT,
MLCT, and LLCT.
phosphorescent emitters. Other examples of phosphorescent
cyclometalated platinum complexes with a fused five−six
metallacycle have also been reported.8 In the tridentate
C∧N*N-4 and N∧C*N-coordinated5 platinum complexes, the
use of the amine linker, specifically the aniline linker, effectively
prevented the formation of excimers or aggregates that could
cause the self-quenching of the emission. This can be reasoned
by the perpendicular orientation of the N-phenyl ring with
respect to the coordination plane, which makes the
intermolecular interaction more difficult. The absence of the
self-quenching can be advantageous when the emitter is used as
a triplet dopant in an OLED device where the emitter is doped
into the host at relatively high concentration.8c The photo-
luminescence quantum efficiencies of the N∧C*N-coordinated5
platinum complexes are comparable to those of highly emissive
N∧C∧N-coordinated platinum complexes,9 while C∧N*N-
coordinated4 platinum complexes displayed higher efficiency
than those of more conventional C∧N∧N-coordinated10
complexes. One explanation for the better emission efficiency
INTRODUCTION
■
Phosphorescent materials based on cyclometalated platinum
complexes have recently attracted a great deal of attention
because of their potential in chemical,1 biological,2 and
optoelectronic applications, particularly as phosphorescent
emitter in OLED (organic light-emitting diode) devices.3
Ligand design remains a main tool of manipulating the
photophysical properties, especially the phosphorescence
efficiency of the complexes for those applications. Recently,
we have developed a series of highly efficient phosphorescent
cyclometalated platinum complexes with a fused five−six-
membered metallacycle, which include tridentate C∧N*N-4 and
N∧C*N-coordinated5 platinum complexes and tetradentate
C∧N*N∧C-,6 N∧C*C∧N-,6 and C∧C*N∧N-coordinated7 plati-
num complexes, where X∧Y and X*Y (X, Y = C, or N)
represents a bidentate coordination to the metal center through
a five-membered metallacycle and through a six-membered
metallacycle, respectively. The ligand design involves the use of
an amine linker to extend the more conventional five-
membered metallacycle into a six-membered ring. These
complexes have displayed generally high photoluminescence
quantum yields, and some of them are among the brightest
Received: March 25, 2013
Published: October 2, 2013
© 2013 American Chemical Society
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Chart 1. Schematic Representation of N∧C∧N, C∧N∧N, N∧C*N, C∧N*N, and C*N∧N-Coordinated Platinum Complexes
Scheme 1. Synthesis of the Ligands and the Complexes
a
Reagents and conditions: (a) Pd(dba)2 (4%), DPPF (4%), NaOtBu (1.2 equiv), toluene, reflux; (b) K2PtCl4 (1 equiv), AcOH, reflux; (c)
phenylacetylene, CuI, triethylamine, dichloromethane, room temperature; (d) CuI (10%), trans-N,N′-dimethylcyclohexanediamine (25%), K2CO3
(2.1 equiv), toluene, reflux.
is based on the geometrical change of the coordination from a
fused five−five-membered metallacycle to a less strained five−
six-membered ring; as a result, the square geometry for a
platinum(II) complexes could be achieved through the increase
of the biting angle from 160° in the C∧N∧N and N∧C∧N-
coordinated complexes to greater than 170° in the C∧N*N and
N∧C*N-coordinated platinum complexes. Such explanation has
been proposed for other complexes particularly for the
complexes with a fused six-six-membered metallacycle.11
More recently, the adverse effect of a geometrical change
from five−five-membered to six-six-membered ring was
reported on the platinum complexes.11a
In order to substantiate the effect of the geometry change
from five−five to five−six-membered metallacycle on the
photophysical properties, it is necessary to consider all possible
variations of coordination patterns of cyclometalated platinum
complexes that form a five-six-membered metallacycle through
coordinating to one carbon and two nitrogen coordinating
atoms, namely C∧N*N, N∧C*N, and C*N∧N-coordinated
complexes as shown in Chart 1. Herein we report the synthesis,
structure, and photophysical properties of C*N∧N-coordinated
platinum complexes, which are also compared with the
corresponding C∧N∧-coordinated complexes.
RESULTS AND DISCUSSION
■
Synthesis. Synthesis of the ligands and complexes is shown
in Scheme 1. L1 was prepared by the palladium-catalyzed C−N
cross-coupling reaction12 of diphenylamine with 6-bromo-2,2′-
bipyridine in 89% yield. Complexation of L1 with K2PtCl4 in
acetic acid under reflux gave the complex 1a in 74% yield,
which was converted to 1b in 95% yield by reacting with
phenylacetylene in the presence of copper iodide and
triethylamine. Ligand L2 was prepared by sequential
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Figure 1. Perspective drawing of 1a and 1b (molecule A), 2b (molecules A and B), and 3a and 3b (molecule A). Ellipsoids are at the 50% probability
level. The π···π contacting distances between two molecules in the crystal packing are shown on the right for 1a, 1b, 2b, and 3a. There was no π···π
interaction in the crystal packing of 3b.
palladium-12 and copper-catalyzed13 C−N bond formation
reactions. The preparation of ligand L3 was achieved by the
copper-catalyzed cross coupling13 of 2-bromo-6-phenylpyridine
with pyrazole. Complexes 2a, 2b, 3a, and 3b were prepared in
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Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1a, 1b, 2b, 3a, and 3b
complex 1a
complex 1b
complex 2b
Pt(1)−C(1)
Pt(1)−N(2)
Pt(1)−N(3)
Pt(1)−Cl(1)
N(1)−C(7)
N(1)−C(6)
N(1)−C(17)
N(2)−Pt(1)−Cl(1)
C(1)−Pt(1)−N(3)
N(2)−Pt(1)−N(3)
C(1)−Pt(1)−Cl(1)
C(1)−Pt(1)−N(2)
N(3)−Pt(1)−Cl(1)
1.996(2)
1.9993(19)
2.091(2)
2.3140(6)
1.377(3)
1.429(3)
1.451(3)
171.97(6)
174.85(9)
81.47(8)
94.26(7)
93.74(9)
90.51(6)
Pt(1)−C(1A)
Pt(1)−N(2A)
Pt(1)−-N(3A)
Pt(1)−C(23A)
N(1A)−C(7A)
N(1A)−C(6A)
N(1A)−C(17A)
C(23A)−Pt(1)−N(2A)
C(1A)−Pt(1)−N(3A)
C(1A)−Pt(1)−N(2A)
C(23A)−Pt(1)−N(3A)
C(23A)−Pt(1)−C(1A)
N(2A)−Pt(1)−N(3A)
1.988(3)
2.040(2)
Pt(1)−C(1A)
Pt(1)−N(2A)
Pt(1)−N(4A)
Pt(1)−C(21A)
N(1A)−C(7A)
N(1A)−C(6A)
N(1A)−C(15A)
C(21A)−Pt(1)−N(2A)
C(1A)−Pt(1)−N(4A)
C(1A)−Pt(1)−N(2A)
C(21A)−Pt(1)−N(4A)
C(21A)−Pt(1)−C(1A)
N(2A)−Pt(1)−N(4A)
1.976(9)
2.043(7)
2.078(8)
1.946(11)
1.393(12)
1.451(11)
1.459(11)
2.071(2)
1.950(3)
1.381(3)
1.442(3)
1.456(3)
173.64(10)
172.27(10)
93.67(10)
93.99(10)
92.06(11)
80.53(9)
170.7(4)
171.4(3)
93.6(3)
93.3(4)
93.9(4)
79.7(3)
complex 3a
complex 3b
Pt(1)−C(1)
Pt(1)−N(1)
Pt(1)−N(3)
Pt(1)−Cl(1)
C(1)−Pt(1)−N(3)
N(1)−Pt(1)−Cl(1)
N(1)−Pt(1)−N(3)
N(1)−Pt(1)−C(1)
C(1)−Pt(1)−Cl(1)
N(3)−Pt(1)−Cl(1)
1.989(2)
Pt(1)−C(1A)
1.985(9)
1.985(8)
2.098(8)
1.950(10)
159.8(3)
175.7(3)
78.0(3)
1.9539(15)
2.0986(18)
2.3101(9)
160.86(6)
177.76(4)
79.13(5)
Pt(1)−N(1A)
Pt(1)−N(3A)
Pt(1)−C15(A)
C(1A)−Pt(1)−N(3A)
C(15A)−Pt(1)−N(1A)
N(1A)−Pt(1)−N(3A)
N(1A)−Pt(1)−C(1A)
C(15A)−Pt(1)−C(1A)
C(15A)−Pt(1)−N(3A)
81.73(6)
81.8(4)
99.92(5)
98.3(4)
99.22(4)
101.8(3)
similar ways to those described for 1a and 1b, respectively. All
ligands and complexes have been characterized by proton and
carbon-13 NMR spectroscopy and satisfactory elemental
analysis.
coordination plane with a dihedral angle of 81.8°. A π···π
interaction was found in the crystal packing, and the distance
between the coordination planes of the adjacent two molecules
was measured to be 3.450 Å. The stacking apparently takes a
head to tail orientation of the two molecules so that they attract
with each other through electron-deficient pyridine ring and
electron-rich cyclometalated phenyl ring and minimize the
steric effect of the perpendicular N-phenyl ring. There is no
Pt···Pt interaction.
The complex 1b crystallized as orange plates with the solvent
molecule of dichloromethane in the crystal packing. There are
two independent molecules in the unit cell. Molecule A (Figure
1) shows nearly square but distorted planar geometry with the
cyclometalated N-phenyl ring slightly twisted out of the
coordination plane. The other molecule (B) has more distorted
coordination geometry than the A; specifically the cyclo-
metalated N-phenyl ring in the molecule is more twisted out of
the platinum-bipyridine coordination plane. In both molecules
A and B, the ring of the phenylacetylide is only slightly twisted
relative to the coordination plane. No significant π···π or Pt···Pt
interaction could be detected.
In the unit cell of the complex 2b, there are also two
independent molecules. Both molecules display nearly planar
coordination geometry; but the orientation of the phenyl ring
of the phenylacetylide is quite different. In molecule B (Figure
1), the phenyl ring of the phenylacetylide ring is significantly
more twisted forming a dihedral angle of 73.4° with the
coordination plane, while the corresponding angle in the other
molecule (molecule A) is only 24.2°. It is interesting to note
that a relatively strong π···π interaction (3.422 Å) exists
between two of the molecule B. Since both the N-phenyl group
and the phenylacetylide ring are nearly perpendicular to the
interacting coordination plane, they have to bend away from
the plane (Figure 1, 2b (molecule B)). This crystal packing
X-ray Crystal Structures. The molecular structures of the
complexes 1a, 1b, 2b, 3a, and 3b were determined by X-ray
crystallography. The structures are shown in Figure 1. The
crystal data and structure refinement details are provided in the
Supporting Information (Table S1), and selected bond lengths
and angles are listed in Table 1. The lengths of the Pt−C and
Pt−N bonds are comparable to those found in other similar
cyclometalated platinum complexes. The Pt−C (acetylide)
bonds in 1b (1.950(3) Å), 2b (1.946(11) Å), and 3b
(1.950(10) Å) are considerably shorter than the Pt−C (aryl)
bonds (1.976(9)−1.988(3) Å). Generally, the strength of
bonds formed between platinum and different types of carbons
increases in the order of sp3, sp2, and sp carbons, which is also
observed in our recent study of cyclometalated platinum
complexes.4,14 The length of the Pt−N bonds varies with their
environments. In 1a and 3a, the Pt−N bond that is trans to a
Pt−C bond is significantly longer than the other Pt−N bond,
indicating a strong structural trans effect15 induced by the
carbon donor. In 1b, as the chloride ligand is replaced with an
acetylide, and the Pt−N bond trans to the sp carbon of the
acetylide becomes longer than that in 1a but is slightly shorter
than the Pt−N bond trans to the sp2 carbon of the metalated
phenyl ring, probably because an sp2 carbon is a stronger
donor. A similar trend is observed in 3a and 3b.
Complex 1a displays nearly square planar coordination
geometry. The trans C(1)−Pt(1)−N(3) bond angle is
174.85(9)°, a significant increase compared with that in the
C∧N∧N-coordinated complexes (160°);9 therefore, the angle
strain caused by the fused five−five-membered ring is relieved
in the C*N∧N-coordinated platinum complex. The N-phenyl
ring not being cyclometalated is nearly perpendicular to the
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Table 2. Photophysical Data for Complexes 1a, 1b, 2a, 2b, 3a, and 3b
a
b
298 K
77 K
d
τ0
ϕ
a
c
complex
λabs (nm) (ε, M−1 cm−1
)
λem (nm)
(μs)
(%)
λem (nm)
568, 611
562, 610
τ (μs)
solid state λem (nm) (τ, μs)
1a
1b
2a
2b
3a
3b
274 (38674), 387 (6682), 469 (4659)
280 (43400), 402 (6067), 468 (4216)
271 (33262), 333 (9984), 417 (4877)
276 (66222), 336 (13702), 415 (7811)
615
<0.5
<0.5
<0.5
8.6
0.1
0.4
5.9 565 (6.8), 604 (9.6)
615
7.5 600 (8.9)
523
<0.1
43
499, 534, 573
497, 534, 575
495, 532, 570
495, 532, 568
19.8 502 (7.8), 532 (8.7), 571 (8.7)
10.5 502 (9.6), 531 (8.3), 570
9.8 534 (9.4), 572 (10.2)
521
270 (17217), 305 (18150), 387 (3016), 496 (60)
504, 540
<0.5
5.2
3.5
277 (20070), 310 (13639), 355 (5093), 411 (4264), 497 (94) 511, 534
62
5.3 548 (10.3)
a
b
c
Measurements were carried out in a solution of dichloromethane. Measurements were carried out in a solution of 2-methyltetrahydrofuran. Solid
d
state spectra were measured with a pure powder sample at 298 K. Quantum yields were measured in deoxygenated dichloromethane with quinine
sulfate in 0.1 N H2SO4 (ϕ = 0.55) as the reference.
feature suggests considerable flexibility of these C*N∧N-
coordinated platinum complexes.
The tridentate C∧N∧N-coordinated platinum complexes 3a
and 3b display nearly perfect planarity; but they deviates
significantly from the square arrangement of the coordinating
groups as indicated by small biting angles of C(1)−Pt(1)−
N(3) (160.86°) and C(1A)−Pt(1)−N(3A) (159.8°) in 3a and
3b, respectively. There are two independent molecules in the
unit cell of 3b and both have very similar geometry. In the
molecule A, the platinum and the cyclometalating ligand forms
the coordination plane and the largest deviation of atoms from
the calculated mean plane is 0.08 Å. The acetylide is not
coplanar with the platinum and the C∧N∧N ligand with
Figure 2. Absorption spectra of 1a, 1b, 2a, and 2b in dichloromethane.
deviations of 0.168 and 0.374 Å for the two sp carbons,
respectively, while the corresponding deviations in molecule B
are 0.293 and 0.556 Å. Interestingly, in both of the molecules A
and B, the phenyl ring of the phenylacetylide is nearly
perpendicular to the coordination plane. The dihedral angle is
86.73° in molecule A and 85.28° in molecule B. A π···π
interaction was found in the crystal packing of 3a and the
distance between the coordination planes of two adjacent
molecules was measured to be 3.414 Å. The stacking takes a
head to tail orientation and the two molecules attracts with
each other through interaction of the electron-deficient moiety
and the electron-rich moiety. There is no Pt···Pt interaction.
In 1a, 1b, and 2b, the amino nitrogen forms strongest bond
with the carbon of the N-pyridine ring, N(1)−C(7), and
weakest bond with the carbon of the N-phenyl ring. This strong
bonding may be attributed to the strong donor−acceptor
conjugation between the lone pair of the nitrogen and the
electron-deficient heteroaromatic ring assembly. The conjuga-
tion between the nitrogen and the phenyl is disrupted by the
orthogonal relationship between the lone pair and the π orbitals
of the phenyl ring. A small but steady increase in N(1)−C(7)
bond length from 1a → 1b → 2b (1.377 → 1.381 →1.393 Å)
might be related to the weakening of the electron acceptor by
introducing the donating phenylacetylide group and poorer
electron accepting pyrazolyl group.
spectra of the corresponding ligands can be assigned as charge
transfer bands. The lowest energy absorptions of 2a and 2b
display a significant blue-shift compared to those of 1a and 1b.
This can be attributed to the replacement of the pyridyl ring by
the pyrazolyl ring that is a poor electron acceptor.5,14 The
introduction of phenylacetylide does not cause drastic change
in the absorption spectra except for the enhancement in the
high energy region assigned to π−π* transitions.
The absorption of 3a and 3b showed some interesting
features (Figure 3). The two complexes showed different low
1
Photophysical Properties. The photophysical data
obtained for all complexes are listed in Table 2. The absorption
and fluorescence emission spectra of ligands L1−L3 are
provided in the Supporting Information (Figures S1 and S2).
Absorption. The absorption spectra of the complexes 1a, 1b,
2a, and 2b are shown in Figure 2. All spectra show strong
absorptions in the high-energy region and weaker absorptions
in the low-energy region. The high-energy intense absorptions
can be assigned as ligand based1π−π* transitions, while the low
energy absorptions that are not present in the absorption
Figure 3. Absorption spectra of 3a and 3b. Inserted are expanded
spectra (×100) showing the lowest energy triplet absorptions.
energy absorption bands. The less intense absorption at around
410 nm for the complex 3b is apparently due to the
introduction of phenylacetylide, while the lowest energy singlet
absorption for 3a is 387 nm. When expanding the absorption
spectra in the low energy region, very weak absorptions at 496
(60 M−1 cm−1) and 497 nm (94 M−1 cm−1) were found for 3a
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and 3b, respectively. These weak absorptions might be assigned
as triplet ligand-centered transitions. Such triplet absorptions
were not observed in the spectra of 1a, 1b, 2a, and 2b. Triplet
ligand-centered absorptions are not frequently reported; but
have been observed for tridentate N∧C∧N-coordinated10a and
tetradentate C∧N*N∧C-coordinated6 platinum complexes.
Solvent effects on the absorption spectra of the six complexes
have been examined and the shifts of low energy absorption
bands with the solvents are listed in Table S2 (Supporting
Information). The low energy absorptions showed remarkable
solvent dependence and shifted to the higher energy by up to
47 nm (2066 cm−1) for 1a as the polarity of the solvents
increases from toluene to acetonitrile, indicating that these low
energy transitions possess significant charge transfer character.
Ambient Emission in Fluid. At room temperature, both
complex 1a and 1b displayed a weak and structureless emission
in the red region (Figure 4). Although the complex 2a with a
Concentration quenching (self-quenching) was observed for
both 2b and 3b in the range of 10−6 to 10−4 M. The intensity of
the emission for both 2b and 3b decreases gradually as the
concentration of the complexes increases. The concentration
quenching may be resulted from the excimer formation rather
than aggregation of the molecule at its ground state since the
absorptions of the complexes obeyed the Beer’s law in the
range of 10−6 to 10−4 M. The observed decay rate was fit to a
Stern−Volmer equation (eq 1),20 where kq is the self-
quenching (excimer formation) rate constant, [Pt] is the
concentration of the platinum complex, and k0 is the decay rate
for the monomer complex.
kobs = kq[Pt] + k0
(1)
The self-quenching rate constants kq were 7.4 × 107 and 2.3
× 109 M−1s−1 for 2b and 3b, respectively. The quenching
constant of 3b is well within the range reported for other
similar emissive platinum complexes.20,6 The quenching
constant for 2b is much smaller than that of 3b, which may
be attributed to the fact that 2b is not as planar as 3b, making it
more difficult to form the excimer or the aggregates through a π
interaction. As revealed in the crystal packing of 2b, the π−π
interaction accompanies further geometric distortion of the
molecules. The monomer emission lifetime (τ0) was extrapo-
lated to be 8.6 and 5.2 μs (1/k0) for 2b and 3b, respectively.
The long lifetime suggest a ligand-centered transition state with
a metal-to-ligand charge transfer (MLCT) admix that promotes
the phosphorescent emission.
The radiative and nonradiative rate constants were estimated
from the luminescent quantum yield (ϕ) and lifetime (τ) of the
phosphorescent complex according to eqs 2 and 3, respectively
kr = ϕτ−1
(2)
Figure 4. Emission spectra of 1a, 1b, 2b, 3a, and 3b in
dichloromethane at 298 K.
knr = kr(ϕ−1 − 1)
(3)
chloride ligand was only weakly emissive (spectrum not shown
in Figure 4), its phenylacetylide derivative 2b was strongly
emissive and emitted bright green light with a quantum yield of
43%. The acetylide ligand is a much stronger donor than the
chloride, therefore inducing a stronger ligand field that raises
the nonradiative MC state to higher energy.16 Much weaker
emission displayed by 1a and 1b may be explained by
considering the energy gap law that rules the lower efficiency
for the lower energy emission. As a comparison to the C*N∧N-
coordinated platinum complex 2a, the C∧N∧N-coordinated
complexes 3a was found to emit green light with an appreciable
quantum yield of 3.5%. When the chloride in 3a was replaced
with phenylacetylide, the quantum yield of 3b jumped to 62%.
It should be mentioned that the phosphorescence efficiency of
C∧N∧N type of cyclometalated platinum complexes has been
reported to be generally low10,17 (<10%). Only in the cases
where extended π-conjugation was introduced to the cyclo-
metalating ligand were higher quantum yields reported to be
attributed to the rigid structure of extended π-conjugation.18 It
has also been suggested that the geometric distortion of the
triplet state of the C∧N∧N-coordinated platinum complex based
on 6-phenyl-2,2′-bipyridyl ligand is the main cause of its low
quantum efficiency.19 Higher quantum efficiency displayed by
3b may be attributed to the more rigid fused five−five-
membered metallacycle and minimal geometric distortion of
the triplet state.
where kr and knr are the radiative and nonradiative rate
constant, respectively. The radiative constants for 2b and 3b
were 4.99 × 104 and 1.3 × 105 s−1, respectively. The
nonradiative decay constants for 2b and 3b were 6.6 × 104
and 6.1 × 104 s−1, respectively. Since both 2b and 3b have
similar nonradiative decay rate constants, the higher quantum
efficiency of 3b is due to its fast radiative decay of the excited
state.
The solvents had very small and uncharacteristic effects on
the emissions of 1a, 1b, 2a, and 2b (Table S2, Supporting
Information). In contrast, the solvent effect on the emissions of
3a and 3b showed some noteworthy tends. In both cases, a
negative solvatochromic effect on the emission was observed,
namely a shift of emission maximum to high energy as the
polarity of the solvents increases; however the magnitude of the
solvatochromic shifts is drastically different and a much greater
solvatochromic shift was observed for 3b. When the solvents
were changed from toluene to acetonitrile, the emission
maximum was shifted from 511 to 505 nm for 3a (232 cm−1
shift) and from 521 to 499 nm for 3b (846 cm−1 shift).
Emission of 3a displays a highly structured spectrum with a
vibronic progression of 1322 cm−1, indicating an LC transition
state involving the aromatic cyclometalated ligand. The larger
solvatochromic shift and smoother band shape of 3b suggested
a transition state of more charge-transfer character that leads to
more significant dipole moment change during the excitation.
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The nature of the charge transfer can be either MLCT or
LLCT (acetylide to the pyrazolylpyridine receptor) or both.
Emission in Rigid Glass at 77 K. All complexes are emissive
in rigid glass at 77 K and estimated lifetimes of the emissions
are in the micro second regime, indicating that they are
phosphorescent emissions. All complexes displayed highly
structured spectra with vibronic progressions of about 1300−
1400 cm−1. The emission can be assigned as primarily LC states
based on the structured spectra and relatively long lifetimes
(5.3−19.8 μs). It is noteworthy that the emission spectra are
almost identical for 1a and 1b, 2a and 2b, or 3a and 3b,
respectively, indicating that the excited states are primarily
localized on the tridentate cyclometalated ligands (Figure 5).
Figure 6. Emission spectra of 1a−3a and 1b−3b in solid state (pure
powder).
Owing to the cylindrical geometry of the carbon−carbon triple
bond, a free rotation of the phenyl ring at ambient temperature
may be expected.
DFT Calculations. The selected molecular orbital energies
and the contribution of platinum to the MOs are provided in
Supporting Information (Table S3). The orbital densities of
selected MOs are provided in the Supporting Information
(Figures S5 and S6). The orbital densities for the HOMO and
LUMO are depicted in Figure 7. The HOMOs of 1a, 2a, and
3a are mainly localized on the platinum and metalated phenyl
ring. The HOMOs of 1b-A, 1b-B, and 2b-A are spread over the
metalated phenyl ring, the platinum, and the phenylacetylide,
whereas for 2b-B there is little contribution from the
phenylacetylide. The HOMOs of 3b-A and 3b-B are localized
on the phenylacetylide and the platinum. All structures have
their LUMOs localized on the electron-deficient heteroar-
omatic rings as expected.
The geometry of the lowest triplet state (T1) was also
optimized for 1a-3a and 1b-3b, which does not show any
significant change compared to the corresponding S0 geometry.
The root-mean-squares of change in geometry between
optimized singlet and triplet states for 1a, 1b-A, 1b-B, 2a,
2b-A, 2b-B, 3a, 3b-A, and 3b-B are 0.0099, 0.0267, 0.0004,
0.0015, 0.5956, 0.0425, 0.0011, 0.0033, and 0.0005 Å,
respectively, which is generally in line with the Huang−Rhys
ratios as discussed before. The selected molecular orbital
energies and the contribution of platinum to the MOs are
provided in the Supporting Information (Table S4). The orbital
densities for selected MOs are depicted in Figures S7 and S8
(Supporting Information). The spin densities of the triplet
states for 1a, 2a, 3a, 1b-A, 2b-A, and 3b-A were calculated and
their contour plots are shown in Figure 8. All structures but 3b-
A have the spin density distributed mostly in the tridentate
ligand with a smaller portion on the platinum, indicative of a
ligand-centered state. In 3b-A, considerable spin density in the
phenyl acetylide indicates a significant ligand-to-ligand charge
transfer (LLCT, phenylacetylide to tridentate ligand) character.
Time-Dependent DFT Calculations. TDDFT calculations
were performed to gain insightful understanding of the nature
of the transitions of complexes. Selected singlet−singlet
transitions are listed in Table S5 (Supporting Information).
The lowest singlet transitions of 1a,b, 2a,b, and 3a,b are listed
in Table 3. The experimental lowest-energy absorptions are
included for comparison. Except for those of 3b-A and 3b-B,
the lowest singlet excited state for all other structures is mainly
Figure 5. Emission spectra of 1a−3a and 1b−3b in rigid glass at 77 K.
The monoanionic ligand chloride or phenylacetylide had little
effect on the LC states. The Huang−Rhys ratios (S = I1−0/I0−0
,
estimated from the relative peak intensities of vibrational
emission bands) of the emissions is smaller for 3a (0.45) and
3b (0.47) than those of 1a (0.6), 1b (0.56), 2a (0.61), and 2b
(0.58). A higher Huang−Rhys ratio suggests larger distortion of
the excited states. Larger rigidochromic shifts were observed for
1a (1345 cm−1), 1b (1533 cm−1), 2a (920 cm−1) and 2b (927
cm−1) than those of 3a (361 cm−1) and 3b (632 cm−1). Based
on the fact that the emissions of 1a and 1b in solution have
nearly identical energy and shape, they can be assigned as a
primarily LC state localized on the tridentate ligand.
Emission in solid state. All complexes are emissive in solid
state at ambient temperature and lifetimes of the emissions are
also in the micro second regime. The emissions of 1a, 1b, 2a,
and 2b are structured and very similar to their emission in rigid
glass except for more intensified vibrational bands (Figure 6).
The emissions of 3a and 3b displayed broad spectra and
showed red shift in the solid state, probably because of the
intermolecular interaction in the solid state.
Theoretical Calculations. In order to further understand
the nature of excited states and the emissions in different
environments, density functional theory (DFT) and time-
dependent DFT (TDDFT) calculations were performed on the
complexes 1a−3a and 1b−3b. Since the X-ray crystallography
revealed two conformational isomers in the crystals of 1b−3b,
the DFT calculations were performed on both conformer A, in
which the phenyl ring of the acetylide is coplanar with the
coordination plane, and conformer B with the phenyl group
being perpendicular. The conformer A was predicted to be
more stable than B, but the calculated energy difference
between the two conformers are very small (<1 kcal/mol).
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Figure 7. Calculated orbital density for the HOMO (magenta) and LUMO (orange) of 1a−3a and 1b−3b at their optimized S0 geometries.
Figure 8. Spin densities of the lowest triplet states (T1) of 1a−3a, 1b-A, 2b-A, and 3b-A.
derived from HOMO→LUMO transitions. It is interesting to
note that 3b has the lowest singlet state derived from mainly
HOMO-1→LUMO transition. According to the orbital density
of those MOs (Figure 7 and Figure S6 in Supporting
Information), the transitions can be assigned as a mixed state
involving one or more of the following charge transfer
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triplet absorptions and the phosphorescent emission maxima
are also provided as a comparison. All the triplet excited states
have multiple one-electron excitation configurations but only
the main components with the coefficient’s absolute value being
equal to or greater than 0.3 are listed in the table. According to
the orbital densities of the MOs (Figures S7 and S8, Supporting
Information), the first triplet state (T1) for 1a−3a can be
characterized as a LC state localized on the tridentate ligand
with an MLCT admixture. The first triplet state of 1b-A and
2b-A can be assigned as a mixed state involving ILCT, LLCT,
and MLCT, but that of 3b-A has much greater LLCT character,
which is very similar to recently reported alkynylplatinum
complexes based on tridentate N-hetereocyclic carbene
ligand.21 The T1 state for 1b-B, 2b-B, and 3b-B can be
assigned as a mixed state involving ILCT and MLCT, similar to
1a−3a. Although the calculated T1 energies of 3a and 3b are
very close to the experimental triplet absorptions and
phosphorescent emissions, those for 1a-b and 2a-b are
significantly overestimated compared to the observed emis-
sions. The assignment of room temperature emissions of 1b−
3b in solution can be complicated by the fact that the different
conformers resulted from the rotation of the phenyl ring of the
phenylacetylide ligand behavior differently. However, the
emissions 1b−3b in the frozen glass can be confidently
assigned as a LC state localized on the tridentate ligand as they
are nearly identical, both in energy and in shape, to the
emissions of 1a−3a, respectively, which implies that the
molecules 1b−3b in the rigid matrix might adopt the
perpendicular conformation B in which the participation of
the phenyl group of the acetylide is prohibited due to its
orthogonal relationship with the coordination plane.
Table 3. TD-DFT g09 m62x/def2-pvtz Lowest Singlet
Excitation States of 1a−3a and 1b−3b at Their Optimized
Geometries of the Ground States
main
energy, eV
(nm)
experiment λabs
compd state
component
oscillator
0.1321
(nm)
1a
S1 HOMO→
3.20 (388)
3.11 (398)
3.19 (389)
3.54 (350)
3.52 (352)
3.56 (348)
3.41 (363)
3.38 (366)
3.42 (362)
469
468
468
417
415
415
387
411
411
LUMO
1b-A
1b-B
2a
S1 HOMO→
0.1648
0.1438
0.1855
0.2320
0.2139
0.0149
0.0157
0.0086
LUMO
S1 HOMO→
LUMO
S1 HOMO→
LUMO
2b-A
2b-B
3a
S1 HOMO→
LUMO
S1 HOMO→
LUMO
S1 HOMO→
LUMO
3b-A
3b-B
S1 HOMO-1→
LUMO
S1 HOMO-1→
LUMO
transitions MLCT (platinum to tridentate ligand), ILCT
(tridentate ligand) and LLCT (phenylacetylide to tridentate
ligand). The order of computed energies (1a,b < 3a,b < 2a,b)
of S0→S1 transitions does not agree exactly with observed
lowest energy absorptions (1a,b < 2a,b < 3a,b) for 1a−3a and
1b−3b. The discrepancy between 2a,b and 3a,b might be
attributed to their different structural types that have different
sensitivity to the computational methods used. The calculated
lowest singlet energies for 1a,b and 2a,b are significantly
overestimated. It should also be pointed out that the 387 nm
may not be the true lowest energy absorption for 3a since there
is a shoulder at around 410 nm in the spectrum (Figure 3).
The calculated energies and main components for the lowest
triplet excited states are listed in Table 4. Experimental lowest
SUMMARY
■
The C*N∧N and C∧N∧N-coordinated platinum complexes
have been synthesized and their structural and photophysical
properties have been investigated. TDDFT calculations have
been carried out to elucidate the nature of excited states and the
origin of phosphorescence emissions. Both 2b and 3b emited
intensely bright green light at room temperature. With this
report, we have completed design, synthesis, structure, and
photophysical characterization of a series of cyclometalated
platinum complexes featuring in a fused five-six-membered
metallacycle, which are designed by introducing an amine linker
to more conventional C∧N∧N and N∧C∧N types of cyclo-
metalating ligands. The design allows many options in
engineering a ligand to achieve desired properties of the
cyclometalated complexes. The synthetic strategies introduced
in these studies, particularly the use of the combination of Pd-
catalyzed C−C and Pd or Cu-catalyzed C−N bond cross-
coupling reactions are generally applicable to the synthesis of
other similar ligands.
Table 4. TD-DFT g09 m62x/def2-pvtz Lowest Triplet
Excitation States of 1a−3a and 1b−3b at Their Optimized
Geometry of the Lowest Triplet State T1
exptl
λabs
(nm)
exptl
λem
(nm)
main
energy, eV
(nm)
compd state component
coeff
1a
T1 HOMO →
0.64
2.25 (552)
615
LUMO
1b-A
T1 HOMO-1 →
0.30
0.57
0.57
0.63
2.29 (542)
615
LUMO
HOMO →
LUMO
1b-B
2a
T1 HOMO →
2.29 (542)
2.57 (483)
2.64 (470)
615
523
521
LUMO
T1 HOMO →
Through these studies on the cyclometalated platinum
complexes based on tridentate CNN and NCN ligands, the
geometrical and electronic effects on the photophysical
properties, particularly the phosphorescence efficiency, can be
summarized as follows. The introduction of five-six-membered
metallacycle improves the square geometry of the platinum
complexes but may increase the flexibility of the molecules,
which has contradictory effects on the quantum efficiency. Such
geometrical modification alone seems insufficient to induce
such a large d orbital splitting that the nonradiative d−d
transition becomes thermally inaccessible, because the
(C∧N*N)PtCl and (C*N∧N)PtCl were found to be less
LUMO
2b-A
T1 HOMO-1 → −0.35
LUMO
HOMO →
0.53
0.61
0.63
0.57
0.60
LUMO
2b-B
3a
T1 HOMO →
2.60 (478)
2.42 (512)
2.56 (484)
2.69 (462)
521
504
511
511
LUMO
T1 HOMO→
496
497
497
LUMO
3b-A
3b-B
T1 HOMO →
LUMO
T1 HOMO-1 →
LUMO
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emissive than their corresponding (C∧N∧N)PtCl. More
importantly, the introduction of an acetylide ligand is still
necessary to achieve high efficiency. In this context, the
superior quantum efficiencies displayed by (N∧C∧N)PtCl and
(N*C∧N)PtCl over those of (C∧N∧N)PtCl, (C∧N*N)PtCl,
and (C*N∧N)PtCl are quite remarkable. On the other hand,
both N∧C∧N and N*C∧N-coordinated platinum complexes
display comparable and generally high quantum efficiency,
which indicates that the positive electronic effect of rectified
square coordination may offset the negative geometrical effect
of increased flexibility. Finally, the N∧C∧N9- and N*C∧N-
coordinated5 platinum complexes have demonstrated decent to
high phosphorescence efficiency in low energy emission beyond
green. In contrast, complexes based on C∧N∧N10,17,18 ligand
that emits efficiently in the orange to red region have not been
reported.
141.70 (2C), 139.86, 129.27 (4C), 127.09, 126.83 (4C), 125.00 (2C),
109.31, 107.18, 102.85. Anal. Calcd for C20H16N4: C, 76.90; H, 5.16;
N, 17.94. Found: C, 76.85; H, 5.14; N, 18.00.
Preparation of L3. This ligand was prepared from 2-bromo-6-
phenylpyridine (702.3 mg, 3 mmol) following general procedure A.
The product was purified by chromatography on silica gel with a
mixture of dichloromethane and hexanes (2:1 v/v to 4:1 v/v) to
1
provide a colorless solid, 298.7 mg, yield 45%: H NMR (500 MHz,
CDCl3) δ 8.73 (d, J = 2 Hz, 1H), 8.06 (dd, J = 8.5 Hz, 1.5 Hz, 2H),
7.92 (d, J = 7.5 Hz, 1H), 7.82 (t, J = 8.5 Hz, 1H), 7.76 (d, J = 1.5 Hz,
1H), 7.59 (dd, J = 7.5 Hz, 1 Hz, 1H), 7.50−7.40 (m, 3H), 6.47 (dd, J =
3 Hz, 1.5 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 155.19, 151.31,
142.00, 139.46, 138.39, 129.37, 128.77(2C), 127.10, 126.88 (2C),
117.68, 110.62, 107.61. Anal. Calcd for C14H11N3: C, 76.00; H, 5.01;
N, 18.99. Found: C, 76.04; H, 5.03; N, 19.09.
Preparation of Platinum Complex 1a. General Procedure B.
To a 50 mL dry, argon-flushed flask were charged ligand L1 (161.6
mg, 0.5 mmol), K2PtCl4 (207.6 mg, 0.5 mmol), and glacial acetic acid
(20 mL). The mixture was degassed and refluxed under argon for 22 h.
After the mixture was cooled to room temperature, the orange
precipitates were collected by filtration, washed with water, and dried
in air. The crude material was purified by flash chromatography on
silica gel with dichloromethane. The compound was further recrystal-
lized from dichloromethane and hexanes to give an orange solid, 204
EXPERIMENTAL SECTION
■
Synthesis. All reactions involving moisture- and/or oxygen-
sensitive organometallic complexes were carried out under nitrogen
atmosphere and anhydrous conditions. Tetrahydrofuran (THF) and 2-
methyltetrahydrofuran were distilled from sodium and benzophenone
under nitrogen before use. All other anhydrous solvents were
purchased from Aldrich Chemical Co. and were used as received.
All other reagents were purchased from chemical companies and were
used as received. Mass spectra were measured on a Waters
spectrometer. NMR spectra were measured on a Bruker 400 or a
Varian 500 spectrometer. Spectra were taken in CDCl3 or CD2Cl2
1
mg, yield 74%: H NMR (500 MHz, CD2Cl2) δ 9.94 (d, J = 6 Hz,
3
1H), 8.71 (dd, J = 7.5 Hz, 2 Hz, JPt−H = 27 Hz, 1H), 8.16−8.10 (m,
2H), 7.75−7.68 (m, 4H), 7.63−7.59(m, 2H), 7.38 (d, J = 7.55 Hz,
2H), 6.89−6.83 (m, 2H), 6.75 (d, J = 9 Hz, 1H), 6.37 (dd, J = 8 Hz, 2
Hz, 1H); 13C NMR was not obtained due to poor solubility. Anal.
Calcd. for C22H16N3ClPt: C, 47.79; H, 2.92; N, 7.60. Found: C, 48.02;
H, 2.81; N, 7.63.
1
using tetramethylsilane as standard for H NMR chemical shifts and
Complex 2a. This complex was prepared from L2 (78.1 mg, 0.25
mmol) following general procedure B. The crude material was purified
by flash chromatography on silica gel with dichloromethane. The
compound was further recrystallized from dichloromethane and
the solvent peak (CDCl3, 77.0 ppm; CD2Cl2, 53.8 ppm) as standard
for 13C NMR chemical shifts. Elemental analyses were performed at
Atlantic Microlab, Inc., Norcross, GA.
Preparation of L1. To a 25 mL flask were charged 6-bromo-2,2′-
bipyridine (470 mg, 2 mmol), diphenylamine (508 mg, 3 mmol),
Pd(dba)2 (46 mg, 0.08 mmol), DPPF (44 mg, 0.08 mmol), sodium
tert-butoxide (231 mg, 2.4 mmol), and toluene (5 mL), and the
mixture was refluxed for 24 h. After being cooled to room temperature,
the reaction mixture was poured into water, and the aqueous phase
was extracted with ethyl acetate (EtOAc) (3 × 35 mL). The combined
organic extracts were washed with water (50 mL) and then brine (50
mL) and dried over MgSO4. The mixture was filtered, concentrated in
vacuo, and purified by chromatography on silica gel with a mixture of
hexanes and ethyl acetate (7:1 v/v) to provide an off-white solid, 575
1
hexanes to give a yellow solid, 132.5 mg, yield 98%: H NMR (500
MHz, CD2Cl2) δ 8.58 (dd, J = 7 Hz, 3 Hz, 3JPt−H = 28 Hz, 1H), 8.32−
8.31 (m, 2H), 7.71−7.58 (m, 4H), 7.36−7.34 (m, 2H), 7.04 (d, J = 8
Hz, 1H), 6.86−6.83 (m, 3H), 6.38 (d, J = 9 Hz, 1H), 6.28−6.26 (m,
1H). 13C NMR (125 MHz, CD2Cl2) δ144.21, 143.64, 140.89, 139.04,
136.02, 134.22, 131.82(2C), 130.69(2C), 129.49, 128.45, 124.38,
118.05, 115.47, 114.85, 110.37, 100.54 (only partial carbon signals
were observed because of poor solubility). Anal. Calcd for
C20H15N4ClPt: C, 44.33; H, 2.79; N, 10.34. Found: C, 44.38; H,
2.62; N, 10.32.
1
Complex 3a. This complex was prepared from L3 (110.6 mg, 0.5
mmol) following general procedure B. The crude material was purified
by flash chromatography on silica gel with dichloromethane. The
compound was further recrystallized from dichloromethane and
mg, yield 89%: H NMR (400 MHz, CDCl3) δ 8.61(d, J = 4.75 Hz,
1H), 7.99 (dt, J = 8 Hz, 1 Hz, 1H), 7.92 (dd, J = 7.5 Hz, 0.7 Hz, 1H),
7.65 (td, J = 8.0 Hz, 1.8 Hz, 1H), 7.57 (t, J = 8.3 Hz, 1H), 7.37−7.33
(m, 4H), 7.28−7.25 (m, 4H), 7.22−7.19 (m, 1H), 7.18−7.13 (m, 2H),
6.73(dd, J = 8 Hz, 0.7 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ
158.04, 156.29, 154.05, 148.86 (2C), 145.95, 138.30, 136.79, 129.20
(4C), 126.52 (4C), 124.49 (2C), 123.40, 121.09, 113.30, 113.04. Anal.
Calcd. for C22H17N3: C, 81.71; H, 5.30; N, 12.99. Found: C, 81.76; H,
5.39; N, 12.89.
Preparation of L2. General Procedure A. To a 25 mL flask were
charged 6-bromo-N,N-diphenylpyridin-2-amine (218 mg, 0.7 mmol),
pyrazole (68 mg, 1 mmol), K2CO3 (207 mg, 1.5 mmol), CuI (13 mg,
0.07 mmol), trans-n,n′-dimethylcyclohexane-1,2-diamine (25 mg,
0.175 mmol), and toluene (5 mL). The mixture was refluxed for 24
h. After being cooled to room temperature, the reaction mixture was
poured into water, and the aqueous phase was extracted with ethyl
acetate (EtOAc) (3 × 30 mL). The combined organic extracts were
washed with water (50 mL) and brine (50 mL) and dried over MgSO4.
The mixture was filtered, concentrated in vacuo, and purified by
chromatography on silica gel with a mixture of hexanes and ethyl
acetate (7:1 v/v) to provide an off-white solid, 214 mg, yield 98%: 1H
NMR (500 MHz, CDCl3) δ 8.07 (d, J = 2.5 Hz, 1H), 7.65 (d, J = 1.5
Hz, 1H), 7.53 (t, J = 8.5 Hz, 1H), 7.40−7.33 (m, 5H), 7.27−7.25 (m,
4H), 7.18 (t, J = 7.5 Hz, 2H), 6.53 (d, J = 8.5 Hz, 1H), 6.30 (t, J = 2.5
Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 157.15, 149.66, 145.37,
1
hexanes to give a yellow solid, 132.1 mg, yield 57%: H NMR (500
MHz, CD2Cl2) δ 8.21 (d, J = 3 Hz, 1H), 7.94 (t, J = 8.5 Hz, 2H), 7.65
3
(d, J = 7.5 Hz, JPt−H = 30.5 Hz, 1H), 7.44−7.42 (m, 2H), 7.26−7.21
(m, 2H), 7.13 (td, J = 7.5 Hz, 1.5 Hz, 1H), 6.84 (t, J = 3 Hz, 1H). 13C
NMR was not obtained due to poor solubility. Anal. Calcd for
C14H10N3ClPt: C, 37.30; H, 2.24; N, 9.32. Found: C, 37.41; H, 2.27;
N, 9.28.
Preparation of Platinum Complex 1b. General Procedure C.
To a 25 mL dry, argon-flushed flask were charged complex 1a (127
mg, 0.23 mmol), phenylacetylene (76.9 μL, 0.7 mmol), CuI (3.5 mg,
0.018 mmol), Et3N (3 mL), and dichloromethane (20 mL). The
mixture was stirred under argon at room temperature for 27 h. The
crude material was purified by flash chromatography on silica gel with
dichloromethane to give a yellow solid, 135.7 mg, yield 95%: 1H NMR
(500 MHz, CD2Cl2) δ 10.11 (d, J = 5 Hz, 1H), 9.10 (dd, J = 7.5 Hz, 2
Hz, 3JPt−H = 39 Hz, 1H), 8.18 (d, J = 8 Hz, 1H), 8.11 (t, J = 8 Hz, 1H),
7.75−7.58 (m, 6H), 7.55 (d, J = 7.5 Hz, 2H), 7.38 (d, J = 8 Hz, 2H),
7.31 (t, J = 7.5 Hz, 2H), 7.19 (t, J = 8 Hz, 1H), 6.88 (t, J = 7 Hz, 1H),
6.83 (t, J = 7.5 Hz, 1H), 6.78 (d, J = 9 Hz, 1H), 6.44 (d, J = 8.5 Hz,
1H). 13C NMR (125 MHz, CD2Cl2) δ 156.81, 154.60, 150.99, 149.20,
145.26, 144.51, 140.43, 137.80, 135.12, 131.73 (2C), 131.67 (2C),
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131.01 (2C), 129.20 (2C), 128.36 (2C), 126.49, 125.43, 124.03,
122.41, 121.92, 120.75, 119.13, 118.66, 114.56, 100.43. Anal. Calcd for
C30H21N3Pt·0.5CH2Cl2: C, 55.42; H, 3.35; N, 6.36. Found: C, 55.67;
H, 3.25; N, 6.39.
The frame integration was performed using SAINT.28 The resulting
raw data was scaled and absorption corrected using a multiscan
averaging of symmetry equivalent data using SADABS29 for 1a, 1b,
and 3a and using TWINABS30 for 2b and 3b. The structure was
solved by direct methods using the XS program31 except for 2b for
which the SIR92 program32 was used. All non-hydrogen atoms were
obtained from the initial solution. The hydrogen atoms were
introduced at idealized positions and were allowed to ride on the
parent atom. The structural model was fit to the data using full matrix
least-squares based on F2. The calculated structure factors included
corrections for anomalous dispersion from the usual tabulation. The
structure was refined using the XL program from SHELXTL,33 and
graphic plots were produced using the NRCVAX crystallographic
program suite.
Complex 2b. This complex was prepared from 1b (124.6 mg, 0.23
mmol) following general procedure C. The crude material was purified
by flash chromatography on silica gel with dichloromethane to give a
1
yellow solid, 68 mg, yield 49%: H NMR (500 MHz, CD2Cl2) δ 8.93
(dd, J = 7 Hz, 2 Hz, 3JPt−H = 49.5 Hz, 1H), 8.39 (d, J = 2 Hz, 1H), 8.33
(d, J = 3 Hz, 1H), 7.71−7.64 (m, 3H), 7.60 (t, J = 7.5 Hz, 1H), 7.51
(d, J = 7.5 Hz, 2H), 7.36 (d, J = 7.5 Hz, 2H), 7.29 (t, J = 8 Hz, 2H),
7.18 (t, J = 7.5 Hz, 1H), 7.06 (d, J = 5 Hz, 1H), 6.87−6.80 (m, 2H),
6.78 (t, J = 2.5 Hz, 1H), 6.42 (d, J = 9 Hz, 1H), 6.36 (dd, J = 8 Hz, 1.5
Hz, 1H); 13C NMR (125 MHz, CD2Cl2) δ 177.95, 170.58, 148.71,
144.71, 144.21, 142.08, 136.61, 134.43, 134.37, 134.35, 131.85 (2C),
131.75 (2C), 130.82 (2C), 129.31, 128.50, 128.32 (2C), 125.41,
124.11, 122.21, 118.85, 116.53, 114.57, 110.48, 100.24. Anal. Calcd for
C28H20N4ClPt: C, 55.35; H, 3.32; N, 9.22. Found: C, 55.09; H, 3.31;
N, 9.15.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data (CIF) for 1a, 1b·CH2Cl2, 2b, 3a, and 3b.
Crystal data and refinement details for 1a, 1b·CH2Cl2, 2b, 3a,
and 3b. Electronic spectra of the ligands. Solvent effects. Stern−
Volmer plots for 2b and 3b. DFT calculation results. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Complex 3b. This complex was prepared from 1c (45.2 mg, 0.1
mmol) following general procedure C. The crude material was purified
by flash chromatography on silica gel with a mixture of dichloro-
methane and ethyl acetate (50:1 v/v) to give orange solid, 29 mg, yield
56%: 1H NMR (500 MHz, CD2Cl2) δ 8.21(d, J = 3 Hz, 1H), 8.03 (d, J
= 2 Hz, 1H), 7.96−7.92 (m, 2H), 7.51−7.45 (m, 4H), 7.30−7.26 (m,
3H), 7.25−7.05 (m, 3H), 6.75 (t, J = 2.5 Hz, 1H); 13C NMR (125
MHz, CD2Cl2) δ 148.37, 146.51, 144.67, 144.21, 140.85, 140.45,
138.83, 138.18, 134.23, 131.98 (2C), 131.67, 130.37, 128.33 (2C),
125.44, 125.24, 124.00, 115.84, 110.51, 106.32, 105.18. Anal. Calcd for
C22H15N3Pt: C, 51.16; H, 2.93; N, 8.14. Found: C, 51.45; H, 2.85; N,
8.25.
DFT Calculations. All geometry optimizations were performed
with DMol322 using the Tsuneda−Suzumura−Hirao23 exchange-
correlation energy-density functional. The base set was a double
numerical plus double polarization numerical basis set generated by us.
Relativistic effects were included using the all-electron-scattering
theoretic approach to scalar relativistic corrections on bonding.24 All
other calculations were performed with Gaussian 09 using the m062x
exchange-correlation energy-density functional. The basis set for Pt
was the def2-tzvp25 basis set while the 6-311g* basis set was used for
all other atoms. The solvation model COSMO26 was used to simulate
the solvent effect of dichloromethane (with a dielectric constant of
9.08) in the geometric optimization.
Photophysical Experiments. Absorption spectra were recorded
using a Shimadzu 2445 UV/vis spectrophotometer using 1 cm path-
length quartz cuvettes. The steady-state emission spectra were
measured using a PTI QM-4CW system, and an excitation source
with the spectral bandpass of 2 nm was used. Quantum yields were
measured using comparative method at room temperature in a
dichloromethane solution. The solution was deoxygenated by purging
with argon gas for 20 min. A long-necked 1 cm quartz cuvette was
used for measurements. The cuvette was cooled with a dry ice-acetone
bath to prevent solvent loss. The optical density of both sample and
reference solutions was maintained below 0.1 AU at and above the
excitation wavelength. A solution of quinine sulfate27 dihydrate in 0.1
N H2SO4 was used as a reference. Emission spectra in a frozen glass
were recorded in 2-MeTHF at 77 K. Solid state emission spectra were
recorded at room temperature using a powder sample holder.
Phosphorescence lifetime measurements were performed on the
same fluorimeter equipped with variable high rep rate pulsed xenon
source for excitation and were limited to lifetimes >0.4 μs. Delayed
phosphorescent emission spectra of the ligands were measured using a
gated analog detector with R928 red extended PMT in 2-MeTHF at
77 K.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: huos@ecu.edu.
Present Addresses
§Department of Chemistry, Georgia Institute of Technology,
Atlanta, GA 30332.
∥Department of Chemistry, Western University, London, ON
N6A 5B7. Canada.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.H. acknowledges financial support from the Research
Corporation for Science Advancement through a Cottrell
College Science Award. P.D.B. thanks the Department of
Chemistry of North Carolina State University and the State of
North Carolina for funding the purchase of the Apex2
diffractometer.
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